SCTP Headers

SCTP Generic header format

This is a generic overview of how a SCTP packet is laid out. Basically, you have a common header first with information describing the whole packet, and the source and destination ports etc. See more below for information on the common header.

After the common header a variable number of chunks are sent, up to the maximum possible in the MTU. All chunks can be bundled except for INIT, INIT ACK and SHUTDOWN COMPLETE, which must not be bundled. DATA chunks may be broken down to fit inside the MTU of the packets.

SCTP Common and generic headers

Every SCTP packet contains the Common header as seen above. The header contains four different fields and is set for every SCTP packet.

Source port - bit 0-15. This field gives the source port of the packet, which port it was sent from. The same as for TCP and UDP source port.

Destination port - bit 16-31. This is the destination port of the packet, ie., the port that the packet is going to. It is the same as for the TCP and UDP destination port.

Verification Tag - bit 32-63. The verification tag is used to verify that the packet comes from the correct sender. It is always set to the same value as the value received by the other peer in the Initiate Tag during the association initialization, with a few exceptions:

• An SCTP packet containing an INIT chunk must have the Verification tag set to 0.

• A SHUTDOWN COMPLETE chunk with the T-bit set must have the verification tag copied from the verification tag of the SHUTDOWN-ACK chunk.

• Packets containing ABORT chunk may have the verification tag set to the same verification tag as the packet causing the ABORT.

Checksum - bit 64-95. A checksum calculated for the whole SCTP packet based on the Adler-32 algorithm. Read RFC 2960 - Stream Control Transmission Protocol, appendix B for more information about this algorithm.

All SCTP chunks has a special layout that they all adhere to as can be seen above. This isn't an actual header, but rather a formalized way of how they do look.

Type - bit 0-7. This field specifies the chunk type of the packet, for example is it an INIT or SHUTDOWN chunk or what? Each chunk type has a specific number, and is specified in the image below. Here is a complete list of Chunk types:

Chunk Flags - bit 8-15. The chunk flags are generally not used but are set up for future usage if nothing else. They are chunk specific flags or bits of information that might be needed for the other peer. According to specifications, flags are only used in DATA, ABORT and SHUTDOWN COMPLETE packets at this moment. This may change however.

Chunk Length - bit 16-31. This is the chunk length calculated in bytes. It includes all headers, including the chunk type, chunk flags, chunk length and chunk value. If there is no chunk value, the chunk length will be set to 4 (bytes).

Chunk Value - bit 32-n. This is specific to each chunk and may contain more flags and data pertaining to the chunk type. Sometimes it might be empty, in which case the chunk length will be set to 4.

SCTP ABORT chunk

The ABORT chunk is used to abort an association as previously described in the Shutdown and abort section of this chapter. ABORT is issued upon unrecoverable errors in the association such as bad headers or data.

Type - bit 0-7. Always set to 6 for this chunk type.

Reserved - bit 8-14. Reserved for future chunk flags but not used as of writing this. See the SCTP Common and generic headers for more information about the chunk flags field.

T-bit - bit 15. If this bit is set to 0, the sender had a TCB associated with this packet that it has destroyed. If the sender had no TCB the T-bit should be set to 1.

Length - bit 16-31. Sets the length of the chunk in bytes including error causes.

SCTP COOKIE ACK chunk

The COOKIE ACK chunk is used during the initialization of the connection and never anywhere else in the connection. It must precede all DATA and SACK chunks but may be sent in the same packet as the first of these packets.

Type - bit 0-7. Always set to 11 for this type.

Chunk flags - bit 8-15. Not used so far. Should always be set to 0 according to RFC 2960 - Stream Control Transmission Protocol. You should always watch out for this kind of specific behaviour stated by RFC's since it might change in the future, and hence break your firewalls etc. Just the same as happened with IP and ECN. See the SCTP Common and generic headers section for more information.

Length - bit 16-31. Should always be 4 (bytes) for this chunk.

SCTP COOKIE ECHO chunk

The COOKIE ECHO chunk is used during the initialization of the SCTP connection by the initiating party to reply to the cookie sent by the responding party in the State cookie field in the INIT ACK packet. It may be sent together with DATA chunks in the same packet, but must precede the DATA chunks in such case.

