March 11th, 1981
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
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
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.
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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
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.