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Versions: 00 01                                                         
INTERNET-DRAFT                             D. Meyer (Editor)
Category                                       Informational
Expires: August 2004                           February 2004

      Operational Concerns and Considerations for Routing Protocol
              Design -- Risk, Interference, and Fit (RIFT)

Status of this Document

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

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

   This document is a product of the RIFT Design Team.  Comments should
   be addressed to the authors, or the mailing list at

Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.

Meyer, et. al.                                                  [Page 1]

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   The Risk, Interference, and Fit (RIFT) design team was formed to
   document the concerns and considerations surrounding the use of
   Internet routing protocols for functions not directly related to
   routing of IP packets within the Internet and IP networks. This
   document is the output of that activity.

Meyer, et. al.                                                  [Page 2]

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                           Table of Contents

   1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . .   5
   2. Scope of this Work . . . . . . . . . . . . . . . . . . . . . .   5
   3. Problem Statement. . . . . . . . . . . . . . . . . . . . . . .   6
    3.1. Risk, Interference, and Application Fit (RIFT)  . . . . . .   6
     3.1.1. Risk: Software Engineering . . . . . . . . . . . . . . .   7
     3.1.2. Interference: Protocol Specification/Dynamic Behavior  .   7
     3.1.3. Application Fit: Distribution Topology . . . . . . . . .   7
   4. Definitions. . . . . . . . . . . . . . . . . . . . . . . . . .   8
    4.1. Reachability Information. . . . . . . . . . . . . . . . . .   8
    4.2. Layer 3 Routing Information . . . . . . . . . . . . . . . .   8
     4.2.1. Standard Routing Information . . . . . . . . . . . . . .   9
    4.3. Auxiliary (non-routing) Information . . . . . . . . . . . .   9
    4.4. Address Family Identifier (AFI) . . . . . . . . . . . . . .   9
    4.5. Subsequent Address Family Identifier (SAFI) . . . . . . . .  10
    4.6. Network Layer Reachability. . . . . . . . . . . . . . . . .  10
    4.7. Application . . . . . . . . . . . . . . . . . . . . . . . .  10
    4.8. Routing Protocol. . . . . . . . . . . . . . . . . . . . . .  10
    4.9. Fate Sharing. . . . . . . . . . . . . . . . . . . . . . . .  11
   5. Architectural Models . . . . . . . . . . . . . . . . . . . . .  11
    5.1. General Purpose Transport Infrastructure (GPT) Model. . . .  12
    5.2. Special Purpose Transport Infrastructure (SPT) Model. . . .  12
   6. Analyzing Risk and Interference. . . . . . . . . . . . . . . .  13
    6.1. Risk: Code Impact, and Resource Sharing . . . . . . . . . .  13
     6.1.1. Code Impact. . . . . . . . . . . . . . . . . . . . . . .  13
     6.1.2. Resource Sharing . . . . . . . . . . . . . . . . . . . .  14 Resource Sharing and Operating System Level Issues .   14
    6.2. Interference. . . . . . . . . . . . . . . . . . . . . . . .  15
   7. GTP and SPT Models: Risk and Interference. . . . . . . . . . .  15
    7.1. Risk. . . . . . . . . . . . . . . . . . . . . . . . . . . .  15
     7.1.1. Code Impact. . . . . . . . . . . . . . . . . . . . . . .  16
     7.1.2. Resource Sharing . . . . . . . . . . . . . . . . . . . .  17
     7.1.3. Multisession BGP . . . . . . . . . . . . . . . . . . . .  17
    7.2. Interference. . . . . . . . . . . . . . . . . . . . . . . .  19
     7.2.1. Multisession BGP . . . . . . . . . . . . . . . . . . . .  19
   8. Application Fit. . . . . . . . . . . . . . . . . . . . . . . .  19
    8.1. RFC 2547 Style VPNs . . . . . . . . . . . . . . . . . . . .  20
     8.1.1. RFC 2547 and Label Distribution. . . . . . . . . . . . .  21
    8.2. VPWS. . . . . . . . . . . . . . . . . . . . . . . . . . . .  21
     8.2.1. Assertion #1 . . . . . . . . . . . . . . . . . . . . . .  22
     8.2.2. Counter-Assertion #1 . . . . . . . . . . . . . . . . . .  22

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     8.2.3. Assertion #2 . . . . . . . . . . . . . . . . . . . . . .  23
     8.2.4. Counter-Assertion #2 . . . . . . . . . . . . . . . . . .  23 Assertion #2a . . . . . . . . . . . . . . . . . . . .  23 Counter-Assertion #2a . . . . . . . . . . . . . . . .  23
     8.2.5. Assertion #3 . . . . . . . . . . . . . . . . . . . . . .  24
     8.2.6. Counter-Assertion #3 . . . . . . . . . . . . . . . . . .  25
    8.3. VPWS and Per-Wire Attributes. . . . . . . . . . . . . . . .  27
     8.3.1. Assertion #4 . . . . . . . . . . . . . . . . . . . . . .  27
     8.3.2. Counter-Assertion #4:. . . . . . . . . . . . . . . . . .  27
     8.3.3. Assertion #5 . . . . . . . . . . . . . . . . . . . . . .  27
     8.3.4. Counter-Assertion #5 . . . . . . . . . . . . . . . . . .  27
     8.3.5. Assertion #6 . . . . . . . . . . . . . . . . . . . . . .  28
     8.3.6. Counter-Assertion #6 . . . . . . . . . . . . . . . . . .  28
     8.3.7. Assertion #7:. . . . . . . . . . . . . . . . . . . . . .  28
     8.3.8. Counter-Assertion #7:. . . . . . . . . . . . . . . . . .  29
    8.4. VPLS. . . . . . . . . . . . . . . . . . . . . . . . . . . .  29
     8.4.1. Assertion #8 . . . . . . . . . . . . . . . . . . . . . .  29
     8.4.2. Counter-Assertion #8 . . . . . . . . . . . . . . . . . .  29
     8.4.3. Assertion #9 . . . . . . . . . . . . . . . . . . . . . .  30
     8.4.4. Counter-Assertion #9 . . . . . . . . . . . . . . . . . .  30
   9. Operational Implications . . . . . . . . . . . . . . . . . . .  30
    9.1. OAM Functionality . . . . . . . . . . . . . . . . . . . . .  30
     9.1.1. Assertion #10: . . . . . . . . . . . . . . . . . . . . .  30
     9.1.2. Counter-Assertion #10: . . . . . . . . . . . . . . . . .  31
    9.2. Full-Mesh Issues. . . . . . . . . . . . . . . . . . . . . .  31
   10. Conclusions and Recommendations . . . . . . . . . . . . . . .  31
   11. Intellectual Property . . . . . . . . . . . . . . . . . . . .  31
   12. Design Team . . . . . . . . . . . . . . . . . . . . . . . . .  31
   13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  32
   14. Security Considerations . . . . . . . . . . . . . . . . . . .  33
   15. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  33
   16. References. . . . . . . . . . . . . . . . . . . . . . . . . .  34
    16.1. Normative References . . . . . . . . . . . . . . . . . . .  34
    16.2. Informative References . . . . . . . . . . . . . . . . . .  37
   17. Editor's Address. . . . . . . . . . . . . . . . . . . . . . .  38
   18. Full Copyright Statement. . . . . . . . . . . . . . . . . . .  38

Meyer, et. al.                                                  [Page 4]

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1.  Introduction

   The stability of the global Internet routing system has been the
   subject of much research (see e.g., [RVBIB]) and discussion on
   various IETF mailing lists [IETFOL]. Much of the research into the
   routing system has centered around the analysis of the dynamics and
   stability of the Border Gateway Protocol Version 4 [BGP] (hereafter
   referred to as BGP).

