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

Versions: 00










Network Working Group                                         Bob Thomas
Internet Draft                                       Cisco Systems, Inc.
Expiration Date: August 2003
                                                                Alex Raj
                                                     Cisco Systems, Inc.

                                                           February 2003


                        LDP DoD Graceful Restart


                draft-thomas-mpls-ldp-dod-restart-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.

   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

   LDP graceful restart is a mechanism that helps reduce the negative
   effects on MPLS traffic caused by the restart of a Label Switching
   Router's (LSR's) control plane, specifically by the restart of its
   Label Distribution Protocol (LDP) component [RFC3036], on LSRs that
   are capable of preserving MPLS forwarding state across the restart.
   [RFC3478] defines procedures for LDP graceful restart for downstream
   unsolicited label distribution but leaves procedures for downstream
   on demand label distribution a subject for future study.  This
   document defines graceful restart procedures for downstream on demand
   label distribution.



Thomas & Raj                                                    [Page 1]


Internet Draft  draft-thomas-mpls-ldp-dod-restart-00.txt   February 2003


1. Introduction

   LDP graceful restart is a mechanism that helps reduce the negative
   effects on MPLS traffic caused by the restart of a Label Switching
   Router's (LSR's) control plane, specifically by the restart of its
   Label Distribution Protocol (LDP) component [RFC3036], on LSRs that
   are capable of preserving MPLS forwarding state across the restart.

   [RFC3478] defines procedures for LDP graceful restart for downstream
   unsolicited label distribution but leaves procedures for downstream
   on demand label distribution a subject for future study.  This
   document defines graceful restart procedures for downstream on demand
   label distribution.

   This document uses terminology introduced in [RFC3478] and, where
   appropriate, uses procedures specified in [RFC3478].  In addition, in
   places it borrows text from [RFC3478], indicating when it does so.

   As in [RFC3478], for the sake of brevity:

   - The phrase "the control plane" means the "the LDP component of the
     control plane".

   - The phrase "MPLS forwarding state (for an LSP)" or simply
     "forwarding state (for an LSP)" means the mapping:

         < FEC -> push outgoing_label, nexthop >
           at an LSP ingress LSR, or

         < incoming_label -> swap outgoing_label, nexthop >
           at an LSP transit LSR, or

         < incoming_label -> pop, nexthop >
           at an LSP transit LSR, with penultimate hop popping or
           at an LSP egress

   The incoming_label in the forwarding state for an LSP may be a
   platform-wide label or an interface-specific label.  Similarly, the
   outgoing_label learned from the nexthop may be a platform-wide label
   or an interface-specific label.  For the procedures below that search
   MPLS forwarding state for an entry with a given incoming or outgoing
   label when the search key is an interface-specific label the match
   criteria includes an interface match.  The procedures below also
   search MPLS forwarding state for an LSP or LSPs which match a given
   nexthop.  Determining that a given LDP peer is an LSP nexthop may
   require comparing the nexthop with addresses advertised in Address
   messages from the peer.




Thomas & Raj                                                    [Page 2]


Internet Draft  draft-thomas-mpls-ldp-dod-restart-00.txt   February 2003


   For the sake of brevity, the phrase "LDP state (for an LSP)" means
   the mapping:

       < FEC -> { outgoing_label, nexthop, nexthop neighbor LDP ID } >
         at an LSP ingress LSR, or

       < FEC -> { incoming_label, upstream neighbor LDP ID },
                { outgoing_label, nexthop, nexthop neighbor LDP ID } >
         at an LSP transit LSR, or

       < FEC -> { incoming_label, upstream neighbor LDP ID } >
         at an LSP egress LSR

   The text in [RFC3478] applies here and is repeated verbatim for the
   reader's convenience.

     "In the case where a Label Switching Router (LSR) could preserve
     its MPLS forwarding state across restart of its control plane,
     specifically its LDP component [LDP], it is desirable not to
     perturb the LSPs going through that LSR (specifically, the LSPs
     established by LDP).  In this document, we describe a mechanism,
     termed "LDP Graceful Restart", that allows the accomplishment of
     this goal".

   This document extends the mechanisms defined in [RFC3478] to enable
   an LSR using LDP downstream on demand label distribution to
   accomplish the goal as well.

   Continuing with the text from [RFC3478]:

     "The mechanism described ... is applicable to all LSRs, both those
     with the ability to preserve forwarding state during LDP restart
     and those without (although the latter need to implement only a
     subset of the mechanism described ...).  Supporting (a subset of)
     the mechanism described here by the LSRs that can not preserve
     their MPLS forwarding state across the restart would not reduce the
     negative impact on MPLS traffic caused by their control plane
     restart, but it would minimize the impact if their neighbor(s) are
     capable of preserving the forwarding state across the restart of
     their control plane and implement the mechanism described here."

