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RFC 8029

 
 
 

Detecting Multiprotocol Label Switched (MPLS) Data-Plane Failures

Part 2 of 4, p. 17 to 41
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3.2.  Target FEC Stack

   A Target FEC Stack is a list of sub-TLVs.  The number of elements is
   determined by looking at the sub-TLV length fields.

    Sub-Type     Length         Value Field
    --------     ------         -----------
           1          5         LDP IPv4 prefix
           2         17         LDP IPv6 prefix
           3         20         RSVP IPv4 LSP
           4         56         RSVP IPv6 LSP
           5                    Unassigned
           6         13         VPN IPv4 prefix
           7         25         VPN IPv6 prefix
           8         14         L2 VPN endpoint
           9         10         "FEC 128" Pseudowire - IPv4 (deprecated)
          10         14         "FEC 128" Pseudowire - IPv4
          11        16+         "FEC 129" Pseudowire - IPv4
          12          5         BGP labeled IPv4 prefix
          13         17         BGP labeled IPv6 prefix
          14          5         Generic IPv4 prefix
          15         17         Generic IPv6 prefix
          16          4         Nil FEC
          24         38         "FEC 128" Pseudowire - IPv6
          25         40+        "FEC 129" Pseudowire - IPv6

   Other FEC types have been defined and will be defined as needed.

   Note that this TLV defines a stack of FECs, the first FEC element
   corresponding to the top of the label stack, etc.

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   An MPLS echo request MUST have a Target FEC Stack that describes the
   FEC Stack being tested.  For example, if an LSR X has an LDP mapping
   [RFC5036] for 192.0.2.1 (say, label 1001), then to verify that label
   1001 does indeed reach an egress LSR that announced this prefix via
   LDP, X can send an MPLS echo request with a FEC Stack TLV with one
   FEC in it, namely, of type LDP IPv4 prefix, with prefix 192.0.2.1/32,
   and send the echo request with a label of 1001.

   Say LSR X wanted to verify that a label stack of <1001, 23456> is the
   right label stack to use to reach a VPN IPv4 prefix (see
   Section 3.2.5) of 203.0.113.0/24 in VPN foo.  Say further that LSR Y
   with loopback address 192.0.2.1 announced prefix 203.0.113.0/24 with
   Route Distinguisher (RD) RD-foo-Y (which may in general be different
   from the RD that LSR X uses in its own advertisements for VPN foo),
   label 23456, and BGP next hop 192.0.2.1 [RFC4271].  Finally, suppose
   that LSR X receives a label binding of 1001 for 192.0.2.1 via LDP.  X
   has two choices in sending an MPLS echo request: X can send an MPLS
   echo request with a FEC Stack TLV with a single FEC of type VPN IPv4
   prefix with a prefix of 203.0.113.0/24 and an RD of RD-foo-Y.
   Alternatively, X can send a FEC Stack TLV with two FECs, the first of
   type LDP IPv4 with a prefix of 192.0.2.1/32 and the second of type of
   IP VPN with a prefix 203.0.113.0/24 with an RD of RD-foo-Y.  In
   either case, the MPLS echo request would have a label stack of <1001,
   23456>.  (Note: in this example, 1001 is the "outer" label and 23456
   is the "inner" label.)

   If, for example, an LSR Y has an LDP mapping for the IPv6 address
   2001:db8::1 (say, label 2001), then to verify that label 2001 does
   reach an egress LSR that announced this prefix via LDP, LSR Y can
   send an MPLS echo request with a FEC Stack TLV with one LDP IPv6
   prefix FEC, with prefix 2001:db8::1/128, and with a label of 2001.

   If an end-to-end path comprises of one or more tunneled or stitched
   LSPs, each transit node that is the originating point of a new tunnel
   or segment SHOULD reply back notifying the FEC stack change along
   with the new FEC details, for example, if LSR X has an LDP mapping
   for IPv4 prefix 192.0.2.10 on LSR Z (say, label 3001).  Say further
   that LSR A and LSR B are transit nodes along the path, which also
   have an RSVP tunnel over which LDP is enabled.  While replying back,
   A SHOULD notify that the FEC changes from LDP to <RSVP, LDP>.  If the
   new tunnel is a transparent pipe, i.e., the data-plane trace will not
   expire in the middle of the tunnel, then the transit node SHOULD NOT
   reply back notifying the FEC stack change or the new FEC details.  If
   the transit node wishes to hide the nature of the tunnel from the
   ingress of the echo request, then the transit node MAY notify the FEC
   stack change and include Nil FEC as the new FEC.

