Network Working Group C. Partridge
Request for Comments: 1152 BBN Systems and Technologies
April 1990
Workshop Report
Internet Research Steering Group Workshop on
Very-High-Speed Networks
Status of this Memo
This memo is a report on a workshop sponsored by the Internet
Research Steering Group. This memo is for information only. This
RFC does not specify an Internet standard. Distribution of this memo
is unlimited.
Introduction
The goal of the workshop was to gather together a small number of
leading researchers on high-speed networks in an environment
conducive to lively thinking. The hope is that by having such a
workshop the IRSG has helped to stimulate new or improved research in
the area of high-speed networks.
Attendance at the workshop was limited to fifty people, and attendees
had to apply to get in. Applications were reviewed by a program
committee, which accepted about half of them. A few key individuals
were invited directly by the program committee, without application.
The workshop was organized by Dave Clark and Craig Partridge.
This workshop report is derived from session writeups by each of the
session chairman, which were then reviewed by the workshop
participants.
Session 1: Protocol Implementation (David D. Clark, Chair)
This session was concerned with what changes might be required in
protocols in order to achieve very high-speed operation.
The session was introduced by David Clark (MIT LCS), who claimed that
existing protocols would be sufficient to go at a gigabit per second,
if that were the only goal. In fact, proposals for high-speed
networks usually include other requirements as well, such as going
long distances, supporting many users, supporting new services such
as reserved bandwidth, and so on. Only by examining the detailed
requirements can one understand and compare various proposals for
protocols. A variety of techniques have been proposed to permit
protocols to operate at high speeds, ranging from clever
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implementation to complete relayering of function. Clark asserted
that currently even the basic problem to be solved is not clear, let
alone the proper approach to the solution.
Mats Bjorkman (Uppsala University) described a project that involved
the use of an outboard protocol processor to support high-speed
operation. He asserted that his approach would permit accelerated
processing of steady-state sequences of packets. Van Jacobson (LBL)
reported results that suggest that existing protocols can operate at
high speeds without the need for outboard processors. He also argued
that resource reservation can be integrated into a connectionless
protocol such as IP without losing the essence of the connectionless
architecture. This is in contrast to a more commonly held belief
that full connection setup will be necessary in order to support
resource reservation. Jacobson said that he has an experimental IP
gateway that supports resource reservation for specific packet
sequences today.
Dave Borman (Cray Research) described high-speed execution of TCP on
a Cray, where the overhead is most probably the system and I/O
architecture rather than the protocol. He believes that protocols
such as TCP would be suitable for high-speed operation if the windows
and sequence spaces were large enough. He reported that the current
speed of a TCP transfer between the processors of a Cray Y-MP was
over 500 Mbps. Jon Crowcroft (University College London) described
the current network projects at UCL. He offered a speculation that
congestion could be managed in very high-speed networks by returning
to the sender any packets for which transmission capacity was not
available.
Dave Feldmeier (Bellcore) reported on the Bellcore participation in
the Aurora project, a joint experiment of Bellcore, IBM, MIT, and
UPenn, which has the goal of installing and evaluating two sorts of
switches at gigabit speeds between those four sites. Bellcore is
interested in switch and protocol design, and Feldmeier and his group
are designing and implementing a 1 Gbps transport protocol and
network interface. The protocol processor will have special support
for such things as forward error correction to deal with ATM cell
loss in VLSI; a new FEC code and chip design have been developed to
run at 1 Gbps.
Because of the large number of speakers, there was no general
discussion after this session.
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Session 2: High-Speed Applications (Keith Lantz, Chair)
This session focused on applications and the requirements they impose
on the underlying networks. Keith Lantz (Olivetti Research
California) opened by introducing the concept of the portable office
- a world where a user is able to take her work with her wherever she
goes. In such an office a worker can access the same services and
the same people regardless of whether she is in the same building
with those services and people, at home, or at a distant site (such
as a hotel) - or whether she is equipped with a highly portable,
multi-media workstation, which she can literally carry with her
wherever she goes. Thus, portable should be interpreted as referring
to portability of access to services rather than to portability of
hardware. Although not coordinated in advance, each of the
presentations in this session can be viewed as a perspective on the
portable office.
The bulk of Lantz's talk focused on desktop teleconferencing - the
integration of traditional audio/video teleconferencing technologies
with workstation-based network computing so as to enable
geographically distributed individuals to collaborate, in real time,
using multiple media (in particular, text, graphics, facsimile,
audio, and video) and all available computer-based tools, from their
respective locales (i.e., office, home, or hotel). Such a facility
places severe requirements on the underlying network. Specifically,
it requires support for several data streams with widely varying
bandwidths (from a few Kbps to 1 Gbps) but generally low delay, some
with minimal jitter (i.e., isochronous), and all synchronized with
each other (i.e., multi-channel or media synchronization). It
appears that high-speed network researchers are paying insufficient
attention to the last point, in particular. For example, the bulk of
the research on ATM has assumed that channels have independent
connection request and burst statistics; this is clearly not the case
in the context of desktop teleconferencing.
Lantz also stressed the need for adaptive protocols, to accommodate
situations where the capacity of the network is exceeded, or where it
is necessary to interoperate with low-speed networks, or where human
factors suggest that the quality of service should change (e.g.,
increasing or decreasing the resolution of a video image). Employing
adaptive protocols suggests, first, that the interface to the network
protocols must be hardware-independent and based only on quality of
service. Second, a variety of code conversion services should be
available, for example, to convert from one audio encoding scheme to
another. Promising examples of adaptive protocols in the video
domain include variable-rate constant-quality coding, layered or
embedded coding, progressive transmission, and (most recently, at
UC-Berkeley) the extension of the concepts of structured graphics to
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video, such that the component elements of the video image are kept
logically separate throughout the production-to-presentation cycle.
