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Versions: 00 01 02 03 04 05 06 07 RFC 4090
Network Working Group Ping Pan, Ed. (Juniper Networks)
Internet Draft Der-Hwa Gan (Juniper Networks)
Expiration Date: July 2002 George Swallow (Cisco Systems)
Network Working Group Jean Philippe Vasseur (Cisco Systems)
Dave Cooper (Global Crossing)
Alia Atlas (Avici Systems)
Markus Jork (Avici Systems)
Fast Reroute Extensions to RSVP-TE for LSP Tunnels
draft-ietf-mpls-rsvp-lsp-fastreroute-00.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.
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Abstract
This document describes the use of RSVP [RSVP, RSVP-TE] to establish
backup LSP tunnels for local repair of LSP tunnels.
Two methods are presented here. One is to setup one-to-one detour LSPs
according to the requirements defined by the head-end users. The other
is to setup many-to-one bypass LSP using a single tunnel to backup a set
of protected LSPs (making use of label stacking). Both methods can be
used to protect links and nodes during network failure. The described
use of RSVP allows both one-to-one and many-to-one backups to
interoperate.
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1. Introduction
This document describes the use of RSVP [RSVP] to establish backup
LSP tunnels for local repair of LSP tunnels. By the term LSP tunnel
we mean an explicitly routed LSP. In this document, we often refer
to LSPs. In all cases we mean explicitly routed LSPs. Applicability
of the techniques discussed herein to LSPs which dynamically change
their routes such as those used in unicast IGP routing is beyond the
scope of this document.
In order to meet the needs of real-time applications such as voice
over IP, it is highly desirable to be able to re-direct user traffic
onto backup LSP tunnels in 10s of milliseconds. The backup LSPs have
to be placed as close to the failure point as possible, since
reporting failure between nodes may cost significant delay. We use
the term local repair when referring to techniques which accomplish
this, and refer the LSP that is associated to one or more backup
tunnels as a protected LSP. There are two basic strategies for
setting up backup tunnels. These are one-to-one backup and facility
backup. One-to-one backup operates on the basis of a backup LSP for
each protected LSP. The facility backup aims at using a single LSP
to back up a set of protected LSPs.
1.1. One-to-one backup
In the one to one case, a label switched path is established which
intersects the original tunnel somewhere downstream of the point of
link or node failure. For each LSP which is backed up, another
backup LSP is established.
[R1]---[R2]-----[R3]----[R4]---[R5]
\ /
[R6]---[R7]
For example, suppose that in the simple topology above, R1 creates a
tunnel to R5 via the path [R1->R2->R3->R4->R5]. R2 can provide user
traffic protection by creating a partial backup tunnel
[R2->R6->R7->R4] which merges with the original tunnel
[R1->R2->R3->R4->R5] at R4. We refer a partial one-to-one backup
tunnel [R2->R6->R7->R4] as a detour.
To fully protect a LSP that traverses through N nodes, there could be
as many as (N - 1) detours. To minimize processing overhead, it is
desirable to merge detours back to a main LSP wherever possible.
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1.2. Facility backup
A second means of backing up LSPs is to take advantage of the label
stack. Instead of creating a separate LSP for every backed-up LSP, a
single LSP is created which serves to backup up a set of LSPs. We
call such a LSP tunnel a bypass tunnel.
The bypass tunnel must intersect the path of the original LSP(s)
somewhere downstream of the point of local repair. This of course
implies that the set of LSPs being backed up all pass through some
common downstream node. All LSPs which pass through the point of
local repair and through this common node which do not also use the
facilities involved in the bypass tunnel are candidates for this set
of LSPs.
To effect the repair of the protected LSPs, packets belonging to a
LSP are redirected onto the bypass tunnel. An additional label
representing the bypass tunnel is stacked onto the redirected
packets. At the penultimate hop of the bypass tunnel, the label for
the bypass tunnel is popped off the stack, revealing the label which
represents the LSP being backed up.
[R8]
\
[R1]---[R2]----[R3]----[R4]---[R5]
\\ // \
[R6]===[R7] [R9]
In the above example, R2 in this case would build a bypass tunnel
[R2->R6->R7->R4]. The doubled lines represent this tunnel. The
backup path for [R1->R2->R3->R4->R5] again rejoins the original path
at R4, but its path is now [R1->R2->R4->R5] with the bypass tunnel as
the connection between R2 and R4.
In this example, the backup tunnel is a Next-Next-Hop (NNHOP) bypass
tunnel. That is, it bypasses a single node (R3) of the protected
path. NNHOP bypass tunnels may protect against Link (R2-R3) failure
and/or Node (R3) failure as NHOP bypass tunnel only protects against
link failure.
The scalability improvement comes in that this bypass tunnel can also
be used to backup LSPs from any of R1, or R2, R8 to any of R4, R5, or
R9 which traverse the link R2->R3.
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [RFC-WORDS].
The reader is assumed to be familiar with the terminology in [RSVP]
and [RSVP-TE].
LSR - Label Switch Router
LSP - An MPLS Label Switched Path
Local Repair - Techniques used to repair LSP tunnels quickly
when a node or link along the LSPs path fails.
Protected LSP - An LSP is said to be protected at a given hop if
it has one or multiple associated backup tunnels originating
at that hop.
Detour LSP - The LSP that is used to re-route traffic around
a failure in one-to-one backup.
Bypass Tunnel - An LSP that is used to protect a set of LSPs
passing over a common facility.
Backup Tunnel - The LSP that is used to backup up one of the many
LSPs in many-to-one backup.
NHOP Bypass Tunnel - Next-Hop Bypass Tunnel. A backup tunnel
which bypasses a single link of the protected LSP.
NNHOP Bypass Tunnel - Next-Next-Hop Bypass Tunnel. A backup
tunnel which bypasses a single node of the protected LSP.
Backup Path - The LSP that is responsible for backing up one
protection LSP. A backup path refers to either a detour LSP
or a backup tunnel.
PLR - Point of Local Repair. The head-end of a backup tunnel or
a detour LSP.
