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INTERNET-DRAFT                             D. Meyer (Editor)
Category                                       Informational
Expires: July 2004                              January 2004


      Operational Concerns and Considerations for Routing Protocol
              Design -- Risk, Interference, and Fit (RIFT)
                     <draft-ietf-grow-rift-00.txt>




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-
   Drafts.

   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
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   The key words "MUST"", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in 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
   grow@lists.uoregon.edu.


Copyright Notice

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





Meyer, et. al.                                                  [Page 1]


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                                Abstract


   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.











































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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
    4.3. Auxiliary (non-routing) Information . . . . . . . . . . . .   9
    4.4. Address Family Identifier (AFI) . . . . . . . . . . . . . .   9
    4.5. Subsequent Address Family Identifier (SAFI) . . . . . . . .   9
    4.6. Network Layer Reachability. . . . . . . . . . . . . . . . .   9
    4.7. Application . . . . . . . . . . . . . . . . . . . . . . . .  10
    4.8. Routing Protocol. . . . . . . . . . . . . . . . . . . . . .  10
    4.9. Fate Sharing. . . . . . . . . . . . . . . . . . . . . . . .  10
   5. Architectural Models . . . . . . . . . . . . . . . . . . . . .  11
    5.1. General Purpose Transport Infrastructure (GPT) Model. . . .  11
    5.2. Special Purpose Transport Infrastructure (SPT) Model. . . .  12
   6. Analyzing Risk and Interference. . . . . . . . . . . . . . . .  12
    6.1. Risk: Code Impact, and Resource Sharing . . . . . . . . . .  13
     6.1.1. Code Impact. . . . . . . . . . . . . . . . . . . . . . .  13
     6.1.2. Resource Sharing . . . . . . . . . . . . . . . . . . . .  13
      6.1.2.1. Resource Sharing and Operating System Level Issues .   14
    6.2. Interference. . . . . . . . . . . . . . . . . . . . . . . .  14
   7. GTP and SPT Models: Risk and Interference. . . . . . . . . . .  15
    7.1. Risk. . . . . . . . . . . . . . . . . . . . . . . . . . . .  15
     7.1.1. Code Impact. . . . . . . . . . . . . . . . . . . . . . .  15
     7.1.2. Resource Sharing . . . . . . . . . . . . . . . . . . . .  16
     7.1.3. Multisession BGP . . . . . . . . . . . . . . . . . . . .  17
    7.2. Interference. . . . . . . . . . . . . . . . . . . . . . . .  18
     7.2.1. Multisession BGP . . . . . . . . . . . . . . . . . . . .  19
   8. Application Fit. . . . . . . . . . . . . . . . . . . . . . . .  19
    8.1. RFC 2547 Style VPNs . . . . . . . . . . . . . . . . . . . .  19
    8.2. VPWS. . . . . . . . . . . . . . . . . . . . . . . . . . . .  20
    8.3. VPLS. . . . . . . . . . . . . . . . . . . . . . . . . . . .  21
   9. Operational Implications . . . . . . . . . . . . . . . . . . .  22
   10. Other Models. . . . . . . . . . . . . . . . . . . . . . . . .  22
   11. Conclusions and Recommendations . . . . . . . . . . . . . . .  22



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   12. Intellectual Property . . . . . . . . . . . . . . . . . . . .  22
   13. Design Team . . . . . . . . . . . . . . . . . . . . . . . . .  22
   14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  23
   15. Security Considerations . . . . . . . . . . . . . . . . . . .  24
   16. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  24
   17. References. . . . . . . . . . . . . . . . . . . . . . . . . .  25
    17.1. Normative References . . . . . . . . . . . . . . . . . . .  25
    17.2. Informative References . . . . . . . . . . . . . . . . . .  27
   18. Editor's Address. . . . . . . . . . . . . . . . . . . . . . .  29
   19. Full Copyright Statement. . . . . . . . . . . . . . . . . . .  29









































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
   protocol.



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.




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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 Axillary
   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
   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



Meyer, et. al.                                    Section 4.6.  [Page 9]


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   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
   [RFC1997].



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
   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
   protocol.



Meyer, et. al.                                   Section 4.9.  [Page 10]


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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
          protocol?

   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.



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.







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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.
   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.








Meyer, et. al.                                     Section 6.  [Page 12]


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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.

    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
      applications?

    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



Meyer, et. al.                                 Section 6.1.2.  [Page 13]


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   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?