   However, while the theoretical properties of BGP remains a topic of
   great interest, a more recent discussion has focused on effects of
   the addition of new types of Network Layer Reachability Information,
   or NLRI to BGP. In particular, the advent of two BGP attributes,
   Multiprotocol Reachable NLRI (MP_REACH_NLRI), and Multiprotocol
   Unreachable NLRI (MP_UNREACH_NLRI) [RFC2858], have made it possible
   to encode and transport a wide variety of features and their
   associated signaling using the BGP transport infrastructure. Examples
   include include IPv6 [RFC2460], flow specification rules [FLOW], IP
   VPNs [RFC2547BIS], Virtual Private LAN services [VPLS], Virtual
   Private Wire Service [VPWS], and auto-discovery mechanisms for VPNs
   in general [BGPVPN],

   This document outlines the concerns and issues surrounding using the
   BGP infrastructure as a generic feature and signaling transport.
   However, the similar concerns apply to the Interior Gateway Protocols
   (IGPs) in common use (e.g., ISIS [RFC1142] or OSPF [RFC2328]).

   The rest of this document is organized as follows: Section 2 outlines
   the scope of this work. Section 3 introduces the problem statement
   which is the focus of this document, section 4 provides definitions,
   and section 5 outlines the main architectural models that are
   discussed. The remaining sections discuss the the implications of
   those models.

2.  Scope of this Work

   It is the intention of the RIFT design team that this document serve
   as a guide for both protocol designers and network operators. The
   goal is to outline the implications associated with employing
   existing routing protocols to enable additional feature sets and
   functionality, as contrasted with designing new mechanisms to carry
   those feature sets and functionalities.

   The issues, concerns and considerations discussed in this document

Meyer, et. al.                                      Section 2.  [Page 5]

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   focus on the implications for BGP [BGP,RFC1771]. It is important to
   note that similar issues will arise when considering generalizations
   to the information that the IGPs carry.

3.  Problem Statement

   The advent of the MP_REACH_NLRI and MP_UNREACH_NLRI attributes,
   combined with the resulting generalization to the BGP infrastructure,
   have created the opportunity to use BGP to transport a wide variety
   of data types and their associated signaling. The combination of a
   BGP data type and its associated signaling is frequently called an
   "application"; example applications include the IPv4 and IPv6
   [RFC2460] routing systems, flow specification rules [FLOW], auto-
   discovery mechanisms for Layer 3 VPNs [BGPVPN], virtual private LAN
   services [VPLS], and virtual private Wire Service [VPWS].

   More recently, the discussion in the IETF community has focused on
   the use of the BGP as a generalized feature transport infrastructure
   [IETFOL]. The debate has recently intensified due to the emergence of
   a new class of application that uses the BGP infrastructure to
   distribute information that is not directly related to inter-domain
   routing. Examples of such applications include the use of the BGP
   transport infrastructure to provide auto-discovery for IP VPNs
   [RFC2547BIS], the virtual private LAN services mentioned above [VPLS]
   and VPNs in general [BGPVPN].

3.1.  Risk, Interference, and Application Fit (RIFT)

   As mentioned above, much of the debate surrounding these new uses of
   the BGP transport infrastructure has focused on the potential
   tradeoffs between the stability of the Internet routing system, as
   effected by the deployment of new applications, and the desire on the
   part of service providers to rapidly deploy these new applications,
   and to reduce the operational cost by re-using existing protocols.

   These tradeoffs have at times been described in terms of risk,
   interference, and application fit. Risk models the software
   engineering impact of new applications on a generic implementation,
   while interference models the impact of new applications on protocol
   definition and behavior. Finally, application fit models the
   similarity between an application's data and signaling requirements

Meyer, et. al.                                    Section 3.1.  [Page 6]

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   and a specific distribution algorithm. Each is described below.

3.1.1.  Risk: Software Engineering

   Risk attempts to assess the robustness tradeoffs inherent in the
   addition of new applications to a given implementation. That is, risk
   models the impact of generic software engineering issues on a given
   implementation. These issues include the impact of new applications
   on existing implementations and on the fate sharing properties of
   those implementations.

   A second aspect of risk lies in  the trade-off of extending an
   existing protocol versus designing, implementing, and deploying a new

3.1.2.  Interference: Protocol Specification/Dynamic Behavior

   Interference  models the potential for a new application to adversely
   effect the operation of an existing implementation at the protocol
   level, by inadvertently introducing a detrimental dependency of some
   kind. That is, an application is said to "interfere" with an existing
   application if, by virtue of the application's protocol extension(s),
   one or more fundamental properties of the protocol's operation are
   detrimentally altered. For example, could we create a new state which
   introduces an unanticipated deadlock situation to occur? Or could we
   destabilize the distributed behavior of the protocol? Or might we
   simply run out of the attributes or bits available (as happened, for
   example, with RADIUS [RFC2138])?

3.1.3.  Application Fit: Distribution Topology

   Application fit refers to how closely the requirements of the data to
   be distributed match the underlying capabilities of a distribution
   mechanism. For example, it is clearly inefficient to broadcast data
   to all peers that is only required between two peers, just as it is
   inefficient to unicast (replicate) data that is required by all peers
   when a single broadcast would do.

Meyer, et. al.                                  Section 3.1.3.  [Page 7]

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4.  Definitions

4.1.  Reachability Information

   Reachability information refers to information describing some part
   of a network, along with how one can reach it, and perhaps also
   containing attributes of the implied path to the network locale.
   Typically, this information pertains to IP routing information; an
   example of non-IP reachability is VPLS information [VPLS].

4.2.  Layer 3 Routing Information

   Layer 3 routing information represents either link state information
   or network reachability information. Link state information
   represents Layer 3 adjacencies and topology. Link state routing
   protocols, such as OSPF [RFC2328] and ISIS [RFC1142], flood link
   state information throughout an IGP domain, so that each
   participating router maintains an identical copy of a database that
   is computed to reflect the complete Layer 3 topology.

   Layer 3 reachability information expressed as an IP address prefix
   represents the set of destinations (systems) whose IP addresses are
   contained in the IP address prefix.  Distance/path vector routing
   protocols, such as BGP, distribute Layer 3 reachability information
   among routing domains.

