Internet Engineering Task Force D.-H. Gan/R. Guerin/S. Kamat
INTERNET DRAFT Juniper Networks, Inc./IBM/IBM
T. Li/E. Rosen
Juniper Networks, Inc./Cisco
21 November 1997
Setting up Reservations on Explicit Paths using RSVP
draft-guerin-expl-path-rsvp-01.txt
Status of This Memo
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Abstract
This document presents motivations for extensions to RSVP in order to
enable setting up of reservations on explicit routes. The advantages
of providing this support are discussed in the context of MPLS and
QoS routing. An approach to providing these extensions by means of
opaque routing objects in RSVP messages is presented.
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Contents
Status of This Memo i
Abstract i
1. Introduction 1
2. Bandwidth Reservation for Explicit Route in an MPLS Environment 1
3. QoS Routing with Explicit Routes 3
3.1. QoS path management . . . . . . . . . . . . . . . . . . . 4
3.2. Enforcing high level admission control policies . . . . . 6
4. Mechanism for Reservation Set Up on Explicit Paths 7
4.1. Explicit Route Object . . . . . . . . . . . . . . . . . . 7
4.1.1. Subobjects . . . . . . . . . . . . . . . . . . . 7
4.1.2. Applicability . . . . . . . . . . . . . . . . . . 8
4.1.3. Semantics of the Explicit Route Object . . . . . 8
4.1.4. Strict and Loose subobjects . . . . . . . . . . . 9
4.1.5. Loops . . . . . . . . . . . . . . . . . . . . . . 10
4.2. Subobject semantics . . . . . . . . . . . . . . . . . . . 10
4.2.1. Subobject 1: The IPv4 prefix . . . . . . . . . . 10
4.2.2. Subobject 2: The IPv6 address . . . . . . . . . 10
4.2.3. Subobject 32: The autonomous system number . . . 10
4.2.4. Subobject 64: MPLS label switched path
termination . . . . . . . . . . . . . . . 10
4.3. Processing of the Explicit Route Object . . . . . . . . . 11
4.3.1. Selection of the next hop . . . . . . . . . . . . 11
4.3.2. Adding subobjects to the explicit route object . 12
4.3.3. Error subcodes . . . . . . . . . . . . . . . . . 13
5. Conclusions 13
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1. Introduction
The purpose of this document is to introduce and motivate extensions
to RSVP to enable setting up of reservations on explicit routes.
Enabling reservations on explicit routes can be useful in several
different contexts. In particular, it can be used to ensure that
certain flows use a ``label switched'' path as in the MPLS context
[CDF+97] or to facilitate the management of QoS paths computed by a
QoS capable router as in [GKO+97]. In this document, we describe
further these potential benefits, and show how they can be attained
with minimal impact to RSVP. It should be pointed out that the focus
of this document is on unicast flows as there are many other issues
that need to be addressed to consider the use of explicit routes for
multicast flows.
In the context of unicast flows, explicit routes are to be specified
through a new Explicit_Route object in RSVP. This object, like policy
objects, is opaque to RSVP which only needs to ensure its delivery to
routing. Routing is responsible for processing the Explicit_Route
object, and will use the information it contains to construct its
response to a Route_Query from RSVP.
Sections 2 and 3 motivate the need for explicit route support within
RSVP in the context of MPLS and QoS routing respectively. Section
4 describes the specific mechanism of setting up reservations on
explicit paths. This includes specification of a format for the
Explicit_Route object and the interactions between RSVP and routing
in this context.
2. Bandwidth Reservation for Explicit Route in an MPLS Environment
Consider the following topology:
A---B---C---D
| |
E-------F
Let us suppose that this topology exists in the network of an
Internet Service Provider (ISP). We suppose further that node A has
an interface to one of the ISP's subscribers, S1, and node B has
an interface to a different subscriber, S2. Finally, we suppose
that both subscribers are generating packets that are addressed to
destinations reachable only through node D.
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In order to make the best provisioning of its bandwidth, the ISP may
decide that such packets from S1 should follow the route A-B-E-F-D,
while such packets from S2 should follow the route B-C-D. Further,
the ISP may want to reserve resources for each of these "flows", so
that it can schedule transmissions along the respective routes in a
way that corresponds to whatever agreements the ISP has made to the
particular subscribers.
