8 Subsequent SDP offer/answer transactions

29.0793GPPOptimal media routeing within the IP Multimedia Subsystem (IMS)Release 17Stage 3TS

8.1 General

Any UA or IMS-ALG controlling a media stream may initiate a subsequent SDP offer/answer transaction at any time after first establishing the media flow, in order to modify characteristics of the media flow.

NOTE: For example, a media endpoint (IP address) can change, the preferred codec can change, or a server can select to insert a media function such as a tone generator or conference focus.

An IMS-ALG or UA shall handle a received SDP offer as a subsequent SDP offer as specified in clause 8.2 if an initial or second SDP offer/answer transaction has been completed and a received SDP offer contains no OMR specific SDP attributes.

8.2 IMS-ALG and UA subsequent SDP offer procedures

When a IMS-ALG determines according to clause 8.1 that a received SDP offer shall be handled as a subsequent SDP offer, the IMS-ALG with or without a local primary or secondary MR allocated shall perform the following steps:

1) determine if the local primary or secondary MR can be inserted or removed from the media path as specified in clause 8.4;

2) if the IMS-ALG keeps an allocated MR or allocates a new MR and continues with an MR inserted in the media path, the IMS-ALG shall:

– if necessary (e.g. due to a codec change or allocation of a new MR) interact with the MR;

– update the connection address for the media line in the outgoing subsequent SDP offer; and

– forward the modified SDP offer as specified in 3GPP TS 24.229 [4].

3) if the IMS-ALG does not allocate a new MR or deallocated an MR allocated in the initial or second SDP offer / answer transaction in clause 6 and continues without a MR inserted in the media path, forward the incoming subsequent SDP offer as received.

When a UA (e.g. a conference application treating the media resource as an endpoint or an MGCF) determines according to clause 8.1 that a SDP offer shall be sent as a subsequent SDP offer , the UA shall not apply OMR procedures in clause 7 when sending the subsequent SDP offer. If the conditions in clause 8.4 for reallocating a MR are fulfilled, the UA may reallocate a MR.

8.3 IMS-ALG and UA subsequent SDP answer procedures

When an IMS-ALG receives a subsequent SDP answer as the result of a subsequent SDP offer generated according to clauses 8.2, the IMS-ALG with or without a local primary or secondary MR allocated shall perform the following steps:

1) if the IMS-ALG has a local MR allocated:

NOTE: The local MR can have been allocated during the initial or second SDP offer / answer transaction as described in clause 6 or as the result of the subsequent SDP offer generated in clause 8.2.

a) if necessary interact with the MR as described in clauses 6.3; and

b) update connection address in the outgoing subsequent SDP answer for the media line with the MR connection information;

c) forward the modified SDP answer as specified in 3GPP TS 24.229 [4]; and

2) if the IMS-ALG has no local MR allocated:

a) if the local MR was allocated when the subsequent SDP offer was received, release MR resources; and

b) forward the subsequent SDP answer as received.

When a UA not acting as a B2BUA (e.g. a conference application treating the media resource as an endpoint or the MGCF) receives a subsequent SDP answer, the UA shall not apply OMR procedures specified in clause 7.

8.4 Handling media resources during a subsequent SDP offer/answer transaction

An IMS-ALG, an AS acting as B2BUA and adapting IMS-ALG procedures to control an MRF or a UA not acting as a B2BUA (e.g. a conference application treating the media resource as an endpoint or the MGCF) shall not allocate, deallocate or reallocate a MR during the subsequent SDP offer/answer transaction unless any of the following exceptions apply:

– an IMS-ALG may allocate an MR resource if the downstream and upstream media anchoring points are within the same realm and in the same operator network as the IMS-ALG;

– an IMS-ALG may deallocate an MR resource if the downstream and upstream media anchoring points are within the same realm and in the same operator network as the IMS-ALG; or

– an UA may reallocate an MR resource if the downstream and upstream media anchoring point is within the same realm and in the same operator network as the UA.

If an IMS-ALG or UA decides to allocate or reallocate a MR, the IMS-ALG or UA shall interact with the MR as described in clause 6.3 or 7.4.

If a IMS-ALG decides to deallocate a MR, the IMS-ALG shall release the MR resource when receiving the subsequent SDP answer.

Annex A (informative):
Signalling flows