Internet DRAFT - draft-wright-mpls-te-policy


tewg/mpls                                                      S.Wright
Internet Draft                                                BellSouth
Document: draft-wright-mpls-te-policy-00.txt               F.Reichmeyer
Category: Informational                                      IP Highway
                                                    LTS, U. of Maryland

                                                             June, 2000

    Policy-Based Load-Balancing in Traffic-Engineered MPLS Networks

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 [1].

   Internet-Drafts are working documents of the Internet Engineering
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1. Abstract

   This document considers the application of policy based traffic
   engineering techniques to load-balancing in MPLS networks. The
   objective is not to mandate a specific policy, but to (a) provide an
   example of a policy based approach to a traffic engineering problem
   and (b) elucidate a range of potential policies related to load
   balancing as a traffic engineering objective as guidance for the
   development of a Policy Information Base (PIB) for MPLS networks.

2. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   this document are to be interpreted as described in RFC-2119 [2].

3. Introduction

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   Traffic Engineering within IP networks is a broad subject outlined
   within the traffic engineering framework document [3]. This draft
   takes a considerably narrower approach in considering just one
   aspect of traffic engineering in order to scope the work.

   RFC 2702 [4] section 3.2 identifies three fundamental problems in
   traffic engineering over MPLS networks - mapping traffic to
   Forwarding Equivalence Classes (FECs), mapping FEC's to label
   Switched Paths (LSPs), and mapping LSPs to physical topology. This
   draft seeks to address aspects of the first two objectives only.

   Specifically, this draft focuses on:
   a) MPLS as an interesting subset of IP protocols
   b) load balancing as a traffic engineering objective
   c) Policy based approaches for describing the objectives and
   constraints of the traffic engineering optimization.

   The MPLS architecture draft identifies load balancing as a potential
   application of MPLS. The load balancing problem in MPLS networks
   concerns the allocation of traffic between two or more LSPs which
   all have the same origin and destination.

   The notion of policy-based MPLS networks is introduced in [5] and
   expanded in [6], in conformance with the generic policy work from
   the policy working group  (see e.g.[7]) and the COPS material from
   the rap working group (see e.g. [8]).

3.1 Terminology

   Load Balancing vs Link Bundling

   In some cases a pair of LSRs may be connected by several (parallel)
   links. From the MPLS Traffic Engineering point of view, for the
   purpose of scalability, it may be desirable to treat all these links
   as a single IP link - an operation known as Link Bundling ( ref.
   [9]). With load balancing, the load to be balanced is spread across
   multiple LSPs that in general does not require physical topology
   adjacency for the LSRs. The techniques are complementary. Link
   bundling provides a local optimization that is particularly suited
   for aggregating low speed links. Load Balancing is targeted at
   larger scale network optimizations.

   While load balancing is perhaps commonly thought of as applying
   between edge LSRs, it could be applied at ANY LSR that provides the
   requisite multiple LSP tunnels with common endpoints. The Policy
   Enforcement Point is the LSR at the source end of the set of LSPs
   with common endpoints. The arriving traffic to be load balanced may
   be from non-MPLS interfaces or MPLS interfaces. In general, the
   source end of an LSP may act as a merge point for multiple input
   streams of traffic.

3.2. Assumptions / Scope

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   This document considers only policies related to load balancing
   objectives. While policy based approaches may be applicable to other
   traffic engineering objectives, the focus here is on load balancing.

   The set of LSPs over which the load is to be balanced is pre-defined
   and the relevant load balancing policies are then applied to these
   LSPs. Extension of this work to support the creation or deletion of
   LSPs in response to policies with load balancing objectives is for
   further study.

   This version of the document is primarily focused on MPLS networks
   without quantitative guarantees on LSP performance in order to
   reduce the effort involved. Future versions of the document may
   include increased support for the various QoS notions supported in
   MPLS networks. By removing the QoS elements from the discussion at
   this stage, we can concentrate on getting the fundamental operation
   correct, without debating the details of QoS or admission control

   By not considering the QoS notions of MPLS, we simplify the policy
   and functions associated with load balancing. Since only best effort
   LSPs are considered, we simplify the admission control
   considerations in the load balancing process. If the LSPs were
   established with QoS constraints, it would be necessary to determine
   if the traffic flow sent over the LSP as a result of load balancing
   fit the profile of the constraints. This adds complexity to the load
   balancing policy as well as the processing of the LSR performing the
   load balancing.

