nat是网络地址转换的意思。 这部分的源码比较独立而且单一，这里就暂时不分析了。 大家了解基本的功能就行了。
nat下面有upnp和pmp两种网络协议。
### upnp的应用场景(pmp是和upnp类似的协议)
如果用户是通过NAT接入Internet的，同时需要使用BC、电骡eMule等P2P这样的软件，这时UPnP功能就会带来很大的便利。利用UPnP能自动的把BC、电骡eMule等侦听的端口号映射到公网上，以便公网上的用户也能对NAT私网侧发起连接。
主要功能就是提供接口可以把内网的IP+端口 映射为 路由器的IP+端口。 这样就等于内网的程序有了外网的IP地址， 这样公网的用户就可以直接对你进行访问了。 不然就需要通过UDP打洞这种方式来进行访问。
### p2p中的UDP协议
现在大部分用户运行的环境都是内网环境。 内网环境下监听的端口，其他公网的程序是无法直接访问的。需要经过一个打洞的过程。 双方才能联通。这就是所谓的UDP打洞。
在p2p代码里面。 peer代表了一条创建好的网络链路。在一条链路上可能运行着多个协议。比如以太坊的协议(eth)。 Swarm的协议。 或者是Whisper的协议。
peer的结构
type protoRW struct {
Protocol
in chan Msg // receices read messages
closed <-chan struct{} // receives when peer is shutting down
wstart <-chan struct{} // receives when write may start
werr chan<- error // for write results
offset uint64
w MsgWriter
}
// Protocol represents a P2P subprotocol implementation.
type Protocol struct {
// Name should contain the official protocol name,
// often a three-letter word.
Name string
// Version should contain the version number of the protocol.
Version uint
// Length should contain the number of message codes used
// by the protocol.
Length uint64
// Run is called in a new groutine when the protocol has been
// negotiated with a peer. It should read and write messages from
// rw. The Payload for each message must be fully consumed.
//
// The peer connection is closed when Start returns. It should return
// any protocol-level error (such as an I/O error) that is
// encountered.
Run func(peer *Peer, rw MsgReadWriter) error
// NodeInfo is an optional helper method to retrieve protocol specific metadata
// about the host node.
NodeInfo func() interface{}
// PeerInfo is an optional helper method to retrieve protocol specific metadata
// about a certain peer in the network. If an info retrieval function is set,
// but returns nil, it is assumed that the protocol handshake is still running.
PeerInfo func(id discover.NodeID) interface{}
}
// Peer represents a connected remote node.
type Peer struct {
rw *conn
running map[string]*protoRW //运行的协议
log log.Logger
created mclock.AbsTime
wg sync.WaitGroup
protoErr chan error
closed chan struct{}
disc chan DiscReason
// events receives message send / receive events if set
events *event.Feed
}
peer的创建，根据匹配找到当前Peer支持的protomap
func newPeer(conn *conn, protocols []Protocol) *Peer {
protomap := matchProtocols(protocols, conn.caps, conn)
p := &Peer{
rw: conn,
running: protomap,
created: mclock.Now(),
disc: make(chan DiscReason),
protoErr: make(chan error, len(protomap)+1), // protocols + pingLoop
closed: make(chan struct{}),
log: log.New("id", conn.id, "conn", conn.flags),
}
return p
}
peer的启动， 启动了两个goroutine线程。 一个是读取。一个是执行ping操作。
func (p *Peer) run() (remoteRequested bool, err error) {
var (
writeStart = make(chan struct{}, 1) //用来控制什么时候可以写入的管道。
writeErr = make(chan error, 1)
readErr = make(chan error, 1)
reason DiscReason // sent to the peer
)
p.wg.Add(2)
go p.readLoop(readErr)
go p.pingLoop()
// Start all protocol handlers.
writeStart <- struct{}{}
//启动所有的协议。
p.startProtocols(writeStart, writeErr)
// Wait for an error or disconnect.
loop:
for {
select {
case err = <-writeErr:
// A write finished. Allow the next write to start if
// there was no error.
