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Versions: (draft-conta-mpls-fr) 00 01 02 03 04 05 RFC 3034

MPLS Working Group                               A. Conta (3COM)
INTERNET-DRAFT                                   P. Doolan (Ennovate)
                                                 A. Malis (Lucent)
                                                 13 June 2000
                                                 Expires 13 December 2000


             Use of Label Switching on Frame Relay Networks
                             Specification

                       draft-ietf-mpls-fr-05.txt


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
     other groups may also distribute working documents as
     Internet-Drafts.

     Internet-Drafts are draft documents valid for a maximum of six
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     The list of current Internet-Drafts can be accessed at
     http://www.ietf.org/ietf/1id-abstracts.txt

     The list of Internet-Draft Shadow Directories can be accessed at
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Abstract

   This  document  defines  the  model  and   generic   mechanisms   for
   Multiprotocol  Label  Switching on Frame Relay networks. Furthermore,
   it  extends  and  clarifies  portions  of  the  Multiprotocol   Label
   Switching Architecture described in [ARCH] and the Label Distribution
   Protocol (LDP) described in [LDP] relative to Frame  Relay  Networks.
   MPLS  enables  the  use  of  Frame  Relay Switches as Label Switching
   Routers (LSRs).










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Table of Contents


   Status of this Memo.........................................1
   Table of Contents...........................................2
1. Introduction................................................3
2. Terminology.................................................3
3. Special Characteristics of Frame Relay Switches.............5
4. Label Encapsulation.........................................5
5. Frame Relay Label Switching Processing......................7
5.1  Use of DLCIs..............................................7
5.2  Homogeneous LSPs..........................................8
5.3  Heterogeneous LSPs........................................8
5.4  Frame Relay Label Switching Loop Prevention and Control...8
5.4.1   FR-LSRs Loop Control - MPLS TTL Processing.............9
5.4.2   Performing MPLS TTL calculations......................10
5.5  Label Processing by Ingress FR-LSRs......................13
5.6  Label Processing by Core FR-LSRs.........................14
5.7  Label Processing by Egress FR-LSRs.......................14
6  Label Switching Control Component for Frame Relay..........15
6.1  Hybrid Switches (Ships in the Night)  ...................16
7  Label Allocation and Maintenance Procedures ...............16
7.1  Edge LSR Behavior........................................16
7.2  Efficient use of label space-Merging FR-LSRs.............19
7.3  LDP message fields specific to Frame Relay...............20
8  Security Considerations  ..................................22
9  Acknowledgments  ..........................................23
10 References  ...............................................23
11 Authors' Addresses  .......................................24
Appendix A - changes since previous versions..................25





















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1. Introduction

   The  Multiprotocol  Label  Switching  Architecture  is  described  in
   [ARCH]. It is possible to use Frame Relay switches as Label Switching
   Routers.   Such  Frame  Relay  switches  run  network  layer  routing
   algorithms (such as OSPF, IS-IS, etc.), and their forwarding is based
   on the results of these routing algorithms.  No specific Frame  Relay
   routing is needed.

   When a Frame Relay switch  is  used  for  label  switching,  the  top
   (current)  label, on which forwarding decisions are based, is carried
   in the DLCI field of the Frame Relay data  link  layer  header  of  a
   frame.  Additional  information  carried along with the top (current)
   label, but not processed by Frame Relay switching, along  with  other
   labels, if the packet is multiply labeled, are carried in the generic
   MPLS encapsulation defined in [STACK].

   Frame Relay permanent virtual circuits (PVCs) could be configured  to
   carry  label switching based traffic. The DLCIs would be used as MPLS
   Labels and the Frame Relay switches would become  Frame  Relay  Label
   Switching  Routers,  while  the  MPLS  traffic  would be encapsulated
   according to this specification, and  would  be  forwarded  based  on
   network layer routing information.

   The keywords MUST, MUST NOT, MAY, OPTIONAL,   REQUIRED,  RECOMMENDED,
   SHALL,  SHALL  NOT,  SHOULD,  SHOULD  NOT   are  to be interpreted as
   defined in RFC 2119.

   This document is a companion document to [STACK] and [ATM].

2. Terminology


   LSR

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

   LC-FR

        A label switching controlled Frame Relay (LC-FR) interface is  a
        Frame  Relay interface controlled by the label switching control
        component. Packets traversing such an interface carry labels  in
        the DLCI field.

   FR-LSR




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        A FR-LSR is an LSR with  one  or  more  LC-FR  interfaces  which
        forwards frames between two such interfaces using labels carried
        in the DLCI field.

   FR-LSR domain

        A FR-LSR  domain  is  a  set  of  FR-LSRs,  which  are  mutually
        interconnected by LC-FR interfaces.

   Edge Set

        The Edge Set of an FR-LSR domain is the set of LSRs,  which  are
        connected to the domain by LC-FR interfaces.

   Forwarding Encapsulation

        The Forwarding Encapsulation is the type of  MPLS  encapsulation
        (Frame  Relay,  ATM,  Generic)  of  a packet that determines the
        packet's MPLS forwarding, or the network layer encapsulation  if
        that  packet  is  forwarded  based  on  the  network  layer (IP,
        etc...)header.

   Input Encapsulation

        The Input Encapsulation is the type of MPLS encapsulation (Frame
        Relay, ATM, Generic) of a packet when that packet is received on
        an   LSR's   interface,    or    the    network    layer    (IP,
        etc...)encapsulation if that packet has no MPLS encapsulation.

