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rfc:ien:ien155
                                                     INDRA
                                                     Working
                                                     Paper
   INDRA Note 967
   IEN 155
   12th August 1980
   The Yellow Book Transport Service: Principles and Status
                        C. J. Bennett
             ABSTRACT:  This  note  is  a  brief
             summary  of  the  principles of the
             Yellow Book Transport Service. This
             service  is likely to be adopted as
             the  UK  Standard   for   transport
             service,   and   is   being  widely
             promoted   amongst    international
             standardisation bodies.
               Department of Computer Science
                  University College London
   1. Introduction
     A recent INDRA Note  [1],  also  issued  as  an  IEN,
   defined an enhancement of TCP [2] to support the Yellow
   Book  transport  service  [3].  This  note   caused   a
   considerable  furore  in  certain sections of the DARPA
   Internet group, largely because there was  very  little
   awareness  of  what  the Yellow Book was, what its aims
   were, what standing it had, and what relevance  it  had
   to the DARPA Internet. The aim of this IEN is to remedy
   this situation, by providing a  brief  summary  of  the
   Yellow  Book,  and  relating  it to other international
   standards activity.
   2. Status of the Yellow Book
     The term 'Yellow Book' is the popular  name,  derived
   from  the colour of the cover of 'A Network Independent
   Transport  Service'.  This  is  one  of  a  series   of
   documents  defining  a  set  of  protocols and services
   being defined within Britain by Study  Group  Three  of
   the  UK Post Office PSS Users Forum and closely related
   bodies  such  as  the  Department  of  Industry's  Data
   Communications  Protocol  Unit.   The  intent  of these
   documents is to define interim  UK  national  standards
   prior  to  the  adoption  of international standards by
   bodies such as ISO and CCITT later in the 1980s, and by
   so   doing   to   influence   the  direction  of  these
   international standards.
     The other documents in the series are [4-6]: the Blue
   Book  (defining  a  Network  Independent  File Transfer
   Protocol),  the  Green  Book  (defining   a   Character
   Terminal service), and the Red Book (defining a Network
   Independent Job Transfer  and  Manipulation  Protocol).
   The  remaining colours - orange indigo and violet - are
   reserved.  Although the protocols are clearly  related,
   they  are,  with  the  exception  of  the  Green  Book,
   intended to  be  self  contained.   Thus  it  is  quite
   possible   to  implement  the  Blue  Book  FTP  without
   requiring the Yellow Book transport service,  and  such
   an  implementation  has  in  fact been available on the
   ARPANET above both NCP and TCP since mid 1979.
     The Yellow Book is in  principle  accepted  as  a  UK
   standard.  It  was  first  proposed  in  1979,  and the
   current definition, of February 1980, is still a  draft
   text.  However  it  is  expected  that  something  very
   similar to the current Yellow Book will be  adopted  by
   the  PSS  Users  Forum  later  in 1980, and a Transport
   Service    Implementors    group,     consisting     of
   representatives from UK universities, manufacturers and
   users, are currently bringing up implementations.

