Network Working Group Gigabit Working Group
Request for Comments: 1077 B. Leiner, Editor
November 1988
Critical Issues in High Bandwidth Networking
Status of this Memo
This memo presents the results of a working group on High Bandwidth
Networking. This RFCis for your information and you are encouraged
to comment on the issues presented. Distribution of this memo is
unlimited.
ABSTRACT
At the request of Maj. Mark Pullen and Maj. Brian Boesch of DARPA, an
ad-hoc working group was assembled to develop a set of
recommendations on the research required to achieve a ubiquitous
high-bandwidth network as discussed in the FCCSET recommendations for
Phase III.
This report outlines a set of research topics aimed at providing the
technology base for an interconnected set of networks that can
provide highbandwidth capabilities. The suggested research focus
draws upon ongoing research and augments it with basic and applied
components. The major activities are the development and
demonstration of a gigabit backbone network, the development and
demonstration of an interconnected set of networks with gigabit
throughput and appropriate management techniques, and the development
and demonstration of the required overall architecture that allows
users to gain Access to sUCh high bandwidth.
1. Introduction and Summary
1.1. Background
The computer communications world is evolving toward both high-
bandwidth capability and high-bandwidth requirements. The recent
workshop conducted under the auspices of the FCCSET Committee on High
Performance Computing [1] identified a number of areas where
extremely high-bandwidth networking is required to support the
scientific research community. These areas range from remote
graphical visualization of supercomputer results through the movement
of high rate sensor data from space to the ground-based scientific
investigator. Similar requirements exist for other applications,
such as military command and control (C2) where there is a need to
quickly access and act on data oBTained from real-time sensors. The
workshop identified requirements for switched high-bandwidth service
in excess of 300 Mbit/s to a single user, and the need to support
service in the range of a Mbit/s on a low-duty-cycle basis to
millions of researchers. When added to the needs of the military and
commercial users, the aggregate requirement for communications
service adds up to many billions of bits per second. The results of
this workshop were incorporated into a report by the FCCSET [2].
Fortunately, technology is also moving rapidly. Even today, the
installed base of fiber optics communications allows us to consider
aggregate bandwidths in the range of Gbit/s and beyond to limited
geographical regions. Estimates arrived at in the workshop lead one
to believe that there will be available raw bandwidth approaching
terabits per second.
The critical question to be addressed is how this raw bandwidth can
be used to satisfy the requirements identified in the workshop: 1)
provide bandwidth on the order of several Gbit/s to individual users,
and 2) provide modest bandwidth on the order of several Mbit/s to a
large number of users in a cost-effective manner through the
aggregation of their traffic.
Through its research funding, the Defense Advanced Research Projects
Agency (DARPA) has played a central role in the development of
packet-oriented communications, which has been of tremendous benefit
to the U.S. military in terms of survivability and interoperability.
DARPA-funded research has resulted in the ARPANET, the first packet-
switched network; the SATNET, MATNET and Wideband Network, which
demonstrated the efficient utilization of shared-access satellite
channels for communications between geographically diverse sites;
packet radio networks for mobile tactical environments; the Internet
and TCP/IP protocols for interconnection and interoperability between
heterogeneous networks and computer systems; the development of
electronic mail; and many advances in the areas of network security,
privacy, authentication and access control for distributed computing
environments. Recognizing DARPA's past accomplishments and its
desire to continue to take a leading role in addressing these issues,
this document provides a recommendation for research topics in
gigabit networking. It is meant to be an organized compendium of the
critical research issues to be addressed in developing the technology
base needed for such a high bandwidth ubiquitous network.
1.2. Ongoing Activities
The OSTP report referred to above recommended a three-phase approach
to achieving the required high-bandwidth networking for the
scientific and research community. Some of this work is now well
underway. An ad-hoc committee, the Federal Research Internet
Coordinating Committee (FRICC) is coordinating the interconnection of
the current wide area networking systems in the government; notably
those of DARPA, Department of Energy (DoE), National Science
Foundation (NSF), National Aeronautics and Space Administration
(NASA), and the Department of Health and Human Services (HHS). In
accordance with Phases I and II of the OSTP report, this activity
will provide for an interconnected set of networks to support
research and other scholarly pursuits, and provide a basis for future
networking for this community. The networking is being upgraded
through shared increased bandwidth (current plans are to share a 45
Mbit/s backbone) and coordinated interconnection with the rest of the
world. In particular, the FRICC is working with the European
networking community under the auspices of another ad-hoc group, the
Coordinating Committee for Intercontinental Research Networks
(CCIRN), to establish effective US-Europe networking.
However, as the OSTP recommendations note, the required bandwidth for
the future is well beyond currently planned public, private, and
government networks. Achieving the required gigabit networking
capabilities will require a strong research activity. There is
considerable ongoing research in relevant areas that can be drawn
upon; particularly in the areas of high-bandwidth communication
links, high-speed computer switching, and high-bandwidth local area
networks. Appendix A provides some pointers to current research
efforts.
1.3. Document Overview
This report outlines a set of research topics aimed at providing the
technology base for an interconnected set of networks that can
provide the required high-bandwidth capabilities discussed above.
The suggested research focus draws upon ongoing research and augments
it with basic and applied components. The major activities are the
development and demonstration of a Gigabit Backbone network (GB) [3],
the development and demonstration of an interconnected set of
networks with gigabit throughput and appropriate management
techniques, and the development and demonstration of the required
overall architecture that allows users to gain access to such high
bandwidth. Section 2 discusses functional and performance goals
along with the anticipated benefits to the ultimate users of such a
system. Section 3 provides the discussion of the critical research
issues needed to achieve these goals. It is organized into the major
areas of technology that need to be addressed: general architectural
issues, high-bandwidth switching, high-bandwidth host interfaces,
network management algorithms, and network services. The discussion
in some cases contains examples of ongoing relevant research or
potential approaches. These examples are intended to clarify the
issues and not to propose that particular approach. A discussion of
the relationship of the suggested research to other ongoing
activities and optimal methods for pursuing this research is provided
in Section 4.
2. Functional and Performance Goals
In this section, we provide an assessment of the types of services a
GN (four or five orders of magnitude faster than the current
networks) should provide to its users. In instances where we felt
there would be a significant impact on performance, we have provided
an estimate of the amount of bandwidth needed and delay allowable to
provide these services.
2.1. Networking Application Support
It is envisioned that the GN will be capable of supporting all of the
following types of networking applications.
Currently Provided Packet Services
It is important that the network provide the users with the
equivalent of services that are already available in packet-
switched networks, such as interactive data exchange, mail
service, file transfer, on-line access to remote computing
resources, etc., and allow them to eXPand to other more advanced
services to meet their needs as they become available.
Multi-Media Mail
This capability will allow users to take advantage of different
media types (e.g., graphics, images, voice, and video as well as
text and computer data) in the transfer of messages, thereby
increasing the effectiveness of message exchange.
Multi-Media Conferencing
Such conferencing requires the exchange of large amounts of
information in short periods of time. Hence the requirement for
high bandwidth at low delay. We estimate that the bandwidth would
range from 1.5 to 100 Mbit/s, with an end-to-end delay of no more
than a few hundred msec.
Computer-Generated Real-time Graphics
Visualizing computer results in the modern world of supercomputers
requires large amounts of real time graphics. This in turn will
require about 1.5 Mbit/s of bandwidth and no more than several
hundred msec. delay.
High-Speed Transaction Processing
One of the most important reasons for having an ultra-high-speed
network is to take advantage of supercomputing capability. There
are several scenarios in which this capability could be utilized.
For example, there could be instances where a non-supercomputer
may require a supercomputer to perform some processing and provide
some intermediate results that will be used to perform still
further processing, or the exchange may be between several
supercomputers operating in tandem and periodically exchanging
results, such as in a battle management, war gaming, or process
control applications. In such cases, extremely short response
times are necessary to accomplish as many as hundreds of
interactions in real time. This requires very high bandwidth, on
the order of 100 Mbit/s, and minimum delay, on the order of
hundreds of msec.
Wide-Area Distributed Data/Knowledge Base Management Systems
Computer-stored data, information, and knowledge is distributed
around the country for a variety of reasons. The ability to
perform complex queries, updates, and report generation as though
many large databases are one system would be extremely powerful,
yet requires low-delay, high-bandwidth communication for
interactive use. The Corporation for National Research
Initiatives (NRI) has promoted the notion of a National Knowledge
base with these characteristics. In particular, an attractive
approach is to cache views at the user sites, or close by to allow
efficient repeated queries and multi-relation processing for
relations on different nodes. However, with caching, a processing
activity may incur a miss in the midst of a query or update,
causing it to be delayed by the time required to retrieve the
missing relation or portion of relation. To minimize the overhead
for cache Directories, both at the server and client sites, the
unit of caching should be large---say a megabyte or more. In
addition, to maintain consistency at the caching client sites,
server sites need to multicast invalidations and/or updates.
Communication requirements are further increased by replication of
the data. The critical parameter is latency for cache misses and
consistency operations. Taking the distance between sites to be
on average 1/4 the diameter of the country, a one Gbit/s data rate
is required to reduce the transmission time to be roughly the same
as the propagation delay, namely around 8 milliseconds for this
size of unit. Note that this application is supporting far more
sophisticated queries and updates than normally associated with
transaction processing, thus requiring larger amount of data to be
transferred.
