Internet Engineering Task Force T. Narten
Internet-Draft IBM
Intended status: Informational July 5, 2011
Expires: January 6, 2012
Problem Statement for ARMD
draft-narten-armd-problem-statement-00
Abstract
This document examines problems related to the massive scaling of
data centers. Our initial scope is relatively narrow. Specifically,
we focus on address resolution (ARP and ND) within a single L2
broadcast domain, in which all nodes are within the same physical
data center. From an IP perspective, the entire L2 network comprises
one IP subnet or IPv6 "link". Data centers in which a single L2
network spans multiple geographic locations are out-of-scope.
Status of this Memo
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This Internet-Draft will expire on January 6, 2012.
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Copyright (c) 2011 IETF Trust and the persons identified as the
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the Trust Legal Provisions and are provided without warranty as
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Out-of-Scope Topics . . . . . . . . . . . . . . . . . . . . . . 4
4. Address Resolution . . . . . . . . . . . . . . . . . . . . . . 5
5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
8. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 6
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1. Introduction
This document examines problems related to the massive scaling of
data centers. Our initial scope is relatively narrow. Specifically,
we focus on address resolution (ARP and ND) within a single L2
broadcast domain, in which all nodes are within the same physical
data center. From an IP perspective, the entire L2 network comprises
one IP subnet or IPv6 "link". Data centers in which a single L2
network spans multiple geographic locations are out-of-scope.
This document is intended to support the ARMD WG identify its work
areas. The scope of this document intentionally starts out narrow,
mirroring the ARMD WG charter. Expanding the scope requires careful
thought, as the topic of scaling data centers generally has an almost
unbounded potential scope. It is important that this group restrict
itself to considering problems that are widespread and that it has
the ability to solve.
2. Background
Large, flat L2 networks have long been known to have scaling
problems. As the size of an L2 network increases, the level of
broadcast traffic from protocols like ARP increases. Large amounts
of broadcast traffic pose a particular burden because every device
(switch, host and router) must process and possibly act on such
traffic. In addition, large L2 networks can be subject to "broadcast
storms". The conventional wisdom for addressing such problems has
been to say "don't do that". That is, split the L2 network into
multiple separate networks, each operating as its own L3/IP subnet.
Unfortunately, this conflicts in some ways with the current trend of
virtualized systems.
Server virtualization is fast becoming the norm in data centers.
With server virtualization, each physical server supports multiple
virtual servers, each running its own operating system, middleware
and applications. Virtualization is a key enabler of workload
agility, i.e. allowing any server to host any application and
providing the flexibility of adding, shrinking, or moving services
among the physical infrastructure. Server virtualization provides
numerous benefits, including higher utilization, increased data
security, reduced user downtime, and even significant power
conservation, along with the promise of a more flexible and dynamic
computing environment.
The greatest flexibility in VM management occurs when it is possible
to easily move a VM from one place within the data center to another.
Unfortunately, movement of services within a data center is easiest
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when movement takes place within a single IP subnet, that is, within
a single L2 broadcast domain. Typically, when a VM is moved, it
retains such state as its IP address. That way, no changes on the
either the VM itself, or on clients communicating with the VM are
needed. In contrast, if a VM moves to a new IP subnet, its address
must change, and clients may need to be made aware of that change.
From a VM management perspective, life is much simpler if all servers
are on a single large L2 network.
With virtualization, a single server now hosts multiple VMs, each
having its own IP address. Consequently, the number of addresses per
machine (and hence per subnet) is increasing, even if the number of
physical machines stays constant. Today, it is not uncommon to
support 10 VMs per physical server. In a few years, the number will
likely reach 100 VMs per physical server.
In the past, services were static in the sense that they tended to
stay in one physical place. A service installed on a machine would
stay on that machine because the cost of moving a service elsewhere
was generally high. Moreover, services would tend to be placed in
such a way as to encourage communication locality. That is, servers
would be physically located near the services they accessed most
heavily. The network traffic patterns in such environments could
thus be optimized, in some cases keeping significant traffic local to
one network segment. In these more static and carefully managed
environments, it was possible to build networks that approached
scaling limitations, but did not actually cross the threshold.
Today, with VM migration becoming increasing common, traffic patterns
are becoming more diverse and changing. In particular, there can
easily be less locality of network traffic as services are moved for
such reasons as reducing overall power usage (by consolidating VMs
and powering off idle machine) or to move a virtual service to a
physical server with more capacity or a lower load. In today's
changing environments, it is becoming more difficult to engineer
networks as traffic patterns continually shift as VMs move around.
In summary, both the size and density of L2 networks is increasing,
with the increased deployment of VMs putting pressure on creating
ever larger L2 networks. Today, there are already data centers with
120,000 physical machines. That number will only increase going
forward. In addition, traffic patterns within a data center are
changing.
3. Out-of-Scope Topics
At the present time, the following items are out-of-scope for this
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document.
Cloud Computing - Cloud Computing is broad topic with many
definitions. Without a clear (and probably narrow) scoping
of what aspect of Cloud Computing to include in this effort,
it will remain out-of-scope.
L3 Links - ARP and ND operate on individual links. Consequently,
this effort is currently restricted to L2 networks
Geographically Extended Network Segments - Geographically separated
L2 networks introduce their own complexity. For example, the
bandwidth of links may be reduced compared to the local LAN,
and round-trip delays become more of a factor. At the
present time, such scenarios are out-of-scope.
VPNs - It is assumed that L2 VLANs are commonly in use to
segregate traffic. At the present time, it is unclear how
that impacts the problem statement for ARMD. While the limit
of a maximum of 4095 VLANs may be a problem for large data
centers, addressing it is out-of-scope for this document. L3
VPNs, are also out-of-scope, as are all L3 scenarios.
4. Address Resolution
In IPv4, ARP performs address resolution. To determine the link-
layer address of a given IP address, a node broadcasts an ARP
Request. The request is flooded to all portions of the L2 network,
and the node with the requested IP address replies with an ARP
response. ARP is an old protocol, and by current standards, is
sparsely documented. For example, there are no clear requirement for
retransmitting ARP requests in the absence of replies. Consequently,
implementations vary in the details of what they actually implement.
From a scaling perspective, there are two main problems with ARP.
First, it uses broadcast, and any network with a large number of
attached hosts will result in a large amount of broadcast ARP
traffic. The second problem is that it is not feasible to change
host implementations of ARP - current implementations are too widely
entrenched, and any changes to host implementations of ARP would take
years to become sufficient deployed to matter.
5. Summary
This document outlines the scope of the problem the ARMD effort is
intended to address. It intentionally begins with a very narrow
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scope of kind of data center ARMD is focusing on. The scope can be
expanded, but only after identifying shared aspects of data centers
that can be clearly defined and scoped.
6. Acknowledgements
7. IANA Considerations
8. Security Considerations
Author's Address
Thomas Narten
IBM
Email: narten@us.ibm.com
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