Subject: [TLS] compression in DTLS

From: Rob Dugal <[email-masked]>
Date: Tue, 22 Nov 2005 09:20:48 -0500

How is record compression supported in DTLS ?

I don't have a lot of experience with compression but I have integrated 
RFC3749 and RFC3943 into our SSL product.  Based on what I've seen, if 
records are delivered out of sequence or lost then I don't think it will 
be possible to decompress records unless the decompressor assumes that the 
compression history has been reset for each record. This and the fact that 
datagrams are going to be quite small will probably mean that most records
will not compress very much. 

Also, in TLS a compression which results in expansion of the input is ok 
so long as the expansion is no more than 1024 bytes. 
I've seen this occur with compression of random data.
In DTLS this means the compressed data could be much larger than the MTU 
so what does DTLS do?
For handshake messages it could refragment the content into smaller pieces 
and try again until the output is < MTU.
Of course most handshake mesages will not be compressed unless the session 
is re-negotiated.
But what about application data? You cannot re-fragment application data.

-----------------------------------------------
Robert Dugal
Member of Development Group
Certicom Corp.
EMAIL: [email-masked]
PHONE: (905) 501-3848
FAX  : (905) 507-4230
WEBSITE: www.certicom.com

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