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RFC 2205

Resource ReSerVation Protocol (RSVP) -- Version 1 Functional Specification

Pages: 112
Proposed Standard
Updated by:  275039364495594664376780
Part 2 of 4 – Pages 19 to 47
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Top   ToC   RFC2205 - Page 19   prevText
2. RSVP Protocol Mechanisms

   2.1 RSVP Messages

       Previous       Incoming           Outgoing             Next
       Hops           Interfaces         Interfaces           Hops

       _____             _____________________                _____
      |     | data -->  |                     |  data -->    |     |
      |  A  |-----------| a                 c |--------------|  C  |
      |_____| Path -->  |                     |  Path -->    |_____|
              <-- Resv  |                     |  <-- Resv     _____
       _____            |       ROUTER        |           |  |     |
      |     |  |        |                     |           |--|  D  |
      |  B  |--| data-->|                     |  data --> |  |_____|
      |_____|  |--------| b                 d |-----------|
               | Path-->|                     |  Path --> |   _____
       _____   | <--Resv|_____________________|  <-- Resv |  |     |
      |     |  |                                          |--|  D' |
      |  B' |--|                                          |  |_____|
      |_____|  |                                          |

                         Figure 9: Router Using RSVP

      Figure 9 illustrates RSVP's model of a router node.  Each data
      flow arrives from a "previous hop" through a corresponding
      "incoming interface" and departs through one or more "outgoing
      interface"(s).  The same interface may act in both the incoming
      and outgoing roles for different data flows in the same session.
      Multiple previous hops and/or next hops may be reached through a
      given physical interface; for example, the figure implies that D
      and D' are connected to (d) with a broadcast LAN.

      There are two fundamental RSVP message types: Resv and Path.

      Each receiver host sends RSVP reservation request (Resv) messages
      upstream towards the senders.  These messages must follow exactly
      the reverse of the path(s) the data packets will use, upstream to
      all the sender hosts included in the sender selection.  They
      create and maintain "reservation state" in each node along the
      path(s).  Resv messages must finally be delivered to the sender
      hosts themselves, so that the hosts can set up appropriate traffic
      control parameters for the first hop.  The processing of Resv
      messages was discussed previously in Section 1.2.
Top   ToC   RFC2205 - Page 20
      Each RSVP sender host transmits RSVP "Path" messages downstream
      along the uni-/multicast routes provided by the routing
      protocol(s), following the paths of the data.  These Path messages
      store "path state" in each node along the way.  This path state
      includes at least the unicast IP address of the previous hop node,
      which is used to route the Resv messages hop-by-hop in the reverse
      direction.  (In the future, some routing protocols may supply
      reverse path forwarding information directly, replacing the
      reverse-routing function of path state).

      A Path message contains the following information in addition to
      the previous hop address:

      o    Sender Template

           A Path message is required to carry a Sender Template, which
           describes the format of data packets that the sender will
           originate.  This template is in the form of a filter spec
           that could be used to select this sender's packets from
           others in the same session on the same link.

           Sender Templates have exactly the same expressive power and
           format as filter specs that appear in Resv messages.
           Therefore a Sender Template may specify only the sender IP
           address and optionally the UDP/TCP sender port, and it
           assumes the protocol Id specified for the session.

      o    Sender Tspec

           A Path message is required to carry a Sender Tspec, which
           defines the traffic characteristics of the data flow that the
           sender will generate.  This Tspec is used by traffic control
           to prevent over-reservation, and perhaps unnecessary
           Admission Control failures.

      o    Adspec

           A Path message may carry a package of OPWA advertising
           information, known as an "Adspec".  An Adspec received in a
           Path message is passed to the local traffic control, which
           returns an updated Adspec; the updated version is then
           forwarded in Path messages sent downstream.
Top   ToC   RFC2205 - Page 21
      Path messages are sent with the same source and destination
      addresses as the data, so that they will be routed correctly
      through non-RSVP clouds (see Section 2.9).  On the other hand,
      Resv messages are sent hop-by-hop; each RSVP-speaking node
      forwards a Resv message to the unicast address of a previous RSVP

   2.2 Merging Flowspecs

      A Resv message forwarded to a previous hop carries a flowspec that
      is the "largest" of the flowspecs requested by the next hops to
      which the data flow will be sent (however, see Section 3.5 for a
      different merging rule used in certain cases).  We say the
      flowspecs have been "merged".  The examples shown in Section 1.4
      illustrated another case of merging, when there are multiple
      reservation requests from different next hops for the same session
      and with the same filter spec, but RSVP should install only one
      reservation on that interface.  Here again, the installed
      reservation should have an effective flowspec that is the
      "largest" of the flowspecs requested by the different next hops.

      Since flowspecs are opaque to RSVP, the actual rules for comparing
      flowspecs must be defined and implemented outside RSVP proper.
      The comparison rules are defined in the appropriate integrated
      service specification document.  An RSVP implementation will need
      to call service-specific routines to perform flowspec merging.

      Note that flowspecs are generally multi-dimensional vectors; they
      may contain both Tspec and Rspec components, each of which may
      itself be multi-dimensional.  Therefore, it may not be possible to
      strictly order two flowspecs.  For example, if one request calls
      for a higher bandwidth and another calls for a tighter delay
      bound, one is not "larger" than the other.  In such a case,
      instead of taking the larger, the service-specific merging
      routines must be able to return a third flowspec that is at least
      as large as each; mathematically, this is the "least upper bound"
      (LUB).  In some cases, a flowspec at least as small is needed;
      this is the "greatest lower bound" (GLB) GLB (Greatest Lower

      The following steps are used to calculate the effective flowspec
      (Re, Te) to be installed on an interface [RFC 2210].  Here Te is
      the effective Tspec and Re is the effective Rspec.
Top   ToC   RFC2205 - Page 22
      1.   An effective flowspec is determined for the outgoing
           interface.  Depending upon the link-layer technology, this
           may require merging flowspecs from different next hops; this
           means computing the effective flowspec as the LUB of the
           flowspecs.  Note that what flowspecs to merge is determined
           by the link layer medium (see Section 3.11.2), while how to
           merge them is determined by the service model in use [RFC

           The result is a flowspec that is opaque to RSVP but actually
           consists of the pair (Re, Resv_Te), where is Re is the
           effective Rspec and Resv_Te is the effective Tspec.

