Internet DRAFT - draft-dunbar-armd-arp-nd-scaling-bcp
draft-dunbar-armd-arp-nd-scaling-bcp
ARMD L. Dunbar
Internet Draft Huawei
Intended status: Information Track W. Kumari
Expires: July 2012 Google
I. Gashinsky
Yahoo
January 3, 2012
BCP for ARP-ND Scaling for Large Data Centers
draft-dunbar-armd-arp-nd-scaling-bcp-00.txt
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Abstract
This draft is intended to document some simple well established
practices which can scale ARP/ND in data center environment.
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 0.
Table of Contents
1. Introduction ................................................ 3
2. Terminology ................................................. 3
3. Potential Solutions to Scale Address Resolution in D......... 4
3.1. Layer 3 solution........................................ 4
3.2. Commonly practiced Layer 2 solution to scale address
resolution .................................................. 5
3.2.1. When a host needs to communicate with an external peer
......................................................... 5
3.2.2. When the L2/L3 boundary router receives an IP packet
towards a host in one of its subnets: ..................... 6
3.2.3. Hosts in two different subnets served by the router
communicate with each other ............................... 7
3.3. Static ARP/ND entries on switches ....................... 7
3.4. DNS based solution ..................................... 7
3.5. ARP/ND Proxy approaches ................................. 8
3.6. Overlay models .......................................... 9
4. Summary and Recommendations ................................. 10
5. Manageability Considerations ................................ 10
6. Security Considerations ..................................... 10
7. IANA Considerations ......................................... 10
8. Acknowledgments ............................................. 10
9. References .................................................. 10
Authors' Addresses ............................................. 11
Intellectual Property Statement ............................... 11
Disclaimer of Validity ......................................... 12
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1. Introduction
As described in [ARMD-Problems], the increasing trend of rapid
workload shifting and server virtualization in modern data centers
is requiring servers to be loaded (or re-loaded) with different
hosts or applications at different times. Those different hosts
loaded to one physical server may have different IP addresses, or
even be in different IP subnets.
In order to allow a physical server to be re-loaded with hosts in
different subnets, or VMs to be moved to different server racks
without IP address re-configuration, the corresponding networks have
to have multiple broadcast domains (many VLANs) on the interfaces of
L2/L3 boundary routers and ToR switches. Unfortunately, this kind of
network can lead to address resolution scaling issues, especially on
the L2/L3 boundary routers, when the combined number of hosts in all
those subnets is large.
This document describes some potential solutions which can minimize
the ARP/ND scaling issues in a Data Center environment.
2. Terminology
ARP: IPv4 Address Resolution Protocol [RFC826]
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.
DC: Data Center
DA: Destination Address
EOR: End of Row switches in data center.
NA: IPv6's Neighbor Advertisement
ND: IPv6's Neighbor Discovery [RFC4861]
NS: IPv6's Neighbor Solicitation
SA: Source Address
ToR: Top of Rack Switch. It is also known as access switch.
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UNA: IPv6's Unsolicited Neighbor Advertisement
VM: Virtual Machines
3. Potential Solutions to Scale Address Resolution in DC
The following solutions have been indicated by data center operators:
1) layer-3 connectivity to the access switch,
2) practices to scale ARP/ND in layer 2,
3) static ARP/ND entries,
4) DNS based approaches, and
5) Extensions to proxy ARP [RFC1027].
There is no single solution that fits all cases. This section
suggests the best practices for each type of solution.
3.1. Layer 3 solution
This is referring to the network design with Layer 3 to the access
switches.
As described in [ARMD-Problem], many data centers are designed this
way, so that ARP/ND broadcast/multicast messages are confined to a
few ports (interfaces) of the access switches (i.e. ToR switches).
Another variant of the Layer 3 solution is Layer 3 all the way to
servers, or even to the VMs. Then the ARP/ND broadcast/multicast
messages are further confined to the small number of hosts within the
server, or none at all.
Advantage: Both ARP/ND scales well. There is no address resolution
issue in this design.
Disadvantage: The main disadvantage to this solution is that IP
addresses have to be re-configured on switches when a server needs to
be re-loaded with an application in different subnet, or VMs need to
be moved to a different location.
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Recommendation: This solution is more suitable to data centers which
have static workload or network operators who can properly re-
configure IP addresses/subnets on switches before any workload
change. No protocol changes are suggested.
3.2. Commonly practiced Layer 2 solution to scale address resolution
L2/L3 boundary routers can be heavily impacted by the ARP/ND
broadcast/multicast messages in a Layer 2 domain which is mapped to
one or multiple subnets (or VLANs) with combined large number of
hosts in all subnets. This section describes some commonly used
practices in reducing the ARP/ND processing required on L2/L3
boundary (or gateway) routers.