   Output Encapsulation

        The Output Encapsulation  is  the  type  of  MPLS  encapsulation
        (Frame  Relay,  ATM,  Generic)  of  a packet when that packet is
        transmitted on an LSR's interface, or  the  network  layer  (IP,
        etc...)encapsulation if that packet has no MPLS encapsulation.

   Input TTL

        The Input TTL is the MPLS TTL of the top of  the  stack  when  a
        labeled  packet  is received on an LSR interface, or the network
        layer (IP) TTL if the packet is not labeled.

   Output TTL

        The Output TTL is the MPLS TTL of the top of the  stack  when  a
        labeled  packet  is  transmitted  on  an  LSR  interface, or the
        network layer (IP) TTL if the packet is not labeled.




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Additionally, this document uses terminology from [ARCH].


3. Special characteristics of Frame Relay Switches

   While  the  label   switching   architecture   permits   considerable
   flexibility  in  LSR  implementation,  a FR-LSR is constrained by the
   capabilities  of  the  (possibly  pre-existing)  hardware   and   the
   restrictions   on  such  matters  as  frame  format  imposed  by  the
   Multiprotocol Interconnect over Frame Relay [MIFR],  or  Frame  Relay
   standards  [FRF],  etc.... Because of these constraints, some special
   procedures are required for FR-LSRs.

   Some of the key features of Frame Relay switches  that  affect  their
   behavior as LSRs are:

   -    the label swapping function is performed on fields (DLCI) in the
        frame's Frame Relay data link header; this dictates the size and
        placement of the label(s) in a packet.  The  size  of  the  DLCI
        field  can be 10 (default) or 23 bits, and  it can  span two  or
        four bytes in the header.

   -    there is generally no capability to  perform  a  `TTL-decrement'
        function as is performed on IP headers in routers.

   -    congestion control is performed by each node based on parameters
        that  are passed at circuit creation. Flags in the frame headers
        may be set as a consequence  of  congestion,  or  exceeding  the
        contractual parameters of the circuit.

   -    although in a standard switch it may be  possible  to  configure
        multiple   input  DLCIs  to  one  output  DLCI  resulting  in  a
        multipoint-to-point circuit,  multipoint-to-multipoint  VCs  are
        generally not fully supported.



   This document describes ways of applying  label  switching  to  Frame
   Relay switches, which work within these constraints.


4. Label Encapsulation

   By default, all  labeled  packets  should  be  transmitted  with  the
   generic  label  encapsulation  as defined in [STACK], using the frame
   relay null encapsulation mechanism:





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            0                       1                       (Octets)
           +-----------------------+-----------------------+
(Octets)0  |                                               |
           /                 Q.922 Address                 /
           /             (length 'n' equals 2 or 4)        /
           |                                               |
           +-----------------------+-----------------------+
        n  |                       .                       |
           /                       .                       /
           /                  MPLS packet                  /
           |                       .                       |
           +-----------------------+-----------------------+

       "n" is the length of the Q.922  Address  which  can  be  2  or  4
       octets.

       The Q.922 [ITU] representation of a DLCI (in  canonical  order  -
       the  first  bit  is  stored  in  the least significant, i.e., the
       right-most bit of a byte in memory) [CANON]is the following:


            7     6     5     4     3     2     1     0      (bit order)
           +-----+-----+-----+-----+-----+-----+-----+-----+
(octet) 0  |            DLCI(high order)       |  0  |  0  |
           +-----+-----+-----+-----+-----+-----+-----+-----+
        1  |  DLCI(low order)      |  0  |  0  |  0  |  1  |
           +-----+-----+-----+-----+-----+-----+-----+-----+

              10 bits DLCI






















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            7     6     5     4     3     2     1     0      (bit order)
           +-----+-----+-----+-----+-----+-----+-----+-----00
(octet) 0  |            DLCI(high order)       |  0  |  0  |
           +-----+-----+-----+-----+-----+-----+-----+-----
        1  |  DLCI                 |  0  |  0  |  0  |  0  |
           +-----+-----+-----+-----+-----+-----+-----+-----+
        2  |             DLCI                        |  0  |
           +-----+-----+-----+-----+-----+-----+-----+-----+
        3  |       DLCI (low order)            |  0  |  1  |
           +-----+-----+-----+-----+-----+-----+-----+-----+

              23 bits DLCI

   The use of the frame relay null  encapsulation  implies  that  labels
   implicitly encode the network protocol type.

   Rules regarding the  construction  of  the  label  stack,  and  error
   messages returned to the frame source are also described in [STACK].

   The generic encapsulation contains "n" labels for a  label  stack  of
   depth  "n"  [STACK],  where  the  top stack entry carries significant
   values for the EXP, S , and TTL fields [STACK] but not for the label,
   which  is  rather  carried  in the DLCI field of the Frame Relay data
   link header encoded in Q.922 [ITU] address format.


5. Frame Relay Label Switching Processing


5.1  Use of DLCIs

   Label switching is accomplished by associating labels with routes and
   using  the  label value to forward packets, including determining the
   value of any replacement label.  See [ARCH] for further details. In a
   FR-LSR,  the top (current) MPLS label is carried in the DLCI field of
   the Frame Relay data link layer header of the frame.  The  top  label
   carries implicitly information about the network protocol type.

   For two connected FR-LSRs, a full-duplex connection must be available
   for  LDP.   The  DLCI  for  the  LDP VC is assigned a value by way of
   configuration, similar to configuring the DLCI used to run IP routing
   protocols between the switches.

   With the exception of this configured value, the DLCI values used for
   MPLS in the two directions of the link may be treated as belonging to
   two independent spaces, i.e. VCs may be half-duplex,  each  direction
   with its own DLCI.




