SCTP, Stream Control Transmission Protocol

Protocol suite:	TCP/IP.
Protocol type:	Transport layer protocol.
IP Protocol:	132.
SNMP MIBs:	iso.org.dod.internet.mgmt.mib-2.sctpMIB (1.3.6.1.2.1.104).
Working groups:	sigtran, Signaling Transport. tcpm, TCP Maintenance and Minor Extensions. tsvwg, Transport Area Working Group.
Links:	IANA: SCTP parameters. sctp.org.

RFC 2960:

SCTP is designed to transport PSTN signaling messages over IP networks, but is capable of broader applications. SCTP is a reliable transport protocol operating on top of a connectionless packet network such as IP. It offers the following services to its users:

Acknowledged error-free non-duplicated transfer of user data.

Data fragmentation to conform to discovered path MTU size.

Sequenced delivery of user messages within multiple streams, with an option for order-of-arrival delivery of individual user messages.

Optional bundling of multiple user messages into a single SCTP packet.

Network-level fault tolerance through supporting of multi-homing at either or both ends of an association.

The design of SCTP includes appropriate congestion avoidance behavior and resistance to flooding and masquerade attacks.

MAC header

IP header

SCTP header

Data :::

SCTP header:

00	01	02	03	04	05	06	07	08	09	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29	30	31
Source port																Destination port
Verification tag
Checksum
Chunk [1.. n] :::

Source port. 16 bits.
The SCTP sender's port number. It can be used by the receiver in combination with the source IP address, the SCTP destination port and possibly the destination IP address to identify the association to which this packet belongs.

Destination port. 16 bits.
The SCTP port number to which this packet is destined. The receiving host will use this port number to de-multiplex the SCTP packet to the correct receiving endpoint/application.

Verification tag. 32 bits.
The receiver of this packet uses the Verification Tag to validate the sender of this SCTP packet. On transmit, the value of this Verification Tag MUST be set to the value of the Initiate Tag received from the peer endpoint during the association initialization, with the following exceptions:

A packet containing an INIT chunk MUST have a zero Verification Tag.
A packet containing a SHUTDOWN-COMPLETE chunk with the T-bit set MUST have the Verification Tag copied from the packet with the SHUTDOWN-ACK chunk.
A packet containing an ABORT chunk may have the verification tag copied from the packet which caused the ABORT to be sent.

An INIT chunk MUST be the only chunk in the SCTP packet carrying it.

Checksum. 32 bits.
Contains the checksum of this SCTP packet. SCTP uses the Adler-32 algorithm for calculating the checksum.

Chunk. Variable length.

00	01	02	03	04	05	06	07	08	09	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29	30	31
Chunk type								Chunk flags								Chunk length
Chunk data :::

Chunk type. 8 bits, 0 to 255.
Identifies the type of information contained in the Chunk data. The value of 255 is reserved for future use as an extension field.

Chunk types are encoded such that the highest-order two bits specify the action that must be taken if the processing endpoint does not recognize the Chunk type.

00 - Stop processing this SCTP packet and discard it, do not process any further chunks within it.
01 - Stop processing this SCTP packet and discard it, do not process any further chunks within it, and report the unrecognized parameter in an 'Unrecognized Parameter Type' (in either an ERROR or in the INIT ACK).
10 - Skip this chunk and continue processing.
11 - Skip this chunk and continue processing, but report in an ERROR Chunk using the 'Unrecognized Chunk Type' cause of error.

The total length of a chunk MUST be a multiple of 4 bytes. If the length of the chunk is not a multiple of 4 bytes, the sender MUST pad the chunk with all zero bytes and this padding is not included in the chunk length field. The sender should never pad with more than 3 bytes. The receiver MUST ignore the padding bytes.

Chunk type	Description	References
0	DATA, Payload Data.	RFC 2960
1	INIT, Initiation.	RFC 2960
2	INIT ACK, Initiation Acknowledgement.	RFC 2960
3	SACK, Selective Acknowledgement.	RFC 2960
4	HEARTBEAT, Heartbeat Request.	RFC 2960
5	HEARTBEAT ACK, Heartbeat Acknowledgement.	RFC 2960
6	ABORT, Abort.	RFC 2960
7	SHUTDOWN, Shutdown.	RFC 2960
8	SHUTDOWN ACK, Shutdown Acknowledgement.	RFC 2960
9	ERROR, Operation Error.	RFC 2960
10	COOKIE ECHO, State Cookie.	RFC 2960
11	COOKIE ACK, Cookie Acknowledgement.	RFC 2960
12	ECNE, Reserved for Explicit Congestion Notification Echo.	RFC 2960
13	CWR, Reserved for Congestion Window Reduced.	RFC 2960
14	SHUTDOWN COMPLETE, Shutdown Complete.	RFC 2960
15	AUTH, Authentication Chunk.	RFC 4895
16 - 62
63	reserved for IETF-defined chunk extensions.
64 - 126
127	reserved for IETF-defined chunk extensions.
128	ASCONF-ACK, Address Configuration Acknowledgment.	RFC 5061
129 - 131
132	PAD, Padding Chunk.	RFC 4820
133 - 191
192	Forward TSN.	RFC 3758
193	ASCONF, Address Configuration Change Chunk.	RFC 5061
194 - 254
255	reserved for IETF-defined chunk extensions.	RFC 4960

Chunk flags. 8 bits.
The usage of these bits depends on the chunk type as given by the Chunk type. Unless otherwise specified, they are set to zero on transmit and are ignored on receipt.

