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.

 substations  and  generating  plants)  and  from  other  control
 centers, performs a variety of monitoring and control functions,
 and transmits commands to the remote terminals and  coordinating
 information to other control centers."   During  the  course  of
 these operations, the following conditions occur:

      1) Some measurements  are  transmitted  or  requested  from
         remote terminals or control centers every  few  seconds.
         No attempt is necessarily made to recover data lost  due
         to transmission error; the application programs  include
         provisions for  proper  operation  when  input  data  is
         occassionally missing.  [Inward data collection]

      2) Some data items are transferred from  commonly  accessed
         remote sites or multi-utility pool coordination  centers
         on   a   request-response   basis.     [Request-response
         interaction]

      3) In some cases, an application program may  require  that
         some measurements be  made  simultaneously  in  a  large
         number of locations.  In these cases, the control center
         will  broadcast  a   command   to   make   th   affected
         measurements.  [Outward data dissemination]

 In closing, they note that "utility control centers  around  the
 world use data communications in ways similar to  those  in  the
 United States."


 Broadcast and multicast (group  addressed)  communication  using
 connection-oriented services is awkward at best  and  impossible
 at   worst,   notwithstanding   the   occassional   mention   of
 "multi-endpoint  connections"  in  the  Reference  Model.   Some
 characteristics  of  connection-based  data  transfer,  such  as
 sequencing and error recovery, are very difficult to provide  in
 a  broadcast/multicast  environment,  and  may   not   even   be
 desirable; and it is not at  all  easy  to  formulate  a  useful
 definition of broadcast/multicast acknowledgement  that  can  be
 supported by a low-level protocol.  Where group addressing is an
 important application consideration, connectionless data  trans-
 mission is usually the only choice.

 Certain special applications,  such  as  digitized  voice,  data

Connectionless Data Transmission, Rev. 1.00



 telemetry, and remote command  and  control,  involving  a  high
 level  of  data   redundancy   and/or   real-time   transmission
 requirements, may profit from the fact that CDT makes no  effort
 to detect or recover lost or corrupted data.  If the  time  span
 during which an individual datum  is  meaningful  is  relatively
 short, since it is quickly superceded by the next - or if, as in
 digitized voice transmission, the loss or corruption of  one  or
 even several data units is insignificant - the application might
 suffer far more from the delay that would  be  introduced  as  a
 connection-oriented service dealt with a lost or out-of-sequence
 data unit (even if retransmission or other  recovery  procedures
 were not invoked) than it would from the unreported  loss  of  a
 few data units in  the  course  of  a  connectionless  exchange.
 Other special considerations - such as the  undesirability,  for
 security reasons, of  maintaining  connection-state  information
 between data transfers in a military command and control  system
 - add force to the argument that CDT should be available  as  an
 alternative to connection-oriented data transfer.

 Local area networks (LANs) are probably the most fertile  ground
 for connectionless services, which find  useful  application  at
 several layers.  LANs  employ  intrinsically  reliable  physical
 transmission  media  and  techniques  (baseband  and   broadband
 coaxial  cable,  fiber  optics,  etc.)  in  a  restricted  range
 (generally no greater than 1 or 2 kilometers), and are typically
 able to achieve extremely low bit error rates.  In addition, the
 media-access contention  mechanisms  favored  by  LAN  designers
 handle transmission errors as a matter  of  course.   The  usual
 approach to physical interconnection ties all nodes together  on
 a common medium, creating an inherently broadcast environment in
 which every transmission  can  be  received  by  every  station.
 Taking advantage of these characteristics  virtually  demands  a
 connectionless data link service, and in fact most  current  and
 proposed LANs - the Xerox Ethernet[43], the  proposed  IEEE  802
 LAN standard[14,46], and many others - depend on such a service.
 As a bonus,  because  connectionless  services  are  simpler  to
 implement - requiring only two or  three  service  primitives  -
 inexpensive VLSI implementations are often possible.

 In addition, the applications for which LANs are often installed
 tend to be precisely those best handled by CDT.   Consider  this
 list of eight application classes identified  by  the  IEEE  802
 Interface Subcommittee as targets for the 802 LAN standard[46]:

 1.   Periodic   status   reporting   -   telemetry   data   from
 instrumentation, monitoring devices associated  with  static  or
 dynamic physical environments;

 2.  Special event reporting - fire alarms, overload or stressing
 conditions;


Connectionless Data Transmission, Rev. 1.00



 3.  Security control - security door opening and closing, system
 recovery or initialization, access control;

 4.  File transfer;

 5.  Interactive transactions - reservation  systems,  electronic
 messaging and conferencing;

 6.  Interactive information exchange -  communicating  text  and
 word processors, electronic mail, remote job entry;

 7. Office information exchange - store and forward of  digitized
 voice messages, digitized graphic/image handling;

 8.  Real-time stimulus and response  -  universal  product  code
 checkout readers, distributed  point  of  sale  cash  registers,
 military  command  and  control,  and  other   closed-loop   and
 real-time applications.


 Of these, almost all have already  been  identified  as  classic
 examples of applications that have an essentially connectionless
 nature.  Consider this more detailed example  of  (8):  a  local
 area network with a large number of nodes and a large number  of
 services  (e.g.,  file  management,  printing,   plotting,   job
 execution,  etc.)  provided  at  various  nodes.   In   such   a
 configuration, it is impractical to maintain  a  table  at  each
 node giving the address of every  service,  since  changing  the
 location of a single service would require updating the  address
 table at every node.  An alternative is  to  maintain  a  single
 independent "server lookup" service, which performs the function
 of mapping the name of a given  service  to  the  address  of  a
 server providing that service.   The  server-lookup  server  re-
 ceives requests such as, "where is service X?", and returns  the
 address at which an instance of service X is currently  located.
 Communication  with  the  server-lookup  server  is   inherently
 self-contained,  consisting   of   a   single   request/response
 exchange.  Only the highest-level acknowledgement - the response
 from the lookup service giving the requested address - is at all
 significant.  The native reliability of the local  area  network
 ensures a low error rate; if a message should be lost,  no  harm
 is done, since the request will simply be re-sent  if  a  timely
 response does not arrive.  Such an interaction is poorly  model-
 led by the connection-oriented paradigm of opening a connection,
 transferring a stream of data, and closing the  connection.   It
 is perfectly suited to connectionless transmission techniques.


