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Versions: 00 01 02 03 04 05                                             
INTERNET-DRAFT                                               Luyuan Fang
Intended Status: Standards Track                              Vijay Gill
Expires: April 30, 2015                                        Microsoft
                                                           Fabio Chiussi

                                                        October 27, 2014


          MPLS-Based Hierarchical SDN for Hyper-Scale DC/Cloud
                    draft-fang-mpls-hsdn-for-hsdc-00


Abstract

   This document describes Hierarchical SDN (HSDN), an architectural
   solution to scale the Data Center (DC) and Data Center Interconnect
   (DCI) networks to support tens of millions of physical underlay
   endpoints, while efficiently handling both ECMP and any-to-any end-
   to-end Traffic Engineered (TE) traffic. HSDN achieves massive scale
   using surprisingly small forwarding tables in the network nodes and
   brings key simplifications in the control plane as well. The HSDN
   forwarding architecture is based on four main concepts: 1. Dividing
   the DC and DCI in a hierarchically-partitioned structure; 2.
   Assigning groups of Underlay Border Nodes in charge of forwarding
   within each partition; 3. Constructing HSDN MPLS label stacks to
   identify the endpoints according to the HSDN structure; and 4.
   Forwarding using the HSDN MPLS labels.

Status of this Memo

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

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   http://www.ietf.org/shadow.html

Copyright and License Notice

   Copyright (c) 2014 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
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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
     1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.2. DC and DCI Reference Model  . . . . . . . . . . . . . . . .  6
   2. Requirements  . . . . . . . . . . . . . . . . . . . . . . . . .  8
     2.1. MPLS-Based HSDN Design Requirements . . . . . . . . . . . .  8
     2.2. Hardware Requirements . . . . . . . . . . . . . . . . . . .  8
   3. HSDN Architecture - Forwarding Plane  . . . . . . . . . . . . .  9
     3.1. Hierarchical Underlay Partitioning  . . . . . . . . . . . .  9
     3.2. Underlay Partition Border Nodes . . . . . . . . . . . . . . 11
       3.2.1. UPBN and UPBG Naming Convention . . . . . . . . . . . . 13
       3.2.2. HSDN Label Stack  . . . . . . . . . . . . . . . . . . . 14
       3.2.3. HSDN Design Example . . . . . . . . . . . . . . . . . . 14
     3.3. MPLS-Based HSDN Forwarding  . . . . . . . . . . . . . . . . 16
   4. Scalability Analysis  . . . . . . . . . . . . . . . . . . . . . 18
     4.1. LFIB Sizing - ECMP  . . . . . . . . . . . . . . . . . . . . 18
     4.2. LFIB Sizing - TE  . . . . . . . . . . . . . . . . . . . . . 18
   5. HSDN Label Stack Assignment Scheme  . . . . . . . . . . . . . . 20
   6. HSDN Architecture - Control Plane . . . . . . . . . . . . . . . 22
     6.1. The SDN approach  . . . . . . . . . . . . . . . . . . . . . 22
     6.2. Distributed control plane . . . . . . . . . . . . . . . . . 23
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 23
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 23
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     9.1  Normative References  . . . . . . . . . . . . . . . . . . . 23
     9.2  Informative References  . . . . . . . . . . . . . . . . . . 24
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24





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1. Introduction

   With the growth in the demand for cloud services, the end-to-end
   cloud network, which includes Data Center (DC) and Data Center
   Interconnect (DCI) networks, has to scale to support millions to tens
   of millions of underlay network endpoints. These endpoints can be
   bare metal servers, virtualized servers, or physical and virtualized
   network functions and appliances.

   The scalability challenge is twofold: 1. Being able to scale using
   low-cost network nodes while achieving high resource utilization in
   the network; and 2. Being able to scale at low operational and
   computational complexity while supporting Equal-Cost Multi-Path
   (ECMP) and any-to-any Traffic Engineering (TE).

   An important set of scalability issues to resolve comes from the
   potential explosion of the routing tables in the network nodes as the
   number of underlay network endpoints increases. Current commodity
   switches have relatively small routing and forwarding tables. For
   example, the typical Forwarding Information Base (FIBs) and Label
   Forwarding Information Base (LFIBs) tables in current low-cost
   network nodes contain 16K or 32K entries. These small sizes are
   clearly insufficient to support entries for all the endpoints in the
   hyper-scale cloud. Address aggregation is used to ameliorate the
   problem, but the scalability challenges remain, since the dynamic and
   elastic environment in the DC/cloud often brings the need to handle
   finely granular prefixes in the network.

   Other factors contribute to the FIB/LFIB explosion. For example, in a
   typical DC using a fat Clos topology, even the support of ECMP load
   balancing may become an issue if the individual outgoing paths
   belonging to an ECMP group carry different outgoing labels.

   Another key scalability issue to resolve is the complexity of certain
   desired functions that should be supported in the network, the most
   prominent one being TE. Currently, any-to-any server-to-server TE in
   the DC/DCI is simply unfeasible, as path computation and bandwidth
   allocation at scale, an NP-complete problem, becomes rapidly
   unmanageable. Furthermore, the forwarding state needed in the network
   nodes for TE tunnels contributes in a major way to the explosion of
   the LFIBs.

   Other major scalability issues are related to the efficient creation,
   management, and use of tunnels, for example the configuration of
   protection paths for fast restoration.

