Network Working Group                                      L. Dunbar
Internet Draft                                             Futurewei
Intended status: Informational                           B. Sarikaya
Expires: August 21, September 25, 2020                      Denpel Informatique
                                                          T. Herbert
                                                          S. Dikshit
                                                   February 21,
                                                      March 25, 2020

     Virtual Machine Mobility Solutions for L2 and L3 Overlay Networks


   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 a VM (virtual machine) moves from an Old NVE to a 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
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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
      4.2. Task VM Migration in Layer-3 Network.........................7
         4.2.1. Network...........................8
      4.3. Address and Connection Migration in Task Migration...8 Migration........9
   5. Handling Packets in Flight.....................................9 Flight....................................10
   6. Moving Local State of VM......................................10
   7. Handling of Hot, Warm and Cold VM Mobility....................10 Mobility....................11
   8. Other VM Mobility Options.....................................11
   9. VM Lifecycle Management.......................................11 Management.......................................12
   10. Security Considerations......................................11 Considerations......................................12
   11. IANA Considerations..........................................12
   12. Acknowledgments..............................................12
   13. Change Log...................................................12 Log...................................................13
   14. References...................................................12 References...................................................13
      14.1. Normative References....................................13
      14.2. Informative References..................................14

1. Introduction
     This document describes the overlay-based data center networks
     solutions in supporting multitenancy and VM (Virtual Machine)
     mobility. This document is strictly within the DCVPN, as defined
     by the NVO3 Framework [RFC 7365]. The intent is to describe Layer
     2 and Layer 3 Network behavior when VMs are moved from one NVE to
     another. This document assumes that the VMs move is initiated by
     VM management system, i.e. planed move. How and when to move VM
     are out of the scope of this document. RFC7666 already has the
     description of the MIB for VMs controlled by Hypervisor. The
     impact of VM mobility on higher layer protocols and applications
     is outside its scope.
     Many large DCs (Data Centers), especially Cloud DCs, host tasks
     (or workloads) for multiple tenants. A tenant can be a department
     of one organization or an organization. There are
     communication communications
     among tasks belonging to one tenant and
     communication 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 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
     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).  In addition, retaining IP
     addresses after a move is a key requirement [RFC7364].  Such a
     requirement is needed in order to maintain existing transport
     In traditional Layer-3 based networks, retaining IP addresses
     after a move is generally not recommended because the frequent
     move will cause 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
     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
     moves 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
      "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)

      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

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 (VM) mobility should not require changing VMs' IP
   addresses after the move.

   There is "Hot Migration" with transport service continuing, and
   "Cold Migration" with transport service restarted, i.e. the task
   running is stopped on the Old NVE, moved to the New NVE and the task
   is restarted. Not all DCs support "Hot Migration. DCs that only
   support Cold Migration should make their customers aware of the
   potential service interruption during the Cold 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.

4. Overview of the VM Mobility Solutions

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

     This document assumes that the communication with external
     entities are via the NVO3 Gateway as described in RFC8014 (NVO3
     Architecture). RFC 8014 (Section 5.3) has the discussion whether a
     VM move may result in or cannot result in a change to the network
     node providing the NV03 Gateway functionality - if such a change
     is not possible, then the path to the external entity may be hair-
     pinned to the NVO3 Gateway used prior to the VM move.

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, dynamic VM Mobility is highly desirable for large scale
     multi-tenant DCs.

     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 sending 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 promptly. All NVEs need
     to change their caches associating the VM Layer-2 or Medium Access
     Control (MAC) address with the new NVE's IP address as soon as the
     VM is moved. Such a change enables all NVEs to encapsulate the
     outgoing MAC frames with the current target NVE IP 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 for that Old NVE to
     forward 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.  Upon receiving this message, both Old and New
     NVEs should update the 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 if the VM already knows its IPv4 address (most common
     scenario). Next, we describe a case where RARP is used.

     There are some vendor deployments (diskless systems or systems
     without configuration files) wherein the VM's user, i.e. end-user
     client askes for the same MAC address upon migration.  This can be
     achieved by the clients sending RARP request message which carries
     the 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 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 needs to tunnel
     these in-flight packets to the New NVE. NVE to avoid packets loss.