Bennett 1 INDRA 967

                       The Yellow Book
     On the international front, the Yellow Book has  been
   circulated  amongst  groups  concerned  with  transport
   protocol    standardisation    within    CCITT,     ISO
   (International  Standards  Organisation)  and ECMA (the
   European Computer Manufacturers' Association).   It  is
   not the only proposal - ECMA in particular are evolving
   one of their own [7] - and some of the concepts are not
   totally   in   accord   with   the   ideas   of   these
   organisations. For instance, the major stumbling  block
   to its adoption as the transport layer within the ISO 7
   layer architecture [8] is its  addressing  concept,  as
   the  ISO  model  assumes  all  addressing  and  routing
   problems have been resolved by this stage (in my view a
   weakness  of  the  ISO  model).  It is however the most
   completely specified proposal being  circulated  within
   these   forums,   and   should  heavily  influence  the
   international standards which are finally adopted.
   3. The Yellow Book
   3.1 Aim of Yellow Book
     The purpose of the Yellow  Book  is  defined  by  its
   title  -  it  aims  to  define a transport service, for
   point  to  point  synchronised   sequenced   fullduplex
   communication,   independently   of  the  structure  of
   underlying communication media. In particular  it  aims
   to   provide  endpoint  communication  across  multiple
   independent networks.
     The Yellow Book  concept  has  three  major  aspects.
   These  are:  the  notion of multiple-domain addressing,
   the notion of a network independent  service,  and  the
   notion of network dependent enhancements to support it.
   In addition the document defines optional  multiplexing
   and  simple  data structuring facilities which will not
   be discussed further here.
   3.2 Yellow Book Addressing
     The central picture of the internet  world  that  the
   Yellow  Book propounds is that there exists a number of
   mutually independent and noninteracting name spaces, or
   domains,  each  one of which has some system for naming
   all addressable objects in some fashion which is global
   to that naming domain [9]. The finer structure of these
   naming domains is not visible to the Yellow  Book,  and
   for  the purposes of discussion, a naming domain may be
   thought of as a component network.

Bennett 2 INDRA 967

                       The Yellow Book
     Two examples of such domains  are  the  international
   public  data  network  system,  whose  naming domain is
   defined by CCITT  recommendation  X121  [10],  and  the
   DARPA  Catenet  [11], which is a multinetwork structure
   with a superimposed global naming mechanism. Other  may
   be based on large commercial networks such as the SWIFT
   or SITA systems. One of the main points of  controversy
   centres  on  whether  this  picture  is  realistic,  or
   whether instead  some  single  system  (people  usually
   think  of  X121)  will  eventually be used for all data
   networks in the world.
     A gateway visible within the Yellow Book is a  system
   which sits between two or more such naming domains, and
   understands the naming systems in all domains to  which
   it  is  connected.   An  object  in  one  naming domain
   addresses an object in another domain by  specifying  a
   concatenation  of  addresses,  each  component being an
   address within some domain. This effectively defines  a
   route   to   the   destination   through  a  series  of
   intermediate gateways. Thus if the Catenet is connected
   to the international public system by some common host,
   the TCP process could specify the address of a  machine
   in  the  international  public  system by a name of the
   form:
   /<TCP address of YB gateway>/<X121 address of destination>/
   This address has two components, only one of  which  is
   in active use at any given point. The gateway specified
   within such a component can obtain the address  of  the
   source  within  that naming domain, and on this basis a
   reverse path can be built.  Thus the  TCP/X121  gateway
   will construct the component
                  /<TCP address of source>/
   to which the destination will prepend the X121  address
   of the gateway to complete the reverse path:
   /<X121 address of YB gateway>/<TCP address of source>/
   The destination may optionally attach a  local  process
   ID,  to  create what is known as a RECALL address, if a
   function such as a logger is required.
     Technically the most  controversial  aspect  of  this
   addressing  procedure is its use for transferring third
   party references.  Clearly, two parties A and B will in
   general  have  different routes to, and hence different
   addresses for, a third party C. If A needs to pass B  a

