ldpd: fix wrong gtsm count

In linux networking stack, the received mpls packets will be processed
by the host *twice*, one as mpls packet, the other as ip packet, so
its ttl decreased 1.

So, we need release the `IP_MINTTL` value if gtsm is enabled, it is for the
mpls packets of neighbor session caused by the command:
`label local advertise explicit-null`.

This change makes the gtsm mechanism a bit deviation.

Fix PR #8313

Signed-off-by: anlan_cs <[email protected]>

ldpd/neighbor.c

@@ -683,15 +683,23 @@ nbr_gtsm_setup(int fd, int af, struct nbr_params *nbrp)
 
 	switch (af) {
 	case AF_INET:
-		if (sock_set_ipv4_minttl(fd, ttl) == -1)
+		/*
+		 * In linux networking stack, the received mpls packets
+		 * will be processed by the host twice, one as mpls packet,
+		 * the other as ip packet, so its ttl will be decreased 1.
+		 *
+		 * Here, decrease 1 for IP_MINTTL if GTSM is enabled.
+		 */
+		if (sock_set_ipv4_minttl(fd, ttl - 1) == -1)
 			return (-1);
 		ttl = 255;
 		if (sock_set_ipv4_ucast_ttl(fd, ttl) == -1)
 			return (-1);
 		break;
 	case AF_INET6:
 		/* ignore any possible error */
-		sock_set_ipv6_minhopcount(fd, ttl);
+		/* decrease 1 for IP_MINTTL, same to AF_INET */
+		sock_set_ipv6_minhopcount(fd, ttl - 1);
 		ttl = 255;
 		if (sock_set_ipv6_ucast_hops(fd, ttl) == -1)
 			return (-1);