2.2. Types of Traffic and Communications Modes
Different types of traffic may impose different constraints in terms
of throughput, delay, delay dispersion, reliability and sequenced
delivery. Table 1 summarizes some of the main characteristics of
several different types of traffic.
Table 1: Communication Traffic Requirements
+------------------------+-------------+-------------+-------------+
Error-free
Traffic Delay Throughput Sequenced
Type Requirement Requirement Delivery
+------------------------+-------------+-------------+-------------+
Interactive Simulation Low Moderate-High No
+------------------------+-------------+-------------+-------------+
Network Monitoring Moderate Low No
+------------------------+-------------+-------------+-------------+
Virtual Terminal Low Low Yes
+------------------------+-------------+-------------+-------------+
Bulk Transfer High High Yes
+------------------------+-------------+-------------+-------------+
Message Moderate Moderate Yes
+------------------------+-------------+-------------+-------------+
Voice Low, constant Moderate No
+------------------------+-------------+-------------+-------------+
Video Low, constant High No
+------------------------+-------------+-------------+-------------+
Facsimile Moderate High No
+------------------------+-------------+-------------+-------------+
Image Transfer Variable High No
+------------------------+-------------+-------------+-------------+
Distributed Computing Low Variable Yes
+------------------------+-------------+-------------+-------------+
Network Control Moderate Low Yes
+------------------------+-------------+-------------+-------------+
The topology among users can be of three types: point-to-point (one-
to-one connectivity), multicast (one sender and multiple receivers),
and conferencing (multiple senders and multiple receivers). There
are three types of transfers that can take place among users. They
are connection-oriented network service, connectionless network
service, and stream or synchronous traffic. Connection and
connectionless services are asynchronous. A connection-oriented
service assumes and provides for relationships among the multiple
packets sent over the connection (e.g., to a common destination)
while connectionless service assumes each packet is a complete and
separate entity unto itself. For stream or synchronous service a
reservation scheme is used to set up and guarantee a constant and
steady amount of bandwidth between any two subscribers.
2.3. Network Backbone
The GB needs to be of high bandwidth to support a large population of
users, and additionally to provide high-speed connectivity among
certain subscribers who may need such capability (e.g., between two
supercomputers). These users may access the GN from local area
networks (LANs) directly connected to the backbone or via high-speed
intermediate regional networks. The backbone must also minimize
end-to-end delay to support highly interactive high-speed
(supercomputer) activities.
It is important that the LANs that will be connected to the GN be
permitted data rates independent of the data rates of the GB. LAN
speeds should be allowed to change without affecting the GB, and the
GB speeds should be allowed to change without affecting the LANs. In
this way, development of the technology for LANs and the GB can
proceed independently.
Access rate requirements to the GB and the GN will vary depending on
user requirements and local environments. The users may require
access rates ranging from multi-kbit/s in the case of terminals or
personal computers connected by modems up to multi-Mbit/s and beyond
for powerful workstations up to the Gbit/s range for high-speed
computing and data resources.
2.4. Directory Services
Directory services similar to those found in CCITT X.500/ISO DIS 9594
need to be provided. These include mapping user names to electronic
mail addresses, distribution lists, support for authorization
checking, access control, and public key encryption schemes,
multimedia mail capabilities, and the ability to keep track of mobile
users (those who move from place to place and host computer to host
computer). The directory services may also list facilities available
to users via the network. Some examples are databases,
supercomputing or other special-purpose applications, and on-line
help or telephone hotlines.
The services provided by X.500 may require some extension for GN.
For example, there is no provision for multilevel security, and the
approach taken to authentication must be studied to ensure that it
meets the requirements of GN and its user community.
2.5. Network Management and Routing
The objective of network management is to ensure that the network
functions smoothly and efficiently, and consists of the following:
accounting, security, performance monitoring, fault isolation and
configuration control.
Accounting ensures that users are properly billed for the services
that the network provides. Accounting enforces a tariff; a tariff
expresses a usage policy. The network need only keep track of those
items addressed by the tariff, such as allocated bandwidth, number of
packets sent, number of ports used, etc. Another type of accounting
may need to be supported by the network to support resource sharing,
namely accounting analogous to telephone "900" numbers. This
accounting performed by the network on behalf of resource providers
and consumers is a pragmatic solution to the problem of getting the
users and consumers into a financial relationship with each other
which has stymied previous attempts to achieve widespread use of
specialized resources.
Performance monitoring is needed so that the managers can tell how
the network is performing and take the necessary actions to keep its
performance at a level that will provide users with satisfactory
service. Fault isolation using technical control mechanisms is
needed for network maintenance. Configuration management allows the
network to function efficiently.
Several new types of routing will be required by GN. In addition to
true type-of-service, needed to support diverse distributed
applications, real-time applications, interactive applications, and
bulk data transfer, there will be need for traffic controls to
enforce various routing policies. For example, policy may dictate
that traffic from certain users, applications, or hosts may not be
permitted to traverse certain segments of the network.
Alternatively, traffic controls may be used to promote fairness; that
is, to make sure that busy link or network segment isn't dominated by
a particular source or destination. The ability of applications to
reserve network bandwidth in advance of its use, and the use of
strategies such as soft connections, will also require development of
new routing algorithms.
2.6. Network Security Requirements
Security is a critical factor within the GN and one of those features
that are difficult to provide. It is envisioned that both
unclassified and classified traffic will utilize the GN, so
protection mechanisms must be an integral part of the network access
strategy. Features such as authentication, integrity,
confidentiality, access control, and nonrepudiation are essential to
provide trusted and secure communication services for network users.
A subscriber must have assurance that the person or system he is
exchanging information with is indeed who he says he is.
Authentication provides this assurance by verifying that the claimed
source of a query request, control command, response, etc., is the
actual source. Integrity assures that the subscriber's information
(such as requests, commands, data, responses, etc.) is not changed,
intentionally or unintentionally, while in transit or by replays of
earlier traffic. Unauthorized users (e.g., intruders or network
viruses) would be denied use of GN assets through access control
mechanisms which verify that the authenticated source is authorized
to receive the requested information or to initiate the specified
command. In addition, nonrepudiation services can be offered to
assure a third party that the transmitted information has not been
altered. And finally, confidentiality will ensure that the contents
of a message are not divulged to unauthorized individuals.
Subscribers can decide, based upon their own security needs and
particular activities, which of these services are necessary at a
given time.
3. Critical Research Issues
In the section above, we discussed the goals of a research program in
gigabit networking; namely to provide the technology base for a
network that will allow gigabit service to be provided in an
effective way. In this section, we discuss those issues which we
feel are critical to address in a research program to achieve such
goals.
3.1. General Architectural Issues
In the last generation of networks, it was assumed that bandwidth was
the scarce resource and the design of the switch was dictated by the
need to manage and allocate the bandwidth effectively. The most
basic change in the next generation network is that the speeds of the
trunks are rising faster than the speeds of the switching elements.
This change in the balance of speeds has manifested itself in several
ways. In most current designs for local area networks, where
bandwidth is not expensive, the design decision was to trade off
effective use of the bandwidth for a simplified switching technique.
In particular, networks such as Ethernet use broadcast as the normal
distribution method, which essentially eliminates the need for a
switching element.
As we look at still higher speed networks, and in particular networks
in which the bandwidth is still the expensive component, we must
design new options for switching which will permit effective use of
bandwidth without the switch itself becoming the bottleneck.
The central thrust of new research must thus be to explore new
network architectures that are consistent with these very different
speed assumptions.
The development of computer communications has been tremendously
distorted by the characteristics of wide-area networking: normally
high cost, low speed, high error rate, large delay. The time is ripe
for a revolution in thinking, technology, and approaches, analogous
to the revolution caused by VCR technology over 8 and 16 mm. film
technology.
Fiber optics is clearly the enabling technology for high-speed
transmission, in fact, so much so that there is an expectation that
the switching elements will now hold down the data rates. Both
conventional circuit switching and packet switching have significant
problems at higher data rates. For instance, circuit switching
requires increasing delays for FTDM synchronization to handle skew.
In the case of packet switching, traditional approaches require too
much processing per packet to handle the tremendous data flow. The
problem for both switching regimes is the "intelligence" in the
switches, which in turn requires electronics technology.
Besides intelligence, another problem for wide-area networks is
storage, both because it ties us to electronics (for the foreseeable
future) and because it produces instabilities in a large-scale
system. (See, for instance, the work by Van Jacobson on self-
organizing phenomena for self-destruction in the Internet.)
Techniques are required to eliminate dependence on storage, such as
cut-through routing.
Overall, high-speed WANs are the greatest agents of change, the
greatest catalyst both commercially and militarily, and the area ripe
for revolution. Judging by the attributes of current high-speed
network research prototypes, WANs of the future will be photonic,
multi-gigabit networks with enormous throughput, low delay, and low
error rate.
A zero-based budgeting approach is required to develop the new high-
speed internetwork architecture. That is, the time is ripe to
significantly rethink the Internet, building on experience with this
system. Issues of concern are manageability, understanding
evolvability and support for the new communication requirements,
including remote procedure call, real-time, security and fault-
tolerance.
