One document matched: draft-ietf-rmt-gra-arch-00.txt
22 October 1999
Generic Router Assist (GRA) Building Block
Motivation and Architecture
<draft-ietf-rmt-gra-arch-00.txt>
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Abstract
Generic Router Assist (GRA) is a network-based service that enables
end-to-end multicast transport protocols to take advantage of infor-
mation distributed across the network elements in a given multicast
distribution tree. The service consists of a canonical set of simple
functions which network elements may apply to selected packets in the
transport session as they traverse the distribution tree. The choice
of function and the packet parameters to which it is applied can be
defined and customized for a given transport session in a highly con-
trolled fashion that still permits enough flexibility for GRA to be
used to address a wide range of multicast transport problems not
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amenable to end-to-end solution. This document provides the motiva-
tion and an architecture for GRA.
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Table of Contents
1. Introduction ............................................... 4
2. Scope of Generic Router Assist ............................. 6
3. Canonical Services and Functional Models/Examples .......... 11
4. Implementation Considerations .............................. 15
Abbreviations .................................................. 17
References ..................................................... 18
Revision History ............................................... 19
Authors' Addresses ............................................. 19
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1. Introduction
The development of scalable end-to-end multicast protocols poses a
tremendous challenge to network protocol designers. For example the
development of reliable multicast protocols has received considerable
attention in recent years. Most protocols are based on an end-to-end
solution [SRM, RMTP, TKP] and have found the problem of scaling to
1000s or even 100s of receivers daunting. The primary obstacles to
the development of scalable protocols have been "feedback implosion"
and "transmission isolation". The first of these concerns the diffi-
culty of a large multicast application to limit feedback from
receivers to a data source or to each other. The second concerns the
difficulty of limiting the transmission of data to the subset of a
multicast group that requires it.
Several proposals have been made to add functionality to routers for
the purpose of improving the performance of multicast applications,
particularly reliable multicast. Papadopoulos and Parulkar [LSM]
introduced additional forwarding functionality to a router which
would allow each router to identify a ``special outgoing interface''
over which to transmit a particular class of packets. They showed how
this ``turning point functionality'' could be used to improve the
performance of reliable multicast protocols. Levine and Garcia-Luna-
Aceves [LABEL] proposed the addition of ``routing labels'' to routing
tables which could be used to direct packets over specific inter-
faces. One of these, called a distance label, was shown to be quite
useful in reliable multicast for directing requests for repairs to
nearby repair servers. The third and, perhaps most relevant proposal
is the PGM protocol [PGM]. Briefly, PGM is a reliable multicast pro-
tocol which uses negative acknowledgements (NAKs). The PGM protocol
is an end-to-end transport protocol that contains a router component
which performs NAK suppression and retransmission subcasting func-
tionality. This proposal, like others [GMTS, BFS], is primarily
motivated by PGM and the recognition of the benefits of exporting a
set of flexible, simple router-based functionality for the purpose of
multicast protocol design. Such functionality can significantly sim-
plify the design of a large class of scalable multicast transport
protocols.
In this draft, we present Generic Router Assist (GRA) functionality
intended to help protocol designers deal with the two problems of
feedback implosion and transmission isolation. This functionality is
designed to assist in the scaling of receiver feedback information
and in providing subcasting for large multicast groups. It consists
of simple filtering and aggregation functions that reside within
routers.
Signaling protocols are used by hosts to set up and invoke this
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functionality. Briefly, a data source first initializes one or more
desired services on its multicast tree using GRA setup messages. The
GRA-capable routers on the tree then aggregate feedback from
receivers and/or isolate transmissions through the use of filters set
by either the sender or the receivers. For robustness, periodic
transmissions of setup messages on the multicast tree are used to
refresh GRA state in the face of routing changes and other possible
errors. It should be stressed that GRA services are only invoked for
certain packets; data packets are usually not treated any differently
and will not cause any additional processing in routers.
GRA is not intended to provide sophisticated services which are dif-
ficult or impossible to implement in routers. GRA functionality is
implemented at the IP layer and provides unreliable ``best-effort''
services. Transport protocols which make use of GRA must be robust
in the face of failures and the absence of GRA-capable routers in the
network.
