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:

• gray: chunk fields,
• red: fixed parameters,
• green/blue: optional/variable-length parameters that alternate colors.

DATA chunk

Chunk type

always 0 for payload data (DATA)

Chunk flags

There are currently^[update] only 4 flags used

• I — SACK chunk should be sent back without delay.
• U — If set, this indicates that 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.^[4] 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

Chunk type

always 1 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 2.

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

Chunk type

Always 3 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 up to and including this number. Chunks with TSNs above this number have not been received yet – except those included in the optional gap ACK blocks (see below).

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 blocks (i.e. pairs of start and end TSNs) included in this chunk.

Number of duplicate TSNs

Indicates the number of duplicate TSNs reported in this chunk.

Optional parameters appear with alternating background colors of green and blue:

Gap ACK block #N start

Indicates a positive offset (with reference to the cumulative TSN ACK value) to the first TSN of an additional block of TSNs that are acknowledged.

Gap ACK block #N end

Indicates a positive offset (with reference to the cumulative TSN ACK value) to the last TSN of an additional block of TSNs that are acknowledged.

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

Chunk type

For heartbeat (HEARTBEAT), this value is always 4.

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

Chunk type

For heartbeat acknowledgement (HEARTBEAT ACK), this value is always 5.

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

Chunk type

always 6 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

Chunk type

For shutdown (SHUTDOWN), this value is always 7.

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

Chunk type

For shutdown acknowledgement (SHUTDOWN ACK), this value is always 8.

Chunk flags

There are currently no flags used.

Chunk length

This is the chunk length, which has a fixed length of 4.

ERROR chunk

Chunk type

For error (ERROR), this value is always 9.

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

Chunk type

always 10 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

Chunk type

For cookie acknowledgement (COOKIE ACK), this value is always 11.

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

Chunk type

For shutdown complete (SHUTDOWN COMPLETE), this value is always 14.

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.

AUTH chunk

Chunk type

For the authentication chunk (AUTH), this value is always 15.

Chunk flags

There are currently no flags used.

Chunk length

Length of the HMAC + 8.

Fixed parameters:

Shared key identifier

identifies the shared key that was used.

HMAC identifier

identifies the type of HMAC used.^[5]

HMAC

HMAC value. Might not be a multiple of 4 bytes. The SCTP protocol takes care of padding to a 4-byte boundary.^[6]

Optional parameters: none

ASCONF-ACK chunk

Chunk type

always 128 for the address reconfiguration acknowledgement chunk (ASCONF-ACK).

Chunk flags

There are currently no flags used.

Chunk length

Depends on the number and length of ASCONF parameter responses included.

Fixed parameters:

Sequence number

The sequence number of the ASCONF packet being acknowledged.

Optional parameters:

ASCONF parameter response 1..N

Address reconfiguration parameter responses (variable length).

RE-CONFIG chunk

Chunk type

always 130 for the stream reconfiguration chunk (RE-CONFIG).

Chunk flags

There are currently no flags used.

Chunk length

Depends on the number and length of re-configuration parameters.

Fixed parameters:

Re-configuration parameter 1

First stream reconfiguration parameter.

Optional parameters:

Re-configuration parameter 2

Second stream reconfiguration parameter.

At most two re-configuration parameters from those mentioned below may appear in this chunk. Not all combinations are valid, see RFC 6525 for details.

PAD chunk

The PAD chunk was introduced to facilitate path MTU discovery,^[7] by enabling a sender to arbitrarily increase the size of an SCTP packet.

Chunk type

always 132 for the padding chunk (PAD).

Chunk flags

There are currently no flags used.

Chunk length

Depends on the size of padding data. The minimum length is 4 bytes.

Fixed parameters: none

Optional parameters:

Padding data

Arbitrary data – will be ignored and unceremoniously discarded by the receiver.

I-DATA chunk

The I-DATA chunk was introduced to avoid a large message in one stream blocking messages in all other streams from being transmitted: SCTP primarily uses the TSN to achieve reliability. In some cases, the TSN is also needed to distinguish different DATA chunks.^[8] When a message is fragmented, the DATA TSN additionally doubles as a fragment sequence number. This means that all fragments in a message must be sent using consecutive TSNs, effectively blocking all other data. The I-DATA chunk disentangles the different uses of the TSN in DATA chunks.

As DATA and I-DATA chunks are not compatible, they may not both be used in the same association.

Chunk type

always 64 for payload data supporting interleaving (I-DATA).

Chunk flags

There are currently^[update] only 4 flags used

• I — SACK chunk should be sent back without delay.
• U — If set, this indicates this data is an unordered chunk. 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 21, 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 for acknowledgement and retransmission).

Stream identifier

Identifier of the stream that this data chunk belongs to.

Message identifier (MID)

Identifier of the message in this stream. If a message is fragmented then the same value is used for all fragments. For ordered messages, the MID also specifies the order in which the messages should be delivered to the upper layer. Ordered and unordered messages in the same stream use independent MID sequences.

Payload protocol identifier

Application-specific protocol identifier,^[4] only present if the B flag is set. 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.

Fragment sequence number

Fragment number for fragmented packets. Only present if the B flag is not set. If the B flag is set, then the fragment sequence number is implicitly zero, and the payload protocol identifier occupies the same space instead.

Data

Application-specific data.

Optional parameters: none.

FORWARD-TSN chunk

The FORWARD-TSN chunk was introduced to support selective unreliability: it allows the sender to tell the receiver that it will not retransmit some number of chunks, and requests that the receiver consider all these chunks as received.

Chunk type

always 192 for the forward TSN chunk (FORWARD-TSN).

Chunk flags

There are currently no flags used.

Chunk length

Depends on the number of new stream sequence numbers included.

Fixed parameters:

New cumulative transmission sequence number (TSN)

The next TSN that the receiver should expect. Any previous TSNs should be considered received.

Optional parameters:

Stream identifier 1..N

Stream identifiers of streams that were skipped by this chunk.

Stream sequence 1..N

New stream sequence numbers associated with the streams that were skipped.

ASCONF chunk

Chunk type

always 193 for the address reconfiguration chunk (ASCONF).

Chunk flags

There are currently no flags used.

Chunk length

Depends on the type of IP address and the number and lengths of ASCONF parameters included.

Fixed parameters:

Sequence number

The sequence number of the ASCONF packet.

Address parameter

parameter type : Type of address in the address parameter: 5 for IPv4, 6 for IPv6.

Address parameter

parameter length : Length of the address parameter: 8 for IPv4, 20 for IPv6.

Address parameter

IP address : 4 bytes for IPv4, 16 bytes for IPv6.

Optional parameters:

ASCONF parameter 1..N

Address reconfiguration parameters (variable length).

I-FORWARD-TSN chunk

The I-FORWARD-TSN chunk was introduced to be used instead of FORWARD-TSN when I-DATA is used instead of DATA.

Chunk type

always 194 for the forward TSN chunk with support for interleaving (I-FORWARD-TSN).

Chunk flags

There are currently no flags used.

Chunk length

Depends on the number of new stream message identifiers included.

Fixed parameters:

New cumulative transmission sequence number (TSN)

The next TSN that the receiver should expect. Any previous TSNs should be considered received.

Optional parameters:

Stream identifier 1..N

Stream identifiers of streams that were skipped by this chunk.

U

0 if the new message identifier is associated with the ordered messages, 1 if it is associated with the unordered messages in the stream.

Message identifier 1..N

New message identifiers associated with the streams that were skipped.