4. Transporting Labeled Packets over PPP

   to receive packets that must go through the tunnel, but are too large
   to pass through the tunnel unfragmented, then:

      -  The LSR at the transmitting end of the tunnel MUST be able to
         determine the MTU of the tunnel as a whole.  It MAY do this by
         sending packets through the tunnel to the tunnel's receiving
         endpoint, and performing Path MTU Discovery with those packets.

      -  Any time the transmitting endpoint of the tunnel needs to send
         a packet into the tunnel, and that packet has the DF bit set,
         and it exceeds the tunnel MTU, the transmitting endpoint of the
         tunnel MUST send the ICMP Destination Unreachable message to
         the source, with code "Fragmentation Required and DF Set", and
         the Next-Hop MTU Field set as described above.

4. Transporting Labeled Packets over PPP

   The Point-to-Point Protocol (PPP) [6] provides a standard method for
   transporting multi-protocol datagrams over point-to-point links.  PPP
   defines an extensible Link Control Protocol, and proposes a family of
   Network Control Protocols for establishing and configuring different
   network-layer protocols.

   This section defines the Network Control Protocol for establishing
   and configuring label Switching over PPP.

4.1. Introduction

   PPP has three main components:

      1. A method for encapsulating multi-protocol datagrams.

      2. A Link Control Protocol (LCP) for establishing, configuring,
         and testing the data-link connection.

      3. A family of Network Control Protocols for establishing and
         configuring different network-layer protocols.

   In order to establish communications over a point-to-point link, each
   end of the PPP link must first send LCP packets to configure and test
   the data link.  After the link has been established and optional
   facilities have been negotiated as needed by the LCP, PPP must send
   "MPLS Control Protocol" packets to enable the transmission of labeled
   packets.  Once the "MPLS Control Protocol" has reached the Opened
   state, labeled packets can be sent over the link.






   The link will remain configured for communications until explicit LCP
   or MPLS Control Protocol packets close the link down, or until some
   external event occurs (an inactivity timer expires or network
   administrator intervention).

4.2. A PPP Network Control Protocol for MPLS

   The MPLS Control Protocol (MPLSCP) is responsible for enabling and
   disabling the use of label switching on a PPP link.  It uses the same
   packet exchange mechanism as the Link Control Protocol (LCP).  MPLSCP
   packets may not be exchanged until PPP has reached the Network-Layer
   Protocol phase.  MPLSCP packets received before this phase is reached
   should be silently discarded.

   The MPLS Control Protocol is exactly the same as the Link Control
   Protocol [6] with the following exceptions:

      1. Frame Modifications

         The packet may utilize any modifications to the basic frame
         format which have been negotiated during the Link Establishment
         phase.

      2. Data Link Layer Protocol Field

         Exactly one MPLSCP packet is encapsulated in the PPP
         Information field, where the PPP Protocol field indicates type
         hex 8281 (MPLS).

      3. Code field

         Only Codes 1 through 7 (Configure-Request, Configure-Ack,
         Configure-Nak, Configure-Reject, Terminate-Request, Terminate-
         Ack and Code-Reject) are used.  Other Codes should be treated
         as unrecognized and should result in Code-Rejects.

      4. Timeouts

         MPLSCP packets may not be exchanged until PPP has reached the
         Network-Layer Protocol phase.  An implementation should be
         prepared to wait for Authentication and Link Quality
         Determination to finish before timing out waiting for a
         Configure-Ack or other response.  It is suggested that an
         implementation give up only after user intervention or a
         configurable amount of time.









      5. Configuration Option Types

         None.

4.3. Sending Labeled Packets

   Before any labeled packets may be communicated, PPP must reach the
   Network-Layer Protocol phase, and the MPLS Control Protocol must
   reach the Opened state.

   Exactly one labeled packet is encapsulated in the PPP Information