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Versions: 00 01 02 draft-ietf-rtgwg-bgp-pic

Network Working Group                                  A. Bashandy, Ed.
Internet Draft                                              C. Filsfils
Intended status: Informational                            Cisco Systems
Expires: May 2016                                          P. Mohapatra
                                                       Sproute Networks
                                                       November 9, 2015
                   BGP Prefix Independent Convergence
                   draft-bashandy-rtgwg-bgp-pic-02.txt


Abstract

In the network comprising thousands of iBGP peers exchanging millions
of routes, many routes are reachable via more than one path. Given
the large scaling targets, it is desirable to restore traffic after
failure in a time period that does not depend on the number of BGP
prefixes. In this document we proposed an architecture by which
traffic can be re-routed to ECMP or pre-calculated backup paths in a
timeframe that does not depend on the number of BGP prefixes. The
objective is achieved through organizing the forwarding chains in a
hierarchical manner and sharing forwarding elements among the maximum
possible number of routes. The proposed technique achieves prefix
independent convergence while ensuring incremental deployment,
complete transparency and automation, and zero management and
provisioning effort. It is noteworthy to mention that the benefits of
BGP-PIC are hinged on the existence of more than one path whether as
ECMP or primary-backup.

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Table of Contents

   1. Introduction...................................................3
      1.1. Conventions used in this document.........................3
      1.2. Terminology...............................................4
   2. Constructing the Shared Hierarchical Forwarding Chain..........5
      2.1. Databases.................................................5
      2.2. Constructing the forwarding chain from a downloaded route.6
      2.3. Examples..................................................7
         2.3.1. Example 1: Forwarding Chain for iBGP ECMP............7
         2.3.2. Example 2: Primary Backup Paths.....................10
         2.3.3. Example 3: Platforms with Limited Levels of Hierarchy10
   3. Forwarding Behavior...........................................15
   4. Forwarding Chain Adjustment at a Failure......................17
      4.1. BGP-PIC core.............................................17
      4.2. BGP-PIC edge.............................................18
         4.2.1. Adjusting forwarding Chain in egress node failure...19
         4.2.2. Adjusting Forwarding Chain on PE-CE link Failure....19
      4.3. Handling Failures for Flattended Forwarding Chains.......20

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   5. Properties....................................................21
   6. Dependency....................................................23
   7. Security Considerations.......................................24
   8. IANA Considerations...........................................24
   9. Conclusions...................................................25
   10. References...................................................25
      10.1. Normative References....................................25
      10.2. Informative References..................................25
   11. Acknowledgments..............................................26

1. Introduction

   As a path vector protocol, BGP is inherently slow due to the
   serial nature of reachability propagation. BGP speakers exchange
   reachability information about prefixes[2][3] and, for labeled
   address families, namely AFI/SAFI 1/4, 2/4, 1/128, and 2/128, an
   edge router assigns local labels to prefixes and associates the
   local label with each advertised prefix such as L3VPN [8], 6PE
   [9], and Softwire [7] using BGP label unicast technique[4]. A BGP
   speaker then applies the path selection steps to choose the best
   path. In modern networks, it is not uncommon to have a prefix
   reachable via multiple edge routers. In addition to proprietary
   techniques, multiple techniques have been proposed to allow for
   more than one path for a given prefix [6][11][12], whether in the
   form of equal cost multipath or primary-backup. Another more
   common and widely deployed scenario is L3VPN with multi-homed VPN
   sites.

   This document proposes a hierarchical and shared forwarding chain
   organization that allows traffic to be restored to pre-calculated
   alternative equal cost primary path or backup path in a time
   period that does not depend on the number of BGP prefixes. The
   technique relies on internal router behavior that is completely
   transparent to the operator and can be incrementally deployed and
   enabled with zero operator intervention.

   1.1. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
   NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL"
   in this document are to be interpreted as described in RFC-2119
   [1].

   In this document, these words will appear with that interpretation
   only when in ALL CAPS. Lower case uses of these words are not to
   be    interpreted as carrying RFC-2119 significance.





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   1.2. Terminology

   This section defines the terms used in this document. For ease of
   use, we will use terms similar to those used by L3VPN [8]

   o  BGP prefix: It is a prefix P/m (of any AFI/SAFI) that a BGP
      speaker has a path for.

   o  IGP prefix: It is a prefix P/m (of any AFI/SAFI) that is learnt
      via an Interior Gateway Protocol, such as OSPF and ISIS, has a
      path for. The prefix may be learnt directly through the IGP or
      redistributed from other protocol(s)

   o  CE: It is an external router through which an egress PE can
      reach a prefix P/m.

   o  Ingress PE, "iPE": It is a BGP speaker that learns about a
      prefix through another IBGP peer and chooses that IBGP peer as
      the next-hop for the prefix.

   o  Path: It is the next-hop in a sequence of unique connected
      nodes starting from the current node and ending with the
      destination node or network identified by the prefix.

   o  Recursive path: It is a path consisting only of the IP address
      of the next-hop without the outgoing interface. Subsequent
      lookups are needed to determine the outgoing interface.

   o  Non-recursive path: It is a path consisting of the IP address
      of the next-hop and one outgoing interface

   o  Primary path: It is a recursive or non-recursive path that can
      be used all the time. A prefix can have more than one primary
      path

   o  Backup path: It is a recursive or non-recursive path that can
      be used only after some or all primary paths become unreachable

   o  Leaf: A leaf is container data structure for a prefix or local
      label. Alternatively, it is the data structure that contains
      prefix specific information.

   o  IP leaf: Is the leaf corresponding to an IPv4 or IPv6 prefix

   o  Label leaf. It is the leaf corresponding to a locally allocated
      label such as the VPN label on an egress PE [8].