Before describing the details of GRA, we present a simple example in
the context of a PGM-like reliable multicast protocol.
Consider a NAK-based reliable multicast protocol which places the
responsibility of packet loss detection on each receiver. Each time
that a receiver detects a loss (based on a gap in the sequence
numbers of the packets that it receives), it unicasts a request for a
repair (NAK) to the sender. Upon receipt of a NAK for a specific
packet, the sender retransmits the packet to all receivers.
This protocol faces considerable challenges in dealing with multiple
NAKs for the same packet. First, there is the problem of the sender
having to process many NAKs. Second, there is the problem of limiting
the number of retransmissions to the same packet. GRA can be used to
(partially) solve these two problems. Prior to the transfer of any
data, the application sets up a NAK aggregation filter at each GRA-
capable router using a setup message. This filter is set up to
suppress NAKs generated for the same packet. In addition, the router
maintains information regarding the interfaces over which it has
received NAKs so that it can subcast the retransmission on the por-
tion of the multicast tree that contains receivers requiring a
retransmission of the packet.
In Figure 1, we show how GRA can be used to aggregate feedback infor-
mation in a reliable multicast transport protocol. In this figure, a
multicast source (Src 1) transmits to two receivers (Rec 1 and Rec
2). The data packets from Src 1 are treated as regular multicast
packets and forwarded accordingly. On the link between router R1 and
router R2, a data packet is lost. Assuming a NAK based reliable mul-
ticast protocol, this loss causes the receivers to each send a NAK to
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the source for the packet that was lost. In the example, receivers
invoke GRA to send the NAKs to the source. Router R2 treats these
NAKs in a special manner, removing the redundant NAKs to the source.
Therefore, only one NAK arrives at the source. We see from this exam-
ple that only certain types of packets require additional processing
at GRA routers and that the majority of end-to-end packets are for-
warded according to normal multicast forwarding rules (i.e. without
additional router processing).
Src 1
^ | |
| | | data packet
| | |
R1 |
^ | |
| | V
R2 suppresses | | X data packet dropped
duplicate +--->R2 <---+
NAK | --- --- |
| | | |
| | | |
| | | |
R3 R4
GRA ^ | | ^ GRA
NAK | | | | NAK
| | | |
Rec 1 Rec 2
Figure 1. Example of network support for transport protocols.
We conclude the introduction by commenting on the relationship of GRA
to the multicast routing algorithm. GRA works on all types of multi-
cast forwarding trees. However, GRA state is per session state and
requires per session state in routers. If a source-based tree routing
protocol is used to forward multicast packets, then this per session
state will already exist in routers in the form of multicast forward-
ing entries. If a shared tree type multicast routing protocol is
being used, then GRA per session state must be maintained on the
shared tree. This is simply because GRA provides per session func-
tionality.
2. Scope of Generic Router Assist
The types of services implemented with GRA are bounded by constraints
and limitations of routing devices. In this section we explicitly
describe the limitations and constraints of routing devices and
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explain what we believe to be reasonable services to implement in
routers.
We specifically describe router limitations in order to limit the
scope of GRA. We believe that a small set of GRA services in routers
can assist in scaling many of the problems in end-to-end multicast.
Previous proposals[ACTIVE] have proposed complex elements that reside
in routers to provide sophisticated capabilities. We feel that these
are unreasonable for current generation routers. The approach of GRA
is to provide a simple fixed set of services which give the maximum
benefit with the least cost.
2.1. Service Properties
GRA services are performed on subset of packets sent between end-to-
end transport protocols on the multicast distribution tree between a
GRA Source and the set of GRA receivers. Only routers on the distri-
bution tree for a particular GRA source act upon GRA packets. The
advantages of GRA type services can only be realized when the actions
are performed on packets that are directly on the forwarding tree of
a multicast group.
In order to describe the requirements of GRA, we first describe the
properties of what we feel are appropriate services.