Putting this decision into effect in a conventional IP network is
extremely difficult, since it requires that two packets going through
B, with the same destination, be sent on separate routes. Therefore,
ISPs tend to use ATM or Frame Relay networks to provide this level of
bandwidth management. ATM and Frame Relay networks also provide the
capability to support whatever resources reservations are necessary.
MPLS [CDF+97] provides a way for an ISP to obtain this functionality
without the need to resort to ATM or Frame Relay. In MPLS, node A
can apply a "label" to packets from S1 which must pass through D;
node B can apply a label to packets from S2 which must pass through
D. When a labeled packet is transmitted, the label is sent along with
it. Once a packet is labeled, the forwarding decision is based only
on the label, NOT on the contents of the packet header. Thus there
is nothing to prevent packet P1 from traveling the A-B-E-F-D path,
while packet P2 travels the B-C-D path, even if P1 and P2 happen to
have the very same destination address.
Of course, MPLS must incorporate some "path setup" procedure whereby
paths that differ from the "normal" IP routing can get explicitly set
up. MPLS must also incorporate some means of performing resource
reservation along these paths. While a resource reservation protocol
could be designed exclusively for MPLS, it would seem to make most
sense to use RSVP for that purpose; after all, RSVP was designed to
be the resource reservation protocol of the internet.
This requires some modification of RSVP. As currently specified,
there is no way to force an RSVP Path message to follow any path
other than the "normal" path to a particular destination. So if a
different MPLS path were set up for certain flows, there is currently
no way to get the Path message to follow that path.
If RSVP control messages could carry opaque objects that are
meaningful to routing and RSVP's interface to routing is broadened
as in [GKR97] so that RSVP could pass such objects to routing, then
this difficulty can be overcome. The Path messages could carry an
explicit route object. To determine the next hop for the flow, RSVP
would pass the Explicit Route Object (and other opaque objects if
present) to routing, which would pass back the identity of the next
hop, and a modified Explicit Route Object. This would force the Path
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message to follow the path of the corresponding MPLS flow and ensure
that resources are reserved along the MPLS path.
The general ability to carry an opaque routing object in RSVP
messages further enables one to combine the setup of an MPLS path
with resource reservation along the same path. This could be
achieved by having a second opaque routing object carry an MPLS flow
identifier (label) in conjunction with the explicit route object.
The definition of such an MPLS label object is deferred to another
draft. Clearly, this approach has the advantage of avoiding a second
round trip to make reservations along the MPLS path when the path set
up itself must be done first. The need to have a second round trip
seems to simply add latency and complexity, without adding any value.
3. QoS Routing with Explicit Routes
An objective of QoS routing is to choose for each flow the path that
has the best likelihood of meeting the flow's QoS requirements,
while still making efficient use of network resources. In order
to achieve this goal, QoS routing requires knowledge of network
resource availability and of the QoS requirements of the flows.
This information can be provided in a number of ways (e.g., see
[CNRS97, GKR97] for possible approaches) and is then used by a QoS
path selection algorithm to identify an appropriate path for a flow.
The selection of a path and the distribution of the information
needed to make that selection, however, only represent a subset of
the functions needed to support QoS routing. There are two other
important issues that a QoS routing solution must address to meet its
objectives. These are:
1. Management of QoS paths of individual flows, and
2. Enforcing high level admission control policies.
Management of QoS paths includes not only setting up the paths
correctly, but also maintaining or adjusting them in response to
failures and changes in the network. High level (call) admission
policies are needed (see [CNRS97] for a discussion of this issue) to
control how selected paths are being used so as to preserve the long
term efficiency of the network. For example, a suitable path might
be found for a flow, but rejected by the high level admission control
because of its cost to the network, e.g., it is using a large number
of links which could alternatively be used to support several such
other calls to different destinations.
In the rest of this section, we articulate how explicit routes can
facilitate handling of these two issues. However, before doing
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so, we briefly compare, in the context of QoS routing, the use of
explicit routes versus the hop-by-hop routing approach presented in
[GKO+97].