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   o  Pathlist: It is an array of paths used by one or more prefix to
      forward traffic to destination(s) covered by a IP prefix. Each
      path in the pathlist carries its "path-index" that identifies
      its position in the array of paths. A pathlist may contain a
      mix of primary and backup paths

   o  OutLabel-Array: Each labeled prefix is associated with an
      OutLabel-Array. The OutLabel-Array is a list of one or more
      outgoing labels and/or label actions where each label or label
      action has 1-to-1 correspondence to a path in the pathlist. It
      is possible that the number of entries in the OutLabel-array is
      different from the number of paths in the pathlist and the ith
      Outlabel-Array entry is associated with the path whose path-
      index is "i". Label actions are: push the label, pop the label,
      or swap the incoming label with the label in the Outlabel-Array
      entry. The prefix may be an IGP or BGP prefix

   o  Adjacency: It is the layer 2 encapsulation leading to the layer
      3 directly connected next-hop

   o  Dependency: An object X is said to be a dependent or Child of
      object Y if Object Y cannot be deleted unless object X is no
      longer a dependent/child of object Y

   o  Route: It is a prefix with one or more paths associated with
      it.  Hence the minimum set of objects needed to construct a
      route is a leaf and a pathlist.



2. Constructing the Shared Hierarchical Forwarding Chain

   2.1. Databases

   The Forwarding Information Base (FIB) on a router maintains 3 basic
   databases

   o  Pathlist-DB: A pathlist is uniquely identified by the list of
      paths. The Pathlist DB contains the set of all shared pathlists

   o  Leaf-DB: A leaf is uniquely identified by the prefix or the label

   o  Adjacency-DB: An adjacency is uniquely identified by the outgoing
      layer 3 interface and the IP address of the next-hop directly
      connected to the layer 3 interface. Adjacency DB contains the
      list of all adjacencies





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   2.2. Constructing the forwarding chain from a downloaded route

   1. A prefix with a list of paths is downloaded to FIB from BGP. For
      labeled prefixes, an OutLabel-Array and possibly a local label
      (e.g. for a VPN [8] prefix on an egress PE) are also downloaded

   2. If the prefix does not exist, construct a new IP leaf from the
      downloaded prefix. If a local label is allocated, construct a
      label leaf from the local label

   3. Construct an OutLabel-Array and attach the Outlabel array to the
      IP and label leaf

   4. The list of paths attached to the route is looked up in the
      pathlist-DB

   5. If a pathlist PL is found

       a. Retrieve the pathlist

   6. Else

       a. Construct a new pathlist

       b. Insert the new pathlist in the pathlist-DB

       c. Resolve the paths of the pathlist as follows

       d. Recursive path:

           i. Lookup the next-hop in the leaf-DB

          ii. If a leaf with at least one reachable path is found, add
               the path to the dependency list of the leaf

         iii. Otherwise the path remains unresolved and cannot be used
               for forwarding

       e. Non-recursive path

           i. Lookup the next-hop and outgoing interface in the
               adjacency-DB

          ii. If an adjacency is found, add the path to the dependency
               list of adjacency

         iii. Otherwise, create a new adjacency and add the path to
               its dependency list

   7. Attach the leaf(s) as (a) dependent(s) of the pathlist

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   As a result of the above steps, a forwarding chain starting with a
   leaf and ending with one or more adjacency is constructed. It is
   noteworthy to mention that the forwarding chain is constructed
   without any operator intervention at all.

   2.3. Examples

   This section outlines three examples that we will use for
   illustration for the rest of the document. The first two examples
   use a standard multihomed VPN [8] prefix in a BGP-free core running
   LDP [5] or segment routing on MPLS [14]. The third example uses
   inter-AS option C [8] with 2 domains running segment routing [14] or
   LDP [5] in the core

   The topology for the first two examples is depicted in Figure 1.


        +-----------------------------------+
        |                                   |
        |   LDP/Segment-Routing Core        |
        |                                   |
        |                                  ePE2
        |                                   |\
        |                                   | \
        |                                   |  \
        |                                   |   \
       iPE                                  |  CE.......VRF "Blue"
        |                                   |   /       (VPN-P1)
        |                                   |  /        (VPN-P2)
        |                                   | /
        |                                   |/
        |                                  ePE1
        |                                   |
        |                                   |
        |                                   |
        +-----------------------------------+
             Figure 1 VPN prefix reachable via multiple PEs

   The first example is an illustration of ECMP while the second
   example is an illustration of primary-backup paths. The third
   example illustrate how to handle limited hardware capability.

2.3.1. Example 1: Forwarding Chain for iBGP ECMP

   Consider the case of the ingress PE (iPE) in the multi-homed VPN
   prefixes depicted in Figure 1. Suppose the iPE receives route
   advertisements for the VPN prefixes VPN-P1 and VPN-P2 from two
   egress PEs, ePE1 and ePE2 with next-hop BGP-NH1 and BGP-NH2,
   respectively. Assume that ePE1 advertise the VPN labels VPN-L11 and
   VPN-L12 while ePE2 advertise the VPN labels VPN-L21 and VPN-L22 for

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   VPN-P1 and VPN-P2, respectively. Suppose that BGP-NH1 and BGP-NH2
   are resolved via the IGP prefixes IGP-P1 and IGP-P2, which also
   happen to have 2 ECMP paths with IGP-NH1 and IGP-NH2 reachable via
   the interfaces I1 and I2. Suppose that local labels (whether LDP[5]
   or segment routing [14]) on the downstream LSRs for IGP-P1 and IGP-
   P2 are assign the LDP labels LDP-L1 and LDP-L2 to the prefixes IGP-
   P1 and IGP-P2. The forwarding chain on the ingress PE "iPE" for the
   VPN prefixes is depicted in Figure 2.











































