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Internet Engineering Task Force              R. Guerin/S. Kamat/E. Rosen
INTERNET DRAFT                                             IBM/IBM/Cisco
                                                            29 July 1997

          Setting up Reservations on Explicit Paths using RSVP

Status of This Memo

   This document is an Internet-Draft.  Internet Drafts are working
   documents of the Internet Engineering Task Force (IETF), its Areas,
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   This document presents motivations for extensions to RSVP in order to
   enable setting up of reservations on explicit routes.  The advantages
   of providing this support are discussed in the context of MPLS and
   QoS routing.  An approach to providing these extensions by means of
   opaque routing objects in RSVP messages is presented.

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Status of This Memo                                                    i

Abstract                                                               i

 1. Introduction                                                       1

 2. Bandwidth Reservation for Explicit Route in an MPLS Environment    1

 3. QoS Routing with Explicit Routes                                   3
     3.1. QoS path management . . . . . . . . . . . . . . . . . . .    4
     3.2. Enforcing high level admission control policies . . . . .    6

 4. Mechanism for Reservation Set Up on Explicit Paths                 7
     4.1. Explicit Route Object . . . . . . . . . . . . . . . . . .    7
     4.2. Processing of Explicit Route Objects  . . . . . . . . . .    8

 5. Conclusions                                                        8

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1. Introduction

   The purpose of this document is to introduce and motivate extensions
   to RSVP to enable setting up of reservations on explicit routes.
   Enabling reservations on explicit routes can be useful in several
   different contexts.  In particular, it can be used to ensure that
   certain flows use a ``label switched'' path as in the MPLS context
   [CDF+97] or to facilitate the management of QoS paths computed by a
   QoS capable router as in [GKO+97].  In this document, we describe
   further these potential benefits, and show how they can be attained
   with minimal impact to RSVP. It should be pointed out that the focus
   of this document is on unicast flows as there are many other issues
   that need to be addressed to consider the use of explicit routes for
   multicast flows.

   In the context of unicast flows, explicit routes are to be specified
   through a new Explicit_Route object in RSVP. This object, like policy
   objects, is opaque to RSVP which only needs to ensure its delivery to
   routing.  Routing is responsible for processing the Explicit_Route
   object, and will use the information it contains to construct its
   response to a Route_Query from RSVP.

   Sections 2 and 3 motivate the need for explicit route support within
   RSVP in the context of MPLS and QoS routing respectively.  Section
   4 describes the specific mechanism of setting up reservations on
   explicit paths.  This includes specification of a format for the
   Explicit_Route object and the interactions between RSVP and routing
   in this context.

2. Bandwidth Reservation for Explicit Route in an MPLS Environment

   Consider the following topology:

      |       |

   Let us suppose that this topology exists in the network of an
   Internet Service Provider (ISP). We suppose further that node A has
   an interface to one of the ISP's subscribers, S1, and node B has
   an interface to a different subscriber, S2.  Finally, we suppose
   that both subscribers are generating packets that are addressed to
   destinations reachable only through node D.

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   In order to make the best provisioning of its bandwidth, the ISP may
   decide that such packets from S1 should follow the route A-B-E-F-D,
   while such packets from S2 should follow the route B-C-D. Further,
   the ISP may want to reserve resources for each of these "flows", so
   that it can schedule transmissions along the respective routes in a
   way which corresponds to whatever agreements the ISP has made to the
   particular subscribers.

   Putting this decision into effect in a conventional IP network is
   extremely difficult, since it requires that two packets going through
   B, with the same destination, be sent on separate routes.  Therefore,
   ISPs tend to use ATM or Frame Relay networks to provide this level of
   bandwidth management.  ATM and Frame Relay networks also provide the
   capability to support whatever resources reservations are necessary.