Charlie Catlett (National Center for Supercomputing Applications)
continued by analyzing a specific scientific application, simulation
of a thunderstorm, with respect to its network requirements. The
application was analyzed from the standpoint of identifying data flow
and the interrelationships between the computational algorithms, the
supercomputer CPU throughput, the nature and size of the data set,
and the available network services (throughput, delay, etc).
Simulation and the visualization of results typically involves
several steps:
1. Simulation
2. Tessellation (transform simulation data into three-dimensional
geometric volume descriptions, or polygons)
3. Rendering (transform polygons into raster image)
For the thunderstorm simulation, the simulation and tessellation are
currently done using a Cray supercomputer and the resulting polygons
are sent to a Silicon Graphics workstation to be rendered and
displayed. The simulation creates data at a rate of between 32 and
128 Mbps (depending on the number of Cray-2 processors working on the
simulation) and the tessellation output data rate is in typically in
the range of 10 to 100 Mbps, varying with the complexity of the
visualization techniques. The SGI workstation can display 100,000
polygons/sec which for this example translates to up to 10
frames/sec. Analysis tools such as tracer particles and two-
dimensional slices are used interactively at the workstation with
pre-calculated polygon sets.
In the next two to three years, supercomputer speeds of 10-30 GFLOPS
and workstation speeds of up to 1 GFLOPS and 1 million
polygons/second display are projected to be available. Increased
supercomputer power will yield a simulation data creation rate of up
to several Gbps for this application. The increased workstation
power will allow both tessellation and rendering to be done at the
workstation. The use of shared window systems will allow multiple
researchers on the network to collaborate on a simulation, with the
possibility of each scientist using his or her own visualization
techniques with the tessellation process running on his or her
workstation. Further developments, such as network virtual memory,
will allow the tessellation processes on the workstations to access
variables directly in supercomputer memory.
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Terry Crowley (BBN Systems and Technologies) continued the theme of
collaboration, in the context of real-time video and audio, shared
multimedia workspaces, multimedia and video mail, distributed file
systems, scientific visualization, network access to video and image
information, transaction processing systems, and transferring data
and computational results between workstations and supercomputers.
In general, such applications could help groups collaborate by
directly providing communication channels (real-time video, shared
multimedia workspaces), by improving and expanding on the kinds of
information that can be shared (multimedia and video mail,
supercomputer data and results), and by reducing replication and the
complexity of sharing (distributed file systems, network access to
video and image information).
Actual usage patterns for these applications are hard to predict in
advance. For example, real-time video might be used for group
conferencing, for video phone calls, for walking down the hall, or
for providing a long-term shared viewport between remote locations in
order to help establish community ties. Two characteristics of
network traffic that we can expect are the need to provide multiple
data streams to the end user and the need to synchronize these
streams. These data streams will include real-time video, access to
stored video, shared multimedia workspaces, and access to other
multimedia data. A presentation involving multiple data streams must
be synchronized in order to maintain cross-references between them
(e.g., pointing actions within the shared multimedia workspace that
are combined with a voice request to delete this and save that).
While much traffic will be point-to-point, a significant amount of
traffic will involve conferences between multiple sites. A protocol
providing a multicast capability is critical.
Finally, Greg Watson (HP) presented an overview of ongoing work at
the Hewlett-Packard Bristol lab. Their belief is that, while
applications for high-speed networks employing supercomputers are the
the technology drivers, the economic drivers will be applications
requiring moderate bandwidth (say 10 Mbps) that are used by everyone
on the network.
They are investigating how multimedia workstations can assist
distributed research teams - small teams of people who are
geographically dispersed and who need to work closely on some area of
research. Each workstation provides multiple video channels,
together with some distributed applications running on personal
computers. The bandwidth requirements per workstation are about 40
Mbps, assuming a certain degree of compression of the video channels.
Currently the video is distributed as an analog signal over CATV
equipment. Ideally it would all be carried over a single, unified
wide-area network operating in the one-to-several Gbps range.
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They have constructed a gigabit network prototype and are currently
experimenting with uncompressed video carried over the same network
as normal data traffic.
Session 3: Lightwave Technology and its Implications (Ira Richer, Chair)
Bob Kennedy (MIT) opened the session with a talk on network design in
an era of excess bandwidth. Kennedy's research is focused on multi-
purpose networks in which bandwidth is not a scarce commodity,
networks with bandwidths of tens of terahertz. Kennedy points out
that a key challenge in such networks is that electronics cannot keep
up with fiber speeds. He proposes that we consider all-optical
networks (in which all signals are optical) with optoelectronic nodes
or gateways capable of recognizing and capturing only traffic
destined for them, using time, frequency, or code divisions of the
huge bandwidth. The routing algorithms in such networks would be
extremely simple to avoid having to convert fiber-optics into slower
electronic pathways to do switching.
