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Network Working Group                                       B. Carpenter
Internet-Draft                                         Univ. of Auckland
Intended status: Informational                                  S. Jiang
Expires: December 14, 2012                  Huawei Technologies Co., Ltd
                                                              W. Tarreau
                                                           June 12, 2012

  Extending Use of the IPv6 Flow Label for Load Balancing Persistence


   This document describes how the IPv6 flow label could be used in an
   extended role to simplify persistence mechanisms for layer 3/4 load
   distribution and balancing for large server farms.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on December 14, 2012.

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   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Server Load Balancing and the Persistence Problem . . . . . . . 3
   3.  Extended Role of Flow Label . . . . . . . . . . . . . . . . . . 4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 7
   7.  Change log [RFC Editor: Please remove]  . . . . . . . . . . . . 7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 8

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1.  Introduction

   The IPv6 flow label has been redefined [RFC6437] and is now a
   recommended IPv6 node requirement [RFC6434].  Its use for layer 3/4
   server load balancing is described in
   [I-D.carpenter-flow-label-balancing], and the reader is assumed to be
   familiar with that document.  In server load balancing, 'persistence'
   is defined as guaranteeing that a given session will run to
   completion on a single server.  The present document describes
   extensions to the role of the flow label to simplify persistence

   Note in draft: The authors recognize that this proposal is
   incomplete, that it needs considerable thought and discussion, and
   that other approaches might be possible.  However, current approaches
   to persistence in server load balancing are complicated and
   pragmatic, and this new approach, even though it requires changes to
   client applications and proxies as well as to load balancers
   themselves, seems worth discussion.

2.  Server Load Balancing and the Persistence Problem

   The IPv6 flow label is a 20 bit field included in every IPv6 header
   [RFC2460].  It is recommended to be supported in all IPv6 nodes by
   [RFC6434] and it is defined in [RFC6437].  In
   [I-D.carpenter-flow-label-balancing], it is explained how the flow
   label value, set at or near the client of a server farm, may be used
   by a layer 3/4 load balancer as part of a 2-tuple {source address,
   flow label} to efficiently identify packets belonging to a given
   client's application data flow and to direct them to a specific

   A layer 3/4 load balancer has to recognize incoming packets as
   belonging to new or existing client sessions, and choose a target
   server or proxy so as to ensure persistence.  In a simple scenario,
   the 2-tuple {source address, flow label} will be a sufficient label
   for a user data flow to guarantee persistence.  However, there are
   various cases where this does not apply.  Sometimes, multiple
   independent transport connections from the same client need to be
   handled by the same server instance.  This can be an extremely
   difficult task which often requires ugly tricks such as pattern
   matching within a buffered stream, cookie insertion, etc, which most
   current load balancers have to deal with every day.

   A common example is FTP.  For a load balancer, passive mode FTP
   requires parsing the entire control stream (port 21), in order to
   find which incoming packet will initiate a data session on a port

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   chosen by the server.  This expensive process is not always useful,
   due to the fact that sometimes clients fail to connect, or that the
   session is finally not used (e.g., because no transfer needs to be

   The same issue is even more prominent with HTTP/HTTPS: while it is
   costly but not difficult to insert a cookie in an HTTP stream to
   identify the server the user was assigned to, it is very difficult to
   do that for HTTPS, because the stream must be deciphered first.
   Deciphering the stream requires a huge amount of centralized power,
   since the load balancer needs to see the clear stream; this is in
   fact the main reason for SSL proxies in load balancing scenarios.

   An additional complication that arises frequently is when a single
   client inadvertently generates sessions that appear to originate from
   different IP addresses.  This can arise, for example, if an
   enterprise uses a proxy farm for outgoing traffic, or in mobile
   applications where several subsequent requests come from different
   network cells and thus different IP addresses (for instance,
   consulting a bank account in the train).  When two consecutive client
   requests pass through two distinct proxies, a different IP source
   address may be presented to the server load balancer, which then
   cannot rely on address-based persistence.

   In some application scenarios, such an inadvertent change in the
   client IP address may only have consequences for performance, such as
   reloading transaction context into a new server.  In other cases it
   may be more serious and result in a transaction failure that seems
   inexplicable to the user.  A reliable and efficient solution to this
   problem is therefore needed.

3.  Extended Role of Flow Label

   We propose a new model in which the client application has control
   over the outgoing flow label, and assigns the same label to all
   transport connections related to a single application session.  It
   would then be both possible and desirable to use the same flow label
   value for multiple correlated transport sessions from the same
   client.  For this to work, it is also necessary for any proxies to be
   transparent to the flow label value.  Thus, modifications to the
   network stack and the application layer code are needed in both hosts
   and proxies.  In particular, the network API needs to provide a
   method for the application layer to set the flow label value for each
   new outgoing transport session, and to read it for each new incoming
   transport session.  The former is needed for client applications, and
   both features are needed for HTTP proxies in particular.

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   Such a mechanism is not the recommended default host behaviour, but
   it is permitted by [RFC6437], which states that "a flow is not
   necessarily 1:1 mapped to a transport connection".  The assignment of
   flow labels in this case is clearly no longer stateless, but the
   requirement that they be drawn from a uniform distribution should be

   In the case of FTP Passive mode, using a flow label, the client could
   generate an initial flow label value when a file transfer is
   expected, and assign the same flow label to all data connections
   related to the same control connection.  A flow label based load
   balancer would then by definition send the data traffic to the same
   server as the control traffic, and would thus guarantee that the
   sessions are properly associated.