6.1.2.1.  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,
      do applications have to compete for per-process scheduling
      budgets?

    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



Meyer, et. al.                                   Section 6.2.  [Page 14]


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   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"
    [WILLINGER2002].

   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
   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).




Meyer, et. al.                                 Section 7.1.1.  [Page 15]


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      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
      applications?

    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.



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
      level)?

    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



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      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
   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}|)

    where

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


   Note that we assume that

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



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    where

    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)

    where

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

    That is, A(n) is approaches 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
   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,
   etc.




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.






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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 data distribution needs of three
   representative classes of application, namely:

    RFC 2547 Style VPNs
    VPWS
    VPLS



8.1.  RFC 2547 Style VPNs



   First, it is useful to review the distribution mechanisms available
   in BGP, in particular, in i-BGP.  i-BGP has been described loosely as
   a broadcast mechanism since an i-BGP speaker sends information to all
   its peers. This is typically achieved by means of one or more route
   reflectors; a more direct but less scalable means is for each i-BGP
   speaker to have a BGP session with each i-BGP peer.

   However, it is more accurate to characterize i-BGP as a constrained
   broadcast mechanism.  This is because the use of communities in
   conjunction with import and export policies allows an i-BGP speaker
   to effectively limit its communication to a subset of the full set of
   i-BGP peers; the efficiency of constrained broadcast can be improved
   by techniques such as described in [ORF] and [RTCONST].


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




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    (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 certain extended communities called Route Targets.  BGP is
   clearly eminently 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.

   Finally, it is useful to observe that standard BGP path selection
   mechanisms (local pref, MED, AS path length, etc.) can be applied to
   the information in (e).

   The conclusion is that BGP is quite well-suited to this application,
   and, with the addition of mechanisms such as [RTCONST] and [RFC2918],
   the fit is even closer.



8.2.  VPWS



   A VPN based on a Virtual Private Wire Service [VPWS] connects a
   number of sites by virtual wires (or pseudo-wires).  The information
   needed to create such a VPN comprises:

    (a). Membership (auto-discovery)



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    (b). VPN site identification
    (c). Labels
    (d). BGP nexthop
    (e). Path selection attributes, and
    (f). Per-wire information

   The analysis of the first five items is exactly as for RFC 2547 VPNs,
   with the slight change that the definition of a 'part of a VPN' is no
   longer an IP prefix, but is a VPN site identifier, which can be
   viewed as the VPWS prefix.  The distribution requirements and the fit
   with BGP distribution mechanisms is identical to RFC 2547.

   The one major change is the potential for 'per-wire' attributes, such
   as bandwidth for a given site-to-site connection.  This information
   should be distributed on a point-to-point basis.  BGP mechanisms are
   not efficient for point-to-point distribution.  However, it is an
   open question whether such 'per-wire' attributes really need to be
   exchanged, as evidenced by the fact that LDP signaling for pseudo-
   wires [MARTINI] has not defined any such attributes.  If per-wire
   information is indeed not necessary, BGP distribution mechanisms are
   as well-suited for VPWS VPNs as for RFC 2547 VPNs.

   Note that existing BGP path selection mechanisms can be used as is
   for VPWS, and can prove useful for multi-homed sites.



8.3.  VPLS



   A VPLS connects a number of sites by an emulated LAN segment.  The
   information needed to create a VPLS consists of:

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

   The notion of 'VPLS site identification' is analogous to a VPN site
   identifier for VPWS.  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.

   Note that existing BGP path selection mechanisms can be used as is
   for VPLS, and can prove useful for multi-homed sites.



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9.  Operational Implications





10.  Other Models





11.  Conclusions and Recommendations



12.  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
   Director.



13.  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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   (pwhiting@vericenter.com).




14.  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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15.  Security Considerations


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



16.  IANA Considerations


   This document creates a no new requirements on IANA namespaces
   [RFC2434].






































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17.  References

17.1.  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
                   progress.

   [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
                   progress.

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

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

   [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.




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

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

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

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

   [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.

   [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,
                   1998.

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





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

   [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


   [VLPS]          Kompella, K., et. al. "Virtual Private LAN
                   Service", draft-ietf-l2vpn-vpls-bgp-02.txt.
                   Work in progress.

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





17.2.  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,
                   1997.

   [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.



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   [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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18.  Editor's Address


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


19.  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
   English.

   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
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

















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Meyer, et. al.


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