A hop-by-hop routing solution has the benefits of requiring the
least changes to RSVP and possibly offering added flexibility (see
[GKH97] for details), but this does come at a cost. Specifically,
with hop-by-hop routing, there are multiple decision points (each
hop) involved in selecting a path, with each making independent
decisions. As a result, end-to-end control of a path requires
coordination between the multiple decision points, and this can
often be a complex task. For example, even in the context of a link
state routing protocol such as OSPF where all routers in a domain
compute their routes using the same algorithm applied to a common
topology database, no single router has complete knowledge of the
actual path being followed. This is because inconsistencies during
routing transients as well as equal cost multi-path considerations,
independently affect local path selection decisions. Additional
mechanisms are, therefore, needed to coordinate these independent
decisions.
On the other hand, when explicit routes are used, selection of the
entire path is made at a single decision point (the first router in
the path). In the rest of this section, we expand on the benefits of
a single decision point in the context of both QoS path management
and high level call admission.
3.1. QoS path management
In best-effort routing, route changes occur relatively infrequently,
and mostly when local interfaces change state or when routing
updates are received from the routing protocol. With QoS routing,
changes that would result in the selection of a new route for a
given destination and QoS requirements are much more frequent, as
they typically occur each time a metric update is received. If such
changes were to trigger re-routing of existing QoS flows, this would
translate into disruption of service to already established flows.
Furthermore, this could also increase routing instability as such
re-routing may trigger additional metric updates and cause further
re-routing. Keeping a flow's routing state, i.e., the path on which
it has established a reservation, ``pinned'' as long as the path
remains satisfactory for the flow (and the network) is one possible
approach to this problem. Path pinning, however, has a number of
implications for QoS routing.
First, path pinning requires knowledge that the path being pinned is
adequate. This includes several aspects. First and foremost, the
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pinned path should be loop free. When an explicit route is used,
this is readily achieved as the node selecting the explicit route can
ensure it is free of any loop. In contrast, when hop-by-hop routing
is used, the coordination of the multiple decision points involved in
the selection of the path requires not only that all nodes rely on
the same routing algorithm, but also imposes close coupling with RSVP
states to detect the formation of loops (see [GKH97] for details).
Such a coupling adds some complexity, but more importantly, it can
prevent the flow of data on the pinned path until after resources
have been successfully reserved on the entire path (see again [GKH97]
for details). In the case where reservations are successful on only
a portion of the path, this means that the data may not be able to
take advantage of such partial reservations. This is obviously
undesirable, and while this can possibly be remedied (see also
[GKH97] for possible approaches), solutions come at the cost of added
signaling and processing complexity.
Besides being loop free, a pinned path must also be capable of
satisfying the QoS requirements of the flow. Hence, it is important
either to ensure the availability of resources on a pinned path,
or to provide simple mechanisms to unpin it in case the required
resources are not available when they are being requested, e.g., when
an RSVP RESV message is received. Hence, the ability to detect such
conditions and trigger the unpinning of a path is required. This can
be achieved using similar mechanisms in both explicit and hop-by-hop
routing cases using the approach of [GKH97]. Note that unpinning a
path only implies that QoS routing is being queried anew to determine
if the current path is still the correct one, or to find if a new
and better one now exists. In particular, unpinning a path does not
result in removal of existing path or reservation states. This is
because although the existing pinned path may not fully satisfy the
requirements of the flow, it may be the best one currently available.
In that case any (partial) reservation that may exist on the current
path should be maintained as it represents the best possible QoS
available to the flow.
There are other instances where a path needs to be unpinned. For
example, when one of the links or nodes on the path fails. In such
cases, it is important to notify all nodes on the current path, so
that they can unpin it and query QoS routing to possibly find an
alternate path. This can again be achieved using similar mechanisms
in both the explicit route and hop-by-hop routing cases [GKH97].
However, when a reservation is already in place, it is also desirable
to identify links on which resources are already reserved for the
flow. This is important so that these existing reservations be
taken into account when searching for an alternate path, i.e., avoid
the ``stepping on one's shadow'' problem. This is made easier in
the case of explicit route by the knowledge of the entire path.
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Knowledge of the entire path is also useful in the context of high
level admission control, and we now briefly review this issue and the
benefits of explicit routes in that context.
