3.1. SCTP Common Header Field Descriptions

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      Source Port Number       |    Destination Port Number    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       Verification Tag                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           Checksum                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Source Port Number: 16 bits (unsigned integer)

This is the SCTP sender's port number. It can be used by the receiver in combination with the source IP address, the SCTP Destination Port Number, and possibly the destination IP address to identify the association to which this packet belongs. The Source Port Number 0 MUST NOT be used.¶

Destination Port Number: 16 bits (unsigned integer)

This is the SCTP port number to which this packet is destined. The receiving host will use this port number to de-multiplex the SCTP packet to the correct receiving endpoint/application. The Destination Port Number 0 MUST NOT be used.¶

Verification Tag: 32 bits (unsigned integer)

The receiver of an SCTP packet uses the Verification Tag to validate the sender of this packet. On transmit, the value of the Verification Tag MUST be set to the value of the Initiate Tag received from the peer endpoint during the association initialization, with the following exceptions:¶

• A packet containing an INIT chunk MUST have a zero Verification Tag.¶
• A packet containing a SHUTDOWN COMPLETE chunk with the T bit set MUST have the Verification Tag copied from the packet with the SHUTDOWN ACK chunk.¶
• A packet containing an ABORT chunk MAY have the Verification Tag copied from the packet that caused the ABORT chunk to be sent. For details, see Sections 8.4 and 8.5.¶

Checksum: 32 bits (unsigned integer)

This field contains the checksum of the SCTP packet. Its calculation is discussed in Section 6.8. SCTP uses the CRC32c algorithm as described in Appendix A for calculating the checksum.¶

3.2. Chunk Field Descriptions

The figure below illustrates the field format for the chunks to be transmitted in the SCTP packet. Each chunk is formatted with a Chunk Type field, a Chunk Flags field, a Chunk Length field, and a Chunk Value field.¶

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Chunk Type   |  Chunk Flags  |         Chunk Length          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\                                                               \
/                          Chunk Value                          /
\                                                               \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Chunk Type: 8 bits (unsigned integer)

This field identifies the type of information contained in the Chunk Value field. It takes a value from 0 to 254. The value of 255 is reserved for future use as an extension field.¶

The values of Chunk Types defined in this document are as follows:¶

Table 1: Chunk Types

ID Value	Chunk Type
0	Payload Data (DATA)
1	Initiation (INIT)
2	Initiation Acknowledgement (INIT ACK)
3	Selective Acknowledgement (SACK)
4	Heartbeat Request (HEARTBEAT)
5	Heartbeat Acknowledgement (HEARTBEAT ACK)
6	Abort (ABORT)
7	Shutdown (SHUTDOWN)
8	Shutdown Acknowledgement (SHUTDOWN ACK)
9	Operation Error (ERROR)
10	State Cookie (COOKIE ECHO)
11	Cookie Acknowledgement (COOKIE ACK)
12	Reserved for Explicit Congestion Notification Echo (ECNE)
13	Reserved for Congestion Window Reduced (CWR)
14	Shutdown Complete (SHUTDOWN COMPLETE)
15 to 62	Unassigned
63	Reserved for IETF-defined Chunk Extensions
64 to 126	Unassigned
127	Reserved for IETF-defined Chunk Extensions
128 to 190	Unassigned
191	Reserved for IETF-defined Chunk Extensions
192 to 254	Unassigned
255	Reserved for IETF-defined Chunk Extensions

Note: The ECNE and CWR chunk types are reserved for future use of Explicit Congestion Notification (ECN).¶

Chunk Types are encoded such that the highest-order 2 bits specify the action that is taken if the processing endpoint does not recognize the Chunk Type.¶

Table 2: Processing of Unknown Chunks

00	Stop processing this SCTP packet and discard the unrecognized chunk and all further chunks.¶
01	Stop processing this SCTP packet, discard the unrecognized chunk and all further chunks, and report the unrecognized chunk in an ERROR chunk using the 'Unrecognized Chunk Type' error cause.¶
10	Skip this chunk and continue processing.¶
11	Skip this chunk and continue processing, but report it in an ERROR chunk using the 'Unrecognized Chunk Type' error cause.¶

Chunk Flags: 8 bits

The usage of these bits depends on the Chunk Type, as given by the Chunk Type field. Unless otherwise specified, they are set to 0 on transmit and are ignored on receipt.¶

Chunk Length: 16 bits (unsigned integer)

This value represents the size of the chunk in bytes, including the Chunk Type, Chunk Flags, Chunk Length, and Chunk Value fields. Therefore, if the Chunk Value field is zero-length, the Length field will be set to 4. The Chunk Length field does not count any chunk padding. However, it does include any padding of variable-length parameters other than the last parameter in the chunk.¶

Note: A robust implementation is expected to accept the chunk whether or not the final padding has been included in the Chunk Length.¶

Chunk Value: variable length

The Chunk Value field contains the actual information to be transferred in the chunk. The usage and format of this field is dependent on the Chunk Type.¶

The total length of a chunk (including Type, Length, and Value fields) MUST be a multiple of 4 bytes. If the length of the chunk is not a multiple of 4 bytes, the sender MUST pad the chunk with all zero bytes, and this padding is not included in the Chunk Length field. The sender MUST NOT pad with more than 3 bytes. The receiver MUST ignore the padding bytes.¶

SCTP-defined chunks are described in detail in Section 3.3. The guidelines for IETF-defined chunk extensions can be found in Section 15.1 of this document.¶

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Parameter Type         |       Parameter Length        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\                                                               \
/                        Parameter Value                        /
\                                                               \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.3. SCTP Chunk Definitions

This section defines the format of the different SCTP chunk types.¶

 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 = 0    |  Res  |I|U|B|E|            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                              TSN                              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      Stream Identifier S      |   Stream Sequence Number n    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                  Payload Protocol Identifier                  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\                                                               \
/                 User Data (seq n of Stream S)                 /
\                                                               \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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 = 1    |  Chunk Flags  |      Chunk Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         Initiate Tag                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          Advertised Receiver Window Credit (a_rwnd)           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Number of Outbound Streams   |   Number of Inbound Streams   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          Initial TSN                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\                                                               \
/              Optional/Variable-Length Parameters              /
\                                                               \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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 = 5            |          Length = 8           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         IPv4 Address                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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 = 6            |          Length = 20          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
|                         IPv6 Address                          |
|                                                               |
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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 = 9            |          Length = 8           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Suggested Cookie Life-Span Increment (msec.)          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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 = 11           |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                           Host Name                           /
\                                                               \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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 = 12           |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Address Type #1        |        Address Type #2        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            ......                             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+

 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              /
\                                                               \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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 = 7            |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                            Cookie                             /
\                                                               \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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 = 8            |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                    Unrecognized Parameter                     /
\                                                               \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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 = 3    |  Chunk Flags  |         Chunk Length          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      Cumulative TSN Ack                       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          Advertised Receiver Window Credit (a_rwnd)           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Gap Ack Blocks = N  |  Number of Duplicate TSNs = M |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    Gap Ack Block #1 Start     |     Gap Ack Block #1 End      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                                                               /
\                              ...                              \
/                                                               /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    Gap Ack Block #N Start     |     Gap Ack Block #N End      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Duplicate TSN 1                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                                                               /
\                              ...                              \
/                                                               /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Duplicate TSN M                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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 = 4    |  Chunk Flags  |       Heartbeat Length        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\                                                               \
/          Heartbeat Information TLV (Variable-Length)          /
\                                                               \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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 = 5    |  Chunk Flags  |     Heartbeat Ack Length      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\                                                               \
/          Heartbeat Information TLV (Variable-Length)          /
\                                                               \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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 = 6    |  Reserved   |T|            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\                                                               \
/                   zero or more Error Causes                   /
\                                                               \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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 = 7    |  Chunk Flags  |          Length = 8           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      Cumulative TSN Ack                       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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 = 8    |  Chunk Flags  |          Length = 4           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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 = 9    |  Chunk Flags  |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\                                                               \
/                   one or more Error Causes                    /
\                                                               \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          Cause Code           |         Cause Length          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                  Cause-Specific Information                   /
\                                                               \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Cause Code = 1         |       Cause Length = 8        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       Stream Identifier       |          (Reserved)           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Cause Code = 2         |   Cause Length = 8 + N * 2    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                 Number of missing params = N                  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Missing Param Type #1     |     Missing Param Type #2     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    Missing Param Type #N-1    |     Missing Param Type #N     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Cause Code = 3         |       Cause Length = 8        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                 Measure of Staleness (usec.)                  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Cause Code = 4         |       Cause Length = 4        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Cause Code = 5         |         Cause Length          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                     Unresolvable Address                      /
\                                                               \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Cause Code = 6         |         Cause Length          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                      Unrecognized Chunk                       /
\                                                               \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Cause Code = 7         |       Cause Length = 4        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Cause Code = 8         |         Cause Length          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                    Unrecognized Parameters                    /
\                                                               \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Cause Code = 9         |       Cause Length = 8        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                              TSN                              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Cause Code = 10        |       Cause Length = 4        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Cause Code = 11        |         Cause Length          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                       New Address TLVs                        /
\                                                               \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Cause Code = 12        |         Cause Length          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                   Upper Layer Abort Reason                    /
\                                                               \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Cause Code = 13         |        Cause Length         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                    Additional Information                     /
\                                                               \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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 = 10   |  Chunk Flags  |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                            Cookie                             /
\                                                               \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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 = 11   |  Chunk Flags  |          Length = 4           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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 = 14   |  Reserved   |T|          Length = 4           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5.1. Normal Establishment of an Association

The initialization process consists of the following steps (assuming that SCTP endpoint "A" tries to set up an association with SCTP endpoint "Z" and "Z" accepts the new association):¶

A)

"A" first builds a TCB and sends an INIT chunk to "Z". In the INIT chunk, "A" MUST provide its Verification Tag (Tag_A) in the Initiate Tag field. Tag_A SHOULD be a random number in the range of 1 to 4294967295 (see Section 5.3.1 for Tag value selection). After sending the INIT chunk, "A" starts the T1-init timer and enters the COOKIE-WAIT state.¶

B)

"Z" responds immediately with an INIT ACK chunk. The destination IP address of the INIT ACK chunk MUST be set to the source IP address of the INIT chunk to which this INIT ACK chunk is responding. In the response, besides filling in other parameters, "Z" MUST set the Verification Tag field to Tag_A and also provide its own Verification Tag (Tag_Z) in the Initiate Tag field.¶

Moreover, "Z" MUST generate and send along with the INIT ACK chunk a State Cookie. See Section 5.1.3 for State Cookie generation.¶

