RFC 4960 Errata fixes
Subsystems (3)
Marcelo Ricardo Leitner
maintainercover RFC 4960 Errata fixes
This patchset contains fixes for 4 Errata topics from
https://tools.ietf.org/html/draft-ietf-tsvwg-rfc4960-errata-01
Namely, sections:
3.12. Order of Adjustments of partial_bytes_acked and cwnd 3.22. Increase of partial_bytes_acked in Congestion Avoidance 3.26. CWND Increase in Congestion Avoidance Phase 3.27. Refresh of cwnd and ssthresh after Idle Period
Tests performed with netperf using net namespaces, with drop rates at 0%, 0.5% and 1% by netem, IPv4 and IPv6, 10 runs for each combination. I couldn't spot differences on the stats. With and without these patches the results vary in a similar way in terms of throughput and retransmissions.
Tests with 20ms delay and 20ms delay + drops at 0.5% and 1% also had results in a similar way, no noticeable difference.
Looking at cwnd, it was possible to notice slightly lower values being used while still sustaining same throughput profile.
Marcelo Ricardo Leitner (4):
sctp: update order of adjustments of partial_bytes_acked and cwnd
sctp: allow increasing cwnd regardless of ctsn moving or not
sctp: adjust cwnd increase in Congestion Avoidance phase
sctp: adjust ssthresh when transport is idle
net/sctp/transport.c | 54 ++++++++++++++++++++++++++++++++-------------------- 1 file changed, 33 insertions(+), 21 deletions(-)
Signature · 1 line
2.9.4
Marcelo Ricardo Leitner
maintainerPATCH 1/4 sctp: update order of adjustments of partial_bytes_acked and cwnd
RFC4960 Errata 3.12 says RFC4960 is unclear about the order of adjustments applied to partial_bytes_acked and cwnd in the congestion avoidance phase, and that the actual order should be:partial_bytes_acked is reset to (partial_bytes_acked - cwnd). Next, cwnd is increased by MTU.
We were first increasing cwnd, and then subtracting the new value pba, which leads to a different result as pba is smaller than what it should and could cause cwnd to not grow as much.
See-also: https://tools.ietf.org/html/draft-ietf-tsvwg-rfc4960-errata-01#section-3.12
--- net/sctp/transport.c | 15 ++++++++------- 1 file changed, 8 insertions(+), 7 deletions(-)
net/sctp/transport.c +8 -7
@@ -452,17 +452,18 @@ void sctp_transport_raise_cwnd(struct sctp_transport *transport,* chunks acknowledged by the new Cumulative TSN Ack and by * Gap Ack Blocks. * -* When partial_bytes_acked is equal to or greater than cwnd -* and before the arrival of the SACK the sender had cwnd or -* more bytes of data outstanding (i.e., before arrival of the -* SACK, flightsize was greater than or equal to cwnd), -* increase cwnd by MTU, and reset partial_bytes_acked to -* (partial_bytes_acked - cwnd). +* When partial_bytes_acked is equal to or greater than +* cwnd and before the arrival of the SACK the sender +* had cwnd or more bytes of data outstanding (i.e., +* before arrival of the SACK, flightsize was greater +* than or equal to cwnd), partial_bytes_acked is reset +* to (partial_bytes_acked - cwnd). Next, cwnd is +* increased by MTU. (RFC 4960 Errata 3.12) */ pba += bytes_acked;if (pba >= cwnd) { + pba = pba - cwnd; cwnd += pmtu; - pba = ((cwnd < pba) ? (pba - cwnd) :0 ); } pr_debug("%s: congestion avoidance: transport:%p, "
Signature · 1 line
2.9.4
Marcelo Ricardo Leitner
maintainerPATCH 2/4 sctp: allow increasing cwnd regardless of ctsn moving or not
In reply to Marcelo Ricardo LeitnerAs per RFC4960 Errata 3.22, this condition is not needed anymore as it could cause the partial_bytes_acked to not consider the TSNs acked in the Gap Ack Blocks although they were received by the peer successfully.
This patch thus drops the check for new Cumulative TSN Ack Point, leaving just the flight_size < cwnd one.
See-also: https://tools.ietf.org/html/draft-ietf-tsvwg-rfc4960-errata-01#section-3.22
--- net/sctp/transport.c | 17 ++++++++--------- 1 file changed, 8 insertions(+), 9 deletions(-)
net/sctp/transport.c +8 -9
@@ -406,11 +406,10 @@ void sctp_transport_raise_cwnd(struct sctp_transport *transport, asoc->fast_recovery =0 ;/* The appropriate cwnd increase algorithm is performed if, and only -* if the cumulative TSN whould advanced and the congestion window is -* being fully utilized. +* if the congestion window is being fully utilized. +* Note that RFC4960 Errata 3.22 removed the other condition. */ -if (TSN_lte (sack_ctsn, transport->asoc->ctsn_ack_point) || - (flight_size < cwnd)) +if (flight_size < cwnd)return ; ssthresh = transport->ssthresh;
@@ -446,11 +445,11 @@ void sctp_transport_raise_cwnd(struct sctp_transport *transport, flight_size, pba); }else {/* RFC 2960 7.2.2 Whenever cwnd is greater than ssthresh, -* upon each SACK arrival that advances the Cumulative TSN Ack -* Point, increase partial_bytes_acked by the total number of -* bytes of all new chunks acknowledged in that SACK including -* chunks acknowledged by the new Cumulative TSN Ack and by -* Gap Ack Blocks. +* upon each SACK arrival, increase partial_bytes_acked +* by the total number of bytes of all new chunks +* acknowledged in that SACK including chunks +* acknowledged by the new Cumulative TSN Ack and by Gap +* Ack Blocks. (updated by RFC4960 Errata 3.22) * * When partial_bytes_acked is equal to or greater than * cwnd and before the arrival of the SACK the sender
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2.9.4
Marcelo Ricardo Leitner
maintainerPATCH 3/4 sctp: adjust cwnd increase in Congestion Avoidance phase
In reply to Marcelo Ricardo LeitnerRFC4960 Errata 3.26 identified that at the same time RFC4960 states that cwnd should never grow more than 1*MTU per RTT, Section 7.2.2 was underspecified and as described could allow increasing cwnd more than that.
