Subject: [rohc] ROHC-TCP: Proposed changes to the packet formats

From: "Kristofer Sandlund \(LU/EAB\)" <[email-masked]>
Date: Fri, 20 Jan 2006 10:35:09 +0100

Hi all,

A long-overdue walkthrough of the "_replicate" formats in ROHC-TCP
found a few things that I think should be changed in the formats.

Here's a list of proposed changes:

- GRE: The r_flag, which is deprecated in RFC2784 should
not be handled as "irregular", since we have no idea of how future
versions might define this bit (and backward compatibility with 1701 
seems redundant?). Future versions of GRE _could_ define that bit so 
that it adds some extension field to the header, and therefore we 
should only compress packets where this is set to zero.
Same argument goes for the version field of GRE (change it to 
uncompressed_value instead of irregular)

- We decided long ago that replication formats should never use LSB encoding in a replicate format when the base field is changing due to
the risk of race conditions, and I noticed that we had some of these 
encodings in GRE, AH and ESP sequence numbers and therefore suggest 
dropping these and replace them with the irregular format for these 
sequence numbers.

- The IPv6 replicate format always replicates the flow_label, which
seems like quite a bad strategy, since this means that the flow
label must be identical if replication is to be possible.
Therefore, I suggest changing so that the flow label can be replicated
only if it is zero, otherwise flow label is is sent as irregular.

- The two replicate formats for IPv4 did not make much sense. The 
difference between them was only that the second format could send 
ip_id_behavior, df and ttl/hoplimit. Quick inspection showed that the
smaller format had 7 free bits from the discriminator, so I suggest
using 3 of these for behavior and df, and let one more bit be
an indicator flag if the ttl should be replicated or not and therefore
we can merge both these formats.
This is a pure optimization and even though I'm removing one format
it does not remove any capability from the replication.

So, unless I hear objections, these changes will go into the next
revision of the draft.

BR,
	Kristofer

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