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Internet Engineering Task Force (IETF)                         S. AmanteRequest for Comments: 6436                                       Level 3Category: Informational                                     B. CarpenterISSN: 2070-1721                                        Univ. of Auckland                                                                S. Jiang                                                                  Huawei                                                           November 2011Rationale for Update to the IPv6 Flow Label SpecificationAbstract   Various published proposals for use of the IPv6 flow label are   incompatible with its original specification inRFC 3697.   Furthermore, very little practical use is made of the flow label,   partly due to some uncertainties about the correct interpretation of   the specification.  This document discusses and motivates changes to   the specification in order to clarify it and to introduce some   additional flexibility.Status of This Memo   This document is not an Internet Standards Track specification; it is   published for informational purposes.   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).  Not all documents   approved by the IESG are a candidate for any level of Internet   Standard; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6436.Amante, et al.                Informational                     [Page 1]

RFC 6436                    Flow Label Update              November 2011Copyright Notice   Copyright (c) 2011 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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   to this document.  Code Components extracted from this document must   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.Table of Contents1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .22.  Impact of Current Specification  . . . . . . . . . . . . . . .33.  Changes to the Specification . . . . . . . . . . . . . . . . .64.  Discussion . . . . . . . . . . . . . . . . . . . . . . . . . .85.  Security Considerations  . . . . . . . . . . . . . . . . . . .96.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .97.  Informative References . . . . . . . . . . . . . . . . . . . .10Appendix A.  Alternative Approaches  . . . . . . . . . . . . . . .121.  Introduction   The flow label field in the IPv6 header was reserved but left   Experimental by [RFC2460], which mandates only that "Hosts or routers   that do not support the functions of the Flow Label field are   required to set the field to zero when originating a packet, pass the   field on unchanged when forwarding a packet, and ignore the field   when receiving a packet."   The flow label field was normatively specified by [RFC3697].  In   particular, we quote three rules from that RFC:   a.  "The Flow Label value set by the source MUST be delivered       unchanged to the destination node(s)."   b.  "IPv6 nodes MUST NOT assume any mathematical or other properties       of the Flow Label values assigned by source nodes."   c.  "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."Amante, et al.                Informational                     [Page 2]

RFC 6436                    Flow Label Update              November 2011   Additionally,RFC 3697 does not define the method a host should adopt   by default to choose the value of the flow label, if no specific   method is in use.  It was expected that various signaling methods   might be defined for agreeing on values of the flow label, but no   such methods have been standardized, except a pre-existing option in   RSVP [RFC2205].   The flow label is hardly used in practice in widespread IPv6   implementations, although some operating systems do set it   [McGann05].  To some extent, this is due to the main focus being on   basic deployment of IPv6, but the absence of a default method of   choosing the flow label value means that most host implementations   simply set it to zero.  There is also anecdotal evidence that the   rules quoted above have led to uncertainty about exactly what is   possible.  Furthermore, various use cases have been proposed that   infringe one or another of the rules.  None of these proposals has   been accepted as a standard and in practice there is no significant   deployment of any mechanism to set the flow label.   The intention of this document is to explain this situation in more   detail and to motivate changes toRFC 3697 intended to remove the   uncertainties and encourage active usage of the flow label.  It does   not formally updateRFC 3697, but it serves as background material   for [RFC6437].2.  Impact of Current Specification   Rule (a) makes it impossible for the routing system to use the flow   label as any form of dynamic routing tag.  This was a conscious   choice in the early design of IPv6, and there appears to be no   practical possibility of revisiting this decision at this stage in   the deployment of IPv6, which uses conventional routing mechanisms   like those used for IPv4.  However, this rule also makes it   impossible to make any use at all of the flow label unless hosts   choose to set it.  It also forbids clearing the flow label for   security reasons.   This last point highlights the security properties, or rather the   lack thereof, with regards to the flow label.  The flow label field   is always unprotected as it travels through the network, because   there is no IPv6 header checksum, and the flow label is not included   in transport pseudo-header checksums, nor in IPsec checksums.  As a   result, intentional and malicious changes to its value cannot be   detected.  Also, it could be used as a covert data channel, since   apparently pseudo-random flow label values could in fact consist of   covert data [NIST].  If the flow label were to carry quality-of-   service semantics, then like the diffserv code point [RFC2474], it   would not be intrinsically trustworthy across domain boundaries.  AsAmante, et al.                Informational                     [Page 3]