After sending an INIT ACK chunk with the State Cookie parameter, "Z" MUST NOT allocate any resources or keep any states for the new association. Otherwise, "Z" will be vulnerable to resource attacks.¶

C)

Upon reception of the INIT ACK chunk from "Z", "A" stops the T1-init timer and leaves the COOKIE-WAIT state. "A" then sends the State Cookie received in the INIT ACK chunk in a COOKIE ECHO chunk, starts the T1-cookie timer, and enters the COOKIE-ECHOED state.¶

The COOKIE ECHO chunk MAY be bundled with any pending outbound DATA chunks, but it MUST be the first chunk in the packet and, until the COOKIE ACK chunk is returned, the sender MUST NOT send any other packets to the peer.¶

D)

Upon reception of the COOKIE ECHO chunk, endpoint "Z" replies with a COOKIE ACK chunk after building a TCB and moving to the ESTABLISHED state. A COOKIE ACK chunk MAY be bundled with any pending DATA chunks (and/or SACK chunks), but the COOKIE ACK chunk MUST be the first chunk in the packet.¶

Implementation Note: An implementation can choose to send the COMMUNICATION UP notification to the SCTP user upon reception of a valid COOKIE ECHO chunk.¶

E)

Upon reception of the COOKIE ACK chunk, endpoint "A" moves from the COOKIE-ECHOED state to the ESTABLISHED state, stopping the T1-cookie timer. It can also notify its ULP about the successful establishment of the association with a COMMUNICATION UP notification (see Section 11).¶

An INIT or INIT ACK chunk MUST NOT be bundled with any other chunk. They MUST be the only chunks present in the SCTP packets that carry them.¶

An endpoint MUST send the INIT ACK chunk to the IP address from which it received the INIT chunk.¶

The T1-init timer and T1-cookie timer SHOULD follow the same rules given in Section 6.3. If the application provided multiple IP addresses of the peer, there SHOULD be a T1-init and T1-cookie timer for each address of the peer. Retransmissions of INIT chunks and COOKIE ECHO chunks SHOULD use all addresses of the peer similar to retransmissions of DATA chunks.¶

If an endpoint receives an INIT, INIT ACK, or COOKIE ECHO chunk but decides not to establish the new association due to missing mandatory parameters in the received INIT or INIT ACK chunk, invalid parameter values, or lack of local resources, it SHOULD respond with an ABORT chunk. It SHOULD also specify the cause of abort, such as the type of the missing mandatory parameters, etc., by including an error cause in the ABORT chunk. The Verification Tag field in the common header of the outbound SCTP packet containing the ABORT chunk MUST be set to the Initiate Tag value of the received INIT or INIT ACK chunk this ABORT chunk is responding to.¶

Note that a COOKIE ECHO chunk that does not pass the integrity check is not considered an 'invalid mandatory parameter' and requires special handling; see Section 5.1.5.¶

After the reception of the first DATA chunk in an association, the endpoint MUST immediately respond with a SACK chunk to acknowledge the DATA chunk. Subsequent acknowledgements SHOULD be done as described in Section 6.2.¶

When the TCB is created, each endpoint MUST set its internal Cumulative TSN Ack Point to the value of its transmitted Initial TSN minus one.¶

Implementation Note: The IP addresses and SCTP port are generally used as the key to find the TCB within an SCTP instance.¶

Endpoint A                                          Endpoint Z
{app sets association with Z}
(build TCB)
INIT [I-Tag=Tag_A
      & other info]  ------\
(Start T1-init timer)       \
(Enter COOKIE-WAIT state)    \---> (compose Cookie_Z)
                                /-- INIT ACK [Veri Tag=Tag_A,
                               /             I-Tag=Tag_Z,
(Cancel T1-init timer) <------/              Cookie_Z, & other info]

COOKIE ECHO [Cookie_Z] ------\
(Start T1-cookie timer)       \
(Enter COOKIE-ECHOED state)    \---> (build TCB, enter ESTABLISHED
                                      state)
                               /---- COOKIE ACK
                              /
(Cancel T1-cookie timer, <---/
 enter ESTABLISHED state)
{app sends 1st user data; strm 0}
DATA [TSN=init TSN_A
    Strm=0,Seq=0 & user data]--\
(Start T3-rtx timer)            \
                                 \->
                               /----- SACK [TSN Ack=init TSN_A,
                                            Block=0]
(Cancel T3-rtx timer) <------/
                                      ...
                                     {app sends 2 messages;strm 0}
                               /---- DATA
                              /        [TSN=init TSN_Z,
                          <--/          Strm=0,Seq=0 & user data 1]
SACK [TSN Ack=init TSN_Z,      /---- DATA
      Block=0]     --------\  /        [TSN=init TSN_Z +1,
                            \/          Strm=0,Seq=1 & user data 2]
                     <------/\
                              \
                               \------>

5.2. Handle Duplicate or Unexpected INIT, INIT ACK, COOKIE ECHO, and COOKIE ACK Chunks

During the life time of an association (in one of the possible states), an endpoint can receive from its peer endpoint one of the setup chunks (INIT, INIT ACK, COOKIE ECHO, or COOKIE ACK). The receiver treats such a setup chunk as a duplicate and process it as described in this section.¶

Note: An endpoint will not receive the chunk unless the chunk was sent to an SCTP transport address and is from an SCTP transport address associated with this endpoint. Therefore, the endpoint processes such a chunk as part of its current association.¶

The following scenarios can cause duplicated or unexpected chunks:¶

A)

the peer has crashed without being detected, restarted itself, and sent a new INIT chunk trying to restore the association,¶

B)

both sides are trying to initialize the association at about the same time,¶

C)

the chunk is from a stale packet that was used to establish the present association or a past association that is no longer in existence,¶

D)

the chunk is a false packet generated by an attacker, or¶

E)

the peer never received the COOKIE ACK chunk and is retransmitting its COOKIE ECHO chunk.¶

The rules in the following sections are applied in order to identify and correctly handle these cases.¶

Endpoint A                                          Endpoint Z
<-------------- Association is established---------------------->
Tag=Tag_A                                             Tag=Tag_Z
<--------------------------------------------------------------->
{A crashes and restarts}
{app sets up an association with Z}
(build TCB)
INIT [I-Tag=Tag_A'
      & other info]  --------\
(Start T1-init timer)         \
(Enter COOKIE-WAIT state)      \---> (find an existing TCB,
                                      populate TieTags if needed,
                                      compose Cookie_Z with Tie-Tags
                                      and other info)
                                /--- INIT ACK [Veri Tag=Tag_A',
                               /               I-Tag=Tag_Z',
(Cancel T1-init timer) <------/                Cookie_Z]
                                     (leave original TCB in place)
COOKIE ECHO [Veri=Tag_Z',
             Cookie_Z]-------\
(Start T1-init timer)         \
(Enter COOKIE-ECHOED state)    \---> (Find existing association,
                                      Tie-Tags in Cookie_Z match
                                      Tie-Tags in TCB,
                                      Tags do not match, i.e.,
                                      case X X M M above,
                                      Announce Restart to ULP
                                      and reset association).
                               /---- COOKIE ACK
(Cancel T1-init timer, <------/
 Enter ESTABLISHED state)
{app sends 1st user data; strm 0}
DATA [TSN=Initial TSN_A
    Strm=0,Seq=0 & user data]--\
(Start T3-rtx timer)            \
                                 \->
                              /--- SACK [TSN Ack=init TSN_A,Block=0]
(Cancel T3-rtx timer) <------/

5.3. Other Initialization Issues

5.4. Path Verification

During association establishment, the two peers exchange a list of addresses. In the predominant case, these lists accurately represent the addresses owned by each peer. However, a misbehaving peer might supply addresses that it does not own. To prevent this, the following rules are applied to all addresses of the new association:¶

1)

Any addresses passed to the sender of the INIT chunk by its upper layer in the request to initialize an association are automatically considered to be CONFIRMED.¶

2)

For the receiver of the COOKIE ECHO chunk, the only CONFIRMED address is the address to which the packet containing the INIT ACK chunk was sent.¶

3)

All other addresses not covered by rules 1 and 2 are considered UNCONFIRMED and are subject to probing for verification.¶

To probe an address for verification, an endpoint will send HEARTBEAT chunks including a 64-bit random nonce and a path indicator (to identify the address that the HEARTBEAT chunk is sent to) within the Heartbeat Info parameter.¶

Upon receipt of the HEARTBEAT ACK chunk, a verification is made that the nonce included in the Heartbeat Info parameter is the one sent to the address indicated inside the Heartbeat Info parameter. When this match occurs, the address that the original HEARTBEAT was sent to is now considered CONFIRMED and available for normal data transfer.¶

These probing procedures are started when an association moves to the ESTABLISHED state and are ended when all paths are confirmed.¶

In each RTO, a probe MAY be sent on an active UNCONFIRMED path in an attempt to move it to the CONFIRMED state. If during this probing the path becomes inactive, this rate is lowered to the normal HEARTBEAT rate. At the expiration of the RTO timer, the error counter of any path that was probed but not CONFIRMED is incremented by one and subjected to path failure detection, as defined in Section 8.2. When probing UNCONFIRMED addresses, however, the association overall error count is not incremented.¶

The number of packets containing HEARTBEAT chunks sent at each RTO SHOULD be limited by the 'HB.Max.Burst' parameter. It is an implementation decision as to how to distribute packets containing HEARTBEAT chunks to the peer's addresses for path verification.¶

Whenever a path is confirmed, an indication MAY be given to the upper layer.¶

An endpoint MUST NOT send any chunks to an UNCONFIRMED address, with the following exceptions:¶

• A HEARTBEAT chunk including a nonce MAY be sent to an UNCONFIRMED address.¶

• A HEARTBEAT ACK chunk MAY be sent to an UNCONFIRMED address.¶

• A COOKIE ACK chunk MAY be sent to an UNCONFIRMED address, but it MUST be bundled with a HEARTBEAT chunk including a nonce. An implementation that does not support bundling MUST NOT send a COOKIE ACK chunk to an UNCONFIRMED address.¶

• A COOKIE ECHO chunk MAY be sent to an UNCONFIRMED address, but it MUST be bundled with a HEARTBEAT chunk including a nonce, and the size of the SCTP packet MUST NOT exceed the PMTU. If the implementation does not support bundling or if the bundled COOKIE ECHO chunk plus HEARTBEAT chunk (including nonce) would result in an SCTP packet larger than the PMTU, then the implementation MUST NOT send a COOKIE ECHO chunk to an UNCONFIRMED address.¶

6.1. Transmission of DATA Chunks

This section specifies the rules for sending DATA chunks. In particular, it defines zero window probing, which is required to avoid the indefinite stalling of an association in case of a loss of packets containing SACK chunks performing window updates.¶