Type - bit 0-7. The chunk type is always set to 10 for this chunk.

Chunk flags - bit 8-15. This field is not used today. The RFC specifies that the flags should always be set to 0, but this might cause trouble as can be seen in the SCTP Common and generic headers section above, specifically the Chunk flags explanation.

Length - bit 16-31. Specifies the length of the chunk, including type, chunk flags, length and cookie fields in bytes.

Cookie - bit 32-n. This field contains the cookie as sent in the previous INIT ACK chunk. It must be the exact same as the cookie sent by the responding party for the other end to actually open the connection. The RFC 2960 - Stream Control Transmission Protocol specifies that the cookie should be as small as possible to insure interoperability, which is very vague and doesn't say much.

SCTP DATA chunk

DATA chunks are used to send actual data through the stream and have rather complex headers in some ways, but not really worse than TCP headers in general. Each DATA chunk may be part of a different stream, since each SCTP connection can handle several different streams.

Type - bit 0-7. The Type field should always be set to 0 for DATA chunks.

Reserved - bit 8-12. Not used today. Might be applicable for change. See SCTP Common and generic headers for more information.

U-bit - bit 13. The U-bit is used to indicate if this is an unordered DATA chunk. If it is, the Stream Sequence Number must be ignored by the receiving host and send it on to the upper layer without delay or tries to re-order the DATA chunks.

B-bit - bit 14. The B-bit is used to indicate the beginning of a fragmented DATA chunk. If this bit is set and the E (ending) bit is not set, it indicates that this is the first fragment of a chunk that has been fragmented into several DATA chunks.

E-bit - bit 15. The E-bit is used to indicate the ending of a fragmented DATA chunk. If this flag is set on a chunk, it signals to the SCTP receiver that it can start reassembling the fragments and pass them on to the upper layer. If a packet has both the BE-bits set to set to 0, it signals that the chunk is a middle part of a fragmented chunk. If both BE-bits are set to 1 it signals that the packet is unfragmented and requires no reassembly et cetera.

Length - bit 16-31. The length of the whole DATA chunk calculated in bytes,including the chunk type field and on until the end of the chunk.

TSN - bit 32-63. The Transmission Sequence Number (TSN) is sent in the DATA chunk, and the receiving host uses the TSN to acknowledge that the chunk got through properly by replying with a SACK chunk. This is an overall value for the whole SCTP association.

Stream Identifier - bit 64-79. The Stream Identifier is sent along with the DATA chunk to identify which stream the DATA chunk is associated with. This is used since SCTP can transport several streams within a single association.

Stream Sequence Number - bit 80-95. This is the sequence number of the chunk for the specific stream identified by the Stream Identifier. This sequence number is specific for each stream identifier. If a chunk has been fragmented, the Stream Sequence Number must be the same for all fragments of the original chunk.

Payload Protocol Identifier - bit 96-127. This value is filled in by the upper layers, or applications using the SCTP protocol as a way to identify to each other the content of the DATA chunk. The field must always be sent, including in fragments since routers and firewalls, et cetera, on the way might need the information. If the value was set to 0, the value was not set by the upper layers.

User data - bit 128-n. This is the actual data that the chunk is transporting. It can be of variable length, ending on an even octet. It is the data in the stream as specified by the stream sequence number n in the stream S.

SCTP ERROR chunk

The ERROR chunk is sent to inform the other peer of any problems within the current stream. Each ERROR chunk can contain one or more Error Causes, which are more specifically detailed in the RFC 2960 - Stream Control Transmission Protocol document. I will not go into further details here than the basic ERROR chunk, since it would be too much information. The ERROR chunk is not fatal in and of itself, but rather details an error that has happened. It may however be used together with an ABORT chunk to inform the peer of the error before killing the connection.

Type - bit 0-7. This value is always set to 9 for ERROR 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. Specifies the length of the chunk in bytes, including all the Error Causes.

Error causes - bit 32-n. Each ERROR chunk may contain one or more Error Causes, which notifies the opposite peer of a problem with the connection. Each Error Cause follows a specific format, as described in the RFC 2960 - Stream Control Transmission Protocol document. We will not go into them here more than to say that they all contain an Cause Code, cause length and cause specific information field. The following Error Causes are possible:

SCTP HEARTBEAT chunk

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.

