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Versions: 00 01 02                                                      
Internet Engineering Task Force                                 S. Blake
Internet-Draft                                          Extreme Networks
Intended status: Standards Track                        October 27, 2008
Expires: April 30, 2009


         Use of the IPv6 Flow Label as a Transport-Layer Nonce
              to Defend Against Off-Path Spoofing Attacks
                  draft-blake-ipv6-flow-label-nonce-00

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on April 30, 2009.

Abstract

   TCP and other transport-layer protocols are vulnerable to spoofing
   attacks from off-path hosts.  These attacks can be prevented through
   the use of cryptographic authentication.  However, it is difficult to
   use cryptographic authentication in all circumstances.  A variety of
   obfuscation techniques -- such as initial sequence number
   randomization and source port randomization -- increase the effort
   required of an attacker to successfully spoof the packet header
   fields which uniquely identify a transport connection.  This memo
   proposes the use of the IPv6 Flow Label field as a random, per-
   connection nonce value, to add entropy to the set of packet header
   fields used to identify a transport connection.  This mechanism is



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   easily implementable, allows for incremental deployment, and is fully
   compliant with the rules for Flow Label use defined in RFC 3697.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements Language  . . . . . . . . . . . . . . . . . . . .  5
   3.  Additional Requirements on Flow Label Value Generation and
       Use  . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  TCP Considerations . . . . . . . . . . . . . . . . . . . . . .  7
   5.  UDP Considerations . . . . . . . . . . . . . . . . . . . . . .  8
   6.  SCTP Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   7.  DCCP Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   8.  RTP Considerations . . . . . . . . . . . . . . . . . . . . . . 10
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     12.1.  Normative References  . . . . . . . . . . . . . . . . . . 10
     12.2.  Informative References  . . . . . . . . . . . . . . . . . 11
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
   Intellectual Property and Copyright Statements . . . . . . . . . . 14




























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1.  Introduction

   Recent effort has been directed towards identifying and reducing the
   vulnerability of TCP [RFC0793] to a variety of "blind" spoofed packet
   injection attacks from hosts that are off-path (i.e., not able to
   intercept communications between a pair of hosts) [RFC4953][RFC5082]
   [I-D.ietf-tcpm-icmp-attacks] [I-D.ietf-tcpm-tcpsecure]
   [I-D.ietf-tsvwg-port-randomization].  Off-path spoofing attacks
   against TCP require an attacker to correctly guess the 4-tuple of
   header fields <IP source address, TCP source port, IP destination
   address, TCP destination port> uniquely identifying a TCP connection,
   along with a valid (in-receive window) value for the 32-bit TCP
   sequence number.  By correctly guessing values for these fields, an
   attacker is then able to inject ACK, DATA, RST, of SYN segments into
   a TCP connection, enabling throughput reduction, data corruption, or
   connection termination.  Similarly, by correctly guessing values for
   these fields, an attacker is able to forge ICMP messages to a sender,
   with similar negative consequences [I-D.ietf-tcpm-icmp-attacks].

   Increased use of long-duration connections by applications, as well
   as faster access link speeds, increase the ability of attackers to
   transmit a sufficient number of spoof packets to successfully attack
   a connection, especially when either the destination or source ports
   are easily guessable.  Cryptographic authentication mechanisms such
   as the TCP MD5 Authentication Option [RFC2385], TCP Authentication
   Option [I-D.ietf-tcpm-tcp-auth-opt], and IPsec [RFC4301] can secure
   against these attacks, as well as some on-path (man-in-the-middle)
   attacks.  However, key management and computational overhead makes
   the deployment of cryptographic authentication prohibitively
   expensive in some environments and applications.

   Obfuscation techniques can be employed to increase the effort
   required of an attacker.  Initial sequence number randomization
   [RFC1948] [I-D.ietf-tcpm-tcpsecure] can be implemented by both the
   client (the host initiating a connection) and server.  For typical
   window sizes of approximately 32 Kbytes, this technique forces an
   attacker to send approximately 57000 RST packets on average to reset
   a connection [RFC4953].  Source port randomization
   [I-D.ietf-tsvwg-port-randomization] can also be implemented by a
   client to increase the number of guesses an attacker must make to
   successfully attack a connection.  This mechanism can provide an
   additional ~15 bits of entropy (depending on implementation).  Source
   port randomization can also be used with other transport protocols.