   Many additional scalability issues in terms of operational and
   computational complexity need to be resolved in order to scale the



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   control plane and the network state. In particular, the controller-
   centric approach of Software Defined Networks (SDNs), which is
   increasingly being accepted as "the way to build the next generation
   clouds," in order to be scalable, requires appropriate scalability
   solutions in order to take full advantage of the potential benefits
   of SDN.

   Finally, the underlay network architecture should offer certain
   capabilities to facilitate the support of the demand of the overlay
   network.

   In this document, we present Hierarchical SDN (HSDN), a set of
   solutions for all these scalability challenges in the underlay
   network, both in the forwarding and in the control plane. Although
   HSDN can be used in principle with any forwarding technology, it has
   been designed to leverage Multi Protocol Label Switching (MPLS)-based
   forwarding [RFC3031], using label stacks [RFC3032] constructed
   according to the HSDN structure.

   HSDN achieves massive scale using surprisingly small LFIBs in the
   network nodes, while supporting both ECMP and any-to-any end-to-end
   TE traffic. HSDN also brings important simplifications in the control
   plane and in the architecture of the SDN controller.

   The HSDN forwarding architecture is based on four main concepts: 1.
   Dividing the DC and DCI in a hierarchically-partitioned structure; 2.
   Assigning groups of Underlay Border Nodes in charge of forwarding
   within each partition; 3. Constructing HSDN MPLS label stacks to
   identify the end points according to the HSDN structure; and 4.
   Forwarding using the HSDN MPLS labels.

   HSDN is designed to allow the physical decoupling of the control and
   forwarding, and have the LFIBs configured by a controller according
   to the SDN approach. However, it is also meant to support the
   traditional distributed routing and label distribution protocol
   approach, which may be particularly useful during technology
   migration.

1.1. Terminology

   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].








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   Term              Definition
   -----------       --------------------------------------------------
   BGP               Border Gateway Protocol
   DC                Data Center
   DCGW              DC Gateway (Border Leaf)
   DCI               Data Center Interconnect
   DID               Destination Identifier
   ECMP              Equal Cost MultiPathing
   FIB               Forwarding Information Base
   HSDN              Hierarchical SDN
   LDP               Label Distribution Protocol
   LFIB              Label Forwarding Information Base
   LN                Leaf Node
   MPLS              Multi-Protocol Label Switching
   PID               Path Identifier
   SDN               Software Defined Network
   SN                Spine Node
   SVR               Server
   UP                Underlay Partition
   UPBG              Underlay Partition Border Group
   UPBG              Underlay Partition Border Node
   TE                Traffic Engineering
   ToR               Top-of-Rack switch
   TR                Top-of-Rack switch
   VN                Virtual Network
   VM                Virtual Machine
   WAN               Wide Area Network

   In this document, we also use the following terms.

   o  End device: A physical device attached to the DC/DCI network.
      Examples of end devices include bare metal servers, virtualized
      servers, network appliances, etc.

   o  Level: A layer in the hierarchy of underlay partitions in the HSDB
      architecture.

   o  Overlay Network (ON): A virtualized network that provides Layer 2
      or Layer 3 virtual network services to multiple tenants. It is
      implemented over the underlay network.

   o  Path Label (PL): A label used for MPLS-based HSDN forwarding in
      the underlay network.

   o  Row: A row of racks where end devices reside in a DC.

   o  Tier: One of the layers of network nodes in a multi-layer Clos-
      based topology.



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   o  Underlay Network (UN): The physical network that provides the
      connectivity among physical end devices. It provides transport for
      the overlay network traffic.

   o  Underlay Partition (UP): A logical portion of the underlay network
      designed according to the HSDN architecture. Underlay partitions
      are arranged in a hierarchy consisting of multiple levels.

   o  VN Label (VL): A label carrying overlay network traffic. It is
      encapsulated in the underlay network in a stack of path labels
      constructed according to the HSDN forwarding scheme.

1.2. DC and DCI Reference Model

   Here we show the typical structure of the DC and DCI, which we use in
   the rest of this document to describe the HSDN architecture. We also
   introduce a few commonly used terms to assist in the explanation.

   Figure 1 illustrates multiple DCs interconnected by the DCI/WAN.

                     +-------------+
                     |             |
                     |     DC      |
                     |             |
                     +-------------+
                               \-----.
                               (       ')
                           .--(.       '.---.
                          (     '      '     )
                         (       DCI/WAN      )
                          (.                .)
                           (     (        .)  \
                            /'--' '-''---'  +-------------+
                 +-------------+            |             |
                 |             |            |     DC      |
                 |     DC      |            |             |
                 |             |            +-------------+
                 +-------------+

          Figure 1. DCIWAN interconnecting multiple DCs.



   Figure 2 below illustrates the typical structure of a Clos-based DC
   fabric.






