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Versions: 00 01 02 03 draft-ietf-spring-segment-routing-msdc

Network Working Group                                   C. Filsfils, Ed.
Internet-Draft                                           S. Previdi, Ed.
Intended status: Informational                       Cisco Systems, Inc.
Expires: January 21, 2016                                    J. Mitchell
                                                            Unaffiliated
                                                                E. Aries
                                                             P. Lapukhov
                                                            G. Nagarajan
                                                                Facebook
                                                            D. Afanasiev
                                                                  Yandex
                                                              T. Laberge
                                                             E. Nkposong
                                                              M. Nanduri
                                                               Microsoft
                                                               J. Uttaro
                                                                     ATT
                                                                  S. Ray
                                                            Unaffiliated
                                                           July 20, 2015


             BGP-Prefix Segment in large-scale data centers
             draft-filsfils-spring-segment-routing-msdc-03

Abstract

   This document describes the motivation and benefits for applying
   segment routing in the data-center.  It describes the design to
   deploy segment routing in the data-center, for both the MPLS and IPv6
   dataplanes.

Requirements Language

   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 [RFC2119].

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 http://datatracker.ietf.org/drafts/current/.




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   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
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   This Internet-Draft will expire on January 21, 2016.

Copyright Notice

   Copyright (c) 2015 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
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   (http://trustee.ietf.org/license-info) in effect on the date of
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   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   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  . . . . . . . . . . . . . . . . . . .   6
     4.2.  eBGP Labeled Unicast (RFC3107)  . . . . . . . . . . . . .   7
       4.2.1.  Control Plane . . . . . . . . . . . . . . . . . . . .   8
       4.2.2.  Data Plane  . . . . . . . . . . . . . . . . . . . . .   9
       4.2.3.  Network Design Variation  . . . . . . . . . . . . . .  10
       4.2.4.  Global BGP Prefix Segment through the fabric  . . . .  10
       4.2.5.  Incremental Deployments . . . . . . . . . . . . . . .  11
     4.3.  iBGP Labeled Unicast (RFC3107)  . . . . . . . . . . . . .  12
   5.  Applying Segment Routing in the DC with IPv6 dataplane  . . .  12
   6.  Communicating path information to the host  . . . . . . . . .  13
   7.  Addressing the open problems  . . . . . . . . . . . . . . . .  14
     7.1.  Per-packet and flowlet switching  . . . . . . . . . . . .  14
     7.2.  Performance-aware routing . . . . . . . . . . . . . . . .  15
     7.3.  Non-oblivious routing . . . . . . . . . . . . . . . . . .  16
     7.4.  Deterministic network probing . . . . . . . . . . . . . .  16
   8.  Additional Benefits . . . . . . . . . . . . . . . . . . . . .  16
     8.1.  MPLS Dataplane with operational simplicity  . . . . . . .  16
     8.2.  Minimizing the FIB table  . . . . . . . . . . . . . . . .  17
     8.3.  Egress Peer Engineering . . . . . . . . . . . . . . . . .  17



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     8.4.  Incremental Deployments . . . . . . . . . . . . . . . . .  18
     8.5.  Anycast . . . . . . . . . . . . . . . . . . . . . . . . .  18
   9.  Preferred SRGB Allocation . . . . . . . . . . . . . . . . . .  18
   10. Alternative Options . . . . . . . . . . . . . . . . . . . . .  19
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   12. Manageability Considerations  . . . . . . . . . . . . . . . .  20
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  20
   14. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  20
   15. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  20
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     16.1.  Normative References . . . . . . . . . . . . . . . . . .  20
     16.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   Segment Routing (SR), as described in
   [I-D.filsfils-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 and service
   chain 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-case use-cases described in this document should be
   considered in the context of the BGP-based large-scale data-center
   (DC) design described in[I-D.ietf-rtgwg-bgp-routing-large-dc]We
   extend 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
   [I-D.ietf-rtgwg-bgp-routing-large-dc] 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.  In
      a Clos topology, the minimum number of parallel paths between two
      elements is determined by the "width" of the middle stage.  See
      Figure 1 below for an illustration of the concept.

   o  Large-scale data-centers commonly use a routing protocol, such as
      BGP4 [RFC4271] in order to provide endpoint connectivity.
      Recovery after a network failure is therefore driven either by



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      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 middle 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 we'll only look into the
   topologies that have a single path from Tier-1 to Tier-3.  The rest
   could be treated similarly, with a few modifications to the logic.

