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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