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         BGP OutLabel Array
           +---------+
           | VPN-L11 |
      +--->+---------+
      |    | VPN-L21 |
      |    +---------+                  IGP OutLabel Array
      |                                   +---------+
      |                                   | LDP-L11 |
      |                               +-->+---------+
      |                               |   | LDP-L21 |
   VPN-P1------+                      |   +---------+
               |                      |
               |                      |
               |                    IGP-P1-----+
               |                      ^        |
               |                      |        |
               V                      |        V  IGP Pathlist
             +--------+               |     +-------------+
             |BGP-NH1 |---------------+     | IGP-NH1, I1 |------>adj1
       BGP   +--------+                     +-------------+
    Pathlist |BGP-NH2 |----+                | IGP-NH2, I2 |------>adj2
             +--------+    |                +-------------+
               ^           |                   ^
               |           |                   |
               |           |                   |
               |         IGP-P2----------------+
               |           |
               |           |
   VPN-P2------+           |    +---------+
      |                    |    | LDP-L12 |
      |                    +--->+---------+
      |                         | LDP-L22 |
      |                         +---------+
      |    +---------+       IGP OutLabel Array
      |    | VPN-L12 |
      +--->+---------+
           | VPN-L22 |
           +---------+
         BGP OutLabel Array


         Figure 2 Forwarding Chain for VPN Prefixes with iBGP ECMP

   The structure depicted in Figure 2 illustrates the two important
   properties discussed in this memo: sharing and hierarchy.  We can
   see that the both the BGP and IGP pathlists are shared among
   multiple BGP and IGP prefixes, respectively. At the same time, the
   forwarding chain objects depend on each other in a child-parent
   relation instead of being collapsed into a single level.


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2.3.2. Example 2: Primary Backup Paths

   Consider the egress PE ePE1 in the case of the multi-homed VPN
   prefixes in the BGP-free LDP core depicted in Figure 1. Suppose ePE1
   determines that the primary path is the external path but the backup
   path is the iBGP path to the other PE ePE2 with next-hop BGP-NH2.
   ePE2 constructs the forwarding chain depicted in Figure 1. We are
   only showing a single VPN prefix for simplicity. But all prefixes
   that are multihomed to ePE1 and ePE2 share the BGP pathlist


                    BGP OutLabel Array
     VPL-L11            +---------+
   (Label-leaf)---+---->|Unlabeled|
                  |     +---------+
                  |     | VPN-L21 |
                  |     | (swap)  |
                  |     +---------+
                  |           ^
                  |           |      BGP Pathlist
                  |           |       +------------+    Connected route
                  |           |       |   CE-NH    |------>(to the CE)
                  |           |       |path-index=0|
                  |           |       +------------+
                  V           |       |  VPN-NH2   |
     VPN-P1 ------------------+------>|  (backup)  |------>IGP Leaf
   (IP prefix leaf)                   |path-index=1|    (Towards ePE2)
                                      +-----+------+

   Figure 3 : VPN Prefix Forwarding Chain with eiBGP paths on egress PE



   The example depicted in Figure 3 differs from the example in Figure
   2 in two main aspects. First as long as the primary path towards the
   CE (external path) is useable, it will be the only path used for
   forwarding while the OutLabel-Array contains both the unlabeled
   label (primary path) and the VPN label (backup path) advertised by
   the backup path ePE2. The second aspect is presence of the label
   leaf corresponding to the VPN prefix. This label leaf is used to
   match VPN traffic arriving from the core. Note that the label leaf
   shares the OutLabel-Array and the pathlist with the IP prefix.

2.3.3. Example 3: Platforms with Limited Levels of Hierarchy

   This example uses a case of inter-AS option C [8] where there are 3
   levels of hierarchy. Figure 4 illustrates the sample topology. To
   force 3 levels of hierarchy, the ASBRs on the ingress domain (domain
   1) advertise the core routers of the egress domain (domain 2) to the
   ingress PE (iPE) via BGP-LU [4] instead of redistributing then into

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   the IGP of domain 1. The end result is that the ingress PE (iPE) has
   2 levels of recursion for the VPN prefix VPN-P1 and VPN2-P2.


                 Domain 1                  Domain 2
           +----------------+           +-------------+
           |                |           |             |
           |   LDP/SR Core  |           | LDP/SR core |
           |                |           |             |
           |              ASBR11------ASBR21.......PE21\
           |                |   \    /  |    .    .   | \
           |                |    \  /   |     .  .    |  \
           |                |     \/    |      ..     |   \VPN-P1
           |                |     /\    |      . .    |   /
           |                |    /  \   |     .   .   |  /
           |                |   /    \  |    .     .  | /
          iPE             ASBR12------ASBR22.......PE22
           |                |           |             | \
           |                |           |             |  \
           |                |           |             |   \
           |                |           |             |   /VPN-P2
           |                |           |             |  /
           |                |           |             | /
           |              ASBR13------ASBR23.......PE23/
           |                |           |             |
           |                |           |             |
           +----------------+           +-------------+
            <==============  <=========  <============
            Advertise PE2x    Advertise   Redistribute
             Using iBGP-LU    PE2x Using    IGP into
                               eBGP-LU        BGP

               Figure 4 Sample 3-level hierarchy topology



   We will make the following assumptions about connectivity

   o  In "domain 2", both ASBR21 and ASBR22 can reach both PE21 and
      PE22 using the same distance

   o  In "domain 2", only ASBR23 can reach PE23

   o  In "domain 1", iPE (the ingress PE) can reach ASBR1, ASBR12, and
      ASBR13 via IGP using the same distance

   We will make the following assumptions about the labels

   o  The VPN labels advertised by PE21 and PE22 for prefix VPN-P1 are
      VPN-PE21(P1) and VPN-PE22(P1), respectively