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   The allowable ranges of DLCIs, the size of DLCIs, and the support for
   VC  merging  MUST be communicated through LDP messages. Note that the
   range of DLCIs used for labels depends on the size of the DLCI field.


5.2  Homogeneous LSPs

   If <LSR1, LSR2, LSR3> is an LSP, it is possible that LSR1, LSR2,  and
   LSR3  will use the same encoding of the label stack when transmitting
   packet P from LSR1, to LSR2,  and  then  to  LSR3.  Such  an  LSP  is
   homogeneous.


5.3  Heterogeneous LSPs

   If <LSR1, LSR2, LSR3> is an LSP, it is possible that  LSR1  will  use
   one  encoding  of the label stack when transmitting packet P to LSR2,
   but LSR2 will use a different encoding when transmitting a  packet  P
   to  LSR3.   In  general,  the  MPLS  architecture  supports LSPs with
   different label stack encodings on different  hops.  When  a  labeled
   packet  is  received, the LSR must decode it to determine the current
   value of the label stack, then must operate on  the  label  stack  to
   determine  the  new label value of the stack, and then encode the new
   value appropriately before transmitting the  labeled  packet  to  its
   next hop.

   Naturally there will be MPLS networks which contain a combination  of
   Frame Relay switches operating as LSRs, and other LSRs, which operate
   using other MPLS encapsulations,  such  as  the  Generic  (MPLS  shim
   header),  or  ATM  encapsulation.  In such networks there may be some
   LSRs, which have Frame Relay  interfaces  as  well  as  MPLS  Generic
   ("MPLS  Shim")  interfaces.  This  is  one  example  of  an  LSR with
   different label stack encodings on different hops of  the  same  LSP.
   Such  an LSR may swap off a Frame Relay  encoded label on an incoming
   interface and replace it with a label encoded  into  a  Generic  MPLS
   (MPLS shim) header on the outgoing interface.


5.4  Frame Relay Label Switching Loop Prevention and Control

   FR-LSRs SHOULD operate on  loop  free  FR-LSPs  or  LSP  Frame  Relay
   segments.  Therefore,  FR-LSRs  SHOULD use loop detection and MAY use
   loop prevention mechanisms as described in [ARCH], and [LDP].








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5.4.1  FR-LSRs Loop Control - MPLS TTL processing

   The MPLS TTL encoded in the MPLS label stack is a mechanism used to:

   (a) suppress loops;

   (b) limit the scope of a packet.

   When a packet travels along an LSP, it should emerge  with  the  same
   TTL  value  that  it  would  have  had  if  it had traversed the same
   sequence of routers without  having  been  label  switched.   If  the
   packet  travels  along  a hierarchy of LSPs, the total number of LSR-
   hops traversed should be reflected in its TTL value when  it  emerges
   from the hierarchy of LSPs [ARCH].

   The initial value of the MPLS TTL is loaded into a newly pushed label
   stack  entry  from  the  previous TTL value, whether that is from the
   network layer header when no previous label stack existed, or from  a
   pre-existent lower level label stack entry.

   A FR-LSR switching same level labeled packets does not decrement  the
   MPLS TTL. A sequence of such FR-LSR is a "non-TTL segment".

   When a packet emerges from a "non-TTL LSP segment", it should however
   reflect  in  the  TTL  the  number  of  LSR-hops it traversed. In the
   unicast case, this can be achieved by propagating  a  meaningful  LSP
   length or LSP Frame Relay segment length to the FR-LSR ingress nodes,
   enabling the ingress to decrement the  TTL  value  before  forwarding
   packets into a non-TTL LSP segment [ARCH].

   When an ingress FR-LSR determines upon decrementing the MPLS TTL that
   a  particular  packet's TTL will expire before the packet reaches the
   egress of the "non-TTL LSP segment", the FR-LSR MUST not label switch
   the  packet,  but  rather  follow the specifications in [STACK] in an
   attempt to return an error message to the packet's source:


       -    it treats the packet as an expired packet and return an ICMP
            message to its source.

       -    it forwards the packet, as an unlabeled packet, with  a  TTL
            that reflects the IP (network layer) forwarding.


   If the incoming TTL is 1, only the first option applies.

   In the multicast case, a meaningful LSP length or LSP segment  length
   is  propagated  to  the  FR-LSR  egress node, enabling  the egress to



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   decrement the TTL value before forwarding packets out of the  non-TTL
   LSP segment.


5.4.2  Performing MPLS TTL calculations

   The calculation applied to the "input TTL" that  yields  the  "output
   TTL"  depends  on  (i)the  "input encapsulation", (ii)the "forwarding
   encapsulation",   and   (iii)the   "output    encapsulation".     The
   relationship  among (i),(ii), and (iii), can be defined as a function
   "D" of "input encapsulation" (ie), "forwarding  encapsulation"  (fe),
   and "output encapsulation" (oe). Subsequently the calculation applied
   to the "input TTL" to yield the "output TTL" can be described as:

     output TTL = input TTL - D(ie, fe, oe)

   or in a brief notation:

     output TTL = input TTL - d

   where "d" has three possible values: "0","1", or "the number of  hops
   of the LSP segment":

  For unicast transmission:

 +================+=================+=================+=================+
 |                |     Type of     |     Type of     |     Type of     |
 |       d        |      Input      |    Forwarding   |     Output      |
 |                |  Encapsulation  |  Encapsulation  |  Encapsulation  |
 +================+=================+=================+=================+
 |       0        |   Frame Relay   |   Frame Relay   |   Frame Relay   |
 +----------------+-----------------+-----------------+-----------------+
 |       1        |       any       |  Generic MPLS   |  Generic MPLS   |
 +----------------+-----------------+-----------------+-----------------+
 | number of hops |                 |  Generic MPLS   |                 |
 |      of        |       any       |      or         |   Frame Relay   |
 |  LSP segment   |                 |IP(network layer)|                 |
 +================+=================+=================+=================+

   The "number of hops of the LSP segment" is  the  value  of  the  "hop
   count"  that  is  attached  with  the  label  used when the packet is
   forwarded, if LDP [LDP] has provided such a "hop count" value when it
   distributed the label for the LSP, that is the LDP message had a "hop
   count object". If LDP didn't provide a "hop count", or it provided an
   "unknown"  value,  the  default  value  of the "number of hops of the
   segment" is 1.

