Hi all,
I know this is a bit late but I didn't have time earlier to take look at
this draft.
Given that this RFC to be is standards track and RECOMMENDED to replace
current DupAck-based loss detection, it is important that the spec is
clear on its advice to those implementing it. Current text seems to
lack important advice w.r.t congestion control, and even though
the spec tries to decouple loss detection from congestion control
and does not intend to modify existing standard congestion control
some of the examples advice incorrect congestion control actions.
Therefore, I think it is worth to correct the mistakes and take
yet another look at a few implications of this specification.
Sec. 3.4 (and elsewhere when discussing recovering a dropped
retransmission):
It is very useful that RACK-TLP allows for recovering dropped rexmits.
However, it seems that the spec ignores the fact that loss of a
retransmission is a loss in a successive window that requires reacting
to congestion twice as per RFC 5681. This advice must be included in
the specification because with RACK-TLP recovery of dropped rexmit
takes place during the fast recovery which is very different
from the other standard algorithms and therefore easy to miss
when implementing this spec.
Sec 9.3:
In Section 9.3 it is stated that the only modification to the existing
congestion control algorithms is that one outstanding loss probe
can be sent even if the congestion window is fully used. This is
fine, but the spec lacks the advice that if a new data segment is sent
this extra segment MUST NOT be included when calculating the new value
of ssthresh as per the equation (4) of RFC 5681. Such segment is an
extra segment not allowed by cwnd, so it must be excluded from
FlightSize, if the TLP probe detects loss or if there is no ack
and RTO is needed to trigger loss recovery.
In these cases the temporary over-commit is not accounted for as DupAck
does not decrease FlightSize and in case of an RTO the next ACK comes too
late. This is similar to the rule in RFC 5681 and RFC 6675 that prohibits
including the segments transmitted via Limitid Transmit in the
calculation of ssthresh.
In Section 9.3 a few example scenarios are used to illustriate the
intended operation of RACK-TLP.
In the first example a sender has a congestion window (cwnd) of 20
segments on a SACK-enabled connection. It sends 10 data segments
and all of them are lost.
The text claims that without RACK-TLP the ending cwnd would be 4 segments
due to congestion window validation. This is incorrect.
As per RFC 7661 the sender MUST exit the non-validated phase upon an
RTO. Therefore the ending cwnd would be 5 segments (or 5 1/2 segments if
the TCP sender uses the equation (4) of RFC 5681).
The operation with RACK-TLP would inevitably result in congestion
collapse if RACK-TLP behaves as described in the example because
it restores the previous cwnd of 10 segments after the fast recovery
and would not react to congestion at all! I think this is not the
intended behavior by this spec but a mistake in the example.
The ssthresh calculated in the beginning of loss recovery should
be 5 segments as per RFC 6675 (and RFC 5681).
Furthermore, it seems that this example with RACK-TLP refers to using
PRR_SSRB which effectively implements regular slow start in this
case(?). From congestion control point of view this is correct because
the entire flight of data as well as ack clock was lost.
However, as correctly discussed in Sec 2, congestion window must be reset
to 1 MSS when an entire flight of data is and Ack clock is lost. But how
can an implementor know what to do if she/he is not implementing the
experimental PRR algrorithm? This spec articulates specifying an
alternative for DupAck counting, indicating that TLP is used to trigger
Fast Retransmit & Fast Recovery only, not a loss recovery in slow start.
This means that without an additional advise an implementation of this
spec would just halve the cwnd and ssthresh and send a potentially very
large burst of segments in the beginning of the Fast Recovery because
there is no ack clock. So, this spec begs for an advise (MUST) when to
slow start and reset cwnd and when not, or at least a discussion of
this problem and some sort of advise what to do and what to avoid.
And, maybe a recommendation to implement it with PRR?
Another question relates to the use of TLP and adjusting timer(s) upon
timeout. In the same example discussed above, it is clear that PTO
that fires TLP is just a more aggressive retransmit timer with
an alternative data segment to (re)transmit.
Therefore, as per RFC 2914 (BCP 41), Sec 9.1, when PTO expires, it is in
effect a retransmission timout and the timer(s) must be backed-off.