RFC 6436                    Flow Label Update              November 2011   a result, some security specialists believe that flow labels should   be cleared for safety [LABEL-SEC] [NSA].  These points must be   considered when discussing the immutability of the flow label across   domain boundaries.  In fact, the adjective "immutable" is confusing,   since it implies a property that the flow label field does not   actually possess.  It has therefore been abandoned as a descriptive   term in [RFC6437].  It is only used in the present document to   explain why it has been abandoned.   Rule (b) appears to forbid any usage in which the bits of the flow   label are encoded with a specific semantic meaning.  However, the   words "MUST NOT assume" are to be interpreted precisely - if a router   knows by configuration or by signaling that the flow label has been   assigned in a certain way, it can make use of that knowledge.  It is   not made clear by the rule that there is an implied distinction   between stateless models (in which there is no signaling, so no   specific assumption about the meaning of the flow label value can be   made) and stateful models (in which there is signaling and the router   has explicit knowledge about the label).   If the word "alone" is overlooked, rule (c) has sometimes been   interpreted as forbidding the use of the flow label as part of a hash   used by load distribution mechanisms.  In this case too, the word   "alone" needs to be taken into account - a router is allowed to   combine the flow label value with other data in order to produce a   uniformly distributed hash.   Both before and after these rules were laid down, a considerable   number of proposals for use of the flow label were published that   seem incompatible with them.  Numerous examples and an analysis are   presented in [RFC6294].  Those examples propose use cases in which   some or all of the following apply:   o  The flow label may be changed by intermediate systems.   o  It doesn't matter if the flow label is changed, because the      receiver doesn't use it.   o  Some or all bits of the flow label are encoded: they have specific      meanings understood by routers and switches along the path.   o  The encoding is related to the required quality of service, as      well as identifying a flow.   o  The flow label is used to control forwarding or switching in some      way.Amante, et al.                Informational                     [Page 4]

RFC 6436                    Flow Label Update              November 2011   These proposals require either some form of semantics encoding in the   bits of the flow label, or the ability for routers to modify the flow   label, or both.  Thus, they appear to infringe the rules fromRFC3697 quoted above.   We can conclude that a considerable number of researchers and   designers have been stymied byRFC 3697.  On the other hand, some   other proposals discussed in [RFC6294] appear to be compatible withRFC 3697.  Several are based on the originator of a packet choosing a   pseudo-random flow label for each flow, which is one option suggested   inRFC 3697.  Thus, we can also conclude that there is a useful role   for this approach.   If our goal is for the flow label to be used in practice, the   conflict between the various approaches creates a dilemma.  There   appear to be two major options:   1.  Discourage locally defined and/or stateful use of the flow label.       StrengthenRFC 3697 to say that hosts should set a label value,       without necessarily creating state, which would clarify and limit       its possible uses.  In particular, its use for load distribution       and balancing would be encouraged.   2.  Relax the rules to encourage locally defined and/or stateful use       of the flow label.  This approach would make the flow label       completely mutable and would exclude use cases depending on       strict end-to-end immutability.  It would encourage applications       of a pseudo-random flow label, such as load distribution, on a       local basis, but it would exclude end-to-end applications.   There was considerable debate about these options and their variants   during 2010 - 2011, with a variety of proposals in previous versions   of this document and in mailing list discussions.  After these   discussions, there appears to be a view that simplicity should   prevail, and that complicated proposals such as defining quality-of-   service semantics in the flow label, or sub-dividing the flow label   field into smaller sub-fields, will not prove efficient or   deployable, especially in high-speed routers.  There is also a   clearly expressed view that using the flow label for various forms of   stateless load distribution is the best simple application for it.   At the same time, it is necessary to recognize that the strict   immutability rule has drawbacks as noted above.   Even under the rules ofRFC 3697, the flow label is intrinsically   untrustworthy, because modifications en route cannot be detected.   For this reason, even with the current strict immutability rule,   downstream nodes cannot rely mathematically on the value being   unchanged.  In this sense, any use of the flow label must be viewedAmante, et al.                Informational                     [Page 5]

