IEN 178 April 1981
ADDRESSING PROBLEMS IN MULTI-NETWORK SYSTEMS
Carl A. Sunshine
University of Southern California
Information Sciences Institute
4676 Admiralty Way
Marina del Rey, CA 90291
Abstract
To allow uers in different networks to communicate with
each other, development of powerful yet practical naming,
addressing, and routing facilities is essential. Basic
procedures for multi-network systems under control of a single
organization have begun to be used, but a large set of more
sophisticated goals remain to be addressed. This paper
describes several of these more advanced problems including
extendability, multihoming, network partitioning, mobile
hosts, shared access, local site connections, gateway routing,
and overcoming differences in heterogeneous systems.
Note:
There are three figures associated with this document
which may be obtained from the author by sending a message to
<SUNSHINE@ISIF>.
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Introduction
The interconnection of multiple computer networks makes
it possible for ever wider communities of computer users and
applications to interact with each other. A basic set of
problems that must be solved in accomplishing such
interconnections concerns providing naming, addressing, and
routing procedures that are general and convenient yet
practical. These problems are particularly difficult when
networks of different designs and/or operating under different
authorities must be interconnected.
Current multi-network systems are fairly small (tens of
networks maximum) and largely designed by and under control of
a single organization. (We shall call this "homogeneous"
internetworking.) Basic interconnection is supported by
simple hierarchical addressing and routing procedures employed
uniformly throughout the system [1,4,10,13]. Interconnections
of different multi-network systems (heterogeneous
internetworking) are just beginning to be made, largely by ad
hoc means.
Thus, while some of the basic problems have been solved,
a large set of secondary problems will soon be upon us. These
include problems of scale (current methods are impractical for
systems with hundreds or thousands of networks); supporting
more sophisticated functions such as multihoming, network
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partitioning, mobile hosts, and shared access; and overcoming
the different procedures in heterogeneous systems.
This paper describes several of these interesting
problems, and discusses potential solutions. The emphasis is
on developing a feel for the range of problems and solutions
rather than on detailed or formal treatment of any one
problem. In many cases it will be clear that further research
is needed to clarify the problems or to develop and evaluate
better solutions.
Hierarchical Methods
A basic approach to addressing and routing in large
systems is to use hierarchical methods. These methods can be
applied at various levels (e.g., within networks and among
networks). We give a brief summary of the basic principles
involved since these form the background for many of the other
problems.
As the number of subscribers or "hosts" in a single
network increases, it becomes desirable to introduce a number
of switches, each serving a subset of the hosts. These
switches must maintain routing tables which give the best
outgoing link (or set of links) for any destination. The
tables are used to forward incoming packets properly toward
their destination. In datagram networks, a routing decision
based on final destination is made for every packet, while in
virtual circuit nets only the initial call request packet
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requires the full routing decision (subsequent packets of a
call are forwarded over fixed routes kept in other tables).
If every switch maintained routing information for every
destination individually, the routing tables would become very
large. A standard approach is to introduce hierarchical
addressing, where each host is assigned a particular port on a
particular switch, and hence addresses take the form <switch,
port>. Then routing may also be done hierarchically by
sending all packets destined to a given switch over the same
route, ignoring the "low order" portion of the address. Hence
each switch need only maintain routes to other switches,
greatly reducing the number of different destinations, and
hence entries, in the routing tables.
Note that hierarchical routing is one major motivation
for introducing hierarchical addresses, but these two
techniques do not necessarily go together as we shall see
below. Another reason for hierarchical addresses is simply to
distribute the authority for assigning addresses within a
large system [14].
The same techniques may be extended to multi-network
systems by adding another level to the addressing hierarchy so
that addresses take the form <net, switch, port>. With
hierarchical routing, packets are first routed to the
destination network, ignoring the rest of their address, and
then routed within the final network as above. This form of
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hierarchical addressing has been adopted by the public packet
switching networks in CCITT Recommendation X.121, and it
appears that most public networks intend to use hierarchical
routing as well [13,19].
The reduction of routing table size that accompanies
hierarchical routing has its price. The resulting routes may
not always be optimal. If there are two ways to reach a
remote network (as is often the case), one may be better for
some hosts within that network and the other for other hosts.
But there is by design no way to determine this from a local
routing table which carries a single entry for the entire
remote network. An even more serious consequence of strict
hierarchical routing is discussed in the next section.
