Description
The Stream Control Transmission Protocol (SCTP) is an IETF-defined transport layer protocol (RFC 4960) that is mandated by 3GPP for carrying signaling plane protocols across many critical interfaces. It operates at an equivalent layer to TCP and UDP but is designed with telecommunication signaling reliability in mind. Within a 3GPP network, SCTP associations are established between network functions, such as between an eNB and an MME (over S1-MME), a gNB and an AMF (over NG-C), or between Diameter peers (e.g., MME and HSS). It provides a reliable, in-sequence delivery of signaling messages with congestion control.
SCTP works by establishing an association between two endpoints, which can consist of multiple IP addresses per endpoint (multi-homing). Data is transferred in chunks, which are bundled into SCTP packets. A key architectural feature is multi-streaming: within a single SCTP association, multiple independent logical streams exist. A failure or head-of-line blocking in one stream (e.g., due to a lost chunk) does not affect the delivery of chunks in other streams. This is crucial for signaling where different procedures (e.g., attach, handover) can proceed in parallel. The protocol uses a four-way handshake (INIT, INIT-ACK, COOKIE-ECHO, COOKIE-ACK) with state cookies to provide protection against blind SYN flooding attacks.
Key components in its operation include the SCTP endpoint (the network function), the SCTP association (the connection), streams, and the chunk-based packet structure. The protocol ensures reliability through a selective acknowledgment (SACK) mechanism and retransmission timers. Its role in the 3GPP architecture is as the foundational transport for nearly all control plane interfaces in both the Evolved Packet Core (EPC) and 5G Core (5GC), including S1-MME, S6a, S11, NG-C, N11, N12, and many more. It carries application layer protocols like S1-AP, NG-AP, and Diameter, ensuring that signaling messages for mobility management, session management, and authentication are delivered robustly even in the face of network congestion or partial path failures.
Furthermore, SCTP's multi-homing capability allows a network function to be reachable via multiple IP network interfaces. If the primary path fails, traffic can be seamlessly switched to a secondary path without breaking the association, providing network-level redundancy. This makes the core network signaling infrastructure highly resilient, which is a fundamental requirement for carrier-grade telecommunications systems.
Purpose & Motivation
SCTP was adopted by 3GPP to address the shortcomings of TCP for carrying telephony signaling. TCP, while reliable, introduces head-of-line blocking where packet loss for one message delays all subsequent messages in the connection, which is unacceptable for time-critical signaling where independent transactions need to proceed in parallel. Furthermore, TCP is vulnerable to certain denial-of-service attacks (e.g., SYN floods) and lacks native support for multi-homed endpoints, which is desirable for network resilience.
Its creation was motivated by the need for a transport protocol that matched the requirements of SS7 signaling but over an IP-based network. SCTP solves these problems by providing message-oriented delivery (preserving application message boundaries), multi-streaming to eliminate head-of-line blocking, a secure four-way handshake, and built-in multi-homing support. This made it the ideal choice for 3GPP when designing the all-IP core network (starting with UMTS R99 and solidified in EPS), ensuring that signaling for millions of subscribers is both robust and efficient. It provided the necessary foundation for the reliability and availability expected in public land mobile networks.
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (82 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, key clarifications and enhancements were made for the SCTP function, primarily focusing on resolving ambiguities and enabling support for multiple SCTP associations and the use of SCTP ports for multiple Transport Network Layer Associations (TNLAs). The release also introduced a specific Payload Protocol Identifier (PPID) value for Datagram Transport Layer Security (DTLS) over SCTP. Furthermore, the specifications addressed the establishment of SCTP associations and provided support for RAN-initiated multiple SCTP associations.
- Introduction of early data transmission TS 36.413CR1578
- Clarification on SCTP association establishment – for 36.300 TS 36.300CR1186
- Clarification on when UL data transmission in EDT is not considered successful TS 36.300CR1201
- CR on PPID value for DTLS over SCTP TS 38.412CR0010
- Use of SCTP ports for multiple TNLA TS 38.412CR0011
- Ambiguity with multiple SCTP associations in 38.412 TS 38.412CR0012
+ 22 more changes
In Release 16, the key enhancements for SCTP included the introduction of support for multiple SCTP associations in EN-DC scenarios and a defined procedure for SCTP association change upon failure, specifically for the X2 interface. Additionally, a new SCTP Payload Protocol Identifier was allocated for the W1AP application protocol.