RFC 6436                    Flow Label Update              November 2011   as an optimization on a best-effort basis; a packet with a changed   (or zero) flow label value should never cause a hard failure.   The remainder of this document discusses specific modifications to   the standard, which are defined normatively in a companion document   [RFC6437].3.  Changes to the Specification   AlthoughRFC 3697 requires that the flow label be delivered   unchanged, as noted above, it is not included in any transport-layer   pseudo-header checksums nor in IPsec authentication [RFC4302].  BothRFC 2460 andRFC 3697 define the default flow label to be zero.  At   the time of writing, this is the observed value in an overwhelming   proportion of IPv6 packets; the most widespread operating systems and   applications do not set it, and routers do not rely on it.  Thus,   there is no reason to expect operational difficulties if a careful   change is made to the rules ofRFC 3697.   In particular, the facts that the label is not checksummed and rarely   used mean that the "immutability" of the label can be moderated   without serious operational consequences.   The purposes of the proposed changes are to remove the uncertainties   left byRFC 3697, in order to encourage setting of the flow label by   default, and to enable its generic use.  The proposed generic use is   to encourage uniformly distributed flow labels that can be used to   assist load distribution or balancing.  There should be no impact on   existing IETF specifications other thanRFC 3697 and no impact on   currently operational software and hardware.   A secondary purpose is to allow changes to the flow label in a   limited way, to allow hosts that do not set the flow label to benefit   from it nevertheless.  The fact that the flow label may in practice   be changed en route is also reflected in the reformulation of the   rules.   A general description of the changes follows.  The normative text is   to be found in [RFC6437].   The definition of a flow is subtly changed fromRFC 3697 to allow any   node, not just the source node, to set the flow label value.   However, it is recommended that sources should set a uniformly   distributed flow label value in all flows, replacing the less precise   recommendation made inSection 3 of RFC 3697.  Both stateful and   stateless methods of assigning a uniformly distributed value could be   used.Amante, et al.                Informational                     [Page 6]

RFC 6436                    Flow Label Update              November 2011   Flow label values should be chosen such that their bits exhibit a   high degree of variability, thus 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 have a low probability of   guessing 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, an   approximation to a discrete uniform distribution is preferable as the   source of flow label values.  In contrast, an implementation in which   flow labels are assigned sequentially is definitely not recommended,   to avoid guessability.   In practice, it is expected that a uniform distribution of flow label   values will be approximated by use of a hash function or a pseudo-   random number generator.  Either approach will produce values that   will appear pseudo-random to an external observer.Section 3 of RFC 3697 allows nodes to participate in an unspecified   stateful method of flow state establishment.  The changes do not   remove that option, but clarify that stateless models are also   possible and are the recommended default. The specific text on   requirements for stateful models has been reduced to a bare minimum   requirement that they do not interfere with the stateless model.  To   enable stateless load distribution at any point in the Internet, a   node using a stateful model should never send packets whose flow   label values do not conform to a uniform distribution.   The main novelty is that a forwarding node (typically a first-hop or   ingress router) may set the flow label value if the source has not   done so, according to the same recommendations that apply to the   source.  This might place a considerable processing load on ingress   routers that choose to do so, even if they adopted a stateless method   of flow identification and label assignment.   The value of the flow label, once it has been set, must not be   changed.  However, some qualifications are placed on this rule, to   allow for the fact that the flow label is an unprotected field and   might be misused.  No Internet-wide mechanism can depend   mathematically on immutable flow labels.  The new rules require that   flow labels exported to the Internet should always be either zero or   uniformly distributed, but even this cannot be relied on   mathematically.  Use cases need to be robust against non-conformingAmante, et al.                Informational                     [Page 7]

