Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools

Problem, Formatting or Query -  Send Feedback

Was this page helpful?-10+1


Network Working Group A. Terzis Request for Comments: 2746 UCLA Category: Standards Track J. Krawczyk

                                             ArrowPoint Communications
                                                         J. Wroclawski
                                                               MIT LCS
                                                              L. Zhang
                                                          January 2000
                   RSVP Operation Over IP Tunnels

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2000).  All Rights Reserved.


 This document describes an approach for providing RSVP protocol
 services over IP tunnels. We briefly describe the problem, the
 characteristics of possible solutions, and the design goals of our
 approach. We then present the details of an implementation which
 meets our design goals.

1. Introduction

 IP-in-IP "tunnels" have become a widespread mechanism to transport
 datagrams in the Internet. Typically, a tunnel is used to route
 packets through portions of the network which do not directly
 implement the desired service (e.g. IPv6), or to augment and modify
 the behavior of the deployed routing architecture (e.g. multicast
 routing, mobile IP, Virtual Private Net).
 Many IP-in-IP tunneling protocols exist today.  [IP4INIP4] details a
 method of tunneling using an additional IPv4 header.  [MINENC]
 describes a way to reduce the size of the "inner" IP header used in
 [IP4INIP4] when the original datagram is not fragmented.  The generic
 tunneling method in [IPV6GEN] can be used to tunnel either IPv4 or
 IPv6 packets within IPv6.  [RFC1933] describes how to tunnel IPv6

Terzis, et al. Standards Track [Page 1] RFC 2746 RSVP Operation Over IP Tunnels January 2000

 datagrams through IPv4 networks.  [RFC1701] describes a generic
 routing encapsulation, while [RFC1702] applies this encapsulation to
 IPv4.  Finally, [ESP] describes a mechanism that can be used to
 tunnel an encrypted IP datagram.
 From the perspective of traditional best-effort IP packet delivery, a
 tunnel behaves as any other link. Packets enter one end of the
 tunnel, and are delivered to the other end unless resource overload
 or error causes them to be lost.
 The RSVP setup protocol [RFC2205] is one component of a framework
 designed to extend IP to support multiple, controlled classes of
 service over a wide variety of link-level technologies. To deploy
 this technology with maximum flexibility, it is desirable for tunnels
 to act as RSVP-controllable links within the network.
 A tunnel, and in fact any sort of link, may participate in an RSVP-
 aware network in one of three ways, depending on the capabilities of
 the equipment from which the tunnel is constructed and the desires of
 the operator.
    1. The (logical) link may not support resource reservation or QoS
       control at all. This is a best-effort link. We refer to this as
       a best-effort or type 1 tunnel in this note.
    2. The (logical) link may be able to promise that some overall
       level of resources is available to carry traffic, but not to
       allocate resources specifically to individual data flows.  A
       configured resource allocation over a tunnel is an example of
       this.  We refer to this case as a type 2 tunnel in this note.
    3. The (logical) link may be able to make reservations for
       individual end-to-end data flows.  We refer to this case as a
       type 3 tunnel. Note that the key feature that distinguishes
       type 3 tunnels from type 2 tunnels is that in the type 3 tunnel
       new tunnel reservations are created and torn down dynamically
       as end-to-end reservations come and go.
 Type 1 tunnels exist when at least one of the routers comprising the
 tunnel endpoints does not support the scheme we describe here. In
 this case, the tunnel acts as a best-effort link. Our goal is simply
 to make sure that RSVP messages traverse the link correctly, and the
 presence of the non-controlled link is detected, as required by the
 integrated services framework.
 When the two end points of the tunnel are capable of supporting RSVP
 over tunnels, we would like to have proper resources reserved along
 the tunnel.  Depending on the requirements of the situation, this
 might mean that  one client's data flow is placed into a larger
 aggregate reservation  (type 2 tunnels) or that possibly a new,

Terzis, et al. Standards Track [Page 2] RFC 2746 RSVP Operation Over IP Tunnels January 2000

 separate reservation is made for the data flow (type 3 tunnels).
 Note that an RSVP reservation between the two tunnel end points does
 not necessarily mean that all the intermediate routers along the
 tunnel path support RSVP, this is equivalent to the case of an
 existing end-to-end RSVP session transparently passing through non-
 RSVP cloud.
 Currently, however, RSVP signaling over tunnels is not possible.
 RSVP packets entering the tunnel are encapsulated with an outer IP
 header that has a protocol number other than 46 (e.g. it is 4 for
 IP-in-IP encapsulation) and do not carry the Router-Alert option,
 making them virtually "invisible" to RSVP routers between the two
 tunnel endpoints.  Moreover, the current IP-in-IP encapsulation
 scheme adds only an IP header as the external wrapper. It is
 impossible to distinguish between packets that use reservations and
 those that don't, or to differentiate packets belonging to different
 RSVP Sessions while they are in the tunnel, because no distinguishing
 information such as a UDP port is available in the encapsulation.
 This document describes an IP tunneling enhancement mechanism that
 allows RSVP to make  reservations across all IP-in-IP tunnels. This
 mechanism is capable of supporting both type 2 and type 3 tunnels, as
 described above, and requires minimal changes to both RSVP and other
 parts of the integrated services framework.

2. The Design

2.1. Design Goals

 Our design choices are motivated by several goals.
  • Co-existing with most, if not all, current IP-in-IP tunneling


  • Limiting the changes to the RSVP spec to the minimum possible.
  • Limiting the necessary changes to only the two end points of a

tunnel. This requirement leads to simpler deployment, lower

      overhead in the intermediate routers, and less chance of failure
      when the set of intermediate routers is modified due to routing
    * Supporting correct inter-operation with RSVP routers that have
      not been upgraded to handle RSVP over tunnels and with non-RSVP
      tunnel endpoint routers. In these cases, the tunnel behaves as a
      non-RSVP link.