      2.   A service-specific calculation of Path_Te, the sum of all
           Tspecs that were supplied in Path messages from different
           previous hops (e.g., some or all of A, B, and B' in Figure
           9), is performed.

      3.   (Re, Resv_Te) and Path_Te are passed to traffic control.
           Traffic control will compute the effective flowspec as the
           "minimum" of Path_Te and Resv_Te, in a service-dependent

      Section 3.11.6 defines a generic set of service-specific calls to
      compare flowspecs, to compute the LUB and GLB of flowspecs, and to
      compare and sum Tspecs.

   2.3 Soft State

      RSVP takes a "soft state" approach to managing the reservation
      state in routers and hosts.  RSVP soft state is created and
      periodically refreshed by Path and Resv messages.  The state is
      deleted if no matching refresh messages arrive before the
      expiration of a "cleanup timeout" interval.  State may also be
      deleted by an explicit "teardown" message, described in the next
      section.  At the expiration of each "refresh timeout" period and
      after a state change, RSVP scans its state to build and forward
      Path and Resv refresh messages to succeeding hops.

      Path and Resv messages are idempotent.  When a route changes, the
      next Path message will initialize the path state on the new route,
      and future Resv messages will establish reservation state there;
      the state on the now-unused segment of the route will time out.
      Thus, whether a message is "new" or a "refresh" is determined
      separately at each node, depending upon the existence of state at
      that node.
Top   ToC   RFC2205 - Page 23
      RSVP sends its messages as IP datagrams with no reliability
      enhancement.  Periodic transmission of refresh messages by hosts
      and routers is expected to handle the occasional loss of an RSVP
      message.  If the effective cleanup timeout is set to K times the
      refresh timeout period, then RSVP can tolerate K-1 successive RSVP
      packet losses without falsely deleting state.  The network traffic
      control mechanism should be statically configured to grant some
      minimal bandwidth for RSVP messages to protect them from
      congestion losses.

      The state maintained by RSVP is dynamic; to change the set of
      senders Si or to change any QoS request, a host simply starts
      sending revised Path and/or Resv messages.  The result will be an
      appropriate adjustment in the RSVP state in all nodes along the
      path; unused state will time out if it is not explicitly torn

      In steady state, state is refreshed hop-by-hop to allow merging.
      When the received state differs from the stored state, the stored
      state is updated.  If this update results in modification of state
      to be forwarded in refresh messages, these refresh messages must
      be generated and forwarded immediately, so that state changes can
      be propagated end-to-end without delay.  However, propagation of a
      change stops when and if it reaches a point where merging causes
      no resulting state change.  This minimizes RSVP control traffic
      due to changes and is essential for scaling to large multicast

      State that is received through a particular interface I* should
      never be forwarded out the same interface.  Conversely, state that
      is forwarded out interface I* must be computed using only state
      that arrived on interfaces different from I*.  A trivial example
      of this rule is illustrated in Figure 10, which shows a transit
      router with one sender and one receiver on each interface (and
      assumes one next/previous hop per interface).  Interfaces (a) and
      (c) serve as both outgoing and incoming interfaces for this
      session.  Both receivers are making wildcard-style reservations,
      in which the Resv messages are forwarded to all previous hops for
      senders in the group, with the exception of the next hop from
      which they came.  The result is independent reservations in the
      two directions.

      There is an additional rule governing the forwarding of Resv
      messages: state from Resv messages received from outgoing
      interface Io should be forwarded to incoming interface Ii only if
      Path messages from Ii are forwarded to Io.
Top   ToC   RFC2205 - Page 24
                      a |                | c
      ( R1, S1 ) <----->|     Router     |<-----> ( R2, S2 )

             Send                |        Receive
        WF( *{3B}) <-- (a)       |     (c) <-- WF( *{3B})
             Receive             |          Send
        WF( *{4B}) --> (a)       |     (c) --> WF( *{4B})
             Reserve on (a)      |        Reserve on (c)
              __________         |        __________
             |  * {4B}  |        |       |   * {3B} |
             |__________|        |       |__________|

                     Figure 10: Independent Reservations

   2.4 Teardown

      RSVP "teardown" messages remove path or reservation state
      immediately.  Although it is not necessary to explicitly tear down
      an old reservation, we recommend that all end hosts send a
      teardown request as soon as an application finishes.

      There are two types of RSVP teardown message, PathTear and
      ResvTear.  A PathTear message travels towards all receivers
      downstream from its point of initiation and deletes path state, as
      well as all dependent reservation state, along the way.  An
      ResvTear message deletes reservation state and travels towards all
      senders upstream from its point of initiation.  A PathTear
      (ResvTear) message may be conceptualized as a reversed-sense Path
      message (Resv message, respectively).

      A teardown request may be initiated either by an application in an
      end system (sender or receiver), or by a router as the result of
      state timeout or service preemption.  Once initiated, a teardown
      request must be forwarded hop-by-hop without delay.  A teardown
      message deletes the specified state in the node where it is
      received.  As always, this state change will be propagated
      immediately to the next node, but only if there will be a net
      change after merging.  As a result, a ResvTear message will prune
      the reservation state back (only) as far as possible.
Top   ToC   RFC2205 - Page 25
      Like all other RSVP messages, teardown requests are not delivered
      reliably.  The loss of a teardown request message will not cause a
      protocol failure because the unused state will eventually time out
      even though it is not explicitly deleted.  If a teardown message
      is lost, the router that failed to receive that message will time
      out its state and initiate a new teardown message beyond the loss
      point.  Assuming that RSVP message loss probability is small, the
      longest time to delete state will seldom exceed one refresh
      timeout period.

