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Versions: (draft-narten-armd-problem-statement) 00 01 02 03 RFC 6820

Internet Engineering Task Force                                T. Narten
Internet-Draft                                                       IBM
Intended status: Informational                                  M. Karir
Expires: September 13, 2012                           Merit Network Inc.
                                                                  I. Foo
                                                     Huawei Technologies
                                                          March 12, 2012


                       Problem Statement for ARMD
                  draft-ietf-armd-problem-statement-02

Abstract

   This document examines address resolution issues related to the
   massive scaling of data centers.  Our initial scope is relatively
   narrow.  Specifically, it focuses on address resolution (ARP and ND)
   within the data center.

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
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 13, 2012.

Copyright Notice

   Copyright (c) 2012 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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   publication of this document.  Please review these documents
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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Generalized Data Center Design . . . . . . . . . . . . . . . .  6
     4.1.  Access Layer . . . . . . . . . . . . . . . . . . . . . . .  7
     4.2.  Aggregation Layer  . . . . . . . . . . . . . . . . . . . .  7
     4.3.  Core . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.4.  Layer 3 / Layer 2 Topological Variations . . . . . . . . .  8
       4.4.1.  Layer 3 to Access Switches . . . . . . . . . . . . . .  8
       4.4.2.  L3 to Aggregation Switches . . . . . . . . . . . . . .  8
       4.4.3.  L3 in the Core only  . . . . . . . . . . . . . . . . .  8
       4.4.4.  Overlays . . . . . . . . . . . . . . . . . . . . . . .  9
     4.5.  Factors that Affect Data Center Design . . . . . . . . . .  9
       4.5.1.  Traffic Patterns . . . . . . . . . . . . . . . . . . .  9
       4.5.2.  Virtualization . . . . . . . . . . . . . . . . . . . . 10
   5.  Address Resolution in IPv4 . . . . . . . . . . . . . . . . . . 10
   6.  Address Resolution in IPv6 . . . . . . . . . . . . . . . . . . 11
   7.  Problem Itemization  . . . . . . . . . . . . . . . . . . . . . 11
     7.1.  ARP Processing on Routers  . . . . . . . . . . . . . . . . 11
     7.2.  IPv6 Neighbor Discovery  . . . . . . . . . . . . . . . . . 13
     7.3.  MAC Address Table Size Limitations in Switches . . . . . . 14
   8.  Summary  . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 14
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   12. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     12.1. Changes from -01 . . . . . . . . . . . . . . . . . . . . . 15
     12.2. Changes from -00 . . . . . . . . . . . . . . . . . . . . . 15
   13. Informative References . . . . . . . . . . . . . . . . . . . . 15
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16















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1.  Introduction

   This document examines issues related to the massive scaling of data
   centers.  Specifically, we focus on address resolution (ARP in IPv4
   and Neighbor Discovery in IPv6) within the data center.  Although
   strictly speaking the scope of address resolution is confined to a
   single L2 broadcast domain (i.e., ARP runs at the L2 layer below IP),
   the issue is complicated by routers having many interfaces on which
   address resolution must be performed or with IEEE 802.1Q domains,
   where individual VLANs form their own broadcast domains.  Thus, the
   scope of address resolution spans both the L2 link and the devices
   attached to those links.

   This document is a product of the ARMD WG and identifies potential
   issues associated with address resolution in datacenters with massive
   number of hosts.  The scope of this document is intentionally
   relatively narrow as it mirrors the ARMD WG charter.  This document
   aims to list "pain points" that are being experienced in current data
   centers.  The goal of this document is to focus on address resolution
   issues and not other broader issues that might arise in datacenters.


2.  Terminology

   Application:  a software process that runs on either a physical or
      virtual machine, providing a service (e.g., web server, database
      server, etc.)

   Broadcast Domain:  The set of all links, repeaters, and switches that
      are traversed in order to reach all nodes that are members of a
      given L2 domain.  For example, when sending a broadcast packet on
      a VLAN, the domain would include all the links and switches that
      the packet traverses when broadcast traffic is sent.

   Host (or server):  A computer system on the network.  This might be a
      standalone physical host, a hypervisor capable of or running
      multiple VMs or a VM host.  A physical host can support an
      application running on an operating system on the "bare metal" or
      multiple applications running within individual VMs on top of a
      hypervisor.  Traditional non-virtualized systems will have a
      single (or small number of) IP addresses assigned to them.  In
      contrast, a virtualized system will use many IP addresses, one for
      the hypervisor plus one (or more) for each individual VM.