To avoid these problems, routing decisions may based on
more of the address where desirable [5,14]. For example, an
internetwork routing table could be augmented with entries for
individual switches receiving high traffic in a remote
network, while other switches in that network were covered by
a single network level entry. This leads to a selective
increase in the size of routing tables, and requires the
ability to search the tables for variable length portions of
addresses and to update tables with varying levels of detail.
Network Partitions
A network is said to be partitioned when enough links
and/or switches fail so that two or more subsets of its hosts
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are formed which cannot communicate with each other. In an
isolated network there is no remedy for this situation until
sufficient repairs are made to restore connectivity. But if
the partitioned net is part of a multi-network system, there
may be paths through other nets which could connect the
partitions. Unfortunately, these paths are not used within
the strictly hierarchical routing procedures described above.
And even if a "local" packet were sent to a neighboring
network by a switch, it would likely be routed right back into
the same paritition by the other network.
This last point indicates another difficulty. Traffic in
a remote network destined for the partitioned net will be
routed into one or the other partition without consideration
of its within-network switch. (Remember that other networks
see a single best route to this network considered as a
whole.) For some destinations, this will be the wrong
partition and the destination will be unreachable by internal
routes, leading to failure to deliver packets routed that way
from remote nets [14,16].
One solution to this problem is to configure the system
with sufficient robustness that partitions occur very rarely,
and to simply tolerate the above delivery problems when they
occur. This may be satisfactory for commercial systems where
loads and outages are fairly predictable.
In military systems where numerous disruptions are
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anticipated, some means of forcing use of any available
connectivity is desirable [3]. One approach is to treat the
number of networks as dynamic, and turn a partitioned network
into two networks, each of which can be an explicit
destination. This requires rather complex methods of updating
each network's view of the overall topology, and promulgates
knowledge of a partition in one network to all other networks
[8]. Another approach might be to return a special error
message to the neighboring router forcing it to choose another
entry point to the failed network. This
backup-and-try-alternate method has been implemented for call
setup in Telenet [19].
"Fast Track" Routing
It is not only in case of catastrophic events like
partitioning that use of external routes between two points
within the same region may be desirable. If two networks
cover the same geographical area, for example a
store-and-forward ground net and a broadcast satellite net,
performance for some types of traffic may be improved by
exiting the ground net near the source, going through the
satellite net, and returning to the ground net near the
destination. File transfer traffic might obtain higher
throughput in this fashion, for example.
To accomplish this, it is once again necessary to violate
hierarchical routing. Either the network level routing must
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distinguish between destinations best reached directly within
the network and those best reached by going outside, or the
within-network level must be made to view paths through other
networks as a special kind of internal link that is available
[9]. But in the latter case, the network level path status
information must be brought into the internal link status
maintenance procedures, probably a messy business.
Multihoming
A subscriber may want to have multiple connections to a
communication system for reliability or performance reasons.
In the simplest case, several independent physical lines may
be managed as one logical data link to obtain greater
reliability, higher throughput, or lower cost (due to the
idiosyncracies of carrier tariffs). Several such multiline
procedures have been developed, for example in Transpac, and
in X.75. The subscriber still has a single address, and no
further complications are involved.
In order to protect against node failures as well as line
failures, lines to different switches must be used. In this
case the user has two (or more) different addresses. The
multiple addresses may be at any level in the address
hierarchy: (e.g. two addresses within a network, or connected
to two different nets). Multiple lines may also provide
better performance by connecting directly to highly used areas
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of the system and thus avoiding extra hops through the
network.
In order to obtain these benefits, the ability to use
both addresses and to select the optimal address must exist.
This may be accomplished by the source explicitly selecting
one address. But this requires the source to know that there
are multiple addresses for a given destination, to select the
best address for performance, and to switch to an alternate
after a failure. These admittedly weighty burdens could be
aided by a remote directory/routing service.
Alternatively, the packet could carry the multiple
addresses explicitly, allowing each switch to pick the best of
the best routes for each address. This of course adds to
packet length and routing processing load.
Instead of carrying the multiple addresses, the packet
might carry the name (or "logical address") of the destination
[14], leaving it for the switches to lookup and select the
best address at each point. This would reduce packet
complexity, but increase the switch processing demands even
further.
Thus we have a spectrum from high source effort to high
network effort in making use of multiple addresses. In
datagram nets it is probably impractical to require complex
processing of the address on every packet, so more source
effort will probably be required. In virtual circuit nets a
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greater amount of effort can be expended by the net on the
call setup request. Some public nets are already providing
call forwarding facilities where a call to one inoperative or
busy address will automatically be forwarded to an alternate
address.
