Subject: [AVT] Questions on TLS partial encryption applied to RTP and RTCP security encapsulation

From: Lakshminath Dondeti <[email-masked]>
Date: Mon, 06 Mar 2006 02:44:07 +0900

Hi Eric,

After reading the partial encryption spec more closely (this is the 
last one in this suite of drafts for me I think :-)), I have more 
questions on using that technique with DTLS for RTP and RTCP security 
encapsulation.   I guess the questions apply to partial encryption in general.

The partial encryption I-D notes that
"
The InitialClearBytes value contains the number of bytes which will
    be in the clear for each application_data record.  This value will
    obtain for the entire life of this association.
"

That seems to make it difficult for variable length headers to be in 
the clear.  I don't know much about RTP and RTCP, but RTP does allow 
variable number of CSRCs and also optional extension headers.

SRTCP might be ok, although you may need to consider adding an 
explicit packet index as in 3711 in draft-tschofenig-avt-rtp-dtls-00.

Another general question I have is about long RTP/RTCP (or any 
application) packets (e.g., Video) and whether partial encryption 
puts restrictions on packet lengths defined elsewhere (RTCP packets 
can be ~ 2^16 32-bit words in length, although I am not sure whether 
that is ever the case in practice.  My recollection is there is 2^14 
length restriction on DTLS records.  My numbers could be off though).

I am still trying to piece this all together and looking for 
clarifications.  Thanks in advance.

best regards,
Lakshminath 

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