- Introduction of MT Early Data Transmission TS 36.413CR1682
- CR to 36.422 for Supporting mutiple SCTP assoication in EN-DC TS 36.422CR0035
- Introduction of NB-IoT related NG-AP procedures TS 38.413CR0156
- Introduction of Control Plane CIoT 5GS Optimisation for NB-IOT and eMTC TS 38.413CR0173
- CR38.423 on TDD pattern for NR-DC power control cordination for sol1 TS 38.423CR0344
- SCTP association change when current SCTP association is failed (X2) TS 36.422CR0037
+ 13 more changes
In Release 17, the SCTP-related update specifically corrected the E1AP PPID (Payload Protocol Identifier) for DTLS over SCTP, ensuring proper protocol identification for secured transport. This change was implemented to align the specification for Integrated Access and Backhaul (IAB) nodes, as indicated by the grounding context's mention of IAB protocol stack corrections. The modification was procedural, involving the transfer of the relevant specification text for E1AP under change control to maintain consistency.
- Support of 5G ProSe Authorization for NG-AP TS 38.413CR0743
- Exchange of protocol support at target RAN node for NG handover [PROT_SUP] TS 38.413CR0800
- Missing transmission bandwidth configurations in XnAP [NR_FR1_35MHz_45MHz_BW] TS 38.423CR1021
- Correction of references to RRC protocol elements in S1AP TS 36.413CR1908
- Correction on protocol stack for IAB TS 38.401CR0242
- Correction on small data transmission TS 38.401CR0311
+ 7 more changes
In Release 18, the primary update for SCTP was the correction of the Payload Protocol Identifiers (PPIDs) and Destination Port Numbers used over SCTP for several critical interface protocols. Specifically, corrections were made for the S1AP, X2AP, NGAP, XnAP, and F1AP application protocols to ensure proper transport layer identification and routing. These changes ensure reliable and correctly addressed signaling transport for the respective network interfaces.
- Support of Network-Controlled Repeater TS 38.401CR0274
- Correction on Location Reporting Control [LRC-Enh] TS 38.413CR0877
- Network-Controlled Repeaters Authorization TS 38.413CR0890
- Protocol Stack for MTSI UE TS 26.114CR0529
- IANA registration for data channel sub-protocols TS 26.114CR0537
- Mechanism to distinguish two bootstrap data channels with the same stream ID value (Rel-18) TS 26.114CR0560
+ 12 more changes
In Release 19, the SCTP function was updated to introduce Overload Control procedures specifically for the Cx and Dx interfaces. This enhancement provides a defined set of network access control mechanisms to manage signaling load and prevent infrastructure overload. The update formalizes these procedures within the application protocol to regulate input rate transmissions between network elements.
Explore further
Broader topics and technologies where SCTP plays a role.
Defining Specifications
3GPP specifications that define or reference SCTP, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TR 21.905 vj00 | 3GPP Technical Terms and Definitions | Rel-19 |
| TS 23.231 vj00 | SIP-I based CS core network stage 2 | Rel-19 |
| TS 23.333 vj00 | MRFC-MRFP Mp Interface Requirements | Rel-19 |
| TS 24.103 vj00 | Telepresence Protocol for IMS | Rel-19 |
| TS 24.229 vj50 | IMS call control protocol based on SIP and SDP | Rel-19 |
| TS 24.803 vc00 | Telepresence using IMS - Study | Rel-12 |
| TS 25.410 vj00 | Iu Interface Introduction for UTRAN | Rel-19 |
| TS 25.412 vj00 | Iu Interface Signalling Transport Specification | Rel-19 |
| TS 25.422 vj00 | Signalling Transport for Iur Interface | Rel-19 |
| TS 25.426 vj00 | UTRAN Iur/Iub Transport Bearers | Rel-19 |
| TS 25.430 vj00 | Introduction to Iub Interface Specifications | Rel-19 |
| TS 25.432 vj00 | Iub NBAP Signalling Transport Specification | Rel-19 |
| TS 25.450 vj00 | Iupc Interface Introduction for UTRAN Positioning | Rel-19 |