   Hypervisor:  Software running on a host that allows multiple VMs to
      run on the same host.





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   L2 domain:   Layer 2 broadcast domain such as for example the
      IEEE802.1Q domain which is capable of supporting up to 4095 VLANs.
      The notion of an L2 broadcast domain is closely tied to individual
      VLANs.  Broadcast traffic (or flooding to reach all destinations)
      reaches every member of the specific VLAN being used.

   Virtual machine (VM):  A software implementation of a physical
      machine that runs programs as if they were executing on a bare
      machine.  Applications do not know they are running on a VM as
      opposed to running on a "bare" host or server.

   ToR:  Top of Rack Switch.  A switch placed in a single rack to
      aggregate network connectivity to and from hosts in that rack.

   EoR:  End of Row Switch.  A switch used to aggregate network network
      connectivity from multiple racks.


3.  Background

   Large, flat L2 networks have long been known to have scaling
   problems.  As the size of an L2 network increases, the level of
   broadcast traffic from protocols like ARP increases.  Large amounts
   of broadcast traffic pose a particular burden because every device
   (switch, host and router) must process and possibly act on such
   traffic.  In extreme cases, "broadcast storms" can occur where the
   quantity of broadcast traffic reaches a level that effectively brings
   down part or all of a network.  For example, poor implementations of
   loop detection and prevention can create conditions that lead to
   broadcast storms as network conditions change.  The conventional
   wisdom for addressing such problems has been to say "don't do that".
   That is, split large L2 networks into multiple smaller L2 networks,
   each operating as its own L3/IP subnet.  Numerous data center
   networks have been designed with this principle, e.g., with each rack
   placed within its own L3 IP subnet.  By doing so, the broadcast
   domain (and address resolution) is confined to one Top of Rack
   switch, which works well from a scaling perspective.  Unfortunately,
   this conflicts in some ways with the current trend towards dynamic
   work load shifting in data centers and increased virtualization as
   discussed below.

   Workload placement has become a challenging task within data centers.
   Ideally, it is desirable to be able to move workloads around within a
   data center in order to optimize server utilization, add additional
   servers in response to increased demand, etc.  However, servers are
   often pre-configured to run with a given set of IP addresses.
   Placement of such servers is then subject to constraints of the IP
   addressing restrictions of the data center.  For example, servers



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   configured with addresses from a particular subnet could only be
   placed where they connect to the IP subnet corresponding to their IP
   addresses.  If each top of rack switch is acting as a gateway for its
   own subnet, a server can only be connected to the one top of rack
   switch.  This gateway switch represents the Layer 2/Layer 3 boundary.
   A similar constraint occurs in virtualized environments, as discussed
   next.

   Server virtualization is fast becoming the norm in data centers.
   With server virtualization, each physical server supports multiple
   virtual servers, each running its own operating system, middleware
   and applications.  Virtualization is a key enabler of workload
   agility, i.e., allowing any server to host any application and
   providing the flexibility of adding, shrinking, or moving services
   among the physical infrastructure.  Server virtualization provides
   numerous benefits, including higher utilization, increased data
   security, reduced user downtime, and even significant power
   conservation, along with the promise of a more flexible and dynamic
   computing environment.

   The discussion below focuses on VM placement and migration.  Keep in
   mind, however, that even in a non-virtualized environment, many of
   the same issues apply to individual workloads running on standalone
   machines.  For example, when increasing the number of servers running
   a particular workload to meet demand, placement of those workloads
   may be constrained by IP subnet numbering considerations.

   The greatest flexibility in VM and workload management occurs when it
   is possible to place a VM (or workload) anywhere in the data center
   regardless of what IP addresses the VM uses and how the physical
   network is laid out.  In practice, movement of VMs within a data
   center is easiest when VM placement and movement does not conflict
   with the IP subnet boundaries of the data center's network, so that
   the VM's IP address need not be changed to reflect its actual point
   of attachment on the network from an L3/IP perspective.  In contrast,
   if a VM moves to a new IP subnet, its address must change, and
   clients will need to be made aware of that change.  From a VM
   management perspective, management is simplified if all servers are
   on a single large L2 network.