This is not adviced in this specification. Whether it is the TCP RTO
or PTO that should be backed-off is an open question. Otherwise,
if the congestion is persistent and further transmission are also lost,
RACK-TLP would not react to congestion properly but would keep
retransmitting with "constant" timer value because new RTT estimate
cannot be obtained.
On a buffer bloated and heavily congested bottleneck this would easily
result in sending at least one unnecessary retransmission per one
delivered segment which is not advisable (e.g., when there are a huge
number of applications sharing a constrained bottleneck and these
applications are sending only one (or a few) segments and then
waiting for an reply from the peer before sending another request).
Additional notes:
Sec 2.2:
Example 2:
"Lost retransmissions cause a resort to RTO recovery, since
DUPACK-counting does not detect the loss of the retransmissions.
Then the slow start after RTO recovery could cause burst losses
again that severely degrades performance [POLICER16]."
RTO reovery is done in slow start. The last sentence is confusing as
there is no (new) slow-start after RTO recovery (or more precisely
slow start continues until cwnd > ssthresh). Do you mean: if/when slow
start still continues after RTO Recovery has repaired lost segments,
it may cause burst losses again?
Example 3:
"If the reordering degree is beyond DupThresh, the DUPACK-
counting can cause a spurious fast recovery and unnecessary
congestion window reduction. To mitigate the issue, [RFC4653]
adjusts DupThresh to half of the inflight size to tolerate the
higher degree of reordering. However if more than half of the
inflight is lost, then the sender has to resort to RTO recovery."
This seems to be somewhat incorrect description of TCP-NCR specified in
RFC 4653. TCP-NCR uses Extended Limited Transmit that keeps on sending
new data segments on DupAcks that makes it likely to avoid an RTO in
the given example scenario, if not too many of the the new data
segments triggered by Extended Limited Transmit are lost.
Sec. 3.5:
"For example, consider a simple case where one
segment was sent with an RTO of 1 second, and then the application
writes more data, causing a second and third segment to be sent right
before the RTO of the first segment expires. Suppose only the first
segment is lost. Without RACK, upon RTO expiration the sender marks
all three segments as lost and retransmits the first segment. When
the sender receives the ACK that selectively acknowledges the second
segment, the sender spuriously retransmits the third segment."
This seems incorrect. When the sender receives the ACK that selectively
acknowledges the second segment, it is a DupAck as per RFC 6675 and does
not increase cwnd and cwnd remains as 1 MSS and pipe is 1 MSS. So, the
rexmit of the third segment is not allowad until the cumulative ACK of
the first segment arrives.
Best regards,
/Markku
On Mon, 16 Nov 2020, The IESG wrote:
The IESG has received a request from the TCP Maintenance and Minor Extensions
WG (tcpm) to consider the following document: - 'The RACK-TLP loss detection
algorithm for TCP'
<draft-ietf-tcpm-rack-13.txt> as Proposed Standard
The IESG plans to make a decision in the next few weeks, and solicits final
comments on this action. Please send substantive comments to the
last-call@xxxxxxxx mailing lists by 2020-11-30. Exceptionally, comments may
be sent to iesg@xxxxxxxx instead. In either case, please retain the beginning
of the Subject line to allow automated sorting.
Abstract
This document presents the RACK-TLP loss detection algorithm for TCP.
RACK-TLP uses per-segment transmit timestamps and selective
acknowledgements (SACK) and has two parts: RACK ("Recent
ACKnowledgment") starts fast recovery quickly using time-based
inferences derived from ACK feedback. TLP ("Tail Loss Probe")
leverages RACK and sends a probe packet to trigger ACK feedback to
avoid retransmission timeout (RTO) events. Compared to the widely
used DUPACK threshold approach, RACK-TLP detects losses more
efficiently when there are application-limited flights of data, lost
retransmissions, or data packet reordering events. It is intended to
be an alternative to the DUPACK threshold approach.
The file can be obtained via
https://datatracker.ietf.org/doc/draft-ietf-tcpm-rack/
No IPR declarations have been submitted directly on this I-D.
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