path: root/doc/rfc/rfc5160.txt

Network Working Group                                           P. Levis
Request for Comments: 5160                                  M. Boucadair
Category: Informational                                   France Telecom
                                                              March 2008


           Considerations of Provider-to-Provider Agreements
              for Internet-Scale Quality of Service (QoS)

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

IESG Note

   This RFC is not a candidate for any level of Internet Standard.  The
   IETF disclaims any knowledge of the fitness of this RFC for any
   purpose and notes that the decision to publish is not based on IETF
   review apart from IESG review for conflict with IETF work.  The RFC
   Editor has chosen to publish this document at its discretion.  See
   RFC 3932 for more information.

Abstract

   This memo analyzes provider-to-provider Quality of Service (QoS)
   agreements suitable for a global QoS-enabled Internet.  It defines
   terminology relevant to inter-domain QoS models.  It proposes a new
   concept denoted by Meta-QoS-Class (MQC).  This concept could
   potentially drive and federate the way QoS inter-domain relationships
   are built between providers.  It opens up new perspectives for a QoS-
   enabled Internet that retains, as much as possible, the openness of
   the existing best-effort Internet.

















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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Assumptions and Requirements . . . . . . . . . . . . . . . . .  3
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Weaknesses of Provider-to-Provider QoS Agreements Based on
       SP Chains  . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     4.1.  IP Connectivity Services . . . . . . . . . . . . . . . . .  6
     4.2.  Similarity between Provider and Customer Agreements  . . .  6
     4.3.  Liability for Service Disruption . . . . . . . . . . . . .  7
     4.4.  SP Chain Trap Leading to Glaciation  . . . . . . . . . . .  7
   5.  Provider-to-Provider Agreements Based on Meta-QoS-Class  . . .  7
     5.1.  Single Domain Covering . . . . . . . . . . . . . . . . . .  8
     5.2.  Binding l-QCs  . . . . . . . . . . . . . . . . . . . . . .  9
     5.3.  MQC-Based Binding Process  . . . . . . . . . . . . . . . . 10
   6.  The Internet as MQC Planes . . . . . . . . . . . . . . . . . . 12
   7.  Towards End-to-End QoS Services  . . . . . . . . . . . . . . . 13
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 16
     10.2. Informative References . . . . . . . . . . . . . . . . . . 16

1.  Introduction

   Three different areas can be distinguished in IP QoS service
   offerings.  The first area is the single domain where a provider
   delivers QoS services inside the boundaries of its own network.  The
   second area is multiple domains where a small set of providers, with
   mutual business interests, cooperate to deliver QoS services inside
   the boundaries of their network aggregate.  The third area, which has
   very seldom been put forward, is the Internet where QoS services can
   be delivered from almost any source to any destination.  Both
   multiple domains and Internet areas deal with inter-domain aspects.
   However, they differ significantly in many ways, such as the number
   of domains and QoS paths involved, which are much higher and dynamic
   for the Internet area.  Multiple domains and Internet areas are
   therefore likely to differ in their respective solutions.  This memo
   is an attempt to investigate the Internet area from the point of view
   of provider-to-provider agreements.  It provides a framework for
   inter-domain QoS-enabled Internet.

   [MESCAL]provides a set of requirements to be met by any solution
   aiming to solve inter-domain QoS issues.  These requirements are not
   reproduced within this memo.  Readers are invited to refer to
   [MESCAL] for more elaborated description on the requirements.





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   This memo shows that for the sake of scalability, providers need not
   be concerned with what occurs more than one hop away (from their
   Autonomous System) when they negotiate inter-domain QoS agreements.
   They should base their agreements on nothing but their local QoS
   capabilities and those of their direct neighbors.  This analysis
   leads us to define terminology relevant to inter-domain QoS models.
   We also introduce a new concept denoted by Meta-QoS-Class (MQC) that
   drives and federates the way QoS inter-domain relationships are built
   between providers.  The rationale for the MQC concept relies on a
   universal and common understanding of QoS-sensitive applications
   needs.  Wherever end-users are connected, they experience the same
   QoS difficulties and are likely to express very similar QoS
   requirements to their respective providers.  Globally confronted with
   the same customer requirements, providers are likely to design and
   operate similar network capabilities.

