超文本傳輸協議版本 2

IETF HTTP2草案(draft-ietf-httpbis-http2-13)

摘要

本規範描述了一種優化的超文本傳輸協議(HTTP)。HTTP/2通過引進報頭字段壓縮以及多路複用來更有效利用網絡資源、減少感知延遲。另外還介紹了服務器推送規範。

本文檔保持對HTTP/1.1的後向兼容，HTTP的現有的語義保持不變。

1 介紹

The Hypertext Transfer Protocol (HTTP) is a wildly successful protocol. However, the HTTP/1.1 message format ([RFC7230], Section 3) was designed to be implemented with the tools at hand in the 1990s, not modern Web application performance. As such it has several characteristics that have a negative overall effect on application performance today.

超文本傳輸協議（HTTP）是一個非常成功的協議。但是HTTP/1.1 是針對90年代的情況而不是現代web應用的性能而設計的，導致它的一些特點已經對現代應用程序的性能產生負面影響。

In particular, HTTP/1.0 only allows one request to be outstanding at a time on a given connection. HTTP/1.1 pipelining only partially addressed request concurrency and suffers from head-of-line blocking. Therefore, clients that need to make many requests typically use multiple connections to a server in order to reduce latency.

特別是，HTTP/1.0只允許在一個連接上建立一個當前未完成的請求。HTTP/1.1管道只部分處理了請求併發和報頭阻塞的問題。因此客戶端需要發起多次請求通過數次連接服務器來減少延遲。

Furthermore, HTTP/1.1 header fields are often repetitive and verbose, which, in addition to generating more or larger network packets, can cause the small initial TCP [TCP] congestion window to quickly fill. This can result in excessive latency when multiple requests are made on a single new TCP connection.

此外，HTTP/1.1的報頭字段經常重複和冗長。在產生更多或更大的網絡數據包時，可能導致小的初始TCP堵塞窗口被快速填充。這可能在多個請求建立在一個新的TCP連接時導致過度的延遲。

This specification addresses these issues by defining an optimized mapping of HTTP's semantics to an underlying connection. Specifically, it allows interleaving of request and response messages on the same connection and uses an efficient coding for HTTP header fields. It also allows prioritization of requests, letting more important requests complete more quickly, further improving performance.

本協議通過定義一個優化的基礎連接的HTTP語義映射來解決這些問題。具體地，它允許在同一連接上交錯地建立請求和響應消息，並使用高效率編碼的HTTP報頭字段。它還允許請求的優先級，讓更多的重要的請求更快速的完成，進一步提升了性能。

The resulting protocol is designed to be more friendly to the network, because fewer TCP connections can be used in comparison to HTTP/1.x. This means less competition with other flows, and longer-lived connections, which in turn leads to better utilization of available network capacity.

最終協議設計爲對網絡更友好，因爲它相對HTTP/1.x減少了TCP連接。這意味着與其他流更少的競爭以及更長時間的連接,從而更有效地利用可用的網絡容量。

Finally, this encapsulation also enables more efficient processing of messages through use of binary message framing.

最後，這種封裝也通過使用二進制消息幀使信息處理更具擴展性。

2 HTTP / 2協議概述

HTTP/2 provides an optimized transport for HTTP semantics. HTTP/2 supports all of the core features of HTTP/1.1, but aims to be more efficient in several ways.

HTTP/2 提供了HTTP語義的傳輸優化。HTTP/2支持所有HTTP／1.1的核心特徵，並且在不同的方面做的更高效。

The basic protocol unit in HTTP/2 is a frame (Section 4.1). Each frame type serves a different purpose. For example, HEADERS and DATA frames form the basis of HTTP requests and responses (Section 8.1); other frame types like SETTINGS, WINDOW_UPDATE, and PUSH_PROMISE are used in support of other HTTP/2 features.

HTTP/2中基本的協議單位是幀。每個幀都有不同的類型和用途。例如，報頭(HEADERS)和數據(DATA)幀組成了基本的HTTP 請求和響應；其他幀例如設置(SETTINGS),窗口更新(WINDOW_UPDATE), 和推送承諾(PUSH_PROMISE)是用來實現HTTP/2的其他功能。

Multiplexing of requests is achieved by having each HTTP request-response exchanged assigned to a single stream (Section 5). Streams are largely independent of each other, so a blocked or stalled request does not prevent progress on other requests.

請求多路複用是通過在一個流上分配多個HTTP請求響應交換來實現的(章節5)。流在很大程度上是相互獨立的，因此一個請求上的阻塞或終止並不會影響其他請求的處理。

Flow control and prioritization ensure that it is possible to properly use multiplexed streams. Flow control (Section 5.2) helps to ensure that only data that can be used by a receiver is transmitted. Prioritization (Section 5.3) ensures that limited resources can be directed to the most important requests first.

流量控制和優先級能確保正確使用複用流。流量控制(章節5.2)有助於確保只傳播接受者需要使用的數據數據。優先級(章節5.3)能確保有限的資源能優先被重要的請求使用。

HTTP/2 adds a new interaction mode, whereby a server can push responses to a client (Section 8.2). Server push allows a server to speculatively send a client data that the server anticipates the client will need, trading off some network usage against a potential latency gain. The server does this by synthesizing a request, which it sends as a PUSH_PROMISE frame. The server is then able to send a response to the synthetic request on a separate stream.

HTTP/2添加了一種新的交互模式，即服務器能推送消息給客戶端。服務器推送允許服務端預測客戶端需要來發送數據給客戶端，交換網絡的使用來阻止潛在的延遲增長。服務器通過複用一個以PUSH_PROMISE幀發送的請求來實現推送，然後服務端可以在一個單獨的流裏面發送響應給這個合成的請求。

Frames that contain HTTP header fields are compressed (Section 4.3). HTTP requests can be highly redundant, so compression can reduce the size of requests and responses significantly.

幀包含的HTTP報頭字段是壓縮的。HTTP請求有可能是高度冗餘的，因此壓縮能顯著減少請求和響應的大小。

文檔結構

The HTTP/2 specification is split into four parts:

HTTP/2協議被分爲以下四個部分：

Starting HTTP/2 (Section 3) covers how an HTTP/2 connection is initiated.
The framing (Section 4) and streams (Section 5) layers describe the way HTTP/2 frames are structured and formed into multiplexed streams.
Frame (Section 6) and error (Section 7) definitions include details of the frame and error types used in HTTP/2.
HTTP mappings (Section 8) and additional requirements (Section 9) describe how HTTP semantics are expressed using frames and streams.
啓動HTTP/2(章節3)包含了一個HTTP/2連接是如何初始化的。
幀(章節4)和流層(章節5)描述了 HTTP/2流的結構以及如何形成複用流的。
幀(章節6)和錯誤碼(章節7)定義了HTTP/2中使用的流和錯誤類型的詳細內容。
HTTP尋址(章節8)和拓展需求(章節9)描述了HTTP語義化是如何由幀和流表達的。

While some of the frame and stream layer concepts are isolated from HTTP, the intent is not to define a completely generic framing layer. The framing and streams layers are tailored to the needs of the HTTP protocol and server push.

一些幀和流層的概念是與HTTP隔離的，因爲意圖並不是定義一個完全通用的幀層。這些幀和流層是爲了HTTP協議和服務端推送的需求定製的。

約定和術語

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

文檔中出現的關鍵字“必須”,“絕對不能”，“要求”，“應”，“不應”，“應該”，“不應該”，“建議”，“可以”及“可選”可通過在 RFC 2119 的解釋進行理解【RFC2119】

All numeric values are in network byte order. Values are unsigned unless otherwise indicated. Literal values are provided in decimal or hexadecimal as appropriate. Hexadecimal literals are prefixed with 0x to distinguish them from decimal literals.

所有的數值都是按網絡字節順序。除非有另外說明，數值是無符號的。按情況提供十進制或十六進制的文本值。十六進制用前綴0x來區分。

The following terms are used:

client: The endpoint initiating the HTTP/2 connection.connection:A transport-level connection between two endpoints.
connection error: An error that affects the entire HTTP/2 connection.
endpoint: Either the client or server of the connection.
frame: The smallest unit of communication within an HTTP/2 connection, consisting of a header and a variable-length sequence of bytes structured according to the frame type.
peer: An endpoint. When discussing a particular endpoint, "peer" refers to the endpoint that is remote to the primary subject of discussion.
receiver: An endpoint that is receiving frames.
sender: An endpoint that is transmitting frames.
server: The endpoint which did not initiate the HTTP/2 connection.
stream: A bi-directional flow of frames across a virtual channel within the HTTP/2 connection.
stream error: An error on the individual HTTP/2 stream.

文中術語包括：

客戶端：發起HTTP/2請求的端點
連接：在兩個端點之間的傳輸層級別的連接
連接錯誤：整個HTTP/2連接過程中發生的錯誤
端點：連接的客戶端或服務器
幀：HTTP/2.0通信連接中的最小單元，包括根據幀類型結構的字節的報頭和可變長度的序列
對等端：一個端點。當討論特定的端點時，“對等端”指的是討論的主題的遠程端點
接收端：正在接收幀的端點
發送端：正在傳輸幀的端點
服務端：不是啓動HTTP/2連接的端點
流：一個雙向字節幀流穿過HTTP/2連接中的虛擬通道
流錯誤：一個HTTP/2流中的錯誤

3 啓動HTTP/2

An HTTP/2 connection is an application level protocol running on top of a TCP connection ([TCP]). The client is the TCP connection initiator.

一個HTTP/2連接是運行在TCP連接上的應用層協議。客戶端是TCP連接的發起者。

HTTP/2 uses the same "http" and "https" URI schemes used by HTTP/1.1. HTTP/2 shares the same default port numbers: 80 for "http" URIs and 443 for "https" URIs. As a result, implementations processing requests for target resource URIs like http://example.org/foo or https://example.com/bar are required to first discover whether the upstream server (the immediate peer to which the client wishes to establish a connection) supports HTTP/2.

HTTP/2使用與HTTP/1.1相同的"http"和"https" 資源標識符(URI)。使用相同的默認端口："http" 的80端口及“https”的443端口。因此，實現對例如http://example.org/foo或https://example.com/bar目標資源的URI請求處理需要首先確定上游服務端(當前客戶端希望建立連接的對等端)是否支持HTTP/2。

The means by which support for HTTP/2 is determined is different for "http" and "https" URIs. Discovery for "http" URIs is described in Section 3.2. Discovery for "https" URIs is described in Section 3.3.

這意味着檢測“http” 及“https” 的URIs是否支持HTTP/2的方法是不一樣的。檢測"http"URIs在章節3.2中描述。檢測"https"URIs 在章節3.3中描述。

3.1 HTTP/2版本定義

The protocol defined in this document has two identifiers.

The string "h2" identifies the protocol where HTTP/2 uses TLS [TLS12]. This identifier is used in the TLS application layer protocol negotiation extension (ALPN) [TLSALPN] field and any place that HTTP/2 over TLS is identified.

The "h2" string is serialized into an ALPN protocol identifier as the two octet sequence: 0x68, 0x32.
The string "h2c" identifies the protocol where HTTP/2 is run over cleartext TCP. This identifier is used in the HTTP/1.1 Upgrade header field and any place that HTTP/2 over TCP is identified.

在本文檔中定義的協議有兩個標識符。

字符"h2"表示HTTP/2協議使用TLS[TLS]。這種方式用在HTTP/1.1的升級字段、TLS 應用層協議協商擴展字段以及其他需要定義協議的地方。當在定義ALPN協議(序列化的字節)中序列化時。"h2"字符序列化到 ALPN 協議中變成兩個字節序列：0x68，0x32。
字符"h2c" 表示HTTP/2協議運行在明文TCP上。這個標識用在HTTP/1.1 升級報頭字段以及任何TCP是確定的地方。

Negotiating "h2" or "h2c" implies the use of the transport, security, framing and message semantics described in this document.

用到"h2" 或者 "h2c" 表明使用文檔中定義的傳輸、安全、幀及語義化消息。

Only implementations of the final, published RFC can identify themselves as "h2" or "h2c". Until such an RFC exists, implementations MUST NOT identify themselves using these strings.

只有依據最終發表的RFC的實現能使用"h2"或"h2c"進行標明。在此之前，任何實現絕對不能使用這兩個字符進行識別。

Examples and text throughout the rest of this document use "h2" as a matter of editorial convenience only. Implementations of draft versions MUST NOT identify using this string.

本文檔內的例子或者文本只作爲編輯方便使用"h2"。針對草案版本的實現絕對不能使用這個字符進行識別。

Implementations of draft versions of the protocol MUST add the string "-" and the corresponding draft number to the identifier. For example, draft-ietf-httpbis-http2-11 over TLS is identified using the string "h2-11".

依據草案版本實現的協議必須添加字符"-"及相對應的草案版本進行標識。例如，基於TLS 的草案draft-ietf-httpbis-http2-11 需要使用字符"h2-11"進行標識。

Non-compatible experiments that are based on these draft versions MUST append the string "-" and an experiment name to the identifier. For example, an experimental implementation of packet mood-based encoding based on draft-ietf-httpbis-http2-09 might identify itself as "h2-09-emo". Note that any label MUST conform to the "token" syntax defined in Section 3.2.6 of [RFC7230]. Experimenters are encouraged to coordinate their experiments on the [email protected] mailing list.

基於這些草案版本的不兼容的實驗必須在標識符中添加字符"-"及實驗名稱。例如，基於draft-ietf-httpbis-http2-09草案的情緒編碼實驗實現必須使用類似"h2-09-emo"的標識符。需要注意的是任何標籤必須符合[RFC7230]章節3.2.6 定義的"token"語法。鼓勵實驗者提交實驗到[email protected] 的郵件列表中。

3.2 Starting HTTP/2 for "http" URIs 針對"http"啓動HTTP/2

A client that makes a request to an "http" URI without prior knowledge about support for HTTP/2 uses the HTTP Upgrade mechanism (Section 6.7 of [RFC7230]). The client makes an HTTP/1.1 request that includes an Upgrade header field identifying HTTP/2 with the "h2c" token. The HTTP/1.1 request MUST include exactly one HTTP2-Settings (Section 3.2.1) header field.

客戶端無法預知服務端是否支持HTTP/2.0 的情況下使用HTTP升級機制發起“http” URI請求([RFC7230] 章節6.7)。客戶端發起一個http1.1請求，其中包含識別HTTP/2的升級報頭字段與h2c token。HTTP/1.1必須包含一個確切的HTTP2-Settings中的報頭字段。

例如：

GET /default.htm HTTP/1.1
Host: server.example.com
Connection: Upgrade, HTTP2-Settings
Upgrade: h2c
HTTP2-Settings: <base64url encoding of HTTP/2 SETTINGS payload>

Requests that contain an entity body MUST be sent in their entirety before the client can send HTTP/2 frames. This means that a large request entity can block the use of the connection until it is completely sent.

包含主體內容的請求必須在客戶端能發送HTTP/2幀前全部發送。這意味着一個大的請求實體能阻塞連接的使用直到其全部被髮送。

If concurrency of an initial request with subsequent requests is important, a small request can be used to perform the upgrade to HTTP/2, at the cost of an additional round-trip.

如果一個請求的併發後續請求是重要的，那麼可以使用一個小的請求來執行升級到HTTP/2的操作,這樣僅消耗一個額外的往返成本。

A server that does not support HTTP/2 can respond to the request as though the Upgrade header field were absent:

不支持HTTP/2的服務端對請求返回一個不包含升級的報頭字段的響應：

HTTP/1.1 200 OK
Content-Length: 243
Content-Type: text/html
...

A server MUST ignore a "h2" token in an Upgrade header field. Presence of a token with "h2" implies HTTP/2 over TLS, which is instead negotiated as described in Section 3.3.

服務端必須忽略升級報頭字段中的“h2” token。“h2” token基於TLS實現的HTTP/2,協商方法在章節3.3中定義。

A server that supports HTTP/2 can accept the upgrade with a 101 (Switching Protocols) response. After the empty line that terminates the 101 response, the server can begin sending HTTP/2 frames. These frames MUST include a response to the request that initiated the Upgrade.

支持HTTP/2的服務端可以返回一個101(轉換協議)響應來接受升級請求。在101空內容響應終止後，服務端可以開始發送HTTP/2幀。這些幀必須包含一個發起升級的請求的響應。

HTTP/1.1 101 Switching Protocols
Connection: Upgrade
Upgrade: h2c

[ HTTP/2 connection ...

The first HTTP/2 frame sent by the server is a SETTINGS frame (Section 6.5). Upon receiving the 101 response, the client sends a connection preface (Section 3.5), which includes a SETTINGS frame.

第一個被服務端發送的HTTP/2幀是一個設置(SETTINGS)幀。在收到101響應後，客戶端發送一個包含設置(SETTINGS)幀的連接序言。

The HTTP/1.1 request that is sent prior to upgrade is assigned stream identifier 1 and is assigned default priority values (Section 5.3.5). Stream 1 is implicitly half closed from the client toward the server, since the request is completed as an HTTP/1.1 request. After commencing the HTTP/2 connection, stream 1 is used for the response.

（*** 升級前所發送的HTTP/1.1請求被派送到標示流1並將賦予最高優先級。）HTTP/1.1最開始用來升級到2.0的請求用1來標示流並將賦予最高優先級。1流對發送到服務端的客戶端是隱式半封閉的，因爲這個請求已經作爲HTTP/1.1請求完成了。HTTP/2連接開始後, 1流在響應中使用。

3.2.1 HTTP2-Settings Header Field HTTP2-Setting報頭字段

A request that upgrades from HTTP/1.1 to HTTP/2 MUST include exactly one HTTP2-Settings header field. The HTTP2-Settings header field is a hop-by-hop header field that includes parameters that govern the HTTP/2 connection, provided in anticipation of the server accepting the request to upgrade.

從HTTP/1.1升級到HTTP/2的請求必須包含一個確切的HTTP2-Settings報頭字段。HTTP2-Settings 的報頭字段是逐跳報頭字段，它包含管理HTTP/2連接參數。這是從對於服務端接受升級請求的預測中所獲取的。

HTTP2-Settings    = token68

A server MUST reject an attempt to upgrade if this header field is not present. A server MUST NOT send this header field.

服務端未檢測到此報頭字段必須拒絕客戶端的升級嘗試。服務端絕對不能發送此報頭字段。

The content of the HTTP2-Settings header field is the payload of a SETTINGS frame (Section 6.5), encoded as a base64url string (that is, the URL- and filename-safe Base64 encoding described in Section 5 of [RFC4648], with any trailing '=' characters omitted). The ABNF [RFC5234] production for token68 is defined in Section 2.1 of [RFC7235].

HTTP2-Settings報頭字段的內容是設置(SETTINGS)幀的有效載體，使用base64url字符編碼(URL及文件名安全的Base64編碼，編碼描述在[RFC4648] 章節5中，忽略任何“=”字符。) ABNF[RFC5234]產品中對token68的定義在[RFC7235] 章節2.1中。

As a hop-by-hop header field, the Connection header field MUST include a value of HTTP2-Settings in addition to Upgrade when upgrading to HTTP/2.

作爲一個逐跳的報文報頭字段，當升級到HTTP/2時，此連接報頭字段必須包含一個HTTP2設置(HTTP2-Settings)的值來完成升級操作。

A server decodes and interprets these values as it would any other SETTINGS frame. Acknowledgement of the SETTINGS parameters (Section 6.5.3) is not necessary, since a 101 response serves as implicit acknowledgment. Providing these values in the Upgrade request ensures that the protocol does not require default values for the above SETTINGS parameters, and gives a client an opportunity to provide other parameters prior to receiving any frames from the server.

服務端就像對任何其他設置(SETTINGS)幀一樣對這些值進行解碼和解釋。因爲101響應的隱式聲明，對這些設置參數的確認不是必須的。這些升級請求中的值使得協議不需要上述設置參數的默認值，同時使客戶端有機會在從服務端接受任何幀之前提供其他參數。

3.3 Starting HTTP/2 for "https" URIs 針對“https”啓動HTTP/2

A client that makes a request to an "https" URI without prior knowledge about support for HTTP/2 uses TLS [TLS12] with the application layer protocol negotiation extension [TLSALPN].

客戶端在不瞭解服務端是否支持HTTP/2的時候，會使用TSL [TLS12] 於其應用層協議協商擴展 [TLSALPN]。

HTTP/2 over TLS uses the "h2" application token. The "h2c" token MUST NOT be sent by a client or selected by a server.

使用TLS的HTTP/2 使用"h2"程序token。“h2c”token絕對不能由客戶端或者選定的服務端發送。

Once TLS negotiation is complete, both the client and the server send a connection preface (Section 3.5).

TSL協議一旦完成，客戶端和服務端都可以發送連接序言(章節3.5)。

3.4 Starting HTTP/2 with Prior Knowledge 先驗下啓動HTTP/2

A client can learn that a particular server supports HTTP/2 by other means. For example, [ALT-SVC] describes a mechanism for advertising this capability.

客戶端可以通過其他方式判斷服務端是否支持HTTP/2。例如，AltSvc定義一種機制讓HTTP頭字段進行廣播。

A client MAY immediately send HTTP/2 frames to a server that is known to support HTTP/2, after the connection preface (Section 3.5). A server can identify such a connection by the use of the "PRI" method in the connection preface. This only affects the establishment of HTTP/2 connections over cleartext TCP; implementations that support HTTP/2 over TLS MUST use protocol negotiation in TLS [TLSALPN].

客戶端可以對支持HTTP/2的服務端在連接序言(章節3.5)之後立即發送HTTP/2幀。服務端可以通過連接序言中的“PRI”方法來區分這種連接。這隻對基於明文TCP的HTTP/2連接建立有影響；支持HTTP/2的服務端對“https”URI需要支持TLS中的協商擴展。

Prior support for HTTP/2 is not a strong signal that a given server will support HTTP/2 for future connections. It is possible for server configurations to change; for configurations to differ between instances in clustered server; or network conditions to change.

對HTTP/2之前的支持並不表明一個給定的服務器會在以後的連接中一定支持HTTP/2。服務器配置有可能改變或者集羣中不同服務器配置有差異。攔截代理(又叫“透明”代理)是另一個可能得原因。

3.5 HTTP/2 Connection Preface HTTP/2連接序言

Upon establishment of a TCP connection and determination that HTTP/2 will be used by both peers, each endpoint MUST send a connection preface as a final confirmation and to establish the initial SETTINGS parameters for the HTTP/2 connection.

在建立TCP連接並且檢測到HTTP/2會被各個對等端使用後，每個端點必須發送一個連接序言最終確認並作爲建立HTTP/2連接的初始設置參數。

The client connection preface starts with a sequence of 24 octets, which in hex notation are:

客戶端連接序言以24個字節的序列開始，以十六進制表示是：

0x505249202a20485454502f322e300d0a0d0a534d0d0a0d0a

(the string PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n). This sequence is followed by a SETTINGS frame (Section 6.5). The SETTINGS frame MAY be empty. The client sends the client connection preface immediately upon receipt of a 101 Switching Protocols response (indicating a successful upgrade), or as the first application data octets of a TLS connection. If starting an HTTP/2 connection with prior knowledge of server support for the protocol, the client connection preface is sent upon connection establishment.

(字符串PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n)。這個序列後跟着一個設置幀，其可爲空幀。客戶端在收到101轉換協議響應(升級成功指示)後馬上發送客戶端連接序言，或者作爲TLS連接的第一個應用數據字節。如果在預先知道服務器支持HTTP/2的情況下啓動HTTP/2連接，客戶端連接序言在連接建立後發送。

The client connection preface is selected so that a large proportion of HTTP/1.1 or HTTP/1.0 servers and intermediaries do not attempt to process further frames. Note that this does not address the concerns raised in [TALKING].

客戶端連接序言是用來讓大部分的HTTP/1.1或者HTTP/1.0服務端以及中介端不試圖進一步處理幀。注意這並不能處理【討論】中提到的問題。

The server connection preface consists of a potentially empty SETTINGS frame (Section 6.5) that MUST be the first frame the server sends in the HTTP/2 connection.

服務端連接序言包含一個有可能是空的設置（SETTING）幀（章節6.5），它必須在HTTP/2連接中首個發送。

To avoid unnecessary latency, clients are permitted to send additional frames to the server immediately after sending the client connection preface, without waiting to receive the server connection preface. It is important to note, however, that the server connection preface SETTINGS frame might include parameters that necessarily alter how a client is expected to communicate with the server. Upon receiving the SETTINGS frame, the client is expected to honor any parameters established.

爲了避免不必要的延遲，允許客戶端在發送客戶端連接序言之後立即發送其他額外的幀，不需要等待收到服務端連接序言。不過需要注意的是，服務端連接序言設置(SETTINGS)幀可能包含一些關於期望客戶端如何與服務端通信的所必須修改的參數。在收到這些設置(SETTINGS)幀之後，客戶端應當遵守所有設置的參數。

Clients and servers MUST terminate the TCP connection if either peer does not begin with a valid connection preface. A GOAWAY frame (Section 6.8) can be omitted if it is clear that the peer is not using HTTP/2.

如果任何一個端點沒有以一個有效的連接序言開頭，客戶端和服務端必須終止TCP連接。如果端點並沒有使用HTTP/2此時可以省略超時(GOAWAY)幀(章節6.8)。

4 HTTP Frames HTTP幀

Once the HTTP/2 connection is established, endpoints can begin exchanging frames.

HTTP/2連接一旦建立，端點之間可以馬上交換數據幀。

4.1 Frame Format 幀格式

All frames begin with a fixed 8-octet header followed by a payload of between 0 and 16,383 octets.

所有的幀以8字節的報頭開始並且跟着0-16,383字節長度的主體。

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | R |     Length (14)           |   Type (8)    |   Flags (8)   |
 +-+-+-----------+---------------+-------------------------------+
 |R|                 Stream Identifier (31)                      |
 +=+=============================================================+
 |                   Frame Payload (0...)                      ...
 +---------------------------------------------------------------+

The fields of the frame header are defined as:

R :A reserved 2-bit field. The semantics of these bits are undefined and the bits MUST remain unset (0) when sending and MUST be ignored when receiving.
Length : The length of the frame payload expressed as an unsigned 14-bit integer. The 8 octets of the frame header are not included in this value.
Type : The 8-bit type of the frame. The frame type determines the format and semantics of the frame. Implementations MUST ignore and discard any frame that has a type that is unknown.
Flags :An 8-bit field reserved for frame-type specific boolean flags.Flags are assigned semantics specific to the indicated frame type. Flags that have no defined semantics for a particular frame type MUST be ignored, and MUST be left unset (0) when sending.
R: A reserved 1-bit field. The semantics of this bit are undefined and the bit MUST remain unset (0) when sending and MUST be ignored when receiving.
Stream Identifier: A 31-bit stream identifier (see Section 5.1.1). The value 0 is reserved for frames that are associated with the connection as a whole as opposed to an individual stream.

幀報頭字段定義是：

R : 保留的2位字段。這些字節的語義是未定義的，並且在發送的時候必須保持未設置(0)，接收的時候必須被忽略此字段。
Length : 14位無符號整數的幀主體長度。8字節長度的幀報頭信息不計算在此內。
Type ：幀的8位類型。幀類型定義了剩餘的幀報頭和幀主體將如何被解釋。具體實現必須在收到未知幀類型(任何未在文檔中定義的幀)時作爲連接錯誤中的類型協議錯誤(PROTOCOL_ERROR)處理。
Flags ：爲幀類型保留的8字節字段有具體的布爾標識。標識針對確定的幀類型賦予特定的語義。確定幀類型定義語義以外的標示必須被忽略，並且必須在發送的時候保留未設置(0)。
R : 1位的保留字段。這個字段的語義未設置並且必須在發送的時候保持未設置(0),在接受的時候必須被忽略。
Stream Identifier : 31字節的流標識符(見StreamIdentifiers)。0是保留的，標明幀是與連接相關作爲一個整體而不是一個單獨的流。

The structure and content of the frame payload is dependent entirely on the frame type.

