Stream Control Transmission Protocol (SCTP) was initially developed as a transport protocol for transmitting SS7 messages over IP networks, operating at Layer 4 of the OSI model alongside TCP and UDP.
What is SCTP Protocol?
What are Chunks in SCTP Protocol?
SCTP organizes data into units called chunks, each associated with a connection that ensures in-order delivery. Unlike TCP, SCTP allows multiple data blocks from different connections to be transmitted within the same chunk, facilitating efficient multiplexing of data.
SCTP supports out-of-order delivery of urgent packets with higher priority, enhancing flexibility in real-time and critical communication scenarios.
One of SCTP’s notable features is its ability to handle multihoming scenarios, where a single host can possess multiple valid IP addresses. This redundancy enhances network reliability and fault tolerance.
To monitor connection states and ensure reliability, SCTP incorporates frequent heartbeat messages. These messages verify the availability and integrity of the connection between endpoints.
SCTP Example in LTE NAS Signaling.
An illustrative example of SCTP’s operation is seen in Figure below, where SCTP is utilized to transport a NAS signaling message, such as an Attach Request from an eNB to an MME over the S1 interface.
- Initiation of SCTP Stream: Upon receiving the Attach Request via the RRC connection on the Uu interface, the eNB triggers the establishment of a dedicated SCTP stream on the S1 interface.
- SCTP Initiation Message: The process begins with the transmission of an SCTP initiation message by the eNB. This message includes the IP addresses of both the eNB and the MME. Each connection is uniquely identified by a pair of SCTP source and destination port numbers.
SCTP Initiation Procedure
The initiation of an SCTP connection involves a series of steps to establish and maintain communication between network entities, ensuring reliability and continuity of data transmission.
Initialization and Handshake
- SCTP Initiation: When an eNB sends an SCTP initiation message to an MME, it includes IP addresses for both ends of the connection. This initiation must be acknowledged by the MME’s SCTP entity.
- Cookie Echo Exchange: Following initiation, a SCTP cookie echo message is sent by the MME to confirm receipt and validate the connection. This exchange forms part of the heartbeat procedure, periodically verifying the connection’s status and functionality.
SCTP Datagram Transport.
- SCTP Datagram (DTGR): Higher-layer messages within SCTP are transmitted using SCTP datagrams, each containing a Transaction Sequence Number (TSN) alongside source and destination addresses. The TSN is crucial for ensuring error-free delivery and is acknowledged by selective ACK messages.
- Selective ACKnowledgment: Upon receiving an SCTP datagram, the peer entity (MME) sends a selective ACK message confirming successful reception. This process ensures data integrity and informs the sender about the status of transmitted messages.
Maintenance and Error Handling
- Periodic Checks: Throughout the connection’s lifespan, periodic Cookie Echo and Cookie Echo ACK messages are exchanged to maintain connection validity and detect potential issues early.
- Error Detection and Response: If SCTP functionality is compromised—for instance, due to congestion or protocol errors—expected selective ACK messages may be missing or incorrectly sent. This can trigger a NACK (Negative ACKnowledgment) from the UE’s side, potentially leading to connection termination or retries.
- UE Reaction to Missing Messages: If critical messages like an attach accept are not received within a specified time (guarded by timers on the UE), the UE may reattempt sending an attach request multiple times. If unsuccessful after a configurable number of attempts, the UE may revert to an idle state.
Conclusion
SCTP’s robust initialization, datagram transport, and heartbeat mechanisms ensure reliable transmission of signaling messages such as NAS (Non-Access Stratum) across LTE networks. Its error detection and handling capabilities help maintain smooth operation, crucial for maintaining quality of service in telecommunications.