   MQC brings up a simplified view of the Internet QoS capabilities as a
   set of MQC planes.  This memo looks at whether the idea of MQC planes
   can be helpful in certain well-known concrete inter-domain QoS
   issues.  The focus, however, is on the provider-to-provider QoS
   agreement framework, and the intention is not to specify individual
   solutions and protocols for a full inter-domain QoS system.  For
   discussion of a complete architecture based on the notion of parallel
   Internets that extends and generalizes the notion of MQC planes, see
   [AGAVE].

   Note that this document does not specify any protocols or systems.

2.  Assumptions and Requirements

   To avoid a great deal of complexity and scalability issues, we assume
   that provider-to-provider QoS agreements are negotiated only for two
   adjacent domains that are directly accessible to each other.  We also
   assume, because they exchange traffic, that these neighbors are BGP
   [RFC4271] peers.  This pairwise peering is logical, therefore it can
   be supported not only on physical point-to-point connections but also
   on Internet exchange points (IXPs), where many operators connect to
   each other using a layer 2 switch.

   The QoS solutions envisaged in this document are exclusively
   solutions suitable for the global Internet.  As far as Internet-wide
   solutions are concerned, this document assumes that:

   o  Any solutions should apply locally in order to be usable as soon
      as deployed in a small set of domains.






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   o  Any solutions should be scalable in order to allow a global
      deployment to almost all Internet domains, with the ability to
      establish QoS communications between any and all end-users.

   o  Any solutions should also maintain a best-effort service that
      should remain the preeminent service as a consequence of the end-
      to-end argument [E2E].

   o  If there is no path available within the requested QoS to reach a
      destination, this destination must remain reachable through the
      best-effort service.

   This memo does not place any specific requirements on the intra-
   domain traffic engineering policies and the way they are enforced.  A
   provider may deploy any technique to ensure QoS inside its own
   network.  This memo assumes only that QoS capabilities inside a
   provider's network can be represented as local-QoS-Classes (l-QCs).
   When crossing a domain, traffic experiences conditions characterized
   by the values of delay, jitter, and packet loss rate that correspond
   to the l-QC selected for that traffic within that domain.
   Capabilities can differ from one provider to another by the number of
   deployed l-QCs, by their respective QoS characteristics, and also by
   the way they have been implemented and engineered.

3.  Terminology

   (D, J, L)

      D: one-way transit delay [RFC2679], J: one-way transit delay
      variation or jitter [RFC3393], and L: packet loss rate [RFC2680].

   Domain

      A network infrastructure composed of one or several Autonomous
      Systems (AS) managed by a single administrative entity.

   IP connectivity service

      IP transfer capability characterized by a (Destination, D, J, L)
      tuple where Destination is a group of IP addresses and (D, J, L)
      is the QoS performance to get to the Destination.










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   Local-QoS-Class (l-QC)

      A QoS transfer capability across a single domain, characterized by
      a set of QoS performance parameters denoted by (D, J, L).  From a
      Diffserv [RFC2475] perspective, an l-QC is an occurrence of a Per
      Domain Behavior (PDB) [RFC3086].

   L-QC binding

      Two l-QCs from two neighboring domains are bound together once the
      two providers have agreed to transfer traffic from one l-QC to the
      other.

   L-QC thread

      Chain of neighboring bound l-QCs.

   Meta-QoS-Class (MQC)

      An MQC provides the limits of the QoS parameter values that two
      l-QCs must respect in order to be bound together.  An MQC is used
      as a label that certifies the support of a set of applications
      that bear similar network QoS requirements.

   Service Provider (SP)

      An entity that provides Internet connectivity.  This document
      assumes that an SP owns and administers an IP network called a
      domain.  Sometimes simply referred to as provider.

   SP chain

      The chain of Service Providers whose domains are used to convey
      packets for a given IP connectivity service.

4.  Weaknesses of Provider-to-Provider QoS Agreements Based on SP Chains

   The objective of this section is to show, by a sort of reductio ad
   absurdum proof, that within the scope of Internet services, provider-
   to-provider QoS agreements should be based on guarantees that span a
   single domain.

