[Docs] [txt|pdf] [Tracker] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 04 05

   Internet Engineering Task Force                       Dimitry Haskin
   Internet Draft                                          Ram Krishnan
   Expires: May 2001                                  Axiowave Networks

                                                          November 2000


           A Method for Setting an Alternative Label Switched Paths
                            to Handle Fast Reroute

                    draft-haskin-mpls-fast-reroute-05.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 up an alternative label
   switched path to handle fast reroute of traffic upon a failure in a
   primary label switched path in Multi-protocol Label Switching (MPLS)
   network.










Haskin, et al.                                                [Page 1]


Internet Draft  draft-haskin-mpls-fast-reroute-05.txt   November 2000

   Table of Contents

  1.  Introduction.....................................................2
  2.  Alternative Path Arrangement.....................................3
  3.  1:1 protection...................................................6
  4.  1:N protection...................................................6
  5.  Restoration Shortcuts............................................7
  6.  Elementary link level protection scheme..........................8
  7.  Bandwidth Reservation Considerations.............................8
  8.  Intellectual Property Considerations.............................9
  9.  Acknowledgments..................................................9
  10. References.......................................................9
  11. Authors' Addresses...............................................9












  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 with the objective to provide a single failure
   protection in such a manner that facilitates quick restoration
   comparable to 50 milliseconds provided in SONET self-healing rings
   and at the same time 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. Both one-to-one (1:1) protection and many-to-one (1:N)
   protection can be achieved with the proposed approach as described
   in this document.




Haskin, et al.             Expires May 2001                   [Page 2]


Internet Draft  draft-haskin-mpls-fast-reroute-05.txt   November 2000

   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 resources with the path that it intends to
   protect.

   The fast reroute support can be facilitated with additional
   extensions incorporated in the MPLS signaling protocols such as RSVP
   or CR-LDP. These extensions are not defined in this document.


  2. Alternative Path Arrangement

   The main idea behind the presented method is to reverse traffic at
   the point of failure 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.

   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 :   +--------+        +--------+        +--------+   : 67
        :       /               /                 /      \   :
        :      /               /                 /        \  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 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.

Haskin, et al.             Expires May 2001                   [Page 3]


Internet Draft  draft-haskin-mpls-fast-reroute-05.txt   November 2000


   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 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 67 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 67 if the alternative path originates at the
   destination switch of the primary path.

   As soon as a failure along the protected path is detected, an
   operational switch at the ingress of the failed link reroutes
   incoming traffic around the failure or congestion by redirecting
   this traffic into the alternative LSP traversing the switch in the
   reverse direction of the primary LSP according to the procedures
   described in the following sections of the document.




Haskin, et al.             Expires May 2001                   [Page 4]


Internet Draft  draft-haskin-mpls-fast-reroute-05.txt   November 2000

   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.

     - Optionally, 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 the source switch
       detects the reverse traffic flow, it may stop sending traffic
       downstream of the primary path and start sending data traffic
       directly along the final alternative path segment.

       It is fair to note that this 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 be minimal.

   The described in-band signaling of an LSP failure to the source
   switch does not exclude other methods of propagating an error
   condition back to the source.

   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 must be taken that
   the limited diagnostic traffic is not interpreted as an indication
   of a primary path failure that triggers data rerouting at the source
   switch.







Haskin, et al.             Expires May 2001                   [Page 5]


Internet Draft  draft-haskin-mpls-fast-reroute-05.txt   November 2000

  3. 1:1 protection

   If the 1:1 path protection is desired, an individual backup LSP is
   set for each LSP that needs to be protected as described in section
   2. When a switch detects that a downstream link has failed, it
   simply splices the traffic onto the alternative LSP. Referring to
   Figure 1, if the link between the Switch 3 and Switch 5 fails,
   Switch 3 accomplishes the fast reroute by swapping the incoming MPLS
   label 13 of the primary path with the outgoing MPLS label 31 of the
   alternative path. In this example 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.


  4. 1:N protection

   In the case of the 1:N protection a single alternative path can be
   used for protection of more than one LSP between the same source and
   destination switches. The difference in rerouting LSPs the 1:N
   protection case is that, rather than splicing protected traffic into
   the alternative LSP, it may be necessary to use the MPLS label
   stacking to tunnel protected traffic via the backup LSP to the
   destination switch as described below.

   A switch detecting failure of a downstream link, first swaps the
   incoming MPLS label of each protected LSP with the respective
   incoming label that identifies that LSP at the destination switch
   and then pushes the outgoing label of the backup LSP to the top of
   the forwarded MPLS packets. In essence, the protected MPLS packets
   are encapsulated inside of the backup LSP and emerge at the backup
   tunnel tail at the egress switch with their respective labels known
   to that switch.

   Referring to Figure 1 and assuming that global label space is used
   at the destination switch, if the link between the Switch 3 and
   Switch 5 fails, Switch 3 swaps incoming MPLS label 13 of the
   protected LSP with label 57 (incoming label at Switch 7) and then
   encapsulates the resulting packet into the backup tunnel by pushing
   label 31 to the top of the forwarded MPLS packets.

