Notes and Study Materials

SCTP Features



The general features of SCTP are as follows:

Transmission Sequence Number (TSN):


The unit of data in TCP is a byte. Data transfer in TCP is controlled by numbering bytes by using a sequence number. On the other hand, the unit of data in SCTP is a DATA chunk which may or may not have a one-to-one relationship with the message coming from the process because of fragmentation, Data transfer in SCTP is controlled by numbering the data chunks. SCTP uses a transmission sequence number (TSN) to number the data chunks. TSNs are 32 bits long and randomly initialized between 0 and 232 - 1. Each data chunk must carry the corresponding TSN in its header.

Stream Identifier (SI):

In TCP, there is only one stream in each connection. In SCTP, there may be several streams in each association. Each stream in SCTP needs to be identified by using a stream identifier (SI). Each data chunk must carry the SI in its header so that when it arrives at the destination, it can be properly placed in its stream. The 51 is a 16-bit number starting from 0.

Stream Sequence Number (SSI):

When a data chunk arrives at the destination SCTP, it is delivered to the appropriate stream and in the proper order. This means that, in addition to an SI, SCTP defines each data chunk in each stream with a stream sequence number (SSN).




The design of SCTP packet is totally different: data are carried as data chunks, control information is carried as control chunks. Several control chunks and data chunks can be packed together in a packet. A packet in SCTP plays the same role as a segment in TCP. The following figure compares a segment in TCP and a packet in SCTP. Let us briefly list the differences between an SCTP packet and a TCP segment:




1. The control information in TCP is part of the header, the control information in SCTP is included in the control chunks. There are several types of control chunks each is used for a different purpose.

2. The data in a TCP segment treated as one entity. But an SCTP packet can carry several data chunks and each can belong to a different stream.

3. The options section, which can be part of a TCP segment, does not exist in an SCTP packet. Options in SCTP are handled by defining new chunk types.

4. The mandatory part of the TCP header is 20 bytes, while the general header in SCTP is only 12 bytes. The SCTP header is shorter due to the following:

a). An SCTP sequence number (TSN) belongs to each data chunk and hence is located in the chunk's header.

b). The acknowledgment number and window size are part of each control chunk.

c). There is no need for a header length field (shown as HL in the TCP segment) because there are no options to make the length of the header variable; the SCTP header length is fixed (12 bytes).

d). There is no need for an urgent pointer in SCTP.


5. The checksum in TCP is 16 bits; in SCTP, it is 32 bits.

6. The verification tag in SCTP is an association identifier, which does not exist in TCP. In TCP, the combination of IP and port addresses defines a connection. In SCTP we may have multi homing using different IP addresses. A unique verification tag is needed to define each association.

7. TCP includes one sequence number in the header, which defines the number of the first byte in the data section. An SCTP packet can include several different data chunks. TSNs, SIs, and SSNs define each data chunk.

8. Some segments in TCP that carry control information (such as SYN and FIN) need to consume one sequence number. Control chunks in SCTP never use a TSN, SI, or SSN. These three identifiers belong only to data chunks, not to the whole packet.

In SCTP, An association may send many packets, a packet may contain several chunks, and chunks may belong to different streams. For example suppose that process A needs to send 11 messages to process B in three streams. The first four messages are in the first stream, the second three messages are in the second stream, and the last four messages are in the third stream.

Although the process could deliver one message from the first stream and then another from the second, we assume that it delivers all messages belonging to the first stream first, all messages belonging to the second stream next, and finally, all messages belonging to the last stream.
We also assume that the network allows only three data chunks per packet, which means that we need four packets as shown in the following figure.


SCTP_Packets_Data Chunks


Data chunks in stream 0 are carried in the first packet and part of the second packet those in stream 1 are carried in the second and third packets; those in stream 2 are carried in the third and fourth packets.
Note that each data chunk needs three identifiers: TSN, SI, and SSN. TSN is a cumulative number and is used for flow control and error control. SI defines the stream to which the chunk belongs. SSN defines the chunk's order in a particular stream. In our example, SSN starts from 0 for each stream.


Acknowledgment Number:

• TCP acknowledgment numbers are byte-oriented and refer to the sequence numbers. SCTP acknowledgment numbers are chunk-oriented. They refer to the TSN.

• A second difference between TCP and SCTP acknowledgments is the control information. This information is part of the segment header in TCP. To acknowledge segments that carry only control information, TCP uses a sequence number and acknowledgment number (for example, a SYN segment needs to be acknowledged by an ACK segment).

• In SCTP, however, the control information is carried by control chunks, which do not need a TSN. These control chunks are acknowledged by another control chunk of the appropriate type (some need no acknowledgment).



Flow Control:

Like TCP, SCTP implements flow control to avoid overwhelming the receiver.

Error Control:

Like TCP, SCTP implements error control to provide reliability. TSN numbers and acknowledgment numbers are used for error control.

Congestion Control:

Like TCP, SCTP implements congestion control to determine how many data chunks can be injected into the network.


For Further Reading:  

Different Services of SCTP
Packet Format in SCTP
SCTP Association
Flow Control in SCTP
Error Control in SCTP
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