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RFC 6294

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Survey of Proposed Use Cases for the IPv6 Flow Label


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Independent Submission                                             Q. Hu
Request for Comments: 6294                                  B. Carpenter
Category: Informational                                Univ. of Auckland
ISSN: 2070-1721                                                June 2011

          Survey of Proposed Use Cases for the IPv6 Flow Label


   The IPv6 protocol includes a flow label in every packet header, but
   this field is not used in practice.  This paper describes the flow
   label standard and discusses the implementation issues that it
   raises.  It then describes various published proposals for using the
   flow label and shows that most of them are inconsistent with the
   standard.  Methods to address this problem are briefly reviewed.  We
   also question whether the standard should be revised.

Status of This Memo

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

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not 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) 2011 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
   ( 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.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
     1.1.  A Brief History of the Flow Label  . . . . . . . . . . . .  2
     1.2.  The Flow Label and Quality of Service  . . . . . . . . . .  3
   2.  Flow Label Definition and Issues . . . . . . . . . . . . . . .  4
     2.1.  Flow Label Properties  . . . . . . . . . . . . . . . . . .  4
     2.2.  Dependency Prohibition . . . . . . . . . . . . . . . . . .  5
     2.3.  Other Issues . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Documented Proposals for the Flow Label  . . . . . . . . . . .  6
     3.1.  Specify the Flow Label as a Pseudo-Random Value  . . . . .  7
       3.1.1.  End-to-End QoS Provisioning  . . . . . . . . . . . . .  7
       3.1.2.  Load-Balancing . . . . . . . . . . . . . . . . . . . .  8
       3.1.3.  Security Nonce . . . . . . . . . . . . . . . . . . . .  8
     3.2.  Specify QoS Parameters in the Flow Label . . . . . . . . .  8
     3.3.  Use Flow Label Hop-by-Hop to Control Switching . . . . . .  9
     3.4.  Diffserv Use of IPv6 Flow Label  . . . . . . . . . . . . . 12
     3.5.  Other Uses . . . . . . . . . . . . . . . . . . . . . . . . 12
   4.  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
   7.  Informative References . . . . . . . . . . . . . . . . . . . . 14

1.  Introduction

   IPv6 is being introduced to overcome the address shortage of the
   current IPv4 protocol, but it also offers a new feature, i.e., the
   Flow Label field in the IPv6 packet header.  The flow label is not
   encrypted by IPsec and is present in all fragments.  However, it is
   used very little in practice, for reasons discussed below and in
   [Amante11].  After a short introduction, this document summarizes the
   current specification of the IPv6 flow label and some open issues
   about its use in Section 2.  Section 3 describes and analyzes various
   proposals that have been made for its use.  Finally, Section 4
   discusses the implications and attempts to draw conclusions.

   The Flow Label field occupies bits 12 through 31 of the IPv6 packet
   header.  It provides a potential way to mark a packet, identify a
   flow, and look up the corresponding flow state.  This field is always
   present in an IPv6 header, so a phrase such as "a packet with no flow
   label" refers to a packet whose Flow Label field contains 20 zero
   bits, i.e., a flow label whose value is zero.

1.1.  A Brief History of the Flow Label

   The original proposal for a flow label has been attributed to Dave
   Clark [Deering93], who proposed that it should contain a pseudo-
   random value.  A Flow Label field was included in the packet header

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   during the preliminary design of IPv6, which followed an intense
   period of debate about several competing proposals.  The final choice
   was made in 1994 [RFC1752].  In particular, the IETF rejected a
   proposal known as the Common Architecture for Next Generation
   Internet Protocol (CATNIP) [RFC1707], which included so-called 'cache
   handles' to identify the next hop in high-performance routers.  Thus,
   CATNIP introduced the notion of a header field that would be shared
   by all packets belonging to a flow, to control packet forwarding on a
   hop-by-hop basis.  We recognize this today as a precursor of the MPLS
   label [RFC3031].

