The HEARTBEAT chunk is sent by one of the peers to probe and find out if a specific SCTP endpoint address is up. This is sent to the different addresses that was negotiated during the initialization of the association to find out if they are all up.

Type - bit 0-7. The type is always set to 4 for HEARTBEAT chunks.

Chunk flags - bit 8-15. Not used today. Might be applicable for change. See SCTP Common and generic headers for more information.

Length - bit 16-31. The length of the whole chunk, including the Heartbeat Information TLV.

Heartbeat Information TLV - bit 32-n. This is a variable-length parameter as defined inside the RFC 2960 - Stream Control Transmission Protocol document. This is a mandatory parameter for the HEARTBEAT chunks that contains 3 fields, info type = 1, info length and a sender-specific Heartbeat Information parameter. The last field should be a sender-specific information field of some kind, for example a timestamp when the heartbeat was sent and a destination IP address. This is then returned in the HEARTBEAT ACK chunk.

The HEARTBEAT ACK is used to acknowledge that a HEARTBEAT was received and that the connection is working properly. The chunk is always sent to the same IP address as the request was sent from.

Type - bit 0-7. Always set to 5 for HEARTBEAT ACK chunks.

Chunk flags - bit 8-15. Not used today. Might be applicable for change. See SCTP Common and generic headers for more information.

Chunk length - bit 16-31. The length of the HEARTBEAT ACK chunk including the Heartbeat Information TLV, calculated in bytes.

Heartbeat Information TLV - bit 32-n. This field must contain the Heartbeat Information parameter that was sent in the original HEARTBEAT chunk.

The INIT chunk is used to initiate a new association with a destination host, and is the first chunk to be sent by the connecting host. The INIT chunk contains several mandatory fixed length parameters, and some optional variable length parameters. The fixed length mandatory parameters are already in the above headers, and are the Initiate Tag, Advertised Receiver Window Credit, Number of Outbound Streams, Number of Inbound Streams and the Initial TSN parameters. After this comes a couple of optional parameters, they will be listed with the optional parameters paragraph below.

Type - bit 0-7. The type field is always set to 1 for INIT chunks.

Chunk flags - bit 8-15. Not used today. Might be applicable for change. See SCTP Common and generic headers for more information.

Chunk Length - bit 16-31. The chunk length is the length of the whole packet, including everything in the headers, including the optional parameters.

Initiate Tag - bit 32-63. The Initiate Tag is set within the INIT chunk and must be used by the receiver to acknowledge all packets henceforth, within the Verification Tag of the established association. The Initiate Tag may take any value except 0. If the value is 0 anyways, the receiver must react with an ABORT.

Advertised Receiver Window Credit (a_rwnd)- bit 64-95. This is the minimum receiving buffer that the sender of the INIT chunk will allocate for this association, in bytes. This can then be used by the receiver of the a_rwnd, to know how much data it can send out without being SACK'ed. This window should not be lessened, but it might by sending the new a_rwnd in a SACK chunk.

Number of Outbound Streams - bit 96-111. This specifies the maximum number of outbound streams that the connecting host wishes to create to the receiving host. The value must not be 0, and if it is, the receiving host should ABORT the association immediately. There is no negotiation of the minimum number of outbound or inbound streams, it is simply set to the lowest that either host has set in the header.

Number of Inbound Streams - bit 112-127. Specifies the maximum number of inbound connections that the sending peer will allow the receiving host to create in this association. This must not be set to 0, or the receiving host should ABORT the connection. There is no negotiation of the minimum number of outbound or inbound streams, it is simply set to the lowest that either host has set in the header.

Initial TSN - bit 128-159. This value sets the initial Transmit Sequence Number (TSN) that the sender will use when sending data. The field may be set to the same value as the Initiate Tag.

On top of the above mandatory fixed length headers, there are also some optional variable length parameters that might be set, and at least one of the IPv4, IPv6 or Hostname parameters must be set. Only one Hostname may be set, and if a Hostname is set, no IPv4 or IPv6 parameters may be set. Multiple IPv4 and IPv6 parameters may also be set in the same INIT chunk. Also, none of these parameters needs to be set in case the sender only has one address that can be reached, which is where the chunk should be coming from. These parameters are used to set up which addresses may be used to connect to the other end of the association. This is a full list of all the parameters available in the INIT chunk:

Table 2-3. INIT Variable Parameters

Below we describe the three most common Parameters used in the INIT chunk.

The IPv4 parameter is used to send an IPv4 address in the INIT chunk. The IPv4 address can be used to send data through the association. Multiple IPv4 and IPv6 addresses can be specified for a single SCTP association.

Parameter Type - bit 0-15. This is always set to 5 for IPv4 address parameters.

Length - bit 16-31. This is always set to 8 for IPv4 address parameters.

IPv4 Address - bit 32-63. This is an IPv4 address of the sending endpoint.

This parameter is used to send IPv6 addresses in the INIT chunk. This address can then be used to contact the sending endpoint with this association.

Type - bit 0-15. Always set to 6 for the IPv6 parameters.

Length bit 16-31. Always set to 20 for IPv6 parameters.

IPv6 address - bit 32-159. This is an IPv6 address of the sending endpoint that can be used to connect to by the receiving endpoint.

The Hostname parameter is used to send a single hostname as an address. Thea receiving host must then look up the hostname and use any and/or all of the addresses it receives from there. If a hostname parameter is sent, no other IPv4, IPv6 or Hostname parameters may be sent.