💾 Archived View for gemini.bortzmeyer.org › rfc-mirror › rfc5704.txt captured on 2023-05-24 at 20:00:46.
⬅️ Previous capture (2021-11-30)
-=-=-=-=-=-=-
Network Working Group S. Bryant, Ed.
Request for Comments: 5704 M. Morrow, Ed.
Category: Informational For the IAB
November 2009
Uncoordinated Protocol Development Considered Harmful
Abstract
This document identifies problems that may result from the absence of
formal coordination and joint development on protocols of mutual
interest between standards development organizations (SDOs). Some of
these problems may cause significant harm to the Internet. The
document suggests that a robust procedure is required prevent this
from occurring in the future. The IAB has selected a number of case
studies, such as Transport MPLS (T-MPLS), as recent examples to
describe the hazard to the Internet architecture that results from
uncoordinated adaptation of a protocol.
This experience has resulted in a considerable improvement in the
relationship between the IETF and the ITU-T. In particular, this was
achieved via the establishment of the "Joint working team on
MPLS-TP". In addition, the leadership of the two organizations
agreed to improve inter-organizational working practices so as to
avoid conflict in the future between ITU-T Recommendations and IETF
RFCs.
Whilst we use ITU-T - IETF interactions in these case studies, the
scope of the document extends to all SDOs that have an overlapping
protocol interest with the IETF.
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
Bryant, et al. Informational [Page 1]
RFC 5704 Uncoordinated Harmful November 2009
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 BSD License.
Table of Contents
1. Introduction ....................................................2
2. Protocol Design Rules ...........................................3
2.1. Protocol Safety ............................................3
2.2. Importance of Invariants ...................................4
2.3. Importance of Correct Identification .......................4
2.4. The Role of the Design Authority ...........................4
2.5. Ships in the Night .........................................5
3. Examples of Miscoordination .....................................6
3.1. T-MPLS as a Case Study .....................................6
3.2. PPP over SONET/SDH (Synchronous Optical Network /
Synchronous Digital Hierarchy ..............................6
4. Managing Information Flow .......................................7
4.1. Managing Information Flow within an SDO ....................7
4.2. Managing Information Flow between SDOs .....................7
5. MPLS-TP as Best Practice ........................................7
6. IETF Change Process .............................................8
7. Security Considerations .........................................8
8. Acknowledgments .................................................8
9. IAB Members at the Time of This Writing .........................8
10. Emerging Issues ................................................9
11. Conclusion .....................................................9
12. Informative References .........................................9
Appendix A. A Review of the T-MPLS Case ..........................12
A.1. Multiple Definitions of Label 14 ..........................12
A.2. Redefinition of TTL Semantics .............................13
A.3. Reservation of Additional Labels ..........................13
A.4. Redefinition of MPLS EXP Bits .............................14
A.5. The Consequences for the Network Operators ................14
A.6. The Consequences for the SDOs .............................15
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].
Bryant, et al. Informational [Page 2]
RFC 5704 Uncoordinated Harmful November 2009
The purpose of this document is to describe the various problems that
may occur without formal coordination and joint development on
protocols of mutual interest between SDOs. Some of the problems that
arise may cause significant harm to the Internet. In particular, the
IAB considers it 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 and over defining the intended
semantics, interpretation, and actions associated with those code-
points.
There is currently a joint IETF - ITU-T development effort underway,
known as the 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.
2. Protocol Design Rules
This section describes a number of protocol design rules needed to
ensure the safe operation of a network. Whilst these rules will be
familiar to many protocol designers, the rules are restated here to
ensure that assumptions are clear and consistent. Differing
assumptions have been at the root of many miscoordinations and
miscommunications between SDOs in the past.
2.1. Protocol Safety
To understand the reasons why the IAB and IETF regard 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. Many end users and businesses have come to rely on the
Internet as part of their critical infrastructure, thus large-scale
networks, such as the Internet, 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.
Bryant, et al. Informational [Page 3]
RFC 5704 Uncoordinated Harmful November 2009
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 may be difficult or impossible to rid the network of the
long-term consequences of the protocol incompatibility. Therefore,
the following issues are of critical importance.
