Saturday, February 5, 2011

Connectionless SCCP

In these days I'm testing my new implementation of the ITU-T Signalling Connection Control Part (SCCP).

Despite its name, SCCP provides its users with both connection oriented and connectionless services, and thus it plays, in the SS7 suite, the roles played by TCP and UDP in the TCP/IP suite. In addition, it has routing capabilities also.

The connectionless services are provided in terms of two protocol classes, namely class 0 and class 1, which are almost identical except for the sequence control which is provided by class 1 only. This sequence control mechanism relies on the MTP3 Signalling Link Selection (SLS) parameter. At each node, the MTP3 layer ensures that user messages with a given SLS and addressed to the same DPC are, in normal conditions, always routed in the same way. Thus, by delivering a stream of datagrams with the same SLS, these should traverse the SS7 network following the same path, without overtaking one each other.

Here are two capture files showing the same stream of datagrams delivered first using class 0, and then using class 1. One can see that in the first case the SLS changes at each UDT (for load sharing) while in the second it is fixed (for in-sequence delivery).

The capture files can be viewed using Wireshark and following the instructions at the end of this post (DLT=148 set to MTP2).

Coming soon... SCCP connection oriented protocol classes.

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