   While load balancing on a best effort network can be viewed as a
   simple case, the basic methodologies have a wider applicability when
   applied to QoS-based LSP selection. Indeed the load balancing case
   for best effort only traffic has similar problems to that of load
   balancing a particular traffic class  such as that with a particular
   DiffServ PHB.  Bandwidth sharing among classes of service raises
   some more complex issues that also apply to the placement of traffic
   into ER-LSPs. As the available capacity for a particular traffic
   class to a particular destination exceeds the capacity of the (ER-
   )LSP for that traffic, some action must be taken to get more
   bandwidth or control access to the LSP.   The PEP will have to
   perform some per traffic class action with the  likely result that
   the best effort  traffic on the network will become squeezed in
   favor of higher priority traffic.  The lending of bandwidth between
   LSPs is a policy issue that must be addressed.   The location of the
   network congestion will have a bearing on a solution.  A policy
   server can initiate a new LSP and map certain flows to this to avoid
   the congestion point, thereby improving the performance of those
   flows and reducing the congestion problem. This would, however,
   require a congestion detection methodology and inter-PDP

4. Policy-based Traffic Engineering

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   A policy provides a rule of the form:
     IF <condition> THEN <action>

   Policy based networking is one of a number of mechanisms that could
   be used in achieving traffic engineering objectives. While traffic
   engineering may be considered an optimization problem, policy
   approaches provide considerable flexibility in the specification of
   the network optimization objectives and constraints.

4.1 Policy-based Traffic Engineering in the Context of the Traffic
Engineering Framework

   Within the TE framework's taxonomy of traffic engineering systems,
   policies may be :
        dependent on time or network state ( either local or global
        based on algorithms executed offline or online
        stored centrally ( e.g. in a directory) or distributed to an
                engineerable number of policy decision points
        prescriptive or descriptive
        designed for open loop or closed loop network control

   We assume network feedback to be an important part of policy-based
   networking. While network configuration (provisioning) can be
   performed in an open-loop manner, in general, policy-based
   networking should imply a closed-loop mechanism. The distribution
   and performance of the policy system itself is beyond the scope of
   this draft - we assume that adequate resources can be provisioned to
   meet the required policy update frequency etc.

   The traffic engineering framework identifies process model
   components for :
        Modeling, analysis and simulation

   Policies may be used to identify relevant measurements available
   through the network and trigger appropriate actions. The available
   traffic metrics for determining the policy trigger conditions are
   constrained e.g. by generic IP traffic metrics (see e.g. RFC 2679
   [10] or RFC 2722 [11]).

   Policies provide an abstraction of network resources - a model that
   can be designed to achieve traffic engineering objectives. Policies
   can provide a degree of analysis by identifying network problem
   through correlation of various measurements of network state. A
   policy based network is not, however, a network simulation.

   A set of policies can be designed to achieve an optimization of
   network performance through appropriate network provisioning

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4.2 Policy Based Load Balancing

   Load Balancing, as a traffic engineering objective, is not described
   in great detail within the TE framework draft. There is some mention
   of it as an example of a relatively "simple" and "fast" computation
   that may be suitable for online traffic engineering in section
   5.2.It is also identified as a traffic engineering objective within
   section 6.6 on content distribution (webserver) requirements.  In
   general though, load balancing would seem to fit within section 6.3
   of the TE framework draft as an example of traffic mapping.

   The relative simplicity of load balancing algorithms is what makes
   them attractive for the purposes of this draft in illustrating
   policy approaches to traffic engineering in the context of MPLS

   While load balancing optimizations have been proposed for various
   routing protocols, such approaches complicate existing routing
   protocols and tend to optimize towards a fairly limited set of load
   balancing objectives. Extending these towards more flexible /
   dynamic load balancing objectives seems overly complicated. Hence
   this draft takes the approach of building on the policy-based
   networking architecture which provides mechanisms specifically
   designed to support flexible and dynamic administration.

5. Policy-Based MPLS Network Context for Load Balancing
   Figure 1 illustrates a generic policy based network architecture in
   the context of an MPLS network. In this example, we have two LSPs
   established :
   LSP #1 that follows the path(abd)
   LSP #2 that follows the path(acd).

   In this draft we do not consider the problems of establishing the
   LSPs. We assume that a variety of mechanisms may be used for either
   manual(e.g. LSPs provision explicit routes) or automated (e.g. LSPs
   based on topology driven or data driven shortest path routes)
   establishment of the LSPs. Future versions of this or another draft
   may address the issue of establishing LSPs via policy mechanisms
   (e.g. using COPS push.