Rich Gitlin (AT&T Bell Labs) gave a talk on issues and opportunities
in broadband telecommunications networks, with emphasis on the role
of fiber optic and photonic technology. A three-level architecture
for a broadband telecommunications network was presented. The
network is B-ISDN/ATM 150 (Mbps) based and consists of: customer
premises equipment (PBXs, LANs, multimedia terminals) that access the
network via a router/gateway, a Network Node (which is a high
performance ATM packet switch) that serves both as a LAN-to-LAN
interconnect and as a packet concentrator for traffic destined for
CPE attached to other Network Nodes, and a backbone layer that
interconnects the NODES via a Digital Cross-Connect System that
provide reconfigurable SONET circuits between the NODES (the use of
circuits minizes delay and avoids the need for implementation of
peak-transmission-rate packet switching). Within this framework, the
most likely places for near-term application of photonics, apart from
pure transport (ie, 150 Mbps channels in a 2.4 Gbps SONET system),
are in the Cross-Connect (a Wavelength Division Multiplexed based
structure was described) and in next-generation LANs that provide
Gigabit per second throughputs by use of multiple fibers, concurrent
transmission, and new access mechanisms (such as store and forward).
A planned interlocation Bell Labs multimedia gigabit/sec research
network, LuckyNet, was described that attempts to extend many of the
above concepts to achieve its principal goals: provision of a gigabit
per second capability to a heterogeneous user community, the
stimulation of applications that require Gpbs throughput (initial
applications are video conferencing and LAN interconnect), and, to
the extent possible, be based on standards so that interconnection
with other Gigabit testbeds is possible.
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Session 4: High Speed Networks and the Phone System
(David Tennenhouse, Chair)
David Tennenhouse (MIT) reported on the ATM workshop he hosted the
two days previous to this workshop. His report will appear as part
of the proceedings of his workshop.
Wally St. John (LANL) followed with a presentation on the Los Alamos
gigabit testbed. This testbed is based on the High Performance
Parallel Interface (HPPI) and on crossbar switch technology. LANL
has designed its own 16x16 crossbar switch and has also evaluated the
Network Systems 8x8 crossbar switch. Future plans for the network
include expansion to the CASA gigabit testbed. The remote sites (San
Diego Supercomputer Center, Caltech, and JPL) are configured
similarly to the LANL testbed. The long-haul interface is from HPPI
to/from SONET (using ATM if in time).
Wally also discussed some of the problems related to building a
HPPI-SONET gateway:
a) Flow control. The HPPI, by itself, is only readily extensible
to 64 km because of the READY-type flow control used in the
physical layer. The gateway will need to incorporate larger
buffers and independent flow control.
b) Error-rate expectations. SONET is only specified to have a
1E-10 BER on a per hop basis. This is inadequate for long
links. Those in the know say that SONET will be much better
but the designer is faced with the poor BER in the SONET spec.
c) Frame mapping. There are several interesting issues to be
considered in finding a good mapping from the HPPI packet
to the SONET frame. Some are what SONET STS levels will be
available in what time frame, the availability of concatenated
service, and the error rate issue.
Dan Helman (UCSC) talked about work he has been doing with Darrell
Long to examine the interconnection of Internet networks via an ATM
B-ISDN network. Since network interfaces and packet processing are
the expensive parts of high-speed networks, they believe it doesn't
make sense to use the ATM backbone only for transmission; it should
be used for switching as well. Therefore gateways (either shared by
a subnet or integrated with fast hosts) are needed to encapsulate or
convert conventional protocols to ATM format. Gateways will be
responsible for caching connections to recently accessed
destinations. Since many short-lived low-bandwidth connections as
foreseen (e.g., for mail and ftp), routing in the ATM network (to set
up connections) should not be complicated - a form of static routing
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should be adequate. Connection performance can be monitored by the
gateways. Connections are reestablished if unacceptable. All
decision making can be done by gateways and route servers at low
packet rates, rather than the high aggregate rate of the ATM network.
One complicated issue to be addressed is how to transparently
introduce an ATM backbone alongside the existing Internet.
Session 5: Distributed Systems (David Farber, Chair)
Craig Partridge (BBN Systems and Technologies) started this session
by arguing that classic RPC does not scale well to gigabit-speed
networks. The gist of his argument was that machines are getting
faster and faster, while the round-trip delay of networks is staying
relatively constant because we cannot send faster than the speed of
light. As a result, the effective cost of doing a simple RPC,
measured in instruction cycles spent waiting at the sending machine,
will become extremely high (millions of instruction cycles spent
waiting for the reply to an RPC). Furthermore, the methods currently
used to improve RPC performance, such as futures and parallel RPC, do
not adequately solve this problem. Future requests will have to be
made much much earlier if they are to complete by the time they are
needed. Parallel RPC allows multiple threads, but doesn't solve the
fact that each individual sequence of RPCs still takes a very long
time.
Craig went on to suggest that there are at least two possible ways
out of the problem. One approach is to try to do a lot of caching
(to waste bandwidth to keep the CPU fed). A limitation of this
approach is that at some point the cache becomes so big that you have
to keep in consistent with other systems' caches, and you suddenly
find yourself doing synchronization RPCs to avoid doing normal RPCs
(oops!). A more promising approach is to try to consolidate RPCs
being sent to the same machine into larger operations which can be
sent as a single transaction, run on the remote machine, and the
result returned. (Craig noted that he is pursuing this approach in
his doctoral dissertation at Harvard).
Ken Schroder (BBN Systems and Technologies) gave a talk on the
challenges of combining gigabit networks with wide-area heterogeneous
distributed operating systems. Ken feels the key goals of wide area
distributed systems will be to support large volume data transfers
between users of conferencing and similar applications, and to
deliver information to a large number of end users sharing services
such as satellite image databases. These distributed systems will be
motivated by the natural distribution of users, of information and of
expensive special purpose computer resources.
