Re: tcp: rearm RTO timer does not comply with RFC6298

On Thu, Jan 21, 2021 at 2:59 AM Yuchung Cheng [off-list ref] wrote:

On Wed, Jan 20, 2021 at 6:59 AM Neal Cardwell [off-list ref] wrote:

quoted

On Wed, Jan 20, 2021 at 5:50 AM Pengcheng Yang [off-list ref] wrote:

quoted

hi,

I have a doubt about tcp_rearm_rto().

Early TCP always rearm the RTO timer to NOW+RTO when it receives
an ACK that acknowledges new data.

Referring to RFC6298 SECTION 5.3: "When an ACK is received that
acknowledges new data, restart the retransmission timer so that
it will expire after RTO seconds (for the current value of RTO)."

After ER and TLP, we rearm the RTO timer to *tstamp_of_head+RTO*
when switching from ER/TLP/RACK to original RTO in tcp_rearm_rto(),
in this case the RTO timer is triggered earlier than described in
RFC6298, otherwise the same.

Is this planned? Or can we always rearm the RTO timer to
tstamp_of_head+RTO?

Thanks.

This is a good question. As far as I can tell, this difference in
behavior would only come into play in a few corner cases, like:

(1) The TLP timer fires and the connection is unable to transmit a TLP
probe packet. This could happen due to memory allocation failure  or
the local qdisc being full.

(2) The RACK reorder timer fires but the connection does not take the
normal course of action and mark some packets lost and retransmit at
least one of them. I'm not sure how this would happen. Maybe someone
can think of a case.

Yes, and it also happens when an ACK (a cumulative ACK covered out-of-order data)
is received that makes ca_state change from DISORDER to OPEN, by calling tcp_set_xmit_timer().
Because TLP is not triggered under DISORDER and tcp_rearm_rto() is called before the
ca_state changes.

quoted

My sense would be that given how relatively rare (1)/(2) are, it is
probably not worth changing the current behavior, given that it seems
it would require extra state (an extra u32 snd_una_advanced_tstamp? )
to save the time at which snd_una advanced (a cumulative ACK covered
some data) in order to rearm the RTO timer for snd_una_advanced_tstamp
+ rto.

also there's an experimental proposal
https://tools.ietf.org/html/rfc7765

so Linux actually implements that in a limited way that only applies
in specific scenarios.

quoted

neal

Thank you for answering my questions.