There are problems at the destination as well as the
source. To obtain the benefits of multihoming, the
destination must be willing to accept traffic on all
addresses. In virtual circuit nets, all the traffic for a
given call must flow over the same line, so a failure during
the call cannot be recovered by using an alternate address.
The call must be cleared with possible loss of data, and a new
one requested.
Even in some datagram nets, higher level protocols are
sensitive to the addresses of the local and remote hosts [3].
The source address is used to demultiplex incoming packets to
the proper "connection," and packets coming from an alternate
address from that used to establish the connection would not
be recognized properly. To avoid this problem, the (single)
name of the source could be used in the connection tables, but
this would have to be carried in the packet. Alternatively,
the multiple remote addresses could be stored as part of the
connection table so that a packet specifying any one as source
would match properly. These multiple addresses would have to
be supplied as part of the connection establishment, and might
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be profitably used in sending traffic if the original address
failed.
Mobile Hosts
Mobile hosts represent a special case of the multiple
address problem. Of course all hosts are technically mobile
in the sense that they occasionally change their address due
to reconfiguration and movement within the user organization,
or modifications to the network topology. Hence directory
information to associate the name of a host with its current
address is available in most systems, either locally or via
some remote server.
However, the problem of changing addresses becomes
qualitatively different when the host is expected to change
its network attachment point frequently, even in the midst of
previously established connections. Special dynamic routing
and addressing procedures have been developed for ground based
mobile hosts communicating via packet radio within a single
network [6]. As distances are increased and this technology
is transferred to airplanes, crossing network boundaries may
also be anticipated.
One method for "tracking" mobile hosts would be to
maintain a specialized database of their current locations
(perhaps replicated for reliability), as is done within
individual packet radio nets (by the "station"). The mobile
hosts would send updates to this database as needed, and users
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wishing to establish communication could query the database
much as any other directory service. However, they should be
prepared to receive frequent address change notifications in
the course of a connection, either from the mobile host
itself, alternate relay points, or the database. Further
details of such a scheme may be found in [18].
Assuming traffic reaches them, destinations must still be
"desensitized" from the particular source address as discussed
above, since this will change. But there is no fixed set of
alternates to exchange at connection setup time in this case,
so packets probably must carry a unique identifier (name) of
the source as well as its current address. For reliability
purposes, they should probably also carry the name of the
destination in case it is no longer associated with the
address they reach.
Mobile hosts may have multiple addresses at one moment as
well as at different times (e.g., an aircraft may be in
contact with two radio nets). Thus it becomes apparent that
problems can interact with each other, making solutions more
difficult.
Sharing Network Access
The opposite problem to one host having several access
lines to the net is several hosts sharing a single access
line. This may be desirable where the number of physiscal
interfaces or ports to the network is limited, or to share a
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long access line among nearby subscribers. Public networks
provide multidrop interfaces for terminal traffic (X.28), but
not for packet mode traffic (X.25). For packet level devices,
the alternative to providing a fixed and hence inefficient
frequency or time division multiplexor must be some sort of
"intelligent" multiplexor functioning at the packet level of
network access protocols.
Broadcast networks (e.g., Ethernets and ring nets)
inherently provide this capability since every interface hears
all traffic. Each interface is responsible for accepting
appropriate traffic, and can sometimes be set to intercept
traffic for multiple addresses.
Another approach is to use a higher level of protocol to
provide the necessary demultiplexing. The Arpanet access
(Host-IMP) protocol does not allow for shared interfaces, and
the limitation of 4 host interfaces on the original IMPs has
proved troublesome in some cases. The Internet Protocol (IP)
is the next level above particular network access protocols in
the ARPA hierarchy [10,11]. IP addresses are sufficiently
long to support multiple "logical" hosts at the same physical
host port on the Arpanet. The Host-IMP header indicates the
same physical host address for all such packets, and the
higher level IP module at the destination demultiplexes the
packets to the correct logical host. An independent device to
perform this function has been developed based on PDP-11/03
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hardware. This "port expander" effectively turns each IMP
port into 4-8 ports for hosts that use the Internet Protocol
[7].
Networks vs. Gateways as Switches
In most models of hierarchical routing, networks are
assumed to function as "super-switches," just as switching
nodes do within one network. This view would be literally
true if there were a single internet switching node in each
net to which all incoming traffic from other nets was routed,
and which then forwarded the traffic to another network or to
a local host. Figure X shows a small example of a
multi-network system and a routing table at one
network/switch. The routing table gives the cost in internet
hops and the best neighbor net to use to reach each other
network in the system.