   Routers use both types of Layer 3 routing information (link state and
   reachability) to produce IP forwarding tables. That, is, for purposes
   of this discussion, "routing information" relates to the Layer 3
   inter-domain routing data traditionally carried by BGP.

   Finally, if one defines routing information as "information used to
   forward packets", combined with the above definition of reachability
   information, then we can consider information such as described in
   [FLOW] (for example) to be routing information (since it is
   attempting to add a level of granularity to how an 'aggregate' is
   defined). That is, [FLOW] intends to complement to the existing
   routing information, and the flow information is dependent on IP4
   unicast reachability advertised by the same neighbor.

Meyer, et. al.                                    Section 4.2.  [Page 8]

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4.2.1.  Standard Routing Information

   In the most general terms, then, a routing protocol distributes data
   to accomplish the following three functionalities:

    (i).   To govern the routing decision process (e.g., the
           standard BGP decision process)

    (ii).  To constrain the flow of information (for example, with
           BGP communities)

    (iii). To tell the recipient how to get packets to the next hop

   We will refer to information that falls into this class as "standard
   routing information".

4.3.  Auxiliary (non-routing) Information

   Auxiliary Information is any information that is exchanged by routers
   which is neither Layer 3 routing information, nor reachability
   information. IS-IS hostname TLVs are an example of auxiliary
   information [RFC1142].

4.4.  Address Family Identifier (AFI)

   An Address Family contains addresses that share common structure and
   semantics. An Address Family Identifier (AFI) uniquely identifies
   each address family. Several routing protocol messages contain a
   field that represents the AFI. The AFI identifies the address type
   used by another data item contained in that message. The Routing
   Information Protocol (RIP) [RFC2453], Distance Vector Multicast
   Routing Protocol (DVMRP) [RFC1075], and BGP all employ the AFI field.

   For example, the BGP MP_REACH_NLRI and MP_UNREACH_NLRI attributes
   contain an AFI field. These BGP attributes also contain a NLRI field
   that enumerates reachable or unreachable subnetworks corresponding to
   the associated address family. The AFI field indicates the address
   type by which reachable subnetworks are identified. When BGP is used
   to distribute Layer 3 routing information, AFIs can indicate the
   following address types: IPv4, IPv6, VPNv4 [RFC2547BIS]. When BGP is
   used to distribute auxiliary information, AFIs can indicate other

Meyer, et. al.                                    Section 4.4.  [Page 9]

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   address families.

4.5.  Subsequent Address Family Identifier (SAFI)

   A Subsequent Address Family Identifier (SAFI) is part of the BGP
   MP_REACH_NLRI and MP_UNREACH_NLRI attributes. These BGP attributes
   also contain a NLRI field that enumerates reachable or unreachable
   subnetworks. The SAFI augments the AFI, carrying additional
   information regarding networks enumerated in the NLRI field.

4.6.  Network Layer Reachability

   Network Layer Reachability Information, or NLRI is the data described
   by the AFI/SAFI fields [AFI,SAFI]. While these concepts were
   originally described for protocols such as DVMRP [RFC1075], the bulk
   of the generalization of the NLRI described in this document derives
   from the introduction of the MP_REACH_NLRI and MP_UNREACH_NLRI
   attributes to BGP [RFC2858].

4.7.  Application

   The term application is used in this document to refer to the
   combination of a BGP data type and any signaling data that is carried
   by BGP in support of the service the data type carries. The data type
   is typically described in an AFI/SAFI, while the actual data is
   frequently contained in both NLRI and BGP community attributes

4.8.  Routing Protocol

   A routing protocol is composed of two basic components: a data
   distribution algorithm and a decision algorithm. A router typically
   obtains Layer 3 routing information via its data distribution
   algorithm, and it uses this information to produce an IP forwarding
   table (by applying the protocol's decision algorithm to the received
   routing data). Note that it is the use of BGP's data distribution
   algorithm that is the focus of this document.  However, when judging

Meyer, et. al.                                   Section 4.8.  [Page 10]

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   application fit, one may also consider whether the decision
   algorithms suit the application.

4.9.  Fate Sharing

   The fate sharing principle for end to end network protocols was first
   enunciated by Dave Clark [CLARK]. As applied to software systems,
   fate sharing refers to the sharing of common resources among a group
   of applications. In our case, the particular "fate" of most interest
   is the ability of one application, call it application A, to cause an
   application with which it is fate sharing, call it application B, to
   experience one or more faults due to faults in application A. Fate-
   sharing can exist at many levels, including between modules on a
   system, between routing protocols, between sessions of a routing
   protocols such as BGP, or between applications within a routing

5.  Architectural Models

   In this section, we consider the two architectural models which are
   motivated by salient questions considered in this document, namely:

    (i).  Does the BGP distribution protocol suit a particular
          application (i.e., does an application fit the BGP
          distribution protocol)?

    (ii). What are the effects on the global routing system (if
          any) of carrying that application using the BGP distribution

   These questions must be analyzed in terms of the cost of protocol and
   code development, as well as in terms of the operational expense that
   may be incurred by utilizing (or not utilizing) the mechanisms
   already present in BGP.

   Two models, describing alternate viewpoints, are examined in the
   following sections.

Meyer, et. al.                                     Section 5.  [Page 11]

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5.1.  General Purpose Transport Infrastructure (GPT) Model

   The GPT model models BGP data distribution infrastructure as a
   generic application transport mechanism. As such, it focuses on
   application fit, and assumes that the tradeoffs, both in terms of
   risk and interference can be managed in an efficient manner.  As a
   result, the GTP models these issues not in terms of whether the
   application and signaling data that need to be distributed are part
   of some particular class (routing, in this case), but rather whether
   the requirements for the distribution these attributes are similar
   enough to the distribution mechanisms of BGP.  In those cases when
   distribution requirements are sufficiently similar, BGP can be a
   logical candidate for a transport infrastructure. Note that this is
   not because of the nature of information distributed, but rather due
   to the similarity in the transport requirements. There are of course
   other operational considerations that make BGP a logical candidate,
   including its close to ubiquitous deployment in the Internet (as well
   as in intra-nets), its policy capabilities, and operator comfort
   levels with the technology.

5.2.  Special Purpose Transport Infrastructure (SPT) Model

   The SPT model, on the other hand, models the BGP infrastructure as a
   special purpose transport designed specifically to transport inter-
   domain routing information. As such, it is more sensitive to risk and
   interference than to application fit.

   There are two basic arguments supporting the SPT model: The first is
   based on the perceived risk profile involved in adding new
   applications to the BGP transport infrastructure or new features to
   existing BGP applications. The concern here is that changes to BGP
   implementations will cause software quality to degrade, and hence
   destabilize the global routing system.  This position is based upon
   well understood software engineering principles, and is strengthened
   by long-standing experience that there is a direct correlation
   between software features and software stability [MULLER1999]. This
   concern is augmented by the fact that in many cases, the existence of
   the code for these features, even if unused, can also cause
   destabilization in the routing system, since in many cases software
   faults cannot be isolated.