   We therefore analyze provider-to-provider QoS agreements based on
   guarantees that span several domains and emphasize their
   vulnerabilities.  In this case, the basic service element that a
   provider offers to its neighboring providers is called an IP
   connectivity service.  It uses the notion of SP chains.  We first
   define what an IP connectivity service is, and then we point out



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   several weaknesses of such an approach, especially the SP chain trap
   problem that leads to the so-called Internet glaciation era.

4.1.  IP Connectivity Services

   An IP connectivity service is a (Destination, D, J, L) tuple where
   Destination is a group of IP addresses reachable from the domain of
   the provider offering the service, and (D, J, L) is the QoS
   performance to get from this domain to Destination.  Destination is
   typically located in a remote domain.

   Provider-               /--------------SP chain---------------\
   oriented
   view         /--Agreement--\
              +----+       +----+    +----+    +----+       +----+
              |SP  +-------+SP  +----+SP  +----+SP  +- ... -+SP  |
              |n+1 |       |n   |    |n-1 |    |n-2 |       |1   |
              +----+       +----+    +----+    +----+       +----+
   Domain-            -----> packet flow                      /
   oriented                                              Destination
   view                    <----------- Guarantee Scope --------->

                     Figure 1: IP connectivity service

   In Figure 1, Provider SPn guarantees provider SPn+1 the level of QoS
   for crossing the whole chain of providers' domains (SPn, SPn-1,
   SPn-2, ...,SP1).  SPi denotes a provider as well as its domain.  The
   top of the figure is the provider-oriented view; the ordered set of
   providers (SPn, SPn-1, SPn-2, ...,SP1) is called an SP chain.  The
   bottom of the figure is the domain-oriented view.

4.2.  Similarity between Provider and Customer Agreements

   This approach maps end-users' needs directly to provider-to-provider
   agreements.  Providers negotiate agreements to a destination because
   they know customers are ready to pay for QoS guaranteed transfer to
   this destination.  As far as service scope is concerned, the
   agreements between providers will resemble the agreements between
   customers and providers.  For instance, in Figure 1, SPn can sell to
   its own customers the same IP connectivity service it sells to SPn+1.
   There is no clear distinction between provider-to-provider agreements
   and customer-to-provider agreements.

   In order to guarantee a stable service, redundant SP chains should be
   formed to reach the same destination.  When one SP chain becomes
   unavailable, an alternative SP chain should be selected.  In the
   context of a global QoS Internet, that would lead to an enormous
   number of SP chains along with the associated agreements.



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4.3.  Liability for Service Disruption

   In Figure 1, if SPn+1 sees a disruption in the IP connectivity
   service, it can turn only against SPn, its legal partner in the
   agreement.  If SPn is not responsible, in the same way, it can only
   complain to SPn-1, and so on, until the faulty provider is found and
   eventually requested to pay for the service impairment.  The claim is
   then supposed to move back along the chain until SPn pays SPn+1.  The
   SP chain becomes a liability chain.

   Unfortunately, this process is prone to failure in many cases.  In
   the context of QoS solutions suited for the Internet, SP chains are
   likely to be dynamic and involve a significant number of providers.
   Providers (that do not all know each other) involved in the same SP
   chain can be competitors in many fields; therefore, trust
   relationships are very difficult to build.  Many complex and critical
   issues need to be resolved: how will SPn+1 prove to SPn that the QoS
   level is not the level expected for a scope that can expand well
   beyond SPn?  How long will it take to find the guilty domain?  Is SPn
   ready to pay SPn+1 for something it does not control and is not
   responsible for?

4.4.  SP Chain Trap Leading to Glaciation

   In Figure 1, SPn implicitly guarantees SPn+1 the level of QoS for the
   crossing of distant domains like SPn-2.  As we saw in Section 4.2, SP
   chains will proliferate.  A provider is, in this context, likely to
   be part of numerous SP chains.  It will see the level of QoS it
   provides guaranteed by many providers it perhaps has never even heard
   of.