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3.2.1.  LDP IPv4 Prefix

   The IPv4 Prefix FEC is defined in [RFC5036].  When an LDP IPv4 prefix
   is encoded in a label stack, the following format is used.  The value
   consists of 4 octets of an IPv4 prefix followed by 1 octet of prefix
   length in bits; the format is given below.  The IPv4 prefix is in
   network byte order; if the prefix is shorter than 32 bits, trailing
   bits SHOULD be set to zero.  See [RFC5036] for an example of a
   Mapping for an IPv4 FEC.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          IPv4 prefix                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Prefix Length |         Must Be Zero                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.2.  LDP IPv6 Prefix

   The IPv6 Prefix FEC is defined in [RFC5036].  When an LDP IPv6 prefix
   is encoded in a label stack, the following format is used.  The value
   consists of 16 octets of an IPv6 prefix followed by 1 octet of prefix
   length in bits; the format is given below.  The IPv6 prefix is in
   network byte order; if the prefix is shorter than 128 bits, the
   trailing bits SHOULD be set to zero.  See [RFC5036] for an example of
   a Mapping for an IPv6 FEC.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          IPv6 prefix                          |
      |                          (16 octets)                          |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Prefix Length |         Must Be Zero                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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3.2.3.  RSVP IPv4 LSP

   The value has the format below.  The Value fields are taken from RFC
   3209 [RFC3209], Sections 4.6.1.1 and 4.6.2.1.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 IPv4 Tunnel Endpoint Address                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Must Be Zero         |     Tunnel ID                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Extended Tunnel ID                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   IPv4 Tunnel Sender Address                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Must Be Zero         |            LSP ID             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.4.  RSVP IPv6 LSP

   The value has the format below.  The Value fields are taken from RFC
   3209 [RFC3209], Sections 4.6.1.2 and 4.6.2.2.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 IPv6 Tunnel Endpoint Address                  |
      |                                                               |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Must Be Zero         |          Tunnel ID            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Extended Tunnel ID                      |
      |                                                               |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   IPv6 Tunnel Sender Address                  |
      |                                                               |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Must Be Zero         |            LSP ID             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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3.2.5.  VPN IPv4 Prefix

   VPN-IPv4 Network Layer Routing Information (NLRI) is defined in
   [RFC4365].  This document uses the term VPN IPv4 prefix for a
   VPN-IPv4 NLRI that has been advertised with an MPLS label in BGP.
   See [RFC3107].

   When a VPN IPv4 prefix is encoded in a label stack, the following
   format is used.  The Value field consists of the RD advertised with
   the VPN IPv4 prefix, the IPv4 prefix (with trailing 0 bits to make 32
   bits in all), and a prefix length, as follows:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Route Distinguisher                      |
      |                          (8 octets)                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         IPv4 prefix                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Prefix Length |                 Must Be Zero                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The RD is an 8-octet identifier; it does not contain any inherent
   information.  The purpose of the RD is solely to allow one to create
   distinct routes to a common IPv4 address prefix.  The encoding of the
   RD is not important here.  When matching this field to the local FEC
   information, it is treated as an opaque value.

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3.2.6.  VPN IPv6 Prefix

   VPN-IPv6 NLRI is defined in [RFC4365].  This document uses the term
   VPN IPv6 prefix for a VPN-IPv6 NLRI that has been advertised with an
   MPLS label in BGP.  See [RFC3107].

   When a VPN IPv6 prefix is encoded in a label stack, the following
   format is used.  The Value field consists of the RD advertised with
   the VPN IPv6 prefix, the IPv6 prefix (with trailing 0 bits to make
   128 bits in all), and a prefix length, as follows:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Route Distinguisher                      |
      |                          (8 octets)                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         IPv6 prefix                           |
      |                                                               |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Prefix Length |                 Must Be Zero                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The RD is identical to the VPN IPv4 Prefix RD, except that it
   functions here to allow the creation of distinct routes to IPv6
   prefixes.  See Section 3.2.5.  When matching this field to local FEC
   information, it is treated as an opaque value.

