[e2e] lifetime record of a UDP packet ?
mattmathis at google.com
Mon Mar 13 12:15:10 PDT 2017
The old FDDI interfaces would do this too. I once unplugged a cable, went
to lunch and then captured the ~hour old packet, just to prove a point (I
think this was in about 1990). I bet the actual record matches the time to
repair for some optical device...
Since you can construct any number that you have the patience for, it isn't
really a very interesting record. "Accidentally captured in a production
network" is a bit interesting.
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On Mon, Mar 13, 2017 at 8:19 AM, Jon Crowcroft <jon.crowcroft at cl.cam.ac.uk>
> cool - nearest i recall was a bug in SIMPs which lost buffers and they had
> a sort of inverse garbage collector that ran every 18 seconds (I think)
> which looked at the heap of unallocated stuff and decided if things had got
> there that shouldn't be and put them back in the right
> thread/task/processes' pool, so every now and then, instead of taking
> .72sec RTT (geo-synch orbit satellite up and down and up and down), took
> nearly 19 seconds....
> purist might say the TTL should have been decremented 18, but a meta-purist
> might say "ah, but a SIMP isn't an IP router, so its layer 2 only", and a
> post MPLS Person (20 years too late) would argue layer 2 devices should
> decrement TTL too, to prevent loops and to bound the max seg lifetime in
> the net so TCPs duplicate detection still operates correctly etc etc
> i suppose this was around 1981?
> On Mon, Mar 13, 2017 at 2:18 PM, Olav Kvittem <olav.kvittem at uninett.no>
> > Hi,
> > Not very serious - just for old-timer entertainment ;-)
> > I was doing a UDP measurement stream with timestamped and sequence
> > marked packets.
> > On analyzing the stream I found a packet arriving 6737 seconds late -
> > i.e. 1 hour 52 minutes.
> > Is that oldest packet seen on The Internet or not ?-)
> > Did it beat the trip time of the IP over Avian Carriers experiment ?-).
> > I had naively set my reordering window to 10 seconds..
> > It turned out that there was maintenance on an optical link on the path,
> > so the link was taken down
> > at the exact time of this event.
> > Rerouting happened in about 50ms of the link going down and that was
> > So where did the packet spend the time ?
> > An explanation might be that the packet was residing in an output
> > buffer in the router in front of the link
> > until it was up again and then sent on the link!
> > I rule out local clock errors on the measurement devices, as these are
> > NTP-controlled Linux systems and that it also happened to an
> > independent pair of measurement nodes thorough the same router.
> > So this looks like a router bug to me - ref RFC 1918 below.
> > Accordingly the maximum lifetime with initial TTL 64 should be 64
> > I would guess that the router should be flushing the queue at some point
> > during the event.
> > However the router is old and out of support so we are not reporting it.
> > best regards
> > Olav Kvittem
> > >From the Rude/Crude log :
> > ID=2 SEQ=1087254 SRC=18.104.22.168:3002 DST=22.214.171.124:10001
> > Tx=1488765681.242718 Rx=1488765681.254802 SIZE=64
> > ID=2 SEQ=413544 SRC=126.96.36.199:3002 DST=188.8.131.52:10001
> > Tx=1488758944.142720 Rx=1488765681.260987 SIZE=64
> > ID=2 SEQ=1087255 SRC=184.108.40.206:3002 DST=220.127.116.11:10001
> > Tx=1488765681.252718 Rx=1488765681.264785 SIZE=64
> > Tx, RX is Unix time for transmit and receive respectively
> > RFC 1918 :
> > 5.3.1 Time to Live (TTL)
> > The Time-to-Live (TTL) field of the IP header is defined to be a
> > timer limiting the lifetime of a datagram. It is an 8-bit field and
> > the units are seconds. Each router (or other module) that handles a
> > packet MUST decrement the TTL by at least one, even if the elapsed
> > time was much less than a second. Since this is very often the case,
> > the TTL is effectively a hop count limit on how far a datagram can
> > propagate through the Internet.
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