   The IETF decided instead to develop a proposal known as the Simple
   Internet Protocol plus (SIPP) [RFC1710] into IP version 6.  SIPP
   included "labeling of packets belonging to particular traffic 'flows'
   for which the sender requests special handling, such as non-default
   quality of service or 'real-time' service" [RFC1710].  In 1994, this
   used a 28-bit Flow Label field.  In 1995, it was down to 24 bits
   [RFC1883], and it was finally reduced to 20 bits [RFC2460] to
   accommodate the IPv6 Traffic Class, which is fully compatible with
   the IPv4 Type of Service byte.

   There was considerable debate in the IETF about the very purpose of
   the flow label.  Was it to be a handle for fast switching, as in
   CATNIP, or was it to be meaningful to applications and used to
   specify quality of service?  Must it be set by the sending host, or
   could it be set by routers?  Could it be modified en route, or must
   it be delivered with no change?

   Because of these uncertainties, and more urgent work, the flow label
   was consistently ignored by implementors, and today is set to zero in
   almost every IPv6 packet.  In fact, [RFC2460] defined it as
   "experimental and subject to change".  There was considerable
   preliminary work, such as [Metzler00], [Conta01a], [Conta01b], and
   [Hagino01].  The ensuing proposed standard "IPv6 Flow Label
   Specification" (RFC 3697) [RFC3697] intended to clarify this
   situation by providing precise boundary conditions for use of the
   flow label.  However, this has not proved successful in promoting use
   of the flow label in practice, as a result of which 20 bits are
   unused in every IPv6 packet header.

1.2.  The Flow Label and Quality of Service

   Developments in high-speed switch design, and the success of MPLS,
   have largely obviated consideration of the flow label for high-speed
   switching.  Thus, although various use cases for the flow label have
   been proposed, most of them assume that it should be used principally
   to support the provision of quality of service (QoS).  For many
   years, it has been recognized that real-time Internet traffic

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   requires a different QoS from general data traffic, and this remains
   true in the era of network neutrality.  Thus, an alternative to
   uniform best-effort service is needed, requiring packets to be
   classified as belonging to a particular class of service or flow.
   Currently, this leads to a layer violation problem, since a 5-tuple
   is often used to classify each packet.  The 5-tuple includes source
   and destination addresses, port numbers, and the transport protocol
   type, so when we want to forward or process packets, we need to
   extract information from the layer above IP.  This may be impossible
   when packets are encrypted such that port numbers are hidden, or when
   packets are fragmented, so the layer violation is not an academic
   concern.  The flow label, being exempt from IPsec encryption and
   being replicated in packet fragments, avoids this difficulty.  It has
   therefore attracted attention from the designers of new approaches to

2.  Flow Label Definition and Issues

2.1.  Flow Label Properties

   RFC 3697 [RFC3697] standardizes properties of the flow label,
   including the following:

   o  If the packets are not part of any flow, the flow label value is

   o  The 3-tuple {source address, destination address, flow label}
      uniquely identifies which packets belong to which particular flow.

   o  Packets can receive flow-specific treatment if the node has been
      set up with flow-specific state.

   o  The flow label set by the source node must be delivered to the
      destination node; i.e., it is an end-to-end label.

   o  The same pair of source and destination addresses must not use the
      same flow label value again within a timeout of at least
      120 seconds.

   One effect of the second of these rules is to avoid the layer
   violation problem mentioned in Section 1.  By using the 3-tuple, we
   only use the IP layer to classify packets, without needing any
   transport-layer information.  This may reduce the lookup time if
   flow-based treatment is required and will work even with IPsec
   encryption and fragmentation.  Therefore, for traffic needing other
   than best-effort service, such as real-time applications, the flow
   label can be set to different values to represent different flows,
   and each node forwarding or receiving the packets may provide

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   different flow-specific treatments by looking at the flow label
   value.  This is more fine-grained than differentiated services
   (Diffserv) [Carpenter02] [RFC2474] but need not be less efficient.

2.2.  Dependency Prohibition

   An additional important rule in the standard [RFC3697] effectively
   forbids any encoding of meaning in the bits of the flow label.  To be
   exact, the standard states that "IPv6 nodes MUST NOT assume any
   mathematical or other properties of the flow label values assigned by
   source nodes".  This rule is aimed at the case where a packet from a
   source using a particular encoding scheme for the flow label reaches
   a node that is using a different scheme.  If, by chance, the bit
   pattern in the flow label is meaningful in both schemes, the receiver
   would misinterpret the flow label.  Therefore, in the absence of
   other information, the receiver must not assume anything about the
   meaning of the value of the flow label.