   Needless to say in order for this scheme to work, each router in the
   protected path must be aware what labels are used at the egress LSR
   for each protected LSP. Such knowledge can be propagated with the
   appropriate extensions incorporated into signaling protocols such as
   RSVP or CR-LDP.

   A single segment of a tunnel between source and destination switches
   can be used to protect multiple LSP segments that originate and
   terminate on these switches as long as this segment of the backup
   tunnel is completely disjoint from each protected LSP segment except
   for the source and destination switches. In such a case the reverse
   segments of backup path merge into the disjoint segment of the
   backup path at the source switch of the protected LSPs as


Haskin, et al.             Expires May 2001                   [Page 6]


Internet Draft  draft-haskin-mpls-fast-reroute-05.txt   November 2000

   illustrated in Figure 2. In Figure 2, dashed lines represent
   protected LSPs and double-dashed lines represent backup LSP tunnels.


   Figure 2:

     +--+        +---+         +--+
     |  |=======>|LSR|========>|D |
     |  |        +---+         |E |
     |  |    +---+    +---+    |S |
     | S|<===|   |<===|   |<===|T |
     | O|--->|LSR|---> LSR|--->|I |
     | U|    +---+    +---+    |N |
     | R|                      |A |
     | C|    +---+    +---+    |T |
     | E|<===|   |<===|   |<===|I |
     |  |--->|LSR|--->|LSR|--->|O |
     |  |    +---+    +---+    |N |
     +--+                      +--+


  5. Restoration Shortcuts

   Some types of applications require bounded end-to-end transmission
   delays to deliver useful services. A notable example is the Voice
   over IP (VoIP) service which requires end-to-end delays that do not
   exceed 400 ms for an acceptable level of service. VoIP is also a
   prime candidate for the fast reroute services. Since most of the
   voice codecs in use today operate in the range of 20-50 ms latency,
   the network component is left with around 300 ms of the end-to-end
   delay limit.

   Given the above considerations, it is important that, when
   restoration provisions are made for a delay sensitive service,
   transmission delays over an alternative path would not exceed an
   acceptable limit. Since a number of the current network providers
   are capable to guarantee network transport delay that do not exceed
   80 ms on their backbone, it appears that in some cases it will be
   possible to use the proposed restoration technique with a single
   alternative path. It allows for at most 200 ms round trip delay over
   a reverse path segment plus at most 100 ms delay over a disjoint
   backup path segment. However in other cases it may be necessary to
   introduce restoration shortcuts as described below to satisfy the
   VoIP latency requirement during restoration.

   Restoration shortcuts are achieved by allowing selected transit
   routers in the primary LSP to establish one or more 'shortcut'
   alternative LSPs to the egress router as illustrated in Figure 3. In
   this illustration, primary link failures that may occur downstream
   of LSR B are rerouted over the shortcut LSP from LSR B to the
   destination of LSP being backed  up. In illustrated example the
   shortcut LSP merges into the backup LSP at LSR D.



Haskin, et al.             Expires May 2001                   [Page 7]


Internet Draft  draft-haskin-mpls-fast-reroute-05.txt   November 2000

   Figure 3:

     +--+             +---+             +--+
     |  |------------>|LSR|------------>|D |
     |  |             | D |             |E |
     | S|             +---+             |S |
     | O|               ^               |T |
     | U|               |               |I |
     | R|               |               |N |
     | C|    +---+    +---+    +---+    |A |
     | E|<---|LSR|<---|LSR|<---|LSR|<---|T |
     |  |===>| A |===>| B |===>| C |===>|I |
     |  |    +---+    +---+    +---+    |O |
     |  |                               |N |
     +--+                               +--+



  6. 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.


  7. Bandwidth Reservation Considerations

   Generally there is no need to exclusively allocate bandwidth
   resources to the alternate LSP. The holding priority of the primary
   LSP can be used as traffic-triggered 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. What we
   call here the traffic-triggered priority is the preemption priority
   assigned to an LSP that is utilized 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
   alternative 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 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.



Haskin, et al.             Expires May 2001                   [Page 8]


Internet Draft  draft-haskin-mpls-fast-reroute-05.txt   November 2000

  8. Intellectual Property Considerations

   IETF has been informed of possible intellectual property protection
   for some or all of the technologies disclosed in this document.


  9. Acknowledgments

   This document has benefited from discussions with Jim Boyle, Robert
   Boyd, and Alan Hannan. We also thank Ken Schroder, Jeff Parker and
   Yanhe Fan for their comments on the document.


  10. References

  [1]  Rosen, E. et al., "Multiprotocol Label Switching Architecture",
       Internet Draft, draft-ietf-mpls-arch-07.txt, July 2000.

  [2]  Awduche, D. et al., "Requirements for Traffic Engineering over
       MPLS", RFC-2702.


  11.    Authors' Addresses

   Dimitry Haskin
   Axiowave Networks, Inc.
   100 Nickerson Road
   Marlborough, MA 01752
   E-mail: dhaskin@axiowave.com

   Ram Krishnan
   Axiowave Networks, Inc.
   100 Nickerson Road
   Marlborough, MA 01752
   E-mail: ram@axiowave.com





















Haskin, et al.             Expires May 2001                   [Page 9]


Html markup produced by rfcmarkup 1.129c, available from https://tools.ietf.org/tools/rfcmarkup/