To: Linda at ISIF Cc: JHaverty at BBNG
IEN-181
Van Gateway: Some Routing
and Performance Issues
Jack Haverty
Bolt Beranek and Newman Inc.
May 1981
IEN-181 Bolt Beranek and Newman Inc.
Jack Haverty
The VAN gateway is a new facility currently under
development for the internet community. Its intended purpose is
to allow interconnection of the ARPANET and therefore the
Internet with Telenet, but it also introduces a new mechanism
whose failure or misuse can seriously affect the system. The key
problem with use of the VAN gateway is to allow and encourage
using it for legitimate purposes, while preventing utilization by
unauthorized users or as a result of a software or hardware
failure in the networks involved.
There are two aspects to this problem. The first control on
the gateway usage must be to assure that the packets being
handled are legitimate, in that they are associated with
authorized users. This is a specific example of the need for
mechanisms which have been discussed at various times as
"restricted routing" mechanisms.
The second control problem is to assure that the gateway is
being used as intended, with a reasonable level of traffic for
the function being performed. Even if packets are being
processed for authorized users, it is possible for failures
within the routing system or host software, for example, to cause
packets to loop endlessly. Failures of the network protocols
could similarly cause duplicate packets to be sent needlessly.
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In both cases, the concern results from the highly visible
impact which use of Telenet incurs, since charges are computed on
a per-packet basis. However, the same issues are inherent in the
Catenet itself, in that misuse of the network consumes resources
which are then unavailable for legitimate use. Thus the problem
of managing use of the gateway is most critical for the VAN
gateway, but applies as well to all gateways, and in fact to any
shared resource.
In the initial implementation of the VAN gateway, resource
management is being provided by use of tables which enumerate the
authorized users of the gateway. These users are simply the
addresses of the hosts, both on the ARPANET (Catenet) side and on
the Telenet side, which will be acceptable as valid source and
destination addresses of packets which transit the gateway. All
other packets which are received by the gateway will be
discarded.
In the internet architecture, the Telenet side of the
gateway appears as a single network to the internet mechanisms.
The gateway contains a table which maps artificial host addresses
on that network into real 14-digit Telenet/X.25 addresses, in
much the same way as other networks convert internet addresses
into addresses for their particular attached network. X.25
virtual circuits are only permitted between the gateway and X.25
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hosts which are present in this translation table, which
effectively defines the set of authorized gateway users in the
X.25 community.
No similar table is necessary for translation of addresses
on the ARPANET side of the gateway, since this translation is
well defined by the internet protocol. Without any additional
mechanisms, this would permit any ARPANET host to use the VAN
gateway. In addition, since gateways to other networks are
simply ARPANET hosts, this would permit any host on the Catenet
to use the VAN gateway.
To restrict the user community of the VAN gateway, a second
table is provided, which enumerates all internet addresses which
are acceptable as sources or destinations on the ARPANET side of
the gateway. Each internet datagram which arrives from the
ARPANET or Telenet is checked to assure that the source and
destination addresses in the internet header are listed in one of
the two tables which define the set of hosts which are permitted
to use the VAN gateway.
The table entries will be set up directly by DARPA. In
selecting the set of valid hosts, the reliability of the data
presented to the gateway should be considered.
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In particular, we note that there is a significant
difference in the addresses presented at the gateway in the
internet header of each packet. If such an address is in fact on
the ARPANET, the gateway can verify it by comparison with the
address supplied by the IMP in the ARPANET leader of the packet.
For packets sent to the ARPANET, one can similarly expect the IMP
subnet to deliver the packet to the host specified in the ARPANET
leader.
If the address of a packet handled by the gateway is on
another component network of the Catenet, the packet is
necessarily handled through one or more gateways. The internet
structure permits gateways to freely enter the system. Gateways
are in general under the control of the organization which owns
them and/or the attached network. Gateways in general do not
check the addresses in the internet headers of packets which they
process, so it is possible for malfunctioning hardware or
software to emit packets with incorrect addresses. If such
addresses happen to be present in the VAN gateway tables, these
packets will be processed by the VAN gateway.
