March 11th, 1981
                                                             Danny Cohen
                                                               USC / ISI

                         Addressing and Routing

There  are  several  approaches  to  addressing such as hierarchical and
flat, but there is only one approach to routing:  Take the best (usually
the shortest) path.

The choice of addressing scheme which is both manageable and may support
an efficient routing is the subject of this note.

Hierarchical addressing is probably the easiest to manage.  The universe
is divided into some small  number  of  units,  and  each  of  which  is
assigned  an  address  which is some kind of a string, typically numeric
but not necessarily so.

If any of these units contains  more  than  one  member  then  the  same
process  is repeated (iterated).  The address of the subunit is appended
somehow, typically to the end  (but  sometimes  to  the  front)  of  the
address of the parent unit.

This  hierarchy  is  easy  to  manage  such  that addresses are uniquely

Typically in an hierarchy each unit is  capable  of  communicating  both
with its ancestor and with all of its immediate descendants.

A  default  routing  can  always  be easily derived from an hierarchical
address.  This route consists of going up to a common ancestor  (parent)
and then down to the destination.

Hence,  an  hierarchical  address is always a route from the root (sorry
about that).

This  default routing requires only N-1 communication links which is the
minimal connectivity for N nodes.

In this situation the default route is also the only existing route,  up
to trivial changes.

In  reality,  for  efficiency  reasons,  the connectivity is richer than
that.  Typically, brothers are in  direct  communication,  and  a  "top"
(root) may not exist at all.

IEN-179                                                         [Page 2]

Because  of the connectivity richness the routing process is non-trivial
and requires that decisions be made.

Routing is the process of chosing a possible communication  link,  where
there is more than one.

The  other  extreme  of  addressing  is  the  flat  space approach where
locations have totally random dependency  on  their  addresses,  meaning
that  a  fully instantiated table lookup must be used in order to locate
an address.

The important feature of flat and  random  addressing  is  that  it  can
conveniently support mobile users.

A  simple,  however  expensive,  way  for implementing routing when flat
addressing is used is by  providing  all  the  switches  (where  routing
decisions  have  to  be  made)  with  tables containing all the required
information about all the addresses.

In the case of flat addressing it is  required  that  all  addresses  be
globally unique.  It is not required however that all addresses be known
to  all  the  switching nodes, provided that nodes can ask for, and get,
information about addresses which were not known to them.

In the hierarchical case it is possible either  that  all  addresses  be
specified  in  an  absolute  (complete)  mode or in a relative (partial)
mode.  The former requires that all addresses  be  fully  expanded,  and
have  the  same  format  regardless  of  where they are specified (e.g.,
1-213-822-1511x104), and the  latter  supports  short  forms  for  local
addresses, at the possible expense of additional control information for
remote addresses.  The same address shown before may be specified as 104
from  inside  ISI  as 822-1511x104 from the STN at LA, as 9-822-1511x104
from   UCLA,   as   213-822-1511x104    from    Palo    Alto    or    as
010-1-213-822-1511x104  from  London.   Similarly, intra-office memos do
not require long addresses, and intra-country mail does not require  the
country name to be specified in the address.

Any  system which can handle perfectly flat addresses can take advantage
of rich communication connectivity  and  can  also  handle  multi-homing
(destination with more than one address).

An   hierarchical   addressing   scheme  which  uses  only  the  default
hierarchical  routing may have difficulties in handling efficiently rich
connectivity and multi-homing.

However, between these two extremes there is a wide  spectrum  of  other

The  claim  of the rest of this paper is that (i) hierarchical addresses
are easier to manage, and (ii) if you can handle flat  addresses  -  you
can handle hierarchical addresses even better.

IEN-179                                                         [Page 3]

Hierarchical addressing is at least as good (in any respect) as flat one
because  flat  addressing  is a special case of hierarchical addressing,
but not the other way round.

Suppose that  there  is  a  system  which  can  handle  very  well  flat
addressing.   Since the only requirement for the flat addressing is that
addresses are unique, one can assign unique addresses in  any  way,  for
example  in  a very structured way, like hierarchical.  The introduction
of hierarchical addresses to  the  flat  addressing  scheme  should  not
degrade its performance since the uniqueness was not disturbed.