MP - Merge Point. The LSR where one or more backup tunnels rejoin
the path of the protected LSP, downstream of the potential
failure. In the case of one-to-one backup, a Merge Point may
also be an LSR where multiple detours converge and only one
detour is signaled beyond that LSR; this type of merge point
may be referred to as a Detour Merge Point. A MP may also
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be a PLR.
Reroutable LSP - Any LSP for which the head-end LSR requests
for local protection. See Section 9.1 for more detail.
CSPF - Constraint-based Shortest Path First.
3. RSVP Extensions
We propose two additional objects, FAST_REROUTE and DETOUR, that
extend RSVP-TE for fast-reroute signaling. The new objects are
defined to be backward compatible for LSRs that do not recognize them
(Section 3.10 in [RSVP]). Both objects can only be carried in RSVP
Path messages.
The SESSION_ATTRIBUTE and RECORD_ROUTE objects are also extended to
support bandwidth and node protection features:
In many circumstances, it may be desirable for the head-end LSR not
only to signal an LSP as fast reroutable but also to specify to every
PLR along its path that the LSP must be rerouted onto a backup path
offering an equivalent bandwidth.
It may be desirable to signal the need for the fast reroutable LSP to
be node protected along its path. By node protected we mean that each
PLR along the path must protect the reroutable LSP with a detour LSP
or a NNHOP backup tunnel (except for the penultimate hop LSR that
will just require a NHOP backup tunnel). This way the reroutable LSP
is being protected against any link or node failure.
3.1. FAST_REROUTE Object
The FAST_REROUTE object carries the control information, such as
setup and hold priorities and bandwidth. A protected LSP uses the
FAST_REROUTE object to specify the level of protection that is
required during local repair. The FAST_REROUTE object can be used for
both one-to-one and facility backup, and has the following format:
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Class = TBD (use form 11bbbbbb for compatibility)
C-Type = 1
0 1 2 3
+-------------+-------------+-------------+-------------+
| Length (bytes) | Class-Num | C-Type |
+-------------+-------------+-------------+-------------+
| Setup Prio | Hold Prio | Hop-limit | Flags |
+-------------+-------------+-------------+-------------+
| Bandwidth |
+-------------+-------------+-------------+-------------+
| Include-any |
+-------------+-------------+-------------+-------------+
| Exclude-any |
+-------------+-------------+-------------+-------------+
| Include-all |
+-------------+-------------+-------------+-------------+
Setup Priority
The priority of the backup path with respect to taking resources,
in the range of 0 to 7. The value 0 is the highest priority.
Setup Priority is used in deciding whether this session can
preempt another session. See [RSVP-TE] for the usage on priority.
Holding Priority
The priority of the backup path with respect to holding
resources, in the range of 0 to 7. The value 0 is the highest
priority. Holding Priority is used in deciding whether this
session can be preempted by another session. See [RSVP-TE] for
the usage on priority.
Hop-limit
The maximum number of extra hops the backup path is allowed
to take, from current node (a PLR) to a MP, with PLR and MP
excluded in counting. For example, hop-limit of 0 means only
direct links between PLR and MP can be considered.
Flags
0x01 One-to-one Backup Desired
Indicates that the LSP should be protected via the one-
to-one backup mechanism described in Section 5.
This flag can only be set by the head-end LSRs.
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0x02 Facility Backup Desired
Indicates that the LSP should be protected via the facility
backup mechanism described in Section 6. This flag can
only be set by the head-end LSRs.
Bandwidth
Bandwidth estimate (32-bit IEEE floating point integer) in
bytes-per-second.
Exclude-any
A 32-bit vector representing a set of attribute filters associated
with a backup path any of which renders a link unacceptable.
Include-any
A 32-bit vector representing a set of attribute filters associated
with a backup path any of which renders a link acceptable (with
respect to this test). A null set (all bits set to zero)
automatically passes.
Include-all
A 32-bit vector representing a set of attribute filters associated
with a backup path all of which must be present for a link to be
acceptable (with respect to this test). A null set (all bits set
to zero) automatically passes.
The C-Class must be assigned in such a way that, for the LSRs that do
not support the FAST_REROUTE objects, they MUST forward the objects
downstream unchanged.
Some of the existing implementations use the FAST_REROUTE object with
a different C-type value, and slightly different object format (shown
below). For backward compatible purposes, it is documented here for
information purpose.
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C-Type = 7
0 1 2 3
+-------------+-------------+-------------+-------------+
| Length (bytes) | Class-Num | C-Type |
+-------------+-------------+-------------+-------------+
| Setup Prio | Hold Prio | Hop-limit | Reserved |
+-------------+-------------+-------------+-------------+
| Bandwidth |
+-------------+-------------+-------------+-------------+
| Include-any |
+-------------+-------------+-------------+-------------+
| Exclude-any |
+-------------+-------------+-------------+-------------+
3.2. DETOUR Object
The DETOUR object is used in one-to-one backup to setup and identify
detour LSPs. It has the following format:
Class = TBD (to conform 0bbbbbbb format for compatibility)
C-Type = 7
0 1 2 3
+-------------+-------------+-------------+-------------+
| Length (bytes) | Class-Num | C-Type |
+-------------+-------------+-------------+-------------+
| PLR ID 1 |
+-------------+-------------+-------------+-------------+
| Avoid Node ID 1 |
+-------------+-------------+-------------+-------------+
// .... //
+-------------+-------------+-------------+-------------+
| PLR ID n |
+-------------+-------------+-------------+-------------+
| Avoid Node ID n |
+-------------+-------------+-------------+-------------+
PLR ID (1 - n)
IPv4 address identifying the beginning point of detour which
is a PLR. Any local address on the PLR can be used.
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Avoid Node ID (1 - n)
IP address identifying the immediate downstream node that
the PLR is trying to avoid. Router ID of downstream node
is preferred. This field is mandatory, and is used by
the MP for merging rules discussed below.
There could be more than one pair of (PLR_ID, Avoid_Node_ID) entry in
a DETOUR object. If detour merging is desired, after each merging
operation (Section 5.3), the MP should combine all the merged detours
in the subsequent Path messages.
