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Versions: (draft-rajahalme-ipv6-flow-label) 00 01 02 03 04 05 06 07 08 09 RFC 3697

IPv6 Working Group                                          J. Rajahalme
INTERNET-DRAFT                                                     Nokia
<draft-ietf-ipv6-flow-label-05.txt>                             A. Conta
                                                            B. Carpenter
                                                              S. Deering
Expires: August 2003                                       February 2003

                     IPv6 Flow Label Specification

Status of this memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

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   This document specifies the IPv6 Flow Label field, the requirements
   for IPv6 source nodes labeling flows, the requirements for IPv6 nodes
   forwarding labeled packets, and the requirements for flow state
   establishment methods.

   The usage of the Flow Label field enables efficient IPv6 flow
   classification based only on IPv6 main header fields in fixed

1.  Introduction

   A flow is a sequence of packets sent from a particular source to a
   particular unicast, anycast or multicast destination that the source
   desires to label as a flow. A flow could consist of all packets in a
   specific transport connection or a media stream. However, a flow is
   not necessarily 1:1 mapped to a transport connection.

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   Traditionally, flow classifiers have been based on the 5-tuple of the
   source and destination addresses, ports 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
   option headers may be inefficient. Additionally, if classifiers
   depend only on IP layer headers, later introduction of alternative
   transport layer protocols will be easier.

   The 3-tuple of the Flow Label and the Source and Destination Address
   fields enables efficient IPv6 flow classification, where only IPv6
   main header fields in fixed positions are used.

   The minimum level of IPv6 flow support consists of labeling the
   flows. IPv6 source nodes can label known flows (e.g. TCP connections,
   application streams), even if the node itself would not require any
   flow-specific treatment. Doing this enables load spreading and
   receiver oriented resource reservations, for example. Node
   requirements for flow labeling are given in section 3.

   Specific flow state establishment methods and the related service
   models are out of scope for this specification, but the generic
   requirements enabling co-existence of different methods in IPv6 nodes
   are set forth in section 4.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [KEYWORDS].

2.  IPv6 Flow Label Specification

   The 20-bit Flow Label field in the IPv6 header [IPv6] is used by a
   source to label packets of a flow. A Flow Label of zero is used to
   indicate packets not part of any flow. Packet classifiers use the
   triplet of Flow Label, Source Address, and Destination Address fields
   to identify which flow a particular packet belongs to. Packets are
   processed in a flow-specific manner by the nodes that have been set
   up with flow-specific state. The nature of the specific treatment and
   the methods for the flow state establishment are out of scope for
   this specification.

   The Flow Label value set by the source MUST be delivered unchanged to
   the destination node(s).

   IPv6 nodes MUST NOT assume any mathematical or other properties of
   the Flow Label values assigned by source nodes. 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

   Nodes keeping dynamic flow state MUST NOT assume packets arriving 120
   seconds or more after the previous packet of a flow still belong to
   the same flow, unless a flow state establishment method in use

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   defines a longer flow state lifetime or the flow state has been
   explicitly refreshed within the lifetime duration.

   If an IPv6 node is not providing flow-specific treatment, it MUST
   ignore the field when receiving or forwarding a packet.

3.  Flow Labeling Requirements

   To enable Flow Label based classification, source nodes SHOULD assign
   each unrelated transport connection and application data stream to a
   new flow. The source node MAY also take part in flow state
   establishment methods that result in assigning certain packets to
   specific flows. A source node which does not assign traffic to flows
   MUST set the Flow Label to zero.

   To enable applications and transport protocols to define what packets
   constitute a flow, the source node MUST provide means for the
   applications and transport protocols to specify the Flow Label values
   to be used with their flows. The source node SHOULD be able to select
   unused Flow Label values for flows not requesting a specific value to
   be used.

   A source node MUST ensure that it does not reuse Flow Label values it
   is currently using or has recently used when creating new flows. Flow
   Label values previously used with a specific pair of source and
   destination addresses MUST NOT be assigned to new flows with the same
   address pair within 120 seconds of the termination of the previous
   flow. The source node SHOULD provide the means for the applications
   and transport protocols to specify quarantine periods longer than the
   default 120 seconds for individual flows.

