Connection Oriented SCCP

As a continuation of the last post I wrote, in this one I present another couple of capture files showing, this time, brief examples of  SCCP connection oriented services.

The first log shows the complete lifecycle - i.e. establishment / data transfer / release - of a class 2 connection, while the other one shows that of a class 3 connection. The two logs can be viewed with Wireshark configured in the same way as for the connectionless examples.

Protocol class 2 provides a simple connection oriented service with no freals except for segmentation and reassembly, while class 3 adds flow control and detection of losses and mis-sequencing of user messages, although with no error recovery mechanism.

Please note that in the class 3 example I chose a very small window size just to keep it short and simple.

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.

An SCTP association

Among other things, last year I've been working at an implementation of the IETF Stream Control Transmission Protocol (SCTP), which is a new internet transport protocol: an alternative to UDP and TCP, suited to carry signalling over IP.

My implementation is (by now) limited to RFC 4960 features, but this is enough to produce some basic example like the one contained in this capture file, in which one can see a complete (although short) life cycle of an SCTP association: establishment / transfer of user data / shutdown.

The simplest SS7 network

As I wrote in the first post, I am currently working at the ITU-T Signalling Sistem no. 7 (SS7) protocol suite. 

I've already implemented the MTP2 and MTP3 (the latter is fairly complex indeed!) so I can use them to open this blog with an example of a simple SS7 network. The simplest one.

As stated in ITU-T Q.705 recommendation:

"The most elementary signalling network consists of originating and destination signalling points connected by a single signalling link."

... so here it is: 

The example network is made up of the two signalling points A and B, addressed with point code 101 and 202 respectively, and directly connected with one linkset composed of a single link, which I assigned the signalling link code 5.

Here is a capture file showing what happen when the network is put into operation: 

• first, there is the link alignment at the MTP2 level (SIOS,SIO,SIN messages); 
• then, the signalling link test at the MTP3 level (SLTM and SLTA messages);
• as the link is available (reception of the SLTA) there is the SP restart phase, which I shortened to 10 seconds (it should last around 1 minute, really) ended with the delivery of the TRA("traffic restart allowed") message;
• and at last, the exchange of some user data.

The capture can be viewed using Wireshark and configuring it to decode the DLT=148 with the MTP2 dissector (to do so, select Edit->Preferences->Protocols->DLT_USER->Edit Encapsulations Table, and add an entry for DLT=148 with Payload Protocol=MTP2).

Hi All

Hi All, my name is Stefano, I am an IT engineer and I post from Italy.

I am working at the implementation of some communication protocols (currently I am implementing the ITU-T SS7 suite) and my intention is to use this blog to share information about this topic.

Of course there is already a lot of stuff on the web, so I'll try to focus on those things that I myself usually search for when I cope with a new protocol: mainly examples of procedures, message flows, logs and things like these.

Thanks for reading, and stay tuned.
Ste.