The C-Class must be assigned in such a way that, for the LSRs that do
not support the DETOUR objects, the LSRs MUST reject the message and
send a PathErr to notify the PLR.
3.3. SESSION_ATTRIBUTE Modification
To explicitly require bandwidth and node protection, two new flags
are defined in the SESSION_ATTRIBUTE object:
SESSION_ATTRIBUTE
Class = 207
C-Type = 7 (LSP_TUNNEL)
0 1 2 3
+-------------+-------------+-------------+-------------+
| Setup Pri | Holding Pri | Flags | Name Length |
+-------------+-------------+-------------+-------------+
| |
// Session Name (NULL padded display string) //
| |
+-------------+-------------+-------------+-------------+
Current Flags:
Local protection desired: 0x01
This flag permits transit routers to use a local repair mechanism
which may result in violation of the explicit route object.
When a fault is detected on an adjacent downstream link or node,
a transit node can reroute traffic for fast service restoration.
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Label recording desired: 0x02
This flag indicates that label information should be included
when doing a route record.
SE Style desired: 0x04
This flag indicates that the tunnel ingress node may choose to
reroute this tunnel without tearing it down. A tunnel egress node
SHOULD use the SE Style when responding with a Resv message.
When requesting fast reroute, the head-end LSR MUST set
this flag.
New Flags:
Bandwidth protection desired: 0x08
This flag indicates to the PLRs along the protected LSP path
that a backup path with a bandwidth guarantee is desired.
The bandwidth which must be guaranteed is that of the protected
LSP, if no FAST_REROUTE object is included in the PATH message;
if a FAST_REROUTE object is in the PATH message, then the
bandwidth specified in there is that which must be guaranteed.
Node protection desired: 0x10
This flag indicates to the PLRs along a protected LSP path
that they must select a backup path that bypasses at least the
next node of the protected LSP.
3.4. RRO Modification
To record bandwidth and node protection, we define two news flags in
the RRO IPv4 sub-object.
RRO IPv4 sub-object address:
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Type: 0x01 IPv4 address
0 1 2 3
+-------------+-------------+-------------+-------------+
| Type | Length | IPv4 address (4 bytes) |
+-------------+-------------+-------------+-------------+
| IPv4 address (continued) | Prefix Len | Flags |
+-------------+-------------+-------------+-------------+
Current Flags:
Local protection available: 0x01
Indicates that the link downstream of this node is protected
via a local repair mechanism, which can be either one-to-one
or facility backup.
Local protection in use: 0x02
Indicates that a local repair mechanism is in use to maintain
this tunnel (usually in the face of an outage of the link it
was previously routed over, or an outage of the neighboring
node).
New Flags:
Bandwidth protection: 0x04
The PLR will set this when the protected LSP has a backup
path which provides the desired bandwidth, which is that in
the FAST_REROUTE object or the bandwidth of the protected LSP,
if no FAST_REROUTE object was included. The PLR may set this
whenever the desired bandwidth is guaranteed; the PLR MUST set
this flag when the desired bandwidth is guaranteed and the
"bandwidth protection desired" flag was set in the
SESSION_ATTRIBUTE object.
Node protection: 0x08
When set, this indicates that the PLR has a backup path
providing protection against link and node failure on
the corresponding path section. In case the PLR could only
setup a link-protection backup path, the "Local protection
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available" bit will be set but the "Node protection" bit
will be cleared.
3.5. New RRO sub-object: MAX_PROTECTED_BANDWIDTH
This sub-object is carried in the RRO object and is optional. An
implementation MAY support it. An LSR MUST ignore and silently
propagate this sub-object, if it is not understood.
RRO MAX_PROTECTED_BANDWIDTH sub-object:
0 1 2 3
+-------------+-------------+-------------+-------------+
| Type | Length | Flags |
+-------------+-------------+-------------+-------------+
| Bandwidth protection ratio |
+-------------+-------------+-------------+-------------+
Type: 0x04
Length: 32
Flags:
No Flags are currently defined
Bandwidth protection ratio
Let's call T the bypass tunnel selected for the protected
LSP. The bandwidth protection ratio is the sum of
the bandwidths of all the protected LSPs having selected
T as their bypass tunnel / bandwidth of the bypass tunnel T.
The bandwidth protection ratio is a 32-bit IEEE floating
point integer in bytes-per-second.
The minimum value for the protected ratio is 1, which means "the TE
LSP is bandwidth protected".
Note that the PLR must select a backup tunnel in such a way that the
bandwidth protected ratio is 1 for the TE LSP having required
bandwidth protection in the SESSION_ATTRIBUTE object of their Path
message
The bandwidth protected ratio may be used for troubleshooting purpose
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or to trigger appropriate decision the head-end LSR (outside the
scope of this document).
4. Signaling for Backup Path
A number of objectives must be met to obtain a satisfactory signaling
solution. These are summarized as follows:
1. Unambiguously and uniquely identify backup paths
2. Unambiguously associate protected LSPs with their backup paths
3. Work with both global and non-global label spaces
4. Allow for merging of backup paths
5. Maintain RSVP state during and after fail-over.
LSP tunnels are identified by a combination of the SESSION and
SENDER_TEMPLATE objects. The relevant fields are as follows.
IPv4 tunnel end point address
IPv4 address of the egress node for the tunnel.
Tunnel ID
A 16-bit identifier used in the SESSION that remains constant
over the life of the tunnel.
Extended Tunnel ID
A 32-bit identifier used in the SESSION that remains constant
over the life of the tunnel. Normally set to all zeros. Ingress
nodes that wish to narrow the scope of a SESSION to the
ingress-egress pair may place their IPv4 address here as a
globally unique identifier.
IPv4 tunnel sender address
IPv4 address for a sender node
LSP ID
A 16-bit identifier used in the SENDER_TEMPLATE and the
FILTER_SPEC that can be changed to allow a sender to share
resources with itself.
The first three of these are in the SESSION object and are the basic
identification of the tunnel. The last two are in the
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SENDER_TEMPLATE.