   When sending a label binding upstream,  the  "hop  count"  associated



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   with the corresponding binding from downstream, if different than the
   "unknown" value, MUST be incremented by 1, and the result transmitted
   upstream  as  the  hop  count  associated  with  the new binding (the
   "unknown" value is transmitted unchanged).  If the  new  "hop  count"
   value  exceeds  the  "maximum"  value,  the  FR-LSR MUST NOT pass the
   binding  upstream,  but  instead  MUST   send   an   error   upstream
   [LDP][ARCH].

  For multicast transmission:

 +================+=================+=================+=================+
 |                |     Type of     |     Type of     |     Type of     |
 |       d        |      Input      |    Forwarding   |     Output      |
 |                |  Encapsulation  |  Encapsulation  |  Encapsulation  |
 +================+=================+=================+=================+
 |       0        |   Frame Relay   |   Frame Relay   |   Frame Relay   |
 +----------------+-----------------+-----------------+-----------------+
 |                |                 |  Generic MPLS   |                 |
 |       1        |       any       |      or         |   Frame Relay   |
 |                |                 |IP(network layer)|                 |
 +----------------+-----------------+-----------------+-----------------+
 | number of hops |                 |  Generic MPLS   |                 |
 |      of        |  Frame Relay    |      or         |       any       |
 |  LSP segment   |                 |IP(network layer)|                 |
 +================+=================+=================+=================+

   Referring to the "forwarding encapsulation" with the abbreviation "I"
   for  IP  (network  layer),  "G"  for Generic MPLS, and "F" for  Frame
   Relay MPLS, referring to an LSR interface with the  abbreviation  "i"
   if the input or output encapsulation is IP and no MPLS encapsulation,
   "g" when the input or output MPLS encapsulation is Generic MPLS,  "f"
   when  it  is  Frame  Relay,  "a"  when  it  is  ATM,  and furthermore
   considering the symbols "iIf", "gGf",  "fFf",  etc...  as  LSRs  with
   input,  forwarding  and  output encapsulations as referred above, the
   following describes examples of TTL calculations for the  Homogeneous
   and Heterogeneous LSPs discussed in previous sections:

                          Homogeneous LSP
                          ---------------
         IP_ttl = n                             IP_ttl=mpls_ttl-1 = n-6
         --------->iIf                      fIi--------->
                     | mpls_ttl = n-5       ^
                     |                      |
 number of hops     1|     Frame Relay      |5
                     |                      |
                     V   2      3      4    |
                     fFf--->fFf--->fFf--->fFf




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 "iIf" is "ingress LSR" in Frame Relay LSP and
        calculates: mpls_ttl = IP_TTL - number of hops = n-5
 "fIi" is "egress LSR" from Frame Relay LSP, and
        calculates: IP_ttl = mpls_ttl-1 = n-6


                          Heterogeneous LSP
                          -----------------
ingress LSR                                                  egress LSR
IP_ttl = n                                               IP_ttl = n - 15
links   LAN   PPP        FR          ATM    PPP    FR     LAN
 --->iIg-->gGg-->gGf            fGa       aGg-->gGf       fGg-->gIi--->
hops     1     2   |     6      | |   9   |  10   |  13   ^  14    15
                   |1          4| |1     3|       |1     3|
                   V  2     3   | V   2   |       V   2   |
                  fFf-->fFf-->fFf aAa-->aAa       fFf-->fFf
mpls_ttl
       n-1   n-2  (n-2)-4=n-6  (n-6)-3=n-9  n-10  n-13     n-14


"iIg" is "ingress LSR" in LSP; it calculates: mpls_ttl=n-1
"gGf" is "egress LSR" from Generic MPLS segment, and
      "ingress LSR" in Frame Relay segment and calculates: mpls_ttl=n-6
"fGa" "egress LSR" from Frame Relay segment, and
      "ingress LSR" in ATM segment and calculates: mpls_ttl=n-9
"gGf" is "egress LSR" from Generic MPLS segment, and
      "ingress LSR" in Frame Relay segment and calculates: mpls_ttl=n-13
"fGg" is "egress LSR" from Frame Relay segment, and
      ingress LSR" in Generic MPLS segment and calculates: mpls_ttl=n-14
"gIi" is "egress LSR" from  LSP and calculates: IP_ttl=n-15



   And further examples:

             Frame Relay Unicast -- TTL calculated at ingress

(ingress LSR)  1     2        3      4
         x--->---+--->---+--->>--+-->>---x (egress LSR)
   o.ttl=i.ttl-4         |     2      3
                         ^
 hops                   1|
                         |
                         x (ingress LSR)
                           o.ttl=i.ttl-3






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       Frame Relay Multicast -- TTL calculated at egress

             (egress LSR)x  o.ttl=i.ttl-3
 hops                    |
                         ^3
  (ingress LSR)          |            o.ttl=i.ttl-4
         x--->---+--->---+--->---+--->---x (egress LSR)
             1       2       3       4



5.5  Label Processing by Ingress FR-LSRs

   When a packet first enters an MPLS domain, the packet is forwarded by
   normal  network  layer  forwarding operations with the exception that
   the outgoing encapsulation will include an MPLS label  stack  [STACK]
   with  at  least  one  entry.  The frame relay null encapsulation will
   carry information about the network layer protocol implicitly in  the
   label,  which MUST be associated only with that network protocol. The
   TTL field in the top label stack entry is  filled  with  the  network
   layer  TTL  (or  hop  limit)  resulted after network layer forwarding
   [STACK]. The further FR-LSR processing is similar  in  both  possible
   cases:

   (a) the LSP is homogeneous -- Frame Relay only -- and the  FR-LSR  is
   the ingress.