Fixed: by fixed services we mean those of which are statically
part of router software or hardware. We DO NOT mean dynamically
loadable modules. We feel that a fixed set of simple services
will suffice for most of the scaling issues in transport proto-
cols.
Simple: We wish only to include those services that we feel are
reasonable to be implemented in routers. These are services which
can be performed with minimum CPU and memory overhead.
Short Term: We wish to provide services for which state and pro-
cessing overhead is short lived. GRA makes use of soft-state
design principles.
2.2. GRA Requirements
When considering the service and architecture of GRA, we adhere to
the following principles:
1. GRA services should be simple and fixed. They should not
require excessive processing in routing devices nor should they
buffer messages.
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2. GRA services are not substitutes for well-engineered end-to-end
protocol designs. We support the end-to-end design principles of
transport protocols. GRA is an "assist" service which is designed
to assist protocols in scaling aspects.
3. GRA services will not take on active networking attributes such
as dynamically uploadable modules or programming language propo-
sals.
4. GRA services should not directly participate in transport pro-
tocols. GRA should not be required for any transport protocols
nor should any GRA services directly support a particular proto-
col.
5. GRA services should be those which may assist all or a reason-
able subset of transport protocols.
6. GRA services should be used for assisting in *control* packet
operations. GRA services should not be for the majority of pack-
ets in a multicast group.
2.3. Constraints of networking devices
Current generation routers perform processing of packets and execute
routing and signaling protocols. Routers perform fast packet for-
warding on the "forwarding path". This is usually hardware and
software specifically designed for the forwarding of packets (and may
include other functions such as policing, shaping, etc). This is in
contrast to the "control plane" of a router where control protocols
are run under an operating systems. Examples of these types of pro-
tocols are routing, management and signaling protocols.
We now describe the role and impact of GRA services on these two
"planes" of a router.
Forwarding path: A router forwarding path usually consists of spe-
cialized hardware and software which is designed specifically for
the purpose of forwarding packets. Newer routers also have abili-
ties to perform other actions such as marking or policing for ser-
vices such as QoS. In general, the forwarding path of routers is
very limited in the amount of state and complexity of processing
that can be performed. Although we do not rule out GRA services
being implemented in the forwarding path of routers, we do not see
this as feasible at this point in time. This is because of the
more complicated processing rules for GRA packets (in comparison
with basic forwarding operations) and the amount of state that is
sometimes involved in performing GRA operations.
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Control plane: Router control planes run embedded or general
operating systems. On top of these operating systems are imple-
mentations of IP control protocols such as routing, signaling, and
network management. The processing power and memory of a control
plane is highly dependent on the hardware design of the router.
Most routers use general purpose CISC or RISC processors for run-
ning the control plane operating system and protocols. The con-
trol plane is limited in processing by its hardware and the load
generated from the other processes or tasks running. We expect
that most GRA operations will be performed in the control plane
where state and processing power are more readily available. How-
ever, we do stress that routers generally have fairly slow control
plane hardware. This is to keep the cost low and because the pro-
cessing required for control plane protocols is usually low.
2.4. State Constraints
As we evaluate particular services which are candidates for inclusion
in GRA, constraints in router state are an important consideration.
We wish to select services which will not create substantial or
long-lived state.
2.4.1. Session State
Routers which perform multicast forwarding contain per tree forward-
ing state. For trees rooted at multicast sources, this amounts to
per source per group forwarding entries. This state must be kept in
both the forwarding as well as the control plane. GRA services are
per session, or per source. This is simply because GRA services are
per transport session.
The session state of GRA does not impose much additional state to
routers. Per each session, a state block describing the services
desired is required. In addition, other types of state may be
created in the course of a GRA session. The GRA session state is the
only state required for the length of a session; other state is setup
and torn down in smaller intervals.
2.4.2. Packet State
The implementation of particular services may cause per packet state
in GRA routers. Services which cause per packet state should use
small timers for tearing down this state, which could explode in cer-
tain conditions. An example of a service which causes per packet
state is NAK elimination. We feel the use of small timers will
minimize problems in state growth.