   The standard [RFC3697] also states that "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".  The problem this rule is intended to avoid is that if a source
   uses one method of choosing flow labels (e.g., counting up from 1),
   any router that assumes another method (e.g., pseudo-randomness) may
   not perform as intended.

   Note that there is no easy escape from the combination of these two
   prohibitions, which we will call the dependency prohibition.  Unlike
   Diffserv code points, flow labels are not locally significant within
   a single administrative domain; they must be preserved end-to-end.
   In general, a router cannot know whether a particular packet
   originated in a host supporting a specific usage of the flow label.
   Therefore, any method that breaks one or both of these rules will
   only work if there is some way for a router to determine which
   sources use the same scheme as itself.

   The interpretation of the dependency rule can be subtle and is not
   spelled out in [RFC3697].  A node must not assume properties of the
   flow label -- but it may know them by construction or by signaling.
   The bits of the flow label alone are poor material for a hash key --
   but they may be combined with bits from other sources, to provide
   uniformly distributed hash outputs.

2.3.  Other Issues

   [RFC3697] does not discuss how to use the flow label most
   effectively.  This remains the major open issue, but some authors
   propose that the label should be used with reserved bandwidth to

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   achieve customized QoS provision.  Coupled with admission control at
   the edge router, this could limit congestion.  However, as we will
   see below, this is not the only proposed use.

   We now introduce some other open issues.

   o  Unknown flow labels: [RFC1809] proposed that when a router
      receives a datagram with an unknown flow label, it should treat it
      as zero.  However, the standard [RFC3697] is silent on this issue.
      Indeed, some methods of flow state establishment might choose to
      use an unknown label as the trigger for creating flow state.

   o  Deleting old flow labels: When a flow finishes, how does the
      router know the flow label has expired?  Should this be based on a
      timeout, on observation of the transport layer, or on explicit
      signaling?  [RFC3697] defines a timeout (120 seconds) that
      effectively imposes a maximum lifetime on flow label state in a
      router.  This implies that flow labeling is inappropriate for very
      intermittent flows, unless there is some mechanism to refresh
      router state.  In contrast, [Banerjee02] suggested that a router
      should send an ICMP message to the source prior to deleting a
      particular label.  The source node may then send a KEEPALIVE
      message to the router; if it does not, the router will release
      that label.

   o  Choosing when to set the flow label: For what kinds of
      applications should we set up non-zero flow labels?  [RFC1809]
      suggested not setting it for short flows containing few bytes but
      using it for long TCP connections and some real-time applications.

   o  Can we modify the flow label?  [RFC3697] states that the flow
      label must be delivered unchanged.  There are several advantages
      of immutable flow labels, apart from respecting the standard: the
      rule is easy to understand, does not require extra processing in
      routers or a signaling protocol, and allows for very simple host
      implementations.  Also, it is straightforward for hosts and
      routers to simply ignore the flow label.  However, this rule does
      appear to exclude any MPLS-like or CATNIP-like use for optimized
      packet switching.  Some of the proposed mechanisms described below
      contradict this by suggesting that switches should change the flow
      label for routing purposes.

3.  Documented Proposals for the Flow Label

   In the following, we do not intend to recommend or criticize various
   proposals.  This section shows the variety of proposals that have
   been published, and whether they are compatible with the existing
   standard.  These proposals almost all assume that the flow label's

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   main purpose is to support QoS, and their flow label mechanisms are
   entangled with QoS mechanisms.  We describe the proposals in five
   broad, and somewhat overlapping, categories, i.e.,

   1.  using pseudo-random flow label values for various purposes (for
       example, to improve routing performance when retrieving cached
       routing state);

   2.  defining specific QoS requirements as parameters embedded in the
       flow label field;

   3.  using the flow label to control packet switching;

   4.  using the flow label specifically to extend the existing
       differentiated services QoS architecture;

   5.  other uses.

