Golang源码深入-Go1.15.6发起http请求流程-3（http2）-go深入-利志分享

Golang源码深入-Go1.15.6发起http请求流程-3（http2）

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这一篇文章是分享http2的文章，前两篇主要是http1的源码实现，前两篇有兴趣的可以再阅读一下。详情可点击：[Golang源码深入-Go1.15.6发起http请求流程-1](https://zengzhihai.com/study/wiki/type/Z29fc3R1ZHlfaW5mbw==/id/624b0ab3469aea5bbc9a7437 "Golang源码深入-Go1.15.6发起http请求流程-1")和[Golang源码深入-Go1.15.6发起http请求流程-2](https://zengzhihai.com/study/wiki/type/Z29fc3R1ZHlfaW5mbw==/id/6250a2f7469aea1db651b5ea "Golang源码深入-Go1.15.6发起http请求流程-2")。

目前http2协议在很多的web网站中有应用，http2通过多路复用，二进制流，Header压缩等等技术，极大地提高了性能。

http2的源码核心流程图解如下：
https://www.processon.com/diagraming/6250fe4963768946f277cde9

下面我们来分别看下不同的模块的源码：
1：RoundTrip函数
```
// http2的入口函数
func (t *Transport) RoundTrip(req *http.Request) (*http.Response, error) {
	return t.RoundTripOpt(req, RoundTripOpt{})
}
```
2：RoundTripOpt函数，实现处理请求
```
// 实现函数：RoundTripOpt
func (t *Transport) RoundTripOpt(req *http.Request, opt RoundTripOpt) (*http.Response, error) {
    // 判断是否是https，或者客户端手动设置是允许HTTP
	if !(req.URL.Scheme == "https" || (req.URL.Scheme == "http" && t.AllowHTTP)) {
		return nil, errors.New("http2: unsupported scheme")
	}
    // 鉴权scheme和host，返回ip:port的结构
	addr := authorityAddr(req.URL.Scheme, req.URL.Host)
	// http2协议发送请求有重试机制，这个版本默认是重试6次。只有在获取不到连接或者重试次数6次的时候进行重试。
	for retry := 0; ; retry++ {
	    // 这个是获取连接的方法，从连接池拿或者新建连接。
		cc, err := t.connPool().GetClientConn(req, addr)
		if err != nil {
			t.vlogf("http2: Transport failed to get client conn for %s: %v", addr, err)
			return nil, err
		}
		reused := !atomic.CompareAndSwapUint32(&cc.reused, 0, 1)
		// 这个是打印traceLog日志的
		traceGotConn(req, cc, reused)
		// 这个方法是最后处理请求或者响应的方法
		res, gotErrAfterReqBodyWrite, err := cc.roundTrip(req)
		if err != nil && retry <= 6 {
		    // 判断哪些err能重试，并且
			if req, err = shouldRetryRequest(req, err, gotErrAfterReqBodyWrite); err == nil {
				// 第一次直接重试，不进行暂停逻辑。
				if retry == 0 {
					continue
				}
				// 生成一个随机值来，判断过多久之后进行重试。
				backoff := float64(uint(1) << (uint(retry) - 1))
				backoff += backoff * (0.1 * mathrand.Float64())
				// 通过select 来监听重试时间和当前请求执行超时，重试是继续，请求超时则直接返回。
				select {
				case <-time.After(time.Second * time.Duration(backoff)):
					continue
				case <-req.Context().Done():
					return nil, req.Context().Err()
				}
			}
		}
		// 这里是重试次数超过了还是报错，则返回错误。
		if err != nil {
			t.vlogf("RoundTrip failure: %v", err)
			return nil, err
		}
		// 这里是正常请求返回
		return res, nil
	}
}
```
3：函数：GetClientConn，获取处理请求连接。
```
func (p *clientConnPool) GetClientConn(req *http.Request, addr string) (*ClientConn, error) {
	return p.getClientConn(req, addr, dialOnMiss)
}
```
4：函数：getClientConn，实现获取处理请求连接。
```
func (p *clientConnPool) getClientConn(req *http.Request, addr string, dialOnMiss bool) (*ClientConn, error) {
    // 先判断请求是否关闭
	if isConnectionCloseRequest(req) && dialOnMiss {
		// It gets its own connection.
