SDP Support

% set profiles signaling ipSignalingProfile <SIP profile name> ingressIpAttributes flags suppress183For3xxRedirectResponsek <disable | enable>
% set profiles signaling ipSignalingProfile <SIP profile name> ingressIpAttributes flags suppress183WithoutSdp <disable | enable>

Passing Audio Codecs in SDP Offer

The 

The

Late Media INVITE or Re-INVITE Support

Example 1: G711 pass-through call

1. Ingress Offer: G711, G729
2. Egress Offer: G711, G729, AMR, G726
3. Ingress Answer: G711
4. Egress Answer: G711

If the

Example 2: G711 to AMR transcoded call

1. Ingress Offer: G711, G729
2. Egress Offer: G711,G729, AMR, G726
3. Egress Answer: AMR
4. Ingress Answer: G711
   If the 
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   receives an re-INVITE with no SDP  on egress, by default, the
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    generates an offer for AMR. If the “Send All Allowed Codecs For Late Media Invite Or Re-Invite” flag is in enabled state, then the
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    generates a codec list of G711,G729, AMR, and G726 in the offer.

The SBC has an existing flag for late media INVITEs or Re-INVITES which specifies the included codecs in the SDP offer, but the behavior differs based on whether the base call is a pass-through or transcoded call. For pass-through calls, the codecs offered do not include those supported for transcoding. For certain scenarios, the originator of the INVITE/Re-INVITE must know the full capabilities even for a pass-through call. This feature addresses the pass-through case where the full set of codecs is needed.

When the sendSBCSupportedCodecsForLateMediaReInvite flag is enabled, the SBC sends all the codecs supported on a particular leg as an SDP OFFER in the 200 OK response to a late media Re-INVITE. The OFFER sent in the 200 OK contains all the pass-through and transcodable codecs supported on that leg. This behavior is applicable only for late media Re-INVITE requests. The flag does not have any impact on late media INVITE requests.

For example, the

Refer to Common IP Attributes - SIP - CLI or Common IP Attributes - Flags (EMA) for configuration details.

Late Media Pass-through Calls over GW-GW

The SBC supports late media pass-through calls over GW-GW and also handles late media (offer-less) re-INVITEs on either side. This functionality is supported when the flag lateMediaSupport is configured as passthru.

set addressContext <addressContext name> zone <zone name> sipTrunkGroup <sipTrunkGroup name> media lateMediaSupport passthru

When the flag lateMediaSupport is set as passthru, a late media INVITE/re-INVITE received on one leg is relayed to the other leg.

ANAT Support

Alternative Network Address Types (ANAT) semantics for the SDP grouping framework allows the expression of alternative network addresses (for example. different IP versions) for a particular media stream. This ability is useful in environments with both IPv4-only hosts and IPv6-only hosts.

The 

ICE-Lite Support

Bundling of Streams

The

The SDP media-level attribute, "bundle-only", is parsed and used to identify that specific media is only used if bundled and resides within a BUNDLE group. The "bundle-only" attribute is a property attribute with no value.

The use of a BUNDLE group and a BUNDLE address also allows the usage of a single set of Interactive Connectivity Establishment (ICE) candidates for multiple "m=" lines.
If using RTP/RTCP multiplexing, use the same address:port combination for all RTP and RTCP packets.

Single-bundle

The

Stream

• You can tag a stream as 'bundle-only' with the port set to '0'.

• The policers for the stream, which represents the bundle reflect the total bandwidth for all streams within the bundle.
• Only relay scenarios are supported; any interworking causes the streams to unbundle.

Bundle Only

The 

• Offers with "m=" lines tagged as bundle-only with the port set to 0.
• Non-compliant offers which associate bundle-only with a non-zero port, the "m=" line is also accepted; the 
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  treats the stream as a bundle-only stream.
• The 
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  handles media attributes received with a bundle-only stream offer, taking into consideration the stream is present, as well as the output in the outbound offers (For example, media codecs and fmtp options).
• The  handles the media attributes received with a bundle-only stream offer taking into consideration that the stream is present and output in the outbound offers. For example; media codecs, fmtp options)

Bundle Group

The 

• The OFFER contains the BUNDLE attribute followed by a list of streams, which are part of the BUNDLE.
• The OFFER contains a unique address:port combinations for each stream as the peer may not support bundling the streams.

If the peer supports the bundle:

• The ANSWER contains the BUNDLE attribute.
• The ANSWER contains the same address:port combination for all streams within the bundle.

OFFER and ANSWER

The following is an example for OFFER and ANSWER mechanism.