   Both obfuscation schemes are compliant with [RFC0793], and are
   incrementally deployable.  Both schemes used in combination introduce
   somewhat less than 32 bits of entropy (~16 + ~15).  However, as
   access link speeds (and consequently, receive windows) increase in



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   size, the amount of entropy declines just as the number of spoof
   packets an attacker can generate in a given interval increases.
   [I-D.weaver-dnsext-comprehensive-resolver] argues that 40 bits of
   entropy is desirable to make off-path spoofing attacks impractical.

   IPv6 [RFC2460] includes a 20-bit Flow Label field, which can be used
   by hosts to uniquely label a sequence of packets from a host to a
   particular unicast, anycast, or multicast destination.  The tuple of
   <IP source address, IP destination address, Flow Label> uniquely
   identify a particular flow during its lifetime plus a subsequent
   quarantine period.  Rules for the generation and usage of Flow Label
   values are defined in [RFC3697].  Because transport-layer port fields
   may be located at a variable offset within a packet due to IPv6
   extension headers, or may be obscured due to encryption, the Flow
   Label provides a common field in the IPv6 header to facilitate flow
   classification in routers.

   While originally intended to facilitate flow-specific packet handling
   in routers (e.g., QoS, fast switching), the Flow Label can also be
   used by hosts to uniquely label one or more transport connections.
   An originating host can select a random Flow Label value at the
   beginning of a connection, and continue to use it for the
   connection's duration.  The host (or hosts for multicast) at the
   other end of the connection can record this Flow Label value, and use
   it as part of the connection demultiplexing key, while also labeling
   response packets with the same or different Flow Label value.  The
   originating host can similarly record the Flow Label value in
   response packets, and use it as part of its connection demultiplexing
   key.  In this way an additional 20 bits of entropy is added to the
   set of header fields used to identify a transport connection.  When
   used in addition to source port randomization, the total amount of
   entropy is approximately 34-35 bits.  When initial sequence number
   randomization is also used (i.e., in TCP), the entropy is increased
   to > 40 bits, making off-path snooping attacks impractical.

   The concept of labeling transport connections to prevent off-path
   spoofing attacks was proposed in [McGann05], in the context of
   stateful firewalls.  This scheme may be useful for other transport
   protocols such as SCTP [RFC4960], UDP [RFC0768], UDP-Lite [RFC3828],
   DCCP [RFC4340], and RTP [RFC3550].  Host implementations in
   compliance with [RFC3697] which do not allocate multiple flows to a
   single transport connection, will either label all packets in the
   connection with a Flow Label value of 0, or some other constant.
   Therefore, this scheme is incrementally deployable by either peer in
   a connection.

   Section 3 talks about additional requirements on Flow Label
   generation.  Section 4 talks about the use of this scheme with TCP.



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   Section 6 talks about the use of this scheme with SCTP.  Section 5
   talks about the use of this scheme with UDP and UDP-Lite.  Section 7
   talks about the use of this scheme with DCCP.  Section 8 talks about
   the use of this scheme with RTP over UDP or DCCP.


2.  Requirements Language

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


3.  Additional Requirements on Flow Label Value Generation and Use

   [RFC3697] specifies the rules governing use of the IPv6 Flow Label.
   The primary requirements relevant to our purpose are as follows
   (quoting directly):

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

   o  To enable Flow Label based classification, source nodes SHOULD
      assign each unrelated transport connection and application data
      stream to a new flow.

   o  The source node SHOULD be able to select unused Flow Label values
      for flows not requesting a specific value to be used.

   o  A source node MUST ensure that it does not unintentionally reuse
      Flow Label values it is currently using or has recently used when
      creating new flows.

   o  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.

   o  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.

   o  To avoid accidental Flow Label value reuse, the source node SHOULD
      select the new Flow Label value 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



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      history, a randomly generated initial value using techniques that
      produce good randomness properties SHOULD be used.