Terzis, et al. Standards Track [Page 3] RFC 2746 RSVP Operation Over IP Tunnels January 2000

2.2. Basic Approach

 The basic idea of the method described in this document is to
 recursively apply RSVP over the tunnel portion of the path. In this
 new session, the tunnel entry point Rentry sends PATH messages and
 the tunnel exit point Rexit sends RESV messages to reserve resources
 for the end-to-end sessions over the tunnel.
 We discuss next two different aspects of the design: how to enhance
 an IP-in-IP tunnel with RSVP capability, and how to map end-to-end
 RSVP sessions to a tunnel session.

2.2.1. Design Decisions

 To establish a RSVP reservation over a unicast IP-in-IP tunnel, we
 made the following design decisions:
 One or more Fixed-Filter style unicast reservations between the two
 end points of the tunnel will be used to reserve resources for
 packets traversing the tunnel. In the type 2 case, these reservations
 will be configured statically by a management interface. In the type
 3 case, these reservations will be created and torn down on demand,
 as end-to-end reservation requests come and go.
 Packets that do not require reservations are encapsulated in the
 normal way, e. g. being wrapped with an IP header only, specifying
 the tunnel entry point as source and the exit point as destination.
 Data packets that require resource reservations within a tunnel must
 have some attribute other than the IP addresses visible to the
 intermediate routers, so that the routers may map the packet to an
 appropriate reservation.  To allow intermediate routers to use
 standard RSVP filterspec handling, we choose to encapsulate such data
 packets by prepending an IP and a UDP header, and to use UDP port
 numbers to distinguish packets of different RSVP sessions. The
 protocol number in the outer IP header in this case will be UDP.
 Figure 1 shows RSVP operating over a tunnel. Rentry is the tunnel
 entry router which encapsulates data into the tunnel.  Some number of
 intermediate routers forward the data across the network based upon
 the encapsulating IP header added by Rentry.  Rexit is the endpoint
 of the tunnel.  It decapsulates the data and forwards it based upon
 the original, "inner" IP header.

Terzis, et al. Standards Track [Page 4] RFC 2746 RSVP Operation Over IP Tunnels January 2000

   ...........             ...............            .............
             :   _______   :             :   _____    :
             :  |       |  :             :  |     |   :
   Intranet  :--| Rentry|===================|Rexit|___:Intranet
             :  |_______|  :             :  |_____|   :
   ..........:             :   Internet  :            :...........
               Figure 1.  An example IP Tunnel

2.2.2. Mapping between End-to-End and Tunnel Sessions

 Figure 2 shows a simple topology with a tunnel and a few hosts. The
 sending hosts H1 and H3 may be one or multiple IP hops away from
 Rentry; the receiving hosts H2 and H4 may also be either one or
 multiple IP hops away from Rexit.
           H1                                          H2
           :                                            :
           :                                            :
       +--------+     +---+     +---+     +---+     +-------+
       |        |     |   |     |   |     |   |     |       |
 H3... | Rentry |===================================| Rexit |.....  H4
       |        |     |   |     |   |     |   |     |       |
       +--------+     +---+     +---+     +---+     +-------+
          Figure 2: An example end-to-end path with
                    a tunnel in the middle.
 An RSVP session may be in place between endpoints at hosts H1 and H2.
 We refer to this session as the "end-to-end" (E2E for short) or
 "original" session, and to its PATH and RESV messages as the end-to-
 end messages.  One or more RSVP sessions may be in place between
 Rentry and Rexit to provide resource reservation over the tunnel. We
 refer to these as the tunnel RSVP sessions, and to their PATH and
 RESV messages as the tunnel or tunneling messages.  A tunnel RSVP
 session may exist independently from any end-to-end sessions.  For
 example through network management interface one may create a RSVP
 session over the tunnel to provide QoS support for data flow from H3
 to H4, although there is no end-to-end RSVP session between H3 and
 When an end-to-end RSVP session crosses a RSVP-capable tunnel, there
 are two cases to consider in designing mechanisms to support an end-
 to-end reservation over the tunnel: mapping the E2E session to an
 existing tunnel RSVP session (type 2 tunnel), and dynamically
 creating a new tunnel RSVP session for each end-to-end session (type

Terzis, et al. Standards Track [Page 5] RFC 2746 RSVP Operation Over IP Tunnels January 2000

 3 tunnel).  In either case, the picture looks like a recursive
 application of RSVP.  The tunnel RSVP session views the two tunnel
 endpoints as two end hosts with a unicast Fixed-Filter style
 reservation in between.  The original, end-to-end RSVP session views
 the tunnel as a single (logical) link on the path between the
 source(s) and destination(s).
 Note that in practice a tunnel may combine type 2 and type 3
 characteristics. Some end-to-end RSVP sessions may trigger the
 creation of new tunnel sessions, while others may be mapped into an
 existing tunnel RSVP session. The choice of how an end-to-end session
 is treated at the tunnel is a matter of local policy.
 When an end-to-end RSVP session crosses a RSVP-capable tunnel, it is
 necessary to coordinate the actions of the two RSVP sessions, to
 determine whether or when the tunnel RSVP session should be created
 and torn down, and to correctly transfer error and ADSPEC information
 between the two RSVP sessions.  We made the following design
  • End-to-end RSVP control messages being forwarded through a

tunnel are encapsulated in the same way as normal IP packets,

      e.g. being wrapped with the tunnel IP header only, specifying
      the tunnel entry point as source and the exit point as