2.1.  Reference design

                                   Tier-1
                                  +-----+
                                  |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, we assume:




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   o  Each node is its own AS (Node X has AS X)

      *  For simple and efficient route propagation filtering, Nodes 5,
         6, 7 and 8 share the same AS, Nodes 3 and 4 share the same AS,
         Nodes 9 and 10 share the same AS.

      *  For efficient usage of the scarce 2-byte Private Use AS pool,
         different Tier-3 nodes might share the same AS.

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

   o  Each node peers with its neighbors via 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 it's loopback interface
      into BGP and announces it to its neighbors.

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

   In this document, we also refer to the Tier-1, Tier-2 and Tier-3
   switches respectively as Spine, Leaf and ToR (top of rack) switches.
   When a ToR switch acts as a gateway to the "outside world", we call
   it a border switch.

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
   design:

   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 that does not
      perform efficiently when flow life-time 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 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



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      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  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
      nodes.

   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, we are going to demonstrate how these
   problems could be addressed within the framework of Segment Routing.

   First, we will explain 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

   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
   ([I-D.keyupate-idr-bgp-prefix-sid]).

   In this document, we make the network design decision to assume that
   all the nodes are allocated the same SRGB, e.g. [16000, 23999].  This
   is important to fulfill the recommendation for operational
   simplification as explained in [I-D.filsfils-spring-segment-routing].





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   Note well that the use of a common SRGB in all nodes is not a
   requirement, one could use a different SRGB at every node.  However,
   this would make the operation of the DC fabric more complex as the
   label allocated to the loopback of a remote switch is then different
   at every node.  This also may increase the complexity of the
   centralized controller.

   For illustration purpose, when considering an MPLS data-plane, we
   assume that the segment index allocated to prefix 192.0.2.x/32 is X.
   As a result, a local label 1600x is allocated for prefix 192.0.2.x/32
   by each node throughout the DC fabric.

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

4.2.  eBGP Labeled Unicast (RFC3107)

   Referring to Figure 1 and [[I-D.ietf-rtgwg-bgp-routing-large-dc], the
   following design modifications are introduced:

   o  Each node peers with its neighbors via eBGP3107 session

   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 on 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, AS
   and index allocation previously described, the following sections




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   detail the control plane operation and the data plane states for the
   prefix 192.0.2.11/32 (loopback of Node11)

4.2.1.  Control Plane

   Node11 originates 192.0.2.11/32 in BGP and allocates to it the BGP-
   Prefix Segment attribute (index11).

   Node11 sends the following eBGP3107 update to Node10:

   . NLRI:  192.0.2.11/32
   . Label: Implicit-Null
   . Next-hop: Node11's interface address on the link to Node10
   . AS Path: {11}
   . BGP-Prefix Attribute: Index 11

   Node10 receives the above update.  As it is SR capable, Node10 is
   able to interpret the BGP-Prefix Attribute and hence understands that
   it should allocate the label LOCAL-SRGB (16000) + "index" 11 (hence
   16011) to the NLRI instead of allocating an nondeterministic 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 eBGP3107 update to Node7:

   . NLRI:  192.0.2.11/32
   . Label: 16011
   . Next-hop: Node10's interface address on the link to Node7
   . AS Path: {10, 11}
   . BGP-Prefix Attribute: Index 11

   Node7 receives the above update.  As it is SR capable, Node7 is able
   to interpret the BGP-Prefix Attribute 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 eBGP3107 NLRI as the outgoing label
   (the index is only used to derive the local/incoming label).

   Node7 sends the following eBGP3107 update to Node4:

   . NLRI:  192.0.2.11/32
   . Label: 16011
   . Next-hop: Node7's interface address on the link to Node4
   . AS Path: {7, 10, 11}
   . BGP-Prefix Attribute: Index 11




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   Node4 receives the above update.  As it is SR capable, Node4 is able
   to interpret the BGP-Prefix Attribute 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 eBGP3107 NLRI as outgoing label (the index is only used to
   derive the local/incoming label).

   Node4 sends the following eBGP3107 update to Node1:

   . NLRI:  192.0.2.11/32
   . Label: 16011
   . Next-hop: Node4's interface address on the link to Node1
   . AS Path: {4, 7, 10, 11}
   . BGP-Prefix Attribute: Index 11

   Node1 receives the above update.  As it is SR capable, Node1 is able
   to interpret the BGP-Prefix Attribute 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 eBGP3107 NLRI as outgoing label (the index is only used to
   derive the local/incoming label).