幀主體的結構和內容完全取決於幀類型。

4.2 Frame Size 幀大小

The maximum size of a frame payload varies by frame type. The absolute maximum size of a frame payload is 214-1 (16,383) octets, meaning that the maximum frame size is 16,391 octets. All implementations MUST be capable of receiving and minimally processing frames up to this maximum size.

幀主體的最大長度限制因不同的幀類型而不同。最大幀主體的絕對長度是 $2^{14}-1$ (16,383)字節，表示最大的幀長度是16,391字節。所有的實現必須具備接收和處理此最大長度幀的能力。

Certain frame types, such as PING (Section 6.7), impose additional limits on the amount of payload data allowed.

某些幀類型，例如PING(章節6.7)，對主體數據大小有額外的限制。同樣的，一些特定的應用也可能使用額外的大小限制(見HttpExtra)。

If a frame size exceeds any defined limit, or is too small to contain mandatory frame data, the endpoint MUST send a FRAME_SIZE_ERROR error. A frame size error in a frame that could alter the state of the entire connection MUST be treated as a connection error (Section 5.4.1); this includes any frame carrying a header block (Section 4.3) (that is, HEADERS, PUSH_PROMISE, and CONTINUATION), SETTINGS, and any WINDOW_UPDATE frame with a stream identifier of 0.

如果一個幀大小超過設定的限制，或者太小無法包含必須的基礎幀數據，這個端點必須發送一個幀大小錯誤(FRAME_SIZE_ERROR)。如果幀大小錯誤可能修改整個連接狀態，必須作爲一個連接錯誤(章節5.4.1)處理；這包括與0流一起的攜帶報頭區塊(即報文頭(HEADERS)，推送承諾(PUSH_PROMISE)和延續(CONTINUATION)幀)、設置(SETTINGS)以及任何窗口更新(WINDOW_UPDATE)幀。

4.3 Header Compression and Decompression 報頭壓縮和解壓縮

A header field in HTTP/2 is a name with one or more associated values. They are used within HTTP request and response messages as well as server push operations (see Section 8.2).

HTTP/2報文報頭字段是包含一個或多個相關的鍵值對。他們在HTTP請求響應消息及服務器推送操作(見章節8.2)中使用。

Header sets are collections of zero or more header fields. When transmitted over a connection, a header set is serialized into a header block using HTTP Header Compression [COMPRESSION]. The serialized header block is then divided into one or more octet sequences, called header block fragments, and transmitted within the payload of HEADERS (Section 6.2), PUSH_PROMISE (Section 6.6) or CONTINUATION (Section 6.10) frames.

報頭集合是0個或多個報頭字段的集合。當他通過連接傳輸的時候，報頭集合將使用HTTP報頭壓縮序列化到報文報頭塊中。序列化的報頭塊被分割成一個或多個的字節序列，稱爲報頭分區，並在報頭、推送承諾及延續幀的載體中傳送。

HTTP Header Compression does not preserve the relative ordering of header fields. Header fields with multiple values are encoded into a single header field using a special delimiter (see Section 8.1.2.3), this preserves the relative order of values for that header field.

HTTP報文頭壓縮並不保留報頭字段的相關順序。具有多個值的報頭字段使用特定的分割器被編碼分割到一個單獨的報頭區域（見章節8.1.2.3 HeaderOrdering），這保留了該報頭字段中各種值的對應順序。

The Cookie header field [COOKIE] is treated specially by the HTTP mapping (see Section 8.1.2.4).

報文頭Cookie字段被通過HTTP映射特殊處理；見章節8.1.2.4。

A receiving endpoint reassembles the header block by concatenating its fragments, then decompresses the block to reconstruct the header set.

A complete header block consists of either:

a single HEADERS or PUSH_PROMISE frame, with the END_HEADERS flag set, or
a HEADERS or PUSH_PROMISE frame with the END_HEADERS flag cleared and one or more CONTINUATION frames, where the last CONTINUATION frame has the END_HEADERS flag set.

接收端點連接報頭區塊重新組裝，並且解壓縮區塊後重建報頭集合。

一個完整的報頭區塊包含：

一個包含報頭終止標記集合的單獨的報頭HEADERS 或推送承諾PUSH_PROMISE幀，或者
一個報頭終止標記被清除的報頭HEADERS 或推送承諾PUSH_PROMISE幀以及一個或多個延續CONTINUATION幀，最後一個延續CONTINUATION幀擁有報頭終止標記設置。

Header compression is stateful, using a single compression context for the entire connection. Each header block is processed as a discrete unit. Header blocks MUST be transmitted as a contiguous sequence of frames, with no interleaved frames of any other type or from any other stream. The last frame in a sequence of HEADERS or CONTINUATION frames MUST have the END_HEADERS flag set. The last frame in a sequence of PUSH_PROMISE or CONTINUATION frames MUST have the END_HEADERS flag set. This allows a header block to be logically equivalent to a single frame.

報頭壓縮是有狀態的並且在整個連接過程中使用同個壓縮環境。每個報頭區塊作爲離散的單元處理。報頭區塊必須作爲一個連續的幀序列傳輸，沒有任何類型或任何其他流的交錯幀。一個報頭HEADERS或者延續CONTINUATION幀序列的最後一幀必須有報頭終止標記設置。推送承諾PUSH_PROMISE或者延續CONTINUATION幀序列的最後一幀必須具有報頭終止標記設置。

Header block fragments can only be sent as the payload of HEADERS, PUSH_PROMISE or CONTINUATION frames, because these frames carry data that can modify the compression context maintained by a receiver. An endpoint receiving HEADERS, PUSH_PROMISE or CONTINUATION frames MUST reassemble header blocks and perform decompression even if the frames are to be discarded. A receiver MUST terminate the connection with a connection error (Section 5.4.1) of type COMPRESSION_ERROR if it does not decompress a header block.

報頭區塊必須被報頭HEADERS、推送承諾PUSH_PROMISE或延續CONTINUATION的有效載體發送，因爲這些幀中攜帶了能被接收端修改的壓縮上下文數據。端點在接收報頭HEADERS、推送承諾PUSH_PROMISE或延續CONTINUATION幀時必須重新組裝報頭區塊並且執行解壓縮，即便這些幀將被廢棄。如何不能重建報頭區間，接收端必須終止連接並報類型爲解壓縮錯誤的連接錯誤(章節5.4.1)。

5 Streams and Multiplexing 流和多路複用

A "stream" is an independent, bi-directional sequence of frames exchanged between the client and server within an HTTP/2 connection. Streams have several important characteristics:

A single HTTP/2 connection can contain multiple concurrently open streams, with either endpoint interleaving frames from multiple streams.
Streams can be established and used unilaterally or shared by either the client or server.
Streams can be closed by either endpoint.
The order in which frames are sent on a stream is significant. Recipients process frames in the order they are received. In particular, the order of HEADERS, and DATA frames is semantically significant.
Streams are identified by an integer. Stream identifiers are assigned to streams by the endpoint initiating the stream.

流是一個獨立的，客戶端和服務端在HTTP/2連接下交換幀的雙向序列。流有一下幾個重要特點：

一個單獨的HTTP/2連接能夠保持多個同時打開的流，各個端點間從多個流中交換幀。
流可以被被客戶端或者服務端單方面建立使用或分享。
流可以被任何一個連接終端關閉。
在流內發送幀的順序很重要。它們將按被接收的順序處理。特別是報頭及數據幀的順序語義上是有意義的。
流以一個整數標識。標識符有啓動流的終端分配。

5.1 Stream States 流狀態

The lifecycle of a stream is shown in Figure 1.

流的生存週期如StreamStatesFigure所示：

                       +--------+
                 PP    |        |    PP
              ,--------|  idle  |--------.
             /         |        |         \
            v          +--------+          v
     +----------+          |           +----------+
     |          |          | H         |          |
 ,---| reserved |          |           | reserved |---.
 |   | (local)  |          v           | (remote) |   |
 |   +----------+      +--------+      +----------+   |
 |      |          ES  |        |  ES          |      |
 |      | H    ,-------|  open  |-------.      | H    |
 |      |     /        |        |        \     |      |
 |      v    v         +--------+         v    v      |
 |   +----------+          |           +----------+   |
 |   |   half   |          |           |   half   |   |
 |   |  closed  |          | R         |  closed  |   |
 |   | (remote) |          |           | (local)  |   |
 |   +----------+          |           +----------+   |
 |        |                v                 |        |
 |        |  ES / R    +--------+  ES / R    |        |
 |        `----------->|        |<-----------'        |
 |  R                  | closed |                  R  |
 `-------------------->|        |<--------------------'
                       +--------+

   H:  HEADERS frame (with implied CONTINUATIONs)
   PP: PUSH_PROMISE frame (with implied CONTINUATIONs)
   ES: END_STREAM flag
   R:  RST_STREAM frame

$$Figure 2: Stream States$$

Note that this diagram shows stream state transitions and frames that affect those transitions only. In this regard, CONTINUATION frames do not result in state transitions and are effectively part of the HEADERS or PUSH_PROMISE that they follow.

請注意該圖僅展示了流狀態的轉換和幀對這些轉換的影響。在這方面，延續幀不會影響流狀態的轉換，但是對後面跟隨的報頭或者推送承諾是有影響的。

Both endpoints have a subjective view of the state of a stream that could be different when frames are in transit. Endpoints do not coordinate the creation of streams; they are created unilaterally by either endpoint. The negative consequences of a mismatch in states are limited to the "closed" state after sending RST_STREAM, where frames might be received for some time after closing.

當流在傳輸的時候，各個端點對在傳送中的流狀態的主觀認識可能不同。終端並不協調流的創建；它們是被任意終端單方面創建的。不匹配的狀態導致的消極結果是在發送RST_STREAM流之後它們的“關閉”是受限制的，因爲可能在關閉之後幀才被接收。

流有以下狀態：

Streams have the following states:

idle :

All streams start in the "idle" state. In this state, no frames have been exchanged.

所有流以“空閒”狀態開始。在這種狀態下，沒有任何幀的交換。

The following transitions are valid from this state:

Sending or receiving a HEADERS frame causes the stream to become "open". The stream identifier is selected as described in Section 5.1.1. The same HEADERS frame can also cause a stream to immediately become "half closed".
Sending a PUSH_PROMISE frame marks the associated stream for later use. The stream state for the reserved stream transitions to "reserved (local)".
Receiving a PUSH_PROMISE frame marks the associated stream as reserved by the remote peer. The state of the stream becomes "reserved (remote)".

下列傳輸在這種狀態下是有效的：

發送或者接收一個報頭HEADERS幀導致流變成“打開”。流標識符如StreamIdentifiers說明。這個報頭HEADERS幀同樣可能導致流立即變成“半關閉”狀態。
發送一個推送承諾PUSH_PROMISE幀標記相關的流後續再使用。保留流狀態將轉換爲“保留(本地)”。
接收一個推送承諾PUSH_PROMISE幀標記相關的流爲遠程端點預留的流。這些流的狀態變成“保留(遠程)”

reserved (local) :

A stream in the "reserved (local)" state is one that has been promised by sending a PUSH_PROMISE frame. A PUSH_PROMISE frame reserves an idle stream by associating the stream with an open stream that was initiated by the remote peer (see Section 8.2).

在“報留(本地)”狀態的是已經被承諾發送推送承諾PUSH_PROMISE幀的流。一個推送承諾PUSH_PROMISE幀通過使一個流與一個由遠端對等端初始化的打開的流相關聯來保留一個空閒流。

In this state, only the following transitions are possible:

The endpoint can send a HEADERS frame. This causes the stream to open in a "half closed (remote)" state.
Either endpoint can send a RST_STREAM frame to cause the stream to become "closed". This releases the stream reservation.

在這種狀態下，只有下列傳輸是可能的：

端點可以發送報頭HEADERS幀，致使流打開到“半封閉(遠程)”狀態。
任意端點能發送一個RST_STREAM幀來使流變成“關閉”。這將釋放流的保留。

An endpoint MUST NOT send frames other than HEADERS or RST_STREAM in this state.

在這種狀態下一個端絕對不能發送報頭HEADERS幀和RST_STREAM以外的幀。

A PRIORITY frame MAY be received in this state. Receiving any frames other than RST_STREAM, or PRIORITY MUST be treated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

在這種狀態下一個優先級PRIORITY幀可能被接收。接收到任何報頭HEADERS幀、RST_STREAM幀或者優先級PRIORITY幀以外的幀都將被認爲是類型爲協議錯誤PROTOCOL_ERROR的連接錯誤(章節5.4.1)。

reserved (remote) :

A stream in the "reserved (remote)" state has been reserved by a remote peer.

在“保留(遠程)”狀態下的流說明已經被遠程對等端所保留。

In this state, only the following transitions are possible:

Receiving a HEADERS frame causes the stream to transition to "half closed (local)".
Either endpoint can send a RST_STREAM frame to cause the stream to become "closed". This releases the stream reservation.

在這種狀態下，只有下列傳輸是可能的：

接收一個報頭HEADERS幀並致使流轉換到“半封閉(本地)”狀態。
任意一個端點能發送一個RST_STREAM 幀來使流變成“關閉”。這將釋放流的保留。

An endpoint MAY send a PRIORITY frame in this state to reprioritize the reserved stream. An endpoint MUST NOT send any other type of frame other than RST_STREAM or PRIORITY.

這種狀態下任意終端可以發送一個優先級PRIORITY幀來變更保留流的優先級順序。終端絕對不能發送任何RST_STREAM 和優先級PRIORITY以外的幀。

Receiving any other type of frame other than HEADERS or RST_STREAM MUST be treated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

接收任何RST_STREAM 和優先級PRIORITY以外的幀必須作爲類型爲協議錯誤PROTOCOL_ERROR的連接錯誤(章節5.4.1)來處理。

open :

A stream in the "open" state may be used by both peers to send frames of any type. In this state, sending peers observe advertised stream level flow control limits (Section 5.2).

處於“打開”狀態的流可以被兩個對等端來發送任何類型的幀。在這種狀態下，發送數據的對等端檢查被廣播端FlowControl流量控制限制(章節5.2)。

From this state either endpoint can send a frame with an END_STREAM flag set, which causes the stream to transition into one of the "half closed" states: an endpoint sending an END_STREAM flag causes the stream state to become "half closed (local)"; an endpoint receiving an END_STREAM flag causes the stream state to become "half closed (remote)".

在這種狀態下每個終端可以發送一個帶有END_STREAM結束流標記的幀來使流轉換到其中一種“半關閉”狀態：一個終端發送一個結束流END_STREAM標記使流變成“半封閉”狀態；一個終端接收一個結束流END_STREAM標記使流變成“半封閉(遠程)”狀態。帶有結束流END_STREAM標記的報頭HEADERS幀後面可以跟着延續CONTINUATION幀。

Either endpoint can send a RST_STREAM frame from this state, causing it to transition immediately to "closed".

這種狀態下各個終端可以發送一個RST_STREAM幀來使流轉換到"關閉"狀態。

half closed (local) :

A stream that is in the "half closed (local)" state cannot be used for sending frames. Only WINDOW_UPDATE, PRIORITY and RST_STREAM frames can be sent in this state.

“半封閉(本地)”狀態下的流不能發送幀。只有窗口更新(WINDOW_UPDATE)、優先級(PRIORITY)和終止流(RST_STREAM)幀能在這種狀態下發送。

A stream transitions from this state to "closed" when a frame that contains an END_STREAM flag is received, or when either peer sends a RST_STREAM frame.

這種狀態下，當流接收到包含END_STREAM標記的幀或者某個終端發送了RST_STREAM幀，流轉換到“關閉”狀態。帶有結束流END_STREAM標記的報頭HEADERS幀後面可以跟着延續CONTINUATION幀。

A receiver can ignore WINDOW_UPDATE frames in this state, which might arrive for a short period after a frame bearing the END_STREAM flag is sent.

這種狀態下接收端可以忽略窗口更新WINDOW_UPDATE（或優先級PRIORITY？？？）幀。這種類型的幀有可能在結束流END_STREAM標記到達一小段時間後才收到。

PRIORITY frames received in this state are used to reprioritize streams that depend on the current stream.

優先級(PRIORITY)幀可以在這種狀態下接收並用來對依賴當前流的流進行優先級重排序。

half closed (remote) :

A stream that is "half closed (remote)" is no longer being used by the peer to send frames. In this state, an endpoint is no longer obligated to maintain a receiver flow control window if it performs flow control.

"半封閉(遠程)"狀態下的流不再被對等端用來發送幀。這種狀態下，執行流量控制的終端不再承擔接收留空控制窗口的工作。

If an endpoint receives additional frames for a stream that is in this state, other than WINDOW_UPDATE, PRIORITY or RST_STREAM, it MUST respond with a stream error (Section 5.4.2) of type STREAM_CLOSED.

如果終端接收到處於這種狀態下的流發送的額外的幀，除非是延續CONTINUATION幀，否則必須返回類型爲流關閉STREAM_CLOSED的流錯誤(章節5.4.2)。

A stream can transition from this state to "closed" by sending a frame that contains an END_STREAM flag, or when either peer sends a RST_STREAM frame.

這種狀態下，當流發送一個帶有終止流END_STREAM標記的幀或者某個終端發送了一個RST_STREAM幀，流將轉換到“關閉”狀態。

closed :

The "closed" state is the terminal state.

“關閉”狀態是終止狀態。

An endpoint MUST NOT send frames on a closed stream. An endpoint that receives any frame other than PRIORITY after receiving a RST_STREAM MUST treat that as a stream error (Section 5.4.2) of type STREAM_CLOSED. Similarly, an endpoint that receives any frames after receiving a frame with the END_STREAM flag set MUST treat that as a connection error (Section 5.4.1) of type STREAM_CLOSED, unless the frame is permitted as described below.

終端絕對不能通過關閉的流發送幀。終端在收到RST_STREAM後接收的任何幀必須作爲類型爲流關閉STREAM_CLOSED的StreamErrorHandler流錯誤stream error(章節5.4.2)處理。相似的，終端接收到帶有END_STREAM標記設置的數據DATA幀之後的任何幀，或在帶有END_STREAM終止流標記且後面沒有延續CONTINUATION幀的報頭HEADERS幀之後收到任何幀都必須作爲類型爲流關閉STREAM_CLOSED的連接錯誤(章節5.4.1)處理。

WINDOW_UPDATE or RST_STREAM frames can be received in this state for a short period after a DATA or HEADERS frame containing an END_STREAM flag is sent. Until the remote peer receives and processes the frame bearing the END_STREAM flag, it might send frames of these types. Endpoints MUST ignore WINDOW_UPDATE or RST_STREAM frames received in this state, though endpoints MAY choose to treat frames that arrive a significant time after sending END_STREAM as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

在這種情況下，在帶有END_STREAM標記的DATA或HEADERS幀發送之後一小段時間內可以接收WINDOW_UPDATE或者RST_STREAM幀。在遠端對等端接收並處理帶有END_STREAM標記的幀之前，可以發送任意這幾種幀。在這種狀態下終端必須忽略接收到的WINDOW_UPDATE,PRIORITY, 或 RST_STREAM幀，但終端也可以當作類型爲PROTOCOL_ERROR的連接錯誤(章節5.4.1)處理。

PRIORITY frames can be sent on closed streams to prioritize streams that are dependent on the closed stream. Endpoints SHOULD process PRIORITY frame, though they can be ignored if the stream has been removed from the dependency tree (see Section 5.3.4).

關閉的流上可以發送優先級幀用來對依賴當前關閉流的流進行優先級重排序。終端應該處理優先級幀，但當該流已經從依賴樹(章節5.3.4)中移除時可以忽略。

If this state is reached as a result of sending a RST_STREAM frame, the peer that receives the RST_STREAM might have already sent - or enqueued for sending - frames on the stream that cannot be withdrawn. An endpoint MUST ignore frames that it receives on closed streams after it has sent a RST_STREAM frame. An endpoint MAY choose to limit the period over which it ignores frames and treat frames that arrive after this time as being in error.

如果流在發送RST_STREAM幀後轉換到這種狀態，接收到RST_STREAM的對等端可能已經發送或者隊列中準備發送無法取消的幀。終端必須忽略從已經發送RST_STREAM幀的流接收到的幀。終端可以選擇設置忽略幀的超時時間並在超過限制後作爲錯誤處理。

Flow controlled frames (i.e., DATA) received after sending RST_STREAM are counted toward the connection flow control window. Even though these frames might be ignored, because they are sent before the sender receives the RST_STREAM, the sender will consider the frames to count against the flow control window.

在發送RST_STREAM之後收到的流量受限幀(如數據DATA幀)轉向流量控制窗口連接處理。儘管這些幀可以被忽略，因爲他們是在發送端接收到RST_STREAM之前發送的，但發送端會認爲這些幀與流量控制窗口不符。

An endpoint might receive a PUSH_PROMISE frame after it sends RST_STREAM. PUSH_PROMISE causes a stream to become "reserved" even if the associated stream has been reset. Therefore, a RST_STREAM is needed to close an unwanted promised stream.

終端可能在發送RST_STREAM之後接收PUSH_PROMISE幀。即便相關的流已經被重置，推送承諾幀也能使流變成“保留”狀態。因此，需要RST_STREAM來關閉一個不想要的被承諾流。

In the absence of more specific guidance elsewhere in this document, implementations SHOULD treat the receipt of a message that is not expressly permitted in the description of a state as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

本文檔中沒有明確說明的地方，具體實現時接收描述狀態中沒有明確許可的信息都應作爲類型爲協議錯誤(PROTOCOL_ERROR)的連接錯誤(章節5.4.1)來處理。

5.1.1 Stream Identifiers 流標識

Streams are identified with an unsigned 31-bit integer. Streams initiated by a client MUST use odd-numbered stream identifiers; those initiated by the server MUST use even-numbered stream identifiers. A stream identifier of zero (0x0) is used for connection control messages; the stream identifier zero cannot be used to establish a new stream.

流由31位字節的無符號整數標識。客戶端發起的流必須以奇數標示；服務器發起的流必須使用偶數來標示。0(0x0)用來標識連接控制信息流，且絕對不能用來建立一個新流。

HTTP/1.1 requests that are upgraded to HTTP/2 (see Section 3.2) are responded to with a stream identifier of one (0x1). After the upgrade completes, stream 0x1 is "half closed (local)" to the client. Therefore, stream 0x1 cannot be selected as a new stream identifier by a client that upgrades from HTTP/1.1.

HTTP/1.1升級到HTTP/2的請求將收到一個1(0x1)標識的流的響應。升級完成後，0x1流將對客戶端處於“半封閉(本地)”狀態。因此，0x1流不能被從HTTP/1.1升級的客戶端用來作爲一個新的流的標識符。

The identifier of a newly established stream MUST be numerically greater than all streams that the initiating endpoint has opened or reserved. This governs streams that are opened using a HEADERS frame and streams that are reserved using PUSH_PROMISE. An endpoint that receives an unexpected stream identifier MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

一個新建立的流標識符必須數值大於任何終端已經打開或者保留的流標識符。規則適用於使用報頭幀打開的流以及使用推送承諾幀保留的流。終端收到不規範的流標識符必須響應一個類型爲協議錯誤(PROTOCOL_ERROR)的連接錯誤。

The first use of a new stream identifier implicitly closes all streams in the "idle" state that might have been initiated by that peer with a lower-valued stream identifier. For example, if a client sends a HEADERS frame on stream 7 without ever sending a frame on stream 5, then stream 5 transitions to the "closed" state when the first frame for stream 7 is sent or received.

新的流標識符第一次被使用時將隱式關閉所有處於“空閒”狀態下可能已經被對等端初始化而且流標識符數字小於新標識符的流。例如，一個客戶端發送一個流7的報頭幀，那麼在流7發送或者接收幀後從沒有發送幀的流5將轉換爲“關閉”狀態。

Stream identifiers cannot be reused. Long-lived connections can result in an endpoint exhausting the available range of stream identifiers. A client that is unable to establish a new stream identifier can establish a new connection for new streams. A server that is unable to establish a new stream identifier can send a GOAWAY frame so that the client is forced to open a new connection for new streams.

流標識符不能被重複使用。生存期長的連接可能導致流標識符可用範圍耗盡。客戶端不能新建流標識符時可以針對新流建立一個新的連接。服務端不能新建流標識符時可以發送一個超時幀(GOAWAY)強制客戶端對新的流使用新的連接。

5.1.2 Stream Concurrency 流併發

A peer can limit the number of concurrently active streams using the SETTINGS_MAX_CONCURRENT_STREAMS parameter (see Section 6.5.2) within a SETTINGS frame. The maximum concurrent streams setting is specific to each endpoint and applies only to the peer that receives the setting. That is, clients specify the maximum number of concurrent streams the server can initiate, and servers specify the maximum number of concurrent streams the client can initiate.

對等端可以使用設置幀裏面的SETTINGS_MAX_CONCURRENT_STREAMS參數來限制流的併發量。最大併發流設置(章節6.5.2)僅適用於終端並且只對接收到此設置的對等端有效。也就是說：客戶端可以指定服務端能啓動的流最大併發量，而且服務端能指定客戶端能啓動的流最大併發量。終端絕對不能超過對等端設置的限制。

Streams that are in the "open" state, or either of the "half closed" states count toward the maximum number of streams that an endpoint is permitted to open. Streams in any of these three states count toward the limit advertised in the SETTINGS_MAX_CONCURRENT_STREAMS setting. Streams in either of the "reserved" states do not count toward the stream limit.

處於“打開”或者任意一種“半封閉”狀態的流均計入終端被允許啓動的流次數中。處於任意這三種狀態下的流都將計入SETTINGS_MAX_CONCURRENT_STREAMS設置次數中。處於任意一種“保留”狀態下的流不計入打開次數中。

Endpoints MUST NOT exceed the limit set by their peer. An endpoint that receives a HEADERS frame that causes their advertised concurrent stream limit to be exceeded MUST treat this as a stream error (Section 5.4.2). An endpoint that wishes to reduce the value of SETTINGS_MAX_CONCURRENT_STREAMS to a value that is below the current number of open streams can either close streams that exceed the new value or allow streams to complete.

終端絕對不能超過對等端設定的設置。終端接收到報頭幀導致他們廣播的併發流超過限制的必須將這作爲流錯誤(章節5.4.2)處理。終端希望將SETTINGS_MAX_CONCURRENT_STREAMS的值減少到比當前打開的流更小時可以關閉超過新的設置值的流或者允許流結束。

5.2 Flow Control 流量控制

Using streams for multiplexing introduces contention over use of the TCP connection, resulting in blocked streams. A flow control scheme ensures that streams on the same connection do not destructively interfere with each other. Flow control is used for both individual streams and for the connection as a whole.

使用複用流介紹了針對TCP連接的資源爭奪導致的流阻塞。流量控制方案等確保同一連接上的流相互之間不會造成破壞性的干擾。流量控制在單個流及整個連接過程中使用。

HTTP/2 provides for flow control through use of the WINDOW_UPDATE frame (Section 6.9).