In particular, setting the "Extended Tunnel ID" to the original IPv4
sender address allows the PLR to identify to which protected LSP a
message (from MP) corresponds. For example, when a Resv message
arrives at the PLR, the Extended Tunnel ID identifies the original
sender, allowing the PLR to identify the state to be refreshed.
4.1. Identification and association of backup paths
We propose two different approaches to identify backup paths:
- Path Message Specific:
The backup paths use the same SESSION and SENDER_TEMPLATE
objects as the ones used in the protected LSP. However, the Path
messages need to provide enough information that allow the LSRs
to differentiate the backup paths from the protected LSPs.
In case of one-to-one backup, the presence of DETOUR object
in Path messages signifies a backup path, while the presence of
FAST_REROUTE object indicates a protected LSP.
- Sender-Template Specific:
In this approach, the SESSION object and the LSP_ID are copied
from the protected LSP. The IPv4 tunnel sender address is set
to an address of the PLR node. If the head-end of a tunnel is
also acting as the PLR, it must choose an IP address different
from the one used in the SENDER_TEMPLATE of the original LSP
tunnel.
In the path-message-specific approach, when an LSR receives multiple
Path message which have the same Session and Sender Template objects
and which have the same next-hop, that LSR must merge the Path
messages; without this behavior, the multiple RESV messages received
back would not be distinguishable as to which backup path each
belongs to. This merging behavior does reduce the total number of
RSVP states inside the network. One merging example is given in
Section 5.3.
When using the sender-template-specific approach, the protected LSPs
and the backup paths SHOULD use the Shared Explicit (SE) style. This
allows bandwidth sharing between multiple backup paths. The backup
paths and the protected LSP can be merged by the Merge Points, when
the ERO from the MP to the egress is the same on each LSP to be
merged, as specified in [RSVP].
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5. One-to-one backup protection
In this section, we describe an one-to-one backup method that has the
feature to protect both network links and nodes.
To support the one-to-one backup, the users at head-end LSRs must
specify the backup service requirements for the protected LSPs. The
LSRs must be able to interface with CSPF to compute the most suitable
detour route for the protected LSPs. Upon receiving the local
protection requests for a protected LSP, the PLRs must try to
establish the detour LSPs immediately. During network failure, the
PLR must redirect the data packets into the detour LSPs in a timely
fashion.
5.1. Operation Overview
If a one-to-one backup for a protected LSP is explicitly desired, the
head-end LSR SHOULD insert into the Path message a FAST_REROUTE
object, with the "One-to-one Backup desired" flag set. A one-to-one
backup for a protected LSP may also be created based upon a PLR's
local policy if either the "local protection desired" flag is set in
the SESSION_ATTRIBUTE object or a FAST_REROUTE object is included or
both.
When processed at a PLR, the PLR initiates a detour LSP by sending a
new Path message that contains a DETOUR object. Since an LSP cannot
be a protected and a detour LSP at the same time, any Path message
MUST NOT contain both FAST_REROUTE and DETOUR objects,
The LSRs that initiate the detour LSPs SHOULD support both
FAST_REROUTE and DETOUR objects. It is possible that some LSRs along
a protected LSP do not support this standard. If that is the case,
those LSRs will not establish protection for their immediate links or
nodes. Any LSR which does support this standard SHOULD provide
protection.
The LSRs that support the detour LSPs MUST store all received
FAST_REROUTE and/or DETOUR objects for Path refreshes. The LSRs must
process the detour LSPs independent of the protected LSPs to avoid
triggering the LSP loop detection procedure described in [RSVP-TE].
The one-to-one backup can use either path-message-specific or sender-
template-specific to identify the detour LSPs.
When using the sender-template-specific approach, the protected and
detour LSPs should have the "SE Style desired" bit set in the
SESSION_ATTRIBUTE objects. At the MP, the detour LSPs merge into the
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protected LSPs according to the merging rules defined for SE style
reservations in [RSVP].
In the case of one-to-one backup, there is no need for the PLRs to
learn about the backup labels used at the merging points.
5.2. Procedures for the PLR
Upon receiving a Path message that contains a FAST_REROUTE object, a
PLR needs to run CPSF based on the information provided in the
FAST_REROUTE, as well as the downstream interface and nexthop router
information, to compute a detour route. More details on CSPF
computation are described in Section 7.
Once a detour is successfully computed and established, the PLR needs
not to compute the detour routes again, unless (1) the contents of
FAST_REROUTE have changed, or (2) the downstream interface and/or the
nexthop router for a protected LSP have changed.
After a successful detour computation, the PLR generates a Path
message to setup a detour path. The Path consists of the following:
- A DETOUR object that specifies the current PLR ID and
Avoid Node ID. Only one pair of (PLR_ID, Avoid_Node_ID)
permitted.
- An EXPLICIT_ROUTE object toward the egress. The ERO information
comes from the CSPF computation.
- The SENDER_TSPEC object contains the bandwidth information from
the previously received FAST_REROUTE objects.
- The RSVP_HOP object contains the PLR's IP address.
- The detour LSP may generate and process its own RRO object.
- The FAST_REROUTE object MUST NOT be included.
- When using the sender-template-specific approach, the "IPv4
tunnel sender address" in the SENDER_TEMPLATE must be set to
an address belonging to the PLR.
- The detour LSPs MUST use the same reservation style as the
protected LSP. This must be correctly reflected in the
SESSION_ATTRIBUTE object.
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- All other objects SHOULD be identical to those of the protected
LSP.
The PLR MUST not mix the messages for the protected and the detour
LSPs. When a PLR receives Resv, ResvTear and PathErr messages from
the downstream detour destination, the messages MUST not be forwarded
upstream. Similarly, when a PLR receives ResvErr and ResvConf
messages from a protected LSP, it MUST not propagate them onto the
associated detour LSP.
A session tear-down request is normally originated by the sender via
PathTear messages. When a PLR node receives a PathTear message from
upstream, it MUST delete both protected and detour LSPs. The PathTear
messages MUST propagate to both protected and detour LSPs.
