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RFC 6437

IPv6 Flow Label Specification

Pages: 15
Proposed Standard
Obsoletes:  3697
Updates:  22052460

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Internet Engineering Task Force (IETF)                         S. Amante
Request for Comments: 6437                                       Level 3
Obsoletes: 3697                                             B. Carpenter
Updates: 2205, 2460                                    Univ. of Auckland
Category: Standards Track                                       S. Jiang
ISSN: 2070-1721                                                   Huawei
                                                            J. Rajahalme
                                                  Nokia Siemens Networks
                                                           November 2011


                     IPv6 Flow Label Specification

Abstract

This document specifies the IPv6 Flow Label field and the minimum requirements for IPv6 nodes labeling flows, IPv6 nodes forwarding labeled packets, and flow state establishment methods. Even when mentioned as examples of possible uses of the flow labeling, more detailed requirements for specific use cases are out of the scope for this document. The usage of the Flow Label field enables efficient IPv6 flow classification based only on IPv6 main header fields in fixed positions. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in 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 http://www.rfc-editor.org/info/rfc6437.
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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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. IPv6 Flow Label Specification . . . . . . . . . . . . . . . . 4 3. Flow Labeling Requirements in the Stateless Scenario . . . . . 5 4. Flow State Establishment Requirements . . . . . . . . . . . . 7 5. Essential Correction to RFC 2205 . . . . . . . . . . . . . . . 7 6. Security Considerations . . . . . . . . . . . . . . . . . . . 7 6.1. Covert Channel Risk . . . . . . . . . . . . . . . . . . . 8 6.2. Theft and Denial of Service . . . . . . . . . . . . . . . 8 6.3. IPsec and Tunneling Interactions . . . . . . . . . . . . . 10 6.4. Security Filtering Interactions . . . . . . . . . . . . . 11 7. Differences from RFC 3697 . . . . . . . . . . . . . . . . . . 11 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12 9.1. Normative References . . . . . . . . . . . . . . . . . . . 12 9.2. Informative References . . . . . . . . . . . . . . . . . . 12 Appendix A. Example 20-Bit Hash Function . . . . . . . . . . . . 14
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1. Introduction

From the viewpoint of the network layer, a flow is a sequence of packets sent from a particular source to a particular unicast, anycast, or multicast destination that a node desires to label as a flow. From an upper-layer viewpoint, a flow could consist of all packets in one direction of a specific transport connection or media stream. However, a flow is not necessarily 1:1 mapped to a transport connection. Traditionally, flow classifiers have been based on the 5-tuple of the source address, destination address, source port, destination port, and the transport protocol type. However, some of these fields may be unavailable due to either fragmentation or encryption, or locating them past a chain of IPv6 extension headers may be inefficient. Additionally, if classifiers depend only on IP-layer headers, later introduction of alternative transport-layer protocols will be easier. The usage of the 3-tuple of the Flow Label, Source Address, and Destination Address fields enables efficient IPv6 flow classification, where only IPv6 main header fields in fixed positions are used. The flow label could be used in both stateless and stateful scenarios. A stateless scenario is one where any node that processes the flow label in any way does not need to store any information about a flow before or after a packet has been processed. A stateful scenario is one where a node that processes the flow label value needs to store information about the flow, including the flow label value. A stateful scenario might also require a signaling mechanism to inform downstream nodes that the flow label is being used in a certain way and to establish flow state in the network. For example, RSVP [RFC2205] and General Internet Signaling Transport (GIST) [RFC5971] can signal flow label values. The flow label can be used most simply in stateless scenarios. This specification concentrates on the stateless model and how it can be used as a default mechanism. Details of stateful models, signaling, specific flow state establishment methods, and their related service models are out of scope for this specification. The basic requirement for stateful models is set forth in Section 4. The minimum level of IPv6 flow support consists of labeling the flows. A specific goal is to enable and encourage the use of the flow label for various forms of stateless load distribution, especially across Equal Cost Multi-Path (ECMP) and/or Link Aggregation Group (LAG) paths. ECMP and LAG are methods to bond together multiple physical links used to procure the required
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   capacity necessary to carry an offered load greater than the
   bandwidth of an individual physical link.  Further details are in a
   separate document [RFC6438].  IPv6 source nodes SHOULD be able to
   label known flows (e.g., TCP connections and application streams),
   even if the node itself does not require any flow-specific treatment.
   Node requirements for stateless flow labeling are given in Section 3.