2.3. Major Issues

 As IP-in-IP tunnels are being used more widely for network traffic
 management purposes, it is clear we must support type 2 tunnels
 (tunnel reservation for aggregate end-to-end sessions).  Furthermore,
 these type 2 tunnels should allow more than one (configurable,
 static) reservation to be used at once, to support different traffic
 classes within the tunnel. Whether it is necessary to support type 3
 tunnels (dynamic per end-to-end session tunnel reservation) is a
 policy issue that should be left open.  Our design supports both
 If there is only one RSVP session configured over a tunnel, then all
 the end-to-end RSVP sessions (that are allowed to use this tunnel
 session) will be bound to this configured tunnel session.  However
 when more than one RSVP session is in use over an IP tunnel, a second
 design issue is how the association, or binding, between an original
 RSVP reservation and a tunnel reservation is created and conveyed
 from one end of the tunnel to the other. The entry router Rentry and
 the exit router Rexit must agree on these associations so that

Terzis, et al. Standards Track [Page 6] RFC 2746 RSVP Operation Over IP Tunnels January 2000

 changes in the original reservation state can be correctly mapped
 into changes in the tunnel reservation state, and that errors
 reported by intermediate routers to the tunnel end points can be
 correctly transformed into errors reported by the tunnel endpoints to
 the end-to-end RSVP session.
 We require that this same association mechanism work for both the
 case of bundled reservation over a tunnel (type 2 tunnel), and the
 case of one-to-one mapping between original and tunnel reservations
 (type 3 tunnel). In our scheme the association is created when a
 tunnel entry point first sees an end-to-end session's RESV message
 and either sets up a new tunnel session, or adds to an existing
 tunnel session.  This new association must be conveyed to Rexit, so
 that Rexit can reserve resources for the end-to-end sessions inside
 the tunnel. This information includes the identifier and certain
 parameters of the tunnel session, and the identifier of the end-to-
 end session to which the tunnel session is being bound. In our
 scheme, all RSVP sessions between the same two routers Rentry and
 Rexit will have identical values for source IP address, destination
 IP address, and destination UDP port number. An individual session is
 identified primarily by the source port value.
 We identified three possible choices for a binding mechanism:
    1. Define a new RSVP message that is exchanged only between two
       tunnel end points to convey the binding information.
    2. Define a new RSVP object to be attached to end-to-end PATH
       messages at Rentry, associating the end-to-end session with one
       of the tunnel sessions. This new object is interpreted by Rexit
       associating the end-to-end session with one of the tunnel
       sessions generated at Rentry.
    3. Apply the same UDP encapsulation to the end-to-end PATH
       messages as to data packets of the session.  When Rexit
       decapsulates the PATH message, it deduces the relation between
       the source UDP port used in the encapsulation and the RSVP
       session that is specified in the original PATH message.

Terzis, et al. Standards Track [Page 7] RFC 2746 RSVP Operation Over IP Tunnels January 2000

 The last approach above does not require any new design.  However it
 requires additional resources to be reserved for PATH messages (since
 they are now subject to the tunnel reservation).  It also requires a
 priori knowledge of whether Rexit supports RSVP over tunnels by UDP
 encapsulation.  If Rentry encapsulates all the end-to-end PATH
 messages with the UDP encapsulation, but Rexit does not understand
 this encapsulation, then the encapsulated PATH messages will be lost
 at Rexit.
 On the other hand, options (1) and (2) can handle this case
 transparently.  They allow Rexit to pass on end-to-end PATHs received
 via the tunnel (because they are decapsulated normally), while
 throwing away the tunnel PATHs, all without any additional
 configuration.  We chose Option (2) because it is simpler.  We
 describe this object in the following section.
 Packet exchanges must follow the following constraints:
    1. Rentry encapsulates and sends end-to-end PATH messages over the
       tunnel to Rexit where they get decapsulated and forwarded
    2. When a corresponding end-to-end RESV message arrives at Rexit,
       Rexit encapsulates it and sends it to Rentry.
    3. Based on some or all of the information in the end-to-end PATH
       messages, the flowspec in the end-to-end RESV message and local
       policies, Rentry decides if and how to map the end-to-end
       session to a tunnel session.
    4. If the end-to-end session should be mapped to a tunnel session,
       Rentry either sends a PATH message for a new tunnel session or
       updates an existing one.
    5. Rentry sends a E2E Path containing a SESSION_ASSOC object
       associating the end-to-end session with the tunnel session
       above.  Rexit records the association and removes the object
       before forwarding the Path message further.
    6. Rexit responds to the tunnel PATH message by sending a tunnel
       RESV message, reserving resources inside the tunnel.
    7. Rentry UDP-encapsulates arriving packets only if a
       corresponding tunnel session reservation is actually in place
       for the packets.

Terzis, et al. Standards Track [Page 8] RFC 2746 RSVP Operation Over IP Tunnels January 2000

2.3.1. SESSION_ASSOC Object

 The new object, called SESSION_ASSOC, is defined with the following
  |          length               |  class        |     c-type    |
  |                                                               |
  |          SESSION object  (for the end-to-end session)         |
  |                                                               |
  |                                                               |
  |           Sender FILTER-SPEC (for the tunnel session)         |
  |                                                               |
                         SESSION_ASSOC Object
    This field contains the size of the SESSION_ASSOC object in bytes.
    Should be 192.
    Should be sent as zero and ignored on receipt.
 SESSION object
    The end-to-end SESSION contained in the object is to be mapped to
    the tunnel session described by the Sender FILTER-SPEC defined
    This is the tunnel session that the above mentioned end-to-end
    session maps to over the tunnel. As we mentioned above, a tunnel
    session is identified primarily by source port. This is why we use
    a Sender Filter-Spec for the tunnel session, in the place of a
    SESSION object.