Bennett 3 INDRA 967

                       The Yellow Book
   reference to C, he could pass his address for C  and  B
   could  then  use the path A.C to reach C. However, this
   requires A to act as a gateway on the route to  C,  and
   the  route  so  obtained may cause data to pass through
   intermediate gateways twice. Hence a procedure has been
   defined  whereby a third party reference is declared to
   a gateway, which may then attempt to parse  it.  If  it
   can  it  modifies  the reference accordingly, so that a
   more optimal address is actually given to B.
     Although in principle this addressing procedure could
   be  used  with  every  internet  packet for an internet
   datagram system, this would be prohibitively  expensive
   in  terms of processing time. It is therefore envisaged
   that the internet path is a set of concatenated virtual
   calls.
   3.3 Yellow Book Service
     The  Yellow  Book  defines  a  number  of   primitive
   functions  which  the transport service supplies to the
   user at the transport service interface, and cause  the
   generation  of  transport service messages. The service
   definition   provides   the   Yellow   Book's   network
   independence,  as  it  is  possible  to  'enhance'  any
   particular network interface  to  reach  the  level  of
   service of the interface.
     The basic primitives are as follows:
    (i)    CONNECT: This  initiates  an  endpoint  virtual
           circuit.  The  traversal of the CONNECT request
           triggers the construction of the address of the
           reverse path in the fashion described above.
    (ii)   ACCEPT: This signals the successful setting  up
           of   the   source  to  destination  path.  Each
           component network may have its own  method  for
           setting   up   a   call   across  it,  but  the
           CONNECT/ACCEPT exchange verifies that the  path
           has been set up from end to end.
    (iii)  DATA: Data messages are sent from end to end in
           sequence  without  loss  or  duplication in the
           normal virtual circuit fashion.
    (iv)   EXPEDITED: Expedited data messages are priority
           messages  which  jump  ahead of the normal data
           sequence where possible.

Bennett 4 INDRA 967

                       The Yellow Book
    (v)    PUSH:  A  push  forces  the  transmission   and
           delivery   of   data   currently   buffered  at
           intermediate  gateways  or  at  the  source  or
           destination. It corresponds to a TCP EOL.
    (vi)   ADDRESS: This is  a  data  message  marked  for
           address  parsing,  so  that  it may be used for
           transferring   third   party   references    as
           described above.
    (vii)  RESET: This flushes messages currently buffered
           in the pipe, and must be answered by a RESET in
           the reverse direction. It does  not  cause  the
           connection to break.
    (viii) DISCONNECT: The  breaks  the  connection.  Like
           RESET,   it  causes  buffered  messages  to  be
           flushed, and must be answered with  a  matching
           DISCONNECT.
     With each primitive there are associated a number  of
   qualifying attributes. Details of these may be found in
   the Yellow Book.
     The relationships  and  permitted  exchanges  of  the
   various messages are defined in some detail in terms of
   state diagrams for the two  endpoints.  These  diagrams
   give  the  semantics  of  the endpoint protocol for the
   virtual connection.
   3.4 Network Enhancement
     The syntax of the Yellow Book messages,  as  distinct
   from  the  semantics  as defined above, is dependent on
   the underlying network  services  available  from  each
   component  network.  It  may be possible to use network
   services directly to achieve a  Yellow  Book  function;
   there  may be services which can be used in association
   with  Yellow  Book  messages;  there   may   be   local
   advantages to particular encodings.
     Thus to complete the definition of  the  Yellow  Book
   service  definition,  a  syntax  must  defined for each
   component network.  The  Yellow  Book  itself  contains
   annexes  defining  such  enhancements  for  X25 and X21
   networks. An enhancements for ring networks  using  the
   BSP  protocol  has been defined, and IEN 154 contains a
   proposed definition of a TCP enhancement which could be
   used  to  support the Yellow Book. In general, provided