RFC 6436                    Flow Label Update              November 2011   flow label values.  This will also enhance compatibility with any   legacy hosts that set the flow label according toRFC 2460 andRFC3697.   A complication that led to much discussion is the possibility that   hosts inside a particular network domain might use a stateful method   of setting the flow label, and that packets bearing stateful labels   might then erroneously escape the domain and be received by nodes   performing stateless processing, such as load balancing.  This might   result in undesirable operational implications (e.g., congestion,   reordering) for not only the inappropriately flow-labeled packets,   but also well-behaved flow-labeled packets, during forwarding at   various intermediate devices.  It was suggested that border routers   might "correct" this problem by overwriting such labels in packets   leaving the domain.  However, neither domain border egress routers   nor intermediate routers/devices (using a flow label, for example, as   a part of an input key for a load-distribution hash) can determine by   inspection that a value is not part of a uniform distribution.  Thus,   there is no way that such values can be detected and "corrected".   Therefore, the recommendation to choose flow labels from a uniform   distribution also applies to stateful schemes.4.  Discussion   The following are some practical consequences of the above changes:   o  Sending hosts that are not updated will in practice continue to      send all-zero labels.  If there is no label-setting router along      the path taken by a packet, the label will be delivered as zero.   o  Sending hosts conforming to the new specification will by default      choose uniformly distributed labels between 1 and 0xFFFFF.   o  Sending hosts may continue to send all-zero labels, in which case      an ingress router may set uniformly distributed labels between 1      and 0xFFFFF.   o  The flow label is no longer unrealistically asserted to be      strictly immutable; it is recognized that it may, incorrectly, be      changed en route.  In some circumstances, this will break end-to-      end usage, e.g., potential detection of third-party spoofing      attacks [LABEL-SEC].   o  The expected default usage of the flow label is some form of      stateless load distribution, such as the ECMP/LAG usage defined in      [RFC6438].Amante, et al.                Informational                     [Page 8]

RFC 6436                    Flow Label Update              November 2011   o  If the new rules are followed, all IPv6 traffic flows on the      Internet should have zero or uniformly distributed flow label      values.   From an operational viewpoint, existing IPv6 hosts that set a default   (zero) flow label value and ignore the flow label on receipt will be   unaffected by implementations of the new specification.  In general,   it is assumed that hosts will ignore the value of the flow label on   receipt; it cannot be relied on as an end-to-end signal.  However,   this doesn't apply if a cryptographically generated label is being   used to detect attackers [LABEL-SEC].   Similarly, routers that ignore the flow label will be unaffected by   implementations of the specification.   Hosts that set a default (zero) flow label but are in a domain where   routers set a label as recommended inSection 3 will benefit from   whatever flow label handling is used on the path.   Hosts and routers that adopt the recommended mechanism will enhance   the performance of any load balancing devices that include the flow   label in the hash used to select a particular path or server, even   when packets leave the local domain.5.  Security Considerations   See [RFC6437] and [LABEL-SEC] for full discussion.  Some useful   remarks are in [Partridge].6.  Acknowledgements   The authors are grateful to Qinwen Hu for general discussion about   the flow label and for his work in searching the literature.   Valuable comments and contributions were made by Ran Atkinson, Fred   Baker, Steve Blake, Remi Despres, Alan Ford, Fernando Gont, Brian   Haberman, Tony Hain, Joel Halpern, Chris Morrow, Thomas Narten, Pekka   Savola, Mark Smith, Pascal Thubert, Iljitsch van Beijnum, and other   participants in the 6man working group.Amante, et al.                Informational                     [Page 9]