   A second concern is based on interference arguments, notably that the
   increase in complexity of BGP due to the number of data types that it
   carries can also potentially destabilize the global routing system.

Meyer, et. al.                                   Section 5.2.  [Page 12]

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   This concern is based on a wide range of concerns, including the fact
   that the interaction of BGP dynamics and current deployment practices
   are poorly understood, and that the addition of non-routing data
   types may adversely effect convergence and other scaling properties
   of the global routing system.

6.  Analyzing Risk and Interference

   One way to frame the tradeoffs involved in a model's risk profile is
   in terms of the software engineering issues surrounding where an
   implementation might demultiplex among applications. The important
   point here is that an implementation's choice of demultiplexing point
   directly affects the implementation's risk profile due to its effects
   on existing code, and on the system resources it requires to be
   shared among those applications.

6.1.  Risk: Code Impact, and Resource Sharing

   For purposes of this discussion, then, we consider the risk profile
   of the SPT and GPT models with respect to their application
   demultiplexing point. The GPT model typically provides a single point
   for demultiplexing all applications (i.e., the AFI/SAFI). On the
   other hand, the SPT model, provides an application demultiplexing
   point above BGP (typically at the TCP port level). That is, in the
   GPT model, applications typically share a common transport session,
   while the SPT model generally envisions one or more applications per
   transport session (see section 7.1.3 for a discussion of the impact
   of multisession BGP [MULTISESSION,SOFTNOTIFY] on this taxonomy).

   Finally, note that these models can have very different risk profiles
   with respect to code impact and resource sharing. Some of the
   questions relating to risk assessment are considered below.

6.1.1.  Code Impact

   In this section, we outline the high-level questions one might ask in
   assessing the difference in risk between GPT model and the SPT model
   based on their effect on an existing code base.

Meyer, et. al.                                 Section 6.1.1.  [Page 13]

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    o Does the code below the demultiplexing point need to be
      changed when a new application is added?

    o Does the code in existing applications have to be changed when
      a new application is added (that is, to what extent are the
      applications decoupled)?

    o Can the code in separate applications be developed, tested,
      released, debugged and packaged independently from other

    o Is there significant code below the demultiplexing point that
      can be shared among all applications?

6.1.2.  Resource Sharing

   In this section, we outline the high-level questions one might ask in
   assessing the difference in risk between GPT model and the SPT model
   with respect to the requirements and properties of the system
   resource sharing they require. In particular:

    o Do applications have to compete for socket buffers, and hence
      have the potential to block or starve each other (at the TCP
      port level)?

    o Do applications have to compete for possible protocol-level
      transport-related buffers and queues, and hence have the
      potential to starve or block each other at the protocol
      send/receive level?

    o Do applications have to compete for a possible per-connection
      processing time budget, hence have the potential to starve
      each other at the intra-process scheduling level?  Resource Sharing and Operating System Level Issues

   In this section, we outline the high-level questions one might ask in
   assessing the difference in risk between GPT model and the SPT model
   based on the affect on resource sharing at the operating system
   level. In particular:

    o Do applications share a common scheduling context? That is,

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      do applications have to compete for per-process scheduling

    o What is the degree of fate sharing between applications?

6.2.  Interference

   Interference models the potential for an application to affect the
   behavior of an existing application or applications. For example, in
   the case of the Internet routing system, one might ask if a certain
   application "interferes" with IPv4 Unicast routing by affecting some
   aspect of its protocol operation (e.g., convergence time).

   Interference in the Internet routing system has its roots in the
   observation that the routing system itself can be described as highly
   self-dissimilar, with extremely different scales and levels of
   abstraction. Complex systems with this property are susceptible to
   "coupling", which RFC 3439 [RFC3439] defines as follows:

    The Coupling Principle states that as things get larger, they
    often exhibit increased interdependence between components.

    COROLLARY: The more events that simultaneously occur, the larger
    the likelihood that two or more will interact. This phenomenon
    has also been termed "unforeseen feature interaction"

   That is, interference, if and where it occurs, has its roots in
   complexity and is frequently the result of application coupling.

7.  GTP and SPT Models: Risk and Interference

   In this section, we analyze the risk and interference profiles of the
   SPT and GPT models.

7.1.  Risk

   As mentioned above, risk models the robustness tradeoffs around
   generic software architecture and engineering associated with

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   protocol implementations, including the impact on existing protocol
   implementations, and on the fate sharing properties of those
   implementations. In the following sections we consider these
   components of risk for both the GPT and SPT models.

7.1.1.  Code Impact

   In this section, we outline the answers to the questions posed above.

    o Does the code below the demultiplexing point need to be
      changed when a new application is added?

      In theory, such code changes are unlikely to be required in
      the SPT model, as the SPT model envisions that a new
      application will have a new demultiplexing point (port).

      The GPT model does not by definition require new code below
      the demultiplexing point either. Specifically, it should in
      theory be possible to isolate code below the demultiplexing
      point with suitable abstraction and constructs such as
      AFI/SAFI API registries.

    o Does the code in existing applications have to be changed when
      a new application is added (that is, to what extent are the
      applications decoupled)?

    The SPT model envisions application independence with respect to
    demultiplexing point. As such, it is unlikely to require such
    changes. However, it is important to note that good software
    engineering practices encourage code reuse and construction of
    general purpose libraries. As a result, if applications share
    libraries and/or other code, the practical independence
    decreases, and consequently risk increases. The same analysis
    can be made for the GPT model, since in this case we are already
    demultiplexing on the AFI/SAFI fields.

    o Can the code in separate applications be developed, tested,
      released, debugged and packaged independently from other

    While this is theoretically possible in the SPT model (and
    possibly more difficult in the GPT model) practice and
    experience has shown that achieving this type of independence is
    difficult in either model.

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7.1.2.  Resource Sharing

   In this section, we address the questions raised above to assess the
   difference in risk between GPT model and the SPT model based on the
   effect on resource sharing considerations.

    o Do applications have to compete for socket buffers, and hence
      have the potential the to block or starve each other (at the GPT

    The SPT model does not require applications to compete for
    socket level resources. It should also be possible to achieve
    this type of application independence in the GPT model with
    multisession BGP.

    o Do applications have to compete for possible protocol-level
      transport-related buffers and queues, and hence have the
      potential to starve or block each other at the protocol
      send/receive level?

    Again, while the SPT model does not require competition for
    transport-level resources, it should be possible to achieve
    similar behavior with multisession BGP.

    o Do applications have to compete for a possible per-connection
      processing time budget, hence have the potential to starve
      each other at the intra-process scheduling level?

    Applications written to the the SPT model should not require
    this type of resource competition. It should also be possible to
    reduce this type of resource competition with multisession BGP.

    o Do applications have to compete for resources within the
      network (e.g., bandwidth), when the protocol session spans
      multiple hops ?

    Neither the SPT model nor the GPT model (again, with
    multisession BGP) should require competition for network
    resources in this case.

