rfc905.txt

RFC 相关的技术文档

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     +-------------------------------------------------------------+
     |           Primitive            |        Parameter           |
     |--------------------------------|----------------------------|
     |T-CONNECT         request       |   Called Address,          |
     |                  indication    |   Calling Address,         |
     |                                |   Expedited Data option,   |
     |                                |   Quality of Service,      |
     |                                |   TS User-Data.            |
     |--------------------------------|----------------------------|
     |T-CONNECT         response      |   Responding Address,      |
     |                  confirm       |   Quality of Service,      |
     |                                |   Expedited Data option,   |
     |                                |   TS User-Data.            |
     |--------------------------------|----------------------------|
     |T-DATA            request       |   TS User-Data.            |
     |                  indication    |                            |
     |--------------------------------|----------------------------|
     |T-EXPEDITED DATA  request       |   TS User-Data.            |
     |                  indication    |                            |
     |--------------------------------|----------------------------|
     |T-DISCONNECT      request       |   TS User-Data.            |
     |--------------------------------|----------------------------|
     |T-DISCONNECT      indication    |   Disconnect reason,       |
     |                                |   TS User-Data.            |
     +--------------------------------|----------------------------+

                   Table 1. Transport service primitives





     5.2  Service assumed from the network layer

     The protocol specified in this International Standard assumes the
     use of the network service defined in DP 8348.

     Information is transferred to and from  the  NS-provider  in  the
     network service primitives listed in table 2.



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     +---------------------------------------------------------------+
     |        Primitives          |X/Y|       Parameters       |X/Y/Z|
     |----------------------------|---|------------------------|-----|
     |N-CONNECT       request     | X | Called Address,        |  X  |
     |                indication  | X | Calling Address,       |  X  |
     |                response    | X | NS User-Data,          |  Z  |
     |                confirm     | X | QOS parameter set,     |  X  |
     |                            |   | Responding address,    |  Z  |
     |                            |   | Receipt confirmation   |  Y  |
     |                            |   | selection.             |     |
     |----------------------------|---|------------------------|-----|
     |N-DATA          request     | X | NS User-Data,          |  X  |
     |                indication  | X | Confirmation request   |  Y  |
     |----------------------------|---|------------------------|-----|
     |N-DATA ACKNOWLEDGE          |   |                        |     |
     |                request     | Y |                        |     |
     |                indication  | Y |                        |     |
     |----------------------------|---|------------------------|-----|
     |N-EXPEDITED DATA            |   |                        |     |
     |                request     | Y | NS User-Data.          |  Y  |
     |                indication  | Y |                        |     |
     |----------------------------|---|------------------------|-----|
     |N-RESET         request     | X | Originator,            |  Z  |
     |                indication  | X | Reason.                |  Z  |
     |                response    | X |                        |     |
     |                confirm     | X |                        |     |
     |----------------------------|---|------------------------|-----|
     |N-DISCONNECT    request     | X | NS User-Data.          |  Z  |
     |                indication  | X | Originator,            |  Z  |
     |                            |   | Reason.                |  Z  |
     +---------------------------------------------------------------+
                    Table 2. Network service primitives










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     Key:

        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.

     NOTES:

        1 - The parameters listed in  this  table  are  those  in  the
            current network service (first DP 8348).

        2 - The way the parameters are exchanged between the transport
            entity and the NS-provider is a local matter.




     5.3  Functions of the Transport Layer

     5.3.1  Overview of functions

     The functions in the  Transport  Layer  are  those  necessary  to
     bridge  the  gap  between the services available from the Network
     Layer and those to be offered to the TS-users.

     The functions in the  Transport  Layer  are  concerned  with  the
     enhancement  of  quality  of  service,  including aspects of cost
     optimization.

     These functions are grouped below into those used  at  all  times
     during a transport connection and those concerned with connection
     establishment, data transfer and release.

     NOTE - This International Standard does not include the following
     functions  which  are under consideration for inclusion in future
     editions of this standard:

        a)  encryption;



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        b)  accounting mechanisms;

        c)  status exchanges and monitoring of QOS;

        d)  blocking;

        e)  temporary release of network connections;

        f)  alternative checksum algorithm.




     5.3.1.1  Functions used at all times

     The following functions, depending upon the  selected  class  and
     options, are used at all times during a transport connection:

        a)  transmission of TPDUs (see 6.2 and 6.9);

        b)  multiplexing and demultiplexing  (see  6.15),  a  function
            used  to  share a single network connection between two or
            more transport connections;

        c)  error detection (see 6.10, 6.13 and 6.17), a function used
            to  detect  the loss, corruption, duplication, misordering
            or misdelivery of TPDUs;

        d)  error recovery (see 6.12, 6.14, 6.18, 6.19, 6.20, 6.21 and
            6.22),  a  function  used  to  recover  from  detected and
            signalled errors.




