netdev - Re: TCP NewReno and single retransmit

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Date:	Mon, 3 Nov 2014 15:08:28 -0500
From:	Neal Cardwell <ncardwell@...gle.com>
To:	Marcelo Ricardo Leitner <mleitner@...hat.com>
Cc:	Yuchung Cheng <ycheng@...gle.com>, netdev <netdev@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: TCP NewReno and single retransmit

On Mon, Nov 3, 2014 at 11:38 AM, Marcelo Ricardo Leitner
<mleitner@...hat.com> wrote:
> On 31-10-2014 01:51, Yuchung Cheng wrote:
>>>>>           if (tp->snd_una == tp->high_seq && tcp_is_reno(tp)) {
>>>>>                   /* Hold old state until something *above* high_seq
>>>>>                    * is ACKed. For Reno it is MUST to prevent false
>>>>>                    * fast retransmits (RFC2582). SACK TCP is safe. */
>>
>> Or we can just remove this strange state-holding logic?
>>
>> I couldn't find such a "MUST" statement in RFC2582. RFC2582 section 3
>> step 5 suggests exiting the recovery procedure when an ACK acknowledges
>> the "recover" variable (== tp->high_seq - 1).
>>
>> Since we've called tcp_reset_reno_sack() before tcp_try_undo_recovery(),
>> I couldn't see how false fast retransmits can be triggered without
>> this state-holding.
>>
>> Any insights?
>
>
> Nice one, me neither. Neal?

Since there are no literal IETF-style "MUST" statements in RFC2582, I
think the "MUST" in the code here is expressing the design philosophy
behind the author. :-)

AFAICT the "Hold old state" code in tcp_try_undo_recovery() is a
pretty reasonable implementation of a mechanism specified in NewReno
RFCs to deal with the fundamental ambiguity between (1) dupacks for
packets the receiver received above a hole above snd_una, and (2)
dupacks for spurious retransmits below snd_una. I think the motivation
behind the "Hold old state" code is to stay in Recovery so that we do
not treat (2) dupacks as (1) dupacks.

I find RFC 2582 not very clear on this point, and the newest NewReno
RFC, RFC 6582, is also not so clear IMHO. But the RFC 3782 version of
NewReno - https://tools.ietf.org/html/rfc3782 - has a reasonable
discussion of this issue. There is a discussion in
https://tools.ietf.org/html/rfc3782#section-11
of this ambiguity:

   There are two separate scenarios in which the TCP sender could
   receive three duplicate acknowledgements acknowledging "recover" but
   no more than "recover".  One scenario would be that the data sender
   transmitted four packets with sequence numbers higher than "recover",
   that the first packet was dropped in the network, and the following
   three packets triggered three duplicate acknowledgements
   acknowledging "recover".  The second scenario would be that the
   sender unnecessarily retransmitted three packets below "recover", and
   that these three packets triggered three duplicate acknowledgements
   acknowledging "recover".  In the absence of SACK, the TCP sender is
   unable to distinguish between these two scenarios.

AFAICT RFC 3782 uses the term "recover" for the sequence number Linux
calls tp->high_seq. The specification in RFC 3782 Sec 3 -
https://tools.ietf.org/html/rfc3782#section-3 - of the criteria for
entering Fast Recovery say that we shouldn't go into a new recovery if
the cumulative ACK field doesn't cover more than high_seq/"recover":

   1)  Three duplicate ACKs:
       When the third duplicate ACK is received and the sender is not
       already in the Fast Recovery procedure, check to see if the
       Cumulative Acknowledgement field covers more than "recover".  If
       so, go to Step 1A.  Otherwise, go to Step 1B.

   1A) Invoking Fast Retransmit:
       ...
       In addition, record the highest sequence number transmitted in
       the variable "recover", and go to Step 2.

   1B) Not invoking Fast Retransmit:
       ...

The last few slides of this presentation by Sally Floyd also talk
about this point:

  http://www.icir.org/floyd/talks/newreno-Mar03.pdf

neal
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