Stream Control Transmission Protocol

Formal oversight

The IETF Signaling Transport (SIGTRAN) working group defined the protocol (number 132)in 2000, and the IETF Transport Area (TSVWG) working group maintains it. RFC 4960 defines the protocol. RFC 3286 provides an introduction.

Message-based multi-streaming

SCTP applications submit their data to be transmitted in messages (groups of bytes) to the SCTP transport layer. SCTP places messages and control information into separate chunks (data chunks and control chunks), each identified by a chunk header. The protocol can fragment a message into a number of data chunks, but each data chunk contains data from only one user message. SCTP bundles the chunks into SCTP packets. The SCTP packet, which is submitted to the Internet Protocol, consists of a packet header, SCTP control chunks (when necessary), followed by SCTP data chunks (when available).

One can characterize SCTP as message-oriented, meaning it transports a sequence of messages (each being a group of bytes), rather than transporting an unbroken stream of bytes as does TCP. As in UDP, in SCTP a sender sends a message in one operation, and that exact message is passed to the receiving application process in one operation. In contrast, TCP is a stream-oriented protocol, transporting streams of bytes reliably and in order. However TCP does not allow the receiver to know how many times the sender application called on the TCP transport passing it groups of bytes to be sent out. At the sender, TCP simply appends more bytes to a queue of bytes waiting to go out over the network, rather than having to keep a queue of individual separate outbound messages which must be preserved as such.

The term multi-streaming refers to the capability of SCTP to transmit several independent streams of chunks in parallel, for example transmitting web page images together with the web page text. In essence, it involves bundling several connections into a single SCTP association, operating on messages (or chunks) rather than bytes.

TCP preserves byte order in the stream by including a sequence number with each segment. SCTP, on the other hand, assigns a sequence number to each message sent in a stream. This allows independent ordering of messages in different streams. However, message ordering is optional in SCTP; a receiving application may choose to process messages in the order of receipt instead of in the order of sending.

Features

Features of SCTP include:

The designers of SCTP originally intended it for the transport of telephony (Signaling System 7) over Internet Protocol, with the goal of duplicating some of the reliability attributes of the SS7 signaling network in IP. This IETF effort is known as SIGTRAN. In the meantime, other uses have been proposed, for example, the Diameter protocol and Reliable server pooling (RSerPool).

Motivations and adoption

TCP has provided the primary means to transfer data reliably across the Internet. However, TCP has imposed limitations on several applications. From RFC 4960:

Adoption has been slowed by lack of awareness and lack of implementations, particularly in Microsoft Windows.

Multi homing

SCTP provides redundant paths to increase reliability.

Each SCTP end point needs to check reachability of the primary and redundant addresses of the remote end point using a heartbeat. Each SCTP end point needs to ack the heartbeats it receives from the remote end point.

When SCTP sends a message to a remote address, the source interface will only be decided by the routing table of the host (and not by SCTP).

Asymmetric multi homing

In asymmetric multi homing, one of the two end points does not support multi homing.

Local multi homing - Remote single homing

In Local multi homing and Remote single homing, if the remote primary address is not reachable, the SCTP association fails even if an alternate path is possible.

Packet structure

SCTP packets have a simpler basic structure than TCP packets. Each consists of two basic sections:

1. The common header, which occupies the first 12 bytes and is highlighted in blue, and
2. The data chunks, which occupy the remaining portion of the packet. The first chunk is highlighted in green, and the last of N chunks (Chunk N) is highlighted in red.

Each chunk starts with a one byte type identifier, with 15 chunk types currently defined by RFC 4960. Eight flag bits, a two byte length field and the data compose the remainder of the chunk. If the chunk does not form a multiple of 4 bytes (i.e., the length is not a multiple of 4) then it is padded with zeros which are not included in the chunk length. The two byte length field limits each chunk to a 65,535 byte length (including the type, flags and length fields).

Security

Although encryption was not part of the original SCTP design, SCTP was designed with features for improved security, such as 4-way handshake (compared to TCP 3-way handshake) to protect against SYN flooding attacks, and large "cookies" for association verification and authenticity.

Reliability was also a key part of the security design of SCTP. Multihoming enables an association to stay open even when some routes and interfaces are down. This is of particular importance for SIGTRAN as it carries SS7 over an IP network using SCTP, and requires strong resilience during link outages to maintain telecommunication service even when enduring network anomalies.

SCTP is sometimes a good fingerprinting candidate. Some operating systems ship with SCTP support enabled, and, as it is not as well known as TCP or UDP, it is sometimes overlooked in firewall and intrusion detection configurations, thus often permitting probing traffic.

Implementations

The SCTP reference implementation runs on FreeBSD, Mac OS X, Microsoft Windows, and Linux.

The following operating systems implement SCTP:

Third-party drivers:

Userspace library:

The following applications implement SCTP:

RFC history