The GN must be able to deal with two sources of high-bandwidth
requirements. There will be some end devices (computers) connected
more or less directly to the GN because of their individual
requirements for high bandwidth (e.g., supercomputers needing to
drive remote high-bandwidth graphics devices). In addition, the
aggregate traffic due to large numbers of moderate rate users
(estimates are roughly up to a million potential users needing up to
1 Mbit/s at any given time) results in a high-bandwidth requirement
in total on the GN. The statistics of such traffic are different and
there are different possible technical approaches for dealing with
them. Thus, an architectural approach for dealing with both must be
developed.
Overall, the next-generation architecture has to be, first and
foremost, a management architecture. The directions in link speeds,
processor speeds and memory solve the performance problems for many
communication situations so well that manageability becomes the
predominant concern. (In fact, fast communication makes large
systems more prone to performance, reliability, and security
problems.) In many ways, the management system of the internetwork
is the ultimate distributed system. The solution to this tough
problem may well require the best talents from the communications,
operating systems and distributed systems communities, perhaps even
drawing on database and parallelism research.
3.1.1. High-Speed Internet using High-Speed Networks
The GN will need to take advantage of a multitude of different and
heterogeneous networks, all of high speed. In addition to networks
based on the technology of the GB, there will be high-speed LANs. A
key issue in the development of the GN will be the development of a
strategy for interconnecting such networks to provide gigabit service
on an end to end basis. This will involve techniques for switching,
interfacing, and management (as discussed in the sections below)
coupled with an architecture that allows the GN to take full
advantage of the performance of the various high-speed networks.
3.1.2. Network Organization
The GN will need an architecture that supports the need to manage the
system as well as obtain high performance. We note that almost all
human-engineered systems are hierarchically structured from the
standpoint of control, monitoring, and information flow. A
hierarchical design may be the key to manageability in the next-
generation architecture.
One approach is to use a general three-level structure, corresponding
to interadministrational, intraadministrational, and cluster
networks. The first level interconnects communication facilities of
truly separate administrations where there is significant separation
of security, accounting, and goals. The second level interconnects
subadministrations which exist for management convenience in large
organizations. For example, a research group within a university may
function as a subadministration. The cluster level consists of
networks configured to provides maximal performance among hosts which
are in frequent communication, such as a set of diskless workstations
and their common file server. These hosts are typically, but not
necessarily, geographically collocated. For example, two remote
networks may be tightly coupled by a fiber optic link that bridges
between the two physical networks, making them function as one.
Research along these lines should study the interorganizational
characteristics of communications, such as those being investigated
by the IAB Task Force on Autonomous Networks. Based on current
results, we expect that such work would clearly demonstrate that
considerable communication takes place between particular
subadministrations in different administrations; communication
patterns are not strictly hierarchical. For example, there might be
intense direct communication between the experimental physics
departments of two independent universities, or between the computer
support group of one company and the operating system development
group of another. In addition, (sub)administrations may well also
require divisions into public information and private information.
3.1.3. Fault-Tolerant System
Although the GN will be developed as part of an experimental research
program, it will also serve as part of the infrastructure for
researchers who are experimenting with applications which will use
such a network. The GN must have reasonably high availability to
support these research activities. In addition to facilitate the
transfer of this technology to future operational military and
commercial users, it will need to be designed to become highly
reliable. This can be accomplished through diversity of transmission
paths, the development of fault-tolerant switches, use of a
distributed control structure with self-correcting algorithms, and
the protection of network control traffic. The architecture of a GN
should support and allow for all of these things.
3.1.4. Functional Division of Control Between Network Elements
Current protocol architectures use the layered model of functional
decomposition first developed in the early work on ARPANET protocols.
The concept of layering has been a powerful concept which has allowed
dramatic variation in network technologies without requiring the
complete reimplementation of applications. The concept of layering
has had a first-order impact on the development of international
standards for data communication---witness the ISO "Reference Model
for Open Systems Interconnection."
Unfortunately, however, the powerful concept of layering has been
paired, both in the DoD Internet work and the ISO work, with an
extremely weak concept of the interface between layers. The
interface designs are all organized around the idea of commands and
responses plus an error indicator. For example, the TCP service
interface provides the user with commands to set up or close a TCP
connection and commands to send and receive datagrams. The user may
well "know" whether they are using a file transfer service or a
character-at-a- time virtual terminal, but can't tell the TCP. The
underlying network may "know" that failures have reduced the path to
the user's destination to a single 9.6 kbit/s link, but it also can't
tell the TCP implementation.
All of the information that an analyst would consider crucial in
diagnosing system performance is carefully hidden from adjacent
layers. One "solution" often discussed (but rarely implemented) is
to condense all of this information into a few bits of "Type of
Service" or "Quality of Service" request flowing in one direction
only---from application to network. It seems likely that this
approach cannot succeed, both because it applies too much compression
to the knowledge available and because it does not provide two-way
flow.
We believe it to be likely that the next-generation network will
require a much richer interface between every pair of adjacent layers
if adequate performance is to be achieved. Research is needed into
the conceptual mechanisms, both indicators and controls, that can be
implemented at these interfaces and that, when used, will result in
better performance. If real differences in performance can be
observed, then the implementors of every layer will have a strong
incentive to make use of the mechanisms.
We can observe the first glimmers of this sort of coordination
between layers in current work. For example, in the ISO work there
are 5 classes of transport protocol which are supposed to provide a
range of possible matches between application needs and network
capabilities. Unfortunately, it is the case today that the class of
transport protocol is chosen statically, by the implementer, rather
than dynamically. The DARPA Wideband net offers a choice of stream
or datagram service, but typically a given host uses all one or all
the other---again, a static rather than a dynamic choice. The
research that we believe is needed, therefore, is not how to provide
alternatives, but how to provide them and choose among them on a
dynamic, real-time basis.
3.1.5. Different Switch Technologies
One approach to high-performance networking is to design a technology
that is expected to work as a stand-alone demonstration, without
addressing the need for interconnection to other networks. Such an
experiment may be very valuable for rapid exploration of the design
space. However, our experience with the Internet project suggests
that a primary research goal should be the development of a network
architecture that permits the interconnection of a number of
different switching technologies.
The Internet project was successful to a large extent because it
could incorporate a number of new and preexisting network
technologies: various local area networks, store and forward
switching networks, broadcast satellite nets, packet radio networks,
and so on. In this way, it decoupled the use of the protocols from a
particular technology base. In fact, the technology base evolved
rapidly, but the Internet protocols themselves provided a stability
that led to their success.
The next-generation architecture must similarly deal with a diverse
and evolving technology base. We see "fast-packet" switching now
being developed (for example in B-ISDN); we see photonic switching
and wavelength division multiplexing as more advanced technologies.
We must divorce our architecture from dependence on any one of these.
At the host interface, we must divorce the multiplexing of the medium
from the form of data that the host sees. Today the packet is used
both as multiplexing and interface element. In the future, the host
may see the network as a message-passing system, or as memory. At
the same time, the network may use classic packets, wavelength
division, or space division switching.
A number of basic functions must be rethought to provide an
architecture that is not dependent on the underlying switching model.
For example, our transport protocols assume that data will be lost in
units of a packet. If part of a packet is lost, we discard the whole
thing. And if several packets are systematically lost in sequence,
we may not recover effectively. There must be a host-level unit of
error recovery that is independent of the network. This sort of
abstraction must be applied to all the ASPects of service
specification: error recovery, flow control, addressing, and so on.
3.1.6. Network Operations, Monitoring, and Control
There is a hierarchy of progressively more effective and
sophisticated techniques for network management that applies
regardless of network bandwidth and application considerations:
1. Reactive problem management
2. Reactive resource management
3. Proactive problem management
4. Proactive resource management.
Today's network management strategies are primarily reactive rather
than proactive: Problem management is initiated in response to user
complaints about service outages; resource allocation decisions are
made when users complain about deterioration of quality of service.
Today's network management systems are stuck at step 1 or perhaps
step 2 of the hierarchy.
Future network management systems will provide proactive problem
management---problem diagnosis and restoral of service before users
become aware that there was a problem; and proactive resource
management---dynamic allocation of network bandwidth and switching
resources to ensure that an acceptable level of service is
continuously maintained.
The GN management system should be expected to provide proactive
problem and resource management capabilities. It will have to do so
while contending with three important changes in the managed network
environment:
1. More complicated devices under management
2. More diverse types of devices
3. More variety of application protocols.
Performance under these conditions will require that we seriously
re-think how a network management system handles the expected high
volumes of raw management-related data. It will become especially
important for the system to provide thresholding, filtering, and
alerting mechanisms that can save the human operator from drowning in
data, while still permitting access to details when diagnostic or
fault isolation modes are invoked.
The presence of expert assistant capabilities for early fault
detection, diagnosis, and problem resolution will be mandatory.
These capabilities are highly desirable today, but they will be
essential to contend with the complexity and diversity of devices and
applications in the Gigabit Network.
In addition to its role in dealing with complexity, automation
provides the only hope of controlling and reducing the high costs of
daily management and operation of a GN.
Proactive resource management in GNs must be better understood and
practiced, initially as an effort requiring human intervention and
direction. Once this is achieved, it too must become automated to a
high degree in the GN.
3.1.7. Naming and Addressing Strategies
Current networks, both voice (telephone) and data, use addressing
structures which closely tie the address to the physical location on
the network. That is, the address identifies a physical access
point, rather than the higher-level entity (computer, process, human)
attached to that access point. In future networks, this physical
aspect of addressing must be removed.