   Any change in a given agreement is likely to have an impact on
   numerous external agreements that make use of it.  A provider sees
   the degree of freedom to renegotiate, or terminate, one of its own
   agreements being restricted by the large number of external (to its
   domain) agreements that depend on it.  This is what is referred to as
   the "SP chain trap" issue.  This solution is not appropriate for
   worldwide QoS coverage, as it would lead to glaciation phenomena,
   causing a completely petrified QoS infrastructure, where nobody could
   renegotiate any agreement.

5.  Provider-to-Provider Agreements Based on Meta-QoS-Class

   If a QoS-enabled Internet is deemed desirable, with QoS services
   potentially available to and from any destination, any solution must
   resolve the above weaknesses and scalability problems and find
   alternate schemes for provider-to-provider agreements.




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5.1.  Single Domain Covering

   Due to the vulnerabilities of the SP chain approach, we assume
   provider-to-provider QoS agreements should be based on guarantees
   covering a single domain.  A provider guarantees its neighbors only
   the crossing performance of its own domain.  In Figure 2, the
   provider SPn guarantees the provider SPn+1 only the QoS performance
   of the SPn domain.  The remainder of this document will show that
   this approach, bringing clarity and simplicity into inter-domain
   relationships, is better suited for a global QoS Internet than one
   based on SP chains.

     Provider-
     oriented
     view                          /--Agreement--\
                                 +----+       +----+
                                 |SP  +-------+SP  +
                                 |n+1 |       |n   |
                                 +----+       +----+
     Domain-               -----> packet flow
     oriented                                 <---->
     view                                  Guarantee Scope

               Figure 2: provider-to-provider QoS agreement

   It is very important to note that the proposition to limit guarantees
   to only one domain hop applies exclusively to provider-to-provider
   agreements.  It does not in any way preclude end-to-end guarantees
   for communications.

   The simple fact that SP chains do not exist makes the AS chain trap
   problem and the associated glaciation threat vanish.

   The liability issue is restricted to a one-hop distance.  A provider
   is responsible for its own domain only, and is controlled by all the
   neighbors with whom it has a direct contract.















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5.2.  Binding l-QCs

   When a provider wants to contract with another provider, the main
   concern is deciding which l-QC(s) in its own domain it will bind to
   which l-QC(s) in the neighboring downstream domain.  The l-QC binding
   process becomes the basic inter-domain process.

                    Upstream          Downstream
                     domain            domain

                     l-QC21   ----->   l-QC11

                     l-QC22   ----->   l-QC12


                     l-QC23   ----->
                                       l-QC13
                     l-QC24   ----->

                          Figure 3: l-QC Binding

   If one l-QC were to be bound to two (or more) l-QCs, it would be very
   difficult to know which l-QC the packets should select.  This could
   imply a flow classification at the border of the domains based on
   granularity as fine as the application flows.  For the sake of
   scalability, we assume one l-QC should not be bound to several l-QCs
   [Lev].  On the contrary, several l-QCs can be bound to the same l-QC,
   in the way that l-QC23 and l-QC24 are bound to l-QC13 in Figure 3.

   A provider decides the best match between l-QCs based exclusively on:

   - What it knows about its own l-QCs;

   - What it knows about its neighboring l-QCs.

   It does not use any information related to what is happening more
   than one domain away.

   Despite this one-hop, short-sighted approach, the consistency and the
   coherency of the QoS treatment must be ensured on an l-QC thread
   formed by neighboring bound l-QCs.  Packets leaving a domain that
   applies a given l-QC should experience similar treatment when
   crossing external domains up to their final destination.  A provider
   should bind its l-QC with the neighboring l-QC that has the closest
   performance.  The criteria for l-QC binding should be stable along
   any l-QC thread.  For example, two providers should not bind two
   l-QCs to minimize the delay whereas further on, on the same thread,
   two other providers have bound two l-QCs to minimize errors.



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   Constraints should be put on l-QC QoS performance parameters to
   confine their values to an acceptable and expected level on an l-QC
   thread scale.  These constraints should depend on domain size; for
   example, restrictions on delay should authorize a bigger value for a
   national domain than for a regional one.  Some rules must therefore
   be defined to establish in which conditions two l-QCs can be bound
   together.  These rules are provided by the notion of Meta-QoS-Class
   (MQC).