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                                 +----+   +----+
  DCGW                 +---------+ GW +   + GW +---------+
                       |         ++--++\ /++--++         |
                       | +--------|--|--\--\  |          |
                       | |        |  \-/-\-|--|--------+ |
                     +-+-++     +-+--+/   \+--+-+     ++-+-+
  Spine    +---------+ SN +     | SN |     | SN +     + SN +---------+
           |         +----+\    ++--++     ++--++\   /+----+         |
           | +---------------\---+  |       |  +---/---------------+ |
           | |        ...      \    |       |    /      ...        | |
          ++-+-+ +----+  +----+ ++--++     ++--++ +----+  +----+ +-+-++
  Leaf    | LN | | LN |  | LN | | LN |     ||LN | | LN |  | LN | | LN |
          ++--++ ++--++  +---++ ++---+     +---++ ++--++  +---++ ++--++
           |   \ /   |    |   \ /   |       |   \ /   |    |   \ /   |
           |   / \   |    |   / \   |       |   / \   |    |   / \   |
           | / ... \ |    | / ... \ |       | / ... \ |    | / ... \ |
         ++++ +--+ ++++ ++++ +--+ ++++    ++++ +--+ ++++ ++++ +--+ ++++
  ToR    |TR| |TR| |TR| |TR| |TR| |TR|    |TR| |TR| |TR| |TR| |TR| |TR|
         +--+ +--+ +--+ +--+ +--+ +--+    +--+ +--+ +--+ +--+ +--+ +--+
          |    |    |    |    |    |       |    |    |    |    |    |
         +--+ +--+ +--+ +--+ +--+ +--+    +--+ +--+ +--+ +--+ +--+ +--+
  Server +--+ +--+ +--+ +--+ +--+ +--+    +--+ +--+ +--+ +--+ +--+ +--+
  rack   +--+ +--+ +--+ +--+ +--+ +--+    +--+ +--+ +--+ +--+ +--+ +--+
         +--+ +--+ +--+ +--+ +--+ +--+    +--+ +--+ +--+ +--+ +--+ +--+

          Figure 2. Typical Clos-based DC fabric topology.

          Note: Not all links are shown in Figure 2.

   The DC fabric shown in Figure 2 uses what is known as a spine and
   leaf architecture with a multi-stage Clos-based topology
   interconnecting multiple tiers of network nodes. The DC Gateways
   (DCGWs) connect the DC to the DCI/WAN. The DCGW connect to the Spine
   Nodes (SNs), which in turn connect to the Leaf Nodes (LFs). The Leaf
   Nodes connect to the Top-of-Rack switches (ToRs). Each ToR typically
   resides in a rack (hence the name) accommodating a number of servers
   connected to their respective ToR. The servers may be bare metal or
   virtualized.

   Each tier of switches and the connectivity between switches is
   designed to offer a desired capacity and provide sufficient bandwidth
   to the servers and end devices.

   The precise topology and connectivity between the tiers of switches
   depends on the specific design of the DC. More or less tiers of
   switches (spines or leaves) or asymmetric topologies, not shown in
   the figure, may be used. A precise description of the topology and
   its design criteria is out of the scope of this document.



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   What's relevant for this document is the fact that a typical large-
   scale DC topology does not have all the tiers fully connected to the
   adjacent tiers. In other words, not all network nodes in a tier are
   connected to all the network nodes in the adjacent tiers. This is
   especially true for the tiers closer to the endpoints, and is due to
   the sheer number of connections and devices and the physical
   constraints of the DC, which makes it impractical, uneconomical, and
   ultimately unnecessary to use a fully connected Clos-based topology.

   The connectivity is typically organized following an
   aggregation/multiplexing connectivity architecture that consolidates
   traffic from the edges into the leafs and spines.

2. Requirements

2.1. MPLS-Based HSDN Design Requirements

   The following are the key design requirements for HSDN solutions.

   1) MUST support millions to tens of millions of underlay network
      endpoints in the DC/DCI.

   2) MUST use very small LFIB sizes (e.g., 16K or 32K LFIB entries) in
      all network nodes.

   3) MUST support both ECMP and any-to-any, end-to-end, server-to-
      server TE traffic.

   4) MUST support ECMP traffic load balancing using a single forwarding
      entry in the LFIBs per ECMP group.

   5) MUST require IP lookup only at the network edges.

   6) MUST support encapsulation of overlay network traffic, and support
      any network virtualization overlay technology.

   7) MUST support control plane using both SDN controller approach, and
      the traditional distributed control plane approach using any label
      distribution protocols.

2.2. Hardware Requirements

   The following are the hardware requirements to support HSDN.

   1) The server NICs MUST be able to push a HSDN label stack consisting
      of as many path labels as levels in the HSDN hierarchical
      partition (e.g., 3 path labels).




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   2) The network nodes MUST support MPLS forwarding.

   3) The network nodes MUST be able perform ECMP on packets carrying a
      label stack consisting of as many path labels as levels in the
      HSDN hierarchical partition, plus one or more VN label/header for
      the overlay network (e.g., 3 path labels + 1 VN label/header).

3. HSDN Architecture - Forwarding Plane

   As mentioned above, a primary design requirement for HSDN is to
   enable scalability of the forwarding plane to tens of millions of
   network endpoints using very small LFIB sizes in all network nodes in
   the DC/DCI, while supporting both ECMP and any-to-any server-to-
   server TE traffic.

   The driving principle of the HSDN forwarding plane is "divide and
   conquer" by partitioning the forwarding task into local and
   independent forwarding. When designed properly, such an approach
   enables extreme horizontal scaling of the DC/DCI.

   HSDN is based on four concepts:

   1) Dividing the underlay network in a hierarchy of partitions;
   2) Assigning groups of Underlay Partition Border Nodes (UPBN) to each
      partition, in charge of forwarding within the corresponding
      partition;
   3) Constructing HSDN label stacks for the endpoint Forward
      Equivalency Classes (FECs) in accordance with the underlay network
      partition hierarchy;
   4) Configuring the LFIBs in all network nodes and forwarding using
      the label stacks.