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3.2.7.  L2 VPN Endpoint

   VPLS stands for Virtual Private LAN Service.  The terms VPLS BGP NLRI
   and VPLS Edge Identifier (VE ID) are defined in [RFC4761].  This
   document uses the simpler term L2 VPN endpoint when referring to a
   VPLS BGP NLRI.  The RD is an 8-octet identifier used to distinguish
   information about various L2 VPNs advertised by a node.  The VE ID is
   a 2-octet identifier used to identify a particular node that serves
   as the service attachment point within a VPLS.  The structure of
   these two identifiers is unimportant here; when matching these fields
   to local FEC information, they are treated as opaque values.  The
   encapsulation type is identical to the Pseudowire (PW) Type in
   Section 3.2.9.

   When an L2 VPN endpoint is encoded in a label stack, the following
   format is used.  The Value field consists of an RD (8 octets), the
   sender's (of the ping) VE ID (2 octets), the receiver's VE ID (2
   octets), and an encapsulation type (2 octets), formatted as follows:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Route Distinguisher                      |
      |                          (8 octets)                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Sender's VE ID        |       Receiver's VE ID        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Encapsulation Type       |         Must Be Zero          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.8.  FEC 128 Pseudowire - IPv4 (Deprecated)

   See Appendix A.1.1 for details.

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3.2.9.  FEC 128 Pseudowire - IPv4 (Current)

   FEC 128 (0x80) is defined in [RFC8077], as are the terms PW ID
   (Pseudowire ID) and PW Type (Pseudowire Type).  A PW ID is a non-zero
   32-bit connection ID.  The PW Type is a 15-bit number indicating the
   encapsulation type.  It is carried right justified in the field below
   termed "encapsulation type" with the high-order bit set to zero.

   Both of these fields are treated in this protocol as opaque values.
   When matching these fields to the local FEC information, the match
   MUST be exact.

   When a FEC 128 is encoded in a label stack, the following format is
   used.  The Value field consists of the Sender's Provider Edge (PE)
   IPv4 Address (the source address of the targeted LDP session), the
   Remote PE IPv4 Address (the destination address of the targeted LDP
   session), the PW ID, and the encapsulation type as follows:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Sender's PE IPv4 Address                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Remote PE IPv4 Address                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             PW ID                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            PW Type            |          Must Be Zero         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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3.2.10.  FEC 129 Pseudowire - IPv4

   FEC 129 (0x81) and the terms PW Type, Attachment Group Identifier
   (AGI), Attachment Group Identifier Type (AGI Type), Attachment
   Individual Identifier Type (AII Type), Source Attachment Individual
   Identifier (SAII), and Target Attachment Individual Identifier (TAII)
   are defined in [RFC8077].  The PW Type is a 15-bit number indicating
   the encapsulation type.  It is carried right justified in the field
   below PW Type with the high-order bit set to zero.  All the other
   fields are treated as opaque values and copied directly from the FEC
   129 format.  All of these values together uniquely define the FEC
   within the scope of the LDP session identified by the source and
   remote PE IPv4 addresses.

   When a FEC 129 is encoded in a label stack, the following format is
   used.  The Length of this TLV is 16 + AGI length + SAII length + TAII
   length.  Padding is used to make the total length a multiple of 4;
   the length of the padding is not included in the Length field.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Sender's PE IPv4 Address                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Remote PE IPv4 Address                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            PW Type            |   AGI Type    |  AGI Length   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                           AGI Value                           ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   AII Type    |  SAII Length  |      SAII Value               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                    SAII Value (continued)                     ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   AII Type    |  TAII Length  |      TAII Value               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                    TAII Value (continued)                     ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  TAII (cont.) |  0-3 octets of zero padding                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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3.2.11.  FEC 128 Pseudowire - IPv6

   The FEC 128 Pseudowire IPv6 sub-TLV has a structure consistent with
   the FEC 128 Pseudowire IPv4 sub-TLV as described in Section 3.2.9.
   The Value field consists of the Sender's PE IPv6 Address (the source
   address of the targeted LDP session), the Remote PE IPv6 Address (the
   destination address of the targeted LDP session), the PW ID, and the
   encapsulation type as follows:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                     Sender's PE IPv6 Address                  ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                      Remote PE IPv6 Address                   ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             PW ID                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            PW Type            |          Must Be Zero         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Sender's PE IPv6 Address: The source IP address of the target IPv6
   LDP session. 16 octets.

   Remote PE IPv6 Address: The destination IP address of the target IPv6
   LDP session. 16 octets.