Chunk length. 16 bits.
This size of the chunk in bytes including the Chunk type, Chunk flags, Chunk length, and Chunk data fields. Therefore, if the Chunk value field is zero-length, the Length field will be set to 4. This field does not count any padding.

Chunk data. Variable length.
The actual information to be transferred in the chunk. The usage and format of this field is dependent on the Chunk type.

Payload Protocol Identifiers.

Identifier	Description	References
0
1	IUA, ISDN Q.921-User Adaptation.	RFC 4233
2	M2UA, MTP2-User Adaptation.	RFC 3331
3	M3UA, MTP3-User Adaptation.	RFC 3332, RFC 4666
4	SUA, Signalling Connection Control Part User Adaptation Layer.	RFC 2960
5	M2PA, MTP2 User Peer-to-peer Adaptation Layer.	RFC 2960
6	V5UA, V5.2-User Adaptation.	RFC 3807
7	H.248.
8	BICC / Q.2150.3.
9	TALI, Transport Adapter Layer Interface.	RFC 3094
10	DUA.	RFC 4129
11	ASAP, Aggregate Server Access Protocol.	RFC 5352
12	ENRP, Enpoint Name Resolution Protocol.	RFC 5353
13	H.323.
14	Q.IPC / Q.2150.3.
15	SIMCO.
16	DDP Segment Chunk.	RFC 5043
17	DDP Stream Session Control.	RFC 5043
18	S1AP, S1 Application Protocol.
19	RUA.
20	HNBAP.
21	ForCES-HP.
22	ForCES-MP.
23	ForCES-LP.
24	SBc-AP.
25	NBAP.
26
27	X2AP.

Glossary:

RFCs:

[RFC 2960] Stream Control Transmission Protocol.

Category: Standards Track.

[RFC 3257] Stream Control Transmission Protocol Applicability Statement.

Category: Informational.

[RFC 3286] An Introduction to the Stream Control Transmission Protocol (SCTP).

Category: Informational.

[RFC 3309] Stream Control Transmission Protocol (SCTP) Checksum Change.

Category: Standards.
Updates:
RFC 2960.

[RFC 3331] Signaling System 7 (SS7) Message Transfer Part 2 (MTP2) - User Adaptation Layer.

Category: Standards Track.
Defines SCTP Payload Protocol Identifier 2 (M2UA).

[RFC 3332] Signaling System 7 (SS7) Message Transfer Part 3 (MTP3) - User Adaptation Layer (M3UA).

Category: Standards Track.
Defines SCTP Payload Protocol Identifier 3 (M3UA).

[RFC 3436] Transport Layer Security over Stream Control Transmission Protocol.

Category: Standards Track.

[RFC 3554] On the Use of Stream Control Transmission Protocol (SCTP) with IPsec.

Category: Standards Track.

[RFC 3708] Using TCP Duplicate Selective Acknowledgement (DSACKs) and Stream Control Transmission Protocol (SCTP) Duplicate Transmission Sequence Numbers (TSNs) to Detect Spurious Retransmissions.

Category: Experimental.

[RFC 3758] Stream Control Transmission Protocol (SCTP) Partial Reliability Extension.

Category: Standards Track.
Defines SCTP chunk type 192 (Forward TSN).

[RFC 3807] V5.2-User Adaptation Layer (V5UA).

Category: Standards Track.
Defines SCTP Payload Protocol Identifier 6 (V5UA).
Updates:
RFC 3057.

[RFC 3873] Stream Control Transmission Protocol (SCTP) Management Information Base (MIB).

Category: Standards Track.
Defines SNMP MIB iso.org.dod.internet.mgmt.mib-2.sctpMIB (1.3.6.1.2.1.104).

[RFC 4129] Digital Private Network Signaling System (DPNSS)/ Digital Access Signaling System 2 (DASS 2) Extensions to the IUA Protocol.

Category: Standards Track.
Defines SCTP Payload Protocol Identifier 10 (DUA).

[RFC 4138] Forward RTO-Recovery (F-RTO): An Algorithm for Detecting Spurious Retransmission Timeouts with TCP and the Stream Control Transmission Protocol (SCTP).

Category: Experimental.

[RFC 4166] Telephony Signalling Transport over Stream Control Transmission Protocol (SCTP) Applicability Statement.

Category: Informational.

[RFC 4168] The Stream Control Transmission Protocol (SCTP) as a Transport for the Session Initiation Protocol (SIP).

Category: Standards Track.

[RFC 4233] Integrated Services Digital Network (ISDN) Q.921-User Adaptation Layer.

Category: Standards Track.
Defines SCTP Payload Protocol Identifier 1 (IUA).
Obsoletes:
RFC 3057.

[RFC 4460] Stream Control Transmission Protocol (SCTP) Specification Errata and Issues.

Category: Informational.

[RFC 5352] Aggregate Server Access Protocol (ASAP).

Category: Experimental.
Defines ASAP messages 0 through 0x0E.
Defines IP multicast addresses 224.0.1.185 and FF0X:0:0:0:0:0:0:133.
Defines SCTP payload 11.

Publications:

Obsolete RFCs: