SCTP packet structure

Bits Bits 0 - 7 8 - 15 16 - 23 24 - 31
+0 Source port Destination port
32 Verification tag
64 Checksum
96 Chunk 1 type Chunk 1 flags Chunk 1 length
128 Chunk 1 data
… …
… Chunk N type Chunk N flags Chunk N length
… Chunk N data

The Stream Control Transmission Protocol (SCTP) has a simpler basic packet structure than TCP. Each consists of two basic sections:

  1. The common header, which occupies the first 12 bytes. In the diagram to the right, this header is highlighted in blue.
  2. The data chunks, which form the remaining portion of the packet. In the diagram, the first chunk is highlighted in green and the last of N chunks (Chunk N) is highlighted in red. There are several types, including payload data as well as different control messages.

Common header

All SCTP packets require the common header section (shown with a blue background).

Source port 
This field identifies the sending port.
Destination port 
This field identifies the receiving port that hosts use to route the packet to the appropriate endpoint/application.
Verification tag 
A 32-bit random value created during initialization to distinguish stale packets from a previous connection.
Checksum 
SCTP's original design catered for Adler-32; but RFC 3309 changed the protocol to use the CRC32c algorithm. [1][2]

Chunks

Each SCTP packet consists, in addition to the common header, of chunks. Each chunk has a common format, but the contents can vary. One chunk appears in the diagram to the right with the green background.

Chunk type 
An 8-bit value predefined by the IETF to identify the contents of the chunk value field.
Chunk flags 
Eight flag-bits whose definition varies with the chunk type. The default value is zero.
Chunk length 
A 16-bit unsigned value specifying the total length of the chunk in bytes (excludes any padding) that includes chunk type, flags, length, and value fields.
Chunk data 
General purpose data field whose definition varies with the chunk type.

If the chunk length does not equate to a multiple of 4 bytes then the protocol implicitly pads the chunk with trailing zeros.

Optional/variable-length parameter definition
+ Bits 0 - 15 16 - 31
0 Parameter type Parameter length
32 Parameter value

Additionally, each chunk type may define a set of parameters which it includes inside the chunk value field (and, consequently, their length in the chunk length).

Two types of parameter exist:

For optional/variable-length parameters, the parameter type, parameter length, and parameter value fields all behave just like their chunk counterparts. The minimum size of parameter is 4 bytes and this occurs when the parameter value field is empty and the parameter consists only of the type & length fields.

List of chunk types

RFC 2960 defines the following list of chunk types. More detailed information about each type is provided in the following subsections.

Following this table each chunk and its parameters are defined. Please note the following color scheme:

Value Abbreviation Description
0 DATA Payload data
1 INIT Initiation
2 INIT ACK Initiation acknowledgement
3 SACK Selective acknowledgement
4 HEARTBEAT Heartbeat request
5 HEARTBEAT ACK Heartbeat acknowledgement
6 ABORT Abort
7 SHUTDOWN Shutdown
8 SHUTDOWN ACK Shutdown acknowledgement
9 ERROR Operation error
10 COOKIE ECHO State cookie
11 COOKIE ACK Cookie acknowledgement
12 ECNE Explicit congestion notification echo (reserved)
13 CWR Congestion window reduced (reserved)
14 SHUTDOWN COMPLETE Shutdown complete
15-62 N/A Reserved by IETF
63 IETF-defined chunk extensions
64-126 Reserved by IETF
127 IETF-defined chunk extensions
128-190 Reserved by IETF
191 IETF-defined chunk extensions
192-254 Reserved by IETF
255 IETF-defined chunk extensions

DATA chunk

+ Bits 0 - 7 8 - 11 12 13 14 15 16 - 31
0 Chunk type=0 Reserved I U B E Chunk length
32 TSN
64 Stream identifier Stream sequence number
96 Payload protocol identifier
128
Data