This document is specified as if there is a single retransmission timer per destination transport address, but implementations MAY have a retransmission timer for each DATA chunk.¶

The following general rules MUST be applied by the data sender for transmission and/or retransmission of outbound DATA chunks:¶

A)

At any given time, the data sender MUST NOT transmit new data to any destination transport address if its peer's rwnd indicates that the peer has no buffer space (i.e., rwnd is smaller than the size of the next DATA chunk; see Section 6.2.1), except for zero window probes.¶

A zero window probe is a DATA chunk sent when the receiver has no buffer space. This rule allows the sender to probe for a change in rwnd that the sender missed due to the SACK chunks having been lost in transit from the data receiver to the data sender. A zero window probe MUST only be sent when the cwnd allows (see rule B below). A zero window probe SHOULD only be sent when all outstanding DATA chunks have been cumulatively acknowledged and no DATA chunks are in flight. Senders MUST support zero window probing.¶

If the sender continues to receive SACK chunks from the peer while doing zero window probing, the unacknowledged window probes SHOULD NOT increment the error counter for the association or any destination transport address. This is because the receiver could keep its window closed for an indefinite time. Section 6.2 describes the receiver behavior when it advertises a zero window. The sender SHOULD send the first zero window probe after 1 RTO when it detects that the receiver has closed its window and SHOULD increase the probe interval exponentially afterwards. Also note that the cwnd SHOULD be adjusted according to Section 7.2.1. Zero window probing does not affect the calculation of cwnd.¶

The sender MUST also have an algorithm for sending new DATA chunks to avoid silly window syndrome (SWS) as described in [RFC1122]. The algorithm can be similar to the one described in Section 4.2.3.4 of [RFC1122].¶

B)

At any given time, the sender MUST NOT transmit new data to a given transport address if it has cwnd + (PMDCS - 1) or more bytes of data outstanding to that transport address. If data is available, the sender SHOULD exceed cwnd by up to (PMDCS - 1) bytes on a new data transmission if the flightsize does not currently reach cwnd. The breach of cwnd MUST constitute one packet only.¶

C)

When the time comes for the sender to transmit, before sending new DATA chunks, the sender MUST first transmit any DATA chunks that are marked for retransmission (limited by the current cwnd).¶

D)

When the time comes for the sender to transmit new DATA chunks, the protocol parameter 'Max.Burst' SHOULD be used to limit the number of packets sent. The limit MAY be applied by adjusting cwnd temporarily, as follows:¶

if ((flightsize + Max.Burst * PMDCS) < cwnd)
    cwnd = flightsize + Max.Burst * PMDCS

¶

Or, it MAY be applied by strictly limiting the number of packets emitted by the output routine. When calculating the number of packets to transmit, and particularly when using the formula above, cwnd SHOULD NOT be changed permanently.¶

E)

Then, the sender can send as many new DATA chunks as rule A and rule B allow.¶

Multiple DATA chunks committed for transmission MAY be bundled in a single packet. Furthermore, DATA chunks being retransmitted MAY be bundled with new DATA chunks, as long as the resulting SCTP packet size does not exceed the PMTU. A ULP can request that no bundling is performed, but this only turns off any delays that an SCTP implementation might be using to increase bundling efficiency. It does not in itself stop all bundling from occurring (i.e., in case of congestion or retransmission).¶

Before an endpoint transmits a DATA chunk, if any received DATA chunks have not been acknowledged (e.g., due to delayed ack), the sender SHOULD create a SACK chunk and bundle it with the outbound DATA chunk, as long as the size of the final SCTP packet does not exceed the current PMTU. See Section 6.2.¶

When the window is full (i.e., transmission is disallowed by rule A and/or rule B), the sender MAY still accept send requests from its upper layer but MUST transmit no more DATA chunks until some or all of the outstanding DATA chunks are acknowledged and transmission is allowed by rule A and rule B again.¶

Whenever a transmission or retransmission is made to any address, if the T3-rtx timer of that address is not currently running, the sender MUST start that timer. If the timer for that address is already running, the sender MUST restart the timer if the earliest (i.e., lowest TSN) outstanding DATA chunk sent to that address is being retransmitted. Otherwise, the data sender MUST NOT restart the timer.¶

When starting or restarting the T3-rtx timer, the timer value SHOULD be adjusted according to the timer rules defined in Sections 6.3.2 and 6.3.3.¶

The data sender MUST NOT use a TSN that is more than 2³¹ - 1 above the beginning TSN of the current send window.¶

For each stream, the data sender MUST NOT have more than 2¹⁶ - 1 ordered user messages in the current send window.¶

Whenever the sender of a DATA chunk can benefit from the corresponding SACK chunk being sent back without delay, the sender MAY set the I bit in the DATA chunk header. Please note that why the sender has set the I bit is irrelevant to the receiver.¶

Reasons for setting the I bit include, but are not limited to, the following (see Section 4 of [RFC7053] for a discussion of the benefits):¶

• The application requests that the I bit of the last DATA chunk of a user message be set when providing the user message to the SCTP implementation (see Section 11.1).¶

• The sender is in the SHUTDOWN-PENDING state.¶

• The sending of a DATA chunk fills the congestion or receiver window.¶

6.2. Acknowledgement on Reception of DATA Chunks

The SCTP endpoint MUST always acknowledge the reception of each valid DATA chunk when the DATA chunk received is inside its receive window.¶

When the receiver's advertised window is 0, the receiver MUST drop any new incoming DATA chunk with a TSN larger than the largest TSN received so far. Also, if the new incoming DATA chunk holds a TSN value less than the largest TSN received so far, then the receiver SHOULD drop the largest TSN held for reordering and accept the new incoming DATA chunk. In either case, if such a DATA chunk is dropped, the receiver MUST immediately send back a SACK chunk with the current receive window showing only DATA chunks received and accepted so far. The dropped DATA chunk(s) MUST NOT be included in the SACK chunk, as they were not accepted. The receiver MUST also have an algorithm for advertising its receive window to avoid receiver silly window syndrome (SWS), as described in [RFC1122]. The algorithm can be similar to the one described in Section 4.2.3.3 of [RFC1122].¶

The guidelines on the delayed acknowledgement algorithm specified in Section 4.2 of [RFC5681] SHOULD be followed. Specifically, an acknowledgement SHOULD be generated for at least every second packet (not every second DATA chunk) received and SHOULD be generated within 200 ms of the arrival of any unacknowledged DATA chunk. In some situations, it might be beneficial for an SCTP transmitter to be more conservative than the algorithms detailed in this document allow. However, an SCTP transmitter MUST NOT be more aggressive in sending SACK chunks than the following algorithms allow.¶

An SCTP receiver MUST NOT generate more than one SACK chunk for every incoming packet, other than to update the offered window as the receiving application consumes new data. When the window opens up, an SCTP receiver SHOULD send additional SACK chunks to update the window even if no new data is received. The receiver MUST avoid sending a large number of window updates -- in particular, large bursts of them. One way to achieve this is to send a window update only if the window can be increased by at least a quarter of the receive buffer size of the association.¶

Implementation Note: The maximum delay for generating an acknowledgement MAY be configured by the SCTP administrator, either statically or dynamically, in order to meet the specific timing requirement of the protocol being carried.¶

An implementation MUST NOT allow the maximum delay (protocol parameter 'SACK.Delay') to be configured to be more than 500 ms. In other words, an implementation MAY lower the value of 'SACK.Delay' below 500 ms but MUST NOT raise it above 500 ms.¶

Acknowledgements MUST be sent in SACK chunks unless shutdown was requested by the ULP, in which case an endpoint MAY send an acknowledgement in the SHUTDOWN chunk. A SACK chunk can acknowledge the reception of multiple DATA chunks. See Section 3.3.4 for SACK chunk format. In particular, the SCTP endpoint MUST fill in the Cumulative TSN Ack field to indicate the latest sequential TSN (of a valid DATA chunk) it has received. Any received DATA chunks with TSN greater than the value in the Cumulative TSN Ack field are reported in the Gap Ack Block fields. The SCTP endpoint MUST report as many Gap Ack Blocks as can fit in a single SACK chunk such that the size of the SCTP packet does not exceed the current PMTU.¶

The SHUTDOWN chunk does not contain Gap Ack Block fields. Therefore, the endpoint SHOULD use a SACK chunk instead of the SHUTDOWN chunk to acknowledge DATA chunks received out of order.¶

Upon receipt of an SCTP packet containing a DATA chunk with the I bit set, the receiver SHOULD NOT delay the sending of the corresponding SACK chunk, i.e., the receiver SHOULD immediately respond with the corresponding SACK chunk.¶

When a packet arrives with duplicate DATA chunk(s) and with no new DATA chunk(s), the endpoint MUST immediately send a SACK chunk with no delay. If a packet arrives with duplicate DATA chunk(s) bundled with new DATA chunks, the endpoint MAY immediately send a SACK chunk. Normally, receipt of duplicate DATA chunks will occur when the original SACK chunk was lost and the peer's RTO has expired. The duplicate TSN number(s) SHOULD be reported in the SACK chunk as duplicate.¶

When an endpoint receives a SACK chunk, it MAY use the duplicate TSN information to determine if SACK chunk loss is occurring. Further use of this data is for future study.¶

The data receiver is responsible for maintaining its receive buffers. The data receiver SHOULD notify the data sender in a timely manner of changes in its ability to receive data. How an implementation manages its receive buffers is dependent on many factors (e.g., operating system, memory management system, amount of memory, etc.). However, the data sender strategy defined in Section 6.2.1 is based on the assumption of receiver operation similar to the following:¶

A)

At initialization of the association, the endpoint tells the peer how much receive buffer space it has allocated to the association in the INIT or INIT ACK chunk. The endpoint sets a_rwnd to this value.¶

B)

As DATA chunks are received and buffered, decrement a_rwnd by the number of bytes received and buffered. This is, in effect, closing rwnd at the data sender and restricting the amount of data it can transmit.¶

C)

As DATA chunks are delivered to the ULP and released from the receive buffers, increment a_rwnd by the number of bytes delivered to the upper layer. This is, in effect, opening up rwnd on the data sender and allowing it to send more data. The data receiver SHOULD NOT increment a_rwnd unless it has released bytes from its receive buffer. For example, if the receiver is holding fragmented DATA chunks in a reassembly queue, it SHOULD NOT increment a_rwnd.¶

D)

When sending a SACK chunk, the data receiver SHOULD place the current value of a_rwnd into the a_rwnd field. The data receiver SHOULD take into account that the data sender will not retransmit DATA chunks that are acked via the Cumulative TSN Ack (i.e., will drop from its retransmit queue).¶

