Decrement TTL/Hop Limit when forwarding IP packets

If the packet must no longer be forwarded because its TTL/Hop Limit
reaches 0, send an ICMP Time Exceeded error to the source.

Required as per relevant RFCs. See comments in code for RFC references.

Fixes #1085

Tests:
  - ipv4_test.TestForwarding
  - ipv6.TestForwarding
PiperOrigin-RevId: 342323610

1 files changed, 12 insertions, 0 deletions

diff --git a/pkg/tcpip/checker/checker.go b/pkg/tcpip/checker/checker.go
index 8868cf4e3..81f762e10 100644
--- a/pkg/tcpip/checker/checker.go
+++ b/pkg/tcpip/checker/checker.go
@@ -904,6 +904,12 @@ func ICMPv4Payload(want []byte) TransportChecker {
 			t.Fatalf("unexpected transport header passed to checker, got = %T, want = header.ICMPv4", h)
 		}
 		payload := icmpv4.Payload()
+
+		// cmp.Diff does not consider nil slices equal to empty slices, but we do.
+		if len(want) == 0 && len(payload) == 0 {
+			return
+		}
+
 		if diff := cmp.Diff(want, payload); diff != "" {
 			t.Errorf("ICMP payload mismatch (-want +got):\n%s", diff)
 		}
@@ -994,6 +1000,12 @@ func ICMPv6Payload(want []byte) TransportChecker {
 			t.Fatalf("unexpected transport header passed to checker, got = %T, want = header.ICMPv6", h)
 		}
 		payload := icmpv6.Payload()
+
+		// cmp.Diff does not consider nil slices equal to empty slices, but we do.
+		if len(want) == 0 && len(payload) == 0 {
+			return
+		}
+
 		if diff := cmp.Diff(want, payload); diff != "" {
 			t.Errorf("ICMP payload mismatch (-want +got):\n%s", diff)
 		}