Ken pointed to three of the key problems that must be addressed at
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the system level in these environments: how to provide high
utilization; how to manage consistency and synchronization in the
presence of concurrency and non-determinism; and how to construct
scalable system and application services. Utilization is key only to
high performance applications, where current systems would be limited
by the cost of factors such as repeatedly copying messages,
converting data representations and switching between application and
operating system. Concurrency can be used improve performance, but
is also likely to occur in many programs inadvertently because of
distribution. Techniques are required both to exploit concurrency
when it is needed, and to limit it when non-determinism can lead to
incorrect results. Extensive research on ensuring consistency and
resolving resource conflicts has been done in the database area,
however distributed scheduling and the need for high availability
despite partial system failures introduce special problems that
require additional research. Service scalability will be required to
support customer needs as the size of the user community grow. It
will require attention both ensuring that components do not break
when they are subdivided across additional processors to support a
larger user population, and to ensure that performance does to each
user can be affordably maintained as new users are added.
In a bold presentation, Dave Cheriton (Stanford) made a sweeping
argument that we are making a false dichotomy between distributed
operating systems and networks. In a gigabit world, he argued, the
major resource in the system is the network, and in a normal
operating system we would expect such a critical resource to be
managed by the operating system. Or, put another way, the gigabit
network distributed operating system should manage the network.
Cheriton went on to say that if a gigabit distributed operating
system is managing the network, then it is perfectly reasonable to
make the network very dumb (but fast) and put the system intelligence
in the operating systems on the hosts that form the distributed
system.
In another talk on interprocess communication, Jonathan Smith (UPenn)
again raised the problem of network delay limiting RPC performance.
In contrast to Partridge's earlier talk, Smith argued that the
appropriate approach is anticipation or caching. He justified his
argument with a simple cost example. If a system is doing a page
fetch between two systems which have a five millisecond round-trip
network delay between them, the cost of fetching n pages is:
5 msec + (n-1) * 32 usec
Thus the cost of fetching an additional page is only 32 usec, but
underfetching and having to make another request to get a page you
missed costs 5000 usec. Based on these arguments, Smith suggested
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that we re-examine work in virtual memory to see if there are
comfortable ways to support distributed virtual memory with
anticipation.
In the third talk on RPC in the session, Tommy Joseph (Olivetti), for
reasons similar to those of Partridge and Smith, argued that we have
to get rid of RPC and give programmers alternative programming
paradigms. He sketched out ideas for asynchronous paradigms using
causal consistency, in which systems ensure that operations happen in
the proper order, without synchronizing through a single system.
Session 6: Hosts and Host Interfaces (Gary Delp, Chair)
Gary Delp (IBM Research) discussed several issues involved in the
increase in speed of network attachment to hosts of increasing
performance. These issues included:
- Media Access - There are aspects of media access that are
best handled by dedicated silicon, but there are also aspects
that are best left to a general-purpose processor.
- Compression - Some forms of compression/expansion may belong
on the network interface; most will be application-specific.
- Forward Error Correction - The predicted major packet loss
mode is packet drops due to internal network congestion, rather
than bit errors, so forward error correction internal to a
packet may not be useful. On the other hand, the latency cost
of not being able to recover from bit errors is very high.
Some proposals were discussed which suggest that FEC among
packet groups, with dedicated hardware support, is the way
to go.
- Encryption/Decryption - This is a computationally intensive
task. Most agree that if it is done with all traffic, some
form of hardware support is helpful. Where does it fit in the
protocol stack?
- Application Memory Mapping - How much of the host memory
structure should be exposed to the network interface?
Virtual memory and paging complicate this issue considerably.
- Communication with Other Channel Controllers - Opinions were
expressed that ranged from absolutely passive network
interfaces to interfaces that run major portions of the
operating system and bus arbitration codes.
- Blocking/Segmentation - The consensus is that B/S should
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occur wherever the transport layer is processed.
- Routing - This is related to communications with other
controllers. A routing-capable interface can reduce the bus
requirements by a factor of two.
- Intelligent participation in the host structure as a gateway,
router, or bridge.
- Presentation Layer issues - All of the other overheads can be
completely overshadowed by this issue if it is not solved well
and integrated into the overall host architecture. This points
out the need for some standardization of representation (IEEE
floating point, etc.)
Eric Cooper (CMU) summarized some initial experience with Nectar, a
high-speed fiber-optic LAN that has been built at Carnegie Mellon.
Nectar consists of an arbitrary mesh of crossbar switches connected
by means of 100 Mbps fiber-optic links. Hosts are connected to
crossbar switches via communication processor boards called CABs.
The CAB presents a memory-mapped interface to user processes and
off-loads all protocol processing from the host.
Preliminary performance figures show that latency is currently
limited by the number of VME operations required by the host-to-CAB
shared memory interface in the course of sending and receiving a
message. The bottleneck in throughput is the speed of the VME
interface: although processes running on the CABs can communicate
over Nectar at 70 Mbps, processes on the hosts are limited to
approximately 25 Mbps.
Jeff Mogul (DEC Western Research Lab) made these observations:
Although off-board protocol processors have been a popular means to
connect a CPU to a network, they will be less useful in the future.
In the hypothetical workstation of the late 1990s, with a 1000-MIPS
CPU and a Gbps LAN, an off-board protocol processor will be of no
use. The bottleneck will not be the computation required to
implement the protocol, but the cost of moving the packet data into
the CPU's cache and the cost of notifying the user process that the
data is available. It will take far longer (hundreds of instruction
cycles) to perform just the first cache miss (required to get the
packet into the cache) than to perform all of the instructions
necessary to implement IP and TCP (perhaps a hundred instructions).