This patch updates it so partial_bytes_acked is maxed to cwnd if flight_size doesn't reach cwnd, protecting it from such case.
See-also: https://tools.ietf.org/html/draft-ietf-tsvwg-rfc4960-errata-01#section-3.26
--- net/sctp/transport.c | 26 ++++++++++++++++++-------- 1 file changed, 18 insertions(+), 8 deletions(-)
net/sctp/transport.c +18 -8
@@ -405,13 +405,6 @@ void sctp_transport_raise_cwnd(struct sctp_transport *transport, TSN_lte(asoc->fast_recovery_exit, sack_ctsn)) asoc->fast_recovery = 0; -/* The appropriate cwnd increase algorithm is performed if, and only -* if the congestion window is being fully utilized. -* Note that RFC4960 Errata 3.22 removed the other condition. -*/ -if (flight_size < cwnd) -return ; - ssthresh = transport->ssthresh; pba = transport->partial_bytes_acked; pmtu = transport->asoc->pathmtu;
@@ -434,6 +427,14 @@ void sctp_transport_raise_cwnd(struct sctp_transport *transport,if (asoc->fast_recovery)return ; +/* The appropriate cwnd increase algorithm is performed +* if, and only if the congestion window is being fully +* utilized. Note that RFC4960 Errata 3.22 removed the +* other condition on ctsn moving. +*/ +if (flight_size < cwnd) +return ; +if (bytes_acked > pmtu) cwnd += pmtu;else
@@ -451,6 +452,13 @@ void sctp_transport_raise_cwnd(struct sctp_transport *transport,* acknowledged by the new Cumulative TSN Ack and by Gap * Ack Blocks. (updated by RFC4960 Errata 3.22) * +* When partial_bytes_acked is greater than cwnd and +* before the arrival of the SACK the sender had less +* bytes of data outstanding than cwnd (i.e., before +* arrival of the SACK, flightsize was less than cwnd), +* reset partial_bytes_acked to cwnd. (RFC 4960 Errata +* 3.26) +* * When partial_bytes_acked is equal to or greater than * cwnd and before the arrival of the SACK the sender * had cwnd or more bytes of data outstanding (i.e.,
@@ -460,7 +468,9 @@ void sctp_transport_raise_cwnd(struct sctp_transport *transport,* increased by MTU. (RFC 4960 Errata 3.12) */ pba += bytes_acked; -if (pba >= cwnd) { +if (pba > cwnd && flight_size < cwnd) + pba = cwnd; +if (pba >= cwnd && flight_size >= cwnd) { pba = pba - cwnd; cwnd += pmtu; }
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2.9.4
Marcelo Ricardo Leitner
maintainerPATCH 4/4 sctp: adjust ssthresh when transport is idle
In reply to Marcelo Ricardo LeitnerRFC 4960 Errata 3.27 identifies that ssthresh should be adjusted to cwnd because otherwise it could cause the transport to lock into congestion avoidance phase specially if ssthresh was previously reduced by some packet drop, leading to poor performance.
The Errata says to adjust ssthresh to cwnd only once, though the same goal is achieved by updating it every time we update cwnd too. The caveat is that we could take longer to get back up to speed but that should be compensated by the fact that we don't adjust on RTO basis (as RFC says) but based on Heartbeats, which are usually way longer.
See-also: https://tools.ietf.org/html/draft-ietf-tsvwg-rfc4960-errata-01#section-3.27
--- net/sctp/transport.c | 2 ++ 1 file changed, 2 insertions(+)
net/sctp/transport.c +2 -0
@@ -569,6 +569,8 @@ void sctp_transport_lower_cwnd(struct sctp_transport *transport,*/ transport->cwnd =max (transport->cwnd/2 ,4 *asoc->pathmtu); +/* RFC 4960 Errata 3.27.2: also adjust sshthresh */ + transport->ssthresh = transport->cwnd;break ; }
Signature · 1 line
2.9.4
David Miller
maintainerFrom: Marcelo Ricardo Leitner <[email protected]>
Date: Fri, 23 Jun 2017 19:58:39 -0300
Quoted 14 lines
This patchset contains fixes for 4 Errata topics from
https://tools.ietf.org/html/draft-ietf-tsvwg-rfc4960-errata-01
Namely, sections:
3.12. Order of Adjustments ofpartial_bytes_ackedand cwnd 3.22. Increase ofpartial_bytes_ackedin Congestion Avoidance 3.26. CWND Increase in Congestion Avoidance Phase 3.27. Refresh of cwnd and ssthresh after Idle PeriodTests performed with netperf using net namespaces, with drop rates at 0%, 0.5% and 1% by netem, IPv4 and IPv6, 10 runs for each combination. I couldn't spot differences on the stats. With and without these patches the results vary in a similar way in terms of throughput and retransmissions.
Quoted 5 lines
Tests with 20ms delay and 20ms delay + drops at 0.5% and 1% also had results in a similar way, no noticeable difference.
Looking at cwnd, it was possible to notice slightly lower values being used while still sustaining same throughput profile.
Series applied, thank you.