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rfc:rfc3034

Network Working Group A. Conta Request for Comments: 3034 Transwitch Corporation Category: Standards Track P. Doolan

                                                              Ennovate
                                                              A. Malis
                                                 Vivace Networks, Inc.
                                                          January 2001
           Use of Label Switching on Frame Relay Networks
                           Specification

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 (2001).  All Rights Reserved.

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).

Table of Contents

 1. Introduction................................................2
 2. Terminology.................................................3
 3. Special Characteristics of Frame Relay Switches.............4
 4. Label Encapsulation.........................................5
 5. Frame Relay Label Switching Processing......................6
 5.1  Use of DLCIs..............................................6
 5.2  Homogeneous LSPs..........................................7
 5.3  Heterogeneous LSPs........................................7
 5.4  Frame Relay Label Switching Loop Prevention and Control...7
 5.4.1   FR-LSRs Loop Control - MPLS TTL Processing.............7
 5.4.2   Performing MPLS TTL calculations.......................8
 5.5  Label Processing by Ingress FR-LSRs......................12

Conta, et al. Standards Track [Page 1] RFC 3034 Label Switching with Frame Relay January 2001

 5.6  Label Processing by Core FR-LSRs.........................12
 5.7  Label Processing by Egress FR-LSRs.......................13
 6.  Label Switching Control Component for Frame Relay.........13
 6.1  Hybrid Switches (Ships in the Night)  ...................14
 7.  Label Allocation and Maintenance Procedures ..............15
 7.1  Edge LSR Behavior........................................15
 7.2  Efficient use of label space-Merging FR-LSRs.............18
 7.3  LDP message fields specific to Frame Relay...............19
 8.  Security Considerations  .................................21
 9.  Acknowledgments  .........................................21
 10. References  ..............................................22
 11. Authors' Addresses  ......................................23
 12. Full Copyright Statement  ................................24

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].

Conta, et al. Standards Track [Page 2] RFC 3034 Label Switching with Frame Relay January 2001

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
    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.

Conta, et al. Standards Track [Page 3] RFC 3034 Label Switching with Frame Relay January 2001

 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.
 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:
  1. 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.
  1. there is generally no capability to perform a 'TTL-decrement'

function as is performed on IP headers in routers.

  1. 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.

Conta, et al. Standards Track [Page 4] RFC 3034 Label Switching with Frame Relay January 2001

  1. 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:
             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

Conta, et al. Standards Track [Page 5] RFC 3034 Label Switching with Frame Relay January 2001

          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.

Conta, et al. Standards Track [Page 6] RFC 3034 Label Switching with Frame Relay January 2001

 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].

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.

Conta, et al. Standards Track [Page 7] RFC 3034 Label Switching with Frame Relay January 2001

 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:
  1. it treats the packet as an expired packet and return an ICMP

message to its source.

  1. 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
 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

Conta, et al. Standards Track [Page 8] RFC 3034 Label Switching with Frame Relay January 2001

 "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
 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].

Conta, et al. Standards Track [Page 9] RFC 3034 Label Switching with Frame Relay January 2001

 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

"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

Conta, et al. Standards Track [Page 10] RFC 3034 Label Switching with Frame Relay January 2001

                        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
        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

Conta, et al. Standards Track [Page 11] RFC 3034 Label Switching with Frame Relay January 2001

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.

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).

Conta, et al. Standards Track [Page 12] RFC 3034 Label Switching with Frame Relay January 2001

 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".

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].

Conta, et al. Standards Track [Page 13] RFC 3034 Label Switching with Frame Relay January 2001

 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.

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.

Conta, et al. Standards Track [Page 14] RFC 3034 Label Switching with Frame Relay January 2001

 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
 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.

Conta, et al. Standards Track [Page 15] RFC 3034 Label Switching with Frame Relay January 2001

 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:
  1. 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;
  1. 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
 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

Conta, et al. Standards Track [Page 16] RFC 3034 Label Switching with Frame Relay January 2001

 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).
 When an LSR determines that it has lost its LDP session with another
 LSR, the following actions are taken:
  1. MUST discard any binding information learned via this

connection;

  1. 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).

Conta, et al. Standards Track [Page 17] RFC 3034 Label Switching with Frame Relay January 2001

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 FR-LSR to select a new next hop for one of its FECs, it MAY
 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.

Conta, et al. Standards Track [Page 18] RFC 3034 Label Switching with Frame Relay January 2001

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
        2     23
    Len values 1 and 3 are reserved for future use.
 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.

Conta, et al. Standards Track [Page 19] RFC 3034 Label Switching with Frame Relay January 2001

 "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:
 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
        2     23

Conta, et al. Standards Track [Page 20] RFC 3034 Label Switching with Frame Relay January 2001

    Len values 1 and 3 are reserved for future use.
 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].

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.

Conta, et al. Standards Track [Page 21] RFC 3034 Label Switching with Frame Relay January 2001

10. References

 [MIFR]  Bradley, T., Brown, C. and A. Malis, "Multiprotocol
         Interconnect over Frame Relay", RFC 2427, September 1998.
 [ARCH]  Rosen, E., Callon, R. and A. Vishwanathan, "Multi-Protocol
         Label Switching Architecture", RFC 3031, January 2001.
 [LDP]   Andersson, L., Doolan, P., Feldman, N., Fredette, A. and R.
         Thomas, "Label Distribution Protocol", RFC 3036, January
         2001.
 [STACK] Rosen, E., Rehter, Y., Tappan, D., Farinacci, D., Fedorkow,
         G., Li, T. and A. Conta, "MPLS Label Stack Encoding", RFC
         3032, January 2001.
 [ATM]   Davie, B., Lawrence, J., McCloghrie, M., Rosen, E., Swallow,
         G., Rekhter, Y., and P. Doolan, "Use of Label Switching with
         ATM", RFC 3035, January 2001.
 [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.

Conta, et al. Standards Track [Page 22] RFC 3034 Label Switching with Frame Relay January 2001

11. Authors' Addresses

 Alex Conta
 Transwitch Corporation
 3 Enterprise Drive
 Shelton, CT 06484
 Phone: 1-203-929-8810
 EMail: aconta@txc.com
 Paul Doolan
 Ennovate Networks
 60 Codman Hill Rd
 Boxborough MA 01719
 Phone: 1-978-263-2002
 EMail: pdoolan@ennovatenetworks.com
 Andrew G. Malis
 Vivace Networks, Inc.
 2730 Orchard Parkway
 San Jose, CA 95134
 USA
 Phone: 1-408-383-7223
 Fax:   1-408-904-4748
 EMail: Andy.Malis@vivacenetworks.com

Conta, et al. Standards Track [Page 23] RFC 3034 Label Switching with Frame Relay January 2001

12. Full Copyright Statement

 Copyright (C) The Internet Society (2001).  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
 English.
 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
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

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

Conta, et al. Standards Track [Page 24]

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