   In specifying procedures this document distinguishes between:

   - A "restarting LSR" which is one which has lost its LDP state but
     has retained its forwarding state; and

   - A "neighbor LSR" which is one which has retained its LDP state and
     its forwarding state but has lost its LDP session with another LSR.



Thomas & Raj                                                    [Page 3]


Internet Draft  draft-thomas-mpls-ldp-dod-restart-00.txt   February 2003


   The goal of the mechanisms specified in [RFC3478] and this document
   is to enable uninterrupted forwarding across a control plane restart.
   The graceful restart mechanisms enable a restarting LSR to validate
   its forwarding state and recover its LDP state and a neighbor LSR to
   to validate its LDP state.


2. LDP Extensions

   [RFC3478] specifies use of the Fault Tolerant (FT) Session TLV [LDP-
   FT] in the LDP Initialization message.  The specified use of the FT
   Session TLV as described in [RFC3478] applies to the extensions for
   downstream on demand graceful restart as well.

   This document modifies the procedures for Label Request, Label
   Mapping and Label Abort messages as follows for LSRs using graceful
   restart for downstream on demand label distribution.

     - An LSR may include a Label TLV (more specifically, a Generic
       Label, ATM Label or Frame Relay Label TLV, as appropriate) in a
       Label Request message to suggest the label a downstream neighbor
       should use in its Label Mapping message response.

     - If an LSR receives a Label Request message for which it has stale
       LDP state it may use the message to validate the state and it may
       reply with a Label Mapping message.  Normal [RFC3036] procedure
       is to consider such a (redundant) Label Request as a protocol
       error and to ignore it.

     - If an LSR which is not restarting receives a Label Mapping
       message for which it has no LDP state it responds with a Label
       Release message.

     - If an LSR receives a Label Abort message which doesn't match a
       previously received Label Request message it replies with a
       Notification message with Label Request Aborted status code.
       Normal [RFC3-36] procedure is to ignore such a Label Abort.


3. Operations

   The text in [RFC3478] applies here and is repeated verbatim for the
   reader's convenience.

     "An LSR that supports functionality described in this document
     advertises this to its LDP neighbors by carrying the FT Session TLV
     in the LDP Initialization message.




Thomas & Raj                                                    [Page 4]


Internet Draft  draft-thomas-mpls-ldp-dod-restart-00.txt   February 2003


     "This document assumes that in certain situations, as specified in
     section "[Restarting] Egress LSR", in addition to the MPLS
     forwarding state, an LSR can also preserve its IP forwarding state
     across the restart.  Procedures for preserving IP forwarding state
     across the restart are defined in [OSPF-RESTART], [ISIS-RESTART],
     and [BGP-RESTART]."


3.1. Procedures for restarting LSR

   The text in [RFC3478] applies here and is repeated verbatim for the
   reader's convenience.

     "After an LSR restarts its control plane, the LSR MUST check
     whether it was able to preserve its MPLS forwarding state from
     prior to the restart.  If not, then the LSR sets the Recovery Time
     to 0 in the FT Session TLV the LSR sends to its neighbors.

     "If the forwarding state has been preserved, then the LSR starts
     its internal timer, called MPLS Forwarding State Holding timer (the
     value of that timer SHOULD be configurable), and marks all the MPLS
     forwarding state entries as "stale".  At the expiration of the
     timer, all the entries still marked as stale SHOULD be deleted.
     The value of the Recovery Time advertised in the FT Session TLV is
     set to the (current) value of the timer at the point in which the
     Initialization message carrying the FT Session TLV is sent.

     "We say that an LSR is in the process of restarting when the MPLS
     Forwarding State Holding timer is not expired.  Once the timer
     expires, we say that the LSR completed its restart."

   The following procedures apply when an LSR using downstream on demand
   label distribution is in the process of restarting.  The graceful
   restart procedures for downstream unsolicited described in [RFC3478]
   distinquish between the behavior of (LSP) egress and non-egress LSRs.
   The procedures for downstream on demand described here distinguish
   between the behavior of (LSP) ingress, transit and egress LSRs.

   In some situations described below a restarting LSR creates LDP state
   which it marks as stale until it can be fully validated.  When the
   Forwarding State Holding timer expires the restarting LSR should
   delete all LDP state marked as stale.









Thomas & Raj                                                    [Page 5]


Internet Draft  draft-thomas-mpls-ldp-dod-restart-00.txt   February 2003


3.1.1. Restarting Ingress LSR

   Reference diagram:

      LSP -->
      R ------ N --- ...

   On receipt of a Label Mapping message from LSR N:

     LabelMap(FEC, LabelTLV(outgoing_label))

   LSR R searches its forwarding state for an LSP for FEC with the
   specified outgoing_label and nexthop N:

     < FEC -> push outgoing_label, nexthop > or
     < FEC -> pop, nexthop >

   where an outgoing_label of Implicit Null matches a pop entry.  If R
   finds no such forwarding state it is not an ingress for the FEC.