OFFER

v=0
o=alice 2890844526 2890844526 IN IP4 10.32.241.3
s= 
c=IN IP4 10.32.241.3 
t=0 0 
a=group:BUNDLE audio video 
m=audio 10000 RTP/AVP 0  
a=mid:audio
a=rtpmap:0 PCMU/8000 
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid 
m=video 10002 RTP/AVP 31 32
a=mid:video 
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid 

ANSWER

v=0 
o=bob 2808844564 2808844564 IN IP4 biloxi.example.com 
s= 
c=IN IP4 biloxi.example.com 
t=0 0 
a=group:BUNDLE audio video 
m=audio 20000 RTP/AVP 0  
a=mid:audio
a=rtpmap:0 PCMU/8000 
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid 
m=video 20000 RTP/AVP 32 
a=mid:video 
a=rtpmap:32 MPV/90000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid 

Interactions with Security

Bundling is only allowed when security is relayed, and:

• DTLS-SRTP and DTLS-SCTP for data are relayed
• SDES SRTP is relayed

Limitations

• You can only relay (not interwork) bundling.
• Configuration for LI or a call requiring transcoding, DTMF, or security termination is unbundled.
• Bundling is not supported over the gateway protocol.

RTP and RTCP Multiplexing

The

Within a BUNDLE group, the offerer and answerer must enable RTP/RTCP multiplexing for the RTP-based media associated with the BUNDLE group.

When RTP/RTCP multiplexing is enabled, the

• The
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   allows relay of multiplexed RTP and RTCP traffic only when it supports the rtcp-mux  on both sides of the call.
• The "a=rtcp-mux" attribute negotiates multiplexing when RTCP is enabled on both sides of the call.
• The rtcp-mux attribute takes precedence over the "a=rtcp:" attribute when it has to accept SDPs from the peer.
• When the
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  uses rtcp-mux within a bundle, rtcp-mux is enabled for all streams in a bundled or disabled for all.

Interactions with ICE

Using media stream bundling and RTCP multiplexing ensures that only a single ICE transaction on one port is required. This limits the occurrence of NAT traversal issues and reduces the time needed to establish a call.

For the OFFER:

• The ICE data is unique for each stream.
• A candidate is included for RTCP in the initial OFFER and for the RTP.
• A bundle-only "m=" line does not include an ICE candidate or ufrag/pwd.

For the ANSWER:

• The ICE data matches the chosen master stream.
• A single ICE candidate line is included for the RTP component
  
  

RTCP Multiplexing to Non-RTCP Multiplexing Interworking

The RTP contains two components:

• a data transfer protocol, and
• an associated Real-time Control Protocol (RTCP).

With the increased use of Network Address Port Translation (NAPT), using separate UDP ports to transport RTP and RTCP has become problematic considering maintaining multiple NAT bindings is costly.To overcome these scenarios, the 

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supports RTP and RTCP flows on a single port to ease NAT traversal. This functionality is known as RTP and RTCP Multiplexing or RTCP-Mux.

Prior to the 6.1.0 release, the

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supported RTCP-Mux in pass-through scenarios where the 

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 relays media in an end-to-end scenario. This support was sufficient for some limited use cases, such as WRTC-to-WRTC call scenarios, where the endpoints terminated media encryption with no need for the 

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 to interwork with any media attributes. 

The 

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 is enhanced to support interworking between RTCP-Mux to non-RTCP-Mux, as well as in the reverse direction. In this scenario, both egress and ingress packet Service Profiles (PSPs) either support RTCP-Mux or fallback to transport RTP and RTCP streams using separate ports. The rtcpMux flag is added to the PSP to configure RTCP-Mux.When the 

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triggers the RTP interception on an RTCP-Mux call (for example, lawful intercept), the 

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 forks the RTP and RTCP streams to a single port towards the mediation server. If the intercept is performed in an RTCP-Mux session, the stream sent towards the mediation server is also multiplexed with RTP and RTCP sent over the same port.

The

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supports RTCP-Mux with the following functionality:

• Multimedia calls
• Monitoring calls
• Both direct media and OMR call scenarios
• DTLS and SRTP scenarios (irrespective of whether the encryption protocol terminates at the 
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  or relays transparently end-to-end)
• Multiplexing RTCP and interworking with non-RTCP Mux for both pass-through (with or without "RTCP termination" enabled) and transcoded calls
• Bundled streams for WRTC calls
• Tones and Announcements
• Using SRTP and SRTCP ( The
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  supports this in both the cases of SRTP pass-through and SRTP to RTP interworking.)

RTCP bandwidth requirements do not change with the use of RTCP-Mux. The 

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transmits bandwidth attributes (RR/RS) as per the existing configuration.

