Subject: [AVT] SRTP index sync. for initial key derivation

From: "John W. Floroiu" <[email-masked]>
Date: Wed, 28 Nov 2007 17:13:44 +0100

Hello,

According to RFC3711 (section 4.3.1):

    Key derivation SHALL be defined as follows in terms of <label>, an
    8-bit constant (see below), master_salt and key_derivation_rate, as
    determined in the cryptographic context, and index, the packet index
    (i.e., the 48-bit ROC || SEQ for SRTP):

    *  Let r = index DIV key_derivation_rate (with DIV as defined above).

I presume for the initial key derivation, the "index" is the SRTP index 
of the first packet in the SRTP stream.

The question is *how can the SRTP endpoints synchronize their "index" 
values* if the first packet in the SRTP stream gets lost*?*

One can think of the key management to solve this problem, however 
according to section 8:

    For initialization, an interoperable SRTP implementation SHOULD be
    given the SSRC and MAY be given the initial RTP sequence number for
    the RTP stream by key management (thus, key management has a
    dependency on RTP operational parameters).  Sending the RTP sequence
    number in the key management may be useful e.g., when the initial
    sequence number is close to wrapping (to avoid synchronization
    problems), and to communicate the current sequence number to a
    joining endpoint (to properly initialize its replay list).

which basically tells that it is not required the key management to 
exchange the initial sequence numbers.

Any insight would be greatly appreciated.

Regards,
John

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