Subject: [tcpm] TCP tail probe and the initial loss behavior

From: Yuchung Cheng <[email-masked]>
Date: Wed, 6 Nov 2013 13:48:57 -0800

We have new data regarding the question raised in the previous two
meetings: "is tail loss probe useful b/c IW10 is causing more (tail)
losses".

We got some data on TCP initial loss behavior with IW10. There were
15,269 outbound Google server connection traces taken from all over
the world. Bottleneck bandwidths ranged from 64kbps to 40Mbps with a
median of ~1Mbps.  All the connections used IW10. For each of them, we
recorded the ending sequence number, packet number and send time of
the first lost packet (the first retransmitted packet of the
connection) and the repair time (time from initial send until
retransmit acked).

980 connections (6.4%) had one or more losses in the IW10 send.
"lossdist1.pdf" is a histogram of % losses by position in initial
window ("none" means no loss in the first 10 packets sent). Claims
that IW10 "increases the loss rate" are not supported by this data:
The average loss rate for the first 10 packets is 0.64%. The average
loss rate for the last 5 packets of the window is only 0.44% compared
to 0.84% for the first 5 packets (in fact, the loss rate for the first
packet is as large as packets 6 to 10 combined).  This is easier to
see on a plot of the cumulative loss (lossdist2.pdf): the black line
is a linear fit to the first five points and the blue to the last five
points. The lines slopes show that the loss rate consistently
increases faster for the first 5 packets of the window compared to the
last 5.

"lossdist3.pdf" shows the distribution of packet vs. cumulative % of
connections with a first loss at or before that packet. The blue line
is a fit of cum% to packet^0.25. The quality of the fit over the
entire range (and the fact that there's no discontinuity between the
portion of the data that lies within the initial window vs the portion
outside it) suggests that the single connection loss probabilities
have nothing to do with IW (at least for IW <= 10).

We didn't graph loss recovery time since it was simple: 78% of the
initial window losses (761 of 980) were repaired in 1 RTT and the
remaining 22% were repaired in 2 RTT. None of them took a timeout; all
were repaired by Fast Retransmit. (This is in contrast to previous IW4
measurements where 20% of the losses required a timeout to repair).

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From: Anna Brunstrom <[email-masked]>
Date: Thu, 07 Nov 2013 09:14:23 +0100

Hi Yuchung,

Thanks for posting the data, that is very interesting!

If I understand it correctly all percentages are calculated in relation 
to the total number of connection traces? If so that makes it a bit 
difficult to compare the numbers. How many of the connections had 10 or 
more packets?

Could you perhaps create an equivalent of the lossdist1 graph that shows 
the % of losses in position x in relation to the number of connections 
that had a length of x or more packets?

Thanks,
Anna

On 2013-11-06 22:48, Yuchung Cheng wrote:
> _______________________________________________
> tcpm mailing list
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---

From: Yuchung Cheng <[email-masked]>
Date: Sat, 9 Nov 2013 09:13:27 -0800

On Thu, Nov 7, 2013 at 12:14 AM, Anna Brunstrom <[email-masked]> wrote:
> If I understand it correctly all percentages are calculated in relation to
> the total number of connection traces? If so that makes it a bit difficult
yes
> to compare the numbers. How many of the connections had 10 or more packets?
why would that makes it difficult to compare the "initial" loss rate?

all have 10 or more packets so they are relevant to the question we
are trying to answer.

>
> Could you perhaps create an equivalent of the lossdist1 graph that shows the
> % of losses in position x in relation to the number of connections that had
> a length of x or more packets?
>
> Thanks,
> Anna
>
>
>

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From: Anna Brunstrom <[email-masked]>
Date: Sat, 09 Nov 2013 19:20:01 +0100

Hi Yuchung,

It was not clear to me from your mail that all 15,269 connections were 
10 packets or longer. I understood it as a random selection of 
connections with IW10 enabled, in which case I guess many would have 
been shorter than 10 packets.

As all 15,269 connections were 10 packets or longer the numbers are of 
course comparable.

Thanks,
Anna

---

From: Yuchung Cheng <[email-masked]>
Date: Sat, 9 Nov 2013 10:25:56 -0800

On Sat, Nov 9, 2013 at 10:20 AM, Anna Brunstrom <[email-masked]> wrote:
> Hi Yuchung,
>
> It was not clear to me from your mail that all 15,269 connections were 10
> packets or longer. I understood it as a random selection of connections with
> IW10 enabled, in which case I guess many would have been shorter than 10
> packets.
I see. Sorry for not being clear about that in my first email.

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