Network Working Group                                       B. Carpenter
Internet-Draft                                         Univ. of Auckland
Intended status: BCP                                           S. Amante
Expires: April 10, 2011                                          Level 3
                                                         October 7, 2010

  Using the IPv6 flow label for equal cost multipath routing  and link
                         aggregation in tunnels


   The IPv6 flow label has certain restrictions on its use.  This
   document describes how those restrictions apply when using the flow
   label for load balancing by equal cost multipath routing, and for
   link aggregation, particularly for tunneled traffic.

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   This Internet-Draft will expire on April 10, 2011.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Normative Notation  . . . . . . . . . . . . . . . . . . . . . . 6
   3.  Guidelines  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
   4.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 7
   7.  Change log  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 8

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1.  Introduction

   When several network paths between the same two nodes are known by
   the routing system to be equally good (in terms of capacity and
   latency), it may be desirable to share traffic among them.  Two such
   techniques are known as equal cost multipath routing (ECMP) and link
   aggregation (LAG) [IEEE802.1AX].  There are of course numerous
   possible approaches to this, but certain goals need to be met:
   o  Roughly equal share of traffic on each path.
   o  Work-conserving method (no idle time when queue is non-empty).
   o  Minimize or avoid out-of-order delivery for individual traffic

   There is some conflict between these goals: for example, strictly
   avoiding idle time could cause a small packet sent on an idle path to
   overtake a bigger packet from the same flow, causing out-of-order

   One lightweight approach to ECMP or LAG is this: if there are N
   equally good paths to choose from, then form a modulo(N) hash
   [RFC2991] from a consistent set of fields in each packet header, and
   use the resulting value to select a particular path.  If the hash
   function is chosen so that the hash values have a uniform statistical
   distribution, this method will share traffic roughly equally between
   the N paths.  If the header fields included in the hash are
   consistent, all packets from a given flow will generate the same
   hash, so out-of-order delivery will not occur.  Assuming a large
   number of unique flows are involved, it is also probable that the
   method will be work-conserving, since the queue for each link will
   remain non-empty.

   The question with such a method is which IP header fields are chosen
   to identify a flow and, consequently, are used as input keys to a
   modulo(N) hash algorithm.

   In the remainder of this document, we will use the term "flow" to
   represent a sequence of packets that may be identified by either the
   source and destination IP addresses alone {2-tuple} or the source and
   destination IP addresses, protocol and source and destination port
   numbers {5-tuple}.  It should be noted that the latter is more
   specifically referred to as a "microflow" in [RFC2474], but this term
   is not used in connection with the flow label in [RFC3697].

   The question with such a method, then, is which IP header fields to
   include to identify a flow.  A minimal choice in the routing system
   is simply to use a hash of the source and destination IP addresses,
   i.e., the 2-tuple.  This is necessary and sufficient to avoid out-of-
   order delivery, and with a wide variety of sources and destinations,

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   as one finds in the core of the network, sometimes sufficient to
   achieve work-conserving load sharing.  In practice, implementations
   often use the 5-tuple {dest addr, source addr, protocol, dest port,
   source port} as input keys to the hash function, to maximize the
   probability of evenly sharing traffic over the equal cost paths.
   However, including transport layer information as input keys to a
   hash may be a problem for IPv4 fragments [RFC2991].  In addition,
   protocol and destination port numbers in the hash will not only make
   the hash slightly more expensive to compute, but will not
   particularly improve the hash distribution, due to the prevalence of
   well known port numbers and popular protocol numbers.  Ephemeral
   ports, on the other hand, are quite well distributed [Lee10].  In the
   case of IPv6, protocol numbers are particularly inconvenient due to
   the variable placement of and variable length of next-headers.  In
   addition, [RFC2460] recommends that all next-headers, except hop-by-
   hop options, should not be inspected by intermediate nodes in the
   network, presumably to make introduction of new next-headers more

   The situation is different in tunneled scenarios.  Identifying a flow
   inside the tunnel is more complicated, particularly because nearly
   all hardware can only identify flows based on information contained
   in the outermost IP header.  Assume that traffic from many sources to
   many destinations is aggregated in a single IP-in-IP tunnel from
   tunnel end point (TEP) A to TEP B (see figure).  Then all the packets
   forming the tunnel have outer source address A and outer destination
   address B. In all probability they also have the same port and
   protocol numbers.  If there are multiple paths between routers R1 and
   R2, and ECMP or LAG is applied to choose a particular path, the
   5-tuple and its hash will be constant and no load sharing will be
   achieved.  If there is much tunnel traffic, this will result in a
   high probability of congestion on one of the paths between R1 and R2.

      _____           _____               _____           _____
     | TEP |_________| R1  |-------------| R2  |_________| TEP |
     |__A__|         |_____|-------------|_____|         |__B__|
             tunnel          ECMP or LAG         tunnel

   Also, for IPv6, the total number of bits in the 5-tuple is quite
   large (296), as well as inconvenient to extract due to the next-
   header placement.  This may be challenging for some hardware
   implementations, raising the potential that network equipment vendors
   might sacrifice the length of the fields extracted from an IPv6
   header.  The question therefore arises whether the 20-bit flow label
   in IPv6 packets would be suitable for use as input to an ECMP or LAG
   hash algorithm.  If it could be used in place of the port numbers and

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   protocol number in the 5-tuple, the hash calculation would be

   The flow label is left experimental by [RFC2460] but is better
   defined by [RFC3697].  We quote three rules from that RFC:
   1.  "The Flow Label value set by the source MUST be delivered
       unchanged to the destination node(s)."
   2.  "IPv6 nodes MUST NOT assume any mathematical or other properties
       of the Flow Label values assigned by source nodes."
   3.  "Router performance SHOULD NOT be dependent on the distribution
       of the Flow Label values.  Especially, the Flow Label bits alone
       make poor material for a hash key."