A high-speed network interface for a reasonably-priced system must be
designed with this cost structure in mind; it should also eliminate
as many CPU interrupts as possible, since interrupts are also very
expensive. It makes more sense to let a user process busy-wait on a
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network-interface flag register than to suspend it and then take an
interrupt; the normal CPU scheduling mechanism is more efficient than
interrupts if the network interactions are rapid.
David Greaves (Olivetti Research Ltd.) briefly described the need for
a total functionality interface architecture that would allow the
complete elimination of communication interrupts. He described the
Cambridge high-speed ring as an ATM cell-like interconnect that
currently runs at 500-1000 MBaud, and claims that ATM at that speed
is a done deal. Dave Tennenhouse also commented that ATM at high
speeds with parallel processors is not the difficult thing that
several others have been claiming.
Bob Beach (Ultra Technologies) started his talk with the observation
that networking could be really fast if only we could just get rid of
the hosts. He then supported his argument with illustrations of
80-MByte/second transfers to frame buffers from Crays that drop to
half that speed when the transfer is host-to-host. Using null
network layers and proprietary MAC layers, the Ultra Net system can
communicate application-to-application with ISO TP4 as the transport
layer at impressive rates of speed. The key to high-speed host
interconnects has been found to be both large packets and large (on
the order of one megabyte) channel transfer requests. Direct DMA
interfaces exhibit much smaller transfer latencies.
Derek McAuley (University Cambridge Computer Laboratory) described
work of the Fairisle project which is producing an ATM network based
on fast packet switches. A RISC processor (12 MIPS) is used in the
host interface to do segmentation/reassembly/demultiplexing. Line
rates of up to 150 Mbps are possible even with this modest processor.
Derek has promised that performance and requirement results from this
system will be published in the spring.
Bryan Lyles (XEROX PARC) volunteered to give an abbreviated talk in
exchange for discussion rights. He reported that Xerox PARC is
interested in ATM technology and wants to install an ATM LAN at the
earliest possible opportunity. Uses will include such applications
as video where guaranteed quality of service (QOS) is required. ATM
technology and the desire for guaranteed QOS places a number of new
constraints on the host interface. In particular, they believe that
they will be forced towards rate-based congestion control. Because
of implementation issues and burst control in the ATM switches, the
senders will be forced to do rate based control on a cell-by-cell
basis.
Don Tolmie (Los Alamos National Laboratory) described the High-
Performance Parallel Interface (HPPI) of ANSI task group X3T9.3. The
HPPI is a standardized basic building block for implementing, or
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connecting to, networks at the Gbps speeds, be they ring, hub,
cross-bar, or long-haul based. The HPPI physical layer operates at
800 or 1600 Mbps over 25-meter twisted-pair copper cables in a
point-to-point configuration. The HPPI physical layer has almost
completed the standards process, and a companion HPPI data framing
standard is under way, and a Fiber Channel standard at comparable
speeds is also being developed. Major companies have completed, or
are working on, HPPI interfaces for supercomputers, high-end
workstations, fiber-optic extenders, and networking components.
The discussion at the end of the session covered a range of topics.
The appropriateness of outboard protocol processing was questioned.
Several people agreed that outboarding on a Cray (or similar
cost/performance) machines makes economic sense. Van Jacobson
contended that for workstations, a simple memory-mapped network
interface that provides packets visible to the host processor may
well be the ideal solution.
Bryan Lyles reiterated several of his earlier points, asserting that
when we talk about host interfaces and how to build them we should
remember that we are really talking about process-to-process
communication, not CPU-to-CPU communication. Not all processes run
on the central CPU, e.g., graphics processors and multimedia.
Outboard protocol processing may be a much better choice for these
architectures.
This is especially true when we consider that memory/bus bandwidth is
often a bottleneck. When our systems run out of bandwidth, we are
forced towards a NUMA model and multiple buses to localize memory
traffic.
Because of QOS issues, the receiver must be able to tell the sender
how fast it can send. Throwing away cells (packets) will not work
because unwanted packets will still clog the receiver's switch
interface, host interface, and requires processing to throw away.
Session 7: Congestion Control (Scott Shenker, Chair)
The congestion control session had six talks. The first two talks
were rather general, discussing new approaches and old myths. The
other four talks discussed specific results on various aspects of
packet (or cell) dropping: how to avoid drops, how to mitigate their
impact on certain applications, a calculation of the end-to-end
throughput in the presence of drops, and how rate-based flow control
can reduce buffer usage. Thumbnail sketches of the talks follow.
In the first of the general talks, Scott Shenker (XEROX PARC)
discussed how ideas from economics can be applied to congestion
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control. Using economics, one can articulate questions about the
goals of congestion control, the minimal feedback necessary to
achieve those goals, and the incentive structure of congestion
control. Raj Jain (DEC) then discussed eight myths related to
congestion control in high-speed networks. Among other points, Raj
argued that (1) congestion problems will not become less important
when memory, processors, and links become very fast and cheap, (2)
window flow control is required along with rate flow control, and (3)
source-based controls are required along with router-based control.
In the first of the more specific talks, Isidro Castineyra (BBN
Communications Corporation) presented a back-of-the-envelope
calculation on the effect of cell drops on end-to-end throughput.
While at extremely low drop rates the retransmission strategies of
go-back-n and selective retransmission produced similar end-to-end
throughput, at higher drop rates selective retransmission achieved
much higher throughput. Next, Tony DeSimone (AT&T) told us why
high-speed networks are not just fast low-speed networks. If the
buffer/window ratio is fixed, the drop rate decreases as the network
speed increases. Also, data was presented which showed that adaptive
rate control can greatly decrease buffer utilization. Jamal
Golestani (Bellcore) then presented his work on stop-and-go queueing.