HTTP/2 通過使用WINDOW_UPDATE幀類型來提供流量控制(章節6.9)。

5.2.1 Flow Control Principles 流量控制規則

HTTP/2 stream flow control aims to allow for future improvements to flow control algorithms without requiring protocol changes. Flow control in HTTP/2 has the following characteristics:

Flow control is hop-by-hop, not end-to-end.
Flow control is based on window update frames. Receivers advertise how many bytes they are prepared to receive on a stream and for the entire connection. This is a credit-based scheme.
Flow control is directional with overall control provided by the receiver. A receiver MAY choose to set any window size that it desires for each stream and for the entire connection. A sender MUST respect flow control limits imposed by a receiver. Clients, servers and intermediaries all independently advertise their flow control window as a receiver and abide by the flow control limits set by their peer when sending.
The initial value for the flow control window is 65,535 bytes for both new streams and the overall connection.
The frame type determines whether flow control applies to a frame. Of the frames specified in this document, only DATA frames are subject to flow control; all other frame types do not consume space in the advertised flow control window. This ensures that important control frames are not blocked by flow control.
Flow control cannot be disabled.
HTTP/2 defines only the format and semantics of the WINDOW_UPDATE frame (Section 6.9). This document does not stipulate how a receiver decides when to send this frame or the value that it sends. Nor does it specify how a sender chooses to send packets. Implementations are able to select any algorithm that suits their needs.

HTTP/2流流量控制目標在於允許不需要協議改動的情況下改進流量控制算法。HTTP/2中的流量控制有以下特點：

流量控制是逐跳的，而不是頭尾連接的。
流量控制是基於窗口更新幀的。接收端廣播自己準備在流及整個連接過程中接收的字節大小。這是一個信用爲基礎的方案。
流量控制是有方向性的，由接收端全權掌握。接收端可以選擇針對流及整個連接設置任意的窗口大小。發送端必須遵守接收端的流量控制限制。客戶端、服務端及中端代理作爲接收者時都獨立的向外廣播他們各自的流量控制窗口，作爲發送者時遵守接收端的限制。
每個新的流及整個連接的流量控制窗口初始值是65,535字節。
幀類型決定了是否適用流量控制規則。本文檔定義的幀中，只有DATA幀受流量控制；所有其他的幀不受廣播的流量控制窗口影響。這保證了重要的控制幀不因流量控制所阻塞。
流量控制不能被禁用。
HTTP/2只標準化WINDOW_UPDATE幀格式(WINDOW_UPDATE)。它沒有規定接收端是何時發送幀或者發送什麼值，也沒有規定發送端如何選擇發送包。具體實現可以選擇任何滿足需求的算法。

Implementations are also responsible for managing how requests and responses are sent based on priority; choosing how to avoid head of line blocking for requests; and managing the creation of new streams. Algorithm choices for these could interact with any flow control algorithm.

具體實現還負責管理請求和響應是如何基於優先級發送的；如何避免請求頭阻塞以及管理新流的創建。這些算法能夠與任何流量控制算法相互作用。

5.2.2 Appropriate Use of Flow Control 正確使用流量控制

Flow control is defined to protect endpoints that are operating under resource constraints. For example, a proxy needs to share memory between many connections, and also might have a slow upstream connection and a fast downstream one. Flow control addresses cases where the receiver is unable process data on one stream, yet wants to continue to process other streams in the same connection.

流量控制的定義是用來保護端點在資源約束條件下的操作。例如，一個代理需要在很多連接之間共享內存，也有可能有緩慢的上游連接和快速的下游連接。流量控制解決的情況是接收端在一個流上處理數據的同時同樣想繼續處理同個連接上的其他流。

Deployments that do not require this capability can advertise a flow control window of the maximum size, incrementing the available space when new data is received. This effectively disables flow control for that receiver. Conversely, a sender is always subject to the flow control window advertised by the receiver.

調度過程中不需要這種能力時可以廣播一個最大值的流量控制窗口，增加接收新數據時的可用空間。發送數據時總是受接收端廣播的流量控制窗口的管理(見[RFC1323])。

Deployments with constrained resources (for example, memory) can employ flow control to limit the amount of memory a peer can consume. Note, however, that this can lead to suboptimal use of available network resources if flow control is enabled without knowledge of the bandwidth-delay product (see [RFC1323]).

資源約束下(例如內存)的調度可以使用流量來限制一個對等端可以消耗的內存數量。需要注意的是如果在不知道帶寬延遲乘積的時候啓用流量控制可能導致無法最優的利用可用的網絡資源(see RFC1323)。

Even with full awareness of the current bandwidth-delay product, implementation of flow control can be difficult. When using flow control, the receiver MUST read from the TCP receive buffer in a timely fashion. Failure to do so could lead to a deadlock when critical frames, such as WINDOW_UPDATE, are not read and acted upon.

即便是對當前的網絡延遲乘積有充分的認識，流量控制的實現也可能很複雜。當使用流量控制時，接收端必須及時地從TCP接收緩衝區讀取數據。這樣做可能導致在一些例如WINDOW_UPDATE的關鍵幀在HTTP/2不可用時導致死鎖。但是流量控制可以保證約束資源能在不需要減少連接利用的情況下得到保護。

5.3 Stream priority 流優先級

A client can assign a priority for a new stream by including prioritization information in the HEADERS frame (Section 6.2) that opens the stream. For an existing stream, the PRIORITY frame (Section 6.3) can be used to change the priority.

新建流的終端可以在報頭幀(章節6.2)中包含優先級信息來對流標記優先級。對於已存在的流，優先級幀(章節6.3)可以用來改變流優先級。

The purpose of prioritization is to allow an endpoint to express how it would prefer its peer allocate resources when managing concurrent streams. Most importantly, priority can be used to select streams for transmitting frames when there is limited capacity for sending.

優先級的目的是允許終端表達它如何讓對等端管理併發流時分配資源。更重要的是，在發送容量有限時優先級能用來選擇流來傳輸幀。

Explicitly setting the priority for a stream is input to a prioritization process. It does not guarantee any particular processing or transmission order for the stream relative to any other stream. An endpoint cannot force a peer to process concurrent streams in a particular order using priority. Expressing priority is therefore only ever a suggestion.

流的優先級明確設置將輸入到優先級處理過程中。它並不能保證能相當其他相關流有特殊的處理或者傳輸順序。終端並不能使用優先級強制要求對等端按照特定順序處理併發流。因此優先級的表達僅僅是一個建議。

Prioritization information can be specified explicitly for streams as they are created using the HEADERS frame, or changed using the PRIORITY frame. Providing prioritization information is optional, so default values are used if no explicit indicator is provided (Section 5.3.5).

優先級信息可以像它們被創建一樣使用報頭幀或者使用優先級幀來明確指定或者改變。提供優先級信息是可選的，沒有明確指定時使用默認值(章節5.3.5)。

5.3.1 Stream Dependencies 流依賴

Each stream can be given an explicit dependency on another stream. Including a dependency expresses a preference to allocate resources to the identified stream rather than to the dependent stream.

每個流可以顯式依賴其他流。包含一個依賴偏好設置表示分配資源給特定的流而不是所依賴的流。

A stream that is not dependent on any other stream is given a stream dependency of 0x0. In other words, the non-existent stream 0 forms the root of the tree.

不依賴任何流的流的流依賴爲0x0。換句話說，不存在的流標識0組成了樹的根。

A stream that depends on another stream is a dependent stream. The stream upon which a stream is dependent is a parent stream. A dependency on a stream that is not currently in the tree - such as a stream in the "idle" state - results in the stream being given a default priority (Section 5.3.5).

依賴其他流的流是一個有依賴流。被依賴的流是父節點流。被依賴的流如果當前不在依賴樹中——例如處於“空閒”狀態的流——流將會被賦予一個默認的優先級(章節 5.3.5)。

When assigning a dependency on another stream, the stream is added as a new dependency of the parent stream. Dependent streams that share the same parent are not order with respect to each other. For example, if streams B and C are dependent on stream A, and if stream D is created with a dependency on stream A, this results in a dependency order of A followed by B, C, and D in any order.

當指定另一個流的依賴時，這個流將添加到父節點流的子流中。共有相同父節點的流互相之間順序是不固定的。例如，如果B和C依賴流A,而且如果新創建的流D依賴流A,最終依賴樹中的結果就是A被B,C和D以任意順序依賴。

    A                 A
   / \      ==>      /|\
  B   C             B D C

$$Example of Default Dependency Creation$$

An exclusive flag allows for the insertion of a new level of dependencies. The exclusive flag causes the stream to become the sole dependency of its parent stream, causing other dependencies to become dependent on the prioritized stream. In the previous example, if stream D is created with an exclusive dependency on stream A, this results in D becoming the dependency parent of B and C.

專用標誌允許插入一個新的層級的依賴。專用標誌導致插入的流成爲其父節點流唯一的子節點流，使其他依賴流變成依賴此優先流。在前面這個例子中，如果D流是用專用標誌在來創建依賴流A的，那麼將導致D流成爲了B和C的依賴父節點流。

                      A
    A                 |
   / \      ==>       D
  B   C              / \
                    B   C

$$Example of Exclusive Dependency Creation$$

Inside the dependency tree, a dependent stream SHOULD only be allocated resources if all of the streams that it depends on (the chain of parent streams up to 0x0) are either closed, or it is not possible to make progress on them.

在一個依賴樹中，一個有依賴的有應該只有在所有其依賴的父節點流(一直到流 0x0的所有父節點流)都關閉或者無法取得進展的情況下才能被分配資源。

A stream cannot depend on itself. An endpoint MUST treat this as a stream error (Section 5.4.2) of type PROTOCOL_ERROR.

流不能依賴其自身。終端必須把這種情況當作類型爲PROTOCOL_ERROR的流錯誤(章節5.4.2)處理。

5.3.2 Dependency Weighting 依賴權重

All dependent streams are allocated an integer weight between 1 to 256 (inclusive).

所有有依賴的流都會被分配一個1-256(含)的整數來標識權重。

Streams with the same parent SHOULD be allocated resources proportionally based on their weight. Thus, if stream B depends on stream A with weight 4, and C depends on stream A with weight 12, and if no progress can be made on A, stream B ideally receives one third of the resources allocated to stream C.

具有相同父節點的流應該根據權重比例來分配資源。因此，如果B流依賴流A的權重是4，C流依賴A流的權重是12，那麼如果A流上不會有進展了，B流理論上將獲取到相對於C流資源的三分之一。

5.3.3 Reprioritization 優先級重組

Stream priorities are changed using the PRIORITY frame. Setting a dependency causes a stream to become dependent on the identified parent stream.

流的優先級是通過使用優先級幀改變的。設置一個依賴將使流變得依賴某個特定的父節點流。

Dependent streams move with their parent stream if the parent is reprioritized. Setting a dependency with the exclusive flag for a reprioritized stream moves all the dependencies of the new parent stream to become dependent on the reprioritized stream.

如果父節點流的優先級被修改，子節點流優先級也將改變。使用專用標記來重新設置流優先級將改變所有對其有依賴的流變成對新的優先級改變的流有依賴。

If a stream is made dependent on one of its own dependencies, the formerly dependent stream is first moved to be dependent on the reprioritized stream's previous parent. The moved dependency retains its weight.

如果流被設置成依賴其子流，之前依賴這個流的所有流將首先轉成依賴優先級改變的流的之前的父節點流。依賴的改變保持其權重不變。

For example, consider an original dependency tree where B and C depend on A, D and E depend on C, and F depends on D. If A is made dependent on D, then D takes the place of A. All other dependency relationships stay the same, except for F, which becomes dependent on A if the reprioritization is exclusive.

例如，考慮原始依賴樹中B和C依賴A,D和E依賴C,且F依賴D。如果A改成依賴D，那麼D替換A的位置。其他所有的依賴關係保持不變，不過如果優先級修改使用的是專用標記，那麼F將變成依賴A。

    ?                ?                ?                 ?
    |               / \               |                 |
    A              D   A              D                 D
   / \            /   / \            / \                |
  B   C     ==>  F   B   C   ==>    F   A       OR      A
     / \                 |             / \             /|\
    D   E                E            B   C           B C F
    |                                     |             |
    F                                     E             E
               (intermediate)   (non-exclusive)    (exclusive)

5.3.4 Prioritization State Management 優先級狀態管理

When a stream is removed from the dependency tree, its dependencies can be moved to become dependent on the parent of the closed stream. The weights of new dependencies are recalculated by distributing the weight of the dependency of the closed stream proportionally based on the weights of its dependencies.

當流從依賴樹中移走後，依賴它的子流可以轉變成依賴被關閉流的父節點流。新的依賴的權重將根據關閉流的權重以及流自身的權重重新計算。

Streams that are removed from the dependency tree cause some prioritization information to be lost. Resources are shared between streams with the same parent stream, which means that if a stream in that set closes or becomes blocked, any spare capacity allocated to a stream is distributed to the immediate neighbors of the stream. However, if the common dependency is removed from the tree, those streams share resources with streams at the next highest level.

從依賴樹中移除的流導致某些優先級信息丟失。資源在具有相同父流的流之間共享，這意味着如果這個集合中的某個流關閉或者阻塞，任何空閒容量將分配給最近的鄰居流。然而，如果子流的共有依賴被從樹中移除，這些子流將與上一層的流共享資源。

For example, assume streams A and B share a parent, and streams C and D both depend on stream A. Prior to the removal of stream A, if streams A and D are unable to proceed, then stream C receives all the resources dedicated to stream A. If stream A is removed from the tree, the weight of stream A is divided between streams C and D. If stream D is still unable to proceed, this results in stream C receiving a reduced proportion of resources. For equal starting weights, C receives one third, rather than one half, of available resources.

例如，假定A流和B流共有同個父依賴，且C和D流都依賴A流。在A流移除之前，如果A和D流都無法繼續進行，那麼C流就會接收所有分配給流A的資源。如果A流從樹中移除，流A的權重將分配給C流和D流。如果D流依舊無法進行，將導致C流獲取到的資源比例變少。對於同等的初始權重，C流獲取到三分之一而不是二分之一的可用資源。

It is possible for a stream to become closed while prioritization information that creates a dependency on that stream is in transit. If a stream identified in a dependency has had any associated priority information destroyed, then the dependent stream is instead assigned a default priority. This potentially creates suboptimal prioritization, since the stream could be given a priority that is higher than intended.

流有可能在優先級信息在自身創建的依賴還在傳輸的時候變成關閉狀態。如果依賴關係中的一個流存在任何相關的優先級信息被銷燬，那麼依賴它的流將被分配爲默認的優先級。這有可能導致不理想的優先級，因爲流可能被賦予一個高於預期的優先級。

To avoid these problems, an endpoint SHOULD retain stream prioritization state for a period after streams become closed. The longer state is retained, the lower the chance that streams are assigned incorrect or default priority values.

爲了避免這些問題，終端應該在流關閉後的一段時間內保留流優先級信息。狀態被保留的時間越長，流被分配錯誤的或者默認的優先級值的可能性就越小。

This could create a large state burden for an endpoint, so this state MAY be limited. An endpoint MAY apply a fixed upper limit on the number of closed streams for which prioritization state is tracked to limit state exposure. The amount of additional state an endpoint maintains could be dependent on load; under high load, prioritization state can be discarded to limit resource commitments. In extreme cases, an endpoint could even discard prioritization state for active or reserved streams. If a fixed limit is applied, endpoints SHOULD maintain state for at least as many streams as allowed by their setting for SETTINGS_MAX_CONCURRENT_STREAMS.

這可能增加終端的負擔，因此這種狀態可以被限制。終端可能會對跟蹤狀態的關閉的流的個數使用一個固定的上限來限制狀態溢出。終端可能根據負荷來決定保留的額外的狀態的數目；在高負荷下，可以丟棄額外的優先級狀態來限制資源的任務。在極端情況下，終端甚至可以丟棄激活或者保留狀態流的優先級信息。如果使用了固定的限制，終端應當至少保留跟SETTINGS_MAX_CONCURRENT_STREAMS設置一樣大小的流狀態。

An endpoint receiving a PRIORITY frame that changes the priority of a closed stream SHOULD alter the dependencies of the streams that depend on it, if it has retained enough state to do so.

如果有足夠的空間，終端接收到優先級幀修改關閉的流的優先級的應該修改流所的依賴的流的優先級，

5.4 Error Handling 錯誤處理

HTTP/2 framing permits two classes of error:

An error condition that renders the entire connection unusable is a connection error.
An error in an individual stream is a stream error.

A list of error codes is included in Section 7.

HTTP/2框架允許兩類錯誤：

使整個連接不可用的錯誤。
單個流中出現的錯誤。

錯誤碼列表可以在"ErrorCodes"找到。

5.4.1 Connection Error Handling 連接錯誤處理

A connection error is any error which prevents further processing of the framing layer, or which corrupts any connection state.

流錯誤是阻止幀層更進一步進行處理或者破壞任何流狀態的錯誤。

An endpoint that encounters a connection error SHOULD first send a GOAWAY frame (Section 6.8) with the stream identifier of the last stream that it successfully received from its peer. The GOAWAY frame includes an error code that indicates why the connection is terminating. After sending the GOAWAY frame, the endpoint MUST close the TCP connection.

發送流錯誤的終端應當首先發送一個超時(GOAWAY)幀(章節6.8)，並帶有最近的一個成功從對等端接收幀的流的標識符。GOAWAY超時幀包含鏈接終端的錯誤碼。發送GOAWAY後，終端必須關閉TCP連接。

It is possible that the GOAWAY will not be reliably received by the receiving endpoint. In the event of a connection error, GOAWAY only provides a best effort attempt to communicate with the peer about why the connection is being terminated.

超時(GOAWAY)幀有可能不被接收端有效接收。在連接錯誤事件中，超時(GOAWAY)幀是嘗試跟對等端通信告知連接終止原因的最佳實踐。

An endpoint can end a connection at any time. In particular, an endpoint MAY choose to treat a stream error as a connection error. Endpoints SHOULD send a GOAWAY frame when ending a connection, providing that circumstances permit it.

終端可以在任何時候終止一個連接。類似的，終端可以選擇將流錯誤作爲連接錯誤處理。只要環境許可，終端在終止連接時應當發送一個GOAWAY幀。

5.4.2 Stream Error Handling 流錯誤處理

A stream error is an error related to a specific stream that does not affect processing of other streams.

流錯誤是與特定流相關的錯誤，並且不會影響其他流的處理。

An endpoint that detects a stream error sends a RST_STREAM frame (Section 6.4) that contains the stream identifier of the stream where the error occurred. The RST_STREAM frame includes an error code that indicates the type of error.

終端檢測到流錯誤時發送一個帶有錯誤發生時的流標識符的RST_STREAM幀(RST_STREAM,章節6.4)。RST_STREAM幀帶有表示錯誤類型的錯誤碼。

A RST_STREAM is the last frame that an endpoint can send on a stream. The peer that sends the RST_STREAM frame MUST be prepared to receive any frames that were sent or enqueued for sending by the remote peer. These frames can be ignored, except where they modify connection state (such as the state maintained for header compression (Section 4.3), or flow control).

RST_STREAM是終端可以發送一個流的最後一幀。發送RST_STREAM幀的對等端必須準備好接收任何由遠端對等端發送或者準備發送的幀。這些幀可以被忽略，除非連接狀態被修改（例如報頭壓縮(章節4.3)中的狀態）。

Normally, an endpoint SHOULD NOT send more than one RST_STREAM frame for any stream. However, an endpoint MAY send additional RST_STREAM frames if it receives frames on a closed stream after more than a round-trip time. This behavior is permitted to deal with misbehaving implementations.

通常，終端不應該在任何流上發送多個RST_STREAM幀。但是，終端如果在一個關閉的流上超過rtt時間後收到幀，則可以發送的額外的RST_STREAM幀。這種做法是被允許用來處理這種非常規情況。

An endpoint MUST NOT send a RST_STREAM in response to an RST_STREAM frame, to avoid looping.

終端絕不能在收到RST_STREAM幀後響應一個RST_STREAM幀，避免死循環。

5.4.3 Connection Termination 連接終止

If the TCP connection is torn down while streams remain in open or half closed states, then the endpoint MUST assume that those streams were abnormally interrupted and could be incomplete.

如果TCP連接在流仍然保持打開或者半封閉狀態下斷開，那麼終端必須假定這些流是異常終端且不完整的。

5.5 Extending HTTP/2 HTTP/2擴展

HTTP/2 permits extension of the protocol. Protocol extensions can be used to provide additional services or alter any aspect of the protocol, within the limitations described in this section. Extensions are effective only within the scope of a single HTTP/2 connection.

HTTP/2協議允許擴展。協議擴展可用在提供額外的服務或者在此節定義的限制內修改協議的任何方面。擴展只在單個HTTP/2連接範圍內有效。

Extensions are permitted to use new frame types (Section 4.1), new settings (Section 6.5.2), new error codes (Section 7), or new header fields that start with a colon (:). Of these, registries are established for frame types (Section 11.2), settings (Section 11.3) and error codes (Section 11.4).

擴展可以允許使用新的幀類型(章節4.1),新的設置(章節6.5.2)，新的錯誤碼(章節7)，或者新的(:)開頭的報頭字段。這裏面，註冊將會在幀類型(章節11.2)、設置(章節11.3)和錯誤碼(章節11.2)中建立。

Implementations MUST ignore unknown or unsupported values in all extensible protocol elements. Implementations MUST discard frames that have unknown or unsupported types. This means that any of these extension points can be safely used by extensions without prior arrangement or negotiation.

具體實現必須忽略擴展協議元素中未知或者不支持的值。實現必須丟棄包含未知或者不支持的類型的幀。這意味着任意這些擴展點能被擴展安全的使用，不需要事先安排或協商。

However, extensions that could change the semantics of existing protocol components MUST be negotiated before being used. For example, an extension that changes the layout of the HEADERS frame cannot be used until the peer has given a positive signal that this is acceptable. In this case, it could also be necessary to coordinate when the revised layout comes into effect. Note that treating any frame other than DATA frames as flow controlled is such a change in semantics, and can only be done through negotiation.

然而，可能導致現有協議元素語義改變的擴展必須經過協商後才能使用。例如，一個修改報頭幀佈局的擴展使用之前必須收到對等端發送的允許使用的信號。在這種情況下，修改過的幀佈局結構生效前需要通過協商。注意將任意數據幀以外的幀納入流量控制的修改也是語義上的修改，也必須在協商後才能生效。

This document doesn't mandate a specific method for negotiating the use of an extension, but notes that a setting (Section 6.5.2) could be used for that purpose. If both peers set a value that indicates willingness to use the extension, then the extension can be used. If a setting is used for extension negotiation, the initial value MUST be defined so that the extension is initially disabled.

本文檔並不強制要求使用特定的方法來完成使用擴展的協商，但注意可以通過使用設置(章節6.5.2)來實現這個目的。如果對等端雙方都設置了指示願意使用擴展的值，那麼這個擴展就可以被使用。如果某個設置用來協商擴展的使用，那麼必須定義初始值來設置擴展初始時是被禁用的。

6 Frame Definitions 幀定義

This specification defines a number of frame types, each identified by a unique 8-bit type code. Each frame type serves a distinct purpose either in the establishment and management of the connection as a whole, or of individual streams.

本規範定義了一系列的幀類型，每種類型由獨特的8位類型代碼標記。不管是在連接管理或單獨的流中，每種幀都爲了特定的目的而服務。

The transmission of specific frame types can alter the state of a connection. If endpoints fail to maintain a synchronized view of the connection state, successful communication within the connection will no longer be possible. Therefore, it is important that endpoints have a shared comprehension of how the state is affected by the use any given frame.

特定幀類型的傳輸可以修改連接的狀態。如果終端不能保持同步的連接狀態，連接中的通信將失敗。因此，終端對於如何使用給定幀修改狀態達成共識很重要。

6.1 DATA 數據幀

DATA frames (type=0x0) convey arbitrary, variable-length sequences of octets associated with a stream. One or more DATA frames are used, for instance, to carry HTTP request or response payloads.

數據幀（類型=0x0）表示隨意，由伴隨流的可變長度序列組成。例如，一個或多個數據幀被用來攜帶HTTP請求或者響應的載體。

DATA frames MAY also contain arbitrary padding. Padding can be added to DATA frames to obscure the size of messages.

數據幀也可以包含任意填充物。填充物可以添加到數據幀中來隱藏消息的大小。

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | [Pad High(8)] | [Pad Low (8)] |          Data (*)             .
 +---------------+---------------+-------------------------------+
 .                            Data (*)                         ...
 +---------------------------------------------------------------+
 |                           Padding (*)                       ...
 +---------------------------------------------------------------+

The DATA frame contains the following fields:

Pad Length:An 8-bit field containing the length of the frame padding in units of octets. This field is optional and is only present if the PADDED flag is set.
Data:Application data. The amount of data is the remainder of the frame payload after subtracting the length of the other fields that are present.
Padding:Padding octets that contain no application semantic value. Padding octets MUST be set to zero when sending and ignored when receiving.

數據幀包含以下字段：

Pad Length ：包含字節爲單位的幀填充長度的8位字段。這個字段是可選的，並且只在設置了PADDED標記的情況下呈現。
Data ：應用數據。數據量的大小是幀的有效載荷減去其他呈現字段的長度。
Padding ：填充字節不包含任何應用語義值。填充字節必須在發送的時候設置爲0，在接收的時候忽略。

The DATA frame defines the following flags:

END_STREAM (0x1):Bit 1 being set indicates that this frame is the last that the endpoint will send for the identified stream. Setting this flag causes the stream to enter one of the "half closed" states or the "closed" state (Section 5.1).
END_SEGMENT (0x2):Bit 2 being set indicates that this frame is the last for the current segment. Intermediaries MUST NOT coalesce frames across a segment boundary and MUST preserve segment boundaries when forwarding frames.
PADDED (0x8):Bit 4 being set indicates that the Pad Length field is present.

數據幀定義了以下標記：

END_STREAM (0x1) ：位1用來表示當前幀是確定的流發送的最後一幀。設置這個標記時流進入到一種半封閉狀態或者關閉狀態(章節5.1)。
END_SEGMENT (0x2) ：位2表示是當前端的最後一幀。代理端絕對不能跨越多個端的邊界來合併幀，轉發幀的時候代理端必須保持片段的邊界。
PADDED (0x8) ：位4用來表示Pad Length 字段是可見的。

DATA frames MUST be associated with a stream. If a DATA frame is received whose stream identifier field is 0x0, the recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

數據幀絕對需要與流相關聯。如果接收到流標記字段是0x0的數據幀，必須響應一個類型爲協議錯誤的連接錯誤(章節5.4.1)。

DATA frames are subject to flow control and can only be sent when a stream is in the "open" or "half closed (remote)" states. The entire DATA frame payload is included in flow control, including Pad Length and Padding fields if present. If a DATA frame is received whose stream is not in "open" or "half closed (local)" state, the recipient MUST respond with a stream error (Section 5.4.2) of type STREAM_CLOSED.

數據幀遵從流量控制，並且只有在流是打開或者半封閉(遠端)狀態下才能夠被髮送。填充同樣包含在流量控制中。如果數據幀在相關流不是在打開和半封閉(本地)狀態下被接收，接收端必須響應一個類型爲流關閉的流錯誤(章節5.4.2)。

The total number of padding octets is determined by the value of the Pad Length field. If the length of the padding is greater than the length of the remainder of the frame payload, the recipient MUST treat this as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

填充字節的總數取決於Pad Length 的值。如果填充物的大小大於幀有效載荷的大小，接收端必須作爲類型爲協議錯誤的連接錯誤(章節5.4.1)處理。

Note: A frame can be increased in size by one octet by including a Pad Length field with a value of zero.

請注意 ：爲幀的大小加1字節可通過增加一個值爲0的Pad Length 值的方法。

Use of padding is a security feature; as such, its use demands some care, see Section 10.7.

使用填充是一種安全手段；例如，用來滿足特定需求，見章節10.7.

6.2 HEADERS 報頭

The HEADERS frame (type=0x1) carries name-value pairs. It is used to open a stream (Section 5.1). HEADERS frames can be sent on a stream in the "open" or "half closed (remote)" states.

