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Versions: 00                                                            
Internet-Draft                                         Nancy Feldman
Expiration Date: August 1999                                     IBM

                                                      Bilel Jamoussi
                                                     Nortel Networks

                                                    Sridhar Komandur
                                               Ascend Communications

                                                    Arun Viswanathan
                                                 Lucent Technologies

                                                         Tom Worster

                                                       February 1999

                     MPLS using ATM VP Switching


Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   months and may be updated, replaced, or obsoleted by other
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   Drafts as reference material or to cite them other than as "work
   in progress."

   The list of current Internet-Drafts can be accessed at

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   The MPLS Architecture [ARCH] discusses the way in which ATM
   switches may be used as Label Switching Routers.  The MPLS ATM VC
   switching document [ATMVC] details the way in which ATM-LSRs are
   used, specifically in the presence of non-merge and VC-merge
   capable switches.  This document is a companion document to
   [ATMVC]; it extends the ATM procedures for the support of VP-
   switching on ATM-LSRs.  It is assumed the reader is familiar with
   the contents of [ATMVC].

1. Introduction

   The MPLS Architecture [ARCH] discusses the way in which ATM
   switches may be used as Label Switching Routers.  The MPLS ATM VC
   Switching [ATMVC] document details the way in which ATM-LSRs are
   used, specifically in the presence of non-merge and VC-merge
   capable switches.  This document extends the ATM procedures in
   [ATMVC] for supporting the distribution of labels as VPI values.
   This document assumes the reader is familiar with the ATM terms
   and procedures as defined in [ATMVC].

   To connect all ingresses to all the egresses in a switched network
   requires O(n-squared) Label Switched Paths (LSPs).  In order to
   scale to O(n) (rather than O(n-squared)), MPLS makes use of the
   concept of stream merge.  This allows the creation of multipoint-
   to-point LSP trees, in which multiple incoming streams with the
   same FEC may be combined into a single outgoing stream.  To merge
   multiple incoming VCs to a single outgoing VC requires that ATM
   switching hardware have the capability of preventing cell
   interleaving [ARCH].  However, much of the legacy ATM switching
   hardware cannot support VC merging.

   Several legacy ATM switches support what is known as "VP-
   switching".  In ATM switches that support VP-switching, it is
   possible to create connections (cross-connects) that use only the
   VPI field in the ATM header to switch cells, and the VCI field is
   not used in the lookup that determines the outgoing label and/or
   the outgoing interface.  This capability is termed as VP-merge.
   VP-merge is the process by which a switch receives cells on
   several incoming VPIs and transmits them on a single outgoing VPI.
   In this merging style, each ingress LSR in a merged multipoint-to-
   point LSP use a unique VCI value when injecting packets (cells)
   into the multipoint-to-point connection.  At the merging nodes,
   where multiple upstream VPs are merged into a single outgoing VP,
   cells from different sources get interleaved, but the unique VCI
   values used by the sources help to maintain the identity of all
   PDUs so that they may be properly reassembled at the egress node.

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   Another use of VP-switching is in label stacking [LBLSTK].  The
   VPI/VCI label pair in ATM can be likened to be two separate
   labels, such that, the lower label is VCI and the top label is
   VPI.  The cells are switched based on the top label (VPI; that is
   VP-switching), till the top label is popped (that is, the VP
   switching ends), and the cells are switched based on the bottom
   label (that is VCI; or VPI/VCI together).

   Yet another use of VP-switching is in tunneling via ATM networks
   that aren't MPLS capable.  This procedure is described in [ATMVC],
   and is repeated in this document for completeness.

   This document provides procedures for exchanging VPIs as labels to
   create LSPs for use in the situations described above.

2. Specification of Requirements

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

3. Definitions

   The following definitions are from [ATMVC], with appropriate
   modifications for the inclusion of VP-switching:

   A Label Switching Router (LSR) is a device which implements the
   label switching control and forwarding components described in

   A label switching controlled ATM (LC-ATM) interface is an ATM
   interface controlled by the label switching control component.
   When a packet traversing such an interface is received, it is
   treated as a labeled packet.  The packet

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=C6s top label is inferred
   from the contents of the VCI field only, the VPI field only, or
   the combined contents of the VPI and VCI fields.  Any two LDP
   peers which are connected via an LC-ATM interface will use LDP
   negotiations to determine which of these cases is applicable to
   that interface.  Moreover, only one of the above cases can be
   active at a time on an LC-ATM interface.

   An ATM-LSR is a LSR with a number of LC-ATM interfaces which
   forwards cells between these interfaces using labels carried in
   the VCI, VPI, or VPI/VCI field.