Chunk type 
always zero for payload data (DATA).
Chunk flags 
There are currently only 4 flags used
  • I — SACK chunk should be sent back without delay.
  • U — If set, this indicates this data is an unordered chunk and the stream sequence number is invalid. If an unordered chunk is fragmented then each fragment has this flag set.
  • B — If set, this marks the beginning fragment. An unfragmented chunk has this flag set.
  • E — If set, this marks the end fragment. An unfragmented chunk has this flag set.
Chunk length 
The chunk length has a minimum value of 17 as data of size less than one byte is not allowed.
Fixed parameters:
Transmission sequence number (TSN) 
The sequence number for the entire DATA stream (used in fragmentation for reassembly).
Stream identifier 
Identifier of the stream that this data chunk belongs to
Stream sequence number 
Identifier of the sequence number for the message in this stream. If a message is fragmented then this value is maintained for all fragments.
Payload protocol identifier 
Application-specific protocol identifier. SCTP makes no use of this or modification of it. However, devices along the path or the endpoints may use it. A value of 0 indicates that no payload protocol is specified.
Data 
Application-specific data.
Optional parameters: none.

INIT chunk

+ Bits 0 - 7 8 - 15 16 - 31
0 Chunk type = 1 Chunk flags Chunk length
32 Initiate tag
64 Advertised receiver window credit
96 # of outbound streams # of inbound streams
128 Initial TSN
160 Parameter type = 5 Parameter length = 8
192 IPv4 address of sending endpoint
224 Parameter type = 6 Parameter length = 20
256 IPv6 address of sending endpoint
288
320
352
384 Parameter type = 9 Parameter length = 8
416 Suggested cookie life-span increment (milliseconds)
448 Parameter type = 11 Parameter length
480 Null-terminated host name
512 Parameter type = 12 Parameter length
544 Address type #1 Address type #2
576 Address type #3 ...
608 Parameter type = 32768 Parameter length
640 Explicit congestion notification (reserved)
Chunk type 
always one for initiation (INIT).
Chunk flags 
There are currently no flags used.
Chunk length 
This is the chunk length which has a minimum value of 20 when chunk value is empty and no optional parameters are used
Fixed parameters have identical meaning as INIT ACK:
Initiate tag 
Unsigned 32-bit number that is used in every SCTP packet in the verification tag within the common header.
Advertised receiver window credit (a_rwnd) 
Amount of dedicated buffer space for this association that should never be reduced.
# of outbound streams 
Number of outbound streams (from the sender of the INIT) it wishes to use for this association. Zero is an invalid value and the receiver should ABORT the association upon receiving a zero.
# of inbound streams 
Identical to # of outbound streams but number of inbound streams. No negotiation takes place on the established number but the minimum of requested and offered should be used.
Initial TSN 
Initial transmission sequence number to be used and may be any value.
Optional parameters appear with alternating background colors of green and blue:
Parameter type = 5 
This parameter lists all the IPv4 addresses used at the sending endpoint. If it is a multihomed connection then the IP address of each may be included.
Parameter type = 6 
This parameter lists all the IPv6 addresses used at the sending endpoint. If it is a multihomed connection then the IP address of each may be included.
Parameter type = 9 
This parameter provides a suggested life-span increment the receiver should add to its default cookie life-span (in milliseconds).
Parameter type = 11 
This parameter is a hostname as defined in RFC 1123, section 2.1. Actual resolution of this name is outside the scope of SCTP. Additionally, a null terminating character must be included and must be included in the parameter length.
Parameter type = 12 
This parameter lists the address types the sender supports (e.g., IPv4 = 5, IPv6 = 6, hostname = 11).
Parameter type = 32768 
This parameter is reserved for explicit congestion notification support.

INIT ACK chunk

The INIT ACK chunk replicates the INIT chunk except the chunk type is always two.

Mandatory parameters, only in INIT ACK:
Parameter type = 7 (State Cookie)
The state cookie holds the minimal information to recreate the Transmission Control Block, and is signed with the sender's private key. The format of the cookie is not specified.

