Date:	Fri, 29 Jun 2012 15:21:10 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Vlad Yasevich <vyasevich@...il.com>
Cc:	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH v3] sctp: be more restrictive in transport selection on
 bundled sacks

On Fri, Jun 29, 2012 at 03:15:12PM -0400, Vlad Yasevich wrote:
> On 06/29/2012 02:43 PM, Neil Horman wrote:
> >On Fri, Jun 29, 2012 at 02:29:52PM -0400, Vlad Yasevich wrote:
> >>On 06/29/2012 12:34 PM, Neil Horman wrote:
> >>>It was noticed recently that when we send data on a transport, its possible that
> >>>we might bundle a sack that arrived on a different transport.  While this isn't
> >>>a major problem, it does go against the SHOULD requirement in section 6.4 of RFC
> >>>2960:
> >>>
> >>>  An endpoint SHOULD transmit reply chunks (e.g., SACK, HEARTBEAT ACK,
> >>>    etc.) to the same destination transport address from which it
> >>>    received the DATA or control chunk to which it is replying.  This
> >>>    rule should also be followed if the endpoint is bundling DATA chunks
> >>>    together with the reply chunk.
> >>>
> >>>This patch seeks to correct that.  It restricts the bundling of sack operations
> >>>to only those transports which have moved the ctsn of the association forward
> >>>since the last sack.  By doing this we guarantee that we only bundle outbound
> >>>saks on a transport that has received a chunk since the last sack.  This brings
> >>>us into stricter compliance with the RFC.
> >>>
> >>>Vlad had initially suggested that we strictly allow only sack bundling on the
> >>>transport that last moved the ctsn forward.  While this makes sense, I was
> >>>concerned that doing so prevented us from bundling in the case where we had
> >>>received chunks that moved the ctsn on multiple transports.  In those cases, the
> >>>RFC allows us to select any of the transports having received chunks to bundle
> >>>the sack on.  so I've modified the approach to allow for that, by adding a state
> >>>variable to each transport that tracks weather it has moved the ctsn since the
> >>>last sack.  This I think keeps our behavior (and performance), close enough to
> >>>our current profile that I think we can do this without a sysctl knob to
> >>>enable/disable it.
> >>>
> >>>Signed-off-by: Neil Horman<nhorman@...driver.com>
> >>>CC: Vlad Yaseivch<vyasevich@...il.com>
> >>>CC: David S. Miller<davem@...emloft.net>
> >>>Reported-by: Michele Baldessari<michele@...hat.com>
> >>>Reported-by: sorin serban<sserban@...hat.com>
> >>>
> >>>---
> >>>Change Notes:
> >>>V2)
> >>>	* Removed unused variable as per Dave M. Request
> >>>	* Delayed rwnd adjustment until we are sure we will sack (Vlad Y.)
> >>>V3)
> >>>	* Switched test to use pkt->transport rather than chunk->transport
> >>>	* Modified detection of sacka-able transport.  Instead of just setting
> >>>	  and clearning a flag, we now mark each transport and association with
> >>>	  a sack generation tag.  We increment the associations generation on
> >>>	  every sack, and assign that generation tag to every transport that
> >>>	  updates the ctsn.  This prevents us from having to iterate over a for
> >>>	  loop on every sack, which is much more scalable.
> >>>---
> >>>  include/net/sctp/structs.h |    4 ++++
> >>>  include/net/sctp/tsnmap.h  |    3 ++-
> >>>  net/sctp/associola.c       |    1 +
> >>>  net/sctp/output.c          |    9 +++++++--
> >>>  net/sctp/sm_make_chunk.c   |   10 ++++++++++
> >>>  net/sctp/sm_sideeffect.c   |    2 +-
> >>>  net/sctp/transport.c       |    2 ++
> >>>  net/sctp/tsnmap.c          |    6 +++++-
> >>>  net/sctp/ulpevent.c        |    3 ++-
> >>>  net/sctp/ulpqueue.c        |    2 +-
> >>>  10 files changed, 35 insertions(+), 7 deletions(-)
> >>>
> >>>diff --git a/include/net/sctp/structs.h b/include/net/sctp/structs.h
> >>>index e4652fe..fecdf31 100644
> >>>--- a/include/net/sctp/structs.h
> >>>+++ b/include/net/sctp/structs.h
> >>>@@ -912,6 +912,9 @@ struct sctp_transport {
> >>>  		/* Is this structure kfree()able? */
> >>>  		malloced:1;
> >>>
> >>>+	/* Has this transport moved the ctsn since we last sacked */
> >>>+	__u32 sack_generation;
> >>>+
> >>>  	struct flowi fl;
> >>>
> >>>  	/* This is the peer's IP address and port. */
> >>>@@ -1584,6 +1587,7 @@ struct sctp_association {
> >>>  		 */
> >>>  		__u8    sack_needed;     /* Do we need to sack the peer? */
> >>>  		__u32	sack_cnt;
> >>>+		__u32	sack_generation;
> >>>
> >>>  		/* These are capabilities which our peer advertised.  */
> >>>  		__u8	ecn_capable:1,	    /* Can peer do ECN? */