     5.3.1.2  Connection Establishment

     The  purpose  of  connection  establishment  is  to  establish  a
     transport   connection   between  two  TS-users.   The  following
     functions of the transport layer during this phase must match the
     TS-users'  requested quality of service with the services offered
     by the network layer:




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        a)  select network service which best matches the  requirement
            of  the  TS-user  taking  into account charges for various
            services (see 6.5);

        b)  decide whether to multiplex multiple transport connections
            onto a single network connection (see 6.5);

        c)  establish the optimum TPDU size (see 6.5);

        d)  select  the  functions  that  will  be  operational   upon
            entering the data transfer phase (see 6.5);

        e)  map transport addresses onto network addresses;

        f)  provide a  means  to  distinguish  between  two  different
            transport connections (see 6.5);

        g)  transport of TS-user data (see 6.5).




     5.3.1.3  Data Transfer

     The purpose of data transfer is to permit duplex transmission  of
     TSDUs  between  the  two  TS-users  connected  by  the  transport
     connection.   This  purpose  is  achieved  by  means  of  two-way
     simultaneous  communication  and by the following functions, some
     of which are used or not used in accordance with  the  result  of
     the selection performed in connection establishment:

        a)  concatenation and separation (see 6.4), a function used to
            collect  several  TPDUs  into a single NSDU at the sending
            transport  entity  and  to  separate  the  TPDUs  at   the
            receiving transport entity;

        b)  segmenting and reassembling (see 6.3), a function used  to
            segment  a  single  data  TSDU  into multiple TPDUs at the
            sending transport entity and to reassemble them into their
            original format at the receiving transport entity;






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        c)  splitting and recombining (see 6.23), a function  allowing
            the simultaneous use of two or more network connections to
            support the same transport connection;

        d)  flow control (see 6.16), a function used to  regulate  the
            flow  of  TPDUs  between  two  transport  entities  on one
            transport connection;

        e)  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;

        f)  expedited data (see 6.11), a function used to  bypass  the
            flow  control  of  normal  data TPDU.  Expedited data TPDU
            flow is controlled by separate flow control;

        g)  TSDU delimiting (see 6.3), a function  used  to  determine
            the beginning and ending of a TSDU.




     5.3.1.4  Release

     The  purpose  of  release  (see  6.7  and  6.8)  is  to   provide
     disconnection  of  the  transport  connection,  regardless of the
     current activity.




     5.4  Classes and options

     5.4.1  General

     The functions of the Transport Layer  have  been  organized  into
     classes and options.

     A class  defines  a  set  of  functions.   Options  define  those
     functions within a class which may or may not be used.

     This International Standard defines five classes of protocol:



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        a)  Class 0:  Simple Class;

        b)  Class 1:  Basic Error recovery Class;

        c)  Class 2:  Multiplexing Class;

        d)  Class 3:  Error Recovery and Multiplexing Class;

        e)  Class 4:  Error Detection and Recovery Class.

     NOTE - Transport connections  of  classes  2,  3  and  4  may  be
     multiplexed together onto the same network connection.




     5.4.2  Negotiation

     The use of classes and options is  negotiated  during  connection
     establishment.   The  choice  made by the transport entities will
     depend upon:

        a)  the TS-users' requirements expressed via T-CONNECT service
            primitives;

        b)  the quality of the available network services;

        c)  the user required service versus cost ratio acceptable  to
            the TS-user.




     5.4.3  Choice of network connection

     The following  list  classifies  network  services  in  terms  of
     quality  with  respect  to  error  behavior  in  relation to user
     requirements; its main purpose is to  provide  a  basis  for  the
     decision  regarding  which  class of transport protocol should be
     used in conjunction with given network connection:






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        a)  Type A.  Network connection with acceptable residual error
            rate  (for  example  not signalled by disconnect or reset)
            and acceptable rate of signalled errors.

        b)  Type B.   Network  connections  with  acceptable  residual
            error  rate  (for  example  not signalled by disconnect or
            reset) but unacceptable rate of signalled errors.

        c)  Type C.  Network connections  with  unacceptable  residual
            error rate.

     It is assumed that each transport entity is aware of the  quality
     of service provided by particular network connections.




     5.4.4  Characteristics of Class 0

     Class 0 provides the simplest type of transport connection and is
     fully  compatible  with the CCITT recommendation S.70 for teletex
     terminals.

     Class 0 has  been  designed  to  be  used  with  type  A  network
     connections.