5.3.  MQC-Based Binding Process

   An MQC provides the limits of the QoS parameters two l-QCs must
   respect in order to be bound together.  A provider goes through
   several steps to extend its internal l-QCs through the binding
   process.  Firstly, it classifies its own l-QCs based on MQCs.  An MQC
   is used as a label that certifies the support of a set of
   applications that bear similar network QoS requirements.  It is a
   means to make sure that an l-QC has the appropriate QoS
   characteristics to convey the traffic of this set of applications.
   Secondly, it learns about available MQCs advertised by its neighbors.
   To advertise an MQC, a provider must have at least one compliant l-QC
   and should be ready to reach agreements to let neighbor traffic
   benefit from it.  Thirdly, it contracts an agreement with its
   neighbor to send some traffic that will be handled according to the
   agreed MQCs.

   The following attributes should be documented in any specification of
   an MQC.  This is not a closed list, other attributes can be added if
   needed.

   o  A set of applications (e.g., VoIP) the MQC is particularly suited
      for.

   o  Boundaries or intervals of a set of QoS performance attributes
      whenever required.  For illustration purposes, we propose to use
      in this document attribute (D, J, L) 3-tuple.  An MQC could be
      focused on a single parameter (e.g., suitable to convey delay
      sensitive traffic).  Several levels could also be specified
      depending on the size of the network provider; for instance, a
      small domain (e.g., regional) needs lower delay than a large
      domain (e.g., national) to match a given MQC.

   o  Constraints on traffic (e.g., only TCP-friendly).

   o  Constraints on the ratio: network resources for the class /
      overall traffic using this class (e.g., less resources than peak
      traffic).




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   Two l-QCs can be bound together if, and only if, they conform to the
   same MQC.

   Provider-to-provider agreements, as defined here, are uni-
   directional.  They are established for transporting traffic in a
   given direction.  However, from a business perspective, it is likely
   that reverse agreements will also be negotiated for transporting
   traffic in the opposite direction.  Note that uni-directional
   provider-to-provider agreements do not preclude having end-to-end
   services with bi-directional guarantees, when you consider the two
   directions of the traffic separately.

   Two providers negotiating an agreement based on MQC will have to
   agree on several other parameters that are outside the definition of
   MQC.  One such obvious parameter is bandwidth.  The two providers
   agree to exchange up to a given level of QoS traffic.  This bandwidth
   level can then be further renegotiated, inside the same MQC
   agreement, to reflect an increase in the end-user QoS requests.
   Other clauses of inter-domain agreements could cover availability of
   the service, time of repair, etc.

   A hierarchy of MQCs can be defined for a given type of service (e.g.,
   VoIP with different qualities: VoIP for residential and VoIP for
   business).  A given l-QC can be suitable for several MQCs (even
   outside the same hierarchy).  Several l-QCs in the same domain can be
   classified as belonging to the same MQC.  There is an MQC with no
   specific constraints called the best-effort MQC.

   There is a need for some form of standardization to control QoS
   agreements between providers [RFC3387].  Each provider must have the
   same understanding of what a given MQC is about.  We need a global
   agreement on a set of MQC standards.  The number of classes to be
   defined must remain very small to avoid overwhelming complexity.  We
   also need a means to certify that the l-QC classification made by a
   provider conforms to the MQC standards.  So the standardization
   effort should be accompanied by investigations on conformance testing
   requirements.

   The three notions of PDB, Service Class [RFC4594], and MQC are
   related; what MQC brings is the inter-domain aspect:

   - PDB is how to engineer a network;

   - Service Class is a set of traffic with specific QoS requests;

   - MQC is a way to classify the QoS capabilities (l-QCs, through
     Diffserv or any other protocols or mechanisms) in order to reach
     agreements with neighbors.  MQCs are only involved when a provider



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     wants to negotiate an agreement with a neighboring provider.  MQC
     is completely indifferent to the way networks are engineered as
     long as the MQC QoS attribute (e.g., (D, J, L)) values are reached.

6.  The Internet as MQC Planes

   The resulting QoS Internet can be viewed as a set of parallel
   Internets or MQC planes.  Each plane consists of all the l-QCs bound
   according to the same MQC.  An MQC plane can have holes and isolated
   domains because QoS capabilities do not cover all Internet domains.
   When an l-QC maps to several MQCs, it belongs potentially to several
   planes.