Consider, for example, finding the desired party in the telephone
network of today. For a person not at his listed number, finding the
number of the correct telephone may require preliminary calls, in
which advice is given to the person placing the call. This works
well when a human is placing the call, since humans are well equipped
to cope with arbitrary conversations. But if a computer is placing
the call, the process of obtaining the correct address will have to
be incorporated in the architecture as a core service of the network.
Since it is reasonable to expect mobile hosts, hosts that are
connected to multiple networks, and replicated hosts, the issue of
mapping to the physical address must be properly resolved.
To permit the network to maintain the dynamic mapping to current
physical address, it is necessary that high-level entities have a
name (or logical address) that identifies them independently of
location. The name is maintained by the network, and mapped to the
current physical location as a core network service. For example,
mobile hosts, hosts that are connected to multiple networks, and
replicated hosts would have static names whose mapping to physical
addresses (many-to-one, in some cases) would change with time.
Hosts are not the only entities whose physical location varies.
Users' electronic mail addresses change. Within distributed systems,
processes and files migrate from host to host. In a computing
environment where robustness and survivability are important, entire
applications may move about, or they may be redundant.
The needed function must be considered in the context of the mobility
and address resolution rates if all addresses in a global data
network were of this sort. The distributed network directory
discussed elsewhere in this report should be designed to provide the
necessary flexibility, and responsiveness. The nature and
administration of names must also be considered.
Names that are arbitrary or unwieldy would be barely better than the
addresses used now. The name space should be designed so that it can
easily be partitioned among the agencies that will assign names. The
structure of names should facilitate, rather than hinder, the mapping
function. For example, it would be hard to optimize the mapping
function if names were flat and unstructured.
3.2. High-Speed Switching
The term "high-speed switching" refers to changing the switching at a
high rate, rather than switching high-speed links, because the latter
is not difficult at low speeds. (Consider, for example, manual
switching of fiber connections). The switching regime chosen for the
network determines various aspects of its performance, its charging
policies, and even its effective capabilities. As an example of the
latter, it is difficult to expect a circuit-switched network to
provide strong multicast support.
A major area of debate lies in the choice between packet switching
and circuit switching. This is a key research issue for the GN,
considering also the possibility of there being combinations of the
two approaches that are feasible.
3.2.1. Unit of Management vs. Multiplexing
With very high data rates, either the unit of management and
switching must be larger or the speed of the processor elements for
management and switching must be faster. For example, at a gigabit,
a 576 byte packet takes roughly 5 microseconds to be received so a
packet switch must act extremely fast to avoid being the dominant
delay in packet times. Moreover, the storage time for the packet in
a conventional store and forward implementation also becomes a
significant component of the delay. Thus, for packet switching to
remain attractive in this environment, it appears necessary to
increase the size of packets (or switch on packet groups), do so-
called virtual cut-through and use high-speed routing techniques,
such as high-speed route caches and source routing.
Alternatively, for circuit switching to be attractive, it must
provide very fast circuit setup and tear-down to support the bursty
nature of most computer communication. This problem is rendered
difficult (and perhaps impossible for certain traffic loads) because
the delay across the country is so large relative to the data rate.
That is, even with techniques such as so-called fast select,
bandwidth is reserved by the circuit along the path for almost twice
the propagation time before being used.
With gigabit circuit switching, because it is not feasible to
physically switch channels, the low-level switching is likely doing
FTDM on micro-packets, as is currently done in telephony. Performing
FTDM at gigabit data rates is a challenging research problem if the
skew introduced by wide-area communication is to be handled with
reasonable overhead for spacing of this micro-packets. Given the
lead and resources of the telephone companies, this area of
investigation should, if pursued, be pursued cooperatively.
3.2.2. Bandwidth Reservation Algorithms
Some applications, such as real-time video, require sustained high
data rate streams over a significant period of time, such as minutes
if not hours. Intuitively, it is appealing for such applications to
pre-allocate the bandwidth they require to minimize the switching
load on the network and guarantee that the required bandwidth is
available. Research is required to determine the merits of bandwidth
reservation, particular in conjunction with the different switching
technologies. There is some concern to raise that bandwidth
reservation may require excessive intelligence in the network,
reducing the performance and reliability of the network. In
addition, bandwidth reservation opens a new option for denial of
service by an intruder or malicious user. Thus, investigations in
this area need to proceed in concert with work on switching
technologies and capabilities and security and reliability
requirements.
3.2.3. Multicast Capabilities
It is now widely accepted that multicast should be provided as a
user-level service, as described in RFC1054 for IP, for example.
However, further research is required to determine the best way to
support this facility at the network layer and lower. It is fairly
clear that the GN will be built from point-to-point fiber links that
do not provide multicast/broadcast for free. At the most
conservative extreme, one could provide no support and require that
each host or gateway simulate multicast by sending multiple,
individually addressed packets. However, there are significant
advantages to providing very low level multicast support (besides the
obvious performance advantages). For example, multicast routing in a
flooding form provides the most fault-tolerant, lowest-delay form of
delivery which, if reserved for very high priority messages, provides
a good emergency facility for high-stress network applications.
Multicast may also be useful as an approach to defeat traffic
analysis.
Another key issue arises with the distinction between so-called open
group multicast and closed group multicast. In the former, any host
can multicast to the group, whereas in the latter, only members of
the group can multicast to it. The latter is easier to support and
adequate for conferencing, for example. However, for more client-
server structured applications, such as using file/database server,
computation servers, etc. as groups, open multicast is required.
Research is needed to address both forms of multicast. In addition,
security issues arise in controlling the membership of multicast
groups. This issue should be addressed in concert with work on
secure forms of routing in general.
3.2.4. Gateway Technologies
With the wide-area interconnection of local networks by the GN,
gateways are expected to become a significant performance bottleneck
unless significant advances are made in gateway performance. In
addition, many network management concerns suggest putting more
functionality (such as access control) in the gateways, further
increasing their load and the need for greater capacity. This would
then raise the issue of the trade-off between general-purpose
hardware and special-purpose hardware.
On the general-purpose side, it may be feasible to use a general-
purpose multiprocessor based on high-end microprocessors (perhaps as
exotic as the GaAs MIPS) in conjunction with a high-speed block
transfer bus, as proposed as part of the FutureBus standard (which is
extendible to higher speeds than currently commercially planned) and
intelligent high-speed network adaptors. This would also allow the
direct use of hardware, operating systems, and software tools
developed as part of other DARPA programs, such as Strategic
Computing. It also appears to make this gateway software more
portable to commercial machines as they become available in this
performance range.
The specialized hardware approach is based on the assumption that
general-purpose hardware, particularly the interconnection bus,
cannot be fast enough to support the level of performance required.
The expected emphasis is on various interconnection network
techniques. These approaches appear to require greater expense, less
commercial availability and more specialized software. They need to
be critically evaluated with respect to the general-purpose gateway
hardware approach, especially if the latter is using multiple buses
for fault-tolerance as well as capacity extension (in the absence of
failure).
The same general-purpose vs. special-purpose contention is an issue
with operating system software. Conventionally, gateways run
specialized run-time executives that are designed specifically for
the gateway and gateway functions. However, the growing
sophistication of the gateway makes this approach less feasible. It
appears important to investigate the feasibility of using a standard
operating system foundation on the gateways that is known to provide
the required security and reliability properties (as well as real-
time performance properties).
3.2.5. VLSI and Optronics Implementations
It appears fairly clear that gigabit communication will use fiber
optics for at least the near future. Without major advances in
optronics to allow effectively for optical computers, communication
must cross the optical-electronic boundary two or more times. There
are significant cost, performance, reliability, and security benefits
for minimizing the number of such crossings. (As an example of a
security benefit, optics is not prone to electronic surveillance or
jamming while electronics clearly is, so replacing an optic-
electronic-optic node with a pure optic node eliminates that
vulnerability point.)
The benefits of improved technology in optronics is so great that its
application here is purely another motivation for an already active
research area (that deserves strong continued support). Therefore,
we focus here in the issue of matching current (and near-term
expected) optronics capabilities with network requirements.
The first and perhaps greatest area of opportunity is to achieve
totally (or largely) photonic switches in the network switching
nodes. That is, most packets would be switched without crossing the
optics-electronics boundary at all. For this to be feasible, the
switch must use very simple switching logic, require very little
storage and operate on packets of a significant size. The source-
routed packet switches with loopback on blockage of Blazenet
illustrate the type of techniques that appear required to achieve
this goal.
Research is required to investigate the feasibility of optronic
implementation of switches. It appears highly likely that networks
will at some point in the future be totally photonically switched,
having the impact on networking comparable to the effect of
integrated circuits on processors and memories.
A next level of focus is to achieve optical switching in the common
case in gateways. One model is a multiprocessor with an optical
interconnect. Packets associated with established paths through the
gateway are optically switched and processed through the
interconnect. Other packets are routed to the multiprocessor,
crossing into the electronics domain. Research is required to marry
the networking requirements and technology with optronics technology,
pushing the state of the art in both areas in the process.
Given the long-term presence of the optic-electronic boundary,
improvements in technology in this area are also important. However,
it appears that there is already enormous commercial research
activity in this area, particularly within the telephone companies.
This is another area in which collaborative investigation appears far
better than an new independent research effort.