Terzis, et al. Standards Track [Page 9] RFC 2746 RSVP Operation Over IP Tunnels January 2000

2.3.2. NODE_CHAR Object

 There has to be a way (other than through configuration) for Rexit to
 communicate to Rentry the fact that it is a tunnel endpoint
 supporting the scheme described in this document. We have defined for
 this reason a new object, called NODE_CHAR, carrying this
 information. If a node receives this object but does not understand
 it, it should drop it without producing any error report. Objects
 with Class-Num = 10bbbbbb (`b' represents a bit), as defined in the
 RSVP specification [RFC2205], have the characteristics we need. While
 for now this object only carries one bit of information, it can be
 used in the future to describe other characteristics of an RSVP
 capable node that are not part of the original RSVP specification.
 The object NODE_CHAR has the following format:
  |          length               |  class        |     c-type    |
  |                         Reserved                            |T|
    This field contains the size of the NODE_CHAR object in bytes. It
    should be set to eight.
    An appropriate value should be assigned by the IANA. We propose
    this value to be 128.
    Should be sent as zero and ignored on receipt.
 T bit
    This bit shows that the node is a RSVP-tunnel capable node.
 When Rexit receives an end-to-end reservation, it appends a NODE_CHAR
 object with the T bit set, to the RESV object, it encapsulates it and
 sends it to Rentry. When Rentry receives this RESV message it deduces
 that Rexit implements the mechanism described here and so it creates
 or adjusts a tunnel session and associates the tunnel session to the
 end-to-end session via a SESSION_ASSOC object. Rentry should remove
 the NODE_CHAR object, before forwarding the RESV message upstream. If

Terzis, et al. Standards Track [Page 10] RFC 2746 RSVP Operation Over IP Tunnels January 2000

 on the other hand, Rentry does not support the RSVP Tunnels mechanism
 it would simply ignore the NODE_CHAR object and not forward it
 further upstream.

3. Implementation

 In this section we discuss several cases separately, starting from
 the simplest scenario and moving to the more complex ones.

3.1. Single Configured RSVP Session over an IP-in-IP Tunnel

 Treating the two tunnel endpoints as a source and destination host,
 one easily sets up a FF-style reservation in between.  Now the
 question is what kind of filterspec to use for the tunnel
 reservation, which directly relates to how packets get encapsulated
 over the tunnel.  We discuss two cases below.

3.1.1. In the Absence of End-to-End RSVP Session

 In the case where all the packets traversing a tunnel use the
 reserved resources, the current IP-in-IP encapsulation could be used.
 The RSVP session over the tunnel would simply specify a FF style
 reservation (with zero port number) with Rentry as the source address
 and Rexit as the destination address.
 However if only some of the packets traversing the tunnel should
 benefit from the reservation, we must encapsulate the qualified
 packets in IP and UDP. This allows intermediate routers to use
 standard RSVP filterspec handling, without having to know about the
 existence of tunnels.
 Rather than supporting both cases we choose to simplify
 implementations by requiring all data packets using reservations to
 be encapsulated with an outer IP and UDP header. This reduces special
 case checking and handling.

3.1.2. In the Presence of End-to-End RSVP Session(s)

 According to the tunnel control policies, installed through some
 management interface, some or all end-to-end RSVP sessions may be
 allowed to map to the single RSVP session over the tunnel.  In this
 case there is no need to provide dynamic binding information between
 end-to-end sessions and the tunnel session, given that the tunnel
 session is unique and pre-configured, and therefore well-known.
 Binding multiple end-to-end sessions to one tunnel session, however,
 raises a new question of when and how the size of the tunnel
 reservation should be adjusted to accommodate the end-to-end sessions

Terzis, et al. Standards Track [Page 11] RFC 2746 RSVP Operation Over IP Tunnels January 2000

 mapped onto it.  Again the tunnel manager makes such policy decision.
 Several scenarios are possible. In the first, the tunnel reservation
 is never adjusted. This makes the tunnel the rough equivalent of a
 fixed-capacity hardware link. In the second, the tunnel reservation
 is adjusted whenever a new end-to-end reservation arrives or an old
 one is torn down. In the third, the tunnel reservation is adjusted
 upwards or downwards occasionally, whenever the end-to-end
 reservation level has changed enough to warrant the adjustment. This
 trades off extra resource usage in the tunnel for reduced control
 traffic and overhead.
 We call a tunnel whose reservation cannot be adjusted a "hard pipe",
 as opposed to a "soft pipe" where the amount of resources allocated
 is adjustable. Section 5.2 explains how the adjustment can be carried
 out for soft pipes.

3.2. Multiple Configured RSVP Sessions over an IP-in-IP Tunnel

 It is straightforward to build on the case of a single configured
 RSVP session over a tunnel by setting up multiple FF-style
 reservations between the two tunnel endpoints using a management
 interface.  In this case Rentry must carefully encapsulate data
 packets with the proper UDP port numbers, so that packets belonging
 to different tunnel sessions will be distinguished by the
 intermediate RSVP routers.  Note that this case and the one described
 before describe what we call type 2 tunnels.

3.2.1. In the Absence of End-to-End RSVP Session

 Nothing more needs to be said in this case. Rentry classifies the
 packets and encapsulates them accordingly. Packets with no
 reservations are encapsulated with an outer IP header only, while
 packets qualified for reservations are encapsulated with a UDP header
 as well as an IP header. The UDP source port value should be properly
 set to map to the corresponding tunnel reservation the packet is
 supposed to use.

3.2.2. In the Presence of End-to-End RSVP Session(s)

 Since in this case, there is more than one RSVP session operating
 over the tunnel, one must explicitly bind each end-to-end RSVP
 session to its corresponding tunnel session.  As discussed
 previously, this binding will be provided by the new SESSION_ASSOC
 object carried by the end-to-end PATH messages.

Terzis, et al. Standards Track [Page 12] RFC 2746 RSVP Operation Over IP Tunnels January 2000

3.3. Dynamically Created Tunnel RSVP Sessions

 This is the case of a type 3 tunnel. The only differences between
 this case and that of Section 4.2 are that:
  1. The tunnel session is created when a new end-to-end session

shows up.