     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 needs to 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 VM Migration in Layer-3 Network

     ARP/neighbor cache scalability considerations can limit the size
     of Layer-2 based DC networks.  Scaling can be accomplished
     seamlessly in

     Traditional Layer-3 based data center networks by just giving each
     virtual network an IP usually have all
     hosts (tasks) within one subnet and a default route that points attached to
     its NVE.  This means no explosion of ARP/ neighbor cache in VMs
     and NVEs (just one ARP/ neighbor cache entry for the default
     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 NVE. By this
     design, the term Task is used in NVE becomes the context of Layer-3
     migration mainly to have slight emphasis on default route for all hosts (tasks)
     within the moving an entity
     (Task) that is instantiated on a VM or a container.

     Traditional Layer-3 based data center networks require subnet. But this design requires IP address of the task a host
     (task) to change after moving because the move to comply with the prefixes of the
     IP address usually reflect under the locations. It is necessary to have an
     IP based new NVE.

     A VM migration in Layer 3 Network solution that can is to 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

     Hot VM Migration in Layer 3 involves coordination among many
     entities, such as VM management system and NVA. 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.


     RFC7666 has the more detailed description of the State Machine of
     VMs controlled by Hypervisor

4.3. Address and Connection Migration in Task Migration

     Address migration

     The term "Task" is achieved referring to an entity (Task) that is
     instantiated on a VM or a container, in another word, a Task can
     be an "Application" or a "workload" running on a VM or a

     Moving a Task running on a VM attached to one NVE to another VM
     attached to a New NVE is same as follows: moving the VM from one NVE to the
     New NVE. The VM attached to the New NVE needs to be assigned with
     the same address as VM attached to the Old NVE, which is called
     Address Migration in this document. Here is an example of the
     steps involved in Address Migration:

     - Configure IPv4/v6 address on the target Task. VM/NVE.
     - Suspend use of the address on the old Task. NVE.  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. Referring to the VM State Machine described in
     - Push the new NVE-VM mapping to VM.  Communicating other NVEs which have the
        attached VMs communicating with the VM being moved.  All
        relevant NVEs will learn of the new mapping via a control plane either by participating in
        a protocol for mapping propagation or by getting the new
        mapping from a central database such as Domain Name System
        (DNS). their
        corresponding NVA.

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

     The simplest course of action is to drop all TCP connections to
     the VM across a migration.  If the migrations are relatively rare
     events in a data center, impact is relatively small when TCP
     connections are 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
     approach 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, as described in
     Section 5.

5. Handling Packets in Flight

     The Old NVE may receive packets from the VM's ongoing
     communications. These packets should not be lost; 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 its NVA
     assuming that the NVA gets the notification when a VM is moved
     from one NVE to another. It is out of the scope of this document
     on which entity manages the VM move and how NVA gets notified of
     the move. 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.

     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 the Timer for storing the New NVE
     address for the VM expires. The Timer 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 moving
     to the New NVE.

     The mechanism of transferring VM States and file system is out of
     the scope of this document.

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

     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
     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 effectiveness (or benefit) of the option. Warm VM
     mobility.  The larger the duration, the less warm (and hence cold) effective the Warm VM
     mobility option becomes.

     For Hot VM Mobility, once a VM moves to a New NVE, the VM IP
     address does not change and the VM should be able to continue to
     receive packets to its address(es). The VM needs to send a
     gratuitous Address Resolution message or unsolicited Neighbor
     Advertisement message upstream after each move.

8. Other VM Mobility Options
     There is also a Hot Standby option in addition to the Hot
     Mobility, where there are VMs in both primary and secondary NVEs.
     They have identical information and can provide services
     simultaneously as in load-share mode of operation.  If the VM in
     the primary NVE fails, there is no need to actively move the VM to
     the secondary NVE because the VM in the secondary NVE already
     contain identical information.  The Hot Standby option is the
     costliest 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 to 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).

9. VM Lifecycle Management
     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
     needs to manage seamlessly VM migration from one server to

10. 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.

11. IANA Considerations

       This document makes no request to IANA.

12. Acknowledgments

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

13. 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.

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

  . Submitted version - 05 05, 06, 07, and 08 to address IETF LC comments
     from TSV area.

14. References

14.1. 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,

    [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-

    [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-

    [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-

    [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, <>.

    [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-

    [RFC7666] H. Asai, et al, "Management Information Base for Virtual
             Machines Controlled by a Hypervisor", RFC7666, Oct 2015.

   [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-

14.2. 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.

Authors' Addresses

   Linda Dunbar

   Behcet Sarikaya
   Denpel Informatique

   Bhumip Khasnabish

   Tom Herbert

   Saumya Dikshit
   Bangalore, India