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Network Working Group                                       Luca Martini
Internet Draft                                           Nasser El-Aawar
Expiration Date: October 2002               Level 3 Communications, LLC.

Giles Heron                                                   Toby Smith
PacketExchange Ltd.                                Laurel Networks, Inc.

Alex Hamilton                                            Andrew G. Malis
Daniel Tappan                                      Vivace Networks, Inc.
Eric C. Rosen
Cisco Systems, Inc.
                                                              April 2002


Encapsulation Methods for Transport of PPP/HDLC Frames Over IP and MPLS Networks


                draft-martini-ppp-hdlc-encap-mpls-00.txt

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
   groups may also distribute working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

Abstract

   This document describes methods for encapsulating the Protocol Data
   Units (PDUs) of layer 2 protocols such as PPP, or HDLC for transport
   across an MPLS or IP network.







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Table of Contents

    1      Specification of Requirements  ..........................   2
    2      Introduction  ...........................................   2
    3      General encapsulation method  ...........................   3
    3.1    The Control Word  .......................................   3
    3.1.1  Setting the sequence number  ............................   5
    3.1.2  Processing the sequence number  .........................   5
    3.2    MTU Requirements  .......................................   6
    4      Protocol-Specific Details  ..............................   6
    4.1    HDLC  ...................................................   6
    4.2    PPP  ....................................................   6
    5      Using an MPLS Label as the Demultiplexer Field  .........   7
    5.1    MPLS Shim EXP Bit Values  ...............................   7
    5.2    MPLS Shim S Bit Value  ..................................   7
    5.3    MPLS Shim TTL Values  ...................................   8
    6      Security Considerations  ................................   8
    7      Intellectual Property Disclaimer  .......................   8
    8      References  .............................................   8
    9      Author Information  .....................................   9





1. Specification of Requirements

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


2. Introduction

   In an MPLS or IP network, it is possible to use control protocols
   such as those specified in [1] to set up "emulated virtual circuits"
   that carry the the Protocol Data Units of layer 2 protocols across
   the network.  A number of these emulated virtual circuits may be
   carried in a single tunnel.  This requires of course that the layer 2
   PDUs be encapsulated.  We can distinguish three layers of this
   encapsulation:

     - the "tunnel header", which contains the information needed to
       transport the PDU across the IP or MPLS network; this is header
       belongs to the tunneling protocol, e.g., MPLS, GRE, L2TP.




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     - the "demultiplexer field", which is used to distinguish
       individual emulated virtual circuits within a single tunnel; this
       field must be understood by the tunneling protocol as well; it
       may be, e.g., an MPLS label or a GRE key field.

     - the "emulated VC encapsulation", which contains the information
       about the enclosed layer 2 PDU which is necessary in order to
       properly emulate the corresponding layer 2 protocol.

   This document specifies the emulated VC encapsulation for PPP, and
   HDLC.  Although different layer 2 protocols require different
   information to be carried in this encapsulation, an attempt has been
   made to make the encapsulation as common as possible for all layer 2
   protocols. Other layer 2 protocols are described in separate
   documents. [4] [5] [6]

   This document also specifies the way in which the demultiplexer field
   is added to the emulated VC encapsulation when an MPLS label is used
   as the demultiplexer field.

   QoS related issues are not discussed in this draft

   For the purpose of this document R1 will be defined as the ingress
   router, and R2 as the egress router. A layer 2 PDU will be received
   at R1, encapsulated at R1, transported, decapsulated at R2, and
   transmitted out of R2.


3. General encapsulation method

   In most cases, it is not necessary to transport the layer 2
   encapsulation across the  network; rather,  the layer 2 header can be
   stripped at R1, and reproduced at R2.  This is done using information
   carried in the control word (see below), as well as information that
   may already have been signaled from R1 to R2.


3.1. The Control Word

   There are three requirements that may need to be satisfied when
   transporting layer 2 protocols over an IP or MPLS backbone:

        -i. Sequentiality may need to be preserved.
       -ii. Small packets may need to be padded in order to be
            transmitted on a medium where the minimum transport unit is
            larger than the actual packet size.





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      -iii. Control bits carried in the header of the layer 2 frame may
            need to be transported.

   The control word defined here addresses all three of these
   requirements. For some protocols this word is REQUIRED, and for
   others OPTIONAL.  For protocols where the control word is OPTIONAL
   implementations MUST support sending no control word, and MAY support
   sending a control word.

   In all cases the egress router must be aware of whether the ingress
   router will send a control word over a specific virtual circuit.
   This may be achieved by configuration of the routers, or by
   signaling, for example as defined in [1].