2.2. 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
(0..15) [RFC3032], and the notion that a Label Switched Router (LSR)
is free to allocate labels with a value of 16 or greater for its own
use.
2.3. 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 become protocol invariants. 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.
2.4. 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
Bryant, et al. Informational [Page 4]
RFC 5704 Uncoordinated Harmful November 2009
consequences of modification. Such understanding may not always be
possible when protocols are modified by organizations that don't have
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-T 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].
It is a recommendation of this document that the IETF introduces
additional change mechanisms to encompass all of the technical areas
for which it has design authority.
2.5. 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.
Bryant, et al. Informational [Page 5]
RFC 5704 Uncoordinated Harmful November 2009
3. Examples of Miscoordination
There are a variety of examples where lack of inter-SDO coordination
has led to the publication of flawed protocol designs. This section
describes a number of case studies that illustrate coordination
issues.
3.1. T-MPLS as a Case Study
A recent example where another SDO created a protocol based on
misunderstandings of IETF protocols is T-MPLS. T-MPLS was created in
ITU-T in an attempt to design a packet-transport method for
transport-oriented networks. This is an example of the success that
IETF protocols have enjoyed, and ITU-T's interest and selection of
MPLS is a compliment to the IETF work. Quite late in the ITU-T
design and specification cycle, there were a number of liaison
exchanges between the ITU-T and the IETF, where the IETF became
increasingly concerned about incompatibility of IETF MPLS procedures
and technologies with ITU-T T-MPLS [RFC5317]. Extensive discussions
took place regarding interpretation, definition, and
misunderstandings regarding aspects such as MPLS Label 14, MPLS swap
operation, TTL semantics, reservation of additional labels, and EXP
bits. If ITU-T had worked with IETF from the start in developing
T-MPLS, these problems could have been avoided. A detailed analysis
of the T-MPLS case is provided in Appendix A.
3.2. PPP over SONET/SDH (Synchronous Optical Network / Synchronous
Digital Hierarchy)
An example of where the IETF failed to coordinate with the ITU-T is
[RFC1619], which defined PPP over SONET. In the text dealing with
the SONET/SDH Synchronous Payload Envelope (SPE), the document
erroneously stated that "no scrambling is needed during insertion
into the SPE." It was later determined by SONET experts operating in
the ITU-T and in ANSI that this error arose due to an incomplete
understanding of the SONET scrambler. By not using a scrambler, the
protocol was attempting to transport non-transparent data over SONET.
This resulted in lost or misinterpreted data in the Packet over SONET
(PoS) network. This impacted routing, signaling, and end-user data
traffic. Details of this work are described in [PPPoSONET]. This
problem would have been prevented if the IETF had worked with ITU-T
and ANSI in initially developing [RFC1619] . The problem was
resolved by working jointly with ITU-T and ANSI experts to publish
[RFC2615], which mandated the use of scrambling.
Note that [RFC1619] was developed four years before the IETF and
ITU-T agreed on formal procedures for cooperation, as documented in
[RFC2436] (which was later obsoleted by [RFC3356]).
Bryant, et al. Informational [Page 6]
RFC 5704 Uncoordinated Harmful November 2009
4. Managing Information Flow
This section recommends that intra- and inter-SDO information flows
require careful management in order to prevent the accidental
extension of protocols without complete coordination of the work with
the relevant design authority.
4.1. Managing Information Flow within an SDO
One cannot assume that an SDO is completely familiar with the
internal structure, information exchange, or internal processes of
another SDO. Hence, the initial contact point and the subgroup with
which a long-term working relationship is formed has a duty to ensure
that the work is fully notified and coordinated to all relevant
parties within the SDO.
4.2. Managing Information Flow between SDOs
A recommendation is that, as part of their document-approval process,
SDOs should confirm that all protocol parameters, code-points, TLV
identifiers, etc., have been authorized by the appropriate design
authority (e.g., IANA, IETF, etc. in this case) and that SDO approval
from the design authority has been obtained prior to publishing
protocol extensions. This confirmation should be an integral part of
the approval or consent process as verifying that the normative
references are qualified.