		traceGetConn(req, addr)
		const singleUse = true
		cc, err := p.t.dialClientConn(addr, singleUse)
		if err != nil {
			return nil, err
		}
		return cc, nil
	}
	// 获取请求加锁，全局锁
	p.mu.Lock()
	// 循环连接池，key是host+port生成的
	for _, cc := range p.conns[addr] {
	    // 判断当前的连接状态，是否能获取到新请求
		if st := cc.idleState(); st.canTakeNewRequest {
			if p.shouldTraceGetConn(st) {
				traceGetConn(req, addr)
			}
			// 如果能获取到连接，则解锁，返回连接
			p.mu.Unlock()
			return cc, nil
		}
	}
	// 这个判断，反正我是没看懂为啥，感觉是多余
	if !dialOnMiss {
		p.mu.Unlock()
		return nil, ErrNoCachedConn
	}
	traceGetConn(req, addr)
	// 这里是第一次获取连接。
	call := p.getStartDialLocked(addr)
	// 请求解锁
	p.mu.Unlock()
	<-call.done
	return call.res, call.err
}
```
5：getStartDialLocked 获取开始的监听之后生成的第一个链接。
```
// requires p.mu is held.
func (p *clientConnPool) getStartDialLocked(addr string) *dialCall {
    // 判断正在连接，如果存在则返回这个连接dialCall
	if call, ok := p.dialing[addr]; ok {
		// A dial is already in-flight. Don't start another.
		return call
	}
	// 生成新的连接对象，进行返回
	call := &dialCall{p: p, done: make(chan struct{})}
	if p.dialing == nil {
		p.dialing = make(map[string]*dialCall)
	}
	p.dialing[addr] = call
	// 这里是开启协程去建立新连接，给call对象赋值
	go call.dial(addr)
	// 直接返回本身的连接对象
	return call
}
```
6：dial 建立新连接
```
// run in its own goroutine.
func (c *dialCall) dial(addr string) {
	const singleUse = false // shared conn
	// 建立客户端的tcp连接
	c.res, c.err = c.p.t.dialClientConn(addr, singleUse)
	close(c.done)

// 加锁
	c.p.mu.Lock()
	// 这个目前没看懂为什么要删除，目的何在？
	delete(c.p.dialing, addr)
	if c.err == nil {
	    // 初始化p.conns和p.keys 并把当前对象连接存进去
		c.p.addConnLocked(addr, c.res)
	}
	c.p.mu.Unlock()
}
```
7：dialClientConn 建立tcp链接
```
func (t *Transport) dialClientConn(addr string, singleUse bool) (*ClientConn, error) {
    // 获取host
	host, _, err := net.SplitHostPort(addr)
	if err != nil {
		return nil, err
	}
	// 建立tls安全协议的tcp连接。
	tconn, err := t.dialTLS()("tcp", addr, t.newTLSConfig(host))
	if err != nil {
		return nil, err
	}
	// 返回建立的新连接
	return t.newClientConn(tconn, singleUse)
}
```
8：newClientConn 建立基于tls的安全tcp协议的http 2的通信
```
func (t *Transport) newClientConn(c net.Conn, singleUse bool) (*ClientConn, error) {
	cc := &ClientConn{
		t:                     t,
		tconn:                 c,
		readerDone:            make(chan struct{}),
		nextStreamID:          1,
		maxFrameSize:          16 << 10,           // spec default
		initialWindowSize:     65535,              // spec default
		maxConcurrentStreams:  1000,               // "infinite", per spec. 1000 seems good enough.
