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, 7 insertions, 2 deletions

diff --git a/pkg/tcpip/header/ipv6.go b/pkg/tcpip/header/ipv6.go
index 4e7e5f76a..55d09355a 100644
--- a/pkg/tcpip/header/ipv6.go
+++ b/pkg/tcpip/header/ipv6.go
@@ -54,7 +54,7 @@ type IPv6Fields struct {
 	// NextHeader is the "next header" field of an IPv6 packet.
 	NextHeader uint8
 
-	// HopLimit is the "hop limit" field of an IPv6 packet.
+	// HopLimit is the "Hop Limit" field of an IPv6 packet.
 	HopLimit uint8
 
 	// SrcAddr is the "source ip address" of an IPv6 packet.
@@ -171,7 +171,7 @@ func (b IPv6) PayloadLength() uint16 {
 	return binary.BigEndian.Uint16(b[IPv6PayloadLenOffset:])
 }
 
-// HopLimit returns the value of the "hop limit" field of the ipv6 header.
+// HopLimit returns the value of the "Hop Limit" field of the ipv6 header.
 func (b IPv6) HopLimit() uint8 {
 	return b[hopLimit]
 }
@@ -236,6 +236,11 @@ func (b IPv6) SetDestinationAddress(addr tcpip.Address) {
 	copy(b[v6DstAddr:][:IPv6AddressSize], addr)
 }
 
+// SetHopLimit sets the value of the "Hop Limit" field.
+func (b IPv6) SetHopLimit(v uint8) {
+	b[hopLimit] = v
+}
+
 // SetNextHeader sets the value of the "next header" field of the ipv6 header.
 func (b IPv6) SetNextHeader(v uint8) {
 	b[IPv6NextHeaderOffset] = v