   (b) the LSP is heterogeneous --  Frame  Relay,  PPP,  Ethernet,  ATM,
   etc...  segments form the LSP -- and the FR-LSR is the ingress into a
   Frame Relay
    segment.

   For unicast packets, the MPLS TTL  SHOULD  be  decremented  with  the
   number  of  hops of the Frame Relay LSP (homogeneous), or Frame Relay
   segment of the LSP  (heterogeneous).  An  LDP  constructing  the  LSP
   SHOULD  pass  meaningful  information to the ingress FR-LSR regarding
   the number of hops of the "non-TTL segment".

   For multicast packets, the MPLS TTL SHOULD be decremented by  1.   An
   LDP  constructing  the  LSP SHOULD pass meaningful information to the
   egress FR-LSR regarding the number of hops of the "non-TTL segment".

   Next, the MPLS encapsulated packet is passed down to the Frame  Relay
   data  link  driver with the top label as output DLCI. The Frame Relay
   frame carrying the MPLS encapsulated packet  is  forwarded  onto  the
   Frame Relay VC to the next LSR.





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5.6  Label Processing by Core FR-LSRs

   In a FR-LSR, the current (top) MPLS label  is  carried  in  the  DLCI
   field of the Frame Relay data link layer header of the frame. Just as
   in conventional Frame Relay, for a frame arriving  at  an  interface,
   the  DLCI carried by the Frame Relay data link header is looked up in
   the DLCI Information Base, replaced  with  the  correspondent  output
   DLCI,  and  transmitted  on  the outgoing interface (forwarded to the
   next hop node).

   The current label information is also carried in the top of the label
   stack.   In   the  top-level  entry,  all  fields  except  the  label
   information, which is carried and switched in the Frame  Relay  frame
   data link-layer header, are of current significance.


5.7  Label Processing by Egress FR-LSRs

   When reaching the end of a Frame Relay LSP, the FR-LSR pops the label
   stack  [ARCH]. If the label popped is the last label, it is necessary
   to determine the particular network layer  protocol  which  is  being
   carried.  The label stack carries no explicit information to identify
   the network layer protocol. This must be inferred from the  value  of
   the label which is popped from the stack.

   If the label popped is not the last label,  the  previous  top  level
   MPLS TTL is propagated to the new top label stack entry.

   If the FR-LSR is the egress switch of a  Frame  Relay  segment  of  a
   hybrid  LSP, and the end of the Frame Relay segment is not the end of
   the LSP, the MPLS packet will be processed for  forwarding  onto  the
   next segment of the LSP based on the information held in the Next Hop
   Label Forwarding Entry (NHLFE) [ARCH]. The output label is set to the
   value  from  the  NHLFE,  and  the  MPLS  TTL  is  decremented by the
   appropriate value depending the type of the output interface and  the
   type  of  transmit  operation  (see  section  6.3). Further, the MPLS
   packet is forwarded according to  the  MPLS  specifications  for  the
   particular link of the next segment of the LSP.

   For unicast packets, the MPLS TTL SHOULD be decremented by one if the
   output  interface is a generic one, or with the number of hops of the
   next ATM segment of the LSP (heterogeneous), if the output  interface
   is an ATM (non-TTL)  interface.

   For multicast packets, the MPLS TTL  SHOULD  be  decremented  by  the
   number  of  hops of the FR segment being exited.  An LDP constructing
   the LSP SHOULD pass  meaningful  information  to  the  egress  FR-LSR
   regarding the number of hops of the FR "non-TTL segment".



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6.  Label Switching Control Component for Frame Relay


   To support label switching a Frame Relay Switch  MUST  implement  the
   control  component  of  label  switching, which consists primarily of
   label  allocation   and   maintenance   procedures.   Label   binding
   information  MAY  be communicated by several mechanisms, one of which
   is the Label Distribution Protocol (LDP) [LDP].

   Since the label switching control component uses information  learned
   directly  from network layer routing protocols, this implies that the
   switch MUST participate as a peer in  these  protocols  (e.g.,  OSPF,
   IS-IS).

   In some cases, LSRs may use other protocols (e.g. RSVP, PIM, BGP)  to
   distribute  label  bindings. In these cases, a Frame Relay LSR should
   participate in these protocols.

   In the case where Frame Relay circuits are established  via  LDP,  or
   RSVP,  or  others,  with  no involvement from traditional Frame Relay
   mechanisms, it  is  assumed  that  circuit  establishing  contractual
   information    such    as    input/output    maximum    frame   size,
   incoming/outgoing  requested/agreed   throughput,   incoming/outgoing
   acceptable      throughput,     incoming/outgoing     burst     size,
   incoming/outgoing frame rate, used in  transmitting,  and  congestion
   control  MAY  be  passed  to  the  FR-LSRs  through  RSVP,  or can be
   statically configured. It is also assumed that congestion control and
   frame  header  flagging as a consequence of congestion, would be done
   by the FR-LSRs in a similar fashion as for  traditional  Frame  Relay
   circuits. With the goal of emulating a best-effort router as default,
   the default VC parameters, in the absence  of  LDP,  RSVP,  or  other
   mechanisms  participation  to setting such parameters, should be zero
   CIR, so that input policing will set the DE bit in  incoming  frames,
   but no frames are dropped.