		peerMaxHeaderListSize: 0xffffffffffffffff, // "infinite", per spec. Use 2^64-1 instead.
		streams:               make(map[uint32]*clientStream),
		singleUse:             singleUse,
		wantSettingsAck:       true,
		pings:                 make(map[[8]byte]chan struct{}),
	}
	// 把设置的空闲时间赋值给每个新建的连接，并设置空闲定时器
	if d := t.idleConnTimeout(); d != 0 {
		cc.idleTimeout = d
		cc.idleTimer = time.AfterFunc(d, cc.onIdleTimeout)
	}
	// 打印日志
	if VerboseLogs {
		t.vlogf("http2: Transport creating client conn %p to %v", cc, c.RemoteAddr())
	}

// 初始化ClientConn相关的结构
	cc.cond = sync.NewCond(&cc.mu)
	cc.flow.add(int32(initialWindowSize))

// TODO: adjust this writer size to account for frame size +
	// MTU + crypto/tls record padding.
	cc.bw = bufio.NewWriter(stickyErrWriter{c, &cc.werr})
	cc.br = bufio.NewReader(c)
	cc.fr = NewFramer(cc.bw, cc.br)
	cc.fr.ReadMetaHeaders = hpack.NewDecoder(initialHeaderTableSize, nil)
	cc.fr.MaxHeaderListSize = t.maxHeaderListSize()

// TODO: SetMaxDynamicTableSize, SetMaxDynamicTableSizeLimit on
	// henc in response to SETTINGS frames?
	cc.henc = hpack.NewEncoder(&cc.hbuf)

if t.AllowHTTP {
		cc.nextStreamID = 3
	}

if cs, ok := c.(connectionStater); ok {
		state := cs.ConnectionState()
		cc.tlsState = &state
	}

initialSettings := []Setting{
		{ID: SettingEnablePush, Val: 0},
		{ID: SettingInitialWindowSize, Val: transportDefaultStreamFlow},
	}
	if max := t.maxHeaderListSize(); max != 0 {
		initialSettings = append(initialSettings, Setting{ID: SettingMaxHeaderListSize, Val: max})
	}

cc.bw.Write(clientPreface)
	cc.fr.WriteSettings(initialSettings...)
	cc.fr.WriteWindowUpdate(0, transportDefaultConnFlow)
	cc.inflow.add(transportDefaultConnFlow + initialWindowSize)
	cc.bw.Flush()
	if cc.werr != nil {
		cc.Close()
		return nil, cc.werr
	}
    
    // 起一个处理response的协程处理。
	go cc.readLoop()
	return cc, nil
}
```
9：readLoop
```
// readLoop运行在一个一个协程中，处理读数据
func (cc *ClientConn) readLoop() {
    // 初始化读的对象
	rl := &clientConnReadLoop{cc: cc}
	defer rl.cleanup()
	// 读对象执行run方法
	cc.readerErr = rl.run()
	if ce, ok := cc.readerErr.(ConnectionError); ok {
		cc.wmu.Lock()
		cc.fr.WriteGoAway(0, ErrCode(ce), nil)
		cc.wmu.Unlock()
	}
}
```
10：clientConnReadLoop.run
```
func (rl *clientConnReadLoop) run() error {
	cc := rl.cc
	// 如果禁用了长连接，或者开启单个http请求，则设置空闲开关关闭
	rl.closeWhenIdle = cc.t.disableKeepAlives() || cc.singleUse
	gotReply := false // ever saw a HEADERS reply
	gotSettings := false
	readIdleTimeout := cc.t.ReadIdleTimeout
	var t *time.Timer
	if readIdleTimeout != 0 {
		t = time.AfterFunc(readIdleTimeout, cc.healthCheck)
		defer t.Stop()
	}
	// 死循环读取帧数据，为了尽快读完http2协议和返回的数据
	for {
	    // 这里是读帧数据
		f, err := cc.fr.ReadFrame()
		// 如果设置了读数据的超时时间，则要等读完数据之后再进行心跳检测包。所以需要重置定时器
