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Internet Engineering Task Force                     Olivier Bonaventure
INTERNET DRAFT                                          Pierre Francois
                                        Mike Shand
                                                        Stefano Previdi
                                                         February, 2006
                                                   Expires August, 2006

                ISIS extensions for ordered FIB updates

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   This document proposes extensions to allow IS-IS to support the
   ordered convergence defined in [RTG-OFIB] that allows to avoid
   transient forwarding loops during the FIB updates that follow a non-
   urgent topology change.

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1   Introduction

   In large ISP networks, topology changes are frequent events. When a
   link metric changes, the router responsible for the changing link
   floods a new LSP and forces all routers to recompute their routing
   table and update their FIB. This routing convergence is that it can
   lead to packet losses due to transient forwarding loops [RTG-OFIB].

   There are various types of topology changes in IP networks. Sudden
   failures of unprotected links are urgent and should be handled by
   using a normal, fast, convergence of the link state routing protocol.
   However, there are frequent topology changes that are non-urgent. For
   example, operators often need to enable or disable links for short
   periods of time during maintenance operations. Another example are
   the IP networks where traffic engineering is performed by changing
   the ISIS link metrics.  These events are non-urgent and they should
   not cause packet losses or transient forwarding loops. Transient
   forwarding loops can be avoided by forcing the routers to orderly
   update their FIB as proposed in [RTG-OFIB]

   In this document, we propose IS-IS extensions allowing IS-IS routers
   to implement the ordered FIB update described in [RTG-OFIB]. We first
   describe the considered topology changes in section 2. Section 3
   defines the new IS-IS TLVs that are used to support [RTG-OFIB].

2.  Graceful topology changes

   In this section, we describe two types of graceful topology changes.
   The first one is the change of a link metric (increase or decrease).
   The second one is the change of the settings of the overload bit (Set
   to Unset or Unset to Set).

 2.1 Change in link metric

   The first change that we consider is a modification of the metric
   associated to a link. This change can be a metric increase or a
   metric decrease.  A non-urgent link shutdown can be converted into a
   weight change by doing the topology change in two steps. First, the
   metric of the link is set to the highest possible value. This
   triggers an ordered FIB update.  After the FIB update, the link can
   be safely removed without risking transient loops. Similarly, a
   failure of a protected link [IPFRR] [MPLSFRR] can be handled as a
   non-urgent failure.

 2.2 Overload status change

   The second change that we consider is the transition of the overload

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   from Set to Unset or from Unset to Set.

3. Extensions to IS-IS

   In this section, we describe the IS-IS extensions that are required
   to support ordered FIB updates that allow to avoid transient loops.
   There are two types of new TLVs. First, we define a capability sub-
   TLV to allow a router to advertise its ability to support ordered FIB
   updates. Second, we define a TLVs to be used inside the IS-IS Hello
   PDUs to implement the completion messages defined in [RTG-OFIB].

 3.1 Ordered FIB capability sub-TLV

   We define a FIB capability sub-TLV (advertised in the IS-IS
   CAPABILITY TLV defined in [ISIS-CAP] ) to allow routers to determine
   which routers in the network support the ordered FIB updates. The FIB
   capability sub-TLV is composed of 1 octet for the type, 1 octet
   specifying the TLV length and a value field. The FIB capability TLV
   is used to advertise the type of ordered FIB updates supported by
   this router. The FIB capability sub-TLV shall only be flooded inside
   the current area.

       TYPE: TBD (Ordered FIB capability sub-TLV)
       LENGTH: 1 byte
       VALUE: Indicates the version of the ordered FIB update supported
   by the router. This document defines version 1.

   This ordered FIB capability sub-TLV must be sent in a router
   capability TLV where the S bit is set to 0x0 and the D bit is set to
   0x0 according to the leaking rules defined in [ISISCAP].

 3.2 Completion message TLV

   We define a new type of TLV to encode inside the IS-IS Hello PDUs the
   completion messages proposed in [RTG-OFIB]. A IS-IS Hello PDU may
   carry several completion messages. Each completion message is encoded
   as a sub-TLV inside the completion message TLV.

       Type   TBD (Completion TLV)
       Length # of bytes in the value field (variable)
       Value : one or several sub-TLVs defined below

   This document defines three types of sub-TLVs for the completion

 3.2.1 Metric change sub-TLV

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   The metric change sub-TLV shall be used as a completion message when
   the metric of a link changes. This happens when a metric is changed
   manually.  It is also possible to use the mechanisms defined in [RTG-
   OFIB] and this document for link failures. For example, when a link
   is shutdown manually, the router could first advertise the link with
   metric maxmetric-1 as a non-urgent change to let all routers apply
   the ordered FIB update. Later, the link can be safely removed without
   risking transient loops. A router could use the same technique when
   one of its protected links fails.

   We define two types of metric changes sub-TLV that are used inside
   the completion TLV. The first one is used with wide metrics and the
   second one with normal metrics. We expect that most deployments will
   use wide metrics.

   Type   TBD (Wide Metric change sub-TLV)
   Length # of bytes in the value field (21)
                                       No. of bytes
                | 0  0  0  0 0 0 0 |Ack |     1
                | Upstream Node Id      |     7
                | Downstream Node Id    |     7
                | Old metric            |     3
                | New metric            |     3
     Figure 1: New sub-TLV for (wide) metric change completion message

   In the Wide metric change sub-TLV, the rightmost bit of the first
   byte is used to indicate whether it corresponds to a completion
   message (value 0) or an acknowledgment (value 1).  The upstream and
   downstream node Ids are the system identifiers of the link whose
   metric changes. "Old metric" is the previous value of the wide metric
   for this link and "New metric" the current value.