Under certain circumstances, the data receiver MAY drop DATA chunks that it has received but has not released from its receive buffers (i.e., delivered to the ULP). These DATA chunks might have been acked in Gap Ack Blocks. For example, the data receiver might be holding data in its receive buffers while reassembling a fragmented user message from its peer when it runs out of receive buffer space. It MAY drop these DATA chunks even though it has acknowledged them in Gap Ack Blocks. If a data receiver drops DATA chunks, it MUST NOT include them in Gap Ack Blocks in subsequent SACK chunks until they are received again via retransmission. In addition, the endpoint SHOULD take into account the dropped data when calculating its a_rwnd.¶

An endpoint SHOULD NOT revoke a SACK chunk and discard data. Only in extreme circumstances might an endpoint use this procedure (such as out of buffer space). The data receiver SHOULD take into account that dropping data that has been acked in Gap Ack Blocks can result in suboptimal retransmission strategies in the data sender and thus in suboptimal performance.¶

The following example illustrates the use of delayed acknowledgements:¶

Endpoint A                                      Endpoint Z

{App sends 3 messages; strm 0}
DATA [TSN=7,Strm=0,Seq=3] ------------> (ack delayed)
(Start T3-rtx timer)

DATA [TSN=8,Strm=0,Seq=4] ------------> (send ack)
                              /------- SACK [TSN Ack=8,block=0]
(cancel T3-rtx timer)  <-----/

DATA [TSN=9,Strm=0,Seq=5] ------------> (ack delayed)
(Start T3-rtx timer)
                                       ...
                                       {App sends 1 message; strm 1}
                                       (bundle SACK with DATA)
                                /----- SACK [TSN Ack=9,block=0] \
                               /         DATA [TSN=6,Strm=1,Seq=2]
(cancel T3-rtx timer)  <------/        (Start T3-rtx timer)

(ack delayed)
(send ack)
SACK [TSN Ack=6,block=0] -------------> (cancel T3-rtx timer)

If an endpoint receives a DATA chunk with no user data (i.e., the Length field is set to 16), it SHOULD send an ABORT chunk with a "No User Data" error cause.¶

An endpoint SHOULD NOT send a DATA chunk with no user data part. This avoids the need to be able to return a zero-length user message in the API, especially in the socket API as specified in [RFC6458] for details.¶

6.3. Management of Retransmission Timer

An SCTP endpoint uses a retransmission timer T3-rtx to ensure data delivery in the absence of any feedback from its peer. The duration of this timer is referred to as RTO (retransmission timeout).¶

When an endpoint's peer is multi-homed, the endpoint will calculate a separate RTO for each different destination transport address of its peer endpoint.¶

The computation and management of RTO in SCTP follow closely how TCP manages its retransmission timer. To compute the current RTO, an endpoint maintains two state variables per destination transport address: SRTT (smoothed round-trip time) and RTTVAR (round-trip time variation).¶

Endpoint A                                         Endpoint Z
{App begins to send}
Data [TSN=7,Strm=0,Seq=3] ------------> (ack delayed)
(Start T3-rtx timer)
                                        {App sends 1 message; strm 1}
                                        (bundle ack with data)
DATA [TSN=8,Strm=0,Seq=4] ----\     /-- SACK [TSN Ack=7,Block=0]
                               \   /      DATA [TSN=6,Strm=1,Seq=2]
                                \ /     (Start T3-rtx timer)
                                 \
                                / \
(Restart T3-rtx timer)  <------/   \--> (ack delayed)
(ack delayed)
{send ack}
SACK [TSN Ack=6,Block=0] --------------> (Cancel T3-rtx timer)
                                        ..
                                        (send ack)
(Cancel T3-rtx timer)  <-------------- SACK [TSN Ack=8,Block=0]

6.4. Multi-Homed SCTP Endpoints

An SCTP endpoint is considered multi-homed if there is more than one transport address that can be used as a destination address to reach that endpoint.¶

Moreover, the ULP of an endpoint selects one of the multiple destination addresses of a multi-homed peer endpoint as the primary path (see Sections 5.1.2 and 11.1 for details).¶

By default, an endpoint SHOULD always transmit to the primary path, unless the SCTP user explicitly specifies the destination transport address (and possibly source transport address) to use.¶

An endpoint SHOULD transmit reply chunks (e.g., INIT ACK, COOKIE ACK, and HEARTBEAT ACK) in response to control chunks to the same destination transport address from which it received the control chunk to which it is replying.¶

The selection of the destination transport address for packets containing SACK chunks is implementation dependent. However, an endpoint SHOULD NOT vary the destination transport address of a SACK chunk when it receives DATA chunks coming from the same source address.¶

When acknowledging multiple DATA chunks received in packets from different source addresses in a single SACK chunk, the SACK chunk MAY be transmitted to one of the destination transport addresses from which the DATA or control chunks being acknowledged were received.¶

When a receiver of a duplicate DATA chunk sends a SACK chunk to a multi-homed endpoint, it MAY be beneficial to vary the destination address and not use the source address of the DATA chunk. The reason is that receiving a duplicate from a multi-homed endpoint might indicate that the return path (as specified in the source address of the DATA chunk) for the SACK chunk is broken.¶

Furthermore, when its peer is multi-homed, an endpoint SHOULD try to retransmit a chunk that timed out to an active destination transport address that is different from the last destination address to which the chunk was sent.¶

When its peer is multi-homed, an endpoint SHOULD send fast retransmissions to the same destination transport address to which the original data was sent. If the primary path has been changed and the original data was sent to the old primary path before the Fast Retransmit, the implementation MAY send it to the new primary path.¶

Retransmissions do not affect the total outstanding data count. However, if the DATA chunk is retransmitted onto a different destination address, both the outstanding data counts on the new destination address and the old destination address to which the data chunk was last sent is adjusted accordingly.¶

6.5. Stream Identifier and Stream Sequence Number

Every DATA chunk MUST carry a valid stream identifier. If an endpoint receives a DATA chunk with an invalid stream identifier, it SHOULD acknowledge the reception of the DATA chunk following the normal procedure, immediately send an ERROR chunk with cause set to "Invalid Stream Identifier" (see Section 3.3.10), and discard the DATA chunk. The endpoint MAY bundle the ERROR chunk and the SACK chunk in the same packet.¶

The Stream Sequence Number in all the outgoing streams MUST start from 0 when the association is established. The Stream Sequence Number of an outgoing stream MUST be incremented by 1 for each ordered user message sent on that outgoing stream. In particular, when the Stream Sequence Number reaches the value 65535, the next Stream Sequence Number MUST be set to 0. For unordered user messages, the Stream Sequence Number MUST NOT be changed.¶

6.6. Ordered and Unordered Delivery

Within a stream, an endpoint MUST deliver DATA chunks received with the U flag set to 0 to the upper layer according to the order of their Stream Sequence Number. If DATA chunks arrive out of order of their Stream Sequence Number, the endpoint MUST hold the received DATA chunks from delivery to the ULP until they are reordered.¶

However, an SCTP endpoint can indicate that no ordered delivery is required for a particular DATA chunk transmitted within the stream by setting the U flag of the DATA chunk to 1.¶

When an endpoint receives a DATA chunk with the U flag set to 1, it bypasses the ordering mechanism and immediately deliver the data to the upper layer (after reassembly if the user data is fragmented by the data sender).¶

This provides an effective way of transmitting "out-of-band" data in a given stream. Also, a stream can be used as an "unordered" stream by simply setting the U flag to 1 in all DATA chunks sent through that stream.¶

Implementation Note: When sending an unordered DATA chunk, an implementation MAY choose to place the DATA chunk in an outbound packet that is at the head of the outbound transmission queue if possible.¶

The 'Stream Sequence Number' field in a DATA chunk with U flag set to 1 has no significance. The sender can fill the 'Stream Sequence Number' with arbitrary value, but the receiver MUST ignore the field.¶

Note: When transmitting ordered and unordered data, an endpoint does not increment its Stream Sequence Number when transmitting a DATA chunk with U flag set to 1.¶

6.7. Report Gaps in Received DATA TSNs

Upon the reception of a new DATA chunk, an endpoint examines the continuity of the TSNs received. If the endpoint detects a gap in the received DATA chunk sequence, it SHOULD send a SACK chunk with Gap Ack Blocks immediately. The data receiver continues sending a SACK chunk after receipt of each SCTP packet that does not fill the gap.¶

Based on the Gap Ack Block from the received SACK chunk, the endpoint can calculate the missing DATA chunks and make decisions on whether to retransmit them (see Section 6.2.1 for details).¶

Multiple gaps can be reported in one single SACK chunk (see Section 3.3.4).¶

When its peer is multi-homed, the SCTP endpoint SHOULD always try to send the SACK chunk to the same destination address from which the last DATA chunk was received.¶

Upon the reception of a SACK chunk, the endpoint MUST remove all DATA chunks that have been acknowledged by the SACK chunk's Cumulative TSN Ack from its transmit queue. All DATA chunks with TSNs not included in the Gap Ack Blocks that are smaller than the highest-acknowledged TSN reported in the SACK chunk MUST be treated as "missing" by the sending endpoint. The number of "missing" reports for each outstanding DATA chunk MUST be recorded by the data sender to make retransmission decisions. See Section 7.2.4 for details.¶

The following example shows the use of SACK chunk to report a gap.¶

Endpoint A                                    Endpoint Z
{App sends 3 messages; strm 0}
DATA [TSN=6,Strm=0,Seq=2] ---------------> (ack delayed)
(Start T3-rtx timer)

DATA [TSN=7,Strm=0,Seq=3] --------> X (lost)

DATA [TSN=8,Strm=0,Seq=4] ---------------> (gap detected,
                                            immediately send ack)
                                /----- SACK [TSN Ack=6,Block=1,
                               /             Start=2,End=2]
                        <-----/
(remove 6 from out-queue,
 and mark 7 as "1" missing report)

The maximum number of Gap Ack Blocks that can be reported within a single SACK chunk is limited by the current PMTU. When a single SACK chunk cannot cover all the Gap Ack Blocks needed to be reported due to the PMTU limitation, the endpoint MUST send only one SACK chunk. This single SACK chunk MUST report the Gap Ack Blocks from the lowest to highest TSNs, within the size limit set by the PMTU, and leave the remaining highest TSN numbers unacknowledged.¶

6.8. CRC32c Checksum Calculation

When sending an SCTP packet, the endpoint MUST strengthen the data integrity of the transmission by including the CRC32c checksum value calculated on the packet, as described below.¶

After the packet is constructed (containing the SCTP common header and one or more control or DATA chunks), the transmitter MUST:¶

1)

fill in the proper Verification Tag in the SCTP common header and initialize the checksum field to 0,¶