報頭幀(類型=0x1)由鍵值對組成。它用來打開一個流(章節5.1)。報頭幀能在流打開或者半封閉(遠程)的狀態下發送。

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |Pad Length? (8)|
 +-+-------------+-----------------------------------------------+
 |E|                 Stream Dependency? (31)                     |
 +-+-------------+-----------------------------------------------+
 |  Weight? (8)  |
 +-+-------------+-----------------------------------------------+
 |                   Header Block Fragment (*)                 ...
 +---------------------------------------------------------------+
 |                           Padding (*)                       ...
 +---------------------------------------------------------------+

The HEADERS frame payload has the following fields:

Pad Length:An 8-bit field containing the length of the frame padding in units of octets. This field is optional and is only present if the PADDED flag is set.
E:A single bit flag indicates that the stream dependency is exclusive, see Section 5.3. This field is optional and is only present if the PRIORITY flag is set.
Stream Dependency:A 31-bit stream identifier for the stream that this stream depends on, see Section 5.3. This field is optional and is only present if the PRIORITY flag is set.
Weight:An 8-bit weight for the stream, see Section 5.3. Add one to the value to obtain a weight between 1 and 256. This field is optional and is only present if the PRIORITY flag is set.
Header Block Fragment:A header block fragment (Section 4.3).
Padding:Padding octets.

報頭幀主體有以下字段：

Pad Length ： 8位的包含單位爲字節幀填充長度字段。這個字段是可選的並只有在設置了PADDED 標記的情況下才呈現。
E ： 1位的標記用於標識流依賴是否是專用的，見章節5.3。這個字段是可選的，並且只在優先級標記設置的情況下才呈現。
Stream Dependency ： 31位流所依賴的流的標識符的字段，見章節5.3。這個字段是可選的，並且只在優先級標記設置的情況下才呈現。
Weight : 流的8位權重標記，見章節5.3。添加一個1-256的值來存儲流的權重。這個字段是可選的，並且只在優先級標記設置的情況下才呈現。
Header Block Fragment ：報頭塊碎片。
Padding ：填充字節

The HEADERS frame defines the following flags:

END_STREAM (0x1):Bit 1 being set indicates that the header block (Section 4.3) is the last that the endpoint will send for the identified stream. Setting this flag causes the stream to enter one of "half closed" states (Section 5.1).
A HEADERS frame that is followed by CONTINUATION frames carries the END_STREAM flag that signals the end of a stream. A CONTINUATION frame cannot be used to terminate a stream.
END_SEGMENT (0x2):Bit 2 being set indicates that this frame is the last for the current segment. Intermediaries MUST NOT coalesce frames across a segment boundary and MUST preserve segment boundaries when forwarding frames. END_SEGMENT, like END_STREAM, applies to a complete sequence of HEADERS and CONTINUATION frames.
END_HEADERS (0x4):Bit 3 being set indicates that this frame contains an entire header block (Section 4.3) and is not followed by any CONTINUATION frames.A HEADERS frame without the END_HEADERS flag set MUST be followed by a CONTINUATION frame for the same stream. A receiver MUST treat the receipt of any other type of frame or a frame on a different stream as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
PADDED (0x8):Bit 4 being set indicates that the Pad Length field is present.
PRIORITY (0x20):Bit 6 being set indicates that the Exclusive Flag (E), Stream Dependency, and Weight fields are present; see Section 5.3.

報頭幀定義了以下標記：

END_STREAM (0x1) ：位1用來標識這是發送端對確定的流發送的最後報頭區塊(章節4.3)。設置這個標記將使流進入一種半封閉狀態(章節5.1)。後面伴隨帶有END_STREAM標記的延續幀的報頭幀表示流的終止。延續幀不用來用終止流。
END_SEGMENT (0x2) ：位2表示這是當前端的最後一幀。中介端絕對不能跨片段來合併幀，且在轉發幀的時候必須保持片段的邊界。
END_HEADERS (0x4) ：位3表示幀包含了整個的報頭塊(章節4.3)，且後面沒有延續幀。不帶有END_HEADERS標記的報頭幀在同個流上後面必須跟着延續幀。接收端接收到任何其他類型的幀或者在其他流上的幀必須作爲類型爲協議錯誤的連接錯誤處理。
PADDED (0x8) ：位4表示Pad Length字段會呈現。
PRIORITY (0x8) ：位6設置指示專用標記(E),流依賴及權重字段將會呈現；見章節5.3

The payload of a HEADERS frame contains a header block fragment (Section 4.3). A header block that does not fit within a HEADERS frame is continued in a CONTINUATION frame (Section 6.10).

報頭幀的主體包含一個報頭區塊碎片(章節4.3)。報頭區塊大於一個報頭幀的將在延續幀中(章節6.10)繼續傳送。

HEADERS frames MUST be associated with a stream. If a HEADERS frame is received whose stream identifier field is 0x0, the recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

報頭幀必須與一個流相關聯。如果一個接收到一個流標示識0x0得報頭幀，接收端必須響應一個類型爲協議錯誤的連接錯誤(章節5.4.1)。

The HEADERS frame changes the connection state as described in Section 4.3.

報頭幀改變連接狀態在章節4.3此中表述。

The HEADERS frame includes optional padding. Padding fields and flags are identical to those defined for DATA frames (Section 6.1).

報頭幀包含可選的填充段。填充字段和標記同數據幀中描述的相同(章節6.1)。

6.3 PRIORITY 優先級幀

The PRIORITY frame (type=0x2) specifies the sender-advised priority of a stream (Section 5.3). It can be sent at any time for an existing stream, including closed streams. This enables reprioritization of existing streams.

優先級幀(type=0x2)明確了發送者建議的流的優先級(章節5.3)。它可以任意時間在存在的流中發送，包括已閉合的流。這個允許了在已存在的流中重新排列優先級次序。

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |E|                  Stream Dependency (31)                     |
 +-+-------------+-----------------------------------------------+
 |   Weight (8)  |
 +-+-------------+

The payload of a PRIORITY frame contains the following fields:

E: A single bit flag indicates that the stream dependency is exclusive, see Section 5.3.
Stream Dependency: A 31-bit stream identifier for the stream that this stream depends on, see Section 5.3.
Weight: An 8-bit weight for the identified stream dependency, see Section 5.3. Add one to the value to obtain a weight between 1 and 256.

優先級幀實體包含下列字段：

E : 一位的標記，指示流的依賴是專有的，見章節5.3
Stream Dependency ：標識流所依賴的流的31位流標識符，見章節5.3
Weight: 流的依賴的的權重(8位)，見章節5.3。添加一個1-256的權重值。

The PRIORITY frame does not define any flags.

優先級不定義任何標記。

The PRIORITY frame is associated with an existing stream. If a PRIORITY frame is received with a stream identifier of 0x0, the recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

優先級與存在的流相關聯。如果接收端收到流標識爲0的優先級幀，必須響應一個類型爲協議錯誤的連接錯誤(章節5.4.1)。

The PRIORITY frame can be sent on a stream in any of the "reserved (remote)", "open", "half closed (local)", "half closed (remote)", or "closed" states, though it cannot be sent between consecutive frames that comprise a single header block (Section 4.3). Note that this frame could arrive after processing or frame sending has completed, which would cause it to have no effect on the current stream. For a stream that is in the "half closed (remote)" or "closed" - state, this frame can only affect processing of the current stream and not frame transmission.

優先級幀可以在流狀態爲“保留(遠端)”、“打開"、“半封閉(本地)”或者“半封閉(遠程)”狀態下發送,但它不能在由單個報頭區塊組成的連續幀之間發送。需要注意的是這個幀可能在處理或者幀發送已經完成之後纔到達，這可能導致沒有效果。對於處於“半封閉(遠程)”狀態下的流，優先級幀只能影響流的處理而不是傳輸。

The PRIORITY frame is the only frame that can be sent for a stream in the "closed" state. This allows for the reprioritization of a group of dependent streams by altering the priority of a parent stream, which might be closed. However, a PRIORITY frame sent on a closed stream risks being ignored due to the peer having discarded priority state information for that stream.

優先級幀是關閉的流上唯一允許發送的幀。這允許通過修改可能已關閉的父節點流的優先級來重新設置一組有依賴的流的優先級。然而，關閉流上發送的優先級幀可能存在被對等端忽略的風險，因爲對等端可能已經丟棄了這個流的優先級狀態信息。

6.4 RST_STREAM RST_STREAM幀

The RST_STREAM frame (type=0x3) allows for abnormal termination of a stream. When sent by the initiator of a stream, it indicates that they wish to cancel the stream or that an error condition has occurred. When sent by the receiver of a stream, it indicates that either the receiver is rejecting the stream, requesting that the stream be cancelled, or that an error condition has occurred.

RST_STREAM幀(type=0x3)允許流的異常終止。當被流的指示器發送時，它表示期望取消流或者錯誤條件發生。當被接收端的流發送時，它表示接收者希望拒絕流、流被取消或者發生了錯誤。

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Error Code (32)                        |
 +---------------------------------------------------------------+

The RST_STREAM frame contains a single unsigned, 32-bit integer identifying the error code (Section 7). The error code indicates why the stream is being terminated.

RST_STREAM 幀由一個無符號的32位整數標記錯誤碼。錯誤碼指明流被終止的原因。

The RST_STREAM frame does not define any flags.

RST_STREAM 幀未定義任何標記。

The RST_STREAM frame fully terminates the referenced stream and causes it to enter the closed state. After receiving a RST_STREAM on a stream, the receiver MUST NOT send additional frames for that stream. However, after sending the RST_STREAM, the sending endpoint MUST be prepared to receive and process additional frames sent on the stream that might have been sent by the peer prior to the arrival of the RST_STREAM.

RST_STREAM 幀完全終止相關的流並使其轉入關閉狀態。在接收到流的RST_STREAM幀後，接收端絕對不能在流上發送額外的幀。然而，在發送RST_STREAM幀後，發送端必須要準備接收並處理流上的其他幀，因爲對等端有可能在收到RST_STREAM幀前就已經發送這些幀。

RST_STREAM frames MUST be associated with a stream. If a RST_STREAM frame is received with a stream identifier of 0x0, the recipient MUST treat this as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

RST_STREAM 幀必須與流相關聯。如果接收端收到流標示符爲0x0的RST_STREAM 幀，必須作爲類型爲協議錯誤的連接錯誤(章節5.4.1)處理。

RST_STREAM frames MUST NOT be sent for a stream in the "idle" state. If a RST_STREAM frame identifying an idle stream is received, the recipient MUST treat this as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

RST_STREAM幀絕對不能在流處於“空閒”狀態下發送。如果接收端收到流狀態爲空閒的RST_STREAM幀，必須作爲類型爲協議錯誤的連接錯誤(章節5.4.1)處理。

6.5 SETTINGS 設置幀

The SETTINGS frame (type=0x4) conveys configuration parameters that affect how endpoints communicate, such as preferences and constraints on peer behavior. The SETTINGS frame is also used to acknowledge the receipt of those parameters. Individually, a SETTINGS parameter can also be referred to as a "setting".

設置幀(type=0x4)包含影響如何與終端通信的設置參數(例如偏好設置以及對等端的行爲約束)，並且用來確認這些參數的接收。單個的設置參數也可以被認爲是“設置”。

SETTINGS parameters are not negotiated; they describe characteristics of the sending peer, which are used by the receiving peer. Different values for the same parameter can be advertised by each peer. For example, a client might set a high initial flow control window, whereas a server might set a lower value to conserve resources.

設置參數不是通過協商確定的；它們描述發送端的特點，並被接收端使用。相同的參數對不同的對等端設置可能不同。例如，一個客戶端可能設置一個較高的流量控制窗口，而服務器爲了保存資源可能設置一個較低的值。

A SETTINGS frame MUST be sent by both endpoints at the start of a connection, and MAY be sent at any other time by either endpoint over the lifetime of the connection. Implementations MUST support all of the parameters defined by this specification.

設置幀必須由兩個終端在連接開始的時候發送，並且可以由各個終端在連接生存期的任意時間發送。具體實現必須支持本規範定義的所有參數。

Each parameter in a SETTINGS frame replaces any existing value for that parameter. Parameters are processed in the order in which they appear, and a receiver of a SETTINGS frame does not need to maintain any state other than the current value of its parameters. Therefore, the value of a SETTINGS parameter is the last value that is seen by a receiver.

設置幀的所有參數將替換參數中現有值。參數由他們出現的順序來處理，而且接收設置幀並不需要保存當前值以外的任何狀態。因此，設置參數的值是接收端接收到的最後一個值。

SETTINGS parameters are acknowledged by the receiving peer. To enable this, the SETTINGS frame defines the following flag:

設置參數是被接收端公認的。爲了實現這個，設置幀定義了以下標記：

ACK (0x1) ： Bit 1 being set indicates that this frame acknowledges receipt and application of the peer's SETTINGS frame. When this bit is set, the payload of the SETTINGS frame MUST be empty. Receipt of a SETTINGS frame with the ACK flag set and a length field value other than 0 MUST be treated as a connection error (Section 5.4.1) of type FRAME_SIZE_ERROR. For more info, see Settings Synchronization (Section 6.5.3).

ACK (0x1) ：位1表示設置幀已被接收端接收並應用。如果這個位設置了，設置幀的載體必須爲空。接收到字段長度不是0的帶有ACK標記的設置幀必須作爲類型爲幀大小錯誤的連接錯誤(章節5.4.1)處理，更多信息，見同步設置(章節6.5.3)。

SETTINGS frames always apply to a connection, never a single stream. The stream identifier for a SETTINGS frame MUST be zero (0x0). If an endpoint receives a SETTINGS frame whose stream identifier field is anything other than 0x0, the endpoint MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

設置幀總是應用於連接，而不是一個單獨的流。流的設置幀標識必須爲0.如果終端接收到流設置幀標識不是0的設置幀，必須響應一個類型爲協議錯誤的連接錯誤(章節5.4.1)。

The SETTINGS frame affects connection state. A badly formed or incomplete SETTINGS frame MUST be treated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

設置幀影響連接狀態。格式錯誤或者未完成的設置幀必須作爲類型爲協議錯誤的連接錯誤處理。

6.5.1 SettingFormat 設置幀格式

The payload of a SETTINGS frame consists of zero or more parameters, each consisting of an unsigned 16-bit setting identifier and an unsigned 32-bit value.

設置幀載體包含0個或多個參數，每個包含一個無符號的16位標識以及一個無符號的32位值。

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |Identifier (8) |                 Value (32)                  ...
 +---------------+-----------------------------------------------+
 ...Value        |
 +---------------+

6.5.2 Defined SETTINGS Parameters 設置幀參數

The following parameters are defined:

SETTINGS_HEADER_TABLE_SIZE (0x1):Allows the sender to inform the remote endpoint of the maximum size of the header compression table used to decode header blocks. The encoder can select any size equal to or less than this value by using signaling specific to the header compression format inside a header block. The initial value is 4,096 bytes.
SETTINGS_ENABLE_PUSH (0x2):This setting can be use to disable server push (Section 8.2). An endpoint MUST NOT send a PUSH_PROMISE frame if it receives this parameter set to a value of 0. An endpoint that has both set this parameter to 0 and had it acknowledged MUST treat the receipt of a PUSH_PROMISE frame as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.The initial value is 1, which indicates that server push is permitted. Any value other than 0 or 1 MUST be treated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
SETTINGS_MAX_CONCURRENT_STREAMS (0x3):Indicates the maximum number of concurrent streams that the sender will allow. This limit is directional: it applies to the number of streams that the sender permits the receiver to create. Initially there is no limit to this value. It is recommended that this value be no smaller than 100, so as to not unnecessarily limit parallelism.A value of 0 for SETTINGS_MAX_CONCURRENT_STREAMS SHOULD NOT be treated as special by endpoints. A zero value does prevent the creation of new streams, however this can also happen for any limit that is exhausted with active streams. Servers SHOULD only set a zero value for short durations; if a server does not wish to accept requests, closing the connection could be preferable.
SETTINGS_INITIAL_WINDOW_SIZE (0x4):Indicates the sender's initial window size (in bytes) for stream level flow control. The initial value is 65,535.This setting affects the window size of all streams, including existing streams, see Section 6.9.2.Values above the maximum flow control window size of 231 - 1 MUST be treated as a connection error (Section 5.4.1) of type FLOW_CONTROL_ERROR.

定義了以下參數：

SETTINGS_HEADER_TABLE_SIZE (1) : 允許發送端通知遠端終端解碼報頭區塊的報頭壓縮表的最大承載量。這個編碼器可以選擇在報頭區塊中使用特定信號來減少報頭壓縮的大小（？？？）。初始值是4,096個字節。
SETTINGS_ENABLE_PUSH (2) : 這個參數可以用來關閉服務器推送。終端在接收到此參數爲0的情況下絕對不能發送服務器推送承諾幀。終端在已經設置此參數爲0並且承認的情況下必須對接收到的服務器推送作爲類型爲協議錯誤的連接錯誤處理。初始值是1，表示推送是許可的。任何不是0或1的值必須作爲類型爲協議錯誤的連接錯誤處理。
SETTINGS_MAX_CONCURRENT_STREAMS (3) : 標明發送者允許的最大併發流。此限制是定向的：它適用於發送端允許接收端創建的最大併發流的數量。初始化時這個值沒有限制。建議值不要大於100,以免不必要的限制並行。此設置爲0的值不應該被終端認爲是特殊的。0的值阻止了新的流的創建，另外它也適用於被激活的流用盡的任何限制。對於短連接不應該設置此參數爲0；如果服務端不希望接收任何請求，最佳的做法是關閉連接。
SETTINGS_INITIAL_WINDOW_SIZE (4) : 表示發送端對流層流量控制的初始窗口大小(字節單位)。初始值是65,535。這個參數影響了所有流的窗口大小，包括現有的流。見章節6.9.2.流量控制窗口大小值大於2的31次方-1的必須被作爲流量控制錯誤的連接錯誤處理。

An endpoint that receives a SETTINGS frame with any unknown or unsupported identifier MUST ignore that setting.

終端收到其他標記的設置幀必須作爲類型爲協議錯誤的連接錯誤處理。

6.5.3 Settings Synchronization 設置同步

Most values in SETTINGS benefit from or require an understanding of when the peer has received and applied the changed the communicated parameter values. In order to provide such synchronization timepoints, the recipient of a SETTINGS frame in which the ACK flag is not set MUST apply the updated parameters as soon as possible upon receipt.

大部分設置值收益於或者需要了解對等端接收到並且改變了通信過的參數的值的時機。爲了提供這樣一種同步的時間點，接收到沒有設置ACK標記的設置幀必須儘快將更新過的參數適用於接收端上。

The values in the SETTINGS frame MUST be applied in the order they appear, with no other frame processing between values. Once all values have been applied, the recipient MUST immediately emit a SETTINGS frame with the ACK flag set. Upon receiving a SETTINGS frame with the ACK flag set, the sender of the altered parameters can rely upon their application.

設置幀的值必須按照它們出現的順序被使用，在處理值中間不能處理其他幀。一旦所有的值被應用，接收端必須馬上發送一個帶有ACK標記的設置幀。在接收到帶有ACK標記的設置幀後，修改參數的發送端可以認爲修改已生效。

If the sender of a SETTINGS frame does not receive an acknowledgement within a reasonable amount of time, it MAY issue a connection error (Section 5.4.1) of type SETTINGS_TIMEOUT.

如果設置參數的發送端沒有在一定時間內收到確認響應，它可以發出一個類型爲設置超時的連接錯誤(章節5.4.1)。

6.6 PUSH_PROMISE 推送承諾幀

The PUSH_PROMISE frame (type=0x5) is used to notify the peer endpoint in advance of streams the sender intends to initiate. The PUSH_PROMISE frame includes the unsigned 31-bit identifier of the stream the endpoint plans to create along with a set of headers that provide additional context for the stream. Section 8.2 contains a thorough description of the use of PUSH_PROMISE frames.

推送承諾幀(type=0x5)用來在流發送者準備發送流之前告知對等端。推送承諾幀包含了終端準備創建的長流的31位無符號標記以及提供附加上下文的報頭的集合。章節8.2詳細描述了推送承諾幀的使用。

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | [Pad High(8)] | [Pad Low (8)] |X|  Promised Stream ID (31)  ...
 +---------------+---------------+-+-----------------------------+
 ...    Promised Stream ID       | Header Block Fragment (*)   ...
 +-------------------------------+-------------------------------+
 |                   Header Block Fragment (*)                 ...
 +---------------------------------------------------------------+
 |                           Padding (*)                       ...
 +---------------------------------------------------------------+

The PUSH_PROMISE frame payload has the following fields:

Pad Length:An 8-bit field containing the length of the frame padding in units of octets. This field is optional and is only present if the PADDED flag is set.
R:A single reserved bit.
Promised Stream ID:This unsigned 31-bit integer identifies the stream the endpoint intends to start sending frames for. The promised stream identifier MUST be a valid choice for the next stream sent by the sender (see new stream identifier (Section 5.1.1)).
Header Block Fragment: header block fragment (Section 4.3) containing request header fields.
Padding:Padding octets.

報頭幀載體包含以下字段：

Pad High : 填充大小高位。這個字段只有在PAD_HIGH標記設置的情況下才呈現。
Pad Low : 填充大小低位。這個字段只有在PAD_LOW標記設置的情況下才呈現。
X : 單獨的保留位。
Promised Stream ID : 這個無符號31位整數表示終端準備發送的流標記。被承諾的流標記必須對發送端準備發送的下一個流來說是有效選擇。
Header Block Fragment : 包含請求頭字段的報頭區塊碎片(章節5.1.1)。
Padding : 填充字節。

The PUSH_PROMISE frame defines the following flags:

END_HEADERS (0x4): Bit 3 being set indicates that this frame contains an entire header block (Section 4.3) and is not followed by any CONTINUATION frames.A PUSH_PROMISE frame without the END_HEADERS flag set MUST be followed by a CONTINUATION frame for the same stream. A receiver MUST treat the receipt of any other type of frame or a frame on a different stream as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
PADDED (0x8): Bit 4 being set indicates that the Pad Length field is present.

推送承諾幀定義了以下標記：

END_HEADERS (0x4) : 位3 表明幀包含了整個報頭區塊(章節4.3)並且不跟着延續幀。不帶有END_HEADERS標記的推送承諾幀在同個流上面後面必須跟着延續幀。接收端接收到任何其他類型或者其他流觴的幀必須作爲類型爲協議錯誤的連接錯誤(章節5.4.1)處理。
PADDED (0x8) : 位4 表明Pad長度字段是已設置。

PUSH_PROMISE frames MUST be associated with an existing, peer-initiated stream. The stream identifier of a PUSH_PROMISE frame indicates the stream it is associated with. If the stream identifier field specifies the value 0x0, a recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

推送承諾幀必須與現有的由對等端初始化的流相關聯。如果流標識字段爲0x0，接收端必須響應一個類型爲協議錯誤的連接錯誤(章節5.4.1)。

Promised streams are not required to be used in the order they are promised. The PUSH_PROMISE only reserves stream identifiers for later use.

被承諾的流並不需要以被承諾的順序使用。推送承諾只保留接下來會使用的流的標識符。

PUSH_PROMISE MUST NOT be sent if the SETTINGS_ENABLE_PUSH setting of the peer endpoint is set to 0. An endpoint that has set this setting and has received acknowledgement MUST treat the receipt of a PUSH_PROMISE frame as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

如果對等端的SETTINGS_ENABLE_PUSH設置是0那麼推送承諾絕對不能發送。終端已經設置了禁止推送承諾並且收到確認的必須將接收到推送承諾幀作爲類型爲協議錯誤的連接錯誤處理(章節5.4.1)。

Recipients of PUSH_PROMISE frames can choose to reject promised streams by returning a RST_STREAM referencing the promised stream identifier back to the sender of the PUSH_PROMISE.

推送承諾的接收端可以選擇給推送承諾的發送端返回一個與被承諾的流標識符相關的RST_STREAM標記來拒絕接收承諾流。

A PUSH_PROMISE frame modifies the connection state in two ways. The inclusion of a header block (Section 4.3) potentially modifies the state maintained for header compression. PUSH_PROMISE also reserves a stream for later use, causing the promised stream to enter the "reserved" state. A sender MUST NOT send a PUSH_PROMISE on a stream unless that stream is either "open" or "half closed (remote)"; the sender MUST ensure that the promised stream is a valid choice for a new stream identifier (Section 5.1.1) (that is, the promised stream MUST be in the "idle" state).

PUSH_PROMISE通過兩種方式修改連接狀態。這包括一個報頭區塊(章節4.3)可能修改壓縮狀態。PUSH_PROMISE同樣保留流後續使用，導致被推送的流進入到“保留”狀態。發送端絕對不能在流上發送PUSH_PROMISE除非流是“打開”或者“半封閉(遠程)”狀態；發送端絕對要保證被承諾的流對於新的流標示(章節5.1.1)來說是一個有效的選擇(就是說，被承諾的流必須進入"空閒"狀態)。

Since PUSH_PROMISE reserves a stream, ignoring a PUSH_PROMISE frame causes the stream state to become indeterminate. A receiver MUST treat the receipt of a PUSH_PROMISE on a stream that is neither "open" nor "half closed (local)" as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. Similarly, a receiver MUST treat the receipt of a PUSH_PROMISE that promises an illegal stream identifier (Section 5.1.1) (that is, an identifier for a stream that is not currently in the "idle" state) as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

由於PUSH_PROMISE保留了一個流、忽略一個PUSH_PROMISE 幀都會導致流狀態變得不確定。接收端接收到流狀態不是“打開”或者“半封閉(本地)”的流的推送承諾幀必須作爲類型爲協議錯誤的連接錯誤(章節5.4.1)處理。相似的，接收端必須對在一個非法標示(章節5.1.1)的流(即流的標識當前不在空閒狀態)上建立的推送承諾作爲類型爲協議錯誤的連接錯誤(章節5.4.1)處理。

The PUSH_PROMISE frame includes optional padding. Padding fields and flags are identical to those defined for DATA frames (Section 6.1).

PUSH_PROMISE幀填充是可選的。填充字段及標記同數據幀(章節6.1)中定義。

6.7 PING PING幀

The PING frame (type=0x6) is a mechanism for measuring a minimal round trip time from the sender, as well as determining whether an idle connection is still functional. PING frames can be sent from any endpoint.

PING幀(type=0x6)是一種從發送端測量最小的RTT時間的機制，同樣也是一種檢測連接是否可用的方法。PING幀可以被任何終端發送。

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 |                      Opaque Data (64)                         |
 |                                                               |
 +---------------------------------------------------------------+

In addition to the frame header, PING frames MUST contain 8 octets of data in the payload. A sender can include any value it chooses and use those bytes in any fashion.

除了幀報頭之外，PING幀必須在載體中包含一個8字節長度的數據。發送端可以選擇使用任何指並在任何時候使用。

Receivers of a PING frame that does not include an ACK flag MUST send a PING frame with the ACK flag set in response, with an identical payload. PING responses SHOULD be given higher priority than any other frame.

接收到不包含ACK標記的PING幀必須發送一個帶有ACK比標記的PING幀響應，以及一個相同的載荷。PING響應應當設置比其他幀更高的優先級。

The PING frame defines the following flags:

ACK (0x1):Bit 1 being set indicates that this PING frame is a PING response. An endpoint MUST set this flag in PING responses. An endpoint MUST NOT respond to PING frames containing this flag.