7.1.3.  Multisession BGP

   Suppose that one makes the simplifying assumption that a GPT

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   implementation's risk profile is dominated by the probability that an
   error in one AFI/SAFI stream will cause some subset of the other
   AFI/SAFI streams to malfunction (e.g., reset). In this case, risk
   might be characterized as a function of the model and the number of
   AFI/SAFI carried. Given this simplification, the risk profile looks
   loosely like

    Risk = f(Model, |{AFI,SAFI}|)


    f:{GPT, SPT} X |{AFI, SAFI}| -> N

   Note that we assume that

    f(SPT,n) = O(f(GPT,n))


    O(f) = {g:N->R | there exists c > 0 and n such that g(n) < c*f(n)}

   That is, that the SPT risk profile is bounded by the GPT risk
   profile. Clearly, the existence of such an upper bound is an integral
   aspect of any argument favoring the SPT model.

   Note that for the SPT model, we can think of the number of AFI/SAFI
   that a single session carries as a small constant, call it k. k will
   typically be small (close to 1), since by definition the SPT model
   envisions a small number of AFI/SAFI per session (e.g., for AFI/SAFI
   IPv4/unicast and IPv6/unicast, k = 2).

   When formulated in this way, one can see that one objective of
   multisession BGP is to find a value, call it g, such that

    f(GPT, g) ~ f(SPT,k), for small values of k (i.e., k close to 1)


    A(n) ~ B(k) ==> A(n) = B(k) + h(n), h(n) >= 0

    That is, A(n) is approximately equal to B(k)

   In this case, g is the size of the multisession AFI/SAFI grouping,
   and for small values of g, multisession BGP can have a risk profile
   that looks very much like the SPT risk profile.  In particular, for g
   = 1, both models would have similar risk profiles. Of course, there

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   are many other components of risk that that are not considered by
   this analysis, such as collateral issues resulting from the existence
   of faulty shared code, operating system process and memory structure,

7.2.  Interference

   Interference concerns stem from the possibility that application
   coupling can lead to the destabilization of the Internet routing
   system in unanticipated and unexpected ways. In this section we
   consider interference properties of the GPT and SPT models.

7.2.1.  Multisession BGP

   Multisession BGP also seeks to reduce the interference profile of the
   GPT model by eliminating one potential source of interference,
   namely, the potential interference due to presence of multiple
   AFI/SAFIs in a single BGP session. Following the analysis presented
   in section 7.1.3, we can see that for small groupings (described as
   small values of g in section 7.1.3), the interference profiles of
   both models converge.

8.  Application Fit

   In the following sub-sections, application fit is examined from the
   perspective of analyzing the signaling and data distribution needs of
   three representative applications, namely:

    RFC 2547 Style VPNs

   However, before investigating how the BGP data distribution mechanism
   (and its extensions) fit the requirements of these applications, it
   is useful to briefly review the gross characteristics of the BGP data
   distribution infrastructure. In particular, we examine which
   distribution topologies can be naturally built using internal BGP (or

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   iBGP has been described loosely as a broadcast mechanism since an
   iBGP speaker sends information to all its peers. This is typically
   achieved by means of one or more route reflectors (or RRs); a more
   direct but less scalable means is for each iBGP speaker to have a BGP
   session with each iBGP peer. It may, however, be more accurate to
   characterize iBGP as a constrained broadcast mechanism.  This is
   because the use of communities in conjunction with import and export
   policies allows an iBGP speaker to effectively limit its
   communication to a subset of the full set of iBGP peers; the
   efficiency of constrained broadcast can be improved by techniques
   such as described in [ORF] and [RTCONST].

8.1.  RFC 2547 Style VPNs

   There are five classes of information that need to be distributed for
   RFC 2547 style VPNs:

    (a). Membership (auto-discovery)
    (b). Prefixes
    (c). Labels
    (d). BGP nexthop, and
    (e). Path selection attributes

   The first of these, membership or auto-discovery, must be sent to all
   peers, as a BGP speaker does not know a priori which of its peers are
   members of a given VPN.  Membership of a given VPN is recognized by
   the use of extended communities called Route Targets. BGP is well-
   suited for this mode of distribution.

   The next three of these constitute the reachability information.
   They say what part of a given VPN (b) is reachable, and how it is to
   be reached (c and d).  The final piece of information is used for
   selection if there are multiple paths to a given prefix of a VPN, as
   in the case of multi-homing.  All of these pieces of information need
   only be distributed to members of the VPN, i.e., they require a
   constrained broadcast mechanism.  BGP is reasonably well-suited for
   this mode of distribution using import and export NLRI filtering.
   The addition of the mechanism in [RTCONST] makes BGP even better
   suited to this.

   The encoding of this information as defined in [RFC2547BIS] puts all
   of this information in a single NLRI.  This seems to imply that a
   broadcast mechanism has to be used for the distribution of RFC 2547
   VPN information.  However, the combination of [RTCONST] and [RFC2918]
   allow BGP to distribute this information correctly yet efficiently.

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   In summary, there seems to be little argument that the RFC 2547
   application is a routing application. This is because the information
   that gets sent via BGP in RFC 2547 is generally considered to be
   "routing information". That is, the protocol distributes address
   prefixes, along with their next hops (and of course, some additional
   attributes). Finally (and perhaps most importantly), there seems to
   be little argument that the information distributed by the RFC 2547
   application is standard routing information.

8.1.1.  RFC 2547 and Label Distribution

   One issue that is frequently raised with respect to whether or not
   the RFC 2547 VPN application is a routing application surrounds the
   fact that, in the 2547 application, BGP distributes MPLS labels along
   with the routes. The contention then, is that the RFC 2547
   application represents more than just a routing application. However,
   in this case the MPLS label is just a shorthand way of representing
   one or more address prefixes. That is, the assertion is that in this
   case, the label represents "standard routing information".

8.2.  VPWS

   The question of whether VPWS is a "good fit" for the BGP transport
   infrastructure is the source of much discussion (and controversy). In
   this section, we will review both positions and their supporting
   arguments as a series of assertions and counter-assertions (we will
   use this format throughout the rest of this section).

   The key debate with respect to VPWS centers around what set of
   services are being defined, and how they are to be signaled. One way
   to analyze the VPWS application, then is in terms of two of its more
   contentious functionalities, namely:

    (a).   Auto-discovery

           Auto-discovery refers to discovery of the set of nodes
           that belong in a common L2VPN, and

    (b).   Signaling

           Signaling refers to the setup and maintenance of the
           point-to-point pseudo-wires that carry the traffic of
           the L2VPN.

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   The next sections examine the various assertions and counter-
   assertions around auto-discovery and signaling for VPLS.

8.2.1.  Assertion #1

   Assertion #1 states VPWS is not a routing application. Those
   supporting this assertion argue that in the case of VPWS, we are not
   distributing address prefixes, and (importantly) unlike the case of
   RFC 2547 style VPNs, the BGP decision process is not used (or at
   least it is not used in the same way). Further, proponents argue that
   what we are distributing is state information that corresponds to
   point-to-point entities, i.e., pseudo-wires, and thus argues that
   that the VPWS application is completely different.

