[e2e] RES: What's wrong with this picture?

Mon Sep 7 07:37:48 PDT 2009

Hi,

Looking at the first ping command, it seems  that the first packet was lost
(icmp_seq=0), as well as a series of packets after the 15th, before David
decided to ^C the ping execution. The 14 packets that went through
experienced a huge delay, really hard to explain.

So, my first guess on a "wrong thing" - no network should hold a packet for
long 8 or 9 seconds, and yet  deliver it to somewhere. No buffer should be
big enough to hold packets so long .  But I still cannot imagine where the
packets where sitting for such a long time.

Looking at the traceroute:  the measured times do not grow monotonously, and
show hi variance. I would say that the delay came from the first hop
(172.26.248.2?).

A new ping, now targeted at 172.26.248.2, shows a decreasing round trip
delay - a buffer getting empty? - and still shows 1 packet loss.

It seems also that David is behind a NAT, since 172.26.248.2 is a RFC1918
reserved address.

In any case, such long delays cannot be good for stable functioning of TCP
congestion control.

--a.l.g.

De: end2end-interest-bounces at postel.org
[mailto:end2end-interest-bounces at postel.org] Em nome de David P. Reed
Enviada em: domingo, 6 de setembro de 2009 22:00
Para: end2end-interest list
Assunto: [e2e] What's wrong with this picture?

For those who have some idea of how TCP does congestion control, I ask
"what's wrong with this picture?"  And perhaps those who know someone
responsible at the Internet Access Provider involved, perhaps we could
organize some consulting help...

(Hint: the problem relates to a question, "why are there no lost IP
datagrams?", and a second hint is that the ping time this morning was about
193 milliseconds.)

Van Jacobsen, Scott Shenker, and Sally Floyd are not allowed to answer the
question.  (they used to get funding from the IAP involved, but apparently
that company does not listen to them).