  1. There is a one-to-one mapping between the end-to-end and tunnel

RSVP sessions, as opposed to possibly many-to-one mapping that

      is allowed in the case described in Section 4.2.

4. RSVP Messages handling over an IP-in-IP Tunnel

4.1. RSVP Messages for Configured Session(s) Over A Tunnel

 Here one or more RSVP sessions are set up over a tunnel through a
 management interface.  The session reservation parameters never
 change for a "hard pipe" tunnel. The reservation parameters may
 change for a "soft pipe" tunnel. Tunnel session PATH messages
 generated by Rentry are addressed to Rexit, where they are processed
 and deleted.

4.2. Handling of RSVP Messages at Tunnel Endpoints

4.2.1. Handling End-to-End PATH Messages at Rentry

 When forwarding an end-to-end PATH message, a router acting as the
 tunnel entry point, Rentry, takes the following actions depending on
 the end-to-end session mentioned in the PATH message. There are two
 possible cases:
    1. The end-to-end PATH message is a refresh of a previously known
       end-to-end session.
    2. The end-to-end PATH message is from a new end-to-end session.
 If the PATH message is a refresh of a previously known end-to-end
 session, then Rentry refreshes the Path state of the end-to-end
 session and checks to see if this session is mapped to a tunnel
 session. If this is the case, then when Rentry refreshes the end-to-
 end session, it includes in the end-to-end PATH message a
 SESSION_ASSOC object linking this session to its corresponding tunnel
 session It then encapsulates the end-to-end PATH message and sends it
 over the tunnel to Rexit. If the tunnel session was dynamically
 created, the end-to-end PATH message serves as a refresh for the
 local tunnel state at Rentry as well as for the end-to-end session.

Terzis, et al. Standards Track [Page 13] RFC 2746 RSVP Operation Over IP Tunnels January 2000

 Otherwise, if the PATH message is from a new end-to-end session that
 has not yet been mapped to a tunnel session, Rentry creates Path
 state for this new session setting the outgoing interface to be the
 tunnel interface. After that, Rentry encapsulates the PATH message
 and sends it to Rexit without adding a SESSION_ASSOC message.
 When an end-to-end PATH TEAR is received by Rentry, this node
 encapsulates and forwards the message to Rexit. If this end-to-end
 session has a one-to-one mapping to a tunnel session or if this is
 the last one of the many end-to-end sessions mapping to a tunnel
 session, Rentry tears down the tunnel session by sending a PATH TEAR
 for that session to Rexit. If, on the other hand, there are remaining
 end-to-end sessions mapping to the tunnel session, then Rentry sends
 a tunnel PATH message adjusting the Tspec of the tunnel session.

4.2.2. Handling End-to-End PATH Messages at Rexit

 Encapsulated end-to-end PATH messages are decapsulated and processed
 at Rexit. Depending on whether the end-to-end PATH message contains a
 SESSION_ASSOC object or not, Rexit takes the following steps:
    1. If the end-to-end PATH message does not contain a SESSION_ASSOC
       object, then Rentry sets the Non_RSVP flag at the Path state
       stored for this end-to-end sender, sets the global break bit in
       the ADSPEC and forwards the packets downstream. Alternatively,
       if tunnel sessions exist and none of them has the Non_RSVP flag
       set, Rexit can pick the worst-case Path ADSPEC params from the
       existing tunnel sessions and update the end-to-end ADSPEC using
       these values. This is a conservative estimation of the composed
       ADSPEC but it has the benefit of avoiding to set the break bit
       in the end-to-end ADSPEC before mapping information is
       available. In this case the Non_RSVP flag at the end-to-end
       Path state is not set.
    2. If the PATH message contains a SESSION_ASSOC object and no
       association for this end-to-end session already exists, then
       Rexit records the association between the end-to-end session
       and the tunnel session described by the object. If the end-to-
       end PATH arrives early before the tunnel PATH message arrives
       then it creates PATH state at Rexit for the tunnel session.
       When the actual PATH message for the tunnel session arrives it
       is treated as an update of the existing PATH state and it
       updates any information missing. We believe that this situation
       is another transient along with the others existing in RSVP and
       that it does not have any long-term effects on the correct
       operation of the mechanism described here.

Terzis, et al. Standards Track [Page 14] RFC 2746 RSVP Operation Over IP Tunnels January 2000

       Before further forwarding the message to the next hop along the
       path to the destination, Rexit finds the corresponding tunnel
       session's recorded state and turns on Non_RSVP flag in the
       end-to-end Path state if the Non_RSVP bit was turned on for the
       tunnel session.  If the end-to-end PATH message carries an
       ADSPEC object, Rexit performs composition of the
       characterization parameters contained in the ADSPEC. It does
       this by considering the tunnel session's overall (composed)
       characterization parameters as the local parameters for the
       logical link implemented by the tunnel, and composing these
       parameters with those in the end-to-end ADSPEC by executing
       each parameter's defined composition function. In the logical
       link's characterization parameters, the minimum path latency
       may take into account the encapsulation/decapsulation delay and
       the bandwidth estimate can represent the decrease in available
       bandwidth caused by the addition of the extra UDP header.
       ADSPECs and composition functions are discussed in great detail
       in [RFC2210].
       If the end-to-end session has reservation state, while no
       reservation state for the matching tunnel session exists, Rexit
       send a tunnel RESV message to Rentry matching the reservation
       in the end-to-end session.
 If Rentry does not support RSVP tunneling, then Rexit will have no
 PATH state for the tunnel. In this case Rexit simply turns on the
 global break bit in the decapsulated end-to-end PATH message and
 forwards it.

