PDF of this tutorialSCTP gains advantage over TCP by the virtue of its unique features. This section explores how multi-homing, multi-streaming, and other SCTP features contribute to the SCTP advantage.
SCTP Multi-Homing
The multi-homing feature enables SCTP endpoints to support multiple IP addresses. Multi-homing protects an association from potential network failures by steering traffic to alternate IP addresses. During the initiation of an association, SCTP endpoints exchange lists of IP addresses. Therefore, each endpoint can send and receive messages from any of the IP addresses listed at the remote endpoint. For example, one of the listed IP addresses will be designated as the primary address during the initiation. If the primary address repeatedly drops chunks, however, all chunks will be transmitted to an alternate address until a connection to the primary address can be reestablished.
Multi-homing is a step above conventional single-homed data exchange sessions (i.e., TCP). In single-homed environments, loss of session could be triggered by core network failures or by isolation of endpoints. Since multi-homing directs traffic on different paths to separate IP addresses, loss of session due to physical network failure is virtually non-existent in SCTP.
SCTP Multi-Streaming
The multi-streaming feature separates and transmits user data on multiple SCTP streams. These streams are capable of independent, sequenced delivery. Message loss in a particular stream will only hinder delivery within that stream. Therefore, other streams within an association are not affected.
Through multi-streaming, SCTP eliminates unnecessary blocking that often occurs in TCP transmissions. In TCP, a stream is defined as a sequence of bytes that conform to strict in-sequence delivery. In-sequence delivery results in a major drawback known as "head-of-the-queue blocking," where messages within a stream are not allowed to bypass each other. Since SCTP streams are independent, retransmitted and high-priority messages can bypass less significant messages.
SCTP Features
In the three stages of association, SCTP applies mechanisms that set it apart from TCP and UDP.
- Initiation Features—In contrast to the three-way handshake that occurs in TCP, SCTP uses a four-way handshake to initiate an association. This four-way handshake defends against denial-of-service attempts caused by attackers bombarding the SCTP nodes with counterfeit PDUs. In addition, SCTP packets that contain invalid verification tags are identified during initiation and removed from the transmission path. The verification tag values and the cookie mechanism shield the initiation procedure from SYN-type attacks (commonly known as blind attacks) that are commonplace in TCP.
- Data Transmission Features—During data transmission, the chunk-bundling feature allows DATA chunks to be multiplexed with control chunks. The peer endpoint acknowledges the receipt of a data chunk by sending a SACK chunk. SACK chunks contain transmission sequence numbers (TSN) that reveal any gaps in the sequence of data chunks. Within each stream, SCTP packets are also assigned stream sequence numbers (SSN). The SSN determines the sequence of data delivery within each independent stream. If the peer endpoint indicates gaps in the SSN, then the message will not be delivered until the gap is filled.
- Shutdown Features—The SCTP shutdown procedure has some significant advantages over TCP. For instance, a TCP connection is considered "half-open" when one endpoint continues to send data though the peer endpoint is no longer transmitting data. In contrast, SCTP implements a graceful close of an association by exchanging three messages. These messages acknowledge that both endpoints will cease in their transmissions of data.
In addition to understanding these numerous features, it is also important for one to understand the unique SCTP packet structure.
