Network Working Group                                   C. Filsfils, Ed.
Internet-Draft                                                S. Previdi
Intended status: Informational                       Cisco Systems, Inc.
Expires: June 24, November 30, 2018                                       J. Mitchell
                                                            Unaffiliated                                      G. Dawra
                                                                E. Aries
                                                        Juniper Networks
                                                             P. Lapukhov
                                                       December 21, 2017
                                                            May 29, 2018

             BGP-Prefix Segment in large-scale data centers


   This document describes the motivation and benefits for applying
   segment routing in BGP-based large-scale data-centers.  It describes
   the design to deploy segment routing in those data-centers, for both
   the MPLS and IPv6 dataplanes.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   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."

   This Internet-Draft will expire on June 24, November 30, 2018.

Copyright Notice

   Copyright (c) 2017 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Large Scale Data Center Network Design Summary  . . . . . . .   3
     2.1.  Reference design  . . . . . . . . . . . . . . . . . . . .   4
   3.  Some open problems in large data-center networks  . . . . . .   5
   4.  Applying Segment Routing in the DC with MPLS dataplane  . . .   6
     4.1.  BGP Prefix Segment (BGP-Prefix-SID) . . . . . . . . . . .   6
     4.2.  eBGP Labeled Unicast (RFC8277)  . . . . . . . . . . . . .   6
       4.2.1.  Control Plane . . . . . . . . . . . . . . . . . . . .   7
       4.2.2.  Data Plane  . . . . . . . . . . . . . . . . . . . . .   9   8
       4.2.3.  Network Design Variation  . . . . . . . . . . . . . .  10   9
       4.2.4.  Global BGP Prefix Segment through the fabric  . . . .  10
       4.2.5.  Incremental Deployments . . . . . . . . . . . . . . .  11  10
     4.3.  iBGP Labeled Unicast (RFC8277)  . . . . . . . . . . . . .  12  11
   5.  Applying Segment Routing in the DC with IPv6 dataplane  . . .  14  13
   6.  Communicating path information to the host  . . . . . . . . .  14  13
   7.  Addressing the open problems  . . . . . . . . . . . . . . . .  15  14
     7.1.  Per-packet and flowlet switching  . . . . . . . . . . . .  15  14
     7.2.  Performance-aware routing . . . . . . . . . . . . . . . .  16  15
     7.3.  Deterministic network probing . . . . . . . . . . . . . .  17  16
   8.  Additional Benefits . . . . . . . . . . . . . . . . . . . . .  17
     8.1.  MPLS Dataplane with operational simplicity  . . . . . . .  18  17
     8.2.  Minimizing the FIB table  . . . . . . . . . . . . . . . .  18  17
     8.3.  Egress Peer Engineering . . . . . . . . . . . . . . . . .  18  17
     8.4.  Anycast . . . . . . . . . . . . . . . . . . . . . . . . .  19  18
   9.  Preferred SRGB Allocation . . . . . . . . . . . . . . . . . .  19  18
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20  19
   11. Manageability Considerations  . . . . . . . . . . . . . . . .  20  19
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  21  20
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  21  20
   14. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  21  20
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     15.2.  Informative References . . . . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24  23

1.  Introduction

   Segment Routing (SR), as described in
   [I-D.ietf-spring-segment-routing] leverages the source routing
   paradigm.  A node steers a packet through an ordered list of
   instructions, called segments.  A segment can represent any
   instruction, topological or service-based.  A segment can have a
   local semantic to an SR node or global within an SR domain.  SR
   allows to enforce a flow through any topological path while
   maintaining per-flow state only at the ingress node to the SR domain.
   Segment Routing can be applied to the MPLS and IPv6 data-planes.

   The use-cases described in this document should be considered in the
   context of the BGP-based large-scale data-center (DC) design
   described in [RFC7938].  This document extends it by applying SR both
   with IPv6 and MPLS dataplane.

2.  Large Scale Data Center Network Design Summary

   This section provides a brief summary of the informational document
   [RFC7938] that outlines a practical network design suitable for data-
   centers of various scales:

   o  Data-center networks have highly symmetric topologies with
      multiple parallel paths between two server attachment points.  The
      well-known Clos topology is most popular among the operators (as
      described in [RFC7938]).  In a Clos topology, the minimum number
      of parallel paths between two elements is determined by the
      "width" of the "Tier-1" stage.  See Figure 1 below for an
      illustration of the concept.

   o  Large-scale data-centers commonly use a routing protocol, such as
      BGP-4 [RFC4271] in order to provide endpoint connectivity.
      Recovery after a network failure is therefore driven either by
      local knowledge of directly available backup paths or by
      distributed signaling between the network devices.

   o  Within data-center networks, traffic is load-shared using the
      Equal Cost Multipath (ECMP) mechanism.  With ECMP, every network
      device implements a pseudo-random decision, mapping packets to one
      of the parallel paths by means of a hash function calculated over
      certain parts of the packet, typically a combination of various
      packet header fields.