   The control word is defined 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Rsvd  | Flags |0 0|   Length  |     Sequence Number           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   In the above diagram the first 4 bits are reserved for future use.
   They MUST be set to 0 when transmitting, and MUST be ignored upon
   receipt.

   The next 4 bits provide space for carrying protocol specific flags.
   These are defined in the protocol-specific details below.

   The next 2 bits MUST be set to 0 when transmitting.

   The next 6 bits provide a length field, which is used as follows: If
   the packet's length (defined as the length of the layer 2 payload
   plus the length of the control word) is less than 64 bytes, the
   length field MUST be set to the packet's length. Otherwise the length
   field MUST be set to zero. The value of the length field, if non-
   zero, can be used to remove any padding. When the packet reaches the
   service provider's egress router, it may be desirable to remove the
   padding before forwarding the packet.

   The next 16 bits provide a sequence number that can be used to
   guarantee ordered packet delivery. The processing of the sequence
   number field is OPTIONAL.

   The sequence number space is a 16 bit, unsigned circular space. The
   sequence number value 0 is used to indicate an unsequenced packet.





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3.1.1. Setting the sequence number

   For a given emulated VC, and a pair of routers R1 and R2, if R1
   supports packet sequencing then the following procedures should be
   used:

     - the initial packet transmitted on the emulated VC MUST use
       sequence number 1
     - subsequent packets MUST increment the sequence number by one for
       each packet
     - when the transmit sequence number reaches the maximum 16 bit
       value (65535) the sequence number MUST wrap to 1

   If the transmitting router R1 does not support sequence number
   processing, then the sequence number field in the control word MUST
   be set to 0.


3.1.2. Processing the sequence number

   If a router R2 supports receive sequence number processing, then the
   following procedures should be used:

   When an emulated VC is initially set up, the "expected sequence
   number" associated with it MUST be initialized to 1.

   When a packet is received on that emulated VC, the sequence number
   should be processed as follows:

     - if the sequence number on the packet is 0, then the packet passes
       the sequence number check

     - otherwise if the packet sequence number >= the expected sequence
       number and the packet sequence number - the expected sequence
       number < 32768, then the packet is in order.

     - otherwise if the packet sequence number < the expected sequence
       number and the expected sequence number - the packet sequence
       number >= 32768, then the packet is in order.

     - otherwise the packet is out of order.

   If a packet passes the sequence number check, or is in order then, it
   can be delivered immediately. If the packet is in order, then the
   expected sequence number should be set using the algorithm:

   expected_sequence_number := packet_sequence_number + 1 mod 2**16
   if (expected_sequence_number = 0) then expected_sequence_number := 1;



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   Packets which are received out of order MAY be dropped or reordered
   at the discretion of the receiver.

   If a router R2 does not support receive sequence number processing,
   then the sequence number field MAY be ignored.


3.2. MTU Requirements

   The network MUST be configured with an MTU that is sufficient to
   transport the largest encapsulation frames.  If MPLS is used as the
   tunneling protocol, for example, this is likely to be 12 or more
   bytes greater than the largest frame size.  Other tunneling protocols
   may have longer headers and require larger MTUs.  If the ingress
   router determines that an encapsulated layer 2 PDU exceeds the MTU of
   the tunnel through which it must be sent, the PDU MUST be dropped. If
   an egress router receives an encapsulated layer 2 PDU whose payload
   length (i.e., the length of the PDU itself without any of the
   encapsulation headers), exceeds the MTU of the destination layer 2
   interface, the PDU MUST be dropped.


4. Protocol-Specific Details

4.1. HDLC

   HDLC mode provides port to port transport of HDLC encapsulated
   traffic. The HDLC PDU is transported in its entirety, including the
   HDLC address, control and protocol fields, but excluding HDLC flags
   and the FCS. Bit/Byte stuffing is undone. The control word is
   OPTIONAL. If the control word is used then the flag bits in the
   control word are not used, and MUST be set to 0 when transmitting,
   and MUST be ignored upon receipt.

   The HDLC mode is suitable for port to port transport of Frame Relay
   UNI or NNI traffic.  It must be noted, however, that this mode is
   transparent to the FECN, BECN and DE bits.