| TS 25.452 vj00 | Iupc Interface Signalling Transport for PCAP | Rel-19 |
| TS 25.468 vj00 | RANAP User Adaption (RUA) protocol specification | Rel-19 |
| TS 25.469 vj00 | HNBAP Specification for HNB to HNB-GW Interface | Rel-19 |
| TS 25.470 vj00 | PCAP User Adaption (PUA) protocol specification | Rel-19 |
| TS 25.471 vj00 | RNSAP User Adaptation (RNA) for Iurh | Rel-19 |
| TS 25.820 v820 | 3G Home NodeB Study Report | Rel-8 |
| TS 26.114 vj10 | IMS Multimedia Telephony Media Handling | Rel-19 |
| TS 26.223 vj00 | IMS Telepresence Client Specification | Rel-19 |
| TS 26.802 vj20 | Multicast Enhancements for 5G Media Streaming | Rel-19 |
| TR 26.862 vh00 | Immersive Teleconferencing & Telepresence for Remote Terminals | Rel-17 |
| TR 26.923 vj00 | Study on IMS-based Telepresence Media Handling | Rel-19 |
| TS 28.671 vj00 | HNS Network Resource Model Requirements | Rel-19 |
| TS 29.109 vj00 | GAA Bootstrapping Interfaces (Zh, Dz, Zn, Zpn) | Rel-19 |
| TS 29.118 vj10 | MME-VLR Interface for CS Fallback & SMS | Rel-19 |
| TS 29.162 vj00 | IMS-IP Network Interworking | Rel-19 |
| TS 29.163 vj00 | Interworking between 3GPP IM CN and CS networks | Rel-19 |
| TS 29.168 vj00 | SBc-AP Protocol Specification | Rel-19 |
| TS 29.171 vj00 | LCS Application Protocol (LCS-AP) Specification | Rel-19 |
| TS 29.202 vj00 | SS7 Signalling Transport Protocol Architectures | Rel-19 |
| TS 29.205 vj00 | BICC Protocols for Bearer-Independent CS Core Network | Rel-19 |
| TS 29.229 vj10 | Diameter Protocol for Cx/Dx Interfaces | Rel-19 |
| TS 29.232 vj00 | Mc Interface Protocol Profile | Rel-19 |
| TS 29.235 vj00 | SIP-I CS Core Network Interworking | Rel-19 |
| TS 29.329 vj10 | Diameter Protocol for Sh Interface | Rel-19 |
| TS 29.332 vj00 | MGCF-IM-MGW Interface Protocol (Mn) | Rel-19 |
| TS 29.333 vj00 | MRFC-MRFP Mp Interface Protocol | Rel-19 |
| TS 29.334 vj00 | IMS-ALG to IMS-AGW Interface Protocol | Rel-19 |
| TS 29.424 v801 | H.248 Profile for Trunking Media Gateways | Rel-8 |
| TS 29.819 vd00 | Diameter Base Protocol Update Analysis | Rel-13 |
| TS 29.890 vg00 | CT3 5G System Technical Report | Rel-16 |
| TS 29.891 vg00 | CT4 Aspects of 5G System Phase 1 | Rel-16 |
| TS 32.771 vb00 | HNS Network Resource Model IRP Requirements | Rel-11 |
| TS 36.300 vj00 | E-UTRAN Radio Interface Protocol Architecture Overview | Rel-19 |
| TS 36.410 vj00 | S1 Interface: General Aspects and Principles | Rel-19 |
| TS 36.412 vj00 | S1 Signalling Transport Specification | Rel-19 |
| TS 36.413 vj10 | S1 Application Protocol (S1AP) | Rel-19 |
| TS 36.422 vj00 | X2 Signalling Transport Specification | Rel-19 |
| TS 36.442 vj00 | Signalling transport for M2 and M3 interfaces | Rel-19 |
| TS 36.456 vj00 | SLm Interface Introduction | Rel-19 |
| TS 36.458 vj00 | SLm Interface Signalling Transport | Rel-19 |
| TS 36.462 vj00 | Xw Interface Signalling Transport | Rel-19 |
| TS 37.470 vj00 | W1 Interface Introduction for ng-eNB | Rel-19 |
| TS 37.472 vj00 | W1 Interface Signalling Transport Specification | Rel-19 |
| TS 37.480 vj00 | E1 Interface General Aspects and Principles | Rel-19 |
| TS 37.482 vj00 | E1 Signalling Transport Specification | Rel-19 |
| TS 38.401 vj10 | NG-RAN Architecture Specification | Rel-19 |
| TS 38.412 vj00 | NG Signalling Transport | Rel-19 |
| TS 38.413 vj10 | NG Application Protocol (NGAP) | Rel-19 |
| TS 38.420 vj10 | Introduction to Xn interface specifications | Rel-19 |
| TS 38.422 vj00 | Xn Signalling Transport Specification | Rel-19 |
| TS 38.423 vj10 | Xn Application Protocol (XnAP) specification | Rel-19 |
| TS 38.460 vj00 | E1 Interface General Aspects and Principles | Rel-19 |
| TS 38.462 vj00 | E1 Signalling Transport Specification | Rel-19 |
| TS 38.470 vj10 | F1 Interface Introduction | Rel-19 |
| TS 38.472 vj00 | F1 Interface Signalling Transport Specification | Rel-19 |