   When a provider contracts with another provider based on the use of
   MQCs, it simply adds a logical link to the corresponding MQC plane.
   This is basically what current traditional inter-domain agreements
   mean for the existing Internet.  Figure 4a) depicts the physical
   layout of a fraction of the Internet, comprising four domains with
   full-mesh connectivity.

                +----+    +----+               +----+    +----+
                |SP  +----+SP  |               |SP  +----+SP  |
                |1   |    |2   |               |1   |    |2   |
                +-+--+    +--+-+               +-+--+    +----+
                  |   \  /   |                   |      /
                  |    \/    |                   |     /
                  |    /\    |                   |    /
                  |   /  \   |                   |   /
                +-+--+    +--+-+               +-+--+    +----+
                |SP  +----+SP  |               |SP  |    |SP  |
                |4   |    |3   |               |4   |    |3   |
                +----+    +----+               +----+    +----+
                a) physical configuration      b) an MQC plane

                           Figure 4: MQC planes

   Figure 4 b) depicts how these four domains are involved in a given
   MQC plane.  SP1, SP2, and SP4 have at least one compliant l-QC for
   this MQC; SP3 may or may not have one.  A bi-directional agreement
   exists between SP1 and SP2, SP1 and SP4, SP2 and SP4.

   MQC brings a clear distinction between provider-to-provider and
   customer-to-provider QoS agreements.  We expect a great deal of
   difference in dynamicity between the two.  Most provider-to-provider
   agreements should have been negotiated, and should remain stable,
   before end-users can dynamically request end-to-end guarantees.
   Provider agreements do not directly map end-users' needs; therefore,
   the number of provider agreements is largely independent of the



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   number of end-user requests and does not increase as dramatically as
   with SP chains.

   For a global QoS-based Internet, this solution will work only if MQC-
   based binding is largely accepted and becomes a current practice.
   This limitation is due to the nature of the service itself, and not
   to the use of MQCs.  Insofar as we target global services, we are
   bound to provide QoS in as many SP domains as possible.  However, any
   MQC-enabled part of the Internet that forms a connected graph can be
   used for QoS communications and can be extended.  Therefore,
   incremental deployment is possible, and leads to incremental
   benefits.  For example, in Figure 4 b), as soon as SP3 connects to
   the MQC plane it will be able to benefit from the SP1, SP2, and SP4
   QoS capabilities.

   The Internet, as a split of different MQC planes, offers an ordered
   and simplified view of the Internet QoS capabilities.  End-users can
   select the MQC plane that is the closest to their needs, as long as
   there is a path available for the destination.  One of the main
   outcomes of applying the MQC concept is that it alleviates the
   complexity and the management burden of inter-domain relationships.

7.  Towards End-to-End QoS Services

   Building end-to-end QoS paths, for the purpose of QoS-guaranteed
   communications between end-users, is going a step further in the QoS
   process.  The full description of customer-to-provider QoS
   agreements, and the way they are enforced, is outside the scope of
   this memo.  However, in this section, we will list some important
   issues and state whether MQC can help to find solutions.

   Route path selection within a selected MQC plane can be envisaged in
   the same way as the traditional routing system used by Internet
   routers.  Thus, we can rely on the BGP protocol, basically one BGP
   occurrence per MQC plane, for the inter-domain routing process.  The
   resilience of the IP routing system is preserved.  If a QoS path
   breaks somewhere, the routing protocol enables dynamic computation of
   another QoS path, if available, in the proper MQC plane.  This
   provides a first level of QoS infrastructure that could be
   conveniently named "best-effort QoS", even if this is an oxymoron.

   On this basis, features can be added in order to select and control
   the QoS paths better.  For example, BGP can be used to convey QoS-
   related information, and to implement a process where Autonomous
   Systems add their own QoS values (D, J, L) when they propagate an AS
   path.  Then, the AS path information is coupled with the information
   on Delay, Jitter, and Loss, and the decision whether or not to use
   the path selected by BGP can be made, based on numerical values.  For



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   example, for destination N, an AS path (X, Y) is advertised to AS Z.
   During the propagation of this AS path by BGP, X adds the information
   concerning its own delay, say 30 ms, and Y adds the information
   concerning its own delay, say 20 ms.  Z receives the BGP
   advertisement (X, Y, N, 50 ms).  One of Z's customers requests a
   delay of 100 ms to reach N.  Z knows its own delay for this customer,
   say 20 ms.  Z computes the expected maximum delay from its customer
   to N: 70 ms, and concludes that it can use the AS path (X, Y).  The
   QoS value of an AS path could also be disconnected from BGP and
   computed via an off-line process.