      It should be possible to tear down any subset of the established
      state.  For path state, the granularity for teardown is a single
      sender.  For reservation state, the granularity is an individual
      filter spec.  For example, refer to Figure 7.  Receiver R1 could
      send a ResvTear message for sender S2 only (or for any subset of
      the filter spec list), leaving S1 in place.

      A ResvTear message specifies the style and filters; any flowspec
      is ignored.  Whatever flowspec is in place will be removed if all
      its filter specs are torn down.

   2.5 Errors

      There are two RSVP error messages, ResvErr and PathErr.  PathErr
      messages are very simple; they are simply sent upstream to the
      sender that created the error, and they do not change path state
      in the nodes though which they pass.  There are only a few
      possible causes of path errors.

      However, there are a number of ways for a syntactically valid
      reservation request to fail at some node along the path.  A node
      may also decide to preempt an established reservation.  The
      handling of ResvErr messages is somewhat complex (Section 3.5).
      Since a request that fails may be the result of merging a number
      of requests, a reservation error must be reported to all of the
      responsible receivers.  In addition, merging heterogeneous
      requests creates a potential difficulty known as the "killer
      reservation" problem, in which one request could deny service to
      another.  There are actually two killer-reservation problems.

      1.   The first killer reservation problem (KR-I) arises when there
           is already a reservation Q0 in place.  If another receiver
           now makes a larger reservation Q1 > Q0, the result of merging
           Q0 and Q1 may be rejected by admission control in some
           upstream node.  This must not deny service to Q0.
Top   ToC   RFC2205 - Page 26
           The solution to this problem is simple: when admission
           control fails for a reservation request, any existing
           reservation is left in place.

      2.   The second killer reservation problem (KR-II) is the
           converse: the receiver making a reservation Q1 is persistent
           even though Admission Control is failing for Q1 in some node.
           This must not prevent a different receiver from now
           establishing a smaller reservation Q0 that would succeed if
           not merged with Q1.

           To solve this problem, a ResvErr message establishes
           additional state, called "blockade state", in each node
           through which it passes.  Blockade state in a node modifies
           the merging procedure to omit the offending flowspec (Q1 in
           the example) from the merge, allowing a smaller request to be
           forwarded and established.  The Q1 reservation state is said
           to be "blockaded".  Detailed rules are presented in Section

      A reservation request that fails Admission Control creates
      blockade state but is left in place in nodes downstream of the
      failure point.  It has been suggested that these reservations
      downstream from the failure represent "wasted" reservations and
      should be timed out if not actively deleted.  However, the
      downstream reservations are left in place, for the following

      o    There are two possible reasons for a receiver persisting in a
           failed reservation: (1) it is polling for resource
           availability along the entire path, or (2) it wants to obtain
           the desired QoS along as much of the path as possible.
           Certainly in the second case, and perhaps in the first case,
           the receiver will want to hold onto the reservations it has
           made downstream from the failure.

      o    If these downstream reservations were not retained, the
           responsiveness of RSVP to certain transient failures would be
           impaired.  For example, suppose a route "flaps" to an
           alternate route that is congested, so an existing reservation
           suddenly fails, then quickly recovers to the original route.
           The blockade state in each downstream router must not remove
           the state or prevent its immediate refresh.

      o    If we did not refresh the downstream reservations, they might
           time out, to be restored every Tb seconds (where Tb is the
           blockade state timeout interval).  Such intermittent behavior
           might be very distressing for users.
Top   ToC   RFC2205 - Page 27
   2.6 Confirmation

      To request a confirmation for its reservation request, a receiver
      Rj includes in the Resv message a confirmation-request object
      containing Rj's IP address.  At each merge point, only the largest
      flowspec and any accompanying confirmation-request object is
      forwarded upstream.  If the reservation request from Rj is equal
      to or smaller than the reservation in place on a node, its Resv is
      not forwarded further, and if the Resv included a confirmation-
      request object, a ResvConf message is sent back to Rj.  If the
      confirmation request is forwarded, it is forwarded immediately,
      and no more than once for each request.

      This confirmation mechanism has the following consequences:

      o    A new reservation request with a flowspec larger than any in
           place for a session will normally result in either a ResvErr
           or a ResvConf message back to the receiver from each sender.
           In this case, the ResvConf message will be an end-to-end

      o    The receipt of a ResvConf gives no guarantees.  Assume the
           first two reservation requests from receivers R1 and R2
           arrive at the node where they are merged.  R2, whose
           reservation was the second to arrive at that node, may
           receive a ResvConf from that node while R1's request has not
           yet propagated all the way to a matching sender and may still
           fail.  Thus, R2 may receive a ResvConf although there is no
           end-to-end reservation in place; furthermore, R2 may receive
           a ResvConf followed by a ResvErr.

   2.7 Policy Control

      RSVP-mediated QoS requests allow particular user(s) to obtain
      preferential access to network resources.  To prevent abuse, some
      form of back pressure will generally be required on users who make
      reservations.  For example, such back pressure may be accomplished
      by administrative access policies, or it may depend upon some form
      of user feedback such as real or virtual billing for the "cost" of
      a reservation.  In any case, reliable user identification and
      selective admission will generally be needed when a reservation is

      The term "policy control" is used for the mechanisms required to
      support access policies and back pressure for RSVP reservations.
      When a new reservation is requested, each node must answer two
      questions: "Are enough resources available to meet this request?"
Top   ToC   RFC2205 - Page 28
      and "Is this user allowed to make this reservation?"  These two
      decisions are termed the "admission control" decision and the
      "policy control" decision, respectively, and both must be
      favorable in order for RSVP to make a reservation.  Different
      administrative domains in the Internet may have different
      reservation policies.

      The input to policy control is referred to as "policy data", which
      RSVP carries in POLICY_DATA objects.  Policy data may include
      credentials identifying users or user classes, account numbers,
      limits, quotas, etc.  Like flowspecs, policy data is opaque to
      RSVP, which simply passes it to policy control when required.
      Similarly, merging of policy data must be done by the policy
      control mechanism rather than by RSVP itself.  Note that the merge
      points for policy data are likely to be at the boundaries of
      administrative domains.  It may therefore be necessary to carry
      accumulated and unmerged policy data upstream through multiple
      nodes before reaching one of these merge points.

