rfc892.txt
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Primitive Parameters
------------------------------------------------------------------------
T-CONNECT Request To Transport Address, From
Indication Transport Address, Expedited
Data Option, Quality of
Service, TS-User data.
------------------------------------------------------------------------
T-CONNECT Response Responding Address, Quality
Confirmation of Service, Expedited Data
Option, TS-User data.
------------------------------------------------------------------------
T-DATA Request TS-User data.
Indication
------------------------------------------------------------------------
T-EXPEDITED Request TS-User data.
DATA Indication
------------------------------------------------------------------------
T-DISCONNECT Request TS-User data.
------------------------------------------------------------------------
T-DISCONNECT Indication Disconnect reason, TS-User
data.
------------------------------------------------------------------------
Figure 1. Transport Service Primitives
5.2 Service Assumed from the Network Layer
The transport protocol described in this document assumes of
the Network Services described in DP bbbb. The Network Service
primitives used are summarized in Figure 2.
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Primitive X/Y Parameters X/Y/Z
------------------------------------------------------------------------
N-CONNECT Request X Called Address, X
Indication X Calling Address, X
Response X NS-User data, Z
Confirmation X QOS. X
------------------------------------------------------------------------
N-DATA Request X NS-User data, X
Indication X Conf. Request Y
------------------------------------------------------------------------
N-DATA Request Y
ACKNOWLEDGE Indication
------------------------------------------------------------------------
N-EXPEDITED Request Y
DATA Indication NS-User data Y
------------------------------------------------------------------------
N-RESET Request X
Indication X
Response X
Confirmation X
------------------------------------------------------------------------
N-DISCONNECT Request X NS-User data Z
Indication X
------------------------------------------------------------------------
X - The Transport Protocol assumes that this facility is
provided in all networks.
Y - The Transport Protocol assumes that this facility is
provided in some networks and a mechanism is provided
to optionally use the facility.
Z - The Transport Protocol does not use this parameter.
Figure 2. Network Service Primitives
5.3 Functions of the Transport Layer
5.3.1 Connection Oriented Functions
5.3.1.1 Overview of Functions
The functions in the transport layer are at least those
necessary to bridge the gap between the services available from the
network layer and those to be offered to the transport users.
The functions in the transport layer are concerned with the
enhancement of quality of service, including all aspects of cost
optimization. They are described below; the descriptions are grouped
into those concerned with the establishment phase, the data transfer
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phase, and the release phase.
5.3.1.1.1 Establishment Phase
The goal of the establishment phase is to establish a
transport connection, i.e., between two transport users. The
functions of transport layer during this phase must match the
requested class of services with the services provided by the network
layer as follows:
o Select network service which best matches the requirement
of the TS-user taking into account charges for various
services.
o Decide whether to multiplex multiple transport connection
onto a single network connection.
o Establish the optimum TPDU size.
o Select the functions that will be operational upon entering
the data transfer phase.
o Map transport addresses onto network addresses.
o Provide a means to distinguish between two different
transport connections.
o Transportation of user's data.
5.3.1.1.2 Data Transfer Phase
The purpose of the data transfer phase is to permit two-way
simultaneous transport of TSDUs between the session entities connected
by the transport connection. This purpose is achieved by means of
two-way simultaneous communication in the Transport protocol and by
the following functions. Each of these functions is used or not used
in accordance with the result of the selection performed in the
establishment phase.
o Concatenation and Separation
A function used to collect several TPDUs into a single
NSDU; the destination transport entity separates the TPDUs.
o Segmenting and Reassembling
The splitting of a single data TSDU into multiple TPDUs
which are reassembled into their original format at the
destination.
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o Multiplexing and Demultiplexing
A function used to share a single network connection
between two or more transport connections.
o Splitting and Recombing
A function allowing the simultaneous use of two or more
network connections to support the same transport connec-
tion.
o Flow Control
A function used to regulate the flow of TPDUs between two
transport entities on one transport connection.
o Error Detection
A function used to detect the loss, corruption,
duplication, misordering or misdelivery of TPDUs.
o Transport Connection Identification
A means to uniquely identify a transport connection
between the pair of transport entities supporting the
connection during the lifetime of the transport
connection.
o Error Recovery
A function used to recover from detected and signalled
errors.
o Expedited Data
A function used to bypass the flow control of normal data
TPDU. Expedited data TPDUs' flow is controlled by
separate flow control.
o TSDU Delimiting
A function used to determine the beginning and ending of
a TSDU.