During error conditions, the LSRs may send ResvTear messages to fix
problems on the failing path. When a PLR node receives the ResvTear
messages from downstream for a protected LSP, as long as a detour is
up, the ResvTear messages MUST not sent further upstream.
5.3. Procedures for the MP using the Path-Specific Approach
An LSR (that is, a MP) may receive multiple Path messages from
different interfaces with identical SESSION and SENDER_TEMPLATE
objects. Path state merging is REQUIRED.
The merging rule is the following:
For all Path messages that do not have either a FAST_REROUTE or a
DETOUR object, or the MP is the egress of the LSP, no merging is
required. The messages are processed according to [RSVP-TE].
Otherwise, the MP MUST record the Path state as well as their
incoming interface. If the Path messages do not share outgoing
interface and next-hop LSR, the MP must consider them as independent
LSPs, and must not merge them.
For all the Path messages that share the same outgoing interface and
next-hop LSR, the MP runs the following procedure to select one of
them as the final LSP.
1. Eliminate from consideration those that traverse nodes that other
LSPs want to avoid.
2. If one LSP is originated from this node, this must be
the final LSP. Quit.
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3. If only one LSP contains FAST_REROUTE object, this must be the
final LSP. Quit.
4. If there are several LSPs, and not all of them have a DETOUR
object, then eliminate those with DETOUR from final LSP
considerations.
5. If several candidates remain (that is, there are both detour
and protected LSPs), prefer the ones with FAST_REROUTE object.
6. If none found, prefer the ones without DETOUR object. If none
found, prefer the ones with DETOUR object.
7. If several candidate LSPs still remain, it is a local decision
to choose which one will be the final LSP. The decision can
be based on the number of IP hops in ERO, bandwidth requirements,
or others.
Once the final LSP has been identified, the MP MUST only transmit the
Path messages that are corresponding to the final LSP. Other LSPs are
considered merged at this node.
The MP may receive PathTear messages for some of the merging LSPs.
No PathTear message should be propagated downstream until the MP has
received tear-down from all merging LSPs.
When an LSR receives a ResvTear for an LSP and it is not a PLR for
that LSP, then the LSR SHOULD propagate the ResvTear towards the
LSP's ingress. For each backup LSP where the LSR is the merge node,
the ResvTear should also be propagated along the backup LSP towards
the backup LSP's ingress, a PLR.
5.3.1. An Example on Path Message Merging
Consider the following example:
G----H----I--\
| | | \
A----B----C----D----E---F
The protected LSP is A-B-C-D-E-F. After running CSPF, let the detour
ERO from B be B-G-H-I-D-E-F, and the detour ERO from C be C-H-I-E-F.
H will receive Path messages that have the same SESSION and
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SENDER_TEMPLATE from detours for B and C. During merging at H, since
detour C has a shorter ERO path length (that is, ERO is I-E-F, and
path length is 3), H will select it as the final LSP, and only
propagate its Path messages downstream. Upon receiving a Resv (or a
ResvTear) message, H must relay on the messages toward both B and C.
E needs to merge as well, and will select the main LSP, since it has
the FAST_REROUTE object. Thus, the detour LSP terminates at E.
5.3.2. Creating new DETOUR object at MP
If several LSPs are merged, the MP uses the following algorithm to
format its outgoing DETOUR object for the final LSP:
- If final LSP is protected LSP itself (that is, it contains
FAST_REROUTE object), no DETOUR object needed.
- Otherwise, combine all the (PLR_ID, Avoid_Node_ID) pairs from
all the DETOUR objects of all merged LSPs, and create a new object
with all listed. Ordering is insignificant.
5.4. Local reroute of the traffic onto the detour LSP
Detour LSPs are regular LSPs in operation. They are established as
soon as the protected LSPs are up. During local repair, packets
belonging to a protected LSP are simply switched (for example, label
swapping) onto the corresponding detour LSP. At the Merge Point, the
packets arrived from the detour LSP are merged to the final LSP.
In the example above, if there is a node failure at D, C will switch
traffic onto the pre-established detour LSP (C-H-I-E-F). At E, the
traffic switches onto the protected LSP again.
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6. Facility protection using label stacked bypass tunnel
In this section, we describe a method where a single backup tunnel
can be used to protect many LSPs. The LSPs can be protected against
both link and node failures.
Each PLR makes use of one or more NHOP or NNHOP bypass tunnels. Each
bypass tunnel will be used to backup a set of protected LSP. Those
bypass tunnels may be setup initially or may also be dynamically
setup. The users at head-end initiate the fast reroute process by
setting the appropriated fields in the SESSION_ATTRIBUTE and/or
FAST_REROUTE objects in an LSP's Path messages. At each PLR, one
bypass tunnel is selected to reroute an LSP's data packets in case of
network failure. The process of selecting a bypass tunnel for a
protected LSP is performed by the PLR when the LSP is first setup.
During failure, the PLR reroutes the data packets of each protected
LSP onto the bypass tunnel. The control messages of the backed-up
LSPs are also sent over the bypass tunnel. The facility backup uses
the sender-template-specific approach to identify the backup tunnels.
6.1. Discovering downstream labels
When global labels are in use at MPs, the PLR may learn backup labels
in a very efficient manner. The labels are learned during normal
signaling of the protected LSP by observing the contents of the RRO
object in the Resv message.
When a protected LSP is first signaled through a PLR, the PLR can
learn about the incoming labels that are used by all downstream nodes
for this LSP. In particular, it can learn incoming labels used by
downstream MPs, whether they are one hop or multiple hops away from
the PLR. The labels are learned during normal signaling of the
protected LSP by observing the contents of the RRO object in the Resv
message.
Two methods are available for discovering/obtaining the label used at
the merge node. One relies on explicit signaling over the bypass
tunnel prior to any failure of the primary path. If the nodes in the
network use a global-to-the-node label space, then the label can be
discovered by using the RRO object without additional signaling.
When this second method is intended, the head-end router includes an
RRO object and sets the label-recording-requested flag in the
Session_Attribute object. This will cause (as specified in [RSVP-
TE]) all nodes to record their INBOUND labels and to note via a flag
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if the label is global to the node.