if err != nil {
reason = DiscNetworkError
break loop
}
writeStart <- struct{}{}
case err = <-readErr:
if r, ok := err.(DiscReason); ok {
remoteRequested = true
reason = r
} else {
reason = DiscNetworkError
}
break loop
case err = <-p.protoErr:
reason = discReasonForError(err)
break loop
case err = <-p.disc:
break loop
}
}
close(p.closed)
p.rw.close(reason)
p.wg.Wait()
return remoteRequested, err
}
startProtocols方法，这个方法遍历所有的协议。
func (p *Peer) startProtocols(writeStart <-chan struct{}, writeErr chan<- error) {
p.wg.Add(len(p.running))
for _, proto := range p.running {
proto := proto
proto.closed = p.closed
proto.wstart = writeStart
proto.werr = writeErr
var rw MsgReadWriter = proto
if p.events != nil {
rw = newMsgEventer(rw, p.events, p.ID(), proto.Name)
}
p.log.Trace(fmt.Sprintf("Starting protocol %s/%d", proto.Name, proto.Version))
// 等于这里为每一个协议都开启了一个goroutine。 调用其Run方法。
go func() {
// proto.Run(p, rw)这个方法应该是一个死循环。 如果返回就说明遇到了错误。
err := proto.Run(p, rw)
if err == nil {
p.log.Trace(fmt.Sprintf("Protocol %s/%d returned", proto.Name, proto.Version))
err = errProtocolReturned
} else if err != io.EOF {
p.log.Trace(fmt.Sprintf("Protocol %s/%d failed", proto.Name, proto.Version), "err", err)
}
p.protoErr <- err
p.wg.Done()
}()
}
}
回过头来再看看readLoop方法。 这个方法也是一个死循环。 调用p.rw来读取一个Msg(这个rw实际是之前提到的frameRLPx的对象，也就是分帧之后的对象。然后根据Msg的类型进行对应的处理，如果Msg的类型是内部运行的协议的类型。那么发送到对应协议的proto.in队列上面。
func (p *Peer) readLoop(errc chan<- error) {
defer p.wg.Done()
for {
msg, err := p.rw.ReadMsg()
if err != nil {
errc <- err
return
}
msg.ReceivedAt = time.Now()
if err = p.handle(msg); err != nil {
errc <- err
return
}
}
}
func (p *Peer) handle(msg Msg) error {
switch {
case msg.Code == pingMsg:
msg.Discard()
go SendItems(p.rw, pongMsg)
case msg.Code == discMsg:
var reason [1]DiscReason
// This is the last message. We don't need to discard or
// check errors because, the connection will be closed after it.
rlp.Decode(msg.Payload, &reason)
return reason[0]
case msg.Code < baseProtocolLength:
// ignore other base protocol messages
return msg.Discard()
default:
// it's a subprotocol message
proto, err := p.getProto(msg.Code)
if err != nil {
return fmt.Errorf("msg code out of range: %v", msg.Code)
}
select {
case proto.in <- msg:
return nil
case <-p.closed:
return io.EOF
}
}
return nil
}
在看看pingLoop。这个方法很简单。就是定时的发送pingMsg消息到对端。
func (p *Peer) pingLoop() {
ping := time.NewTimer(pingInterval)
defer p.wg.Done()
defer ping.Stop()
for {
select {
case <-ping.C:
if err := SendItems(p.rw, pingMsg); err != nil {
p.protoErr <- err
return
}
ping.Reset(pingInterval)
case <-p.closed:
return
}
}
}
最后再看看protoRW的read和write方法。 可以看到读取和写入都是阻塞式的。
func (rw *protoRW) WriteMsg(msg Msg) (err error) {
if msg.Code >= rw.Length {
return newPeerError(errInvalidMsgCode, "not handled")
}
msg.Code += rw.offset
select {
case <-rw.wstart: //等到可以写入的受在执行写入。 这难道是为了多线程控制么。
err = rw.w.WriteMsg(msg)
// Report write status back to Peer.run. It will initiate
// shutdown if the error is non-nil and unblock the next write
// otherwise. The calling protocol code should exit for errors
// as well but we don't want to rely on that.
rw.werr <- err
case <-rw.closed:
err = fmt.Errorf("shutting down")
}
return err
}
func (rw *protoRW) ReadMsg() (Msg, error) {
select {
case msg := <-rw.in:
msg.Code -= rw.offset
return msg, nil
case <-rw.closed:
return Msg{}, io.EOF
}
}