Re: [TLS] draft-ietf-tls-rfc4347-bis-00.txt

[Eric Rescorla <ekr at networkresonance.com>]

At Tue, 14 Oct 2008 10:04:37 +0200,
Robin Seggelmann wrote:
> 
> Hello all,
> I was just checking the draft for changes relevant to DTLS over SCTP  
> and came across the following new paragraph:
> 
> - For DTLS over TCP or SCTP, which automatically fragment
>    and reassemble datagrams, the upper layer protocol
>    SHOULD be informed that the PMTU is effectively infinite.
> 
> What does 'effectively infinite' mean? TLS limits the message size to  
> 2^14 bytes, so shouldn't this limit also apply to DTLS? If the  
> message size really is arbitrary, doesn't this affect some cipher  
> algorithms? Or should the application then ignore the announced  
> 'infinite' PMTU and limit the message size anyway?


Yes, that's a fair point. OTOH, this is a maximum message size
which is related to, but not identical to, the PMTU. Operationally
the application needs to restrict itself to 2^14 and associated
limits. I'm tempted to simply add a parenthetical "(though of
course applications still MUST NOT write any record that exceeds
2^14 bytes)"

-Ekr


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