For efficiency, this internet switching function is
usually distributed to processors called "gateways" serving
each of the internet links. Instead of being sent through the
net to some central point, the internet traffic can be routed
immediately at its entry point to the best exit point (either
another gateway or the destination host). Figure Y shows the
same internet system with internet links labeled, and a
routing table at the gateway located on one incoming link.
Since the gateway must send packets across its net to a
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particular outgoing link, the routing table now shows the name
of this next link rather than the next net.
Another step in this progression leads to a single
gateway located in the "middle" of each internet link rather
than two separate processors in each net. The gateways take
on the identity of their internet link(s). In this
configuration, it is more realistic to count the network hops
as the cost fucntion rather than the internet links. Hence
each gateway is maintaining a distance (in network hops)
between gateways, and a best next gateway to use for each
destination. In this model, the gateways may be more
realistically viewed as the switching nodes, and the networks
as the links connecting them. This is essentially the dual of
the earlier model as shown in Figure Z. But the destinations
in the routing table are networks, not gateways, making this a
curious sort of hybrid scheme. Hence it is not clear how to
apply the "link state" type of routing procedures used in
single networks (e.g., the Arpanet) to this multi-network
configuration with gateways as switching nodes.
Local Site Connections
Many sites start with a single host connected to a
long-haul net. As the site develops, a few more hosts are
connected, also directly to the long-haul switch. As even
more hosts want to join the net at that site, problems result
from costly or inefficient use of network access procedures.
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Some sort of port expander or intelligent multiplexor devices
as discussed above become attractive.
This addresses the network connection problem, but not
the local traffic requirements which are also growing, and may
easily exceed traffic to remote sites. The network switch is
handling a lot of traffic that never goes any further through
the net. In some cases the port expanders may be capable of
local switching, forming a rudimentary local net.
To handle local traffic more efficiently, an explicit
local net may be desirable. A question then arises as to
whether this net should be "known" to the rest of the internet
system, and connected to it via one or more full-fledged
gateways, or whether it should be "invisible" at the internet
level with its hosts appearing as if they were directly
connected to the long-haul net. In the first case, local
hosts have internet addresses on an explicit local net, while
in the second they have addresses on the long-haul net.
The explicit local net approach has certain advantages
stemming from the explicit identification of the group of
hosts at a site as a network. If the site is connectected to
two or more other nets, then the internet routing mechansims
will automatically choose the best path to the local hosts,
which have only a single address (on their local net).
However, this participation at the internet level can
also be a problem. As the number of sites with local nets
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increases, so will the number of nets and hence the size of
the routing tables and updates which must be propagated all
over the internet system. If the growth continues at a site
so that there are several local nets connected by "local"
gateways, should all of these nets and the local topology be
known throughout the internet system? At some point treating
local nets on a par with long-haul or backbone nets breaks
down.
The invisible local net approach, on the other hand,
avoids problems of proliferating networks at the internet
level. Many port expander or local distribution systems can
perform an internal switching function, relieving the
long-haul net switch of handling local traffic. But sites
with connections to two or more nets will have multiple
addresses for their hosts (one for each net the hosts appear
"directly" connected to), and this causes some difficulties as
discussed above under Multihoming.
The best solution to this tradeoff is not clear. Adding
an additional level to the addressing hierarchy may be a
temporary solution, but it, too, will become strained in time.
This suggests allowing a variable number of levels in the
addressing hierarchy, adding new levels as complexity
increases in some area. But this imposes a rigid ordering of
levels and hence routing, while in reality "higher" and
"lower" may depend on the viewpoint of the user. Further
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research is needed on how internet systems may grow and still
maintain efficient addressing and routing procedures.
Multiple Domains
Most of the previous discussion has assumed a single
compatible "domain" in which network addressing and routing
procedures are carried out uniformly. In the real world we
have already seen the growth of several large domains with
different conventions, including public, mainframe
manufacturer, Defense Department, and local networks. It is
unrealistic and perhaps impossible that these diverse groups
will ever adopt a single addressing scheme, so we must live
with the problem of multiple domains for the foreseeable
future.
One approach is to assume that one domain will make use
of another merely as transport medium between its own
homogeneous components. The used system appears merely as one
of several types of media that the using system can employ via
appropriate access protocols. The using system's packets will
be "encapsulated" in the used system's protocols. Of course
the two domains can make use of each other, achieving
coexistence, if not complete interoperation, by "mutual
encapsulation" [15].
To achieve full interoperability between heterogeneous
systems, each system must recognize the hosts on the other.
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Two basic choices are possible for crossing domain boundaries:
mapping and source routing.