   Among the proposals described in the following five sections, various
   methods are proposed to set up the flow label value.  It should be
   noted that some of these proposals embody novel and perhaps
   controversial approaches to QoS provision, and these cannot readily
   be separated from their use of the flow label.  We give a reasonable
   amount of technical detail for some of the proposals, to show the
   extent to which they propose detailed semantics for the flow label

3.1.  Specify the Flow Label as a Pseudo-Random Value

3.1.1.  End-to-End QoS Provisioning

   As our first example, [Lin06] specifies a 17-bit pseudo-random value.
   The figure below shows the proposed flow label structure.

   o  The Label Flag (LF) bit: 1 means this type of flow label is
      present.  We note that this encoding is incompatible with the
      dependency prohibition in [RFC3697], since a source that does not
      use this method may also set the LF bit.

   o  The Label Type (LT): 2 bits; describes the type of packet.

   o  The Label Number (LN): randomly generated by the source node.

   [Lin06] also describes a signaling process between source, routing,
   and destination nodes based on this label structure and on the IPv6
   Traffic Class byte, in order to reserve and release router resources
   for a given flow within a given class of traffic.  The pseudo-random
   LN value is used to uniquely identify a given flow.

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   Flow Label Specification (figure simplified from [Lin06])

         | 1| 2  |              17 bits        |
         |LF| LT |              LN             |

   LF   0  Disable
        1  Enable
   LT  00  Flow label requested by source
       01  Flow label returned by destination
       10  Flow label for data delivery
       11  Flow label terminates connection
   LN      Random number created by source

3.1.2.  Load-Balancing

   There have been numerous informal discussions of using pseudo-random
   flow labels to allow load-balancing or at least load-sharing.  This
   would be achieved by including the flow label value among the fields
   in each packet header used as input to a modulo(N) hash used to
   select among N alternative paths.  However, concerns about the
   interpretation of the dependency prohibition have generally prevented
   such proposals from being written up until recently [Carpenter11].

3.1.3.  Security Nonce

   Another proposal for a pseudo-random flow label value is [Blake09].
   This states that off-path spoofing attacks have become a big issue
   for TCP and other transport-layer applications, and proposes that in
   IPv6 we should set a random value in the flow label to make the
   packet header more complex and less easy for the attacker to guess.
   The two ends of the session will agree on flow label values during
   the SYN/ACK exchange, but off-path attackers will be unlikely to
   guess the agreed value.  Naturally, on-path attackers who can observe
   the flow labels in use can trivially defeat this protection.  This
   proposal does not involve using the flow label value to retrieve
   routing state.

3.2.  Specify QoS Parameters in the Flow Label

   [Prakash04] proposes to utilize the flow label to indicate required
   QoS parameters in detail.  It uses the first few bits of the Flow
   Label field as codes to support different approaches, as summarized
   in the following table.  Again, this is incompatible with the
   dependency prohibition in [RFC3697], since a source that does not use
   this method may also set the first two bits to non-zero.

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   Classification for various approaches (from [Prakash04])

    Bit Pattern   Approach
    00            No QoS requirement (Default QoS value)
    01            Pseudo-Random value used for the value of Flow-Label
    10            Support for Direct Parametric Representation
    1100          Support for the DiffServ Model
    1101          Reserved for future use
    111           Reserved for future use

   This method allows a pseudo-random option but also adds options for a
   direct QoS request and for Diffserv.  In the direct QoS parameters
   approach, 18 bits are used to encode requirements for one-way delay,
   IP delay variation, bandwidth, and one-way packet loss.  The proposal
   appears to assume that the Resource Reservation Protocol (RSVP)
   [RFC2205] mechanisms are used to actually implement these QoS

   This proposal allows the use of the flow label for various important
   QoS models, so the end user and service provider can choose the most
   suitable model for their situation; [Prakash04] claims that "The
   proposed approach results in a simple, scalable, modular and generic
   implementation to provide for QoS using the IPv6 flow label field".