3.2.1. When a host needs to communicate with an external peer:
When the external peer is in a different subnet, the originating host
needs to send ARP/ND requests to its default gateway router to get
router's MAC address. If there are many subnets enabled on the
gateway router with large combined number of hosts in all those
subnets, the gateway router has to process a very large number of
ARP/ND requests, which is CPU intensive.
Solution: For IPv4 networks, a common practice to alleviate this
problem is to have the L2/L3 boundary router (or gateway router) send
periodic gratuitous ARP messages, so that all the connected hosts can
refresh their ARP caches. As the result, most hosts, if not all,
won't send ARP messages to gateway routers when they need to
communicate with external hosts.
However, IPv6 hosts are still required to send ND messages, via
unicast, to their default gateway router even with their gateway
routers periodically sending Unsolicited Neighbor Advertisement. This
is due to IPv6 requiring bi-directional path validation before a data
packet can be sent.
Advantage: Reduction of ARP requests to be processed by L2/L3
boundary router for IPv4.
Disadvantage: No reduction of ND processing on L2/L3 boundary router
for IPv6 traffic.
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Recommendation: Use for IPv4-only networks, or change the ND protocol
to allow data frames to be sent without requiring bidirectional frame
validation.
3.2.2. When the L2/L3 boundary router receives an IP packet towards a
host in one of its subnets:
When the source address is in a different subnet and the target is
not in router's ARP/ND cache, the router usually holds the packet and
triggers an ARP/ND request to make sure the target actually exists in
its L2 domain. The router may need to send multiple ARP/ND requests
until either a timeout is reached or an ARP/ND reply is received.
After this the gateway router can forward the data packets towards
the target's MAC address. This process is not only CPU intensive but
also buffer intensive.
Solution: For IPv4 network, a common practice to alleviate this
problem is by an L2/L3 boundary router (or gateway router) snooping
ARP messages, so that its ARP cache can be refreshed with active
hosts in its L2 domain. As a result, there is an increased likelihood
of the router's ARP cache having the IP-MAC entry when it receives
data frames from external subnets.
For IPv6 hosts, routers are supposed to send ND unicast even if it
has snooped UNA/NS/NA from those hosts. Therefore, this practice
doesn't help IPv6 very much.
Advantage: Reduction of ARP requests which routers have to send upon
receiving IPv4 packets and the amount of IPv4 data frames from
external subnets which routers have to hold.
Disadvantage: The amount of ND processing on routers for IPv6 traffic
is not reduced. Even for IPv4, Routers still need to hold data
packets from external subnets and trigger ARP requests if the targets
of the data packets either don't exist or are not very active.
Recommendation: Do not use with IPv6 or make protocol changes to
IPv6's ND. For IPv4, if there are higher chance of routers receiving
data packets towards non-existing or inactive targets, alternative
approaches should be considered.
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3.2.3. Hosts in two different subnets served by the router communicate
with each other
The router will be hit twice under this scenario. Once for the
originating host in subnet-A initiating ARP/ND request to the gateway
(3.2.1 above); and the second for the gateway to initiate ARP/ND
requests to the target in subnet-B (3.2.2 above).
Again, practices described in 3.2.1 and 3.2.2 can alleviate problems
in IPv4 network, but don't help very much for IPv6.
Advantage: reduction of ARP processing on L2/L3 boundary routers for
IPv4 traffic.
Disadvantage: For IPv6 traffic, there is no reduction of ND
processing on L2/L3 boundary routers.
Recommendation: do not use with IPv6 or consider other approaches.
3.3. Static ARP/ND entries on switches
In a data center environment, applications placement to servers,
racks, and rows may be orchestrated by Server (or VM) Management
System(s). Therefore it is possible for static ARP/ND entries to be
downloaded to switches, routers or servers.
Advantage: This methodology has been used to reduce ARP/ND
fluctuations in large scale deployments.
Disadvantage: There is no well defined mechanism for switches to get
static ARP/ND entries, to get prompt update of static ARP/ND entries
when changes occur, or to perform certain steps when switches go
through reset.
Recommendation: The IETF should create a well-defined mechanism (or
protocols) for switches or servers to get static ARP/ND entries.
3.4. DNS based solution
This solution is best suited to environments where applications
resolve the address of things they need to connect to via DNS, and
periodically refresh these addresses. While this solution is very
well known, and extensively used, it is mainly appropriate for
stateless services, or for services that have a large number of short
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lived connections. While elegant, it may not be appropriate for
generic host migration.
. When a VM is to be moved to a new location, here are the steps in
getting the IP addresses:
Instantiate the service on a VM in a distant rack. The new VM
gets a new IP address
Change the address of the service in DNS
Wait for the DNS TTL to expire. While you are waiting, watch the
number of connections to the new VM increase and the number of
connections to the old VM decrease.
Wait a little longer. When the number of connections to the old
VM reaches zero, shut down the old VM.