   In this section, we explain in detail each of these concepts.
   Scalability analysis for both ECMP and TE is presented in Section 4.
   In Section 5, we describe a possible label stack assignment scheme
   for HSDN.

3.1. Hierarchical Underlay Partitioning

   HSDN is based on dividing the DC/DCI underlay network into logical
   partitions arranged in a multi-level hierarchy.

   The HSDN hierarchical partitioning is illustrated in Figure 3.








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           +----------------------------------------------------------+
  UP       |                          UP0                             |
  Level 0  |                                                          |
           |+---------------------------------+   +------------------+|
           ||                                 |...|                  ||
           +|---------------------------------|---|------------------|+
  UP        |             UP1-0               |   |       UP1-N      |
  Level 1   |                                 |   |                  |
            |+-------------+   +-------------+|   |   +-------------+|
            ||             |...|             ||   |...|             ||
            +|-------------|---|-------------|+   +---|-------------|+
  UP         |   UP2-0-0   |   |   UP2-0-N   |        |  UP2-N-N    |
  Level 2    |             |   |             |        |             |
             +-----+ +-----+   +-----+ +-----+        +-----+ +-----+
             | SVR | | SVR |   | SVR | | SVR |        | SVR | | SVR |
             |-----|-|-----|   |-----|-|-----|        |-----|-|-----|
  Overlay    |VM|VM| |VM|VM|   |VM|VM| |     |        |VM|VM| |     |
  Level      |-----| |-----|   |-----| |     |        |-----| |     |
             |VM|VM| |VM|VM|   |VM|VM| |     |        |VM|VM| |     |
             +-----+ +-----+   +-----+ +-----+        +-----+ +-----+

   Figure 3. HSDN underlay network hierarchical partitioning of DC/DCI.


   The hierarchy consists of multiple levels of Underlay Partitions
   (UPs). For simplicity, we describe HSDN using three levels of
   partitioning, but more or less levels can be used, depending on the
   size and architecture of the overall network, using similar design
   principles (as shown below, three levels of partitions are sufficient
   to achieve scalability to tens of millions servers using very small
   LFIBs).

   The levels of partitions are nested into a hierarchical structure. At
   each level, the combination of all partitions covers the entire
   DC/DCI topology. In general, within each level, the UPs do not
   overlap, although there may be design scenarios in which overlapping
   UPs within a level may be used. The top level (Level 0) consists of a
   single underlay partition UP0 (the HSDN concept can be extended to
   multi-partitioned Level 0).

   We use the following naming convention for the UPs:

   -  Partitions at Level i are referred to as UPi (e.g., UP0 for Level
      0, UP1 for Level 1, UP2 for Level2, and so on).

   -  Within each level, partitions are identified by a rightmost
      sequential number (starting from 1) referring to the corresponding
      level and a set of sequential number(s) for each partition in a



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      higher level that the specific partition is nested into.

      For example, at Level 1, there are N partitions, referred to as
      UP1-1 to UP1-N.

      Similarly, at Level 2, there are M partitions for each Level 1
      partitions, for a total of NxM partitions. For example, the Level
      2 partitions nested into Level 1 partition UP1-1 are UP2-1-1 to
      UP2-1-M, while the ones nested into UP1-N are UP2-N-1 to UP2-N-M.

   -  Note that for simplicity in illustrating the partitioning, we
      assume a symmetrical arrangement of the partitions, where the
      number of partitions nested into each partition at a higher level
      is the same (e.g., all UP1 partitions have M UP2 partitions). In
      practice, this is rarely the case, and the naming convention can
      be adapted accordingly for different numbers of partitions.

   The following considerations complete the description of Figure 3.

   o  The servers (bare metal or virtualized) are attached to the bottom
      UP level (in our case, Level 2). A similar naming convention as
      the one used for the partitions may be used.

   o  In Figure 3, we also show an additional Overlay Level. This
      corresponds to the virtualized overlay network (if any) providing
      Virtual Networks (VN) connecting Virtual Machines (VMs) and other
      overlay network endpoints. Overlay network traffic is encapsulated
      by the HSDN underlay network. The operation of the Overlay Level
      is out of scope of this document.

   The UPs are designed to contain one or more tiers of switches in the
   DC topology or nodes in the DCI. The key design criteria in defining
   the partitions at each layer is that they need to follow the
   "natural" connectivity implemented in the DC/DCI topology. An example
   is given below to further clarify how the partitions are designed.

3.2. Underlay Partition Border Nodes

   Once the HSDN hierarchical partitioning is defined, Underlay
   Partition Border Nodes (UPBNs) are assigned to each UP. This is
   illustrated in Figure 4.










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  +++++++++++++++++++++++++++++++++++++++++++++++++
  |                     UP0              <--------|-----+
  |                                               |     |
  | +++++++++++++++++++++++++++++++++++++++++++++ |     |
  | |   +-----------------------------------+   | |     |
  | |   | +---------+  UPBG1-i  +---------+ |   | |     |
  | |   | | UPBN1-i |    ...    | UPBN1-i | |   | |     |
  | |   | +---------+           +---------+ |   | |     |
  | |   +-----------------------------------+   | |     |
  ++|+++++++++++++++++++++++++++++++++++++++++++|++     |
    |                   UP1              <------|----+  |   +---------+
    | +++++++++++++++++++++++++++++++++++++++++ |    |  +---|   LP0   |
    | | +-----------------------------------+ | |    |      +---------+
    | | | +---------+ UPBG2-i-j +---------+ | | |    +------|   LP1   |
    | | | |UPBN2-i-j|    ...    |UPBN2-i-j| | | |           +---------+
    | | | +---------+           +---------+ | | |    +------|   LP2   |
    | | +-----------------------------------+ | |    |      +---------+
    ++|+++++++++++++++++++++++++++++++++++++++|++    |  +---|   VL    |
      |                 UP2              <----|------+  |   +---------+
      |          +---------------+            |         |
      |          |   SVR-i-j-k   |            |         |
      |          | +-----------+ |            |         |
      |          | |    NVE    |<-------------|---------+
      |          | +-----+-----+ |            |
      +++++++++++| | VM  | VM  | |+++++++++++++
                 | +-----+-----+ |
                 | | VM  | VM  | |
                 +-+-----+-----+-+