   Similarly, [Lee04] defines the Flow Label field in five parts, with
   the first 3 bits used as an approach type.  The authors define two
   approaches: a "random" scheme and a "hybrid" scheme.  If the first 3
   bits equal "001", the flow label will be used as the random
   identifier of the flow, but if they equal "101", the remaining bits
   will include a hybrid QoS requirement for this packet, subdivided
   into traffic type (stringent or best-effort), bandwidth, buffer, and
   delay requirements.  Once again, the dependency prohibition in
   [RFC3697] is broken.  This proposal also includes throughput
   monitoring and dynamic capacity allocation.  Effectively, this
   proposal uses the flow label both to signal Intserv-like QoS
   requirements and to classify traffic into Diffserv-like virtual
   label-switched paths.  Packets with a "random" flow label are mapped
   into a generic (best-effort) virtual path.

3.3.  Use Flow Label Hop-by-Hop to Control Switching

   [Chakravorty08b] and [Chakravorty08a] describe an architectural
   framework called "IPv6 Label Switching Architecture" (6LSA).  In
   6LSA, network components identify a flow by looking at the Flow Label
   field in the IPv6 packet header; all packets with the same flow label
   must receive the same treatment and be sent to the same next hop.
   However, 6LSA resembles MPLS by considering that a label only has

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   meaning between 6LSA routers and setting the flow label at each hop.
   If the original source sets a non-zero flow label, there is no
   mechanism to restore it before delivery: a fundamental breach of
   [RFC3697].  The authors of [Chakravorty08b] did at one stage discuss
   using an IPv6 hop-by-hop option to correct this problem, but this has
   not been documented.  This is a more serious incompatibility than
   simply breaking the dependency prohibition.

   Unlike traditional routing algorithms, but like MPLS, 6LSA packets
   are classified into a Forwarding Equivalence Class (FEC), and routers
   forward packets on different paths by looking at the FEC.  Like
   previous solutions, this solution divides the Flow Label field into
   three parts.  The first 3 bits identify the FEC, which will help the
   router or 6LSA nodes to group the IP packets that receive the same
   forwarding treatment and forward them on the same virtual path.  The
   following 4 bits describe the application type, and the final 13 bits
   (defined by each node or a group of nodes) specify the hop-specific
   label.  From the table below, we can see the FEC has 6 major
   categories, each with up to 16 subcategories.

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   Flow Label Specification (shortened from [Chakravorty08b])

   | FEC (First 3 Bits)       | Next 4 Bits | Purpose                  |
   | No FEC (000)             | 0000        |                          |
   | Domain Specific (000)    | 0001 - 1111 |                          |
   | ------------------------ |             |                          |
   | VPN (001)                | 0001        | (IPSec - Tunnel Mode)    |
   |                          | 0010        | (IPSec - Transport Mode) |
   |                          | 0011        | (Special Encryption)     |
   |                          | 0100        | (VRF)                    |
   |                          | 0101        | (End Network Specific)   |
   |                          | 0110 - 1111 | (Reserved)               |
   | ------------------------ |             |                          |
   | TE Subset/               | 0001        | (DiffServ)               |
   | QoS Enhancement (010)    | 0010        | (RSVP)                   |
   . . .
   |                          | 1111        | (Reserved)               |
   | ------------------------ |             |                          |
   | Encapsulation (011)      | 0001        | (IPv6 in IPv6)           |
   |                          | 0010        | (IPv4 in IPv6)           |
   |                          | 0011        | (Other in IPv6)          |
   |                          | 0100        | (Enterprise Specific)    |
   |                          | 0101 - 1111 | (Reserved)               |
   | ------------------------ |             |                          |
   | Enterprise Specific(111) | 0000 - 1111 | (Reserved)               |

   The authors claim that fast switching using 20-bit labels instead of
   128-bit IPv6 addresses will provide memory and processing savings, as
   well as network management advantages.  "It also allows a network
   management entity updating available label tables, across the network
   to reduce man-in-the-middle attacks [sic]" [Chakravorty08b].

   We note that a similar proposal for QoS-based switching of IPv6
   packets [Roberts05] is designed to use a hop-by-hop option, which has
   not so far been allocated by the IETF.  Proposals related to this
   have been discussed by the Telecommunications Industry Association
   and the ITU-T [Adams08].