   These rules, especially the last one, have caused designers to
   hesitate about using the flow label in support of ECMP or LAG.  The
   fact is today that most nodes set a zero value in the flow label, and
   the first rule definitely forbids the routing system from changing
   the flow label once a packet has left the source node.  Considering
   normal IPv6 traffic, the fact that the flow label is typically zero
   means that it would add no value to an ECMP or LAG hash.  But neither
   would it do any harm to the distribution of the hash values.  If the
   community at some stage agrees to set pseudo-random flow labels in
   the majority of traffic flows, this would add to the value of the

   However, in the case of an IP-in-IPv6 tunnel, the TEP is itself the
   source node of the outer packets.  Therefore, a TEP may freely set a
   flow label in the outer IPv6 header of the packets it sends into the
   tunnel.  In particular, it may follow the [RFC3697] suggestion to set
   a pseudo-random value.

   The second two rules quoted above need to be seen in the context of
   [RFC3697], which assumes that routers using the flow label in some
   way will be involved in some sort of method of establishing flow
   state: "To enable flow-specific treatment, flow state needs to be
   established on all or a subset of the IPv6 nodes on the path from the
   source to the destination(s)."  The RFC should perhaps have made
   clear that a router that has participated in flow state establishment
   can rely on properties of the resulting flow label values without
   further signaling.  If a router knows these properties, rule 2 is
   irrelevant, and it can choose to deviate from rule 3.

   In the tunneling situation sketched above, routers R1 and R2 can rely
   on the flow labels set by TEP A and TEP B being assigned by a known
   method.  This allows a safe ECMP or LAG method to be based on the
   flow label without breaching [RFC3697].

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2.  Normative Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Guidelines

   We assume that the routers supporting ECMP or LAG (R1 and R2 in the
   above figure) are unaware that they are handling tunneled traffic.
   If it is desired to include the IPv6 flow label in an ECMP or LAG
   hash in the tunneled scenario shown above, the following guidelines
   o  Inner packets MUST be encapsulated in an outer IPv6 packet whose
      source and destination addresses are those of the tunnel end
      points (TEPs).
   o  The flow label in the outer packet SHOULD be set by the sending
      TEP to a pseudo-random 20-bit value in accordance with [RFC3697].
      The same flow label value MUST be used for all packets in a single
      user flow, as determined by the IP header fields of the inner
      *  Note that this rule is a SHOULD rather than a MUST, to permit
         individual implementers to take an alternative approach if they
         wish to do so.  Such an alternative MUST conform to [RFC3697].
   o  The sending TEP MUST classify all packets into flows, once it has
      determined that they should enter a given tunnel, and then write
      the relevant flow label into the outer IPv6 header.  A user flow
      could be identified by the ingress TEP most simply by its
      {destination, source} address pair (coarse) or by its 5-tuple
      {dest addr, source addr, protocol, dest port, source port} (fine).
      This is an implementation detail in the sending TEP.
      *  It might be possible to make this classifier stateless, by
         using a suitable 20 bit hash of the inner IP header's 2-tuple
         or 5-tuple as the pseudo-random flow label value.
   o  At intermediate router(s) that perform load distribution of
      tunneled packets whose source address is a TEP, the hash algorithm
      used to determine the outgoing component-link in an ECMP and/or
      LAG toward the next-hop MUST minimally include the triple {dest
      addr, source addr, flow label} to meet the [RFC3697] rules.
      *  Intermediate router(s) MAY also include {protocol, dest port,
         source port} as input keys to the ECMP and/or LAG hash
         algorithms, to provide sufficient entropy in cases where the
         flow-label is currently set to zero.

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4.  Security Considerations

   The flow label is not protected in any way and can be forged by an
   on-path attacker.  Off-path attackers are unlikely to guess a valid
   flow label if a pseudo-random value is used.  In either case, the
   worst an attacker could do against ECMP or LAG is to attempt to
   selectively overload a particular path.  For further discussion, see

5.  IANA Considerations

   This document requests no action by IANA.

6.  Acknowledgements

   This document was suggest by corridor discussions at IETF76.  Joel
   Halpern made crucial comments on an early version.  We are grateful
   to Qinwen Hu for general discussion about the flow label.  Valuable
   comments and contributions were made by Jarno Rajahalme, Brian
   Haberman, Sheng Jiang, and others.

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

7.  Change log

   draft-carpenter-flow-ecmp-03: clarifications after further comments,

   draft-carpenter-flow-ecmp-02: updated after IETF77 discussion,
   especially adding LAG, changed to BCP language, added second author,

   draft-carpenter-flow-ecmp-01: updated after comments, 2010-02-18

   draft-carpenter-flow-ecmp-00: original version, 2010-01-19

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6

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              (IPv6) Specification", RFC 2460, December 1998.

   [RFC3697]  Rajahalme, J., Conta, A., Carpenter, B., and S. Deering,
              "IPv6 Flow Label Specification", RFC 3697, March 2004.

8.2.  Informative References

              Institute of Electrical and Electronics Engineers, "Link
              Aggregation", IEEE Standard 802.1AX-2008, 2008.

   [Lee10]    Lee, D., Carpenter, B., and N. Brownlee, "Observations of
              UDP to TCP Ratio and Port Numbers", Fifth International
              Conference on Internet Monitoring and Protection ICIMP
              2010, May 2010, <

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              December 1998.

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

Authors' Addresses

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


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   Shane Amante
   Level 3 Communications, LLC
   1025 Eldorado Blvd
   Broomfield, CO  80021


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