[cisco-voip] srtp packets

[daniel at ohnesorge.me]

Section 3.1 of RFC3711 states:

    The "Encrypted Portion" of an SRTP packet consists of the encryption
    of the RTP payload (including RTP padding when present) of the
    equivalent RTP packet.  The Encrypted Portion MAY be the exact size
    of the plaintext or MAY be larger.  Figure 1 shows the RTP payload
    including any possible padding for RTP [RFC3550].

    None of the pre-defined encryption transforms uses any padding; for
    these, the RTP and SRTP payload sizes match exactly.  New transforms
    added to SRTP (following Section 6) may require padding, and may
    hence produce larger payloads.  RTP provides its own padding format
    (as seen in Fig. 1), which due to the padding indicator in the RTP
    header has merits in terms of compactness relative to paddings using
    prefix-free codes.  This RTP padding SHALL be the default method for
    transforms requiring padding.  Transforms MAY specify other padding
    methods, and MUST then specify the amount, format, and processing of
    their padding.  It is important to note that encryption transforms
    that use padding are vulnerable to subtle attacks, especially when
    message authentication is not used [V02].  Each specification for a
    new encryption transform needs to carefully consider and describe the
    security implications of the padding that it uses.  Message
    authentication codes define their own padding, so this default does
    not apply to authentication transforms.

Hope that helps.

On 2017-02-16 14:58, cisco.voip wrote:

> All, can somebody tell me the typical srtp packet size and format vs 
> rtp packet size and format of a g711 encoded call.
> I cannot find these number anywhere.
> Thanks
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