Note

The rtcpMux flag is visible only when the rtcp control is enabled on the PSP.

The following table describes the RTCP-Mux to non-RTCP-Mux interworking scenarios for offer/answer:

RTCP-Mux to non-RTCP-Mux Interworking

Interworking Scenarios	Offer from Ingress	Offer to Egress	Answer from Egress	Answer to Ingress	RTCP Flow Ingress Port(s) → Egress Port(s)
RTCP-Mux enabled on both PSPs	“a=rtcp-mux"	“a=rtcp-mux”	“a=rtcp-mux”	“a=rtcp-mux”	(P1)→(P1)
	“a=rtcp-mux”	“a=rtcp-mux"	No Mux	“a=rtcp-mux”	(P1)→(P1,P2)
	No Mux	“a=rtcp-mux”	“a=rtcp-mux”	No Mux	(P1,P2)→(P1)
	No Mux	“a=rtcp-mux”	No Mux	No Mux	(P1,P2)→(P1,P2)
RTCP-Mux enabled on ingress only	“a=rtcp-mux”	No Mux	No Mux	“a=rtcp-mux”	(P1)→(P1,P2)
	No Mux	No Mux	No Mux	No Mux	(P1,P2)→(P1,P2)
RTCP-Mux enabled on egress only	“a=rtcp-mux”	“a=rtcp-mux”	“a=rtcp-mux”	No Mux	(P1,P2)→(P1)
	“a=rtcp-mux”	“a=rtcp-mux”	No Mux	No Mux	(P1,P2)→(P1,P2)
	No Mux	“a=rtcp-mux”	“a=rtcp-mux”	No Mux	(P1,P2)→(P1)
	No Mux	“a=rtcp-mux”	No Mux	No Mux	(P1,P2)→(P1,P2)
RTCP-Mux disabled on both PSPs	“a=rtcp-mux”	No Mux	No Mux	No Mux	(P1,P2)→(P1,P2)
	No Mux	No Mux	No Mux	No Mux	(P1,P2)→(P1,P2)

In the offer, ““a=rtcp-mux”” is signaled in the SDP in all the media "m=" lines for whichit requires RTP/RTCP mux. If the answerer wishes to multiplex RTP and RTCP on a single port, it must generate an answer containing an "“a=rtcp-mux”" attribute for each media where it uses RTP and RTCP-Mux. If the answerer rejects the usage of RTP/RTCP-Mux, it must not associate an SDP "rtcp-mux" or SDP "rtcp" attribute in the answer. Based on the port number, the answerer uses the next higher (odd) destination port number for sending RTCP packets associated with the corresponding "m=" line towards the offerer.

RTCP-Mux to non-RTCP-Mux interworking for ICE Calls

When using ICE where the 

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sends the RTP and RTCP are sent on separate ports, it checks the connectivity for each component. Some of these connectivity checks can be avoided by multiplexing RTP and RTCP on the same port, thus reducing the ICE overhead.

When the 

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offers the “a=rtcp-mux” attribute, it generates the offer that contains "a=candidate:" lines for both RTP and RTCP along with an "a=rtcp:" line (provided that the sendRtcpPortInSdp flag is enabled) to designate a fallback port for RTCP in case the answerer does not support RTP and RTCP-Mux. This response is performed for each media stream when it desire the RTP and RTCP-Mux.

Note

With an upgrade to

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software that supports RTCP-Mux, an existing user running WRTC calls must enable the control rtcpMux on both the PSPs to maintain backward compatibility. This is required when the sessions transparently relay DTLS and RTCP-Mux attributes using DTLS-SRTP relay control (dtlsSrtpRelay) in the PSP and the flag sdpAttributesSelectiveRelay is enabled.

Once the RTCP-Mux is negotiated, the subsequent reINVITE(s) sent from the 

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to the same endpoint does not re-negotiate the RTCP-Mux. However, the RTCP-Mux is re-negotiated with the target endpoints for the redirect calls or for call transfer. The RTCP-Mux is also negotiated on a per leg basis when the endpoint triggers the negotiation or the SDP change includes an address change. Thus, the 

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never restarts ICE on its own and waits for the endpoint to trigger an ICE restart. If it add the new media stream  in an existing session, ICE is negotiated on that stream. If the existing streams are updated for codec changes, ICE is not re-negotiated.

RTCP-Mux re-negotiation is consistent for both for the 

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ICE and non-ICE scenarios.

Limitation

• The
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  does not support pass-through for the rtcp-mux.

Asymmetric PRACK interworking

The 

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supports PRACK functionalities either on the ingress or on the egress leg and denotes it with the phrase "Asymmetric PRACK Interworking". The flag sdp100relIwkForPrack supports Asymmetric PRACK interworking for the late media calls.