      Carrying user-provided policy data in Resv messages presents a
      potential scaling problem.  When a multicast group has a large
      number of receivers, it will be impossible or undesirable to carry
      all receivers' policy data upstream.  The policy data will have to
      be administratively merged at places near the receivers, to avoid
      excessive policy data.  Further discussion of these issues and an
      example of a policy control scheme will be found in [PolArch96].
      Specifications for the format of policy data objects and RSVP
      processing rules for them are under development.

   2.8 Security

      RSVP raises the following security issues.

      o    Message integrity and node authentication

           Corrupted or spoofed reservation requests could lead to theft
           of service by unauthorized parties or to denial of service
           caused by locking up network resources.  RSVP protects
           against such attacks with a hop-by-hop authentication
           mechanism using an encrypted hash function.  The mechanism is
           supported by INTEGRITY objects that may appear in any RSVP
           message.  These objects use a keyed cryptographic digest
           technique, which assumes that RSVP neighbors share a secret.
           Although this mechanism is part of the base RSVP
           specification, it is described in a companion document
Top   ToC   RFC2205 - Page 29
           Widespread use of the RSVP integrity mechanism will require
           the availability of the long-sought key management and
           distribution infrastructure for routers.  Until that
           infrastructure becomes available, manual key management will
           be required to secure RSVP message integrity.

      o    User authentication

           Policy control will depend upon positive authentication of
           the user responsible for each reservation request.  Policy
           data may therefore include cryptographically protected user
           certificates.  Specification of such certificates is a future

           Even without globally-verifiable user certificates, it may be
           possible to provide practical user authentication in many
           cases by establishing a chain of trust, using the hop-by-hop
           INTEGRITY mechanism described earlier.

      o    Secure data streams

           The first two security issues concerned RSVP's operation.  A
           third security issue concerns resource reservations for
           secure data streams.  In particular, the use of IPSEC (IP
           Security) in the data stream poses a problem for RSVP:  if
           the transport and higher level headers are encrypted, RSVP's
           generalized port numbers cannot be used to define a session
           or a sender.

           To solve this problem, an RSVP extension has been defined in
           which the security association identifier (IPSEC SPI) plays a
           role roughly equivalent to the generalized ports [RFC 2207].

   2.9 Non-RSVP Clouds

      It is impossible to deploy RSVP (or any new protocol) at the same
      moment throughout the entire Internet.  Furthermore, RSVP may
      never be deployed everywhere.  RSVP must therefore provide correct
      protocol operation even when two RSVP-capable routers are joined
      by an arbitrary "cloud" of non-RSVP routers.  Of course, an
      intermediate cloud that does not support RSVP is unable to perform
      resource reservation.  However, if such a cloud has sufficient
      capacity, it may still provide useful realtime service.

      RSVP is designed to operate correctly through such a non-RSVP
      cloud.  Both RSVP and non-RSVP routers forward Path messages
      towards the destination address using their local uni-/multicast
      routing table.  Therefore, the routing of Path messages will be
Top   ToC   RFC2205 - Page 30
      unaffected by non-RSVP routers in the path.  When a Path message
      traverses a non-RSVP cloud, it carries to the next RSVP-capable
      node the IP address of the last RSVP-capable router before
      entering the cloud.  An Resv message is then forwarded directly to
      the next RSVP-capable router on the path(s) back towards the

      Even though RSVP operates correctly through a non-RSVP cloud, the
      non-RSVP-capable nodes will in general perturb the QoS provided to
      a receiver.  Therefore, RSVP passes a `NonRSVP' flag bit to the
      local traffic control mechanism when there are non-RSVP-capable
      hops in the path to a given sender.  Traffic control combines this
      flag bit with its own sources of information, and forwards the
      composed information on integrated service capability along the
      path to receivers using Adspecs [RFC 2210].

      Some topologies of RSVP routers and non-RSVP routers can cause
      Resv messages to arrive at the wrong RSVP-capable node, or to
      arrive at the wrong interface of the correct node.  An RSVP
      process must be prepared to handle either situation.  If the
      destination address does not match any local interface and the
      message is not a Path or PathTear, the message must be forwarded
      without further processing by this node.  To handle the wrong
      interface case, a "Logical Interface Handle" (LIH) is used.  The
      previous hop information included in a Path message includes not
      only the IP address of the previous node but also an LIH defining
      the logical outgoing interface; both values are stored in the path
      state.  A Resv message arriving at the addressed node carries both
      the IP address and the LIH of the correct outgoing interface, i.e,
      the interface that should receive the requested reservation,
      regardless of which interface it arrives on.

      The LIH may also be useful when RSVP reservations are made over a
      complex link layer, to map between IP layer and link layer flow

   2.10 Host Model

      Before a session can be created, the session identification
      (DestAddress, ProtocolId [, DstPort]) must be assigned and
      communicated to all the senders and receivers by some out-of-band
      mechanism.  When an RSVP session is being set up, the following
      events happen at the end systems.
Top   ToC   RFC2205 - Page 31
      H1   A receiver joins the multicast group specified by
           DestAddress, using IGMP.

      H2   A potential sender starts sending RSVP Path messages to the

      H3   A receiver application receives a Path message.

      H4   A receiver starts sending appropriate Resv messages,
           specifying the desired flow descriptors.

      H5   A sender application receives a Resv message.

      H6   A sender starts sending data packets.

      There are several synchronization considerations.

      o    H1 and H2 may happen in either order.

      o    Suppose that a new sender starts sending data (H6) but there
           are no multicast routes because no receivers have joined the
           group (H1).  Then the data will be dropped at some router
           node (which node depends upon the routing protocol) until
           receivers(s) appear.

      o    Suppose that a new sender starts sending Path messages (H2)
           and data (H6) simultaneously, and there are receivers but no
           Resv messages have reached the sender yet (e.g., because its
           Path messages have not yet propagated to the receiver(s)).
           Then the initial data may arrive at receivers without the
           desired QoS.  The sender could mitigate this problem by
           awaiting arrival of the first Resv message (H5); however,
           receivers that are farther away may not have reservations in
           place yet.

      o    If a receiver starts sending Resv messages (H4) before
           receiving any Path messages (H3), RSVP will return error
           messages to the receiver.

