This subclause specifies the standards for signalling transport to be used across SGs interface. SGs interface is a logical interface between the MME and the VLR. All the SGsAP messages described in the present document require an SCTP association between the MME and the VLR.

The MME and the VLR shall support IPv6 (see RFC 2460) and/or IPv4 (see RFC 791). The IP layer of SGs only supports point-to-point transmission for delivering SGsAP messages.

SCTP (see RFC 4960) shall be supported as the transport layer of SGsAP messages. Semi-permanent SCTP associations shall be established between the MME and VLR, i.e. the SCTP associations shall remain up under normal circumstances. The MME shall establish the SCTP association. Transport network redundancy can be achieved by SCTP multi-homing between two end-points, of which one or both is assigned with multiple IP addresses. SCTP end-points shall support a multi-homed remote SCTP end-point. For SCTP endpoint redundancy, an SCTP endpoint (in the MME or VLR) may send an INIT, at any time for an already established SCTP association, which the other SCTP endpoint shall handle as defined in RFC 4960. MME and VLR shall support a configuration with a single SCTP association per MME/VLR pair. Configurations with multiple SCTP endpoints per MME/VLR pair may be supported. Within the SCTP association established between one MME and one VLR, both MME and VLR shall reserve several stream identifiers, based on the INIT message exchange for the sole use of SGsAP procedures. The registered port number for SGsAP is 29118. The payload protocol identifier to be used for SGsAP is 0.