		if t != nil {
			t.Reset(readIdleTimeout)
		}
		if err != nil {
			cc.vlogf("http2: Transport readFrame error on conn %p: (%T) %v", cc, err, err)
		}
		// 如果是流数据读取错误，则会重置流数据进行读取
		if se, ok := err.(StreamError); ok {
			if cs := cc.streamByID(se.StreamID, false); cs != nil {
				cs.cc.writeStreamReset(cs.ID, se.Code, err)
				cs.cc.forgetStreamID(cs.ID)
				if se.Cause == nil {
					se.Cause = cc.fr.errDetail
				}
				rl.endStreamError(cs, se)
			}
			continue
		// 如果是其他错误，则直接返回错误
		} else if err != nil {
			return err
		}
		if VerboseLogs {
			cc.vlogf("http2: Transport received %s", summarizeFrame(f))
		}
		if !gotSettings {
		    // 协议参数配置错误，则会报PROTOCOL_ERROR
			if _, ok := f.(*SettingsFrame); !ok {
				cc.logf("protocol error: received %T before a SETTINGS frame", f)
				return ConnectionError(ErrCodeProtocol)
			}
			gotSettings = true
		}
		maybeIdle := false // whether frame might transition us to idle

switch f := f.(type) {
		// 处理http2头部数据流
		case *MetaHeadersFrame:
			err = rl.processHeaders(f)
			maybeIdle = true
			gotReply = true
		// 处理http数据流
		case *DataFrame:
			err = rl.processData(f)
			maybeIdle = true
		// 告诉远端停止在当前连接上创建流
		case *GoAwayFrame:
			err = rl.processGoAway(f)
			maybeIdle = true
		// 协议数据被重置RST的流
		case *RSTStreamFrame:
			err = rl.processResetStream(f)
			maybeIdle = true
		// 检验点对点的设置参数配置数据流
		case *SettingsFrame:
			err = rl.processSettings(f)
		// 用于初始化服务器流
		case *PushPromiseFrame:
			err = rl.processPushPromise(f)
		// 用于实现流控制
		case *WindowUpdateFrame:
			err = rl.processWindowUpdate(f)
		// 心跳检测流
		case *PingFrame:
			err = rl.processPing(f)
		default:
			cc.logf("Transport: unhandled response frame type %T", f)
		}
		if err != nil {
			if VerboseLogs {
				cc.vlogf("http2: Transport conn %p received error from processing frame %v: %v", cc, summarizeFrame(f), err)
			}
			return err
		}
		// 如果获取完数据，并且这个连接空闲，并且空闲开关是关闭，则会直接关闭当前连接。
		if rl.closeWhenIdle && gotReply && maybeIdle {
			cc.closeIfIdle()
		}
	}
}
```
11：processData：处理http2的数据流函数
```
func (rl *clientConnReadLoop) processData(f *DataFrame) error {
	cc := rl.cc
	cs := cc.streamByID(f.StreamID, f.StreamEnded())
	data := f.Data()
	// 如果当前连接不存在
	if cs == nil {
		cc.mu.Lock()
		neverSent := cc.nextStreamID
		cc.mu.Unlock()
		// 如果当前的流数据Id大于等于下一个，则表示结束或者协议有异常
		if f.StreamID >= neverSent {
			// We never asked for this.
			cc.logf("http2: Transport received unsolicited DATA frame; closing connection")
			return ConnectionError(ErrCodeProtocol)
		}
		// We probably did ask for this, but canceled. Just ignore it.
		// TODO: be stricter here? only silently ignore things which
		// we canceled, but not things which were closed normally
		// by the peer? Tough without accumulating too much state.