Note that when global labels are used, no Path message need be sent
via the bypass tunnel prior to failure.
When MPs use per-interface-label spaces, the PLR must send Path
messages (for each Reroutable LSP) via the bypass tunnel prior to the
failure in order to discover the appropriate MP label. The signaling
procedures for this are identical to those in section 6.3 below.
6.2. Procedures for the PLR before fast-reroute
When a protected LSP in first signaled, all the PLRs along the path
which determine to create a backup tunnel via a bypass tunnel should
perform the following:
- If the "Local protection desired" bit is set in the
SESSION_ATTRIBUTE and there is no Fast_Reroute object, or
there is a Fast_Reroute object with the Facility-Backup-Desired
flag set, the PLR should select or create a bypass tunnel for
the reroutable LSP.
- If the PLR can find a NNHOP bypass tunnel, the PLR MUST set
the "Node protection" bit and the "Local protection available"
flags of its IPv4 or IPv6 RRO subobject if an RRO object is
included in the Resv message.
- If the PLR cannot find a NNHOP bypass tunnel, but can find
a NHOP bypass tunnel, the PLR must clear the "Node protection"
bit and must set the "local protection available" flags in
the RRO object of the Resv message,
- If the PLR can find a bypass tunnel with bandwidth guarantee,
the PLR must set the "Bandwidth protection" flag in the
above mentioned RRO subobject.
- If the PLR cannot find a bypass tunnel with the requested
bandwidth guarantee, the PLR must clear the "Bandwidth
protection" flag in the above mentioned RRO subobject.
Based on this additional information the head-end may take
appropriate actions.
Note that when global labels are used, no Path message need be sent
via the bypass tunnel prior to failure.
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6.3. Procedures for the PLR during fast-reroute
When the PLR detects a link or/and node failure condition, it needs
to reroute the data traffic onto the bypass tunnel and to start
sending the control traffic for the protected LSP onto the bypass
tunnel.
The backup tunnel is identified as follows:
- The SESSION and SESSION_ATTRIBUTE are unchanged.
- The IPv4 tunnel sender address of the SENDER_TEMPLATE is changed
(set to an address belonging to the PLR).
- The RSVP_HOP object must contain the IPv4 source address
(and LIH) of the bypass tunnel. Consequently, the MP will send
messages back to the PLR with HOP objects containing this same
IPv4 address.
- The PLR must generate an EXPLICIT_ROUTE object toward the egress.
Detailed ERO processing is described below.
- The RRO object may need to be updated, as described below.
Messages sent by PLR via the backup tunnel include Path, PathTear,
and ResvConf. Messages sent by MP via the same RSVP_HOP object
contents include Resv, and ResvTear.
6.3.1. Processing backup tunnel's ERO
Procedures for ERO processing are described in [RSVP-TE]. If normal
ERO processing rules are followed by the Merge Point, and the PLR
sends a Path message via the backup tunnel, the Merge Point would
examine the first sub-object and likely reject it (Bad initial sub-
object).
This is because the ERO may contain the IP address of a bypassed node
(in the case of a NNHOP Backup Tunnel), or of an interface which is
currently down (in the case of a NHOP Backup Tunnel). For this
reason, the PLR must update the ERO before sending Path messages onto
Backup Tunnels.
This is done by operating on the original ERO:
Sub-objects belonging to abstract nodes which precede the Merge Point
are removed, along with the first Sub-object belonging to the MP. A
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Sub-object identifying the Backup Tunnel destination is then added.
More specifically, the PLR must:
- remove all the sub-objects proceeding the first address belonging
to the MP.
- replace this first MP address with the IP destination address of
the backup tunnel.
The procedure described above ensures successful ERO processing at
the Merge Point.
6.3.2. Processing backup tunnel's RRO
During fast reroute, for each protected LSP containing an RRO object,
the PLR must update the RRO by inserting an IPv4 sub-object with the
IPv4 address of the backup tunnel source address in the Path
messages.
For each rerouted LSP in the backup tunnel, the PLR must update the
RRO object in Resv messages sent upstream in the following manner:
- In the IPv4 or IPv6 sub-object inserted by this node, set the
"Local protection available" and "Local protection in use" flags
according to the current state of the local repair mechanism.
- Update the label sub-object recording the INBOUND label
(same label value as the one sent the Resv message).
6.4. Procedures for state maintenance during fast-reroute
We will describe how state is maintained using an example:
[R8]
\
[R1]---[R2]-X--[R3]----[R4]---[R5]
\\ // \
[R6]===[R7] [R9]
We assume that:
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- a bypass tunnel is set up and follows the R2-R6-R7-R4 path;
- PLR (R2) performs 1:N protection;
- various protected LSPs exist and follow the R2-R3-R4 segment;
- link R2-R3 fails, and all protected LSPs are rerouted via
the bypass tunnel.
Note that the same procedure as the one described bellow would apply
in case of a node (R3) failure.
6.4.1. Path state
Path state for every locally repaired LSPs is refreshed downstream by
the PLR. These Path messages use a new SENDER_TEMPLATE value (the
IPv4 tunnel sender address is set to a PLR address), and are sent
onto the bypass tunnel with changed PHOP, ERO and RRO.
When a local link fails, there could be some protected LSPs using
this link. At this point, the LSR MUST NOT remove the state (Path
and Resv) and send PathTear and ResvErr messages that are
corresponding to these LSPs immediately. We always assume that these
LSPs may have been repaired upstream, and new Path messages will soon
arrive via the bypass tunnels.
However, the state will be removed if they have not been refreshed by
a PLR after the soft-state lifetime has expired.
6.4.2. Resv state
Resv state is refreshed by the MP by sending Resv messages to the IP
destination contained in the PHOP object of the Path message received
via the bypass tunnel.
The PLR receives these Resv messages, refreshes the original state
(corresponding to the protected LSP), and hence continues refreshing
the state upstream of the PLR to the head-end.
