[AVT] question on draft-mattsson-srtp-store-and-forward-00.txt

[David McGrew <mcgrew at cisco.com>]

Hello,

Section 3.2.1 says that "The answering machine, acting as a streaming  
server when sending the data to the callee, will not use the exact  
same RTP headers on the outgoing SRTP traffic as was used on the  
incoming SRTP traffic. SRTP as specified in [RFC3711] will not work in  
this case, since parts of the RTP header is input to the encryption/ 
authentication transforms."

However, some parts of the RTP header would need to be stored along  
with the payloads.  Since the answering machine is storing ciphertext  
payloads in this case, it would need to store and play back the RTP  
timestamp field, since otherwise the answering machine could not  
effectively determine what values to put into this field.  The same  
considerations would probably apply to the PT field.

Since some parts of the header necessarily need to be stored as part  
of the solution, why not store the entire header, then replay the SRTP  
session from the answering machine exactly as it was sent to it?    
This is a simple solution that avoids the need for RTP endpoints to  
implement new SRTP transforms and options.  It can be implemented by  
the answering machine's RTP/SRTP stack, without any effect on  
signaling or any need for explicit support in any other RTP/SRTP stack.

best regards,

David


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