8.2.2.  Counter-Assertion #1

   Counter-Assertion #1 states that VPWS is a routing application. More
   specifically, this position is outlined in [VPLS] (section 3.4),

    "It is often desired to multi-home a VPLS site, i.e., to connect
    it to multiple PEs, perhaps even in different ASes. In such a
    case, the PEs connected to the same site can either be
    configured with the same VE ID or with different VE IDs. In the
    latter case, it is mandatory to run STP on the CE device, and
    possibly on the PEs, to construct a loop-free VPLS topology.

    In the case where the PEs connected to the same site are
    assigned the same VE ID, a loop-free topology is constructed by
    routing mechanisms, in particular, by BGP path selection. When a
    BGP speaker receives two equivalent NLRIs (see below for the
    definition), it applies standard path selection criteria such as
    Local Preference and AS Path Length to determine which NLRI to
    choose; it MUST pick only one.

    If the chosen NLRI is subsequently withdrawn, the BGP speaker
    applies path selection to the remaining equivalent VPLS NLRIs to
    pick another; if none remain, the forwarding information
    associated with that NLRI is removed."

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8.2.3.  Assertion #2

   Assertion #2 states that auto-discovery for VPWS requires some form
   of constrained broadcast. There doesn't seem to be much controversy
   that auto-discovery does require some sort of constrained broadcast
   mechanism (which we don't want to be limited to a single AS), and we
   may want to be able to optimize it by using a RP (rendezvous point)
   like mechanism. BGP route reflectors (RR) provide a convenient and
   ubiquitously deployed candidate RP. In this case (RR as RP), the fit
   is good since auto-discovery, like routing, requires an n-party
   protocol where each party has no a priori knowledge of the existence
   or identity of the other n-1 parties.

8.2.4.  Counter-Assertion #2

   There is no real counter-position to Assertion #2, as it simply
   states that VPWS auto-discovery requires some form of constrained
   broadcast (about which there is some controversy; see Assertion #2a
   below).  Assertion #2a

   Assertion #2a states that auto-discovery is not needed for VPWS.
   Further, the Assertion #2a states that there is not a validated need
   for VPWS auto-discovery, since auto-discovery is useful only when
   creating full mesh layer 2 topologies, which are undesirable due to
   their (well-understood) poor scaling properties; hence auto-discovery
   for VPWS is not useful.  Counter-Assertion #2a

   <what is the counter-assertion here? auto-discovery is useful for
   partial mesh? cite?>

   In summary, with the exception of Assertion #2a, the major

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   controversy surrounding VPWS is in signaling piece of the
   application. The "VPWS is not a routing application" camp argues that
   the VPWS signaling requirements do not fit the BGP distribution
   infrastructure, while the "VPWS is a routing application" camp
   believes that BGP is a good fit. The next sections examine these

8.2.5.  Assertion #3

   Assertion #3 states that VPWS applications are not a good fit for
   BGP. This argument is based on the assertion that BGP is poorly
   suited to the VPWS signaling requirements because pseudo-wires are
   inherently point-to-point (see, for example [L2VPNSIG]). Further, the
   assertion is that VPWS signaling is qualitatively different than in
   routing or auto-discovery, in which each piece of information must be
   distributed to the n participants. The conclusion here is that BGP's
   distribution mechanisms are a poor match for VPWS signaling. Another
   way to think about this is that BGP generally works from a single
   database, and then applies some filtering on a per-connection basis;
   this only makes sense if most of the information is going to go to a
   lot of places.

   For example, suppose that a RR is used for VPWS signaling, and there
   is the need to set up n pseudo-wires. In this case, instead of
   sending n setup messages, one sends one large "meta-setup" message
   with all the info that would have been in the n setup messages. That
   is, let

    n = number of pseudo-wires
    l = the size of the per-wire label information
    k = the size of the per-wire information

    In this case, the meta-setup message will be of size O((l + k) * n).

   After receiving the setup message, the RR then must send the n
   messages that could have been sent by the endpoint (note that this is
   almost true; the endpoint would have to send n messages of size (l +
   k), but the RR will have to send n copies of the larger setup

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8.2.6.  Counter-Assertion #3

   Counter-Assertion #3 states that the VPWS application is a good fit
   for BGP (see, for example [L2VPNT]). In particular, this camp
   suggests that a RR really only needs to distribute the label-range
   [LABELRANGE], so the setup message isn't really n times as large, but
   rather is analyzed as follows:

    Let  n = number of pseudo-wires
         m = the size of the label-range data
         k = the size of the per-wire information

    Then the messages will be of size O(m + (k * n)), and most
    importantly for the label-range argument:

      O(m + (k * n)) < O((l + k) * n)

    That is, the label-range concept reduces the size of the
    messages that need to be sent to and by the RR.

   However, some will argue that the label-range concept is efficient if
   and only if:

    (a).   A large enough label range is preallocated to accommodate
           all the systems you might ever want to add to the
           VPLS/VPWS (assuming that service interruption is not
           acceptable), and

    (b).   There is no per-wire information other than labels that
           needs to distributed

   In these cases, the label range approach can reduce the size of the
   setup messages as analyzed above. However, the counter argument is
   that any such reduction will become a second-order effect as soon as
   some other piece of per-wire status or configuration (e.g., MTU)
   information must be distributed. In addition, the idea of pre-
   allocating  a large enough label range to accommodate future
   expansion, while saving bits in the setup messages, has other costs
   which may be large. In particular:

    (a).   Until the future expansion takes place (if it ever does),
           one may be wasting quite a lot of labels (noting that
           that each label you distribute requires you to allocate a
           piece of high-speed memory in your forwarding engine;
           putting some of it aside for possible later use seems
           very costly. Each one you put aside is, e.g., one less
           RFC2547 route you can support).

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           However, if you don't preallocate enough contiguous label
           space for future expansion, then if the expansion occurs
           you must start adding additional labels or label ranges,
           and your setup messages start getting longer anyway (in
           theory, you could just carve a new set of label ranges,
           instead of adding new ones; counter-position: if you did
           that, you'd have to bring down your whole VPWS (and
           possibly VPLS) every time you add a new endpoint).

    (b).   Fragmentation of the label space, which can result from
           this preallocation, has real impact on label switching
           implementations (as the MPLS architecture explicitly
           leaves it to the implementation to develop its own label
           assignment strategies). So if, for example, a hardware
           designer thinks s/he can improve performance by using,
           say, prime numbered labels first, s/he should have the
           ability to design her/his system in this way. If an
           application is going to come along and demand that labels
           be assigned in contiguous groups, implementations which
           are perfectly conformant to the architecture may not be
           able to support that application.

    (c).   For diagnosis of network problems, the label-range
           approach may have the additional issue that the operator
           may not know (a priori) which label(s) were assigned to
           which endpoint(s).