// But at least return their flow control:
		if f.Length > 0 {
			cc.mu.Lock()
			cc.inflow.add(int32(f.Length))
			cc.mu.Unlock()

cc.wmu.Lock()
			cc.fr.WriteWindowUpdate(0, uint32(f.Length))
			cc.bw.Flush()
			cc.wmu.Unlock()
		}
		return nil
	}
	if !cs.firstByte {
		cc.logf("protocol error: received DATA before a HEADERS frame")
		rl.endStreamError(cs, StreamError{
			StreamID: f.StreamID,
			Code:     ErrCodeProtocol,
		})
		return nil
	}
	if f.Length > 0 {
	    // 判断不是头协议
		if cs.req.Method == "HEAD" && len(data) > 0 {
			cc.logf("protocol error: received DATA on a HEAD request")
			rl.endStreamError(cs, StreamError{
				StreamID: f.StreamID,
				Code:     ErrCodeProtocol,
			})
			return nil
		}
		// Check connection-level flow control.
		// 从数据流读data的数据
		cc.mu.Lock()
		if cs.inflow.available() >= int32(f.Length) {
			cs.inflow.take(int32(f.Length))
		} else {
			cc.mu.Unlock()
			return ConnectionError(ErrCodeFlowControl)
		}
		// Return any padded flow control now, since we won't
		// refund it later on body reads.
		var refund int
		if pad := int(f.Length) - len(data); pad > 0 {
			refund += pad
		}
		// Return len(data) now if the stream is already closed,
		// since data will never be read.
		didReset := cs.didReset
		if didReset {
			refund += len(data)
		}
		// 这里是读取到数据之后，更新读取当前的tcp流数据
		if refund > 0 {
			cc.inflow.add(int32(refund))
			cc.wmu.Lock()
			cc.fr.WriteWindowUpdate(0, uint32(refund))
			if !didReset {
				cs.inflow.add(int32(refund))
				cc.fr.WriteWindowUpdate(cs.ID, uint32(refund))
			}
			cc.bw.Flush()
			cc.wmu.Unlock()
		}
		cc.mu.Unlock()

// 这里是读取数据，然后最后读到的数据写到当前连接的read buf中
		if len(data) > 0 && !didReset {
			if _, err := cs.bufPipe.Write(data); err != nil {
				rl.endStreamError(cs, err)
				return err
			}
		}
	}

// 判断当前读出流数据是不是结束的帧，如果是则会把最后的响应结果写到cs.resc，
	if f.StreamEnded() {
		rl.endStream(cs)
	}
	return nil
}
```
12：roundTrip函数，处理请求头，和响应的函数
```
func (cc *ClientConn) roundTrip(req *http.Request) (res *http.Response, gotErrAfterReqBodyWrite bool, err error) {
    // 检测连接协议头是否有异常
	if err := checkConnHeaders(req); err != nil {
		return nil, false, err
	}
	// 停止空闲连接的定时器。
	if cc.idleTimer != nil {
		cc.idleTimer.Stop()
	}

// 处理请求头信息
	trailers, err := commaSeparatedTrailers(req)
	if err != nil {
		return nil, false, err
	}
	hasTrailers := trailers != ""

cc.mu.Lock()
	if err := cc.awaitOpenSlotForRequest(req); err != nil {
		cc.mu.Unlock()
		return nil, false, err
	}
    // 计算请求的长度
	body := req.Body
	contentLen := actualContentLength(req)
	hasBody := contentLen != 0

// TODO(bradfitz): this is a copy of the logic in net/http. Unify somewhere?
	// 请求未压缩，则设置gzip压缩
	var requestedGzip bool
	if !cc.t.disableCompression() &&
		req.Header.Get("Accept-Encoding") == "" &&
		req.Header.Get("Range") == "" &&
		req.Method != "HEAD" {
		// Request gzip only, not deflate. Deflate is ambiguous and
		// not as universally supported anyway.