This is a simple stalling algorithm implemented at the switches which
guarantees no dropped packets and greatly reduces delay jitter. The
algorithm requires prior bandwidth reservation and some flow control
on sources, and is compatible with basic FIFO queues. In the last
talk, Victor Frost (University of Kansas) discussed the impact of
different dropping policies on the perceived quality of a voice
connection. When the source marks the drop priority of cells and the
switch drops low priority cells first, the perceived quality of the
connection is much higher than when cells are dropped randomly.
Session 8: Switch Architectures (Dave Sincoskie, Chair)
Dave Mills (University of Delaware) presented work on a project now
under way at the University of Delaware to study architectures and
protocols for a high-speed network and packet switch capable of
operation to the gigabit regime over distances spanning the country.
It is intended for applications involving very large, very fast, very
bursty traffic typical of supercomputing, remote sensing, and
visualizing applications. The network is assumed to be composed of
fiber trunks, while the switch architecture is based on a VLSI
baseband crossbar design which can be configured for speeds from 25
Mbps to 1 Gbps.
Mills' approach involves an externally switched architecture in which
the timing and routing of flows between crossbar switches are
determined by sequencing tables and counters in high-speed memory
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local to each crossbar. The switch program is driven by a
reservation-TDMA protocol and distributed scheduling algorithm
running in a co-located, general-purpose processor. The end-to-end
customers are free to use any protocol or data format consistent with
the timing of the network. His primary interest in the initial
phases of the project is the study of appropriate reservation and
scheduling algorithms. He expect these algorithms to have much in
common with the PODA algorithm used in the SATNET and WIDEBAND
satellite systems and to the algorithms being considered for the
Multiple Satellite System (MSS).
John Robinson (JR, BBN Systems and Technologies) gave a talk called
Beyond the Butterfly, which described work on a design for an ATM
cell switch, known as MONET. The talk described strategies for
buffering at the input and output interfaces to a switch fabric
(crossbar or butterfly). The main idea was that cells should be
introduced to the switch fabric in random sequence and to random
fabric entry ports to avoid persistent traffic patterns having high
cell loss in the switch fabric, where losses arise due to contention
at output ports or within the switch fabric (in the case of a
butterfly). Next, the relationship of this work to an earlier design
for a large-scale parallel processor, the Monarch, was described. In
closing, JR offered the claim that this class of switch is realizable
in current technology (barely) for operation over SONET OC-48 2.4
Gbps links.
Dave Sincoskie (Bellcore) reported on two topics: recent switch
construction at Bellcore, and high-speed processing of ATM cells
carrying VC or DG information. Recent switch design has resulted in
a switch architecture named SUNSHINE, a Batcher-banyan switch which
uses recirculation and multiple output banyans to resolve contention
and increase throughput. A paper on this switch will be published at
ISS '90, and is available upon request from the author. One of the
interesting traffic results from simulations of SUNSHINE shows that
per-port output queues of up to 1,000 cells (packets) may be
necessary for bursty traffic patterns. Also, Bill Marcus (at
Bellcore) has recently produced Batcher-banyan (32x32) chips which
test up to 170Mb/sec per port.
The second point in this talk was that there is little difference in
the switching processing of Virtual Circuit (VC) and Datagram (DG)
traffic that which has been previously broken into ATM cells at the
network edge. The switch needs to do a header translation operation
followed by some queueing (not necessarily FIFO). The header
translation of the VC and DG cells differs mainly in the memory
organization of the address translation tables (dense vs. sparse).
The discussion after the presentations seemed to wander off the topic
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of switching, back to some of the source-routing vs. network routing
issues discussed earlier in the day.
Session 9: Open Mike Night (Craig Partridge, Chair)
As an experiment, the workshop held an open mike session during the
evening of the second day. Participants were invited to speak for up
to five minutes on any subject of their choice. Minutes of this
session are sketchy because the chair found himself pre-occupied by
keeping speakers roughly within their time limits.
Charlie Catlett (NSCA) showed a film of the thunderstorm simulations
he discussed earlier.
Dave Cheriton (Stanford) made a controversial suggestion that perhaps
one could manage congestion in the network simply by using a steep
price curve, in which sending large amounts of data cost
exponentially more than sending small amounts of data (thus leading
people only to ask for large bandwidth when they needed it, and
having them pay so much, that we can afford to give it to them).
Guru Parulkar (Washington University, St. Louis) argued that the
recent discussion on appropriateness of existing protocol and need
for new protocols (protocol architecture) for gigabit networking
lacks the right focus. The emphasis of the discussion should be on
what is the right functionality for gigabit speeds, which is simpler
per packet processing, combination of rate and window based flow
control, smart retransmission strategy, appropriate partitioning of
work among host cpu+os, off board cpu, and custom hardware, and
others. It is not surprising that the existing protocols can be
modified to include this functionality. By the same token, it is not
surprising that new protocols can be designed which take advantage of
lessons of existing protocols and also include other features
necessary for gigabit speeds.
Raj Jain (DEC) suggested we look at new ways to measure protocol
performance, suggesting our current metrics are insufficiently
informative.
Dan Helman (UCSC) asked the group to consider, more carefully, who
exactly the users of the network will be. Large consumers? or many
small consumers?
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Session 10: Miscellaneous Topics (Bob Braden, Chair)
As its title implies, this session covered a variety of different
topics relating to high-speed networking.