   If R finds finds the forwarding state it creates LDP state for the
   FEC:

     < FEC, { outgoing_label, nexthop, nexthop neighbor LDP ID } >

   and marks it stale.  Next it sends a Label Request message to N with
   a Label TLV suggesting outgoing_label:

     LabelReq(MsgId_R, FEC, LabelTLV(outgoing_label))

   On receipt of a matching Label Mapping message from N:

     LabelMap(FEC, MsgIdTLV(MsgId_R), LabelTLV(neighbor_label))

   R updates its LDP state for the LSP and clears the stale mark:

     < FEC, { neighbor_label, nexthop, nexthop neighbor LDP ID } >

   If the received neighbor_label matches outgoing_label in the
   forwarding state R clears the stale mark from the LSP forwarding
   state.  Otherwise, R replaces the LSP forwarding state, as
   appropriate, with:

     < FEC -> push neighbor_label, nexthop > or
     < FEC -> pop, nexthop >

   If R's Forwarding State Holding timer expires before it receives the
   reply to its Label Request message it deletes its forwarding state



Thomas & Raj                                                    [Page 6]


Internet Draft  draft-thomas-mpls-ldp-dod-restart-00.txt   February 2003


   for the LSP as per [RFC3478], clears the stale mark from its LDP
   state for the LSP, sends a Label Abort message to N, and follows the
   [RFC3036] procedures for Label Abort.


3.1.2. Restarting Egress LSR

   Reference diagram:

          LSP -->
      --- N ------ R

   On receipt of a Label Request message from neighbor LSR N for a FEC
   for which restarting LSR R is an egress and which includes a
   suggested label:

     LabelReq(MsgId_N, FEC, LabelTLV(incoming_label))

   LSR R checks whether the suggested label is compatible with its
   configuration for generating labels for the FEC.  (An LSR may be
   configured to use a non-Null label, Explicit Null or Implicit Null
   for a FEC for which it is an egress.)  If the suggested label is not
   compatible, R behaves as if the message had no Label TLV and treats
   it as specifed by [RFC3036].

   If the suggested label is compatible with its configuration R
   attempts to locate forwarding state for the LSP using the suggested
   incoming_label.  If incoming_label is non-Null R searches its MPLS
   forwarding state for an entry:

     < incoming_label -> pop, nexthop >

   and if incoming_label is either Explicit or Implicit Null R searches
   its IP forwarding state for an entry for the FEC.

   If R cannot locate forwarding state corresponding to the Label
   Request message, it behaves as if the message had no Label TLV and
   treats it as specified by [RFC3036].

   If R succeeds in locating forwarding state for the FEC it:

     1. Updates its LDP state for the LSP:

          < FEC, { incoming_label, upstream neighbor LDP ID } >

     2. Clears the stale mark from the LSP forwarding state if
        incoming_label is non-NULL;




Thomas & Raj                                                    [Page 7]


Internet Draft  draft-thomas-mpls-ldp-dod-restart-00.txt   February 2003


     3. Replies to N with a Label Mapping message:

          LabelMap(FEC, MSGIdTLV(MsgId_N), LabelTLV(incoming_label)


3.1.3. Restarting Transit LSR

   Reference diagram:

                    --- LSP -->
      ... --- Nu ------ R ------ Nd --- ...

   Restarting LSR R must complete graceful restart procedures with
   upstream neighbor LSR Nu and downstream neighbor LSR Nd to recover a
   transit LSP.  The order in which the procedures with Nu and Nd begin
   depends upon when the respective LDP sessions are established and the
   first LDP messages for the LSP are received by R.

   The procedure with Nu begins with the receipt of a Label Request
   message with a suggested incoming_label from Nu:

     LabelReq(MsgId_Nu, FEC, LabelTLV(incoming_label))

   LSR R attempts to locate forwarding state for the LSP using the
   suggested incoming_label:

     < incoming_label -> swap outgoing_label, nexthop >, or
     < incoming_label -> pop, nexthop >

   If R cannot locate forwarding state corresponding to the Label
   Request message, it behaves as if the message had no Label TLV and
   treats it as specified by [RFC3036].

   If R succeeds in locating the forwarding state it updates its LDP
   state for the LSP to include incoming_label and marks it stale:

     < FEC, { incoming_label, neighbor Nu LDP ID } >

   At this point R's interaction with Nu for the LSP is said to be in
   the "waiting for downstream" state and its interaction with Nu must
   wait until it has a session with Nd and it completes the following
   procedure for clearing the stale mark on the LSP LDP state.