   With virtualization, a single physical server can host 10 (or more)
   VMs, each having its own IP (and MAC) addresses.  Consequently, the
   number of addresses per machine (and hence per subnet) is increasing,
   even when the number of physical machines stays constant.  Today, it
   is not uncommon to support 10s of VMs per physical server.  In a few
   years, the numbers will likely be even higher.

   In the past, services were static in the sense that they tended to



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   stay in one physical place.  A service installed on a machine would
   stay on that machine because the cost of moving a service elsewhere
   was generally high.  Moreover, services would tend to be placed in
   such a way as to facilitate communication locality.  That is, servers
   would be physically located near the services they accessed most
   heavily.  The network traffic patterns in such environments could
   thus be optimized, in some cases keeping significant traffic local to
   one network segment.  In these more static and carefully managed
   environments, it was possible to build networks that approached
   scaling limitations, but did not actually cross the threshold.

   Today, with the proliferation of VMs, traffic patterns are becoming
   more diverse and less predictable.  In particular, there can easily
   be less locality of network traffic as services are moved for such
   reasons as reducing overall power usage (by consolidating VMs and
   powering off idle machine) or to move a virtual service to a physical
   server with more capacity or a lower load.  In today's changing
   environments, it is becoming more difficult to engineer networks as
   traffic patterns continually shift as VMs move around.

   In summary, both the size and density of L2 networks is increasing.
   In addition, increasingly dynamic workloads and the increased usage
   of VMs is creating pressure for ever larger L2 networks.  Today,
   there are already data centers with over 100,000 physical machines
   and many times that number of VMs.  These number will only increase
   going forward.  In addition, traffic patterns within a data center
   are also constantly changing.  Ultimately, the issues described in
   this document might be observed at any scale depending on the
   particular design of the datacenter.  In the next section we describe
   a generalized design which can allow us to more easily describe the
   L2 scaling issues.


4.  Generalized Data Center Design

   There are many different ways in which data centers might be
   designed.  The designs are usually engineered to suit the particular
   application that is being deployed in the data center.  For example,
   a massive web server farm might be engineered in a very different way
   than a general-purpose multi-tenant cloud hosting service.  However
   in most cases the designs can be abstracted into a typical three-
   layer model consisting of an Access Layer, an Aggregation Layer and
   the Core.  The access layer generally refers to the Layer 2 switches
   that are closest to the physical or virtual severs, the aggregation
   layer serves to interconnect multiple access layer devices.  The Core
   switches connect the aggregation switches to the larger network core.
   Figure 1 shows a generalized Data Center design, which captures the
   essential elements of various alternatives.



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                  +-----+-----+     +-----+-----+
                  |   Core0   |     |    Core1  |      Core
                  +-----+-----+     +-----+-----+
                        /    \        /       /
                       /      \----------\   /
                      /    /---------/    \ /
                    +-------+           +------+
                  +/------+ |         +/-----+ |
                  | Aggr11| + --------|AggrN1| +      Aggregation Layer
                  +---+---+/          +------+/
                    /     \            /      \
                   /       \          /        \
                 +---+    +---+      +---+     +---+
                 |T11|... |T1x|      |TN1|     |TNy|  Access Layer
                 +---+    +---+      +---+     +---+
                 |   |    |   |      |   |     |   |
                 +---+    +---+      +---+     +---+
                 |   |... |   |      |   |     |   |
                 +---+    +---+      +---+     +---+  Server racks
                 |   |... |   |      |   |     |   |
                 +---+    +---+      +---+     +---+
                 |   |... |   |      |   |     |   |
                 +---+    +---+      +---+     +---+

   Figure 1: Typical Layered Architecture in DC

                                 Figure 1

4.1.  Access Layer

   The Access switches provide connectivity directly to/from physical
   and virtual servers.  The access switches might be placed either on
   top-of-rack (ToR) or at end-of-row (EoR) physical configuration.  A
   server rack may have a single uplink to one access switch, or may
   have dual uplinks to two different access switches.

4.2.  Aggregation Layer

   In a typical data center, aggregation switches interconnect many ToR
   switches.  Usually there are multiple parallel aggregation switches,
   serving the same group of ToRs to achieve load sharing.  It is no
   longer uncommon to see aggregation switches interconnecting hundreds
   of ToR switches in large data centers.