PING frames are not associated with any individual stream. If a PING frame is received with a stream identifier field value other than 0x0, the recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

PING幀定義了以下標記：

ACK (0x1) ：位1表示PING幀是一個PING響應。終端必須在PING響應中設置此標記。終端絕對不能對包含此標記的PING幀做出響應。

PING幀捕魚任何獨立的流相關聯。如果收到流標示字段不是0x0的PING幀，接收端必須響應一個類型爲協議錯誤的連接錯誤。

Receipt of a PING frame with a length field value other than 8 MUST be treated as a connection error (Section 5.4.1) of type FRAME_SIZE_ERROR.

接收到字段長度不是8的PING幀必須作爲類型爲幀大小錯誤的連接錯誤處理。

6.8 GOAWAY 超時幀

The GOAWAY frame (type=0x7) informs the remote peer to stop creating streams on this connection. GOAWAY can be sent by either the client or the server. Once sent, the sender will ignore frames sent on any new streams with identifiers higher than the included last stream identifier. Receivers of a GOAWAY frame MUST NOT open additional streams on the connection, although a new connection can be established for new streams.

超時幀(type=0x7)通知遠端對等端不要在這個連接上建立新流。超時幀可以由客戶端或者服務端發送。一旦發送，發動端將忽略當前連接上新的和標示符大於上一個流的幀的發送。接收端接收到超時幀後絕對不能在這個連接上打開新的流，但是可以針對新的流創建一個新的連接。

The purpose of this frame is to allow an endpoint to gracefully stop accepting new streams, while still finishing processing of previously established streams. This enables administrative actions, like server maintainence.

這個幀的目的是允許終端優雅的停止接收新的流，但仍可以繼續完成之前已經建立的流的處理。這使得管理員行爲可用，如服務器維護。

There is an inherent race condition between an endpoint starting new streams and the remote sending a GOAWAY frame. To deal with this case, the GOAWAY contains the stream identifier of the last stream which was or might be processed on the sending endpoint in this connection. If the receiver of the GOAWAY has sent data on streams with a higher stream identifier than what is indicated in the GOAWAY frame, those streams are not or will not be processed. The receiver of the GOAWAY frame can treat the streams as though they had never been created at all, thereby allowing those streams to be retried later on a new connection.

在終端啓動新的流及遠端發送超時幀之間有一個內在的競爭條件。爲了處理這種情況，超時幀帶有當前連接中發送終端處理的最後一個流的標識。如果超時幀的接收端使用了比指定的流更新的流，它們將不會被髮送端處理，而且接收端可以認爲這些流根本沒有被創建(因此接收端可以稍後在新的連接上重新創建這些流)。

Endpoints SHOULD always send a GOAWAY frame before closing a connection so that the remote can know whether a stream has been partially processed or not. For example, if an HTTP client sends a POST at the same time that a server closes a connection, the client cannot know if the server started to process that POST request if the server does not send a GOAWAY frame to indicate what streams it might have acted on.

終端在關閉一個連接之前總是應當發送一個超時幀，這樣遠端就能知道一個流是否已被部分處理。例如，如果一個HTTP客戶端在服務端關閉連接的時候發送了一個POST請求，如果服務端不發送一個指示它在哪裏停止工作的超時幀，客戶端將不知道這個POST請求是否已開始被處理。對於不規範的對等端，終端可以選擇不發送超時幀的情況下關閉連接。

An endpoint might choose to close a connection without sending GOAWAY for misbehaving peers.

對於行爲不規範的對等端，終端可以選擇不發送超時幀直接關閉連接。

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |X|                  Last-Stream-ID (31)                        |
 +-+-------------------------------------------------------------+
 |                      Error Code (32)                          |
 +---------------------------------------------------------------+
 |                  Additional Debug Data (*)                    |
 +---------------------------------------------------------------+

The GOAWAY frame does not define any flags.

超時幀沒有定義任何標記。

The GOAWAY frame applies to the connection, not a specific stream. An endpoint MUST treat a GOAWAY frame with a stream identifier other than 0x0 as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

超時幀適用於連接而不是特定的流。終端接收到流標識符不是0x0的超時幀必須作爲類型爲協議錯誤的連接錯誤(章節5.4.1)處理。

The last stream identifier in the GOAWAY frame contains the highest numbered stream identifier for which the sender of the GOAWAY frame might have taken some action on, or might yet take action on. All streams up to and including the identified stream might have been processed in some way. The last stream identifier can be set to 0 if no streams were processed.

超時幀中最後一個流的標識包含了接收端接收到並可能已經進行某些處理的流的標識的最大值。所有小於或等於此指定標識符的流都可能通過某種方式被處理。如果沒有流被處理，最後流的標識符設置爲0。

Note：In this context, "processed" means that some data from the stream was passed to some higher layer of software that might have taken some action as a result.

注意：這個案例中，“已處理”表示流中的某些數據已經被傳到軟件的更高的層並可能被進行某些處理。

If a connection terminates without a GOAWAY frame, the last stream identifier is effectively the highest possible stream identifier.

如果連接在沒有超時幀的情況下終止，這個值有效的是最大的流標識符。

On streams with lower or equal numbered identifiers that were not closed completely prior to the connection being closed, re-attempting requests, transactions, or any protocol activity is not possible, with the exception of idempotent actions like HTTP GET, PUT, or DELETE. Any protocol activity that uses higher numbered streams can be safely retried using a new connection.

連接關閉前小於或等於標識符上的流沒有完全關閉的，重試請求，交換，或者任何協議活動都是不可能的(例如HTTP GET,PUT,或者刪除等等冪行爲例外)。任何使用更高的流數值的協議行爲可以在新的連接上安全地重試。

Activity on streams numbered lower or equal to the last stream identifier might still complete successfully. The sender of a GOAWAY frame might gracefully shut down a connection by sending a GOAWAY frame, maintaining the connection in an open state until all in-progress streams complete.

小於或等於最後流標識符上的流的活動可能仍然能成功完成。超時幀的發送端可能通過發送超時幀優雅地關閉了連接，保持連接在打開狀態直到正在處理的流全部處理完成。

An endpoint MAY send multiple GOAWAY frames if circumstances change. For instance, an endpoint that sends GOAWAY with NO_ERROR during graceful shutdown could subsequently encounter an condition that requires immediate termination of the connection. The last stream identifier from the last GOAWAY frame received indicates which streams could have been acted upon. Endpoints MUST NOT increase the value they send in the last stream identifier, since the peers might already have retried unprocessed requests on another connection.

如果環境改變終端可能發送多個超時幀。例如，終端發送帶有NO_ERROR標記的超時幀來優雅關閉時隨後可能遇到的情況需要理解終止連接。從最後一個超時幀接收到的最後的流標識符標識表示這些流可能已經被處理了。終端絕對不能增加他們最後發送的流標識的值，因爲對等端可能已經有在其他連接上未處理的重試請求。

A client that is unable to retry requests loses all requests that are in flight when the server closes the connection. This is especially true for intermediaries that might not be serving clients using HTTP/2. A server that is attempting to gracefully shut down a connection SHOULD send an initial GOAWAY frame with the last stream identifier set to 231-1 and a NO_ERROR code. This signals to the client that a shutdown is imminent and that no further requests can be initiated. After waiting at least one round trip time, the server can send another GOAWAY frame with an updated last stream identifier. This ensures that a connection can be cleanly shut down without losing requests.

服務端關閉連接時，終端無法重試請求的將丟失所有正在發送的請求。尤其是針對中介端無法使用HTTP/2服務客戶端的時候。無負擔嘗試優雅關閉連接時應當發送一個攜帶2^{31} -1 大小的流標識符和一個錯誤碼的初始超時幀。這個信號對客戶端來說意味着即將關閉連接並且不能建立更多請求了。在等待至少一個RTT時間之後，服務端能發送另外一個帶有更新後的最終流標識符超時幀。這個確保了連接能徹底的關閉而不用丟失請求。

After sending a GOAWAY frame, the sender can discard frames for streams with identifiers higher than the identified last stream. However, any frames that alter connection state cannot be completely ignored. For instance, HEADERS, PUSH_PROMISE and CONTINUATION frames MUST be minimally processed to ensure the state maintained for header compression is consistent (see Section 4.3); similarly DATA frames MUST be counted toward the connection flow control window. Failure to process these frames can cause flow control or header compression state to become unsynchronized.

在發送超時幀後，發送端能丟棄流標識符大於最終流標識的流的幀。然而，任何修改流狀態的幀不能被全部忽略。例如，報頭幀、推送承諾幀和延續幀必須被最低限度的處理來保證維持的報頭壓縮是連續的(章節4.3);類似的數據幀必須被計入連接流程控制窗口中。這些處理失敗可能導致流量控制或者報頭壓縮狀態不同步。

The GOAWAY frame also contains a 32-bit error code (Section 7) that contains the reason for closing the connection.

超時幀同樣包含一個32位的錯誤碼（章節7），裏面包含了關閉連接的原因。

Endpoints MAY append opaque data to the payload of any GOAWAY frame. Additional debug data is intended for diagnostic purposes only and carries no semantic value. Debug information could contain security- or privacy-sensitive data. Logged or otherwise persistently stored debug data MUST have adequate safeguards to prevent unauthorized access.

終端可以在超時幀載體上附加不透明數據。額外的調試數據僅用來診斷沒有語義值。調試信息可以包含安全或者隱私敏感的數據。登錄或者其他持續存儲的數據必須有足夠的保障措施，以防止未經授權的訪問。

6.9 WINDOW_UPDATE 窗口更新幀

The WINDOW_UPDATE frame (type=0x8) is used to implement flow control; see Section 5.2 for an overview.

WINDOW_UPDATE幀(type=0x8)用來實現流量控制；概述見章節5.2。

Flow control operates at two levels: on each individual stream and on the entire connection.

流量控制在兩種層面上操作：每個單獨的流或者整個連接。

Both types of flow control are hop-by-hop; that is, only between the two endpoints. Intermediaries do not forward WINDOW_UPDATE frames between dependent connections. However, throttling of data transfer by any receiver can indirectly cause the propagation of flow control information toward the original sender.

所有類型的流量控制都是逐跳的；就是說，只在兩個終端之間作用。中介端不在依賴的連接上轉接WINDOW_UPDATE幀。接收端對數據的顯示可以直接導致流量控制信息的傳播轉到原始發送端。

Flow control only applies to frames that are identified as being subject to flow control. Of the frame types defined in this document, this includes only DATA frames. Frames that are exempt from flow control MUST be accepted and processed, unless the receiver is unable to assign resources to handling the frame. A receiver MAY respond with a stream error (Section 5.4.2) or connection error (Section 5.4.1) of type FLOW_CONTROL_ERROR if it is unable to accept a frame.

流量控制只適用於確定受流量控制影響的幀。文檔中定義的幀類型中，只包括數據幀。不受流量控制的幀必須被接收和處理，除非接收端無法爲幀分配資源。接收端如果無法接收幀，可以響應一個流錯誤(章節5.4.2)或者類型爲流量控制錯誤的連接錯誤(章節5.4.1)。

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |X|              Window Size Increment (31)                     |
 +-+-------------------------------------------------------------+

The payload of a WINDOW_UPDATE frame is one reserved bit, plus an unsigned 31-bit integer indicating the number of bytes that the sender can transmit in addition to the existing flow control window. The legal range for the increment to the flow control window is 1 to 231 - 1 (0x7fffffff) bytes.

WINDOW_UPDATE幀的載體是一個保留字節，加上一個無符號31爲整數表明發送端除了現有的流量控制窗口可以發送的字節數。留空控制窗口有效的增量範圍是$1 至 2^{31}-1$(0x7fffffff) 字節。

The WINDOW_UPDATE frame does not define any flags.

WINDOW_UPDATE幀沒有定義任何標記。

The WINDOW_UPDATE frame can be specific to a stream or to the entire connection. In the former case, the frame's stream identifier indicates the affected stream; in the latter, the value "0" indicates that the entire connection is the subject of the frame.

WINDOW_UPDATE可以專指某個流或者整個連接。在前者的情況下，幀的流標識符指的是被影響的流；在後者情況下，值"0"表示整個連接都受這個幀的影響。

WINDOW_UPDATE can be sent by a peer that has sent a frame bearing the END_STREAM flag. This means that a receiver could receive a WINDOW_UPDATE frame on a "half closed (remote)" or "closed" stream. A receiver MUST NOT treat this as an error, see Section 5.1.

WINDOW_UPDATE可以由一個已經發送帶有END_STREAM標記的幀的對等端來發送。這意味着接收端可以在“半封閉(遠程)”或者“關閉”的流上接收WINDOW_UPDATE幀。接收端絕對不能作爲錯誤處理，見章節5.1

A receiver that receives a flow controlled frame MUST always account for its contribution against the connection flow control window, unless the receiver treats this as a connection error (Section 5.4.1). This is necessary even if the frame is in error. Since the sender counts the frame toward the flow control window, if the receiver does not, the flow control window at sender and receiver can become different.

接收端收到受流量控制的幀必須總是計算流量對整個連接流量控制的影響量，除非接收端將這作爲連接錯誤處理。即使幀出錯這也是必須的。因爲發送端將這個幀計入了流量控制窗口，如果接收端沒有這樣做，發送端和接收端的流量控制會不相同。

6.9.1 The Flow Control Window 流量控制窗口

Flow control in HTTP/2 is implemented using a window kept by each sender on every stream. The flow control window is a simple integer value that indicates how many bytes of data the sender is permitted to transmit; as such, its size is a measure of the buffering capacity of the receiver.

HTTP/2中流量控制是通過每個發送端在每個流上攜帶一個窗口來實現的。流量控制窗口是一個簡單的整數值，指示發送端被允許傳輸的字節數；因此，它的大小是接收端的緩存能力的衡量。

Two flow control windows are applicable: the stream flow control window and the connection flow control window. The sender MUST NOT send a flow controlled frame with a length that exceeds the space available in either of the flow control windows advertised by the receiver. Frames with zero length with the END_STREAM flag set (that is, an empty DATA frame) MAY be sent if there is no available space in either flow control window.

流量控制窗口對流和連接的流量控制窗口都適用。發送端絕對不能發送超出接收端廣播的流量控制窗口大小的可用空間長度的受流量控制影響的幀。在各個流量控制窗口中沒有可用空間時，可以發送帶有END_STREAM標記的長度爲0的幀(例如，空數據幀)。

For flow control calculations, the 8 byte frame header is not counted.

流量控制計算中，8字節的幀報頭不被計入。

After sending a flow controlled frame, the sender reduces the space available in both windows by the length of the transmitted frame.

在發送一個流量控制幀後，發送端在各個窗口中可用空間中減去發送的幀長度。

The receiver of a frame sends a WINDOW_UPDATE frame as it consumes data and frees up space in flow control windows. Separate WINDOW_UPDATE frames are sent for the stream and connection level flow control windows.

接收端發送一個WINDOW_UPDATE幀當它接受信息並釋放了流量控制窗口的空間。單獨的WINDOW_UPDATE幀用於流及連接層面的流量控制窗口中。

A sender that receives a WINDOW_UPDATE frame updates the corresponding window by the amount specified in the frame.

發送端收到WINDOW_UPDATE後按幀中指定的大小更新到正確的窗口。

A sender MUST NOT allow a flow control window to exceed 231 - 1 bytes. If a sender receives a WINDOW_UPDATE that causes a flow control window to exceed this maximum it MUST terminate either the stream or the connection, as appropriate. For streams, the sender sends a RST_STREAM with the error code of FLOW_CONTROL_ERROR code; for the connection, a GOAWAY frame with a FLOW_CONTROL_ERROR code.

發送端絕對不允許流量控制窗口超過2^{31}-1字節。如果發送端接收到WINDOW_UPDATE使得流量控制窗口超過這個最大值，它必須適當地終止這個流或者這個連接。對於流，發送端發送一個帶有流量控制錯誤的錯誤碼的ST_STREAM幀；對於連接，發送一個帶有流量控制錯誤碼的超時幀。

Flow controlled frames from the sender and WINDOW_UPDATE frames from the receiver are completely asynchronous with respect to each other. This property allows a receiver to aggressively update the window size kept by the sender to prevent streams from stalling.

發送端發送的受流量控制的幀以及接收端收到的WINDOW_UPDATE幀是完全相互異步的。這種屬性讓接收端積極的更新發送端攜帶的窗口大小來防止流停轉。

6.9.2 Initial Flow Control Window Size 流量控制窗口初始值

When an HTTP/2 connection is first established, new streams are created with an initial flow control window size of 65,535 bytes. The connection flow control window is 65,535 bytes. Both endpoints can adjust the initial window size for new streams by including a value for SETTINGS_INITIAL_WINDOW_SIZE in the SETTINGS frame that forms part of the connection preface. The connection flow control window can only be changed using WINDOW_UPDATE frames.

HTTP/2連接初次建立時，新的流創建的初始化流量控制大小是65,535字節。連接的流量控制大小是65,535字節。兩個終端都能通過在組成連接序言的設置幀中攜帶一個SETTINGS_INITIAL_WINDOW_SIZE設置調整新流的初始化窗口大小。連接的流量控制窗口只能被WINDOWS_UPDATE幀修改。

Prior to receiving a SETTINGS frame that sets a value for SETTINGS_INITIAL_WINDOW_SIZE, an endpoint can only use the default initial window size when sending flow controlled frames. Similarly, the connection flow control window is set to the default initial window size until a WINDOW_UPDATE frame is received.

在收到設置幀指定SETTINGS_INITIAL_WINDOW_SIZE前，終端只能只有流量控制的默認窗口值。類似的，連接的流量控制窗口初始化時也是默認值知道收到WINDOW_UPDATE幀。

A SETTINGS frame can alter the initial flow control window size for all current streams. When the value of SETTINGS_INITIAL_WINDOW_SIZE changes, a receiver MUST adjust the size of all stream flow control windows that it maintains by the difference between the new value and the old value.

設置幀可以針對所有當前的流修改流量控制初始化大小。當SETTINGS_INITIAL_WINDOW_SIZE值改變時，接收端必須將根據新舊值調整其保留的所有流的窗口大小設置幀不能修改連接的流量控制窗口。

A change to SETTINGS_INITIAL_WINDOW_SIZE can cause the available space in a flow control window to become negative. A sender MUST track the negative flow control window, and MUST NOT send new flow controlled frames until it receives WINDOW_UPDATE frames that cause the flow control window to become positive.

SETTINGS_INITIAL_WINDOW_SIZE的修改可能導致流量控制窗口的可用空間爲負數。發送端必須跟蹤負數的流量控制窗口，並且絕對不能發送新的受流量控制的幀直到接收到窗口更新幀使得流量控制窗口變成正數。

For example, if the client sends 60KB immediately on connection establishment, and the server sets the initial window size to be 16KB, the client will recalculate the available flow control window to be -44KB on receipt of the SETTINGS frame. The client retains a negative flow control window until WINDOW_UPDATE frames restore the window to being positive, after which the client can resume sending.

例如，如果客戶端在建立的連接上立即發送60KB的數據，而終端將初始的窗口大小設置成16KB,客戶端將重新計算流量控制窗口的可用空間爲-44KB。終端將保持一個負數的流量控制窗口直到窗口更新幀恢復窗口到正數，這個時候客戶端才能恢復數據發送。

A SETTINGS frame cannot alter the connection flow control window.

設置幀不能修改鏈接流量控制窗口。

An endpoint MUST treat a change to SETTINGS_INITIAL_WINDOW_SIZE that causes any flow control window to exceed the maximum size as a connection error (Section 5.4.1) of type FLOW_CONTROL_ERROR.

終端必須將SETTINGS_INITIAL_WINDOW_SIZE的修改導致流量控制窗口超過最大值的情況作爲類型爲流量控制錯誤的連接錯誤(章節 5.4.1)處理。

6.9.3 Reducing the Stream Window Size 減少流量窗口大小

A receiver that wishes to use a smaller flow control window than the current size can send a new SETTINGS frame. However, the receiver MUST be prepared to receive data that exceeds this window size, since the sender might send data that exceeds the lower limit prior to processing the SETTINGS frame.

接收端希望使用比當前大小更小的流量控制窗口可以發送一個新的設置幀。然而，接收端必須準備好接收超過窗口大小的數據，因爲發送端在處理設置幀之前發送了低於最低下限的數據。

After sending a SETTINGS frame that reduces the initial flow control window size, a receiver has two options for handling streams that exceed flow control limits:

在發送減小初始化流量控制窗口大小的設置幀後，接收端有兩種選擇處理流超過流量限制的情況：

The receiver can immediately send RST_STREAM with FLOW_CONTROL_ERROR error code for the affected streams.
The receiver can accept the streams and tolerate the resulting head of line blocking, sending WINDOW_UPDATE frames as it consumes data.
接收端可以針對受影響的流立即發送帶有流量控制錯誤錯誤碼的RST_STREAM幀。
接收端如果在消耗數據可以接受流並且忍受報頭阻塞的結果，併發送WINDOW_UPDATE幀。

6.10 CONTINUATION 延續幀

The CONTINUATION frame (type=0x9) is used to continue a sequence of header block fragments (Section 4.3). Any number of CONTINUATION frames can be sent on an existing stream, as long as the preceding frame is on the same stream and is a HEADERS, PUSH_PROMISE or CONTINUATION frame without the END_HEADERS flag set.

延續幀(type=0x9)用來延續一個報頭區塊(章節4.3)碎片序列。在現有流上可以發送到一個已存在延續幀，只要相同流上的前一陣是報頭幀、推送承諾幀或者不帶有END_HEADERS標記的延續幀。

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                   Header Block Fragment (*)                 ...
 +---------------------------------------------------------------+

The CONTINUATION frame payload contains a header block fragment (Section 4.3).

延續幀實體包含一個報頭區塊碎片(章節4.3)。

The CONTINUATION frame defines the following flag:

END_HEADERS (0x4):Bit 3 being set indicates that this frame ends a header block (Section 4.3).If the END_HEADERS bit is not set, this frame MUST be followed by another CONTINUATION frame. A receiver MUST treat the receipt of any other type of frame or a frame on a different stream as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

延續幀載體有以下字段：

END_HEADERS (0x4) : 位3設置指示這個幀的報頭區塊的終止(章節4.3)。如果END_HEADERS位沒有被設置，這個幀必須跟着另一個延續幀。接收到必須將收到其他類型的幀或者其他流上的幀錯位類型爲協議錯誤的連接錯誤(章節5.4.1)處理。

The CONTINUATION frame changes the connection state as defined in Section 4.3.

延續幀改變連接狀態如章節4.3中定義。

CONTINUATION frames MUST be associated with a stream. If a CONTINUATION frame is received whose stream identifier field is 0x0, the recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

延續幀必須與流相關聯。如果延續幀的相關流表示字段是0x0，終端必須響應一個類型爲協議錯誤的連接錯誤。

A CONTINUATION frame MUST be preceded by a HEADERS, PUSH_PROMISE or CONTINUATION frame without the END_HEADERS flag set. A recipient that observes violation of this rule MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

延續幀必須跟在不帶有END_HEADERS設置的報頭幀、推送承諾幀或延續幀後面。終端接收到不符合此規則的必須響應一個類型爲協議錯誤的連接錯誤(章節5.4.1)。

7 Error Codes 錯誤碼

Error codes are 32-bit fields that are used in RST_STREAM and GOAWAY frames to convey the reasons for the stream or connection error.

錯誤碼是32位字段，用在RST_STREAM和超時幀中用來標識流或者鏈接錯誤的原因。

Error codes share a common code space. Some error codes apply only to either streams or the entire connection and have no defined semantics in the other context.

錯誤碼共享一個功能的代碼空間。一些錯誤代碼只適用於特定的條件，在某些幀類型沒有定義的語義。

The following error codes are defined:

NO_ERROR (0x0):The associated condition is not as a result of an error. For example, a GOAWAY might include this code to indicate graceful shutdown of a connection.
PROTOCOL_ERROR (0x1):The endpoint detected an unspecific protocol error. This error is for use when a more specific error code is not available.
INTERNAL_ERROR (0x2):The endpoint encountered an unexpected internal error.
FLOW_CONTROL_ERROR (0x3):The endpoint detected that its peer violated the flow control protocol.
SETTINGS_TIMEOUT (0x4):The endpoint sent a SETTINGS frame, but did not receive a response in a timely manner. See Settings Synchronization (Section 6.5.3).
STREAM_CLOSED (0x5):The endpoint received a frame after a stream was half closed.
FRAME_SIZE_ERROR (0x6):The endpoint received a frame that was larger than the maximum size that it supports.
REFUSED_STREAM (0x7):The endpoint refuses the stream prior to performing any application processing, see Section 8.1.4 for details.
CANCEL (0x8):Used by the endpoint to indicate that the stream is no longer needed.
COMPRESSION_ERROR (0x9):The endpoint is unable to maintain the header compression context for the connection.
CONNECT_ERROR (0xa):The connection established in response to a CONNECT request (Section 8.3) was reset or abnormally closed.
ENHANCE_YOUR_CALM (0xb):The endpoint detected that its peer is exhibiting a behavior that might be generating excessive load.
INADEQUATE_SECURITY (0xc):The underlying transport has properties that do not meet minimum security requirements (see Section 9.2).

定義了以下錯誤碼：

NO_ERROR (0) ：相關的條件並不是錯誤的結果。例如超時幀可以攜帶此錯誤碼指示連接的平滑關閉。
PROTOCOL_ERROR (1) : 終端檢測到一個不確定的協議錯誤。這個錯誤用在一個更具體的錯誤碼不可用的時候。
INTERNAL_ERROR (2) : 終端遇到意外的內部錯誤。
FLOW_CONTROL_ERROR (3) : 終端檢測到對等端違反了流量控制協議。
SETTINGS_TIMEOUT (4) : 終端發送了設置幀，但是沒有及時收到響應。見Settings Synchronization。
STREAM_CLOSED (5) : 終端在流半封閉的時候收到幀。
FRAME_SIZE_ERROR (6) : 終端收到大小超過最大尺寸的幀。
REFUSED_STREAM (7) : 終端拒絕流在它執行任何應用處理之前，詳見Reliability（章節 8.1.4）
CANCEL (8) : 終端使用這個標示某個流不再需要。
COMPRESSION_ERROR (9) : 終端無法維持報頭壓縮上下文的連接
CONNECT_ERROR (10) : 響應某個連接請求建立的連接被服爲異常關閉。
ENHANCE_YOUR_CALM (11) : 終端檢測出對等端在表現出可能會產生過大負荷的行爲。
INADEQUATE_SECURITY (12) ：基礎傳輸包含屬性不滿足文檔或者終端申明的最小要求。

Unknown or unsupported error codes MUST NOT trigger any special behavior. These MAY be treated by an implementation as being equivalent to INTERNAL_ERROR.

未知或者不支持的錯誤碼絕對不能觸發任何特殊的行爲。這可以被作爲等同於INTERNAL_ERROR來實現。

8 HTTP Message Exchanges HTTP消息交換

HTTP/2 is intended to be as compatible as possible with current uses of HTTP. This means that, from the application perspective, the features of the protocol are largely unchanged. To achieve this, all request and response semantics are preserved, although the syntax of conveying those semantics has changed.

HTTP/2的目的是儘可能的兼容目前使用的HTTP。這意味着，從服務端或者客戶端應用的角度來看，該協議的特點是不變的。爲了實現這點，所有響應與請求的語義都將保留，儘管包含這些語義的語法已經改變。

Thus, the specification and requirements of HTTP/1.1 Semantics and Content [RFC7231], Conditional Requests [RFC7232], Range Requests [RFC7233], Caching [RFC7234] and Authentication [RFC7235] are applicable to HTTP/2. Selected portions of HTTP/1.1 Message Syntax and Routing [RFC7230], such as the HTTP and HTTPS URI schemes, are also applicable in HTTP/2, but the expression of those semantics for this protocol are defined in the sections below.