   Control and state information for the circuits based on MPLS  MAY  be
   communicated through LDP.

   Support  of  label  switching  on  a  Frame  Relay  switch   requires
   conformance  only  to  [FRF]  (framing,  bit-stuffing,  headers, FCS)
   except for section 2.3 (PVC control signaling procedures,  aka  LMI).
   Q.933  signaling for PVCs and/or SVCs is not required. PVC and/or SVC
   signaling may be used for non-MPLS (standard Frame Relay) PVCs and/or
   SVCs  when  both  are  running  on  the  same  interface  as MPLS, as
   discussed in the next section.






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6.1  Hybrid Switches (Ships in the Night)


   The existence of the label switching control  component  on  a  Frame
   Relay switch does not preclude the ability to support the Frame Relay
   control component defined by the ITU and Frame  Relay  Forum  on  the
   same   switch  and  the  same  interfaces  (NICs).  The  two  control
   components, label switching and  those  defined  by  ITU/Frame  Relay
   Forum, would operate independently.

   Definition of how such a device operates is beyond the scope of  this
   document.   However,  only  a small amount of information needs to be
   consistent between the two control components, such as  the  portions
   of the DLCI space which are available to each component.


7.  Label Allocation and Maintenance Procedures


   The mechanisms and message formats of a Label  Distribution  Protocol
   are documented in [ARCH] and [LDP].  The "downstream-on-demand" label
   allocation and maintenance mechanism discussed in this  section  MUST
   be used by FR-LSRs that do not support VC merging, and it MAY also be
   used by FR-LSRs that do support VC merging (note that this  mechanism
   applies to hop-by-hop routed traffic):


7.1   Edge LSR Behavior

   Consider a member of the Edge Set of a FR-LSR domain. Assume that, as
   a result of its routing calculations, it selects a FR-LSR as the next
   hop of a certain route (FEC), and that the next hop is reachable  via
   a  LC-Frame  Relay  interface.  Assume that the next-hop FR-LSR is an
   "LDP-peer" [ARCH][LDP]. The Edge LSR sends an LDP  "request"  message
   for  a label binding from the next hop, downstream LSR. When the Edge
   LSR receives in response from the downstream LSR  the  label  binding
   information  in  an LDP "mapping" message, the label is stored in the
   Label Information Base (LIB) as an outgoing label for that  FEC.  The
   "mapping"   message   may  contain  the  "hop  count"  object,  which
   represents the number of hops a packet will take to cross the  FR-LSR
   domain  to  the Egress FR-LSR when using this label. This information
   may be stored for TTL calculation. Once this is done, the LSR may use
   MPLS forwarding to transmit packets in that FEC.

   When a member of the Edge Set of the FR-LSR domain  receives  an  LDP
   "request"  message  from  a  FR-LSR  for  a  FEC,  it means it is the
   Egress-FR-LSR. It allocates a label, creates a new entry in its Label
   Information  Base  (LIB),  places  that  label  in the incoming label



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   component of the entry, and returns (via  LDP)  a  "mapping"  message
   containing  the  allocated  label  back upstream to the LDP peer that
   originated the request.  The  "mapping"  message  contains  the  "hop
   count" object value set to 1.

   When a routing calculation causes an Edge LSR to change the next  hop
   for  a  route,  and the former next hop was in the FR-LSR domain, the
   Edge LSR should notify the former next  hop  (via  an  LDP  "release"
   message)  that  the  label  binding  associated  with the route is no
   longer needed.

   When a Frame Relay-LSR  receives  an  LDP  "request"  message  for  a
   certain  route  (FEC) from an LDP peer connected to the FR-LSR over a
   LC-FR interface, the FR-LSR takes the following actions:

      -    it allocates a label,  creates  a  new  entry  in  its  Label
           Information Base (LIB), and places that label in the incoming
           label component of the entry;

      -    it propagates the "request",  by  sending  an  LDP  "request"
           message to the next hop LSR, downstream for that route (FEC);


   In the "ordered control" mode [ARCH], the FR-LSR will  wait  for  its
   "request"  to  be  responded from downstream with a "mapping" message
   before returning the "mapping" upstream in response  to  a  "request"
   ("ordered  control"  approach  [ARCH]).  In  this  case,  the  FR-LSR
   increments the hop count it received from downstream  and  uses  this
   value in the "mapping" it returns upstream.

   Alternatively, the FR-LSR may return  the  binding  upstream  without
   waiting for a binding from downstream ("independent control" approach
   [ARCH]). In this case, it uses a reserved value for hop count in  the
   "mapping", indicating that it is 'unknown'. The correct value for hop
   count will be returned later, as described below.

   Since both the "ordered" and "independent" control has advantages and
   disadvantages,  this  is  left as an implementation, or configuration
   choice.

   Once the FR-LSR receives in response the  label  binding  in  an  LDP
   "mapping"  message  from  the  next hop, it places the label into the
   outgoing label component of the LIB entry.

   Note that a FR-LSR, or a member of the edge set of a  FR-LSR  domain,
   may  receive  multiple binding requests for the same route (FEC) from
   the same FR-LSR. It must generate a new "mapping" for each  "request"
   (assuming  adequate  resources  to  do  so),  and retain any existing



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   mapping(s).  For  each  "request"  received,  a  FR-LSR  should  also
   generate  a  new  binding "request" toward the next hop for the route
   (FEC).