   If we use QoS routing, we can incorporate the (D, J, L) information
   in the BGP decision process, but that raises the issue of composing
   performance metrics in order to select appropriate paths [Chau].
   When confronted by multiple incompatible objectives, the local
   decisions made to optimize the targeted parameters could give rise to
   a set of incomparable paths, where no path is strictly superior to
   the others.  The existence of provider-to-provider agreements based
   on MQC offers a homogenous view of the QoS parameters, and should
   therefore bring coherency, and restrict the risk of such non-optimal
   choices.

   A lot of end-to-end services are bi-directional, so one must measure
   the composite performance in both directions.  Many inter-domain
   paths are asymmetric, and usually, some providers involved in the
   forward path are not in the reverse path, and vice versa.  That means
   that no assumptions can be made about the reverse path.  Although
   MQC-based provider-to-provider agreements are likely to be negotiated
   bi-directionally, they allow the inter-domain routing protocol to
   compute the forward and the reverse paths separately, as usual.  The
   only constraint MQC puts on routing is that the selected paths must
   use the chosen MQCs throughout the selected domains.  There might be
   a different MQC requirement in the reverse direction than in the
   forward direction.  To address this problem, we can use application-
   level communication between the two parties (customers) involved in
   order to specify the QoS requirements in both directions.

   We can go a step further in the control of the path to ensure the
   stability of QoS parameters such as, e.g., enforcing an explicit
   routing scheme, making use of RSVP-TE/MPLS-TE requests [RFC3209],
   before injecting the traffic into an l-QC thread.  However,
   currently, several problems must be resolved before ready and
   operational solutions for inter-domain route pinning, inter-domain
   TE, fast failover, and so forth, are available.  For example, see the
   BGP slow convergence problem in [Kushman].

   Multicast supports many applications such as audio and video
   distribution (e.g., IPTV, streaming applications) with QoS



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   requirements.  Along with solutions at the IP or Application level,
   such as Forward Error Correction (FEC), the inter-domain multicast
   routing protocol with Multiprotocol Extensions for BGP-4 [RFC4760],
   could be used to advertise MQC capabilities for multicast source
   reachability.  If an inter-domain tree that spans several domains
   remains in the same MQC plane, it would be possible to benefit from
   the consistency and the coherency conferred by MQC.

   Note that the use of some QoS parameters to drive the route selection
   process within an MQC plane may induce QoS deterioration since the
   best QoS-inferred path will be selected by all Autonomous System
   Border Routers (ASBRs) involved in the inter-domain path computation
   (i.e., no other available routes in the same MQC plane will have a
   chance to be selected).  This problem was called the QoS Attribute-
   rush (QA-rush) in [Grif].  This drawback may be avoided if all
   involved ASes in the QoS chain implement some out-of-band means to
   control the inter-domain QoS path consistency (MQC compliance).

   To conclude this section, whatever the protocols we want to use, and
   however tightly we want to control QoS paths, MQC is a concept that
   can help to resolve problems [WIP], without prohibiting the use of
   any particular mechanism or protocol at the data, control, or
   management planes.

8.  Security Considerations

   This document describes a framework and not protocols or systems.
   Potential risks and attacks will depend directly on the
   implementation technology.  Solutions to implement this proposal must
   detail security issues in the relevant protocol documentation.

   Particular attention should be paid to giving access to MQC resources
   only to authorized traffic.  Unauthorized access can lead to denial
   of service when the network resources get overused [RFC3869].

9.  Acknowledgements

   This work is funded by the European Commission, within the context of
   the MESCAL (Management of End-to-End Quality of Service Across the
   Internet At Large) and AGAVE (A liGhtweight Approach for Viable End-
   to-end IP-based QoS Services) projects.  The authors would like to
   thank all the other partners for the fruitful discussions.

