Initiation Acknowledgement (INIT ACK)

The INIT ACK chunk is used to acknowledge the initiation of an SCTP association.

The parameter part of INIT ACK is formatted similarly to the INIT chunk. It uses two extra variable parameters: The State Cookie and the Unrecognized Parameter:

The format of the INIT ACK chunk is shown below:

0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Type = 2 | Chunk Flags | Chunk Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Initiate Tag | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Advertised Receiver Window Credit | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Number of Outbound Streams | Number of Inbound Streams | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Initial TSN | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

\ \ / Optional/Variable-Length Parameters / \ \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Initiate Tag: 32 bits (unsigned integer)

The receiver of the INIT ACK records the value of the Initiate Tag parameter. This value MUST be placed into the Verification Tag field of every SCTP packet that the INIT ACK receiver transmits within this association.

The Initiate Tag MUST NOT take the value 0. See Section 5.3.1 for more on the selection of the Initiate Tag value.

If the value of the Initiate Tag in a received INIT ACK chunk is found to be 0, the receiver MUST treat it as an error and close the association by transmitting an ABORT.

Advertised Receiver Window Credit (a_rwnd): 32 bits (unsigned integer)

This value represents the dedicated buffer space, in number of bytes, the sender of the INIT ACK has reserved in association with this window. During the life of the association this buffer space SHOULD not be lessened (i.e. dedicated buffers taken away from this association).

Number of Outbound Streams (OS): 16 bits (unsigned integer)

Defines the number of outbound streams the sender of this INIT ACK chunk wishes to create in this association. The value of 0 MUST NOT be used.

Note: A receiver of an INIT ACK with the OS value set to 0 SHOULD destroy the association discarding its TCB.

Number of Inbound Streams (MIS) : 16 bits (unsigned integer)

Defines the maximum number of streams the sender of this INIT ACK chunk allows the peer end to create in this association. The value 0 MUST NOT be used.

Note: There is no negotiation of the actual number of streams but instead the two endpoints will use the min(requested, offered).

See Section 5.1.1 for details.

Note: A receiver of an INIT ACK with the MIS value set to 0 SHOULD destroy the association discarding its TCB.

Initial TSN (I-TSN) : 32 bits (unsigned integer)

Defines the initial TSN that the INIT-ACK sender will use. The valid range is from 0 to 4294967295. This field MAY be set to the value of the Initiate Tag field.

Fixed Parameters Status Initiate Tag Mandatory Advertised Receiver Window Credit Mandatory Number of Outbound Streams Mandatory Number of Inbound Streams Mandatory Initial TSN Mandatory

Variable Parameters Status Type Value State Cookie Mandatory 7

IPv4 Address (Note 1) Optional 5 IPv6 Address (Note 1) Optional 6 Unrecognized Parameters Optional 8

Reserved for ECN Capable (Note 2) Optional 32768 (0x8000) Host Name Address (Note 3) Optional 11

Note 1: The INIT ACK chunks can contain any number of IP address parameters that can be IPv4 and/or IPv6 in any combination.

Note 2: The ECN capable field is reserved for future use of Explicit Congestion Notification.

Note 3: The INIT ACK chunks MUST NOT contain more than one Host Name address parameter. Moreover, the sender of the INIT ACK MUST NOT combine any other address types with the Host Name address in the INIT ACK. The receiver of the INIT ACK MUST ignore any other address types if the Host Name address parameter is present.

IMPLEMENTATION NOTE: An implementation MUST be prepared to receive a INIT ACK that is quite large (more than 1500 bytes) due to the

variable size of the state cookie AND the variable address list. For example if a responder to the INIT has 1000 IPv4 addresses it wishes to send, it would need at least 8,000 bytes to encode this in the INIT ACK.

In combination with the Source Port carried in the SCTP common header, each IP Address parameter in the INIT ACK indicates to the receiver of the INIT ACK a valid transport address supported by the sender of the INIT ACK for the lifetime of the association being initiated.

If the INIT ACK contains at least one IP Address parameter, then the source address of the IP datagram containing the INIT ACK and any additional address(es) provided within the INIT ACK may be used as destinations by the receiver of the INIT-ACK. If the INIT ACK does not contain any IP Address parameters, the receiver of the INIT-ACK MUST use the source address associated with the received IP datagram as its sole destination address for the association.

The State Cookie and Unrecognized Parameters use the Value format as defined in Section 3.2.1 and are described below.

The other fields are defined the same as their counterparts in the INIT chunk.

3.3.3.1 Optional or Variable Length Parameters State Cookie

Parameter Type Value: 7

Parameter Length: variable size, depending on Size of Cookie Parameter Value:

This parameter value MUST contain all the necessary state and parameter information required for the sender of this INIT ACK to create the association, along with a Message Authentication Code (MAC). See Section 5.1.3 for details on State Cookie definition.

Unrecognized Parameters:

Parameter Type Value: 8

Parameter Length: Variable Size.

Parameter Value:

This parameter is returned to the originator of the INIT chunk when the INIT contains an unrecognized parameter which has a value that indicates that it should be reported to the sender.

This parameter value field will contain unrecognized parameters copied from the INIT chunk complete with Parameter Type, Length and Value fields.