4.2.3. Handling End-to-End RESV Messages at Rexit

 When forwarding a RESV message upstream, a router serving as the exit
 router, Rexit, may discover that one of the upstream interfaces is a
 tunnel.  In this case the router performs a number of tests.
 Step 1: Rexit must determine if there is a tunnel session bound to
 the end-to-end session given in the RESV message.  If not, the tunnel
 is treated as a non-RSVP link, Rexit appends a NODE_CHAR object with
 the T bit set, to the RESV message and forwards it over the tunnel
 interface (where it is encapsulated as a normal IP datagram and
 forwarded towards Rentry).
 Step 2: If a bound tunnel session is found, Rexit checks to see if a
 reservation is already in place for the tunnel session bound to the
 end-to-end session given in the RESV message. If the arriving end-
 to-end RESV message is a refresh of existing RESV state, then Rexit
 sends the original RESV through tunnel interface (after adding the
 NODE_CHAR object). For dynamic tunnel sessions, the end-to-end RESV

Terzis, et al. Standards Track [Page 15] RFC 2746 RSVP Operation Over IP Tunnels January 2000

 message acts as a refresh for the tunnel session reservation state,
 while for configured tunnel sessions, reservation state never
 If the arriving end-to-end RESV message causes a change in the end-
 to-end RESV flowspec parameters, it may also trigger an attempt to
 change the tunnel session's flowspec parameters.  In this case Rexit
 sends a tunnel session RESV, including a RESV_CONFIRM object.
 In the case of a "hard pipe" tunnel, a new end-to-end reservation or
 change in the level of resources requested by an existing reservation
 may cause the total resource level needed by the end-to-end
 reservations to exceed the level of resources reserved by the tunnel
 reservation. This event should be treated as an admission control
 failure, identically to the case where RSVP requests exceed the level
 of resources available over a hardware link. A RESV_ERR message with
 Error Code set to 01 (Admission Control failure), should be sent back
 to the originator of the end-to-end RESV message.
 If a RESV CONFIRM response arrives, the original RESV is encapsulated
 and sent through the tunnel. If the updated tunnel reservation fails,
 Rexit must send a RESV ERR to the originator of the end-to-end RESV
 message, using the error code and value fields from the ERROR_SPEC
 object of the received tunnel session RESV ERR message. Note that the
 pre-existing reservations through the tunnel stay in place. Rexit
 continues refreshing the tunnel RESV using the old flowspec.
 Tunnel session state for a "soft pipe" may also be adjusted when an
 end-to-end reservation is deleted.  The tunnel session gets reduced
 whenever one of the end-to-end sessions using the tunnel goes away
 (or gets reduced itself). However even when the last end-to-end
 session bound to that tunnel goes away, the configured tunnel session
 remains active, perhaps with a configured minimal flowspec.
 Note that it will often be appropriate to use some hysteresis in the
 adjustment of the tunnel reservation parameters, rather than
 adjusting the tunnel reservation up and down with each arriving or
 departing end-to-end reservation.  Doing this will require the tunnel
 exit router to keep track of the resources allocated to the tunnel
 (the tunnel flowspec) and the resources actually in use by end-to-end
 reservations (the sum or statistical sum of the end-to-end
 reservation flowspecs) separately.
 When an end-to-end RESV TEAR is received by Rexit, it encapsulates
 and forwards the message to Rentry. If the end-to-end session had
 created a dynamic tunnel session, then a RESV TEAR for the
 corresponding tunnel session is send by Rexit.

Terzis, et al. Standards Track [Page 16] RFC 2746 RSVP Operation Over IP Tunnels January 2000

4.2.4. Handling of End-to-End RESV Messages at Rentry.

 If the RESV message received is a refresh of an existing reservation
 then Rentry updates the reservation state and forwards the message
 upstream. On the other hand, if this is the first RESV message for
 this end-to-end session and a NODE_CHAR object with the T bit set is
 present, Rentry should initiate the mapping between this end-to-end
 session and some (possibly new) tunnel session. This mapping is based
 on some or all of the contents of the end-to-end PATH message, the
 contents of the end-to-end RESV message, and local policies. For
 example, there could be different tunnel sessions based on the
 bandwidth or delay requirements of end-to-end sessions)
 If Rentry decides that this end-to-end session should be mapped to an
 existing configured tunnel session, it binds this end-to-end session
 to that tunnel session.
 If this end-to-end RSVP session is allowed to set up a new tunnel
 session, Rentry sets up tunnel session PATH state as if it were a
 source of data by starting to send tunnel-session PATH messages to
 Rexit, which is treated as the unicast destination of the data. The
 Tspec in this new PATH message is computed from the original PATH
 message by adjusting the Tspec parameters to include the tunnel
 overhead of the encapsulation of data packets. In this case Rentry
 should also send a PATH message from the end-to-end session this time
 containing the SESSION_ASSOC object linking the two sessions. The
 receipt of this PATH message by Rexit will trigger an update of the
 end-to-end Path state which in turn will have the effect of Rexit
 sending a tunnel RESV message, allocating resources inside the
 The last case is when the end-to-end session is not allowed to use
 the tunnel resources. In this case no association is created between
 this end-to-end session and a tunnel session and no new tunnel
 session is created.
 One limitation of our scheme is that the first RESV message of an
 end-to-end session determines the mapping between that end-to-end
 session and its corresponding session over the tunnel. Moreover as
 long as the reservation is active this mapping cannot change.

Terzis, et al. Standards Track [Page 17] RFC 2746 RSVP Operation Over IP Tunnels January 2000

5. Forwarding Data

 When data packets arrive at the tunnel entry point Rentry, Rentry
 must decide whether to forward the packets using the normal IP-in-IP
 tunnel encapsulation or the IP+UDP encapsulation expected by the
 tunnel session.  This decision is made by determining whether there
 is a resource reservation (not just PATH state) actually in place for
 the tunnel session bound to the arriving packet, that is, whether the
 packet matches any active filterspec.
 If a reservation is in place, it means that both Rentry and Rexit are
 RSVP-tunneling aware routers, and the data will be correctly
 decapsulated at Rexit.
 If no tunnel session reservation is in place, the data should be
 encapsulated in the tunnel's normal format, regardless of whether
 end-to-end PATH state covering the data is present.

