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Versions: 00 01 02 draft-carpenter-flow-label-balancing

V6OPS                                                       B. Carpenter
Internet-Draft                                         Univ. of Auckland
Intended status: Informational                                  S. Jiang
Expires: September 7, 2012                  Huawei Technologies Co., Ltd
                                                              W. Tarreau
                                                           March 6, 2012

          Using the IPv6 Flow Label for Server Load Balancing


   This document describes how the IPv6 flow label can be used in
   support of layer 3/4 load distribution and balancing for large server

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on September 7, 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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   the Trust Legal Provisions and are provided without warranty as

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Role of the Flow Label . . . . . . . . . . . . . . . . . . . .  5
   3.  Possible extended role . . . . . . . . . . . . . . . . . . . .  8
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   7.  Change log [RFC Editor: Please remove] . . . . . . . . . . . . 11
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12

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1.  Introduction

   The IPv6 flow label has been redefined [RFC6437] and 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 a brief introduction to
   load balancing techniques and then describes how the flow label might
   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.  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.
      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

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      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.
      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 are
      not affected at all 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.
          - 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.

2.  Role of the Flow Label

   The IPv6 flow label is a 20 bit field included in every IPv6 header
   [RFC2460] and it is defined in [RFC6437].  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 [RFC6437], 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.

   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 proxy for the source port number, found at a fixed position
   in the layer 3 header.  Thus, 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, there are two possibilities:
      1.  The ingress router at the server site could implement the
          stateless mechanism in Section 3 of [RFC6437] 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 to identify the packets
          belonging to the same flow.
      2.  Load balancers will use the flow label value as described
          below if it is set, but use the transport header in the
          traditional way otherwise.
      In either case, the idea is that as the use of the flow label
      becomes more prevalent, load balancers will reap a growing
      performance benefit.
   o  The 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, instead of searching for the transport
      port number later in the header.  Note that they do not need to
      consider the destination address as it is always the same, i.e.,
      the server address.

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      Stateless layer 3/4 load balancers would simply apply a hash
      algorithm to the 2-tuple {source address, flow label} on all
      packets, while stateful load balancers would apply their usual
      load distribution algorithm to the first packet of a session, and
      store the { 2-tuple, server } association in a table so that all
      packets belonging to the same session are forwarded to the same
      server.  However, 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  An SSL proxy should forward the flow identifier between the
      ciphered side and the clear side.  Being able to forward data used
      for persistence is very important, as it's the only way to stack
      multiple layers of network components without losing information.
   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
   rare event, it would not damage the overall load balancing effect.
   Moreover, it is very likely that there will be many more servers than
   possible flow label values at most locations (1 million possible
   values), so it is already expected that many different flow label
   values will end up on the same server for a given IP address.  In the
   case where many thousands of clients are hidden behind the same
   large-scale NAT with a single 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 valid reason to implement NAT [RFC4864].  The statistical
   assumption is valid for sites that implement network prefix
   translation [RFC6296], since this technique provides a different
   address for each client.

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3.  Possible extended role

   A particular aspect of the session persistence issue is when 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
   load balancers have to deal with every day.  If the client
   application has control over the outgoing flow label, then it can
   itself assign the same label to all transport connections related to
   a single application session.

   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
   chosen by the server.  This does not always work well due to the fact
   that sometimes clients don't connect, or that the session is finally
   not used (e.g., because no transfer needs to be performed).

   Using a flow label, the client could generate an initial random flow
   identifier 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.  Such a
   mechanism is permitted by [RFC6437], although it is not the
   recommended default.

   The same need 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.  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 both HTTP and HTTPS, without
   having to open the stream payload at all nor to inspect anything
   beyond layer 3, which clearly is not possible today.

   An additional complication that can arise 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 thus different IP addresses (for instance, consulting
   banking account in the train).  When two consecutive client requests

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   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.  It would be possible and desirable in
   principle to use the same flow label value for correlated sessions
   from the same client, if the proxies were transparent to the flow
   label value.

   In some application scenarios, an inadvertent change in the client IP
   address may have only minor consequences, such as reloading
   transaction context into a new server.  In other cases it may be more
   serious and result in a transaction failure.  For this reason, a
   reliable solution in which the load balancer would use the flow label
   value on its own would be advantageous.

   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.

   Such extensions to the role of the flow label in load balancing are
   theoretically very attractive, but would require a major refresh of
   client 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 nothing though that what is important to save server-side
   resources is wide enough adoption.  Most of todays 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.

4.  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

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   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 cookies 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, still all these methods are
   currently used because the benefits for load balancing and
   persistence hugely outweight the risks.

   Specifically, [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 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.

   The suggestion in Section 3 of using the flow label on its own as a
   session handle is somewhat problematic.  It should never be used in
   applications nor where any form of resource sharing is not desired.
   For instance, it is not conceivable that an application would
   identify a user session by its flow label value due to the inevitable
   collisions.  Using the flow label on its own should only be performed
   where resource sharing is inevitable and desired (for instance, load
   balancing) and by components explicitely designed for this task,
   taking into account all the risks exposed here with solid protections
   against mis-use, and acceptable fallbacks for the remaining
   situations where the flow label values will not be usable.

   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

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   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

5.  IANA Considerations

   This document requests no action by IANA.

6.  Acknowledgements

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

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

7.  Change log [RFC Editor: Please remove]

   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.

   [RFC6437]  Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
              "IPv6 Flow Label Specification", RFC 6437, November 2011.

8.2.  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.

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

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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