5. 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
led 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 ITU-T recommendations
that describe MPLS-TP in which the technical definition is provided
through normative references to IETF RFCs.
Although the JWT approach allowed the IETF and the ITU-T to agree on
a method of resolving the T-MPLS problem, this approach had a
significant resource cost. The JWT required considerable protocol-
design expertise and IETF management time to agree on a suitable
technical solution. A lightweight process, starting with close
Bryant, et al. Informational [Page 7]
RFC 5704 Uncoordinated Harmful November 2009
coordination during the requirements phase and continuing during the
development phase, would likely reduce the resources needed to an
acceptable level in the future.
6. IETF Change Process
There is an MPLS-change-process [RFC4929] . However, the IETF has
not yet defined a change process that is applicable to all of its
work areas. The problems described in this document indicate that
the IETF needs to develop a universal change process. The MPLS-
change-process would seem to be a suitable starting point.
7. Security Considerations
The uncoordinated development of protocols 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.
8. Acknowledgments
The authors wish to acknowledge Loa Andersson for his contribution to
this work.
9. IAB Members at the Time of This Writing
Marcelo Bagnulo
Gonzalo Camarillo
Stuart Cheshire
Vijay Gill
Russ Housley
John Klensin
Olaf Kolkman
Gregory Lebovitz
Andrew Malis
Danny McPherson
David Oran
Jon Peterson
Dave Thaler
Bryant, et al. Informational [Page 8]
RFC 5704 Uncoordinated Harmful November 2009
10. Emerging Issues
Although we have used T-MPLS as a case study, there are other ongoing
ITU-T projects and core IETF specifications that could be the subject
of either improved coordination or new conflicts, depending on
whether or not the principles outlined in this document are adhered
to by the IETF and ITU. Two current examples are [Y.2015] and
[Q.Flowsig]. New areas with potential for cooperation or conflict
are emerging from the work of ITU-T SG13 Question 7, "IPv6" -- for
example: [Y.ipv6split] and [Y.ipv6migration].
Improved coordination, of course, is not limited to the relationship
between IETF and ITU-T. This issue is present between a set of SDOs.
The IETF - ITU-T relationship has simply been used because there is a
recent example where a potential disaster was, through much hard
work, not only prevented but turned into a net gain for all.
11. Conclusion
It is important that all SDOs developing standards that affect the
operation of the Internet learn the lessons that arise from cases
cited in this document. Uncoordinated parallel work between SDOs
creates significant problems with potentially damaging operation
impact to those that deploy and use the Internet. Both inter- and
intra-SDO information flow needs to be well managed to ensure that
all impacted parties are aware of work items. Finally, the IETF
needs to develop a universal change process that encompasses all of
its work areas.
12. Informative References
[E.164] ITU-T, "ITU Recommendation E.164: The international
public telecommunication numbering plan",
February 2005.
[LS26] ITU-T Study Group 15, "Cooperation Between IETF and
ITU-T on the Development of MPLS-TP", Geneva,
December 2008, <https://datatracker.ietf.org/
documents/LIAISON/file596.pdf>.
[PPPoSONET] Manchester, J., et al., "PPP over SONET/SDH", Work in
Progress, October 1997.
[Q.Flowsig] ITU-T Study Group 11, Question 5, "Signalling protocols
and procedures relating to flow state aware access QoS
control in an NGN", Draft Recommendation.
Bryant, et al. Informational [Page 9]
RFC 5704 Uncoordinated Harmful November 2009
[RFC1393] Malkin, G., "Traceroute Using an IP Option", RFC 1393,
January 1993.
[RFC1619] Simpson, W., "PPP over SONET/SDH", RFC 1619, May 1994.
[RFC2436] Brett, R., Bradner, S., and G. Parsons, "Collaboration
between ISOC/IETF and ITU-T", RFC 2436, October 1998.
[RFC2615] Malis, A. and W. Simpson, "PPP over SONET/SDH",
RFC 2615, June 1999.
[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.