4.2.2.  Data Plane

   Referring to Figure 1Referring 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 192.0.2.11/32 at Nodes 1, 4, 7 and 10.

              -----------------------------------------------
              Incoming label    | outgoing label | Outgoing
              or IP destination |                | Interface
              ------------------+----------------+-----------
                   16011        |      16011     | ECMP{3, 4}
                192.0.2.11/32   |      16011     | ECMP{3, 4}
              ------------------+----------------+-----------

                     Figure 3: Node1 Forwarding Table

              -----------------------------------------------
              Incoming label    | outgoing label | Outgoing
              or IP destination |                | Interface
              ------------------+----------------+-----------
                   16011        |      16011     | ECMP{7, 8}
                192.0.2.11/32   |      16011     | ECMP{7, 8}
              ------------------+----------------+-----------

                     Figure 4: Node4 Forwarding Table



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              -----------------------------------------------
              Incoming label    | outgoing label | Outgoing
              or IP destination |                | Interface
              ------------------+----------------+-----------
                   16011        |      16011     |    10
                192.0.2.11/32   |      16011     |    10
              ------------------+----------------+-----------

                     Figure 5: Node7 Forwarding Table

              -----------------------------------------------
              Incoming label    | outgoing label | Outgoing
              or IP destination |                | Interface
              ------------------+----------------+-----------
                   16011        |      POP       |    11
                192.0.2.11/32   |      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 Nodes 4, 7 and 10.  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 EPE use-case,
   [I-D.filsfils-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 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 segment (label) allocation throughout the DC fabric.

   A few examples follow:

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

   o  Traffic-engineered path to Node11: an application on a host behind
      Node1 might want to restrict its traffic to paths via the Spine



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      switch Node5.  The application achieves this by sending its
      packets with a label stack of {16005, 16011}. BGP Prefix segment
      16005 directs the packet up to Node5 along the path (Node1, Node3,
      Node5).  BGP Prefix Segment 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 Segment
   attribute and let us show how the fabric connectivity is preserved.

   From a signaling viewpoint, nothing would change: if Node7 does not
   understand the BGP-Prefix Segment attribute, 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 192.0.2.11/32
   (e.g. 123456) instead of the "hinted" label as instructed by the BGP
   Prefix Segment attribute.  The neighbors of Node7 adapt automatically
   as they always use the label in the BGP3107 NLRI as outgoing label.

   Node4 does understand the BGP-Prefix Segment attribute and hence
   allocates the indexed label in the SRGB (16011) for 192.0.2.11/32.

   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 LSP is preserved because the
   outgoing label is always derived from the received label within the
   BGP3107 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









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

                     Figure 8: Node4 Forwarding Table

   The BGP-Prefix Segment functionality can thus be deployed
   incrementally one node at a time.

   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 (RFC3107)

   The same exact design as eBGP3107 is used with the following
   modifications:

      All switches share the same AS

      iBGP3107 reflection with nhop-self is used instead of eBGP3107

      For simple and efficient route propagation filtering, Nodes 5, 6,
      7 and 8 share the same Cluster ID, Nodes 3 and 4 share the same
      Cluster ID, Nodes 9 and 10 share the same Cluster ID.

      AIGP metric ([RFC7311]) is likely applied to the BGP prefix
      segments as part of a large-scale multi-domain design such as
      Seamless MPLS [I-D.ietf-mpls-seamless-mpls].

      The control-plane behavior is mostly the same as described in the
      previous section: the only difference is that the eBGP3107 path
      propagation is simply replaced by an iBGP3107 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 I-D.ietf-rtgwg-bgp-routing-large-dc
   [I-D.ietf-rtgwg-bgp-routing-large-dc] is reused with one single
   modification.  We highlight it using the example of the reachability
   to Node11 via spine switch Node5.

   Spine5 originates 2001:DB8::5/128 with the attached BGP Prefix
   Attribute adverting the support of the Segment Routing extension



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   header (SRH, [I-D.previdi-6man-segment-routing-header]) for IPv6
   packets destined to segment 2001:DB8::5.

   Tor11 originates 2001:DB8::11/128 with the attached BGP Prefix
   Attribute adverting the support of the Segment Routing extension
   header (SRH, [I-D.previdi-6man-segment-routing-header]) 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 SR
   extension 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, we note 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.

   We note, 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.





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7.  Addressing the open problems

   This section demonstrates how the problems describe above 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.