   The procedure with nexthop LSR Nd begins with the receipt of a Label
   Mapping from Nd for the LSP:

     LabelMap(FEC, LabelTLV(outgoing_label))




Thomas & Raj                                                    [Page 8]


Internet Draft  draft-thomas-mpls-ldp-dod-restart-00.txt   February 2003


   When it receives the message R searches its forwarding state for an
   LSR with the specified outgoing_label and nexthop Nd:

     < incoming_label -> swap outgoing_label, nexthop >, or
     < incoming_label -> pop, nexthop >

   If R cannot locate forwarding state corresponding to the Label
   Mapping it ignores the message.

   If R succeeds in locating the forwarding state it updates its LDP
   state for the LSP to include:

     < FEC, { outgoing_label, neighbor Nd LDP ID } >

   and marks it stale.

   At this point R's interaction with Nd must wait until it has a
   session with Nu and the interaction with Nu for the LSP is in the
   "waiting for downstream" state.

   When the interaction with Nu is in "waiting for downstream" state R
   sends Nd a Label Request message with a Label TLV suggesting
   outgoing_label:

     LabelReq(MsgId_R, FEC, LabelTLV(outgoing_label))

   On receipt of a Label Mapping message from Nd:

     LabelMap(FEC, MsgIdTLV(MsgId_R), LabelTLV(neighbor_label))

   that matches the Label Request message R:

     1. Checks if neighbor_label matches the outgoing_label in the LSP
        forwarding state.  If so, it clears the stale mark from the
        forwarding state.  Otherwise, it replaces the forwarding state,
        as appropriate, with:

          < incoming_label -> swap neighbor_label, nexthop >, or
          < incoming_label -> pop, nexthop >

     2. Updates its LDP state for the LSP and clears the stale mark:

          < FEC, { incoming_label, neighbor Nu LDP ID } >
                 { neighbor_label, nexthop, neighbor Nd LDP ID } ]

   At this point R may complete its interaction with Nu for the LSP by
   replying to Nu's Label Request message with a Label Mapping message:




Thomas & Raj                                                    [Page 9]


Internet Draft  draft-thomas-mpls-ldp-dod-restart-00.txt   February 2003


     LabelMap(FEC, MsgIdTLV(MsgId_Nu), LabelTLV(incoming_label))

   If R's Forwarding State Holding timer expires before it receives the
   reply from Nd to its Label Request message it deletes its forwarding
   state for the LSP as per [RFC3478], sends a Label Abort message to
   Nd, and deletes its LDP state for the LSP.


3.2. Restart of LDP communication with neighbor LSR

   An LSP is said to be "fully established from the point of view of an
   LSR" if the LSR has installed forwarding state for the LSP.

   When an LSR detects that an LDP downstream on demand session with a
   neighbor capable of preserving MPLS forwarding state across the
   restart has gone down, the LSR handles the LDP state learned by the
   session as follows:

   - It marks the LDP state for LSPs that are fully established from its
     point of view as "stale";

   - It handles the LDP state for LSPs not fully established as if the
     neighbor was incapable of preserving MPLS forwarding state across
     the restart; that is, it follows [RFC3036] procedures for session
     loss for these LSPs.

   The text in [RFC3478] regarding neighbor LSR behavior when an LDP
   session with a neighbor is lost and later re-established applies here
   and is repeated verbatim for the reader's convenience.

     "After detecting that the LDP session with the neighbor went down,
     the LSR tries to re-establish LDP communication with the neighbor
     following the usual LDP procedures.

     "The amount of time the LSR keeps its stale label-FEC bindings is
     set to the lesser of the FT Reconnect Timeout, as was advertised by
     the neighbor, and a local timer, called the Neighbor Liveness
     Timer.  If within that time the LSR still does not establish an LDP
     session with the neighbor, all the stale bindings SHOULD be
     deleted.  The Neighbor Liveness Timer is started when the LSR
     detects that its LDP session with the neighbor went down.  The
     value of the Neighbor Liveness timer SHOULD be configurable.

     "If the LSR re-establishes an LDP session with the neighbor within
     the lesser of the FT Reconnect Timeout and the Neighbor Liveness
     Timer, and the LSR determines that the neighbor was not able to
     preserve its MPLS forwarding state, the LSR SHOULD immediately
     delete all the stale label-FEC bindings received from that



Thomas & Raj                                                   [Page 10]


Internet Draft  draft-thomas-mpls-ldp-dod-restart-00.txt   February 2003


     neighbor.  If the LSR determines that the neighbor was able to
     preserve its MPLS forwarding state (as was indicated by the non-
     zero Recovery Time advertised by the neighbor), the LSR SHOULD
     further keep the stale label-FEC bindings, received from the
     neighbor, for as long as the lesser of the Recovery Time advertised
     by the neighbor, and a local configurable value, called Maximum
     Recovery Time, allows."