   When a routing calculation causes a FR-LSR to change the next hop for
   a  route  (FEC), the FR-LSR should notify the former next hop (via an
   LDP "release" message) that the label  binding  associated  with  the
   route is no longer needed.

   When a LSR receives a notification that a particular label binding is
   no  longer  needed,  the LSR may deallocate the label associated with
   the binding, and destroy the binding. This mode is the  "conservative
   label  retention  mode"  [ARCH].  In the case where a FR-LSR receives
   such notification and destroys the binding, it should notify the next
   hop  for  the route that the label binding is no longer needed.  If a
   LSR does not  destroy  the  binding  (the  FR-LSR  is  configured  in
   "liberal  label  retention  mode"  [ARCH]), it may re-use the binding
   only if it receives a request for the same route with  the  same  hop
   count  as  the  request  that  originally  caused  the  binding to be
   created.

   When a route changes, the label bindings are re-established from  the
   point  where  the  route  diverges  from  the  previous  route.  LSRs
   upstream  of  that  point  are  (with  one  exception,  noted  below)
   oblivious  to  the change.  Whenever a LSR changes its next hop for a
   particular route, if the new next hop is a FR-LSR or a member of  the
   edge  set reachable via a LC-FR interface, then for each entry in its
   LIB associated with the route the LSR  should  request  (via  LDP)  a
   binding from the new next hop.

   When a FR-LSR receives a label binding from a downstream neighbor, it
   may  already  have  provided  a  corresponding label binding for this
   route  to  an  upstream  neighbor,  either  because   it   is   using
   "independent  control"  or because the new binding from downstream is
   the result of a routing change. In this case, it should  extract  the
   hop  count  from  the new binding and increment it by one. If the new
   hop count is different from that which was previously conveyed to the
   upstream neighbor (including the case where the upstream neighbor was
   given the value  'unknown')  the  FR-LSR  must  notify  the  upstream
   neighbor  of the change. Each FR-LSR in turn increments the hop count
   and passes it upstream until it reaches the ingress Edge LSR.

   Whenever a FR-LSR originates a label binding request to its next  hop
   LSR  as  a  result  of receiving a label binding request from another
   (upstream) LSR, and the request to the next hop LSR is not satisfied,
   the  FR-LSR  should  destroy  the  binding created in response to the
   received request, and notify the requester  (via  an  LDP  "withdraw"
   message).



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   When an LSR determines that it has lost its LDP session with  another
   LSR, the following actions are taken:

        -    MUST discard  any  binding  information  learned  via  this
             connection;

        -    For any label bindings that were created  as  a  result  of
             receiving label binding requests from the peer, the LSR may
             destroy these bindings (and  deallocate  labels  associated
             with these binding).



7.2   Efficient use of label space - Merging FR-LSRs

   The above discussion assumes that an edge LSR will request one  label
   for  each  prefix in its routing table that has a next hop in the FR-
   LSR domain. In fact, it  is  possible  to  significantly  reduce  the
   number  of  labels  needed by having the edge LSR request instead one
   label for several routes. Use of many-to-one mappings between  routes
   (address   prefixes)  and  labels  using  the  notion  of  Forwarding
   Equivalence Classes (as described in [ARCH]) provides a mechanism  to
   conserve the number of labels.

   Note that conserving label space (VC merging) may  be  restricted  in
   case  the frame traffic requires Frame Relay fragmentation. The issue
   is that Frame Relay fragments must be transmitted in  sequence,  i.e.
   fragments  of  distinct  frames  must  not  be  interleaved.  If  the
   fragmenting FR-LSR  ensures  the  transmission  in  sequence  of  all
   fragments  of  a  frame, without interleaving with fragments of other
   frames, then label conservation (VC merging) can be performed.

   When label conservation is used, when a  FR-LSR  receives  a  binding
   request  from  an upstream LSR for a certain FEC, and it does already
   have an outgoing label binding for that FEC,  it  does  not  need  to
   issue  a  downstream  binding  request.  Instead,  it may allocate an
   incoming label, and return that label in a binding  to  the  upstream
   requester.  Packets  received  from the requester, with that label as
   top label, will be forwarded  after  replacing  the  label  with  the
   existing  outgoing label for that FEC. If the FR-LSR does not have an
   outgoing label binding for that FEC, but  does  have  an  outstanding
   request  for  one, it need not issue another request. This means that
   in a label conservation case,  a  FR-LSR  must  respond  with  a  new
   binding  for  every  upstream  request,  but it may  need to send one
   binding request downstream.

   In case of label conservation, if  a  change  in  the  routing  table
   causes  a FR-LSR to select a new next hop for one of its FECs, it MAY



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   release the binding for that route from the former next  hop.  If  it
   doesn't already have a corresponding binding for the new next hop, it
   must request one (note that the choice depends on the label retention
   mode [ARCH]).

   If a new binding is obtained, which contain a hop count that  differs
   from  that  of  the  old binding, the FR-LSR must process the new hop
   count: increment by 1, if different than "unknown",  and  notify  the
   upstream  neighbors  who  have label bindings for this FEC of the new
   value. To ensure that loops will be detected, if the  new  hop  count
   exceeds  the  "maximum"  value, the label values for this FEC must be
   withdrawn  from  all  upstream  neighbors  to  whom  a  binding   was
   previously sent.