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   o  The VPN labels advertised byPE22 and PE23 for prefix VPN-P2 are
      VPN-PE22(P2) and VPN-PE23(P2), respectively

   o  The labels for advertised to iPE by ASBR11 using BGP-LU [4] for
      the egress PEs PE21 and PE22 are LASBR11(PE21) and LASBR11(PE22),
      respectively.

   o  The labels for advertised by ASBR12 to iPE using BGP-LU [4] for
      the egress PEs PE21 and PE22 are LASBR12(PE21) and LASBR12(PE22),
      respectively

   o  The label for advertised by ASBR11 to iPE using BGP-LU [4] for
      the egress PE PE23 is LASBR13(PE23)

   o  The local labels of the next hops from the ingress PE iPE towards
      ASBR11, ASBR12, and ASBR13 in the core of domain 1 are L11, L12,
      and L13, respectively.

   The diagram in Figure 5 illustrates the forwarding chain assuming
   that the forwarding hardware in iPE supports 3 levels of hierarchy.
   The leaves corresponding to the ABSRs on domain 1 (ASBR11, ASBR12,
   and ASBR13) are at the bottom of the hierarchy. There are few
   important points

   o  Because the hardware supports the required depth of hierarchy,
      the sizes of a pathlist equal the size of the label array
      associated with the leaves using this pathlist

   o  The index inside the pathlist entry indicates the label that will
      be picked from the Outlabel-array if that path is chosen by the
      forwarding engine hashing function.




















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   Outlabel Array                                     Outlabel Array
     For VPN-P1                                         For VPN-P2
   +------------+    +-------+            +-------+   +------------+
   |VPN-PE21(P1)|<---| VPN-P1|            | VPN-P2|-->|VPN-PE22(P2)|
   +------------+    +---+---+            +---+---+   +------------+
   |VPN-PE22(P1)|        |                    |       |VPN-PE23(P2)|
   +------------+        |                    |       +------------+
                         |                    |
                         V                    V
                    +---+---+            +---+---+
                    | 0 | 1 |            | 0 | 1 |
                    +-|-+-\-+            +-/-+-\-+
                      |    \              /     \
                      |     \            /       \
                      |      \          /         \
                      |       \        /           \
                      v        \      /             \
                 +-----+       +-----+             +-----+
            +----+ PE21|       |PE22 +-----+       | PE23+-----+
            |    +--+--+       +-----+     |       +--+--+     |
            v       |            /         v          |        v
   +-------------+  |           /   +-------------+   | +-------------+
   |LASBR11(PE21)|  |          /    |LASBR11(PE22)|   | |LASBR13(PE23)|
   +-------------+  |         /     +-------------+   | +-------------+
   |LASBR12(PE21)|  |        /      |LASBR12(PE22)|   | Outlabel Array
   +-------------+  |       /       +-------------+   |    For PE23
   Outlabel Array   |      /        Outlabel Array    |
       For PE21     |     /           For PE22        |
                    |    /                            |
                    |   /                             |
                    |  /                              |
                    v /                               v
                +---+---+  Shared Pathlist          +---+  Pathlist
                | 0 | 1 | For PE21 and PE22         | 0 |  For PE23
                +-|-+-\-+                           +-|-+
                  |    \                              |
                  |     \                             |
                  |      \                            |
                  |       \                           |
                  v        \                          v
     +---+     +------+    +------+    +---+      +------+    +---+
     |L11|<--->|ASBR11|    |ASBR12+--->|L12|      |ASBR13+--->|L13|
     +---+     +------+    +------+    +---+      +------+    +---+

       Figure 5 : Forwarding Chain for hardware supporting 3 Levels






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   Now suppose the hardware on iPE (the ingress PE) supports 2 levels
   of hierarchy only. In that case, the 3-levels forwarding chain in
   Figure 5 needs to be "flattended" into 2 levels only.


   Outlabel Array                                 Outlabel Array
     For VPN-P1                                    For VPN-P2
   +------------+    +-------+      +-------+    +------------+
   |VPN-PE21(P1)|<---| VPN-P1|      | VPN-P2|--->|VPN-PE22(P2)|
   +------------+    +---+---+      +---+---+    +------------+
   |VPN-PE22(P1)|        |              |        |VPN-PE23(P2)|
   +------------+        |              |        +------------+
                         |              |
                         |              |
                         |              |
          Flattened      |              |  Flattened
          pathlist       V              V   pathlist
                    +===+===+        +===+===+===+     +=============+
           +--------+ 0 | 1 |        | 0 | 0 | 1 +---->|LASBR11(PE22)|
           |        +=|=+=\=+        +=/=+=/=+=\=+     +=============+
           v          |    \          /   /     \      |LASBR12(PE22)|
    +=============+   |     \  +-----+   /       \     +=============+
    |LASBR11(PE21)|   |      \/         /         \    |LASBR13(PE23)|
    +=============+   |      /\        /           \   +=============+
    |LASBR12(PE21)|   |     /  \      /             \
    +=============+   |    /    \    /               \
                      |   /      \  /                 \
                      |  /       +  +                  \
                      |  +       |  |                   \
                      |  |       |  |                    \
                      v  v       v  v                     \
          +---+     +------+    +------+    +---+     +------+    +---+
          |L11|<--->|ASBR11|    |ASBR12+--->|L12|     |ASBR13+--->|L13|
          +---+     +------+    +------+    +---+     +------+    +---+


     Figure 6 : Flattening 3 levels to 2 levels of Hierarchy on iPE



   Figure 6 represents one way to "flatten" a 3 levels hierarchy into
   two levels. There are few important points.

   o  The flattened pathlists have label arrays associated with them.
      The size of the label array associated with the flattened
      pathlist equals the size of the pathlist. Hence it is possible
      that an implementation includes these label arrays in the
      flattened pathlist itself