           The receiver may simply choose to ignore such error messages,
           or it may avoid them by waiting for Path messages before
           sending Resv messages.

      A specific application program interface (API) for RSVP is not
      defined in this protocol spec, as it may be host system dependent.
      However, Section 3.11.1 discusses the general requirements and
      outlines a generic interface.
Top   ToC   RFC2205 - Page 32
3. RSVP Functional Specification

   3.1 RSVP Message Formats

      An RSVP message consists of a common header, followed by a body
      consisting of a variable number of variable-length, typed
      "objects".  The following subsections define the formats of the
      common header, the standard object header, and each of the RSVP
      message types.

      For each RSVP message type, there is a set of rules for the
      permissible choice of object types.  These rules are specified
      using Backus-Naur Form (BNF) augmented with square brackets
      surrounding optional sub-sequences.  The BNF implies an order for
      the objects in a message.  However, in many (but not all) cases,
      object order makes no logical difference.  An implementation
      should create messages with the objects in the order shown here,
      but accept the objects in any permissible order.

      3.1.1 Common Header

                0             1              2             3
         | Vers | Flags|  Msg Type   |       RSVP Checksum       |
         |  Send_TTL   | (Reserved)  |        RSVP Length        |

         The fields in the common header are as follows:

         Vers: 4 bits

              Protocol version number.  This is version 1.

         Flags: 4 bits

              0x01-0x08: Reserved

                   No flag bits are defined yet.

         Msg Type: 8 bits

              1 = Path

              2 = Resv
Top   ToC   RFC2205 - Page 33
              3 = PathErr

              4 = ResvErr

              5 = PathTear

              6 = ResvTear

              7 = ResvConf

         RSVP Checksum: 16 bits

              The one's complement of the one's complement sum of the
              message, with the checksum field replaced by zero for the
              purpose of computing the checksum.  An all-zero value
              means that no checksum was transmitted.

         Send_TTL: 8 bits

              The IP TTL value with which the message was sent.  See
              Section 3.8.

         RSVP Length: 16 bits

              The total length of this RSVP message in bytes, including
              the common header and the variable-length objects that

      3.1.2 Object Formats

         Every object consists of one or more 32-bit words with a one-
         word header, with the following format:

                0             1              2             3
         |       Length (bytes)      |  Class-Num  |   C-Type    |
         |                                                       |
         //                  (Object contents)                   //
         |                                                       |
Top   ToC   RFC2205 - Page 34
         An object header has the following fields:


              A 16-bit field containing the total object length in
              bytes.  Must always be a multiple of 4, and at least 4.


              Identifies the object class; values of this field are
              defined in Appendix A.  Each object class has a name,
              which is always capitalized in this document.  An RSVP
              implementation must recognize the following classes:


                   A NULL object has a Class-Num of zero, and its C-Type
                   is ignored.  Its length must be at least 4, but can
                   be any multiple of 4.  A NULL object may appear
                   anywhere in a sequence of objects, and its contents
                   will be ignored by the receiver.


                   Contains the IP destination address (DestAddress),
                   the IP protocol id, and some form of generalized
                   destination port, to define a specific session for
                   the other objects that follow.  Required in every
                   RSVP message.


                   Carries the IP address of the RSVP-capable node that
                   sent this message and a logical outgoing interface
                   handle (LIH; see Section 3.3).  This document refers
                   to a RSVP_HOP object as a PHOP ("previous hop")
                   object for downstream messages or as a NHOP (" next
                   hop") object for upstream messages.


                   Contains the value for the refresh period R used by
                   the creator of the message; see Section 3.7.
                   Required in every Path and Resv message.
Top   ToC   RFC2205 - Page 35

                   Defines the reservation style plus style-specific
                   information that is not in FLOWSPEC or FILTER_SPEC
                   objects.  Required in every Resv message.


                   Defines a desired QoS, in a Resv message.


                   Defines a subset of session data packets that should
                   receive the desired QoS (specified by a FLOWSPEC
                   object), in a Resv message.


                   Contains a sender IP address and perhaps some
                   additional demultiplexing information to identify a
                   sender.  Required in a Path message.


                   Defines the traffic characteristics of a sender's
                   data flow.  Required in a Path message.


                   Carries OPWA data, in a Path message.


                   Specifies an error in a PathErr, ResvErr, or a
                   confirmation in a ResvConf message.


                   Carries information that will allow a local policy
                   module to decide whether an associated reservation is
                   administratively permitted.  May appear in Path,
                   Resv, PathErr, or ResvErr message.

                   The use of POLICY_DATA objects is not fully specified
                   at this time; a future document will fill this gap.
Top   ToC   RFC2205 - Page 36

                   Carries cryptographic data to authenticate the
                   originating node and to verify the contents of this
                   RSVP message.  The use of the INTEGRITY object is
                   described in [Baker96].


                   Carries an explicit list of sender hosts towards
                   which the information in the message is to be
                   forwarded.  May appear in a Resv, ResvErr, or
                   ResvTear message.  See Section 3.4.


                   Carries the IP address of a receiver that requested a
                   confirmation.  May appear in a Resv or ResvConf


              Object type, unique within Class-Num.  Values are defined
              in Appendix A.

         The maximum object content length is 65528 bytes.  The Class-
         Num and C-Type fields may be used together as a 16-bit number
         to define a unique type for each object.

         The high-order two bits of the Class-Num is used to determine
         what action a node should take if it does not recognize the
         Class-Num of an object; see Section 3.10.

