Subject: [AVT] Packet fragmentation

From: [email-masked]
Date: Fri, 8 Mar 2002 07:34:03 -0800 (PST)

Hello experts,

Can you point me to resources (or other mailing lists that I should aske this
question) that talk about performance degradation on the network for packets
that undergo fragmentation.

In my naive understanding, in a scenario where an application on a machine
sends a UDP packet that is larger than the network MTU size, the packet 
will be fragmented at the sending machine. Assuming that the network MTU
in the middle between the sender and the receiver does not change, the only
other component affected by the fragmentation is the receiver. The network
components should ideally be just passing the fragments along with no extra
overhead.

I understand the loss of a single fragment results in the loss of the entire
packet. However, I have heard that such traffic suffers from other performance
overheads in the other components in the network. Is it because:
- in a typical network, MTU sizes change a lot in different segments?
- there are many legacy routers that are buggy?
- NATs?

Any other reasons, pointers to existing work would be really appreciated.

Thanks.
Ram

_______________________________________________
Audio/Video Transport Working Group
[email-masked]
https://www1.ietf.org/mailman/listinfo/avt

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From: Marshall Eubanks <[email-masked]>
Date: Fri, 08 Mar 2002 12:36:38 -0500

Hello;

    Kordale;

> components should ideally be just passing the fragments along with no extra

This is what it _supposed_ to happen. Some networks will just drop 
fragmented packets. Some routers will take a fragmented packet out
of the forwarding path and turn them over to the CPU to process, thus
burdening the router and significantly slowing processing.

TCP can find out what the MTU is; UDP cannot in IPv4. In my humble 
opinion, multicast packet sizes should be kept below 1500 bytes to avoid 
transport layer fragmentation. (I.e., fragmentation should be done at 
the application layer.)

> - there are many legacy routers that are buggy?

I do not think that NAT's are an issue here.

Regards
Marshall Eubanks

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From: Ram Kordale <[email-masked]>
Date: Fri, 08 Mar 2002 18:24:42 -0800

Hello Marshall,

Thank you very much for your response.

The behaviour of the routers that you talk about - are they due to bugs in 
current or legacy routers?

Also, I did not understand your note on the effect of UDP vs TCP on 
fragmentation.

Ram

At 12:36 PM 3/8/2002 -0500, Marshall Eubanks wrote:
>transport layer fragmentation. (I.e., fragmentation should be done at the 
>application layer.)
>
>