 Network interconnection (internetworking) can be  facilitated  -
 especially when networks of different types are involved, as  is
 often the case - if the internetwork service is  connectionless;

Connectionless Data Transmission, Rev. 1.00



 and a number of related activities, such  as  gateway-to-gateway
 communication,  exhibit  the   request-response,   inward   data
 collection, and outward data dissemination characteristics  that
 are well supported by CDT.   One  of  the  best  examples  of  a
 connectionless internetwork service is described in  a  document
 published by the  National  Bureau  of  Standards  (Features  of
 Internetwork  Protocol[29],  which  includes  a  straightforward
 discussion of the merits of the connectionless approach:

         "The  greatest   advantage   of   connectionless
         service at the  internet  level  is  simplicity,
         particularly in  the  gateways.   Simplicity  is
         manifested in terms of smaller and less  compli-
         cated computer code and smaller computer storage
         requirements.  The gateways and  hosts  are  not
         required  to  maintain  state  information,  nor
         interpret call request and call clear  commands.
         Each     data-unit      can      be      treated
         independently...Connectionless service assumes a
         minim[al]   service    from    the    underlying
         subnetworks.   This  is  advantageous   if   the
         networks are diverse.  Existing internet  proto-
         cols which are intended for interconnection of a
         diverse variety  of  networks  are  based  on  a
         connectionless  service  [for  example  the  PUP
         Internetwork  protocol[44],  the  Department  of
         Defence Standard Internet Protocol[31], and  the
         Delta-t protocol developed at Lawrence Livermore
         Laboratory[45]]."

 The principle motivating the development of internetwork  servi-
 ces and protocols that make few assumptions about the nature  of
 the individual network services (the "lowest common denominator"
 approach) was formulated by Carl  Sunshine  as  the  "local  net
 independence principle"[39]: "Each local net  shall  retain  its
 individual address space, routing  algorithms,  packet  formats,
 protocols, traffic controls, fees, and other network  character-
 istics to the greatest extent  possible."   The  simplicity  and
 robustness of connectionless internetworking  systems  guarantee
 their widespread use as the number of different network types  -
 X.25 networks, LANs,  packet  radio  networks,  other  broadcast
 networks, and satellite networks - increases and  the  pressures
 to interconnect them grow.



 4  CDT and the OSI Reference Model


 As a concept, connectionless data transmission  complements  the
 concept of connection-oriented data transfer throughout the  OSI

Connectionless Data Transmission, Rev. 1.00



 architecture.  As a basis for deriving standard OSI services and
 protocols, however, it has a greater impact on  some  layers  of
 the Reference Model than on others.   Careful  analysis  of  the
 relative  merits  of  connectionless   and   connection-oriented
 operation at each layer is necessary to control  the  prolifera-
 tion of incompatible or useless options and preserve  a  balance
 between the power of the complementary concepts and the stabili-
 zing objective of the OSI standardization effort.

 Figure 5 illustrates the layered OSI hierarchy  as  it  is  most
 commonly represented (it shows two instances of  the  hierarchy,
 representing the relationship between  two  OSI  systems).   The
 following sections discuss the CDT concept  in  the  context  of
 each of the seven layers.


 4.1  Physical Layer


 The duality of connections and connectionless service is  diffi-
 cult  to  demonstrate  satisfactorily  at  the  physical  layer,
 largely because the concept of a physical "connection"  is  both
 intuitive and colloquial.  The physical layer is responsible for
 generating and interpreting signals represented for the  purpose
 of transmission  by  some  form  of  physical  encoding  (be  it
 electrical, optical, acoustic, etc.), and a physical connection,
 in the most general sense (and restricting our consideration, as
 does the Reference Model itself, to  telecommunications  media),
 is a signal pathway through a medium or a combination of  media.
 Is  a  packet   radio   broadcast   network,   then,   using   a
 "connectionless" physical service?  No explicit  signal  pathway
 through a  medium  or  media  is  established  before  data  are
 transmitted.  On the other hand, it can easily be argued that  a
 physical connection is established with the introduction of  two
 antennae into the "ether"; and if the antennae are aimed at each
 other and designed to handle microwave transmission, the impres-
 sion that a physical connection exists is strengthened.  Whether
 or not one recognizes the possibility of connectionless physical
 services - other than purely  whimsical  ones  -  will  probably
 continue to depend on one's point of  view,  and  will  have  no
 effect on the development of actual telecommunication systems.


 4.2  Data Link Layer


 Many data link technologies -  particularly  those  coming  into
 popular use with the growth of local area networking -  are  far
 easier  to  understand  and  work  with  when  the   traditional
 connection-oriented concepts  (embodied,  for  example,  in  the
 widely-used HDLC, SDLC, and ADCCP standards) are replaced by the












         ,---------------------,            ,---------------------,
         |                     |            |                     |
Level 7  |  Application Layer  |<---------->|  Application Layer  |
         |                     |            |                     |
         |----------|----------|            |----------|----------|
         |                     |            |                     |
Level 6  | Presentation Layer  |<---------->| Presentation Layer  |
         |                     |            |                     |
         |----------|----------|            |----------|----------|