Multi-Streaming
SCTP supports streams as depicted in the following figure which depicts an SCTP association that supports three streams.
![](img/GUID-5AFDBD05-8B45-4D18-908D-D7A2A113D8AE-default.png)
The multiple stream mechanism is designed to solve the head-of-the-line blocking problem of TCP. Therefore, messages from different multiplexed flows do not block one another.
A stream can be thought of as a sub-layer between the transport layer and the upper layer. SCTP supports multiple logical streams to improve data transmission throughput. As shown in the above figure, SCTP allows multiple unidirectional streams within an association. This multiplexing/de-multiplexing capability is called multi-streaming and it is achieved by introducing a field called Stream Identifier contained in every DATA chunk) that is used to differentiate segments in different streams.
SIP transactions are mapped into SCTP streams as described in Section 5.1 of RFC 4168. In what it describes as the simplest way, the RFC suggests (keyword SHOULD) that all SIP messages be transmitted via Stream 0 with the U bit set to 1.
On the transmit side, the current SCTP implementation follows the RFC 4168 recommendation. On the receiving side, a SIP entity must be prepared to receive SIP messages over any stream.