          Figure 4. UBPNs, UBPGs, and label stack assignment.


   The UPBNs belong to two partitions in adjacent levels in the
   hierarchy and they constitute the entry points for traffic from the
   higher level partition destined to the corresponding lower level
   partition. As such, they constitute the forwarding end destinations
   within each partition.

   In order to provide sufficient capacity and support traffic load
   balancing between the levels in the hierarchy, multiple UPBNs are
   assigned to each partition. The UPBNs for each partition are grouped
   into an Underlay Partition Border Group (UPBG). As shown below, using
   an appropriate Label Stack Assignment scheme all UPBNs in a UPBG can
   be made identical for ECMP traffic forwarding (i.e., the ECMP entries
   in the LFIBs in all UPBNs in a UPBG are identical). Thus, for ECMP
   traffic load balancing, all UPBNs belong to the same FEC as far as
   the higher level partition is concerned. For TE traffic, a desired
   UPBN within a UPBG group may need to be specified, and thus the UPBNs



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   in a UPBG are not forwarding-wise equivalent.

   In practice, the UPs are designed by finding the most advantageous
   way to partition the DC Clos-based topology and the DCI topology.
   Within the DC, the UPBNs in each level are subsets of the network
   nodes in one of the tiers that form the multi-stage Clos
   architecture. In general, the UPs may internally contain tiers of
   network nodes that are not UPBNs. A specific design example to
   further illustrate the HSDN partitioning is provided below.

   As explained in more detail below, for forwarding purposes, by
   partitioning the DC/DCI in this manner and using HSDN forwarding, the
   UPBNs need to have entries in their LFIBs only to reach destinations
   in the two partitions to which they belong to (their own
   corresponding partition and the higher-layer partition to which they
   nested to). The network nodes inside the UPs only need to have
   entries in their LFIB to reach the destinations in their partition.

   From these considerations, a first design heuristic for choosing the
   partitioning structure is to keep the number of partitions nested at
   each level into the higher level relatively small for all levels. For
   the lowest level, the number of endpoints (servers) in each partition
   should also be kept to manageable levels. Clearly, the design
   tradeoff is between the size and the number of partitions at each
   level. Fortunately, for most practical deployments, it is relatively
   simple to find a good tradeoff that achieves the desired
   scalability.

3.2.1. UPBN and UPBG Naming Convention

   We use a similar naming convention for the UPBNs and UPBGs as the one
   used for the UPs:

   -  UPBNi is a UPBN between partitions at Level(i) and Level(i-1).
      Similarly for UPBG.

   -  Within each level, the UPBNs are identified by a set of sequential
      number(s) equal to the corresponding sequential number(s) of the
      corresponding partition within that level.

      For example, at Level 1, UPBN1-1 corresponds to partition UP1-1,
      and connects UP0 with UP1-1. UPBN1-N corresponds to partition UP1-
      N and connects UP0 with UP1-N, and so on. Similarly for UPBG.

      At Level 2, UPBN2-1-1 corresponds to partition UP2-1-1 and
      connects UP1-1 with UP2-1-1, and so on. Similarly for UPBG.

      Note that the UPBNs within an UPBGs can be further distinguished



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      using an appropriate naming convention, which is not shown here.

3.2.2. HSDN Label Stack

   In MPLS-Based HSDN, an MPLS label stack is defined and used for
   forwarding. The key notion in HSDN is that the label stack is defined
   and the labels are assigned in accordance with the hierarchical
   partitioned structure defined above.

   The label stack, shown in Figure 4 above, is constructed as follows.

   -  The label stack contains as many Path Labels (PLs) as levels in
      the partition hierarchy.

   -  Each PL in the label stack is associated to a corresponding level
      in the partition hierarchy and is used for forwarding at that
      level.

      In the scenario of Figure 4, PL0 is associated to Level 0 and is
      used to forward to destination in UP0, PL1 is associated to Level
      1 and is used to forward to destinations in any UP1 partitions,
      and PL2 is associated to Level 2 and is used to forward to
      destinations in any UP2 partitions.

   -  A VN Label (VL) is also shown in the label stack in Figure 4. This
      label is associated to the Overlay Level and is used to forward in
      the overlay network. The VL is simply encapsulated in the label
      stack and transported in the HSDN underlay network. The details of
      the VL processing within the overlay network are out of scope of
      this document.

   Each endpoint in the DC/DCI is identified by a corresponding label
   stack. For a given endpoint, the label stack is constructed in such a
   way that the PLO specifies the UP1 to which the endpoint is attached
   to, the PL1 specifies the UP1 to which the endpoint is attached to,
   and the PL2 specifies the FEC in the UP2 corresponding to the
   endpoint.