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4. Use of VPI/VCIs

   Label switching is accomplished by associating labels with
   Forwarding Equivalence Classes, and using the label value to
   forward packets.  An ATM-LSR that is performing VP switching
   carries the label in the VPI field.

   We say that two LSRs are "directly connected" over an LC-ATM
   interface if all cells transmitted out that interface by one LSR
   will reach the other, and there are no ATM switches between the
   two LSRs.

   When two LSRs are directly connected via an LC-ATM interface, they
   jointly control the allocation of VPIs/VCIs on the interface
   connecting them.  They may agree to use the VCI, VPI, or VPI/VCI
   field to encode a single label.

   The default VPI/VCI value for the non-MPLS connection is VPI 0,
   VCI 32.  Other values can be configured, as long as both parties
   are aware of the configured value.

   The VPI value 0 MUST NOT be used with VP switching.

   Any VCI value in the range 0-65535 inclusive MAY be used as a
   label with VP switching.

   With the exception of these reserved values, the VPI/VCI values
   used in the two directions of the link MAY be treated as
   independent spaces.

   The allowable ranges of VPIs and VCIs are communicated through

4.1.  VP-merge Connection

   Directly connected ATM-LSRs may use the VPI field to carry the
   MPLS label.  The choice of this mode of operation and the valid
   VPI ranges must be negotiated via LDP.  However, the VPI value of
   0 MUST NOT be used as an MPLS label indicator.  In addition, if
   two LDP peers using VP-merge are connected via a LC-ATM interface,
   the VPI value 0 is reserved for non-MPLS connections.  This non-
   MPLS connection is used to carry LDP packets between the two
   peers, and MAY also be used (but is not required to be used) for
   other unlabeled packets (such as OSPF packets, etc.).  The
   LLC/SNAP encapsulation of RFC 1483 [RFC1483] MUST be used on the
   non-MPLS connection.

4.2.  Connections via an ATM Virtual Path

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   Sometimes it can be useful to treat two LSRs as adjacent (in some
   LSP) across an LC-ATM interface, even though the connection
   between them is made through an ATM "cloud" via an ATM Virtual
   Path.  In this case, the VPI field is not available to MPLS, and
   the label MUST be encoded entirely within the VCI field.

   In this case, the default VCI value of the non-MPLS connection
   between the LSRs is 32.  The VPI is set to whatever is required to
   make use of the Virtual Path.

   A VPI/VCI value whose VCI part is in the range 0-32 inclusive MUST
   NOT be used as the encoding of a label.

   With the exception of these reserved values, the VPI/VCI values
   used in the two directions of the link MAY be treated as
   independent spaces.

   The allowable ranges of VPI/VCIs are communicated through LDP.  If
   more than one VPI is used for label switching, the allowable range
   of VCIs may be different for each VPI, and each range is
   communicated through LDP.

5. Label Distribution and Maintenance Procedures

   In general, an ATM-LSR configured for VP-merge MAY use either
   "ordered control" OR "independent control" label distribution, as
   well as "unsolicited downstream" OR "downstream-on-demand" label
   allocation. (see [ARCH, LDP]).

   Due to the limited number of VPs that may be created in a VP-merge
   environment, there may be a shortage of VPs if the number of IP-
   prefixes in the domain is greater than the maximum number of VPs
   available (as determined by the allocated VPI bits).  This can be
   mitigated in the following ways:

       -  All ATM-LSRs in a domain may be configured for BGP or link st=
          egress aggregation.  In this case, a single LSP is sustained =
          all prefixes which share a common egress point, thereby
          significantly reducing the number of necessary VPs.

       -  All ATM-LSRs in a domain may be configured for "ordered contr=
          label distribution and "unsolicited downstream" label allocat=
          This allows the egress nodes to control which LSPs are create=
d in
          the domain, thereby permitting a limited VP space to be alloc=

       -  All ATM-LSRs in a domain SHOULD be configured for Conservativ=
          Label Retention (see [ARCH]).  This eliminates the use and
          maintenance of unnecessary labels.

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5.1.  VP-merge-capable ATM Switches

   VP-merge is the process by which a switch receives cells on
   several incoming VPIs and transmits them on a single outgoing VPI.
   The ATM-LSRs neither looks into the VCI field for switching the
   cells nor modify its content on the outgoing interface.  VP-merge
   MAY be used only when the ENTIRE ATM-LSR domain is capable of VP-
   merge.  If the entire ATM-LSR domain is not capable of VP-merge,
   then one of the applicable procedures described for either non-
   merge or VC-merge must be used (see [ATMVC]).