7.3   LDP messages specific to Frame Relay

   The Label Distribution Protocol [LDP] messages exchanged between  two
   Frame   Relay  "LDP-peer"  LSRs  may  contain  Frame  Relay  specific
   information such as:

   "Frame Relay Label Range":

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Reserved    |Len|               Minimum DLCI                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Reserved        |               Maximum DLCI                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   with the following fields:

   Reserved
     This fields are reserved. They must be set to zero on  transmission
     and must be ignored on receipt.

   Len
     This field specifies the number of bits of the DLCI. The  following
     values are supported:

          Len  DLCI bits

          0     10
          1     23







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   Minimum DLCI
     This 23 bit field is the binary value of the lower bound of a block
     of  Data  Link  Connection Identifiers (DLCIs) that is supported by
     the originating FR-LSR. The Minimum DLCI should be right  justified
     in this field and the preceding bits should be set to 0.

   Maximum DLCI
     This 23 bit field is the binary value of the upper bound of a block
     of  Data  Link  Connection Identifiers (DLCIs) that is supported by
     the originating FR-LSR. The Maximum DLCI should be right  justified
     in this field and the preceding bits should be set to 0.

   "Frame Relay Merge":

          0 1 2 3 4 5 6 7
         +-+-+-+-+-+-+-+-+
         | Reserved    |M|
         +-+-+-+-+-+-+-+-+

   with the following fields:

   Merge
     One bit field that specifies the merge capabilities of the FR-LSR:

     Value                  Meaning

       0                    Merge NOT supported
       1                    Merge supported

     A FR-LSR that supports  VC  merging  MUST  ensure  that  fragmented
     frames  from  distinct  incoming  DLCIs  are not interleaved on the
     outgoing DLCI.

   Reserved
     This field is reserved. It must be set to zero on transmission  and
     must be ignored on receipt.

   and "Frame Relay Label":

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Reserved    |Len|                       DLCI                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   with the following fields:





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   Reserved
     This field is reserved. It must be set to zero on transmission  and
     must be ignored on receipt.

   Len
     This field specifies the number of bits of the DLCI. The  following
     values are supported:

          Len  DLCI bits

          0     10
          1     23


   DLCI
     The binary value of the Frame Relay Label. The  significant  number
     of  bits  (10 or 23) of the label value are to be encoded  into the
     Data Link Connection Identifier (DLCI) field when part of the Frame
     Relay data link header (see Section 4.).


8.  Security Considerations


   This section looks at the security aspects of:

         (a) frame traffic,

         (b) label distribution.

   MPLS encapsulation  has  no  effect  on  authenticated  or  encrypted
   network  layer  packets, that is IP packets that are authenticated or
   encrypted will incur no change.

   The MPLS protocol has no mechanisms of its  own  to  protect  against
   misdirection of packets or the impersonation of an LSR by accident or
   malicious intent.

   Altering by accident or forgery an existent label in the  DLCI  field
   of  the  Frame Relay data link layer header of a frame or one or more
   fields in a potentially following label stack affects the  forwarding
   of that frame.

   The label distribution  mechanism can  be  secured  by  applying  the
   appropriate  level  of  security  to the underlying protocol carrying
   label information - authentication or encryption -  see [LDP].





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9.  Acknowledgments

   The initial version of this  document  was  derived  from  the  Label
   Switching over ATM document [ATM].

   Thanks for the extensive reviewing and constructive comments from (in
   alphabetical  order)  Dan  Harrington,  Milan Merhar, Martin Mueller,
   Eric Rosen. Also thanks to George Swallow for the suggestion  to  use
   null encapsulation, and to Eric Gray for his reviewing.

   Also thanks to Nancy Feldman and Bob Thomas for  their  collaboration
   in including the LDP messages specific to Frame Relay LSRs.

10. References

   [MIFR] T. Bradley, C. Brown,  A.  Malis  "Multiprotocol  Interconnect
   over Frame Relay" RFC 2427, September 1998.


   [ARCH] E. Rosen, R. Callon, A.  Vishwanathan,  "Multi-Protocol  Label
   Switching Architecture", Work in Progress, July 1998.


   [LDP] L. Anderson, P. Doolan, N. Feldman,  A.  Fredette,  R.  Thomas,
   "Label Distribution Protocol", Work in Progress, August 1998.


   [STACK] E. Rosen et al, "Label  Switching:  Label  Stack  Encodings",
   Work in Progress, September 1998


   [ATM] B. Davie et al. "Use of Label  Switching  with  ATM",  Work  in
   Progress, July 1998.


   [ITU] International Telecommunications Union, "ISDN Data  Link  Layer
   Specification  for  Frame Mode Bearer Services", ITU-T Recommendation
   Q.922, 1992.


   [FRF] Frame Relay  Forum,  User-to-Network  Implementation  Agreement
   (UNI), FRF 1.1, January 19, 1996









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11.Authors' Addresses

   Alex Conta
   3COM
   100 3COM Drive
   Marlborough, MA 01752
   +1 508 323-2297
   E-mail: Alex_Conta@ne.3com.com

   Paul Doolan
   Ennovate Networks
   60 Codman Hill Rd
   Boxborough MA 01719
   +1 978 263-2002
   E-mail: pdoolan@ennovatenetworks.com

   Andrew Malis
   Lucent Technologies
   1 Robbins Rd
   Westford, MA 01886
   +1 978 952-7414
   E-mail: amalis@lucent.com





























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Appendix A - Changes since previous versions

   From "version 02 to 03"
       - Replace "cloud" with "domain",
       - Update references to other documents,
       - Change definitions in "Terminology" section,
       - Add more definitions to "Terminology" section,
       - Make editorial changes to text and figures,
       - Change "Performing TTL calculations" section,
       - Add more reviewers in "Acknowledgments" section,
       - Add Appendix A - changes.








































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