3.2. Enforcing high level admission control policies
As pointed out in the framework document for QoS routing [CNRS97],
some form of higher level admission control and administrative
control of routing behavior may be necessary within an AS. This
is because QoS routing has to balance the sometimes conflicting
requirements of high network resource utilization and improved
chances of successful resource reservation for individual flows.
For example, when current load in the network suggests a QoS path
that is much longer than the ``usual'' path, admitting the flow
along such a path may actually deny service to later flows that
would have been admissible along segments of this long path. Hence,
this could negatively affect the overall network utilization. In
such situations, a high level admission control policy may find it
desirable not to admit the flow based on routing decision alone. One
possible approach is to compare the length of the path returned by
QoS routing to that of a ``usual'' path, and decide whether or not
to use the path depending on this comparison as well as possibly
other factors such as overall network load. Conversely, if a flow
has been already set up and later a much more efficient path becomes
available, it might be desirable to reroute the flow along the new
path. This is particularly true if the current path only supports a
fraction of the desired reservation, while the new path may be able
to accommodate the complete reservation.
In all such instances, these decisions are greatly facilitated when
a single entity is responsible for determining and controlling
the entire path. Hence, such controls are more readily performed
when routing is done using explicit routes instead of hop-by-hop
routing. This is not to say that they are not feasible with
hop-by-hop routing, but distributed decisions and knowledge generally
complicate such tasks. For example, transient inconsistent routing
information at multiple routers can lead to the pinning of a long
but loop-free path, without any single router on the path being
aware of the problem. Hence, it becomes difficult to identify and
rectify such bad routing choices. Solutions to this problem require
the introduction of additional signaling information to coordinate
information and decisions across the nodes on the path, e.g., a
policy object carried in PATH messages that specifies a limit on
the acceptable path length. This would in turn add to the overall
signaling and processing overhead, and may all but eliminate the
potentially greater simplicity of hop-by-hop routing. On the other
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hand, the single decision point of explicit routes avoids most of
these problems.
4. Mechanism for Reservation Set Up on Explicit Paths
4.1. Explicit Route Object
As stated earlier, explicit routes are to be specified through a new
Explicit_route object in RSVP. RSVP PATH messages will carry this
object. The format of the explicit route object is described below.
0 1 2 3
+-------------+-------------+-------------+-------------+
| Length (bytes) | Class-Num | C-Type |
+-------------+-------------+-------------+-------------+
| |
// (Object contents) //
| |
+-------------+-------------+-------------+-------------+
Class-Num
The Class-Num indicates that the object is POLICY_DATA.
C-Type
The C-Type for an Explicit Route Object is XXX [TBD].
If a PATH message contains multiple explicit route objects, only the
first object is meaningful. Subsequent explicit route objects may be
ignored and should not be propagated.
4.1.1. Subobjects
The contents of an explicit route object are a series of variable
length data items called subobjects. Each subobject has the form:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+------//--------------+
|L| Type | Length | (Subobject contents) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+------//--------------+
L
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The L bit is an attribute of the subobject. The L bit is
set if the subobject represents a loose hop in the explicit
route. If the bit is not set, the subobject represents a
strict hop in the explicit route.
Type
The Type indicates the type of contents of the subobject.
Currently defined values are:
0 Reserved
1 IPv4 prefix
2 IPv6 prefix
32 Autonomous system number
64 MPLS label switched path termination
Length
The Length contains the total length of the subobject in
bytes, including the L, Type and Length fields. The Length
must always be a multiple of 4, and at least 4.
4.1.2. Applicability
The Explicit Route Object is intended to only be used for unicast
situations. Applications of explicit routing to multicast are a
topic for further research.
The Explicit Route Object is only to be used when all routers along
the explicit route support RSVP and the Explicit Route Object. The
mechanisms for determining that such support is present are beyond
the scope of this document.
4.1.3. Semantics of the Explicit Route Object
An explicit route is a particular path in the network topology.
Typically, the explicit route is computed by a node, with the intent
of directing traffic down that path.
An explicit route is described as a list of groups of nodes along the
explicit route. Certain operations to be performed along the path
can also be encoded in the explicit route.
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In addition to the ability to identify specific nodes along the
path, an explicit route can identify a group of nodes that must be
traversed along the path. This capability allows the routing system
a significant amount of local flexibility in fulfilling a request
for an explicit route. In turn, this allows the generator of the
explicit route to have imperfect information about the details of the
path.