   We wish to use the Flow Label value as an unguessable nonce.  Hence,
   the following additional requirements are implied:

   o  Source hosts MUST assign each unrelated transport connection and
      application data stream to a new flow.

   o  Source hosts MUST be able to select unused Flow Label values for
      flows not requesting a specific value to be used.  The selected
      Flow Label value must remain constant for the duration of the
      flow.

   o  The Flow Label value MUST be practically unguessable, in a manner
      similar to the TCP source port or initial sequence number when
      they are randomized.  A random number generator with good
      randomness properties (i.e., uniformly distributed) as specified
      in [RFC4086] MUST be used to generate Flow Label values not
      explicitly requested by an application.

   o  Flow Label state for a transport connection or application data
      stream must be cleaned-up by the destination host(s) as part of
      the transport connection/application data stream state clean-up.

   o  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 X seconds of the termination of the
      previous flow, where X is the maximum of either 120 seconds, or
      the duration for which transport connection state might linger at
      a destination host after traffic flow has ceased (e.g., TIME-WAIT
      state in TCP).

   For this application of the Flow Label field, it would not pose a
   problem if multiple flows from a source host in unrelated transport
   connections/application data streams coincidentally shared the same
   Flow Label value, as long as the other previous requirements are
   adhered to.  However, the prohibition in [RFC3697] against
   simultaneous reuse of Flow Label values MUST be observed.

   Transport-specific requirements on Flow Label use are provided in the
   subsequent sections.  However, as a general requirement, if a packet
   is received on a transport connection/application data stream with an
   unexpected Flow Label value, the packet MUST be silently discarded.
   If excessive Flow Label errors are received, the event SHOULD be
   logged.

   ICMPv6 error messages contain the IPv6 header of the packet



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   triggering the error [RFC4443].  A host receiving an ICMPv6 error
   message can validate the Flow Label value in the message payload to
   protect against ICMPv6 spoofing attacks [I-D.ietf-tcpm-icmp-attacks].

   Use of the Flow Label value as an unguessable nonce is incrementally
   deployable, whether a source host fails to support setting the Flow
   Label to a non-zero value, or a destination host fails to check its
   value.  However, a Flow Label value of 0 is easily guessable, so
   resistance to spoofing attacks is not improved.  Hosts SHOULD NOT
   rely on the mechanisms defined in this document when operating in
   high-threat environments.

   The additional requirements given here for Flow Label generation and
   use are not in conflict with the requirements in [RFC3697].
   Therefore, additional applications of the Flow Label field (e.g.,
   special QoS handling) can be applied simultaneously with the use of
   the Flow Label field as a transport-layer nonce.


4.  TCP Considerations

   Unidirectional traffic in a TCP connection is assumed to constitute a
   single flow, and hence MUST be assigned a unique Flow Label value by
   the source host.  A single TCP connection MUST NOT be treated by a
   source host as consisting of multiple flows.  Given the Flow Label
   value's additional use as a packet classification field in routers
   (for QoS or other purposes), there is no compelling reason to sub-
   divide traffic within a TCP connection for classification purposes.

   For this application of the Flow Label field, it would not pose a
   problem if multiple TCP connections from a source host to a
   particular destination host reused the same Flow Label value.
   However, because of the additional uses of the Flow Label field, a
   host MUST NOT assign the same Flow Label value to multiple TCP
   connections.  It is not assumed that both directions of traffic flow
   in a TCP connection must share the same Flow Label value, nor it is
   prohibited.

   A host originating a TCP connection (client) selects a unique Flow
   Label value for the connection, which it stores as the
   OUTGOING_FLOW_ID in its Transport Control Block (TCB) for this
   connection.  This Flow Label value is included in the first (and
   subsequent) SYN packet(s) sent to the destination host (server).  The
   server receiving the first SYN packet records the Flow Label value in
   its TCB for this connection as the INCOMING_FLOW_ID.  The server then
   selects a unique Flow Label value for the connection, which it stores
   as the OUTGOING_FLOW_ID in the connection's TCB.  It includes this
   Flow Label value in the first (and subsequent) SYN-ACK packet(s) sent



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   to the client.  The client receiving the SYN-ACK packet from the
   server records the Flow Label value in its TCB for this connection as
   the INCOMING_FLOW_ID.  It sends all additional packets of the
   connection to the server using OUTGOING_FLOW_ID, and checks all
   incoming packets of the connection from the server to ensure that
   they include INCOMING_FLOW_ID.  The server performs identical
   processing.  Any packets received with a Flow Label value that does
   not match INCOMING_FLOW_ID MUST be silently discarded.  If a
   significant number of such packets are received, the event SHOULD be
   logged.