   MPLS [CDF+97] provides a way for an ISP to obtain this functionality
   without the need to resort to ATM or Frame Relay.  In MPLS, node A
   can apply a "label" to packets from S1 which must pass through D;
   node B can apply a label to packets from S2 which must pass through
   D. When a labeled packet is transmitted, the label is sent along with
   it.  Once a packet is labeled, the forwarding decision is based only
   on the label, NOT on the contents of the packet header.  Thus there
   is nothing to prevent packet P1 from traveling the A-B-E-F-D path,
   while packet P2 travels the B-C-D path, even if P1 and P2 happen to
   have the very same destination address.

   Of course, MPLS must incorporate some "path setup" procedure whereby
   paths that differ from the "normal" IP routing can get explicitly set
   up.  MPLS must also incorporate some means of performing resource
   reservation along these paths.  While a resource reservation protocol
   could be designed exclusively for MPLS, it would seem to make most
   sense to use RSVP for that purpose; after all, RSVP was designed to
   be the resource reservation protocol of the internet.

   This requires some modification of RSVP. As currently specified,
   there is no way to force an RSVP Path message to follow any path
   other than the "normal" path to a particular destination.  So if a
   different MPLS path were set up for certain flows, there is currently
   no way to get the Path message to follow that path.

   If RSVP control messages could carry opaque objects that are
   meaningful to routing and RSVP's interface to routing is broadened
   as in [GKR97] so that RSVP could pass such objects to routing, then
   this difficulty can be overcome.  The Path messages could carry an
   explicit route object.  To determine the next hop for the flow, RSVP
   would pass the Explicit Route Object (and other opaque objects if
   present) to routing, which would pass back the identity of the next
   hop, and a modified Explicit Route Object.  This would force the Path

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   message to follow the path of the corresponding MPLS flow and ensure
   that resources are reserved along the MPLS path.

   The general ability to carry an opaque routing object in RSVP
   messages further enables one to combine the setup of an MPLS path
   with resource reservation along the same path.  This could be
   achieved by having a second opaque routing object carry an MPLS flow
   identifier (label) in conjunction with the explicit route object.
   Clearly, this approach has the advantage of avoiding a second round
   trip to make reservations along the MPLS path when the path set up
   itself must be done first.  The need to have a second round trip
   seems to simply add latency and complexity, without adding any value.

3. QoS Routing with Explicit Routes

   An objective of QoS routing is to choose for each flow the path that
   has the best likelihood of meeting the flow's QoS requirements,
   while still making efficient use of network resources.  In order
   to achieve this goal, QoS routing requires knowledge of network
   resources availability and of the QoS requirements of the flows.
   This information can be provided in a number of ways (e.g., see
   [CNRS97, GKR97] for possible approaches) and is then used by a QoS
   path selection algorithm to identify an appropriate path for a flow.
   The selection of a path and the distribution of the information
   needed to make that selection, however, only represent a subset of
   the functions needed to support QoS routing.  There are two other
   important issues that a QoS routing solution must address to meet its
   objectives.  These are:

    1. Management of QoS paths of individual flows, and

    2. Enforcing high level admission control policies.

   Management of QoS paths includes not only setting up the paths
   correctly, but also maintaining or adjusting them in response to
   failures and changes in the network.  High level (call) admission
   policies are needed (see [CNRS97] for a discussion of this issue) to
   control how selected paths are being used so as to preserve the long
   term efficiency of the network.  For example, a suitable path might
   be found for a flow, but rejected by the high level admission control
   because of its cost to the network, e.g., it is using a large number
   of links which could alternatively be used to support several such
   other calls to different destinations.

