Network Working Group                                       B. Carpenter
Internet-Draft                                         Univ. of Auckland
Intended status: Informational                                  S. Jiang
Expires: June 8, 2013                       Huawei Technologies Co., Ltd
                                                              W. Tarreau
                                                        December 5, 2012

          Using the IPv6 Flow Label for Server Load Balancing


   This document describes how the IPv6 flow label as currently
   specified can be used to enhance 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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   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on June 8, 2013.

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

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Summary of Flow Label Specification  . . . . . . . . . . . . .  3
   3.  Summary of Load Balancing Techniques . . . . . . . . . . . . .  4
   4.  Applying the Flow Label to L3/L4 Load Balancing  . . . . . . .  7
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   8.  Change log [RFC Editor: Please remove] . . . . . . . . . . . . 11
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12

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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 load
   sharing in multipath routing has been specified [RFC6438].  Another
   scenario in which the flow label could be used is in load
   distribution for large server farms.  Load distribution is a slightly
   more general term than load balancing, but the latter is more
   commonly used.  This document starts with brief introductions to the
   flow label and to load balancing techniques, and then describes how
   the flow label can be used to enhance layer 3/4 load balancers in

   The motivation for this approach is to improve the performance of
   most types of layer 3/4 load balancers, especially for traffic
   including multiple IPv6 extension headers and in particular for
   fragmented packets.  Fragmented packets, often the result of
   customers reaching the load balancer via a VPN with a limited MTU,
   are a common performance problem.

2.  Summary of Flow Label Specification

   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].  There is additional
   background material in [RFC6436] and [RFC6294].  According to its
   definition, the flow label should be set to a constant value for a
   given traffic flow (such as an HTTP connection), and that value will
   belong to a uniform statistical distribution, making it potentially
   valuable for load balancing purposes.

   Any device that has access to the IPv6 header has access to the flow
   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.  In fact, the logic of finding the
   transport header is always more complex for IPv6 than for IPv4, due
   to the absence of an Internet Header Length field in IPv6.
   Additionally, if packets are fragmented, the flow label will be
   present in all fragments, but the transport header will only be in
   one packet.  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 RFC 6437, source hosts should set the flow label, but if
   they do not (i.e. its value is zero), forwarding nodes (such as the
   first-hop router) may set it instead.  In both cases, the flow label

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   value must be constant for a given transport session, normally
   identified by the IPv6 and Transport header 5-tuple.  By default, the
   flow label value should be calculated by a stateless algorithm.  The
   resulting value should form part of a statistically uniform
   distribution, regardless of which node sets it.

   It is recognised that at the time of writing, very few traffic flows
   include a non-zero flow label value.  The mechanism described below
   is one that can be added to existing load balancing mechanisms, so
   that it will become effective as more and more flows contain a non-
   zero label.  If the flow label is in fact set to zero, it will not
   affect the information entropy of the IPv6 header.  Even if the flow
   label is chosen from an imperfectly uniform distribution, it will
   nevertheless increase the header entropy.  These facts allow for
   progressive introduction of load balancing based on the flow label.

   A careful reading of RFC 6437 shows that for a given source accessing
   a well-known TCP port at a given destination, the flow label is in
   effect a substitute for the source port number, found at a fixed
   position in the layer 3 header.

   The flow label is defined as an end-to-end component of the IPv6
   header, but there are three qualifications to this:

   1.  Until the RFC 6437 standard is widely implemented as recommended
       by RFC 6434, the flow label will often be set to the default
       value of zero.
   2.  Because of the recommendation to use a stateless algorithm to
       calculate the label, there is a 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.
   3.  The flow label field is in an unprotected part of the IPv6
       header, which means that intentional or unintentional changes to
       its value cannot be trivially detected by a receiver.

   The first two points are addressed below in Section 4 and the third
   in Section 5.

3.  Summary of Load Balancing Techniques

   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, and the actual load balancing algorithm (the choice
   of which server to use for a new client session) is irrelevant to
   this discussion.

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   o  The simplest method is simply using the DNS to return different
      server addresses for a single name such as 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.
      Routing may be configured such that the different addresses are
      handled by different ingress routers.  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
   o  Finally the "front end" of many load balancing systems is a layer
      3/4 load balancer.  While it can sometimes be a dedicated
      hardware, it also happens to be a standard function of some
      network switches or routers (eg: using ECMP, [RFC2991]).  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 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 therefore needs to inspect each incoming
      packet to identify the session.  There are two common types of
      layer 3/4 load balancers, the totally stateless ones which only
      act on packets, generally involving a per-packet hashing of easy-
      to-find information such as the source address and/or port into a
      server number, and the stateful ones which take the routing
      decision on the very first packets of a session and maintain the
      same direction for all packets belonging to the same session.

