Liaison Response To: ITU-T Study Group 15 Source: Sigtran WG Chair Subject: New Recommendation G.769 Dear Sirs, On behalf of the Sigtran WG, I would like to thank SG 15 for its liaison concerning the approval of Recommendation G.769. After discussion of the liaison over our mailing list we have the following comments on the protocol stack diagrams provided in the liaison: 1. Regarding the Configuration 1 shown, this was documented in RFC 2719 as a possible configuration, but no accompanying protocol was defined in this group, as it represents the use of a non-standard interworking function between MTP2 and SCTP. 2. Regarding Configuration 2 shown, this is felt to be not a desirable configuration, as M2UA is defined primarily for "backhaul" purposes and not for transport of messages between two signaling gateways as shown. 3. Configuration 3 appears to be the closest fit to the work that has been going on in the Sigtran WG. The M2PA (currently in WG Last Call) has been defined for the usage that is shown in the Configuration. Text comparing the M2PA and M2UA usage has been extracted from the M2PA draft (draft-ietf-sigtran-m2pa-07.txt) and attached to this liaison for information. The WG is also considering development of an extension to the M3UA user adaptation protocol that would support signaling gateway-to-signaling gateway application of the M3UA, which may be another option that SG 15 may wish to consider. Information on the usage of different adaptation layers can also be found in the Telephony Signalling over SCTP Applicability Statement (draft-ietf-sigtran-signalling-over-sctp-applic-08.txt) that is currenlty under review by our Area Directors and IESG as an Informational document. Best regards, Lyndon Ong Chair, IETF Sigtran WG lyong@ciena.com Attachment 1 : Extract from M2Pav7 1.9 Differences Between M2PA and M2UA The MTP2 User Adaptation Layer (M2UA) [M2UA] also adapts the MTP3 layer to the SCTP/IP stack. It does so through a backhauling architecture [RFC2719]. This section intends to clarify some of the differences between the M2PA and M2UA approaches. A possible M2PA architecture is shown in Figure 3. Here the IPSP's MTP3 uses its underlying M2PA as a replacement for MTP2. Communication between the two layers MTP3/M2PA is defined by the same primitives as in SS7 MTP3/MTP2. M2PA performs functions similar to MTP2. ******** SS7 *************** IP ******** * SEP *--------* SG *--------* IPSP * ******** *************** ******** +------+ +-------------+ +------+ | SCCP | | SCCP | | SCCP | +------+ +-------------+ +------+ | MTP3 | | MTP3 | | MTP3 | +------+ +------+------+ +------+ | MTP2 | | MTP2 | M2PA | | M2PA | +------+ +------+------+ +------+ | MTP1 | | MTP1 | SCTP | | SCTP | | | | +------+ +------+ | | | | IP | | IP | +------+ +------+------+ +------+ Figure 3. M2PA in IP Signaling Gateway A comparable architecture for M2UA is shown in Figure 4. In M2UA, the MGC's MTP3 uses the SG's MTP2 as its lower SS7 layer. Likewise, the SG's MTP2 uses the MGC's MTP3 as its upper SS7 layer. In SS7, communication between the MTP3 and MTP2 layers is defined by primitives. In M2UA, the MTP3/MTP2 communication is defined as M2UA messages and sent over the IP connection. ******** SS7 *************** IP ******** * SEP *--------* SG *--------* MGC * ******** *************** ******** +------+ +------+ | SCCP | | SCCP | +------+ +------+ | MTP3 | (NIF) | MTP3 | +------+ +------+------+ +------+ | MTP2 | | MTP2 | M2UA | | M2UA | +------+ +------+------+ +------+ | MTP1 | | MTP1 | SCTP | | SCTP | | | | +------+ +------+ | | | | IP | | IP | +------+ +------+------+ +------+ NIF - Nodal Interworking Function Figure 4. M2UA in IP Signaling Gateway M2PA and M2UA are similar in that: a. Both transport MTP3 data messages. b. Both present an MTP2 upper interface to MTP3. Differences between M2PA and M2UA include: a. M2PA: IPSP processes MTP3/MTP2 primitives. M2UA: MGC transports MTP3/MTP2 primitives between the SG's MTP2 and the MGC's MTP3 (via the NIF) for processing. b. M2PA: SG-IPSP connection is an SS7 link. M2UA: SG-MGC connection is not an SS7 link. It is an extension of MTP to a remote entity. c. M2PA: SG is an SS7 node with a point code. M2UA: SG is not an SS7 node and has no point code. d. M2PA: SG can have upper SS7 layers, e.g., SCCP. M2UA: SG does not have upper SS7 layers since it has no MTP3. e. M2PA: relies on MTP3 for management procedures. M2UA: uses M2UA management procedures. Potential users of M2PA and M2UA should be aware of these differences when deciding how to use them for SS7 signaling transport over IP networks.