   To use VP-merge in an ATM-LSR domain, each of the LSRs in the Edge
   Set MUST be assigned a unique VCI value.  VP-Merge capable Ingress
   Edge LSRs MUST provide a means of configuring a unique VCI value.
   While other unique VCI allocation mechanisms may be possible, a
   description of such mechanisms is outside the scope of this
   document.  The egress  must be cognizant of the VCI ranges
   allocated at the ingresses, so that it aware of the VCI values it
   must reassemble on.

5.2.  Limitations of VP-merge

   VP-merge, besides its usefulness, comes with several limitations.
   These limitations must be carefully considered before deploying a
   network that uses VP-merge.

   All ATM-LSRs in an ATM-LSR domain must be capable of VP-merge.
   That is, VP-merge does not interoperate with VC-merge or non-merge

     -  There should be sufficient number of VPI bits supported by the
        ATM-LSRs in use.  If hybrid mode (ships in the night mode) is u=
        some LC-ATM can have only a maximum of 8-bits for VPI.

     -  LSRs in an ATM-LSR domain using VP-merge must be directly
        connected.  That is, the LC-ATM interface of a VP-merge ATM-LSR=

        directly connect to another VP-merge ATM-LSR without any
        ATM switches.  Therefore, two VP-merge domains cannot be connec=
        via a public ATM network except through the IP layer.

     -  To use VP-merge, each Ingress Edge ATM-LSR MUST be assigned a
        unique VCI value.  Currently, the only specified method to assi=
        these VCI values is via configuration.

6. Egress aggregation with VP-merge

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   Aggregation is the concept of merging multiple FECs with a common
   destination into a single FEC, sharing the same LSP.  Aggregation
   may be achieved through information available in BGP or link-state
   routing protocols, where a set of IP destinations are known to
   traverse the same IP path to a common egress point.  However, some
   route protocols, such as RIP, do not provide knowledge of the
   egress point.  In these situations, if egress aggregation is
   desired, the egress LSR must be configured to advertise the same
   label for all (or a set of) IP prefixes reachable through that

   In the case of aggregation via BGP or a link-state routing
   protocol, the egress node is advertised in the FEC TLV "IP Prefix"
   with a mask of /32 [LDP].  In the other form of egress

ation, hence referred to as "generic" egress aggregation,
   each prefix may be advertised individually as a separate FEC via
   the TLV "IP Prefix", yet given the same label.  However, when VP-
   merging is used, this latter form of generic egress aggregation
   could cause cell interleaving problems if each destination does
   not follow the exact same IP-path through the network.  Thus, an
   LSR desiring generic egress aggregation must use the FEC
   Aggregation-List TLV (see section 6.1).

   In the Aggregation-List TLV, each set of IP prefixes which are to
   share a common LSP are distinguished by a unique identifier.  The
   aggregating egress LSR transmits a Mapping Message with an
   Aggregation-List TLV containing a unique identifier, commonly the
   router-id of the egress LSR, as well as a list of IP prefixes that
   are to share the same label (as found in the Label TLV; see
   [LDP]).  Each LSR which receives the Aggregation-List from a peer
   verifies in the Forwarding Information Base (FIB) that each IP
   prefix in the given list uses that peer as its nexthop; any IP
   prefix which fails the test MUST be pruned off of the aggregation
   list.  The LSR then forwards the remaining list to its upstream

   The Aggregation-List is tightly coupled with its peer and assigned
   label.  Once it has been received and accepted, any further
   Mapping Message containing that aggregation identifier MUST be
   received from the same peer, and MUST contain the same label.  An
   LSR MUST NOT accept an Aggregation-List with the same unique
   identifier from any other peer, or with a different label, until
   the original Aggregation-List has been cancelled via the Withdraw
   Message [LDP].  By constraining the Aggregation-List to associate
   with only one peer and label, it guarantees that all IP prefixes
   to a common egress LSR will share the same LSP, and thus avoids
   any cell interleaving problems.

   Note that a Withdraw Message containing an Aggregation-List TLV
   with zero prefixes is treated as a "wildcard", and removes ALL

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   prefixes associated with that aggregation list.

   Generic egress aggregation MUST be used in conjunction with
   "ordered" label distribution and SHOULD be used with "unsolicited
   downstream" label allocation.