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2.4.3. Buffering
As part of the requirements for GRA services, we specify that the
buffering of packets is unacceptable. GRA services may drop or
modify packets in transit, but they should never buffer packets.
We feel that the buffering of packets in routing devices is unaccept-
able due to the unpredictable behavior of such a service. In addi-
tion we feel that this is an unreasonable service for routers to sup-
port without a significant payback in end-to-end scaling.
2.5. Processing Constraints
GRA services may require differing amount of computation. As a gen-
eral rule, GRA services should require minimal computation and packet
manipulation.
2.5.1. Computation
We expect most GRA services to require minimal computation. Services
which require minimal computation are reasonable to implement in
routing devices and minimize security risk. Examples of operations
which we feel are appropriate are:
Comparison: comparison operations may be performed on GRA packets
against the GRA session state in the router in order to determine
whether a service should be invoked. Examples of comparison
operations are: equal to, less than, greater than, etc.
Basic Predicates: When receiving a GRA packet, a router may be
required to update its GRA session state. The operations required
for updating the state should remain simple. Example of simple
operations are addition, subtraction, etc.
In order to limit the computational complexity of GRA services, we
recommend that GRA operations remain singular. The combination of
predicates and operations creates problems in both router processing
and in GRA specifications themselves. We wish to avoid problems in
operator and action precedence.
2.5.2. Buffer Operations
The possibility exists to define services which require extensive
packet manipulation by GRA routers. We feel that extensive manipula-
tion of packets is expensive and therefore unreasonable for routers.
GRA services should be able to be invoked without extensive manipula-
tion of packets. Services which update or overwrite fields are
acceptable. Services which require the formation of new packets or
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aggregate information into new packets are unacceptable.
2.6. Examples of "Reasonable" Services
In this section we briefly describe three GRA services which we feel
are appropriate and why we feel they meet the above requirements.
The two example services are those that provide general functions
which do not incur large amounts of state or processing.
Elimination: Elimination is the selected dropping of redundant
signaling. An example of elimination is NAK suppression in a
reliable multicast protocol. Elimination is a service which
requires little computational overhead. It does however, require
per packet (or per sequence block) state. This state can be con-
trolled by the use of short soft state timers.
Subcasting: Subcasting is the forwarding of a multicast packet to
a subset of the multicast tree. Subcasting is useful for a
variety of multicast protocols. Subcasting does not require a
large amount of processing. The state required is an identifier
for the subtree and a list of outgoing interfaces. This state is
similar to multicast forwarding tree state.
3. Canonical Services and Functional Models/Examples
While a variety of mechanisms must come together to enable a specific
GRA service in a distribution tree (session path messages to estab-
lish session parameters and neighbour information, a control protocol
to define, enable, and disable specific filtering services, etc.),
the basic mechanism of GRA consists of router based services that can
be described in the language of filters, keys, and conditional func-
tions (binary predicates and their outcomes).
Based upon the examples (of predicate elimination and sub-casting) in
the previous sections, this section expands the model and terminology
of filters to describe the full flexibility of GRA. The intent here
is to generalize the mechanism fully enough to be able to accommodate
currently unspecified requirements for multicast transport services
as they emerge.
Revisiting the example of predicate elimination, note that the
receipt of a loss report at a router implies several things. The
packet type implies that a type of filter should be established for
the transport session, that the filter key is (a sequence number) of
a particular length at a particular offset, that the loss report in
hand should be forwarded, that the interface upon which the loss
report was received along with the count of parity packets requested
should be recorded and associated with the key in the filter, and
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that subsequent matching loss reports on any interface should be
eliminated unless they request a larger number of parity packets than
has been requested on any interface. The filter itself has other
implied characteristics. It eliminates only for a certain interval
after forwarding the initiating or any subsequent loss report, and it
has an implied lifetime after which it is locally expired by the
router.
Similarly in the example of sub-casting, the receipt of a retransmis-
sion at a router implies several things. The packet type implies
that the packet should be matched against an existing filter type
based on a key of a particular length and at a particular offset,
that the packet itself should be forwarded on all interfaces with
non-zero counts recorded as state associated with the key, that the
corresponding count should be decremented by 1 per interface until it
reaches 0, and that the key's state should be discarded once all
counts are zero. By implication, unmatched retransmissions should
not be forwarded.