   The following is a schematic of a five-stage Clos topology, with four
   devices in the "Tier-1" stage.  Notice that number of paths between
   Node1 and Node12 equals to four: the paths have to cross all of
   Tier-1 devices.  At the same time, the number of paths between Node1
   and Node2 equals two, and the paths only cross Tier-2 devices.  Other
   topologies are possible, but for simplicity only the topologies that
   have a single path from Tier-1 to Tier-3 are considered below.  The
   rest could be treated similarly, with a few modifications to the

2.1.  Reference design

                                  |NODE |
                               +->|  5  |--+
                               |  +-----+  |
                       Tier-2  |           |   Tier-2
                      +-----+  |  +-----+  |  +-----+
        +------------>|NODE |--+->|NODE |--+--|NODE |-------------+
        |       +-----|  3  |--+  |  6  |  +--|  9  |-----+       |
        |       |     +-----+     +-----+     +-----+     |       |
        |       |                                         |       |
        |       |     +-----+     +-----+     +-----+     |       |
        | +-----+---->|NODE |--+  |NODE |  +--|NODE |-----+-----+ |
        | |     | +---|  4  |--+->|  7  |--+--|  10 |---+ |     | |
        | |     | |   +-----+  |  +-----+  |  +-----+   | |     | |
        | |     | |            |           |            | |     | |
      +-----+ +-----+          |  +-----+  |          +-----+ +-----+
      |NODE | |NODE | Tier-3   +->|NODE |--+   Tier-3 |NODE | |NODE |
      |  1  | |  2  |             |  8  |             | 11  | |  12 |
      +-----+ +-----+             +-----+             +-----+ +-----+
        | |     | |                                     | |     | |
        A O     B O            <- Servers ->            Z O     O O

                      Figure 1: 5-stage Clos topology

   In the reference topology illustrated in Figure 1, It is assumed:

   o  Each node is its own AS (Node X has AS X). 4-byte AS numbers are
      recommended ([RFC6793]).

      *  For simple and efficient route propagation filtering, Node5,
         Node6, Node7 and Node8 use the same AS, Node3 and Node4 use the
         same AS, Node9 and Node10 use the same AS.

      *  In case of 2-byte autonomous system numbers are used and for
         efficient usage of the scarce 2-byte Private Use AS pool,
         different Tier-3 nodes might use the same AS.

      *  Without loss of generality, these details will be simplified in
         this document and assume that each node has its own AS.

   o  Each node peers with its neighbors with a BGP session.  If not
      specified, eBGP is assumed.  In a specific use-case, iBGP will be
      used but this will be called out explicitly in that case.

   o  Each node originates the IPv4 address of its loopback interface
      into BGP and announces it to its neighbors.

      *  The loopback of Node X is 192.0.2.x/32.

   In this document, the Tier-1, Tier-2 and Tier-3 nodes are referred to
   respectively as Spine, Leaf and ToR (top of rack) nodes.  When a ToR
   node acts as a gateway to the "outside world", it is referred to as a
   border node.

3.  Some open problems in large data-center networks

   The data-center network design summarized above provides means for
   moving traffic between hosts with reasonable efficiency.  There are
   few open performance and reliability problems that arise in such

   o  ECMP routing is most commonly realized per-flow.  This means that
      large, long-lived "elephant" flows may affect performance of
      smaller, short-lived "mouse" flows and reduce efficiency of per-
      flow load-sharing.  In other words, per-flow ECMP does not perform
      efficiently when flow lifetime distribution is heavy-tailed.
      Furthermore, due to hash-function inefficiencies it is possible to
      have frequent flow collisions, where more flows get placed on one
      path over the others.

   o  Shortest-path routing with ECMP implements an oblivious routing
      model, which is not aware of the network imbalances.  If the
      network symmetry is broken, for example due to link failures,
      utilization hotspots may appear.  For example, if a link fails
      between Tier-1 and Tier-2 devices (e.g.  Node5 and Node9), Tier-3
      devices Node1 and Node2 will not be aware of that, since there are
      other paths available from perspective of Node3.  They will
      continue sending roughly equal traffic to Node3 and Node4 as if
      the failure didn't exist which may cause a traffic hotspot.

   o  The absence of path visibility leaves transport protocols, such as
      TCP, with a "blackbox" view of the network.  Some TCP metrics,
      such as SRTT, MSS, CWND and few others could be inferred and
      cached based on past history, but those apply to destinations,
      regardless of the path that has been chosen to get there.  Thus,
      for instance, TCP is not capable of remembering "bad" paths, such
      as those that exhibited poor performance in the past.  This means
      that every new connection will be established obliviously (memory-
      less) with regards to the paths chosen before, or chosen by other

   o  Isolating faults in the network with multiple parallel paths and
      ECMP-based routing is non-trivial due to lack of determinism.
      Specifically, the connections from HostA to HostB may take a
      different path every time a new connection is formed, thus making
      consistent reproduction of a failure much more difficult.  This
      complexity scales linearly with the number of parallel paths in
      the network, and stems from the random nature of path selection by
      the network devices.

   Further in this document (Section 7), it is demonstrated how these
   problems could be addressed within the framework of Segment Routing.

   First, it will be explained how to apply SR in the DC, for MPLS and
   IPv6 data-planes.

4.  Applying Segment Routing in the DC with MPLS dataplane

4.1.  BGP Prefix Segment (BGP-Prefix-SID)

   A BGP Prefix Segment is a segment associated with a BGP prefix.  A
   BGP Prefix Segment is a network-wide instruction to forward the
   packet along the ECMP-aware best path to the related prefix.

