[107034] in North American Network Operators' Group
RE: IP Fragmentation (correction)
daemon@ATHENA.MIT.EDU (John Lee)
Wed Aug 20 14:37:36 2008
From: John Lee <john@internetassociatesllc.com>
To: John Lee <john@internetassociatesllc.com>, Glen Kent
<glen.kent@gmail.com>, OPS Gurus <nanog@merit.edu>
Date: Wed, 20 Aug 2008 13:34:55 -0500
Errors-To: nanog-bounces@nanog.org
Correction.
TTL needs to be set to sufficiently large number of hops to allow the packe=
t to get through the number of hops and the timers need to be set to allow =
the packet to transit the network and the low speed links before timing out=
and retransmitting the packet.
John (ISDN) Lee
________________________________________
From: John Lee [john@internetassociatesllc.com]
Sent: Wednesday, August 20, 2008 2:10 PM
To: Glen Kent; OPS Gurus
Subject: RE: IP Fragmentation
Glen,
With the v4 networks that I have worked on in the past, they did not do end=
to end MTU discovery before sending packets. The TTL had to be set appropr=
iately so that if you had low speed links, for example, the packet and resp=
onse would get through in time. On our DS3 (T3) and OC-3c packet links we d=
id 4k, 9k, and 16k packet sizes for video and file transfers.
At the other end of the spectrum are civilian and military systems with tac=
tical links, both wired and radio, with low bit rates and header compressio=
n on IP and TCP packets. Speeds range from 300 -9,600 bps, 16k, 32k, 64k an=
d Nx64k bps links that can do packet fragmentation and adding proprietary E=
CC codes for the radio links. Some systems strip the IP packet and use stan=
dard or non-standard link layer protocols across the mediums. Some of these=
systems are store and forward so that the computer/router that is connecte=
d to the low speed link will ack the packet for the high speed network conn=
ection and buffer it up until it can be sent on the lower speed system.
IMHO current IPv6 protocols ignore the lower end segment by specifying the =
lowest MTU for the circuit be the MTU for the entire circuit and not allow =
fragmentation. I do not see this as an efficient use of high speed network =
resources and local link management can handle fragmentation just fine.
John (ISDN) Lee
A slightly different History Channel.
________________________________________
From: Glen Kent [glen.kent@gmail.com]
Sent: Wednesday, August 20, 2008 12:13 PM
To: OPS Gurus
Subject: IP Fragmentation
Hi,
Do transit routers in the wild actually get to do IP fragmentation
these days? I was wondering if routers actually do it or not, because
the source usually discovers the path MTU and sends its data with the
least supported MTU. Is this true?
Even if this is, then this would break for multicast IP. The source
cannot determine which receivers would get interested in the traffic
and what capacities the links connecting them would support. So, a
source would send IP packets with some size, and theres a chance that
one of the routers *may* have to fragment those IP packets before
passing it on to the next router.
I would wager that the vendors and operators would want to avoid IP
fragmentation since thats usually done in SW (unless you've got a very
powerful ASIC or your box is NP based).
Thanks,
Glen