   The load balancing operation is performed at the LSR containing the
   ingress of the LSPs to be load balanced. This LSR ((a) in Figure 1)
   is acting as the Policy Enforcement Point for load-balancing
   policies related to these LSPs. In this context the load-balancing
   problem concerns the selection of suitable policies to control the
   admission of flows to both LSPs.

   The admission decision for an LSP is reflected in the placement of
   that LSP as the Next Hop Forwarding Label Entry  (NHFLE) within the
   appropriate routing tables within the LSR. Normally, there is only
   one NHLFE corresponding to each FEC, however there are some
   circumstances where multiple NHLFEs may exist for an FEC.

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   The conditions for the policies applying to the set of LSPs to be
   load balanced should be consistent. For example, if the condition
   used to allocate flows between LSPs is the source address range,
   then the set of policies applied to the set of LSPs should account
   for the disposition of the entire source address range.

                  +   Policy   +
                  + Management +
                  +    Tool    +
                    |\       |\
                    |        |
                    |        | (E.g., JAVA, LDAP)
                    |       ++++++++++++++
                    |       +   Policy   +
                    |       + Repository +
                    |       +            +
                    |       ++++++++++++++
                    |        |\
                    |        |
                    |        | (e.g. LDAP)
                  +   Policy   +
                  +  Decision  +
                  +    Point   +
                   /   / |    \
                  /   /  |     +-------+
                 /   /   |              \   (e.g. COPS, SNMP)
    +++++++++++++++  | ++++++++++++++   +++++++++++++++
    + ELSR(a)     +----+ LSR (b)    +---+ ELSR (d)    +    
    +++++++++++++++  | ++++++++++++++   +++++++++++++++
                  \  |                    /
                   \ +--\                /
                    \  ++++++++++++++   /
                     \-+ LSR (c)    +--/

   Figure 1 LSR as PEP

   For policy-based MPLS networks, traffic engineering policies would
   also be able to utilize for both conditions and actions the
   parameters available in the standard MPLS MIBs i.e.

        MPLS Traffic Engineering MIB [12],
        MPLS LSR MIB [13]
        MPLS Packet Classifier MIB [14]

   MIB elements for additional traffic metrics are for further study
   beyond the scope of this internet draft.

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5.1 Load Balancing at Edge of MPLS Domain

   Flows admitted to an LSP at the edge of an MPLS domain are described
   by the set of Forwarding Equivalence Classes (FECs) that are mapped
   to the LSPs in the FEC to NHLFE (FTN) table.

   The load-balancing operation may be considered as redefining the
   FECs to send traffic along the appropriate path. Rather than sending
   all the traffic along a single LSP, the load balancing policy
   operation results in the creation of new FECs which effectively
   partition the traffic flow among the LSPs in order to achieve some
   load balance objective.

   Consider as an example, two simple point-to-point LSPs, (a) and (b),
   with the same source and destination, over which we are to load
   balance some aggregate FEC (z). The aggregate FEC (z) is the union
   of FEC (a) and FEC (b). The load balancing policy may adjust the FEC
   (a) and FEC (b) definitions such that the aggregate FEC (z) is

5.2 Load Balancing at interior of MPLS Domain

   Flows admitted to an LSP at the interior of an MPLS domain are
   described by the set of labels that are mapped to the LSPs in the
   Incoming label Map (ILM).

   A Point-to-Point LSP that simply transits an LSR at the interior of
   an MPLS domain does not have an LSP ingress at this transit LSR.

   Merge points of a Multipoint-to-Point LSP may be considered as
   ingress points for the next link of the LSP.

   A label stacking operation may be considered as an ingress point to
   a new LSP.

   The above conditions, which put multiple LSPs onto different LSPs,
   may require balancing at the interior node.  The FEC of an incoming
   flow may be inferred from its label. Hence load-balancing policies
   may operate based on incoming labels to segregate traffic rather
   than requiring the ability to walk up the incoming label stack to
   the packet header in order to reclassify the packet. The result is a
   coarse load balancing of LSPs (not flows) onto one of a number of
   LSPs from the LSR to the egress LSR

5.3 Load Balancing with multiple NHLFEs

   The MPLS Architecture identifies that the NHLFE may have multiple
   entries for one FEC. Load balancing is suggested as an example
   rationale for this, but it is not the only interpretation of
   multiple NHLFE entries. Other interpretations may exist and the

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   potential interaction with load balancing should be considered.
   Multiple NHLFEs may be present to represent:
   (a) the Incoming FEC / label set is to be multicast
   (b) when route selection based on the EXP field in addition to the
   label is required.

