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Versions: 00 01 02 03 RFC 6294

Independent Submission                                             Q. Hu
Internet-Draft                                              B. Carpenter
Intended status: Informational                         Univ. of Auckland
Expires: April 3, 2011                                September 30, 2010


          Survey of proposed use cases for the IPv6 flow label
                      draft-hu-flow-label-cases-02

Abstract

   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 Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on April 3, 2011.

Copyright Notice

   Copyright (c) 2010 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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   include Simplified BSD License text as described in Section 4.e of



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  A brief history of the flow label  . . . . . . . . . . . .  3
     1.2.  The flow label and quality of service  . . . . . . . . . .  4
   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
     3.2.  Specify QoS parameters in the flow label . . . . . . . . .  8
     3.3.  Use flow label hop-by-hop to control switching . . . . . . 10
     3.4.  Diffserv use of IPv6 flow label  . . . . . . . . . . . . . 12
     3.5.  Other uses . . . . . . . . . . . . . . . . . . . . . . . . 12
   4.  Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
   8.  Change log . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   9.  Informative References . . . . . . . . . . . . . . . . . . . . 15
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18

























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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
   very little used in practice, for reasons discussed below and in [1].
   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, and then Section 3 describes and analyses various
   proposals that have been made for its use.  Finally, Section 4
   discusses the implications and attempts to draw conclusions.

1.1.  A brief history of the flow label

   The original proposal for a flow label has been attributed to Dave
   Clark [2], who proposed that it should contain a pseudo-random value.
   A flow label field was included in the packet header during the
   preliminary design of IPv6, which followed an intense period of
   debate about several competing proposals.  The final choice was made
   in 1994 [3].  In particular, the IETF rejected a proposal known as
   CATNIP [4], 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, on a hop-by-hop basis.  We recognize this today
   as a precursor of the MPLS label [5].  However, the IETF decided
   instead to develop a proposal known as SIPP 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" [6].  In 1994, this was a
   28-bit flow label field.  In 1995 it was down to 24 bits [7] and it
   was finally reduced to 20 bits [8] 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, [8] defined it as "experimental and subject to change."
   There was considerable preliminary work such as [9], [10], [11] and
   [12].  The ensuing proposed standard "IPv6 Flow Label Specification"
   (RFC 3697) [13] 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



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   practice, which can still be described quite accurately as a waste of
   space in every IPv6 packet.

1.2.  The flow label and quality of service

   Various use cases for the flow label have been proposed, many of them
   assuming 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 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 QoS.


2.  Flow label definition and issues

2.1.  Flow label properties

   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.

   RFC 3697 [13] standardizes properties of the flow label, including:
   o  If the packets are not part of any flow, the flow label value is
      zero.
   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 must not use the same flow
      label value again within a timeout of at least 120 seconds.




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   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
   different flow-specific treatments by looking at the flow label
   value.  This is more fine-grained than differentiated services
   (Diffserv) [14], [15] but need not be less efficient.

2.2.  Dependency prohibition

   An additional important rule in the standard [13] 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 [13] also states "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) will be misled.

   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.

2.3.  Other issues

   [13] does not discuss how to use flow label most effectively.  This
   remains the major open issue, but some authors propose that the label



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   should be used with reserved bandwidth to 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: [16] proposed that when a router receives a
      datagram with an unknown flow label, it should treat it as zero.
      However, the standard [13] 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
      signalling?  The standard defines a timeout (120 seconds) after
      which a receiving node should discard a previously recorded flow
      label if there is no more traffic.  However, this will be
      unsatisfactory in the case of a very intermittent flow.  In
      contrast, [17] suggested that a router should send an ICMP message
      to the source to delete a particular label.  The source node can
      then either send a KEEPALIVE message to the router, or it can
      allow the router to release that label.
   o  Choosing when to set the flow label: For what kinds of application
      should we set up non-zero flow labels? [16] suggested not setting
      it for short flows containing few bytes, but using it for long TCP
      connections and some real-time applications.  However, this does
      not really define clear use cases.
   o  Can we modify the flow label? [13] 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
      signalling 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 authors have objected to this feature,
      suggesting that switches should change the flow label for routing
      purposes.  We will describe these and other proposed mechanisms in
      next section.