$ ping lcs.mit.edu
PING lcs.mit.edu (128.30.2.121) 56(84) bytes of data.
64 bytes from zermatt.csail.mit.edu (128.30.2.121): icmp_seq=1 ttl=44
time=6330 ms
64 bytes from zermatt.csail.mit.edu (128.30.2.121): icmp_seq=2 ttl=44
time=6005 ms
64 bytes from zermatt.csail.mit.edu (128.30.2.121): icmp_seq=3 ttl=44
time=8509 ms
64 bytes from zermatt.csail.mit.edu (128.30.2.121): icmp_seq=4 ttl=44
time=9310 ms
64 bytes from zermatt.csail.mit.edu (128.30.2.121): icmp_seq=5 ttl=44
time=8586 ms
64 bytes from zermatt.csail.mit.edu (128.30.2.121): icmp_seq=6 ttl=44
time=7765 ms
64 bytes from zermatt.csail.mit.edu (128.30.2.121): icmp_seq=7 ttl=44
time=7168 ms
64 bytes from zermatt.csail.mit.edu (128.30.2.121): icmp_seq=8 ttl=44
time=10261 ms
64 bytes from zermatt.csail.mit.edu (128.30.2.121): icmp_seq=9 ttl=44
time=10624 ms
64 bytes from zermatt.csail.mit.edu (128.30.2.121): icmp_seq=10 ttl=44
time=9625 ms
64 bytes from zermatt.csail.mit.edu (128.30.2.121): icmp_seq=11 ttl=44
time=9725 ms
64 bytes from zermatt.csail.mit.edu (128.30.2.121): icmp_seq=12 ttl=44
time=8725 ms
64 bytes from zermatt.csail.mit.edu (128.30.2.121): icmp_seq=13 ttl=44
time=9306 ms
64 bytes from zermatt.csail.mit.edu (128.30.2.121): icmp_seq=14 ttl=44
time=8306 ms
^C
--- lcs.mit.edu ping statistics ---
24 packets transmitted, 14 received, 41% packet loss, time 33174ms
rtt min/avg/max/mdev = 6005.237/8589.365/10624.776/1334.140 ms, pipe 11
$ traceroute lcs.mit.edu
traceroute to lcs.mit.edu (128.30.2.121), 30 hops max, 60 byte packets
 1  * * *
 2  172.26.248.2 (172.26.248.2)  693.585 ms  693.415 ms  712.282 ms
 3  * * *
 4  172.16.192.18 (172.16.192.18)  712.700 ms  1356.680 ms  1359.469 ms
 5  12.88.7.205 (12.88.7.205)  1361.306 ms  673.642 ms  673.541 ms
 6  cr84.cgcil.ip.att.net (12.122.152.134)  673.442 ms  673.371 ms  673.742
ms
 7  cr2.cgcil.ip.att.net (12.123.7.250)  655.126 ms  654.186 ms  554.690 ms
 8  * * ggr2.cgcil.ip.att.net (12.122.132.133)  912.385 ms
 9  192.205.33.210 (192.205.33.210)  909.925 ms  911.335 ms  911.204 ms
10  ae-31-53.ebr1.Chicago1.Level3.net (4.68.101.94)  569.740 ms  569.605 ms
907.409 ms
11  ae-1-5.bar1.Boston1.Level3.net (4.69.140.93)  369.680 ms  344.495 ms
345.252 ms
12  ae-7-7.car1.Boston1.Level3.net (4.69.132.241)  355.645 ms  641.866 ms
641.367 ms
13  MASSACHUSET.car1.Boston1.Level3.net (4.53.48.98)  636.598 ms  636.797 ms
635.755 ms
14  B24-RTR-2-BACKBONE-2.MIT.EDU (18.168.1.23)  635.766 ms  634.794 ms
866.430 ms
15  MITNET.TRANTOR.CSAIL.MIT.EDU (18.4.7.65)  758.305 ms  822.244 ms
821.202 ms
16  trantor.kalgan.csail.mit.edu (128.30.0.246)  833.699 ms  1055.548 ms
1116.813 ms
17  zermatt.csail.mit.edu (128.30.2.121)  1114.838 ms  539.951 ms  620.681
ms
[david at whimsy ~]$ ping 172.26.248.2
PING 172.26.248.2 (172.26.248.2) 56(84) bytes of data.
64 bytes from 172.26.248.2: icmp_seq=1 ttl=254 time=1859 ms
64 bytes from 172.26.248.2: icmp_seq=2 ttl=254 time=1363 ms
64 bytes from 172.26.248.2: icmp_seq=3 ttl=254 time=1322 ms
64 bytes from 172.26.248.2: icmp_seq=4 ttl=254 time=1657 ms
64 bytes from 172.26.248.2: icmp_seq=5 ttl=254 time=1725 ms
64 bytes from 172.26.248.2: icmp_seq=6 ttl=254 time=1740 ms
64 bytes from 172.26.248.2: icmp_seq=7 ttl=254 time=1838 ms
64 bytes from 172.26.248.2: icmp_seq=8 ttl=254 time=1738 ms
64 bytes from 172.26.248.2: icmp_seq=9 ttl=254 time=1517 ms
64 bytes from 172.26.248.2: icmp_seq=10 ttl=254 time=978 ms
64 bytes from 172.26.248.2: icmp_seq=11 ttl=254 time=715 ms
64 bytes from 172.26.248.2: icmp_seq=12 ttl=254 time=678 ms
64 bytes from 172.26.248.2: icmp_seq=13 ttl=254 time=638 ms
64 bytes from 172.26.248.2: icmp_seq=14 ttl=254 time=761 ms
^C
--- 172.26.248.2 ping statistics ---
15 packets transmitted, 14 received, 6% packet loss, time 14322ms
rtt min/avg/max/mdev = 638.651/1324.002/1859.725/455.200 ms, pipe 2
$

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