6.1.  Aggregation-List Encoding

   The FEC TLV is defined in [LDP].  It is composed of FEC element
   types.  The following element is used for the Aggregation List:

         FEC Element       Type      Value
         type name

         Aggregation-List  0x04      Variable

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   | Agg-List (4)  |     Address Family            |     PreCount  |
   |                     Aggregate Unique-Id                       |
   |   Prefix Len   |  Prefix ...
    .... (variable length)

      Address Family

          Two octet quantity containing a value from ADDRESS FAMILY
          NUMBERS in [RFC1700] that encodes the address family for
          the address prefix in the Prefix field.


          One octet number of address prefixes in this TLV, which
          comprise the set of prefixes that are to be aggregated

      Aggregate Unique-Id

          Four octet unique identifier assigned by the LSR
          initiating the aggregation.  This value MAY be the
          originating LSRs unique router-id.

      Prefix Len

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          One octet unsigned integer containing the length in bits
          of the address prefix that follows.


          A variable length field containing an address prefix whose
          length, in bits, was specified in the previous (Prefix
          Len) field.  The last prefix in this TLV must be padded to
          a byte boundary.

7. Stacking

   ATM switches typically support the use of the VPI and VCI fields
   as a two level hierarchy. This capability may be used to support
   label stacking in MPLS. In a VCI/VPI label stacking scenario,
   aggregation happens as LSPs in VCs in the outer cloud are
   multiplexed onto VPs to be sent across the inner cloud. When a VP
   reaches its egress from the inner cloud it is demultiplexed and
   the VC LSPs are segregated for distribution to their destinations.
   In this stacking model the VP LSPs are point-to-point and VC merge
   may be deployed in the outer cloud.

   Procedures for using VP switching in this mode will be discussed
   in a future revision.

8. Encapsulation

   The MPLS encapsulation to be used when sending labeled packets to
   or from ATM-LSRs is as specified in [ATMVC].

9. Optional Loop Detection: Distributing Path Vectors

   The optional loop detection procedures are as specified in
   [ATMVC].  Rules specific to VC-merge are also applicable to VP-

10.   TTL Manipulation

   The TTL handling procedures are as specified in [ATMVC].

11.   Security Considerations

   The use of the procedures and encapsulations specified in this
   document does not have any security impact other than that which
   may generally be present in the use of any MPLS procedures or

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   encapsulations (see [ARCH]).

12.   Intellectual Property Considerations

   The IETF has been notified of intellectual property rights claimed
   in regard to some or all of the specification contained in this
   document.  For more information consult the online list of claimed

13.   Acknowledgements

   This document shamelessly borrows from the ATM companion document

14.   References

   [ARCH]    E. Rosen, A. Viswanathan, R. Callon, "Multiprotocol
             Label Switching Architecture", Work in Progress,
             February 1999.

   [ATMVC]   B. Davie, J. Lawrence, K. McCloghrie, Y. Rekhter, E.
             Rosen, G. Swallow, P. Doolan, "MPLS using ATM VC
             Switching", Work in Progress, November, 1998.

   [LBLSTK]  E. Rosen, Y. Rekhter, D. Tappan, D. Farinacci, G.
             Fedorkow, T. Li, A. Conta, "MPLS Label Stack Encoding",
             Work in Progress,   September 1998.

   [LDP]     L. Anderson, P. Doolan, N. Feldman, A. Fredette, B.
             Thomas, "LDP Specification", Work in Progress, January

   [RFC1483] J. Heinanen, "Multiprotocol Encapsulation over ATM
             Adaptation Layer 5", RFC 1483, Telecom Finland, July

   [RFC1700] J. Reynolds, J.Postel, "Assigned Numbers", October

   [RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
             Requirement Levels", RFC 2119, BCP 14, March 1997.

15.   Authors' Addresses

   Nancy Feldman
   IBM Corp.

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   30 Saw Mill River Rd.
   Hawthorne, NY 10532
   Phone: +1 914-784-3254
   Email: nkf@us.ibm.com

   Bilel Jamoussi
   Nortel Networks
   P O Box 3511 Station C
   Ottawa, ON K1Y 4H7
   Phone: +1 613 765-4814
   Email: jamoussi@NortelNetworks.com

   Sridhar Komandur
   Ascend Communications
   1 Robbins Rd
   Westford, MA 01886
   Phone: +1 978-952-7164
   Email: sridhar.komandur@ascend.com

   Arun Viswanathan
   Lucent Technologies
   101 Crawford Cornet Rd.
   Holmdel, NJ 07733
   Phone: +1 732-332-5163
   Email: arunv@lucent.com

   Tom Worster
   199 W Newton St.
   Boston, MA 02116 USA
   Phone: +1 617 247 2624
   Email: tom.worster@gdc.com

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