   PW ID: Same as FEC 128 Pseudowire IPv4 in Section 3.2.9.

   PW Type: Same as FEC 128 Pseudowire IPv4 in Section 3.2.9.

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3.2.12.  FEC 129 Pseudowire - IPv6

   The FEC 129 Pseudowire IPv6 sub-TLV has a structure consistent with
   the FEC 129 Pseudowire IPv4 sub-TLV as described in Section 3.2.10.
   When a FEC 129 is encoded in a label stack, the following format is
   used.  The length of this TLV is 40 + AGI (Attachment Group
   Identifier) length + SAII (Source Attachment Individual Identifier)
   length + TAII (Target Attachment Individual Identifier) length.
   Padding is used to make the total length a multiple of 4; the length
   of the padding is not included in the Length field.

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                   Sender's PE IPv6 Address                    ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                    Remote PE IPv6 Address                     ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            PW Type            |   AGI Type    |  AGI Length   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                           AGI Value                           ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   AII Type    |  SAII Length  |      SAII Value               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                    SAII Value (continued)                     ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   AII Type    |  TAII Length  |      TAII Value               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                    TAII Value (continued)                     ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  TAII (cont.) |  0-3 octets of zero padding                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Sender's PE IPv6 Address: The source IP address of the target IPv6
   LDP session. 16 octets.

   Remote PE IPv6 Address: The destination IP address of the target IPv6
   LDP session. 16 octets.

   The other fields are the same as FEC 129 Pseudowire IPv4 in
   Section 3.2.10.

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3.2.13.  BGP Labeled IPv4 Prefix

   BGP labeled IPv4 prefixes are defined in [RFC3107].  When a BGP
   labeled IPv4 prefix is encoded in a label stack, the following format
   is used.  The Value field consists of the IPv4 prefix (with trailing
   0 bits to make 32 bits in all) and the prefix length, as follows:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          IPv4 prefix                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Prefix Length |                 Must Be Zero                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.14.  BGP Labeled IPv6 Prefix

   BGP labeled IPv6 prefixes are defined in [RFC3107].  When a BGP
   labeled IPv6 prefix is encoded in a label stack, the following format
   is used.  The value consists of 16 octets of an IPv6 prefix followed
   by 1 octet of prefix length in bits; the format is given below.  The
   IPv6 prefix is in network byte order; if the prefix is shorter than
   128 bits, the trailing bits SHOULD be set to zero.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          IPv6 prefix                          |
      |                          (16 octets)                          |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Prefix Length |         Must Be Zero                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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3.2.15.  Generic IPv4 Prefix

   The value consists of 4 octets of an IPv4 prefix followed by 1 octet
   of prefix length in bits; the format is given below.  The IPv4 prefix
   is in network byte order; if the prefix is shorter than 32 bits, the
   trailing bits SHOULD be set to zero.  This FEC is used if the
   protocol advertising the label is unknown or may change during the
   course of the LSP.  An example is an inter-AS LSP that may be
   signaled by LDP in one Autonomous System (AS), by RSVP-TE [RFC3209]
   in another AS, and by BGP between the ASes, such as is common for
   inter-AS VPNs.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          IPv4 prefix                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Prefix Length |         Must Be Zero                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.16.  Generic IPv6 Prefix

   The value consists of 16 octets of an IPv6 prefix followed by 1 octet
   of prefix length in bits; the format is given below.  The IPv6 prefix
   is in network byte order; if the prefix is shorter than 128 bits, the
   trailing bits SHOULD be set to zero.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          IPv6 prefix                          |
      |                          (16 octets)                          |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Prefix Length |         Must Be Zero                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.17.  Nil FEC

   At times, labels from the reserved range, e.g., Router Alert and
   Explicit-null, may be added to the label stack for various diagnostic
   purposes such as influencing load-balancing.  These labels may have
   no explicit FEC associated with them.  The Nil FEC Stack is defined
   to allow a Target FEC Stack sub-TLV to be added to the Target FEC
   Stack to account for such labels so that proper validation can still
   be performed.

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   The Length is 4.  Labels are 20-bit values treated as numbers.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 Label                 |          MBZ          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Label is the actual label value inserted in the label stack; the MBZ
   fields MUST be zero when sent and ignored on receipt.

3.3.  Downstream Mapping (Deprecated)

   See Appendix A.2 for more details.