      3.1.3 Path Messages

         Each sender host periodically sends a Path message for each
         data flow it originates.  It contains a SENDER_TEMPLATE object
         defining the format of the data packets and a SENDER_TSPEC
         object specifying the traffic characteristics of the flow.
         Optionally, it may contain may be an ADSPEC object carrying
         advertising (OPWA) data for the flow.

         A Path message travels from a sender to receiver(s) along the
         same path(s) used by the data packets.  The IP source address
         of a Path message must be an address of the sender it
         describes, while the destination address must be the
         DestAddress for the session.  These addresses assure that the
         message will be correctly routed through a non-RSVP cloud.
Top   ToC   RFC2205 - Page 37
         The format of a Path message is as follows:

           <Path Message> ::= <Common Header> [ <INTEGRITY> ]

                                     <SESSION> <RSVP_HOP>


                                    [ <POLICY_DATA> ... ]

                                    [ <sender descriptor> ]

           <sender descriptor> ::= <SENDER_TEMPLATE> <SENDER_TSPEC>

                                    [ <ADSPEC> ]

         If the INTEGRITY object is present, it must immediately follow
         the common header.  There are no other requirements on
         transmission order, although the above order is recommended.
         Any number of POLICY_DATA objects may appear.

         The PHOP (i.e., RSVP_HOP) object of each Path message contains
         the previous hop address, i.e., the IP address of the interface
         through which the Path message was most recently sent.  It also
         carries a logical interface handle (LIH).

         Each RSVP-capable node along the path(s) captures a Path
         message and processes it to create path state for the sender
         defined by the SENDER_TEMPLATE and SESSION objects.  Any
         POLICY_DATA, SENDER_TSPEC, and ADSPEC objects are also saved in
         the path state.  If an error is encountered while processing a
         Path message, a PathErr message is sent to the originating
         sender of the Path message.  Path messages must satisfy the
         rules on SrcPort and DstPort in Section 3.2.

         Periodically, the RSVP process at a node scans the path state
         to create new Path messages to forward towards the receiver(s).
         Each message contains a sender descriptor defining one sender,
         and carries the original sender's IP address as its IP source
         address.  Path messages eventually reach the applications on
         all receivers; however, they are not looped back to a receiver
         running in the same application process as the sender.

         The RSVP process forwards Path messages and replicates them as
         required by multicast sessions, using routing information it
         obtains from the appropriate uni-/multicast routing process.
         The route depends upon the session DestAddress, and for some
Top   ToC   RFC2205 - Page 38
         routing protocols also upon the source (sender's IP) address.
         The routing information generally includes the list of zero or
         more outgoing interfaces to which the Path message is to be
         forwarded.  Because each outgoing interface has a different IP
         address, the Path messages sent out different interfaces
         contain different PHOP addresses.  In addition, ADSPEC objects
         carried in Path messages will also generally differ for
         different outgoing interfaces.

         Path state for a given session and sender may not necessarily
         have a unique PHOP or unique incoming interface.  There are two
         cases, corresponding to multicast and unicast sessions.

         o    Multicast Sessions

              Multicast routing allows a stable distribution tree in
              which Path messages from the same sender arrive from more
              than one PHOP, and RSVP must be prepared to maintain all
              such path state.  The RSVP rules for handling this
              situation are contained in Section 3.9.  RSVP must not
              forward (according to the rules of Section 3.9) Path
              messages that arrive on an incoming interface different
              from that provided by routing.

         o    Unicast Sessions

              For a short period following a unicast route change
              upstream, a node may receive Path messages from multiple
              PHOPs for a given (session, sender) pair.  The node cannot
              reliably determine which is the right PHOP, although the
              node will receive data from only one of the PHOPs at a
              time.  One implementation choice for RSVP is to ignore
              PHOP in matching unicast past state, and allow the PHOP to
              flip among the candidates.  Another implementation choice
              is to maintain path state for each PHOP and to send Resv
              messages upstream towards all such PHOPs.  In either case,
              the situation is a transient; the unused path state will
              time out or be torn down (because upstream path state
              timed out).

      3.1.4 Resv Messages

         Resv messages carry reservation requests hop-by-hop from
         receivers to senders, along the reverse paths of data flows for
         the session.  The IP destination address of a Resv message is
         the unicast address of a previous-hop node, obtained from the
         path state.  The IP source address is an address of the node
         that sent the message.
Top   ToC   RFC2205 - Page 39
         The Resv message format is as follows:

           <Resv Message> ::= <Common Header> [ <INTEGRITY> ]

                                   <SESSION>  <RSVP_HOP>


                                   [ <RESV_CONFIRM> ]  [ <SCOPE> ]

                                   [ <POLICY_DATA> ... ]

                                   <STYLE> <flow descriptor list>

           <flow descriptor list> ::=  <empty> |

                            <flow descriptor list> <flow descriptor>

         If the INTEGRITY object is present, it must immediately follow
         the common header.  The STYLE object followed by the flow
         descriptor list must occur at the end of the message, and
         objects within the flow descriptor list must follow the BNF
         given below.  There are no other requirements on transmission
         order, although the above order is recommended.

         The NHOP (i.e., the RSVP_HOP) object contains the IP address of
         the interface through which the Resv message was sent and the
         LIH for the logical interface on which the reservation is

         The appearance of a RESV_CONFIRM object signals a request for a
         reservation confirmation and carries the IP address of the
         receiver to which the ResvConf should be sent.  Any number of
         POLICY_DATA objects may appear.

