W-Note-16                                                     BBN


                       Wideband Net Working Note #16
                       Internet Experiment Note #162

                           John A. Pershing Jr.

                       Bolt Beranek and Newman Inc.
                             50 Moulton Street
                          Cambridge, Mass. 02238
                              (617) 491-1850

                               October 1980

The online version of this note does not contain 2 figures.  Hardcopy
versions including the figures may be obtained from the author at the
above address, or by a request via computer mail to JPershing@BBNA.


     W-Note-16                                                     BBN


       This  note  proposes  a model for addressing and routing in the

     DARPA  Wideband  Satellite  Experiment.    The  purpose  of   the

     organization  described  herein  is  twofold:    (1)  to hide the

     physical structure of the Wideband Net from The Internet and  its

     gateways;  and  (2)  to unify the transport and routing functions

     performed within the Wideband Net.

       Certain terms, defined in a glossary at the end of  this  memo,

     are  used  with specific, somewhat non-standard meanings in order

     to avoid ambiguities.

     1.  Current Organization

       The Wideband Satellite Experiment involves the  development  of

     the   PSAT   satellite   network,  several  local  networks,  and

     connections to several existing networks  such  as  the  ARPANET.

     Currently,  no  plan  exists  for the organization of these parts

     into a unified communication medium.  So far, the Wideband Net is

     a collection of independent, sovereign networks,  each  with  its

     own  transport  protocol,  addressing, and routing schemes.  This

     sovereignty of the constituent networks is a feature which should

     be preserved as much as possible,  so  that  the  development  of

     local  and  satellite  network  technologies  can proceed without

     artificial constraints.


     W-Note-16                                                     BBN

       Figure  1  illustrates  a hypothetical organization sometime in

     the future, after the network has expanded somewhat.   Each  site

     has    one    PSAT,   and   perhaps   a   Voice   Funnel   and/or

     mini-concentrator; additionally, sites have  a  number  of  local

     networks  with  various interconnections.  Hosts may be connected

     directly to a PSAT, a Voice Funnel, or to a local  network;  some

     hosts may have connections to two or more nets, etc.

       A uniform plan for communication in the Wideband Net will avoid

     ad  hoc  schemes  involving  specialized interface machines which

     transform one net's local protocol and addressing  into  that  of

     another  net.    Using  specialized  gateways in combination with

     source routing will not take full advantage of the  topology  and

     dynamics  of the situation.  (This is particularly evident if the

     link indicated by the dotted line exists, since  only  the  Voice

     Funnel  at  the right side of the figure knows the status of both

     the dotted link and the  satellite  link,  and  only  that  Voice

     Funnel  is  able  to  choose  the appropriate link.)  Such ad hoc

     schemes are inflexible, inefficient in terms of  manpower  (since

     large  amounts  of special-purpose code must be implemented), and

     do not allow the Wideband Net to be readily integrated  into  the


       A  "simple" approach is to consider every component in Figure 1

     to be a member of The Internet,  assigning  an  Internet  Network

     Number  to  each of the constituent nets, and relying on internet


     W-Note-16                                                     BBN

            Figure 1: Current Organization of the Wideband Net

     gateways  for  routing.    However,  even this simple diagram has

     twelve nets, not counting the ARPANET  (the  Voice  Funnels  must

     also  behave as networks so that the directly attached hosts have

     well-defined internet addresses),  and  there  are  16  cross-net

     connections   requiring   Internet  gateways.    It  is  probably


     W-Note-16                                                     BBN

     unreasonable  for  us to consume so many network numbers from the

     address space of 255; furthermore, a  proliferation  of  Internet

     Networks  places  an  unreasonable  burden  on  the  gateways and

     networks of the Internet,  both  in  terms  of  table  space  and

     routing  update  traffic  (since  all  gateways  must  track  all


     2.  Proposed Structure

       This memo  proposes  that  the  networks  within  the  Wideband

     Satellite  Experiment collectively behave as one network from the

     point of view of the rest of  The  Internet,  as  illustrated  in

     Figure  2.  All components of the Wideband Net share one Internet

     network number.   Internally,  the  Wideband  Net  looks  like  a

     catenet; this structure was suggested by Vint Cerf in IEN-48 [1].

