One document matched: draft-haskin-mpls-fast-reroute-00.txt
Internet Engineering Task Force Dimitry Haskin
Internet Draft Ram Krishnan
Expires: December 1999 Nexabit Networks
Robert Boyd
Alan Hannan
Frontier GlobalCenter
June 1999
A Method for Setting an Alternative Label Switched Paths
to Handle Fast Reroute
draft-haskin-mpls-fast-reroute-00.txt
Status
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups. Note that other groups
may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet- Drafts as reference material
or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
This document describes a method for setting an alternative label
switched path to handle fast data packet reroute upon a failure in a
primary label switched path in Multi-protocol Label Switching (MPLS)
network.
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1. Introduction
The ability to quickly reroute traffic around a failure or congestion
in a label switched path (LSP) can be important in mission critical
MPLS networks. When an established label switched path becomes unusable
(e.g. due to a physical link or switch failure) data may need to be re-
routed over an alternative path. Such an alternative path can be
established after a primary path failure is detected or, alternatively,
it can be established beforehand in order to reduce the path switchover
time.
Pre-established alternative paths are essential where packet loss due
to an LSP failure is undesirable. Since it may take a significant time
for a device on a label switched path to detect a distant link failure,
it may continue sending packets along the primary path. As soon as
such packets reach a switch that is aware of the failure, packets must
be immediately rerouted by the switch to an alternative path away from
the failure if loss of data is to be avoided. Since it is impossible
to predict where failure may occur along an LSP tunnel, it might
involve complex computations and extensive signaling to establish
alternative paths to protect the entire tunnel. In the extreme, to
fully protect an LSP tunnel, alternative paths might be established at
each intermediate switch along the primary LSP.
This document defines a method for setting alternative label switched
paths in such a matter that minimizes alternative path computation
complexity and signaling requirements. It also can provide in-band
means for quick detection of link and switch failures or congestion
along a primary path without resorting to an out of band signaling
mechanism.
In order for the presented method to work, it is important that network
topology and policy allow the establishment of a backup LSP between the
endpoint switches of the protected LSP tunnel such that, with the
exception of the tunnel endpoint switches, the backup LSP does not
share any links or switches with the path that it intends to protect.
2. Alternative Path Arrangement
The main idea behind the presented method is to reverse traffic at the
point of the protected LSP back to the source switch of the protected
LSP such that the traffic flow can be then redirected via a parallel
LSP between source and destination switches of the protected LSP
tunnel.
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Referring to Figure 1, there is an MPLS network consisting of 7
interconnected switches.
Figure 1:
+--------+ 24 +--------+ 46 +--------+
+-->| Switch |------->| Switch |------->| Switch |---+
: | 2 |--------| 4 |--------| 6 | :
: | | | | | | :
12 : +--------+ +--------+ +--------+ :
: / / / \ :
: / / / \ V
+--------+ 31 +--------+ 53 +--------+ 75 +--------+
| Switch |<-------| Switch |<-------| Switch |<......| Switch |
| 1 |--------| 3 |--------| 5 |-------| 7 |
=>| |=======>| |=======>| |======>| |=>
+--------+ 13 +--------+ 35 +--------+ 57 +--------+
The following terminology is used for purpose of describing the method:
A portion of a label switched path that is to be protected by an
alternative path is referred as 'protected path segment'. Only
failures within the protected path segment, which may at its extreme
include the entire primary path, are subject to fast reroute to the
alternative path. A primary LSP between switches 1 and 7 is shown by a
double-dashed links labeled 13, 35, and 57. Arrows indicate direction
of the data traffic.
The switch at the ingress endpoint of the protected path segment is
referred as 'the source switch'. Switch 1 in Figure 1 is the source
switch in our example of a protected path.
The switch at the egress endpoint of the protected path segment is
referred as 'the destination switch'. Switch 7 in Figure 1 is the
destination switch in our example of a protected path.
The switches between the source switch and the destination switch along
the protected path are referred as protected switches.
The switch immediately preceding the destination switch along the
protected path segment is referred as the last hop switch. Switch 5 in
Figure 1 is the last hop switch for the protected path.
The essence of the presented method is that an alternative
unidirectional label switched path is established in the following way:
The initial segment of the alternative LSP runs between the last hop
switch and the source switch in the reverse direction of the protected
path traversing through every protected switch between the last hop
switch and the source switch. The dashed line between switches 5 and 1
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illustrates such a segment of the alternative path. Alternatively, the
initial LSP segment can be set from the destination switch to the
source switch in the reverse direction of the protected path traversing
through every protected switch between the destination switch and the
source switch. The dashed line between switches 7 and 1 illustrates the
initial path segment that is set in this way.