   When the LSR determines that the neighbor has preserved its MPLS
   forwarding state it starts an internal timer called the LDP Recovery
   timer.

   Other than to specify the procedure when the Neighbor Liveness Timer
   expires [RFC3478] need not address LSR behavior following session
   loss prior to re-establishment.  The procedures for downstream on
   demand specify neighbor LSR behavior for individual LSPs during this
   period as well as following session re-establishment.


3.2.1. Neighbor Ingress LSR

   Reference Diagram:

      LSP -->
      N ------ P --- ...

   Prior to session re-establishment if the nexthop for an LSP FEC
   changes from peer LSR P LSR N clears the stale mark from the LSP LDP
   state and follows the [RFC3036] procedures for a nexthop change with
   the following difference.  Where the procedures require N to send a
   Label Release message to Pd N omits the message.

   Following session establishment with downstream peer LSR P, neighbor
   LSR N scans its LDP state for LSPs for which it is the ingress LSR
   and which use P as nexthop:

     < FEC, { outgoing_label, nexthop, nexthop neighbor LDP ID } >

   For each such LSP N sends P a Label Request message with a Label TLV
   suggesting outgoing_label:

     LabelReq(MsgId_N, FEC, LabelTLV(outgoing label))

   On receipt of a matching Label Mapping message from P:

     LabelMap(FEC, MsgIdTLV(MsgId_N), LabelTLV(neighbor_label))

   N updates its LDP state for the LSP and clears the stale mark:



Thomas & Raj                                                   [Page 11]


Internet Draft  draft-thomas-mpls-ldp-dod-restart-00.txt   February 2003


     < FEC, { neighbor_label, nexthop, nexthop neighbor LDP ID } >

   If the received neighbor_label matches outgoing_label in the LSP
   forwarding state N takes no further action; otherwise, it replaces
   the LSP forwarding state, as appropriate, with:

     < FEC -> push neighbor_label, nexthop > or
     < FEC -> pop, nexthop >

   If N's LDP Recovery timer expires before it receives the reply to its
   Label Request message it deletes its forwarding state for the LSP,
   removes the outgoing_label from the LDP state for the LSP, clears the
   stale mark from the LDP state, sends a Label Abort message to P, and
   follows the [RFC3036] procedures for Label Abort.


3.2.2. Neighbor Egress LSR

   Reference Diagram:

              LSP -->
      ... --- P ------ N

   Prior to session re-establishment if the nexthop for the LSP FEC
   (i.e., the FEC route) is lost N follows the [RFC3036] procedures for
   a change in FEC nexthop with the following difference.  Where the
   procedures require N send a Label Withdraw message to upstream peer
   LSR Pu it omits the message and behaves as if it had sent the message
   and Pu had replied with a Label Release message.

   Following session establishment with upstream LSR P neighbor LSR N
   scans its LDP state for LSPs for which it is an egress and P is the
   previous hop:

     [ FEC, { incoming label, upstream neighbor LDP ID } ]

   For each such LSP it sends a Label Mapping to P:

     LabelMap(FEC, LabelTLV(incoming label)

   On receipt of a Label Request message with a suggested label from P:

     LabelReq(MsgId_P, FEC, LabelTLV(incoming_label);

   neighbor LSR N attempts to locate LDP state using the FEC and
   suggested incoming_label:

   If N cannot locate the LDP state message, it behaves as if the



Thomas & Raj                                                   [Page 12]


Internet Draft  draft-thomas-mpls-ldp-dod-restart-00.txt   February 2003


   message had no Label TLV and treats it as specified by [RFC3036].

   If N succeeds in locating the LDP state, it clears the stale mark
   from the LDP state and replies to P with a Label Mapping message:

     LabelMap(FEC, MsgIdTLV(MsgId_P), LabelTLV(incoming label)

   If N's LDP Recovery timer expires before it receives a Label Request
   message from P for the LSP it deletes its LDP state for the LSP and
   any MPLS forwarding state for the LSP.


3.2.3. Neighbor Transit LSR

   There are 3 cases for a neighbor LSR acting as a transit LSR.

   Case 1: LDP session with upstream peer LSR Pu is lost and recovered:

     Reference Diagram:

                       -- LSP -->
        ... --- Pu ------ N --- ...

     Prior to session re-establishment the following could occur for an
     LSP for which upstream LSR Pu is the previous hop:

       - N receives a Label Withdraw message from the LSP nexthop LSR
         (Pd).  In this case N follows the procedures specified in
         [RFC3036] for receipt of a Label Withdraw with the following
         differences.  Regardless of the control method used to
         establish the LSP N behaves as if it had used ordered control.
         Where the procedures require N send a Label Withdraw message to
         upstream peer LSR Pu it omits the message and behaves as if it
         had sent the message and Pu had replied with a Label Release
         message.