Jim Kurose (University of Massachussetts) described his studies of
scheduling and discard policies for real-time (constrained delay)
traffic. He showed that by enforcing local deadlines at switches
along the path, it is possible to significantly reduce overall loss
for such traffic. Since his results depend upon the traffic model
assumptions, he ended with a plea for work on traffic models, stating
that Poisson models can sometimes lead to results that are wrong by
many orders of magnitude.
Nachum Shacham (SRI International) discussed the importance of error
correction schemes that can recover lost cells, and as an example
presented a simple scheme based upon longitudinal parity. He also
showed a variant, diagonal parity, which allows a single missing cell
to be recreated and its position in the stream determined.
Two talks concerned high-speed LANs. Biswanath Muhkerjee (UC Davis)
surveyed the various proposals for fair scheduling on unidirectional
bus networks, especially those that are distance insensitive, i.e.,
that can achieve 100% channel utilization independent of the bus
length and data rate. He described in particular his own scheme,
which he calls p-i persistant.
Howard Salwen (Proteon), speaking in place of Mehdi Massehi of IBM
Zurich who was unable to attend, also discussed high-speed LAN
technologies. At 100 Mbps, a token ring has a clear advantage, but
at 1 Gbps, the speed of light kills 802.6, for example. He briefly
described Massehi's reservation-based scheme, CRMA (Cyclic-
Reservation Multiple-Access).
Finally, Yechiam Yemeni (YY, Columbia University) discussed his work
on a protocol silicon compiler. In order to exploit the potential
parallelism, he is planning to use one processor per connection.
The session closed with a spirited discussion of about the relative
merits of building an experimental network versus simulating it.
Proponents of simulation pointed out the high cost of building a
prototype and limitation on the solution space imposed by a
particular hardware realization. Proponents of building suggested
that artificial traffic can never explore the state space of a
network as well as real traffic can, and that an experimental
prototype is important for validating simulations.
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Session 11: Protocol Architectures (Vint Cerf, Chair)
Nick Maxemchuk (AT&T Bell Labs) summarized the distinctions between
circuit switching, virtual circuits, and datagrams. Circuits are
good for (nearly) constant rate sources. Circuit switching dedicates
resources for the entire period of service. You have to set up the
resource allocation before using it. In a 1.7 Gbps network, a 3000-
mile diameter consumes 10**7 bytes during the circuit set-up round-
trip time, and potentially the same for circuit teardown. Some
service requirements (file transfer, facsimile transmission) are far
smaller than the wasted 2*10**7 bytes these circuit management delays
impose. (Of course, these costs are not as dramatic if the allocated
bandwidth is less than the maximum possible.)
Virtual circuits allow shared use of bandwidth (multiplexing) when
the primary source of traffic is idle (as in Voice Time Assigned
Speech Interpolation). The user notifies the network of planned
usage.
Datagrams (DG) are appropriate when there is no prior knowledge of
use statistics or usage is far less than the capacity wasted during
circuit or virtual circuit set-up. One can adaptively route traffic
among equivalent resources.
In gigabit ATMs, the high service speed and decreased cell size
increases the relative burstiness of service requests. All of these
characteristics combine to make DG service very attractive.
Maxemchuk then described a deflection routing notion in which traffic
would be broken into units of fixed length and allowed into the
network when capacity was available and routed out by any available
channel, with preference being given to the channel on the better
path. This idea is similar to the hot potato routing of Paul Baran's
1964 packet switching design. With buffering (one buffer), Maxemchuk
achieved a theoretical 90% utilization. Large reassembly buffers
provide for better throughput.
Maxemchuk did not have an answer to the question: how do you make
sure empty "slots" are available where needed? This is rather like
the problem encountered by D. Davies at the UK National Physical
Laboratory in his isarythmic network design in which a finite number
of crates are available for data transport throughout the network.
Guru Parulkar (Washington University, St. Louis) presented a broad
view of an Internet architecture in which some portion of the system
would operate at gigabit speeds. In his model, internet, transport,
and application protocols would operate end to end. The internet
functions would be reflected in gateways and in the host/net
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interface, as they are in the current Internet. However, the
internet would support a new type of service called a congram which
aims at combining strengths of both soft connection and datagram.
In this architecture, a variable grade of service would be provided.
Users could request congrams (UCON) or the system could set them up
internally (Picons) to avoid end-to-end setup latency. The various
grades of service could be requested, conceptually, by asserting
various required (desired) levels of error control, throughput,
delay, interarrival jitter, and so on. Gateways based on ATM
switches, for example, would use packet processors at entry/exit to
do internet specific per packet processing, which may include
fragmentation and reassembly of packets (into and out of ATM cells).
At the transport level, Parulkar argued for protocols which can
provide application-oriented flow and error control with simple per
packet processing. He also mentioned the notion of a generalized RPC
(GRPC) in which code, data, and execution might be variously local or
remote from the procedure initiator. GRPC can be implemented using
network level virtual storage mechanisms.
The basic premise of Raj Yavatkar's presentation (University of
Kentucky) was that processes requiring communication service would
specify their needs in terms of peak and average data rate as well as
defining burst parameters (frequency and size). Bandwidth for a
given flow would be allocated at the effective data rate that is
computed on the basis of flow parameters. The effective data rate
lies somewhere between the peak and average data rate based on the
burst parameters. Statistical multiplexing would take up the gap
between peak and effective rate when a sudden burst of traffic
arrives. Bounds on packet loss rate can be computed for a given set
of flow parameters and corresponding effective data rate.
