Prioritizing Coder List In Sdp Offer; Srtp-Rtp And Srtp-Srtp Transcoding - AudioCodes Mediant 3000 User Manual

28.4.7 Prioritizing Coder List in SDP Offer

In addition to restricting the use of coders with Allowed coders, you can prioritize the
coders listed in the SDP offer. This feature is referred to as Coder Preference. This is done
on both SBC legs:

Incoming SDP offer: The device arranges the coder list according to the order in the
Allowed Coders Group table. The coders listed higher up in the table take preference
over ones listed lower down in the table. This feature is enabled by setting the
'Allowed Coders Mode' parameter in the IP Profile table to Preference or Restriction
and Preference. If set to Preference, in addition to the Allowed coders that are listed
first in the SDP offer, the original coders received in the SDP are retained and listed
after the Allowed coders. Thus, this mode does not necessarily restrict coder use to
Allowed coders, but uses (prefers) the Allowed coders whenever possible.

Outgoing SDP offer: If only Allowed coders are used, the coders are arranged in the
SDP offer as described above. However, if Extension coders are also used, the coder
list is arranged according to the 'SBC Preferences Mode' parameter in the IP Profile
table. This parameter can be configured to add the Extension coders after the Allowed
coders (i.e., at the end of the list - default) according to their order in the Coders
Group table, or arrange Allowed and Extension coders according to their position in
the Coders Group table.

28.4.8 SRTP-RTP and SRTP-SRTP Transcoding

The device supports transcoding between SRTP and RTP. The device can also enforce
specific SBC legs to use SRTP and/or RTP. The device's handling of SRTP/RTP is
configured using the IP Profile parameter, SBCMediaSecurityBehaviour, which provides
the following options:

SBC passes the media as is, regardless of whether it's RTP or SRTP (default).

SBC legs negotiate only SRTP media lines (m=); RTP media lines are removed from
the incoming SDP offer\answer.

SBC legs negotiate only RTP media lines; SRTP media lines are removed from the
incoming offer\answer.

Each SDP offer\answer is extended (if not already) to two media lines for RTP and
SRTP.
If after SDP offer\answer negotiation, one SBC leg uses RTP while the other uses SRTP,
then the device performs RTP-SRTP transcoding. To translate between RTP and SRTP,
the following prerequisites must be met:

At least one supported SDP "crypto" attribute.

The EnableMediaSecurity parameter must be set to 1.
Transcoding where both legs are configured for SRTP is typically required to trans-encrypt
and trans-decrypt. This is relevant when the MKI and Symmetric MKI parameters are
enabled. In other words, both sides need to both encrypt and decrypt the outgoing and
incoming SRTP packets, respectively.
Channel (DSP) resources are not required for RTP and SRTP transcoding.
User's Manual 500
Mediant 3000
Document #: LTRT-89738