   We are grateful to Brian Carpenter, Jon Crowcroft, and Juergen
   Quittek for their helpful comments and suggestions for improving this
   document.





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10.  References

10.1.  Normative References

   [RFC2679]  Almes, G., Kalidindi, S., and M. Zekauskas, "A One-way
              Delay Metric for IPPM", RFC 2679, September 1999.

   [RFC2680]  Almes, G., Kalidindi, S., and M. Zekauskas, "A One-way
              Packet Loss Metric for IPPM", RFC 2680, September 1999.

   [RFC3393]  Demichelis, C. and P. Chimento, "IP Packet Delay Variation
              Metric for IP Performance Metrics (IPPM)", RFC 3393,
              November 2002.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              January 2006.

10.2.  Informative References

   [AGAVE]    Boucadair, et al., "Parallel Internets Framework", IST
              AGAVE project public deliverable D1.1, September 2006.

   [Chau]     Chau, C., "Policy-based routing with non-strict
              preferences", Proceedings of the ACM SIGCOMM 2006
              Conference on Applications, Technologies, Architectures,
              and Protocols for Computer Communications, Pisa, Italy, pp
              387-398, September 2006.

   [E2E]      Saltzer, J H., Reed, D P., and D D. Clark, "End-To-End
              Arguments in System Design", ACM Transactions in Computer
              Systems, Vol 2, Number 4, pp 277-288, November 1984.

   [Grif]     Griffin, D., Spencer, J., Griem, J., Boucadair, M.,
              Morand, P., Howarth, M., Wang, N., Pavlou, G., Asgari, A.,
              and P. Georgatsos, "Interdomain routing through QoS-class
              planes [Quality-of-Service-Based Routing Algorithms for
              Heterogeneous Networks]",  IEEE Communications
              Magazine, Vol 45, Issue 2, pp 88-95, February 2007.

   [Kushman]  Kushman, N., Kandula, S., and D. Katabi, "Can You Hear Me
              Now?! It Must Be BGP", ACM Journal of Computer and
              Communication Review CCR, November 2007.

   [Lev]      Levis, P., Asgari, H., and P. Trimintzios, "Consideration
              on Inter-domain QoS and Traffic Engineering issues Through
              a Utopian Approach", SAPIR-2004 workshop of ICT-2004, (C)
              Springer-Verlag, August 2004.



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   [MESCAL]   Flegkas, et al., "Specification of Business Models and a
              Functional Architecture for Inter-domain QoS Delivery",
              IST MESCAL project public deliverable D1.1, May 2003.

   [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
              and W. Weiss, "An Architecture for Differentiated
              Services", RFC 2475, December 1998.

   [RFC3086]  Nichols, K. and B. Carpenter, "Definition of
              Differentiated Services Per Domain Behaviors and Rules for
              their Specification", RFC 3086, April 2001.

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, December 2001.

   [RFC3387]  Eder, M., Chaskar, H., and S. Nag, "Considerations from
              the Service Management Research Group (SMRG) on Quality of
              Service (QoS) in the IP Network", RFC 3387,
              September 2002.

   [RFC3869]  Atkinson, R., Ed., Floyd, S., Ed., and Internet
              Architecture Board, "IAB Concerns and Recommendations
              Regarding Internet Research and Evolution", RFC 3869,
              August 2004.

   [RFC4594]  Babiarz, J., Chan, K., and F. Baker, "Configuration
              Guidelines for DiffServ Service Classes", RFC 4594,
              August 2006.

   [RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4", RFC 4760,
              January 2007.

   [WIP]      Deleuze, G. and F. Guattari, "What Is Philosophy?",
              Columbia University Press ISBN: 0231079893, April 1996.















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Authors' Addresses

   Pierre Levis
   France Telecom
   42 rue des Coutures
   BP 6243
   Caen Cedex 4  14066
   France

   EMail: pierre.levis@orange-ftgroup.com


   Mohamed Boucadair
   France Telecom
   42 rue des Coutures
   BP 6243
   Caen Cedex 4  14066
   France

   EMail: mohamed.boucadair@orange-ftgroup.com































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