[RFC3356] Fishman, G. and S. Bradner, "Internet Engineering Task
Force and International Telecommunication Union -
Telecommunications Standardization Sector Collaboration
Guidelines", RFC 3356, August 2002.
[RFC3429] Ohta, H., "Assignment of the 'OAM Alert Label' for
Multiprotocol Label Switching Architecture (MPLS)
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.
Bryant, et al. Informational [Page 10]
RFC 5704 Uncoordinated Harmful November 2009
[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.
[RFC5654] Niven-Jenkins, B., Brungard, D., Betts, M., Sprecher,
N., and S. Ueno, "Requirements of an MPLS Transport
Profile", RFC 5654, September 2009.
[Y.1711-2002] ITU-T Study Group 13, "ITU-T Recommendation Y.1711: OAM
mechanism for MPLS networks", November 2002.
[Y.1711-2004] ITU-T Study Group 13, "ITU-T Recommendation Y.1711: OAM
mechanism for MPLS networks", February 2004.
[Y.1711am1] ITU-T Study Group 13, "ITU-T Recommendation Y.1711
Amendment 1: New Function Type Codes", October 2005.
[Y.1711cor1] ITU-T Study Group 13, "ITU-T Recommendation Y.1711
(2004) corrigendum 1", February 2005.
[Y.2015] ITU-T Study Group 13, Question 5, "General Requirements
for ID/Locator Separation in NGN".
[Y.ipv6migration]
ITU-T, "ITU draft Y.ipv6migration: Roadmap for IPv6
migration from NGN operators perspective", 2009.
[Y.ipv6split] ITU-T, "ITU draft Y.ipv6split: Framework of ID/LOC
separation in IPv6-based NGN", 2009.
Bryant, et al. Informational [Page 11]
RFC 5704 Uncoordinated Harmful November 2009
Appendix A. A Review of the T-MPLS Case
T-MPLS was created in ITU-T in an attempt to design an MPLS-based
packet-transport method for transport-oriented networks. This
appendix describes the technical issues that the IETF identified with
the T-MPLS documents and their consequences.
A.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 Multiprotocol
Label Switching Architecture (MPLS) Operation and Maintenance (OAM)
Functions".
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 [Y.1711am1].
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 the 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
authority.
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.
Bryant, et al. Informational [Page 12]
RFC 5704 Uncoordinated Harmful November 2009
The MPLS architecture [RFC3031], defines a swap operation as an
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.
A.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 shares its fate
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.
A.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, whilst other LSRs would perform normal forwarding on
them.
Bryant, et al. Informational [Page 13]
RFC 5704 Uncoordinated Harmful November 2009
A.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 use by formally changing the name to
Traffic Class (TC).
A.5. The Consequences for the Network Operators
Transport network operators need a way to realize the statistical
gain inherent in packet networking while retaining the familiar
operational structure of their SONET/SDH networks. T-MPLS was an
attempt to provide that functionality. However, creating an
incompatible variant of MPLS without tight coordination with IETF
created a number of problems for network operators.
Firstly, those operators that deployed T-MPLS in production networks
will need to address the risk and complexity of transitioning their
network to MPLS-TP. Secondly, there has been a consequential delay
of the necessary enhancements to MPLS to meet their needs [RFC5654]
as the IETF and ITU-T executed a redevelopment of MPLS-based
transport network protocols.
Fortunately, the two organizations are now working together to design
the necessary enhancements
The resulting technology, MPLS-TP, brings significant benefits to
all. However, this has not been without cost. Had things continued,
we are not sure what would have happened -- at the least, the larger
Bryant, et al. Informational [Page 14]
RFC 5704 Uncoordinated Harmful November 2009
MPLS community would have been denied the benefit of better OAM, and
the transport community would have been denied the many benefits of
an interoperable solution.
A.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.
Authors' Addresses
Stewart Bryant (editor)
EMail: stbryant@cisco.com
Monique Morrow (editor)
EMail: mmorrow@cisco.com
Internet Architecture Board
EMail: iab@iab.org
Bryant, et al. Informational [Page 15]