         The BNF above defines a flow descriptor list as simply a list
         of flow descriptors.  The following style-dependent rules
         specify in more detail the composition of a valid flow
         descriptor list for each of the reservation styles.

         o    WF Style:

                <flow descriptor list> ::=  <WF flow descriptor>

                <WF flow descriptor> ::= <FLOWSPEC>
Top   ToC   RFC2205 - Page 40
         o    FF style:

                <flow descriptor list> ::=

                          <FLOWSPEC>  <FILTER_SPEC>  |

                          <flow descriptor list> <FF flow descriptor>

                <FF flow descriptor> ::=

                          [ <FLOWSPEC> ] <FILTER_SPEC>

              Each elementary FF style request is defined by a single
              (FLOWSPEC, FILTER_SPEC) pair, and multiple such requests
              may be packed into the flow descriptor list of a single
              Resv message.  A FLOWSPEC object can be omitted if it is
              identical to the most recent such object that appeared in
              the list; the first FF flow descriptor must contain a

         o    SE style:

                <flow descriptor list> ::= <SE flow descriptor>

                <SE flow descriptor> ::=

                                       <FLOWSPEC> <filter spec list>

                <filter spec list> ::=  <FILTER_SPEC>

                                  |  <filter spec list> <FILTER_SPEC>

         The reservation scope, i.e., the set of senders towards which a
         particular reservation is to be forwarded (after merging), is
         determined as follows:

         o    Explicit sender selection

              The reservation is forwarded to all senders whose
              SENDER_TEMPLATE objects recorded in the path state match a
              FILTER_SPEC object in the reservation.  This match must
              follow the rules of Section 3.2.
Top   ToC   RFC2205 - Page 41
         o    Wildcard sender selection

              A request with wildcard sender selection will match all
              senders that route to the given outgoing interface.

              Whenever a Resv message with wildcard sender selection is
              forwarded to more than one previous hop, a SCOPE object
              must be included in the message (see Section 3.4 below);
              in this case, the scope for forwarding the reservation is
              constrained to just the sender IP addresses explicitly
              listed in the SCOPE object.

              A Resv message that is forwarded by a node is generally
              the result of merging a set of incoming Resv messages
              (that are not blockaded; see Section 3.5).  If one of
              these merged messages contains a RESV_CONFIRM object and
              has a FLOWSPEC larger than the FLOWSPECs of the other
              merged reservation requests, then this RESV_CONFIRM object
              is forwarded in the outgoing Resv message.  A RESV_CONFIRM
              object in one of the other merged requests (whose
              flowspecs are equal to, smaller than, or incomparable to,
              the merged flowspec, and which is not blockaded) will
              trigger the generation of an ResvConf message containing
              the RESV_CONFIRM.  A RESV_CONFIRM object in a request that
              is blockaded will be neither forwarded nor returned; it
              will be dropped in the current node.

      3.1.5 Path Teardown Messages

         Receipt of a PathTear (path teardown) message deletes matching
         path state.  Matching state must have match the SESSION,
         SENDER_TEMPLATE, and PHOP objects.  In addition, a PathTear
         message for a multicast session can only match path state for
         the incoming interface on which the PathTear arrived.  If there
         is no matching path state, a PathTear message should be
         discarded and not forwarded.

         PathTear messages are initiated explicitly by senders or by
         path state timeout in any node, and they travel downstream
         towards all receivers.  A unicast PathTear must not be
         forwarded if there is path state for the same (session, sender)
         pair but a different PHOP.  Forwarding of multicast PathTear
         messages is governed by the rules of Section 3.9.
Top   ToC   RFC2205 - Page 42
         A PathTear message must be routed exactly like the
         corresponding Path message.  Therefore, its IP destination
         address must be the session DestAddress, and its IP source
         address must be the sender address from the path state being
         torn down.

             <PathTear Message> ::= <Common Header> [ <INTEGRITY> ]

                                         <SESSION> <RSVP_HOP>

                                        [ <sender descriptor> ]

             <sender descriptor> ::= (see earlier definition)

         A PathTear message may include a SENDER_TSPEC or ADSPEC object
         in its sender descriptor, but these must be ignored.  The order
         requirements are as given earlier for a Path message, but the
         above order is recommended.

         Deletion of path state as the result of a PathTear message or a
         timeout must also adjust related reservation state as required
         to maintain consistency in the local node.  The adjustment
         depends upon the reservation style.  For example, suppose a
         PathTear deletes the path state for a sender S.  If the style
         specifies explicit sender selection (FF or SE), any reservation
         with a filter spec matching S should be deleted; if the style
         has wildcard sender selection (WF), the reservation should be
         deleted if S is the last sender to the session.  These
         reservation changes should not trigger an immediate Resv
         refresh message, since the PathTear message has already made
         the required changes upstream.  They should not trigger a
         ResvErr message, since the result could be to generate a shower
         of such messages.

      3.1.6 Resv Teardown Messages

         Receipt of a ResvTear (reservation teardown) message deletes
         matching reservation state.  Matching reservation state must
         match the SESSION, STYLE, and FILTER_SPEC objects as well as
         the LIH in the RSVP_HOP object.  If there is no matching
         reservation state, a ResvTear message should be discarded.  A
         ResvTear message may tear down any subset of the filter specs
         in FF-style or SE-style reservation state.

         ResvTear messages are initiated explicitly by receivers or by
         any node in which reservation state has timed out, and they
         travel upstream towards all matching senders.
Top   ToC   RFC2205 - Page 43
         A ResvTear message must be routed like the corresponding Resv
         message, and its IP destination address will be the unicast
         address of a previous hop.

             <ResvTear Message> ::= <Common Header> [<INTEGRITY>]

                                         <SESSION> <RSVP_HOP>

                                         [ <SCOPE> ] <STYLE>

                                         <flow descriptor list>

             <flow descriptor list> ::= (see earlier definition)

         FLOWSPEC objects in the flow descriptor list of a ResvTear
         message will be ignored and may be omitted.  The order
         requirements for INTEGRITY object, sender descriptor, STYLE
         object, and flow descriptor list are as given earlier for a
         Resv message, but the above order is recommended.  A ResvTear
         message may include a SCOPE object, but it must be ignored.