   When a server implements a SYN cache and/or SYN cookies, the Flow
   Label value used in the SYN-ACK packet MUST be consistent with the
   Flow Label value used in subsequent packets [RFC4987].  For the SYN
   cache case, this can be handled easily by including INCOMING_FLOW_ID
   and OUTGOING_FLOW_ID as part of each cache entry.  For SYN cookies,
   one approach to satisfying the requirement without storing state is
   to use the Flow Label value received in the SYN (INCOMING_FLOW_ID) as
   the Flow Label value in the SYN-ACK (OUTGOING_FLOW_ID).  This
   approach leaves a window of vulnerability to spoofing before the
   connection is established.

   [RFC0793] specifies that a connection should remain in TIME_WAIT
   state for 2 * MSL (Maximum Segment Lifetime) seconds.  [RFC0793]
   specifies MSL as 120 seconds, although many implementations default
   to a lower value.  The Flow Label value quarantine period MUST be no
   less than the maximum of either 2 * MSL for the connection, or 120
   seconds.

   The specified behavior at the client and server will work even if
   either the client or server fails to set a non-zero outgoing Flow
   Label value, or check the incoming Flow Label value.  However,
   resistance to spoofing attacks is not improved.  Further, no
   mechanism for detecting whether a peer is supporting the Flow Label
   nonce is defined.  Therefore, some cryptographic authentication
   mechanism SHOULD BE used when operating in a high-threat environment
   [RFC2385][I-D.ietf-tcpm-tcp-auth-opt] [RFC4301].


5.  UDP Considerations

   UDP is a connectionless protocol, which is also vulnerable to
   spoofing attacks.  Source port randomization can be utilized with
   UDP, but UDP does not have sequence numbers, so it is arguably more
   vulnerable than TCP with source port and initial sequence number
   randomization.  With the exception of connected sockets, UDP/IP
   stacks (usually) do not maintain sufficient state to maintain
   INCOMING_FLOW_ID or OUTGOING_FLOW_ID values for each application data



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   stream between a source host and a destination host or multicast
   group.  Therefore, Flow Label generation and validation must happen
   at the application layer.

   For purposes of discussion, we define a UDP connection as a flow of
   traffic matching the tuple <IP source address, UDP source port, IP
   destination/group address, UDP destination port>.  Note that a UDP
   connection consists of unidirectional traffic flow between a pair of
   hosts, or between a host and the receivers of a multicast group.  UDP
   applications SHOULD assign each connection to a unique flow, and
   hence SHOULD assign each connection a unique Flow Label value.  One
   exception is where multiple application data streams are multiplexed
   onto the same address/port pairs.  In this case UDP applications MUST
   assign application data streams to unique flows (as appropriate for
   the intended QoS or other handling), and MUST use application-layer
   demultiplexing to associate incoming data streams with flows.
   Maintenance of INCOMING_FLOW_ID and OUTGOING_FLOW_ID values for each
   flow MUST be provided by the application.  Note that an alternative
   to multiplexing multiple application data streams onto the same
   address/port pair is to utilize different source and/or destination
   port values for each data stream.

   There is no standard sockets API for either setting the Flow Label
   value in outgoing packets, nor retrieving it in incoming packets
   [RFC3493][RFC3542].  There is also no standard sockets API for
   specifying that a non-zero Flow Label value be used in outgoing
   packets.  Therefore the requirements above cannot be satisfied,
   except where a non-standard API is available, or the functionality is
   provided automatically within the UDP/IP stack.  It would be
   worthwhile to define a standard sockets API for Flow Label
   management.