3.  Documented proposals for the flow label

   In the following, we do not intend to recommend or criticise various
   proposals.  This section shows the variety of proposals that have
   been published, and whether they are compatible with the existing
   standard.  Most published proposals for the flow label assume that
   its main purpose is to support QoS, and their flow label mechanisms



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   are entangled with QoS mechanisms.  We describe the proposals in five
   broad categories, i.e.,
      (1) use pseudo-random flow label values for various purposes (for
      example, to improve routing performance when retrieving cached
      routing state)
      (2) define specific QoS requirements as parameters embedded in the
      flow label field;
      (3) use the flow label to control packet switching;
      (4) use the flow label to extend the existing differentiated
      services QoS architecture;
      (5) other uses.

   Across these categories, we observe various options to set up the
   flow label value, described in the following sections.  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.

3.1.  Specify the flow label as a pseudo-random value

   As our first example, [18] 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 [13], since a source that does not use
      this method may also set the LF bit.
   o  The Label type (LT): 2 bits which describe the type of packet.
   o  The Label Number (LN): which is randomly generated by the source
      node.

   [18] also describes a signalling 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 [18])

         +--+----+-----------------------------+
         | 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


   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
   dependency prohibition have generally prevented such proposals from
   being written up until recently [19].

   Another proposal for a pseudo-random flow label value is [20].  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

   [21] 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
   following table.  Again, this is incompatible with the dependency
   prohibition in [13], 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 [21])

     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 RSVP [22] mechanisms are used to actually
   implement these QoS parameters.

   This proposal allows use of flow label for various important QoS
   models, so the end user and service provider can choose the most
   suitable model for their situation; [21] claims that "this proposal
   is simple, scalable, modular and generic implementation to provide
   for QoS using the IPv6 flow label field".

   Similarly, [23] 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 [13] 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.

   The authors simulated this architecture for a network of fourteen
   nodes, using the NS-2 simulator with a weighted fair queueing
   extension to support their "hybrid" scheme.  Their results indicate
   that the "hybrid" scheme improves capacity utilization for QoS
   traffic and performance for real-time traffic, rather in the manner
   of differentiated services, whereas competing traffic using the
   "random" scheme simply experiences best effort service.  It must be
   considered, however, that NS-2 does not accurately simulate the
   performance of typical high-performance routers.



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3.3.  Use flow label hop-by-hop to control switching

   [24] and [25] 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 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 [13].  The authors of [24] 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, the authors divide 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 forwarding 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 [24])

   +--------------------------+-------------+--------------------------+
   | 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]" [24].

   We note that a similar proposal for QoS-based switching of IPv6
   packets [26] 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 [27].

   We also note that router lookup efficiency was a major concern at the
   time when Clark first proposed a flow label [2], but with the advent
   of very large scale integrated circuits capable of rapid lookup in a
   routing table, most vendors no longer express such concern.






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3.4.  Diffserv use of IPv6 flow label

   [17] 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) [28]; the last bit is
   reserved for future use.  This method too breaks the dependency
   prohibition in [13].

   [29] 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 [15] to
   indicate that the flow label is a switching tag.  In this way, a
   router can determine whether the flow label conforms to [13] or to
   [29].  In [30], 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

   We are not aware of any proposals combining the flow label with the
   other two Internet QoS architectures (Integrated Services [31] and
   Next Steps in Signaling (NSIS) [32]), except for recognition that the
   flow label can be used as a packet filter [22].

   [33] 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 [13].

   There has been some discussion of possible flow label use in both the
   ROLL (Routing Over Low power and Lossy networks) [34] 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 [35].  Similarly, MEXT has defined a new mobility
   option to support flow bindings [36], rather than using the IPv6 flow
   label field.






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4.  Discussion

   Three aspects of the current standard [13] have caused problems to
   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
   [37], [38].  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 metioned above do appear to respect the
   rules of RFC 3697.  These are [20], [33], [19], [29] and [30].

   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
   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



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   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 current 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 [13].


6.  IANA Considerations

   This document requests no action by IANA.


7.  Acknowledgements

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

   This document was produced using the xml2rfc tool [39].