   The BGP Prefix Segment is defined as the BGP-Prefix-SID Attribute in
   [I-D.ietf-idr-bgp-prefix-sid] which contains an index.  Throughout
   this document the BGP Prefix Segment Attribute is referred as the
   BGP-Prefix-SID and the encoded index as the label-index.

   In this document, the network design decision has been made to assume
   that all the nodes are allocated the same SRGB (Segment Routing
   Global Block), e.g. [16000, 23999].  This provides operational
   simplification as explained in Section 9, but this is not a

   For illustration purpose, when considering an MPLS data-plane, it is
   assumed that the label-index allocated to prefix 192.0.2.x/32 is X.
   As a result, a local label (16000+x) is allocated for prefix
   192.0.2.x/32 by each node throughout the DC fabric.

   When IPv6 data-plane is considered, it is assumed that Node X is
   allocated IPv6 address (segment) 2001:DB8::X.

4.2.  eBGP Labeled Unicast (RFC8277)

   Referring to Figure 1 and [RFC7938], the following design
   modifications are introduced:

   o  Each node peers with its neighbors via a eBGP session with
      extensions defined in [RFC8277] (named "eBGP8277" throughout this
      document) and with the BGP-Prefix-SID attribute extension as
      defined in [I-D.ietf-idr-bgp-prefix-sid].

   o  The forwarding plane at Tier-2 and Tier-1 is MPLS.

   o  The forwarding plane at Tier-3 is either IP2MPLS (if the host
      sends IP traffic) or MPLS2MPLS (if the host sends MPLS-
      encapsulated traffic).

   Figure 2 zooms into a path from server A to server Z within the
   topology of Figure 1.

                      +-----+     +-----+     +-----+
          +---------->|NODE |     |NODE |     |NODE |
          |           |  4  |--+->|  7  |--+--|  10 |---+
          |           +-----+     +-----+     +-----+   |
          |                                             |
      +-----+                                         +-----+
      |NODE |                                         |NODE |
      |  1  |                                         | 11  |
      +-----+                                         +-----+
        |                                              |
        A                    <- Servers ->             Z

          Figure 2: Path from A to Z via nodes 1, 4, 7, 10 and 11

   Referring to Figure 1 and Figure 2 and assuming the IP address with
   the AS and label-index allocation previously described, the following
   sections detail the control plane operation and the data plane states
   for the prefix (loopback of Node11)

4.2.1.  Control Plane

   Node11 originates in BGP and allocates to it a BGP-
   Prefix-SID with label-index: index11 [I-D.ietf-idr-bgp-prefix-sid].

   Node11 sends the following eBGP8277 update to Node10:

   . IP Prefix:
   . Label: Implicit-Null
   . Next-hop: Node11's interface address on the link to Node10
   . AS Path: {11}
   . BGP-Prefix-SID: Label-Index 11

   Node10 receives the above update.  As it is SR capable, Node10 is
   able to interpret the BGP-Prefix-SID and hence understands that it
   should allocate the label from its own SRGB block, offset by the
   Label-Index received in the BGP-Prefix-SID (16000+11 hence 16011) to
   the NLRI instead of allocating a non-deterministic label out of a
   dynamically allocated portion of the local label space.  The
   implicit-null label in the NLRI tells Node10 that it is the
   penultimate hop and must pop the top label on the stack before
   forwarding traffic for this prefix to Node11.

   Then, Node10 sends the following eBGP8277 update to Node7:

   . IP Prefix:
   . Label: 16011
   . Next-hop: Node10's interface address on the link to Node7
   . AS Path: {10, 11}
   . BGP-Prefix-SID: Label-Index 11

   Node7 receives the above update.  As it is SR capable, Node7 is able
   to interpret the BGP-Prefix-SID and hence allocates the local
   (incoming) label 16011 (16000 + 11) to the NLRI (instead of
   allocating a "dynamic" local label from its label manager).  Node7
   uses the label in the received eBGP8277 NLRI as the outgoing label
   (the index is only used to derive the local/incoming label).

   Node7 sends the following eBGP8277 update to Node4:

   . IP Prefix:
   . Label: 16011
   . Next-hop: Node7's interface address on the link to Node4
   . AS Path: {7, 10, 11}
   . BGP-Prefix-SID: Label-Index 11

   Node4 receives the above update.  As it is SR capable, Node4 is able
   to interpret the BGP-Prefix-SID and hence allocates the local
   (incoming) label 16011 to the NLRI (instead of allocating a "dynamic"
   local label from its label manager).  Node4 uses the label in the
   received eBGP8277 NLRI as outgoing label (the index is only used to
   derive the local/incoming label).

   Node4 sends the following eBGP8277 update to Node1:

   . IP Prefix:
   . Label: 16011
   . Next-hop: Node4's interface address on the link to Node1
   . AS Path: {4, 7, 10, 11}
   . BGP-Prefix-SID: Label-Index 11

   Node1 receives the above update.  As it is SR capable, Node1 is able
   to interpret the BGP-Prefix-SID and hence allocates the local
   (incoming) label 16011 to the NLRI (instead of allocating a "dynamic"
   local label from its label manager).  Node1 uses the label in the
   received eBGP8277 NLRI as outgoing label (the index is only used to
   derive the local/incoming label).

