V6OPS B. Carpenter
Internet-Draft Univ. of Auckland
Intended status: Informational S. Jiang
Expires: April 15, 2012 Huawei Technologies Co., Ltd
October 13, 2011
Using the IPv6 Flow Label for Server Load Balancing
draft-carpenter-v6ops-label-balance-00
Abstract
This document describes how the IPv6 flow label can be used in
support of layer 3/4 load balancing for large server farms.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Role of the Flow Label . . . . . . . . . . . . . . . . . . . . 5
3. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
6. Change log [RFC Editor: Please remove] . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
The IPv6 flow label has been redefined [I-D.ietf-6man-flow-3697bis]
and its use for load balancing in multipath routing has been
specified [I-D.ietf-6man-flow-ecmp]. Another scenario in which the
flow label could be used is in load balancing for large server farms.
This document starts with a brief introduction to load balancing
techniques and then describes how the flow label can be used to
enhance layer 3/4 flow balancers in particular.
Load balancing for server farms is achieved by a variety of methods,
often used in combination [Tarreau]. The flow label is not relevant
to all of them. Also, the actual load balancing algorithm (the
choice of server for a new client session) is irrelevant to this
discussion.
o The simplest method is simply using the DNS to return different
server addresses for a single name such as www.example.com to
different users. Typically this is done by rotating the order in
which different addresses are listed by the relevant authoritative
DNS server, assuming that the client will pick the first one. The
flow label can have no impact on this method and it is not
discussed further.
o Another method, for HTTP servers, is to operate a layer 7 reverse
proxy in front of the server farm. The reverse proxy will present
a single IP address to the world, communicated to clients by a
single AAAA record. For each new client session (an incoming TCP
connection and HTTP request), it will pick a particular server and
proxy the session to it. Hopefully the act of proxying will be
cheap compared to the act of serving the required content. The
proxy must retain TCP state and proxy state for the duration of
the session. This TCP state could, potentially, include the
incoming flow label value.
o A component of some load balancing systems is an SSL reverse proxy
farm. The individual SSL proxies handle all cryptographic aspects
and exchange raw HTTP with the actual servers. Thus, from the
load balancing point of view, this really looks just like a server
farm, except that it's specialised for HTTPS. Each proxy will
retain SSL and TCP and maybe HTTP state for the duration of the
session, and the TCP state could potentially include the flow
label.
o Finally the "front end" of many load balancing systems is a layer
3/4 load balancer. In this case, it is the layer 3/4 load
balancer whose IP address is published as the primary AAAA record
for the service. All client sessions will pass through this
device. According to the precise scenario, it will spread new
sessions across the actual application servers, across an SSL
proxy farm, or across a set of layer 7 proxies. In all cases, the
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layer 3/4 load balancer has to recognize incoming packets as
belonging to new or existing client sessions, and choose the
target server or proxy so as to ensure persistence. 'Persistence'
is defined as guaranteeing that a given session will run to
completion on a single server. The layer 3/4 load balancer,
whatever method it uses for forwarding the session, is certain to
inspect the source address and the protocol and port numbers in
each incoming packet. At the same time, it could inspect and make
use of the flow label.
Layer 3/4 load balancers use various techniques to actually reach
their target server.
- All servers are configured with the same IP address, they are
all on the same LAN, and the load balancer sends directly to their
individual MAC addresses.
- Each server has its own IP address, and the balancer uses an IP-
in-IP tunnel to reach it.
- Each server has its own IP address, and the balancer performs
NAPT (address and port translation).
The following diagram, inspired by [Tarreau], shows a maximum layout.
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___________________________________________
( )
( Clients in the Internet )
(___________________________________________)
|
------------
| Ingress |
| router |
------------
____________|_____________
| |
|DNS-based load splitting|
| |
------------ ------------
|L3/L4 ASIC| |L3/L4 ASIC|
| balancer | | balancer |
------------ ------------
| load |
| spreading |
__________|________________________|___________
| | | |
------------ ------------ -------- --------
|HTTP proxy|...|HTTP proxy| | SSL |...| SSL |
| balancer | | balancer | | proxy| | proxy|
------------ ------------ -------- --------
____|_____________|_____________|_________|_____
| | | | |
-------- -------- -------- -------- --------
|HTTP | |HTTP | |HTTP | |HTTP | |HTTP |
|server| |server| |server| |server| |server|
-------- -------- -------- -------- --------
From the previous paragraphs, we can identify several points in this
diagram where the flow label may be relevant:
1. L3/L4 load balancers.
2. SSL proxies.
3. HTTP proxies.
2. Role of the Flow Label
The IPv6 flow label is included in every IPv6 header [RFC2460] and it
is defined in [I-D.ietf-6man-flow-3697bis]. According to this
definition, it should be set to a constant value for a given traffic
flow (such as an HTTP connection), but until the standard is widely
implemented it will often be set to the default value of zero. Any
device that has access to the IPv6 header has access to the flow
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label, and it is at a fixed position in every IPv6 packet. In
contrast, transport layer information, such as the port numbers, is
not always in a fixed position, since it follows any IPv6 extension
headers that may be present. Therefore, within the lifetime of a
given transport layer connection, the flow label can be a more
convenient "handle" than the port number for identifying that
particular connection.
