rfc787.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

         |                     |            |                     |
Level 5  |    Session Layer    |<---------->|     Session Layer   |
         |                     |            |                     |
         |----------|----------|            |----------|----------|
         |                     |            |                     |
Level 4  |   Transport Layer   |<---------->|   Transport Layer   |
         |                     |            |                     |
         |----------|----------|            |----------|----------|
         |                     |            |                     |
Level 3  |    Network Layer    |<---------->|    Network Layer    |
         |                     |            |                     |
         |----------|----------|            |----------|----------|
         |                     |            |                     |
Level 2  |   Data Link Layer   |<---------->|   Data Link Layer   |
         |                     |            |                     |
         |----------|----------|            |----------|----------|
         |                     |            |                     |
Level 1  |    Physical Layer   |<---------->|    Physical Layer   |
         |                     |            |                     |
         '---------------------'            '---------------------'





     FIGURE 5 - Layered Hierarchy of Open Systems Interconnection

Connectionless Data Transmission, Rev. 1.00



 concept  of  connectionless  data  transmission.   The  previous
 discussion of local area networking has already made  the  point
 that the high-speed, short-range, intrinsically reliable  broad-
 cast transmission media used to interconnect stations  in  local
 area networks are complemented  both  functionally  and  concep-
 tually by connectionless data link techniques.

 One of the  organizations  currently  developing  a  local  area
 network data link layer standard  -  the  Data  Link  and  Media
 Access (DLMAC) subcommittee of IEEE 802 -  has  recognized  both
 the need to retain compatibility with existing long-haul techni-
 ques and the unique advantages of CDT for local area networks by
 proposing that two data link procedures be defined for the  IEEE
 802 standard.

 In one procedure, information frames are unnumbered and  may  be
 sent at any time by any station  without  first  establishing  a
 connection.  The intended receiver  may  accept  the  frame  and
 interpret it, but is under no  obligation  to  do  so,  and  may
 instead discard the frame with no notice to the sender.  Neither
 is the sender notified if  no  station  recognizes  the  address
 coded  into  the  frame,  and  there  is  no   receiver.    This
 "connectionless" procedure, of course,  assumes  the  "friendly"
 environment and higher-layer acceptance of  responsibility  that
 are   usually   characteristic    of    local    area    network
 implementations.

 The other procedure provides all of  the  sequencing,  recovery,
 and    other     guarantees     normally     associated     with
 connection-oriented link procedures.  It is in fact very similar
 to the ISO standard HDLC balanced asynchronous mode procedure.

 Data  link  procedures  designed  for  transmission  media  that
 (unlike those used in local area networks)  suffer  unacceptable
 error rates are almost universally connection-based, since it is
 generally  more  efficient   to   recover   the   point-to-point
 bit-stream errors detectable by  connection-oriented  data  link
 procedures at the data link layer (with its comparatively  short
 timeout intervals) than at a higher layer.


 4.3  Network Layer


 Connectionless network service is useful for many  of  the  same
 reasons that were  identified  in  the  previous  discussion  of
 network interconnection: it greatly simplifies  the  design  and
 implementation of systems; makes few assumptions about  underly-
 ing services; and is more efficient than  a  connection-oriented
 service when higher layers  perform  whatever  sequencing,  flow
 control, and error recovery is required by user applications (in

Connectionless Data Transmission, Rev. 1.00



 fact, internetwork services are provided by the Network  Layer).
 CDT  also   facilitates   dynamic   routing   in   packet-   and
 message-switched networks,  since  each  data  unit  (packet  or
 message) can be directed along the most appropriate  "next  hop"
 unencumbered   by   connection-mandated   node   configurations.
 Examples of more or less connectionless  network  layer  designs
 and implementations abound: Zilog's  Z-net  (which  offers  both
 "reliable"   and   "unreliable"   service   options);   DECNET's
 "transport layer" (which corresponds to the OSI Network  layer);
 Livermore Lab's Delta-t protocol (although it  provides  only  a
 reliable   service,   performing   error   checking,   duplicate
 detection, and acknowledgement); the User Datagram protocol[48];
 and the  Cyclades  network  protocol[38].   In  fact,  even  the
 staunchly  connection-oriented   X.25   public   data   networks
 (Canada's Datapac is the  best  example)  generally  emply  what
 amounts to  a  connectionless  network-layer  service  in  their
 internal packet switches, which enables them to perform flexible
 dynamic routing on a packet-by-packet basis.


 4.4  Transport Layer


 The connectionless transport service is important  primarily  in
 systems that distinguish  the  Transport  layer  and  everything
 below it as providing something generically named the "Transport
 Service", and abandon or severely compromise  adherence  to  the
 OSI architecture above the Transport layer.  In such  systems  a
 connectionless transport service may  be  needed  for  the  same
 reasons that other (more OSI-respecting) systems need a  connec-
 tionless application service.  Otherwise, the purpose of  defin-
 ing a connectionless transport service is to enable a  uniformly
 connectionless service to  be  passed  efficiently  through  the
 Transport layer to higher layers.


 4.5  Session Layer


 The whole notion of a session which binds  presentation-entities
 into a relationship of  some  temporal  duration  is  inherently
 connection-oriented.  The purpose of defining  a  connectionless
 session service, therefore, is to enable a uniformly connection-
 less service to be passed efficiently through the session  layer
 to higher layers.  In this  sense,  the  connectionless  session
 service stands in precisely the same relationship to the connec-
 tionless transport service as a session-connection stands  to  a
 transport-connection.

