Support for connection tracking of TCP packets.

Connection tracking is used to track packets in prerouting and
output hooks of iptables. The NAT rules modify the tuples in
connections. The connection tracking code modifies the packets by
looking at the modified tuples.

1 files changed, 17 insertions, 0 deletions

diff --git a/pkg/tcpip/header/tcp.go b/pkg/tcpip/header/tcp.go
index 13480687d..21581257b 100644
--- a/pkg/tcpip/header/tcp.go
+++ b/pkg/tcpip/header/tcp.go
@@ -594,3 +594,20 @@ func AddTCPOptionPadding(options []byte, offset int) int {
 	}
 	return paddingToAdd
 }
+
+// Acceptable checks if a segment that starts at segSeq and has length segLen is
+// "acceptable" for arriving in a receive window that starts at rcvNxt and ends
+// before rcvAcc, according to the table on page 26 and 69 of RFC 793.
+func Acceptable(segSeq seqnum.Value, segLen seqnum.Size, rcvNxt, rcvAcc seqnum.Value) bool {
+	if rcvNxt == rcvAcc {
+		return segLen == 0 && segSeq == rcvNxt
+	}
+	if segLen == 0 {
+		// rcvWnd is incremented by 1 because that is Linux's behavior despite the
+		// RFC.
+		return segSeq.InRange(rcvNxt, rcvAcc.Add(1))
+	}
+	// Page 70 of RFC 793 allows packets that can be made "acceptable" by trimming
+	// the payload, so we'll accept any payload that overlaps the receieve window.
+	return rcvNxt.LessThan(segSeq.Add(segLen)) && segSeq.LessThan(rcvAcc)
+}