7.1.  Per-packet and flowlet switching

   With the ability to choose paths on the host, one may go from per-
   flow load-sharing in the network to per-packet or per-flowlet (see
   [KANDULA04] for information on flowlets).  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 VL2 (see
   [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 switches, 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's illustrate this assuming that
   HostA has an elephant flow to Z 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 nhop (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 switch Node1).  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 A is made aware of 4
   disjoint paths via intermediate segment 16005, 16006, 16007 and 16008
   (the BGP prefix SID's of the 4 spine switches) 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,



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   16011} and {16008, 16011}. This would spread the load of the elephant
   flow through all the ECMP paths available in the fabric and rebalance
   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 switch Node5 using label stack {16005, 16011}. The
   application A.1 may collect information on packet loss, deduced from
   TCP retransmissions and other signals (e.g.  RTT increases).  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).

   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 detect
   the path(s) being affected and steer their 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.

   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 latter 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.








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7.3.  Non-oblivious routing

   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 switch 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 switch 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, as described in
   Section 7.2.

7.4.  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 inSection 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 [I-D.ietf-rtgwg-bgp-routing-large-dc], no new
   signaling protocol is introduced.  The Prefix Segment is a
   lightweight extension to BGP Labelled Unicast (RFC3107 [RFC3107]).
   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
   [I-D.ietf-rtgwg-bgp-routing-large-dc]



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   A key element of the operational simplicity is the deployment of the
   design with a single and consistent SRGB across the DC fabric.

   At every node in the fabric, the same label is associated to a given
   BGP prefix segment and hence a notion of global prefix segment
   arises.

   When a controller programs HostA to send traffic to HostZ via the
   normally available BGP ECMP paths, the controller uses label 16011
   associated with the ToR switch connected to the HostZ.  The
   controller does not need to pick the label based on the ToR that the
   source host is connected to.

   In a classic BGP Labelled Unicast design applied to the DC fabric
   illustrated in Figure 1, the ToR Node1 connected to HostA would most
   likely allocate a different label for 192.0.2.11/32 than the one
   allocated by ToR Node2.  As a consequence, the controller would need
   to adapt the SR policy to each host, based on the ToR switch that
   they are connected to.  This adds state maintenance and
   synchronization problems.  All of this unnecessary complexity is
   eliminated if a single consistent SRGB is utilized across the fabric.

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 at the source ToR switch by pushing the BGP-
   Prefix Segment of the destination ToR for intra-DC traffic or border
   switch 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
   [I-D.filsfils-spring-segment-routing-central-epe].

   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 switch (say Node12 in
   Figure 1 instead of another border switch Node11) and a specific
   egress peer of Node12 (say peer AS 9999 of local PeerNode segment



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   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 switch Node12.  Once the flow gets to
   border switch Node12, the active segment is 9999 (thanks to PHP on
   the upstream neighbor of Node12).  This EPE PeerNode segment forces
   border switch Node12 to forward the packet to peer AS 9999, without
   any IP lookup at the border switch.  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 switch Node12 is not required if all FIB lookups are
   avoided there by using EPE.

8.4.  Incremental Deployments

   As explained in Section 4.2.5, this design can be deployed
   incrementally.

8.5.  Anycast

   The design presented in this document preserves the availability and
   load-balancing properties of the base design presented in
   [I-D.filsfils-spring-segment-routing].

   For example, one could assign an anycast loopback 192.0.2.20/32 and
   associate segment index 20 to it on the border switches 11 and 12 (in
   addition to their node-specific loopbacks).  Doing so, the EPE
   controller could express a default "go-to-the- Internet via any
   border switch" policy as segment list {16020}. Indeed, from any host
   in the DC fabric or from any ToR switch, 16020 steers the packet
   towards the border switches 11 or 12 leveraging ECMP where available
   along the best paths to these switches.

9.  Preferred SRGB Allocation

   In the MPLS case, we do not recommend to use different SRGBs at each
   node.

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

   From an operation viewpoint, it is much simpler to have the same
   global label at every node for the same destination (the MPLS



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   troubleshooting is then similar to the IPv6 troubleshooting where
   this global property is a given).

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

   Let us consider two applications A and B respectively connected to
   ToR1 and ToR2.  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 Spine5 and spine 8.

   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
   FB2.

   Let us assume a design where the SRGB is different at every node:
   SRGB of Node K starts at value K*1000 and the SRGB length is 1000
   (e.g.  ToR1's SRGB is [1000, 1999], ToR2's SRGB is [2000, 2999]...).