The explicit route is encoded as a series of subobjects contained in
an explicit route object. Each subobject may identify a group of
nodes in the explicit route or may be an operation to be performed
along the path. An explicit route is then a path including all
of the identified groups of nodes, with the specified operations
occurring along the path.
To simplify the discussion, we call each group of nodes an abstract
node. Thus, we can also say that an explicit route is a path
including all of the abstract nodes, with the specified operations
occurring along that path.
As an example, consider an explicit route that consists solely of
autonomous system number subobjects. Each subobject corresponds to
an autonomous system in the network topology. Each autonomous system
is an abstract node. In this case, the explicit route is a path
including each of the specified autonomous systems. There may be
multiple hops within each autonomous system.
4.1.4. Strict and Loose subobjects
The L bit in the subobject is a one-bit attribute. If the L bit is
set, then the value of the attribute is `loose.' Otherwise, the
value of the attribute is `strict.' For brevity, we say that if
the value of the subobject attribute is `loose' then it is a `loose
subobject.' Otherwise, it's a `strict subobject.' Further, we say
that the abstract node of a strict or loose subobject is a strict
or a loose node, respectively. Loose and strict nodes are always
interpreted relative to their prior abstract nodes.
The path between a strict node and its prior node MUST include only
network nodes from the strict node and its prior abstract node.
The path between a loose node and its prior node MAY include other
network nodes which are not part of the strict node or its prior
abstract node.
The L bit has no meaning in operation subobjects.
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4.1.5. Loops
While the explicit route object is of finite length, the existence
of loose nodes implies that it is possible to construct forwarding
loops during transients in the underlying routing protocol. This may
be detected by the originator of the explicit route through the use
of another opaque route object called the Record Route object. The
Record Route object is used to collect detailed path information and
is useful for loop detection as well as diagnostic purposes. The
definition of Record Route object is deferred to another draft.
4.2. Subobject semantics
4.2.1. Subobject 1: The IPv4 prefix
The contents of an IPv4 prefix subobject are a 4 octet IPv4 address,
1 octet of prefix length, and 1 octet of padding. The abstract node
represented by this subobject is the set of nodes which have an IP
address which lies within this prefix. Note that a prefix length of
32 indicates a single IPv4 node.
The length of the IPv4 prefix subobject is 8 octets. The contents of
the 1 octet of padding must be zero on transmission and must not be
checked on receipt.
4.2.2. Subobject 2: The IPv6 address
TBD
4.2.3. Subobject 32: The autonomous system number
The contents of an autonomous system (AS) number subobject are a
2 octet autonomous system number. The abstract node represented
by this subobject is the set of nodes belonging to the autonomous
system.
The length of the AS number subobject is 4 octets.
4.2.4. Subobject 64: MPLS label switched path termination
The contents of an MPLS label switched path termination subobject
are 2 octets of padding. The subobject is an operation subobject.
This object is only meaningful if there is a Label Object in the PATH
message.
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If a Label Object is present in the PATH message, then this PATH
message is being used to establish a Label Switched Path. In this
case, this subobject indicates that the prior abstract node should
remove one level of label from all packets following this Label
Switched Path.
The length of the MPLS label termination subobject is 4 octets.
4.3. Processing of the Explicit Route Object
4.3.1. Selection of the next hop
A PATH message containing an explicit route object must determine
the next hop for this path. Selection of this next hop may involve
a selection from a set of possible alternatives. The mechanism for
making a selection from this set is implementation dependent and is
outside of the scope of this specification. Selection of particular
paths is also outside of the scope of this specification, but it is
assumed that each node will make a best effort attempt to determine
a loop-free path. Note that such best efforts may be overridden by
local policy.
To determine the next hop for the path, a node performs the following
steps:
1) The node receiving the RSVP message must first evaluate the first
subobject. If the node is not part of the abstract node described by
the first subobject, it has received the message in error, and should
return a "Bad initial subobject" error. If the first subobject is an
operation subobject, the message is in error, and the system should
return a "Bad Explicit Routing Object" error. If there is no first
subobject, the message is also in error and the system should return
a "Bad Explicit Routing Object" error.