4.2.2.  Data Plane

   Referring to Figure 1, and assuming all nodes apply the same
   advertisement rules described above and all nodes have the same SRGB
   (16000-23999), here are the IP/MPLS forwarding tables for prefix at Node1, Node4, Node7 and Node10.

              Incoming label    | outgoing label | Outgoing
              or IP destination |                | Interface
                   16011        |      16011     | ECMP{3, 4}
         |      16011     | ECMP{3, 4}

                     Figure 3: Node1 Forwarding Table

              Incoming label    | outgoing label | Outgoing
              or IP destination |                | Interface
                   16011        |      16011     | ECMP{7, 8}
         |      16011     | ECMP{7, 8}

                     Figure 4: Node4 Forwarding Table

              Incoming label    | outgoing label | Outgoing
              or IP destination |                | Interface
                   16011        |      16011     |    10
         |      16011     |    10

                     Figure 5: Node7 Forwarding Table

              Incoming label    | outgoing label | Outgoing
              or IP destination |                | Interface
                   16011        |      POP       |    11
         |      N/A       |    11

                          Node10 Forwarding Table

4.2.3.  Network Design Variation

   A network design choice could consist of switching all the traffic
   through Tier-1 and Tier-2 as MPLS traffic.  In this case, one could
   filter away the IP entries at Node4, Node7 and Node10.  This might be
   beneficial in order to optimize the forwarding table size.

   A network design choice could consist in allowing the hosts to send
   MPLS-encapsulated traffic based on the Egress Peer Engineering (EPE)
   use-case as defined in [I-D.ietf-spring-segment-routing-central-epe].
   For example, applications at HostA would send their Z-destined
   traffic to Node1 with an MPLS label stack where the top label is
   16011 and the next label is an EPE peer segment
   ([I-D.ietf-spring-segment-routing-central-epe]) at Node11 directing
   the traffic to Z.

4.2.4.  Global BGP Prefix Segment through the fabric

   When the previous design is deployed, the operator enjoys global BGP-
   Prefix-SID and label allocation throughout the DC fabric.

   A few examples follow:

   o  Normal forwarding to Node11: a packet with top label 16011
      received by any node in the fabric will be forwarded along the
      ECMP-aware BGP best-path towards Node11 and the label 16011 is
      penultimate-popped at Node10 (or at Node 9).

   o  Traffic-engineered path to Node11: an application on a host behind
      Node1 might want to restrict its traffic to paths via the Spine
      node Node5.  The application achieves this by sending its packets
      with a label stack of {16005, 16011}. BGP Prefix SID 16005 directs
      the packet up to Node5 along the path (Node1, Node3, Node5).  BGP-
      Prefix-SID 16011 then directs the packet down to Node11 along the
      path (Node5, Node9, Node11).

4.2.5.  Incremental Deployments

   The design previously described can be deployed incrementally.  Let
   us assume that Node7 does not support the BGP-Prefix-SID and let us
   show how the fabric connectivity is preserved.

   From a signaling viewpoint, nothing would change: even though Node7
   does not support the BGP-Prefix-SID, it does propagate the attribute
   unmodified to its neighbors.

   From a label allocation viewpoint, the only difference is that Node7
   would allocate a dynamic (random) label to the prefix
   (e.g. 123456) instead of the "hinted" label as instructed by the BGP-
   Prefix-SID.  The neighbors of Node7 adapt automatically as they
   always use the label in the BGP8277 NLRI as outgoing label.

   Node4 does understand the BGP-Prefix-SID and hence allocates the
   indexed label in the SRGB (16011) for

   As a result, all the data-plane entries across the network would be
   unchanged except the entries at Node7 and its neighbor Node4 as shown
   in the figures below.

   The key point is that the end-to-end Label Switched Path (LSP) is
   preserved because the outgoing label is always derived from the
   received label within the BGP8277 NLRI.  The index in the BGP-Prefix-
   SID is only used as a hint on how to allocate the local label (the
   incoming label) but never for the outgoing label.

                Incoming label     | outgoing | Outgoing
                or IP destination  |  label   | Interface
                     12345         |  16011   |   10

                     Figure 7: Node7 Forwarding Table

                Incoming label     | outgoing | Outgoing
                or IP destination  |  label   | Interface
                     16011         |  12345   |   7

                     Figure 8: Node4 Forwarding Table

   The BGP-Prefix-SID can thus be deployed incrementally one node at a

   When deployed together with a homogeneous SRGB (same SRGB across the
   fabric), the operator incrementally enjoys the global prefix segment
   benefits as the deployment progresses through the fabric.

4.3.  iBGP Labeled Unicast (RFC8277)

   The same exact design as eBGP8277 is used with the following

      All nodes use the same AS number.

      Each node peers with its neighbors via an internal BGP session
      (iBGP) with extensions defined in [RFC8277] (named "iBGP8277"
      throughout this document).

      Each node acts as a route-reflector for each of its neighbors and
      with the next-hop-self option.  Next-hop-self is a well known
      operational feature which consists of rewriting the next-hop of a
      BGP update prior to send it to the neighbor.  Usually, it's a
      common practice to apply next-hop-self behavior towards iBGP peers
      for eBGP learned routes.  In the case outlined in this section it
      is proposed to use the next-hop-self mechanism also to iBGP
      learned routes.