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6.5. Local reroute of the traffic onto the bypass tunnel
To perform Local Repair, packets belonging to a protected LSP are
sent on the corresponding backup tunnel in case of local failure.
An additional label (representing the bypass tunnel) is pushed onto
the stack. At the penultimate hop of the bypass tunnel, the
additional label is popped off the stack. The packet thus arrives at
the Merge Point with the same top-level label it would have carried
when arriving prior to failure (although it would have arrived on a
different interface prior to failure).
7. Procedures for detour and bypass tunnel computation
To setup the detours described in Section 5 and the bypass tunnels in
Section 6, CSPF may be used to find the optimal route. Before CSPF
computation, the following information should be collected at a PLR:
- The list of downstream nodes that the protected LSP passes
through. This information is readily available from the
RECORD_ROUTE objects during LSP setup. Note, a protected LSP's
ERO may not provide adequate information since the LSP could
be a loose routed path.
- The downstream links/nodes that we want to protect against. Once
again, this information is learnt from the RECORD_ROUTE objects.
- The upstream uni-directional links that the protected LSP passes
through, this information is learnt from the RECORD_ROUTE
objects. This information is only needed for setting up
one-to-one protection in path-message-specific approach.
- The LSP resource information, such as bandwidth. Such information
can be found in the FAST_REROUTE objects.
When applying a CSPF algorithm to compute the backup route, the
following constraints should be satisfied:
- The source address of the backup LSP is the current PLR,
For setting detours (Section 5), the destination MUST be
the tail-end of the protected LSP, whereas for setting up
bypass tunnels (Section 6), the destination MUST be the address
of the MP.
- When setting up one-to-one protection using the path-specific
approach, a detour MUST not traverse the upstream links of
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the protected LSP in the same direction. This prevents the
possibility of early merging of the detour into the protected
LSP.
- The backup LSP cannot traverse the downstream nodes and links
that we are trying to protect against. However, if the PLR
is the penultimate hop, avoid traversing downstream link only.
The detour LSP/bypass tunnel may also be SRLG disjoint from
the protected section (see the note at the end of this section).
- The backup path must satisfy the resource requirements of the
protected LSP.
If such computation succeeds, the PLR should trigger RSVP to
establish a backup path. The PLR may schedule a re-computation at a
later time. The backup path should be as short as possible, and must
merge back into the protected LSP at its MP. If for any reason, the
PLR is unable to bring up a backup path, it must schedule a retry at
a later time.
The PLR has the option to apply other constraints during the CSPF
computation. For example, a simple method can be to terminate the
computation as soon as a backup path is found. On the other hand, an
implementation may wish to continue exhaustive search to discover an
optimal path with lowest cost (or highest available bandwidth).
The PLR also has the option to re-compute the backup path
periodically even after the backup is up and running to ensure
continuous adaptation to the latest network conditions. However,
during the replacement of a functional backup path with a more
optimal one, the protected LSP may not have any backup path available
for a short interval. Except, if the PLR supports both one-to-one
and facility backup schemes, the protected LSP could be protected by
multiple backup LSPs. In this case, the LSP is fully protected at all
time.
Nevertheless, the exact CSPF algorithms to be used to compute back-up
tunnels or detour LSPs are beyond the scope of this document. Both
[OSPF-TE] and [ISIS-TE] may provide more insight on this subject.
Note also that the backup tunnel path computation may be performed by
a centralized path computation server or may use some distributed
backup path computation algorithms.
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7.1. Notion of diverse routing
Two TE LSPs are said link diverse if and only if their paths do not
have any link in common. Two TE LSPs are said node diverse if and
only if their paths do not have any node in common. It is
straightforward to demonstrate that two node diverse paths are also
link diverse.
To be effective a backup tunnel must imperatively be diversely routed
from the protected LSP path section it is protecting. That is, a one-
hop NHOP backup tunnel path must not contain the protected link. In
the example provided in Section 6, the backup LSP path must not
contain the R2-R3 link. A NNHOP backup tunnel must not contain the
protected link nor the PLR's next hop. In the first example provided
in Section 1, the backup tunnel must not traverse the R2-R3 link nor
the R3 node.
The notion of SRLG diverse path also exists. A set of links
constitute a SRLG ("Shared Risk Link Group") if they share a resource
whose failure may affect all the links in the set. So the backup
tunnel may be SRLG disjoint from the protected LSP path section it is
protecting.
Note that in the case of Path protection, the whole paths of the
protected LSP and the backup tunnel must be entirely link/node
diverse.
Well-known algorithms can be used to compute link/node/SRLG diversely
routed paths.
8. Network Failure Detection, Notification and Troubleshooting
8.1. Notification of local repair
In many situations, the route used during a Local Repair will be less
than optimal. The point of the Local Repair is to keep high priority
and loss sensitive traffic flowing while a more optimal re-routing of
the tunnel can be effected by the head-end of the tunnel. Thus the
head-end needs to know of the failure so it may re-signal an LSP
which is optimal.
To provide this notification, the PLR SHOULD send a Path Error
message with error code of "Notify" (Error code =25) and an error
value field of ss00 cccc cccc cccc where ss=00 and the sub-code = 3
("Tunnel locally repaired") (see [RSVP-TE])
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Note also that in the case of inter-area TE LSP (TE LSP spanning
areas), the head-end LSR will exclusively rely on the Path Error
message to be informed that the LSP has suffered a failure if the
failure occurs in another area than the area it belongs to. In the
case of a failure occurring in the head-end area or in the case of
intra-area TE LSP, the head-end could also detect the TE LSP failure
through the IGP notification.
8.2. Failure detection mechanisms
Link failure detection can be performed through layer-2 failure
detection mechanism. Node failure detection can be done through IGP
loss of adjacency or RSVP hellos messages extensions as per defined
in [RSVP-TE]. However, it is beyond the scope of this document to
define and describe the exact mechanisms on failure detection.
When a network failure is detected, the PLR MUST immediately switch
traffic from the protected LSP to the backup path. At the same time,
the PLR MAY send a PathErr messages toward the head-end LSR to notify
the failure condition. The PLR MUST send a RESV with an updated RRO
which indicates that local protection is in use.