The second and final segment of the alternative path is set between the
source switch and the destination switch along a transmission path that
does not utilize any protected switches. It is not an intention of this
document to specify procedures for calculating such a path. The dashed
line between Switches 1 and 7 through Switches 2, 4, and 6 illustrates
the final segment of the alternative path.
The initial and final segments of the alternative path are linked to
form an entire alternative path from the last hop switch to the
destination switch. In Figure 1 the entire alternative path consists of
the LSP links labeled 53, 31, 12, 24, 46, and 46 if the alternative
path originates at the last hop switch. Alternatively, the entire
alternative path consists of the LSP links labeled 75, 53, 31, 12, 24,
46, and 46 if the alternative path originates at the destination switch
of the primary path.
As soon as a link failure or congestion along the protected path is
detected an operational switch at ingress of failed link reroutes
incoming traffic around of the failure or congestion by linking
upstream portion of the primary path to the downstream portion of the
alternative path. Thus if the link between Switches 3 and 5 fails, the
primary and alternative paths are linked at Switch 3 forming the
following label switched path for the traffic flow:
13->31->12->24->46->67.
The presented method of setting the alternative label switched path has
the following benefits:
- Path computation complexity is greatly reduced. Only a single
additional path between the source and destination switches of the
protected path segment needs to be calculated. Moreover, both
primary and alternative path computations can be localized at a
single switch avoiding problems that can arise when computations
are distributed among multiple switches.
- The amount of LSP setup signaling is minimized. With small
extensions to RSVP or LDP (described in separated documents), a
single switch at ingress of the protected path can initiate label
allocations for both primary and alternative paths.
- Presence of traffic on the alternative path segment that runs in
the reverse direction of the primary path can be used as an
indication of a failure or congestion of a downstream link along
the primary path. As soon as a switch along the primary path sees
the reverse traffic flow, it may stop sending traffic downstream
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of the primary path by initiating an immediate rerouting of data
traffic to the alternative path. As the result of this "crank
back" process the source switch may start sending data traffic
directly along the final alternative path segment. It is fair to
note that the crank-back technique increases the likelihood of
data packet reordering during the path rerouting process.
Therefore benefits of the reducing the alternative path latency
should be weighed against possible problems associated with short
term packet reordering. On a positive side, if multiple microflows
are aggregated in a single protected LSP tunnel, only a very
limited number of microflows may be affected by such packet
reordering. Additionally, the impact of reordering on any single
microflow may tend to be minimal.
It also can be noted that if the alternative label switched path is
originated at the destination switch of the primary path, it forms a
'loop-back' LSP that originates and terminates at this switch.
Therefore in this case it is possible to verify integrity of the entire
alternative path by simply sending a probe packet from the destination
switch along the alternative path and asserting that the packet arrives
back to the destination switch. When this technique is used to assert
the path integrity, the care has to be taken that the limited
diagnostic traffic is not interpreted as an indication of a primary
path failure that triggers data rerouting at the intermediate switches.
3. Elementary link level protection scheme
If only link-level protection is desired, an alternative path between
link endpoints can be set up to protect each link. Such a scheme can be
viewed as a degenerate case of this proposal in which the link
endpoints constitute the source and destination endpoints in the
described approach.
4. Bandwidth Reservation Considerations
Generally there is no need to specifically allocate bandwidth resources
to the alternate LSP. The Traffic-triggered priority of the primary LSP
can be used as resource preemption priority for the alternate LSP in
case the primary LSP fails and traffic is switched to the alternate LSP
as described in this document. The traffic-triggered priority is the
priority assigned to traffic belonging to an LSP, only when there is
traffic present on that LSP. When there is no traffic, other LSPs
sharing the interface should get full access to bandwidth and other
system resources. Consequently, if the traffic-triggered priority of
the primary LSP is greater than the holding priorities of the other
LSPs using an interface in the alternate path, the alternate LSP can
preempt bandwidth and other system resources as soon as traffic gets
rerouted via the alternate LSP. This enables high-priority LSPs, which
are being rerouted, to preempt resources from lower priority LSPs
without explicit bandwidth reservation for the alternate path. Of
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course, if bandwidth efficiency is not an issue, bandwidth resources
can be explicitly reserved for the alternate LSP also.
An extension to existing signaling protocols such as RSVP and LDP may
be needed to indicate that traffic-triggered resource preemption is
requested for a particular LSP as opposed to the setup priority
preemption.
5. Intellectual Property Considerations
Nexabit Networks may seek patent or other intellectual property
protection for some or all of the technologies disclosed in this
document. In the event that Nexabit Networks obtains such patent
rights, Nexabit Networks intends to license them on reasonable and non-
discriminatory terms in accordance with the intellectual property
rights procedures of the IETF standards process.
6. Acknowledgments
This document has benefited from discussions with Jim Boyle.
