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Versions: 00 01 02 rfc5704                                              
Network Working Group                                     S. Bryant, Ed.
Internet-Draft                                            M. Morrow, Ed.
Intended status: Informational                      On behalf of the IAB
Expires: December 10, 2009                                  June 8, 2009

        "Uncoordinated Protocol Development Considered Harmful"

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   This document identifies various types of damage that may occur
   without formal coordination and joint development on protocols of
   mutual interest between standards development organizations.  The IAB
   has selected T-MPLS as recent case study to describe hazard to both
   the MPLS and Internet architecture as a result of uncoordinated
   adaptation of a protocol.

   This experience has resulted in a considerable improvement in the
   relationship between the two organisations.  In particular, this was
   achieved via the establishment of the "Joint working team on MPLS-TP"
   which was the first ever joint ITU/IETF group.  In addition, the
   leadership of the two organisations have agreed to improve inter-
   organizational working practices so as to avoid conflict in future
   between ITU-T Recommendations and IETF RFCs.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Protocol Safety  . . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  Importance of Invariants . . . . . . . . . . . . . . . . .  5
     2.2.  Importance of Correct Identification . . . . . . . . . . .  5
     2.3.  The Role of the Design Authority . . . . . . . . . . . . .  6
     2.4.  Ships in the Night . . . . . . . . . . . . . . . . . . . .  6
   3.  T-MPLS As a Case Study . . . . . . . . . . . . . . . . . . . .  7
     3.1.  Multiple Definitions of Label 14 . . . . . . . . . . . . .  7
     3.2.  Redefinition of TTL Semantics  . . . . . . . . . . . . . .  8
     3.3.  Reservation of Additional Labels . . . . . . . . . . . . .  8
     3.4.  Redefinition of MPLS EXP Bits  . . . . . . . . . . . . . .  9
     3.5.  The Consequences for the Network Operators . . . . . . . .  9
     3.6.  The Consequences for the SDOs  . . . . . . . . . . . . . .  9
   4.  MPLS-TP As Best Practice . . . . . . . . . . . . . . . . . . . 10
   5.  IANA considerations  . . . . . . . . . . . . . . . . . . . . . 11
   6.  Security considerations  . . . . . . . . . . . . . . . . . . . 12
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 13
   8.  Generalization from this Case Study  . . . . . . . . . . . . . 14
   9.  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 15
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 16
     10.2. Informative references . . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19

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1.  Introduction

   The uncoordinated adaptation of a protocol, parameter, or code-point
   by a standards development organization (SDO), either through the
   allocation of a code-point without following the formal registration
   procedures, or by unilaterally modifying the semantics of the
   protocol or intended use of the code-point itself, poses a risk of
   harm to the Internet [RFC4775].

   The purpose of this document is to describe the various types of
   damages that may occur without formal coordination and joint
   development on protocols of mutual interest between SDOs.  In
   particular, the IAB considers an essential principle of the protocol
   development process that only one SDO maintains design authority for
   a given protocol, with that SDO having ultimate authority over the
   allocation of protocol parameter code-points; defining the intended
   semantics, interpretation, and actions associated with those code-

   There is currently a joint IETF - ITU-T development effort underway,
   known as MPLS Transport Profile (MPLS-TP), which is progressing
   rapidly to extend MPLS in a way that is consistent with the MPLS
   architecture, and fully satisfies the requirements of the transport
   network provider [LS26].  By way of a case study we will refer to the
   design and standardization process of the ITU-T protocol known as
   Transport MPLS (T-MPLS).  Development of T-MPLS was abandoned
   [RFC5317] by ITU-T Study Group 15 due to inherent conflicts with the
   IETF MPLS design and in particular with the Internet architecture.
   These conflicts arose due to the lack of coordination with the IETF
   as the design authority for MPLS.

   The goal of this document is to demonstrate the importance of a
   coordinated approach to successful collaboration between SDOs, and to
   explain a model for inter-SDO collaborative protocol development that
   is being used successfully by the ITU-T and IETF.