In the mapping approach, each domain provides a set of
otherwise unused internal addresses which it maps to
particular addresses in other domains. Traffic addressed to
one of these "pseudo-addresses" is routed to an interface or
gateway to the appropriate other domain, at which point the
pseudo-address is converted into an address in the other
domain. In the simplest case, this requires only bilateral
agreements between domains, but it may also be extended across
intermediaries with further collaboration.
A disadvantage of this approach is that the number of
external addresses available is limited to those for which
mappings have been previously defined and installed.
Typically only a small fraction of remote parties are
supported. Another disadvantage is that the same party has
different addresses in different domains--the directory of
names to addresses has many entries for each name, one for
each domain supporting that party. The major advantage is
that for those names supported, the users may address remote
parties in exactly the same fashion as local ones, with no
additional procedures.
In source routing [14,17,5], the source specifies a route
to reach the destination consisting of the addresses of
successive inter-domain gateways, and ending with the final
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destination. Each address in this list is interpreted within
a domain where it is meaningful, and then removed so that the
next address is available in the next domain transitted.
Using this method, the full range of remote parties is
accessable, and the inter-domain gateways do not have to
maintain any predefined mappings or perform address
conversions. The burden is shifted to the source which must
know enough about the overall topology and address formats to
construct a successful source route. Of course packet headers
become bigger, and packet processing increases to accomodate
the variable length source routes. Once again, the "address"
of a given party varies from one domain to another, but it is
now possible to combine this information--if the directory
gives the source route to X from domain A, and a user in
domain B knows a route to domain A, he can concatenate them to
get a route to X from B (although it may not be an optimal
route).
It is often useful to collect a return route at the same
time the source route is being consumed. This allows the
destination to reply. In general the return route is not
simply the inverse of the source route. The return addresses
are added as the packet enters each domain, while the
successive destination addresses are removed as the packet
exits each domain (see [17] for a detailed example).
The "network independent" transport protocol [2]
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developed by the British PSS Users Forum is one of the first
to explicitly deal with the problem of multiple domains. They
suggest essentially a source routing mechanism. There are
additional provisions for translating explicitly identified
address information transmitted as data between end users.
The protocol assumes a route setup procedure as part of call
establishment so that the source route need only be carried in
the call request packet.
The public networks have also provided for a limited form
of source routing in the Call User Data field of X.25 call
request packets. This field may be used by the destination
DTE as additional address information for subsequent steps in
a call. This mechanism was used to allow international calls
between Canadian and US public networks before the
hierarchical X.121 numbering plan was put into effect [12].
The Call User Data field is also beginning to be used in an ad
hoc fashion to provide addressing within various private
and/or local nets connected to public nets.
The Arpa Internet Protocol also supports a source routing
option, but addresses within the route are all expected to be
IP format addresses [11].
Conclusions
We have identified a number of problems that must be
considered in going beyond the simple network interconection
techniques that are in use today. The significance of these
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problems is just beginning to be widely percieved. Some
preliminary solutions have been proposed, but little practical
experience exists. Much work remains to be done in clarifying
the problems, and in developing and evaluating solutions.
Acknowledgements
Many of the concepts presented in this paper have been
discussed over several years as part of the ARPA Internet
project. Much of the credit for developing and clarifying
these ideas belongs to my colleagues at ISI and the other
sites engaged in this project.
References
Note: Several of the references listed below are Internet
Experiment Notes, unpublished memos written for the ARPA
Internet project.
[1] D. R. Boggs, J. F. Shoch, E. A. Taft, and R. M. Metcalfe,
"Pup: An Internetwork Architecture," IEEE Trans. on
Communications 28, 4, April 1980, pp. 612-623.
[2] British Post Office PSS User Forum, A Network Independent
Transport Service, February 1980.
[3] V. G. Cerf, Internet Addressing and Naming in a Tactical
Environment, Internet Experiment Note 110, August 1979.
[4] V. G. Cerf and P. T. Kirstein, "Issues in Packet-Network
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[5] D. D. Clark and D. Cohen, A Proposal for Addressing and
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[7] H. A. Nelson, J. E. Mathis, and J. M. Lieb, The ARPANET
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[8] R. Perlman, Flying Packet Radios and Network Partitions,
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[10] J. B. Postel, "Internetwork Protocol Approaches," IEEE
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[14] J. F. Shoch, "Inter-Network Naming, Addressing, and
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[15] J. F. Shoch, D. Cohen, and E. A. Taft, "Mutual
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[16] C. A. Sunshine, "Interconnection of Computer Networks,"
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