   If both multicast and load balancing functions are required, it is
   necessary to disambiguate the scope of the operations. The way we
   have defined the load balancing operation, it partitions a set of
   input traffic (defined as FECs or Labels) across a set of output
   LSPs. One (or more) of the arriving FECs may be multicast to both
   the set of load balanced LSPs as well as other LSPs. This would
   imply that the packet replication (multicast) function occurs before
   the load balancing.

   When the route selection is based on the EXP field, this appears to
   simply be a special case of the policy based load-balancing
   approach. It is suggested replicating NHLFEs for this purpose be
   deprecated and the more generic policy based approach be used to
   specify an FEC/ label space partition based on the EXP field.

   In this draft, we take the perspective that the load balancing
   function is more properly considered as part of the classification
   function. This allows us to preserve a mapping of an FEC into one
   NHLFE for unicast. While classification of incoming flows into FECs
   is commonly thought of as an operation on some tuple of packet
   headers, this is not the only basis for classification - router
   state can also be used. For example, the source port of a flow may
   be a useful basis on which to discriminate flows. Equally, a "random
   number" generated within the router may be attractive as the basis
   for allocating flows for a load balancing objective. Some algorithm
   within the router, which may include some hash function on the
   packet headers, may generate the "random number".

6. MPLS Policies for Load Balancing

   MPLS load balancing partitions an incoming stream of traffic across
   multiple LSPs. The load balancing policy, as well as the ingress LSR
   where the policy is enforced, must be able to distinctly identify
   LSPs. We do not, in this draft, address the issue of how LSPs are
   established. It is assumed, however, that the PDP that installs the
   load balancing policy, has knowledge of the existing LSPs and is
   able to identify them in policy rules. One way to achieve this is
   through the binding of a label to an LSP.

   An example MPLS load-balancing policy may state, for the simple case
   of balancing across two LSPs, If traffic matches classifier C,
   then forward on LSP L1, else forward on LSP L2. Classification
   can be done on a number of parameters, such as packet header fields,
   incoming labels, etc. The classification conditions of an MPLS load-
   balancing policy are thus effectively constrained to be able to

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   specify the FEC in terms that can be resolved into MPLS packet
   classification MIB parameters.

   Forwarding traffic on an LSP can be achieved by tagging the traffic
   with the appropriate label corresponding to the LSP. MPLS load-
   balancing policy actions typically result in the definition of a new
   aggregate FEC to be forwarded down a specific LSP. This would
   typically be achieved by appropriate provisioning of the FEC and
   routing tables (FTN and ILM).

   The basis for partitioning the traffic can be static or dynamic.
   Dynamic load balancing can be based on a dynamic administrative
   control (e.g. Time of Day), or it can form a closed control loop
   with some measured network parameter.

   Static Partitioning of the Load can be based on information carried
   within the packet header (e.g. source / destination addresses,
   Source / destination port numbers, packet size, protocol ID etc.).
   Static partitioning can also be based on other information available
   at the LSR (e.g. the arriving physical interface). However if load
   partition is truly static, or at least very slowly changing (say < 1
   change / day ?) then the need for a policy based control of this
   provisioning information maybe debatable and a direct manipulation
   of the LSR MIB may suffice.

   A control-loop based load-balancing scheme seeks to balance the load
   close to some objective, subject to error in the measurements and
   delays in the feedback loop etc. The objective may be based on a
   fraction of the input traffic to be sent down a link (e.g. 20% down
   LSP (abd) and 80% down LSP (acd)) in which case some measurement of
   the input traffic is required. The objective may also be based on
   avoiding congestive loss in which case some loss metric is required.

   The metrics required for control loop load balancing may be derived
   from information available locally at the upstream LSR, or may be
   triggered by events distributed elsewhere in the network. In the
   latter case, the metrics must be delivered to the Policy Decision
   Point.  Obviously, locally derived trigger conditions would be
   expected to avoid the propagation delays etc. associated with the
   general distributed case. Frequent notification of the state of
   these metrics increases network traffic which may be undesirable.
   This draft does not seek to provide guidance on the appropriate rate
   of notification for metric updates.

   Consider the case of a single large flow that must be load balanced
   across a set of links. In this case policies based solely on the
   packet headers may be inadequate and some other approach (e.g. based
   on a random number generated within the router) may be required.
   Note that sequence integrity of the aggregate FEC forwarded over a
   set of load balancing LSPs may not be preserved under such a regime.

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7. Security Considerations

   The policy system and the MPLS system both have their inherent
   security issues, which this document does not attempt to resolve.