    (d).  Finally, one may argue that label-range allocation is
          sub-optimal for non-full mesh topologies, since all peers
          of the VPN must hear about the a label-range withdraw, and
          (in a non-full mesh topology), not all peers need to know
          about it.

   In any event, one may argue that the scaling benefits of using a RR
   in routing is that the RR pre-digests all the received info; it runs
   the (BGP) decision process, and only forwards the results of the
   decision process, rather than forwarding all the raw data. In the
   case of VPWS (and possibly VPLS), the argument is that this advantage
   is absent (i.e., we don't run BGP path selection), and as a result,
   the RR doesn't help with scaling in the same way it does with
   routing. Of course, the counter position is that some form of BGP
   path selection is used; see discussion above). Finally, one may argue
   that using the RR will introduce some latency into the label withdraw

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8.3.  VPWS and Per-Wire Attributes

   While several per-wire attributes have been defined (see [L2TPV3],
   for example), the need for per-wire attributes for VPWS remains
   controversial. The following sections examine those controversies.

8.3.1.  Assertion #4

   Assertion #4 is that VPWS requires various per-wire parameters. These
   may include (but are not limited to) MTU, whether to use sequencing
   capabilities, bandwidth capabilities, and QoS. In addition, during
   the lifetime of a pseudo-wire, there are per-wire status indications
   that may need to be passed to the other endpoint.

8.3.2.  Counter-Assertion #4:

   Counter-Assertion #4 states that it has not been demonstrated that
   VPWS needs per-wire attributes as few (per-wire attributes) have as
   yet been defined (see, e.g., [MARTINI]).

8.3.3.  Assertion #5

   Assertion #5 states that  passing per-wire attributes through an RR
   will likely be inefficient. The argument here is that in the event
   that per-wire attributes are required,  passing these (per-wire)
   attributes through a RR will be sub-optimal as the RR will forward
   the status to all the VPWS members, not just to the one endpoint that
   is interested in it. For attributes like sequence numbers, it may
   even more difficult as one has to make sure the sequence numbers
   resynchronize properly when the pseudo-wire flaps. This seems
   somewhat difficult to achieve through a BGP RR.

8.3.4.  Counter-Assertion #5

   The counter assertion here is that, since few (or no) per-wire

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   attributes have been defined (counter-assertion #4), the fact that it
   is inefficient to use a RR for distribution is irrelevant.

8.3.5.  Assertion #6

   Assertion #6 states that, while still an open issue, pseudo-wire
   congestion control may require regular point-to-point control message
   exchanges, something which BGP would seem ill-equipped to handle.

8.3.6.  Counter-Assertion #6

   In this case, the counter assertion is that since few (or no) per-
   wire attributes have been defined (see counter-assertion #4), and
   further, since congestion control for pseudo-wires is still an open
   issue, arguing fit is premature.

8.3.7.  Assertion #7:

   Assertion #7 states that the primary motivation for VPWS is to
   deliver existing service models (i.e., Frame Relay and ATM) over a
   packet infrastructure (this is as opposed to some new service). In
   this case, common deployments involve partial mesh topologies (more
   specifically multiple hub and spoke connections, with some hub to hub
   connectivity that makes sense for the enterprise traffic profile). In
   addition, some of the connections in such deployments require per-
   wire characteristics (e.g., guaranteed throughput for voice, etc).

   In other words, the argument here is that a VPWS service designed to
   support so-called legacy services (Frame Relay and ATM) will require
   point-to-point signaling due to existing topologies and the need for
   per-wire attributes. Further, for new VPWS services that require
   full-mesh auto-provisioning, the "Colored Pools PW Provisioning
   Model" [L2VPN] suggests a method to support such provisioning while
   retaining the point-to-point signaling required to support per-wire

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8.3.8.  Counter-Assertion #7:

   <what is the counter-assertion here?>

8.4.  VPLS

   A VPLS service connects a number of sites by an emulated LAN segment.
   In the next sections, we examine whether VPLS maybe be considered to
   be a routing application, and hence whether BGP is a good fit for its
   distribution requirements.

8.4.1.  Assertion #8

   Assertion #8 states that VPLS is a routing application, since the
   notion of "VPLS site identification" is analogous to a VPN site
   identifier for VPWS (which this camp also views as a routing
   application). As a result, the analysis of the distribution needs of
   these five items is exactly as for RFC 2547 VPNs, and the conclusion
   is that BGP is reasonably well-suited for this application, and with
   the addition of [RTCONST] and [REFRESH], the fit is even better.
   Finally, note that existing BGP path selection mechanisms can be used
   as is for VPLS, and can prove useful for multi-homed sites.

8.4.2.  Counter-Assertion #8

   Counter-Assertion #8 states that VPLS is not a routing application.
   In particular, the contention here is that while the VPLS NLRI are
   used to identify that a particular PE belongs to a particular VPLS
   instance (as described in Assertion #8),the path which data traffic
   follows will depend on the route to that PE, and that route is
   determined by the ordinary IP routing. As a result, it is not
   relevant which  neighbor a VPLS NLRI was received from, and hence is
   not routing.

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8.4.3.  Assertion #9

   Assertion #9 is that constrained or true broadcast is not valuable
   for VPLS, since the same label can not be used by all peers. In
   particular, the same label can not be used by all peers since MAC
   address learning is performed in the data plane.

8.4.4.  Counter-Assertion #9

   <what is the counter-assertion here? label ranges?>

9.  Operational Implications

   In this section we examine the operational implications of the
   various choices in the design spaces described in this document.

9.1.  OAM Functionality

   A service provider (SP) may want to know exactly where a particular
   pseudo-wire leaves its domain, and in addition may want to keep
   various counts and bits of status at that point. Further, the SP may
   want to be able to do data path testing to that point. That is, a SP
   may want point-to-point pseudo-wire state to be maintained at its
   border routers.

9.1.1.  Assertion #10:

   Assertion #10 states that it may be difficult for service providers
   to maintain point-to-point pseudo-wire state at their border routers
   with the proposed BGP signaling mechanisms. This is because those
   mechanisms provide no way to ensure that a pseudo-wire data path will
   leave the network at a node which has state information for that

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9.1.2.  Counter-Assertion #10:

   <counter assertion?>

9.2.  Full-Mesh Issues

10.  Conclusions and Recommendations

11.  Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; neither does it represent that it
   has made any effort to identify any such rights.  Information on the
   IETF's procedures with respect to rights in standards-track and
   standards-related documentation can be found in BCP-11 [RFC2028].
   Copies of claims of rights made available for publication and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementors or users of this
   specification can be obtained from the IETF Secretariat.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive

12.  Design Team

   The design team that produced this document consisted of Daniel
   Awduche (awduche@awduche.com), Ron Bonica (Ronald.P.Bonica@mci.com),
   Hank Kilmer (hank@rem.com), Kireeti Kompella (kireeti@juniper.net),
   Chris Lewis (chrlewis@cisco.com), Danny McPherson (danny@tcb.net),
   David Meyer (dmm@1-4-5.net) and Peter Whiting

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13.  Acknowledgments

   David Ball, Peter Gutierrez, Susan Harris, Pedro Marques, Eric Rosen,
   Pekka Savola, and Mark Townsley have all made many insightful
   comments on earlier versions of this document.

