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.

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                         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.

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                         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

     /<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

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                         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

     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.

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                         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

       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

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                         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

     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

       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

       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

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                         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

     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

     [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

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                         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

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