4.3.  Core

   Core switches connect multiple aggregation switches and act as the
   data center gateway to external networks or interconnect to different



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   sets of racks within one data center.

4.4.  Layer 3 / Layer 2 Topological Variations

4.4.1.  Layer 3 to Access Switches

   In this scenario the L3 domain is extended all the way to the Access
   Switches.  Each rack enclosure consists of a single Layer 2 domain,
   which is confined to the rack.  In general, there are no significant
   ARP/ND scaling issues in this scenario as the Layer 2 domain cannot
   grow very large.  This topology is ideal for scenarios where servers
   attached to a particular access switch generally run applications
   that are are confined to using a single subnet.  These applications
   aren't moved (migrated) to other racks which might be attached to
   different access switches (and different IP subnets).  A small server
   farm or very static compute cluster might be best served via this
   design.

4.4.2.  L3 to Aggregation Switches

   When the Layer 3 domain only extends to aggregation switches, hosts
   in any of the IP subnets configured on the aggregation switches can
   be reachable via Layer 2 through any access switches if access
   switches enable all the VLANs.  This topology allows for a great deal
   of flexibility as servers attached to one access switch can be re-
   loaded with applications with different IP prefix and VMs can now
   migrate between racks without IP address changes.  The drawback of
   this design however is that multiple VLANs have to be enabled on all
   access switches and all ports of aggregation switches.  Even though
   layer 2 traffic are still partitioned by VLANs, the fact that all
   VLANs are enabled on all ports can lead to broadcast traffic on all
   VLANs to traverse all links and ports, which is same effect as one
   big Layer 2 domain.  In addition, internal traffic itself might have
   to cross different Layer 2 boundaries resulting in significant ARP/ND
   load at the aggregation switches.  This design provides the best
   flexibility/Layer 2 domain size trade-off.  A moderate sized data
   center might utilize this approach to provide high availability
   services at a single location.

4.4.3.  L3 in the Core only

   In some cases where a wider range of VM mobility is desired (i.e.
   greater number of racks among which VMs can move without IP address
   change), the Layer 3 routed domain might be terminated at the core
   routers themselves.  In this case VLANs can span across multiple
   groups of aggregation switches, which allow hosts to be moved among
   more number of server racks without IP address change.  This scenario
   results in the largest ARP/ND performance impact as explained later.



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   A data center with very rapid workload shifting may consider this
   kind of design.

4.4.4.  Overlays

   There are several approaches regarding how overlay networks can make
   very large layer 2 network scale and enable mobility.  Overlay
   networks using various Layer 2 or Layer 3 mechanisms allow interior
   switches/routers to mask host addresses.  This can help the data
   center designer to control the size of the L2 domain.  However, the
   Overlay Edge switches/routers which perform the network address
   encapsulation/decapsulation must ultimately perform a L2 address
   resolution and could still potentially face scaling issues at that
   point.

   A potential problem that arises in a large data center is when a
   large number of hosts communicate with their peers in different
   subnets, all these hosts send (and receive) data packets to their
   respective L2/L3 boundary nodes as the traffic flows are generally
   bi-directional.  This has the potential to further highlight any
   scaling problems.  These L2/L3 boundary nodes have to process ARP/ND
   requests sent from originating subnets and resolve physical addresses
   (MAC) in the target subnets for what are generally bi-directional
   flows.  Therefore, For maximum flexibility in managing the data
   center workload, it is often desirable to use overlays to place
   related groups of hosts in the same topological subnet to avoid the
   L2/L3 boundary translation.  The use of overlays in the data center
   network can be a useful design mechanism to help manage a potential
   bottleneck at the Layer 2 / Layer 3 boundary by redefining where that
   boundary exists.

4.5.  Factors that Affect Data Center Design

4.5.1.  Traffic Patterns

   Expected traffic patterns play an important role in designing the
   appropriately sized Access, Aggregation and Core networks.  Traffic
   patterns also vary based on the expected use of the Data Center.
   Broadly speaking it is desirable to keep as much traffic as possible
   on the Access Layer in order to minimize the bandwidth usage at the
   Aggregation Layer.  If the expected use of the data center is to
   serve as a large web server farm, where thousands of nodes are doing
   similar things and the traffic pattern is largely in and out a large
   data center, an access layer with EoR switches might be used as it
   minimizes complexity, allows for servers and databases to be located
   in the same Layer 2 domain and provides for maximum density.