   In this case, not only the controller would need to collect and store
   all of these different SRGB's, 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.  Alternative Options

   In order to support all the requirements and get consensus, the BGP
   Prefix SID attribute has been extended to allow this design.

   Specifically, the ORIGINATOR_SRGB TLV in the BGP Prefix SID signals
   the SRGB of the switch that originated the BGP Prefix Segment.

   This allows to determine the local label allocated by any switch for
   any BGP Prefix Segment, despite the lack of a consistent unique SRGB
   in the domain.





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11.  IANA Considerations

   TBD

12.  Manageability Considerations

   TBD

13.  Security Considerations

   TBD

14.  Contributors

   Benjamin Black, Arjun Arjun Sreekantiah and Keyur Patel have
   contributed to the content of this document.

15.  Acknowledgements

   The authors would like to thank Acee Lindem for his review.

16.  References

16.1.  Normative References

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

   [RFC3107]  Rekhter, Y. and E. Rosen, "Carrying Label Information in
              BGP-4", RFC 3107, May 2001.

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

   [RFC7311]  Mohapatra, P., Fernando, R., Rosen, E., and J. Uttaro,
              "The Accumulated IGP Metric Attribute for BGP", RFC 7311,
              August 2014.

16.2.  Informative References

   [GREENBERG09]
              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.







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   [I-D.filsfils-spring-segment-routing]
              Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
              Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R.,
              Ytti, S., Henderickx, W., Tantsura, J., and E. Crabbe,
              "Segment Routing Architecture", draft-filsfils-spring-
              segment-routing-04 (work in progress), July 2014.

   [I-D.filsfils-spring-segment-routing-central-epe]
              Filsfils, C., Previdi, S., Patel, K., Aries, E.,
              shaw@fb.com, s., Ginsburg, D., and D. Afanasiev, "Segment
              Routing Centralized Egress Peer Engineering", draft-
              filsfils-spring-segment-routing-central-epe-04 (work in
              progress), July 2015.

   [I-D.ietf-mpls-seamless-mpls]
              Leymann, N., Decraene, B., Filsfils, C., Konstantynowicz,
              M., and D. Steinberg, "Seamless MPLS Architecture", draft-
              ietf-mpls-seamless-mpls-07 (work in progress), June 2014.

   [I-D.ietf-rtgwg-bgp-routing-large-dc]
              Lapukhov, P., Premji, A., and J. Mitchell, "Use of BGP for
              routing in large-scale data centers", draft-ietf-rtgwg-
              bgp-routing-large-dc-03 (work in progress), June 2015.

   [I-D.keyupate-idr-bgp-prefix-sid]
              Patel, K., Previdi, S., Filsfils, C., Sreekantiah, A.,
              Ray, S., and H. Gredler, "Segment Routing Prefix SID
              extensions for BGP", draft-keyupate-idr-bgp-prefix-sid-04
              (work in progress), July 2015.

   [I-D.previdi-6man-segment-routing-header]
              Previdi, S., Filsfils, C., Field, B., and I. Leung, "IPv6
              Segment Routing Header (SRH)", draft-previdi-6man-segment-
              routing-header-06 (work in progress), May 2015.

   [KANDULA04]
              Sinha, S., Kandula, S., and D. Katabi, "Harnessing TCP's
              Burstiness with Flowlet Switching", 2004.

Authors' Addresses

   Clarence Filsfils (editor)
   Cisco Systems, Inc.
   Brussels
   BE

   Email: cfilsfil@cisco.com




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   Stefano Previdi (editor)
   Cisco Systems, Inc.
   Via Del Serafico, 200
   Rome  00142
   Italy

   Email: sprevidi@cisco.com


   Jon Mitchell
   Unaffiliated

   Email: jrmitche@puck.nether.net


   Ebben Aries
   Facebook
   US

   Email: exa@fb.com


   P. Lapukhov
   Facebook
   US

   Email: petr@fb.com


   G. Nagarajan
   Facebook
   US

   Email: gaya@fb.com


   Dmitry Afanasiev
   Yandex
   RU

   Email: fl0w@yandex-team.ru


   Tim Laberge
   Microsoft

   Email: tim.laberge@microsoft.com




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   Edet Nkposong
   Microsoft

   Email: edetn@microsoft.com


   Mohan Nanduri
   Microsoft

   Email: mnanduri@microsoft.com


   James Uttaro
   ATT

   Email: ju1738@att.com


   Saikat Ray
   Unaffiliated

   Email: raysaikat@gmail.com





























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