Stream Control Transmission Protocol (SCTP)
An Introduction
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Prof. David Bernstein
James Madison University
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| Computer Science Department |
| bernstdh@jmu.edu |
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Introduction
- Specification:
- Purpose:
- Provide a mechanism for applications to reliably send and receive
messages
- Assumptions:
- Access to a potentially unreliable datagram service
- Applications are associated with ports
Properties
- Connection/Session-Oriented:
- Handshaking
- State information is maintained
- Reliable:
- Error-free non-duplicated transfer of messages
- Message-Oriented:
- Transmission of complete messages (with the possibility of
requiring a particular order-of-arrival)
- Each message can contain one or more chunks (containing
either control information or user DATA)
- Controlled:
- Flow control
- Congestion control
- Multi-Homing:
- A single endpoint can support multiple IP addresses
for redundancy purposes (i.e., to improve the probability of
retransmissions reaching the destination)
Reliability in SCTP
- Requirement:
- "SCTP provides a reliable transport service, ensuring that
data is transported across the network without error and
in sequence."
- Satisfying this Requirement:
- Selective ACK - Receipt of DATA chunks is acknowledged
by sending SACK chucks (that include the cumulative
Transmission Sequence Number range received and
non-cumulative TSNs that indicate gaps)
- Aknowledgment (ACK) from the recipient (and retransmission
if the ACK is not received in a given amount of time)
- Use checksums
Flow and Congestion Control in SCTP
- Requirement:
- Satisfying this Requirement:
The Demultiplexing Process
- Recall:
- SCTP needs to know what process/application to
deliver segments to
- There may be more than one active connection at the
same time
- The Socket:
- As in TCP, the connection can be characterized by
a four-tuple consisting of the sending
IP address and port and receiving IP address and port
Moving through the Stack/Layers
- Moving Down:
- The SCTP segment becomes the payload of the IP datagram
- Moving Up:
- The payload of the SCTP datagram is handed off to the
process bound to the destination port
Required Functionality of Service Providers
- Create a "welcoming channel" that waits for a connection
- Receive connection-establishment requests
- Send and/or receive messages
- Close the connection
SCTP Packet Format
- The Common Header:
- Source Port (16 bits)
- Destination Port (16 bits)
- Verification Tag (32 bits) - agreed upon during handshaking
- Checksum (32 bits)
- Chunk Field Descriptions:
- Chunk Type (8 bits)
- Chunk Flags (8 bits)
- Chunk Length (16 bits)
SCTP Packet Format (cont.)
- Some Chunk Types:
- 0 - Payload Data (DATA)
- 1 - Initiation (INIT)
- 2 - Initiation Acknowledgment (INIT ACK)
- 3 - Selective Acknowledgment (SACK)
- 4 - Heartbeat Request (HEARTBEAT)
- 5 - Heartbeat Acknowledgment (HEARTBEAT ACK)
- 7 - Shutdown (SHUTDOWN)
- 8 - Shutdown (SHUTDOWN ACK)
- 10 - State Cookie (COOKIE ECHO)
- 11 - Cookie Acknowledgment (COOKIE ACK)
- DATA Chunks:
- Type (8 bits)
- Reserved (5 bits)
- Unordered Flag (1 bit)
- Beginning Fragment Flag (1 bit)
- Ending Fragment Flag (1 bit)
- Length (16 bits)
- Transmission Sequence Number/TSN (32 bits)
- "Stream" Identifier (16 bits)
- "Stream" Sequence Number (16 bits)
- Payload Protocol Identifier (for use by applications)
- User Data
- Other Chunks:
Establishing a Connection (Four-Way Handshake)
- Client sends INIT to server
- Includes the client's Verification Tag
- Server responds with INIT ACK
- Includes the client's Verification Tag and the
server's verification Tag
- Includes a cookie
- The server does not keep any state information at this point
- Client responds with a COOKIE ECHO
- Includes the cookie received from the server
- Server responds with a COOKIE ACK
- The server now starts to maintain state information
- When the client receives the COOKIE ACK it also starts to maintain
state information
Closing a Connection (Three-Way Procedure)
- Peer A sends SHUTDOWN
- Peer Z receives SHUTDOWN and when there are no more outstanding
DATA chunks, responds with a SHUTDOWN ACK
- Peer A receives the SHUTDOWN ACK and sends a SHUTDOWN COMPLETE
- When peer A receives the SHUTDOWN ACK it deletes all state
information (assuming it is in the SHUTDOWN-ACK-STATE
state)
- When peer Z receives the SHUTDOWN COMPLETE it
deletes all state information