Re: [tcpm] Last Call: <draft-ietf-tcpm-rack-13.txt> (The RACK-TLPloss detection algorithm for TCP) to Proposed Standard

[Markku Kojo <[e-mail]>]

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
> [e-mail] mailing lists by 2020-11-30. Exceptionally, comments may
> be sent to [e-mail] 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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