       - The nexthop for the LSP FEC changes.  In this case N follows
         the [RFC3036] procedures for a change in FEC nexthop with the
         following difference.  Where the procedures require N send a
         Label Withdraw message to upstream peer LSR Pu it omits the
         message and behaves as if it had sent the message and Pu had
         replied with a Label Release message.

     Following session establishment with upstream peer LSR Pu neighbor
     LSR N scans its LDP state for LSPs for which LSR Pu is the previous
     hop.  For each such LSP N behaves as described in section "Neighbor
     Egress LSR" with the following difference.




Thomas & Raj                                                   [Page 13]


Internet Draft  draft-thomas-mpls-ldp-dod-restart-00.txt   February 2003


     If N's LDP Recovery timer expires before it receives a Label
     Request message from Pu for the LSP it clears the stale mark from
     its LDP state for the LSP and behaves as if it had received a Label
     Release message from Pu and follows the [RFC3036] Label Release
     procedures.

   Case 2: LDP session with downstream peer LR Pd is lost and recovered:

     Reference Diagram:

            --- LSP -->
        ... --- N ------ Pd --- ...

     Prior to session re-establishment the following could occur for an
     LSP for which downstream LSR Pd is the nexthop:

       - The LSP previous hop LSR (Pu) releases its label for the LSP.
         In this case neighbor LSR N follows the [RFC3036] procedures
         for Label Release with the following difference.  Where the
         procedures require N to send a Label Release message to Pd N
         omits the message.

       - The nexthop for an LSP FEC changes.  In this case N clears the
         stale mark from the LSP LDP state and follows the [RFC3036]
         procedures for a nexthop change with the following difference.
         Where the procedures require N to send a Label Release message
         to Pd N omits the message.

     Following session establishment with downstream peer LSR Pd,
     neighbor LSR N scans its LDP state for LSPs for which Pd is
     nexthop.  For each such LSP N behaves as described in section
     "Neighbor Ingress LSR" with the following difference.

     If N's LDP Recovery timer expires before it receives the reply to
     its Label Request message it behaves as follows:

       1. It deletes its forwarding state for the LSP and clears the
          stale mark from its LDP state for the LSP.

       2. It sends a Label Abort message to Pd and follows the [RFC3036]
          procedures for Label Abort.

       3. With respect to its upstream neighbors it behaves as if it had
          received a Label Withdraw message from Pd and sends Label
          Withdraws to some or all of its upstream neighbors as
          specified by [RFC3036].





Thomas & Raj                                                   [Page 14]


Internet Draft  draft-thomas-mpls-ldp-dod-restart-00.txt   February 2003


   Case 3: LDP sessions with upstream peer LSR Pu and downstream peer
           LSR Pd are lost and recovered.

     Reference Diagram:

                     ---> LSP -->
        ... --- Pu ------ N ------ Pd --- ...

     Neighbor LSR N must complete graceful restart procedures with
     upstream peer LSR Pu and downstream peer LSR Pd to recover a
     transit LSP.  The order in which the procedures with Pu and Pd
     begin depends upon when the respective LDP sessions are established
     and the first LDP messages for the LSP are received by N.

     Prior to session re-establishment with Pd if the nexthop for an LSP
     FEC changes from downstream peer LSR Pd, neighbor LSR N follows the
     [RFC3036] procedures for a change in FEC nexthop with the following
     differences.  If the session with Pu has not been re-established
     where the [RFC3036] procedures require N send a Label Withdraw
     message to upstream peer LSR Pu N omits the message and behaves as
     if it had sent the message and Pu had replied with a Label Release
     message; and, where the procedures require N to send a Label
     Release message to Pd N omits the message.

     Following session establishment with upstream LSR Pu neighbor LSR N
     scans its LDP state for LSPs for which Pu is the previous hop:

       < FEC, { incoming label, upstream neighbor LDP ID } >

     For each such LSP it sends a Label Mapping to Pu:

       LabelMap(FEC, LabelTLV(incoming label)

     On receipt of a Label Request message with a suggested label from
     Pu:

       LabelReq(MsgId_Pu, FEC, LabelTLV(incoming_label);

     N attempts to locate LDP state using the FEC and suggested
     incoming_label:

     If N cannot locate the LDP state message, it behaves as if the
     message had no Label TLV and treats it as specified by [RFC3036].

     If N succeeds in locating the LDP state its interaction with Pu for
     the LSP is said to be in the "waiting for downstream" state and its
     interaction with Pu for the LSP must wait until it has a session
     with Pd and it completes the following procedure for clearing the



Thomas & Raj                                                   [Page 15]


Internet Draft  draft-thomas-mpls-ldp-dod-restart-00.txt   February 2003


     stale mark on the LSP LDP state.