The impact of this situation on the policy for allowing use
of the VAN gateway is that hosts on networks other than the
ARPANET are to be considered somewhat less reliable in terms of
enforcement of the usage policy. The mechanisms in the initial
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VAN gateway implementation will provide some degree of control
over the use of the gateway, but these mechanisms are not to be
considered appropriate or complete in the general sense, and they
are not proof against failures. These mechanisms are intended
only as an interim measure.
We suggest that further development work on the internet
gateway system, of which the VAN gateway is a component, should
address the problems of resource control at the internet system
level. Any mechanism which restricts the usage of a gateway must
be designed in conjunction with other network mechanisms, such as
routing, flow control, load sharing, and error control.
As an example, we can consider a hypothetical configuration
in which two VAN gateways are connected to Telenet, one from
ARPANET, and the other from SATNET, to support traffic between
Telenet users and hosts on ARPANET or SATNET. Only these
user/hosts would be listed in the VAN gateway tables.
Since the VAN gateways are participants in the internet
routing mechanisms, failure of the gateway between ARPANET and
SATNET would cause the system to recognize the path through
TELENET, as a transit network, as a viable route for ARPANET-
SATNET traffic. However, this traffic would be discarded at the
VAN gateway because the addresses are not listed in its tables.
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This scenario will be avoided by preventing the two VAN
gateways from being "neighbors" for routing purposes, which is
acceptable only in restricted configurations. The general
problem results from the usage restrictions at the VAN gateway,
which makes the path a valid route for one class of packets, but
invalid for other classes. The current routing mechanism cannot
handle this situation.
In addition, the effect of the usage restrictions on future
mechanisms for handling partitioned networks, and load sharing of
gateway paths, must be investigated.
We have two mechanisms to propose for consideration as
mechanisms to attack this problem. The first is a resource
control model which is a result of the TIP Login work. The
second involves the use of performance models, which monitor use
of resources to determine if unexpected behavior occurs. These
two mechanisms can be introduced to work more effectively in the
VAN gateway problem.
The architecture for the TIP Login system identifies several
abstract modules which interact to implement the resource control
functions. A "Control Point" is the module which directly
controls the use of a resource. It is responsible for detecting
an attempt to use some resource, collecting such information
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which identifies who is trying to use the resource and what they
are trying to do, and then permitting or denying use of the
resource. The decision concerning whether or not a particular
usage is allowed is made by a "Decision Module." This module
takes the information supplied by the Control Point, and applies
the algorithm which defines the resource usage policy.
Typically, and particularly in the Tip Login case, the Decision
Module will obtain more information about the particular user
and/or resource involved in the decision, by using a distributed
database system.
In the TIP Login system, the Control Point is at each TIP.
Decision Modules are located in special-purpose hosts. The
database system is present in those hosts as well as on larger
database-maintenance hosts, where tools to manipulate and modify
the database exist. Typically the Decision Module identifies the
particular individual attempting to use a TIP, and retrieves a
record of information specific to that individual, which defines
his authorizations (or lack thereof).
Much of this mechanism should prove to be useful as a basis
for control of gateways as well. In such a system, the control
points would be at the gateways. Decision Modules might also be
at the gateways, if decisions must be made on each packet.
Decisions might be based on source or destination addresses, or
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on the identify of the individual responsible for the packet. We
suggest that this approach be considered for further research.
A problem which is not being addressed currently is the
second control problem mentioned earlier, namely the monitoring
of the use of some resource by an authorized user, to guarantee
that the resource is being used as intended. In the VAN gateway
case, for example, a malfunctioning TCP might cause many
unnecessary packets to be handled, but since they are associated
with authorized addresses, no control is applied. In addition to
the obvious cost and performance penalties, lack of monitoring
precludes the use of policies which grant limited use of
resources to, for example, allow some users to handle only low-
throughput traffic, or low priority traffic. We believe that the
use of performance models, embedded within the gateways and for
hosts, is a promising direction for attacking this problem.
Limitations of the current LSI-11 implementation of the VAN
gateway are likely to preclude any significant testing of these
approaches. We have been pursuing these ideas as research
issues, which have surfaced during the current implementation
efforts.
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