   We also note that router lookup efficiency was a major concern at the
   time when Clark first proposed a flow label [Deering93], but with the
   advent of very large scale integrated circuits capable of rapid
   lookup in a routing table, most vendors no longer express such

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3.4.  Diffserv Use of IPv6 Flow Label

   [Banerjee02] uses the Flow Label field as a replacement for the IPv6
   Traffic Class field; this proposal suggests the incoming flow label
   can indicate the QoS requirement by matching a Diffserv classifier.
   The authors have used the first three bits to indicate this, and the
   following 16 bits to indicate a Differentiated Services Per-Hop
   Behavior Identification code (Diffserv PHB-ID) [RFC3140]; the last
   bit is reserved for future use.  This method too breaks the
   dependency prohibition in [RFC3697].

   [Beckman07a] blends the flow label as an MPLS-like switching tag with
   Diffserv.  Unlike 6LSA, the method attempts to bypass the dependency
   prohibition by using one bit in the Diffserv Code Point [RFC2474] to
   indicate that the flow label is a switching tag.  In this way, a
   router can determine whether the flow label conforms to [RFC3697] or
   to [Beckman07a].  In [Beckman07b], the same author proposes using the
   flow label as a way of compressing IPv6 headers by hashing the
   addresses into the flow label, again using the Diffserv Code Point to
   mark the packets accordingly.

3.5.  Other Uses

   The Integrated Services QoS architecture [RFC1633] specifies that the
   flow label may be used as a packet filter [RFC2205].  At least one
   implementation supported this [Braden10].

   We are not aware of any proposals combining the flow label with the
   Next Steps in Signaling (NSIS) [RFC4080] architecture.

   [Donley11] proposes a use case whereby certain flows encapsulated in
   a particular type of IPv4-in-IPv6 tunnel would be distinguished at
   the remote end of the tunnel by a specific flow label value.  This
   would allow a service provider to deliver a tailored quality of
   service.  This usage appears to be completely compatible with

   There has been some discussion of possible flow label use in both the
   ROLL (Routing Over Low power and Lossy networks) [RPL-07] and MEXT
   (Mobility EXTensions for IPv6) working groups of the IETF.  Such uses
   tend to encode specific local meanings or routing-related tags in the
   label, so they appear to infringe the dependency prohibition or the
   immutability of the Flow Label field.  The ROLL group has indeed most
   recently opted not to use the Flow Label field for these reasons,
   despite having to add the undesirable overhead of an IPv6 hop-by-hop
   option instead [RPL].  Similarly, MEXT has defined a new mobility
   option to support flow bindings [RFC6089] rather than using the IPv6
   Flow Label field.

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4.  Conclusion

   Three aspects of the current standard [RFC3697] have caused problems
   for many designers:

   1.  The immutability of labels

   2.  "Nodes MUST NOT assume any mathematical or other properties of
       the Flow Label"

   3.  "Router performance SHOULD NOT be dependent on the distribution
       of the Flow Label values"

   Taken together, these rules essentially forbid any encoding of the
   semantics of a flow, or of any information about its path, in the
   flow label.  This was intentional, in accordance with the stateless
   nature of the Internet architecture and with the end-to-end principle
   [Saltzer84], [RFC3724].  It was also felt that QoS encoding via
   Diffserv was sufficient and that the requirement for high-speed
   switching could be met by MPLS.  But this means that the majority of
   the proposals described above breach the standard and the intent of
   the standard.  The authors often appear to be using the flow label
   either as an MPLS-like switching handle or as an encoded QoS signal.

   In contrast, a few documents mentioned above do appear to respect the
   rules of RFC 3697.  These are [Blake09], [Donley11], [Carpenter11],
   [Beckman07a], and [Beckman07b].  Additionally, [Lin06] would have
   joined this list if it had not assigned three flag bits in the Flow
   Label field.  Although predating RFC 3697, the Integrated Services
   usage [RFC2205] also seems to be compatible.