   A Data Center that is expected to host a multi-tenant cloud hosting



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   service might have some completely unique requirements.  In order to
   isolate inter-customer traffic smaller Layer 2 domains might be
   preferred and though the size of the overall Data Center might be
   comparable to the previous example, the multi-tenant nature of the
   cloud hosting application requires a smaller more compartmentalized
   Access layer.  A multi-tenant environment might also require the use
   of Layer 3 all the way to the Access Layer ToR switch.

   Yet another example of an application with a unique traffic pattern
   is a high performance compute cluster where most of the traffic is
   expected to stay within the cluster but at the same time there is a
   high degree of crosstalk between the nodes.  This would once again
   call for a large Access Layer in order to minimize the requirements
   at the Aggregation Layer.

4.5.2.  Virtualization

   Using virtualization in the Data Center further serves to increase
   the possible densities that can be achieved.  Virtualization also
   further complicates the requirements on the Access Layer as that
   determines the scope of server migrations or failover of servers on
   physical hardware failures.

   Virtualization also can place additional requirements on the
   Aggregation switches in terms of address resolution table size and
   the scalability of any address learning protocols that might be used
   on those switches.  The use of virtualization often also requires the
   use of additional VLANs for High Availability beaconing which would
   need to span across the entire virtualized infrastructure.  This
   would require the Access Layer to span as wide as the virtualized
   infrastructure.


5.  Address Resolution in IPv4

   In IPv4 over Ethernet, ARP provides the function of address
   resolution.  To determine the link-layer address of a given IP
   address, a node broadcasts an ARP Request.  The request is delivered
   to all portions of the L2 network, and the node with the requested IP
   address replies with an ARP response.  ARP is an old protocol, and by
   current standards, is sparsely documented.  For example, there are no
   clear requirement for retransmitting ARP requests in the absence of
   replies.  Consequently, implementations vary in the details of what
   they actually implement [RFC0826][RFC1122].

   From a scaling perspective, there are a number of problems with ARP.
   First, it uses broadcast, and any network with a large number of
   attached hosts will see a correspondingly large amount of broadcast



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   ARP traffic.  The second problem is that it is not feasible to change
   host implementations of ARP - current implementations are too widely
   entrenched, and any changes to host implementations of ARP would take
   years to become sufficiently deployed to matter.  That said, it may
   be possible to change ARP implementations in hypervisors, L2/L3
   boundary routers, and/or ToR access switches, to leverage such
   techniques as Proxy ARP [RFC1027] and/or OpenFlow [OpenFlow] infused
   directory assistance approaches.  Finally, ARP implementations need
   to take steps to flush out stale or otherwise invalid entries.
   Unfortunately, existing standards do not provide clear implementation
   guidelines for how to do this.  Consequently, implementations vary
   significantly, and some implementations are "chatty" in that they
   just periodically flush caches every few minutes and rerun ARP.


6.  Address Resolution in IPv6

   Broadly speaking, from the perspective of address resolution, IPv6's
   Neighbor Discovery (ND) behaves much like ARP, with a few notable
   differences.  First, ARP uses broadcast, whereas ND uses multicast.
   Specifically, when querying for a target IP address, ND maps the
   target address into an IPv6 Solicited Node multicast address.  From
   an L2 perspective, sending to a multicast vs. broadcast address may
   result in the packet being delivered to all nodes, but most (if not
   all) nodes will filter out the (unwanted) query via filters installed
   in the NIC -- hosts will never see such packets.  Thus, whereas all
   nodes must process every ARP query, ND queries are processed only by
   the nodes to which they are intended.


7.  Problem Itemization

   This section articulates some specific problems or "pain points" that
   are related to large data centers.  It is a future activity to
   determine which of these areas can or will be addressed by ARMD or
   some other IETF WG.

7.1.  ARP Processing on Routers

   One pain point with large L2 broadcast domains is that the routers
   connected to the L2 domain need to process "a lot of" ARP traffic.
   Even though the vast majority of ARP traffic may well not be aimed at
   that router, the router still has to process enough of the ARP
   request to determine whether it can safely be ignored.  The ARP
   algorithm specifies that a recipient must update its ARP cache if it
   receives an ARP query from a source for which it has an entry
   [RFC0826].




