SCTP Characteristics

Stream Control Transmission Protocol (SCTP) is a relatively new protocol in the game, but since it is growing in usage and complements the TCP and UDP protocols, I have chosen to add this section about it. It has an even higher reliability than TCP, and at the same time a lower overhead from protocol headers.

SCTP has a couple of very interesting features that can be interesting. For those who wish to learn more about this, read the RFC 3286 - An Introduction to the Stream Control Transmission Protocol and RFC 2960 - Stream Control Transmission Protocol document. The first document is an introduction to SCTP and should be very interesting to people who are still in need of more information. The second document is the actual specification for the protocol, which might be less interesting unless you are developing for the protocol or are really interested.

The protocol was originally developed for Telephony over IP, or Voice over IP (VoIP), and has some very interesting attributes due to this. Industry grade VoIP requires very high reliability for one, and this means that a lot of resilience has to be built into the system to handle different kind of problems. The following is a list of the basic features of SCTP.

This list could be made even longer, but I will not. Most of this information is gathered from the RFC 3286 - An Introduction to the Stream Control Transmission Protocol document, so read on there for more information.


In SCTP we talk about chunks, not packets or windows anymore. An SCTP frame can contain several different chunks since the protocol is message oriented. A chunk can either be a control or a data chunk. Control chunks is used to control the session, and data chunks are used to send actual data.

Initialization and association

Each connection is initialized by creating an association between the two hosts that wants to talk to each other. This association is initialized when a user needs it. It is later used as needed.

The initialization is done through 4 packets. First an INIT chunk is sent, which is replied to with an INIT ACK containing a cookie, after this the connection can start sending data. However, two more packets are sent in the initialization. The cookie is replied to with a COOKIE ECHO chunk, which is finally replied to with a COOKIE ACK chunk.

Data sending and control session

SCTP can at this point send data. In SCTP there are control chunks and data chunks, as previously stated. Data chunks are sent using DATA chunks, and DATA chunks are acknowledged by sending a SACK chunk. This works practically the same as a TCP SACK. SACK chunks are control chunks.

On top of this, there are some other control chunks that can be seen. HEARTBEAT and HEARTBEAT ACK chunks for one, and ERROR chunks for another. HEARTBEATs are used to keep the connection alive, and ERROR is used to inform of different problems or errors in the connection, such as invalid stream id's or missing mandatory parameters et cetera.

Shutdown and abort

The SCTP connection is finally closed by either an ABORT chunk or by a graceful SHUTDOWN chunk. SCTP doesn't have a half-closed state as TCP, in other words one side can not continue sending data while the other end has closed its sending socket.

When the user/application wants to close the SCTP socket gracefully, it tells the protocol to SHUTDOWN. SCTP then sends all the data still in its buffers, and then sends a SHUTDOWN chunk. When the other end receives the SHUTDOWN, it will stop accepting data from the application and finish sending all the data. Once it has gotten all the SACK's for the data, it will send a SHUTDOWN ACK chunk and once the closing side has received this chunk, it will finally reply with a SHUTDOWN COMPLETE chunk. The whole session is now closed.

Another way of closing a session is to ABORT it. This is an ungraceful way of removing an SCTP association. When a connecting party wants to remove an SCTP association instantaneously, it sends an ABORT chunk with all the right values signed. All data in the buffers et cetera will be discarded and the association will then be removed. The receiving end will do the same after verifying the ABORT chunk.