7. References
[1] Rosen, E. et al., "Multiprotocol Label Switching Architecture",
Internet Draft, draft-ietf-mpls-arch-05.txt, April 1999.
[2] Awduche, D. et al., "Requirements for Traffic Engineering over
MPLS", Internet Draft, draft-ietf-mpls-traffic-eng-00.txt, October
1998.
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7. Authors' Addresses
Dimitry Haskin
Nexabit Networks, Inc.
200 Nickerson Road
Marlborough, MA 01752
E-mail: dhaskin@nexabit.com
Ram Krishnan
Nexabit Networks, Inc.
200 Nickerson Road
Marlborough, MA 01752
E-mail: ram@nexabit.com
Robert Boyd
Frontier GlobalCenter
1154 East Arques Avenue,
Sunnyvale, CA 94086
E-mail: rboyd@globalcenter.net
Alan Hannan
Frontier GlobalCenter
1154 East Arques Avenue,
Sunnyvale, CA 94086
Email: alan@globalcenter.net
Haskin, et al. Expires December 1999 [Page 7]
Internet Engineering Task Force Ram Krishnan
Internet Draft Dimitry Haskin
Expires: December 1999 Nexabit Networks
June 1999
Extensions to RSVP to Handle Establishment of Alternate
Label-Switched Paths for Fast Re-route
draft-krishnan-mpls-reroute-rsvpext-00.txt
Status
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet- Drafts as
reference material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
This document describes the extensions that enables RSVP to
support creation of an alternative label switched path to handle
fast data packet reroute upon failure in a primary label switched
path in an Multi-protocol Label Switching (MPLS) network as
described in [3]. As such, this draft is a companion draft to [3].
The proposed extensions present no backward compatibility issues.
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1. Introduction
A mechanism to establish an alternate label-switched path (LSP) that
is used for quickly re-routing traffic in the event of a network
element failure or congestion along the primary LSP is described in
[3]. Only one alternate LSP needs to be created in this approach as
opposed to an approach that requires multiple alternate LSPs to be
created at each intermediate switch along the LSP. Such an approach
reduces computational complexity and the associated signalling
overhead. It is required that the alternate backup LSP does not
share any network elements (links or label-switched routers (LSR))
with the exception of the source and destination LSRs of the primary
LSP.
This document defines objects necessary for signalling the creation
and establishment of the alternate LSP when the primary and
alternative LSPs are initiated from a single router using RSVP [4].
2. Description of the Approach
The main idea behind the approach in [3]is to redirect traffic at
the point of failure in the primary LSP back to the source end-point
of the primary LSP in the reverse direction after which the traffic
flow is sent along via the alternate disjoint LSP between source and
destination switches of the protected primary LSP.
Referring to Figure 1, there is an MPLS network consisting of 7
interconnected switches.
Figure 1:
+--------+ 24 +--------+ 46 +--------+
+-->| Switch |------->| Switch |------->| Switch |---+
: | 2 |--------| 4 |--------| 6 | :
: | | | | | | :
12 : +--------+ +--------+ +--------+ :
: / / / \ :
: / / / \ V
+--------+ 31 +--------+ 53 +--------+ 75 +--------+
| Switch |<-------| Switch |<-------| Switch |<......| Switch |
| 1 |--------| 3 |--------| 5 |-------| 7 |
=>| |=======>| |=======>| |======>| |=>
+--------+ 13 +--------+ 35 +--------+ 57 +--------+
A primary LSP between switches 1 and 7 is shown by a double-dashed
links labeled 13, 35, and 57. Arrows indicate direction of the data
traffic.
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The initial segment of the alternative LSP runs between the
destination LSR and the source LSR in the reverse direction of the
primary path traversing through every switch between the last hop
switch and the source LSR. The dashed line between switches 5 and 1
illustrates such a segment of the alternative path.
The second and final segment of the alternative path is set between
the source switch and the destination switch along a transmission
path disjoint from the primary LSP. The dashed line between Switches
1 and 7 through Switches 2, 4, and 6 illustrates the final segment
of the alternative LSP.
The initial and final segments of the alternative path are linked to
form an entire alternative path from the last hop switch to the
destination switch. In Figure 1 the entire alternative path consists
of the LSP links labeled 53, 31, 12, 24, 46, and 46 if the
alternative path originates at the last hop switch.
As soon as a link failure or congestion along the protected path is
detected an operational switch at ingress of failed link reroutes
incoming traffic around of the failure or congestion by linking
upstream portion of the primary path to the downstream portion of
the alternative path. Thus if the link between Switches 3 and 5
fails, the primary and alternative paths are linked at Switch 3
forming the following label switched path for the traffic flow:
13->31->12->24->46->67.