VLSI technology is an established technology with active research
support. The GN effort does not appear to require major new
initiatives in the VLSI area, yet one should be open to significant
novel opportunities not identified here.
3.2.6. High-Speed Transfer Protocols
To achieve the desired speeds, it will be necessary to rethink the
form of protocols.
1. The simple idea of a stateless gateway must be replaced by a
more complex model in which the gateway understands the
desired function of the end point and applies suitable
optimizations to the flow.
2. If multiplexing is done in the time domain, the elements of
multiplexing are probably so small that no significant
processing can be performed on each individually. They must
be processed as an aggregate. This implies that the unit of
multiplexing is not the same as the unit of processing.
3. The interfaces between the structural layers of the
communication system must change from a simple
command/response style to a richer system which includes
indications and controls.
4. An approach must be developed that couples the memory
management in the host and the structure of the transmitted
data, to allow efficient transfers into host memory.
The result of rethinking these problems will be a new style of
communications and protocols, in which there is a much higher degree
of shared responsibility among the components (hosts, switches,
gateways). This may have little resemblance to previous work either
in the DARPA or commercial communities.
3.3. High-Speed Host Interfaces
As networks get faster, the most significant bottleneck will turn out
to be the packet processing overhead in the host. While this does
not restrict the aggregate rates we can achieve over trunks, it
prevents delivery of high data rate flows to the host-based
applications, which will prevent the development of new applications
needing high bandwidth. The host bottleneck is thus a serious
impediment to networked use of supercomputers.
To build a GN we need to create new ways for hosts and their high
bandwidth peripherals to connect to networks. We believe that
pursuing research in the ways to most effectively isolate host and
LAN development paths from the GN is the most productive way to
proceed. By decoupling the development paths, neither is restricted
by the momentary performance of capability bottlenecks of the other.
The best context in which to view this separation is with the notion
of a network front end (NFE). The NFE can take the electronic input
data at many data rates and transform it into gigabit light data
appropriately packetized to traverse the GN. The NFE can accept
inputs from many types of gateways, hosts, host peripherals, and LANS
and provide arbitration and path set-up facilities as needed. Most
importantly, the NFE can perform protocol arbitration to retain
upward compatibility with the existing Internet protocols while
enabling those sophisticated network input sources to execute GN
specific high-throughput protocols. Of course, this introduces the
need for research into high-speed NFEs to avoid the NFE becoming a
bottleneck.
3.3.1. VLSI and Optronics Implementations
In a host interface, unless the host is optical (an unlikely prospect
in the near-term), the opportunities for optronic support are
limited. In fact, with a serial-to-parallel conversion on reception
stepping the clock rate down by a factor of 32 (assuming a 32-bit
data path on the host interface), optronic speeds are not required in
the immediate future.
One exception may be for encryption. Current VLSI implementations of
standard encryption algorithms run in the 10 Mbit/s range. Optronic
implementation of these encryption techniques and encryption
techniques specifically oriented to, or taking advantage of, optronic
capabilities appears to be an area of some potential (and enormous
benefit if achieved).
The potential of targeted VLSI research in this area appears limited
for similar reasons discussed above with its application in high-
speed switching. The major benefits will arise from work that is
well-motivated by other research (such as high-performance
parallelism) and by strong commercial interest. Again, we need to be
open to imaginative opportunities not foreseen here while keeping
ourselves from being diverted into low-impact research without
further insights being put forward.
3.3.2. High-Performance Transport Protocols
Current transport protocols exhibit some severe problems for maximal
performance, especially for using hardware support. For example, TCP
places the checksum in the packet header, forcing the packet to be
formed and read fully before transmission begins. ISO TP4 is even
worse, locating the checksum in a variable portion of the header at
an indeterminate offset, making hardware implementation extremely
difficult.
The current Internet has thrived and grown due to the existence of
TCP implementations for a wide variety of classes of host computers.
These various TCP implementations achieve robust interoperability by
a "least common denominator" approach to features and options. Some
applications have arisen in the current Internet, and analogs can be
envisioned for the GN environment, which need qualities of service
not generally supported by the ubiquitous generic TCP, and therefore
special purpose transport protocols have been developed. Examples
include special purpose transport protocols such as UDP (user
datagram protocol), RDP (reliable datagram protocol), LDP
(loader/debugger protocol), NETBLT (high-speed block transfer
protocol), NVP (network voice protocol) and PVP (packet video
protocol). Efforts are also under way to develop a new generic
transport protocol VMTP (versatile message transaction protocol)
which will remedy some of deficiencies of TCP, without the need to
resort to special purpose protocols for some applications. Research
is needed in this area to understand how transport level protocols
should be constructed for a GN which provide adequate qualities of
service and ease of implementation.
A new transport protocol of reasonable success can be expected to
last for ten years more. Therefore, a new protocol should not be
over optimized for current networks and must not ignore the
functional deficiencies of current protocols. These deficiencies are
essential to remedy before it is feasible to deploy even current
distributed systems technology for military and commercial
applications.
Forward Error Correction (FEC) is a useful approach when the
bandwidth/delay ratio of the physical medium is high, as can be
expected in transcontinental photonic links. A degenerate form of
FEC is to simply transmit multiple copies of the data; this allows
one to trade bandwidth for delay and reliability, without requiring
much intelligence. In fact, it is generally true that reliability,
bandwidth, and delay are interrelated and an improvement in one
generally comes at the expense of the others for a given technology.
Research is required to find appropriate operating points in networks
using transmission components which offer extremely high bandwidth
with very good bit-error-rate performance.
3.3.3. Network Adaptors
With the promised speed of networks, the future network adaptor must
be viewed as a memory interconnect, tying the memory in one host to
another, at least if the data rate and the low latency made possible
by the network is to be realized at the host-to-host or process-to-
process level. The challenge is too great to be met by just
implementing protocols in custom VLSI.
Research is required to investigate the impact of network
interconnection on a machine architecture and to define and evaluate
new network adaptor architectures. Of key importance is integration
of network adaptor into the operating system so that process-to-
process communications performance matches that offered by the
network. In particular, we conjecture that the transport level will
be implemented largely, if not entirely, in the network adaptor,
providing the host with reliable memory-to-memory transfer at memory
speeds with a minimum of interrupt processing bus overhead and packet
processing.
Drawing an analogy to RISC technology again, maximal performance
requires a well-designed and coordinated protocol, software, and
hardware (network adaptor) design. Current standard protocols are
significantly flawed for hardware compatibility, suggesting a need
for considerable further research on high-performance protocol
design.
3.3.4. Host Operating System Software
Conventionally, communication has been an add-on to an operating
system. With the GN, the network may well become the fastest
"peripheral" connected to most nodes. High-performance process-to-
process (or application to application) communication will not be
achieved until the operating system is well designed for fast access
to and from the network. For example, incorporating templates of the
network packet header directly in the process descriptor may allow a
process to initiate communications with minimal overhead. Similarly,
memory mapping can be used to eliminate copies between data arriving
from the network and it being delivered to the applications. With a
GN, an extra copy forced by the operating system may easily double
the perceived transfer time for a packet between applications.
Besides matching data transfer mechanisms, operating systems must be
well-matched in security design to that supported by the host
interface and network as well. Otherwise, all but the most trivial
additional security actions by the operating system in common case
communication can easily eliminate the performance benefits of the
GN. For example, if the host has to do further encryption or
decryption, the throughput is likely to be at least halved and the
latency doubled.
Research effort is required to further refine operating systems for
the level of performance offered by the GN. This effort may well be
best realized with coupling existing efforts in distributed systems
with the GN activities, as opposed to starting new separate efforts.
3.4. Advanced Network Management Algorithms
An important emphasis for research into network management should be
on decentralized approaches. The ratio of propagation delay across
the country to data rates in a GN appear to be too great to deal
effectively with resource management centrally when traffic load is
bursty and unstable (and if it is not, one might argue there is no
problem). In addition, important principles of fault containment and
minimal privilege for reliability and security suggest that a
centralized management approach is infeasible. In particular,
compromising the security of one portion of the network should not
compromise the security of the whole network. Similarly, a failure
or fault should affect at most a local region of the network.
The challenge is clearly to provide decentralized management
techniques that lead to good global behavior in the normal case and
acceptable behavior in expected worst-case failures, traffic
variations and security intrusions.
3.4.1. Control Flow vs. Data Flow
Network operational communications can be separated into flow of user
data and flow of management/control data. However, the user data
must contain some amount of control data. One question that needs to
be explored in light of changes in communications and computing costs
and performance is the trade-off between these two flows. An example
of a potential approach is to use data units which contain predefined
path indicators. The switch can perform a simple table look-up which
maps the path indicator onto the preferred outbound link and
transmits the packet immediately. There is a path set-up packet
which fills in the appropriate tables. Path set-up occurs before the
first data packet flows and then, while data is flowing, to improve
the routes during the lifetime of the connection. This concept has
been discussed in the Internet engineering group under the name of
soft connections.
We note that separating the data flow from the control flow in the GN
has security and reliability advantages as well. We could encrypt
most of the packet header to provide confidentiality within the GN
and to limit the ability of intruders to perform traffic analysis.
