对于read write的封装还是有问题

tcpip/stack/nic.go

@@ -129,7 +129,7 @@ func (n *NIC) addAddressLocked(protocol tcpip.NetworkProtocolNumber, addr tcpip.
 	// 如果不存在 就会调用arp协议发广播来定位这个ip对应的设备
 	if n.linkEP.Capabilities()&CapabilityResolutionRequired != 0 {
 		if _, ok := n.stack.linkAddrResolvers[protocol]; ok {
-			ref.linkCache = n.stack
+			ref.linkCache = n.stack // 对于loopback驱动而言 他的缓存就是nil 不开启
 		}
 	}
 

tcpip/stack/route.go

@@ -75,9 +75,7 @@ func (r *Route) Capabilities() LinkEndpointCapabilities {
 // 如果需要地址解析，则返回ErrNoLinkAddress和通知通道，以阻止顶级调用者。
 // 地址解析完成后，通道关闭（不管成功与否）。
 func (r *Route) Resolve(waker *sleep.Waker) (<-chan struct{}, *tcpip.Error) {
-	if !r.IsResolutionRequired() {
-		// Nothing to do if there is no cache (which does the resolution on cache miss) or
-		// link address is already known.
+	if !r.IsResolutionRequired() { // 没有配置地址解析
 		return nil, nil
 	}
 
@@ -109,9 +107,7 @@ func (r *Route) RemoveWaker(waker *sleep.Waker) {
 	r.ref.linkCache.RemoveWaker(r.ref.nic.ID(), nextAddr, waker)
 }
 
-// IsResolutionRequired returns true if Resolve() must be called to resolve
-// the link address before the this route can be written to.
-// 本地缓存有东西 而且不知道远端的MAC
+// 检查是否允许了地址解析 首先检查是否配置了mac缓存 然后检查目标mac是否已经存在
 func (r *Route) IsResolutionRequired() bool {
 	return r.ref.linkCache != nil && r.RemoteLinkAddress == ""
 }
@@ -119,6 +115,7 @@ func (r *Route) IsResolutionRequired() bool {
 // WritePacket writes the packet through the given route.
 func (r *Route) WritePacket(hdr buffer.Prependable, payload buffer.VectorisedView,
 	protocol tcpip.TransportProtocolNumber, ttl uint8) *tcpip.Error {
+	// 路由对应的IP的WritePacket
 	err := r.ref.ep.WritePacket(r, hdr, payload, protocol, ttl)
 	if err == tcpip.ErrNoRoute {
 		r.Stats().IP.OutgoingPacketErrors.Increment()

tcpip/stack/stack.go

@@ -580,11 +580,13 @@ func (s *Stack) RemoveAddress(id tcpip.NICID, addr tcpip.Address) *tcpip.Error {
 }
 
 // FindRoute 路由查找实现，比如当tcp建立连接时，会用该函数得到路由信息
+// 注意仅仅包含 SrcMAC SrcIp DstIp 没有 DstMAC
 func (s *Stack) FindRoute(id tcpip.NICID, localAddr, remoteAddr tcpip.Address,
 	netProto tcpip.NetworkProtocolNumber) (Route, *tcpip.Error) {
 	s.mu.RLock()
 	defer s.mu.RUnlock()
 
+	// 遍历协议栈的路由表
 	for i := range s.routeTable {
 		if (id != 0 && id != s.routeTable[i].NIC) || // 检查是否是对应的网卡
 			(len(remoteAddr) != 0 && !s.routeTable[i].Match(remoteAddr)) {
@@ -609,9 +611,10 @@ func (s *Stack) FindRoute(id tcpip.NICID, localAddr, remoteAddr tcpip.Address,
 		if len(remoteAddr) == 0 {
 			// If no remote address was provided, then the route
 			// provided will refer to the link local address.
-			remoteAddr = ref.ep.ID().LocalAddress // 发回自己? TODO
+			remoteAddr = ref.ep.ID().LocalAddress // 本地环回
 		}
 
+		// 构建一个路由 包括 目标ip 目标mac 本地ip 本地mac
 		r := makeRoute(netProto, ref.ep.ID().LocalAddress, remoteAddr, nic.linkEP.LinkAddress(), ref)
 		r.NextHop = s.routeTable[i].Gateway
 		logger.GetInstance().Info(logger.IP, func() {