   A scheme to assign the PL labels in the HSDN label stack is described
   below.

3.2.3. HSDN Design Example

   We use an example to further explain the HSDN design criteria to
   define the hierarchically-partitioned structure of the DC/DCI. We use
   the same design example in the Scalability Analysis section below to
   show the LFIB sizing with ECMP and TE traffic.




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   To summarize some of the design heuristics for the HSDN underlay
   partitions:

   -  The UPs should be designed to follow the "natural" connectivity
      topology in the DC/DCI.

   -  The number of partitions at each level nested into the higher
      level should be relatively small (since they are FEC entries in
      the LFIBs in the network nodes in the corresponding levels).

   -  The number of endpoints (servers) in each partition in the lowest
      level should be relatively small (since they are FEC entries in
      the LFIBs in the network nodes in the lowest level).

   -  The number of levels should be kept small (since it corresponds to
      the number of path labels in the stack).

   -  The number of tiers in each partition in each level should be kept
      small. This is due to the multiplicative fanout effect for TE
      traffic (explained below), which has a major impact on the LFIB
      size needed to support any-to-any server-to-server TE.

   The HSDN forwarding plane design consists in finding the best
   tradeoff among these contrasting objectives. Although the optimal
   design choices ultimately depend on the specific deployment, here we
   describe an illustrative example.

   As shown below, a three-level HSDN hierarchy is sufficient to scale
   the DC/DCI to tens of millions of servers.

   With three levels, a possible design choice for the UP1s is to have
   each UP1 correspond to a DC. With this choice, the UP0 corresponds to
   the DCI and the UPBN1s are the DCGWs in each DC (the UPBG1s group the
   DCGWs in each DC).

   Once the UP1s are chosen this way, a possible design choice for the
   UP2s is to have each UP2 correspond to a group of racks, where each
   group of racks may correspond to a portion of a row of racks, an
   entire row of racks, or multiple rows of racks. The specific best
   choice of how many racks should be in a group of racks corresponding
   to each UP2 ultimately depends on the specific connectivity, the
   number of servers per racks.

   While precise numbers depend on the specific technologies used in
   each deployment, here and in the Scalability Analysis section below
   we want to give some ideas of the scaling capabilities of HSDN. For
   this purpose, we use some hypothetical yet reasonable numbers to
   characterize the partitioning design example.



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   Assume the following: a) 20 DCs connected via the DCI/WAN; b) 50
   servers per rack; c) 20 racks per group of racks; d) 50 groups of
   racks per DC.

   With these numbers, there are 500K servers per DC, for a total of 10M
   underlay network endpoints in the DC/DCI.

   In the HSDN structure in this example, there are 20 UP1s, 500 UP2s
   per UP1, and 1000 servers per UP2.

3.3. MPLS-Based HSDN Forwarding

   The hierarchically partitioned structure and the corresponding label
   stack are used in HSDN to scale the forwarding plane horizontally
   while using LFIBs of surprising small sizes in the network nodes.

   As explained above, each label in the HSDN label stack is associated
   with one of the levels in the hierarchy and is used to forward to
   destinations in the underlay partitions at that level.

   We describe the life of a packet in the HSDN DC/DCI. We use the
   specific design example described in Section 3.2.3 above to help in
   the explanation, but of course the forwarding would be similar for
   other design choices.

   We first describe the behavior for non-TE traffic. In the HSDN
   DC/DCI, for a packet that needs to be forwarded to a specific
   endpoint in the underlay network, the outer label PL0 specifies which
   UP1 contains the endpoint. Let's refer to this UP1 as UP1-a. For ECMP
   traffic, the PL0 binding is with a FEC corresponding to the UPBG1-a
   associated with UP1-a. Note that all the endpoints reachable via UP1-
   a are forwarded using the same FEC entry for Level 0 in the
   hierarchical partitioning.

   Once the packet reaches one of the network nodes UPBN1-a in the
   UPBG1-a group (the upstream network nodes perform ECMP load
   balancing, thus the packet may enter UP1-a via any of the UPBN1-a
   nodes), the PL0 is popped and the PL1 is used for forwarding in the
   UP1-a. To be precise, because of penultimate hop popping, it is the
   network node immediately upstream of the chosen UPBN1-a that pops the
   label P0).

   The PL1 is used within UP1-a to reach the UP2 which contains the
   endpoint. Let's refer to this UP2 as UP2-a. In the UP2 network nodes
   the PL1 binding is with a FEC corresponding to the UPBG2-a associated
   with UP2-a. Similarly as above, note that all the endpoints reachable
   via UP2-a are forwarded using the same FEC entry for Level 1 in the
   hierarchical partitioning.



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   Once the packet reaches one of the network nodes UPBN2-a in the
   UPBG2-a group (once again, the upstream network nodes perform ECMP
   load balancing, so the packet may transit to any of the UPBN2-a
   nodes), the PL1 is popped and the PL2 is used for the rest of the
   forwarding (again, to be precise, the penultimate network node
   upstream of UPBN2-a is the one popping the PL1 label).

   The PL2 is used within UP2-a to reach the desired endpoint. Note that
   the UPBN2 nodes and the network nodes in the UP2s have entries in
   their LFIBs only to reach endpoints within their UP2. They can reach
   endpoints in other UP2s by using a FEC entry corresponding to the UP2
   containing the destination endpoint, identified by PL1.