   This document replaces [RFC3697] and Section 6 and Appendix A of
   [RFC2460].  A rationale for the changes made is documented in
   [RFC6436].  The present document also includes a correction to
   [RFC2205] concerning the flow label.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

2. IPv6 Flow Label Specification

The 20-bit Flow Label field in the IPv6 header [RFC2460] is used by a node to label packets of a flow. A Flow Label of zero is used to indicate packets that have not been labeled. Packet classifiers can use the triplet of Flow Label, Source Address, and Destination Address fields to identify the flow to which a particular packet belongs. Packets are processed in a flow-specific manner by nodes that are able to do so in a stateless manner or that have been set up with flow-specific state. The nature of the specific treatment and the methods for flow state establishment are out of scope for this specification. Flow label values should be chosen such that their bits exhibit a high degree of variability, making them suitable for use as part of the input to a hash function used in a load distribution scheme. At the same time, third parties should be unlikely to be able to guess the next value that a source of flow labels will choose. In statistics, a discrete uniform distribution is defined as a probability distribution in which each value in a given range of equally spaced values (such as a sequence of integers) is equally likely to be chosen as the next value. The values in such a distribution exhibit both variability and unguessability. Thus, as specified in Section 3, an approximation to a discrete uniform distribution is preferable as the source of flow label values. Intentionally, there are no precise mathematical requirements placed on the distribution or the method used to achieve such a distribution.
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   Once set to a non-zero value, the Flow Label is expected to be
   delivered unchanged to the destination node(s).  A forwarding node
   MUST either leave a non-zero flow label value unchanged or change it
   only for compelling operational security reasons as described in
   Section 6.1.

   There is no way to verify whether a flow label has been modified en
   route or whether it belongs to a uniform distribution.  Therefore, no
   Internet-wide mechanism can depend mathematically on unmodified and
   uniformly distributed flow labels; they have a "best effort" quality.
   Implementers should be aware that the flow label is an unprotected
   field that could have been accidentally or intentionally changed en
   route (see Section 6).  This leads to the following formal rule:

   o  Forwarding nodes such as routers and load distributors MUST NOT
      depend only on Flow Label values being uniformly distributed.  In
      any usage such as a hash key for load distribution, the Flow Label
      bits MUST be combined at least with bits from other sources within
      the packet, so as to produce a constant hash value for each flow
      and a suitable distribution of hash values across flows.
      Typically, the other fields used will be some or all components of
      the usual 5-tuple.  In this way, load distribution will still
      occur even if the Flow Label values are poorly distributed.

   Although uniformly distributed flow label values are recommended
   below, and will always be helpful for load distribution, it is unsafe
   to assume their presence in the general case, and the use case needs
   to work even if the flow label value is zero.

   As a general practice, packet flows should not be reordered, and the
   use of the Flow Label field does not affect this.  In particular, a
   Flow label value of zero does not imply that reordering is
   acceptable.

3. Flow Labeling Requirements in the Stateless Scenario

This section defines the minimum requirements for methods of setting the flow label value within the stateless scenario of flow label usage. To enable Flow-Label-based classification, source nodes SHOULD assign each unrelated transport connection and application data stream to a new flow. A typical definition of a flow for this purpose is any set of packets carrying the same 5-tuple {dest addr, source addr, protocol, dest port, source port}. It should be noted that a source node always has convenient and efficient access to this 5-tuple, which is not always the case for nodes that subsequently forward the packet.
Top   ToC   RFC6437 - Page 6
   It is desirable that flow label values should be uniformly
   distributed to assist load distribution.  It is therefore RECOMMENDED
   that source hosts support the flow label by setting the flow label
   field for all packets of a given flow to the same value chosen from
   an approximation to a discrete uniform distribution.  Both stateful
   and stateless methods of assigning a value could be used, but it is
   outside the scope of this specification to mandate an algorithm.  The
   algorithm SHOULD ensure that the resulting flow label values are
   unique with high probability.  However, if two simultaneous flows are
   assigned the same flow label value by chance and have the same source
   and destination addresses, it simply means that they will receive the
   same flow label treatment throughout the network.  As long as this is
   a low-probability event, it will not significantly affect load
   distribution.

   A possible stateless algorithm is to use a suitable 20-bit hash of
   values from the IP packet's 5-tuple.  A simple example hash function
   is described in Appendix A.

   An alternative approach is to use a pseudo-random number generator to
   assign a flow label value for a given transport session; such a
   method will require minimal local state to be kept at the source node
   by recording the flow label associated with each transport socket.

   Viewed externally, either of these approaches will produce values
   that appear to be uniformly distributed and pseudo-random.

   An implementation in which flow labels are assigned sequentially is
   NOT RECOMMENDED, as it would then be simple for on-path observers to
   guess the next value.

   A source node that does not otherwise set the flow label MUST set its
   value to zero.

   A node that forwards a flow whose flow label value in arriving
   packets is zero MAY change the flow label value.  In that case, it is
   RECOMMENDED that the forwarding node sets the flow label field for a
   flow to a uniformly distributed value as just described for source
   nodes.

   o  The same considerations apply as to source hosts setting the flow
      label; in particular, the preferred case is that a flow is defined
      by the 5-tuple.  However, there are cases in which the complete
      5-tuple