5.3.1.1.3 Release Phase
A function to provide a disconnection of the transport
connection, regardless of the current activity.
5.3.1.2 Classes and Options
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A class defines a set of functions. In this protocol five
classes are defined:
o Class 0: Simple Class
o Class 1: Basic Error Recovery Class
o Class 2: Multiplexing Class
o Class 3: Error Recovery and Multiplexing Class
o Class 4: Error Detection and Recovery Class.
Note that with the exception of classes 0 and 1, transport
connections of different class may be multiplexed together
onto the same network connection.
5.3.1.2.2 Options within Classes
Options define potential functions which may be used within
a class.
5.3.1.2.3 Negotiation
Classes and options within classes are negotiated during
the connection establishment phase.
5.3.1.2.4 Choice of the Class of Protocol
The choice will be made by the transport entities according
to:
o the users requirement expressed via T-CONNECT service
primitives. In particular, for the choice of the
class of protocol, the following rules apply:
- if the TS-User requests either transmission of
user data during the connection phase, or use of
Expedited data transfer, then Class 0 cannot be
selected.
- if the TS-User requests use of Expedited data
transfer, then Class 2 with the non-explicit
flow control option cannot be selected.
o the quality of the available Network services;
o the user required service versus cost ratio
acceptable for the transport user.
The following is a classification of network services in terms
of quality with respect to error behavior relative to the user
requirements. Its main purpose is to provide a basis for the decision
regarding which class of transport connection should be used on top of
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a given network connection.
Type A: Network connection with acceptable residual error
rate (for example not signalled by 'clear' or 'reset')
and acceptable rate of signalled failures.
Type B: Network connections with acceptable residual error
rate (for example not signalled by 'clear' or 'reset')
but unacceptable rate of signalled failures.
Type C: Network connections with residual error rate not
acceptable to the TS-user.
It is assumed that each transport entity is aware of the
quality of service provided by particular Network connections.
5.3.1.3 Potential Functions
The protocol described in this document does not include the
following set of functions which have been identified as potential
transport layer functions:
o provision for encryption
o provision for accounting mechanisms
o provision for status exchanges and monitoring of quality
of service
o provision for blocking
o temporary release of network connections
5.4 Model of the Transport Layer
TSAP TSAP
Transport Protocol Transport Protocol
Entity Entity
NSAP ------- NSAP -------
| (NSAP) | (NSAP)
| | | |
| |-------------------------|--------
| |
-----------------------------------
A Transport Protocol entity within the Transport Layer
communicates with a Transport User through a TSAP by means of the
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service primitives as defined by the transport service definition DP
aaaa. Service primitives will cause or be the result of Transport
Protocol Data Unit exchanges between the peer Transport Protocol
entities supporting a Transport Connection. These protocol exchanges
are effected using the services of the Network Layer as defined by the
Network Service Definition DP bbbb through one or more NSAPs.
Transport connection endpoints are identified in end systems
by an internal, implementation dependent, mechanism so that the
Transport User and the Transport Protocol entity can refer to each
Transport connection.
Section Two - Transport Protocol Specification
6. Protocol Mechanisms
Several functions are described as 'inherent' or 'pervasive'.
Inherent functions must be invoked for every transport connection.
Pervasive functions are optional, but if one is invoked for the first
transport connection over a network connection, it must also be invoked
for any and all other transport connections which use that network
connection during its lifetime.
6.1 Assignment to Network Connection
Purpose: Assignment of transport connections to network
connections.
Network Service Primitives:
N-CONNECT
N-DISCONNECT
Description:
This function is inherent.
Before a transport connection can be created or used, it must
be assigned to one (or more if splitting function is being used)
network connection(s). Both transport entities involved must become
aware of this assignment. A transport connection may be assigned to a
suitable existing network connection; one or more new network
connections may also be created for the purpose.
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