     The  various  subnets are interconnected by gateways; each subnet

     maintains its own autonomy, and hosts that are only  involved  in

     local communication can ignore the catenet (and Internet) aspects

     of the arrangement.

       We  will  adopt an internal addressing and routing scheme which

     is transparent to the Internet Protocol  (IP) [2],  so  that  all

     hosts  on  the  Wideband  Net  will  have  well-defined  Internet

     addresses; we adopt IP as the Wideband Net's  "catenet  transport

     protocol",  and  superimpose a fine-structure on the 24-bit local

     address part of the (32-bit) Internet  address  (see  Figure  3).


     W-Note-16                                                     BBN

            Figure 2: Proposed Organization of the Wideband Net

     The high-order 8 bits specify a subnet of the Wideband Net (e.g.,

     a  specific  LexNet), and the remaining 16 bits specify the local

     address on that net.


     W-Note-16                                                     BBN

     |      28.      | Subnet Number |    Reserved for Subnet Use    |
      <-------------> <--------------------------------------------->
         Internet                  Internet Local Address
      Network Number

          Figure 3: Internet Address, as used in the Wideband Net

       Since the model we are adopting is one that has been thoroughly

     explored  by  the  Internet  community, many of the problems have

     been solved and many issues have  already  been  resolved.    For

     example,  the  gateways  between  these subnets are much like the

     Internet Gateways.  These gateways will  pass  IP  packets  among

     subnets  of  the  Wideband  Net,  stripping  the  local transport

     protocol layer from incoming packets, making  routing  decisions,

     and  wrapping  outgoing  packets  in the protocol of the next net

     which the packet will have to traverse.

       This scheme  does  not  depend  on  the  participation  of  all

     attached  hosts  --  only  the gateways are critical, and, as the

     network becomes richly interconnected, individual gateways  cease

     to be critical to the proper operation of the net.

       Other catenet transport protocols could be chosen, or one could

     be devised for use within the Wideband Net.  However, hosts would

     still  need  to  implement  IP  in  order to communicate over The


     W-Note-16                                                     BBN

     Internet, so such a "local" catenet protocol would be essentially

     excess  baggage.   By adopting IP as the Wideband Net's protocol,

     there is also a good chance of being able to  use  existing  code

     with only minor modifications.

       The issue of what is an Internet network (with its own assigned

     Internet  network  number)  and  what  is  a  subnet is more of a

     managerial problem than a technical one.  The relevant  issue  is

     the  partitioning of The Internet, in terms of name space and the

     burden of the routing algorithm on the gateways, as  well  as  in

     terms  of managerial responsibility.  The proposed scheme for the

     Wideband Net is quite flexible in this regard.   If  one  of  the

     subnets  becomes  an  Internet  network,  then  the gateways will

     perform somewhat different routing with respect to that  network,

     and   the  other  hosts  of  the  Wideband  Net  will  be  mostly

     indifferent to the change (except that the addresses of the hosts

     on the "promoted" subnet will change).  Presumably, any  Wideband

     Net  gateways  which were connected to the net will be "promoted"

     to Internet gateways.

       While we believe that  this  proposed  organization  should  be

     adopted,  it  has several problems.  However, these problems also

     exist in The Internet and  are  treated  more  fully  in  various

     Internet Experiment Notes; they are only briefly mentioned here.

       The name space is becoming crowded; using 8 bits for the subnet

     number  and  16  bits  for  the local host address is perhaps the


     W-Note-16                                                     BBN

     wrong  partition.    However,  this  choice  seems  to  be a good

     tradeoff between the potential size of the Wideband Net (measured

     in subnets) and the addressing  requirements  of  any  individual

     subnet.  Eventually, the issue of name-space size will have to be

     addressed by the Internet community as a whole.