因此，HTTP/1.1語義與內容、有條件的請求、範圍請求、緩存與驗證定義的規範與要求同樣適用於HTTP/2。HTTP/1.1消息語法與路由選定的內容，例如HTTP與HTTPS URI方案，也同樣適用於HTTP/2，但是表達這些協議的語義在下面的章節定義。

8.1 HTTP Request/Response Exchange HTTP 請求/響應交換

A client sends an HTTP request on a new stream, using a previously unused stream identifier (Section 5.1.1). A server sends an HTTP response on the same stream as the request.

客戶端在一個新的流上發起HTTP請求，使用以前未使用的流標示。服務端在同個流上發起HTTP請求。

An HTTP message (request or response) consists of:

one HEADERS frame (followed by zero or more CONTINUATION frames) containing the message headers (see [RFC7230], Section 3.2), and
zero or more DATA frames containing the message payload (see [RFC7230], Section 3.3), and
optionally, one HEADERS frame, followed by zero or more CONTINUATION frames containing the trailer-part, if present (see [RFC7230], Section 4.1.2).

一個HTTP消息(請求或相應)包含：

一個報頭幀，後面跟着0個或多個延續幀(包含消息報頭;見RFC7230),以及
0個或多個數據幀(包含消息載荷，見RFC7230章節3.3),以及
一個可選的版頭，後面跟着0個或多個延續幀(如果存在，包含尾部部分，見RFC7230章節4.1.2)

The last frame in the sequence bears an END_STREAM flag, noting that a HEADERS frame bearing the END_STREAM flag can be followed by CONTINUATION frames that carry any remaining portions of the header block.

序列中的最後一幀具有END_STREAM標記，但是包含END_STREAM標記的報頭幀後面可以跟着包含任意報報頭分的延續幀。

Other frames (from any stream) MUST NOT occur between either HEADERS frame and any CONTINUATION frames that might follow.

其他幀(來自任何流的)絕對不能出現在任意報頭幀和延續幀(如果存在)之間,也不能出現在延續幀中間。

Otherwise, frames MAY be interspersed on the stream between these frames, but those frames do not carry HTTP semantics. In particular, HEADERS frames (and any CONTINUATION frames that follow) other than the first and optional last frames in this sequence do not carry HTTP semantics.

否則，流上的這些幀可能被打散，但是那些幀並不包含HTTP語義。特別是，報頭幀(及任何跟在後面的延續幀)序列中第一幀及可選的最後一幀以外的並不包含HTTP語義。

Trailing header fields are carried in a header block that also terminates the stream. That is, a sequence starting with a HEADERS frame, followed by zero or more CONTINUATION frames, where the HEADERS frame bears an END_STREAM flag. Header blocks after the first that do not terminate the stream are not part of an HTTP request or response.

報頭區塊中的報頭尾部字段同樣終止流。就是說，一個報頭幀開始的序列，後面跟着0個或者多個帶有END_STREAM標記的延續幀。第一個不終止流後面的報頭區塊不是當前HTTP請求與響應的一部分。

An HTTP request/response exchange fully consumes a single stream. A request starts with the HEADERS frame that puts the stream into an "open" state. The request ends with a frame bearing END_STREAM, which causes the stream to become "half closed (local)" for the client and "half closed (remote)" for the server. A response starts with a HEADERS frame and ends with a frame bearing END_STREAM, which places the stream in the "closed" state.

一個HTTP 請求/響應的數據交換在同一個流上進行。一個請求由是流進入打開狀態的報頭幀開始，並由一個攜帶使流對客戶端進入半封閉的END_STREAM標記的幀結束，另外可選的後面可以跟着延續幀，使流進入關閉狀態。一個響應從HEADERS幀開始並且結束於一個放在封閉狀態中支撐END_STREAM的幀

8.1.1 Informational Responses 響應信息

The 1xx series of HTTP response status codes ([RFC7231], Section 6.2) are not supported in HTTP/2.

1xx系列的HTTP響應狀態碼在HTTP/2中不支持。

The most common use case for 1xx is using an Expect header field with a 100-continue token (colloquially, "Expect/continue") to indicate that the client expects a 100 (Continue) non-final response status code, receipt of which indicates that the client should continue sending the request body if it has not already done so.

1xx最常見的用法是使用一個帶有100-繼續token(通俗的，“期待/繼續”)的期望報頭字段來表明客戶端期望一個100(繼續)非最終響應的狀態碼，收到這個表示客戶端如果沒有發送完應當繼續發送請求正文。

Typically, Expect/continue is used by clients wishing to avoid sending a large amount of data in a request body, only to have the request rejected by the origin server, thereby leaving the connection potentially unusable.

通常來說，期望/繼續幀被客戶端用來希望避免在請求正文中發送大量數據，只用來讓請求被源服務器拒絕(因此讓連接可能不可用)。

HTTP/2 does not enable the Expect/continue mechanism; if the server sends a final status code to reject the request, it can do so without making the underlying connection unusable.

HTTP/2不支持期望/繼續機制；如果服務端發送一個最終狀態碼來拒絕請求，它可以在不需要使當前連接不可用的情況下做到這個。

Note that this means HTTP/2 clients sending requests with bodies may waste at least one round trip of sent data when the request is rejected. This can be mitigated by restricting the amount of data sent for the first round trip by bandwidth-constrained clients, in anticipation of a final status code.

需要注意的是這意味着，HTTP/2客戶端發送帶有請求正文的請求在連接被拒絕時可能浪費至少一個發送數據的RTT時間。這可以通過限制帶寬受限的客戶端第一個RTT的數據大小來緩和，通過預期的最終狀態碼實現。

Other defined 1xx status codes are not applicable to HTTP/2. For example, the semantics of 101 (Switching Protocols) aren't suitable to a multiplexed protocol. Likewise, 102 (Processing) [RFC2518] is no longer necessary to ensure connection liveness, because HTTP/2 has a separate means of keeping the connection alive. The use of the 102 (Processing) status code for progress reporting has since been deprecated and is not retained.

其他定義的1xx狀態碼也不適用於HTTP/2。例如，101(轉換協議)的語義不適用於多路複用協議。同樣的，102(處理中)也不在需要確保連接的活躍性，因爲HTTP/2有單獨的保持連接可用的方式。

This difference between protocol versions necessitates special handling by intermediaries that translate between them:

An intermediary that translates HTTP/1.1 requests to HTTP/2 MUST generate a 100 (Continue) response if a received request includes and Expect header field with a 100-continue token ([RFC7231], Section 5.1.1), unless it can immediately generate a final status code. It MUST NOT forward the 100-continue expectation in the request header fields.
An intermediary that translates HTTP/2 to HTTP/1.1 MAY add an Expect header field with a 100-continue expectation when forwarding a request that has a body; see [RFC7231], Section 5.1.1 for specific requirements.
An intermediary that gateways HTTP/2 to HTTP/1.1 MUST discard all other 1xx informational responses.

這些不同協議版本之間的差異需要被中介端在轉換時特殊處理：

轉換HTTP/1.1到HTTP/2的中介網關如果收到請求包含帶有100-繼續token(RFC7231,章節5.1.1)的期望報頭字段，必須生成一個100(繼續)響應，除非它能馬上生成一個最終狀態碼。絕對不能轉發請求報頭中的100-繼續期望字段。
轉換HTTP/2到HTTP/1.1的中介網關在轉發一個帶有正文的請求時可以添加一個帶有100-繼續的期望報頭字段。特定要求見RFC7231.
轉換HTTP/2到HTTP/1.1的中介網關必須丟棄所有1xx以外的響應信息。

8.1.2 HTTP Header Fields HTTP報頭字段

HTTP header fields carry information as a series of key-value pairs. For a listing of registered HTTP headers, see the Message Header Field Registry maintained at https://www.iana.org/assignments/message-headers.

HTTP報頭字段攜帶一系列鍵-值對的信息。對於註冊的HTTP報頭列表，見https://www.iana.org/assignments/message-headers中消息報頭字段註冊。

While HTTP/1.x used the message start-line (see [RFC7230], Section 3.1) to convey the target URI and method of the request, and the status code for the response, HTTP/2 uses special pseudo-headers beginning with ':' character (ASCII 0x3a) for this purpose.

HTTP/1.x使用消息起始線(見RFC7230,章節3.1)來傳達URI目標和請求的方法、以及響應的狀態碼，HTTP/2使用特殊的':'(ASCII 0x3a)開頭的僞頭字符來實現這個目的。

Just as in HTTP/1.x, header field names are strings of ASCII characters that are compared in a case-insensitive fashion. However, header field names MUST be converted to lowercase prior to their encoding in HTTP/2. A request or response containing uppercase header field names MUST be treated as malformed (Section 8.1.2.5).

正如HTTP/1.x中，報頭字段名稱是ASCII字符，且不區分大小寫。然而，HTTP/2中報頭字段名稱必須轉成使用同樣編碼的小寫字符。帶有大寫報頭字段的請求或者響應必須被認爲是不規範的(章節8.1.2.5)。

This means that an intermediary transforming an HTTP/1.x message to HTTP/2 will need to remove any header fields nominated by the Connection header field, along with the Connection header field itself. Such intermediaries SHOULD also remove other connection-specific header fields, such as Keep-Alive, Proxy-Connection, Transfer-Encoding and Upgrade, even if they are not nominated by Connection.

這意味着中介端轉換一個HTTP/1.x消息到HTTP/2需要移除由連接報頭字段指定的任何報頭字段，包含連接報頭字段本身。這樣的中介端同樣應當移除其他連接特定的報頭字段，例如Keep-Alive、Proxy-Connection、Transfer-Encoding和Upgrade，即便它們不是由連接指定的。

One exception to this is the TE header field, which MAY be present in an HTTP/2 request, but when it is MUST NOT contain any value other than "trailers".

一個例外是TE報頭字段，這個可能在 HTTP/2 請求中保留，但是它不能包含“trailers”以外的值。

Note: : HTTP/2 purposefully does not support upgrade to another protocol. The handshake methods described in Section 3 are believed sufficient to negotiate the use of alternative protocols.

Note: : HTTP/2不支持升級到其他協議。3章節中描述的握手協議被認爲足夠用來作爲替代協議使用。

8.1.2.1 Request Header Fields 請求報頭字段

HTTP/2 defines a number of pseudo header fields starting with a colon ':' character that carry information about the request target:

The :method header field includes the HTTP method ([RFC7231], Section 4).
The :scheme header field includes the scheme portion of the target URI ([RFC3986], Section 3.1).:scheme is not restricted to http and https schemed URIs. A proxy or gateway can translate requests for non-HTTP schemes, enabling the use of HTTP to interact with non-HTTP services.
The :authority header field includes the authority portion of the target URI ([RFC3986], Section 3.2). The authority MUST NOT include the deprecated userinfo subcomponent for http or https schemed URIs.To ensure that the HTTP/1.1 request line can be reproduced accurately, this header field MUST be omitted when translating from an HTTP/1.1 request that has a request target in origin or asterisk form (see [RFC7230], Section 5.3). Clients that generate HTTP/2 requests directly SHOULD instead omit the Host header field. An intermediary that converts an HTTP/2 request to HTTP/1.1 MUST create a Host header field if one is not present in a request by copying the value of the :authority header field.
The :path header field includes the path and query parts of the target URI (the path-absolute production from [RFC3986] and optionally a '?' character followed by the query production, see [RFC3986], Section 3.3 and [RFC3986], Section 3.4). This field MUST NOT be empty; URIs that do not contain a path component MUST include a value of '/', unless the request is an OPTIONS request in asterisk form, in which case the :path header field MUST include '*'.

HTTP/2定義了一個以字符“:”開頭的報頭域，包含目標請求的信息：

:method 報頭字段包含了HTTP方法
:scheme字段包含了目標URI方案部分。:scheme並不是被限制於http和https類的URIS。代理或者網路關口可以轉化非HTTP體系的請求使其可以於非HTTP得請求互動
:authority報頭字段包含了目標URI的權限部分。這個權限絕對不能包含http:或者https: URIs的廢棄的用戶信息子成份。爲了保證HTTP/1.1請求行能被精確複製，當原始請求有請求目標或者星號形式(見[ http-p1 ]，5.3節)的HTTP/1.1請求進行轉換時這個字段必須被忽略。客戶端直接生成HTTP/2請求的相反應該忽略Host報頭字段。如果其中一個請求沒有Host字段，中介端將HTTP/2請求轉換爲HTTP/1.1請求的時候必須複製:authority字段的值來生成Host字段。
:path字段包含目標URI的路徑及查詢部分(絕對路徑由[RFC3986]以及可選的‘?’字符後面跟着查詢詞組成見xx)。這個字段絕對不能爲空；URI不包含path組件的必須包含一個'/'值，除非請求是一個星號形式的可選請求：這種情況下:path報頭字段必須包含""。是一個星號形式的可選請求：這種情況下:path報頭字段必須包含""。

8.1.2.2 Response Header Fields 響應報頭字段

A single :status header field is defined that carries the HTTP status code field (see [RFC7231], Section 6). This header field MUST be included in all responses, otherwise the response is malformed (Section 8.1.2.5).

定義了一個單獨的:status 報頭字段攜帶了HTTP狀態碼信息(見RFC7231,章節6)。這個報頭字段必須包含在所有的響應中，否則響應就是不規範的(章節8.1.2.5)。

HTTP/2 does not define a way to carry the version or reason phrase that is included in an HTTP/1.1 status line.

HTTP/2沒有定義一種方式攜帶HTTP/1.1狀態行中的版本或原因短語信息。

8.1.2.3 Header Field Ordering 報頭字段順序

HTTP Header Compression [COMPRESSION] does not preserve the order of header fields, because the relative order of header fields with different names is not important. However, the same header field can be repeated to form a list (see [RFC7230], Section 3.2.2), where the relative order of header field values is significant. This repetition can occur either as a single header field with a comma-separated list of values, or as several header fields with a single value, or any combination thereof. Therefore, in the latter case, ordering needs to be preserved before compression takes place.

HTTP報頭壓縮並不保留報頭字段的順序，因爲不同名稱的字段的相對位置並不重要。然而，當相同字段重複組成一個列表的時候(見RFC7230，章節3.2.2)，報頭字段值的相對位置就有意義。這種重複會出現在以逗號分隔的單個報頭字段值列表中，或者作爲幾個報頭字段的單一值，或他們的任何組合。因此，再後者情況下，報頭需在再壓縮前保留字段順序。

To preserve the order of multiple occurrences of a header field with the same name, its ordered values are concatenated into a single value using a zero-valued octet (0x0) to delimit them.

爲了保留同個名稱的報頭字段出現多次的順序，他們的順序使用單個以零值字節(0x0)分隔的值連接來保留。

After decompression, header fields that have values containing zero octets (0x0) MUST be split into multiple header fields before being processed.

解壓縮後，報頭字段包含有0字節的必須分割成多個報頭字段之後才能進一步處理。

For example, the following HTTP/1.x header block:

例如，下面的HTTP/1.x報頭區塊：

              Content-Type: text/html
              Cache-Control: max-age=60, private
              Cache-Control: must-revalidate

contains three Cache-Control directives; two directives in the first Cache-Control header field, and the third directive in the second Cache-Control field. Before compression, they would need to be converted to a form similar to this (with 0x0 represented as '\0'):

包含三個緩存控制指令；兩個在第一個緩存控制報頭字段，一個在第二個緩存控制字段。在壓縮前，它們需要轉成類似格式("\0"表示0x0):

cache-control: max-age=60, private\0must-revalidate
              content-type: text/html

Note here that the ordering between Content-Type and Cache-Control is not preserved, but the relative ordering of the Cache-Control directives - as well as the fact that the first two were comma-separated, while the last was on a different line - is.

注意這裏內容類型與緩存控制的順序是不保留的，但是緩存控制指令的相對順序——事實上也就是前兩個是用逗號分隔，而後一個是在不一樣的行——是保留的。

Header fields containing multiple values MUST be concatenated into a single value unless the ordering of that header field is known to be not significant.

報頭字段包含多個值必須組成單個值，除非報頭字段的順序是不重要的。

The special case of set-cookie - which does not form a comma-separated list, but can have multiple values - does not depend on ordering. The set-cookie header field MAY be encoded as multiple header field values, or as a single concatenated value.

特殊情況是設置cookie——不需要形成一個逗號分隔的列表，但是可以有多個值——不需要依賴順序。設置cookie字段可以被編碼成多行報頭字段值，或者單個的連接值。

8.1.2.4 Compressing the Cookie Header Field 報頭Cookie字段壓縮

The Cookie header field [COOKIE] can carry a significant amount of redundant data.

Cookie報頭字段可以攜帶大量的冗餘數據。

The Cookie header field uses a semi-colon (";") to delimit cookie-pairs (or "crumbs"). This header field doesn't follow the list construction rules in HTTP (see [RFC7230], Section 3.2.2), which prevents cookie-pairs from being separated into different name-value pairs. This can significantly reduce compression efficiency as individual cookie-pairs are updated.

Cookie字段使用“;”來分割cookie-對(或叫麪包屑)。這報頭字段不遵循HTTP中的構建規則(見RFC7230，章節3.2.2)，以防cookie-對被分隔成不同的鍵值對。單個cookie-對更新的時候能顯著提升壓縮效率。

To allow for better compression efficiency, the Cookie header field MAY be split into separate header fields, each with one or more cookie-pairs. If there are multiple Cookie header fields after decompression, these MUST be concatenated into a single octet string using the two octet delimiter of 0x3B, 0x20 (the ASCII string "; ").

爲了更好的壓縮效率，Cookie字段可以被分隔成多個報頭字段，每個包含一個或者多個cookie對。如果解壓後有多個Cookie報頭字段，他們必須由兩個字節的0x3B, 0x20(ASCII";")連接成單個字段。

The Cookie header field MAY be split using a zero octet (0x0), as defined in Section 8.1.2.3. When decoding, zero octets MUST be replaced with the cookie delimiter ("; ").

Coookie報頭字段也可以使用0值字節(0x0)來分割，同章節8.1.2.3中定義。當解碼的時候，0值字節必須被替換成Cookie的分隔符(";")。

Therefore, the following sets of Cookie header fields are semantically equivalent, though the final form might appear in a different order after compression and decompression.

因此，下列Cookie報頭字段集合在語言上是相等，儘管壓縮和編碼後在最終形式上可能看起來不同。

  cookie: a=b; c=d; e=f

  cookie: a=b\0c=d; e=f

  cookie: a=b
  cookie: c=d
  cookie: e=f

8.1.2.5 Malformed Messages 不規範的消息

A malformed request or response is one that uses a valid sequence of HTTP/2 frames, but is otherwise invalid due to the presence of prohibited header fields, the absence of mandatory header fields, or the inclusion of uppercase header field names.

不規範的請求或者響應是一個使用了有效序列的HTTP/2幀，但是使用了禁止的報頭字段、必須字段缺失或者字段名稱使用了大寫。

A request or response that includes an entity body can include a content-length header field. A request or response is also malformed if the value of a content-length header field does not equal the sum of the DATA frame payload lengths that form the body.

包含實體的請求或者響應可以保護一個實體內容長度的報頭字段。如果內容長度報頭字段的值不等於組成實體的數據幀載荷長度，同樣是不規範的。

Intermediaries that process HTTP requests or responses (i.e., any intermediary not acting as a tunnel) MUST NOT forward a malformed request or response.

中介端處理HTTP請求或者響應(除了用來作爲隧道的所有中介端)絕對不能轉發一個不規範的請求或者響應。

Implementations that detect malformed requests or responses need to ensure that the stream ends. For malformed requests, a server MAY send an HTTP response prior to closing or resetting the stream. Clients MUST NOT accept a malformed response. Note that these requirements are intended to protect against several types of common attacks against HTTP; they are deliberately strict, because being permissive can expose implementations to these vulnerabilities.

中介端檢測到不規範的請求或者響應必須保證流已經終止。對於不規範的請求，服務端可以發送之前提到的響應來關閉或者重置流。客戶端絕對不能接收一個不規範的響應。請注意，這些要求是爲了防止一些針對HTTP的常見攻擊；故意這麼嚴格，是因爲允許這些情況的話可能會暴露這些漏洞的實現。

8.1.3 Examples 示例

This section shows HTTP/1.1 requests and responses, with illustrations of equivalent HTTP/2 requests and responses.

這個部分介紹了HTTP/1.1的請求與響應，並帶有HTTP/2請求與響應的插圖。

An HTTP GET request includes request header fields and no body and is therefore transmitted as a single HEADERS frame, followed by zero or more CONTINUATION frames containing the serialized block of request header fields. The HEADERS frame in the following has both the END_HEADERS and END_STREAM flags set; no CONTINUATION frames are sent:

一個帶有請求報頭字段但沒有正文的HTTP GET請求將被轉換成一個單獨的報頭幀，後面跟着0個或者多個包含序列化的報頭字段區塊的延續幀。序列中最後一個報頭幀將有END_HEADERS和END_STREAM標記。

  GET /resource HTTP/1.1           HEADERS
  Host: example.org          ==>     + END_STREAM
  Accept: image/jpeg                 + END_HEADERS
                                       :method = GET
                                       :scheme = https
                                       :path = /resource
                                       host = example.org
                                       accept = image/jpeg

Similarly, a response that includes only response header fields is transmitted as a HEADERS frame (again, followed by zero or more CONTINUATION frames) containing the serialized block of response header fields.

相似的，只帶有報頭字段的響應將被轉換成一個報頭幀(同樣的，後面跟着0個護着多個延續幀)，且包含序列化的響應報頭字段區塊。序列中最後一個報頭幀將有END_HEADERS和END_STREAM標記。

  HTTP/1.1 304 Not Modified        HEADERS
  ETag: "xyzzy"              ==>     + END_STREAM
  Expires: Thu, 23 Jan ...           + END_HEADERS
                                       :status = 304
                                       etag: "xyzzy"
                                       expires: Thu, 23 Jan ...

An HTTP POST request that includes request header fields and payload data is transmitted as one HEADERS frame, followed by zero or more CONTINUATION frames containing the request header fields, followed by one or more DATA frames, with the last CONTINUATION (or HEADERS) frame having the END_HEADERS flag set and the final DATA frame having the END_STREAM flag set:

帶有報頭和載荷數據的POST HTTP請求將被轉換成一個報頭幀，後面跟着0個或者多個帶有請求報頭字段的延續幀，同時後面跟着一個或者多個數據幀。延續幀或者報頭幀的最後一幀有END_HEADERS標記，最後一個數據幀擁有END_STREAM標記。

 POST /resource HTTP/1.1          HEADERS
  Host: example.org          ==>     - END_STREAM
  Content-Type: image/jpeg           + END_HEADERS
  Content-Length: 123                  :method = POST
                                       :scheme = https
  {binary data}                        :path = /resource
                                       :authority = example.org
                                       content-type = image/jpeg
                                       content-length = 123

                                   DATA
                                     + END_STREAM
                                   {binary data}

Note that data contributing to any given header field could be spread between header block fragments. The allocation of header fields to frames in this example is illustrative only.

需要注意的是任何給定的報頭字段的相關數據都可能被分割到報頭區塊碎片中。這個例子中的報頭字段配置僅僅作爲說明。

A response that includes header fields and payload data is transmitted as a HEADERS frame, followed by zero or more CONTINUATION frames, followed by one or more DATA frames, with the last DATA frame in the sequence having the END_STREAM flag set:

帶有報頭字段及載荷數據的響應將被轉換成一個報頭幀，後面跟着0個或多個延續幀，另外後面跟着一個或多個數據幀，序列中的最後一個數據幀擁有END_STREAM標記。

  HTTP/1.1 200 OK                  HEADERS
  Content-Type: image/jpeg   ==>     - END_STREAM
  Content-Length: 123                + END_HEADERS
                                       :status = 200
  {binary data}                        content-type = image/jpeg
                                       content-length = 123

                                   DATA
                                     + END_STREAM
                                   {binary data}

Trailing header fields are sent as a header block after both the request or response header block and all the DATA frames have been sent. The HEADERS frame starting the trailers header block has the END_STREAM flag set.

所有的請求或者響應報頭區塊以及所有的數據幀發送之後，尾報頭字段作爲一個報頭區塊發送。帶有尾部的報頭/延續幀序列包含一個帶有END_HEADERS及END_STREAM標記的終止幀。

  HTTP/1.1 200 OK                  HEADERS
  Content-Type: image/jpeg   ==>     - END_STREAM
  Transfer-Encoding: chunked         + END_HEADERS
  Trailer: Foo                         :status        = 200
                                       content-length = 123
  123                                  content-type   = image/jpeg
  {binary data}                        trailer        = Foo
  0
  Foo: bar                         DATA
                                     - END_STREAM
                                   {binary data}

                                   HEADERS
                                     + END_STREAM
                                     + END_HEADERS
                                       foo: bar

8.1.4 Request Reliability Mechanisms in HTTP/2 HTTP/2響應可靠性機制

In HTTP/1.1, an HTTP client is unable to retry a non-idempotent request when an error occurs, because there is no means to determine the nature of the error. It is possible that some server processing occurred prior to the error, which could result in undesirable effects if the request were reattempted.

在HTTP/1.1中，HTTP客戶端在發送錯誤時不能重試一個非冪等的請求，因爲沒有方式來確定錯誤的性質。也許在錯誤出現前服務端已經對信息有所處理，以至於如果重新嘗試發送會得到一些不想要的結果。

HTTP/2 provides two mechanisms for providing a guarantee to a client that a request has not been processed:

The GOAWAY frame indicates the highest stream number that might have been processed. Requests on streams with higher numbers are therefore guaranteed to be safe to retry.
The REFUSED_STREAM error code can be included in a RST_STREAM frame to indicate that the stream is being closed prior to any processing having occurred. Any request that was sent on the reset stream can be safely retried.

HTTP/2提供了兩種機制來確保讓客戶端知道請求沒有被處理:

超時幀指示了流可能被處理的最大流流標示。在更大數字的流上的請求可以保證安全的重試。
RST_STREAM幀中可以包含REFUSED_STREAM錯誤碼來指示流由於之前的處理正在關閉。重置流上的任何請求都可以安全重試。

Requests that have not been processed have not failed; clients MAY automatically retry them, even those with non-idempotent methods.

未經處理的請求且沒有失敗；客戶可以自動重試，甚至包括那些非冪等元的方法。

A server MUST NOT indicate that a stream has not been processed unless it can guarantee that fact. If frames that are on a stream are passed to the application layer for any stream, then REFUSED_STREAM MUST NOT be used for that stream, and a GOAWAY frame MUST include a stream identifier that is greater than or equal to the given stream identifier.

服務端絕對不能表示一個流未被處理除非它能確保這個事實。如果流上的幀被傳遞給應用層的任何流，絕對不能在這個流上使用REFUSED_STREAM，而且一個超時幀必須包含一個大於或等於給定流表示的標識符。

In addition to these mechanisms, the PING frame provides a way for a client to easily test a connection. Connections that remain idle can become broken as some middleboxes (for instance, network address translators, or load balancers) silently discard connection bindings. The PING frame allows a client to safely test whether a connection is still active without sending a request.

除了這些機制，PING幀給客戶的提供了一種方式來簡單測試連接。保持空閒的連接可能被一些中間件(例如網絡地址翻譯或負載均衡器)靜默丟棄連接綁定而打破。PING幀允許客戶端在無需發送請求的情況下安全地測試連接是否依舊激活。

8.2 Server Push 服務端推送

HTTP/2 enables a server to pre-emptively send (or "push") one or more associated responses to a client in response to a single request. This feature becomes particularly helpful when the server knows the client will need to have those responses available in order to fully process the response to the original request.