2)

calculate the CRC32c checksum of the whole packet, including the SCTP common header and all the chunks (refer to Appendix A for details of the CRC32c algorithm), and¶

3)

put the resultant value into the checksum field in the common header and leave the rest of the bits unchanged.¶

When an SCTP packet is received, the receiver MUST first check the CRC32c checksum as follows:¶

1)

Store the received CRC32c checksum value aside.¶

2)

Replace the 32 bits of the checksum field in the received SCTP packet with 0 and calculate a CRC32c checksum value of the whole received packet.¶

3)

Verify that the calculated CRC32c checksum is the same as the received CRC32c checksum. If it is not, the receiver MUST treat the packet as an invalid SCTP packet.¶

The default procedure for handling invalid SCTP packets is to silently discard them.¶

Any hardware implementation SHOULD permit alternative verification of the CRC in software.¶

6.9. Fragmentation and Reassembly

An endpoint MAY support fragmentation when sending DATA chunks, but it MUST support reassembly when receiving DATA chunks. If an endpoint supports fragmentation, it MUST fragment a user message if the size of the user message to be sent causes the outbound SCTP packet size to exceed the current PMTU. An endpoint that does not support fragmentation and is requested to send a user message such that the outbound SCTP packet size would exceed the current PMTU MUST return an error to its upper layer and MUST NOT attempt to send the user message.¶

An SCTP implementation MAY provide a mechanism to the upper layer that disables fragmentation when sending DATA chunks. When fragmentation of DATA chunks is disabled, the SCTP implementation MUST behave in the same way an implementation that does not support fragmentation, i.e., it rejects calls that would result in sending SCTP packets that exceed the current PMTU.¶

Implementation Note: In this error case, the SEND primitive discussed in Section 11.1.5 would need to return an error to the upper layer.¶

If its peer is multi-homed, the endpoint SHOULD choose a DATA chunk size smaller than or equal to the AMDCS.¶

Once a user message is fragmented, it cannot be re-fragmented. Instead, if the PMTU has been reduced, then IP fragmentation MUST be used. Therefore, an SCTP association can fail if IP fragmentation is not working on any path. Please see Section 7.3 for details of PMTU discovery.¶

When determining when to fragment, the SCTP implementation MUST take into account the SCTP packet header as well as the DATA chunk header(s). The implementation MUST also take into account the space required for a SACK chunk if bundling a SACK chunk with the DATA chunk.¶

Fragmentation takes the following steps:¶

1)

The data sender MUST break the user message into a series of DATA chunks. The sender SHOULD choose a size of DATA chunks that is smaller than or equal to the AMDCS.¶

2)

The transmitter MUST then assign, in sequence, a separate TSN to each of the DATA chunks in the series. The transmitter assigns the same Stream Sequence Number to each of the DATA chunks. If the user indicates that the user message is to be delivered using unordered delivery, then the U flag of each DATA chunk of the user message MUST be set to 1.¶

3)

The transmitter MUST also set the B/E bits of the first DATA chunk in the series to 10, the B/E bits of the last DATA chunk in the series to 01, and the B/E bits of all other DATA chunks in the series to 00.¶

An endpoint MUST recognize fragmented DATA chunks by examining the B/E bits in each of the received DATA chunks and queue the fragmented DATA chunks for reassembly. Once the user message is reassembled, SCTP passes the reassembled user message to the specific stream for possible reordering and final dispatching.¶

If the data receiver runs out of buffer space while still waiting for more fragments to complete the reassembly of the message, it SHOULD dispatch part of its inbound message through a partial delivery API (see Section 11), freeing some of its receive buffer space so that the rest of the message can be received.¶

6.10. Bundling

An endpoint bundles chunks by simply including multiple chunks in one outbound SCTP packet. The total size of the resultant SCTP packet MUST be less that or equal to the current PMTU.¶

If its peer endpoint is multi-homed, the sending endpoint SHOULD choose a size no larger than the PMTU of the current primary path.¶

When bundling control chunks with DATA chunks, an endpoint MUST place control chunks first in the outbound SCTP packet. The transmitter MUST transmit DATA chunks within an SCTP packet in increasing order of TSN.¶

Note: Since control chunks are placed first in a packet and since DATA chunks are transmitted before SHUTDOWN or SHUTDOWN ACK chunks, DATA chunks cannot be bundled with SHUTDOWN or SHUTDOWN ACK chunks.¶

Partial chunks MUST NOT be placed in an SCTP packet. A partial chunk is a chunk that is not completely contained in the SCTP packet; i.e., the SCTP packet is too short to contain all the bytes of the chunk as indicated by the chunk length.¶

An endpoint MUST process received chunks in their order in the packet. The receiver uses the Chunk Length field to determine the end of a chunk and beginning of the next chunk, taking account of the fact that all chunks end on a 4-byte boundary. If the receiver detects a partial chunk, it MUST drop the chunk.¶

An endpoint MUST NOT bundle INIT, INIT ACK, or SHUTDOWN COMPLETE chunks with any other chunks.¶

7.1. SCTP Differences from TCP Congestion Control

Gap Ack Blocks in the SCTP SACK chunk carry the same semantic meaning as the TCP SACK. TCP considers the information carried in the SACK as advisory information only. SCTP considers the information carried in the Gap Ack Blocks in the SACK chunk as advisory. In SCTP, any DATA chunk that has been acknowledged by a SACK chunk, including DATA that arrived at the receiving end out of order, is not considered fully delivered until the Cumulative TSN Ack Point passes the TSN of the DATA chunk (i.e., the DATA chunk has been acknowledged by the Cumulative TSN Ack field in the SACK chunk). Consequently, the value of cwnd controls the amount of outstanding data, rather than (as in the case of non-SACK TCP) the upper bound between the highest acknowledged sequence number and the latest DATA chunk that can be sent within the congestion window. SCTP SACK leads to different implementations of Fast Retransmit and Fast Recovery than non-SACK TCP. As an example, see [FALL96].¶

The biggest difference between SCTP and TCP, however, is multi-homing. SCTP is designed to establish robust communication associations between two endpoints, each of which might be reachable by more than one transport address. Potentially different addresses might lead to different data paths between the two endpoints; thus, ideally, one needs a separate set of congestion control parameters for each of the paths. The treatment here of congestion control for multi-homed receivers is new with SCTP and might require refinement in the future. The current algorithms make the following assumptions:¶

• The sender usually uses the same destination address until being instructed by the upper layer to do otherwise; however, SCTP MAY change to an alternate destination in the event an address is marked inactive (see Section 8.2). Also, SCTP MAY retransmit to a different transport address than the original transmission.¶

• The sender keeps a separate congestion control parameter set for each of the destination addresses it can send to (not each source-destination pair but for each destination). The parameters SHOULD decay if the address is not used for a long enough time period. [RFC5681] specifies this period of time as a retransmission timeout.¶

• For each of the destination addresses, an endpoint does slow start upon the first transmission to that address.¶

Note: TCP guarantees in-sequence delivery of data to its upper-layer protocol within a single TCP session. This means that when TCP notices a gap in the received sequence number, it waits until the gap is filled before delivering the data that was received with sequence numbers higher than that of the missing data. On the other hand, SCTP can deliver data to its upper-layer protocol, even if there is a gap in TSN if the Stream Sequence Numbers are in sequence for a particular stream (i.e., the missing DATA chunks are for a different stream) or if unordered delivery is indicated. Although this does not affect cwnd, it might affect rwnd calculation.¶

7.2. SCTP Slow-Start and Congestion Avoidance

The slow-start and congestion avoidance algorithms MUST be used by an endpoint to control the amount of data being injected into the network. The congestion control in SCTP is employed in regard to the association, not to an individual stream. In some situations, it might be beneficial for an SCTP sender to be more conservative than the algorithms allow; however, an SCTP sender MUST NOT be more aggressive than the following algorithms allow.¶

Like TCP, an SCTP endpoint uses the following three control variables to regulate its transmission rate.¶

• Receiver advertised window size (rwnd, in bytes), which is set by the receiver based on its available buffer space for incoming packets.¶

  Note: This variable is kept on the entire association.¶

• Congestion control window (cwnd, in bytes), which is adjusted by the sender based on observed network conditions.¶

  Note: This variable is maintained on a per-destination-address basis.¶

• Slow-start threshold (ssthresh, in bytes), which is used by the sender to distinguish slow-start and congestion avoidance phases.¶

  Note: This variable is maintained on a per-destination-address basis.¶

SCTP also requires one additional control variable, partial_bytes_acked, which is used during the congestion avoidance phase to facilitate cwnd adjustment.¶

Unlike TCP, an SCTP sender MUST keep a set of the control variables cwnd, ssthresh, and partial_bytes_acked for EACH destination address of its peer (when its peer is multi-homed). When calculating one of these variables, the length of the DATA chunk, including the padding, SHOULD be used.¶

Only one rwnd is kept for the whole association (no matter if the peer is multi-homed or has a single address).¶

ssthresh = max(cwnd / 2, 4 * PMDCS)
cwnd = ssthresh
partial_bytes_acked = 0

ssthresh = max(cwnd / 2, 4 * PMDCS)
cwnd = PMDCS
partial_bytes_acked = 0

7.3. PMTU Discovery

[RFC8899], [RFC8201], and [RFC1191] specify "Packetization Layer Path MTU Discovery", whereby an endpoint maintains an estimate of PMTU along a given Internet path and refrains from sending packets along that path that exceed the PMTU, other than occasional attempts to probe for a change in the PMTU. [RFC8899] is thorough in its discussion of the PMTU discovery mechanism and strategies for determining the current end-to-end PMTU setting as well as detecting changes in this value.¶

An endpoint SHOULD apply these techniques and SHOULD do so on a per-destination-address basis.¶

There are two important SCTP-specific points regarding PMTU discovery:¶

1)

SCTP associations can span multiple addresses. An endpoint MUST maintain separate PMTU estimates for each destination address of its peer.¶

2)

The sender SHOULD track an AMDCS that will be the smallest PMDCS discovered for all of the peer's destination addresses. When fragmenting messages into multiple parts, this AMDCS SHOULD be used to calculate the size of each DATA chunk. This will allow retransmissions to be seamlessly sent to an alternate address without encountering IP fragmentation.¶