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   One common router implementation architecture has ARP processing
   handled in a "slow path" software processor rather than directly by a
   hardware ASIC as is the case when forwarding packets.  Such a design
   significantly limits the rate at which ARP traffic can be processed.
   Current implementations today can support in the low thousands of ARP
   packets per second, which is several orders of magnitude lower than
   the rate at which packets can be forwarded by ASICs.

   To further reduce the ARP load, some routers have implemented
   additional optimizations in their ASIC fast paths.  For example, some
   routers can be configured to discard ARP requests for target
   addresses other than those assigned to the router.  That way, the
   router's software processor only receives ARP requests for addresses
   it owns and must respond to.  This can significantly reduce the
   number of ARP requests that must be processed by the router.

   Another optimization concerns reducing the number of ARP queries
   targeted at routers, whether for address resolution or to validate
   existing cache entries.  Some routers can be configured to send out
   periodic gratuitous ARPs.  Upon receipt of a gratuitous ARP,
   implementations mark the associated entry as "fresh", resetting the
   revalidate timer to its maximum setting.  Consequently, sending out
   periodic gratuitous ARPs can effectively prevent nodes from needing
   to send ARP requests intended to revalidate stale entries for a
   router.  The net result is an overall reduction in the number of ARP
   queries routers receive.  Gratuitous ARPs can also pre-populate ARP
   caches on neighboring devices, further reducing ARP traffic.

   Finally, another area concerns how routers process IP packets for
   which no ARP entry exists.  Such packets must be held in a queue
   while address resolution is performed.  Once an ARP query has been
   resolved, the packet is forwarded on.  Again, the processing of such
   packets is handled in the "slow path".  This effectively limits the
   rate at which a router can process ARP "cache misses" and is viewed
   as a problem in some deployments today.  Additionally, If no response
   is received, the router has to send the ARP/ND query multiple times.
   If no response is received after a number of ARP/ND requests, the
   router needs to drop all those data packets.  This process can be CPU
   intensive.

   Although address-resolution traffic remains local to one L2 network,
   some data center designs terminate L2 subnets at individual
   aggregation switches/routers (e.g., see Section 4.4.2).  Such routers
   can be connected to a large number of interfaces (e.g., 100 or more).
   While the address resolution traffic on any one interface may be
   manageable, the aggregate address resolution traffic across all
   interfaces can become problematic.




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   Another variant of the above issue has individual routers servicing a
   relatively small number of interfaces, with the individual interfaces
   themselves serving very large subnets.  Once again, it is the
   aggregate quantity of ARP traffic seen across all of the router's
   interfaces that can be problematic.  This "pain point" is essentially
   the same as the one discussed above, the only difference being
   whether a given number of hosts are spread across a few large IP
   subnets or many smaller ones.

   When hosts in two different subnets under the same L2/L3 boundary
   router need to communicate with each other, the L2/L3 router not only
   has to initiate ARP/ND requests to the target's Subnet, it also has
   to process the ARP/ND requests from the originating subnet.  This
   process further adds to the overall ARP processing load.

7.2.  IPv6 Neighbor Discovery

   Though IPv6's Neighbor Discovery behaves much like ARP there are
   several notable differences which result in a different set of
   potential issues.  From a L2 perspective there is the simple
   difference between sending to a multicast versus broadcast address
   which results in ND queries only being processed by the nodes to
   which they are intended.

   Another key difference concerns revalidating stale ND entries.  ND
   requires that nodes periodically re-validate any entries they are
   using, to ensure that bad entries are timed out quickly enough that
   TCP does not terminate a connection.  Consequently, some
   implementations will send out "probe" ND queries to validate in-use
   ND entries as frequently as every 35 seconds [RFC4861].  Such probes
   are sent via unicast (unlike in the case of ARP).  However, on larger
   networks, such probes can result in routers receiving many such
   queries.  Unfortunately, the IPv4 mitigation technique of sending
   gratuitous ARPs does not work in IPv6.  The ND specification
   specifically specifies that gratuitous ND "updates" cannot cause an
   ND entry to be marked "valid".  Rather, such entries are marked
   "probe", which causes the receiving node to (eventually) generate a
   probe back to the sender, which in this case is precisely the
   behavior that the router is trying to prevent!