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2.  Protocol Safety

   To understand the reasons why the IAB and the IETF regards
   uncoordinated use of code-points and/or protocol modification as
   posing a risk of harm to the Internet, it is necessary to recap some
   important principles of protocol design in large scale networks such
   as the Internet.  Large scale networks represent significant economic
   value.  Any outage in a large scale network due to a protocol failure
   will therefore result in significant commercial and political damage.
   When two incompatible protocols, or forms of the same protocol, are
   deployed without coordination, there is a serious risk that this may
   be catastrophic to the stability or security of the network.
   Furthermore, the scale and distributed nature of the Internet is such
   that it is impossible to coordinate a network wide restart to rid the
   network of the long- term consequences of the protocol
   incompatibility.  Therefore, the following issues are of critical

2.1.  Importance of Invariants

   Invariants are core properties that are consistent across the network
   and do not change over extremely long time-scales.  Protocol
   designers use such invariants as axioms in designing protocols.  A
   protocol often places an absolute reliance on an invariant to resolve
   a design corner case.  One example of an invariance in IP that was
   inherited in the design of MPLS is the invariant that a time to live
   (TTL) value is monotonically decreased and that a packet with TTL<=1
   will not be forwarded.  This is a safety mechanism to mitigate the
   damaging effects of packet forwarding loops.  Another example is the
   way that MPLS applies special semantics to the reserved label set
   [RFC3032] (0..15), and the notion that a Label Switched Router (LSR)
   is free to allocate labels with a value of 16 or greater for its own

2.2.  Importance of Correct Identification

   A special type of invariant is the allocation of a code-point.  A
   code-point may be used to identify a packet type or a component
   within a packet.  Without these identifiers, a packet is an opaque
   sequence of bits.  A packet parser operates by first identifying the
   code-point and then using the semantics associated with that code-
   point to interpret other components within the packet.  Once a code-
   point is defined the interpretation of associated data and the
   consequential actions becomes a protocol invariant.  Subsequent
   protocol development must adhere to those invariants.  The semantics
   for an allocated code point must never change.  If a future
   enhancement requires different semantics, interpretation, or action,
   then a new code point must be obtained.

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2.3.  The Role of the Design Authority

   A code-point such as an IEEE Ethertype is allocated to a design
   authority such as the IETF.  It is this design authority that
   establishes how information identified by the code-point is to be
   interpreted to associate appropriate invariants.  Modification and
   extension of a protocol requires great care.  In particular it is
   necessary to understand the exact nature of the invariants and the
   consequences of modification.  This may not always be the case when
   protocols are modified by organizations without the experience of the
   original designers, or the design authority expert pool.
   Furthermore, since there can only safely be a single interpretation
   of the information identified by a code-point, there must be a unique
   authority responsible for authorizing and documenting the semantics
   of the information and consequential protocol actions.

   In the case of IP and MPLS technologies, the design authority is the
   IETF.  The IETF has an internal process for evolving and maintaining
   the protocols for which it is the design authority.  The IETF also
   has change processes in place [RFC4929] to work with other SDOs that
   require enhancements to its protocols and architectures.  Similarly,
   the ITU has design authority for Recommendation E.164 [E.164] and
   allocates the relevant code points, even though the IETF has design
   authority for the protocols ("ENUM") used to access E.164 numbers
   through the Internet DNS [RFC3245].

2.4.  Ships in the Night

   It may be tempting for a designer to assert that the protocol
   extensions it proposes are safe even though it breaks the invariants
   of the original protocol because these protocol variants will operate
   as ships in the night.  That is, these protocol variants will never
   simultaneously exist in the same network domain and will never need
   to inter-work.  This is a fundamentally unsound assumption for a
   number of reasons.  First, it is infeasible to ensure that the two
   instances will never be interconnected under any circumstances.
   Second, even if the operators that deploy the protocols apply
   appropriate due diligence to ensure that the two instances do not
   conflict, it is infeasible to ensure that such conflicting protocols
   will not be interconnected under fault conditions.

   This assumption of ships in the night is particularly hazardous when
   the instances of the protocol share the same identifying code-point.
   This is because a system is unable to determine which variant of the
   protocol it has received, and hence how to correctly interpret the
   associated information or to determine what protocol actions may be
   safely executed.

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3.  T-MPLS As a Case Study

   In order to illustrate the issues that arise from uncoordinated
   protocol development called out above we will use the ITU-T T-MPLS
   protocol as a recent example.