Connectionless Data Transmission, Rev. 1.00



 4.6  Presentation Layer


 Very much the same  considerations  apply  to  the  Presentation
 layer as apply to the Session layer.


 4.7  Application Layer


 The most obvious reason to define a  connectionless  application
 service - to give  user  application  processes  access  to  the
 connectionless services of the architecture - is  not  the  only
 one.  The application layer performs functions  that  help  user
 application processes to converse regarding the meaning  of  the
 information they exchange, and is also responsible  for  dealing
 with the overall system management aspects of the OSI operation.
 Over  and  above  the  many  user-application  requirements  for
 connectionless service, it may be profitably employed by  system
 management functions that monitor and report on  the  status  of
 resources in the local open system; by application layer manage-
 ment functions that need to interact in a request-response  mode
 with similar functions in  other  systems  to  perform  security
 access control; and by user application process  functions  that
 monitor the status of activities in progress.


 The potential availability of two complementary services at each
 layer of the architecture raises an obvious question  -  how  to
 choose between them?  It should be  clear  at  this  point  that
 unilateral exclusion of  one  or  the  other,  although  it  may
 simplify the situation for some applications, is not  a  general
 solution to the problem.  There are actually two  parts  to  the
 question: how  to  select  an  appropriate  set  of  cooperative
 services for all seven layers during the design of a  particular
 open system; and, if one or more layers of the system will offer
 both connection-oriented and  connectionless  services,  how  to
 provide for the dynamic selection of one or the other in a given
 circumstance.

 The second part is easiest to dispose of, since actual systems -
 as opposed to the more abstract set of  services  and  protocols
 collected under the banner of  OSI  -  will  generally  be  con-
 structed in such a way as  to  combine  services  cooperatively,
 with some attention paid to the way in which they will  interact
 to meet specific goals.  Although two services may  be  provided
 at a given layer, logical combinations of services for different
 applications will generally be assembled according to relatively
 simple rules established during the design of the system.

 Evaluating the requirements of the applications  a  system  must

Connectionless Data Transmission, Rev. 1.00



 support and the characteristics of the preferred  implementation
 technologies will also answer  the  first  question.   A  system
 designed primarily to transport large  files  over  a  long-haul
 network would probably use  only  connection-oriented  services.
 One designed to collect data from widely scattered  sensors  for
 processing at a central  site  might  provide  a  connectionless
 application  service  but  use  a  connection-oriented   network
 service to achieve compatibility with  a  public  data  network.
 Another system, built around a local area network bus  or  ring,
 might use a connectionless data link service regardless  of  the
 applications   supported;   if   several   LANs   sere   to   be
 interconnected, perhaps with other network types, it might  also
 employ a connectionless internetwork service.

 The definition of OSI standard services and protocols,  however,
 must consider the general case, so as to accomodate a wide range
 of  actual-system  configurations.   The  motivating   principle
 should be to achieve a balance between the two  goals  of  power
 and simplicity.  The service  definition  for  each  layer  must
 include both connection-oriented  and  connectionless  services;
 otherwise, the utility of  a  service  at  one  layer  could  be
 negated by the unavailability of a corresponding  service  else-
 where in the  hierarchy.   However,  the  role  played  by  each
 service may be radically different from one layer to  the  next.
 The Presentation, Session, and Transport layers,  for  instance,
 need to support their respective  connectionless  services  only
 because the Application layer, which must provide a  connection-
 less service to user applications, cannot do so  effectively  if
 they do not.  Recognizing these role  variations  opens  up  the
 possibility of restoring a measure of the simplicity lost in the
 introduction of choice  at  each  layer  by  limiting,  not  the
 choices, but the places in the hierarchy where  conversion  from
 one choice to the other - connection to connectionless, or  vice
 versa - is allowed (see figure 6).  At this stage in the  devel-
 opment of the CDT concept, it appears that there are  exscellent
 reasons for allowing such a conversion  to  take  place  in  the
 Application, Transport, and Network layers (and in the Data Link
 layer, if some physical interconnection strategies are deemed to
 be connectionless).  In the other layers, the provision  of  one
 kind of service to the next-higher layer must always  be  accom-
 plished by using the same kind of service  from  the  next-lower
 layer (see figure 7).  (This principle of  like-to-like  mapping
 is not related to  multiplexing;  it  refers  to  service  types
 (connection-oriented  and   connectionless),   not   to   actual
 services.) Adopting such a restriction would contribute  to  the
 achievement of the balance mentioned  above,  without  excluding
 those combinations of  services  that  have  demonstrated  their
 usefulness.




                ^                              ^   (N+1)-LAYER
                |                              |
                |                              |
----------------o------------------------------o----------------
                |                              |
   ,-------------------------,    ,-------------------------,
   | Offers a connectionless |    |   Offers a connection-  |
   |       (N)-service       |    |   oriented (N)-service  |
   |            |            |    |            |            |
   |        (N)-LAYER        | OR |        (N)-LAYER        |
   |            |            |    |            |            |
   |   Uses a connection-    |    |  Uses a connectionless  |
   | oriented (N-1)-service  |    |      (N-1)-service      |
   '-------------------------'    '-------------------------'
                |                              |
----------------o------------------------------o----------------
                |                              |
                |                              |
                v                              v   (N-1)-LAYER


               FIGURE 6 - Service Type Conversion







                ^                              ^   (N+1)-LAYER
                |                              |
                |                              |
----------------o------------------------------o----------------
                |                              |
   ,-------------------------,    ,-------------------------,
   | Offers a connectionless |    |   Offers a connection-  |