BGP: Mandatory option for channels

Allow to mark channel to be mandatory, and do not allow BGP sessions if
no common AFI/SAFI is established.

doc/bird.sgml

@@ -2565,6 +2565,15 @@ be used in explicit configuration.
 <p>BGP channels have additional config options (together with the common ones):
 
 <descrip>
+	<tag><label id="bgp-mandatory">mandatory <m/switch/</tag>
+	When local and neighbor sets of configured AFI/SAFI pairs differ,
+	capability negotiation ensures that a common subset is used. For
+	mandatory channels their associated AFI/SAFI must be negotiated
+	(i.e., also announced by the neighbor), otherwise BGP session
+	negotiation fails with <it/'Required capability missing'/ error.
+	Regardless, at least one AFI/SAFI must be negotiated in order to BGP
+	session be successfully established. Default: off.
+
 	<tag><label id="bgp-next-hop-keep">next hop keep <m/switch/|ibgp|ebgp</tag>
 	Do not modify the Next Hop attribute and advertise the current one
 	unchanged even in cases where our own local address should be used

proto/bgp/bgp.h

@@ -136,6 +136,7 @@ struct bgp_channel_config {
   ip_addr next_hop_addr;		/* Local address for NEXT_HOP attribute */
   u8 next_hop_self;			/* Always set next hop to local IP address (NH_*) */
   u8 next_hop_keep;			/* Do not modify next hop attribute (NH_*) */
+  u8 mandatory;				/* Channel is mandatory in capability negotiation */
   u8 missing_lladdr;			/* What we will do when we don' know link-local addr, see MLL_* */
   u8 gw_mode;				/* How we compute route gateway from next_hop attr, see GW_* */
   u8 secondary;				/* Accept also non-best routes (i.e. RA_ACCEPTED) */

proto/bgp/config.Y

@@ -29,7 +29,7 @@ CF_KEYWORDS(BGP, LOCAL, NEIGHBOR, AS, HOLD, TIME, CONNECT, RETRY, KEEPALIVE,
 	SECURITY, DETERMINISTIC, SECONDARY, ALLOW, BFD, ADD, PATHS, RX, TX,
 	GRACEFUL, RESTART, AWARE, CHECK, LINK, PORT, EXTENDED, MESSAGES, SETKEY,
 	STRICT, BIND, CONFEDERATION, MEMBER, MULTICAST, FLOW4, FLOW6, LONG,
-	LIVED, STALE, IMPORT, IBGP, EBGP)
+	LIVED, STALE, IMPORT, IBGP, EBGP, MANDATORY)
 
 %type <i> bgp_nh
 %type <i32> bgp_afi
@@ -223,6 +223,7 @@ bgp_channel_item:
  | NEXT HOP ADDRESS ipa { BGP_CC->next_hop_addr = $4; }
  | NEXT HOP SELF bgp_nh { BGP_CC->next_hop_self = $4; }
  | NEXT HOP KEEP bgp_nh { BGP_CC->next_hop_keep = $4; }
+ | MANDATORY bool { BGP_CC->mandatory = $2; }
  | MISSING LLADDR SELF { BGP_CC->missing_lladdr = MLL_SELF; }
  | MISSING LLADDR DROP { BGP_CC->missing_lladdr = MLL_DROP; }
  | MISSING LLADDR IGNORE { BGP_CC->missing_lladdr = MLL_IGNORE; }

proto/bgp/packets.c

@@ -573,6 +573,42 @@ err:
   return;
 }
 
+static int
+bgp_check_capabilities(struct bgp_conn *conn)
+{
+  struct bgp_proto *p = conn->bgp;
+  struct bgp_caps *local = conn->local_caps;
+  struct bgp_caps *remote = conn->remote_caps;
+  struct bgp_channel *c;
+  int count = 0;
+
+  /* This is partially overlapping with bgp_conn_enter_established_state(),
+     but we need to run this just after we receive OPEN message */
+
+  WALK_LIST(c, p->p.channels)
+  {
+    const struct bgp_af_caps *loc = bgp_find_af_caps(local,  c->afi);
+    const struct bgp_af_caps *rem = bgp_find_af_caps(remote, c->afi);
+
+    /* Find out whether this channel will be active */
+    int active = loc && loc->ready &&
+      ((rem && rem->ready) || (!remote->length && (c->afi == BGP_AF_IPV4)));
+
+    /* Mandatory must be active */
+    if (c->cf->mandatory && !active)
+      return 0;
+
+    if (active)
+      count++;
+  }
+
+  /* We need at least one channel active */
+  if (!count)
+    return 0;
+
+  return 1;
+}
+
 static int
 bgp_read_options(struct bgp_conn *conn, byte *pos, int len)
 {
@@ -683,6 +719,10 @@ bgp_rx_open(struct bgp_conn *conn, byte *pkt, uint len)
   if (!id || (p->is_internal && id == p->local_id))
   { bgp_error(conn, 2, 3, pkt+24, -4); return; }
 
+  /* RFC 5492 4 - check for required capabilities */
+  if (p->cf->capabilities && !bgp_check_capabilities(conn))
+  { bgp_error(conn, 2, 7, NULL, 0); return; }
+
   struct bgp_caps *caps = conn->remote_caps;
 
   if (caps->as4_support)