2) If there is no second subobject, this indicates the end of the
explicit route. The explicit route object should be removed from
the PATH message. This node may or may not be the end of the path.
Processing continues with section 4.3.2, where a new explicit route
object may be added to the PATH message.
3) Next, the node evaluates the second subobject. If the subobject
is an operation subobject, the node records the subobject, deletes it
from the explicit route object and continues processing with step 2,
above. Note that this changes the third subobject into the second
subobject in subsequent processing. The precise operations to be
performed by this node must be defined by the operation subobject.
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4) If node is also a part of the abstract node described by the
second subobject, then the node deletes the first subobject and
continues processing with step 2, above. Note that this makes the
second subobject into the first subobject of the next iteration.
5) The node determines if it is topologically adjacent to the
abstract node described by the second subobject. If so, the node
selects a particular next hop which is a member of the abstract node.
The node then deletes the first subobject and continues processing
with section 4.3.2.
6) Next, the node selects a next hop within the abstract node of the
first subobject that is along the path to the abstract node of the
second subobject. If no such path exists then there are two cases:
6a) If the second subobject is a strict subobject, then there is an
error and the node should return a "Bad strict node" error.
6b) Otherwise, if the second subobject is a loose subobject, then the
node selects any next hop that is along the path to the next abstract
node. If no path exists, then there is an error, and the node should
return a "Bad loose node" error.
7) Finally, the node replaces the first subobject with any subobject
that denotes an abstract node containing the next hop. This is
necessary so that when the explicit route is received by the next
hop, it will be accepted.
4.3.2. Adding subobjects to the explicit route object
After selecting a next hop, the node may alter the explicit route in
the following ways.
If, as part of executing the algorithm in section 4.3.1, the explicit
route object is removed, the node may add a new explicit route
object.
Otherwise, if the node is a member of the abstract node for the first
subobject, then a series of subobjects may be inserted before the
first subobject or may replace the first subobject. Each subobject
in this series must denote an abstract node that is a subset of the
current abstract node.
Alternately, if the first subobject is a loose subobject, an
arbitrary series of subobjects may be inserted prior to the first
subobject.
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4.3.3. Error subcodes
In the processing described above, certain errors need to be reported
as part of a ``Routing problem'' PathErr message. This section
defines the subcodes for the errors described above.
Value Error
1 Bad Explicit Routing Object
2 Bad strict node
3 Bad loose node
4 Bad initial subobject
5. Conclusions
This document provides a motivation for supporting opaque routing
objects in RSVP to enable setting up resource reservations on
explicit routes. The benefits of this approach in the contexts of
MPLS and QoS routing were expounded and a mechanism for supporting
this feature was discussed.
References
[CDF+97] R. Callon, P. Doolan, N. Feldman, A. Fredette, G. Swallow,
and A. Viswanathan. A Framework for Multi-Protocol
Label Switching (draft-ietf-mpls-framework-00.txt).
INTERNET-DRAFT, Internet Engineering Task Force, May 1997.
[CNRS97] E. Crawley, R. Nair, B. Rajagopalan, and H. Sandick.
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Internet Draft RSVP on Explicit Paths 21 November 1997
Authors' Address
Der-Hwa Gan
Juniper Networks, Inc.
385 Ravendale Dr.
Mountain View, CA 94043
Email: dhg@juniper.net
Phone: +1 650 526 8074
Fax: +1 650 526 8001
Roch Guerin
IBM T.J. Watson Research Center
P.O. Box 704
Yorktown Heights, NY 10598
EMAIL: guerin@watson.ibm.com
VOICE +1 914 784-7038
FAX +1 914 784-6205
Sanjay Kamat
IBM T.J. Watson Research Center
P.O. Box 704
Yorktown Heights, NY 10598
EMAIL: sanjay@watson.ibm.com
VOICE +1 914 784-7402
FAX +1 914 784-6205
Tony Li
Juniper Networks, Inc.
385 Ravendale Dr.
Mountain View, CA 94043
Email: tli@juniper.net
Phone: +1 650 526 8006
Fax: +1 650 526 8001
Eric Rosen
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
EMAIL: erosen@cisco.com
Guerin, et al. Expires 26 May 1998 [Page 14]