8.1. Endpoint Failure Detection

An endpoint SHOULD keep a counter on the total number of consecutive retransmissions to its peer (this includes data retransmissions to all the destination transport addresses of the peer if it is multi-homed), including the number of unacknowledged HEARTBEAT chunks observed on the path that is currently used for data transfer. Unacknowledged HEARTBEAT chunks observed on paths different from the path currently used for data transfer SHOULD NOT increment the association error counter, as this could lead to association closure even if the path that is currently used for data transfer is available (but idle). If the value of this counter exceeds the limit indicated in the protocol parameter 'Association.Max.Retrans', the endpoint SHOULD consider the peer endpoint unreachable and SHALL stop transmitting any more data to it (and thus the association enters the CLOSED state). In addition, the endpoint SHOULD report the failure to the upper layer and optionally report back all outstanding user data remaining in its outbound queue. The association is automatically closed when the peer endpoint becomes unreachable.¶

The counter used for endpoint failure detection MUST be reset each time a DATA chunk sent to that peer endpoint is acknowledged (by the reception of a SACK chunk). When a HEARTBEAT ACK chunk is received from the peer endpoint, the counter SHOULD also be reset. The receiver of the HEARTBEAT ACK chunk MAY choose not to clear the counter if there is outstanding data on the association. This allows for handling the possible difference in reachability based on DATA chunks and HEARTBEAT chunks.¶

8.2. Path Failure Detection

When its peer endpoint is multi-homed, an endpoint SHOULD keep an error counter for each of the destination transport addresses of the peer endpoint.¶

Each time the T3-rtx timer expires on any address, or when a HEARTBEAT chunk sent to an idle address is not acknowledged within an RTO, the error counter of that destination address will be incremented. When the value in the error counter exceeds the protocol parameter 'Path.Max.Retrans' of that destination address, the endpoint SHOULD mark the destination transport address as inactive, and a notification SHOULD be sent to the upper layer.¶

When an outstanding TSN is acknowledged or a HEARTBEAT chunk sent to that address is acknowledged with a HEARTBEAT ACK chunk, the endpoint SHOULD clear the error counter of the destination transport address to which the DATA chunk was last sent (or HEARTBEAT chunk was sent) and SHOULD also report to the upper layer when an inactive destination address is marked as active. When the peer endpoint is multi-homed and the last chunk sent to it was a retransmission to an alternate address, there exists an ambiguity as to whether or not the acknowledgement could be credited to the address of the last chunk sent. However, this ambiguity does not seem to have significant consequences for SCTP behavior. If this ambiguity is undesirable, the transmitter MAY choose not to clear the error counter if the last chunk sent was a retransmission.¶

Note: When configuring the SCTP endpoint, the user ought to avoid having the value of 'Association.Max.Retrans' larger than the summation of the 'Path.Max.Retrans' of all the destination addresses for the remote endpoint. Otherwise, all the destination addresses might become inactive while the endpoint still considers the peer endpoint reachable. When this condition occurs, how SCTP chooses to function is implementation specific.¶

When the primary path is marked inactive (due to excessive retransmissions, for instance), the sender MAY automatically transmit new packets to an alternate destination address if one exists and is active. If more than one alternate address is active when the primary path is marked inactive, only ONE transport address SHOULD be chosen and used as the new destination transport address.¶

8.3. Path Heartbeat

By default, an SCTP endpoint SHOULD monitor the reachability of the idle destination transport address(es) of its peer by sending a HEARTBEAT chunk periodically to the destination transport address(es). The sending of HEARTBEAT chunks MAY begin upon reaching the ESTABLISHED state and is discontinued after sending either a SHUTDOWN chunk or SHUTDOWN ACK chunk. A receiver of a HEARTBEAT chunk MUST respond to a HEARTBEAT chunk with a HEARTBEAT ACK chunk after entering the COOKIE-ECHOED state (sender of the INIT chunk) or the ESTABLISHED state (receiver of the INIT chunk), up until reaching the SHUTDOWN-SENT state (sender of the SHUTDOWN chunk) or the SHUTDOWN-ACK-SENT state (receiver of the SHUTDOWN chunk).¶

A destination transport address is considered "idle" if no new chunk that can be used for updating path RTT (usually including first transmission DATA, INIT, COOKIE ECHO, or HEARTBEAT chunks, etc.) and no HEARTBEAT chunk has been sent to it within the current heartbeat period of that address. This applies to both active and inactive destination addresses.¶

The upper layer can optionally initiate the following functions:¶

A)

Disable heartbeat on a specific destination transport address of a given association,¶

B)

Change the 'HB.interval',¶

C)

Re-enable heartbeat on a specific destination transport address of a given association, and¶

D)

Request the sending of an on-demand HEARTBEAT chunk on a specific destination transport address of a given association.¶

The endpoint SHOULD increment the respective error counter of the destination transport address each time a HEARTBEAT chunk is sent to that address and not acknowledged within one RTO.¶

When the value of this counter exceeds the protocol parameter 'Path.Max.Retrans', the endpoint SHOULD mark the corresponding destination address as inactive if it is not so marked and SHOULD also report to the upper layer the change in reachability of this destination address. After this, the endpoint SHOULD continue sending HEARTBEAT chunks on this destination address but SHOULD stop increasing the counter.¶

The sender of the HEARTBEAT chunk SHOULD include in the Heartbeat Information field of the chunk the current time when the packet is sent and the destination address to which the packet is sent.¶

Implementation Note: An alternative implementation of the heartbeat mechanism that can be used is to increment the error counter variable every time a HEARTBEAT chunk is sent to a destination. Whenever a HEARTBEAT ACK chunk arrives, the sender SHOULD clear the error counter of the destination that the HEARTBEAT chunk was sent to. This, in effect, would clear the previously stroked error (and any other error counts as well).¶

The receiver of the HEARTBEAT chunk SHOULD immediately respond with a HEARTBEAT ACK chunk that contains the Heartbeat Information TLV, together with any other received TLVs, copied unchanged from the received HEARTBEAT chunk.¶

Upon the receipt of the HEARTBEAT ACK chunk, the sender of the HEARTBEAT chunk SHOULD clear the error counter of the destination transport address to which the HEARTBEAT chunk was sent and mark the destination transport address as active if it is not so marked. The endpoint SHOULD report to the upper layer when an inactive destination address is marked as active due to the reception of the latest HEARTBEAT ACK chunk. The receiver of the HEARTBEAT ACK chunk SHOULD also clear the association overall error count (as defined in Section 8.1).¶

The receiver of the HEARTBEAT ACK chunk SHOULD also perform an RTT measurement for that destination transport address using the time value carried in the HEARTBEAT ACK chunk.¶

On an idle destination address that is allowed to heartbeat, it is RECOMMENDED that a HEARTBEAT chunk is sent once per RTO of that destination address plus the protocol parameter 'HB.interval', with jittering of +/- 50% of the RTO value and exponential backoff of the RTO if the previous HEARTBEAT chunk is unanswered.¶

A primitive is provided for the SCTP user to change the 'HB.interval' and turn on or off the heartbeat on a given destination address. The 'HB.interval' set by the SCTP user is added to the RTO of that destination (including any exponential backoff). Only one heartbeat SHOULD be sent each time the heartbeat timer expires (if multiple destinations are idle). It is an implementation decision on how to choose which of the candidate idle destinations to heartbeat to (if more than one destination is idle).¶

When tuning the 'HB.interval', there is a side effect that SHOULD be taken into account. When this value is increased, i.e., the time between the sending of HEARTBEAT chunks is longer, the detection of lost ABORT chunks takes longer as well. If a peer endpoint sends an ABORT chunk for any reason and the ABORT chunk is lost, the local endpoint will only discover the lost ABORT chunk by sending a DATA chunk or HEARTBEAT chunk (thus causing the peer to send another ABORT chunk). This is to be considered when tuning the heartbeat timer. If the sending of HEARTBEAT chunks is disabled, only sending DATA chunks to the association will discover a lost ABORT chunk from the peer.¶

8.4. Handle "Out of the Blue" Packets

An SCTP packet is called an "Out of the Blue" (OOTB) packet if it is correctly formed (i.e., passed the receiver's CRC32c check; see Section 6.8), but the receiver is not able to identify the association to which this packet belongs.¶

The receiver of an OOTB packet does the following:¶

1)

If the OOTB packet is to or from a non-unicast address, a receiver SHOULD silently discard the packet. Otherwise,¶

2)

If the OOTB packet contains an ABORT chunk, the receiver MUST silently discard the OOTB packet and take no further action. Otherwise,¶

3)

If the packet contains an INIT chunk with a Verification Tag set to 0, it SHOULD be processed as described in Section 5.1. If, for whatever reason, the INIT chunk cannot be processed normally and an ABORT chunk has to be sent in response, the Verification Tag of the packet containing the ABORT chunk MUST be the Initiate Tag of the received INIT chunk, and the T bit of the ABORT chunk has to be set to 0, indicating that the Verification Tag is not reflected. Otherwise,¶

4)

If the packet contains a COOKIE ECHO chunk as the first chunk, it MUST be processed as described in Section 5.1. Otherwise,¶

5)

If the packet contains a SHUTDOWN ACK chunk, the receiver SHOULD respond to the sender of the OOTB packet with a SHUTDOWN COMPLETE chunk. When sending the SHUTDOWN COMPLETE chunk, the receiver of the OOTB packet MUST fill in the Verification Tag field of the outbound packet with the Verification Tag received in the SHUTDOWN ACK chunk and set the T bit in the Chunk Flags to indicate that the Verification Tag is reflected. Otherwise,¶

6)

If the packet contains a SHUTDOWN COMPLETE chunk, the receiver SHOULD silently discard the packet and take no further action. Otherwise,¶

7)

If the packet contains an ERROR chunk with the "Stale Cookie" error cause or a COOKIE ACK chunk, the SCTP packet SHOULD be silently discarded. Otherwise,¶

8)

The receiver SHOULD respond to the sender of the OOTB packet with an ABORT chunk. When sending the ABORT chunk, the receiver of the OOTB packet MUST fill in the Verification Tag field of the outbound packet with the value found in the Verification Tag field of the OOTB packet and set the T bit in the Chunk Flags to indicate that the Verification Tag is reflected. After sending this ABORT chunk, the receiver of the OOTB packet MUST discard the OOTB packet and MUST NOT take any further action.¶

8.5. Verification Tag

The Verification Tag rules defined in this section apply when sending or receiving SCTP packets that do not contain an INIT, SHUTDOWN COMPLETE, COOKIE ECHO (see Section 5.1), ABORT, or SHUTDOWN ACK chunk. The rules for sending and receiving SCTP packets containing one of these chunk types are discussed separately in Section 8.5.1.¶

When sending an SCTP packet, the endpoint MUST fill in the Verification Tag field of the outbound packet with the tag value in the Initiate Tag parameter of the INIT or INIT ACK chunk received from its peer.¶

When receiving an SCTP packet, the endpoint MUST ensure that the value in the Verification Tag field of the received SCTP packet matches its own tag. If the received Verification Tag value does not match the receiver's own tag value, the receiver MUST silently discard the packet and MUST NOT process it any further, except for those cases listed in Section 8.5.1 below.¶