3.1.  Multiple Definitions of Label 14

   To appreciate why the use of MPLS Reserved Label 14 by the T-MPLS
   protocol represented a safety concern to the Internet, it is
   important to understand the current standards that use MPLS Reserved
   Label 14.

   The governing standard on the use of MPLS Reserved Label 14 is
   [RFC3429], "Assignment of the 'OAM Alert Label' for Multi-protocol
   Label Switching Architecture (MPLS) Operation and Maintenance (OAM)

   Label 14 is assigned to a specific protocol, namely, ITU-T
   Recommendation [Y.1711-2002].

   ITU-T Recommendation [Y.1711-2002] has been superseded by newer
   versions, specifically: - [Y.1711-2004], [Y.1711cor1] and

   All three documents are currently in-force as ITU-T Recommendations.

   The problem is that the changes made to Y.1711 were never reflected
   in an update to RFC 3429 which only defines the protocol as specified
   by the now superseded 2002 Recommendation.  So for example, MPLS
   equipment responding to an MPLS packet containing Label 14 would
   expect to see the 2002 version of Y.1711 [Y.1711-2002] protocol and
   would not recognize any of the new function codes specified in Y.1711
   Amendment 1.  This problem arises because Y.1711 does not have a
   version field, and RFC 3429 offers no other method to disambiguate
   non-interoperable versions of Y.1711.  Having a version number in the
   protocol permits an implementer to codify non-interoperability.
   Furthermore, the IETF as the authority over Label 14 semantics has
   the final say over maintaining the interoperability of the protocol
   employing that code-point, unless the IETF explicitly delegates such

   With regard to T-MPLS there was a lack of coordination between the
   ITU-T and the IETF over the redefinition of the semantics of MPLS
   label 14, which resulted in protocol definitions that conflicted with
   the IETF MPLS Architecture.

   The MPLS architecture [RFC3031], defines a swap operation as an

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   atomic function that replaces the top label in an MPLS label stack
   with another label which provides context for the next hop LSR.
   However the ITU-T Recommendations that specified the new OAM
   functions defined by Label 14 redefined the label swap operation as a
   POP, followed by a PUSH, thereby causing all LSRs to inspect the
   label stack for the presence of Label 14 at each hop.  This proposed
   new behaviour conflicts with the IETF definition of a swap operation.
   The behaviour proposed in these specifications would have had major
   consequences for deployed hardware designs.  The outcome would have
   been that the equipments built according to the two different
   specifications would have been incompatible.  It is important that
   the atomic procedure defined in [RFC3031] is kept unchanged.

3.2.  Redefinition of TTL Semantics

   The standard method of delivering an OAM message to an entity on a
   label switched path (LSP) such that the OAM message fate shares with
   the data traffic is to use TTL expiry.  The IETF's Internet Protocol
   (IP) utilizes this mechanism for traceroute [RFC1393], as does MPLS
   ping [RFC4379].

   At one stage, the T-MPLS designers considered a multi-level OAM
   design in which the OAM packet was steered to its target by a process
   in which some nodes increased the TTL as they forwarded the OAM
   packet to its next hop.  TTL is a safety device in the IETF IP and
   MPLS architecture that prevents a packet from continuously looping
   under fault conditions.  Thus the proposed extension to support an
   enhanced OAM mechanism violated an MPLS design invariant specifically
   introduced to ensure safe operation of the Internet by preventing
   persistent forwarding loops.

3.3.  Reservation of Additional Labels

   The IETF MPLS protocol uses a small number of reserved labels
   [RFC3032] as a mechanism to provide additional context and to trigger
   some special processing operations in the forwarder.  All other
   labels used for forwarding use semantics defined by the forwarding
   equivalence class (FEC).  In an early implementation of T-MPLS the
   designers determined that they needed some additional labels to alert
   the forwarder that the packet needed special processing.  Thus a
   conflict was thereby introduced between the behaviour of an IETF MPLS
   LSR, and LSRs that operate according to the specification in the
   ITU-T Recommendation.  Thus some LSRs would attribute special
   semantics to labels 16..31, whist other LSRs would perform normal
   forwarding on them.