3.4.  Downstream Detailed Mapping TLV

   The Downstream Detailed Mapping object is a TLV that MAY be included
   in an MPLS echo request message.  Only one Downstream Detailed
   Mapping object may appear in an echo request.  The presence of a
   Downstream Detailed Mapping object is a request that Downstream
   Detailed Mapping objects be included in the MPLS echo reply.  If the
   replying router is the destination (Label Edge Router) of the FEC,
   then a Downstream Detailed Mapping TLV SHOULD NOT be included in the
   MPLS echo reply.  Otherwise, the replying router SHOULD include a
   Downstream Detailed Mapping object for each interface over which this
   FEC could be forwarded.  For a more precise definition of the notion
   of "downstream", see Section 3.4.2, "Downstream Router and
   Interface".

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |               MTU             | Address Type  |    DS Flags   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |               Downstream Address (4 or 16 octets)             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         Downstream Interface Address (4 or 16 octets)         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Return Code  | Return Subcode|        Sub-TLV Length         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       .                                                               .
       .                      List of Sub-TLVs                         .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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   The Downstream Detailed Mapping TLV format is derived from the
   deprecated Downstream Mapping TLV format (see Appendix A.2.)  The key
   change is that variable length and optional fields have been
   converted into sub-TLVs.

   Maximum Transmission Unit (MTU)

      The MTU is the size in octets of the largest MPLS frame (including
      label stack) that fits on the interface to the downstream LSR.

   Address Type

      The Address Type indicates if the interface is numbered or
      unnumbered.  It also determines the length of the Downstream IP
      Address and Downstream Interface fields.  The Address Type is set
      to one of the following values:

       Type #        Address Type
       ------        ------------
            1        IPv4 Numbered
            2        IPv4 Unnumbered
            3        IPv6 Numbered
            4        IPv6 Unnumbered

   DS Flags

      The DS Flags field is a bit vector of various flags with the
      following format:

        0 1 2 3 4 5 6 7
       +-+-+-+-+-+-+-+-+
       | Rsvd(MBZ) |I|N|
       +-+-+-+-+-+-+-+-+

      Two flags are defined currently, I and N.  The remaining flags
      MUST be set to zero when sending and ignored on receipt.

       Flag  Name and Meaning
       ----  ----------------
          I  Interface and Label Stack Object Request

             When this flag is set, it indicates that the replying
             router SHOULD include an Interface and Label Stack
             Object in the echo reply message.

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          N  Treat as a Non-IP Packet

             Echo request messages will be used to diagnose non-IP
             flows.  However, these messages are carried in IP
             packets.  For a router that alters its ECMP algorithm
             based on the FEC or deep packet examination, this flag
             requests that the router treat this as it would if the
             determination of an IP payload had failed.

   Downstream Address and Downstream Interface Address

      IPv4 addresses and interface indices are encoded in 4 octets; IPv6
      addresses are encoded in 16 octets.

      If the interface to the downstream LSR is numbered, then the
      Address Type MUST be set to IPv4 or IPv6, the Downstream Address
      MUST be set to either the downstream LSR's Router ID or the
      interface address of the downstream LSR, and the Downstream
      Interface Address MUST be set to the downstream LSR's interface
      address.

      If the interface to the downstream LSR is unnumbered, the Address
      Type MUST be IPv4 Unnumbered or IPv6 Unnumbered, the Downstream
      Address MUST be the downstream LSR's Router ID, and the Downstream
      Interface Address MUST be set to the index assigned by the
      upstream LSR to the interface.

      If an LSR does not know the IP address of its neighbor, then it
      MUST set the Address Type to either IPv4 Unnumbered or IPv6
      Unnumbered.  For IPv4, it must set the Downstream Address to
      127.0.0.1; for IPv6, the address is set to 0::1.  In both cases,
      the interface index MUST be set to 0.  If an LSR receives an Echo
      Request packet with either of these addresses in the Downstream
      Address field, this indicates that it MUST bypass interface
      verification but continue with label validation.

      If the originator of an echo request packet wishes to obtain
      Downstream Detailed Mapping information but does not know the
      expected label stack, then it SHOULD set the Address Type to
      either IPv4 Unnumbered or IPv6 Unnumbered.  For IPv4, it MUST set
      the Downstream Address to 224.0.0.2; for IPv6, the address MUST be
      set to FF02::2.  In both cases, the interface index MUST be set to
      0.  If an LSR receives an echo request packet with the all-routers
      multicast address, then this indicates that it MUST bypass both
      interface and label stack validation but return Downstream Mapping
      TLVs using the information provided.