According to [I-D.ietf-6man-flow-3697bis], source hosts should set
the flow label, but if they do not (i.e. its value is zero),
forwarding nodes may do so instead. In both cases, the flow label
value must be constant for a given transport session, normally
identified by the IPv6 and Transport header 5-tuple. The flow label
should be calculated by a stateless algorithm. The value should form
part of a statistically uniform distribution, making it suitable as
part of a hash function used for load distribution. Because of using
a stateless algorithm to calculate the label, there is a very low
(but non-zero) probability that two simultaneous flows from the same
source to the same destination have the same flow label value despite
having different transport protocol port numbers.
The suggested model for using the flow label in a load balancing
mechanism is as follows.
o It is clearly better if the original source, e.g. an HTTP client,
sets the flow label. However, if the flow label of an incoming
packet is zero, the ingress router at the server site should
implement the stateless mechanism in Section 3 of
[I-D.ietf-6man-flow-3697bis] to set the flow label value to an
appropriate value. This relieves the subsequent load balancers of
the need to fully analyse the IPv6 and Transport header 5-tuple.
o The L3/L4 load balancers use the 2-tuple {source address, flow
label} as the session key for whatever load distribution algorithm
they support, instead of searching for the transport port number
later in the header. This means they can ignore all IPv6
extension headers, which should simplify their design and lead to
a performance benefit.
o The SSL proxies may do the same. However, since they have to
process the transport layer in any case, this might not lead to
any performance benefit.
o The HTTP proxies may do the same. However, since they have to
process the transport and application layers in any case, this
might not lead to any performance benefit.
Note that in the unlikely event of two simultaneous flows from the
same source having the same flow label value, the two flows would end
up assigned to the same server, where they would be distinguished as
normal by their port numbers. Since this would be a statistically
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rare event, it would not damage the overall load balancing effect.
3. Security Considerations
Security aspects of the flow label are discussed in
[I-D.ietf-6man-flow-3697bis]. As noted there, a malicious source or
man-in-the-middle could disturb load balancing by manipulating flow
labels.
Specifically, [I-D.ietf-6man-flow-3697bis] states that "stateless
classifiers should not use the flow label alone to control load
distribution, and stateful classifiers should include explicit
methods to detect and ignore suspect flow label values." The former
point is answered by also using the source address. The latter point
is more complex. If the risk is considered serious, the ingress
router mentioned above should verify incoming flows with non-zero
flow label values. If a flow from a given source address and port
number does not have a constant flow label value, it is suspect and
should be dropped.
4. IANA Considerations
This document requests no action by IANA.
5. Acknowledgements
Valuable comments and contributions were made by
This document was produced using the xml2rfc tool [RFC2629].
6. Change log [RFC Editor: Please remove]
draft-carpenter-v6ops-label-balance-00: original version, 2011-10-13.
7. References
7.1. Normative References
[I-D.ietf-6man-flow-3697bis]
Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
"IPv6 Flow Label Specification",
draft-ietf-6man-flow-3697bis-07 (work in progress),
July 2011.
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[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
7.2. Informative References
[I-D.ietf-6man-flow-ecmp]
Carpenter, B. and S. Amante, "Using the IPv6 flow label
for equal cost multipath routing and link aggregation in
tunnels", draft-ietf-6man-flow-ecmp-05 (work in progress),
July 2011.
[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
June 1999.
[Tarreau] Tarreau, W., "Making applications scalable with load
balancing", 2006, <http://1wt.eu/articles/2006_lb/>.
Authors' Addresses
Brian Carpenter
Department of Computer Science
University of Auckland
PB 92019
Auckland, 1142
New Zealand
Email: brian.e.carpenter@gmail.com
Sheng Jiang
Huawei Technologies Co., Ltd
Q14, Huawei Campus
No.156 Beiqing Road
Hai-Dian District, Beijing 100095
P.R. China
Email: jiangsheng@huawei.com
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