     Following session establishment with downstream peer LSR Pd,
     neighbor LSR N scans its LDP state for LSPs which use Pd as
     nexthop:

       < FEC, { ... } >
              { outgoing_label, nexthop, nexthop neighbor LDP ID } >

     Each such LSP the interaction with Pd is considered to be in
     "waiting for upstream" state.  At this point for a given LSP N must
     wait until the interaction with Pu is in "waiting for downstream"
     state.

     When the interaction with Pu is in "waiting for downstream" state
     and the interaction with Pd is in "waiting for upstream" state N
     sends Pd a Label Request message with a Label TLV suggesting
     outgoing_label:

       LabelReq(MsgId_N, FEC, LabelTLV(outgoing label))

     On receipt of a Label Mapping message from Pd:

       LabelMap(FEC, MsgIdTLV(MsgId_N), LabelTLV(neighbor_label))

     that matches the Label Request message N:

       1. Checks if the received neighbor_label from Pd matches the
          outgoing_label in the LSP forwarding state.  If not N replaces
          the LSP forwarding state with:

            <incoming_label -> neighbor_label, nexthop>

       2. Updates its LDP state for the LSP and clears the stale mark:

            < FEC, { neighbor_label, nexthop, nexthop Pd LDP ID } >

     At this point, N may complete its interaction with Nu for the LSP
     by replying to Pu's Label Request message with a Label Mapping
     message:

       LabelMap(FEC, MsgIdTLV(MsgId_Pu), LabelTLV(incoming label)

     If N's LDP Recovery timer expires before it receives the reply from
     Pd to its Label Request message it deletes its forwarding state for
     the LSP as per [RFC3478], sends a Label Abort message to Pd, and
     deletes its LDP state for the LSP.




Thomas & Raj                                                   [Page 16]


Internet Draft  draft-thomas-mpls-ldp-dod-restart-00.txt   February 2003


3.3. Non-Merging LSRs

   The forwarding state for a given FEC with N LSPs on a non-merging
   restarting LSR is:

     [ incoming_label_1 -> swap outgoing_label_1, nexthop ]
     [ incoming_label_2 -> swap outgoing_label_2, nexthop ]
     ...
     [ incoming_label_N -> swap outgoing_label_N, nexthop ]

   In some situations it may be important for a restarting transit LSR
   to preserve the existing LSPs for a given FEC; that is, to ensure
   that following completion of graceful restart each incoming label is
   (re)connected to the same outgoing label for each LSP.

   The procedures described in section "Restarting Transit LSR" ensure
   that the LSR recovers each LSP for the FEC correctly.  Label Request
   messages from the upstream neighbor LSR identify the FEC LSPs by
   incoming_label.  This enables the LSR to determine the corresponding
   outgoing_label for the LSP from the forwarding state.  Similarly
   Label Mapping messages from the downstream neighbor LSR identify the
   FEC LSPs by outgoing_label.  This enables the LSR to determining the
   incoming_label for the LSP from the forwarding state.


3.4. Mixed DU / DoD operation

   An LSR may use downstream unsolicited label distribution with some
   neighbors and downstream on demand with others.  A situation of
   interest is where a restarting Transit LSR uses different methods
   with its upstream and downstream neighbors for a given LSP.

   The discussion below assumes all labels are Generic Labels.  In
   situations where the label types are different (e.g., the downstream
   on demand session distributes ATM Labels and the downstream
   unsolicited session distributes Generic Labels) the forwarding state
   and the way LSR R manages it are implementation dependent and are
   likely to be be different in detail, but are similar in kind, to
   situations where all labels are Generic.

   For this situation the LSP LDP state to be recovered is:

     < FEC, { incoming_label, neighbor Nu LDP ID } >
            { outgoing_label, nexthop, neighbor Nd LDP ID } >

   and the LSP forwarding state to be validated is:





Thomas & Raj                                                   [Page 17]


Internet Draft  draft-thomas-mpls-ldp-dod-restart-00.txt   February 2003


     < incoming_label -> outgoing_label, nexthop >, or
     < incoming_label -> pop, nexthop >

   To fully recover the LSP R must establish sessions with Nu and Nd and
   complete restart procedures with each.  In general R will not know
   until a session is established whether it is downstream on demand or
   downstream unsolicited.

   There are two cases to consider:

   Case 1: Upstream session is downstream on demand;
           downstream session is downstream unsolicited.

                      --- LSP -->
        ... --- Nu ------ R ------ Nd --- ...
                    DoD       DU

     The graceful restart procedures for restarting LSR R's session with
     Nu are those described above in section "Restarting Egress LSR"
     without the configuration check.  Successful completion of the
     procedures with Nu results in the recovery of part of the LSP LDP
     state:

       < FEC, { incoming_label, upstream neighbor  LDP ID } >

     and validation of the incoming_label part of the forwarding state.