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   Return Code

      The Return Code is set to zero by the sender of an echo request.
      The receiver of said echo request can set it in the corresponding
      echo reply that it generates to one of the values specified in
      Section 3.1 other than 14.

      If the receiver sets a non-zero value of the Return Code field in
      the Downstream Detailed Mapping TLV, then the receiver MUST also
      set the Return Code field in the echo reply header to "See DDMAP
      TLV for Return Code and Return Subcode" (Section 3.1).  An
      exception to this is if the receiver is a bud node [RFC4461] and
      is replying as both an egress and a transit node with a Return
      Code of 3 ("Replying router is an egress for the FEC at stack-
      depth <RSC>") in the echo reply header.

      If the Return Code of the echo reply message is not set to either
      "See DDMAP TLV for Return Code and Return Subcode" (Section 3.1)
      or "Replying router is an egress for the FEC at stack-depth
      <RSC>", then the Return Code specified in the Downstream Detailed
      Mapping TLV MUST be ignored.

   Return Subcode

      The Return Subcode is set to zero by the sender.  The receiver can
      set this field to an appropriate value as specified in
      Section 3.1: The Return Subcode is filled in with the stack-depth
      for those codes that specify the stack-depth.  For all other
      codes, the Return Subcode MUST be set to zero.

      If the Return Code of the echo reply message is not set to either
      "See DDMAP TLV for Return Code and Return Subcode" (Section 3.1)
      or "Replying router is an egress for the FEC at stack-depth
      <RSC>", then the Return Subcode specified in the Downstream
      Detailed Mapping TLV MUST be ignored.

   Sub-TLV Length

      Total length in octets of the sub-TLVs associated with this TLV.

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3.4.1.  Sub-TLVs

   This section defines the sub-TLVs that MAY be included as part of the
   Downstream Detailed Mapping TLV.

            Sub-Type    Value Field
           ---------   ------------
             1         Multipath data
             2         Label stack
             3         FEC stack change

3.4.1.1.  Multipath Data Sub-TLV

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |Multipath Type |       Multipath Length        |Reserved (MBZ) |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      |                  (Multipath Information)                      |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The multipath data sub-TLV includes Multipath Information.

   Multipath Type

      The type of the encoding for the Multipath Information.

      The following Multipath Types are defined in this document:

      Key   Type                  Multipath Information
      ---   ----------------      ---------------------
       0    no multipath          Empty (Multipath Length = 0)
       2    IP address            IP addresses
       4    IP address range      low/high address pairs
       8    Bit-masked IP         IP address prefix and bit mask
              address set
       9    Bit-masked label set  Label prefix and bit mask

      Type 0 indicates that all packets will be forwarded out this one
      interface.

      Types 2, 4, 8, and 9 specify that the supplied Multipath
      Information will serve to exercise this path.

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   Multipath Length

      The length in octets of the Multipath Information.

   MBZ

      MUST be set to zero when sending; MUST be ignored on receipt.

   Multipath Information

      Encoded multipath data (e.g., encoded address or label values),
      according to the Multipath Type.  See Section 3.4.1.1.1 for
      encoding details.

3.4.1.1.1.  Multipath Information Encoding

   The Multipath Information encodes labels or addresses that will
   exercise this path.  The Multipath Information depends on the
   Multipath Type.  The contents of the field are shown in the table
   above.  IPv4 addresses are drawn from the range 127/8; IPv6 addresses
   are drawn from the range 0:0:0:0:0:FFFF:7F00:0/104.  Labels are
   treated as numbers, i.e., they are right justified in the field.  For
   Type 4, ranges indicated by address pairs MUST NOT overlap and MUST
   be in ascending sequence.