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3.4.  Redefinition of MPLS EXP Bits

   The early MPLS documents defined the form of the MPLS label stack
   entry [RFC3032].  This includes a three-bit field called the "EXP
   field".  The exact use of this field was not defined by these
   documents, except to state that it was to be "reserved for
   experimental use".

   Although the intended use of the EXP field was as a "Class of
   Service" (CoS) field, it was not named a CoS field by these early
   documents because the use of such a CoS field was not considered to
   be sufficiently defined.  Today a number of standards documents
   define its usage as a CoS field [RFC3270], [RFC5129], and hardware is
   deployed that assumes this exclusive usage.

   The T-MPLS designers, unaware of the historic reason for the
   "provisional" naming of this field assumed that they were available
   for any experimental use and re-purposed them to indicate the
   maintenance entity level (a hierarchical maintenance mechanism).

   The intended use of the EXP field was subsequently carried in
   [RFC5462], which reinforces this by formally changing the name to
   Traffic Class (TC).

3.5.  The Consequences for the Network Operators

   Two consequences arose for the users of the T-MPLS protocol.
   Firstly, those who deployed development versions of the protocol were
   exposing their network to hazards explicitly guaranteed by the IETF
   MPLS protocol suite to not occur.  Indeed those operating T-MPLS in
   production networks in anticipation of later adoption of the MPLS
   Transport Profile (MPLS-TP) may still experience those hazards; and
   vendors continuing to distribute T-MPLS equipment expose their
   customers to the same hazards.

   The second consequence was to delay the necessary enhancements to
   MPLS to meet their needs [I-D.ietf-mpls-tp-requirements] as the IETF
   and ITU-T executed a redevelopment of MPLS-based transport network

3.6.  The Consequences for the SDOs

   The process of resolution required considerable resources and
   introduced a great deal of conflict between the IETF and the ITU-T,
   much of which was exposed to public scrutiny, to the detriment of
   both organizations.  In particular this conflict resolution process
   consumed the very resources required to develop an optimal
   architecture for MPLS in transport networks.

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4.  MPLS-TP As Best Practice

   In order to bridge the gap between the two organizations, the IETF
   and the ITU-T established a joint working team (JWT) to assess
   whether it was possible to enhance existing MPLS standards to provide
   a best in class solution for transport networks.  The outcome of this
   investigation is reported in [RFC5317].

   The JWT proposed a design that was acceptable to both SDOs.  This has
   lead to the creation of the MPLS-TP project.  This is a joint project
   in which the ITU-T experts work with the IETF on protocol development
   tasks; and IETF members work within the ITU-T to understand
   requirements, and to assist in the creation of the ITU-T
   recommendations that describe MPLS-TP in which the technical
   definition is provided through normative references to IETF RFCs.

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5.  IANA considerations

   There are no requests for IANA allocation of code-points in this
   document, nor are any other IANA actions required.

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6.  Security considerations

   The uncoordinated development of protocols is poses a risk of harm to
   the Internet and must not be permitted.  The enhancement or
   modification of a protocol can increase attack surfaces considerably
   and may therefore compromise the security or stability of the
   Internet.  The IETF has a review process that combines cross area
   review with specialist security review by experts familiar with the
   development and deployment context of the Internet protocol suite.
   In particular, because of the Internet infrastructure's reliance on
   the IP and MPLS protocol suites, this security review process must be
   exercised with considerable diligence.  Failure to apply this review
   process exposes the Internet to increased risk along both security
   and stability vectors.

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7.  Acknowledgments

   The authors wish to acknowledge Loa Andersson and the members of the
   2009/2010 Internet Architecture Board.  Additionally we wish to
   acknowledge Malcolm Johnson, Director, ITU Telecommunication
   Standardization Bureau and Yoichi Maeda ITU-T Study Group 15 Chair,
   for their review of this draft and their contribution of text.