SCTP 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.

SCTP INIT 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:

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.

Type - bit 0-15. This is always set to 11 for Hostname Parameters.

Length - bit 16-31. The length of the whole parameter, including type, length and hostname field. The Hostname field is variable length. The length is counted in bytes.

Hostname - bit 32-n. A variable length parameter containing a hostname. The hostname is resolved by the receiving end to get the addresses that can be used to contact the sending endpoint.

SCTP INIT ACK chunk

The INIT ACK chunk is sent in response to a INIT chunk and contains basically the same headers, but with values from the recipient of the original INIT chunk. In addition, it has two extra variable length parameters, the State Cookie and the Unrecognized Parameter parameters.

Type - bit 0-7. This header is always set to 2 for INIT 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 chunk length is the length of the whole packet, including everything in the headers, and the optional parameters.

Initiate Tag - bit 32-63. The receiver of the Initiate Tag of the INIT ACK chunk must save this value and copy it into the Verification Tag field of every packet that it sends to the sender of the INIT ACK chunk. The Initiate Tag must not be 0, and if it is, the receiver of the INIT ACK chunk must close the connection with an ABORT.

Advertised Receiver Window Credit (a_rwnd) - bit 64-95. The dedicated buffers that the sender of this chunk has located for traffic, counted in bytes. The dedicated buffers should never be lowered to below this value.

Number of Outbound Streams - bit 96-111. How many outbound streams that the sending host wishes to create. Must not be 0, or the receiver of the INIT ACK should ABORT the association. 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. How many inbound streams that the sending endpoint is willing to accept. Must not be 0, or the receiver of the INIT ACK should ABORT the association. 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 is set to the Initial Transmission Sequence Number (I-TSN) which will be used by the sending party in the association to start with.

After this point, the INIT ACK chunk continues with optional variable-length parameters. The parameters are exactly the same as for the INIT chunk, with the exception of the addition of the State Cookie and the Unrecognized Parameters parameter, and the deletion of the Supported Address Types parameter. The list in other words look like this:

The State Cookie is used in INIT ACK to send a cookie to the other host, and until the receiving host has replied with a COOKIE ECHO chunk, the association is not guaranteed. This is to prevent basically the same as a SYN attack in TCP protocol.

Type - bit 0-15. Always set to 7 for all State Cookie parameters.

Length - bit 16-31. The size of the whole parameter, including the type, length and State Cookie field in bytes.

State Cookie - bit 31-n. This parameter contains a cookie of variable length. For a description on how this cookie is created, see the RFC 2960 - Stream Control Transmission Protocol document.

SCTP SACK chunk

The SACK chunk is used to tell the sender of DATA chunks which chunks has been received and where there has been a gap in the stream, based on the received TSN's. Basically, the SACK chunk acknowledges that it has received data up to a certain point (the Cumulative TSN Ack parameter), and then adds Gap Ack Blocks for all of the data that it has received after the Cumulative TSN Ack point. A SACK chunk must not be sent more than once for every DATA chunk that is received.

Type - bit 0-7. This header is always set to 3 for SACK 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 chunk, including everything in the headers and all the parameters.

Cumulative TSN Ack - bit 32-63. This is the Cumulative TSN Ack parameter, which is used to acknowledge data. The DATA chunk receiver will use this field to tell the sending host that it has received all data up to this point of the association. After this point, all data that has not been specifically acknowledged by the Gap Ack Blocks will, basically, be considered unaccounted for.

Advertised Receiver Window Credit (a_rwnd) - bit 64-95. The a_rwnd field is basically the same as the a_rwnd in the INIT and INIT ACK chunks, but can be used to raise or lower the a_rwnd value. Please read more in the RFC 2960 - Stream Control Transmission Protocol document about this.

Number of Gap Ack Blocks - bit 96-111. The number of Gap Ack Blocks listed in this chunk. Each Gap Ack Block takes up 32 bits in the chunk.

Number of Duplicate TSNs - bit 112-127. The number of DATA chunks that has been duplicated. Each duplicated TSN is listed after the Gap Ack Blocks in the chunk, and each TSN takes 32 bits to send.

