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Internet Engineering Task Force (IETF)                      B. Carpenter
Request for Comments: 7098                             Univ. of Auckland
Category: Informational                                         S. Jiang
ISSN: 2070-1721                             Huawei Technologies Co., Ltd
                                                              W. Tarreau
                                              HAProxy Technologies, Inc.
                                                            January 2014

      Using the IPv6 Flow Label for Load Balancing in Server Farms


   This document describes how the currently specified IPv6 flow label
   can be used to enhance layer 3/4 (L3/4) load distribution and
   balancing for large server farms.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2014 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
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Summary of Flow Label Specification . . . . . . . . . . . . .   2
   3.  Summary of Server Farm Load-Balancing Techniques  . . . . . .   4
   4.  Applying the Flow Label to Layer 3/4 Load Balancing . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  12

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.  In the context of a server farm, both terms refer to
   mechanisms that distribute the workload of a server farm among
   different servers in order to optimize performance.  Server load
   balancing commonly applies to HTTP traffic, but most of the
   techniques described would apply to other upper-layer applications as
   well.  This document starts with brief introductions to the flow
   label and to server load-balancing techniques, and then describes how
   the flow label can be used to enhance load balancers operating on IP
   packets and TCP sessions, commonly known as layer 3/4 load balancers.

   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 [RFC6437] is a 20-bit field included in every
   IPv6 header [RFC2460].  It is recommended to be supported in all IPv6
   nodes by [RFC6434].  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.

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   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;
   however, 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 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 recognized 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.  Even if the flow label is chosen from an imperfectly
   uniform distribution, it will nevertheless increase the information
   entropy of the IPv6 header as a whole.  This allows for progressive
   introduction of load balancing based on the flow label.

   If the recommendations in Section 3 of RFC 6437 are followed for
   traffic from a given source accessing a well-known TCP port at a
   given destination, the flow label can act as a substitute for the
   port numbers as far as a load balancer is concerned, and it can be
   found at a fixed position in the layer 3 header even if extension
   headers are present.

   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 IPv6 flow label specification in RFC 6437 is widely
       implemented as recommended by RFC 6434, the flow label will often
       be set to the default value of zero.

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   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 easily detected by a receiver.

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

3.  Summary of Server Farm Load-Balancing Techniques

   Load balancing for server farms is achieved by a variety of methods,
   often used in combination [Tarreau].  This section gives a general
   overview of common methods, although the flow label is not relevant
   to all of them.  The actual load-balancing algorithm (the choice of
   which server to use for a new client session) is irrelevant to this
   discussion.  We give examples for HTTP, but analogous techniques may
   be used for other application protocols.

   o  The simplest method is using the DNS to return different server
      addresses for a single name such as www.example.com to different
      users.  This is typically done by rotating the order in which
      different addresses within the server site are listed by the
      relevant authoritative DNS server, on the assumption that the
      client will pick the first one.  Routing may be configured such
      that the different addresses are handled by different ingress
      routers.  Several variants of this load-balancing mechanism exist,
      such as expecting some clients to use all the advertised addresses
      when multiple connections are involved, or directing the traffic
      to multiple sites, also known as global load balancing.  None of
      these mechanisms are in the scope of this document, and the
      proposal in this document does not affect their usability nor aim
      to replace them, so they will not be 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.  The act of proxying should be more
      efficient and less resource-intensive than 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.

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   o  A component of some load-balancing systems is an SSL reverse proxy
      farm.  The individual SSL proxies handle all cryptographic aspects
      and exchange unencrypted 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 specialized 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 be a dedicated device, it is also
      a standard function of some network switches or routers (e.g.
      using Equal-Cost Multipath Routing (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.  Depending on the specific
      scenario, the balancer will assign new sessions among the actual
      application servers, across an SSL proxy farm, or among a set of
      layer 7 proxies.  In all cases, the layer 3/4 load balancer has to
      classify incoming packets very quickly and choose the target
      server or proxy so as to ensure persistence.  'Persistence' is
      defined as the guarantee that a given client session will run to
      completion on a single server.  The layer 3/4 load balancer
      therefore needs to inspect each incoming packet to classify it.
      There are two common types of layer 3/4 load balancers, the
      totally stateless ones which only act on single 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 that 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.  Clearly, both types of
      layer 3/4 balancers could inspect and make use of the flow label

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

      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.  In this case,
             return packets from the server to the client are sent back
             without passing through the balancer, a technique known as
             direct server return, but we are not concerned here with
             the return packets.

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          +  All servers are configured with the same IP address,
             treated locally as an anycast address by layer 3 ECMP

          +  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 directly affected by use of the flow label.
          It should be noted that there may be difficulty correlating an
          ICMPv6 "Packet too big" response with the session it refers
          to, but that is out of the scope of the present document.