SACK chunk

+ Bits 0 - 7 8 - 15 16 - 31
0 Chunk type = 3 Chunk flags Chunk length
32 Cumulative TSN ACK
64 Advertised receiver window credit
96 Number of gap ACK blocks = N Number of duplicate TSNs = X
128 Gap ACK block #1 start Gap ACK block #1 end
... ... ...
96+N*32 Gap ACK block #N start Gap ACK block #N end
128+N*32 Duplicate TSN #1
... ...
96+N*32+X*32 Duplicate TSN #X
Chunk type 
always three for selective acknowledgment (SACK).
Chunk flags 
There are currently no flags used.
Chunk length 
This is the chunk length which has a minimum value of 16 when no gaps or duplicates are sent.
Fixed parameters:
Cumulative TSN ACK 
Acknowledges all sequence numbers received in sequence so is the sequence number of the last received byte. The immediate value after this one has not been received yet.
Advertised receiver window credit 
Amount of dedicated buffer space for this association that should never be reduced.
Number of gap ACK blocks 
Indicates the number of gap ACK block start and gap ACK block end pairs included.
Number of duplicate TSNs 
Indicates the number of duplicate TSNs the endpoint has received.
Optional parameters appear with alternating background colors of green and blue:
Gap ACK block #n start 
Indicates the TSN (added to the cumulative TSN ACK) of a block of TSNs to acknowledge.
Gap ACK block #n end 
Indicates the TSN (added to the cumulative TSN ACK) of a block of TSNs to acknowledge.
Duplicate TSN #x 
A TSN that was received more than once. A TSN will appear in this list for each time it is received after the first time.

HEARTBEAT chunk

+ Bits 0 - 7 8 - 15 16 - 31
0 Chunk type = 4 Chunk flags Chunk length
32 Parameter type = 1 Parameter length
64+ Heartbeat info
Chunk type 
For heartbeat (HEARTBEAT), this value is always four.
Chunk flags 
There are currently no flags used.
Chunk length 
This is the chunk length which has a minimum value of 8 with no parameter value added.
Fixed parameters: None
Optional parameters are shown with alternating background colors of green and blue:
Parameter type = 1 
This parameter contains the sender-specific heartbeat info

HEARTBEAT ACK chunk

+ Bits 0 - 7 8 - 15 16 - 31
0 Chunk type = 5 Chunk flags Chunk length
32 Parameter type = 1 Parameter length
64+ Heartbeat info
Chunk type 
For heartbeat acknowledgement (HEARTBEAT ACK), this value is always five.
Chunk flags 
There are currently no flags used.
Chunk length 
This is the chunk length which has a minimum value of 8 with no parameter value added.
Fixed parameters: None
Optional parameters are shown with alternating background colors of green and blue:
Parameter type = 1 
This parameter contains the sender-specific heartbeat info received in the request.

ABORT chunk

+ Bits 0 - 7 8 - 14 15 16 - 31
0 Chunk type = 6 Reserved T Chunk length
32 Zero or more error causes
Chunk type 
always six for abort (ABORT).
Chunk flags 
There is currently only one flag used:
T 
Set if the sender sent its own Verification Tag (that receiver should check); not set if the sender sent peer's Verification Tag (which should be checked anyway).
Chunk length 
This is the chunk length which has a minimum value of 4 with no error causes given.
Optional parameters (the error causes) are defined in the ERROR chunk.

SHUTDOWN chunk

+ Bits 0 - 7 8 - 15 16 - 31
0 Chunk type = 7 Chunk flags Chunk length
32 Cumulative TSN ACK
Chunk type 
For shutdown (SHUTDOWN), this value is always seven.
Chunk flags 
There are currently no flags used.
Chunk length 
This is the chunk length which has a fixed length of 8.
Fixed parameters:
Cumulative TSN ACK 
Contains the last TSN received in sequence by the sender.

SHUTDOWN ACK chunk

+ Bits 0 - 7 8 - 15 16 - 31
0 Chunk type = 8 Chunk flags Chunk length = 4
Chunk type 
For shutdown acknowledgement (SHUTDOWN ACK), this value is always eight.
Chunk flags 
There are currently no flags used.
Chunk length 
This is the chunk length which has a fixed length of 4.