   Type 8 allows a more dense encoding of IP addresses.  The IP prefix
   is formatted as a base IP address with the non-prefix low-order bits
   set to zero.  The maximum prefix length is 27.  Following the prefix
   is a mask of length 2^(32 - prefix length) bits for IPv4 and
   2^(128 - prefix length) bits for IPv6.  Each bit set to 1 represents
   a valid address.  The address is the base IPv4 address plus the
   position of the bit in the mask where the bits are numbered left to
   right beginning with zero.  For example, the IPv4 addresses
   127.2.1.0, 127.2.1.5-127.2.1.15, and 127.2.1.20-127.2.1.29 would be
   encoded as follows:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 1 1 1 1 1 1 1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 1 1 1 1 1 1 1 1 1 1 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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   Those same addresses embedded in IPv6 would be encoded as follows:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 1 1 1 1 1 1 1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 1 1 1 1 1 1 1 1 1 1 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type 9 allows a more dense encoding of labels.  The label prefix is
   formatted as a base label value with the non-prefix low-order bits
   set to zero.  The maximum prefix (including leading zeros due to
   encoding) length is 27.  Following the prefix is a mask of length
   2^(32 - prefix length) bits.  Each bit set to one represents a valid
   label.  The label is the base label plus the position of the bit in
   the mask where the bits are numbered left to right beginning with
   zero.  Label values of all the odd numbers between 1152 and 1279
   would be encoded as follows:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 1 0 0 0 0 0 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   If the received Multipath Information is non-null, the labels and IP
   addresses MUST be picked from the set provided.  If none of these
   labels or addresses map to a particular downstream interface, then
   for that interface, the type MUST be set to 0.  If the received
   Multipath Information is null (i.e., Multipath Length = 0, or for
   Types 8 and 9, a mask of all zeros), the type MUST be set to 0.

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   For example, suppose LSR X at hop 10 has two downstream LSRs, Y and
   Z, for the FEC in question.  The received X could return Multipath
   Type 4, with low/high IP addresses of 127.1.1.1->127.1.1.255 for
   downstream LSR Y and 127.2.1.1->127.2.1.255 for downstream LSR Z.
   The head end reflects this information to LSR Y.  Y, which has three
   downstream LSRs, U, V, and W, computes that 127.1.1.1->127.1.1.127
   would go to U and 127.1.1.128-> 127.1.1.255 would go to V.  Y would
   then respond with 3 Downstream Detailed Mapping TLVs: to U, with
   Multipath Type 4 (127.1.1.1->127.1.1.127); to V, with Multipath Type
   4 (127.1.1.127->127.1.1.255); and to W, with Multipath Type 0.

   Note that computing Multipath Information may impose a significant
   processing burden on the receiver.  A receiver MAY thus choose to
   process a subset of the received prefixes.  The sender, on receiving
   a reply to a Downstream Detailed Mapping with partial information,
   SHOULD assume that the prefixes missing in the reply were skipped by
   the receiver and MAY re-request information about them in a new echo
   request.

   The encoding of Multipath Information in scenarios where a few LSRs
   apply Entropy-label-based load-balancing while other LSRs are non-EL
   (IP-based) load balanced will be defined in a different document.

   The encoding of Multipath Information in scenarios where LSRs have
   Layer 2 ECMP over Link Aggregation Group (LAG) interfaces will be
   defined in a different document.

3.4.1.2.  Label Stack Sub-TLV

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Downstream Label                |    Protocol   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                                                               .
      .                                                               .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Downstream Label                |    Protocol   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Label Stack sub-TLV contains the set of labels in the label stack
   as it would have appeared if this router were forwarding the packet
   through this interface.  Any Implicit Null labels are explicitly
   included.  The number of label/protocol pairs present in the sub-TLV
   is determined based on the sub-TLV data length.  When the Downstream
   Detailed Mapping TLV is sent in the echo reply, this sub-TLV MUST be
   included.

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   Downstream Label

      A downstream label is 24 bits, in the same format as an MPLS label
      minus the TTL field, i.e., the MSBit of the label is bit 0, the
      LSBit is bit 19, the TC field [RFC5462] is bits 20-22, and S is
      bit 23.  The replying router SHOULD fill in the TC field and S
      bit; the LSR receiving the echo reply MAY choose to ignore these.