   The Policy system provides a mechanism to configure the LSPs within
   LSRs. Any thing that can be configured can also be incorrectly
   configured with potentially disastrous results. The policy system
   can help to secure the MPLS system by providing appropriate controls
   on the LSP life cycle. Conversely, if the security of the Policy
   system is compromised, then this may impact any MPLS systems
   controlled by that policy system.

   Use of the COPS protocol within the policy system between the
   PEP/PDP allows the use of message level security for authentication,
   replay protection and message integrity. Existing protocols such as
   IPSEC or TLS can also be used to authenticate and secure the
   channel. The COPS protocol also provides a reliable transport
   mechanism with a session keep-alive.

   The MPLS network is not expected to impact the security of the
   Policy system.

   Further security considerations of Policy-Enabled MPLS networks is
   for further study.

8. Conclusions / Further Work

   Policy-based networking approaches provide a flexible mechanism that
   can be utilized to support certain traffic engineering objectives.
   In this draft we have illustrated the potential role for traffic
   engineering policy approaches to load balancing in MPLS networks.
   In future drafts we expect to extend this work to cover more
   sophisticated notions of QoS in addition to "best effort" MPLS
   services, and into MPLS PIB proposals.

   The authors of this draft believe that portions of this material
   should be incorporated into working group drafts within the scope of
   the tewg, mpls and possibly other working groups.

9. References

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

   [2]  Bradner,S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.

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   [3] Awduche, D., Chiu, A., Elwalid, A., Widjaja, I., Xiao, X., "A
      Framework for Internet Traffic Engineering", work-in-progress,
      draft-ietf-tewg-framewrk-01.txt, May 2000.

   [4] Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M., McManus, J.,
      "Requirements for Traffic Engineering over MPLS", RFC 2702,
      September 1999.

   [5]  Rosen, E., Viswanathan, A., Callon, R., "Multiprotocol Label
      Switching Architecture", work-in-progress, draft-ietf-mpls-arch-
      06.txt, August 1999.

   [6]  Wright,S., Herzog,S., Reichmeyer,F., Jaeger,R., "Requirements
      for Policy Enabled MPLS", work-in-progress, draft-wright-policy-
      mpls-00.txt, March 2000.

   [7]  Moore, B., Elleson, E., Strassner, J., "Policy Framework Core
      Information Model -- Version 1 Specification" work-in-progress,

   [8]  Yavatkar, R., Pendarakis, D., Guerin, R., " A Framework for
      Policy-based Admission Control" RFC2753, January 2000.

   [9]  Kompella, K., Rekhter, Y., " Link Bundling in MPLS Traffic
      Engineering", work-in-progress, draft-kompella-mpls-bundle-
      00.txt, February 2000.

   [10] Almes, G., Kalidindi, Zekauskas, M., "A One-way Delay Metric
      for IPPM", RFC 2769, September 1999.

   [11] Brownlee, N., Mills, C., Ruth, G., "Traffic Flow Measurement:
      Architecture", RFC 2722, October 1999.

   [12]  Srinivasan, C., Viswanathan, A., Nadeau, T., "MPLS Traffic
      Engineering Management Information Base Using SMIv2", work-in-
      progress, draft-ietf-mpls-te-mib-03.txt, March 2000.

   [13] Srinivasan, C., Viswanathan,A., Nadeau,T.,"MPLS Label Switch
      Router Management Information Base Using SMIv2", work-in-
      progress,  draft-ietf-mpls-lsr-mib-04.txt, April 2000.

   [14] Nadeau,T., Srinivasan, C., Viswanathan,A., "Multiprotocol label
      Switching Packet Classification Management Information Base Using
      SMIv2", work-in-progress, draft-nadeau-mpls-packet-classifier-
      mib-00.txt, March 2000.

10.  Acknowledgments

   Indra Widjaja provided several comments on an earlier draft.

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11. Author's Addresses

   Steven Wright
   Science & Technology
   BellSouth Telecommunications
   41G70 BSC
   675 West Peachtree St. NE.
   Atlanta, GA 30375
   Phone +1 (404) 332-2194

   Francis Reichmeyer
   IPHighway, Inc.
   55 New York Avenue
   Framingham, MA 01701
   Phone +1 (201) 655-8714

   Robert Jaeger
   Laboratory for Telecommunications Science,
   2800 Powder Mill Road, Bldg 601, Room 131
   Adelphi, MD 20783
   Phone +1 (301) 688-1420

   Mark Gibson
   Nortel Networks
   Harlow laboratories, London Rd.,
   Harlow, Essex UK CM17 9NA
   Phone: +44(0)1279 402621

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