8.3. Troubleshooting of local repair
For troubleshooting purposes, an RRO object may be inserted in the
Path message sent by the head-end. The previously described
mechanisms do not require the Path message to carry an RRO object.
On the other hand, the RRO object MUST be inserted in the Resv
message for the protected LSP if the "Local protection desired" bit
of the SESSION_ATTRIBUTE has been set in the corresponding Path
message, or if FAST_REROUTE object is present in Path messages.
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9. Interoperability considerations
The following guidelines are useful when running one-to-one and/or
facility backups.
9.1. Requesting local-protection and recognizing those requests
The head-end LSR of a protected LSP MUST either set the "Local
protection desired" flag in the SESSION_ATTRIBUTE object, or include
the FAST_REROUTE object, or both. A PLR MUST consider that a PATH
message with either a set "Local protection desired" flag in the
SESSION_ATTRIBUTE object, or the presence of the FAST_REROUTE object,
or both to be a request for local protection.
A PLR SHOULD consider the constraints signaled via a received
FAST_REROUTE object, or a received SESSION_ATTRIBUTE object
(Bandwidth and Node protection constraints on the bypass tunnel can
also be specified by setting the "Bandwidth protection desired" and
"Node protection desired" bits in the SESSION_ATTRIBUTE object), when
determining the backup path to use. If signaled backup constraints
and bandwidth are desired, the PATH message SHOULD contain the
FAST_REROUTE object.
A head-end LSR MUST set the "Label recording desired" flag in the
SESSION_ATTRIBUTE object if a backup tunnel through a bypass tunnel
is desired.
If local protection was not requested for the current LSP of a tunnel
and it is then desired for that tunnel, the head-end LSR MUST send a
new Path message reflecting the change ("Local protection desired"
flag set in the SESSION_ATTRIBUTE object or include a FAST_REROUTE
object). When a node detects a change in the SESSION_ATTRIBUTE object
it SHOULD forward the Path message immediately.
9.2. Backups for local protection
A PLR that recognizes that local protection is required on a
protected LSP MUST try to protect the LSP's data path immediately, by
either setting up an one-to-one detour LSP or a bypass tunnel.
When a network has a mix of PLRs that support either one-to-one
backup, or facility backup, or both, it is up to the network
operators to decide which backup mechanism to use.
When using both schemes, the PLR has the option to backup data
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traffic on an one-to-one detour LSP, as well as on a bypass tunnel.
In case of a network failure, the PLR can re-reroute traffic using
one of the two backup path initially. If the backup path failed also,
the other backup path can be used to re-reroute user traffic.
If no established detour LSP or backup tunnel exists, or the detour
LSP and the backup tunnel is in "DOWN" state, the PLR MUST clear the
"local protection available" flag in its IPv4 (or IPv6) address
subobject of the RRO and SHOULD send the updated RESV. When a detour
LSP or backup tunnel is established, the PLR MUST set the "local
protection available" flag and the appropriated "bandwidth
protection" and "node protection" bits, and SHOULD send the updated
Resv.
10. Security Considerations
This document does not introduce new security issues. The security
considerations pertaining to the original RSVP protocol [RSVP] remain
relevant.
11. IANA Guidelines
IANA [RFC-IANA] will assign RSVP C-class numbers for FAST_ROUTE and
DETOUR objects. Currently, in production networks, FAST_REROUTE uses
C-class 205, and DETOUR uses C-class 63.
12. Intellectual Property Considerations
Cisco Systems and Juniper Networks may have intellectual property
rights claimed in regard to some of the specification contained in
this document
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13. Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
14. Acknowledgments
We acknowledge the helpful comments from Arthi Ayyangar, Riza Cetin,
Rob Goguen, Carol Iturralde, Kireeti Kompella, Manoj Leelanivas,
Yakov Rekhter and Nischal Sheth.
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Internet Draft draft-ietf-mpls-rsvp-lsp-fastreroute-00.txt January 2002
15. References
[RSVP] R. Braden, Ed., et al, "Resource ReSerVation protocol (RSVP)
-- version 1 functional specification," RFC2205, September 1997.
[RSVP-TE] D. Awduche, et al, "RSVP-TE: Extensions to RSVP for LSP
tunnels", RFC3029, December 2001.
[RFC-WORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[OSPF-TE] Katz, Yeung, Traffic Engineering Extensions to OSPF, draft-
katz-yeung-ospf-traffic-05.txt, June 2001.
[ISIS-TE] Smit, Li, IS-IS extensions for Traffic Engineering, draft-
ietf-isis-tr affic-03.txt, June 2001.
[RFC-IANA] T. Narten and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", RFC 2434.
16. Author Information
Ping Pan
Juniper Networks
1194 N.Mathilda Ave
Sunnyvale, CA 94089
e-mail: pingpan@juniper.net
phone: +1 408 745 3704
Der-Hwa Gan
Juniper Networks
1194 N.Mathilda Ave
Sunnyvale, CA 94089
e-mail: dhg@juniper.net
phone: +1 408 745 2074
George Swallow
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA 01824
email: swallow@cisco.com
phone: +1 978 244 8143
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Internet Draft draft-ietf-mpls-rsvp-lsp-fastreroute-00.txt January 2002
Jean Philippe Vasseur
Cisco Systems, Inc.
11, rue Camille Desmoulins
92782 Issy les Moulineaux Cedex 9,
France
email: jvasseur@cisco.com
phone: +33 689108267
Dave Cooper
Global Crossing
960 Hamlin Court
Sunnyvale, CA 94089
email: dcooper@gblx.net
phone: +1 916 415 0437
Alia Atlas
Avici Systems
101 Billerica Avenue
N. Billerica, MA 01862
email: aatlas@avici.com
phone: +1 978 964 2070
Markus Jork
Avici Systems
101 Billerica Avenue
N. Billerica, MA 01862
email: mjork@avici.com
phone: +1 978 964 2142
draft-ietf-mpls-rsvp-lsp-fastreroute-00.txt ^L[Page 33]
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