Populate link address cache at dispatch

This allows the stack to learn remote link addresses on incoming
packets, reducing the need to ARP to send responses.

This also reduces the number of round trips to the system clock,
since that may also prove to be performance-sensitive.

Fixes #739.

PiperOrigin-RevId: 265815816

1 files changed, 11 insertions, 8 deletions

diff --git a/pkg/tcpip/stack/nic.go b/pkg/tcpip/stack/nic.go
index 04b63d783..89b4c5960 100644
--- a/pkg/tcpip/stack/nic.go
+++ b/pkg/tcpip/stack/nic.go
@@ -531,6 +531,13 @@ func (n *NIC) leaveGroup(addr tcpip.Address) *tcpip.Error {
 	return nil
 }
 
+func handlePacket(protocol tcpip.NetworkProtocolNumber, dst, src tcpip.Address, localLinkAddr, remotelinkAddr tcpip.LinkAddress, ref *referencedNetworkEndpoint, vv buffer.VectorisedView) {
+	r := makeRoute(protocol, dst, src, localLinkAddr, ref, false /* handleLocal */, false /* multicastLoop */)
+	r.RemoteLinkAddress = remotelinkAddr
+	ref.ep.HandlePacket(&r, vv)
+	ref.decRef()
+}
+
 // DeliverNetworkPacket finds the appropriate network protocol endpoint and
 // hands the packet over for further processing. This function is called when
 // the NIC receives a packet from the physical interface.
@@ -558,6 +565,8 @@ func (n *NIC) DeliverNetworkPacket(linkEP LinkEndpoint, remote, _ tcpip.LinkAddr
 
 	src, dst := netProto.ParseAddresses(vv.First())
 
+	n.stack.AddLinkAddress(n.id, src, remote)
+
 	// If the packet is destined to the IPv4 Broadcast address, then make a
 	// route to each IPv4 network endpoint and let each endpoint handle the
 	// packet.
@@ -566,10 +575,7 @@ func (n *NIC) DeliverNetworkPacket(linkEP LinkEndpoint, remote, _ tcpip.LinkAddr
 		n.mu.RLock()
 		for _, ref := range n.endpoints {
 			if ref.isValidForIncoming() && ref.protocol == header.IPv4ProtocolNumber && ref.tryIncRef() {
-				r := makeRoute(protocol, dst, src, linkEP.LinkAddress(), ref, false /* handleLocal */, false /* multicastLoop */)
-				r.RemoteLinkAddress = remote
-				ref.ep.HandlePacket(&r, vv)
-				ref.decRef()
+				handlePacket(protocol, dst, src, linkEP.LinkAddress(), remote, ref, vv)
 			}
 		}
 		n.mu.RUnlock()
@@ -577,10 +583,7 @@ func (n *NIC) DeliverNetworkPacket(linkEP LinkEndpoint, remote, _ tcpip.LinkAddr
 	}
 
 	if ref := n.getRef(protocol, dst); ref != nil {
-		r := makeRoute(protocol, dst, src, linkEP.LinkAddress(), ref, false /* handleLocal */, false /* multicastLoop */)
-		r.RemoteLinkAddress = remote
-		ref.ep.HandlePacket(&r, vv)
-		ref.decRef()
+		handlePacket(protocol, dst, src, linkEP.LinkAddress(), remote, ref, vv)
 		return
 	}