Set IPv4 ID on all non-atomic datagrams

RFC 6864 imposes various restrictions on the uniqueness of the IPv4
Identification field for non-atomic datagrams, defined as an IP datagram that
either can be fragmented (DF=0) or is already a fragment (MF=1 or positive
fragment offset). In order to be compliant, the ID field is assigned for all
non-atomic datagrams.

Add a TCP unit test that induces retransmissions and checks that the IPv4
ID field is unique every time. Add basic handling of the IP_MTU_DISCOVER
socket option so that the option can be used to disable PMTU discovery,
effectively setting DF=0. Attempting to set the sockopt to anything other
than disabled will fail because PMTU discovery is currently not implemented,
and the default behavior matches that of disabled.

PiperOrigin-RevId: 320081842

1 files changed, 12 insertions, 0 deletions

diff --git a/pkg/tcpip/transport/tcp/endpoint.go b/pkg/tcpip/transport/tcp/endpoint.go
index bd3ec5a8d..caac6ef57 100644
--- a/pkg/tcpip/transport/tcp/endpoint.go
+++ b/pkg/tcpip/transport/tcp/endpoint.go
@@ -1589,6 +1589,13 @@ func (e *endpoint) SetSockOptInt(opt tcpip.SockOptInt, v int) *tcpip.Error {
 		e.UnlockUser()
 		e.notifyProtocolGoroutine(notifyMSSChanged)
 
+	case tcpip.MTUDiscoverOption:
+		// Return not supported if attempting to set this option to
+		// anything other than path MTU discovery disabled.
+		if v != tcpip.PMTUDiscoveryDont {
+			return tcpip.ErrNotSupported
+		}
+
 	case tcpip.ReceiveBufferSizeOption:
 		// Make sure the receive buffer size is within the min and max
 		// allowed.
@@ -1896,6 +1903,11 @@ func (e *endpoint) GetSockOptInt(opt tcpip.SockOptInt) (int, *tcpip.Error) {
 		v := header.TCPDefaultMSS
 		return v, nil
 
+	case tcpip.MTUDiscoverOption:
+		// Always return the path MTU discovery disabled setting since
+		// it's the only one supported.
+		return tcpip.PMTUDiscoveryDont, nil
+
 	case tcpip.ReceiveQueueSizeOption:
 		return e.readyReceiveSize()