Stub out support for TCP_MAXSEG.

Adds support to set/get the TCP_MAXSEG value but does not
really change the segment sizes emitted by netstack or
alter the MSS advertised by the endpoint. This is currently
being added only to unblock iperf3 on gVisor. Plumbing
this correctly requires a bit more work which will come
in separate CLs.

PiperOrigin-RevId: 257859112

2 files changed, 20 insertions, 1 deletions

diff --git a/pkg/tcpip/header/ipv4.go b/pkg/tcpip/header/ipv4.go
index 7da4c4845..7b8034de4 100644
--- a/pkg/tcpip/header/ipv4.go
+++ b/pkg/tcpip/header/ipv4.go
@@ -85,6 +85,10 @@ const (
 	// units, the header cannot exceed 15*4 = 60 bytes.
 	IPv4MaximumHeaderSize = 60
 
+	// MinIPFragmentPayloadSize is the minimum number of payload bytes that
+	// the first fragment must carry when an IPv4 packet is fragmented.
+	MinIPFragmentPayloadSize = 8
+
 	// IPv4AddressSize is the size, in bytes, of an IPv4 address.
 	IPv4AddressSize = 4
 
diff --git a/pkg/tcpip/header/tcp.go b/pkg/tcpip/header/tcp.go
index 1141443bb..82cfe785c 100644
--- a/pkg/tcpip/header/tcp.go
+++ b/pkg/tcpip/header/tcp.go
@@ -176,6 +176,21 @@ const (
 
 	// TCPProtocolNumber is TCP's transport protocol number.
 	TCPProtocolNumber tcpip.TransportProtocolNumber = 6
+
+	// TCPMinimumMSS is the minimum acceptable value for MSS. This is the
+	// same as the value TCP_MIN_MSS defined net/tcp.h.
+	TCPMinimumMSS = IPv4MaximumHeaderSize + TCPHeaderMaximumSize + MinIPFragmentPayloadSize - IPv4MinimumSize - TCPMinimumSize
+
+	// TCPMaximumMSS is the maximum acceptable value for MSS.
+	TCPMaximumMSS = 0xffff
+
+	// TCPDefaultMSS is the MSS value that should be used if an MSS option
+	// is not received from the peer. It's also the value returned by
+	// TCP_MAXSEG option for a socket in an unconnected state.
+	//
+	// Per RFC 1122, page 85: "If an MSS option is not received at
+	// connection setup, TCP MUST assume a default send MSS of 536."
+	TCPDefaultMSS = 536
 )
 
 // SourcePort returns the "source port" field of the tcp header.
@@ -306,7 +321,7 @@ func ParseSynOptions(opts []byte, isAck bool) TCPSynOptions {
 	synOpts := TCPSynOptions{
 		// Per RFC 1122, page 85: "If an MSS option is not received at
 		// connection setup, TCP MUST assume a default send MSS of 536."
-		MSS: 536,
+		MSS: TCPDefaultMSS,
 		// If no window scale option is specified, WS in options is
 		// returned as -1; this is because the absence of the option
 		// indicates that the we cannot use window scaling on the