Reference document

Connection-oriented Transport protocol


This Recommendation | International Standard specifies

  • five classes of procedures when operating over the connection-mode network service
    • class 0 simple class;
    • class 1 basic error recovery class;
    • class 2 multiplexing class;
    • class 3 error recovery and multiplexing class;
    • class 4 error detection and recovery class;
  • for the connection-mode transfer of data and control information from one transport entity to a peer transport entity;
  • one class (class 4) of procedure when operating over the connectionless-mode network service;
  • the means of negotiating the class of procedures to be used by the transport entities;
  • the structure and encoding of the transport protocol data units used for the transfer of data and control information.

The procedures are defined in terms of

  • the interactions between peer transport entities through the exchange of transport protocol data units;
  • the interactions between a transport entity and the transport service user in the same system through the exchange of transport service primitives;
  • the interactions between a transport entity and the network service provider through the exchange of network service primitives.

These procedures are defined in the main text of this Recommendation 1 International Standard supplemented by state tables in Annex A.

These procedures are applicable to instances of communication between systems which support the Transport Layer of the OS1 Reference Model and which wish to interconnect in an open systems environment.

This Recommendation | International Standard specifies, in clause 14, conformance requirements for systems implementing these procedures and provides the PICS proforma in compliance with the relevant requirements, and in accordance with the relevant guidance, given in CCITT Rec. X.291 and ISO/IEC 9646-2. It does not contain tests which can be used to demonstrate this conformance.

Responsible Party





Flag Date RFC Version
added 1998-11-10 0.1
changed 2000-07-17 0.2
changed 2003-07-31 0.5
changed 2005-09-15 0.7
changed 2010-11-01 5
changed 2013-08-12 8
changed 2017-01-14 9-19 10
changed 2022-12-20 14-62 15
changed 2023-03-23 14-73 15