		// See: https://zlib.net/zlib_faq.html#faq39
		//
		// Note that we don't request this for HEAD requests,
		// due to a bug in nginx:
		//   http://trac.nginx.org/nginx/ticket/358
		//   https://golang.org/issue/5522
		//
		// We don't request gzip if the request is for a range, since
		// auto-decoding a portion of a gzipped document will just fail
		// anyway. See https://golang.org/issue/8923
		requestedGzip = true
	}

// we send: HEADERS{1}, CONTINUATION{0,} + DATA{0,} (DATA is
	// sent by writeRequestBody below, along with any Trailers,
	// again in form HEADERS{1}, CONTINUATION{0,})
	// 处理请求头信息encode
	hdrs, err := cc.encodeHeaders(req, requestedGzip, trailers, contentLen)
	if err != nil {
		cc.mu.Unlock()
		return nil, false, err
	}

cs := cc.newStream()
	cs.req = req
	cs.trace = httptrace.ContextClientTrace(req.Context())
	cs.requestedGzip = requestedGzip
	bodyWriter := cc.t.getBodyWriterState(cs, body)
	cs.on100 = bodyWriter.on100

defer func() {
		cc.wmu.Lock()
		werr := cc.werr
		cc.wmu.Unlock()
		if werr != nil {
			cc.Close()
		}
	}()

cc.wmu.Lock()
	endStream := !hasBody && !hasTrailers
	// 发送头部请求
	werr := cc.writeHeaders(cs.ID, endStream, int(cc.maxFrameSize), hdrs)
	cc.wmu.Unlock()
	traceWroteHeaders(cs.trace)
	cc.mu.Unlock()

if werr != nil {
		if hasBody {
			req.Body.Close() // per RoundTripper contract
			bodyWriter.cancel()
		}
		cc.forgetStreamID(cs.ID)
		// Don't bother sending a RST_STREAM (our write already failed;
		// no need to keep writing)
		traceWroteRequest(cs.trace, werr)
		return nil, false, werr
	}

var respHeaderTimer <-chan time.Time
	if hasBody {
		bodyWriter.scheduleBodyWrite()
	} else {
		traceWroteRequest(cs.trace, nil)
		if d := cc.responseHeaderTimeout(); d != 0 {
			timer := time.NewTimer(d)
			defer timer.Stop()
			respHeaderTimer = timer.C
		}
	}

readLoopResCh := cs.resc
	bodyWritten := false
	ctx := req.Context()

handleReadLoopResponse := func(re resAndError) (*http.Response, bool, error) {
		res := re.res
		if re.err != nil || res.StatusCode > 299 {
			// On error or status code 3xx, 4xx, 5xx, etc abort any
			// ongoing write, assuming that the server doesn't care
			// about our request body. If the server replied with 1xx or
			// 2xx, however, then assume the server DOES potentially
			// want our body (e.g. full-duplex streaming:
			// golang.org/issue/13444). If it turns out the server
			// doesn't, they'll RST_STREAM us soon enough. This is a
			// heuristic to avoid adding knobs to Transport. Hopefully
			// we can keep it.