   In the rest of this section, we articulate how explicit routes can
   facilitate handling of these two issues.  However, before doing
   so, we briefly compare, in the context of QoS routing, the use of

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   explicit routes versus the hop-by-hop routing approach presented in

   A hop-by-hop routing solution has the benefits of requiring the
   least changes to RSVP and possibly offering added flexibility (see
   [GKH97] for details), but this does come at a cost.  Specifically,
   with hop-by-hop routing, there are multiple decision points (each
   hop) involved in selecting a path, with each making independent
   decisions.  As a result, end-to-end control of a path requires
   coordination between the multiple decision points, and this can
   often be a complex task.  For example, even in the context of a link
   state routing protocol such as OSPF where all routers in a domain
   compute their routes using the same algorithm applied to a common
   topology database, no single router has complete knowledge of the
   actual path being followed.  This is because inconsistencies during
   routing transients as well as equal cost multi-path considerations,
   independently affect local path selection decisions.  Additional
   mechanisms are, therefore, needed to coordinate these independent

   On the other hand, when explicit routes are used, selection of the
   entire path is made at a single decision point (the first router in
   the path).  In the rest of this section, we expand on the benefits of
   a single decision point in the context of both QoS path management
   and high level call admission.

3.1. QoS path management

   In best-effort routing, route changes occur relatively infrequently,
   and mostly when local interfaces change state or when routing
   updates are received from the routing protocol.  With QoS routing,
   changes that would result in the selection of a new route for a
   given destination and QoS requirements are much more frequent, as
   they typically occur each time a metric update is received.  If such
   changes were to trigger re-routing of existing QoS flows, this would
   translate into disruption of service to already established flows.
   Furthermore, this could also increase routing instability as such
   re-routing may trigger additional metric updates and cause further
   re-routing.  Keeping a flow's routing state, i.e., the path on which
   it has established a reservation, ``pinned'' as long as the path
   remains satisfactory for the flow (and the network) is one possible
   approach to this problem.  Path pinning, however, has a number of
   implications for QoS routing.

   First, path pinning requires knowledge that the path being pinned is
   adequate.  This includes several aspects.  First and foremost, the
   pinned path should be loop free.  When an explicit route is used,

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   this is readily achieved as the node selecting the explicit route can
   ensure it is free of any loop.  In contrast, when hop-by-hop routing
   is used, the coordination of the multiple decision points involved in
   the selection of the path requires not only that all nodes rely on
   the same routing algorithm, but also imposes close coupling with RSVP
   states to detect the formation of loops (see [GKH97] for details).
   Such a coupling adds some complexity, but more importantly, it can
   prevent the flow of data on the pinned path until after resources
   have been successfully reserved on the entire path (see again [ref]
   for details).  In the case where reservations are successful on only
   a portion of the path, this means that the data may not be able to
   take advantage of such partial reservations.  This is obviously
   undesirable, and while this can possibly be remedied (see also
   [GKH97] for possible approaches), solutions come at the cost of added
   signaling and processing complexity.

   Besides being loop free, a pinned path must also be capable of
   satisfying the QoS requirements of the flow.  Hence, it is important
   either to ensure the availability of resources on a pinned path,
   or to provide simple mechanisms to unpin it in case the required
   resources are not available when they are being requested, e.g., when
   an RSVP RESV message is received.  Hence, the ability to detect such
   conditions and trigger the unpinning of a path is required.  This can
   be achieved using similar mechanisms in both the explicit route and
   hop-by-hop routing cases, and is further detailed in the next section
   which expands on the approach of [GKH97].  Note that unpinning a path
   only implies that QoS routing is being queried anew to determine if
   the current path is still the correct one, or to find if a new and
   better one now exists.  In particular, unpinning a path does not
   result in removal of existing path or reservation states.  This is
   because although the existing pinned path may not fully satisfy the
   requirements of the flow, it may be the best one currently available.
   In that case any (partial) reservation that may exist on the current
   path should be maintained as it represents the best possible QoS
   available to the flow.

   There are other instances where a path needs to be unpinned.  For
   example, when one of the links or nodes on the path fails.  In such
   cases, it is important to notify all nodes on the current path, so
   that they can unpin it and query QoS routing to possibly find an
   alternate path.  This can again be achieved using similar mechanisms
   in both the explicit route and hop-by-hop routing cases [GKH97].
   However, when a reservation is already in place, it is also desirable
   to identify links on which resources are already reserved for the
   flow.  This is important so that these existing reservations be
   taken into account when searching for an alternate path, i.e., avoid
   the ``stepping on one's shadow'' problem.  This is made easier in
   the case of explicit route by the knowledge of the entire path.