   What would other designers need to do, if they wish to respect
   RFC 3697?  There appear to be two choices.  One is to simply accept
   the existing rules at face value, as in the proposals just listed.
   This limits the application of the flow label.  It can, for example,
   be used as a nonce or as part of the material for a hash used to
   share load among alternate paths.  It cannot be the only material for
   such a hash, because of the dependency prohibition.  The flow label
   could also be used consistently with RFC 3697, if an application
   designer so chose, as a way to associate all packets belonging to a
   given application session between two hosts, across multiple
   transport sessions.  This, however, would presumably exclude using
   the flow label to govern routing decisions in any way, and would have
   widespread implications that have never been explored.

   The other choice, for designers who wish to use the flow label to
   control switching or QoS directly, is to bypass the rules within a
   given domain (a set of cooperating nodes) in a way that nodes outside

Top      ToC       Page 14 
   the domain cannot detect.  In this case, any deviation from RFC 3697
   has no possible effect outside the domain in question.

   An example scheme to emulate the immutability of labels is as
   follows.  Within the domain, all hosts set the label to zero, the
   routers set and interpret the label in any way they wish, and the
   last-hop router always sets the label back to zero.  If a packet
   arrives from outside the domain with a non-zero label, there is a
   method (such as a special Diffserv code point) to mark this packet so
   that its label would be ignored and delivered unchanged.  An
   alternative approach would be to define a hop-by-hop option to carry
   the original flow label across the domain, so that it could be
   changed within the domain but restored before forwarding the packet
   beyond the domain.

   If a domain allows mutable labels in such a way, it may safely ignore
   the dependency prohibition, and it may set the bits in the label
   according to locally defined rules.  Within the domain, the label
   could be used as desired, and most of the proposed designs discussed
   above could be "rescued".

   However, given the considerable number of designers who have proposed
   solutions incompatible with RFC 3697, the relatively few designs
   using the standard rules, and the failure of designs such as ROLL and
   MEXT to make use of the flow label, it seems reasonable to ask
   whether the RFC 3697 standard has value.

5.  Security Considerations

   The flow label is not protected in any way and can be forged by an
   on-path attacker.  Off-path attackers may be able to guess a valid
   flow label unless a pseudo-random value is used.  Specific usage
   models for the flow label need to allow for these exposures.  For
   further discussion, see [RFC3697].

6.  Acknowledgements

   An invaluable review of this document was performed by Bob Braden.
   Helpful comments were made by Sheng Jiang.

7.  Informative References

   [Adams08]  Adams, J., Joung, J., and J. Song, "Progress and future
              development of Flow State Aware standards, and a proposal
              for alerting nodes or end-systems on data related to a
              flow", Work in Progress, June 2008.

Top      ToC       Page 15 
   [Amante11] Amante, S., Carpenter, B., and S. Jiang, "Rationale for
              update to the IPv6 flow label specification", Work
              in Progress, May 2011.

              Banerjee, R., Malhotra, S., and M. M, "A Modified
              Specification for use of the IPv6 Flow Label for providing
              An efficient Quality of Service using a hybrid approach",
              Work in Progress, April 2002.

              Beckman, M., "IPv6 Dynamic Flow Label Switching (FLS)",
              Work in Progress, February 2007.

              Beckman, M., "IPv6 Header Compression via Addressing
              Mitigation Protocol (IPv6 AMP)", Work in Progress,
              November 2006.

   [Blake09]  Blake, S., "Use of the IPv6 Flow Label as a Transport-
              Layer Nonce to Defend Against Off-Path Spoofing Attacks",
              Work in Progress, October 2009.

   [Braden10] Braden, R., "Private Communication", 2010.

              Carpenter, B. and K. Nichols, "Differentiated Services in
              the Internet", Proc IEEE 90 (9) 1479-1494, 2002.

              Carpenter, B. and S. Amante, "Using the IPv6 flow label
              for equal cost multipath routing and link aggregation in
              tunnels", Work in Progress, May 2011.

              Chakravorty, S., "Challenges of IPv6 Flow Label
              implementation", Proc IEEE MILCOM2008, 2008.

              Chakravorty, S., Bush, J., and J. Bound, "IPv6 Label
              Switching Architecture", Work in Progress, July 2008.

   [Conta01a] Conta, A. and B. Carpenter, "A proposal for the IPv6 Flow
              Label Specification", Work in Progress, July 2001.