Gap Ack Block #1 Start - bit 128-143. This is the first Gap Ack Block in the SACK chunk. If there are no gaps in the received DATA chunk TSN numbers, there will be no Gap Ack Blocks at all. However, if DATA chunks are received out of order or some DATA chunks where lost during transit to the host, there will be gaps. The gaps that has been seen will be reported with Gap Ack Blocks. The Gap Ack Block start point is calculated by adding the Gap Ack Block Start parameter to the Cumulative TSN value. The calculated value is the start of the block.

Gap Ack Block #1 End - bit 144-159. This value reports the end of the first Gap Ack Block in the stream. All the DATA chunks with the TSN between the Gap Ack Block Start and the Gap Ack Block End has been received. The Gap Ack Block End value is added to the Cumulative TSN, just as the Start parameter, to get the actual last TSN of the block chunks to be Acknowledged.

Gap Ack Block #N Start - bits variable. For every Gap Ack Block counted in the Number of Gap Ack Blocks parameter, one Gap Ack Block is added, until the final N block. Ie, if Number of Gap Ack Blocks = 2, then there will be two Gap Ack Blocks in the SACK chunk. This is the last one simply, and contains the same type of value as the Gap Ack Block #1 Start.

Gap Ack Block #N End - bits variable. Same as for the Gap Ack Block #N End, but for the end of the gap.

Duplicate TSN #1 - bits variable. These fields report a duplicate TSN, in which case we have already received a specific chunk, but receive the same TSN several times more. This can either be router glitches (retransmitting already sent data) or a case of retransmission from the sending endpoint, or a score of other possibilities. Each instance of a duplicate TSN should be reported once. For example, if 2 duplicate TSN's has been received after acknowledging the first one, each of these duplicate TSN's should be sent sent in the next SACK message that is being sent. If even more duplicate TSN's should appear after this second SACK is sent, the new duplicates should be added in the next SACK, and so on.

Duplicate TSN #X - bits variable. This is the last duplicate TSN parameter, containing the same type of information as the first parameter.

SCTP SHUTDOWN chunk

The SHUTDOWN chunk is issued when one of the endpoints of a connection wants to close the current association. The sending party must empty all of its sending buffers before sending the SHUTDOWN chunk, and must not send any more DATA chunks afterwards. The receiver must also empty its sending buffers and must then send the responding SHUTDOWN ACK chunk.

Type - bit 0-7. This header is always set to 7 for SHUTDOWN 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 the Cumulative TSN Ack parameter. The length of the SHUTDOWN chunk should always be 8.

Cumulative TSN Ack - bit 32-63. This is a Cumulative TSN Ack field, just the same as in the SACK chunk. The Cumulative TSN Ack acknowledges the last TSN received in sequence from the opposite endpoint. This parameter does not, nor can the rest of the SHUTDOWN chunk either, acknowledge Gap Ack Blocks. The lack of a Gap Ack Block in the SHUTDOWN chunk that was acknowledged before should not be interpreted as if the previously acknowledged block was lost again.

SCTP SHUTDOWN ACK chunk

The SHUTDOWN ACK chunk is used to acknowledge a SHUTDOWN chunk that has been received. Before the SHUTDOWN ACK chunk is sent, all data in the sending buffers should be sent, but the buffers must not accept any new data from the application. SCTP does not support half-open connections as TCP does.

Type - bit 0-7. This header is always set to 8 for SHUTDOWN 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 chunk length is the length of the whole chunk. The length of the SHUTDOWN ACK chunk should always be 4.

SCTP SHUTDOWN COMPLETE chunk

The SHUTDOWN COMPLETE chunk is sent, by the originating host of the SHUTDOWN, in response to the SHUTDOWN ACK chunk. It is sent to acknowledge that the association is finally closed.

Type - bit 0-7. Always set to 14 for SHUTDOWN COMPLETE chunks.

Reserved - bit 8-14. Not used today. Might be applicable for change. See SCTP Common and generic headers for more information.

T-bit - bit 15. The T-bit is not set to signal that the sending host had a Transmission Control Block (TCB) associated with this connection and that it destroyed. If the T-bit was set, it had no TCB to destroy.

Length - bit 16-31. This is always set to 4 for SHUTDOWN COMPLETE chunks, since the chunk should never be any larger, as long as no updates to the standards are made.