BGP: Improve handling of hold and keepalive timers

The effective keepalive time now scales relative to the negotiated
hold time, to maintain proportion between the keepalive time and the
hold time. This avoids issues when both keepalive and hold times
were configured, the hold time was negotiated to a smaller value,
but the keepalive time stayed the same.

Add new options 'min hold time' and 'min keepalive time', which reject
session attempts with too small hold time.

Improve validation of config options an their documentation.

Thanks to Alexander Zubkov and Sergei Goriunov for suggestions.

1 files changed, 18 insertions, 2 deletions

diff --git a/proto/bgp/packets.c b/proto/bgp/packets.c
index c464e9c7..afcacffa 100644
--- a/proto/bgp/packets.c
+++ b/proto/bgp/packets.c
@@ -847,9 +847,25 @@ bgp_rx_open(struct bgp_conn *conn, byte *pkt, uint len)
   if (bgp_read_options(conn, pkt+29, pkt[28], len-29) < 0)
     return;
 
+  /* RFC 4271 4.2 - hold time must be either 0 or at least 3 */
   if (hold > 0 && hold < 3)
   { bgp_error(conn, 2, 6, pkt+22, 2); return; }
 
+  /* Compute effective hold and keepalive times */
+  uint hold_time = MIN(hold, p->cf->hold_time);
+  uint keepalive_time = p->cf->keepalive_time ?
+    (p->cf->keepalive_time * hold_time / p->cf->hold_time) :
+    hold_time / 3;
+
+  /* Keepalive time might be rounded down to zero */
+  if (hold_time && !keepalive_time)
+    keepalive_time = 1;
+
+  /* Check effective values against configured minimums */
+  if ((hold_time < p->cf->min_hold_time) ||
+      (keepalive_time < p->cf->min_keepalive_time))
+  { bgp_error(conn, 2, 6, pkt+22, 2); return; }
+
   /* RFC 6286 2.2 - router ID is nonzero and AS-wide unique */
   if (!id || (p->is_internal && id == p->local_id))
   { bgp_error(conn, 2, 3, pkt+24, -4); return; }
@@ -947,8 +963,8 @@ bgp_rx_open(struct bgp_conn *conn, byte *pkt, uint len)
   }
 
   /* Update our local variables */
-  conn->hold_time = MIN(hold, p->cf->hold_time);
-  conn->keepalive_time = p->cf->keepalive_time ? : conn->hold_time / 3;
+  conn->hold_time = hold_time;
+  conn->keepalive_time = keepalive_time;
   conn->as4_session = conn->local_caps->as4_support && caps->as4_support;
   conn->ext_messages = conn->local_caps->ext_messages && caps->ext_messages;
   p->remote_id = id;