Account for endpoint header size when calculating Route MTU.

If r.mtu has been set, as it is after
#11857, we skip calculating the proper MTU
via endpoint.MTU and use r.mtu instead. However nothing in the callstack from
tcpip/stack/stack.go:FindRoute to setting r.mtu actually adjusts the size to be
able to hold the endpoint header. Hence we need to adjust it.

PiperOrigin-RevId: 843416148

Author: konstantin-s-bogom

pkg/tcpip/network/arp/arp.go

@@ -128,6 +128,11 @@ func (e *endpoint) MTU() uint32 {
 	return lmtu - uint32(e.MaxHeaderLength())
 }
 
+// EndpointHeaderSize returns the size necessary for the ARP header.
+func (e *endpoint) EndpointHeaderSize() uint32 {
+	return header.ARPSize
+}
+
 func (e *endpoint) MaxHeaderLength() uint16 {
 	return e.nic.MaxHeaderLength() + header.ARPSize
 }

pkg/tcpip/network/ipv4/ipv4.go

@@ -426,6 +426,11 @@ func (e *endpoint) MTU() uint32 {
 	return networkMTU
 }
 
+// EndpointHeaderSize returns the size necessary for the IPv4 header.
+func (e *endpoint) EndpointHeaderSize() uint32 {
+	return header.IPv4MinimumSize
+}
+
 // MaxHeaderLength returns the maximum length needed by ipv4 headers (and
 // underlying protocols).
 func (e *endpoint) MaxHeaderLength() uint16 {

pkg/tcpip/network/ipv6/ipv6.go

@@ -728,6 +728,11 @@ func (e *endpoint) MTU() uint32 {
 	return networkMTU
 }
 
+// EndpointHeaderSize returns the size necessary for the IPv6 header.
+func (e *endpoint) EndpointHeaderSize() uint32 {
+	return header.IPv6MinimumSize
+}
+
 // MaxHeaderLength returns the maximum length needed by ipv6 headers (and
 // underlying protocols).
 func (e *endpoint) MaxHeaderLength() uint16 {

pkg/tcpip/stack/registration.go

@@ -872,6 +872,9 @@ type NetworkEndpoint interface {
 	// minus the network endpoint max header length.
 	MTU() uint32
 
+	// EndpointHeaderSize returns the size of this endpoint header.
+	EndpointHeaderSize() uint32
+
 	// MaxHeaderLength returns the maximum size the network (and lower
 	// level layers combined) headers can have. Higher levels use this
 	// information to reserve space in the front of the packets they're

pkg/tcpip/stack/route.go

@@ -245,6 +245,14 @@ func makeRoute(netProto tcpip.NetworkProtocolNumber, gateway, localAddr, remoteA
 }
 
 func makeRouteInner(netProto tcpip.NetworkProtocolNumber, localAddr, remoteAddr tcpip.Address, outgoingNIC, localAddressNIC *nic, localAddressEndpoint AssignableAddressEndpoint, loop PacketLooping, mtu uint32) *Route {
+	if mtu != 0 {
+		adjusted := mtu - outgoingNIC.getNetworkEndpoint(netProto).EndpointHeaderSize()
+		if adjusted > mtu {
+			mtu = 0
+		} else {
+			mtu = adjusted
+		}
+	}
 	r := &Route{
 		routeInfo: routeInfo{
 			NetProto:         netProto,

pkg/tcpip/stack/stack_test.go

@@ -169,6 +169,10 @@ func (f *fakeNetworkEndpoint) HandlePacket(pkt *stack.PacketBuffer) {
 	f.dispatcher.DeliverTransportPacket(transProtoNum, pkt)
 }
 
+func (f *fakeNetworkEndpoint) EndpointHeaderSize() uint32 {
+	return fakeNetHeaderLen
+}
+
 func (f *fakeNetworkEndpoint) MaxHeaderLength() uint16 {
 	return f.nic.MaxHeaderLength() + fakeNetHeaderLen
 }