9.1. Abort of an Association

When an endpoint decides to abort an existing association, it MUST send an ABORT chunk to its peer endpoint. The sender MUST fill in the peer's Verification Tag in the outbound packet and MUST NOT bundle any DATA chunk with the ABORT chunk. If the association is aborted on request of the upper layer, a "User-Initiated Abort" error cause (see Section 3.3.10.12) SHOULD be present in the ABORT chunk.¶

An endpoint MUST NOT respond to any received packet that contains an ABORT chunk (also see Section 8.4).¶

An endpoint receiving an ABORT chunk MUST apply the special Verification Tag check rules described in Section 8.5.1.¶

After checking the Verification Tag, the receiving endpoint MUST remove the association from its record and SHOULD report the termination to its upper layer. If a "User-Initiated Abort" error cause is present in the ABORT chunk, the Upper Layer Abort Reason SHOULD be made available to the upper layer.¶

9.2. Shutdown of an Association

Using the SHUTDOWN primitive (see Section 11.1), the upper layer of an endpoint in an association can gracefully close the association. This will allow all outstanding DATA chunks from the peer of the shutdown initiator to be delivered before the association terminates.¶

Upon receipt of the SHUTDOWN primitive from its upper layer, the endpoint enters the SHUTDOWN-PENDING state and remains there until all outstanding data has been acknowledged by its peer. The endpoint accepts no new data from its upper layer but retransmits data to the peer endpoint if necessary to fill gaps.¶

Once all its outstanding data has been acknowledged, the endpoint sends a SHUTDOWN chunk to its peer, including in the Cumulative TSN Ack field the last sequential TSN it has received from the peer. It SHOULD then start the T2-shutdown timer and enter the SHUTDOWN-SENT state. If the timer expires, the endpoint MUST resend the SHUTDOWN chunk with the updated last sequential TSN received from its peer.¶

The rules in Section 6.3 MUST be followed to determine the proper timer value for T2-shutdown. To indicate any gaps in TSN, the endpoint MAY also bundle a SACK chunk with the SHUTDOWN chunk in the same SCTP packet.¶

An endpoint SHOULD limit the number of retransmissions of the SHUTDOWN chunk to the protocol parameter 'Association.Max.Retrans'. If this threshold is exceeded, the endpoint SHOULD destroy the TCB and SHOULD report the peer endpoint unreachable to the upper layer (and thus the association enters the CLOSED state). The reception of any packet from its peer (i.e., as the peer sends all of its queued DATA chunks) SHOULD clear the endpoint's retransmission count and restart the T2-shutdown timer, giving its peer ample opportunity to transmit all of its queued DATA chunks that have not yet been sent.¶

Upon reception of the SHUTDOWN chunk, the peer endpoint does the following:¶

• enter the SHUTDOWN-RECEIVED state,¶

• stop accepting new data from its SCTP user, and¶

• verify, by checking the Cumulative TSN Ack field of the chunk, that all its outstanding DATA chunks have been received by the SHUTDOWN chunk sender.¶

Once an endpoint has reached the SHUTDOWN-RECEIVED state, it MUST ignore ULP shutdown requests but MUST continue responding to SHUTDOWN chunks from its peer.¶

If there are still outstanding DATA chunks left, the SHUTDOWN chunk receiver MUST continue to follow normal data transmission procedures defined in Section 6, until all outstanding DATA chunks are acknowledged; however, the SHUTDOWN chunk receiver MUST NOT accept new data from its SCTP user.¶

While in the SHUTDOWN-SENT state, the SHUTDOWN chunk sender MUST immediately respond to each received packet containing one or more DATA chunks with a SHUTDOWN chunk and restart the T2-shutdown timer. If a SHUTDOWN chunk by itself cannot acknowledge all of the received DATA chunks (i.e., there are TSNs that can be acknowledged that are larger than the cumulative TSN and thus gaps exist in the TSN sequence) or if duplicate TSNs have been received, then a SACK chunk MUST also be sent.¶

The sender of the SHUTDOWN chunk MAY also start an overall guard timer T5-shutdown-guard to bound the overall time for the shutdown sequence. At the expiration of this timer, the sender SHOULD abort the association by sending an ABORT chunk. If the T5-shutdown-guard timer is used, it SHOULD be set to the RECOMMENDED value of 5 times 'RTO.Max'.¶

If the receiver of the SHUTDOWN chunk has no more outstanding DATA chunks, the SHUTDOWN chunk receiver MUST send a SHUTDOWN ACK chunk and start a T2-shutdown timer of its own, entering the SHUTDOWN-ACK-SENT state. If the timer expires, the endpoint MUST resend the SHUTDOWN ACK chunk.¶

The sender of the SHUTDOWN ACK chunk SHOULD limit the number of retransmissions of the SHUTDOWN ACK chunk to the protocol parameter 'Association.Max.Retrans'. If this threshold is exceeded, the endpoint SHOULD destroy the TCB and SHOULD report the peer endpoint unreachable to the upper layer (and thus the association enters the CLOSED state).¶

Upon the receipt of the SHUTDOWN ACK chunk, the sender of the SHUTDOWN chunk MUST stop the T2-shutdown timer, send a SHUTDOWN COMPLETE chunk to its peer, and remove all record of the association.¶

Upon reception of the SHUTDOWN COMPLETE chunk, the endpoint verifies that it is in the SHUTDOWN-ACK-SENT state; if it is not, the chunk SHOULD be discarded. If the endpoint is in the SHUTDOWN-ACK-SENT state, the endpoint SHOULD stop the T2-shutdown timer and remove all knowledge of the association (and thus the association enters the CLOSED state).¶

An endpoint SHOULD ensure that all its outstanding DATA chunks have been acknowledged before initiating the shutdown procedure.¶

An endpoint SHOULD reject any new data request from its upper layer if it is in the SHUTDOWN-PENDING, SHUTDOWN-SENT, SHUTDOWN-RECEIVED, or SHUTDOWN-ACK-SENT state.¶

If an endpoint is in the SHUTDOWN-ACK-SENT state and receives an INIT chunk (e.g., if the SHUTDOWN COMPLETE chunk was lost) with source and destination transport addresses (either in the IP addresses or in the INIT chunk) that belong to this association, it SHOULD discard the INIT chunk and retransmit the SHUTDOWN ACK chunk.¶

Note: Receipt of a packet containing an INIT chunk with the same source and destination IP addresses as used in transport addresses assigned to an endpoint but with a different port number indicates the initialization of a separate association.¶

The sender of the INIT or COOKIE ECHO chunk SHOULD respond to the receipt of a SHUTDOWN ACK chunk with a stand-alone SHUTDOWN COMPLETE chunk in an SCTP packet with the Verification Tag field of its common header set to the same tag that was received in the packet containing the SHUTDOWN ACK chunk. This is considered an OOTB packet as defined in Section 8.4. The sender of the INIT chunk lets T1-init continue running and remains in the COOKIE-WAIT or COOKIE-ECHOED state. Normal T1-init timer expiration will cause the INIT or COOKIE chunk to be retransmitted and thus start a new association.¶

If a SHUTDOWN chunk is received in the COOKIE-WAIT or COOKIE ECHOED state, the SHUTDOWN chunk SHOULD be silently discarded.¶

If an endpoint is in the SHUTDOWN-SENT state and receives a SHUTDOWN chunk from its peer, the endpoint SHOULD respond immediately with a SHUTDOWN ACK chunk to its peer and move into the SHUTDOWN-ACK-SENT state, restarting its T2-shutdown timer.¶

If an endpoint is in the SHUTDOWN-ACK-SENT state and receives a SHUTDOWN ACK, it MUST stop the T2-shutdown timer, send a SHUTDOWN COMPLETE chunk to its peer, and remove all record of the association.¶

11.1. ULP-to-SCTP

The following sections functionally characterize a ULP/SCTP interface. The notation used is similar to most procedure or function calls in high-level languages.¶

The ULP primitives described below specify the basic functions that SCTP performs to support inter-process communication. Individual implementations define their own exact format and provide combinations or subsets of the basic functions in single calls.¶

INITIALIZE ([local port],[local eligible address list])
-> local SCTP instance name

ASSOCIATE(local SCTP instance name,
initial destination transport addr list, outbound stream count)
-> association id [,destination transport addr list]
[,outbound stream count]

SHUTDOWN(association id) -> result

ABORT(association id [, Upper Layer Abort Reason]) -> result

SEND(association id, buffer address, byte count [,context]
[,stream id] [,life time] [,destination transport address]
[,unordered flag] [,no-bundle flag] [,payload protocol-id]
[,sack-immediately flag]) -> result

SETPRIMARY(association id, destination transport address,
[source transport address]) -> result

RECEIVE(association id, buffer address, buffer size [,stream id])
-> byte count [,transport address] [,stream id]
[,stream sequence number] [,partial flag] [,payload protocol-id]

STATUS(association id) -> status data

CHANGE HEARTBEAT(association id, destination transport address,
new state [,interval]) -> result

REQUESTHEARTBEAT(association id, destination transport address)
-> result

GETSRTTREPORT(association id, destination transport address)
-> srtt result

SETFAILURETHRESHOLD(association id, destination transport address,
failure threshold) -> result

SETPROTOCOLPARAMETERS(association id,
[destination transport address,] protocol parameter list)
-> result

RECEIVE_UNSENT(data retrieval id, buffer address, buffer size
[,stream id] [, stream sequence number] [,partial flag]
[,payload protocol-id])

RECEIVE_UNACKED(data retrieval id, buffer address, buffer size,
[,stream id] [,stream sequence number] [,partial flag]
[,payload protocol-id])

DESTROY(local SCTP instance name)

11.2. SCTP-to-ULP

It is assumed that the operating system or application environment provides a means for the SCTP to asynchronously signal the ULP process. When SCTP does signal a ULP process, certain information is passed to the ULP.¶

Implementation Note: In some cases, this might be done through a separate socket or error channel.¶

12.1. Security Objectives

As a common transport protocol designed to reliably carry time-sensitive user messages, such as billing or signaling messages for telephony services, between two networked endpoints, SCTP has the following security objectives:¶

• availability of reliable and timely data transport services¶
• integrity of the user-to-user information carried by SCTP¶

12.2. SCTP Responses to Potential Threats

SCTP could potentially be used in a wide variety of risk situations. It is important for operators of systems running SCTP to analyze their particular situations and decide on the appropriate counter-measures.¶

Operators of systems running SCTP might consult [RFC2196] for guidance in securing their site.¶