RFC 6436                    Flow Label Update              November 20117.  Informative References   [FLOWSWITCH]  Beckman, M.,"IPv6 Dynamic Flow Label Switching (FLS)",                 Work in Progress, March 2007.   [LABEL-SEC]   Gont, F.,"Security Assessment of the IPv6 Flow Label",                 Work in Progress, November 2010.   [McGann05]    McGann, O. and D. Malone, "Flow Label Filtering                 Feasibility", European Conference on Computer Network                 Defence , 2005.   [NIST]        Frankel, S., Graveman, R., Pearce, J., and M. Rooks,                 "Guidelines for the Secure Deployment of IPv6",                 National Institute of Standards and Technology Special                 Publication 800-119, 2010, <http://csrc.nist.gov/publications/nistpubs/800-119/sp800-119.pdf>.   [NSA]         Potyraj, C., "Firewall Design Considerations for IPv6",                 National Security Agency I733-041R-2007, 2007,                 <http://www.nsa.gov/ia/_files/ipv6/I733-041R-2007.pdf>.   [Partridge]   Partridge, C., Arsenault, A., and S. Kent, "Information                 Assurance and the Transition to IP Version 6 (IPv6)",                 Military Communications Conference (MILCOM 2007)  ,                 2007, <http://www.ir.bbn.com/documents/articles/MILCOM_Paper_from_Proceedings.pdf>.   [RFC2205]     Braden, B., Zhang, L., Berson, S., Herzog, S., and S.                 Jamin, "Resource ReSerVation Protocol (RSVP) -- Version                 1 Functional Specification",RFC 2205, September 1997.   [RFC2460]     Deering, S. and R. Hinden, "Internet Protocol, Version                 6 (IPv6) Specification",RFC 2460, December 1998.   [RFC2474]     Nichols, K., Blake, S., Baker, F., and D. Black,                 "Definition of the Differentiated Services Field (DS                 Field) in the IPv4 and IPv6 Headers",RFC 2474,                 December 1998.   [RFC3697]     Rajahalme, J., Conta, A., Carpenter, B., and S.                 Deering, "IPv6 Flow Label Specification",RFC 3697,                 March 2004.   [RFC4302]     Kent, S., "IP Authentication Header",RFC 4302,                 December 2005.Amante, et al.                Informational                    [Page 10]

RFC 6436                    Flow Label Update              November 2011   [RFC6294]     Hu, Q. and B. Carpenter, "Survey of Proposed Use Cases                 for the IPv6 Flow Label",RFC 6294, June 2011.   [RFC6437]     Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,                 "IPv6 Flow Label Specification",RFC 6437, November                 2011.   [RFC6438]     Carpenter, B. and S. Amante, "Using the IPv6 Flow Label                 for Equal Cost Multipath Routing and Link Aggregation                 in Tunnels",RFC 6438, November 2011.Amante, et al.                Informational                    [Page 11]

RFC 6436                    Flow Label Update              November 2011Appendix A.  Alternative Approaches   A model was discussed in an earlier version of this document which   defined a notion of 'flow label domain' analogous to a differentiated   services domain [RFC2474].  This model would have encouraged local   usage of the flow label as an alternative to any form of generic use,   but it required complex rules for the behavior of domain boundary   routers, and proved controversial in discussion.   Two even more complex alternative approaches were also considered and   rejected.   The first was to distinguish locally significant flow labels from   those conforming toRFC 3697 by setting or clearing the most   significant bit (MSB) of the flow label.  This led to quite   complicated rules, seems impossible to make fully self-consistent,   and was not considered practical.   The second was to use a specific differentiated services code point   (DSCP) [RFC2474] in the Traffic Class octet instead of the MSB of the   flow label itself, to flag a locally defined behavior.  A more   elaborate version of this was proposed in [FLOWSWITCH].  There are   two issues with that approach.  One is that DSCP values are   themselves only locally significant, inconsistent with the end-to-end   nature of the original flow label definition.  Secondly, it seems   unwise to meld the semantics of differentiated services, which are   currently deployed, with the unknown future semantics of flow label   usage.  However, this approach, while not recommended, does not   appear to violate any basic principles if applied strictly within a   single differentiated services domain.Amante, et al.                Informational                    [Page 12]

RFC 6436                    Flow Label Update              November 2011Authors' Addresses   Shane Amante   Level 3 Communications, LLC   1025 Eldorado Blvd   Broomfield, CO  80021   USA   EMail: shane@level3.net   Brian Carpenter   Department of Computer Science   University of Auckland   PB 92019   Auckland,   1142   New Zealand   EMail: brian.e.carpenter@gmail.com   Sheng Jiang   Huawei Technologies Co., Ltd   Q14, Huawei Campus   No.156 Beiqing Road   Hai-Dian District, Beijing  100095   P.R. China   EMail: jiangsheng@huawei.comAmante, et al.                Informational                    [Page 13]