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   o  Because of "flattening", the size of a flattened pathlist may not
      be equal to the size of the label arrays of leaves using the
      flattened pathlist.

   o  The indices inside a flattened pathlist still indicate the label
      index in the Outlabel-Arrays of the leaves using that pathlist.
      Because the size of the flattened pathlist may be different from
      the size of the label arrays of the leaves, the indices may be
      repeated

   o  Let's take a look at the flattened pathlist used by the prefix
      "VPN-P2", The pathlist associated with the prefix "VPN-P2" has
      three entries.

       o The first and second entry have index "0". This is because
          both entries correspond to PE22. Hence when hashing performed
          by the forwarding engine results in using first or the second
          entry in the pathlist, the forwarding engine will pick the
          correct VPN label "VPN-PE22(P2)", which is the label
          advertised by PE22 for the prefix "VPN-P2"

       o The third entry has the index "1". This is because the third
          entry corresponds to PE23. Hence when the hashing is
          performed by the forwarding engine results in using the third
          entry in the flattened pathlist, the forwarding engine will
          pick the correct VPN label "VPN-PE22(P2)", which is the label
          advertised by "PE23" for the prefix "VPN-P2"


3. Forwarding Behavior

   When a packet arrives at a router, it matches a leaf. A labeled
   packet matches a label leaf while an IP packet matches an IP prefix
   leaf. The forwarding engines walks the forwarding chain starting
   from the leaf until the walk terminates on an adjacency. Thus when a
   packet arrives, the chain is walked as follows:

   1. Lookup the leaf based on the destination address or the label at
      the top of the packet

   2. Retrieve the parent pathlist of the leaf

   3. Pick the outgoing path from the list of resolved paths in the
      pathlist. The method by which the outgoing path is picked is
      beyond the scope of this document (i.e. flow-preserving hash
      exploiting entropy within the MPLS stack and IP header). Let the
      "path-index" of the outgoing path be "i".




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   4. If the prefix is labeled, use the "path-index" "i" to retrieve
      the ith label "Li" stored the ith entry in the OutLabel-Array and
      apply the label action of the label on the packet (e.g. for VPN
      label on the ingress PE, the label action is "push").

   5. Move to the parent of the chosen path "i"

   6. If the chosen path "i" is recursive, move to its parent prefix
      and go to step 2

   7. If the chosen path "i" is non-recursive move to its parent
      adjacency

   8. Encapsulate the packet in the L2 string specified by the
      adjacency and send the packet out.

   Let's applying the above forwarding steps to the example described
   in Figure 1 Section 2.3.1.  Suppose a packet arrives at ingress PE
   iPE from an external neighbor. Assume the packet matches the VPN
   prefix VPN-P1. While walking the forwarding chain, the forwarding
   engine applies a hashing algorithm to choose the path and the
   hashing at the BGP level yields path 0 while the hashing at the IGP
   level yields path 1. In that case, the packet will be sent out of
   interface I1 with the label stack "LDP-L12,VPN-L21".

   Now let's try and apply the above steps to the flattened forwarding
   chain illustrated in Figure 6.

   o  Suppose a packet arrives at "iPE" and matches the VPN prefix
      "VPN-P2"

   o  The forwarding engine walks to the parent of the "VPN_P2", whiuch
      is the flattened pathlist and applies a hashing algorithm to pick
      a path

   o  Suppose the hashing by the forwarding engine picks the second
      entry in the flattened pathlist associated with the leaf "VPN-
      P2".

   o  Because the second entry has the index "0", the label "VPN-
      PE22(P2)" is pushed on the packet

   o  At the same time, the forwarding engine picks the second label
      from the Outlabel-Array associated with the flattened pathlist.
      Hence the next label that is pushed is "LASBR12(PE22)"

   o  The forwarding engine now moves to the parent of the flattened
      pathlist corresponding tgo the second entry. The parent is the
      IGP label leaf corresponding to "ASBR12"


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   o  So the packet is forwarded towards the ASBR "ASBR12" and the
      SR/LDP label at the top will be "L12"

   The packet is arriving at iPE reaches its destination as follows

   o  iPE sends the packet along the shortest path towards  ASBR12 with
      the following label stack starting from the top: {L12,
      LASBR12(PE22), VPN-PE22(P2)}.

   o  The penultimate hop of ASBR12 pops the top label "L12". Hence the
      packet arrives at ASBR12 with the label stack {LASBR12(PE22),
      VPN-PE22(P2)} where "LASBR12(PE22)" is the top label.

   o  ASBR12 swaps "LASBR12(PE22)" with the label "LASBR22(PE22)",
      which is the label advertised by ASBR22 for the PE22 (the egress
      PE).

   o  ASBR22 receives the packet with "LASBR22(PE22)" at the top.

   o  Hence ASBR22 swaps "LASBR22(PE22)" with the LDP/SR label of PE22,
      pushes the label of the next-hop towards PE22 in domain 2, and
      sends the packet along the shortest path towards PE22.

   o  The penultimate hop of PE22 pops the top label. Hence PE22
      receives the packet with the top label VPN-PE22(P2) at the top

   o  PE22 pops "VPN-PE22(P2)" and sends the packet as a pure IP packet
      towards the destination VPN-PE22.

4. Forwarding Chain Adjustment at a Failure

   The hierarchical and shared structure of the forwarding chain
   explained in Section 2 allows modifying a small number of
   forwarding chain objects to re-route traffic to a pre-calculated
   equal-cost or backup path without the need to modify the possibly
   very large number of BGP prefixes. In this section, we go over
   various core and edge failure scenarios to illustrate how FIB
   manager can utilize the forwarding chain structure to achieve prefix
   independent convergence.