Advantage: DNS is existing technology and this is a well-known,
commonly practiced technique.
Disadvantage: This approach is not suitable for multi-tenant
scenarios, or when the data center operators does not have full
control of the applications.
Recommendation: Limited use to where the data-center operators are in
control of the entire application and runs the DNS. More appropriate
for service migration than host / VM migration..
3.5. ARP/ND Proxy approaches
RFC1027 specifies one ARP proxy approach. Since RFC1027, which was
published in 1987, there have been many variants of ARP proxy being
deployed. The term "ARP Proxy" is a loaded phrase, with different
interpretations depending on vendors and / or environments.
RFC1027's ARP Proxy is for a Gateway to return its own MAC address on
behalf of the target host. Another technique, also called "ARP
Proxy" is for a ToR switch to snoop ARP requests and return the
target hosts MAC if it knows it. .
Advantage: Proxy ARP [RFC1027] and its variants have allowed multi-
subnet ARP traffic for over a decade.
Disadvantage: Proxy ARP protocol [RFC1027] was developed prior to the
concepts of VLANs and for hosts which don't support subnets, and does
not provide the scaling.
Recommendation: Revise RFC1027 with VLAN support and scalability for
the Data Center Environment.
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3.6. Overlay models
There are several drafts on using overlay networks to scale large
layer 2 networks and enable mobility (e.g. draft-wkumari-dcops-l3-
vmmobility-00, draft-mahalingam-dutt-dcops-vxlan-00). TRILL and
IEEE802.1ah (Mac-in-Mac) are other types of overlay network to scale
Layer 2.
Overlay networks hide the hosts' addresses form the interior switches
and routers. The Overlay Edge nodes which perform the network address
encapsulation/decapsulation still see all remote hosts addresses
which communicate with hosts attached locally.
For a large data center with tens of thousands of applications
communicating with peers outside the data center, all those
applications' IP addresses are visible to external peers. When a
great number of VMs move freely within a data center, all those VMs'
IP addresses might not be aggregated very nicely on gateway routers,
causing forwarding table size exploding.
When the Gateway router receives a data frame from external peers
destined to a target within the data center, routers need to resolve
target's MAC address and the Overlay Edge node's address in order to
perform the proper overlay encapsulation.
Therefore, the overlay network will have a bottleneck at the Gateway
router(s) in processing resolving target hosts' physical address (MAC
or IP) and overlay edge address within the data center.
Here are some approaches being used to minimize the problem:
1. Use static mapping as described in Section 3.3.
2. Have multiple gateway nodes (i.e. routers), with each handling
a subset of hosts addresses which are visible to external peers,
e.g. Gateway #1 handles a set of prefixes, Gateway #2 handles
another subset of prefixes, etc. This architecture assumes that
each gateway have enough downstream ports to be connected to all
server racks.
If each server rack is allowed to instantiate hosts/applications
with any IP addresses, or allowing any VM to move anywhere
without re-configuring IP/MAC addresses, each gateway has to
resolve addresses which are potentially located on any server
rack. The address resolution processing for each gateway can
still be very heavy.
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4. Summary and Recommendations
This memo describes some best practices which can alleviate impact
of address resolution to L2/L3 gateway routers.
In the Data Center, no single solution fits all deployments. This
memo has summarized five different technologies and the advantages
and disadvantages about all of these practices.
In some of these scenarios, the best practices could be improved by
creating and/or extending existing IETF protocols. These protocol
change recommendations are:
Extend IPv6 ND method,
Create a "download" static ARP/ND entry protocol,
Revise Proxy ARP [1027] for use in the data center.
5. Manageability Considerations
This text gives recommendations for best practices in order to
improve manageability of DC.
6. Security Considerations
Security will be addressed in a separate document.
7. IANA Considerations
None.
8. Acknowledgments
We want to acknowledge the following people for their valuable inputs
to this draft: K.K.Ramakrishnan.
This document was prepared using 2-Word-v2.0.template.dot.
9. References
[ARP] D.C. Plummer, "An Ethernet address resolution protocol."
RFC826, Nov 1982.
[DC-ARCH] Karir,et al, "draft-karir-armd-datacenter-reference-arch"
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[ARMD-Problem] Narten, "draft-ietf-armd-problem-statement" in
progress, Oct 2011.
[Gratuitous ARP] S. Cheshire, "IPv4 Address Conflict Detection", RFC
5227, July 2008.
Authors' Addresses
Linda Dunbar
Huawei Technologies
5340 Legacy Drive, Suite 175
Plano, TX 75024, USA
Phone: (469) 277 5840
Email: ldunbar@huawei.com
Warren Kumari
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
US
Email: warren@kumari.net
Igor Gashinsky
Yahoo
45 West 18th Street 6th floor
New York, NY 10011
Email: igor@yahoo-inc.com
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