   Protocol

      This specifies the label distribution protocol for the Downstream
      label.  Protocol values are taken from the following table:

      Protocol #        Signaling Protocol
      ----------        ------------------
               0        Unknown
               1        Static
               2        BGP
               3        LDP
               4        RSVP-TE

3.4.1.3.  FEC Stack Change Sub-TLV

   A router MUST include the FEC stack change sub-TLV when the
   downstream node in the echo reply has a different FEC Stack than the
   FEC Stack received in the echo request.  One or more FEC stack change
   sub-TLVs MAY be present in the Downstream Detailed Mapping TLV.  The
   format is as below.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Operation Type | Address Type  | FEC-tlv length|  Reserved     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Remote Peer Address (0, 4, or 16 octets)            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   .                         FEC TLV                               .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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   Operation Type

      The operation type specifies the action associated with the FEC
      stack change.  The following operation types are defined:

            Type #     Operation
            ------     ---------
            1          Push
            2          Pop

   Address Type

      The Address Type indicates the remote peer's address type.  The
      Address Type is set to one of the following values.  The length of
      the peer address is determined based on the address type.  The
      address type MAY be different from the address type included in
      the Downstream Detailed Mapping TLV.  This can happen when the LSP
      goes over a tunnel of a different address family.  The address
      type MAY be set to Unspecified if the peer address is either
      unavailable or the transit router does not wish to provide it for
      security or administrative reasons.

           Type #   Address Type   Address length
           ------   ------------   --------------
           0        Unspecified    0
           1        IPv4           4
           2        IPv6           16

   FEC TLV Length

      Length in octets of the FEC TLV.

   Reserved

      This field is reserved for future use and MUST be set to zero.

   Remote Peer Address

      The remote peer address specifies the remote peer that is the next
      hop for the FEC being currently traced.  If the operation type is
      PUSH, the remote peer address is the address of the peer from
      which the FEC being pushed was learned.  If the operation type is
      pop, the remote peer address MAY be set to Unspecified.

      For upstream-assigned labels [RFC5331], an operation type of pop
      will have a remote peer address (the upstream node that assigned
      the label), and this SHOULD be included in the FEC stack change

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      sub-TLV.  The remote peer address MAY be set to Unspecified if the
      address needs to be hidden.

   FEC TLV

      The FEC TLV is present only when the FEC-tlv length field is non-
      zero.  The FEC TLV specifies the FEC associated with the FEC stack
      change operation.  This TLV MAY be included when the operation
      type is pop.  It MUST be included when the operation type is PUSH.
      The FEC TLV contains exactly one FEC from the list of FECs
      specified in Section 3.2.  A Nil FEC MAY be associated with a PUSH
      operation if the responding router wishes to hide the details of
      the FEC being pushed.

   FEC stack change sub-TLV operation rules are as follows:

   a.  A FEC stack change sub-TLV containing a PUSH operation MUST NOT
       be followed by a FEC stack change sub-TLV containing a pop
       operation.

   b.  One or more pop operations MAY be followed by one or more PUSH
       operations.

   c.  One FEC stack change sub-TLV MUST be included per FEC stack
       change.  For example, if 2 labels are going to be pushed, then
       one FEC stack change sub-TLV MUST be included for each FEC.

   d.  A FEC splice operation (an operation where one FEC ends and
       another FEC starts, MUST be performed by including a pop type FEC
       stack change sub-TLV followed by a PUSH type FEC stack change
       sub-TLV.

   e.  A Downstream Detailed Mapping TLV containing only one FEC stack
       change sub-TLV with pop operation is equivalent to IS_EGRESS
       (Return Code 3, Section 3.1) for the outermost FEC in the FEC
       stack.  The ingress router performing the LSP traceroute MUST
       treat such a case as an IS_EGRESS for the outermost FEC.

3.4.2.  Downstream Router and Interface

   The notion of "downstream router" and "downstream interface" should
   be explained.  Consider an LSR X.  If a packet that was originated
   with TTL n>1 arrived with outermost label L and TTL=1 at LSR X, X
   must be able to compute which LSRs could receive the packet if it was
   originated with TTL=n+1, over which interface the request would
   arrive and what label stack those LSRs would see.  (It is outside the
   scope of this document to specify how this computation is done.)  The
   set of these LSRs/interfaces consists of the downstream routers/

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   interfaces (and their corresponding labels) for X with respect to L.
   Each pair of downstream router and interface requires a separate
   Downstream Detailed Mapping to be added to the reply.

   The case where X is the LSR originating the echo request is a special
   case.  X needs to figure out what LSRs would receive the MPLS echo
   request for a given FEC Stack that X originates with TTL=1.

   The set of downstream routers at X may be alternative paths (see the
   discussion below on ECMP) or simultaneous paths (e.g., for MPLS
   multicast).  In the former case, the Multipath Information is used as
   a hint to the sender as to how it may influence the choice of these
   alternatives.



(page 41 continued on part 3)

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