To configure PRACK on both the legs, enable the existing flag endToEndPrack. For more information on endToEndPrack, refer to Flags - CLI.

Supported Call Flow Scenarios:

The configurations for the existing flags associated with the flag sdp100relIwkForPrack, and the supported call flow scenarios, are summarized below:

Parameter configurations for supported call flows

rel100Support flag on the ingress Trunk Group	rel100Support flag on the egress Trunk Group	lateMediaSupport flag on the ingress Trunk Group	sdp100relIwkForPrack flag on the egress Trunk Group	Relevant Call Flow Scenario
enabled	disabled | enabled	passthru	enabled	Scenario 1
enabled	disabled | enabled	convert	enabled	Scenario 2
disabled	enabled	passthru	enabled	Scenario 3

For more information on the existing flags lateMediaSupport and rel100Support, refer to the following pages:

• lateMediaSupport:
  • SIP Trunk Group - Media - CLI
  • SIP Trunk Group - Media
• rel100Support:
  • SIP Trunk Group - Signaling - CLI
  • SIP Trunk Group - Signaling

Scenario 1: When PRACK is supported only on the ingress leg of the call, and the lateMediaSupport flag is set to passthru

Asymmetric PRACK Interworking - Supported Call Flow Scenario 1

 

Scenario 2: When PRACK is supported only on the ingress leg of the call, and the lateMediaSupport flag is set to convert

Asymmetric PRACK Interworking - Supported Call Flow Scenario 2

 

Scenario 3: When PRACK is supported only on the egress leg of the call, and the lateMediaSupport flag is set to passthru

Asymmetric PRACK Interworking - Supported Call Flow Scenario 3

Multiple m-lines Support in SDP

The 

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can process multiple audio m-lines, or a combination of audio and image m-lines, from the SDP.  The 

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 uses multiple m-lines (media streams) in call recording scenarios or several fax transmission types.

When the 

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supports both the ingress and egress legs of a call when multiple m-lines are present, the 

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determines a core stream among the m-lines. The core stream is the first m-line within the SDP, which contains a codec supported by the SBC. The SBC applies the media parameters specified by merging the ingress and egress Packet Service Profile (PSP) parameters, such as transcoding policy, to the core stream. For all other non-core audio or image media streams, the SBC allocates resources for the stream and transparently relays the m-lines. If the first stream containing a supported codec is for fax (m=image), then the SBC treats the call as fax and relays all other media streams.

Support for multiple m-lines is enabled or disabled using the multipleAudioStreamsSupport option within media parameters in SIP trunk groups.

If you enable multiple m-line support, you also have the option to enable a separate media option in SIP trunk groups, which disables all SRTP (SAVP) streams present in incoming SDP. This disabling enables deployments to disallow SRTP on the ingress call leg, but not on the egress leg. Enabling the option disallowSrtpStream means that for all SAVP streams in the incoming SDP, the 

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sets the port to zero, thus disabling the SAVP stream. The SBC handles any other streams in the SDP according to the multiple m-line handling just described. If the incoming SDP contains only one or more SAVP streams, the 

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rejects the call. The option disallowSrtpStream can only be enabled when the multipleAudioStreamsSupport option is enabled.

The multiple m-line support applies only to cases where the m-lines present consist of all audio streams or a combination of audio and image media streams. The 

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maintains its default behavior if the non-core streams are video or other non-audio, non-image streams. To apply this feature, enable the multiple m-line support on both ingress and egress SIP Trunk Groups.

When the multipleAudioStreamsSupport option is disabled, the SBC default behavior takes place.

For more information, refer to:

• SIP Trunk Group - Media - CLI
• SIP Trunk Group - Media

Offer-Answer Timer Configuration

In specific call scenarios, the 

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treats the Offer-Answer (OA) as a MODIFY Offer-Answer cycle, but the peer treats it as an INITIAL Offer-Answer cycle. According to RFC 3261, the response from the peer is expected within 300 seconds. The

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, however, assumes a 20-second response, and therefore any delay in the response from the peer which exceeds of 20 seconds causes call failure.

Currently, the internal Offer-Answer (OA) timer value is fixed and cannot be configured. To overcome this limitation, the 

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is enhanced with a new parameter offerAnswerTimer to configure this OA timer.

Note

During switchover, the active calls use the old timer value at the start of the call. After switchover, the new calls use the latest configured value.

Call flow

The following figure depicts the call message flow diagram in the Offer-Answer cycle:

Message Flow Diagram in Offer-Answer Cycle