rfc905.txt

RFC 相关的技术文档

     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 data unit (TPDU) transfer

     6.2.1  Purpose

     The TPDU transfer procedure is used  in  all  classes  to  convey
     transport  protocol  data  units  in  user data fields of network
     service primitives.





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     6.2.2  Network Service Primitives

     The procedure uses the following network service primitives:

        a)  N-DATA;

        b)  N-EXPEDITED DATA




     6.2.3  Procedure

     The  transport  protocol  data  units  (TPDUs)  defined  for  the
     protocol are listed in 4.2.

     When the network expedited variant has been selected for class 1,
     the transport entities shall transmit and receive ED and EA TPDUs
     as NS-user data parameters of N-EXPEDITED DATA primitives.

     In all other cases, transport entities shall transmit and receive
     TPDUs as NS-user data parameters of N-DATA primitives.

     When  a  TPDU  is  put  into  an  NS-user  data  parameter,   the
     significance  of the bits within an octet and the order of octets
     within a TPDU shall be as defined in 13.2.

     NOTE - TPDUs may be concatenated (see 6.4).




     6.3  Segmenting and reassembling

     6.3.1  Purpose

     The segmenting and reassembling procedure is used in all  classes
     to map TSDUs onto TPDUs.








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     6.3.2  TPDUs and parameter used

     The procedure makes use of the following TPDU and parameter:

        DT TPDUs;

           - End of TSDU.




     6.3.3  Procedure

     A transport entity shall map a TSDU on to an ordered sequence  of
     one  or more DT TPDUs.  This sequence shall not be interrupted by
     other DT TPDUs on the same transport connection.

     All DT TPDUs except the last DT TPDU in a sequence  greater  than
     one shall have a length of data greater than zero.

     NOTES

        1.  The EOT parameter of a DT TPDU indicates  whether  or  not
            there are subsequent DT TPDUs in the sequence.

        2.  There is no requirement that the DT TPDUs shall be of  the
            maximum length selected during connection establishment.




     6.4  Concatenation and separation

     6.4.1  Purpose

     The procedure for concatenation