ERROR chunk

+ Bits 0 - 7 8 - 15 16 - 31
0 Chunk type = 9 Chunk flags Chunk length
32 Parameter type = 1 Parameter length = 8
64 Stream identifier Zeros (reserved)
96 Parameter type = 2 Parameter length
128 Number of missing parameters = N
160 Missing parameter type #1 Missing parameter type #2
192 ... Missing parameter type #N
224 Parameter type = 3 Parameter length = 8
256 Measure of staleness (microseconds)
288 Parameter type = 4 Parameter length = 4
320 Parameter type = 5 Parameter length
352 Unresolvable address
384 Parameter type = 6 Parameter length
416 Unrecognized chunk
448 Parameter type = 7 Parameter length = 4
480 Parameter type = 8 Parameter length
512 Unrecognized parameters
544 Parameter type = 9 Parameter length = 8
576 TSN value
608 Parameter type = 10 Parameter length = 4
Chunk type 
For error (ERROR), this value is always nine.
Chunk flags 
There are currently no flags used.
Chunk length 
This is the chunk length which has a minimum value of 8 when only one error is sent with no parameter value. The size is 4 bytes plus the size of all error causes.
Fixed parameters: None.
Optional parameters are shown with alternating background colors of green and blue:
Parameter type = 1 
This parameter identifies that the sender received an invalid stream-identifier.
Parameter type = 2 
This parameter indicates that the sender received an INIT or INIT ACK chunk with missing mandatory parameters.
Parameter type = 3 
This parameter indicates receipt of a valid state cookie but it was stale by a given number of microseconds.
Parameter type = 4 
This parameter indicates the sender is out of resources; this usually accompanies an ABORT chunk.
Parameter type = 5 
This parameter identifies an address that the sender could not resolve (possibly because it does not support the address type); this usually accompanies an ABORT chunk.
Parameter type = 6 
This parameter identifies an unrecognized chunk when the chunk type's most-significant bits are 01 or 11.
Parameter type = 7 
This parameter identifies a mandatory parameter in an INIT or INIT ACK chunk has an invalid value.
Parameter type = 8 
This parameter is directed to the originator of an INIT ACK chunk that contained an unrecognized parameter.
Parameter type = 9 
This parameter indicates a DATA chunk contained no user data; this usually accompanies an ABORT chunk.
Parameter type = 10 
This parameter indicates the sender received a COOKIE ECHO while the endpoint was in a SHUTDOWN-ACK-SENT state.

COOKIE ECHO chunk

+ Bits 0 - 7 8 - 15 16 - 31
0 Chunk type = 10 Chunk flags Chunk length
32+ Cookie
Chunk type 
always ten for cookie echo (COOKIE ECHO).
Chunk flags 
There are currently no flags used.
Chunk length 
This is the chunk length.
Chunk value 
Contains the cookie data.

COOKIE ACK chunk

+ Bits 0 - 7 8 - 15 16 - 31
0 Chunk type = 11 Chunk flags Chunk length = 4
Chunk type 
For cookie acknowledgement (COOKIE ACK), this value is always eleven.
Chunk flags 
There are currently no flags used.
Chunk length 
This is the chunk length and is always 4.

ECNE chunk

Not defined yet.

CWR chunk

Not defined yet.

SHUTDOWN COMPLETE chunk

+ Bits 0 - 7 8 - 14 15 16 - 31
0 Chunk type = 14 Reserved T Chunk length = 4
Chunk type 
For shutdown complete (SHUTDOWN COMPLETE), this value is always fourteen.
Chunk flags 
There is currently only one flag defined
T 
Set if the sender didn't have a TCB; not set if the sender had one (that it destroyed).
Chunk length 
This is the chunk length which has a fixed length of 4.

Notes

  1. ↑ Castagnoli, G.; S. Braeuer; M. nHerrman (June 1993). "Optimization of Cyclic Redundancy-Check Codes with 24 and 32 Parity Bits". IEEE Transactions on Communications 41 (6): 883. doi:10.1109/26.231911.. Castagnoli's et al. work on algorithmic selection of CRC polynomials
  2. ↑ Koopman, P. (June 2002). "32-Bit Cyclic Redundancy Codes for Internet Applications". The International Conference on Dependable Systems and Networks: 459. doi:10.1109/DSN.2002.1028931.. Verification of Castagnoli's results by exhaustive search and some new good polynomials

References

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