       Since  the  fine  structure of the Wideband Net is not known to

     The Internet, it is possible that  Internet  Gateways  will  make

     non-optimal  routing  decisions with respect to the Wideband Net.

     This is the penalty which must be paid for trying to minimize the

     number of Internet Networks.  If the Internet Gateways which  are

     connected  to  the  Wideband  Net  are also Wideband Net gateways

     (that is, they participate in Wideband Net  routing  as  well  as

     Internet routing), then they may be able to fine-tune the routing

     of   Internet  packets  through  the  use  of  advisory  messages

     exchanged with the other Internet Gateways.

     3.  Transport Protocol Layers

       For purposes of discussion, we define  the  notion  of  catenet

     adjacency.    Two  hosts on some catenet are adjacent if they are

     connected to the same constituent network.  Conversely, two hosts

     are non-adjacent if they are connected to  different  constituent

     networks  of  some  catenet.   Note that hosts with interfaces on

     more than one network (such as a gateway) may  be  both  adjacent

     and  non-adjacent  to  a  given  host;  in  fact,  such a host is

     non-adjacent to itself by this definition.


     W-Note-16                                                     BBN

       The  transport,  addressing,  and  routing schemes suggested in

     this memo are intended to be used by non-adjacent  hosts  on  the

     Wideband  Net.    Of course this does not prohibit adjacent hosts

     from communicating with these protocols; however, such hosts have

     the option of communicating using protocols which  are  local  to

     the  network  to  which  they  are  attached (and, for efficiency

     reasons, such hosts will probably exercise this option).

       Datagrams which are to be transmitted through the Wideband  Net

     are  wrapped  in  a layer of catenet protocol, which is common to

     the entire Wideband Net, followed by a (possibly null)  layer  of

     protocol  which  is  dictated  by the particular network that the

     datagram  is  traversing.    The  catenet  protocol   header   is

     considered  to  be  part  of the datagram, and is preserved (with

     only  minor  changes)  as  the  datagram  traverses  the  various

     constituent  networks.   The layer of local protocol is volatile,

     and will be discarded as soon as the datagram exits  the  network

     defining that particular local protocol.

       For  compatibility with The Internet, the catenet protocol used

     is the DoD Standard Internet Protocol  (IP) [2].    This  permits

     hosts  of  the  Wideband  Net  to communicate with hosts on other

     Internet Networks without  resorting  to  yet  another  layer  of

     protocol.    Initially,  no internet gateways will be provided on

     the Wideband Net, so that  a  restricted  subset  of  IP  may  be

     implemented  by  the  various  hosts.    The  restrictions of the

     initial implementation follow:


     W-Note-16                                                     BBN

       o  Fragmentation  and  Reassembly:  Not implemented.  Total
          length may not exceed 576 octets; we  believe  that  all
          participating hosts, gateways, and networks will support
          Internet datagrams of this length without fragmentation.
          The  "Flags"  field will always be set to '010' (binary)
          to inhibit fragmentation; the  "Fragment  Offset"  field
          will always be zero.

       o  Source/Destination  Addresses:    The  Internet  Network
          Number (first 8 bits) will  always  be  '28'  (decimal).
          The Source/Destination Local Address (remaining 24 bits)
          is further interpreted as being 8 bits of network number
          and 16 bits of local address on that network.

       o  Options:    Strictly  optional.    Any options which are
          present must be accounted for  by  the  Internet  Header
          Length  (IHL)  and  Header  Checksum fields; however, no
          host is required to interpret any options.

       The local protocol is dependent on the particular network(s) to

     which a host is attached --  there  is  potentially  a  different

     local  protocol  for  every  constituent network.  In addition, a

     single network may  have  more  than  one  link-level  protocols,

     depending  on  the  particular  type of port to which the host is

     attached.  Issues of local protocols are of  no  concern  to  the

     purposes  of  this  memo;  the  implementor  is  referred  to the

     (possibly nonexistent) documentation on the local protocol of the

     network(s) to which a particular host is to be interfaced.