   To avoid accidental Flow Label value reuse, the source node SHOULD
   select new Flow Label values in a well-defined sequence (e.g.
   sequential or pseudo-random) and use an initial value that avoids
   reuse of recently used Flow Label values each time the system
   restarts. The initial value SHOULD be derived from a previous value
   stored in non-volatile memory, or in the absence of such history, a
   randomly generated initial value using techniques that produce good
   randomness properties [RND] SHOULD be used.

4.  Flow State Establishment Requirements

   To enable flow-specific treatment, flow state needs to be established
   on all or a subset of the IPv6 nodes on the path from the source to
   the destination(s). The methods for the state establishment, as well
   as the models for flow-specific treatment will be defined in separate

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   To enable co-existence of different methods in IPv6 nodes, the
   methods MUST meet the following basic requirements:

   (1)  The method MUST provide the means for flow state clean-up from
        the IPv6 nodes providing the flow-specific treatment. Signaling
        based methods where the source node is involved are free to
        specify flow state lifetimes longer than the default 120

   (2)  Flow state establishment methods MUST be able to recover from
        the case where the requested flow state cannot be supported.

5.  Security Considerations

   This section considers security issues raised by the use of the Flow
   Label, primarily the potential for denial-of-service attacks, and the
   related potential for theft of service by unauthorized traffic
   (Section 5.1). Section 5.2 addresses the use of the Flow Label in the
   presence of IPsec including its interaction with IPsec tunnel mode
   and other tunneling protocols. We also note that inspection of
   unencrypted Flow Labels may allow some forms of traffic analysis by
   revealing some structure of the underlying communications.

5.1  Theft and Denial of Service

   The goal of the Flow Label is to allow different levels of service to
   be provided for traffic streams on a common network infrastructure. A
   variety of techniques may be used to achieve this, but the end result
   will be that some packets receive different (e.g., better or worse)
   service than others. The mapping of network traffic to the flow-
   specific treatment is triggered by the IP addresses and Flow Label
   value of the IPv6 header, and hence an adversary may be able to
   obtain better service by modifying the IPv6 header or by injecting
   packets with false addresses and labels. Taken to its limits, such
   theft-of-service becomes a denial-of-service attack when the modified
   or injected traffic depletes the resources available to forward it
   and other traffic streams.

   Since flows are identified by the 3-tuple of the Flow Label and the
   Source and Destination Address, the risk of theft or denial of
   service introduced by the Flow Label is closely related to the risk
   of theft or denial of service by address spoofing. An adversary who
   is in a position to forge an address is also likely to be able to
   forge a label, and vice versa.

   Forging a non-zero Flow Label on packets that originated with a zero
   label, or modifying or clearing a label, could only occur if an
   intermediate system such as a router was compromised, or through some
   other form of man-in-the-middle attack. However, the risk is limited
   to traffic receiving better or worse quality of service than
   intended. For example, if Flow Labels are altered or cleared at

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   random, flow classification will no longer happen as intended, and
   the altered packets will receive default treatment. If a complete 3-
   tuple is forged, the altered packets will be classified into the
   forged flow and will receive the corresponding quality of service;
   this will create a denial of service attack subtly different from one
   where only the addresses are forged. Because it is limited to a
   single flow definition, e.g. to a limited amount of bandwidth, such
   an attack will be more specific and at a finer granularity than a
   normal address-spoofing attack.

   Since flows are identified by the complete 3-tuple, ingress filtering
   [INGR] will mitigate part of the risk. If the source address of a
   packet is validated by ingress filtering, there can be a degree of
   trust that the packet has not transited a compromised router, to the
   extent that ISP infrastructure may be trusted. However, this gives no
   assurance that another form of man-in-the-middle attack has not

   Applications in a sending host may or may not be entitled to set a
   non-zero Flow Label. Policy and authorization mechanisms for this may
   be required; for example, in a multi-user host, only some users may
   be entitled to set the Flow Label. Such authorization issues are
   outside the scope of this specification.

