9. Termination of Association

SCTP provides two mechanisms for association termination: graceful shutdown and forced abort.

9.1. Abort of an Association

The ABORT chunk is used to immediately terminate an association. This is an abnormal termination with no guarantee of data delivery.

9.1.1. Sending ABORT

An endpoint MAY send an ABORT chunk in the following situations:

• Protocol violation detected
• Unrecoverable error received
• Application layer requests immediate termination
• Resource exhaustion

ABORT Chunk Format:

ABORT chunk contains:
- Chunk Type = 6
- T bit: Indicates Verification Tag usage
- Error Causes (optional): Describes abort reason

9.1.2. Receiving ABORT

Upon receiving an ABORT chunk, an endpoint MUST:

1. Stop sending any new data to the association
2. Discard all unsent and unacknowledged data
3. Report association abort to upper layer
4. Destroy the TCB
5. Send no response

Important: ABORT is unidirectional; the receiver MUST NOT send any response.

9.1.3. ABORT Causes

Common ABORT causes include:

Cause Code	Name	Description
1	Invalid Stream Identifier	Invalid stream identifier
2	Missing Mandatory Parameter	Missing mandatory parameter
3	Stale Cookie Error	Stale cookie
4	Out of Resource	Resource shortage
5	Unresolvable Address	Unresolvable address
6	Unrecognized Chunk Type	Unrecognized chunk type
7	Invalid Mandatory Parameter	Invalid mandatory parameter
8	Unrecognized Parameters	Unrecognized parameters
9	No User Data	No user data
10	Cookie Received While Shutting Down	Cookie received while shutting down
11	Restart with New Addresses	Restart with new addresses
12	User Initiated Abort	User-initiated abort
13	Protocol Violation	Protocol violation

9.1.4. T Bit Usage

The T bit controls Verification Tag selection in ABORT chunk:

T=0:

- Use normal Verification Tag
- Used for ABORT in normal association

T=1:

- Use Verification Tag from received packet
- Used for response to "Out of the Blue" packets
- Or when unsure of correct Tag

9.2. Shutdown of an Association

SHUTDOWN is a normal, orderly association termination process ensuring all data is reliably delivered.

9.2.1. Shutdown Process Overview

Normal shutdown involves exchange of three chunks:

Endpoint A                                     Endpoint Z

(No more data to send)
         --------[SHUTDOWN]--------->
                                     (Transmit remaining data)
         <------[SHUTDOWN ACK]-------
         -----[SHUTDOWN COMPLETE]---->
         
Both sides destroy TCB

9.2.2. Initiating SHUTDOWN

When ULP requests association closure:

Sender Behavior:

1. Stop accepting new user data
2. Complete transmission of all unsent data
3. Wait for all unacknowledged data to be acknowledged
4. Send SHUTDOWN chunk with Cumulative TSN Ack
5. Start T2-shutdown timer

SHUTDOWN Chunk Format:

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

9.2.3. Receiving SHUTDOWN

Upon receiving a SHUTDOWN chunk, an endpoint MUST:

1. Stop accepting new user data
2. Verify all data up to Cumulative TSN Ack has been sent
3. If unsent data exists, continue transmission
4. Once all data is acknowledged, send SHUTDOWN ACK
5. Start T2-shutdown timer

State Transition:

ESTABLISHED -> SHUTDOWN-RECEIVED

9.2.4. Sending SHUTDOWN ACK

SHUTDOWN ACK Chunk Format:

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    |     Flags     |      Length = 4               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Sending Timing:

• All received data delivered to ULP
• All local data acknowledged by peer

9.2.5. Receiving SHUTDOWN ACK

Upon receiving SHUTDOWN ACK, an endpoint MUST:

1. Send SHUTDOWN COMPLETE chunk
2. Stop T2-shutdown timer
3. Report association closure to ULP
4. Destroy TCB

State Transition:

SHUTDOWN-SENT -> CLOSED

9.2.6. SHUTDOWN COMPLETE

SHUTDOWN COMPLETE Chunk Format:

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                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

T Bit Usage:

• T=0: Normal case, use local Tag
• T=1: Use when responding to unexpected SHUTDOWN ACK

Receiving SHUTDOWN COMPLETE:

1. Stop T2-shutdown timer
2. Report association closure to ULP
3. Destroy TCB

State Transition:

SHUTDOWN-ACK-SENT -> CLOSED

9.2.7. T2-shutdown Timer

Purpose: Ensure SHUTDOWN process doesn't wait indefinitely.

Timeout Handling:

If in SHUTDOWN-SENT state:
  - Retransmit SHUTDOWN chunk
  - Double RTO
  - Restart T2-shutdown timer
  
If in SHUTDOWN-ACK-SENT state:
  - Retransmit SHUTDOWN ACK chunk
  - Double RTO
  - Restart T2-shutdown timer

If retransmissions exceed Association.Max.Retrans:
  - Report error to ULP
  - Destroy TCB

9.2.8. Data Handling During Shutdown

In SHUTDOWN-PENDING state:

• Continue normal transmission and reception of data
• Wait for all unacknowledged data to be acknowledged

In SHUTDOWN-SENT state:

• Do not accept new user data
• May receive peer's data
• Send SACK to acknowledge received data

In SHUTDOWN-RECEIVED state:

• Do not accept new user data
• Continue transmitting unsent data
• Receive and acknowledge peer's data

In SHUTDOWN-ACK-SENT state:

• Do not accept new user data
• Do not transmit new data
• Retransmit SHUTDOWN ACK in response to duplicate SHUTDOWN

9.2.9. SHUTDOWN vs ABORT Comparison

Feature	SHUTDOWN	ABORT
Data Guarantee	All sent data acknowledged	No data guarantee
Termination Speed	Slower (with handshake)	Immediate
Resource Cleanup	Graceful cleanup	Immediate cleanup
Use Case	Normal closure	Error or emergency
Response Required	Requires peer response	Unidirectional, no response

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Summary

SCTP provides flexible association termination mechanisms:

1. SHUTDOWN: For normal closure, guarantees data integrity
2. ABORT: For exceptional situations, immediate termination

Best Practices:

• Prefer SHUTDOWN for normal closure
• Use ABORT only for errors or resource issues
• Handle T2-shutdown timer timeouts properly
• Implement appropriate retransmission mechanisms