6. Details

6.1. Selecting UDP port numbers

 There may be multiple end-to-end RSVP sessions between the two end
 points Rentry and Rexit. These sessions are distinguished by the
 source UDP port. Other components of the session ID, the source and
 destination IP addresses and the destination UDP port, are identical
 for all such sessions.
 The source UDP port is chosen by the tunnel entry point Rentry when
 it establishes the initial PATH state for a new tunnel session. The
 source UDP port associated with the new session is then conveyed to
 Rexit by the SESSION_ASSOC object.
 The destination UDP port used in tunnel sessions should the one
 assigned by IANA (363).

6.2. Error Reporting

 When a tunnel session PATH message encounters an error, it is
 reported back to Rentry. Rentry must relay the error report back to
 the original source of the end-to-end session.
 When a tunnel session RESV request fails, an error message is
 returned to Rexit. Rexit must treat this as an error in crossing the
 logical link (the tunnel) and forward the error message back to the
 end host.

Terzis, et al. Standards Track [Page 18] RFC 2746 RSVP Operation Over IP Tunnels January 2000

6.3. MTU Discovery

 Since the UDP encapsulated packets should not be fragmented, tunnel
 entry routers must support tunnel MTU discovery as discussed in
 section 5.1 of [IP4INIP4]. Alternatively, the Path MTU Discovery
 mechanism discussed in RFC 2210 [RFC2210] can be used.

6.4. Tspec and Flowspec Calculations

 As multiple End-to-End sessions can be mapped to a single tunnel
 session, there is the need to compute the aggregate Tspec of all the
 senders of those End-to-End sessions. This aggregate Tspec will the
 Tspec of the representative tunnel session. The same operation needs
 to be performed for flowspecs of End-to-End reservations arriving at
 The semantics of these operations are not addressed here.  The
 simplest way to do them is to compute a sum of the end-to-end Tspecs,
 as is defined in the specifications of the Controlled-Load and
 Guaranteed services (found at [RFC2211] and [RFC2212] respectively).
 However, it may also be appropriate to compute the aggregate
 reservation level for the tunnel using a more sophisticated
 statistical or measurement-based computation.

7. IPSEC Tunnels

 In the case where the IP-in-IP tunnel supports IPSEC (especially ESP
 in Tunnel-Mode with or without AH) then the Tunnel Session uses the
 [RSVPESP] for the PATH and RESV messages.
 Data packets are not encapsulated with a UDP header since the SPI can
 be used by the intermediate nodes for classification purposes.
 Notice that user oriented keying must be used between Rentry and
 Rexit, so that different SPIs are assigned to data packets that have
 reservation and "best effort" packets, as well as packets that belong
 to different Tunnel Sessions if those are supported.

8. RSVP Support for Multicast and Multipoint Tunnels

 The mechanisms described above are useful for unicast tunnels.
 Unicast tunnels provide logical point-to-point links in the IP
 infrastructure, though they may encapsulate and carry either unicast
 or multicast traffic between those points.

Terzis, et al. Standards Track [Page 19] RFC 2746 RSVP Operation Over IP Tunnels January 2000

 Two other types of tunnels may be imagined.  The first of these is a
 "multicast" tunnel.  In this type of tunnel, packets arriving at an
 entry point are encapsulated and transported (multicast) to -all- of
 the exit points.  This sort of tunnel might prove useful for
 implementing a hierarchical multicast distribution network, or for
 emulating efficiently some portion of a native multicast distribution
 A second possible type of tunnel is the "multipoint" tunnel. In this
 type of tunnel, packets arriving at an entry point are normally
 encapsulated and transported to -one- of the exit points, according
 to some route selection algorithm.
 This type of tunnel differs from all previous types in that the '
 shape' of the usual data distribution path does not match the 'shape'
 of the tunnel.  The topology of the tunnel does not by itself define
 the data transmission function that the tunnel performs.  Instead,
 the tunnel becomes a way to express some shared property of the set
 of connected tunnel endpoints.  For example, the "tunnel" may be used
 to create and embed a logical shared broadcast network within some
 larger network.  In this case the tunnel endpoints are the nodes
 connected to the logical shared broadcast network.  Data traffic may
 be unicast between two such nodes, broadcast to all connected nodes,
 or multicast between some subset of the connected nodes.  The tunnel
 itself is used to define a domain in which to manage routing and
 resource management - essentially a virtual private network.
 Note that while a VPN of this form can always be implemented using a
 multicast tunnel to emulate the broadcast medium, this approach will
 be very inefficient in the case of wide area VPNs, and a multipoint
 tunnel with appropriate control mechanisms will be preferable.
 The following paragraphs provide some brief commentary on the use of
 RSVP in these situations. Future versions of this note will provide
 more concrete details and specifications.
 Using RSVP to provide resource management over a multicast tunnel is
 relatively straightforward. As in the unicast case, one or more RSVP
 sessions may be used, and end-to-end RSVP sessions may be mapped onto
 tunnel RSVP sessions on a many-to-one or one-to-one basis. Unlike the
 unicast, case, however, the mapping is complicated by RSVP's
 heterogeneity semantics. If different receivers have made different
 reservation requests, it may be that the RESV messages arriving at
 the tunnel would logically map the receiver's requests to different
 tunnel sessions. Since the data can actually be placed into only one
 session, the choice of session must be reconciled (merged) to select
 the one that will meet the needs of all applications. This requires a
 relatively simple extension to the session mapping mechanism.

