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ARMD BOF                                                      L. Dunbar
Internet Draft                                                 S. Hares
Intended status: Informational                                   Huawei
Expires: September 2011                                    M. Sridharan
                                                      N. Venkataramaiah
                                                          B. Schliesser
                                                          Cisco Systems
                                                         March 14, 2011

        Address Resolution for Large Data Center Problem Statement

Status of this Memo

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   document must include Simplified BSD License text as described in
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   Modern data center networks face a number of scale challenges.  One
   such challenge for so-called "massive" data center networks is
   address resolution, such as is provided by ARP and/or ND.  This
   document describes the problem of address resolution in massive data
   centers.  It discusses the network impact of various data center
   technologies including server virtualization, illustrates reasons
   why it is still desirable to have multiple hosts on the same Layer 2
   data center network, and describes potential address resolution
   problems this type of Layer 2 network will face.

Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC-2119.

Table of Contents

   1. Introduction...................................................3
   2. Terminology....................................................4
   3. Layer 2 Requirements in the Data Center........................5
      3.1. Layer 2 Requirement for VM Migration......................5
      3.2. Layer 2 Requirement for Load Balancing....................5
      3.3. Layer 2 Requirement for Active/Standby VMs................6
   4. Cloud and Internet Data Centers with Virtualized Servers.......6
   5. ARP Issues in the Data Center..................................7
   6. ARPs & VM Migration............................................9
   7. Limitations of VLANs/Smaller Subnets in the Cloud Data Center.10
   8. Why IETF Needs To Develop Solutions Instead of IEEE 802.......10
   9. Conclusion and Recommendation.................................10
   10. Manageability Considerations.................................11
   11. Security Considerations......................................11
   12. IANA Considerations..........................................11
   13. Acknowledgments..............................................11
   14. References...................................................11
   Authors' Addresses...............................................12
   Intellectual Property Statement..................................12
   Disclaimer of Validity...........................................13

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1. Introduction

   Modern data center networks face a number of scale challenges,
   especially as they reach sizes and densities that are "massive"
   relative to historical norms.  One such challenge is the effective
   and efficient performance of address resolution, such as is provided
   by ARP and/or ND.

   The fundamental issue challenging address resolution in massive data
   centers is the need to grow both the number and density of logical
   Layer 2 segments while retaining flexibility in the physical
   location of host attachment.  This problem has historically been
   bounded by physical limits on data center size, as well as practical
   considerations in the physical placement of server resources.
   However, the increasing popularity of server virtualization
   technology (e.g. in support of "cloud" computing), the trend toward
   building physically massive data center facilities, and the logical
   extension of network segments across traditional geographic
   boundaries is driving an increase of the number of addresses in the
   modern data center network.

1.1. Server Virtualization

   Server virtualization allows the sharing of the underlying physical
   machine (server) resources among multiple virtual machines, each
   running its own operating system.  Server virtualization is the key
   enabler to data center workload agility, i.e. allowing any server to
   host any applications 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.  However, server
   virtualization also stresses the data center network by enabling the
   creation of many more network hosts (accompanied by their network
   interfaces and addresses) within the same physical footprint.

   Further, in order to maximize the benefits of server virtualization,
   VM placement algorithms (e.g. based on efficiency, capacity,
   redundancy, security, etc) may be designed in such a way that
   increases both the range and density of Layer 2 segments.  For
   instance, these algorithms may satisfy the processing requirements
   of each VM while requiring the minimal number of physical servers
   and switching devices, simultaneously spreading the VM hosts across
   a diverse and redundant infrastructure.  Such an algorithm may
   potentially result in a large number of diverse Layer 2 segments

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   attached to each physical host, as well as a larger number and range
   of data center-wide Layer 2 segments.  With this, and similar types
   of VM assignment algorithm, subnets tend to extend throughout the
   network and ARP/ND traffic associated with each subnet is likely to
   traverse a significant number of links and switches in the network.

1.2. Physically Massive Facilities

   Regardless of server virtualization technology, in recent years the
   physical facility of a data center has been seen to grow larger.
   There are inherent efficiencies in constructing larger data center
   buildings, infrastructure, and networks.  As data center operators
   pursue these physical efficiencies, the address resolution problem
   described by this document becomes more prevalent.  Physically
   massive data centers may face address resolution scale challenges
   simply due to their physical capacity.  Combined with server
   virtualization, the host and address density of these facilities is
   historically unmatched.

1.3. Geographically Extended Network Segments

   The modern data center network is influenced by the demands of
   flexibility due to cloud computing, demands of redundancy due to
   regulatory or enterprise uptime requirements, as well as demands on
   topology due to security and/or performance.  In support of these
   demands and others, VPN and physical network extensions (including
   both Layer 3 and Layer 2 extensions) increase the data center
   network scope beyond physical and/or geographical boundaries.

   As such, the number of addresses that are present on a single Layer
   2 segment may be greater than the number of hosts physically or
   logically present within the data center itself.  Combined with
   physically massive data center facilities and server virtualization,
   this trend results in a potential for massive numbers of addresses
   per Layer 2 segment, beyond any historical norm, truly challenging
   address resolution protocols such as ARP and/or ND.

2. Terminology

   Aggregation Switch: A Layer 2 switch interconnecting ToR switches

   Bridge:  IEEE802.1Q compliant device. In this draft, Bridge is used
            interchangeably with Layer 2 switch.

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   CUG:     Closed User Group

   DC:      Data Center

   DA:      Destination Address

   EOR:     End of Row switches in data center.

   FDB:     Filtering Database for Bridge or Layer 2 switch

   SA:      Source Address

   ToR:     Top of Rack Switch. It is also known as access switch.

   VM:      Virtual Machines

   VPN:     Virtual Private Network

3. Layer 2 Requirements in the Data Center

3.1. Layer 2 Requirement for VM Migration

   VM migration refers to moving virtual machines from one physical
   server to another.  Current technology even allows for the real-time
   migration of VM hosts in a "live" state.  Seamlessly moving VMs
   within a resource pool is the key to achieve efficient server
   utilization and data center agility.

   One of the key requirements for VM migration is the VM maintaining
   the same IP address and MAC address after moving to the new
   location, so that its operation can be continued in the new
   location.  This requirement is even more stringent in the case of
   "live" migrations, for which ongoing stateful connections must be
   maintained.  Thus, in absence of new technology, VMs can only be
   migrated among servers on the same Layer 2 network.

3.2. Layer 2 Requirement for Network Services

   Many network services such as firewalls and load balancers must be
   in-line with network traffic in order to function correctly.  As
   such, Layer 2 networks often provide a form of traffic engineering
   for steering traffic through these devices for a given subnet or

   Further, even in some cases where the network service need not be
   in-line for all traffic, it must be connected on a common Layer 2
   segment in order to function.  One such common application is load

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   balancing (providing a single Internet service from multiple
   servers) with Layer 2 Direct Server Return. While a traditional load
   balancer typically sits in-line between the client and the hosts
   that provide the services to the client, for applications with
   relative smaller amount of traffic going into servers and relative
   large amount of traffic from servers, it is sometimes desirable to
   allow reply data from servers go directly to clients without going
   through the Load Balancer. In this kind of design it is necessary
   for Load Balancer and the cluster of hosts to be on same Layer 2
   network so that they communicate with each other via their MAC

3.3. Layer 2 Requirement for Active/Standby VMs

   For redundant servers (or VMs) serving redundant instances of the
   same applications, both Active and Standby servers (VMs) need to
   share keep-alive messages between them.  Further, the mechanism for
   failing over from Active to Standby may be facilitated by assumption
   of a shared MAC address and/or some kind of ARP/ND announcement.
   When the Active server fails/is taken out of service, the switch
   over to the Standby would be transparent if they are on the same
   Layer 2 network.

4. Cloud and Internet Data Centers with Virtualized Servers

   Cloud Computing service often allows subscribers to create their own
   virtual hosts and virtual subnets which are housed within the cloud
   providers' data centers.  Network service providers may also extend
   existing VPNs to connect with VMs that are hosted by servers in the
   provider's data center(s).  This is often realized by grouping hosts
   belonging to one subscriber's VPN into distinct segregated subnets
   in the data center(s).  This design for a multi-tenant data center
   network typically requires the secure segregation of different
   customers' VMs and hosts.

   Further, these client subnets in the data center could have client-
   specific IP addresses, which could lead to possible overlaps in
   address spaces.  In this scenario, it is very critical to segregate
   traffic among different client subnets (or VPNs) in data center.  As
   a result, within a cloud data center there may be a larger number of
   distinct Layer 2 segments as well as a larger demand for host
   density within each Layer 2 segment.

  Cloud/Internet Data Centers have the following special properties:

  . Massive number of hosts

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     Consider a typical tree structured Layer 2 network, with one or
     two aggregation switches connected to a group of Top of Rack (ToR)
     switches and each ToR switch connected to a group of physical
     servers. The number of servers connected in this network is
     limited to the port count of the ToR switches. For example, if a
     ToR switch has 20 downstream ports, there are only 20 servers or
     hosts connected to it. If the aggregation switch has 256 ports
     connecting to ToR switches, there could be up to 20*256=5120 hosts
     connected to one aggregation switch when the servers are not

     When servers are virtualized, one server can support tens or
     hundreds of VMs. Hypothetically, if one server supports up to 100
     VMs, the same ToR switches and Aggregation switch as above would
     need to support up to 512000 hosts. Even if there is enough
     bandwidth on the links to support the traffic volume from all
     those VMs, other issues associated with Layer 2, like frequent ARP
     broadcast by hosts and flooding due to unknown DA, create
     challenges to the network.

  . Massive number of client subnets or Closed User Groups co-existing
     in the data center, with each subnet having their own IP addresses

       In the example of VPN (L2VPN or L3VPN)extended with virtual
       machines residing in Service Provider data centers, each VPN
       would require an unique subnet for its associated VMs in the
       data center. Due to large number of VPNs being deployed today,
       those types of services can require a large number of subnets to
       be supported by the data center.

  . Hosts (VMs) migrate from one location to another

       When data center is virtualized, physical resource and logical
       hosts/contents are separated. One application could be loaded to
       any Virtual Machines on any servers, and could be migrated to
       different locations during the continuous process of minimizing
       the physical resources consumed in data center(s).

       As discussed earlier, this migration requires the VMs to
       maintain the same IP and MAC addresses. The association to their
       corresponding subnet (or VPN) should not be changed either.

5. ARP/ND Issues in the Data Center

   Traditional Layer 2 networks placed hosts belonging to one subnet
   (or VLAN) closely together, so that broadcast messages among hosts
   in the subnet are confined to the access switches. However this kind

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   of network design puts a lot of constraints on where VMs can be
   placed and can lead to very unbalanced utilization of data center

   In data center with virtualized servers, data center administrators
   may want to leverage the flexibility of server virtualization to
   place VMs in such a way that satisfies the processing requirements
   of each VM but require the minimal number of physical servers and
   switching devices. When those types of VM placement algorithms are
   used, hosts can be attached and re-attached at any location on the
   network. IPv4 hosts use ARP (Address Resolution Protocol-RFC826) to
   find the corresponding MAC address of a target host. IPv4 ARP is a
   protocol that uses the Ethernet broadcast service for discovering a
   host's MAC address from its IP address. For host A to find the MAC
   address of a host B on the same subnet with IP Address B-IP, host A
   broadcasts an ARP query packet containing B  as well as its own IP
   address (A ) on its Ethernet interface. All hosts in the same subnet
   receive the packet. Host B, whose IP address is B , replies (via
   unicast) to inform A of its MAC address. A will also record the
   mapping between B  and B-MAC.

   Even though all hosts maintain the MAC to target IP address mapping
   locally to avoid repetitive ARP broadcast message for the same
   target IP address, hosts age out their learnt MAC to IP mapping very
   frequently. For Microsoft Windows (Versions XP and Server 2003), the
   default ARP cache policy is to discard entries that have not been
   used in at least two minutes, and for cache entries that are in use,
   to retransmit an ARP request every 10 minutes. So hosts send out ARP
   very frequently.

   In addition to broadcast messages sent from hosts, Layer 2 switches
   also flood received data frames if the destination MAC address is

   The flooding and broadcast have worked well in the past when hosts
   belonging to one subnet (or VLAN) are placed closely together. A
   common scenario is for Layer 2 networks to limit the number of hosts
   in one subnet to be less then 200, so that broadcast storms and
   flooding can be restricted to a smaller domain when all the hosts
   are confined to small number of ports on access switches. When
   subnets are spanning across multiple ToR switches or EoR switches,
   each subnet's broadcast messages and flooding will be exposed to the
   backbone links and switches of entire Data Center network. Then, the
   network will experience the similar problems as one big flat Layer 2
   network. With large number of hosts in data centers with virtualized
   servers, the amount of broadcast messages and flooding over the
   backbone links can take away huge amount of bandwidth.

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   As indicated in Reference [Scaling Ethernet], Carnegie Mellon did a
   study on the number of ARP queries received at a workstation on
   CMU's School of Computer Science LAN over a 12 hour period on August
   9, 2004. At peak, the host received 1150 ARPs per second, and on
   average, the host received 89 ARPs per second. During the data
   collection, 2,456 hosts were observed sending ARP queries. The
   report expects that the amount of ARP traffic will scale linearly
   with the number of hosts on the LAN. For 1 million hosts, it is
   expected to have 468,240 ARPs per second or 239 Mbps of ARP traffic
   at peak, which is more than enough to overwhelm a standard 100 Mbps
   LAN connection. Ignoring the link capacity, forcing servers to
   handle an extra half million packets per second to inspect each ARP
   packet would impose a prohibitive computational burden.

6. ARPs & VM Migration

  In general, there are more flooding and more ARP messages when VMs
  migrate.VM migration in Layer 2 environments will require updating
  the Layer 2 (MAC) FDB in the individual switches in the data center
  to ensure accurate forwarding. Consider a case where a VM migrates
  across racks.  The migrated VM often sends out a gratuitous ARP
  broadcast when it comes up at the new location. This is flooded by
  the TOR switch at the new rack to the entire network. The TOR at the
  old rack is not aware of the migration until it receives this
  gratuitous ARP. So it continues to forward frames to the port where
  it learnt the VM's MAC address from before, leading to black holing
  of traffic.  The duration of this black holing period may depend upon
  the topology. It may be longer if the VM has moved to a rack in a
  different data center connected to this data center over Layer 2.

  During transition periods, some hosts might be temporarily taken out
  of service. Then, there will be lots of ARP request broadcast
  messages repetitively transmitted from hosts to those temporarily out
  of service hosts. Since there is no response from those target hosts,
  switches do not learn their path, which will cause ARP messages from
  various hosts being flooded across the network.

  Simple VLAN partitioning is no longer enough to segregate traffic
  among tens of thousands of subnets (or Closed User Groups) within a
  data center. Some types of encapsulation have to be used, like MAC-
  in-MAC or TRILL, to further isolate the traffic belonging to
  different subnets. When encapsulation is performed by TOR, VMs
  migration can cause more broadcast messages and more data frames
  being flooded in the network due to new TOR not knowing the
  destination address of the outer header of the encapsulation.

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  Therefore, it is very critical to have some types of ARP optimization
  or extended ARP reply for TOR switches which perform the
  encapsulation. This can involve knowledge of the target TOR address,
  so that the amount of flooding among TOR switches due to unknown
  destination can be dramatically reduced.

7. Limitations of VLANs/Smaller Subnets in the Cloud Data Center

   Large data centers might need to support more subnets or VLANs than
   4095. So, simple VLAN partitioning is no longer enough to segregate
   traffic among all those subnets. To enforce traffic segregation
   among all those subnets, some types of encapsulation have to be

   As the result of continuous VM migration, hosts in one subnet (VLAN)
   may start with being close together and gradually being relocated to
   various places.

   When one physical server is supporting more than 100 Virtual
   Machines, i.e. >100 hosts, it may start with serving hosts belonging
   to smaller number of VLANs. But gradually, as VM migration proceeds,
   hosts belonging to different VLANs may end up being loaded to VMs on
   this server. Consider a case when there are 50 subnets (VLANs)
   enabled on the switch port to the server, the server has to handle
   all the ARP broadcast messages on all 50 subnets (VLANs). The amount
   of ARP to be processed by each server is still too much.

8. Why IETF Needs To Develop Solutions Instead of IEEE 802

   ARP involves IP to MAC mapping, which traditionally has been
   standardized by IETF, e.g. RFC826.

9. Conclusion and Recommendation

   When there are tens of thousands of VMs in one Data Center or
   multiple data centers interconnected by a common Layer 2 network,
   Address Resolution has to be enhanced to support large scale data
   center and service agility

   Therefore, we recommend that the IETF engage in the study of this
   address resolution scale problem and, if appropriate, the
   development of interoperable solutions for address resolution in
   massive data center networks.

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10. Manageability Considerations

   This document does not add additional manageability considerations.

11. Security Considerations

   This document discusses a number of topics with their own security
   concerns, such as address resolution mechanisms including ARP and/or
   ND as well as multi-tenant data center networks, but creates no
   additional requirement for security.

12. IANA Considerations

   This document creates no additional IANA considerations.

13. Acknowledgments

   Many thanks to T. Sridhar for his contributions to the text.

14. References

   [ARP]    D.C. Plummer, "An Ethernet address resolution protocol."
             RFC826, Nov 1982.

   [Microsoft Windows] "Microsoft Windows Server 2003 TCP/IP
             implementation details."
             2003/technologies/networking/tcpip03.mspx, June 2003.

   [Scaling Ethernet] Myers, et. al., " Rethinking the Service Model:
             Scaling Ethernet to a Million Nodes", Carnegie Mellon
             University and Rice University

   [Cost of a Cloud] Greenberg, et. al., "The Cost of a Cloud: Research
             Problems in Data Center Networks"

   [Gratuitous ARP]  S. Cheshire, "IPv4 Address Conflict Detection",
             RFC 5227, July 2008.

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Authors' Addresses

   Linda Dunbar
   Huawei Technologies
   1700 Alma Drive, Suite 500
   Plano, TX 75075, USA
   Phone: (972) 543 5849
   Email: ldunbar@huawei.com

   Sue Hares
   Huawei Technologies
   2330 Central Expressway,
   Santa Clara, CA 95050, USA
   Email: shares@huawei.com

   Murari Sridharan
   Microsoft Corporation

   Narasimhan Venkataramaiah
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052-6399 USA
   Phone : 425-707-4328
   Email : narave@microsoft.com

   Benson Schliesser
   Cisco Systems, Inc.
   Email: bschlies@cisco.com

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   permission for the use of such proprietary rights by implementers or
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