5.2  IPsec and Tunneling Interactions

   The IPsec protocol, as defined in [IPSec, AH, ESP], does not include
   the IPv6 header's Flow Label in any of its cryptographic calculations
   (in the case of tunnel mode, it is the outer IPv6 header's Flow Label
   that is not included). Hence modification of the Flow Label by a
   network node has no effect on IPsec end-to-end security, because it
   cannot cause any IPsec integrity check to fail. As a consequence,
   IPsec does not provide any defense against an adversary's
   modification of the Flow Label (i.e., a man-in-the-middle attack).

   IPsec tunnel mode provides security for the encapsulated IP header's
   Flow Label. A tunnel mode IPsec packet contains two IP headers: an
   outer header supplied by the tunnel ingress node and an encapsulated
   inner header supplied by the original source of the packet. When an
   IPsec tunnel is passing through nodes performing flow classification,
   the intermediate network nodes operate on the Flow Label in the outer
   header. At the tunnel egress node, IPsec processing includes removing
   the outer header and forwarding the packet (if required) using the
   inner header. The IPsec protocol requires that the inner header's
   Flow Label not be changed by this decapsulation processing to ensure
   that modifications to label cannot be used to launch theft- or
   denial-of-service attacks across an IPsec tunnel endpoint. This
   document makes no change to that requirement; indeed it forbids
   changes to the Flow Label.

   When IPsec tunnel egress decapsulation processing includes a
   sufficiently strong cryptographic integrity check of the encapsulated

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   packet (where sufficiency is determined by local security policy),
   the tunnel egress node can safely assume that the Flow Label in the
   inner header has the same value as it had at the tunnel ingress node.

   This analysis and its implications apply to any tunneling protocol
   that performs integrity checks. Of course, any Flow Label set in an
   encapsulating IPv6 header is subject to the risks described in the
   previous section.


   The discussion on the topic in the IPv6 WG mailing list has been
   instrumental for the definition of this specification. The authors
   want to thank Steve Blake, Jim Bound, Francis Dupont, Robert Elz,
   Tony Hain, Robert Hancock, Bob Hinden, Christian Huitema, Frank
   Kastenholz, Thomas Narten, Charles Perkins, Hesham Soliman, Michael
   Thomas, and Margaret Wasserman for their contributions.

Normative References

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

   [KEYWORDS]  Bradner, S., "Key words for use in RFCs to indicate
               requirement levels", BCP 14, RFC 2119, March 1997.

   [RND]       Eastlake, D., Crocker, S., Schiller, J., "Randomness
               Recommendations for Security", RFC 1750, December 1994.

Informative References

   [AH]        Kent, S., Atkinson, R., "IP Authentication Header", RFC
               2402, November 1998.

   [ESP]       Kent, S., Atkinson, R., "IP Encapsulating Security
               Payload (ESP)", RFC 2406, November 1998.

   [INGR]      Ferguson, P., "Network Ingress Filtering: Defeating
               Denial of Service Attacks which employ IP Source Address
               Spoofing", RFC 2827, May 2000.

   [IPSec]     Kent, S., Atkinson, R., "Security Architecture for the
               Internet Protocol", RFC 2401, November 1998.

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Authors' Addresses

   Jarno Rajahalme
   Nokia Research Center
   P.O. Box 407
   E-mail: jarno.rajahalme@nokia.com

   Alex Conta
   Transwitch Corporation
   3 Enterprise Drive
   Shelton, CT 06484
   Email: aconta@txc.com

   Brian E. Carpenter
   IBM Zurich Research Laboratory
   Saeumerstrasse 4 / Postfach
   8803 Rueschlikon
   Email: brian@hursley.ibm.com

   Steve Deering
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA 95134-1706
   Email: deering@cisco.com

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Expiration Date

   This memo is filed as <draft-ietf-ipv6-flow-label-05.txt> and expires
   in August 2003.

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