HTTP/2允許服務端針對客戶端一個單獨的請求，主動的發送(或推送)一個或者多個相關的響應。這種特定在服務端知道客戶端需要這些響應來完整的處理最初的請求的時候特別有用。

Pushing additional responses is optional, and is negotiated between individual endpoints. The SETTINGS_ENABLE_PUSH setting can be set to 0 to indicate that server push is disabled.

推送額外的響應是可選的，並且由各個單獨的終端之間協商。SETTINGS_ENABLE_PUSH設置設置爲0來標識服務端推送是無效的。

Because pushing responses is effectively hop-by-hop, an intermediary could receive pushed responses from the server and choose not to forward those on to the client. In other words, how to make use of the pushed responses is up to that intermediary. Equally, the intermediary might choose to push additional responses to the client, without any action taken by the server.

因爲推送的響應是有效地逐跳，中介端接從服務端接收到推送響應的可以選擇不轉發這些到客戶端。也就是說，如何使用推送響應取決於這些中介端。相等的，中介可能選擇不推送的額外的響應給客戶端，不需要服務端進行任何操作。

A client cannot push. Thus, servers MUST treat the receipt of a PUSH_PROMISE frame as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. Clients MUST reject any attempt to change the SETTINGS_ENABLE_PUSH setting to a value other than 0 by treating the message as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

客戶端不能推送。因此，服務端收到客戶端的PUSH_PROMISE幀必須作爲連接錯誤(章節5.4.1)處理。客戶端必須拒絕任何嘗試修改SETTINGS_ENABLE_PUSH設置值爲0以外的值，並將這消息作爲類型爲協議錯誤的連接錯誤(章節5.4.1)處理。

A server can only push responses that are cacheable (see [RFC7234], Section 3); promised requests MUST be safe (see [RFC7231], Section 4.2.1) and MUST NOT include a request body.

服務端只能推送可以被緩存的響應(見RFC7234,章節3);被承諾的請求必須是安全的(見RFC7231,章節4.2.1)，而且絕對不能包含一個請求主體。

8.2.1 Push Requests 推送請求

Server push is semantically equivalent to a server responding to a request; however, in this case that request is also sent by the server, as a PUSH_PROMISE frame.

服務端推送語義上等同於服務端響應一個請求；然而，這種情況下請求也是由服務端發送的，作爲一個PUSH_PROMISE幀。

The PUSH_PROMISE frame includes a header block that contains a complete set of request header fields that the server attributes to the request. It is not possible to push a response to a request that includes a request body.

PUSH_PROMISE包含了一個報頭區塊，含有完整的服務端屬性請求報頭字段。不可能對帶有請求實體的請求進行推送。

Pushed responses are always associated with an explicit request from the client. The PUSH_PROMISE frames sent by the server are sent on that explicit request's stream. The PUSH_PROMISE frame also includes a promised stream identifier, chosen from the stream identifiers available to the server (see Section 5.1.1).

推送的響應總是與客戶端的一個明確的請求相關。服務端在這個明確的請求流上發送PUSH_PROMISE幀。PUSH_PROMISE幀一般包含被承諾的流標識符，從可用的服務端流標識符中選擇(見章節5.1.1)。

The header fields in PUSH_PROMISE and any subsequent CONTINUATION frames MUST be a valid and complete set of request header fields (Section 8.1.2.1). The server MUST include a method in the :method header field that is safe and cacheable. If a client receives a PUSH_PROMISE that does not include a complete and valid set of header fields, or the :method header field identifies a method that is not safe, it MUST respond with a stream error (Section 5.4.2) of type PROTOCOL_ERROR.

在PUSH_PROMISE或者任何其他延續的幀中的報頭字段必須是完整的請求報頭字段(章節8.1.2.1)。服務端必須在:method字段中包含一個安全而且可緩存的方法。如何客戶端收到不包含完整而且有效的報頭字段的PUSH_PROMISE幀、或者:method表示的方法不是安全的，客戶端必須響應一個類型爲協議錯誤的流錯誤(章節5.4.2)。

The server SHOULD send PUSH_PROMISE (Section 6.6) frames prior to sending any frames that reference the promised responses. This avoids a race where clients issue requests prior to receiving any PUSH_PROMISE frames.

服務端應當在發送任何被承諾的響應之前發送一個PUSH_PROMISE幀(章節6.6)。這避免了客戶端在收到任何PUSH_PROMISE幀前發出請求而出現的競賽。

For example, if the server receives a request for a document containing embedded links to multiple image files, and the server chooses to push those additional images to the client, sending push promises before the DATA frames that contain the image links ensures that the client is able to see the promises before discovering embedded links. Similarly, if the server pushes responses referenced by the header block (for instance, in Link header fields), sending the push promises before sending the header block ensures that clients do not request them.

例如：如果服務端收到文檔請求包含多個嵌入式的圖像鏈接，而且服務端選擇推送那些額外的圖像給客戶端，再數據幀前發送push promises能確保客戶端能夠在發現內嵌鏈接前看到這些承諾。類似的，如果服務端推送與報頭區塊(例如，在Link報頭域)相關的響應，再發送報頭區塊前推送承諾能確保客戶端不請求它們。

PUSH_PROMISE frames MUST NOT be sent by the client.

PUSH_PROMISE幀絕對不能由客戶端發送。

PUSH_PROMISE frames can be sent by the server in response to any client-initiated stream, but the stream MUST be in either the "open" or "half closed (remote)" state with respect to the server. PUSH_PROMISE frames are interspersed with the frames that comprise a response, though they cannot be interspersed with HEADERS and CONTINUATION frames that comprise a single header block.

PUSH_PROMISE可以由服務端在任意由客戶端打開的流上發送。他們必須在對服務端狀態爲“打開”或者“半封閉(遠端)”的流上發送。PUSH_PROMISE幀由響應幀穿插組成，不過他們不能由包含單個報頭區塊的報頭幀和延續幀組成。

Sending a PUSH_PROMISE frame creates a new stream and puts the stream into the “reserved (local)” state for the server and the “reserved (remote)” state for the client.

發送一個推送承諾幀創建了一個新的流並且對服務端將流轉到“保留(本地)”狀態、對客戶端轉“保留(遠端)”的狀態。

8.2.2 Push Responses 推送響應

After sending the PUSH_PROMISE frame, the server can begin delivering the pushed response as a response (Section 8.1.2.2) on a server-initiated stream that uses the promised stream identifier. The server uses this stream to transmit an HTTP response, using the same sequence of frames as defined in Section 8.1. This stream becomes "half closed" to the client (Section 5.1) after the initial HEADERS frame is sent.

發送PUSH_PROMISE幀後，服務端可以開始接收被推送進來的響應作爲一個由服務端發起的使用被承諾流標識的流的響應(章節8.1.2.2)。服務端使用這些流傳送一個HTTP響應，使用HttpSequence中定義的相同的幀序列。在初始化報頭幀發送後，流對客戶端(章節5.1)變爲“半封閉”狀態。

Once a client receives a PUSH_PROMISE frame and chooses to accept the pushed response, the client SHOULD NOT issue any requests for the promised response until after the promised stream has closed.

一旦客戶端接收到PUSH_PROMISE幀並且選擇接受推送的響應，客戶端不應該對被承諾的響應發起人和請求，直到被承諾的流被關閉爲止。

If the client determines, for any reason, that it does not wish to receive the pushed response from the server, or if the server takes too long to begin sending the promised response, the client can send an RST_STREAM frame, using either the CANCEL or REFUSED_STREAM codes, and referencing the pushed stream's identifier.

如果客戶端以任何理由決定不希望接受服務端推送的響應，或者服務端花費太長時間纔開始發送承諾的響應，客戶端可以發送一個RST_STREAM幀，使用CANCEL或者REFUSED_STREAM碼來關聯被推送的流標識符。

A client can use the SETTINGS_MAX_CONCURRENT_STREAMS setting to limit the number of responses that can be concurrently pushed by a server. Advertising a SETTINGS_MAX_CONCURRENT_STREAMS value of zero disables server push by preventing the server from creating the necessary streams. This does not prohibit a server from sending PUSH_PROMISE frames; clients need to reset any promised streams that are not wanted.

客戶端可以使用SETTINGS_MAX_CONCURRENT_STREAMS設置來限制服務端推送的響應的併發量。廣播值爲0的SETTINGS_MAX_CONCURRENT_STREAMS能防止服務端創建必需的流。這不能禁止服務端發送PUSH_PROMISE幀；客戶端需要重置任何不需要的被承諾的流。

Clients receiving a pushed response MUST validate that the server is authorized to provide the response, see Section 10.1. For example, a server that offers a certificate for only the example.com DNS-ID or Common Name is not permitted to push a response for https://www.example.org/doc.

客戶端收到推送響應必須驗證服務端是授權提供響應的，見章節10.1。例如，服務端只帶有針對example.com的DNS的授權證書是不允許給https://www.example.org/doc推送給響應的。

The response for a PUSH_PROMISE stream begins with a HEADERS frame, which immediately puts the stream into the “half closed (remote)” state for the server and “half closed (local)” state for the client, and ends with a frame bearing END_STREAM, which places the stream in the "closed" state.

對推送承諾的幀的響應以一個報頭幀開始，並馬上將流轉變成對服務端的“半封閉(遠端)”狀態以及對客戶端的“半封閉(本地)”狀態，並且以一個帶有END_STREAM的幀結尾，這個將把流轉到“關閉”狀態。

Note:The client never sends a frame with the END_STREAM flag for a server push.

注意：客戶端從不會在服務端推送的時候發送一個帶有END_STREAM標記的幀。

8.3 The CONNECT Method CONNECT方法

In HTTP/1.x, the pseudo-method CONNECT ([RFC7231], Section 4.3.6) is used to convert an HTTP connection into a tunnel to a remote host. CONNECT is primarily used with HTTP proxies to establish a TLS session with an origin server for the purposes of interacting with https resources.

在HTTP/1.x中，僞方法連接(RFC7231，章節4.3.6)用來轉換HTTP連接成隧道到遠端主機。連接主要用HTTP代理爲使用HTTPS資源相互作用的目的源服務器建立TLS會話。

In HTTP/2, the CONNECT method is used to establish a tunnel over a single HTTP/2 stream to a remote host, for similar purposes. The HTTP header field mapping works as mostly as defined in Request Header Fields (Section 8.1.2.1), with a few differences. Specifically:

在HTTP/2中，連接方法用來在一個單一的HTTP/2流上建立一個通向遠端主機的隧道，目的類似。HTTP報頭字段尋址工作大部分同請求報頭字段(章節8.1.2.1)中定義，有一部分不同。具體爲：

The :method header field is set to CONNECT.
The :scheme and :path header fields MUST be omitted.
The :authority header field contains the host and port to connect to (equivalent to the authority-form of the request-target of CONNECT requests, see [RFC7230], Section 5.3).
:method是連接中包含:method報頭字段。
:scheme和:path報頭字段必須被忽略。
:authority報頭字段包含主機及連接的端口(相當於authority形式的請求目標連接請求，見RFC7230章節5.3)。

A proxy that supports CONNECT establishes a TCP connection [TCP] to the server identified in the :authority header field. Once this connection is successfully established, the proxy sends a HEADERS frame containing a 2xx series status code to the client, as defined in [RFC7231], Section 4.3.6.

支持CONNECT的代理服務器建立一個TCP連接到服務器，如同:authority報頭字段中所定義的。一旦連接建立成功，代理髮送一個報頭幀包含一個2xx序列狀態碼到客戶端，如RFC7231中定義，章節4.3.6。

After the initial HEADERS frame sent by each peer, all subsequent DATA frames correspond to data sent on the TCP connection. The payload of any DATA frames sent by the client are transmitted by the proxy to the TCP server; data received from the TCP server is assembled into DATA frames by the proxy. Frame types other than DATA or stream management frames (RST_STREAM, WINDOW_UPDATE, and PRIORITY) MUST NOT be sent on a connected stream, and MUST be treated as a stream error (Section 5.4.2) if received.

初始化報頭幀由各個對等端發送後，所有隨後的與數據對應的數據幀在TCP連接上發送。客戶端發送的數據幀載荷由代理轉換再發送給TCP服務器；從TCP服務器接收到的數據由代理組裝成數據幀。數據幀或者流管理幀以外的幀(RST_STREAM、WINDOW_UPDATE和PRIORITY)絕對不能再建立的流上發送，如果收到這樣的幀必須作爲流錯誤(章節5.4.2)處理。

The TCP connection can be closed by either peer. The END_STREAM flag on a DATA frame is treated as being equivalent to the TCP FIN bit. A client is expected to send a DATA frame with the END_STREAM flag set after receiving a frame bearing the END_STREAM flag. A proxy that receives a DATA frame with the END_STREAM flag set sends the attached data with the FIN bit set on the last TCP segment. A proxy that receives a TCP segment with the FIN bit set sends a DATA frame with the END_STREAM flag set. Note that the final TCP segment or DATA frame could be empty.

TCP連接可以被各個對等端關閉。數據幀上的END_STREAM標記被認爲與TCP FIN比特相同。客戶端在收到帶有END_STREAM標記的幀後被期望應該發送一個帶有 END_STREAM標記的數據幀。代理接收到帶有END_STREAM 標記的數據幀將在發送這些數據的時候在最後的TCP段上設置FIN位。帶有接收到帶有FIN位的TCP端發送一個帶有END_STREAM標記的數據幀。注意最後的TCP端或者數據幀可以爲空。

A TCP connection error is signaled with RST_STREAM. A proxy treats any error in the TCP connection, which includes receiving a TCP segment with the RST bit set, as a stream error (Section 5.4.2) of type CONNECT_ERROR. Correspondingly, a proxy MUST send a TCP segment with the RST bit set if it detects an error with the stream or the HTTP/2 connection.

TCP連接錯誤由RST_STREAM來標記。代理對外TCP連接中的任何錯誤，包括接收到設置了RST位的TCP段，作爲類型爲連接錯誤的流錯誤(章節5.4.2)處理。相應的，代理如果檢測到流或者HTTP/2連接的錯誤必須發送一個設置了RST位的TCP段。

9 Additional HTTP Requirements/Considerations 額外HTTP要求/考慮

This section outlines attributes of the HTTP protocol that improve interoperability, reduce exposure to known security vulnerabilities, or reduce the potential for implementation variation.

這段概況了HTTP協議的屬性，包括提高互操作性、減少暴露已知的安全漏洞，或者減少執行變動的可能。

9.1 Connection Management 連接管理

HTTP/2 connections are persistent. For best performance, it is expected clients will not close connections until it is determined that no further communication with a server is necessary (for example, when a user navigates away from a particular web page), or until the server closes the connection.

HTTP/2連接是永久性的。爲了最佳的性能，它期待直到確定與服務端的進一步溝通不再必要的時候，客戶端纔會關閉連接(例如，當用戶導航到其他特定的網頁)，或者直到服務端關閉連接。

Clients SHOULD NOT open more than one HTTP/2 connection to a given host and port pair, where host is derived from a URI, a selected alternative service [ALT-SVC], or a configured proxy.

客戶端不應該再給定的目的地上打開多個HTTP/2連接，目的地是由給定的URI確定的IP地址及TCP端口【這裏我們需要小心Alt-Svc】，或者配置的代理的IP和端口。

A client can create additional connections as replacements, either to replace connections that are near to exhausting the available stream identifier space (Section 5.1.1), to refresh the keying material for a TLS connection, or to replace connections that have encountered errors (Section 5.4.1).

客戶端可以創建額外的連接作爲替代，或者取代快要用盡可用流標識空間(章節5.1.1)的連接，或者替換遇到錯誤(章節5.4.1)的連接。

A client MAY open multiple connections to the same IP address and TCP port using different Server Name Indication [TLS-EXT] values or to provide different TLS client certificates, but SHOULD avoid creating multiple connections with the same configuration.

客戶端可以使用不相同的服務端名稱標識值或者提供不一樣的TLS客戶端證書對相同IP地址及TCP端口打開多個連接，但應該避免對相同的配置上創建多個連接。關於客戶端證書相關的更多信息.

Servers are encouraged to maintain open connections for as long as possible, but are permitted to terminate idle connections if necessary. When either endpoint chooses to close the transport-level TCP connection, the terminating endpoint SHOULD first send a GOAWAY (Section 6.8) frame so that both endpoints can reliably determine whether previously sent frames have been processed and gracefully complete or terminate any necessary remaining tasks.

服務端被孤立儘可能長的保持打開的連接，但在必要下允許關閉空閒的連接。當任意一個終端決定關閉傳輸層的TCP連接，決定關閉的終端應首先發送一個GOAWAY幀這樣兩個終端都能可靠的確定之前發送的幀是否已經被處理及優雅的完成或者終止任何必要的剩餘任務。

9.1.1 Connection Reuse 複用連接

Clients MAY use a single server connection to send requests for URIs with multiple different authority components as long as the server is authoritative (Section 10.1). For http resources, this depends on the host having resolved to the same IP address.

客戶端可以使用單個服務端連接來發送多個不同認證組件的URIs請求，只要服務端是認證的(章節 10.1)。對於http資源來說，這個取決於對同個IP地址已經解析的主機端。

For https resources, connection reuse additionally depends on having a certificate that is valid for the host in the URI. That is the use of server certificate with multiple subjectAltName attributes, or names with wildcards. For example, a certificate with a subjectAltName of *.example.com might permit the use of the same connection for a.example.com and b.example.com.

對於https資源，複用連接還另外需要一個對於URI中的主機已經驗證過的證書。這是多subjectAltName熟悉或者使用通配符的名稱的服務端證書的使用。例如，一個帶有*.example.com的subjectAltName的證書將允許a.example.com和b.example.com.使用同個連接。

In some deployments, reusing a connection for multiple origins can result in requests being directed to the wrong origin server. For example, TLS termination might be performed by a middlebox that uses the TLS Server Name Indication (SNI) [TLS-EXT] extension to select the an origin server. This means that it is possible for clients to send confidential information to servers that might not be the intended target for the request, even though the server has valid authentication credentials.

在某些部署環境中，多個源的複用連接可能導致請求被指向錯誤的源服務器。例如，代理可能使用TLS服務端名稱指示(SNI)[TLS-EXT]擴展選擇一個源服務器來執行TLS終止。這意味着可能客戶端未像預期那樣發送機密信息到請求的服務器，儘管服務端有驗證的權限證書。

A server that does not wish clients to reuse connections can indicate that it is not authoritative for a request by sending a 421 (Not Authoritative) status code in response to the request (see Section 9.1.2).

服務端不希望客戶端複用連接的可以在響應(章節9.1.2)中發送一個421(未驗證)的狀態碼來指示請求是未驗證的。

9.1.2 The 421 (Not Authoritative) Status Code 421(未驗證)狀態碼

The 421 (Not Authoritative) status code indicates that the current origin server is not authoritative for the requested resource, in the sense of [RFC7230], Section 9.1 (see also Section 10.1).

421(未驗證)狀態碼標識當前源服務器對請求的資源未驗證，詳見RFC7230，章節9.1(同見章節10.1)。

Clients receiving a 421 (Not Authoritative) response from a server MAY retry the request - whether the request method is idempotent or not - over a different connection. This is possible if a connection is reused (Section 9.1.1) or if an alternative service is selected ([ALT-SVC]).

客戶端接收到服務端發送的421(未驗證)響應可以在不同的連接上重試請求-不管這個請求是不是冪等的。這在連接是複用的(章節9.1.1)或可選的服務被選擇(ALT-SVC)下是可能的。

This status code MUST NOT be generated by proxies.

這個狀態碼絕對不能由代理端生成。

A 421 response is cacheable by default; i.e., unless otherwise indicated by the method definition or explicit cache controls (see Section 4.2.2 of [RFC7234]).

421響應緩存是默認的；舉例：除非被定義的方法或顯示的緩存控制另有說明(RFC7234, 章節4.2.2)。

9.2 Use of TLS Features 使用TLS功能

Implementations of HTTP/2 MUST support TLS 1.2 [TLS12] for HTTP/2 over TLS. The general TLS usage guidance in [TLSBCP] SHOULD be followed, with some additional restrictions that are specific to HTTP/2.

實現HTTP/2必須支持TLS 1.2。通用的TLS用法指導應該遵循，同時加上對HTTP/2的特定支持。

9.2.1 TLS Features TLS功能

The TLS implementation MUST support the Server Name Indication (SNI) [TLS-EXT] extension to TLS. HTTP/2 clients MUST indicate the target domain name when negotiating TLS.

TLS實現必須支持服務端名稱標識(SNI)的TLS擴展。HTTP/2客戶端再協商TLS的時候必須標明目標域名名稱。

The TLS implementation MUST disable compression. TLS compression can lead to the exposure of information that would not otherwise be revealed [RFC3749]. Generic compression is unnecessary since HTTP/2 provides compression features that are more aware of context and therefore likely to be more appropriate for use for performance, security or other reasons.

TLS實現必須禁止壓縮。TLS壓縮可能導致信息暴露。通用的壓縮是不必要的，因爲HTTP/2提供的壓縮功能更加上下文，因爲可能是更符合使用性能、安全或者其他原因。

The TLS implementation MUST disable renegotiation. An endpoint MUST treat a TLS renegotiation as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. Note that disabling renegotiation can result in long-lived connections becoming unusable due to limits on the number of messages the underlying cipher suite can encipher.

TLS實現必須禁用協商。終端必須將TLS協商作爲類型爲協議錯誤的連接錯誤(章節5.4.1)處理。需要注意的是由於密碼套件可以加密的消息的次數限制，禁用協商可導致長期連接變成不可用。

A client MAY use renegotiation to provide confidentiality protection for client credentials offered in the handshake, but any renegotiation MUST occur prior to sending the connection preface. A server SHOULD request a client certificate if it sees a renegotiation request immediately after establishing a connection.

客戶端可以使用協商來對客戶端握手平局提供機密保護，但是任何協商必須在發送連接序言之前。服務端如果看到連接建立後馬上協商應該請求客戶端證書。

This effectively prevents the use of renegotiation in response to a request for a specific protected resource. A future specification might provide a way to support this use case.

這有效的防止了通過協商來獲取一個特定的受保護的資源的請求。未來的規範可能會提供一種方式來支持這種需求。

9.2.2 TLS Cipher Suites TLS加密套件

The set of TLS cipher suites that are permitted in HTTP/2 is restricted. HTTP/2 MUST only be used with cipher suites that have ephemeral key exchange, such as the ephemeral Diffie-Hellman (DHE) [TLS12] or the elliptic curve variant (ECDHE) [RFC4492]. Ephemeral key exchange MUST have a minimum size of 2048 bits for DHE or security level of 128 bits for ECDHE. Clients MUST accept DHE sizes of up to 4096 bits. HTTP MUST NOT be used with cipher suites that use stream or block ciphers. Authenticated Encryption with Additional Data (AEAD) modes, such as the Galois Counter Model (GCM) mode for AES [RFC5288] are acceptable.

HTTP/2中授權使用的TLS加密套件是不對外公開的。HTTP/2必須僅在支持密鑰交換的加密套件下使用，如短暫的Diffie-Hellman（DHE）或橢圓曲線的變體（ecdhe）。交換的密鑰必須具有最小尺寸的2048位（DHE）或128位的安全級別（ecdhe）。客戶端必須接受多達4096位DHE尺寸。HTTP絕對不能使用用流或區塊密碼的加密套件。使用額外數據(AEAD)模式的認證加密，例如針對AES[ RFC5288 ] 的Galois Counter Model (GCM)模式是允許的。

Clients MAY advertise support of other cipher suites in order to allow for connection to servers that do not support HTTP/2 to complete without the additional latency imposed by using a separate connection for fallback.

客戶端可以廣播對其他加密套件的支持來允許不支持HTTP/2的服務端的連接能在不需要額外的延遲(因爲需要使用單獨的連接來降級)下完成。

An implementation SHOULD NOT negotiate a TLS connection for HTTP/2 without also negotiating a cipher suite that meets these requirements. Due to implementation limitations, it might not be possible to fail TLS negotiation. An endpoint MUST immediately terminate an HTTP/2 connection that does not meet these minimum requirements with a connection error (Section 5.4.1) of type INADEQUATE_SECURITY.

實現中對於HTTP/2不應該在沒有使用符合需求的加密套件協商的情況下進行TLS協商。由於實施的限制，不可能基於所有需求來使TLS協商失敗。終端必須終止不符合TLS最小需求的TLS會話上建立的HTTP/2連接，並作爲類型爲INADEQUATE_SECURITY的連接錯誤(章節5.4.1)處理。

9.3 GZip Content-Encoding 內部編碼Gzip壓縮

客戶端必須支持HTTP響應體的Gzip壓縮。不管接收白頭字段的編碼的值，服務端可以發送Gzip編碼響應。一個壓縮的響應還必須承擔適當的內容編碼報頭字段。

10 Security Considerations 安全性考慮

10.1 Server Authority 服務端認證

A client is only able to accept HTTP/2 responses from servers that are authoritative for those resources. This is particularly important for server push (Section 8.2), where the client validates the PUSH_PROMISE before accepting the response.

客戶端只有經過權限驗證才能獲取HTTP/2響應的資源。這在服務器推送中(章節8.2)尤爲重要，客戶端在接收響應前驗證PUSH_PROMISE幀。

HTTP/2 relies on the HTTP/1.1 definition of authority for determining whether a server is authoritative in providing a given response, see [RFC7230], Section 9.1. This relies on local name resolution for the "http" URI scheme, and the authenticated server identity for the "https" scheme (see [RFC2818], Section 3).

HTTP/2依據HTTP/1.1權限定義來檢測服務端是否有權限提供給定的響應，見RFC2818 章節3.這依賴於本地“HTTP”URI方案的域名解析，以及服務端提供的“https”方案驗證。

A client MUST discard responses provided by a server that is not authoritative for those resources.

客戶端絕對不能以任何形式使用服務端提供的客戶端沒有權限的資源。

10.2 Cross-Protocol Attacks 跨協議攻擊

In a cross-protocol attack, an attacker causes a client to initiate a transaction in one protocol toward a server that understands a different protocol. An attacker might be able to cause the transaction to appear as valid transaction in the second protocol. In combination with the capabilities of the web context, this can be used to interact with poorly protected servers in private networks.

在跨協議攻擊中，攻擊者使客戶端在一種協議中向解析另一種協議的服務器啓動一個交易。攻擊者可能能夠使交易看起來在第二種協議中是合法的。結合對web上下文的利用，這個可以針對保護不力的服務器在祕密網絡下進行互動。

Completing a TLS handshake with an ALPN identifier for HTTP/2 can be considered sufficient protection against cross protocol attacks. ALPN provides a positive indication that a server is willing to proceed with HTTP/2, which prevents attacks on other TLS-based protocols.

在ALPN和TLS合作的情況下對於阻止HTTP/2被協議攻擊是足夠的。ALPN提供了一個積極的指示說明服務器願意處理HTTP/2,這有助於阻止基於TLS協議的攻擊。

The encryption in TLS makes it difficult for attackers to control the data which could be used in a cross-protocol attack on a cleartext protocol.

TLS中的加密使得攻擊者很難控制明文協議中能被用來進行跨協議攻擊的數據。

The cleartext version of HTTP/2 has minimal protection against cross-protocol attacks. The connection preface (Section 3.5) contains a string that is designed to confuse HTTP/1.1 servers, but no special protection is offered for other protocols. A server that is willing to ignore parts of an HTTP/1.1 request containing an Upgrade header field in addition to the client connection preface could be exposed to a cross-protocol attack.