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      Clearly, both types of layer 3/4 balancers could inspect and make
      use of the flow label value.

      Our focus is on how the balancer identifies a particular flow.
      For clarity, note that two aspects of layer 3/4 load balancers
      could not be affected by use of the flow label to identify

      1.  Balancers use various techniques to redirect traffic to a
          specific 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 (network address and port translation) to
          deliver the client's packets to that address.

          The choice between these methods is not affected by use of the
          flow label.

      2.  A layer 3/4 balancer must correctly handle Path MTU Discovery
          by forwarding relevant ICMPv6 packets in both directions.
          This too is not affected by use of the flow label.

   The following diagram, inspired by [Tarreau], shows a maximum layout.

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       (                                           )
       (          Clients in the Internet          )
              |                            |
         ------------                ------------
         | Ingress  |                | Ingress  |
         | router   |                | router   |
         ------------                ------------
                |     load splitting     |
                |                        |
                |                        |
           ------------             ------------
           | L3/4 ASIC|             | L3/4 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 might be relevant:

   1.  Layer 3/4 load balancers.
   2.  SSL proxies.
   3.  HTTP proxies.

   However, usage by the proxies seems unlikely to be cost-effective, so
   in this document we focus only on layer 3/4 balancers.

4.  Applying the Flow Label to L3/L4 Load Balancing

   The suggested model for using the flow label in a load balancing
   mechanism is as follows:

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   o  We are only concerned with IPv6 traffic in which the flow label
      value has been set at or near the source according to [RFC6437].
      If the flow label of an incoming packet is zero, load balancers
      will continue to use the transport header in the traditional way.
      As the use of the flow label becomes more prevalent according to
      RFC 6434, load balancers, and therefore users, will reap a growing
      performance benefit.
   o  If the flow label of an incoming packet is non-zero, layer 3/4
      load balancers can use the 2-tuple {source address, flow label} as
      the session key for whatever load distribution algorithm they
      support.  If any IPv6 extension headers, including fragment
      headers, are present, this will be significantly quicker than
      searching for the transport port numbers later in the packet.
      Moreover, the transport layer information such as the source port
      is not repeated in fragments, which generally prevents stateless
      load balancers from supporting fragmented traffic since they
      generally cannot reassemble fragments.

      A stateless layer 3/4 load balancer would simply apply a hash
      algorithm to the 2-tuple {source address, flow label} on all
      packets, in order to select the same target server consistently
      for a given flow.

      A stateful layer 3/4 load balancer would apply its usual load
      distribution algorithm to the first packet of a session, and store
      the {2-tuple, server} association in a table so that subsequent
      packets belonging to the same session are forwarded to the same
      server.  Thus, for all subsequent packets of the session, it can
      ignore all IPv6 extension headers, which should lead to a
      performance benefit.  Whether this benefit is valuable will depend
      on engineering details of the specific load balancer.

      Layer 3/4 balancers that redirect the incoming packets by NAPT are
      not expected to obtain any saving of time by using the flow label,
      because they must in any case follow the extension header chain in
      order to locate and modify the port number and transport checksum.
      The same would apply to balancers that perform TCP state tracking
      for any reason.
   o  Note that correct handling of ICMPv6 for Path MTU Discovery
      requires the layer 3/4 balancer to keep state for the client
      source address, independently of either the port numbers or the
      flow label.
   o  SSL and HTTP proxies, if present, should forward the flow label
      value towards the server.  This has no performance benefit, but is
      consistent with the general RFC 6437 model for the flow label.

   It should be noted that the performance benefit, if any, depends
   entirely on engineering trade-offs in the design of the L3/L4

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   balancer.  An extra test is needed (is the label non-zero?), but all
   logic for handling extension headers can be omitted except for the
   first packet of a new flow.  Since the only state to be stored is the
   2-tuple and the server identifier, storage requirements will be
   reduced.  Additionally, the method will work for fragmented traffic
   and for flows where the transport information is missing (unknown
   transport protocol) or obfuscated (e.g., IPsec).  Traffic reaching
   the load balancer via a VPN is particularly prone to the
   fragmentation issue, due to MTU size issues.  For some load balancer
   designs, these are very significant advantages.

   In the unlikely event of two simultaneous flows from the same source
   address 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
   rare event, it would not damage the overall load balancing effect.
   Moreover, it is very likely that there will be many more flow label
   values than servers at most sites (1 million possible label values),
   so it is already expected that multiple flow label values will end up
   on the same server for a given IP address.