                               |  Tier-1   |
                               |  +-----+  |
                               |  |NODE |  |
                               |  |  5  |  |
                    Cluster-2  |  +-----+  |  Cluster-3
                   +---------+ |           | +---------+
                   | Tier-2  | |           | |  Tier-2 |
                   | +-----+ | |  +-----+  | | +-----+ |
                   | |NODE | | |  |NODE |  | | |NODE | |
                   | |  3  | | |  |  6  |  | | |  9  | |
                   | +-----+ | |  +-----+  | | +-----+ |
                   |         | |           | |         |
                   |         | |           | |         |
                   | +-----+ | |  +-----+  | | +-----+ |
                   | |NODE | | |  |NODE |  | | |NODE | |
                   | |  4  | | |  |  7  |  | | |  10 | |
                   | +-----+ | |  +-----+  | | +-----+ |
                   +---------+ |           | +---------+
                               |           |
                               |  +-----+  |
                               |  |NODE |  |
             Tier-3            |  |  8  |  |         Tier-3
         +-----+ +-----+       |  +-----+  |      +-----+ +-----+
         |NODE | |NODE |       +-----------+      |NODE | |NODE |
         |  1  | |  2  |                          | 11  | |  12 |
         +-----+ +-----+                          +-----+ +-----+

         Figure 9: iBGP Sessions with Reflection and Next-Hop-Self

      For simple and efficient route propagation filtering and as
      illustrated in Figure 9:

         Node5, Node6, Node7 and Node8 use the same Cluster ID (Cluster-
         Node3 and Node4 use the same Cluster ID (Cluster-2)

         Node9 and Node10 use the same Cluster ID (Cluster-3)

      The control-plane behavior is mostly the same as described in the
      previous section: the only difference is that the eBGP8277 path
      propagation is simply replaced by an iBGP8277 path reflection with
      next-hop changed to self.

      The data-plane tables are exactly the same.

5.  Applying Segment Routing in the DC with IPv6 dataplane

   The design described in [RFC7938] is reused with one single
   modification.  It is highlighted using the example of the
   reachability to Node11 via spine node Node5.

   Node5 originates 2001:DB8::5/128 with the attached BGP-Prefix-SID for
   IPv6 packets destined to segment 2001:DB8::5

   Node11 originates 2001:DB8::11/128 with the attached BGP-Prefix-SID
   advertising the support of the SRH for IPv6 packets destined to
   segment 2001:DB8::11.

   The control-plane and data-plane processing of all the other nodes in
   the fabric is unchanged.  Specifically, the routes to 2001:DB8::5 and
   2001:DB8::11 are installed in the FIB along the eBGP best-path to
   Node5 (spine node) and Node11 (ToR node) respectively.

   An application on HostA which needs to send traffic to HostZ via only
   Node5 (spine node) can do so by sending IPv6 packets with a Segment
   Routing header (SRH, [I-D.ietf-6man-segment-routing-header]).  The
   destination address and active segment is set to 2001:DB8::5.  The
   next and last segment is set to 2001:DB8::11.

   The application must only use IPv6 addresses that have been
   advertised as capable for SRv6 segment processing (e.g. for which the
   BGP prefix segment capability has been advertised).  How applications
   learn this (e.g.: centralized controller and orchestration) is
   outside the scope of this document.

6.  Communicating path information to the host

   There are two general methods for communicating path information to
   the end-hosts: "proactive" and "reactive", aka "push" and "pull"
   models.  There are multiple ways to implement either of these
   methods.  Here, it is noted that one way could be using a centralized
   controller: the controller either tells the hosts of the prefix-to-
   path mappings beforehand and updates them as needed (network event
   driven push), or responds to the hosts making request for a path to
   specific destination (host event driven pull).  It is also possible
   to use a hybrid model, i.e., pushing some state from the controller
   in response to particular network events, while the host pulls other
   state on demand.

   It is also noted, that when disseminating network-related data to the
   end-hosts a trade-off is made to balance the amount of information
   Vs.  the level of visibility in the network state.  This applies both
   to push and pull models.  In the extreme case, the host would request
   path information on every flow, and keep no local state at all.  On
   the other end of the spectrum, information for every prefix in the
   network along with available paths could be pushed and continuously
   updated on all hosts.

7.  Addressing the open problems

   This section demonstrates how the problems described above (in
   section 3) could be solved using the segment routing concept.  It is
   worth noting that segment routing signaling and data-plane are only
   parts of the solution.  Additional enhancements, e.g., such as the
   centralized controller mentioned previously, and host networking
   stack support are required to implement the proposed solutions.  Also
   the applicability of the solutions described below are not restricted
   to the data-center alone, the same could be re-used in context of
   other domains as well

7.1.  Per-packet and flowlet switching

   A flowlet is defined as a burst of packets from the same flow
   followed by an idle interval.  [KANDULA04] developed a scheme that
   uses flowlets to split traffic across multiple parallel paths in
   order to optimize traffic load sharing.

   With the some ability to choose paths on the host, one may go from per-
   flow load-sharing in the network to per-packet or per-flowlet.  The
   host may select different segment routing instructions either per
   packet, or per flowlet, and route them over different paths.  This
   allows for solving the "elephant flow" problem in the data-center and
   avoiding link imbalances.

