Subject: [rohc] Does ROHC break PMTU?

From: Gorry Fairhurst <[email-masked]>
Date: Tue, 09 Nov 2004 19:38:02 -0500

This issue was raised in PMTUD WG by Magnus Westerlund, and I've expanded it
with a few thoughts below:

1) ROHC by its nature can produce a variable-sized network-layer packet
overhead, given a fixed sized link frame (or segmentation/reassembly payload
if you are using transparent fragmentation), this appears to result in  a
variable size Link MTU. Carsten pointed out that ROHC can expand headers
also in some cases, not just compress them.

2) Path MTU discovery (typically with TCP, but we could talk about UDP, etc)
probes for bandwidth. If it probes over a path with varying MTU the effects
are likely to have undesirable side-effects with PMTUD.

3) Is this an issue?
This depends on how (where) you perform Link MTU checks when a link (header)
compression scheme is used. Two options come to mind:

* If the Link were to check the Link MTU of each packet to be forwarded
based on the actual limit of the compressed size (see 1 above), then the
link MTU could vary from packet to packet within a flow.

* If the Link instead checked the minimum uncompressed size that was
guaranteed, and represented this as the Link MTU, there would be no problem.

I vote for the second one - any thoughts from others on the list, about what
should be done (or what is done)?

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Is this written anywhere in a ROHC document?

- My recollection was only the TCP Behaviour document talked about MTU, and
I don't recall this provided any guidance on this.

Gorry 

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From: Carsten Bormann <[email-masked]>
Date: Tue, 9 Nov 2004 23:23:24 -0500

> guaranteed, and represented this as the Link MTU, there would be no 

I think the fun issue here comes up with tunnels, where ROHC itself 
would have to do PMTU discovery to do 2.
Most non-tunnel ROHC links have link MTUs in excess of the "Internet 
MTU" (1500--).
(802 will provide new challenges, but still will have a predictable 
link MTU.)
Note also that ROHC does have a segmentation protocol, so if all else 
fails, a packet can be segmented to avoid an ICMP too big.

Gruesse, Carsten