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14.  Security Considerations

   This document specifies neither a protocol nor an operational
   practice, and as such, it creates no new security considerations.

15.  IANA Considerations

   This document creates a no new requirements on IANA namespaces

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16.  References

16.1.  Normative References

   [AFI]           http://www.iana.org/assignments/address-family-numbers

   [BGP]           Rekhter, Y, T.Li, and S. Hares, "A Border Gateway
                   Protocol 4 (BGP-4)", draft-ietf-idr-bgp4-23.txt.
                   Work in progress.

   [BGPVPN]        Ould-Brahim, H., E. Rosen, and Y. Rekhter, "Using
                   BGP as an Auto-Discovery Mechanism for
                   Provider-provisioned VPNs",
                   draft-ietf-l3vpn-bgpvpn-auto-00.txt. Work in

   [CLARK]         Clark, D., "Design Philosophy of the DARPA Internet
                   Protocols", Computer Communication Review, volume
                   25, number 1, January 1995. ISSN # 0146-4833.

   [EXTCOMM]       Sangali, S., D. Tappan, and Y. Rekhter, "BGP
                   Extended Communities Attribute",
                   draft-ietf-idr-bgp-ext-communities-06.txt. Work
                   in progress.

   [FLOW]          Marques, P, et. al., "Dissemination of flow
                   specification rules",
                   draft-marques-idr-flow-spec-00.txt. Work in

   [LABELRANGE]    What is the cite here?

   [L2VPN]         Andersson, L. and E. Rosen, "L2VPN Framework",
                   draft-ietf-l2vpn-l2-framework-03.txt. Work in

   [L2VPNSIG]      Rosen, E. and V. Rodoaca, "Provisioning Models
                   and Endpoint Identifiers in L2VPN Signaling",
                   draft-ietf-l2vpn-signaling-00.txt. Work in

   [L2VPNT]        Kompella, K. (Editor), "Layer 2 VPNs Over
                   Tunnels", draft-kompella-l2vpn-l2vpn-00.txt.
                   Work in Progress.

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   [L2TPv3]        Lau, J., M. Townsley and I. Goyret (Editors),
                   "Layer Two Tunneling Protocol (Version
                   3)", draft-ietf-l2tpext-l2tp-base-11.txt. Work in

   [MARTINI]       Martini, L., E.Rosen, and T. Smith, "Pseudowire
                   Setup and Maintenance using LDP",
                   draft-ietf-pwe3-control-protocol-05.txt. Work in

   [MULLER1999]    Muller, R. et. al., "Control System Reliability
                   Requires Careful Software Installation
                   Procedures", International Conference on
                   Accelerator and Largeand Large Experimental
                   Physics Systems, 1999, Trieste, Italy.

   [MULTISESSION]  Scudder, J. and C. Appanna, "Multisession BGP,
                   draft-scudder-bgp-multisession-00.txt. Work in

   [ORF]           Chen, E., and Rekhter, Y., "Cooperative Route
                   Filtering Capability for BGP-4",
                   draft-ietf-idr-route-filter-09.txt. Work in

   [RTCONST]       Bonica, R. et al, "Constrained VPN route
                   draft-marques-ppvpn-rt-constrain-01.txt. Work in

   [SOFTNOTIFY}    Nalawade, G., K. Patel, J. Scudder, and D. Ward,
                   "BGPv4 Soft-Notification Message",
                   draft-nalawade-bgp-soft-notify-00.txt., Work in

   [RFC1075]       Waitzman, D., C. Partridge, and S. Deering,
                   "Distance Vector Multicast Routing Protocol", RFC
                   1075, November, 1988.

   [RFC1142]       Oran, D. Editor, "OSI IS-IS Intra-domain Routing
                   Protocol", RFC 1142, February, 1990.

   [RFC1771]       Rekhter, Y., and T. Li, "A Border Gateway
                   Protocol 4 (BGP-4)", RFC 1771, March 1995.

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   [RFC1958]       Carpenter, B., "Architectural principles of the
                   Internet", Editor. RFC 1958, June 1996.

   [RFC1997]       Chandra, R., P. Traina, and T. Li,  "BGP
                   Communities Attribute", RFC 1997, August, 1996.

   [RFC2138]       Rigney, C., et. al., "Remote Authentication Dial
                   In User Service (RADIUS)", RFC 2138, April, 1997.

   [RFC2328]       Moy, J., "OSPF Version 2", RFC 2328, April, 1998.

   [RFC2453]       Malkin, G., "RIP Version 2", RFC 2453, November,

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

   [RFC2547BIS]    Rosen, E., et. al., "BGP/MPLS IP VPNs",
                   draft-ietf-l3vpn-rfc2547bis-00.txt. Work in

   [RFC2858]       Bates, T., et. al., "Multiprotocol Extensions
                   for BGP-4", RFC 2858, June 2000.

   [RFC2918]       Chen, E., "Route Refresh Capability for BGP-4",
                   RFC 2918, September 2000.

   [RFC3036]       Anderson, L., et. al., "LDP Specification", RFC
                   3036, January 2001.

   [RFC3439]       Bush, R. and D. Meyer, "Some Internet
                   Architectural Guidelines and Philosophy", RFC
                   3439, December, 2002.

   [SAFI]          http://www.iana.org/assignments/safi-namespace

   [VPLS]          Kompella, K., et. al. "Virtual Private LAN
                   Service", draft-ietf-l2vpn-vpls-bgp-01.txt.
                   Work in progress.

   [VPWS]          Kompella, K. et.al. "Layer 2 VPNs Over Tunnels",
                   draft-kompella-ppvpn-l2vpn-04.txt. Work in

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16.2.  Informative References

   [IETFOL]        https://www1.ietf.org/mailman/listinfo/routing-discussion

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

   [RFC2026]       Bradner, S., "The Internet Standards Process --
                   Revision 3", RFC 2026/BCP 9, October, 1996.

   [RFC2028]       Hovey, R. and S. Bradner, "The Organizations
                   Involved in the IETF Standards Process", RFC
                   2028/BCP 11, October, 1996.

   [RFC2434]       Narten, T., and H. Alvestrand, "Guidelines for
                   Writing an IANA Considerations Section in RFCs",
                   RFC 2434/BCP 26, October 1998.

   [RVBIB]         http://www.routeviews.org/papers

   [WILLINGER2002] Willinger, W., and J. Doyle, "Robustness and the
                   Internet: Design and evolution", 2002.

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17.  Editor's Address

   David Meyer
   Email: dmm@1-4-5.net

18.  Full Copyright Statement

   Copyright (C) The Internet Society (2004). All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an

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Meyer, et. al.                                    Section 18.  [Page 39]