Bennett 5 INDRA 967

                       The Yellow Book
   these enhancements support the Yellow  Book  semantics,
   their  specification  is  a matter local to the network
   concerned.  The gateway is expected to map the  formats
   of  the  messages  as  appropriate,  and to trigger any
   local supporting action which takes place.
     The network enhancements will normally be based on  a
   virtual  call  interface  such  as  TCP  or  X25.  This
   considerably  simplifies  their  definition.  In  cases
   where   such   an   interface  is  not  available,  the
   enhancement will be considerably more complex to ensure
   the  proper  sequencing  and  completeness  of the data
   transferred.
   3.5 Summary
     Compared to the DARPA Catenet an Internet based  upon
   the  Yellow Book is a much more loosely coupled system.
   There  is  no  global  address  space  imposed  on  the
   Internet, and it is assumed that it will be possible to
   make  use  of  local  network  facilities.  Thus  local
   network  independence  is  retained  to  a  far greater
   degree than the Catenet achieves. The agreement that is
   required   is   to  implement  a  gateway  between  two
   networks, but this is essentially a bilateral agreement
   between  the network administrations concerned and does
   not  require  any  global  agreement  as   to   gateway
   structure.
     There is of course  a  price  to  be  paid  for  this
   independence.  The system is essentially a virtual call
   system, and therefore has none of  the  advantages  (or
   disadvantages!) to be obtained from the dynamic routing
   and flexible  recovery  mechanisms  possible  within  a
   datagram   system   such  as  the  Catenet.  While  the
   addressing scheme is extremely  flexible  in  terms  of
   responsiveness  to  topology  changes  and  growth,  it
   throws the burden of knowledge of the internet topology
   onto  the  higher  level processes, and the third party
   problem points out  that  it  is  easy  to  use  it  to
   disadvantage.
     Although the Yellow Book proposals can be regarded as
   fairly  firm,  the  details of the service provided are
   still subject to debate. Clearly it has been influenced
   to  some  extent  by X25 - particularly as to the RESET
   and  DISCONNECT  functions  (the  CONNECT/ACCEPT  is  a
   consequence   of  the  addressing  scheme).  The  major
   technical debate centres on the ADDRESS primitives  and
   on  whether  multiplexing  should  be  embodied  as  an

Bennett 6 INDRA 967

                       The Yellow Book
   essential Yellow Book service.
     Yellow Book and TCP are both attempting to provide  a
   transport  service,  but  they  are  doing  so  in very
   different environments. The  difference  is  such  that
   there is no fundamental conflict in the two approaches.
   Within the Yellow Book environment the DARPA Catenet is
   a  single  subsystem.   The  TCP  enhancement needed to
   support a full  Yellow  Book  may  appear  to  be  just
   another  user  protocol  to TCP. Higher level protocols
   may have their  transport  service  interfaces  defined
   semantically  in  such a way that they can be installed
   above TCP, for  'local'  Catenet  operation,  or  above
   Yellow  Book,  for  more  general operation, with equal
   ease.
   4. References
   [1] - C. J. Bennett. Realization  of  the  Yellow  Book
         Transport  Service Above TCP. IEN 153, July 1980.
         (Revised and reissued as IEN 154 in August 1980)
   [2] - Information  Sciences  Institute:   "Transmission
         Control Protocol" IEN 129. January 1980.
   [3] - PSS User Forum  Study  Group  Three:  "A  Network
         Independent   Transport   Service"   SG3/CP(80)2.
         February 1980.
   [4] - High Level Protocol Group: "A Network Independent
         File  Transfer  Protocol"  HLP/CP(78)1.  December
         1977. (Revision expected October 1980)
   [5] - PSS User  Forum  Study  Group  Three:  "Character
         Terminal Protocols on PSS." BIG/CP(79)11. October
         1979 (Under revision).
   [6] - Data Communication Protocol Unit  Working  Group:
         "A   Network   Independent   Job   Transfer   and
         Manipulation  Protocol."  DCPU/JTMP(80)1.   April
         1980.
   [7] - ECMA: "Standard ECMA  Transport  Protocol:  Third
         Draft." ECMA/TC24/80/17. January 1980.
   [8] - ISO/TC97/SC16:  "Open  Systems  Interconnection".
         ISO/TC97/SC16/N227 September 1979.
   [9] - P. F.  Linington,  V.  Hathway:  "The  Addressing

Bennett 7 INDRA 967

                       The Yellow Book
         Requirements  of   a   Transport   Service."   in
         "Kommunikation     in    verteilten    Systemen",
         Springer-Verlag Press, Berlin, December 1979.
   [10] - CCITT: "International Numbering Plan for  Public
         Data Networks." Recommendation X121. Geneva 1978.
   [11] - Information  Sciences  Institute.  "Internetwork
         Datagram Protocol" IEN 128 January 1980.
   Copies of references 3-6 may be obtained from:
         Dr P. F. Linington
         Data Communication Protocols Unit
         8 Corn Exchange St
         Cambridge
         UNITED KINGDOM

Bennett 8 INDRA 967

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