			bodyWriter.cancel()
			cs.abortRequestBodyWrite(errStopReqBodyWrite)
			if hasBody && !bodyWritten {
				<-bodyWriter.resc
			}
		}
		if re.err != nil {
			cc.forgetStreamID(cs.ID)
			return nil, cs.getStartedWrite(), re.err
		}
		res.Request = req
		res.TLS = cc.tlsState
		return res, false, nil
	}

// 通过for select 来监听读请求详情结果，之前起的读协程读取response的响应结果会写回readLoopResCh := cs.resc
	for {
		select {
		// 处理响应结果
		case re := <-readLoopResCh:
			return handleReadLoopResponse(re)
		// 响应头超时
		case <-respHeaderTimer:
			if !hasBody || bodyWritten {
				cc.writeStreamReset(cs.ID, ErrCodeCancel, nil)
			} else {
				bodyWriter.cancel()
				cs.abortRequestBodyWrite(errStopReqBodyWriteAndCancel)
				<-bodyWriter.resc
			}
			cc.forgetStreamID(cs.ID)
			return nil, cs.getStartedWrite(), errTimeout
		// 这个是客户端请求的超时处理
		case <-ctx.Done():
			if !hasBody || bodyWritten {
				cc.writeStreamReset(cs.ID, ErrCodeCancel, nil)
			} else {
				bodyWriter.cancel()
				cs.abortRequestBodyWrite(errStopReqBodyWriteAndCancel)
				<-bodyWriter.resc
			}
			cc.forgetStreamID(cs.ID)
			return nil, cs.getStartedWrite(), ctx.Err()
		// 请求主动取消
		case <-req.Cancel:
			if !hasBody || bodyWritten {
				cc.writeStreamReset(cs.ID, ErrCodeCancel, nil)
			} else {
				bodyWriter.cancel()
				cs.abortRequestBodyWrite(errStopReqBodyWriteAndCancel)
				<-bodyWriter.resc
			}
			cc.forgetStreamID(cs.ID)
			return nil, cs.getStartedWrite(), errRequestCanceled
		// 处理报文被重置
		case <-cs.peerReset:
			// processResetStream already removed the
			// stream from the streams map; no need for
			// forgetStreamID.
			return nil, cs.getStartedWrite(), cs.resetErr
		// body写入的状态监听
		case err := <-bodyWriter.resc:
			bodyWritten = true
			// Prefer the read loop's response, if available. Issue 16102.
			select {
			case re := <-readLoopResCh:
				return handleReadLoopResponse(re)
			default:
			}
			if err != nil {
				cc.forgetStreamID(cs.ID)
				return nil, cs.getStartedWrite(), err
			}
			if d := cc.responseHeaderTimeout(); d != 0 {
				timer := time.NewTimer(d)
				defer timer.Stop()
				respHeaderTimer = timer.C
			}
		}
	}
}
```

相关学习的知识整理：
1：http2的帧，FrameHeader基础协议是9个字节，结构如下：
```
type FrameHeader struct {
    //标志位，默认是1个位
	valid bool

// 帧类型，默认是1个字节，8位
	Type FrameType

// 帧类型标记，默认1个字节 8位
	Flags Flags

// 帧的长度 默认三个字节，24位，最大表示16MB
	Length uint32

// 帧的唯一ID，标识符，0x00值被保留给与连接相关联的帧作为一个整体，而不是单独的流。是31位
	StreamID uint32
}
```
2：http2的数据协议帧结构如下：
```
type DataFrame struct {
	FrameHeader
	data []byte
}
```
3：http2的头部协议实现结构是HeadersFrame，这个主要是实现http2的协议响应。
4：关于如何通过本地实现http2的服务实现，并且通过代码debug来查看http2的请求过程，笔者也是通过先实现功能，然后通过debug，单步调试查看源码的执行流程，我这里整理一份代码，有兴趣可以参考如下链接：[golang实现http2.0服务端，客户端完整案例](https://zengzhihai.com/study/wiki/type/Z29fc3R1ZHlfaW5mbw==/id/625a23df469aea5437eb0c8f "golang实现http2.0服务端，客户端完整案例")。

总结：
1：http2其实是tcp的长连接，并在tcp协议上封装了一层http2协议。
2：http2的FrameHeader的数据结构的定义解决了tcp的粘包问题。
3：http2默认底层实现重试为6次
4：http2通过多路复用，二进制流，Header压缩等等技术，极大地提高了性能。

参考文献：
https://httpwg.org/http2-spec/draft-ietf-httpbis-http2bis.html#name-data

注意：
笔者本着严谨的态度，http2流程中的很多细节并未详细提及或讲述，请读者酌情参考。

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