     4.  Addressing

       All hosts on the Wideband Net have at least one unique Internet

     address.  Since the 8-bit  Internet  Network  Number  is  already

     specified  by  the Internet protocols (the Wideband Net is number

     28 decimal), this leaves a 24-bit address space  for  use  within

     the Wideband Net.


     W-Note-16                                                     BBN

       Since  the Wideband Net is organized as a catenet, the "network

     number / local host number" strategy employed by The Internet  is

     also   employed   within   the  Wideband  Net.    Each  distinct,

     constituent network is assigned a unique, 8-bit  network  number;

     and  each host is assigned a unique, 16-bit local address on each

     network  to  which  it  is  attached.    These  two  numbers  are

     concatenated to produce the 24-bit Wideband Net Address.

     5.  Routing and Gateways

       Routing  in  the Wideband Net is implemented in the same manner

     as routing in The Internet [4].   The  constituent  networks  are

     connected  by  gateways;  these  gateways  implement  the routing

     function.  One of the primary functions of the Voice  Funnels  is

     to  implement  this gateway function for each network to which it

     is attached.  If a network is to be attached to the Wideband  Net

     at   some   place   besides  a  Voice  Funnel,  then  it  is  the

     responsibility of that network to provide a gateway.

       The IP server in each host needs  to  know  very  little  about

     routing  in  order to function properly.  It is assumed that each

     host knows its own address (that is, its network number  and  its

     address on that network) and the address of at least one adjacent

     gateway.    Also,  the  IP server must be able to produce a local

     network protocol header from a 24-bit Wideband Net address  which

     specifies this local network.


     W-Note-16                                                     BBN

       When  a  host's IP server is called on to deliver a datagram to

     an adjacent host it wraps the datagram first  in  an  IP  header,

     then in a local header which is addressed to that host, and sends

     the  result  directly  to  its destination.  When an IP server is

     called on to  deliver  to  a  non-adjacent  host,  it  wraps  the

     datagram  first  in an IP header, then in a local header which is

     addressed to an adjacent gateway and delivers it.  That  gateway,

     in  cooperation with the other gateways of the Wideband Net, will

     deliver the datagram to its destination.

       In addition to transmitting outgoing datagrams, IP servers will

     receive datagrams from their network(s).  These should be  routed

     internally  as  appropriate;  this  is  probably dependent on the

     "Protocol" field of the IP header (but note that  a  protocol  of

     '3'  indicates an advisory message from a gateway directed at the

     IP server itself).

       If an IP server is to function optimally, it must keep track of

     ALL adjacent gateways  which  are  up.    Additionally,  it  must

     maintain  a  cache  of  those  non-adjacent hosts to which it has

     recently sent datagrams, along with  the  gateway  through  which

     these  datagrams  were  forwarded.    When  it is about to send a

     datagram, the IP server first checks the cache.  If the addressee

     is found in the cache and if the gateway named in  the  cache  is

     still  up,  then  the datagram is forwarded to that same gateway.

     Otherwise, a gateway  is  chosen  arbitrarily  through  which  to

     forward the datagram.


     W-Note-16                                                     BBN

       Occasionally, a routing advisory message will be sent to the IP

     server  by  a  gateway.   Currently, one of two advisories may be

     received:  "destination unreachable", meaning  that  it  will  be

     futile  to  send  more datagrams to some host for a while (e.g. a

     couple of minutes); and "redirect", meaning  that  a  non-optimal

     gateway  was used, and that further datagrams should be forwarded

     through the gateway specified in the redirect message.   When  an

     IP  server  receives  a  routing  advisory,  it should update its

     cache, and perhaps notify one or more processes as appropriate.