8.  Change log

   draft-hu-flow-label-cases-00: first I-D version, 2010-04-19

   draft-hu-flow-label-cases-01: updated following review comments,



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   2010-08-18

   draft-hu-flow-label-cases-02: updated following more review comments,
   2010-09-30


9.  Informative References

   [1]   Amante, S., Carpenter, B., and S. Jiang, "Update to the IPv6
         flow label specification", draft-carpenter-6man-flow-update-04
         (work in progress), September 2010.

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

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

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

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

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

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

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

   [9]   Metzler, J. and S. Hauth, "An end-to-end usage of the IPv6 flow
         label", draft-metzler-ipv6-flowlabel-00 (work in progress),
         November 2000.

   [10]  Conta, A. and B. Carpenter, "A proposal for the IPv6 Flow Label
         Specification", draft-conta-ipv6-flow-label-02 (work in
         progress), July 2001.

   [11]  Conta, A. and J. Rajahalme, "Amodel for Diffserv use of the
         IPv6 Flow Label Specification",
         draft-conta-diffserv-ipv6-fl-classifier-01 (work in progress),
         November 2001.

   [12]  Hagino, J., "Socket API for IPv6 flow label field",
         draft-itojun-ipv6-flowlabel-api-01 (work in progress),



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         April 2001.

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

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

   [15]  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.

   [16]  Partridge, C., "Using the Flow Label Field in IPv6", RFC 1809,
         June 1995.

   [17]  Banerjee, R., "A Modified Specification for use of the IPv6
         Flow Label for providing An  efficient Quality of Service using
         hybrid approach", draft-banerjee-flowlabel-ipv6-qos-03 (work in
         progress), April 2002.

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

   [19]  Carpenter, B. and S. Amante, "Using the IPv6 flow label for
         equal cost multipath routing and link aggregation in tunnels",
         draft-carpenter-flow-ecmp-02 (work in progress), April 2010.

   [20]  Blake, S., "Use of the IPv6 Flow Label as a Transport-Layer
         Nonce to Defend Against Off-Path Spoofing Attacks",
         draft-blake-ipv6-flow-label-nonce-02 (work in progress),
         October 2009.

   [21]  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), 2004.

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

   [23]  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.

   [24]  Chakravorty, S., Bush, J., and J. Bound, "IPv6 Label Switching
         Architecture", draft-chakravorty-6lsa-03 (work in progress),
         July 2008.




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

   [26]  Roberts, L. and J. Harford, "In-Band QoS Signaling for IPv6",
         draft-roberts-inband-qos-ipv6-00 (work in progress), July 2005.

   [27]  song, j., Adams, J., and J. Joung, "Progress and future
         development of Flow State Aware standards, and a  proposal for
         alerting nodes or end-systems on data related to a flow",
         draft-adams-tsvwg-flow-signalling-codepoint-00 (work in
         progress), June 2008.

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

   [29]  Beckman, M., "IPv6 Dynamic Flow Label Switching (FLS)",
         draft-martinbeckman-ietf-ipv6-fls-ipv6flowswitching-03 (work in
         progress), March 2007.

   [30]  Beckman, M., "IPv6 Header Compression via Addressing Mitigation
         Protocol (IPv6 AMP)",
         draft-martinbeckman-ietf-ipv6-amp-ipv6hcamp-01 (work in
         progress), March 2007.

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

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

   [33]  Donley, C. and K. Erichsen, "Using the Flow Label with Dual-
         Stack Lite", draft-donley-softwire-dslite-flowlabel-00 (work in
         progress), July 2010.

   [34]  Winter, T. and P. Thubert, "RPL: IPv6 Routing Protocol for Low
         power and Lossy Networks", draft-ietf-roll-rpl-07 (work in
         progress), March 2010.

   [35]  Winter, T., Thubert, P., and R. Team, "RPL: IPv6 Routing
         Protocol for Low power and Lossy Networks",
         draft-ietf-roll-rpl-11 (work in progress), July 2010.

   [36]  Tsirtsis, G., Soliman, H., Montavont, N., Giaretta, G., and K.
         Kuladinithi, "Flow Bindings in Mobile IPv6 and NEMO Basic
         Support", draft-ietf-mext-flow-binding-10 (work in progress),
         September 2010.




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

   [38]  Kempf, J., Austein, R., 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.

   [39]  Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
         June 1999.


Authors' Addresses

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

   Email: qhu009@aucklanduni.ac.nz


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

   Email: brian.e.carpenter@gmail.com




















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