   In the case that many thousands of clients are hidden behind the same
   large-scale NAPT (network address and port translator) with a single
   shared IP address, the assumption of low probability of conflicts
   might become incorrect, unless flow label values are random enough to
   avoid following similar sequences for all clients.  This is not
   expected to be a factor for IPv6 anyway, since there is no need to
   implement large-scale NAPT with address sharing [RFC4864].  The
   statistical assumption is valid for sites that implement network
   prefix translation [RFC6296], since this technique provides a
   different address for each client.

5.  Security Considerations

   Security aspects of the flow label are discussed in [RFC6437].  As
   noted there, a malicious source or man-in-the-middle could disturb
   load balancing by manipulating flow labels.  This risk already exists
   today where the source address and port are used as hashing key in
   layer 3/4 load balancers, as well as where a persistence cookie is
   used in HTTP to designate a server.  It even exists on layer 3
   components which only rely on the source address to select a
   destination, making them more DDoS-prone.  Nevertheless, all these
   methods are currently used because the benefits for load balancing
   and persistence hugely outweigh the risks.  The flow label does not
   significantly alter this situation.

   Specifically, the specification [RFC6437] states that "stateless

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   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 site ingress
   router or the layer 3/4 balancer should use a suitable heuristic to
   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.  This would
   deal with both intentional and accidental changes to the flow label.

   RFC 6437 notes in its Security Considerations that if the covert
   channel risk is considered significant, a firewall might rewrite non-
   zero flow labels.  As long as this is done as described in RFC 6437,
   it will not invalidate the mechanisms described above.

   The flow label may be of use in protecting against distributed denial
   of service (DDOS) attacks against servers.  As noted in RFC 6437, a
   source should generate flow label values that are hard to predict,
   most likely by including a secret nonce in the hash used to generate
   each label.  The attacker does not know the nonce and therefore has
   no way to invent flow labels which will all target the same server,
   even with knowledge of both the hash algorithm and the load balancing
   algorithm.  Still, it is important to understand that it is always
   trivial to force a load balancer to stick to the same server during
   an attack, so the security of the whole solution must not rely on the
   unpredicatability of the flow label values alone, but should include
   defensive measures like most load balancers already have against
   abnormal use of source address or session cookies.

   New flows are assigned to a server according to any of the usual
   algorithms available on the load balancer (e.g., least connections,
   round robin, etc.).  The association between the flow label value and
   the server is stored in a table (often called stick table) so that
   future connections using the same flow label can be sent to the same
   server.  This method is more robust against a loss of server and also
   makes it harder for an attacker to target a specific server, because
   the association between a flow label value and a server is not known

6.  IANA Considerations

   This document requests no action by IANA.

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7.  Acknowledgements

   Valuable comments and contributions were made by Fred Baker, Lorenzo
   Colitti, Joel Jaeggli, Gurudeep Kamat, Julia Renouard, Julius Volz,
   and others.

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

8.  Change log [RFC Editor: Please remove]

   draft-carpenter-flow-label-balancing-02: updates based on external
   review, 2012-12-05.

   draft-carpenter-flow-label-balancing-01: update following comments,

   draft-carpenter-flow-label-balancing-00: restructured after IETF83,

   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.

9.  References

9.1.  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,
              "IPv6 Flow Label Specification", RFC 6437, November 2011.

9.2.  Informative References

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

   [RFC2991]  Thaler, D. and C. Hopps, "Multipath Issues in Unicast and

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              Multicast Next-Hop Selection", RFC 2991, November 2000.

   [RFC4864]  Van de Velde, G., Hain, T., Droms, R., Carpenter, B., and
              E. Klein, "Local Network Protection for IPv6", RFC 4864,
              May 2007.

   [RFC6294]  Hu, Q. and B. Carpenter, "Survey of Proposed Use Cases for
              the IPv6 Flow Label", RFC 6294, June 2011.

   [RFC6296]  Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix
              Translation", RFC 6296, June 2011.

   [RFC6436]  Amante, S., Carpenter, B., and S. Jiang, "Rationale for
              Update to the IPv6 Flow Label Specification", RFC 6436,
              November 2011.

   [RFC6438]  Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
              for Equal Cost Multipath Routing and Link Aggregation in
              Tunnels", RFC 6438, November 2011.

   [Tarreau]  Tarreau, W., "Making applications scalable with load
              balancing", 2006, <>.

Authors' Addresses

   Brian Carpenter
   Department of Computer Science
   University of Auckland
   PB 92019
   Auckland,   1142
   New Zealand


   Sheng Jiang
   Huawei Technologies Co., Ltd
   Q14, Huawei Campus
   No.156 Beiqing Road
   Hai-Dian District, Beijing  100095
   P.R. China


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


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