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Internet Engineering Task Force (IETF)                      B. CarpenterRequest for Comments: 7098                             Univ. of AucklandCategory: Informational                                         S. JiangISSN: 2070-1721                             Huawei Technologies Co., Ltd                                                              W. Tarreau                                              HAProxy Technologies, Inc.                                                            January 2014Using the IPv6 Flow Label for Load Balancing in Server FarmsAbstract   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; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7098.Copyright Notice   Copyright (c) 2014 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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.Carpenter, et al.             Informational                     [Page 1]

RFC 7098          Flow Label for Server Load Balancing      January 2014Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .22.  Summary of Flow Label Specification . . . . . . . . . . . . .23.  Summary of Server Farm Load-Balancing Techniques  . . . . . .44.  Applying the Flow Label to Layer 3/4 Load Balancing . . . . .85.  Security Considerations . . . . . . . . . . . . . . . . . . .106.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .117.  References  . . . . . . . . . . . . . . . . . . . . . . . . .127.1.  Normative References  . . . . . . . . . . . . . . . . . .127.2.  Informative References  . . . . . . . . . . . . . . . . .121.  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.Carpenter, et al.             Informational                     [Page 2]

RFC 7098          Flow Label for Server Load Balancing      January 2014   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 toRFC 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 inSection 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 inRFC 6437 is widely       implemented as recommended byRFC 6434, the flow label will often       be set to the default value of zero.Carpenter, et al.             Informational                     [Page 3]

RFC 7098          Flow Label for Server Load Balancing      January 2014   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 inSection 4 and the third   inSection 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.Carpenter, et al.             Informational                     [Page 4]

RFC 7098          Flow Label for Server Load Balancing      January 2014   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      label.   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      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 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.Carpenter, et al.             Informational                     [Page 5]

RFC 7098          Flow Label for Server Load Balancing      January 2014          +  All servers are configured with the same IP address,             treated locally as an anycast address by layer 3 ECMP             routing.          +  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".)Carpenter, et al.             Informational                     [Page 6]

RFC 7098          Flow Label for Server Load Balancing      January 2014        ___________________________________________       (                                           )       (          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.Carpenter, et al.             Informational                     [Page 7]

RFC 7098          Flow Label for Server Load Balancing      January 20144.  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 toRFC 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.  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      fragments.      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 multipleCarpenter, et al.             Informational                     [Page 8]

RFC 7098          Flow Label for Server Load Balancing      January 2014      services if these services rely on transport-layer information,      since the goal of using the flow label is to avoid looking up that      information.      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      significant.   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 inRFC 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 rareCarpenter, et al.             Informational                     [Page 9]

RFC 7098          Flow Label for Server Load Balancing      January 2014   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.Carpenter, et al.             Informational                    [Page 10]

RFC 7098          Flow Label for Server Load Balancing      January 2014RFC 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 inRFC 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 inRFC 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.Carpenter, et al.             Informational                    [Page 11]

RFC 7098          Flow Label for Server Load Balancing      January 20147.  References7.1.  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/>.Carpenter, et al.             Informational                    [Page 12]

RFC 7098          Flow Label for Server Load Balancing      January 2014Authors' 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   France   EMail: willy@haproxy.comCarpenter, et al.             Informational                    [Page 13]