And, by separating the control flow, we can encrypt all the control
exchanges between switches and the host front ends thereby offering
confidentiality and integrity. No unauthorized entity will be able
to alter or examine the control traffic. By employing a path set-up
procedure, we can assure that the GN NFE-to-NFE path is functioning
and also include user-specific requirements in the route. For
example, we could request a certain bandwidth allocation and simplify
the job of the switches in handling flow control. We could also set
up backup paths in case the output link will be busy for so many
microseconds that the packet cannot be stored until the link is
freed.
3.4.2. Resource Management Algorithms
Most current networks deliver one quality of service. X.25 networks
deliver a reliable byte-stream. Most LANs deliver a best-effort
unreliable service. There are few networks today that can support
multiple types of service, and allocate their resources among them.
Indeed, for many networks, such as best-effort unreliable service,
there is little management of resources at all. The next generation
of network will require a much more controlled allocation of
resources.
There will be a much wider range of desired types of service, with
current services such as remote procedure call mixing with new
services such as video streams. Unless these are separately
recognized and controlled, there is little reason to believe that
effective service can be delivered unless the network is very lightly
loaded.
In order to support multiple types of service, two things must
happen, both a change from current practice. First, the application
must describe to the network what type of service is required.
Second, the network must use this information to make resource
allocation decisions. Both of these practices present difficulties.
Past experience suggests that application code is not prepared to
know or specify what service it needs. By custom, operating systems
provide a virtual world, and the applications in this world are
unaware of the relation between this and the reality of time and
space. Resource requests must be in real terms. Allocation of
resources in the network is difficult, because it requires that
decisions be made in the network, but as network packet throughput
increases, there is less time for decisions.
The resolution of this latter conflict is to observe that decisions
must be made on larger units than the unit of multiplexing such as
the packet. This in turn implies that packets must be visible to the
network as being part of a sequence, as opposed to the pure datagram
model previously exploited. As suggested earlier in this report,
research is required to support this more complex form of switch
without compromising robustness.
To permit the application to specify the service it needs, it will be
necessary to propose some abstraction of service class. By clever
design of this abstraction, it should be possible to allow the
application to describe its needs effectively. For example, an
application such as file transfer or mail has two modes of operation;
bulk data transfer and remote procedure call. The application may
not be able to predict when it will be in which mode, but if it just
describes both of them, the system may be able to adapt by observing
its current operation.
Experimentation needs to be done to determine a suitable service
specification interface. This experimentation could be done in the
context of the current protocols, and could thus be undertaken at
once.
3.4.3. Adaptive Protocols
Network operating conditions can vary quickly and over a wide range.
This is true of the current Internet, and is likely to affect the GN
too. Protocols that can adapt to changing circumstances would
provide more even and robust service than is currently possible. For
example, when error rates increased, a protocol implementation might
decide to use smaller packets, thus reducing the burden caused by
retransmissions.
The environment in which a protocol operates can be described in
terms of the service it is getting from the next lower layer. A
protocol implementation can adapt to changes in that service by
tuning its internal mechanisms (time-outs, retransmission strategies,
etc.). Therefore, to design adaptive protocols, we must understand
the interaction between protocol layers and the mechanisms used
within them. There has been some work done in this area. For
example, the SATNET measurement task force has looked at the
interactions between the protocol used by the SIMP, IP, and TCP.
What is needed is a more complete characterization of the
interactions at various layer boundaries, and the development of
appropriate protocol designs and mechanisms to provide for necessary
adaptations and renegotiations.
3.4.4. Error Recovery Mechanisms
Being large and complex, GNs will experience a variety of faults such
as link or nodal failure, excessive buffer overflow due to faulty
flow and congestion control, and partial failure of switching fabric.
These failures, which also exist in today's networks, will have a
stronger effect in GNs where a large amount of data will be "stored"
in transit and, to expedite the switching, nodes will apply only
minimal processing to the packets traversing them. In source
routing, for example, a link failure may cause the loss of all
packets sent until the source is notified about the change in
topology. The longer is the delay in recovering from failures, the
higher is the degradation in performance observed by the users.
To minimize the effects of failures, GNs will need to employ error
recovery mechanisms whereby the network detects failures and error
conditions, reconfigures itself to adapt to the new network state,
and notifies peripheral devices of the new configuration. Such
protocols, which have to be developed, will respond quickly, will be
decentralized or distributed to minimize the possibility of fatal
failures, and will complement, rather than replicate, the error
correction mechanisms of the end-to-end protocols, and the two must
operate in coordinated manner. To this end, the peripheral devices
will have to be knowledgeable about the intranet recovery mechanisms
and interact continuously with them to minimize the effect on the
connections they manage.
3.4.5. Flow Control
As networks become faster, two related problems arise. First,
existing flow control mechanisms such as windows do not work well,
because the window must be opened to such an extent to achieve
desired bandwidth that effective flow control cannot be achieved.
Second, especially for long-haul networks, the larger number of bits
in transit at one time becomes so large that most computer messages
will fit into one window. This means that traditional congestion
control schemes will cease to work well.
What is needed is a combination of two approaches, both new. First,
for messages that are small (most messages generated by computers
today will be small, since they will fit into one round-trip time of
future networks), open-loop controls on flow and congestion are
needed. For longer messages (voice or video streams, for example),
some explicit resource commitment will be required.
3.4.6. Latency Control and Real-Time Operations
Currently, there are several distinct approaches to latency control.
First, there are some networks which are physically short, more like
multiprocessor buses. Applications in these networks are built
assuming that delays will be short.
Second, there are networks where the physical length is not
constrained by the design and may differ by orders of magnitude,
depending on the scope of the network. Most general purpose networks
fall in this category. In these networks, one of two things happens.
Either the application takes special steps to deal with variable
latency, such as echo suppression in voice networks, or these
applications are not supported.
For most applications today, the latency in the network is not an
obvious issue so long as the network is not overloaded (which leads
to losses and long queues), because the protocol overhead masks the
variation in the network latency. This balance will change. The
latency due to the speed of light will obviously remain the same, but
the overhead will drop (of necessity if we are to achieve high
performance) which will leave speed of light and queueing as the most
critical sources of delay.
This conclusion implies that if queueing delay can be controlled, it
will be possible to build networks with stable and controlled
latency. If applications exist that require this class of service,
it can be supported. Either the network must be underloaded, so that
queues do not develop at all, or a specific class of service must be
supported in which resources are allocated to stabilize the delay.
If this service is provided, it will still leave the application with
delays that can vary by several orders of magnitude, depending on the
physical size of the network. Research at the application level will
be required to see how applications can be designed to cope with this
variation.
3.4.7. High-Speed Internetworking and Administrational Domains
Internetworking recognized that the value of communication services
increases significantly with wider interconnection but ignored
management and the role of administrations. As a consequence we see
that:
1. The Internet is more or less unmanageable, as evidenced by
performance, reliability, and security problems.
2. The Internet is being stressed by administrators that are
building networks to match their organization rather than the
geography. An example is a set of Ethernets at different
company locations operating as a single Internet network but
geographically dispersed and connected by satellite or leased
lines.
The next generation of internetworking must focus on administration
and management. Internetworking must support cohesion within an
administration and a healthy separation between administrations. To
illustrate by analogy, the American and Soviet embassies in Mexico
City are geographically closer to each other than to their respective
home countries but further in administrational distance, including
security, accounting, etc. The emerging revolution in WANs makes
this issue that much more critical. The amount of communication to
exchange the state of systems is bound to increase enormously. The
potential cost of failures and security violations is frightening.
A promising approach appears to be high-level gateways that guard
between administrations and require negotiations to set up access
paths between administrations. These paths are set up, and labeled
with agreements on authorization, security, accounting, and possible
resource limits. These administrative virtual circuits provide
transparency to the physical and geographical interconnection, but
need not support more than datagram packet delivery. One view is
that of communication contracts with high-level gateways acting as
contract monitors at each end. The key is the focus on controlled
interadministrational connectivity, not the conventional protocol
concerns.
Focus is required on developing an (inter)network management
architecture and the specifics of high-level gateways. The
structures of such gateways will have to take advantage of advances
in multi-processor architectures to handle the processing load.
Moreover, a key issue is being able to optimize communication between
administrations once the contract is in place, but without losing
control. Related is the issue of allowing high-speed interconnection
within a single administration, although geographical dispersed.
Another issue is fault-tolerance. High-level gateways contain state
information whose loss typically disrupts communication. How does
one minimize this problem?
A key goal of these administrational gateways has to be failure
containment: How to protect against external (to administration)
problems and how to prevent local problems imposing liability on
others. A particular area of concern is the self-organizing problems
of large-scale systems, observed by Van Jacobson in the Internet.
Gateways must serve to damp out oscillations and control wide load
swings. Rate control appears to be a key area to investigate as a
basis for buffer management and for congestion control, as well as to
control offered load.
Given the speed of new networks, and the sophistication of the
gateways suggested above, another key area to investigate is the
provision of high-speed network interface adaptors.
3.4.8. Policy-Based Algorithms
Networks of today generally select routes based on minimizing some
measure such as delay. However, in the real world, route selection
will commonly be constrained at the global level by policy issues,
such as access rights to resources and accounting and billing for
usage.
It is difficult for connectionless protocols such as Internet to deal
with policy controls, because a lack of state in the gateway implies
that a separate policy decision must be made for each packet in
isolation. As networks get faster, the cost of this processing will
be intolerable. One possible approach, discussed above, is to move
to a more sophisticated model in which there is knowledge in the
gateways of the ongoing flows. Alternatively, it may be possible to
design gateways that simply cache recent policy evaluations and apply
them to successive packets.