Top      ToC       Page 16 
   [Conta01b] Conta, A. and J. Rajahalme, "A model for Diffserv use of
              the IPv6 Flow Label Specification", Work in Progress,
              November 2001.

              Deering, S., "SIPP Overview and Issues", Minutes of the
              Joint Sessions of the SIP and PIP Working Groups,
              November 1993.

   [Donley11] Donley, C. and K. Erichsen, "Using the Flow Label with
              Dual-Stack Lite", Work in Progress, January 2011.

   [Hagino01] Hagino, J., "Socket API for IPv6 flow label field", Work
              in Progress, April 2001.

   [Lee04]    Lee, I. and S. Kim, "A QoS Improvement Scheme for Real-
              Time Traffic Using IPv6 Flow Labels", Lecture Notes in
              Computer Science Vol. 3043, 2004.

   [Lin06]    Lin, C., Tseng, P., and W. Hwang, "End-to-End QoS
              Provisioning by Flow Label in IPv6", JCIS , 2006.

              Metzler, J. and S. Hauth, "An end-to-end usage of the IPv6
              flow label", Work in Progress, November 2000.

              Prakash, B., "Using the 20 bit flow label field in the
              IPv6 header to indicate desirable quality of service on
              the internet", University of Colorado (M.Sc. Thesis),

   [RFC1633]  Braden, R., Clark, D., and S. Shenker, "Integrated
              Services in the Internet Architecture: an Overview",
              RFC 1633, June 1994.

   [RFC1707]  McGovern, M. and R. Ullmann, "CATNIP: Common Architecture
              for the Internet", RFC 1707, October 1994.

   [RFC1710]  Hinden, R., "Simple Internet Protocol Plus White Paper",
              RFC 1710, October 1994.

   [RFC1752]  Bradner, S. and A. Mankin, "The Recommendation for the IP
              Next Generation Protocol", RFC 1752, January 1995.

Top      ToC       Page 17 
   [RFC1809]  Partridge, C., "Using the Flow Label Field in IPv6",
              RFC 1809, June 1995.

   [RFC1883]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 1883, December 1995.

   [RFC2205]  Braden, R., Ed., Zhang, L., Berson, S., Herzog, S., and S.
              Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
              Functional Specification", RFC 2205, September 1997.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [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.

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031, January 2001.

   [RFC3140]  Black, D., Brim, S., Carpenter, B., and F. Le Faucheur,
              "Per Hop Behavior Identification Codes", RFC 3140,
              June 2001.

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

   [RFC3724]  Kempf, J., Ed., Austein, R., Ed., and IAB, "The Rise of
              the Middle and the Future of End-to-End: Reflections on
              the Evolution of the Internet Architecture", RFC 3724,
              March 2004.

   [RFC4080]  Hancock, R., Karagiannis, G., Loughney, J., and S. Van den
              Bosch, "Next Steps in Signaling (NSIS): Framework",
              RFC 4080, June 2005.

   [RFC6089]  Tsirtsis, G., Soliman, H., Montavont, N., Giaretta, G.,
              and K. Kuladinithi, "Flow Bindings in Mobile IPv6 and
              Network Mobility (NEMO) Basic Support", RFC 6089,
              January 2011.

   [RPL]      Winter, T., Ed., Thubert, P., Ed., Brandt, A., Clausen,
              T., Hui, J., Kelsey, R., Levis, P., Pister, K., Struik,
              R., and J. Vasseur, "RPL: IPv6 Routing Protocol for Low
              power and Lossy Networks", Work in Progress, March 2011.

Top      ToC       Page 18 
   [RPL-07]   Winter, T., Ed. and P. Thubert, Ed., "RPL: IPv6 Routing
              Protocol for Low power and Lossy Networks", Work
              in Progress, March 2010.

              Roberts, L. and J. Harford, "In-Band QoS Signaling for
              IPv6", Work in Progress, July 2005.

              Saltzer, J., Reed, D., and D. Clark, "End-To-End Arguments
              in System Design", ACM TOCS 2 (4) 277-288, 1984.

Authors' Addresses

   Qinwen Hu
   Department of Computer Science
   University of Auckland
   PB 92019
   Auckland  1142
   New Zealand


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