   4.1. BGP-PIC core

   This section describes the adjustments to the forwarding chain when
   a core link or node fails but the BGP next-hop remains reachable.

   There are two case: remote link failure and attached link failure.
   Node failures are treated as link failures.

   When a remote link or node fails, IGP on the ingress PE receives
   advertisement indicating a topology change so IGP re-converges to

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   either find a new next-hop and outgoing interface or remove the path
   completely from the IGP prefix used to resolve BGP next-hops. IGP
   and/or LDP download the modified IGP leaves with modified outgoing
   labels for labeled core. FIB manager modifies the existing IGP leaf
   by executing the steps outlined in Section 2.2.

   When a local link fails, FIB manager detects the failure almost
   immediately. The FIB manager marks the impacted path(s) as unuseable
   so that only useable paths are used to forward packets. Note that in
   this particular case there is actually no need even to backwalk to
   IGP leaves to adjust the OutLabel-Arrays because FIB can rely on the
   path-index stored in the useable paths in the loadinfo to pick the
   right label.

   It is noteworthy to mention that because FIB manager modifies the
   forwarding chain starting from the IGP leaves only, BGP pathlists
   and leaves are not modified. Hence traffic restoration occurs within
   the time frame of IGP convergence, and, for local link failure,
   within the timeframe of local detection. Thus it is possible to
   achieve sub-50 msec convergence as described in [10] for local link
   failure



   Let's apply the procedure to the forwarding chain depicted in Figure
   2 Section 2.3.1. Suppose a remote link failure occurs and impacts
   the first ECMP IGP path to the remote BGP nhop. Upon IGP
   convergence, the IGP pathlist of the BGP nhop is updated to reflect
   the new topology (one path instead of two). As soon as the IGP
   convergence is effective for the BGP nhop entry, the new forwarding
   state is immediately available to all dependent BGP prefixes. The
   same behavior would occur if the failure was local such as an
   interface going down. As soon as the IGP convergence is complete for
   the BGP nhop IGP route, all its BGP depending routes benefit from
   the new path. In fact, upon local failure, if LFA protection is
   enabled for the IGP route to the BGP nhop and a backup path was pre-
   computed and installed in the pathlist, upon the local interface
   failure, the LFA backup path is immediately activated (sub-50msec)
   and thus protection benefits all the depending BGP traffic through
   the hierarchical forwarding dependency between the routes.

   4.2. BGP-PIC edge

   This section describes the adjustments to the forwarding chains as a
   result of edge node or edge link failure






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4.2.1. Adjusting forwarding Chain in egress node failure

   When an edge node fails, IGP on neighboring core nodes send route
   updates indicating that the edge node is no longer reachable. IGP
   running on the iBGP peers instructs FIB to remove the IP and label
   leaves corresponding to the failed edge node from FIB. So FIB
   manager performs the following steps:

   o  FIB manager deletes the IGP leaf corresponding to the failed edge
      node

   o  FIB manager backwalks to all dependent BGP pathlists and marks
      that path using the deleted IGP leaf as unresolved

   o  Note that there is no need to modify BGP leaves because each path
      in the pathlist carries its path index and hence the correct
      outgoing label will be picked. So for example the forwarding
      chain depicted in Figure 2, if the 1st path becomes unresolved,
      then the forwarding engine will only use the second path path for
      forwarding. Yet the pathindex of that single resolved path will
      still be 1 and hence the label VPN-L21 or VPN-L22 will be pushed

4.2.2. Adjusting Forwarding Chain on PE-CE link Failure

   Suppose the link between an edge router and its external peer fails.
   There are two scenarios (1) the edge node attached to the failed
   link performs next-hop self and (2) the edge node attached to the
   failure advertises the IP address of the failed link as the next-hop
   attribute to its iBGP peers.

   In the first case, the rest of iBGP peers will remain unaware of the
   link failure and will continue to forward traffic to the edge node
   until the edge node attached to the failed link withdraws the BGP
   prefixes. If the destination prefixes are multi-homed to another
   iBGP peer, say ePE2, then FIB manager on the edge router detecting
   the link failure performs the following tasks

   o  FIB manager backwalks to the BGP pathlists marks the path through
      the failed link to the external peer  as unresolved

   o  Hence traffic will be forwarded used the backup path towards ePE2

   o  For labeled traffic

       o The Outlabel-Array attached to the BGP leaves already
          contains an entry corresponding to the path towards ePE2.

       o The label entry in OutLabel-Arrays corresponding to the
          internal path to ePE2 has swap action and the label
          advertised by ePE2

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       o For an arriving label packet (e.g. VPN), the top label is
          swapped with the label advertised by ePE2

   o  For unlabeled traffic, packets is simply redirected towards ePE2.
      To avoid loops, ePE2 MUST treat any core facing path as a backup
      path, otherwise ePE2 may redirect traffic arriving from the core
      back to ePE1 causing a loop.

   In the second case where the edge router uses the IP address of the
   failed link as the BGP next-hop, the edge router will still perform
   the previous steps. But, unlike the case of next-hop self, IGP on
   failed edge node informs the rest of the iBGP peers that IP address
   of the failed link is no longer reachable. Hence the FIB manager on
   iBGP peers will delete the IGP leaf corresponding to the IP prefix
   of the failed link. The behavior of the iBGP peers will be identical
   to the case of edge node failure outlined in Section 4.2.1.

   It is noteworthy to mention that because the edge link failure is
   local to the edge router, sub-50 msec convergence can be achieved as
   described in [10].