Routing based on policy is particularly difficult because a route
must be globally consistent to be useful; otherwise it may loop.
This implies that the every policy decision must be propagated
globally. Since there can be expected to be a large number of
policies, this global passing of information might easily lead to an
information explosion.
There are at least two solutions. One is to restrict the possible
classes of policy. Another is to use some form of source route, so
that the route consistent with some set of policies is computed at
one point only, and then attached to the packet. Both of these
approaches have problems. A two-pronged research program is needed,
in which mechanisms are proposed, and at the same time the needed
policies are defined.
The same trade-off can be seen for accounting and billing. A single
accounting metric, such as "bytes times distance", could be proposed.
This might be somewhat simple to implement, but would not permit the
definition of individual billing policies, as is now done in the
parts of the telephone system. The current connectionless transport
architectures such as TCP/IP or the connectionless ISO configuration
using TP4 do not have good tools for accounting for traffic, or for
restricting traffic from certain resources. Building these tools is
difficult in a connectionless environment, because an accounting or
control facility must deal with each packet in isolation, which
implies a significant processing burden as part of packet forwarding.
This burden is an increasing problem as switches are expected to
operate faster.
The lack of these tools is proving a significant problem for network
design. Not only are accounting and control needed to support
management requirements, they are needed as a building block to
support enforcement of such things as multiple qualities of service,
as discussed above.
Network accounting is generally considered to be simply a step that
leads to billing, and thus is often evaluated in terms of how simple
or difficult it will be to implement. Yet an accounting and billing
procedure is a mechanism for implementing a policy considered to be
desirable for reasons beyond the scope of accounting per se. For
example, a policy might be established either to encourage or
discourage network use, while fully recovering operational cost. A
policy of encouraging use could be implemented by a relatively high
monthly attachment charge and a relatively low per-packet charge. A
policy of discouraging use could be implemented by a low monthly
charge and a high per-packet charge.
Network administrators have a relatively small number of variables
with which to implement policy objectives. Nevertheless, these
variables can be combined in a number of innovative ways. Some of
the possibilities include:
1. Classes of users (e.g., large or small institutions, for-
profit or non-profit).
2. Classes of service.
3. Time varying (e.g., peak and off-peak).
4. Volume (e.g., volume discounts, or volume surcharges).
5. Access charges (e.g., per port, or port * [bandwidth of
port]).
6. Distance (e.g., circuit-miles, airline miles, number of hops).
Generally, an accounting procedure can be developed to support
voluntary user cooperation with almost any single policy objective.
Difficulties most often arise when there are multiple competing
policy objectives, or when there is no clear policy at all.
Another aspect of accounting and billing procedures which must be
carefully considered is the cost of accumulating and processing the
data on which billing is based. Of particular concern is collection
of detailed data on a per-packet basis. As network circuit data
rates increase, the number of instructions which must be executed on
a per-packet basis can become the limiting factor in system
throughput. Thus, it may be appropriate to prefer accounting and
billing policies and procedures which minimize the difficulty of
collecting data, even if this approach requires a compromise of other
objectives. Similarly, node memory required for data collection and
any network bandwidth required for transmission of the data to
administrative headquarters are factors which must be traded off
against the need to process user packets.
3.4.9. Priority and Preemption
The GN should support multiple levels of priority for traffic and the
preemption of network resources for higher priority use. Network
control traffic should be given the highest priority to ensure that
it is able to pass through the network unimpeded by congestion caused
by user-level traffic. There may be additional military uses for
multiple levels of priority which correspond to rank or level of
importance of a user or the mission criticality of some particular
data.
The use of and existence of priority levels may be different for
different types of traffic. For example, datagram traffic may not
have multiple priority levels. Because the network's transmission
speed is so high and traffic bursts may be short, it may not make
sense to do any processing in the switches to deal with different
priority levels. Priority will be more important for flow- (or
soft-connection-) oriented data or hard connections in terms of
permitting higher priority connections to be set up ahead of lower
priority connections. Preemption will permit requests for high
priority connections to reclaim network resources currently in use by
lower priority traffic.
Networks such as the Wideband Satellite Network, which supports
datagram and stream traffic, implement four priority levels for
traffic with the highest reserved for network control functions and
the other three for user traffic. The Wideband Network supports
preemption of lower priority stream allocations by higher priority
requests. An important component of the use of priority and
preemption is the ability to notify users when requests for service
have been denied, or allocations have been modified or disrupted.
Such mechanisms have been implemented in the Wideband Network for
streams and dynamic multicast groups.
Priority and preemption mechanisms for a GN will have to be
implemented in an extremely simple way so that they can take effect
very quickly. It is likely that they will have to built into the
hardware of the switch fabric.
3.5. User and Network Services
As discussed in Section 2 above, there will need to be certain
services provided as part of the network operation to the users
(people) themselves and to the machines that connect to the network.
These services, which include such capabilities as white and yellow
pages (allowing users to determine what the appropriate network
identification is for other users and for network-available computing
resources) and distributed fault identification and isolation, are
needed in current networks and will continue to be required in the
networks of the future. The speed of the GN will serve to accentuate
this requirement, but at the same time will allow for new
architectures to be put in place for such services. For example,
Ethernet speeds in the local environment have allowed for more usable
services to be provided.
3.5.1. Impact of High Bandwidth
One issue that will need to be addressed is the impact on the user of
such high-bandwidth capabilities. Users are already becoming
saturated by information in the modern information-rich environment.
(Many of us receive more than 50 electronic mail messages each day,
each requiring some degree of human attention.) Methods will be
needed to allow users to cope with this ever-expanding access to
data, or we will run the risk of users turning back to the relative
peace and quiet of the isolated Office.
3.5.2. Distributed Network Directory
A distributed network directory can support the user-level directory
services and the lower-level name-to-address mapping services
described elsewhere in this report. It can also support distributed
systems and network management facilities by storing additional
information about named objects. For example, the network directory
might store node configurations or security levels.
Distributing the directory eases and decentralizes the administrative
burdens and provides a more robust and survivable implementation.
One approach toward implementing a distributed network directory
would be to base it upon the CCITT X.500/ISO DIS 9594 standard. This
avoids starting from ground zero and has the advantage of
facilitating interoperability with other communications networks.
However, research and development will be required even if this path
is chosen.
One area in which research and development are required is in the
services supplied by the distributed network directory. The X.500
standard is very general and powerful, but so far specific provisions
have been made only for storing information about network users and
applications. As mentioned elsewhere, multilevel security is not
addressed by X.500, and the approach taken toward authentication must
be carefully considered in view of DoD requirements. Also, X.500
assumes that administration of the directory will be done locally and
without the need for standardization; this may not be true of GN or
the larger national research network.
The model and algorithms used by a distributed network directory
constitute a second area of research. The model specified by X.500
must be extended into a framework that provides the necessary
flexibility in terms of services, responsiveness, data management
policies, and protocol layer utilization. Furthermore, the internal
algorithms and mechanisms of X.500 must be extended in a number of
areas; for example, to support redundancy of the X.500 database,
internal consistency checking, fuller sharing of information about
the distribution of data, and defined access-control mechanisms.
4. Avenues of Approach
Ongoing research and commercial activities provide an opportunity for
more rapidly attacking some of the above research issues. At the
same time, there needs to be attention paid to the overall technical
approach used to allow multiple potential solutions to be explored
and allow issues to be attacked in parallel.
4.1. Small Prototype vs. Nationwide Network
The central question is how far to jump, and how far can the current
approaches get. That is, how far will connectionless network service
get us, how far will packet switching get us, and how far do we want
to go. If our goal is a Gbit/s net, then that is what we should
build. Building a 100 Mbit/s network to achieve a GN is analogous to
climbing a tree to get to the moon. It may get you closer, but it
will never get you there.
There are currently some network designs which can serve as the basis
for a GN prototype. The next step is some work by experts in
photonics and possibly high-speed electronics to explore ease of
implementation. Developing a prototype 3-5 node network at a Gbit/s
data rate is realistic at this point and would demonstrate wide-area
(40 km or more) Gbit/s networking.
DARPA should consider installing a Gbit/s cross-country set of
connected links analogous to the NSF backbone in 2 years. A Gbit/s
link between the east and west coasts would open up a whole new
generation of (C3I), distributed computing, and parallel computing
research possibilities and would reestablish DARPA as the premier
network research funding agency in the country. This will require
getting "dark" fiber from one or more of the common carriers and some
collaboration with these organizations on repeaters, etc. With this
collaboration, the time to a commercial network in the Gbit/s range
would be substantially reduced, and the resulting nationwide GN would
give the United States an enormous technical and economic advantage
over countries without it.
Demonstrating a high-bandwidth WAN is not enough, however. As one
can see from the many research issues identified above, it will be
necessary to pursue via study and experiment the issues involved in
interconnecting high-bandwidth networks into a high-bandwidth
internet. These experiments can be done through use of a new
generation of internet, even if it requires starting at lower speeds
(e.g., T1 through 100 Mbit/s). Appropriate care must be given,
however, to assure that the capabilities that are demonstrated are
applicable to the higher bandwidths (Gbit/s) as they emerge.
4.2. Need for Parallel Efforts/Approaches
Parallel efforts will therefore be required for two major reasons.