   The following two observations help in further clarifying the
   forwarding operation above.

   -  The PL0 is used for forwarding from the source to the UPBN1-a. For
      a packet originating from an endpoint attached to a certain UP2,
      say UP2-b, nested to a different UP1, say UP1-b, PL0 is used for
      forwarding in all network nodes that the packet transits until it
      reaches the UPBN1-a. This includes network nodes in UP2-b and UP1-
      b (i.e., "on the way out" from UP2). It also includes one of the
      UPBN1-b nodes. Note, however, that the PL0 is not popped at the
      UPBN1-b, since it is used for forwarding to the destination UPBN1-
      a.

   -  Not all packets carry a three-label MPLS stack. For example, a
      packet originating from the endpoint in UP2-b and destined to an
      endpoint in the same UP2-b only carries PL2. Similarly a packet
      originating from the endpoint in UP2-b and destined to an endpoint
      in a different UP2 nested in the same UP1-b only carries PL1 and
      PL2.

   In the case of TE traffic, the use of the different labels in the
   label stack is similar as what described above for ECMP traffic.
   However, the labels are bound to FECs identifying a specific path
   within each UPs that is traversed. Therefore, TE traffic contributes
   for additional entries in the LFIBs in the network nodes. By properly
   designing the UPs, the number of LFIB entries can be kept relatively
   small.

   HSDN, by superimposing a hierarchically-partitioned structure and
   using a label stack constructed according to such a structure, is
   able to impose a forwarding scheme that is aggregated by
   construction. This translates in dramatic reductions in the size of
   the LFIBs in the network nodes, since each node only needs to know a
   limited portion of the forwarding space.




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   HSDN supports any label assignment scheme to generate the labels in
   the label stack. However, if a label assignment scheme that is
   consistent with the HSDN structure is used, additional
   simplifications of the LFIBs and the control plane can be achieved.

   In Section 5 below, we present one example of such a scheme, where
   the labels in the label stack represent the "physical" location of
   the endpoint, expressed according to the HSDN structure. For TE
   traffic, the labels represent a specific path towards the desired
   destination through the HSDN structure.

   In the Scalability Analysis section and in the Control Plane section
   below we assume that such a Label Assignment scheme is used.

   In HSDN, the LFIBs in the network nodes can be configured in such a
   way that all the paths in the DC/DCI are pre-established. This can be
   achieved using surprisingly small LFIB sizes.

4. Scalability Analysis

   In this section, we compute the maximum size of the LFIBs for non-
   TE/ECMP traffic and any-to-any server-to-server TE traffic.

4.1. LFIB Sizing - ECMP

   For ECMP traffic, at each level, all destinations belonging to the
   same partition at a lower level are forwarded using the same FEC
   entry in the LFIB, which identifies the destination UPBG for that
   level, or the destination endpoint at the lower level. Since the UPs
   are designed in such a way to keep the number of destinations small
   in all UPs, and the network nodes only need to know how to reach
   destinations in their own UP and in the adjacent UP at the higher
   level in the hierarchy, this translate to the fact that hyper scale
   of the DC/DCI can be achieved with very small LFIB sizes in the all
   individual network nodes.

   The worst case for the LFIB size occurs at one of the network nodes
   that serve as UPBNs for one of the levels of UPs in the hierarchy.
   The level where the LFIB size occurs depend on the specific choice of
   the partitioning design.

   To be completed.

4.2. LFIB Sizing - TE

   As noted above, TE traffic may add a considerable number of entries
   to LFIB, since it creates one new FEC per TE tunnel to each
   destination.



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   HSDN provides a solution to this problem. In fact, HSDN can support
   any-to-any server-to-server "TE Max Case" with small LFIB sizes. In
   TE Max Cases, all sources are connected to all destinations (e.g.,
   server to server) with TE tunnels, the tunnels using all possible
   distinct paths in the network. TE Max Case gives therefore an upper
   bound to the number of TE tunnels (and consequently, LFIB entries) in
   the network.

   Again, in HSDN, since the UPs are designed in such a way to be
   relatively small, the number of paths in each partition can be kept
   to a manageable number.

   In a Clos Topology (the analysis can be extended to generic
   topologies), the number of paths in a UP with N destination can be
   easily computed. The number of paths (and the maximum number of LFIB
   entries is equal to the products of the switch fanout in each tier
   traversed from the source to the destination in that UP. We refer to
   this as the TE Fanout Multiplicative Effect, which is illustrated in
   Figure 5.

   Total # LFIB Entries for TE Max Case = N * F1 * F2 * ... * F(M-1)

   Where Fi is the fanout of a switch in each tier traversed to the
   destination, M is the number of tiers in the UP, and N is the number
   of destinations in the UP.

   Once again, by properly designing the UPs, the TE Fanout
   Multiplicative Effect can be kept under control, since the path
   computation is local for each of the UPs.






















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                                  +-------+
           Source                 |  Src  |
                                  | Node  |
                                  +-------+  F1=3
                                 /    |    \
                                /     |     \
                        +-------+ +-------+ +-------+
           Tier 1       |       | |       | |       |
                        |  Node | |  Node | |  Node |
                        +-+-----+ +---+---+ +-----+-+  F2=3
                          |   \ \ /   |   \ / /   |
                          |    \/ \---|---/ \/    |
                          |   / \ /---|---\ / \   |
                          | /  / \    |    / \  \ |
                        +-+-----+ +---+---+ +-----+-+
           Tier 2       |       | |       | |       |
                        |  Node | |  Node | |  Node |
                        +-------+ +-------+ +-------+  F3=2
                          \      \ /     \ /     /
                           \     / \     / \    /
                            \   /    / \    \  /
                             +-------+ +-------+
                             |       | |       |
           Tier 3            | Node  | | Node  |
                             +-------+ +-------+  F4=1
                                 \        /
                                  \      /
                                  +-------+
                                  | Dest  |
           Destination            | Node  |
                                  +-------+

            Figure 5. Fan out multiplicative effect with TE.