   Let's try to apply the case of next-hop self to the forwarding chain
   depicted in Figure 3. After failure of the link between ePE1 and CE,
   the forwarding engine will route traffic arriving from the core
   towards VPN-NH2 with path-index=1. A packet arriving from the core
   will contain the label VPN-L11 at top. The label VPN-L11 is swaped
   with the label VPN-L21 and the packet is forwarded towards ePE2

   4.3. Handling Failures for Flattended Forwarding Chains

   As explained in the Example in Section 2.3.3, if the number of
   hierarchy levels of a platform cannot support the number of
   hierarchy levels of a recursive dependency, the instantiated
   forwarding chain is constructed by flattening two or more levels.
   Hence a 3 levels chain in Figure 5 is flattened into the 2 levels
   chain in Figure 6.

   While reducing the benefits of BGP-PIC, flattening one hierarchy
   into a shallower hierarchy does not always result in a complete loss
   of the benefits of the BGP-PIC. To illustrate this fact suppose
   ASBR12 is no longer reachable. If the platform supports the full
   hierarchy depth, the forwarding chain is depicted in Figure 5 and
   hence the FIB manager needs to backwalk one level to the pathlist
   shared by "PE21" and "PE222" and adjust it. If the platform supports
   2 levels of hierarchy, then a useable forwarding chain is the one
   depicted in Figure 6. In that case, if ASBR12 is no longer
   reachable, the FIB manager has to backwalk to the two flattened
   pathlists and update both of them.



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   Hence if the platform supports the "unflattened" forwarding chain,
   then a single pathlist needs to be updated while if the platform
   supports a shallower forwarding chain, then two pathlists need to be
   updated. In the latter case, convergence is still independent of the
   number of leaves due to the fact that the flattened pathlists
   continue to be shared among possibly a large number of leaves



5. Properties

   5.1 Coverage

   All the possible failures, except CE node failure, are covered,
   whether they impact a local or remote IGP path or a local or remote
   BGP nhop as described in Section 4.  This section provides details
   for each failure and now the hierarchical and shared FIB structure
   proposed in this document allows recovery that does not depend on
   number of BGP prefixes

   5.1.1 A remote failure on the path to a BGP nhop

   Upon IGP convergence, the IGP leaf for the BGP nhop is updated upon
   IGP convergence and all the BGP depending routes leverage the new
   IGP forwarding state immediately.

   This BGP resiliency property only depends on IGP convergence and is
   independent of the number of BGP prefixes impacted.

   5.1.2 A local failure on the path to a BGP nhop

   Upon LFA protection, the IGP leaf for the BGP nhop is updated to use
   the precomputed LFA backup path and all the BGP depending routes
   leverage this LFA protection.

   This BGP resiliency property only depends on LFA protection and is
   independent of the number of BGP prefixes impacted.

   5.1.3 A remote iBGP nhop fails

   Upon IGP convergence, the IGP leaf for the BGP nhop is deleted and
   all the depending BGP Path-Lists are updated to either use the
   remaining ECMP BGP best-paths or if none remains available to
   activate precomputed backups.

   This BGP resiliency property only depends on IGP convergence and is
   independent of the number of BGP prefixes impacted.

   5.1.4 A local eBGP nhop fails


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   Upon local link failure detection, the adjacency to the BGP nhop is
   deleted and all the depending BGP Path-Lists are updated to either
   use the remaining ECMP BGP best-paths or if none remains available
   to activate precomputed backups.

   This BGP resiliency property only depends on local link failure
   detection and is independent of the number of BGP prefixes impacted.

   5.2 Performance

   When the failure is local (a local IGP nhop failure or a local eBGP
   nhop failure), a pre-computed and pre-installed backup is activated
   by a local-protection mechanism that does not depend on the number
   of BGP destinations impacted by the failure. Sub-50msec is thus
   possible even if millions of BGP routes are impacted.

   When the failure is remote (a remote IGP failure not impacting the
   BGP nhop or a remote BGP nhop failure), an alternate path is
   activated upon IGP convergence. All the impacted BGP destinations
   benefit from a working alternate path as soon as the IGP convergence
   occurs for their impacted BGP nhop even if millions of BGP routes
   are impacted.

   5.2.1 Perspective

   The following table puts the BGP PIC benefits in perspective
   assuming

   o  1M impacted BGP prefixes

   o  IGP convergence ~ 500 msec

   o  local protection ~ 50msec

   o  FIB Update per BGP destination ~ 100usec conservative,

                                     ~ 10usec optimistic

   o  BGP Convergence per BGP destination ~ 200usec conservative,

                                          ~ 100usec optimistic



                                 Without PIC                With PIC

   Local IGP Failure             10 to 100sec                50msec

   Local BGP Failure            100 to 200sec                50msec


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   Remote IGP Failure            10 to 100sec               500msec

   Local BGP Failure            100 to 200sec               500msec

   Upon local IGP nhop failure or remote IGP nhop failure, the existing
   primary BGP nhop is intact and usable hence the resiliency only
   depends on the ability of the FIB mechanism to reflect the new path
   to the BGP nhop to the depending BGP destinations. Without BGP PIC,
   a conservative back-of-the-envelope estimation for this FIB update
   is 100usec per BGP destination. An optimistic estimation is 10usec
   per entry.

   Upon local BGP nhop failure or remote BGP nhop failure, without the
   BGP PIC mechanism, a new BGP Best-Path needs to be recomputed and
   new updates need to be sent to peers. This depends on BGP processing
   time that will be shared between best-path computation, RIB update
   and peer update. A conservative back-of-the-envelope estimation for
   this is 200usec per BGP destination. An optimistic estimation is
   100usec per entry.



   5.3 Automated

   The BGP PIC solution does not require any operator involvement. The
   process is entirely automated as part of the FIB implementation.