First is the need to pursue alternative approaches (e.g., different
strategies for high-bandwidth switching, different addressing
techniques, etc). This is the case for most research programs, but
it is made more difficult here by the costs of prototyping. Thus, it
is necessary that appropriate review take place in the decisions as
to which efforts are supported through prototyping.
In addition, it will be necessary to pursue the different aspects of
the program in parallel. It will not be possible to wait until the
high-bandwidth network is available before starting on prototyping
the high-bandwidth internet. Thus, a phased and evolutionary
approach will be needed.
4.3. Collaboration with Common Carriers
Computer communication networks in the United States today
practically ignore the STN (the Switched Telephone Network), except
for buying raw bandwidth through it. However, advances in network
performance are based on improvements in the underlying communication
media, including satellite communication, fiber optics, and photonic
switching.
In the past we used "their" transmission under "our" switching. An
alternative approach is to utilize the common-carrier switching
capabilities as an integral part of the networking architecture. We
must take an objective scientific and economic look and reevaluate
this question.
Another place for cooperation with the common carriers is in the area
of network addressing. Their addressing scheme ("numbering plan")
has a few advantages such as proven service to 300 million users [4].
On the other hand, the common carriers have far fewer administrative
domains (area codes) than the current plethora of locally
administered local area networks in the internet system.
It is likely that future networks will eventually be managed and
operated by commercial communications providers. A way to maximize
technology transfer from the research discussed here to the
marketplace is to involve the potential carriers from the start.
However, it is not clear that the goals of commercial communications
providers, who have typically been most interested in meeting the
needs of 90+ percent of the user base, will be compatible with the
goals of the research described here. Thus, while we recommend that
the research program involve an appropriate amalgam of academia and
industry, paying particular attention to involvement of the potential
system developers and operators, we also caution that the specific
and unique goals of the DARPA program must be retained.
4.4. Technology Transfer
As we said above, it is our belief that future networks will
ultimately be managed and operated by commercial communications
providers. (Note that this may not be the common carriers as we know
them today, but may be value-added networks using common carrier
facilities.) The way to assure technology transfer, in our belief, is
to involve the potential system developers from the start. We
therefore believe that the research program would benefit from an
appropriate amalgam of university and industry, with provision for
close involvement of the potential system developers and operators.
4.5. Standards
The Internet program was a tremendous success in influencing national
and international standards. While there were changes to the
protocols, the underlying technology and approaches used by CCITT and
ISO in the standardization of packet-switched networks clearly had
its roots in the DARPA internet. Nevertheless, this has had some
negative impact on the research program, as the evolution of the
standards led to pressure to adopt them in the research environment.
Thus, it appears that there is a "catch-22" here. It is desirable
for the technology base developed in the research program to have
maximal impact on the standards activities. This is expedited by
doing the research in the context of the standards environment.
However, standards by their very nature will always lag behind the
research environment.
The only reasonable approach, therefore, appears to be an occasional
"checkpointing" of the research environment, where the required
conversions take place to allow a new plateau of standards to be used
for future evolution and research. A good example is conducting
future research in mail using X.400 and X.500 where possible.
5. Conclusions
We hope that this document has provided a useful compendium of those
research issues critical to achieving the FCCSET phase III
recommendations. These problems interact in a complex way. If the
only goal of a new network architecture was high speed, reasonable
solutions would not be difficult to propose. But if one must achieve
higher speeds while supporting multiple services, and at the same
time support the establishment of these services across
administrative boundaries, so that policy concerns (e.g., access
control) must be enforced, the interactions become complex.
APPENDIX
A. Current R and D Activities
In this appendix, we provide pointers to some ongoing activities in
the research and development community of which the group was aware
relevant to the goal of achieving the GN. In some cases, a short
abstract is provided of the research. Neither the order of the
listing (which is random) nor the amount of detail provided is meant
to indicate in any way the significance of the activity. We hope
that this set of pointers will be useful to anyone who chooses to
pursue the research issues discussed in this report.
1. Grumman (at Bethpage) is working on a three-year DARPA
contract, started in January 1988 to develop a 1.6 Gbit/s LAN,
for use on a plane or ship, or as a "building block". It is
really raw transport capacity running on two fibers in a
token-ring like mode. First milestone (after one year?) is to
be a 100 Mbit/s demonstration.
2. BBN Laboratories, as part of its current three-year DARPA
Network-Oriented Systems contract, has proposed design
concepts for a 10-100 Gbit/s wide area network. Work under
this effort will include wavelength division multiplexing,
photonic switching, self-routing packets, and protocol design.
3. Cheriton (Stanford) research on Blazenet, a high-bandwidth
network using photonic switching.
4. Acampora (Bell Labs) research on the use of wavelength
division multiplexing for building a shared optical network.
5. Yeh is reserching a VLSI approach to building high-bandwidth
parallel processing packet switch.
6. Bell Labs is working on a Metropolitan Area Network called
"Manhattan Street Net." This work, under Dr. Maxemchuck, is
similar to Blazenet. It is in the prototype stage for a small
number of street intersections; ultimately it is meant to be
city-wide. Like Blazenet, is uses photonic switching 2 x 2
lithium niobate block switches.
7. Ultra Network Technologies is a Silicon Valley company which
has a (prototype) Gbit/s fiber link which connects backplanes.
This is based on the ISO-TP4 transport protocol.
8. Jonathan Turner, Washington University, is working on a
Batcher-Banyan Multicast Net, based on the "SONET" concept,
which provides 150 Mbit/s per pipe.
9. David Sincowskie, Bellcore, is working with Batcher-Banyan
design and has working 32x32 switches.
10. Stratacom has a commercial product which is really a T1 voice
switch implemented internally by a packet switch, where the
packet is 192 bits (T1 frame). This switch can pass 10,000
packets per second.
11. Stanford NAB provides 30-50 Mbit/s throughput on 100 Mbit/s
connection using Versatile Message Transaction Protocol (VMTP)
[see RFC1045]
12. The December issue of IEEE Journal on Selected Areas in
Communications, provides much detail concerning interconnects.
13. Ultranet Technology has a 480 Mbit/s connection using modified
ISO TP4.
14. At MIT, Dave Clark has an architecture proposal of interest.
15. At CMU, the work of Eric Cooper is relevant.
16. At Protocol Engines, Inc., Greg Chesson is working on an XTP-
based system.
17. Larry Landweber at Wisconsin University is doing relevant
work.
18. Honeywell is doing relevant work for NASA.
19. Kung at CMU is working on a system called "Nectar" based on a
STARLAN on fiber connecting dissimilar processors.
20. Burroughs (now Unisys) has some relevant work within the IEEE
802.6 committee.
21. Bellcore work in "Switched Multimedia Datanet Service" (SMDS)
is relevant (see paper supplied by Dave Clark).
22. FDDI-2, a scheme for making TDMA channel allocations at 200
Mbit/s.
23. NRI, Kahn-Farber Proposal to NSF, is a paper design for high-
bandwidth network.
24. Barry Goldstein work, IBM-Yorktown.
25. Bell Labs S-Net, 1280 Mbit/s prototype.
26. Fiber-LAN owned by Bell South and SECOR, a pre-prototype 575
Mbit/s Metro Area Net.
27. Bellcore chip implementation of FASTNET (1.2 Gbit/s).
28. Scientific Computer Systems, San Diego, 1.4 Gbit/s prototype.
29. BBN Monarch Switch, Space Division pre-prototype, chips being
fabricated, 64 Mbit/s per path.
30. Proteon, 80 Mbit/s token ring.
31. Toronto University, 150 Mbit/s "tree"--- really a LAN.
32. NSC Hyperchannel II, reputedly available at 250 Mbit/s.
33. Tobagi at Stanford working on EXPRESSNET; not commercially
available.
34. Columbia MAGNET-- 150 Mbit/s.
35. Versatile Message Transaction Protocol (VMTP).
36. ST integrated with IP.
37. XTP (Chesson).
38. Stanford Transport Gateway.
39. X.25/X.75.
40. Work of the Internet Activities Board.
B. Gigabit Working Group Members
Member Affiliation
Gordon Bell Ardent Computers
Steve Blumenthal BBN Laboratories
Vint Cerf Corporation for National Research Initiatives
David Cheriton Stanford University
David Clark Massachusetts Institute of Technology
Barry Leiner (Chairman) Research Institute for Advanced Computer Science
Robert Lyons Defense Communication Agency
Richard Metzger Rome Air Development Center
David Mills University of Delaware
Kevin Mills National Bureau of Standards
Chris Perry MITRE
Jon Postel USC Information Sciences Institute
Nachum Shacham SRI International
Fouad Tobagi Stanford University
End Notes
[1] Workshop on Computer Networks, 17-19 February 1987, San Diego,
CA.
[2] "A Report to the Congress on Computer Networks to Support
Research in the United States: A Study of Critical Problems and
Future Options", White House Office of Scientific and Technical
Policy (OSTP), November 1987.
[3] We distinguish in the report between development of a backbone
network providing gigabit capacity, the GB, and an
interconnected set of high-speed networks providing high-
bandwidth service to the user, the Gigabit Network (GN).
[4] Incidentally, they already manage to serve 150 million
subscribers in an 11-digit address-space (about 1:600 ratio).
We have a 9.6-digit address-space and are running into troubles
with much less than 100,000 users (less than 1:30,000 ratio).