Terzis, et al. Standards Track [Page 20] RFC 2746 RSVP Operation Over IP Tunnels January 2000

 Use of RSVP to support multipoint tunnels is somewhat more difficult.
 In this case, the goal is to give the tunnel as a whole a specific
 level of resources. For example, we may wish to emulate a "logical
 shared 10 megabit Ethernet" rather than a "logical shared Ethernet".
 However, the problem is complicated by the fact that in this type of
 tunnel the data does not always go to all tunnel endpoints. This
 implies that we cannot use the destination address of the
 encapsulated packets as part of the packet classification filter,
 because the destination address will vary for different packets
 within the tunnel.
 This implies the need for an extension to current RSVP session
 semantics in which the Session ID (destination IP address) is used
 -only- to identify the session state within network nodes, but is not
 used to classify packets.  Other than this, the use of RSVP for
 multipoint tunnels follows that of multicast tunnels. A multicast
 group is created to represent the set of nodes that are tunnel
 endpoints, and one or more tunnel RSVP sessions are created to
 reserve resources for the encapsulated packets. In the case of a
 tunnel implementing a simple VPN, it is most likely that there will
 be one session to reserve resources for the whole VPN. Each tunnel
 endpoint will participate both as a source of PATH messages and a
 source of (FF or SE) RESV messages for this single session,
 effectively creating a single shared reservation for the entire
 logical shared medium. Tunnel endpoints MUST NOT make wildcard
 reservations over multipoint tunnels.

9. Extensions to the RSVP/Routing Interface

 The RSVP specification [RFC2205] states that through the RSVP/Routing
 Interface, the RSVP daemon must be able to learn the list of local
 interfaces along with their IP addresses. In the RSVP Tunnels case,
 the RSVP daemon needs also to learn which of the local interface(s)
 is (are) IP-in-IP tunnel(s) having the capabilities described here.
 The RSVP daemon can acquire this information, either by directly
 querying the underlying network and physical layers or by using any
 existing interface between RSVP and the routing protocol properly
 extended to provide this information.

Terzis, et al. Standards Track [Page 21] RFC 2746 RSVP Operation Over IP Tunnels January 2000

10. Security Considerations

 The introduction of RSVP Tunnels raises no new security issues other
 than those associated with the use of RSVP and tunnels. Regarding
 RSVP, the major issue is the need to control and authenticate access
 to enhanced qualities of service. This requirement is discussed
 further in [RFC2205]. [RSVPCRYPTO] describes the mechanism used to
 protect the integrity of RSVP messages carrying the information
 described here.  The security issues associated with IP-in-IP tunnels
 are discussed in [IPINIP4] and [IPV6GEN].

11. IANA Considerations

 IANA should assign a Class number for the NODE_CHAR object defined in
 Section 3.3.2. This number should be in the 10bbbbbb range. The
 suggested value is 128.

12. Acknowledgments

 We thank Bob Braden for his insightful comments that helped us to
 produce this updated version of the document.

13. References

 [ESP]        Atkinson, R., "IP Encapsulating Security Payload (ESP)",
              RFC 1827, August 1995.
 [IP4INIP4]   Perkins, C., "IP Encapsulation within IP", RFC 2003,
              October 1996.
 [IPV6GEN]    Conta, A. and S. Deering, "Generic Packet Tunneling in
              IPv6 Specification", RFC 2473, December 1998.
 [MINENC]     Perkins, C., "Minimal Encapsulation within IP", RFC
              2004, October 1996.
 [RFC1701]    Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic
              Routing Encapsulation (GRE)", RFC 1701, October 1994.
 [RFC1702]    Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic
              Routing Encapsulation over IPv4 Networks", RFC 1702,
              October 1994.
 [RFC1933]    Gilligan, R. and E. Nordmark, "Transition Mechanisms for
              IPv6 Hosts and Routers", RFC 1933, April 1996.

Terzis, et al. Standards Track [Page 22] RFC 2746 RSVP Operation Over IP Tunnels January 2000

 [RFC2210]    Wroclawski, J., "The Use of RSVP with IETF Integrated
              Services", RFC 2210, September 1997.
 [RFC2211]    Wroclawski, J., "Specification of the Controlled-Load
              Network Element Service", RFC 2211, September 1997.
 [RFC2212]    Shenker, S., Partridge, C. and R. Guerin, "Specification
              of the Guaranteed Quality of Service", RFC 2212,
              September 1997.
 [RFC2205]    Braden, R., Zhang, L., Berson, S., Herzog, S. and S.
              Jamin, "Resource ReSerVation Protocol (RSVP) -- Version
              1 Functional Specification", RFC 2205, September 1997.
 [RSVPESP]    Berger, L. and T. O'Malley, "RSVP Extensions for IPSEC
              Data Flows", RFC 2207, September 1997.
 [RSVPCRYPTO] Baker, F., Lindell, B. and M. Talwar, "RSVP
              Cryptographic Authentication", RFC 2747, January 2000.

Terzis, et al. Standards Track [Page 23] RFC 2746 RSVP Operation Over IP Tunnels January 2000

14. Authors' Addresses

 John Krawczyk
 ArrowPoint Communications
 50 Nagog Park
 Acton, MA 01720
 Phone: 978-206-3027
 John Wroclawski
 MIT Laboratory for Computer Science
 545 Technology Sq.
 Cambridge, MA  02139
 Phone: 617-253-7885
 Fax:   617-253-2673
 Lixia Zhang
 4531G Boelter Hall
 Los Angeles, CA  90095
 Phone: 310-825-2695
 Andreas Terzis
 4677 Boelter Hall
 Los Angeles, CA 90095
 Phone: 310-267-2190

Terzis, et al. Standards Track [Page 24] RFC 2746 RSVP Operation Over IP Tunnels January 2000

15. Full Copyright Statement

 Copyright (C) The Internet Society (2000).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an


 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Terzis, et al. Standards Track [Page 25]

/data/webs/external/dokuwiki/data/pages/rfc/rfc2746.txt · Last modified: 2000/01/27 19:50 (external edit)