12.3. SCTP Interactions with Firewalls

It is helpful for some firewalls if they can inspect just the first fragment of a fragmented SCTP packet and unambiguously determine whether it corresponds to an INIT chunk (for further information, please refer to [RFC1858]). Accordingly, we stress the requirements, as stated in Section 3.1, that (1) an INIT chunk MUST NOT be bundled with any other chunk in a packet and (2) a packet containing an INIT chunk MUST have a zero Verification Tag. The receiver of an INIT chunk MUST silently discard the INIT chunk and all further chunks if the INIT chunk is bundled with other chunks or the packet has a non-zero Verification Tag.¶

12.4. Protection of Non-SCTP-capable Hosts

To provide a non-SCTP-capable host with the same level of protection against attacks as for SCTP-capable ones, all SCTP implementations MUST implement the ICMP handling described in Section 10.¶

When an SCTP implementation receives a packet containing multiple control or DATA chunks and the processing of the packet would result in sending multiple chunks in response, the sender of the response chunk(s) MUST NOT send more than one packet containing chunks other than DATA chunks. This requirement protects the network for triggering a packet burst in response to a single packet. If bundling is supported, multiple response chunks that fit into a single packet MAY be bundled together into one single response packet. If bundling is not supported, then the sender MUST NOT send more than one response chunk and MUST discard all other responses. Note that this rule does not apply to a SACK chunk, since a SACK chunk is, in itself, a response to DATA chunks, and a SACK chunk does not require a response of more DATA chunks.¶

An SCTP implementation MUST abort the association if it receives a SACK chunk acknowledging a TSN that has not been sent.¶

An SCTP implementation that receives an INIT chunk that would require a large packet in response, due to the inclusion of multiple "Unrecognized Parameter" parameters, MAY (at its discretion) elect to omit some or all of the "Unrecognized Parameter" parameters to reduce the size of the INIT ACK chunk. Due to a combination of the size of the State Cookie parameter and the number of addresses a receiver of an INIT chunk indicates to a peer, it is always possible that the INIT ACK chunk will be larger than the original INIT chunk. An SCTP implementation SHOULD attempt to make the INIT ACK chunk as small as possible to reduce the possibility of byte amplification attacks.¶

14.1. Parameters Necessary for the SCTP Instance

Associations:

A list of current associations and mappings to the data consumers for each association. This might be in the form of a hash table or other implementation-dependent structure. The data consumers might be process identification information, such as file descriptors, named pipe pointer, or table pointers dependent on how SCTP is implemented.¶

Secret Key:

A secret key used by this endpoint to compute the MAC. This SHOULD be a cryptographic quality random number with a sufficient length. Discussion in [RFC4086] can be helpful in selection of the key.¶

Address List:

The list of IP addresses that this instance has bound. This information is passed to one's peer(s) in INIT and INIT ACK chunks.¶

SCTP Port:

The local SCTP port number to which the endpoint is bound.¶

14.2. Parameters Necessary per Association (i.e., the TCB)

Peer Verification Tag:

Tag value to be sent in every packet and is received in the INIT or INIT ACK chunk.¶

My Verification Tag:

Tag expected in every inbound packet and sent in the INIT or INIT ACK chunk.¶

State:

COOKIE-WAIT, COOKIE-ECHOED, ESTABLISHED, SHUTDOWN-PENDING, SHUTDOWN-SENT, SHUTDOWN-RECEIVED, SHUTDOWN-ACK-SENT.¶

Note: No "CLOSED" state is illustrated, since, if an association is "CLOSED", its TCB SHOULD be removed.¶

Peer Transport Address List:

A list of SCTP transport addresses to which the peer is bound. This information is derived from the INIT or INIT ACK chunk and is used to associate an inbound packet with a given association. Normally, this information is hashed or keyed for quick lookup and access of the TCB.¶

Primary Path:

This is the current primary destination transport address of the peer endpoint. It might also specify a source transport address on this endpoint.¶

Overall Error Count:

The overall association error count.¶

Overall Error Threshold:

The threshold for this association that, if the Overall Error Count reaches, will cause this association to be torn down.¶

Peer Rwnd:

Current calculated value of the peer's rwnd.¶

Next TSN:

The next TSN number to be assigned to a new DATA chunk. This is sent in the INIT or INIT ACK chunk to the peer and incremented each time a DATA chunk is assigned a TSN (normally, just prior to transmit or during fragmentation).¶

Last Rcvd TSN:

This is the last TSN received in sequence. This value is set initially by taking the peer's Initial TSN, received in the INIT or INIT ACK chunk, and subtracting one from it.¶

Mapping Array:

An array of bits or bytes indicating which out-of-order TSNs have been received (relative to the Last Rcvd TSN). If no gaps exist, i.e., no out-of-order packets have been received, this array will be set to all zero. This structure might be in the form of a circular buffer or bit array.¶

Ack State:

This flag indicates if the next received packet is to be responded to with a SACK chunk. This is initialized to 0. When a packet is received, it is incremented. If this value reaches 2 or more, a SACK chunk is sent and the value is reset to 0. Note: This is used only when no DATA chunks are received out of order. When DATA chunks are out of order, SACK chunks are not delayed (see Section 6).¶

Inbound Streams:

An array of structures to track the inbound streams, normally including the next sequence number expected and possibly the stream number.¶

Outbound Streams:

An array of structures to track the outbound streams, normally including the next sequence number to be sent on the stream.¶

Reasm Queue:

A reassembly queue.¶

Receive Buffer:

A buffer to store received user data that has not been delivered to the upper layer.¶

Local Transport Address List:

The list of local IP addresses bound in to this association.¶

Association Maximum DATA Chunk Size:

The smallest Path Maximum DATA Chunk Size of all destination addresses.¶

14.3. Per Transport Address Data

For each destination transport address in the peer's address list derived from the INIT or INIT ACK chunk, a number of data elements need to be maintained, including:¶

Error Count:

The current error count for this destination.¶

Error Threshold:

Current error threshold for this destination, i.e., what value marks the destination down if error count reaches this value.¶

cwnd:

The current congestion window.¶

ssthresh:

The current ssthresh value.¶

RTO:

The current retransmission timeout value.¶

SRTT:

The current smoothed round-trip time.¶

RTTVAR:

The current RTT variation.¶

partial bytes acked:

The tracking method for increase of cwnd when in congestion avoidance mode (see Section 7.2.2).¶

state:

The current state of this destination, i.e., DOWN, UP, ALLOW-HEARTBEAT, NO-HEARTBEAT, etc.¶

PMTU:

The current known PMTU.¶

PMDCS:

The current known PMDCS.¶

Per Destination Timer:

A timer used by each destination.¶

RTO-Pending:

A flag used to track if one of the DATA chunks sent to this address is currently being used to compute an RTT. If this flag is 0, the next DATA chunk sent to this destination is expected to be used to compute an RTT and this flag is expected to be set. Every time the RTT calculation completes (i.e., the DATA chunk is acknowledged), clear this flag.¶

last-time:

The time to which this destination was last sent. This can be used to determine if the sending of a HEARTBEAT chunk is needed.¶

14.4. General Parameters Needed

Out Queue:

A queue of outbound DATA chunks.¶

In Queue:

A queue of inbound DATA chunks.¶

15.1. IETF-Defined Chunk Extension

The assignment of new chunk type codes is done through an IETF Review action, as defined in [RFC8126]. Documentation for a new chunk MUST contain the following information:¶

a)

A long and short name for the new chunk type.¶

b)

A detailed description of the structure of the chunk, which MUST conform to the basic structure defined in Section 3.2.¶

c)

A detailed definition and description of intended use of each field within the chunk, including the chunk flags if any. Defined chunk flags will be used as initial entries in the chunk flags table for the new chunk type.¶

d)

A detailed procedural description of the use of the new chunk type within the operation of the protocol.¶

The last chunk type (255) is reserved for future extension if necessary.¶

For each new chunk type, IANA creates a registration table for the chunk flags of that type. The procedure for registering particular chunk flags is described in Section 15.2.¶

15.2. IETF-Defined Chunk Flags Registration

The assignment of new chunk flags is done through an RFC Required action, as defined in [RFC8126]. Documentation for the chunk flags MUST contain the following information:¶

a)

A name for the new chunk flag.¶

b)

A detailed procedural description of the use of the new chunk flag within the operation of the protocol. It MUST be considered that implementations not supporting the flag will send 0 on transmit and just ignore it on receipt.¶

IANA selects a chunk flags value. This MUST be one of 0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40, or 0x80, which MUST be unique within the chunk flag values for the specific chunk type.¶

15.3. IETF-Defined Chunk Parameter Extension

The assignment of new chunk parameter type codes is done through an IETF Review action, as defined in [RFC8126]. Documentation of the chunk parameter MUST contain the following information:¶

a)

Name of the parameter type.¶

b)

Detailed description of the structure of the parameter field. This structure MUST conform to the general Type-Length-Value format described in Section 3.2.1.¶

c)

Detailed definition of each component of the parameter value.¶

d)

Detailed description of the intended use of this parameter type and an indication of whether and under what circumstances multiple instances of this parameter type can be found within the same chunk.¶

e)

Each parameter type MUST be unique across all chunks.¶

15.4. IETF-Defined Additional Error Causes

Additional cause codes can be allocated through a Specification Required action as defined in [RFC8126]. Provided documentation MUST include the following information:¶

a)

Name of the error condition.¶

b)

Detailed description of the conditions under which an SCTP endpoint issues an ERROR (or ABORT) chunk with this cause code.¶

c)

Expected action by the SCTP endpoint that receives an ERROR (or ABORT) chunk containing this cause code.¶

d)

Detailed description of the structure and content of data fields that accompany this cause code.¶

The initial word (32 bits) of a cause code parameter MUST conform to the format shown in Section 3.3.10, that is:¶

• first 2 bytes contain the cause code value¶

• last 2 bytes contain the length of the error cause.¶

15.5. Payload Protocol Identifiers

The assignment of payload protocol identifiers is done using the First Come First Served policy, as defined in [RFC8126].¶

Except for value 0, which is reserved to indicate an unspecified payload protocol identifier in a DATA chunk, an SCTP implementation will not be responsible for standardizing or verifying any payload protocol identifiers. An SCTP implementation simply receives the identifier from the upper layer and carries it with the corresponding payload data.¶

The upper layer, i.e., the SCTP user, SHOULD standardize any specific protocol identifier with IANA if it is so desired. The use of any specific payload protocol identifier is out of the scope of this specification.¶

15.6. Port Numbers Registry

SCTP services can use contact port numbers to provide service to unknown callers, as in TCP and UDP. An IESG-appointed Expert Reviewer supports IANA in evaluating SCTP port allocation requests, according to the procedure defined in [RFC8126]. The details of this process are defined in [RFC6335].¶