   Note that traditional ECMP routing could be easily simulated with on-
   host path selection, using method proposed in [GREENBERG09].  The
   hosts would randomly pick a Tier-2 or Tier-1 device to "bounce" the
   packet off of, depending on whether the destination is under the same
   Tier-2 nodes, or has to be reached across Tier-1.  The host would use
   a hash function that operates on per-flow invariants, to simulate
   per-flow load-sharing in the network.

   Using Figure 1 as reference, let us illustrate this concept assuming
   that HostA has an elephant flow to HostZ called Flow-f.

   Normally, a flow is hashed on to a single path.  Let's assume HostA
   sends its packets associated with Flow-f with top label 16011 (the
   label for the remote ToR, Node11, where HostZ is connected) and Node1
   would hash all the packets of Flow-F via the same next-hop (e.g.
   Node3).  Similarly, let's assume that leaf Node3 would hash all the
   packets of Flow-F via the same next-hop (e.g.: spine node Node5).
   This normal operation would restrict the elephant flow on a small
   subset of the ECMP paths to HostZ and potentially create imbalance
   and congestion in the fabric.

   Leveraging the flowlet proposal, assuming HostA is made aware of 4
   disjoint paths via intermediate segment 16005, 16006, 16007 and 16008
   (the BGP prefix SID's of the 4 spine nodes) and also made aware of
   the prefix segment of the remote ToR connected to the destination
   (16011), then the application can break the elephant flow F into
   flowlets F1, F2, F3, F4 and associate each flowlet with one of the
   following 4 label stacks: {16005, 16011}, {16006, 16011}, {16007,
   16011} and {16008, 16011}. This would spread the load of the elephant
   flow through all the ECMP paths available in the fabric and re-
   balance the load.

7.2.  Performance-aware routing

   Knowing the path associated with flows/packets, the end host may
   deduce certain characteristics of the path on its own, and
   additionally use the information supplied with path information
   pushed from the controller or received via pull request.  The host
   may further share its path observations with the centralized agent,
   so that the latter may keep up-to-date network health map to assist
   other hosts with this information.

   For example, an application A.1 at HostA may pin a TCP flow destined to
   HostZ via Spine node Node5 using label stack {16005, 16011}. The
   application A.1 may collect information on packet loss, deduced from
   TCP retransmissions and loss or other signals (e.g.  RTT increases).
   metrics.  A.1 may additionally publish this information to a
   centralized agent, e.g. after a flow completes, or periodically for
   longer lived flows.  Next, using both local and/or global performance
   data, application A.1 as well as other applications sharing the same
   resources in the DC fabric may pick up the best path for the new
   flow, or update an existing path (e.g.: when informed of congestion
   on an existing path).  The mechanisms for collecting the flow
   metrics, their publishing to a centralized agent and the decision
   process at the centralized agent and the application/host to pick a
   path through the network based on this collected information is
   outside the scope of this document.

   One particularly interesting instance of performance-aware routing is
   dynamic fault-avoidance.  If some links or devices in the network
   start discarding packets due to a fault, the end-hosts could probe
   and detect the path(s) that are affected and hence steer the affected
   flows away from the problem spot.  Similar logic applies to failure
   cases where packets get completely black-holed, e.g., when a link
   goes down and the failure is detected by the host while probing the

   For example, an application A.1 informed about 5 paths to Z {16005,
   16011}, {16006, 16011}, {16007, 16011}, {16008, 16011} and {16011}
   might use the last one by default (for simplicity).  When performance
   is degrading, A.1 might then start to pin TCP flows to each of the 4
   other paths (each via a distinct spine) and monitor the performance.
   It would then detect the faulty path and assign a negative preference
   to the faulty path to avoid further flows using it.  Gradually, over
   time, it may re-assign flows on the faulty path to eventually detect
   the resolution of the trouble and start reusing the path.  The
   mechanisms for monitoring performance for a specific flow and for the
   various paths and the deduction of optimal paths to improve the same
   for the flow are outside the scope of this document.

   By leveraging Segment Routing, one avoids issues associated with
   oblivious ECMP hashing.  For example, if in the topology depicted on
   Figure 1 a link between spine node Node5 and leaf node Node9 fails,
   HostA may exclude the segment corresponding to Node5 from the prefix
   matching the servers under Tier-2 devices Node9.  In the push path
   discovery model, the affected path mappings may be explicitly pushed
   to all the servers for the duration of the failure.  The new mapping
   would instruct them to avoid the particular Tier-1 node until the
   link has recovered.  Alternatively, in pull path, the centralized
   controller may start steering new flows immediately after it
   discovers the issue.  Until then, the existing flows may recover
   using local detection of the path issues.

7.3.  Deterministic network probing

   Active probing is a well-known technique for monitoring network
   elements' health, constituting of sending continuous packet streams
   simulating network traffic to the hosts in the data-center.  Segment
   routing makes possible to prescribe the exact paths that each probe
   or series of probes would be taking toward their destination.  This
   allows for fast correlation and detection of failed paths, by
   processing information from multiple actively probing agents.  This
   complements the data collected from the hosts routing stacks as
   described in Section 7.2.

   For example, imagine a probe agent sending packets to all machines in
   the data-center.  For every host, it may send packets over each of
   the possible paths, knowing exactly which links and devices these
   packets will be crossing.  Correlating results for multiple
   destinations with the topological data, it may automatically isolate
   possible problem to a link or device in the network.

