M2 signalling bearer provides the following functions:
Provision of reliable transfer of M2-AP message over M2 interface
Provision of networking and routeing function
Provision of redundancy in the signalling network
The protocol stack for M2 signalling bearer is shown in figure 4.1.1 and details on each protocol are described in the following clauses.
Figure 4.1.1: M2 signalling bearer protocol stack
The transport network layer is based on IP transport, comprising SCTP on top of IP.
The support of any suitable data link layer protocol, e.g. PPP, Ethernet, etc., shall not be prevented.
The eNB and MCE shall support IPv6 (IETF RFC 2460 ) and/or IPv4 (IETF RFC 791 ).
The IP layer of M2 only supports point-to-point transmission for delivering M2-AP messages.
The eNB and MCE shall support the Diffserv Code Point marking as described in IETF RFC 2474 .
SCTP (IETF RFC 4960 ) shall be supported as the transport layer of M2 signalling bearer. The Payload Protocol Identifier assigned by IANA to be used by SCTP for the application layer protocol M2-AP is 43.
SCTP refers to the Stream Control Transmission Protocol developed by the Sigtran working group of the IETF for the purpose of transporting various signalling protocols over IP network.
There shall be only one SCTP association established between one MCE and eNB pair.
The eNB shall establish the SCTP association. The SCTP Destination Port number value assigned by IANA to be used for M2-AP is 36443.
Within the SCTP association established between one MCE and eNB pair:
a single pair of stream identifiers shall be reserved for the use of M2-AP elementary procedures.
Transport network redundancy may 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 INIT may be sent from MCE or eNB, at any time for an already established SCTP association, which shall be handled as defined in IETF RFC 4960  in clause 5.2.