The SCTP (IETF RFC 4960 ) is used to support the exchange of X2 Application Protocol (X2AP) signalling messages between two eNBs
A single SCTP association per X2-C interface instance shall be used, except when the X2 GW is used.
Only when the X2 GW is used to connect one eNB to one or more specific eNB(s), all X2-C interface instances from that eNB to the relevant eNB(s) are carried over the same SCTP association from that eNB to the X2 GW, and over separate SCTP association(s) from the X2 GW to the relevant eNB(s).
For X2-C interface instance(s) over one SCTP association one pair of stream identifiers shall be used for X2-C common procedures and only a few pairs of stream identifiers should be used for X2-C dedicated procedures.
Source-eNB communication context identifiers that are assigned by the source-eNB for X2-C dedicated procedures, and target-eNB communication context identifiers that are assigned by the target-eNB for X2-C dedicated procedures, shall be used to distinguish UE specific X2-C signalling transport bearers. The communication context identifiers are conveyed in the respective X2AP messages.