8.  Additional Benefits

8.1.  MPLS Dataplane with operational simplicity

   As required by [RFC7938], no new signaling protocol is introduced.
   The BGP-Prefix-SID is a lightweight extension to BGP Labeled Unicast
   [RFC8277].  It applies either to eBGP or iBGP based designs.

   Specifically, LDP and RSVP-TE are not used.  These protocols would
   drastically impact the operational complexity of the Data Center and
   would not scale.  This is in line with the requirements expressed in

   Provided the same SRGB is configured on all nodes, all nodes use the
   same MPLS label for a given IP prefix.  This is simpler from an
   operation standpoint, as discussed in Section 9

8.2.  Minimizing the FIB table

   The designer may decide to switch all the traffic at Tier-1 and Tier-
   2's based on MPLS, hence drastically decreasing the IP table size at
   these nodes.

   This is easily accomplished by encapsulating the traffic either
   directly at the host or the source ToR node by pushing the BGP-
   Prefix-SID of the destination ToR for intra-DC traffic, or the BGP-
   Prefix-SID for the the border node for inter-DC or DC-to-outside-
   world traffic.

8.3.  Egress Peer Engineering

   It is straightforward to combine the design illustrated in this
   document with the Egress Peer Engineering (EPE) use-case described in

   In such case, the operator is able to engineer its outbound traffic
   on a per host-flow basis, without incurring any additional state at
   intermediate points in the DC fabric.

   For example, the controller only needs to inject a per-flow state on
   the HostA to force it to send its traffic destined to a specific
   Internet destination D via a selected border node (say Node12 in
   Figure 1 instead of another border node, Node11) and a specific
   egress peer of Node12 (say peer AS 9999 of local PeerNode segment
   9999 at Node12 instead of any other peer which provides a path to the
   destination D).  Any packet matching this state at host A would be
   encapsulated with SR segment list (label stack) {16012, 9999}.  16012
   would steer the flow through the DC fabric, leveraging any ECMP,
   along the best path to border node Node12.  Once the flow gets to
   border node Node12, the active segment is 9999 (because of PHP on the
   upstream neighbor of Node12).  This EPE PeerNode segment forces
   border node Node12 to forward the packet to peer AS 9999, without any
   IP lookup at the border node.  There is no per-flow state for this
   engineered flow in the DC fabric.  A benefit of segment routing is
   the per-flow state is only required at the source.

   As well as allowing full traffic engineering control such a design
   also offers FIB table minimization benefits as the Internet-scale FIB
   at border node Node12 is not required if all FIB lookups are avoided
   there by using EPE.

8.4.  Anycast

   The design presented in this document preserves the availability and
   load-balancing properties of the base design presented in

   For example, one could assign an anycast loopback and
   associate segment index 20 to it on the border Node11 and Node12 (in
   addition to their node-specific loopbacks).  Doing so, the EPE
   controller could express a default "go-to-the-Internet via any border
   node" policy as segment list {16020}. Indeed, from any host in the DC
   fabric or from any ToR node, 16020 steers the packet towards the
   border Node11 or Node12 leveraging ECMP where available along the
   best paths to these nodes.

9.  Preferred SRGB Allocation

   In the MPLS case, it is recommend to use same SRGBs at each node.

   Different SRGBs in each node likely increase the complexity of the
   solution both from an operational viewpoint and from a controller

   From an operation viewpoint, it is much simpler to have the same
   global label at every node for the same destination (the MPLS
   troubleshooting is then similar to the IPv6 troubleshooting where
   this global property is a given).

   From a controller viewpoint, this allows us to construct simple
   policies applicable across the fabric.

   Let us consider two applications A and B respectively connected to
   Node1 and Node2 (ToR nodes).  A has two flows FA1 and FA2 destined to
   Z.  B has two flows FB1 and FB2 destined to Z.  The controller wants
   FA1 and FB1 to be load-shared across the fabric while FA2 and FB2
   must be respectively steered via Node5 and Node8.

   Assuming a consistent unique SRGB across the fabric as described in
   the document, the controller can simply do it by instructing A and B
   to use {16011} respectively for FA1 and FB1 and by instructing A and
   B to use {16005 16011} and {16008 16011} respectively for FA2 and

   Let us assume a design where the SRGB is different at every node and
   where the SRGB of each node is advertised using the Originator SRGB
   TLV of the BGP-Prefix-SID as defined in
   [I-D.ietf-idr-bgp-prefix-sid]: SRGB of Node K starts at value K*1000
   and the SRGB length is 1000 (e.g.  Node1's SRGB is [1000, 1999],
   Node2's SRGB is [2000, 2999], ...).

   In this case, not only the controller would need to collect and store
   all of these different SRGB's (e.g., through the Originator SRGB TLV
   of the BGP-Prefix-SID), furthermore it would need to adapt the policy
   for each host.  Indeed, the controller would instruct A to use {1011}
   for FA1 while it would have to instruct B to use {2011} for FB1
   (while with the same SRGB, both policies are the same {16011}).

   Even worse, the controller would instruct A to use {1005, 5011} for
   FA1 while it would instruct B to use {2011, 8011} for FB1 (while with
   the same SRGB, the second segment is the same across both policies:
   16011).  When combining segments to create a policy, one need to
   carefully update the label of each segment.  This is obviously more
   error-prone, more complex and more difficult to troubleshoot.

