On Tue, 2009-05-12 at 14:25 +0530, Kapil Saxena wrote: > I have a MGW that has SIP interface towards UE and ISUP towards MSC. > > For a SIP->ISUP call: > > 1. MGW receives SIP INVITE from UE and sends IAM to MSC-A. > 2. MGW receives backward ACM message with "called party status = > free" from MSC-A. > 3. MGW sends 180 Ringing towards SIP UE. > 4. Called Party (on MSC-A) rings but does not answer > 5. Call is now forwarded by MSC-A (this is the case of call > forwarding after alerting the user) towards MSC-B. Thus, MSC-A sends > IAM to MSC-B > 6. MSC-B sends ACM message with "called party status = free" towards > MSC-A > 7. MSC-A transforms it to CPG (Alerting) and sends it towards MGW. > 8. My question is: what MGW should do with 2nd CPG (Alerting) > message? RFC 3398 (ISUP<->SIP interworking) mandates to send out 180 > but as MGW has already sent out 180 (see #3 above), so it can not send > duplicate 180 with same "tag".
Certainly, it is legal in SIP to send more than one 1xx message with the same to-tag. As a matter of implementation, the MGW may prefer not to do so, as it may not carry any additional information. More difficulty arises if the offered media from MSC-B is different than the offered media from MSC-A, because that would require that the second 180 have different SDP than the first 180, and that is not allowed if they have the same tags. Another implementation approach would be for the MGW to construct the second 180 using a different to-tag than the first 180. That would give it the freedom to apply different SDP to the second 180. I expect that there are a lot of possible implementations of such an MGW. Dale _______________________________________________ Sip-implementors mailing list [email protected] https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors
