Virtual Machine Mobility Solutions for L2 and L3 Overlay Networks
draft-ietf-nvo3-vmm-05

Network Working Group                                      L. Dunbar
Internet Draft                                             Futurewei
Intended status: Informational                           B. Sarikaya
Expires: Dec 2019                                Denpel Informatique
                                                       B.Khasnabish
                                                        Independent
                                                          T. Herbert
                                                               Intel
                                                          S. Dikshit
                                                           Aruba-HPE
                                                     August 22, 2019

     Virtual Machine Mobility Solutions for L2 and L3 Overlay Networks
                          draft-ietf-nvo3-vmm-05

Abstract

   This document describes virtual machine mobility solutions commonly
   used in data centers built with overlay-based network. This document
   is intended for describing the solutions and the impact of moving
   VMs (or applications) from one Rack to another connected by the
   Overlay networks.

   For layer 2, it is based on using an NVA (Network Virtualization
   Authority) - NVE (Network Virtualization Edge) protocol to update
   ARP (Address Resolution Protocol) table or neighbor cache entries
   after VM (virtual machine) moves from Old NVE to the New NVE.  For
   Layer 3, it is based on address and connection migration after the
   move.

Status of this Memo

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

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79. This document may not be modified,
   and derivative works of it may not be created, except to publish it
   as an RFC and to translate it into languages other than English.

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Table of Contents

   1. Introduction...................................................3
   2. Conventions used in this document..............................4
   3. Requirements...................................................5
   4. Overview of the VM Mobility Solutions..........................6
      4.1. VM Migration in Layer 2 Network...........................6

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      4.2. Task Migration in Layer-3 Network.........................7
         4.2.1. Address and Connection Migration in Task Migration...8
   5. Handling Packets in Flight.....................................9
   6. Moving Local State of VM......................................10
   7. Handling of Hot, Warm and Cold VM Mobility....................10
   8. VM Operation..................................................11
   9. Security Considerations.......................................12
   10. IANA Considerations..........................................12
   11. Acknowledgments..............................................12
   12. Change Log...................................................12
   13. References...................................................13
      13.1. Normative References....................................13
      13.2. Informative References..................................14

1. Introduction
     This document describes the overlay-based data center networks
     solutions in supporting multitenancy and VM (Virtual Machine)
     mobility. Many large DCs, especially Cloud DCs, host tasks (or
     workloads) for multiple tenants, which can be multiple departments
     of one organization or multiple organizations. There is
     communication among tasks belonging to one tenant and
     communications among tasks belonging to different tenants or with
     external entities.
     Server Virtualization, which is being used in almost all of
     today's data centers, enables many VMs to run on a single physical
     computer or compute server sharing the processor/memory/storage.
     Network connectivity among VMs is provided by the network
     virtualization edge (NVE) [RFC8014].  It is highly desirable
     [RFC7364] to allow VMs to be moved dynamically (live, hot, or cold
     move) from one server to another for dynamic load balancing or
     optimized work distribution.
     There are many challenges and requirements related to VM mobility
     in large data centers, including dynamic attaching/detaching VMs
     to/from Virtual Network Edges (VNEs).  Retaining IP addresses
     after a move is a key requirement [RFC7364].  Such a requirement
     is needed in order to maintain existing transport connections.
     In traditional Layer-3 based networks, retaining IP addresses
     after a move is generally not recommended because the frequent
     move will cause non-aggregated IP addresses (a.k.a. fragmented IP
     addresses), which introduces complexity in IP address management.

     In view of many VM mobility schemes that exist today, there is a
     desire to document comprehensive VM mobility solutions that cover

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     both IPv4 and IPv6. The large Data Center networks can be
     organized as one large Layer-2 network geographically distributed
     in several buildings/cities or Layer-3 networks with large number
     of host routes that cannot be aggregated as the result of frequent
     move from one location to another without changing their IP
     addresses.  The connectivity between Layer 2 boundaries can be
     achieved by the network virtualization edge (NVE) functioning as
     Layer 3 gateway routing across bridging domain such as in
     Warehouse Scale Computers (WSC).

2. Conventions used in this document

      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] and [RFC8014].

      This document uses the terminology defined in [RFC7364].  In
      addition, we make the following definitions:

      VM:    Virtual Machine

      Tasks:  Task is a program instantiated or running on a virtual
               machine or container.  Tasks in virtual machines or
               containers can be migrated from one server to another.
               We use task, workload and virtual machine
               interchangeably in this document.

      Hot VM Mobility: A given VM could be moved from one server to
               another in running state.

     Warm VM Mobility:  In case of warm VM mobility, the VM states are
               mirrored to the secondary server (or domain) at a
               predefined (configurable) regular intervals.  This
               reduces the overheads and complexity, but this may also
               lead to a situation when both servers may not contain
               the exact same data (state information)

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      Cold VM Mobility:  A given VM could be moved from one server to
               another in stopped or suspended state.

      Old NVE:  refers to the old NVE where packets were forwarded to
               before migration.

      New NVE: refers to the new NVE after migration.

      Packets in flight: refers to the packets received by the Old NVE
               sent by the correspondents that have old ARP or neighbor
               cache entry before VM or task migration.

      Users of VMs in diskless systems or systems not using
               configuration files are called end user clients.

      Cloud DC:  Third party data centers that host applications,
               tasks or workloads owned by different organizations or
               tenants.

3. Requirements

   This section states requirements on data center network virtual
   machine mobility.

   Data center network should support both IPv4 and IPv6 VM mobility.

   Virtual machine mobility should not require changing their IP
   addresses after the move.

   There is "Hot Migration" with transport service continuing, and
   there is a "Cold Migration" with transport service restarted, i.e.
   stop the task running on the Old NVE and move to the New NVE before
   restart as described in the Task Migration.

   VM mobility solutions/procedures should minimize triangular routing
   except for handling packets in flight.

   VM mobility solutions/procedures should not need to use tunneling
   except for handling packets in flight.

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4. Overview of the VM Mobility Solutions

     Layer 2 and Layer 3 mobility solutions are described respectively
     in the following sections.

4.1. VM Migration in Layer 2 Network

     Being able to move VMs dynamically, from one server to another,
     makes it possible for dynamic load balancing or work distribution.
     Therefore, it is highly desirable for large scale multi-tenants
     data centers.

     In a Layer-2 based approach, VM moving to another server does not
     change its IP address, but this VM is now under a new NVE,
     previously communicating NVEs will continue to send their packets
     to the Old NVE.  To solve this problem, Address Resolution
     Protocol (ARP) cache in IPv4 [RFC0826] or neighbor cache in IPv6
     [RFC4861] in the NVEs need to be updated. NVEs need to change
     their caches associating the VM Layer-2 or Medium Access Control
     (MAC) address with the NVE's IP address. Such a change enables
     NVEs to encapsulate the outgoing MAC frames with the current
     target NVE address. It may take some time to refresh ARP/ND cache
     when a VM is moved to a New NVE.  During this period, a tunnel is
     needed so that Old NVE can forwards packets destined to the VM to
     the New NVE.

     In IPv4, the VM immediately after the move should send a
     gratuitous ARP request message containing its IPv4 and Layer 2 MAC
     address in its new NVE.  This message's destination address is the
     broadcast address.  Old NVE receives this message. Both Old and
     New NVEs should update VM's ARP entry in the central directory at
     the NVA, to update its mappings to record the IPv4 address & MAC
     address of the moving VM along with the new NVE IPv4 address.  An
     NVE-to-NVA protocol is used for this purpose [RFC8014].

     Reverse ARP (RARP) which enables the host to discover its IPv4
     address when it boots from a local server [RFC0903], is not used
     by VMs because the VM already knows its IPv4 address. Next, we
     describe a case where RARP is used.

     There are some vendor deployments (diskless systems or systems
     without configuration files) wherein VM users, i.e. end-user
     clients ask for the same MAC address upon migration.  This can be
     achieved by the clients sending RARP request message which carries
     the old MAC address looking for an IP address allocation.  The
     server, in this case the new NVE needs to communicate with NVA,
     just like in the gratuitous ARP case to ensure that the same IPv4

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     address is assigned to the VM.  NVA uses the MAC address as the
     key in the search of ARP cache to find the IP address and informs
     this to the new NVE which in turn sends RARP reply message.  This
     completes IP address assignment to the migrating VM.

     Other NVEs communicating with this VM could have the old ARP
     entry. If any VMs in those NVEs need to communicate with the VM
     attached to the New NVE, old ARP entries might be used.  Thus, the
     packets are delivered to the Old NVE.  The Old NVE MUST tunnel
     these in-flight packets to the New NVE.

     When an ARP entry for those VMs times out, their corresponding
     NVEs should access the NVA for an update.

     IPv6 operation is slightly different:

     In IPv6, after the move, the VM immediately sends an unsolicited
     neighbor advertisement message containing its IPv6 address and
     Layer-2 MAC address to its new NVE. This message is sent to the
     IPv6 Solicited Node Multicast Address corresponding to the target
     address which is the VM's IPv6 address. The NVE receiving this
     message should send request to update VM's neighbor cache entry in
     the central directory of the NVA.  The NVA's neighbor cache entry
     should include IPv6 address of the VM, MAC address of the VM and
     the NVE IPv6 address.  An NVE-to-NVA protocol is used for this
     purpose [RFC8014].

     Other NVEs communicating with this VM might still use the old
     neighbor cache entry.  If any VM in those NVEs need to communicate
     with the VM attached to the New NVE, it could use the old neighbor
     cache entry. Thus, the packets are delivered to the Old NVE.  The
     Old NVE MUST tunnel these in-flight packets to the New NVE.

     When a neighbor cache entry in those VMs times out, their
     corresponding NVEs should access the NVA for an update.

4.2. Task Migration in Layer-3 Network

     Layer-2 based data center networks become quickly prohibitive
     because ARP/neighbor caches don't scale.  Scaling can be
     accomplished seamlessly Layer-3 data center networks by just
     giving each virtual network an IP subnet and a default route that
     points to NVE.  This means no explosion of ARP/ neighbor cache in
     VMs and NVEs (just one ARP/ neighbor cache entry for default

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     route) and there is no need to have Ethernet header in
     encapsulation [RFC7348] which saves at least 16 bytes.

     Even though the term VM and Task are used interchangeably in this
     document, the term Task is used in the context of Layer-3
     migration mainly to have slight emphasis on the moving an entity
     (Task) that is instantiated on a VM or a container.

     Traditional Layer-3 based data center networks require IP address
     of the task to change after moving because the prefixes of the IP
     address usually reflect the locations. It is necessary to have an
     IP based VM migration solution that can allow IP addresses staying
     the same after moving to different locations. The Identifier
     Locator Addressing or ILA [I-D.herbert-nvo3-ila] is one of such
     solutions.

     Because broadcasting is not available in Layer-3 based networks,
     multicast of neighbor solicitations in IPv6 would need to be
     emulated.

     Cold task migration, which is a common practice in many data
     centers, involves the following steps:

     - Stop running the task.
     - Package the runtime state of the job.
     - Send the runtime state of the task to the New NVE where the
        task is to run.
     - Instantiate the task's state on the new machine.
     - Start the tasks for the task continuing from the point at which
        it was stopped.

     Address migration and connection migration in moving tasks or VMs
     are addressed next.

 4.2.1. Address and Connection Migration in Task Migration

     Address migration is achieved as follows:

     - Configure IPv4/v6 address on the target Task.
     - Suspend use of the address on the old Task.  This includes
        handling established connections.  A state may be established
        to drop packets or send ICMPv4 or ICMPv6 destination
        unreachable message when packets to the migrated address are
        received.

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     - Push the new mapping to VM.  Communicating VMs will learn of
        the new mapping via a control plane either by participation in
        a protocol for mapping propagation or by getting the new
        mapping from a central database such as Domain Name System
        (DNS).

     Connection migration involves reestablishing existing TCP
     connections of the task in the new place.

     The simplest course of action is to drop TCP connections across a
     migration.  It the migrations are relatively rare events, it is
     conceivable that TCP connections could be automatically closed in
     the network stack during a migration event.  If the applications
     running are known to handle this gracefully (i.e. reopen dropped
     connections) then this may be viable.

     More involved approach to connection migration entails pausing the
     connection, packaging connection state and sending to target,
     instantiating connection state in the peer stack, and restarting
     the connection.  From the time the connection is paused to the
     time it is running again in the new stack, packets received for
     the connection could be silently dropped.  For some period of
     time, the old stack will need to keep a record of the migrated
     connection.  If it receives a packet, it can either silently drop
     the packet or forward it to the new location, similarly as in
     Section 5.

5. Handling Packets in Flight

     The Old NVE may receive packets from the VM's ongoing
     communications and these packets should not be lost, and they
     should be sent to the New NVE to be delivered to the VM.  The
     steps involved in handling packets in flight are as follows:

     Preparation Step:  It takes some time, possibly a few seconds for
     a VM to move from its Old NVE to a New NVE. During this period, a
     tunnel needs to be established so that the Old NVE can forward
     packets to the New NVE. Old NVE gets New NVE address from NVA in
     the request to move the VM. The Old NVE can store the New NVE
     address for the VM with a timer. When the timer expired, the entry
     for the New NVE for the VM can be deleted.

     Tunnel Establishment - IPv6:  Inflight packets are tunneled to the
     New NVE using the encapsulation protocol such as VXLAN in IPv6.

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     Tunnel Establishment - IPv4:  Inflight packets are tunneled to the
     New NVE using the encapsulation protocol such as VXLAN in IPv4.

     Tunneling Packets - IPv6:  IPv6 packets received for the migrating
     VM are encapsulated in an IPv6 header at the Old NVE.  New NVE
     decapsulates the packet and sends IPv6 packet to the migrating VM.

     Tunneling Packets - IPv4:  IPv4 packets received for the migrating
     VM are encapsulated in an IPv4 header at the Old NVE. New NVE
     decapsulates the packet and sends IPv4 packet to the migrating VM.

     Stop Tunneling Packets:  When Old NVE stops receiving packets
     destined to the VM that has just moved to the New NVE. The Timer
     for storing the New NVE address for the VM should be long enough
     for all other NVEs that need to communicate with the VM to get
     their NVE-VM cache entries updated.

6. Moving Local State of VM
     In addition to the VM mobility related signaling (VM Mobility
     Registration Request/Reply), the VM state needs to be transferred
     to the New NVE.  The state includes its memory and file system if
     the VM cannot access the memory and the file system after moved to
     the New NVE.  Old NVE opens a TCP connection with New NVE over
     which VM's memory state is transferred.

     File system or local storage is more complicated to transfer.  The
     transfer should ensure consistency, i.e. the VM at the New NVE
     should find the same file system it had at the Old NVE.  Pre-
     copying is a commonly used technique for transferring the file
     system.  First the whole disk image is transferred while VM
     continues to run.  After the VM is moved any changes in the file
     system are packaged together and sent to the New NVE Hypervisor
     which reflects these changes to the file system locally at the
     destination.

7. Handling of Hot, Warm and Cold VM Mobility
     Both Cold and Warm VM mobility (or migration) refers to the VM
     being completely shut down at the Old NVE before restarted at the
     New NVE. Therefore, all transport services to the VM are
     restarted.

     Upon starting at the New NVE, the VM should send an ARP or
     Neighbor Discovery message. Cold VM mobility also allows the Old
     NVE and all communicating NVEs to time out ARP/neighbor cache
     entries of the VM.  It is necessary for the NVA to push the

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     updated ARP/neighbor cache entry to NVEs or for NVEs to pull the
     updated ARP/neighbor cache entry from NVA.

     The Cold VM mobility can be facilitated by cold standby entity
     receiving scheduled backup information. The cold standby entity
     can be a VM or can be other form factors which is beyond the scope
     of this document. The cold mobility option can be used for non-
     critical applications and services that can tolerate interrupted
     TCP connections.

     The Warm VM mobility refers the backup entities receive backup
     information at more frequent intervals.  The duration of the
     interval determines the warmth of the option.  The larger the
     duration, the less warm (and hence cold) the Warm VM mobility
     option becomes.

     There is also a Hot Standby option in addition to the Hot
     Mobility, where there are VMs in both primary and secondary NVEs
     and they identical information and can provide services
     simultaneously as in load-share mode of operation.  If the VMs in
     the primary NVE fails, there is no need to actively move the VMs
     to the secondary NVE because the VMs in the secondary NVE already
     contain identical information.  The hot standby option is the most
     costly mechanism, and hence this option is utilized only for
     mission-critical applications and services.  In hot standby
     option, regarding TCP connections, one option is to start with and
     maintain TCP connections to two different VMs at the same time.
     The least loaded VM responds first and pickup providing service
     while the sender (origin) still continues to receive Ack from the
     heavily loaded (secondary) VM and chooses not use the service of
     the secondary responding VM.  If the situation (loading condition
     of the primary responding VM) changes the secondary responding VM
     may start providing service to the sender (origin).

8. VM Operation
     Once VM moves to a New NVE, VM IP address does not change and VM
     should be able to continue to receive packets to its address(es).

     VM needs to send a gratuitous Address Resolution message or
     unsolicited Neighbor Advertisement message upstream after each
     move.

     The VM lifecycle management is a complicated task, which is beyond
     the scope of this document. Not only it involves monitoring server
     utilization, balanced distribution of workload, etc., but also

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     needs to manage seamlessly VM migration from one server to
     another.

9. Security Considerations
     Security threats for the data and control plane for overlay
     networks are discussed in [RFC8014].  There are several issues in
     a multi-tenant environment that create problems.  In Layer-2 based
     overlay data center networks, lack of security in VXLAN,
     corruption of VNI can lead to delivery to wrong tenant.  Also, ARP
     in IPv4 and ND in IPv6 are not secure, especially if we accept
     gratuitous versions.  When these are done over a UDP
     encapsulation, like VXLAN, the problem is worse since it is
     trivial for a non-trusted entity to spoof UDP packets.

     In Layer-3 based overlay data center networks, the problem of
     address spoofing may arise.  An NVE may have untrusted tasks
     attached. This usually happens in cases like the VMs (tasks)
     running third party applications.  This requires the usage of
     stronger security mechanisms.

10. IANA Considerations

       This document makes no request to IANA.

11. Acknowledgments

   The authors are grateful to Bob Briscoe, David Black, Dave R.
   Worley, Qiang Zu, Andrew Malis for helpful comments.

12. Change Log

  . submitted version -00 as a working group draft after adoption

  . submitted version -01 with these changes: references are updated,
       o added packets in flight definition to Section 2

  . submitted version -02 with updated address.

  . submitted version -03 to fix the nits.

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  . submitted version -04 in reference to the WG Last call comments.

  . Submitted version - 05 to address IETF LC comments from TSV area.

13. References

13.1. Normative References

   [RFC0826]  Plummer, D., "An Ethernet Address Resolution Protocol: Or
             Converting Network Protocol Addresses to 48.bit Ethernet
             Address for Transmission on Ethernet Hardware", STD 37,
             RFC 826, DOI 10.17487/RFC0826, November 1982,
             <https://www.rfc-editor.org/info/rfc826>.

    [RFC0903]  Finlayson, R., Mann, T., Mogul, J., and M. Theimer, "A
             Reverse Address Resolution Protocol", STD 38, RFC 903,
             DOI 10.17487/RFC0903, June 1984, <https://www.rfc-
             editor.org/info/rfc903>.

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

    [RFC2629]  Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
             DOI 10.17487/RFC2629, June 1999,  <https://www.rfc-
             editor.org/info/rfc2629>.

    [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
             "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
             DOI 10.17487/RFC4861, September 2007,  <https://www.rfc-
             editor.org/info/rfc4861>.

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    [RFC7348]  Mahalingam, M., Dutt, D., Duda, K., Agarwal, P.,
             Kreeger,  L., Sridhar, T., Bursell, M., and C. Wright,
             "Virtual  eXtensible Local Area Network (VXLAN): A
             Framework for Overlaying Virtualized Layer 2 Networks over
             Layer 3 Networks", RFC 7348, DOI 10.17487/RFC7348, August
             2014, <https://www.rfc-editor.org/info/rfc7348>.

    [RFC7364]  Narten, T., Ed., Gray, E., Ed., Black, D., Fang, L.,
             Kreeger, L., and M. Napierala, "Problem Statement:
             Overlays for Network Virtualization", RFC 7364,  DOI
             10.17487/RFC7364, October 2014,  <https://www.rfc-
             editor.org/info/rfc7364>.

    [RFC8014]  Black, D., Hudson, J., Kreeger, L., Lasserre, M., and T.
             Narten, "An Architecture for Data-Center Network
             Virtualization over Layer 3 (NVO3)", RFC 8014,  DOI
             10.17487/RFC8014, December 2016, <https://www.rfc-
             editor.org/info/rfc8014>.

13.2. Informative References

    [I-D.herbert-nvo3-ila] Herbert, T. and P. Lapukhov, "Identifier-
             locator addressing for IPv6", draft-herbert-nvo3-ila-04
             (work in progress), March 2017.

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Authors' Addresses

   Linda Dunbar
   Futurewei
   Email: ldunbar@futurewei.com

   Behcet Sarikaya
   Denpel Informatique
   Email: sarikaya@ieee.org

   Bhumip Khasnabish
   Independent
   55 Madison Avenue, Suite 160
   Morristown, NJ  07960
   Email: vumip1@gmail.com

   Tom Herbert
   Intel
   Email: tom@herbertland.com

   Saumya Dikshit
   Aruba-HPE
   Bangalore, India
   Email: saumya.dikshit@hpe.com

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