10.  IANA Considerations

   This document does not make any IANA request.

11.  Manageability Considerations

   The design and deployment guidelines described in this document are
   based on the network design described in [RFC7938].

   The deployment model assumed in this document is based on a single
   domain where the interconnected DCs are part of the same
   administrative domain (which, of course, is split into different
   autonomous systems).  The operator has full control of the whole
   domain and the usual operational and management mechanisms and
   procedures are used in order to prevent any information related to
   internal prefixes and topology to be leaked outside the domain.

   As recommended in [I-D.ietf-spring-segment-routing], the same SRGB
   should be allocated in all nodes in order to facilitate the design,
   deployment and operations of the domain.

   When EPE ([I-D.ietf-spring-segment-routing-central-epe]) is used (as
   explained in Section 8.3, the same operational model is assumed.  EPE
   information is originated and propagated throughout the domain
   towards an internal server and unless explicitly configured by the
   operator, no EPE information is leaked outside the domain boundaries.

12.  Security Considerations

   This document proposes to apply Segment Routing to a well known
   scalability requirement expressed in [RFC7938] using the BGP-Prefix-
   SID as defined in [I-D.ietf-idr-bgp-prefix-sid].

   It has to be noted, as described in Section 11 that the design
   illustrated in [RFC7938] and in this document, refer to a deployment
   model where all nodes are under the same administration.  In this
   context, it is assumed that the operator doesn't want to leak outside
   of the domain any information related to internal prefixes and
   topology.  The internal information includes prefix-sid and EPE
   information.  In order to prevent such leaking, the standard BGP
   mechanisms (filters) are applied on the boundary of the domain.

   Therefore, the solution proposed in this document does not introduce
   any additional security concerns from what expressed in [RFC7938] and
   [I-D.ietf-idr-bgp-prefix-sid].  It is assumed that the security and
   confidentiality of the prefix and topology information is preserved
   by outbound filters at each peering point of the domain as described
   in Section 11.

13.  Acknowledgements

   The authors would like to thank Benjamin Black, Arjun Sreekantiah,
   Keyur Patel, Acee Lindem and Anoop Ghanwani for their comments and
   review of this document.

14.  Contributors
   Gaya Nagarajan


   Gaurav Dawra
   Cisco Systems


   Dmitry Afanasiev


   Tim Laberge


   Edet Nkposong Inc.


   Mohan Nanduri


   James Uttaro


   Saikat Ray

   Jon Mitchell


15.  References

15.1.  Normative References

              Previdi, S., Filsfils, C., Lindem, A., Sreekantiah, A.,
              and H. Gredler, "Segment Routing Prefix SID extensions for
              BGP", draft-ietf-idr-bgp-prefix-sid-07 draft-ietf-idr-bgp-prefix-sid-21 (work in progress),
              October 2017.
              May 2018.

              Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing
              Architecture", draft-ietf-spring-segment-routing-14 draft-ietf-spring-segment-routing-15 (work
              in progress), December 2017. January 2018.

              Filsfils, C., Previdi, S., Dawra, G., Aries, E., and D.
              Afanasiev, "Segment Routing Centralized BGP Egress Peer
              Engineering", draft-ietf-spring-segment-routing-central-
              epe-10 (work in progress), December 2017.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,

   [RFC7938]  Lapukhov, P., Premji, A., and J. Mitchell, Ed., "Use of
              BGP for Routing in Large-Scale Data Centers", RFC 7938,
              DOI 10.17487/RFC7938, August 2016,

   [RFC8277]  Rosen, E., "Using BGP to Bind MPLS Labels to Address
              Prefixes", RFC 8277, DOI 10.17487/RFC8277, October 2017,

15.2.  Informative References

              Greenberg, A., Hamilton, J., Jain, N., Kadula, S., Kim,
              C., Lahiri, P., Maltz, D., Patel, P., and S. Sengupta,
              "VL2: A Scalable and Flexible Data Center Network", 2009.

              Previdi, S., Filsfils, C., Raza, K., Leddy, J., Field, B.,
    , d.,, d., Matsushima, S., Leung, I., Linkova, J., Aries, E., Kosugi,
              T., Vyncke, E., Lebrun, D., Steinberg, D., and R. Raszuk,
              d., "IPv6 Segment Routing Header
              (SRH)", draft-ietf-6man-
              segment-routing-header-07 draft-ietf-6man-segment-routing-header-13 (work in
              progress), July 2017.

              Sinha, S., Kandula, S., and D. Katabi, "Harnessing TCP's
              Burstiness with Flowlet Switching", 2004. May 2018.

   [RFC6793]  Vohra, Q. and E. Chen, "BGP Support for Four-Octet
              Autonomous System (AS) Number Space", RFC 6793,
              DOI 10.17487/RFC6793, December 2012,

Authors' Addresses

   Clarence Filsfils (editor)
   Cisco Systems, Inc.


   Stefano Previdi
   Cisco Systems, Inc.


   Jon Mitchell

   Gaurav Dawra

   Ebben Aries
   Juniper Networks
   1133 Innovation Way
   Sunnyvale  CA 94089


   Petr Lapukhov