   It should be noted that ND does not require the sending of probes in
   all cases.  Section 7.3.1 of [RFC4861] describes a technique whereby
   hints from TCP can be used to verify that an existing ND entry is
   working fine and does not need to be revalidated.







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7.3.  MAC Address Table Size Limitations in Switches

   L2 switches maintain L2 MAC address forwarding tables for all sources
   and destinations traversing through the switch.  These tables are
   populated through learning and are used to forward L2 frames to their
   correct destination.  The larger the L2 domain, the larger the tables
   have to be.  While in theory a switch only needs to keep track of
   addresses it is actively using, switches flood broadcast frames
   (e.g., from ARP), multicast frames (e.g., from Neighbor Discovery)
   and unicast frames to unknown destinations.  Switches add entries for
   the source addresses of such flooded frames to their forwarding
   tables.  Consequently, MAC address table size can become a problem as
   the size of the L2 domain increases.  The table size problem is made
   worse with VMs, where a single physical machine now hosts ten (or
   more) VMs, since each has its own MAC address that is visible to
   switches.

   When layer 3 extends all the way to access switches (see Section
   4.4.1), the size of MAC address tables in switches is not generally a
   problem.  When layer 3 extends only to aggregation switches (see
   Section 4.4.2), however, MAC table size limitations can be a real
   issue.


8.  Summary

   This document has outlined a number of problems or issues related to
   address resolution in large data centers.  In particular we have
   described different scenarios where such issues might arise, what
   these potential issues are, and what the various fundamental factors
   are that cause them.  It is hoped that describing specific pain
   points will facilitate a discussion as to whether and how to best
   address them.


9.  Acknowledgments

   This document has been significantly improved by comments from Benson
   Schliesser, Linda Dunbar and Sue Hares.  Igor Gashinsky deserves
   additional credit for highlighting some of the ARP-related pain
   points and for clarifying the difference between what the standards
   require and what some router vendors have actually implemented in
   response to operator requests.


10.  IANA Considerations

   This document makes not request of IANA.



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11.  Security Considerations

   This documents lists existing problems or pain points with address
   resolution in data centers.  This document does not create any
   security implications nor does it have any security implications.
   The security vulnerabilities in ARP are well known and this document
   does not change or mitigate them in any way.


12.  Change Log

12.1.  Changes from -01

   1.  Wordsmithing and editorial improvements.

12.2.  Changes from -00

   1.  Merged draft-karir-armd-datacenter-reference-arch-00.txt into
       this document.

   2.  Added section explaining how ND differs from ARP and the
       implication on address resolution "pain".


13.  Informative References

   [DATA1]    Cisco, Systems., "Data Center Design - IP Infrastructure",
              October 2009.

   [DATA2]    Juniper, Networks., "Government Data Center Network
              Reference Architecture", 2010.

   [OpenFlow]
              McKeown, N., Anderson, T., Balakrishnan, H., Parulkar, G.,
              Peterson, L., Rexford, J., Shenker, S., and J. Turner,
              "OpenFlow: Enabling Innovation in Campus Networks",
              March 2008.

   [RFC0826]  Plummer, D., "Ethernet Address Resolution Protocol: Or
              converting network protocol addresses to 48.bit Ethernet
              address for transmission on Ethernet hardware", STD 37,
              RFC 826, November 1982.

   [RFC1027]  Carl-Mitchell, S. and J. Quarterman, "Using ARP to
              implement transparent subnet gateways", RFC 1027,
              October 1987.

   [RFC1122]  Braden, R., "Requirements for Internet Hosts -



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              Communication Layers", STD 3, RFC 1122, October 1989.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

   [STUDY]    Rees, J. and M. Karir, "ARP Traffic Study", NANOG 52, URL
              http://www.nanog.org/meetings/nanog52/presentations/Tuesda
              y/Karir-4-ARP-Study-Merit Network.pdf, June 2011.


Authors' Addresses

   Thomas Narten
   IBM

   Email: narten@us.ibm.com


   Manish Karir
   Merit Network Inc.

   Email: mkarir@merit.edu


   Ian Foo
   Huawei Technologies

   Email: Ian.Foo@huawei.com






















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