         A ResvTear message will cease to be forwarded at the node where
         merging would have suppressed forwarding of the corresponding
         Resv message.  Depending upon the resulting state change in a
         node, receipt of a ResvTear message may cause a ResvTear
         message to be forwarded, a modified Resv message to be
         forwarded, or no message to be forwarded.  These three cases
         can be illustrated in the case of the FF-style reservations
         shown in Figure 6.

         o    If receiver R2 sends a ResvTear message for its
              reservation S3{B}, the corresponding reservation is
              removed from interface (d) and a ResvTear for S3{B} is
              forwarded out (b).

         o    If receiver R1 sends a ResvTear for its reservation
              S1{4B}, the corresponding reservation is removed from
              interface (c) and a modified Resv message FF( S1{3B} ) is
              immediately forwarded out (a).

         o    If receiver R3 sends a ResvTear message for S1{B}, there
              is no change in the effective reservation S1{3B} on (d)
              and no message is forwarded.
Top   ToC   RFC2205 - Page 44
      3.1.7 Path Error Messages

         PathErr (path error) messages report errors in processing Path
         messages.  They are travel upstream towards senders and are
         routed hop-by-hop using the path state.  At each hop, the IP
         destination address is the unicast address of a previous hop.
         PathErr messages do not modify the state of any node through
         which they pass; they are only reported to the sender

           <PathErr message> ::= <Common Header> [ <INTEGRITY> ]

                                      <SESSION> <ERROR_SPEC>

                                      [ <POLICY_DATA> ...]

                                     [ <sender descriptor> ]

           <sender descriptor> ::= (see earlier definition)

         The ERROR_SPEC object specifies the error and includes the IP
         address of the node that detected the error (Error Node
         Address).  One or more POLICY_DATA objects may be included
         message to provide relevant information.  The sender descriptor
         is copied from the message in error.  The object order
         requirements are as given earlier for a Path message, but the
         above order is recommended.

      3.1.8 Resv Error Messages

         ResvErr (reservation error) messages report errors in
         processing Resv messages, or they may report the spontaneous
         disruption of a reservation, e.g., by administrative

         ResvErr messages travel downstream towards the appropriate
         receivers, routed hop-by-hop using the reservation state.  At
         each hop, the IP destination address is the unicast address of
         a next-hop node.
Top   ToC   RFC2205 - Page 45
           <ResvErr Message> ::= <Common Header> [ <INTEGRITY> ]

                                      <SESSION>  <RSVP_HOP>

                                      <ERROR_SPEC>  [ <SCOPE> ]

                                      [ <POLICY_DATA> ...]

                                    <STYLE> [ <error flow descriptor> ]

         The ERROR_SPEC object specifies the error and includes the IP
         address of the node that detected the error (Error Node
         Address).  One or more POLICY_DATA objects may be included in
         an error message to provide relevant information (e.g.,, when a
         policy control error is being reported).  The RSVP_HOP object
         contains the previous hop address, and the STYLE object is
         copied from the Resv message in error.  The use of the SCOPE
         object in a ResvErr message is defined below in Section 3.4.
         The object order requirements are as given for Resv messages,
         but the above order is recommended.

         The following style-dependent rules define the composition of a
         valid error flow descriptor; the object order requirements are
         as given earlier for flow descriptor.

         o    WF Style:

                  <error flow descriptor> ::= <WF flow descriptor>

         o    FF style:

                  <error flow descriptor> ::= <FF flow descriptor>

              Each flow descriptor in a FF-style Resv message must be
              processed independently, and a separate ResvErr message
              must be generated for each one that is in error.

         o    SE style:

                  <error flow descriptor> ::= <SE flow descriptor>

              An SE-style ResvErr message may list the subset of the
              filter specs in the corresponding Resv message to which
              the error applies.
Top   ToC   RFC2205 - Page 46
         Note that a ResvErr message contains only one flow descriptor.
         Therefore, a Resv message that contains N > 1 flow descriptors
         (FF style) may create up to N separate ResvErr messages.

         Generally speaking, a ResvErr message should be forwarded
         towards all receivers that may have caused the error being
         reported.  More specifically:

         o    The node that detects an error in a reservation request
              sends a ResvErr message to the next hop node from which
              the erroneous reservation came.

              This ResvErr message must contain the information required
              to define the error and to route the error message in
              later hops.  It therefore includes an ERROR_SPEC object, a
              copy of the STYLE object, and the appropriate error flow
              descriptor.  If the error is an admission control failure
              while attempting to increase an existing reservation, then
              the existing reservation must be left in place and the
              InPlace flag bit must be on in the ERROR_SPEC of the
              ResvErr message.

         o    Succeeding nodes forward the ResvErr message to next hops
              that have local reservation state.  For reservations with
              wildcard scope, there is an additional limitation on
              forwarding ResvErr messages, to avoid loops; see Section
              3.4.  There is also a rule restricting the forwarding of a
              Resv message after an Admission Control failure; see
              Section 3.5.

              A ResvErr message that is forwarded should carry the
              FILTER_SPEC(s) from the corresponding reservation state.

         o    When a ResvErr message reaches a receiver, the STYLE
              object, flow descriptor list, and ERROR_SPEC object
              (including its flags) should be delivered to the receiver

      3.1.9 Confirmation Messages

         ResvConf messages are sent to (probabilistically) acknowledge
         reservation requests.  A ResvConf message is sent as the result
         of the appearance of a RESV_CONFIRM object in a Resv message.
Top   ToC   RFC2205 - Page 47
         A ResvConf message is sent to the unicast address of a receiver
         host; the address is obtained from the RESV_CONFIRM object.
         However, a ResvConf message is forwarded to the receiver hop-
         by-hop, to accommodate the hop-by-hop integrity check

           <ResvConf message> ::= <Common Header> [ <INTEGRITY> ]

                                      <SESSION> <ERROR_SPEC>


                                      <STYLE> <flow descriptor list>

           <flow descriptor list> ::= (see earlier definition)

         The object order requirements are the same as those given
         earlier for a Resv message, but the above order is recommended.

         The RESV_CONFIRM object is a copy of that object in the Resv
         message that triggered the confirmation.  The ERROR_SPEC is
         used only to carry the IP address of the originating node, in
         the Error Node Address; the Error Code and Value are zero to
         indicate a confirmation.  The flow descriptor list specifies
         the particular reservations that are being confirmed; it may be
         a subset of flow descriptor list of the Resv that requested the

(page 47 continued on part 3)

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