   To be completed.

5. HSDN Label Stack Assignment Scheme

   HSDN can use any scheme to assign the labels in the label stack.
   However, if a label assignment scheme which assigns labels in a way
   consistent with the HSDN structure, important simplifications can be
   achieved in the control plane and in the LFIBs.

   For non-TE FECs, the HSDN label assignment scheme assigns labels
   according to the "physical" location of the endpoint in the HSDN
   structure. Continuing our design example from above, for an endpoint
   X in UP2-a, PL0 would identify the DC in which the endpoint is



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   located, PL1 would identify the group of racks in which the endpoint
   is located within the DC, and PL2 would identify the endpoint within
   the group of servers within the DC.

   For TE FECs, the HSDN label assignment scheme assigns labels to
   identify a specific path in each UP that is traversed. In our
   example, for a specific TE tunnel to endpoint X, PL0 would identify
   the specific path that should be followed in the DCI, PL1 would
   identify the path that should be followed within the DC to reach the
   group of racks, and PL2 would identify the path to reach the endpoint
   within the group of racks (if there are multiple paths).

   In order to assign labels to both non-TE traffic and TE traffic, HSDN
   uses a label format in which the labels are divided into two logical
   sub-fields, one identifying the destination within the UP, called
   Destination Identifier (DID), and one identifying the path, called
   Path Identifier (PID). The Path Identifier is only relevant for TE
   traffic, and can be zero for non-TE traffic. The HSDN Label format is
   illustrated in Figure 6.
































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           0                          d                       19
           +++++++++++++++++++++++++++++++++++++++++++++++++++++
           |  Destination Identifier  |     Path Identifier    |
           +++++++++++++++++++++++++++++++++++++++++++++++++++++
           |<-------- d bits -------->|<-----(20-d bits)------>|
           LSB                                               MSB

                    Figure 6. HSDN Label format.


   Depending on the LFIB configuration, the two MSBs may be reserved for
   identifying the layer (i.e., whether the label is PL0, PL1, or PL2)
   to resolve ambiguity (not shown in Figure 6).

   By properly designing the UPs, this label assignment scheme can
   support the desired scalability and the support of end-to-end TE
   traffic.

   Note that by using this type of label assignment scheme important
   benefits can be achieved, including:

   -  The LFIBs become rather "static," since the FECs are tied to
      "physical" locations and paths, which change infrequently. This
      simplifies the use of the SDN approach to configure the LFIBs via
      a controller.

   -  All paths in each ECMP group use the same outgoing labels. This
      guarantees that a single LFIB entry can be used for each ECMP
      group.

   The label stack needs to be imposed at the entry points. For an
   endpoint, this implies that the server NIC must be able to push a
   three-label stack of path labels (in addition to possibly one
   additional VL label for the overlay network).

6. HSDN Architecture - Control Plane

   HSDN has been designed to support the controller-centric SDN approach
   in a scalable fashion. HSDN also supports the traditional distributed
   control plane approach.

   HSDN introduces important simplifications in the control plane and in
   the network state as well.

6.1. The SDN approach

   In the controller-centric SDN approach, the SDN controller configures
   the LFIBs in all the network nodes. With HSDN, the hierarchical



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   partitioned structure offers a natural framework for a distributed
   implementation of the SDN controller, since the control plane in each
   UP is largely independent from other UPs.

   For example, a possible architecture uses a SDN controller for each
   UP. Such SDN partition controller is in charge of configuring the
   LFIBs in the network nodes in the corresponding UP.

   The SDN partition controller may also be in charge of TE computation.
   With proper design of the UPs, TE path computation algorithms which
   perform partition-local computation while approach global optimality
   can be used.

   To be completed.

6.2. Distributed control plane

   HSDN can also use the traditional distributed routing protocol
   approach to distribute HSDN labels, for example using BGP [RFC3107].

   To be completed.

7.  Security Considerations

   When the SDN approach is used, the protocols used to configure the
   LFIBs in the network nodes MUST be mutually authenticated.

   For general MPLS/GMPLS security considerations, refer to [RFC5920].

   Given the potentially very large scale and the dynamic nature in the
   cloud/DC environment, the choice of key management mechanisms need to
   be further studied.

   To be completed.

8.  IANA Considerations

   TBD.

9.  References

9.1  Normative References

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

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031, January 2001.



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   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, January 2001.

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


9.2  Informative References

   [RFC5920]  Fang, L., Ed., "Security Framework for MPLS and GMPLS
              Networks", RFC 5920, July 2010.


Authors' Addresses

   Luyuan Fang
   Microsoft
   5600 148th Ave NE
   Redmond, WA 98052
   Email: lufang@microsoft.com

   Vijay Gill
   Microsoft
   5600 148th Ave NE
   Redmond, WA 98052
   Email: vgill@microsoft.com

   Fabio Chiussi
   Seattle, WA
   Email: fabiochiussi@gmail.com




















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