   The salient points enabling this automation are:

   o  Extension of the BGP Best Path to compute more than one primary
      ([11]and [12]) or backup BGP nhop ([6] and [13]).

   o  Sharing of BGP Path-list across BGP destinations with same
      primary and backup BGP nhop

   o  Hierarchical indirection and dependency between BGP Path-List and
      IGP-Path-List

   5.4 Incremental Deployment

   As soon as one router supports BGP PIC solution, it benefits from
   all its benefits without any requirement for other routers to
   support BGP PIC.



6. Dependency

   This section describes the required functionality in the forwarding
   and control planes to support BGP-PIC described in this document

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   6.1 Hierarchical Hardware FIB

   BGP PIC requires a hierarchical hardware FIB support: for each BGP
   forwarded packet, a BGP leaf is looked up, then a BGP Pathlist is
   consulted, then an IGP Pathlist, then an Adjacency.

   An alternative method consists in "flattening" the dependencies when
   programming the BGP destinations into HW FIB resulting in
   potentially eliminating both the BGP Path-List and IGP Path-List
   consultation. Such an approach decreases the number of memory
   lookup's per forwarding operation at the expense of HW FIB memory
   increase (flattening means less sharing hence duplication), loss of
   ECMP properties (flattening means less pathlist entropy) and loss of
   BGP PIC properties.

   6.2 Availability of more than one primary or secondary BGP next-hops

   When the primary BGP next-hop fails, BGP PIC depends on the
   availability of a pre-computed and pre-installed secondary BGP next-
   hop in the BGP Pathlist.

   The existence of a secondary next-hop is clear for the following
   reason: a service caring for network availability will require two
   disjoint network connections hence two BGP nhops.

   The BGP distribution of the secondary next-hop is available thanks
   to the following BGP mechanisms: Add-Path [11], BGP Best-External
   [6], diverse path [12], and the frequent use in VPN deployments of
   different VPN RD's per PE. It is noteworthy to mention that the
   availability of another BGP path does not mean that all failure
   scenarios can be covered by simply forwarding traffic to the
   available secondary path. The discussion of how to cover various
   failure scenarios is beyond the scope of this document

   6.3 Pre-Computation of a secondary BGP nhop

   [13] describes how a secondary BGP next-hop can be precomputed on a
   per BGP destination basis.



7. Security Considerations

   No additional security risk is introduced by using the mechanisms
   proposed in this document

8. IANA Considerations

   No requirements for IANA


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9. Conclusions

   This document proposes a hierarchical and shared forwarding chain
   structure that allows achieving prefix independent convergence,
   and in the case of locally detected failures, sub-50 msec
   convergence. A router can construct the forwarding chains in a
   completely transparent manner with zero operator intervention. It
   supports incremental deployment.

10. References

   10.1. Normative References

   [1]   Bradner, S., "Key words for use in RFCs to Indicate
         Requirement Levels", BCP 14, RFC 2119, March 1997.

   [2]   Rekhter, Y., Li, T., and S. Hares, "A Border Gateway Protocol
         4 (BGP-4), RFC 4271, January 2006

   [3]   Bates, T., Chandra, R., Katz, D., and Rekhter Y.,
         "Multiprotocol Extensions for BGP", RFC 4760, January 2007

   [4]   Y. Rekhter and E. Rosen, " Carrying Label Information in BGP-
         4", RFC 3107, May 2001

   [5]   Andersson, L., Minei, I., and B. Thomas, "LDP Specification",
         RFC 5036, October 2007

   10.2. Informative References

   [6]   Marques,P., Fernando, R., Chen, E, Mohapatra, P., Gredler, H.,
         "Advertisement of the best external route in BGP", draft-ietf-
         idr-best-external-05.txt, January 2012.

   [7]   Wu, J., Cui, Y., Metz, C., and E. Rosen, "Softwire Mesh
         Framework", RFC 5565, June 2009.

   [8]   Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
         Networks (VPNs)", RFC 4364, February 2006.

   [9]   De Clercq, J. , Ooms, D., Prevost, S., Le Faucheur, F.,
         "Connecting IPv6 Islands over IPv4 MPLS Using IPv6 Provider
         Edge Routers (6PE)", RFC 4798, February 2007

   [10]  O. Bonaventure, C. Filsfils, and P. Francois. "Achieving sub-
         50 milliseconds recovery upon bgp peering link failures, "
         IEEE/ACM Transactions on Networking, 15(5):1123-1135, 2007




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   [11]  D. Walton, E. Chen, A. Retana, J. Scudder, "Advertisement of
         Multiple Paths in BGP", draft-ietf-idr-add-paths-10.txt,
         October 2014

   [12]  R. Raszuk, R. Fernando, K. Patel, D. McPherson, K. Kumaki,
         "Distribution of diverse BGP paths", RFC 6774.txt, November
         2012

   [13]  P. Mohapatra, R. Fernando, C. Filsfils, and R. Raszuk, "Fast
         Connectivity Restoration Using BGP Add-path", draft-pmohapat-
         idr-fast-conn-restore-03, Jan 2013

   [14]  C. Filsfils, S. Previdi, A. Bashandy, B. Decraene, S.
         Litkowski, M. Horneffer, R. Shakir, J. Tansura, E. Crabbe
         "Segment Routing with MPLS data plane", draft-ietf-spring-
         segment-routing-mpls-02 (work in progress), October 2015



11. Acknowledgments

   Special thanks to Neeraj Malhotra, Yuri Tsier for the valuable
   help

   Special thanks to Bruno Decraene for the valuable comments

   This document was prepared using 2-Word-v2.0.template.dot.



Authors' Addresses

   Ahmed Bashandy
   Cisco Systems
   170 West Tasman Dr, San Jose, CA 95134, USA
   Email: bashandy@cisco.com

   Clarence Filsfils
   Cisco Systems
   Brussels, Belgium
   Email: cfilsfil@cisco.com

   Prodosh Mohapatra
   Sproute Networks
   Email: mpradosh@yahoo.com






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