Stream Control Transmission Protocol (SCTP) is a reliable transport protocol that runs on top of a packet service without potentially unreliable connection such as IP. It was developed specifically for applications and signaling offers recognized without unduplicated transfer error datagrams (messages). Detection of data corruption, data loss and data duplication is performed using checksums and sequence numbers.

A selective retransmission mechanism is applied to correct the loss or corruption of data. The decisive difference is TCP multi-homing and the concept of multiple streams in a connection. Where in a TCP flow is called a sequence of bytes, a SCTP stream represents a sequence of messages. SCTP tries to combine the advantages of UDP and TCP, but avoid their drawbacks; it is defined in IETF RFC 4960.

SCTP is used on several network internal control plane interfaces, with these SCTP applications −

  • S1-MME: between eNodeB and MME
  • SBc: between MME and SBc.
  • S6a: between MME and HSS
  • S6d: between SGSN and HSS
  • SGs: between MSC/VLR and MME
  • S13: between MME and EIR

S1 Application Protocol

Two categories of procedures across S1-MME exist: UE associated and non-associated UE. Furthermore two classes of messages are defined: Class1 is with the class 2 is answered. Class 1 and related procedures initiator/response messages are listed in the table below; procedures for Class 2 message names are largely identical to the procedure names, and the table below (second table) lists only these.

Elementary Procedure Initiating Message Response(if successful)
Handover preparation Handover required Handover command
Hand resource allocation Handover req. Path switch req. ack.
Patch Switch Request Path Switch Req. Path switch ack.
Handover Cancellation Handover cancel Handover cancel ack.
E-RAB Setup E-RAB setup Req. E-RAB setup Resp.
E-RAB modify E-RAB modify req. E-RAB modify Resp.
E-RAB release E-RAB command release E-RAB command Resp.
Initial context setup Initial context setup req. Initial context setup Resp.
Reset Reset Reset Ack.
S1 setup S1 setup req. S1 setup Resp.
UE context release UE context release command UE context release complete
UE context modification UE context modification req. UE context modification resp.
eNodeB configuration ENB configuration update ENB configuration update ack.
MME Configuration MME configuration update MME configuration update ack.
Write- Replace warning Write-Replace warning req. Write-Replace warning resp.

Elementary Procedure

  • Handover Notification
  • E-RAB release indication
  • Paging
  • Initial UE message
  • Downlink NAS transport
  • Uplink NAS transport
  • NAS non delivery indication
  • Deactivate Trace
  • Trace start
  • Trace failure indication
  • Location reporting failure indication
  • Location reporting control
  • Location report
  • Cell Traffic Trace
  • Error indication
  • UE Context release request
  • Downlink S1 CDMA2000 tunneling
  • uplink S1 CDMA2000 tunneling
  • UE capability info indication
  • eNodeB status transfer
  • MME status transfer
  • Overload Start
  • Overload Stop
  • eNodeB direct information Transfer
  • MME direct information transfer
  • eNodeB configuration transfer
  • MME configuration

X2 Application Protocol

X2 application protocol has much in common with the S1-AP; same categorization in class 1 and class 2 messages is made. The setup message is much smaller, corresponding to the specialized function of X2.

Procedure Initiating Message Class Response(if successful)
Handover preparation Handover req. 1 Handover req. ack.
Reset Reset req. 1 Reset resp.
X2 setup X2 setup 1 X2 setup resp.
eNodeB Configuration update ENB Configuration update 1 ENB Configuration update ack.
Resource Status Reporting Initiation Resource Status req. 1 Resource Status resp.
Load Indication Load Information 2
Handover Cancel Handover Cancel 2
SN status transfer SN status transfer 2
UE context release UE context release 2
Resource Status Resource Status 2
Reporting update
Error Indication Error Indication 2