rfc905.txt

RFC 的详细文档！










        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.




     5.4.5  Characteristics of Class 1

     Class 1  provides  a  basic  transport  connection  with  minimal
     overheads.

     The main  purpose  of  the  class  is  to  recover  from  network
     disconnect or reset.

     Selection of this class is usually based on reliability criteria.
     Class  1  has  been  designed  to  be  used  with  type B network
     connections.






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     5.4.6  Characteristics of Class 2

     5.4.6.1  General

     Class 2 provides a way to multiplex several transport connections
     onto  a  single network connection.  This class has been designed
     to be used with type A network connections.




     5.4.6.2  Use of explicit flow control

     The objective is to provide flow control to help avoid congestion
     at transport-connection-end-points and on the network connection.
     Typical use is when traffic is  heavy  and  continuous,  or  when
     there  is  intensive  multiplexing.   Use  of  flow  control  can
     optimize response times and resource utilization.




     5.4.6.3  Non-use of explicit flow control

     The objective is to provide a  basic  transport  connection  with
     minimal  overheads  suitable  when  explicit disconnection of the
     transport connection is desirable.  The option would typically be
     used for unsophisticated terminals, and when no multiplexing onto
     network  connections  is  required.   Expedited  data  is   never
     available.




     5.4.7  Characteristics of Class 3

     Class 3 provides the characteristics of Class 2 plus the  ability
     to  recover  from network disconnect or reset.  Selection of this
     class is usually based upon reliability criteria.   Class  3  has
     been designed to be used with type B network connections.






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     5.4.8  Characteristics of Class 4

     Class 4  provides  the  characteristics  of  Class  3,  plus  the
     capability  to  detect  and  recover from errors which occur as a
     result of the  low  grade  of  service  available  from  the  NS-
     provider.   The  kinds  of  errors  to be detected include:  TPDU
     loss, TPDU delivery out of sequence, TPDU  duplication  and  TPDU
     corruption.   These  errors  may  affect control TPDUs as well as
     data TPDUs.

     This class also provides for increased throughput capability  and
     additional  resilience  against network failure. Class 4 has been
     designed to be used with type C network connections.




     5.5  Model of the transport layer

     A transport entity communicates with its TS-users through one  or
     more  TSAPs  by means of the service primitives as defined by the
     transport service definition DP 8072.   Service  primitives  will
     cause  or be the result of transport protocol data unit exchanges
     between  the  peer  transport  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 8348 through one or more NSAPs.

     Transport connection endpoints are identified in end  systems  by
     an  internal, implementation dependent, mechanism so that the TS-
     user and  the  transport  entity  can  refer  to  each  transport
     connection.














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               +------+                        +------+
     ----------| TSAP |------------------------| TSAP |----------
               +------+                        +------+
                   |                               |
            +---------------+               +---------------+
            | Transport     |               | Transport     |
            |       entity  |               |       entity  |
            +---------------+               +---------------+
                   |                               |
                   |                               |
               +------+                        +------+
     ----------| NSAP |------------------------| NSAP |----------
               +------+                        +------+
                   |                               |
                   +-------------------------------+

                  Figure 2 . Model of the transport layer



     NOTE - For purpose of illustration, this figure  shows  only  one
     TSAP  and  one  NSAP  for  each  transport  entity.   In  certain
     instances, more than one TSAP and/or more than one  NSAP  may  be
     associated with a particular transport entity.


















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     SECTION TWO.  TRANSPORT PROTOCOL SPECIFICATION




     6  ELEMENTS OF PROCEDURE

     This clause contains elements of procedure which are used in  the
     specification  of  protocol  classes  in  clauses 7 to 12.  These
     elements are not meaningful on their own.

     The procedures define the transfer of TPDUs whose  structure  and
     coding  is  specified  in  clause  13.   Transport entities shall
     accept and respond to any TPDU received in a valid NSDU  and  may
     issue  TPDUs  initiating specific elements of procedure specified
     in this clause.

     NOTE - Where network service primitives and TPDUs and  parameters
     used  are  not significant for a particular element of procedure,
     they have not been included in the specification.




     6.1  Assignment to network connection

     6.1.1  Purpose

     The  procedure  is  used  in  all  classes  to  assign  transport
     connections to network connections.




     6.1.2  Network service primitives

     The  procedure  makes  use  of  the  following  network   service
     primitives:

        a)  N-CONNECT;

        b)  N-DISCONNECT.




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     6.1.3  Procedure

     Each  transport  connection  shall  be  assigned  to  a   network
     connection.  The initiator may assign the transport connection to
     an existing network connection of which it is the owner or  to  a
     new  network  connection  (see  Note 1) which it creates for this
     purpose.

     The  initiator  shall  not  assign  or  reassign  the   transport
     connection  to  an  existing  network  connection if the protocol
     class(es)  proposed  or  the  class  in  use  for  the  transport
     connection are incompatible with the current usage of the network
     connection with respect to multiplexing (see Note 2).

     During the resynchronization (see 6.14)  and  reassignment  after
     failure  (see 6.12) procedures, a transport entity may reassign a
     transport connection to another network  connection  joining  the
     same  NSAPs,  provided  that  it  is  the  owner  of  the network
     connection and that the transport connection is assigned to  only
     one network connection at any given time.

     During the splitting procedure (see 6.23), a transport entity may
     assign   a   transport   connection  to  any  additional  network
     connection joining the same NSAPs, provided that it is the  owner
     of  the  network  connection and that multiplexing is possible on
     the network connection.

     The responder becomes aware of the assignment when it receives

        a)  a CR TPDU during the  connection  establishment  procedure
            (see 6.5); or

        b)  an RJ TPDU or a retransmitted CR or  DR  TPDU  during  the
            resynchronization   (see   6.14)  and  reassignment  after
            failure (see 6.12) procedures; or

        c)  any TPDU when splitting (see 6.23) is used.









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     NOTES

        1.  When a new network connection is created, the  quality  of
            service  requested  is  a  local  matter, although it will
            normally be  related  to  the  requirements  of  transport
            connection(s) expected to be assigned to it.

        2.  An existing network connection may also  not  be  suitable
            if,  for example, the quality of service requested for the
            transport  connection  cannot  be  attained  by  using  or
            enhancing the network connection.

        3.  A  network  connection  with  no  transport  connection(s)
            assigned   to   it,   may   be   available  after  initial
            establishment, or because all of the transport connections
            previously  assigned  to  it  have  been  released.  It is
            recommended  that  only  the  owner  of  such  a   network
            connection   should   release   it.   Furthermore,  it  is
            recommended that it not be released immediately after  the
            transmission of the final TPDU of a transport connection -
            either a DR TPDU in response to CR TPDU or a  DC  TPDU  in
            response  to DR TPDU.  An appropriate delay will allow the
            TPDU  concerned  to  reach  the  other  transport   entity
            allowing  the freeing of any resources associated with the
            transport connection concerned.

        4.  After the  failure  of  a  network  connection,  transport
            connections which were previously multiplexed together may
            be assigned to different  network  connections,  and  vice
            versa.




     6.2  Transport protocol