> >>>diff --git a/include/net/sctp/tsnmap.h b/include/net/sctp/tsnmap.h
> >>>index e7728bc..2c5d2b4 100644
> >>>--- a/include/net/sctp/tsnmap.h
> >>>+++ b/include/net/sctp/tsnmap.h
> >>>@@ -117,7 +117,8 @@ void sctp_tsnmap_free(struct sctp_tsnmap *map);
> >>>  int sctp_tsnmap_check(const struct sctp_tsnmap *, __u32 tsn);
> >>>
> >>>  /* Mark this TSN as seen.  */
> >>>-int sctp_tsnmap_mark(struct sctp_tsnmap *, __u32 tsn);
> >>>+int sctp_tsnmap_mark(struct sctp_tsnmap *, __u32 tsn,
> >>>+		     struct sctp_transport *trans);
> >>>
> >>>  /* Mark this TSN and all lower as seen. */
> >>>  void sctp_tsnmap_skip(struct sctp_tsnmap *map, __u32 tsn);
> >>>diff --git a/net/sctp/associola.c b/net/sctp/associola.c
> >>>index 5bc9ab1..6c66adb 100644
> >>>--- a/net/sctp/associola.c
> >>>+++ b/net/sctp/associola.c
> >>>@@ -271,6 +271,7 @@ static struct sctp_association *sctp_association_init(struct sctp_association *a
> >>>  	 */
> >>>  	asoc->peer.sack_needed = 1;
> >>>  	asoc->peer.sack_cnt = 0;
> >>>+	asoc->peer.sack_generation=0;
> >>>
> >>>  	/* Assume that the peer will tell us if he recognizes ASCONF
> >>>  	 * as part of INIT exchange.
> >>>diff --git a/net/sctp/output.c b/net/sctp/output.c
> >>>index f1b7d4b..0de6cd5 100644
> >>>--- a/net/sctp/output.c
> >>>+++ b/net/sctp/output.c
> >>>@@ -240,14 +240,19 @@ static sctp_xmit_t sctp_packet_bundle_sack(struct sctp_packet *pkt,
> >>>  	 */
> >>>  	if (sctp_chunk_is_data(chunk)&&   !pkt->has_sack&&
> >>>  	!pkt->has_cookie_echo) {
> >>>-		struct sctp_association *asoc;
> >>>  		struct timer_list *timer;
> >>>-		asoc = pkt->transport->asoc;
> >>>+		struct sctp_association *asoc = pkt->transport->asoc;
> >>>+
> >>>  		timer =&asoc->timers[SCTP_EVENT_TIMEOUT_SACK];
> >>>
> >>>  		/* If the SACK timer is running, we have a pending SACK */
> >>>  		if (timer_pending(timer)) {
> >>>  			struct sctp_chunk *sack;
> >>>+
> >>>+			if (pkt->transport->sack_generation !=
> >>>+			    pkt->transport->asoc->peer.sack_generation)
> >>>+				return retval;
> >>>+
> >>>  			asoc->a_rwnd = asoc->rwnd;
> >>>  			sack = sctp_make_sack(asoc);
> >>>  			if (sack) {
> >>>diff --git a/net/sctp/sm_make_chunk.c b/net/sctp/sm_make_chunk.c
> >>>index a85eeeb..ffa2a8e 100644
> >>>--- a/net/sctp/sm_make_chunk.c
> >>>+++ b/net/sctp/sm_make_chunk.c
> >>>@@ -736,6 +736,7 @@ struct sctp_chunk *sctp_make_sack(const struct sctp_association *asoc)
> >>>  	__u16 num_gabs, num_dup_tsns;
> >>>  	struct sctp_tsnmap *map = (struct sctp_tsnmap *)&asoc->peer.tsn_map;
> >>>  	struct sctp_gap_ack_block gabs[SCTP_MAX_GABS];
> >>>+	struct sctp_transport *trans;
> >>>
> >>>  	memset(gabs, 0, sizeof(gabs));
> >>>  	ctsn = sctp_tsnmap_get_ctsn(map);
> >>>@@ -805,6 +806,15 @@ struct sctp_chunk *sctp_make_sack(const struct sctp_association *asoc)
> >>>  		sctp_addto_chunk(retval, sizeof(__u32) * num_dup_tsns,
> >>>  				 sctp_tsnmap_get_dups(map));
> >>>
> >>>+	/*
> >>>+	 * Once we have a sack generated, clear the moved_tsn information
> >>>+	 * from all the transports
> >>>+	 */
> >>>+	if (!asoc->peer.sack_generation)
> >>>+		list_for_each_entry(trans,&asoc->peer.transport_addr_list,
> >>>+				    transports)
> >>>+			trans->sack_generation = UINT_MAX;
> >>>+	((struct sctp_association *)asoc)->peer.sack_generation++;
> >>
> >>Two points here:
> >>1) The commend no longer matches the code
> >Crud, missed that, I'll fix it.
> >
> >>2) Why special case the peer.sack_generations == 0 and set the
> >>transport to UNIT_MAX?
> >>
> >To avoid wrapping problems leading to erroneous bundling errors.  Consider a
> >long lived connection with two trasports (A and B).
> >
> >If all traffic is sent on A for a long time (generating UINT_MAX sacks), and the
> >peer chooses that moment to send data on transport B, its possible that we will
> >bundle a sack with that data chunk erroneously, because the associations
> >sack_generation has wrapped, and now matches with the transports, even though we
> >never received data on transport B.  The special casing ensures that we never
> >hit that problem.
> >
> 
> But you just move this condition to the UINT_MAX value instead.  If
> we use the alternate transport at the time that sack_generation ==
> UINT_MAX, we may pick the wrong transport.
> 
Yes, i noticed that as I was fixing the comment, thank you!

> You may want to consider value 0 reserved as UNUSED and make
> peer.sack_generation start at 1 and wrap to 1.
> 
Thats exactly what I just did :)
Neil

> -vlad
> 
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