bgpd: implement SendHoldTimer

As described by
https://www.ietf.org/archive/id/draft-spaghetti-idr-bgp-sendholdtimer-04.html

Since this replicates the HoldTime check on the receiver that is already
part of the protocol, I do not believe it necessary to wait for IETF
progress on this draft.  It's just replicating an existing element of
the protocol at the other side of the session.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>

bgpd/bgp_errors.c

@@ -181,6 +181,12 @@ static struct log_ref ferr_bgp_warn[] = {
 		.description = "BGP is in the process of building NLRI information for a peer and has discovered an inconsistent internal state",
 		.suggestion = "Gather log files and open an Issue, restart FRR",
 	},
+	{
+		.code = EC_BGP_SENDQ_STUCK_WARN,
+		.title = "BGP has been unable to send anything to a peer for an extended time",
+		.description = "The BGP peer does not seem to be receiving or processing any data received from us, causing updates to be delayed.",
+		.suggestion = "Check connectivity to the peer and that it is not overloaded",
+	},
 	{
 		.code = END_FERR,
 	}
@@ -480,6 +486,12 @@ static struct log_ref ferr_bgp_err[] = {
 		.description = "BGP when using a v6 peer requires a v6 LL address to be configured on the outgoing interface as per RFC 4291 section 2.1",
 		.suggestion = "Add a v6 LL address to the outgoing interfaces as per RFC",
 	},
+	{
+		.code = EC_BGP_SENDQ_STUCK_PROPER,
+		.title = "BGP is shutting down a peer due to being unable to send anything for an extended time",
+		.description = "No BGP updates were successfully sent to the peer for more than twice the holdtime.",
+		.suggestion = "Check connectivity to the peer and that it is not overloaded",
+	},
 	{
 		.code = END_FERR,
 	}

bgpd/bgp_errors.h

@@ -102,6 +102,8 @@ enum bgp_log_refs {
 	EC_BGP_INVALID_BGP_INSTANCE,
 	EC_BGP_INVALID_ROUTE,
 	EC_BGP_NO_LL_ADDRESS_AVAILABLE,
+	EC_BGP_SENDQ_STUCK_WARN,
+	EC_BGP_SENDQ_STUCK_PROPER,
 };
 
 extern void bgp_error_init(void);

bgpd/bgp_io.c

@@ -298,6 +298,7 @@ static uint16_t bgp_write(struct peer *peer)
 	unsigned int iovsz;
 	unsigned int strmsz;
 	unsigned int total_written;
+	time_t now;
 
 	wpkt_quanta_old = atomic_load_explicit(&peer->bgp->wpkt_quanta,
 					       memory_order_relaxed);
@@ -430,19 +431,22 @@ static uint16_t bgp_write(struct peer *peer)
 	}
 
 done : {
+	now = bgp_clock();
 	/*
 	 * Update last_update if UPDATEs were written.
 	 * Note: that these are only updated at end,
 	 *       not per message (i.e., per loop)
 	 */
 	if (uo)
-		atomic_store_explicit(&peer->last_update, bgp_clock(),
+		atomic_store_explicit(&peer->last_update, now,
 				      memory_order_relaxed);
 
 	/* If we TXed any flavor of packet */
-	if (update_last_write)
-		atomic_store_explicit(&peer->last_write, bgp_clock(),
+	if (update_last_write) {
+		atomic_store_explicit(&peer->last_write, now,
 				      memory_order_relaxed);
+		peer->last_sendq_ok = now;
+	}
 }
 
 	return status;

bgpd/bgp_packet.c

@@ -122,8 +122,45 @@ void bgp_packet_set_size(struct stream *s)
  */
 static void bgp_packet_add(struct peer *peer, struct stream *s)
 {
+	intmax_t delta;
+	uint32_t holdtime;
+
 	frr_with_mutex(&peer->io_mtx) {
+		/* if the queue is empty, reset the "last OK" timestamp to
+		 * now, otherwise if we write another packet immediately
+		 * after it'll get confused
+		 */
+		if (!stream_fifo_count_safe(peer->obuf))
+			peer->last_sendq_ok = bgp_clock();
+
 		stream_fifo_push(peer->obuf, s);
+
+		delta = bgp_clock() - peer->last_sendq_ok;
+		holdtime = atomic_load_explicit(&peer->holdtime,
+						memory_order_relaxed);
+
+		/* Note that when we're here, we're adding some packet to the
+		 * OutQ.  That includes keepalives when there is nothing to
+		 * do, so there's a guarantee we pass by here once in a while.
+		 *
+		 * That implies there is no need to go set up another separate
+		 * timer that ticks down SendHoldTime, as we'll be here sooner
+		 * or later anyway and will see the checks below failing.
+		 */
+		if (delta > 2 * (intmax_t)holdtime) {
+			flog_err(
+				EC_BGP_SENDQ_STUCK_PROPER,
+				"%s has not made any SendQ progress for 2 holdtimes, terminating session",
+				peer->host);
+			BGP_EVENT_ADD(peer, TCP_fatal_error);
+		} else if (delta > (intmax_t)holdtime &&
+			   bgp_clock() - peer->last_sendq_warn > 5) {
+			flog_warn(
+				EC_BGP_SENDQ_STUCK_WARN,
+				"%s has not made any SendQ progress for 1 holdtime, peer overloaded?",
+				peer->host);
+			peer->last_sendq_warn = bgp_clock();
+		}
 	}
 }
 

bgpd/bgpd.h

@@ -1533,6 +1533,11 @@ struct peer {
 	/* timestamp when the last msg was written */
 	_Atomic time_t last_update;
 
+	/* only updated under io_mtx.
+	 * last_sendq_warn is only for ratelimiting log warning messages.
+	 */
+	time_t last_sendq_ok, last_sendq_warn;
+
 	/* Notify data. */
 	struct bgp_notify notify;