Suppose  further  that  all  the addresses are examined at every switch.
All addresses which result in the same routing  decisions  by  both  the
flat  addressing  scheme  and by the hierarchical addressing are marked,
and the  entries  corresponding  to  them  are  removed  from  the  flat
addressing tables.

This  causes  no  degradation  of  the flat addressing scheme (which was
assumed to  be  a  good  one  to  start  with)  since  the  hierarchical
addressing  yields  the same routing decision for these addresses.  This
process repeats at all levels, and only the "GCD" addresses are stored.

Hence, after removing these addresses the size of the  tables  decreases
in  all  switches  which probably may yield some performance improvement
and increased capacities.  In fact,  the  resulting  scheme  is  now  an
hierarchical  scheme  with tables of exceptions which are handled in the
best known way.  The format of the exception table is discussed later.

Note that this results in uneven distribution of knowledge. At different
switches different set of destinations are known.  For example, all  the
traffic  to the East coast may be treated in the same way when being far
away from there, but as the message approaches its  destination,  it  is
likely that the granularity of the address examination changes such that
more  details are used.  When leaving Tokyo the only part of the address
which was used for  the  routing  was  the  fact  that  the  message  is
addressed  to  the  States.  After  arriving  at  California, the Boston
traffic may be treated separately from the Washington traffic, later the
Boston traffic is examined even closer to distinguish the Cambridge from
the Lexington traffic, and later the MIT traffic is  isolated  from  the
Harvard  and  the  BBN  traffic. Upon entering MIT the traffic is sorted
further to the appropriate local MIT net,  and  later  to  its  terminal

This  scheme allows the switches in Tokyo NOT to know about the internal
structure of the addressing  in  the  USA  in  general  and  at  MIT  in
particular.  It  also helps the switches in California to take advantage
of special HU (high utilization) trunks to the Boston area, if any.

Since  this  hybrid  contains  both  the  flat  and   the   hierarchical
addressing, multi homing can be handled as well as by flat addressing.

IEN-179                                                         [Page 4]

Another  variant of this scheme is not to start with full tables of flat
addresses (which are impossible to manage) but to treat the system as if
it was a pure hierarchical one, but in addition to maintain a  table  of
exception which is dynamically updated.

The  entries  in  these  exception tables should include both addresses,
their granularity level and the associated routing decision.    If,  for
example,  addresses  always consist of a sequence of bytes (of any size)
the granularity level may be measured by the number of bytes  which  are

For  example,  a  telephone exchange in Marina del Rey may know that the
traffic for 1-213-828X and 1-213-821X is  local,  all  the  traffic  for
addresses  2X  and 4X (Europe) should be routed to a certain cable which
goes directly to Europe, the traffic to 1-212X be routed to a some cable
which goes directly to NY, the traffic to either 1-617-49X or 1-617-253X
should be routed to the HU trunk which goes to Cambridge, and  similarly
the traffic to 1-202-694-3049 should be given to a certain line which is
connected  to  a  certain  destination,  somewhere.  All the rest of the
traffic should be routed to downtown  LA  where  smarter  computers  can
solve the routing.

In  the  above  example  the  various  entries  have different levels of
granularity, varying from 1 (in the case of Europe) to 11 (in  the  last

Such  a  scheme  yields  tables of reasonable size, can benefit from any
structure  and  logic/regularity  (as  opposed  to  randomness)  of  the
communication  subsystem, can handle multihoming, and has the capability
to utilize special irregular HU lines and shortcuts.

Note that in this scheme the addressing may be highly structured  (e.g.,
in  a  hierarchical  way)  but  the  routing is not necessarily so.  The
routing is hierarchical only when this is known to be the  right  thing,
or  when nothing else is known and the hierarchical routing is used as a

In summary, this is a way to benefit from the simplicity of hierarchical
routing whenever possible, while being able to use  the  flat-addressing
features  when  needed,  on  a  case-by-case  basis, without the need to
maintain tables of unreasonable size.