This presentation led to a discussion about deliberate disciplining
of inter-process communication demands to match the requested flow
(service) profile. This point was made in response to the
observation that we often have little information about program
behavior and might have trouble estimating the network service
requirements of any particular program.
Architectural Discussion
An attempt was made to conduct a high-level discussion on various
architectural questions. The discussion yielded a variety of
opinions:
1. The Internet would continue to exist in a form similar
to its current incarnation, and gateways would be required,
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at least to interface the existing facilities to the high
speed packet switching environment.
2. Strong interest was expressed by some participants in access
to raw (naked ATM) services. This would permit users
to construct their own gigabit nets, at the IP level, at any
rate. The extreme view of this was taken by David Cheriton
who would prefer to have control over routing decisions and
other behavior of the ATM network.
3. The speed of light problem (latency, round-trip delay)
is not going to go away and will have serious impact on
control of the system. The optimistic view was taken,
for example, by Craig Partridge and Van Jacobson, who felt
that many of the existing network and communications
management mechanisms used in the present Internet protocols
would suffice, if suitably implemented, at higher speeds.
A less rosy view was taken by David Clark who observed
(as did others) that many transactions would be serviced in
much less than one round-trip time, so that any end-to-end
controls would be largely useless.
4. For applications requiring fixed, periodic service,
reservation of resource seemed reasonably attractive to many
participants, as long as the service period dominated the
set-up time (round-trip delay) by an appreciable
margin.
5. There was much discussion throughout the workshop of
congestion control and flow control. Although these
problems were not new, they took on somewhat newer
character in the presence of much higher round-trip delays
(measured in bits outstanding). One view is that end-to-end
flow control is needed, in any case, to moderate sources
sending to limited bandwidth receivers. End-to-end flow
control may not, however, be sufficient to protect the
interior of the network from congestion problems, so
additional, intra-network means are needed to cope with
congestion hot spots. Eventually such conditions
have to be reflected to the periphery of the network to
moderate traffic sources.
6. There was disagreement on the build or simulate
question. One view was in favor of building network
components so as to collect and understand live application
data. Another view held that without some careful
simulation, one might have little idea what to build
(for example, Sincoskie's large buffer pool requirement was
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not apparent until the system was simulated).
Comments from Workshop Evaluation Forms
At the end of the IRSG workshop, we asked attendees to fill out an
evaluation form. Of the fifty-one attendees, twenty-nine (56%)
turned in a form.
The evaluation form asked attendees to answer two questions:
#1. Do you feel that having attended this workshop will help you
in your work on high speed networks during the next year?
#2. What new ideas, questions, or issues, did you feel were
brought up in the workshop?
In this section we discuss the answers we got to both questions.
Question #1
There was a satisfying unanimity of opinion on question #1. Twenty-
four attendees answered yes, often strongly (e.g., Absolutely and
very much so). Of the remaining five respondents, three said they
expected it to have some effect on their research and only two said
the workshop would have little or no effect.
Some forms had some additional notes about why the workshop helped
them. Several people mentioned that there was considerable benefit
to simply meeting and talking with people they hadn't met before. A
few other people noted that the workshop had broadened their
perspective, or improved their understanding of certain issues. A
couple of people noted that they'd heard ideas they thought they
could use immediately in their research.
Question #2
Almost everyone listed ideas they'd seen presented at the conference
which were new to them.
It is clear that which new ideas were important was a matter of
perspective - the workshop membership was chosen to represent a broad
spectrum of specialties, and people in different specialities were
intrigued by different ideas. However, there were some general
themes raised in many questionnaires:
(1) Limitations of our traffic models. This particular subject
was mentioned, in some form, on many forms. The attendees
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generally felt we didn't understand how network traffic would
behave on a gigabit network, and were concerned that people
might design (or worse, standardize) network protocols for
high speed networks that would prove inadequate when used
with real traffic. Questions were raised about closed-loop
vs. open-loop traffic models and the effects of varying types
of service. This concern led several people to encourage the
construction of a high-speed testbed, so we can actually see
some real traffic.
(2) Congestion control. Despite the limitations of our traffic
models, respondents felt that congestion control at both
switching elements and network wide was going to be even more
important than today, due to the wider mix of traffic that
will appear on gigabit networks. Most forms mentioned at
least one of the congestion control talks as a containing a
new idea. The talks by Victor Frost, Jamal Golestani and
Scott Shenker received the most praise. Some attendees were
also interested in methods for keeping the lower-layer
switching fabric from getting congested and mentioned the
talks by Robinson and Maxemchuk as of interest.
(3) Effects of fixed delay. While the reviews were by no means
unanimous, many people had come to the conclusion that the
most serious problem in gigabit networking was not bandwidth,
but delay. The workshop looked at this issue in several
guises, and most people listed at least one aspect of fixed
delay as a challenging new problem. Questions that people
mentioned include:
- How to avoid a one round-trip set up delay, for less than one
round-trip time's worth of data?
- How to recover from error without retransmission (and thus
additional network delays)? Several people were intrigued by
Nachum Shacham's work on error detection and recovery.
- Should we use window flow-control or rate-based flow control
when delays were long?
- Can we modify the idea of remote procedure calls to deal with
the fact that delays are relatively long?
A couple of attendees noted that while some of these problems looked
similar to those of today, the subtle differences caused by operating a
network at gigabit speeds led them to believe the actual approaches to
solving these problems would have to be very different from those of
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today.
Security Considerations
Security issues are not discussed in this memo.
Author's Address
Craig Partridge
Bolt Beranek and Newman Inc.
50 Moulton Street
Cambridge, MA 02138
Phone: (617) 873-2459
EMail:
[email protected]
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