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   Knowledge of the entire path is also useful in the context of high
   level admission control, and we now briefly review this issue and the
   benefits of explicit routes in that context.

3.2. Enforcing high level admission control policies

   As pointed out in the framework document for QoS routing [CNRS97],
   some form of higher level admission control and administrative
   control of routing behavior may be necessary within an AS. This
   is because QoS routing has to balance the sometimes conflicting
   requirements of high network resource utilization and improved
   chances of successful resource reservation for individual flows.
   For example, when current load in the network suggests a QoS path
   that is much longer than the ``usual'' path, admitting the flow
   along such a path may actually deny service to later flows that
   would have been admissible along segments of this long path.  Hence,
   this could negatively affect the overall network utilization.  In
   such situations, a high level admission control policy may find it
   desirable not to admit the flow based on routing decision alone.  One
   possible approach is to compare the length of the path returned by
   QoS routing to that of a ``usual'' path, and decide whether or not
   to use the path depending on this comparison as well as possibly
   other factors such as overall network load.  Conversely, if a flow
   has been already set up and later a much more efficient path becomes
   available, it might be desirable to reroute the flow along the new
   path.  This is particularly true if the current path only supports a
   fraction of the desired reservation, while the new path may be able
   to accommodate the complete reservation.

   In all such instances, these decisions are greatly facilitated when
   a single entity is responsible for determining and controlling
   the entire path.  Hence, such controls are more readily performed
   when routing is done using explicit routes instead of hop-by-hop
   routing.  This is not to say that they are not feasible with
   hop-by-hop routing, but distributed decisions and knowledge generally
   complicate such tasks.  For example, transient inconsistent routing
   information at multiple routers can lead to the pinning of a long
   but loop-free path, without any single router on the path being
   aware of the problem.  Hence, it becomes difficult to identify and
   rectify such bad routing choices.  Solutions to this problem require
   the introduction of additional signaling information to coordinate
   information and decisions across the nodes on the path, e.g., a
   policy object carried in PATH messages that specifies a limit on
   the acceptable path length.  This would in turn add to the overall
   signalling and processing overhead, and may all but eliminate the
   potentially greater simplicity of hop-by-hop routing.  On the other

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   hand, the single decision point of explicit routes avoids most of
   these problems.

4. Mechanism for Reservation Set Up on Explicit Paths

4.1. Explicit Route Object

   As stated earlier, explicit routes are to be specified through a
   new Explicit_Route object in RSVP. RSVP PATH messages will carry
   this object.  The format of the Explicit_Route object as shown below
   consists of a (strict/loose) source route and a pointer indicating
   the current position within the source route.

        |    Code     |    Length   |  Pointer    |             |
        |           IPv4 Hop 0 (src) Address (4 bytes)          |
        //                                                      //
        |           IPv4 Hop N (dst) Address (4 bytes)          |

   This is a new object class, with the C-type indicating IPv4.  The
   value of the Code field will indicate whether the sequence of IP
   addresses represents a loose or a strict source route.

   Semantics of Explicit Route Object correspond to the semantics of
   the IP Strict or Loose Source Route Option as specified in RFC
   1812, "Requirements for IP Version 4 Routers".  As in the case of
   processing of IP header source route option processing, routing at
   intermediate gateways will overwrite the items in the address list
   with their local network addresses, in effect recording the actual
   traversed route.  This facilitates avoidance of loops which may arise
   in the case of a loose source route.

   As described in Section 2 when MPLS is enabled, it may also be
   desirable to combine the setup of an MPLS with the reservation of
   resources along that path.  This can be achieved through the use
   of other opaque routing objects, that will carry the MPLS flow
   information required for the associated path setup.  Details on the
   formats of those object are deferred to other documents dedicated to
   the setup of MPLS flows.