   The following diagram, inspired by [Tarreau], shows a layout with
   various methods in use together.  (Below, "ASIC" stands for
   "Application-Specific Integrated Circuit".)

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       (                                           )
       (          Clients in the Internet          )
              |                            |
         ------------ DNS-based      ------------
         | Ingress  | load splitting | Ingress  |
         | router   | affects        | router   |
         ------------ routing        ------------
                |                        |
                |                        |
                |                        |
           ------------             ------------
           | 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 affect performance,
   because they must in any case process the application-layer header,
   so in this document we focus only on layer 3/4 balancers.

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4.  Applying the Flow Label to Layer 3/4 Load Balancing

   The suggested model for using the flow label to enhance an layer 3/4
   load-balancing mechanism is as follows:

   o  We are only concerned with IPv6 traffic in which the flow label
      value has been set 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

   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.  Alternatively, they might use the 3-tuple {dest address,
      source address, flow label}, especially if the server farm
      supports multiple server IP addresses, but using the 3-tuple 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

      A stateless layer 3/4 load balancer would simply apply a hash
      algorithm to the 2-tuple or 3-tuple on all packets in order to
      select the same target server consistently for a given flow.
      Needless to say, the hash algorithm has to be well chosen for its
      purpose, but this problem is common to several forms of stateless
      load balancing.  The discussion in [RFC6438] applies.

      A stateful layer 3/4 load balancer would apply its usual load
      distribution algorithm to the first packet of a session, and store
      the {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.

      Note that such a balancer will not identify new transport sessions
      from the same source that use the same flow label; they will be
      delivered to the same server.  This is like the behavior of
      existing hash-based layer 4 balancers that always send similarly
      hashed packets to the same destination.  However, a global state
      table in a flow label balancer cannot be shared between multiple

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      services if these services rely on transport-layer information,
      since the goal of using the flow label is to avoid looking up that

      A related issue is that the balancer will not detect FIN/ACK
      sequences at the end of sessions.  Therefore, it will rely on
      inactivity timers to delete session state.  However, all existing
      balancers must maintain such timers to deal with hung sessions,
      and the practical impact on memory utilization is unlikely to be

   o  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 have no choice but to 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 usually has no performance
      benefit, but it is consistent with the general model for the flow
      label described in RFC 6437.

   It should be noted that the performance benefit, if any, depends
   entirely on engineering trade-offs in the design of the layer 3/4
   balancer.  An extra test is needed to check if the label is non-zero,
   but if there is a non-zero label, all logic for handling extension
   headers can be skipped except for the first packet of a new flow.
   Since the identifying state to be stored is only the 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.  There are approximately one million
   possible flow label values, and if the rules for flow label
   generation [RFC6437] are followed, this would be a statistically rare

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   event, and would not damage the overall load-balancing effect.
   Moreover, with a million possible label values, it is very likely
   that there will be many more flow label values than servers at most
   sites, so it is already expected that multiple flow label values will
   end up on the same server for a given client IP address.

   In the case that many thousands of clients are hidden behind the same
   large-scale NAPT 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 probability of conflicts is low 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 a 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 that 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 IPv6 flow label specification [RFC6437] 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 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.

   A malicious source or man-in-the-middle could generate a flow in
   which the flow label is constant but the transport port numbers in
   some packets are invalid.  Such packets, if load-balanced only on the
   basis of the flow label, could reach the target server and create a
   single-source DoS attack on its TCP engine.

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   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 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 that 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 unpredictability
   of the flow label values alone, but should include defensive measures
   like most load balancers already have against abnormal use of source
   addresses 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 2-tuple {source
   address, flow label} and the server is stored in a table (often
   called stick table) so that future traffic from the same source 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 externally.

   In the case that a stateless hash function is used to assign client
   packets to specific servers, it may be advisable to use a
   cryptographic hash function of some kind, to ensure that an attacker
   cannot predict the behavior of the load balancer.

6.  Acknowledgements

   Valuable comments and contributions were made by Fred Baker, Olivier
   Bonaventure, Ben Campbell, Lorenzo Colitti, Linda Dunbar, Donald
   Eastlake, Joel Jaeggli, Gurudeep Kamat, Warren Kumari, Julia
   Renouard, Julius Volz, and others.

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7.  References

7.1.  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.

7.2.  Informative References

   [RFC2991]  Thaler, D. and C. Hopps, "Multipath Issues in Unicast and
              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, <http://1wt.eu/articles/2006_lb/>.

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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
   HAProxy Technologies, Inc.
   R&D Network Products
   3 rue du petit Robinson
   78350 Jouy-en-Josas

   EMail: willy@haproxy.com

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