From these examples some generalities emerge which, once they are
extricated from their specific semantics, can be re-assembled in a
variety of useful ways to provide router-based assistance for a
broader class of transport services.
3.1. General Model
The general model is one in which packets carry one of a tightly con-
strained set of signals that alert the router to apply a pre-defined
filtering service.
The packet-borne signal conveys
a filter type,
an associated transaction,
a key,
and possible packet variables
(the parity count in the case of parity loss reports).
Canonical filtering services in routers can be defined by
a filter type,
associated supported transactions,
predicated on specific conditions,
and three functions gated on that predicate whether TRUE or FALSE:
f(p): how to dispose of the packet,
f(s): how to transform the key's state, and
f(v): how to transform the outgoing interface (OIF) list
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associated with the key.
All of these as well as the offsets and lengths of the key and any
packet variables for each supported transaction constitute the defin-
ition of the filtering service.
3.2. An Example Using the General Model
Given this model, the handling of parity NAKs and parity retransmis-
sions in PGM can be described as a predicate eliminating and subcast-
ing filter.
Let IIF be the interface on which a packet is received. Let COUNT be
the number of parity packets requested and recorded either in the
loss report (RQST COUNT), as interface state (OIF COUNT), or as key
state (HIGH COUNT). Let ET be the elimination timer for a particular
key's state, and LT be the life timer for a particular key's state.
The filtering service in the router supports two transactions, one
inbound (RCVR_UPDATE) and one outbound (FORWARD).
3.2.1. RCVR_UPDATE
For RCVR_UPDATE, in addition to a transport session identifier, the
following are defined for the signal in the packet:
ELIM_SUBC is the filter type
RCVR_UPDATE is the transaction
SQN is the key
COUNT is a packet-borne variable
For RCVR_UPDATE, the following are defined for the filtering service
in the router:
In case the key fails to match existing key state:
predicate: NOOP
f(v): OIF COUNT for IIF = MAX(RQST COUNT, 0)
f(s): start ET, start LT, HIGH COUNT = RQST COUNT
f(p): reverse forward to upstream neighbour
In case the key matches existing key state:
predicate: ET is running or RQST COUNT LEQ HIGH COUNT
In case the predicate is TRUE:
f(v): OIF COUNT for IIF = MAX(RQST COUNT, OIF COUNT for IIF)
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f(s): restart LT
f(p): discard
In case the predicate is FALSE:
f(v): OIF COUNT for IIF = MAX(RQST COUNT, OIF COUNT for IIF)
f(s): restart ET, HIGH COUNT = RQST COUNT
f(p): reverse forward to upstream neighbour
3.2.2. FORWARD
For FORWARD, in addition to a transport session identifier, the fol-
lowing are defined for the signal in the packet:
ELIM_SUBC is the filter type
FORWARD is the transaction
SQN is the key
For FORWARD, the following are defined for the filtering service in
the router:
In case the key fails to match existing key state:
predicate: NOOP
f(v): NOOP
f(s): NOOP
f(p): discard
In case the key matches existing key state:
predicate: for all OIF COUNTs, OIF COUNT NE 0
In case the predicate is TRUE:
f(v): decrement OIF COUNT
f(s): NOOP
f(p): forward on OIF
In case the predicate is FALSE:
f(v): NOOP
f(s): NOOP
f(p): discard
Associated with the filtering service would be an additional house-
keeping function which would discard a key's state if either LT
expired or all the OIF COUNTs were zero.
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The point of this example (eventually, "these examples") is to demon-
strate that this model for GRA can accommodate a highly functional
set of router-based services which, given a transport-layer-
independent implementation, could be provided by routers for general
deployment by transport protocols engineered to signal the routers in
a generic way. Now that we have established the context and a model
for GRA, the next section discusses implementation considerations in
the interest of making the mechanisms of GRA more concrete, and
highlighting their practical and performance consequences to tradi-
tional multicast forwarding.