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4.2. Processing of Explicit Route Objects

   Certain additional processing is involved when dealing with explicit
   route objects as compared to processing a similar source route
   option in an IP datagram.  First, in case the source route object
   suggests more than one possible outgoing interfaces, routing is free
   to choose one from among them by any local means.  However, it must
   remember which one it chose, and make the same choice in response
   to subsequent route queries from RSVP with the same explicit route
   object.  Second, in the event of errors in processing the route
   object, in stead of generating ICMP messages as in the case of IP
   datagram processing, routing will notify RSVP of an error.  This
   would trigger a PathErr message to the source with a new error code
   ("Cannot satisfy source route").

   During the processing of the PATH message, RSVP will extract any
   routing objects (if present) and pass them to routing.  Routing
   will parse these objects implementing their associated semantics
   and return new opaque objects to replace the original ones in the
   outgoing messages.  As specified in [GKR97] this is achieved by means
   of Route_Query and Route_Response interfaces:

    -  Route_Query interface is of the form:

       Route_Query( flow_id, Network header, Transport Header,
       notify_flag, sender_TSPEC, ADSPEC, opaque_object1,
       opaque_object2, ...)

    -  Route_Reply is of the form:  Route_Reply(flow_id, notify_flag,
       outgoing_interface_mask, opaque_object1, opaque_object2, ...)

5. Conclusions

   This document provides a motivation for supporting opaque routing
   objects in RSVP to enable setting up resource reservations on
   explicit routes.  The benefits of this approach in the contexts of
   MPLS and QoS routing were expounded and a mechanism for supporting
   this feature was discussed.


   [CDF+97] R. Callon, P. Doolan, N. Feldman, A. Fredette, G. Swallow,
            and A. Viswanathan.  A Framework for Multi-Protocol
            Label Switching (draft-ietf-mpls-framework-00.txt).
            INTERNET-DRAFT, Internet Engineering Task Force, May 1997.

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   [CNRS97] E. Crawley, R. Nair, B. Rajagopalan, and H. Sandick.
            A Framework for QoS-based Routing in the Internet
            (draft-ietf-qosr-framework-00.txt).  INTERNET-DRAFT,
            Internet Engineering Task Force, March 1997.

   [GKH97]  R. Guerin, S. Kamat, and S. Herzog.  QoS Path Management
            with RSVP, (draft-guerin-qos-path-mgt-rsvp-00.txt).
            INTERNET-DRAFT, Internet Engineering Task Force, March

   [GKO+97] R. Guerin, S. Kamat, A. Orda, T. Przygienda, and
            D. Williams.  QoS Routing Mechanisms and OSPF Extensions,
            (draft-guerin-qos-routing-ospf-01.txt).  INTERNET-DRAFT,
            Internet Engineering Task Force, March 1997.

   [GKR97]  R. Guerin, S. Kamat, and E. Rosen.  Extended RSVP-Routing
            Interface (draft-guerin-ext-rsvp-rtng-intf-00.txt).
            INTERNET-DRAFT, Internet Engineering Task Force, July 1997.

Authors' Address

 Roch Guerin
 IBM T.J. Watson Research Center
 P.O. Box 704
 Yorktown Heights, NY 10598
 EMAIL: guerin@watson.ibm.com
 VOICE   +1 914 784-7038
 FAX     +1 914 784-6205

 Sanjay Kamat
 IBM T.J. Watson Research Center
 P.O. Box 704
 Yorktown Heights, NY 10598
 EMAIL: sanjay@watson.ibm.com
 VOICE   +1 914 784-7402
 FAX     +1 914 784-6205

 Eric Rosen
 Cisco Systems, Inc.
 250 Apollo Drive
 Chelmsford, MA, 01824
 EMAIL: erosen@cisco.com

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