   One application where the use of the Flow Label as a nonce might be
   beneficial is in protection against DNS cache poisoning attacks
   [I-D.weaver-dnsext-comprehensive-resolver].  If DNS clients assign
   each DNS transaction to a unique flow, and if DNS servers sends
   responses with an outgoing Flow Label value equal to the incoming
   Flow Label value received in the request, then the client can
   validate with high-probability that the request was generated by the
   targeted server, since the UDP source port, DNS transaction ID, and
   Flow Label provide approximately 51 bits of entropy.

   Use of the Flow Label with UDP-Lite will be discussed in a subsequent
   revision of this document.







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6.  SCTP Considerations

   Use of the Flow Label with SCTP will be discussed in a subsequent
   revision of this document.


7.  DCCP Considerations

   Use of the Flow Label with DCCP will be discussed in a subsequent
   revision of this document.


8.  RTP Considerations

   Use of the Flow Label with RTP applications will be discussed in a
   subsequent revision of this document.


9.  Acknowledgements

   [McGann05] proves that the concept of using the Flow Label as a
   transport-layer nonce pre-dates this document (although the author
   only discovered this paper after the writing of this document
   commenced).  If others are aware of when and where this concept might
   have been discussed previously, please contact the author.

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


10.  IANA Considerations

   This memo includes no request to IANA.


11.  Security Considerations

   All drafts are required to have a security considerations section.


12.  References

12.1.  Normative References

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              August 1980.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.



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

   [RFC1948]  Bellovin, S., "Defending Against Sequence Number Attacks",
              RFC 1948, May 1996.

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

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

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

   [RFC3828]  Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., and
              G. Fairhurst, "The Lightweight User Datagram Protocol
              (UDP-Lite)", RFC 3828, July 2004.

   [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
              Requirements for Security", BCP 106, RFC 4086, June 2005.

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340, March 2006.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006.

   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol",
              RFC 4960, September 2007.

12.2.  Informative References

   [RFC2385]  Heffernan, A., "Protection of BGP Sessions via the TCP MD5
              Signature Option", RFC 2385, August 1998.

   [RFC2629]  Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
              June 1999.

   [RFC3493]  Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
              Stevens, "Basic Socket Interface Extensions for IPv6",
              RFC 3493, February 2003.

   [RFC3542]  Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
              "Advanced Sockets Application Program Interface (API) for
              IPv6", RFC 3542, May 2003.



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   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC4953]  Touch, J., "Defending TCP Against Spoofing Attacks",
              RFC 4953, July 2007.

   [RFC4987]  Eddy, W., "TCP SYN Flooding Attacks and Common
              Mitigations", RFC 4987, August 2007.

   [RFC5082]  Gill, V., Heasley, J., Meyer, D., Savola, P., and C.
              Pignataro, "The Generalized TTL Security Mechanism
              (GTSM)", RFC 5082, October 2007.

   [I-D.ietf-tcpm-icmp-attacks]
              Gont, F., "ICMP attacks against TCP",
              draft-ietf-tcpm-icmp-attacks-03 (work in progress),
              March 2008.

   [I-D.ietf-tcpm-tcpsecure]
              Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's
              Robustness to Blind In-Window Attacks",
              draft-ietf-tcpm-tcpsecure-10 (work in progress),
              July 2008.

   [I-D.ietf-tsvwg-port-randomization]
              Larsen, M. and F. Gont, "Port Randomization",
              draft-ietf-tsvwg-port-randomization-02 (work in progress),
              August 2008.

   [I-D.ietf-tcpm-tcp-auth-opt]
              Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", draft-ietf-tcpm-tcp-auth-opt-01
              (work in progress), July 2008.

   [I-D.weaver-dnsext-comprehensive-resolver]
              Weaver, N., "Comprehensive DNS Resolver Defenses Against
              Cache Poisoning",
              draft-weaver-dnsext-comprehensive-resolver-00 (work in
              progress), September 2008.

   [McGann05]
              McGann, O. and D. Malone, "Flow Label Filtering
              Feasibility", European Conference on Computer Network
              Defence, December 2005.







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Author's Address

   Steven Blake
   Extreme Networks
   Pamlico Building One, Suite 100
   3306/08 E. NC Hwy 54
   RTP, NC  27709
   USA

   Phone: +1 919 884 3211
   Email: sblake@extremenetworks.com








































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Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
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