4. Implementation Considerations
4.1. Signalling Protocol - GRA service indicators
A consideration of the implementation issues attending GRA strongly
determines the class of functions that may be realized with this
mechanism. These considerations relate to both security of the
router and performance in the forwarding path. The former will be
dealt with in another section. This section outlines the time and
space scaling consequences of GRA to the forwarding path.
To be a generic network layer service, it's clear that some minimal
indicator is required in the network layer to signal the presence of
GRA signal on a packet. As has been previously noted, remember that
the GRA indicator is typically NOT borne by basic data packets.
These are switched without exception in the usual forwarding path.
In this section, packets bearing a GRA indicator will be referred to
as GRA packets.
A network layer indicator frees forwarding engines from the burden of
having to walk into and parse transport headers on every switched
packet. It's highly efficient to encode the GRA indicator on the
network layer header since that header is typically already within
the grasp of the forwarding engine. In PGM, this indicator is imple-
mented with an IP Router Alert option, but a single bit in the basic
IP header would function just as well (even better, actually, for
legacy routers).
Once GRA packets can be detected in the forwarding path, the next
step is to locate and parse the GRA parameters: the filter type, the
transaction, the key, and the variables from above. These should be
encoded as TLVs located somewhere between the end of the network
layer header and the beginning of the transport layer payload or SDU.
It's tempting in the case of the key and the packet variables to con-
sider encoding their offsets and lengths in a TLV that could be used
to locate the actual values themselves in the transport header or,
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more wildly, in the SDU, but this would amount to providing a near
programmatic directive to the network element which is fraught with
risk. So note that, in the example of elimination above, the number
of loss reports requested would, in this model, be encoded both in
its natural place in the transport header, and also as a GRA-specific
variable in a separate GRA TLV.
An alternative is to establish the location and length of keys and
packet variables as attributes of the filtering service defined in
the network element itself so that the only vulnerability to the net-
work elements would derive from abuse of the setup protocol in the
control plane in place of vulnerability in the forwarding path or
data plane.
The location of GRA TLVs before, inside, or after the transport layer
header is at the crux of whether GRA is regarded as integral to a
specific layer or as a shim layer between layers. The answer to the
question determines not just where to locate the TLVs but also where
to implement the service in host protocol stacks.
As for forwarding-time implementation of GRA services, we take it as
a requirement for this specification that some services must be
light-weight enough to be candidates for optimized implementations in
hardware or firmware, while others may be sufficiently complex to
warrant software implementations. Soft-state-based services that do
not maintain per-packet state may be in the first class; services
that maintain per-packet state and therefor require a
sorted/searchable data structure may be in the second class.
Beyond these very GRA-specific issues are the more general control
protocol issues of how to interoperate network elements with
disparate GRA capabilities, and all of the specifics of defining,
enabling, and disabling filters themselves. These issues are best
treated once a solid model of the GRA mechanism itself is esta-
blished.
4.2. Control Protocol - Filter definition, enabling, and disabling
4.3. Control Protocol - Session path messages and neighbour informa-
tion
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Abbreviations
IIF Incoming Interface
NAK Negative Acknowledgement
NOOP No Operation
OIF Outgoing Interface
SDU Service Data Unit
SQN Sequence Number
TLV Type Length Value
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(GMTS)", Nortel Networks Technical Report, December 1998.
[KKT] S. Kasera, J. Kurose, D. Towsley. ``A Comparison of Server-
Based and Receiver-Based Local Recovery Approaches for Scalable Reli-
able Multicast'', Proc. INFOCOM'98, 1998.
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[SRM] S. Floyd, V. Jacobson, S. McCanne, C. Lin, L. Zhang. ``A Reli-
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Cain/Speakman/Towsley [Page 18]
INTERNET-DRAFT Generic Router Assist 22 October 1999
Revision History
draft-ietf-rmt-gra-00.txt October 1999
Original draft.
Authors' Addresses
Brad Cain
bcain@nortelnetworks.com
Tony Speakman
speakman@cisco.com
Don Towsley
towsley@cs.umass.edu
Cain/Speakman/Towsley [Page 19]
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