HTTP/2明文版本對於跨協議攻擊具有很小的保護措施。連接序言(章節3.5)包含一個字符串，是用來迷惑HTTP/1.1服務器的，但是對於其他版本協議沒有提供特殊保護。服務端願意忽略包含升級字段的HTTP/1.1請求部分的可以認爲是一次跨協議攻擊。

10.3 中介端封裝攻擊

HTTP/2 header field names and values are encoded as sequences of octets with a length prefix. This enables HTTP/2 to carry any string of octets as the name or value of a header field. An intermediary that translates HTTP/2 requests or responses into HTTP/1.1 directly could permit the creation of corrupted HTTP/1.1 messages. An attacker might exploit this behavior to cause the intermediary to create HTTP/1.1 messages with illegal header fields, extra header fields, or even new messages that are entirely falsified.

HTTP/2報頭名稱和值編碼成帶有長度前綴的字節序列。這使得HTTP/2能夠攜帶任何字符串的字節作爲報頭字段的名稱或值。中介端直接轉換HTTP/2請求或響應到HTTP/1.1時可以允許HTTP/1.1消息創建的損壞。攻擊者可以利用這個行爲讓中介端創建HTTP/1.1消息時帶有非法報頭字段、額外報頭字段，甚至是完全僞造的新消息。

Header field names or values that contain characters not permitted by HTTP/1.1, including carriage return (ASCII 0xd) or line feed (ASCII 0xa) MUST NOT be translated verbatim by an intermediary, as stipulated in [RFC7230], Section 3.2.4.

報頭字段名稱或值帶有不被HTTP/1.1允許的字符，包括回車(U+000D)或者換行(U+000A),絕對不能被中介端逐行解析，見[HTTP-p1]定義，章節3.2.4.

Translation from HTTP/1.x to HTTP/2 does not produce the same opportunity to an attacker. Intermediaries that perform translation to HTTP/2 MUST remove any instances of the obs-fold production from header field values.

從HTTP/1.x轉換到HTTP/2不會被攻擊者利用出現類似的情況。執行轉換的中介端必須移除摺疊obs的任何實例。

10.4 推送響應的緩存

Pushed responses do not have an explicit request from the client; the request is provided by the server in the PUSH_PROMISE frame.

推送響應並沒有一個來自客戶端的明確請求；請求是服務端從PUSH_PROMISE幀中提供的。

Caching responses that are pushed is possible based on the guidance provided by the origin server in the Cache-Control header field. However, this can cause issues if a single server hosts more than one tenant. For example, a server might offer multiple users each a small portion of its URI space.

緩存響應推送可能是基於原始服務器的緩存控制報頭字段的指導。然而，如果服務端主機包含多個用戶可能會導致問題。例如，服務端可能爲多個用戶每個提供小部分的URI空間。

Where multiple tenants share space on the same server, that server MUST ensure that tenants are not able to push representations of resources that they do not have authority over. Failure to enforce this would allow a tenant to provide a representation that would be served out of cache, overriding the actual representation that the authoritative tenant provides.

當多個用戶共享同一臺服務器時，該服務器必須確保用戶不能推送沒有權限使用的資源。如果不能確保這個將導致用戶可能提供超出緩存以外的內容，覆蓋用戶實際有權限提供的內容。

Pushed responses for which an origin server is not authoritative (see Section 10.1) are never cached or used.

沒有權限的源服務器(見10.1章節)推送響應將不會被緩存或者使用。

10.5 拒絕服務的注意事項

An HTTP/2 connection can demand a greater commitment of resources to operate than a HTTP/1.1 connection. The use of header compression and flow control depend on a commitment of resources for storing a greater amount of state. Settings for these features ensure that memory commitments for these features are strictly bounded.

HTTP/2連接可以要求使用比HTTP/1.1連接更大的資源。報頭壓縮和流量控制的使用取決於承諾的資源存儲更大量的狀態。這些功能的設置確保這些承諾的內存是嚴格限制的。處理能力不能在相同的高速緩存中被守護。

The number of PUSH_PROMISE frames is not constrained in the same fashion. A client that accepts server push SHOULD limit the number of streams it allows to be in the "reserved (remote)" state. Excessive number of server push streams can be treated as a stream error (Section 5.4.2) of type ENHANCE_YOUR_CALM.

推送承諾幀的數量並不是以相同方式約束的。客戶端收到服務端推送應該限制允許轉成“保留(遠端)”狀態的流的數量。服務端推送流數量超出可以作爲類型爲ENHANCE_YOUR_CALM的流錯誤(章節5.4.2)處理。

Processing capacity cannot be guarded as effectively as state capacity.

處理能力不能像狀態容量一樣被有效的保護。

The SETTINGS frame can be abused to cause a peer to expend additional processing time. This might be done by pointlessly changing SETTINGS parameters, setting multiple undefined parameters, or changing the same setting multiple times in the same frame. WINDOW_UPDATE or PRIORITY frames can be abused to cause an unnecessary waste of resources.

設置幀可能被濫用導致對等端花費額外的處理時間。這可能是毫無意義改變設置參數、設置多個未定義的參數，或者在同個幀中多次修改同個值。WINDOW_UPDATE 或 PRIORITY幀也可能被濫用導致資源的不必要的浪費。服務端可能在沒有權限爲客戶端產生過量工作時錯誤地假定源服務器的ALTSVC幀。

Large numbers of small or empty frames can be abused to cause a peer to expend time processing frame headers. Note however that some uses are entirely legitimate, such as the sending of an empty DATA frame to end a stream.

大量的小或空的貞可能被濫用導致對等端花費額外的時間處理報頭幀。但需要注意的是有些使用是完全合法的，例如發送空數據幀到結束流。

Header compression also offers some opportunities to waste processing resources; see Section 8 of [COMPRESSION] for more details on potential abuses.

報頭壓縮也可能導致處理資源的浪費；見[COMPRESSION]章節8查更多潛在濫用的細節。

Limits in SETTINGS parameters cannot be reduced instantaneously, which leaves an endpoint exposed to behavior from a peer that could exceed the new limits. In particular, immediately after establishing a connection, limits set by a server are not known to clients and could be exceeded without being an obvious protocol violation.

設置參數的限制不能瞬間降低，這使終端對對等端暴露的行爲可能超出新的限制。特別是，連接連接後瞬間，服務端設置的限制並不被客戶端知道，而且客戶端可能在不明顯違反協議的情況下超出限制。

All these features - i.e., SETTINGS changes, small frames, header compression - have legitimate uses. These features become a burden only when they are used unnecessarily or to excess.

所有的這些功能，即設置的修改、小幀、報頭壓縮使用都是合法的。他們只有在不必要或者多餘的使用時纔會成爲負擔。

An endpoint that doesn't monitor this behavior exposes itself to a risk of denial of service attack. Implementations SHOULD track the use of these features and set limits on their use. An endpoint MAY treat activity that is suspicious as a connection error (Section 5.4.1) of type ENHANCE_YOUR_CALM.

終端不監測這種行爲可能暴露其遭受拒絕服務攻擊的風險。具體實現應當跟蹤這些功能的使用並限制它們的使用。終端可以對待這些可疑的活動作爲類型爲ENHANCE_YOUR_CALM的連接錯誤(章節5.4.1)處理。

10.5.1 Limits on Header Block Size

A large header block (Section 4.3) can cause an implementation to commit a large amount of state. In servers and intermediaries, header fields that are critical to routing, such as :authority, :path, and :scheme are not guaranteed to be present early in the header block. In particular, values that are in the reference set cannot be emitted until the header block ends.

一個較大的報頭區塊(章節4.3)可以導致具體實現中提交大量的狀態。在服務端和中介端，報頭字段對路由至關重要，例如:authority, :path, 和:scheme並沒有保證在報頭區塊中靠前呈現。特別是，引用集合中的值只有在報頭區塊結束才能被提交。

This can prevent streaming of the header fields to their ultimate destination, and forces the endpoint to buffer the entire header block. Since there is no hard limit to the size of a header block, an endpoint could be forced to exhaust available memory.

這可能阻止流的報頭字段到達其最終目的地，並強制終端來緩存整個報頭區塊。由於對報頭區塊沒有硬性規定限制，終端可能被強制耗盡可用內存。

A server that receives a larger header block than it is willing to handle can send an HTTP 431 (Request Header Fields Too Large) status code [RFC6585]. A client can discard responses that it cannot process. The header block MUST be processed to ensure a consistent connection state, unless the connection is closed.

服務端接收到大於它願意處理的報頭區塊可以發送一個HTTP 431(請求報頭字段過大)狀態碼 RFC6585。終端可以丟棄其不能處理的響應。報頭區塊必須被處理以確保連續的連接狀態，除非連接是關閉的。

10.6 Use of Compression

HTTP/2 enables greater use of compression for both header fields (Section 4.3) and entity bodies. Compression can allow an attacker to recover secret data when it is compressed in the same context as data under attacker control.

HTTP/2允許更大的使用報頭字段(見4.3章節)和響應主體(見9.3章節)的壓縮。壓縮可以使攻擊者在相同上下文數據壓縮的攻擊控制下恢復祕密數據。

There are demonstrable attacks on compression that exploit the characteristics of the web (e.g., [BREACH]). The attacker induces multiple requests containing varying plaintext, observing the length of the resulting ciphertext in each, which reveals a shorter length when a guess about the secret is correct.

明文網絡(例如[BREACH])下有針對壓縮的明顯攻擊。攻擊者誘導包含不同明文的多個請求，觀察各個得到的密文的長度，當密碼猜測是正確時就揭示了更短的長度。

Implementations communicating on a secure channel MUST NOT compress content that includes both confidential and attacker-controlled data unless separate compression dictionaries are used for each source of data. Compression MUST NOT be used if the source of data cannot be reliably determined.

具體實現在一個安全通道通信時絕對不能壓縮保密的和受攻擊者控制的內容，除非對於各個數據源的壓縮字典是不同的。絕對不能使用數據源可靠性不確定的壓縮數據。

Further considerations regarding the compression of header fields are described in [COMPRESSION].

關於報頭uziduan壓縮的更多表述參照[COMPRESSION]

10.7 填充的使用

Padding within HTTP/2 is not intended as a replacement for general purpose padding, such as might be provided by TLS [TLS12]. Redundant padding could even be counterproductive. Correct application can depend on having specific knowledge of the data that is being padded.

HTTP/2的填充不打算作爲通用填充的替換，如可能在TLS [TLS12]提供的。多餘的填充可能會適得其反。正確的應用依靠的是對於填充的數據有具體的認知。

To mitigate attacks that rely on compression, disabling or limiting compression might be preferable to padding as a countermeasure.

爲了減輕依靠壓縮的攻擊危險，禁用或者限制壓縮可能是作爲填充的最好對策。

Padding can be used to obscure the exact size of frame content, and is provided to mitigate specific attacks within HTTP. For example, attacks where compressed content includes both attacker-controlled plaintext and secret data (see for example, [BREACH]).

填充可以用來混淆幀內容的實際大小，而且減少HTTP中的特殊攻擊。例如，壓縮的內容包含攻擊者控制的明文和祕密數據的攻擊(見BREACH)。

Use of padding can result in less protection than might seem immediately obvious. At best, padding only makes it more difficult for an attacker to infer length information by increasing the number of frames an attacker has to observe. Incorrectly implemented padding schemes can be easily defeated. In particular, randomized padding with a predictable distribution provides very little protection; similarly, padding payloads to a fixed size exposes information as payload sizes cross the fixed size boundary, which could be possible if an attacker can control plaintext.

使用填充可能導致比明顯看起來的更少的保護。最好情況下，通過增加攻擊者需要觀察的幀的數量，填充能使得攻擊者更難推斷長度信息。但不正確的實現填充方案可能被輕鬆破解。特別是，帶有可預測分佈的隨機填充提供保護非常小；或者填充的載荷是一個固定大小的公開信息做未來分佈在固定大小的邊界，這可能導致攻擊者能控制明文的情況下破解。

Intermediaries SHOULD retain padding for DATA frames, but MAY drop padding for HEADERS and PUSH_PROMISE frames. A valid reason for an intermediary to change the amount of padding of frames is to improve the protections that padding provides.

中介端不應該移除填充，但中介如果是想提高填充的保護可以移除填充添加不同數量的填充量。

10.8 隱私注意事項

Several characteristics of HTTP/2 provide an observer an opportunity to correlate actions of a single client or server over time. This includes the value of settings, the manner in which flow control windows are managed, the way priorities are allocated to streams, timing of reactions to stimulus, and handling of any optional features.

HTTP/2的一些特點導致觀察者有機會觀察單個客戶端或服務器的行爲。這包含設置的值、流量控制窗口的管理方式、優先分配流的方法、反應時間以及任何可選功能的處理。

As far as this creates observable differences in behavior, they could be used as a basis for fingerprinting a specific client, as defined in Section 1.8 of [HTML5].

至於這個可觀察到的行爲的差異，他們可以作爲特定的客戶端的識別的基礎，見HTML5 章節1.8中定義。

11 IANA Considerations

A string for identifying HTTP/2 is entered into the "Application Layer Protocol Negotiation (ALPN) Protocol IDs" registry established in [TLSALPN].

標識HTTP/2的字符串已經收錄到TLSALPN中建立的"Application Layer Protocol Negotiation (ALPN) Protocol IDs"註冊表中。

This document establishes a registry for frame types, settings, and error codes. These new registries are entered into a new "Hypertext Transfer Protocol (HTTP) 2 Parameters" section.

本文檔建立了幀類型、設置和錯誤碼的記錄。這些新的記錄被收錄到新的"Hypertext Transfer Protocol (HTTP) 2 Parameters" 章節中。

This document registers the HTTP2-Settings header field for use in HTTP; and the 421 (Not Authoritative) status code.

本文檔記錄了HTTP使用的HTTP2-Settings報頭字段；以及421(未驗證)狀態碼。

This document registers the PRI method for use in HTTP, to avoid collisions with the connection preface (Section 3.5).

爲了避免與連接序言(章節3.5)衝突，本文檔註冊了在HTTP中使用的PRI方法。

11.1 HTTP/2識別字符串的註冊

This document creates two registrations for the identification of HTTP/2 in the "Application Layer Protocol Negotiation (ALPN) Protocol IDs" registry established in [TLSALPN].

本文檔在"應用層協議協商(ALPN)協議IDs"中的[TLSALPN]註冊列表創建了兩種HTTP/2標識符註冊。

The "h2" string identifies HTTP/2 when used over TLS:

Protocol:HTTP/2 over TLSIdentification Sequence:0x68 0x32 ("h2")Specification:This documentThe "h2c" string identifies HTTP/2 when used over cleartext TCP:Protocol:HTTP/2 over TCPIdentification Sequence:0x68 0x32 0x63 ("h2c")Specification:This document

使用TLS時“h2”字符串標識HTTP/2:

協議: TLS上的HTTP/2
標識序列： 0x68 0x32 ("h2")
定義：本文檔
"h2c"字符串標識使用明文TCP時的HTTP/2: TCP上的HTTP/2
協議： TCP上的HTTP/2
標識序列： 0x68 0x32 0x63 ("h2c")
定義：本文檔

11.2 Frame Type Registry

This document establishes a registry for HTTP/2 frame types codes. The "HTTP/2 Frame Type" registry manages an 8-bit space. The "HTTP/2 Frame Type" registry operates under either of the "IETF Review" or "IESG Approval" policies [RFC5226] for values between 0x00 and 0xef, with values between 0xf0 and 0xff being reserved for experimental use.

本文檔建立了HTTP/2幀類型碼的註冊表。"HTTP/2 Frame Type"註冊表管理一個8位的空間。" “HTTP/2 Frame Type” 註冊表在 “IETF Review” 或者 “IESG Approval”政策下操作0x00 到 0xef的值， 0xf0 到 0xff的值是預留給實驗用的。

New entries in this registry require the following information:

Frame Type:A name or label for the frame type.Code:The 8-bit code assigned to the frame type.Specification:A reference to a specification that includes a description of the frame layout, it's semantics and flags that the frame type uses, including any parts of the frame that are conditionally present based on the value of flags.

這個註冊表中新的元素需要提供以下信息：

幀類型：幀類型的名稱或者標記
編碼：幀類型的8位編碼標識
定義：包含幀使用的佈局、語義及標記的描述的定義參考規範，包含任何幀根據標記值有條件呈現的部分。

The entries in the following table are registered by this document.

下面表格中的實體已經被本文檔所註冊。

Frame Type	Code	Section
DATA	0x0	Section 6.1
HEADERS	0x1	Section 6.2
PRIORITY	0x2	Section 6.3
RST_STREAM	0x3	Section 6.4
SETTINGS	0x4	Section 6.5
PUSH_PROMISE	0x5	Section 6.6
PING	0x6	Section 6.7
GOAWAY	0x7	Section 6.8
WINDOW_UPDATE	0x8	Section 6.9
CONTINUATION	0x9	Section 6.10

11.3 Settings Registry

This document establishes a registry for HTTP/2 settings. The "HTTP/2 Settings" registry manages a 16-bit space. The "HTTP/2 Settings" registry operates under the "Expert Review" policy [RFC5226] for values in the range from 0x0000 to 0xefff, with values between and 0xf000 and 0xffff being reserved for experimental use.

本文檔爲HTTP/2設置建立了註冊表。"HTTP/2 Settings"註冊表管理了一個16位的空間。"HTTP/2 Settings"註冊表在"Expert Review"政策(RFC5226)下操作0x0000 到 0xefff的值，0xf000 到 0xffff的值是爲實驗保留的。

New registrations are advised to provide the following information:

Name:A symbolic name for the setting. Specifying a setting name is optional.
Code:The 16-bit code assigned to the setting.
Initial Value:An initial value for the setting.
Specification:A stable reference to a specification that describes the use of the setting.

新的註冊建議提供以下信息：

名稱 : 設置的語義名稱。定義一個設置的名稱是可選的
代碼: 16位的指定設置的編碼
初始值: 設置的初始值
說明：描述設置使用的穩定的參考定義

An initial set of setting registrations can be found in Section 6.5.2.

設置註冊表的初始集合可以在6.5.2章節中找到。

Name	Code	Initial Value	Specification
HEADER_TABLE_SIZE	0x1	4096	Section 6.5.2
ENABLE_PUSH	0x2	1	Section 6.5.2
MAX_CONCURRENT_STREAMS	0x3	(infinite)	Section 6.5.2
INITIAL_WINDOW_SIZE	0x4	65535	Section 6.5.2

11.4 錯誤碼註冊

This document establishes a registry for HTTP/2 error codes. The "HTTP/2 Error Code" registry manages a 32-bit space. The "HTTP/2 Error Code" registry operates under the "Expert Review" policy [RFC5226].

本文檔建立了HTTP/2錯誤碼註冊表。“HTTP/2錯誤碼”註冊表管理一個32位空間。“HTTP/2錯誤碼”註冊表在“專家評審”政策下操作[RFC5226]。

Registrations for error codes are required to include a description of the error code. An expert reviewer is advised to examine new registrations for possible duplication with existing error codes. Use of existing registrations is to be encouraged, but not mandated.

錯誤碼的註冊表需要包括錯誤碼的描述信息。專家評審應該檢查新的錯誤代碼以防止可能重複現有錯誤代碼。建議使用已有的註冊表，但是不是必須的。

New registrations are advised to provide the following information:

Name:A name for the error code. Specifying an error code name is optional.
Code:The 32-bit error code value.
Description:A brief description of the error code semantics, longer if no detailed specification is provided.
Specification:An optional reference for a specification that defines the error code.

The entries in the following table are registered by this document.

新的註冊應提供以下信息：

錯誤碼：32位錯誤碼值
名稱：錯誤碼名稱。指定一個錯誤碼的名稱是可選的。
描述：描述錯誤碼的適用條件
規範：定義錯誤碼規範的可選參考。

錯誤碼等級初始表可以在第7章節中找到。

Name	Code	Description	Specification
NO_ERROR	0x0	Graceful shutdown	Section 7
PROTOCOL_ERROR	0x1	Protocol error detected	Section 7
INTERNAL_ERROR	0x2	Implementation fault	Section 7
FLOW_CONTROL_ERROR	0x3	Flow control limits exceeded	Section 7
SETTINGS_TIMEOUT	0x4	Settings not acknowledged	Section 7
STREAM_CLOSED	0x5	Frame received for closed stream	Section 7
FRAME_SIZE_ERROR	0x6	Frame size incorrect	Section 7
REFUSED_STREAM	0x7	Stream not processed	Section 7
CANCEL	0x8	Stream cancelled	Section 7
COMPRESSION_ERROR	0x9	Compression state not updated	Section 7
CONNECT_ERROR	0xa	TCP connection error for CONNECT method	Section 7
ENHANCE_YOUR_CALM	0xb	Processing capacity exceeded	Section 7
INADEQUATE_SECURITY	0xc	Negotiated TLS parameters not acceptable	Section 7

11.5 HTTP2設置報頭字段註冊

This section registers the HTTP2-Settings header field in the Permanent Message Header Field Registry [BCP90].

Header field name:HTTP2-Settings
Applicable protocol:http
Status:standard
Author/Change controller:IETF
Specification document(s):Section 3.2.1 of this document
Related information:This header field is only used by an HTTP/2 client for Upgrade-based negotiation.

本章節註冊永久消息報頭字段註冊[BCP90]中的HTTP2設置報頭字段。

報頭字段名稱： HTTP2-Settings
應用層協議： http
狀態：標準
作者/修改操作者：LETF
文檔定義: 本文檔3.2.1章節
相關信息：本字段只在HTTP/2客戶端基於升級的協商中使用。

11.6 PRI方法註冊

This section registers the PRI method in the HTTP Method Registry ([RFC7231], Section 8.1).

Method Name:PRISafe : NoIdempotent : NoSpecification document(s) : Section 3.5 of this documentRelated information: This method is never used by an actual client. This method will appear to be used when an HTTP/1.1 server or intermediary attempts to parse an HTTP/2 connection preface.

本章節註冊HTTP方法註冊[HTTP-p2]中的PRI方法。

方法名稱： PRI
- 安全：否
冪等元：否
文檔定義: 本文檔3.5章節
相關信息：這個方法從不會被確切的客戶端使用。該方法只在HTTP/1.1服務端或者中介端試圖解析HTTP/2連接序言中使用。

11.7 The 421 Not Authoritative HTTP Status Code

This document registers the 421 (Not Authoritative) HTTP Status code in the Hypertext Transfer Protocol (HTTP) Status Code Registry ([RFC7231], Section 8.2).

Status Code:421
Short Description: Not Authoritative
Specification: Section 9.1.2 of this document

本文檔在Hypertext Transfer Protocol (HTTP)狀態碼註冊表(RFC7231,見章節8.2)中註冊了HTTP 421(未驗證)狀態碼。

狀態碼:421
簡短描述: 未驗證權限
定義: 本文檔章節9.1.2

12. Acknowledgements

This document includes substantial input from the following individuals:

本文檔包含以下個人的大量貢獻：

Adam Langley, Wan-Teh Chang, Jim Morrison, Mark Nottingham, Alyssa Wilk, Costin Manolache, William Chan, Vitaliy Lvin, Joe Chan, Adam Barth, Ryan Hamilton, Gavin Peters, Kent Alstad, Kevin Lindsay, Paul Amer, Fan Yang, Jonathan Leighton (SPDY contributors).
Gabriel Montenegro and Willy Tarreau (Upgrade mechanism).
William Chan, Salvatore Loreto, Osama Mazahir, Gabriel Montenegro, Jitu Padhye, Roberto Peon, Rob Trace (Flow control).
Mike Bishop (Extensibility).
Mark Nottingham, Julian Reschke, James Snell, Jeff Pinner, Mike Bishop, Herve Ruellan (Substantial editorial contributions).
Alexey Melnikov was an editor of this document during 2013.
A substantial proportion of Martin's contribution was supported by Microsoft during his employment there.

原文： https://github.com/fex-team/http2-spec/blob/master/HTTP2%E4%B8%AD%E8%8B%B1%E5%AF%B9%E7%85%A7%E7%89%88%2806-29%29.md

http2協議翻譯

超文本傳輸協議版本 2

IETF HTTP2草案(draft-ietf-httpbis-http2-13)

摘要

1 介紹

2 HTTP / 2協議概述

文檔結構

約定和術語

3 啓動HTTP/2

3.1 HTTP/2版本定義

3.2 Starting HTTP/2 for "http" URIs 針對"http"啓動HTTP/2

3.2.1 HTTP2-Settings Header Field HTTP2-Setting報頭字段

3.3 Starting HTTP/2 for "https" URIs 針對“https”啓動HTTP/2

3.4 Starting HTTP/2 with Prior Knowledge 先驗下啓動HTTP/2

3.5 HTTP/2 Connection Preface HTTP/2連接序言

4 HTTP Frames HTTP幀

4.1 Frame Format 幀格式

4.2 Frame Size 幀大小

4.3 Header Compression and Decompression 報頭壓縮和解壓縮

5 Streams and Multiplexing 流和多路複用

5.1 Stream States 流狀態

5.1.1 Stream Identifiers 流標識

5.1.2 Stream Concurrency 流併發

5.2 Flow Control 流量控制

5.2.1 Flow Control Principles 流量控制規則

5.2.2 Appropriate Use of Flow Control 正確使用流量控制

5.3 Stream priority 流優先級

5.3.1 Stream Dependencies 流依賴

5.3.2 Dependency Weighting 依賴權重

5.3.3 Reprioritization 優先級重組

5.3.4 Prioritization State Management 優先級狀態管理

5.4 Error Handling 錯誤處理

5.4.1 Connection Error Handling 連接錯誤處理

5.4.2 Stream Error Handling 流錯誤處理

5.4.3 Connection Termination 連接終止

5.5 Extending HTTP/2 HTTP/2擴展

6 Frame Definitions 幀定義

6.1 DATA 數據幀

6.2 HEADERS 報頭

6.3 PRIORITY 優先級幀

6.4 RST_STREAM RST_STREAM幀

6.5 SETTINGS 設置幀

6.5.1 SettingFormat 設置幀格式

6.5.2 Defined SETTINGS Parameters 設置幀參數

6.5.3 Settings Synchronization 設置同步

6.6 PUSH_PROMISE 推送承諾幀

6.7 PING PING幀

6.8 GOAWAY 超時幀

6.9 WINDOW_UPDATE 窗口更新幀

6.9.1 The Flow Control Window 流量控制窗口

6.9.2 Initial Flow Control Window Size 流量控制窗口初始值

6.9.3 Reducing the Stream Window Size 減少流量窗口大小

6.10 CONTINUATION 延續幀

7 Error Codes 錯誤碼

8 HTTP Message Exchanges HTTP消息交換

8.1 HTTP Request/Response Exchange HTTP 請求/響應交換

8.1.1 Informational Responses 響應信息

8.1.2 HTTP Header Fields HTTP報頭字段

8.1.2.1 Request Header Fields 請求報頭字段

8.1.2.2 Response Header Fields 響應報頭字段

8.1.2.3 Header Field Ordering 報頭字段順序

8.1.2.4 Compressing the Cookie Header Field 報頭Cookie字段壓縮

8.1.2.5 Malformed Messages 不規範的消息

8.1.3 Examples 示例

8.1.4 Request Reliability Mechanisms in HTTP/2 HTTP/2響應可靠性機制

8.2 Server Push 服務端推送

8.2.1 Push Requests 推送請求

8.2.2 Push Responses 推送響應

8.3 The CONNECT Method CONNECT方法

9 Additional HTTP Requirements/Considerations 額外HTTP要求/考慮

9.1 Connection Management 連接管理

9.1.1 Connection Reuse 複用連接

9.1.2 The 421 (Not Authoritative) Status Code 421(未驗證)狀態碼

9.2 Use of TLS Features 使用TLS功能

9.2.1 TLS Features TLS功能

9.2.2 TLS Cipher Suites TLS加密套件

9.3 GZip Content-Encoding 內部編碼Gzip壓縮

10 Security Considerations 安全性考慮

10.1 Server Authority 服務端認證

10.2 Cross-Protocol Attacks 跨協議攻擊