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WIP: ggsn: Support announcing APN MTU over ICMPv6 RA

Related: OS#6298
Related: SYS#7122
Change-Id: I8bb67915dd5f39ad9ffb80e5aaf9af1e7d70c96c
---
M ggsn/ggsn.c
M lib/icmpv6.c
M lib/icmpv6.h
3 files changed, 33 insertions(+), 3 deletions(-)

git pull ssh://gerrit.osmocom.org:29418/osmo-ggsn refs/changes/70/38470/1

diff --git a/ggsn/ggsn.c b/ggsn/ggsn.c
index 4dc0c94..19b0132 100644
--- a/ggsn/ggsn.c
+++ b/ggsn/ggsn.c
@@ -774,7 +774,7 @@
 		/* daddr: all-routers multicast addr */
 		if (IN6_ARE_ADDR_EQUAL(&ip6h->ip6_dst, &all_router_mcast_addr))
 			return handle_router_mcast(pdp->gsn, pdp, &peer->addr.v6,
-						&apn->v6_lladdr, pack, len);
+						&apn->v6_lladdr, apn->cfg.mtu, pack, len);
 		break;
 	case 4:
 		peer = pdp_get_peer_ipv(pdp, false);
diff --git a/lib/icmpv6.c b/lib/icmpv6.c
index b6994cd..e74fb70 100644
--- a/lib/icmpv6.c
+++ b/lib/icmpv6.c
@@ -94,11 +94,13 @@
  *  \returns callee-allocated message buffer containing router advertisement */
 static struct msgb *icmpv6_construct_ra(const struct in6_addr *saddr,
 				 const struct in6_addr *daddr,
-				 const struct in6_addr *prefix)
+				 const struct in6_addr *prefix,
+				 uint32_t mtu)
 {
 	struct msgb *msg = msgb_alloc_headroom(512,128, "IPv6 RA");
 	struct icmpv6_radv_hdr *ra;
 	struct icmpv6_opt_prefix *ra_opt_pref;
+	struct icmpv6_opt_mtu *ra_opt_mtu;
 
 	OSMO_ASSERT(msg);
 
@@ -135,6 +137,25 @@
 	ra_opt_pref->res2 = 0;
 	memcpy(ra_opt_pref->prefix, prefix, sizeof(ra_opt_pref->prefix));
 
+	/* RFC4861 Section 4.6.4, MTU */
+	ra_opt_mtu = (struct icmpv6_opt_mtu *) msgb_put(msg, sizeof(*ra_opt_mtu));
+	ra_opt_mtu->hdr.type = 5;	/* RFC4861 4.6.4 */
+	ra_opt_mtu->hdr.len = 1;	/* RFC4861 4.6.4 */
+	ra_opt_mtu->reserved = 0;
+	ra_opt_mtu->mtu = htonl(mtu);
+
+	/* The Prefix is contained in the Prefix Information Option of
+	 * the Router Advertisements and shall have the A-flag set
+	 * and the L-flag cleared */
+	ra_opt_pref->a = 1;
+	ra_opt_pref->l = 0;
+	ra_opt_pref->res = 0;
+	/*  The lifetime of the prefix shall be set to infinity */
+	ra_opt_pref->valid_lifetime = htonl(GGSN_AdvValidLifetime);
+	ra_opt_pref->preferred_lifetime = htonl(GGSN_AdvPreferredLifetime);
+	ra_opt_pref->res2 = 0;
+	memcpy(ra_opt_pref->prefix, prefix, sizeof(ra_opt_pref->prefix));
+
 	/* checksum */
 	ra->hdr.csum = icmpv6_prepend_ip6hdr(msg, saddr, daddr);
 
@@ -194,6 +215,7 @@
 int handle_router_mcast(struct gsn_t *gsn, struct pdp_t *pdp,
 			const struct in6_addr *pdp_prefix,
 			const struct in6_addr *own_ll_addr,
+			uint32_t mtu,
 			const uint8_t *pack, unsigned len)
 {
 	const struct ip6_hdr *ip6h = (struct ip6_hdr *)pack;
@@ -230,7 +252,7 @@
 		/* Send router advertisement from GGSN link-local
 		 * address to MS link-local address, including prefix
 		 * allocated to this PDP context */
-		msg = icmpv6_construct_ra(own_ll_addr, &ip6h->ip6_src, pdp_prefix);
+		msg = icmpv6_construct_ra(own_ll_addr, &ip6h->ip6_src, pdp_prefix, mtu);
 		/* Send the constructed RA to the MS */
 		gtp_data_req(gsn, pdp, msgb_data(msg), msgb_length(msg));
 		msgb_free(msg);
diff --git a/lib/icmpv6.h b/lib/icmpv6.h
index 5bed5c6..1040600 100644
--- a/lib/icmpv6.h
+++ b/lib/icmpv6.h
@@ -81,6 +81,13 @@
 	uint8_t prefix[16];
 } __attribute__ ((packed));
 
+/* RFC4861 Section 4.6.4 */
+struct icmpv6_opt_mtu {
+	struct icmpv6_opt_hdr hdr;
+	uint16_t reserved;
+	uint32_t mtu;
+} __attribute__ ((packed));
+
 uint16_t icmpv6_prepend_ip6hdr(struct msgb *msg, const struct in6_addr *saddr,
 				  const struct in6_addr *daddr);
 
@@ -89,6 +96,7 @@
 int handle_router_mcast(struct gsn_t *gsn, struct pdp_t *pdp,
 			const struct in6_addr *pdp_prefix,
 			const struct in6_addr *own_ll_addr,
+			uint32_t mtu,
 			const uint8_t *pack, unsigned len);
 
 struct icmpv6_radv_hdr *icmpv6_validate_router_adv(const uint8_t *pack, unsigned len);

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