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8.  Generalization from this Case Study

   Although we have used T-MPLS as a case study, there is potential
   conflict between other ongoing ITU-T projects and core IETF
   specifications, for example [Y.2015] , [Q.Flowsig] .  There are new
   areas of potential conflict also emerging from the other work of
   ITU-T SG13 Question 7, "IPv6" for example: [Y.ipv6split] and

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9.  Conclusion

   It is important that all SDOs developing standards that effect the
   operation of the Internet learn the lessons that arise from the
   T-MPLS experience.  Uncoordinated parallel work between SDOs creates
   significant problems with potentially damaging operation impact to
   those that deploy and use the Internet.  SDOs need to work jointly on
   new projects in such a way that the safety of the Internet is assured
   and the expertise of the organizations concerned is applied

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10.  References

10.1.  Normative References

10.2.  Informative references

   [E.164]    ITU-T, "ITU Recommendation E.164: The international public
              telecommunication numbering plan", February 2005.

              Niven-Jenkins, B., Brungard, D., Betts, M., Sprecher, N.,
              and S. Ueno, "MPLS-TP Requirements",
              draft-ietf-mpls-tp-requirements-08 (work in progress),
              May 2009.

   [LS26]     ITU-T Study Group 15, "Cooperation Between IETF and ITU-T
              on the Development of MPLS-TP, Geneva, 1-12 December 2008,

              ITU-T Study Group 11, "ITU-T Study Group 11, Question 5,
              Signalling protocols and procedures relating to flow state
              aware access QoS control in an NGN; draft

   [RFC1393]  Malkin, G., "Traceroute Using an IP Option", RFC 1393,
              January 1993.

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031, January 2001.

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, January 2001.

   [RFC3245]  Klensin, J. and IAB, "The History and Context of Telephone
              Number Mapping (ENUM) Operational Decisions: Informational
              Documents Contributed to ITU-T Study Group 2 (SG2)",
              RFC 3245, March 2002.

   [RFC3270]  Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen,
              P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi-
              Protocol Label Switching (MPLS) Support of Differentiated
              Services", RFC 3270, May 2002.

   [RFC3429]  Ohta, H., "Assignment of the 'OAM Alert Label' for
              Multiprotocol Label Switching Architecture (MPLS)

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              Operation and Maintenance (OAM) Functions", RFC 3429,
              November 2002.

   [RFC4379]  Kompella, K. and G. Swallow, "Detecting Multi-Protocol
              Label Switched (MPLS) Data Plane Failures", RFC 4379,
              February 2006.

   [RFC4775]  Bradner, S., Carpenter, B., and T. Narten, "Procedures for
              Protocol Extensions and Variations", BCP 125, RFC 4775,
              December 2006.

   [RFC4929]  Andersson, L. and A. Farrel, "Change Process for
              Multiprotocol Label Switching (MPLS) and Generalized MPLS
              (GMPLS) Protocols and Procedures", BCP 129, RFC 4929,
              June 2007.

   [RFC5129]  Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
              Marking in MPLS", RFC 5129, January 2008.

   [RFC5317]  Bryant, S. and L. Andersson, "Joint Working Team (JWT)
              Report on MPLS Architectural Considerations for a
              Transport Profile", RFC 5317, February 2009.

   [RFC5462]  Andersson, L. and R. Asati, "Multiprotocol Label Switching
              (MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
              Class" Field", RFC 5462, February 2009.

              ITU-T Study Group 13, "ITU-T Recommendation Y.1711 "OAM
              mechanism for MPLS networks", November 2002".

              ITU-T Study Group 13, "ITU-T Recommendation Y.1711 "OAM
              mechanism for MPLS networks", February 2004".

              ITU-T Study Group 13, "ITU-T Recommendation Y.1711
              Amendment 1, New Function Type Codes, October 2005.".

              ITU-T Study Group 13, "ITU-T Recommendation Y.1711 (2004)
              corrigendum 1, February 2005.".

   [Y.2015]   ITU-T Study Group 13, "ITU-T Study Group 13, Question 5,
              "General Requirements for ID/Locator Separation in NGN"".

              ITU-T, "ITU draft Y.ipv6migration : Roadmap for IPv6

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              migration from NGN operators perspective", 2009.

              ITU-T, "ITU draft Y.ipv6split : Framework of ID/LOC
              separation in IPv6-based NGN", 2009.

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Authors' Addresses

   Stewart Bryant (editor)
   On behalf of the IAB

   Email: stbryant@cisco.com

   Monique Morrow (editor)
   On behalf of the IAB

   Email: mmorrow@cisco.com

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