     6.  Unresolved Issues

       Since the currently planned constituents of  the  Wideband  Net

     (the  PSAT  Net,  LexNets,  and Voice Funnels) are all capable of

     supporting broadcasting, it might be worthwhile to consider  some

     form of broadcasting as a basic Wideband Net service.  This would

     probably take two forms:  group addressing, essentially extending

     the  group  concept  of  the  PSAT Net to allow broadcasting to a

     designated group of  hosts  of  the  Wideband  Net;  and  general

     broadcasting,  directed at all hosts on a particular subnet or at

     all hosts of the Wideband Net.

       The ST protocol  should  also  be  supported  directly  by  the

     Wideband  Net's  gateways.    ST  will  allow the capacity of the

     various subnets to be more fully utilized.   It  should  also  be

     made  to  take  advantage  of the broadcast nature of the various



     W-Note-16                                                     BBN

     7.  Glossary

       In  order  to  avert the ambiguities inherent in this two-level

     structure, this memo uses a two-level  nomenclature,  defined  as


     network:        A  physical  communication  service  in which all
                     attached hosts communicate with all  other  hosts
                     using  a  uniform, local set of link, addressing,
                     and transport protocols; i.e. the "usual" meaning
                     of the  word.    To  quote  Cerf [1],  "the  term
                     'local'  is  used in a loose sense here, since it
                     means 'peculiar to the particular network' rather
                     than 'a network of limited geographical  extent'.
                     A  satellite-based  network,  such  as  the  ARPA
                     packet satellite network, therefore  has  'local'
                     characteristics  (e.g.  broadcast operation) even
                     though it spans many thousands  of  square  miles
                     geographically  speaking."  A network must appear
                     to be homogeneous from the "outside looking  in";
                     however,  this  does  not necessarily preclude an
                     internal structure.

     Internet Network:
                     A communicating system of hosts  and/or  networks
                     belonging  to  The  Internet,  and  which  can be
                     uniquely identified by an 8-bit "network  number"
                     assigned  by  the  number  czar [3].    This  may
                     correspond to part of a network, one network,  or
                     a  concatenation of many networks.  Note that the
                     Wideband Net is an Internet Network.

     catenet:        A collection of two or more networks, arbitrarily
                     interconnected  by   gateways,   in   which   the
                     communicating  hosts  have  agreed,  a-priori, on
                     some canonical "catenet protocol" which  is  used
                     for datagram transport.

     The Internet:   The  collection  of Internet Networks, along with
                     their Internet Gateways.  Hosts on  The  Internet
                     communicate  using  version 4 of the DoD Standard
                     Internet Protocol [2].

     gateway:        A logical host which is connected to two or  more
                     networks,   and   which   can   forward  "catenet
                     protocol" datagrams arriving from  any  of  these


     W-Note-16                                                     BBN

                     networks  to  the  appropriate  outgoing network.
                     Many adjectives may be applied  to  "gateway"  as
                     needed.    A hidden gateway is one whose presence
                     is generally unknown by the hosts attached to the
                     networks adjacent to the gateway.  A  non-routing
                     (or  static-routing)  gateway  makes  its routing
                     decisions  based  on  a-priori   information;   a
                     routing  gateway exchanges information with other
                     routing gateways in order  to  be  able  to  make
                     dynamic adjustments to its routing information as
                     the conditions of the catenet change.

     Internet Gateway:
                     A  gateway  connected  to  two  or  more Internet
                     Networks, which can forward internet datagrams.


     W-Note-16                                                     BBN


     [1]   Cerf.
           The Catenet Model for Internetworking.
           Internet Experiment Note 48, July, 1978.

     [2]   Postel, ed.
           The DoD Standard Internet Protocol.
           Internet Experiment Note 128, January, 1980.

     [3]   Postel, ed.
           Assigned Numbers.
           Request for Comments 770, September, 1980.

     [4]   Strasisar.
           How to Build a Gateway.
           Internet Experiment Note 109, August, 1979.