   For a multiprotocol transaction, if a web client (browser) used the
   same flow label for any protocol targetting a given host (or domain),
   this could be used by load balancers to reach the same server for
   several protocols (such as HTTP and HTTPS), without having to inspect
   the stream payload at all nor to inspect anything beyond layer 3,
   which is unavoidable today.

   In the case of an inadvertent change in the client IP address, most
   likely due to an intervening proxy, the use of the same flow label
   for an entire application session would allow a load balancer to
   reliably route all packets for the session to the same server without
   any additional packet inspection or cookie insertion.  This would
   improve both efficiency and reliability for all parties concerned.

   This proposal means that the layer 3/4 load balancer identifies a
   session by using the flow label value alone, rather than the 2-tuple
   {source address, flow label} as described in
   [I-D.carpenter-flow-label-balancing].  However, it is of minor
   importance if two independent client sessions are directed to the
   same server because they happen to have the same flow label.  They
   will in any case be treated separately by the server, and
   statistically there will be a negligible effect on load balancing,
   since there are a million different possible flow labels to spread
   traffic across the server farm.

   Using the flow label in this way would also greatly simplify the
   logging of user sessions.  A very common task is to match logs from
   various equipments to follow a user's activity and decide whether it
   indicates a bug, user error or attack.  Logging a flow label would of
   course help because it's easier to find the beginning and end of a
   session and decide whether it's legitimate or not.  In the case of
   two simultaneous application sessions using the same flow label value
   by chance, the two logs would be intermingled, so the analysis would

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   be more complex, but still quite feasible.

   Such extensions to the role of the flow label in load balancing are
   theoretically very attractive, but would require a major refresh of
   client and proxy software as well as of load balancers themselves.
   It amounts to considering an entire application session, in a broad
   sense, as a single flow for the purposes of RFC 6437.

   It is worth noting though that what is important to save server-side
   resources is wide enough adoption.  Most of today's load balanced
   traffic is HTTP originating from a handful of browsers which are
   regularly upgraded for security considerations.  Once a mechanism is
   adopted, it can quickly be deployed and become the general case.

   The difficulty of the upgrade path is then on the server side.  The
   first step would consist in having layer 7 load balancers be able to
   consider the flow label to avoid costly layer 7 analysis each time it
   is possible.  This means that if a non-null flow label is seen, then
   the load balancer would consider it, otherwise it would fall back to
   its default behaviour.  The second step would consist in having front
   layer 3/4 load balancers bypass the layer 7 load balancer farms when
   the flow label is found.  This point would greatly offload layer 7
   load balancers.

   Finally we observe that both clients and server farms would benefit
   from this approach, in terms of performance and reliability.  Thus,
   although both parties (and proxy operators) would need to upgrade
   software, it is in their own interest to do so.

4.  Security Considerations

   The security considerations of [RFC6437] and
   [I-D.carpenter-flow-label-balancing] apply.

   Using the flow label on its own as a session handle has limitations.
   It has no security properties and must not be used in any way as an
   identifier or authenticator; it does not have enough bits to be used
   as a nonce.  Its value should never be used in the application layer
   nor where any form of resource sharing is not desired.  For instance,
   it is not acceptable that an application would identify a user
   session by its flow label value, due to the inevitable collisions and
   the risk of forgery.  The flow label value on its own should only be
   used where resource sharing is intended (for instance, load
   balancing) and by components explicitly designed for this task,
   taking into account all the risks exposed in the above security
   references, with solid protection against mis-use, and acceptable
   fallbacks for remaining situations where the flow label values are

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   The setting of the flow label by an application is necessarily a
   stateful process, so that the application can store the label value
   for re-use in all transport sessions that are part of the same
   application transaction.  Therefore, any firewall that chooses to
   rewrite the label, to avoid a perceived covert channel risk, must do
   so in such a way that a given incoming label value is always
   rewritten to the same outgoing value.

5.  IANA Considerations

   This document requests no action by IANA.

6.  Acknowledgements

   Valuable comments and contributions were made by...

   This document was produced using the xml2rfc tool [RFC2629].

7.  Change log [RFC Editor: Please remove]

   draft-tarreau-extend-flow-label-balancing-00: extended role extracted
   after IETF83, 2012-06-12.

   draft-carpenter-v6ops-label-balance-02: clarified after WG
   discussions, 2012-03-06.

   draft-carpenter-v6ops-label-balance-01: updated with community
   comments, additional author, 2012-01-17.

   draft-carpenter-v6ops-label-balance-00: original version, 2011-10-13.

8.  References

8.1.  Normative References

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC6434]  Jankiewicz, E., Loughney, J., and T. Narten, "IPv6 Node
              Requirements", RFC 6434, December 2011.

   [RFC6437]  Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,

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              "IPv6 Flow Label Specification", RFC 6437, November 2011.

8.2.  Informative References

              Carpenter, B., Jiang, S., and W. Tarreau, "Using the IPv6
              Flow Label for Server Load Balancing",
              draft-carpenter-flow-label-balancing-01 (work in
              progress), June 2012.

   [RFC2629]  Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
              June 1999.

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

   Willy Tarreau
   R&D Produits reseau
   3 rue du petit Robinson
   78350 Jouy-en-Josas

   Email: w@1wt.eu

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