3. Extensions to RSVP for Alternate LSP Establishment
Clearly, a label-switched path needs to be set up similar to the
establishment of the primary LSP. The presence of LABEL-REQ object
in the PATH message and LABEL object in the RESV message enables the
downstream-on-demand label allocation policy by which the labels are
exchanged among the neighbors. As shown in Figure 1, the alternate
LSP is composed of two components: the disjoint segment between the
source end-point and the destination end-point of the primary LSP
(12->24->46-67 in the example) and the segment in the reverse
direction between the destination end-point and the source end-point
traversing the same network elements as the primary LSP (75->53-
>31).
3.1 Establishing the Disjoint Segment of the Alternate LSP.
No new RSVP objects are necessary for establishing the disjoint
segment of the alternate LSP. Procedures similar to the creation of
the primary LSP can be used to establish this disjoint segment. As
mentioned earlier, care should be exercised to make sure that this
segment of the alternate path is completely disjoint from the
primary LSP. For instance, the disjoint segment can be explicitly
specified using the Explicit Route Object (ERO) in the PATH message
[4].
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3.2 Establishing the Reverse Segment of the Alternate LSP.
New RSVP objects are required in the PATH and RESV messages to
establish the reverse segment of the alternate LSP.
A new Flag option is defined in the Flags field of the SESSION-
ATTRIBUTE object that specifies Fast-reroute based on reverse-path
setup.
Flags
0x08 = Fast reroute based on reverse-path alternate LSP. When this
flag is set, all transit LSRs set up an alternate LSP based on the
mechanism specified in this document.
Two new optional objects are required: a REVERSE-LABEL-REQ object in
the RESV message and REVERSE-LABEL object in the PATH message are
used for setting up the reverse segment of the alternate LSP. The
term REVERSE refers to the establishment of an alternate LSP in the
reverse direction of the primary LSP. The function and format of
these objects are similar to the LABEL-REQ and the LABEL object used
to set-up the primary LSP.
When the destination end-point of the primary LSP receives a PATH
message consisting of the SESSION-ATTRIBUTE object, it includes the
optional REVERSE-LABEL-REQ object in the corresponding RESV message
if Fast-reroute is enabled in the SESSION-ATTRIBUTE object. Each LSR
in the path of the primary LSP allocates a label for the reverse
segment of the alternate LSP and stores the label in the PSB for
inclusion in the corresponding PATH message. An LSR that receives a
RESV message with the REVERSE-LABEL-REQ object should allocate and
include the REVERSE-LABEL object in the corresponding PATH message,
unless it is unable to allocate a label in the specified label range
in the REVERSE-LABEL-REQ object. In that case, the LSR should send a
PATHERR message with the appropriate error codes.
REVERSE-LABEL object
REVERSE-LABEL Class = [TBD] C-Type = 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length(bytes) | Class-Num | C-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// (Object contents) //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (top label) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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The contents of the REVERSE-LABEL object are a stack of labels, and
the top of the stack is in the right four octets of the contents.
REVERSE-LABEL-REQ object
REVERSE-LABEL-REQ Class = [TBD] C-Type = 1
The format of the REVERSE-LABEL-REQ object is similar to that of the
LABEL-REQ object with the exception of the Class number. Three
possible C-Types are supported: Label request without a label range,
Label request with an ATM label range and a Label request with a
Frame Relay label range.
The source end-point of the LSP allocates a label in the PATH
message for the reverse segment of the alternate LSP, in response to
a label request from its downstream neighbor. This label is used as
the incoming label in its cross-connect table while the outgoing
label used by the source end-point is allocated by its immediate
downstream neighbor in the disjoint segment of the alternate LSP.
The proposed extensions are backward compatible with those LSRs that
do not recognize the optional REVERSE-LABEL_REQ and REVERSE-LABEL
objects.
4. References
[1] Rosen, E. et al., "Multiprotocol Label Switching Architecture",
Internet Draft, draft-ietf-mpls-arch-05.txt, April 1999.
[2] Awduche, D. et al., "Requirements for Traffic Engineering over
MPLS", Internet Draft, draft-ietf-mpls-traffic-eng-00.txt, October
1998.
[3] Haskin, D. et al., öA Method for Setting an Alternate Label-
Switched Paths to Handle Fast Re-routeö, work in progress, draft-
haskin-fast-reroute-00.txt, May 1999.
[4] Awduche, D. et al., öExtensions to RSVP for LSP tunnelsö, work
in progress, draft-ietf-mpls-rsvp-lsp-tunnel-01.txt, March 1999.
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5. Authors' Addresses
Ram Krishnan
Nexabit Networks, Inc.
200 Nickerson Road
Marlborough, MA 01752
E-mail: ram@nexabit.com
Dimitry Haskin
Nexabit Networks, Inc.
200 Nickerson Road
Marlborough, MA 01752
E-mail: dhaskin@nexabit.com
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