Subject: [tcpm] Tail loss probe discussions @IETF-84

From: Nandita Dukkipati <[email-masked]>
Date: Fri, 7 Sep 2012 12:32:59 -0700

Michael's email reminded me to send out my summary of discussions
related to TLP at last IETF. Because of tight TCPM agenda, all
questions were offline. Folks can correct me if I am recounting the
discussions incorrectly ;)

[Richard Scheffenegger]

* For TLP loss plugging, if timestamps are available, could they be
used to check which transmission plugged the hole?
    - That's correct. Draft should include this.

* "rescue retransmission" introduced in 3517bis deals with loss events
such as AL*SL*. TLP covers wider range of circumstances such as AL*.
How often is "AL*" really a AL*S*L* that can be recovered by regular
SACK recovery. Reference to 3517bis and rescue retransmissions would
be good.
  - We implemented rescue retransmission on our Web servers, but
didn't see much performance improvement. When the last segment is
lost, it is more likely that a number of contiguous segments preceding
the segment are also lost, i.e. AL* is common. Timeouts that occur in
the fast recovery are rare.

* Wouldn't it be worthwhile to look into an adaptive re-ACK scheme,
where client retransmits the last ACK when for a certain period no
additional data segment was received?
  - But that in itself is not sufficient to trigger fast recovery at
sender. The receiver has no SACK blocks to report in the event of tail
loss. Just receiving duplicate ACKs without SACK blocks is not enough
to trigger fast recovery (at least true for Linux SACK).

[Anantha Ramaiah]

* How many consecutive probes do you send?
    - Draft specifies at most two back-to-back probes.
* Can you take help from the applications to indicate end of
transaction and somehow use that for smarter tail loss recovery?
* Wanted to see a description/diagram of relation between PTO (probe
timeout) and RTO.

[Michael Welzl and tcpm chairs]

* There is another draft in tcpm related to RTO -
http://www.ietf.org/id/draft-hurtig-tcpm-rtorestart-02.txt
Would be great to understand the relation between RTO-restart and TLP
draft. Both draft authors think these I-Ds are orthogonal; more
discussion needed on this front.

[Ilpo Järvinen]
Linux specific.
* Discussed about reviving the idea of transmitting just 1-byte probe.
As per Ilpo it may be a Linux bug that a SACK of one segment triggers
fast recovery but that a SACK of 1-byte disn't. Potentially if we fix
this bug, we will not require any extra loss detection algorithm.
* Apparently there was something like TLP by the thin-stream folks.
Even posted to net-dev, but patch was confusing to review. Need to
look this up.
* We discussed on whether TLP probes should increment retrans_out. If
we do that then sender can no longer be in Open state, so probably
best not to increment retrans_out.

Plan to address these questions in the next rev. of I-D.

Nandita