     Completion of the [RFC3478] procedures with Nd results in recovery
     of part of the LDP state for the LSP:

       < FEC, { incoming_label },
              { outgoing_label, downstream neighbor LDP ID } >

     and full validation of the LSP LDP state.

   Case 2: Upstream session is downstream unsolicited;
           downstream session is downstream on demand.

                      --- LSP -->
        ... --- Nu ------ R ------ Nd --- ...
                     DU      DoD

     Following establishment of its session with DU upstream neighbor
     Nu, restarting LSR R considers all LSPs that transit Nu-R-Nd (which
     it has yet to identify from it preserved forwarding state and
     interactions with Nd) to be in "waiting for downstream" state, and
     it follows the [RFC3478] procedures for its session with Nu.




Thomas & Raj                                                   [Page 18]


Internet Draft  draft-thomas-mpls-ldp-dod-restart-00.txt   February 2003


     After establishment of R's session with DoD downstream neighbor Nd,
     on reciept of a Label Mapping message from Nd:

       LabelMap(FEC, LabelTLV(outgoing_label))

     LSR R searches its forwarding state for an LSR with the specified
     outgoing_label and nexthop Nd:

       < incoming_label -> swap outgoing_label, nexthop >, or
       < incoming_label -> pop, nexthop >

     If R cannot locate forwarding state corresponding to the Label
     Mapping it ignores the message.  If R succeeds in locating the
     forwarding state it updates its LDP state for the LSP to include:

       < FEC, { incoming_label, ... },
              { outgoing_label, neighbor Nd LDP ID } >

     At this point R's interaction with Nd must wait until it has a
     session with Nu and the interaction with Nu for the LSP is in the
     "waiting for downstream" state.

     When the interaction with Nu for the LSP is in "waiting for
     downstream" state R:

       1. Sends Nd a Label Request message with a Label TLV suggesting
          outgoing_label:

            LabelReq(MsgId_R, FEC, LabelTLV(outgoing_label))

       2. On receipt of a Label Mapping message from Nd:

            LabelMap(FEC, MsgIdTLV(MsgId_R), LabelTLV(neighbor_label))

          that matches the Label Request message R sends Nu a Label
          Mapping message:

            LabelMap(FEC, LabelTLV(incoming_label))

       3. Clears the stale mark from the LSP forwarding state.











Thomas & Raj                                                   [Page 19]


Internet Draft  draft-thomas-mpls-ldp-dod-restart-00.txt   February 2003


3.5. LSP attributes and loop detection

   [RFC3036] procedures for handling Hop Count and Path Vector TLVs in
   Label Mapping and Label Request messages apply following LDP session
   re-establishment during the period while Forwarding State and LDP
   Recovery timers are running.  This includes the procedures for the
   optional loop detection mechanism specified for use with downstrean
   on demand label distribution.


4. Security Considerations

   This document introduces no security issues beyond those previously
   identified by [RFC3036] and [RFC3478].


5. Acknowledgements

   The authors would like to thank Bin Mo, Bibhuti Kar, Iftekhar
   Hussain, Richard Lam and Bob Fish for their input.


6. References

   [BGP-RESTART]  "Graceful Restart Mechanism for BGP", draft-ietf-idr-
                  restart-03.txt, work in progress.

   [ISIS-RESTART] "Restart signaling for ISIS", draft-ietf-isis-
                  restart-01.txt, work in progress.

   [LDP-FT]       Farrell A., et al, "Fault Tolerance for the Label
                  Distribution Protocol (LDP)", draft-ietf-mpls-ldp-ft-
                  06.txt, September 2002, work in progress.

   [OSPF-RESTART] "Hitless OSPF Restart", draft-ietf-ospf-hitless-
                  restart-01.txt, work in progress.

   [RFC3036]      Andersson, L., et al, "LDP Specification, RFC3036",
                  January 2001.

   [RFC3478]      Leelanivas, M., et al, "Graceful Restart Mechanism for
                  LDP", draft-ietf-mpls-ldp-restart-06.txt, October
                  2002, work in progress.








Thomas & Raj                                                   [Page 20]


Internet Draft  draft-thomas-mpls-ldp-dod-restart-00.txt   February 2003


7. Authors' Addresses

   Alex Raj                                  Bob Thomas
   Cisco Systems, Inc                        Cisco Systems, Inc.
   300 Apollo Dr.                            300 Apollo Dr.
   Chelmsford, MA 01824                      Chelmsford, MA 01824
   email: araj@cisco.com                     email: rhthomas@cisco.com












































Thomas & Raj                                                   [Page 21]


Html markup produced by rfcmarkup 1.122, available from https://tools.ietf.org/tools/rfcmarkup/