   A similar sub-TLV is also defined for the normal metric. As current
   implementations of IS-IS only support the default metric, we do not
   consider changes in the delay, error or expense metrics.

   Type   TBD (Normal Metric change sub-TLV)
   Length # of bytes in the value field (17)
                                        No. of bytes

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                | 0  0  0  0 0 0 0 |Ack |     1
                | Upstream Node Id      |     7
                | Downstream Node Id    |     7
                | Old Default metric    |     1
                | New Default metric    |     1

    Figure 2: New sub-TLV for (normal) metric change completion message

   The Default metric shall be encoded as in normal LSPs.

 3.2.2 Overload change sub-TLV

   The overload change sub-TLV is used to encode the changes to the
   Overload Bit.

   Type   TBD (Overload change sub-TLV)
   Length # of octets in the value field (10)
                                      No. of bytes

                |Old|New| 0 0 0 0 0 Ack |     1
                | Node Id               |     7

               Figure 3: New sub-TLV for Overload bit change

   The first octet contains three useful bits. The leftmost bit is the
   old value of the overload bit, the next bit is the new value of the
   overload bit and rightmost bit is the acknowledgment bit. The Node Id
   is the system identifier of the node whose overload bit change

 3.3 Utilization of the completion messages

   The TLVs defined in sections 3.2 and 3.3 allow to implement the
   completion messages specified in [RTG-OFIB]. Each sub-TLV corresponds
   to a completion message for one topological change. The completion
   TLV in one HELLO PDU may carry several sub-TLVs and thus several
   completion messages. A router supporting the ordered FIB update

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   defined in this document SHOULD send HELLO PDUs containing the
   completion messages according to the rules defined in [RTG-OFIB].

   To ensure a reliable delivery of the completion messages, an
   acknowledgment scheme is used. Upon reception of a HELLO PDU
   containing a completion message (rightmost bit of the first byte set
   to 0 in the sub-TLV), a router MUST ensure that the next HELLO PDU
   that it will send to this neighbor will contain, in the completion
   TLV, the same sub-TLV but with the rightmost bit of the first byte
   set to one. This sub-TLV is used to acknowledge the completion
   message. A router MAY send a HELLO PDU immediately in response to a
   receive completion message or wait for the next scheduled Hello
   transmission. A router MAY retransmit the same completion message in
   several HELLO PDUs to ensure that they are correctly received by the

5. Security considerations

   This document raises no new security issues for IS-IS. For the
   ordered FIB capability sub-TLV, the discussion in [ISISCAP] is
   applicable.  The authentication mechanisms defined in [RFC] can be
   used to authenticate the Hello PDUs that carry the completion
   messages if required.

6. IANA considerations

   IANA will assign a new IS-IS sub-TLV code-point for the order FIB
   capability sub-TLV that can appear inside the Router Capability TLV
   defined in [ISISCAP].

   IANA will assign a new IS-IS TLV code-point for the completion TLV.
   This TLV can only appear inside Hello PDUs.

   Name                                  Value  IIH   LSP  SNP
   ----------------------                -----  ---   ---  ---
   Completion messages TLV                TBD    y     n    n

   IANA will maintain a registry with the sub-TLVs defined for the
   completion messages TLV. This document defines three sub-TLVs :

   Name                                  Value
   ----------------------                -----
   Normal metric change sub-TLV           TBD
   Wide metric change sub-TLV             TBD
   Overload change sub-TLV                TBD

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7.   Conclusion

   In this document, we have proposed a set of IS-IS TLVs that allows
   routers using IS-IS to support the ordered FIB convergence
   and the completion messages proposed in [RTG-OFIB].


   [IS-IS]   "Intermediate system to Intermediate system routeing
             information exchange protocol for use in conjunction
             with the Protocol for providing the Connectionless-mode
             Network Service (ISO 8473)," ISO/IEC 10589:2002,
             Second Edition.
   [IPFRR]   M. Shand, S. Bryant, IP Fast Reroute Framework,
             draft-ietf-rtgwg-ipfrr-framework-04.txt, October 2005
   [MPLSFRR] Pan, P. et al, "Fast Reroute Extensions to
             RSVP-TE for LSP Tunnels", RFC 4090.
   [PFOB05]  P. Francois, O. Bonaventure. Avoiding transient loops
             during IGP convergence. IEEE INFOCOM 2005, March 2005,
             Miami, Fl., USA
   [RTG-OFIB] P. Francois, O. Bonaventure, M. Shand, S. Previdi, S. Bryant,
           Loop-free convergence using ordered FIB updates, Internet draft,
           draft-francois-ordered-fib-00.txt, work in progress, February 2006
   [ISISCAP]  J.-P. Vasseur, S. Previdi, M. Shand, IS-IS extensions for
   advertising router capabilities, Internet draft,
   draft-vasseur-isis-caps-06.txt, February 2004, work in progress
   [RFC3567] T. Li, R. Atkinson, Intermediate System to Intermediate
   System (IS-IS), Cryptographic Authentication, July 2003, RFC3567

Authors Addresses

   Olivier Bonaventure, Pierre Francois
   Universite catholique de Louvain
   Email: FirstName.LastName@uclouvain.be
   URL  : http://www.info.ucl.ac.be/people/OBO/

   Stefano Previdi, Mike Shand
   Cisco Systems
   Email: {sprevidi,mshand}@cisco.com

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