4.2. PPP

   PPP mode provides point to point transport of PPP encapsulated
   traffic, as specified in [3]. The PPP PDU is transported in its
   entirety, including the protocol field (whether compressed using PFC
   or not), but excluding any media-specific framing information, such
   as HDLC address and control fields or FCS.  Since media-specific
   framing is not carried the following options will not operate
   correctly if the PPP peers attempt to negotiate them:



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     - Frame Check Sequence (FCS) Alternatives
     - Address-and-Control-Field-Compression (ACFC)
     - Asynchronous-Control-Character-Map (ACCM)

   Note also that VC LSP Interface MTU negotiation as specified in [1]
   is not affected by PPP MRU advertisement.  Thus if a PPP peer sends a
   PDU with a length in excess of that negotiated for the VC LSP that
   PDU will be discarded by the ingress router.

   The control word is OPTIONAL. If the control word is used then the
   flag bits in the control word are not used, and MUST be set to 0 when
   transmitting, and MUST be ignored upon receipt.


5. Using an MPLS Label as the Demultiplexer Field

   To use an MPLS label as the demultiplexer field, a 32-bit label stack
   entry [2] is simply prepended to the emulated VC encapsulation, and
   hence will appear as the bottom label of an MPLS label stack.  This
   label may be called the "VC label".  The particular emulated VC
   identified by a particular label value must be agreed by the ingress
   and egress LSRs, either by signaling (e.g, via the methods of [1]) or
   by configuration. Other fields of the label stack entry are set as
   follows.


5.1. MPLS Shim EXP Bit Values

   If it is desired to carry Quality of Service information, the Quality
   of Service information SHOULD be represented in the EXP field of the
   VC label.  If more than one MPLS label is imposed by the ingress LSR,
   the EXP field of any labels higher in the stack SHOULD also carry the
   same value.


5.2. MPLS Shim S Bit Value

   The ingress LSR, R1, MUST set the S bit of the VC label to a value of
   1 to denote that the VC label is at the bottom of the stack.












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5.3. MPLS Shim TTL Values

   The ingress LSR, R1, SHOULD set the TTL field of the VC label to a
   value of 2.



6. Security Considerations

   This document specifies only encapsulations, and not the protocols
   used to carry the encapsulated packets across the network.  Each such
   protocol may have its own set of security issues, but those issues
   are not affected by the encapsulations specified herein.


7. Intellectual Property Disclaimer

   This document is being submitted for use in IETF standards
   discussions.


8. References

   [1] "Transport of Layer 2 Frames Over MPLS", draft-martini-
   l2circuit-trans-mpls-09.txt. ( work in progress )

   [2] "MPLS Label Stack Encoding", E. Rosen, Y. Rekhter, D. Tappan, G.
        Fedorkow, D. Farinacci, T. Li, A. Conta. RFC3032

   [3] "The Point-to-Point Protocol (PPP)", RFC 1661.

   [4] "Encapsulation Methods for Transport of ATM Cells/Frame Over IP
   and MPLS Networks", draft-martini-atm-encap-mpls-00.txt. ( work in
   progress )

   [5] "Encapsulation Methods for Transport of Ethernet Frames Over IP
   and MPLS Networks", draft-martini-ethernet-encap-mpls-00.txt. ( work
   in progress )

   [6] "Encapsulation Methods for Transport of Frame-Relay Over IP and
   MPLS Networks", draft-martini-frame-encap-mpls-00.txt. ( work in
   progress )









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9. Author Information


   Luca Martini
   Level 3 Communications, LLC.
   1025 Eldorado Blvd.
   Broomfield, CO, 80021
   e-mail: luca@level3.net


   Nasser El-Aawar
   Level 3 Communications, LLC.
   1025 Eldorado Blvd.
   Broomfield, CO, 80021
   e-mail: nna@level3.net


   Giles Heron
   PacketExchange Ltd.
   The Truman Brewery
   91 Brick Lane
   LONDON E1 6QL
   United Kingdom
   e-mail: giles@packetexchange.net


   Dan Tappan
   Cisco Systems, Inc.
   250 Apollo Drive
   Chelmsford, MA, 01824
   e-mail: tappan@cisco.com


   Alex Hamilton,
   Cisco Systems Inc.
   285 W. Tasman , MS-SJCI/3/4,
   San Jose, CA, 95134
   e-mail: tahamilt@cisco.com


   Eric Rosen
   Cisco Systems, Inc.
   250 Apollo Drive
   Chelmsford, MA, 01824
   e-mail: erosen@cisco.com






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Internet Draft  draft-martini-ppp-hdlc-encap-mpls-00.txt      April 2002



   Toby Smith
   Laurel Networks, Inc.
   Omega Corporate Center
   1300 Omega Drive
   Pittsburgh, PA 15205
   e-mail: tob@laurelnetworks.com


   Andrew G. Malis
   Vivace Networks, Inc.
   2730 Orchard Parkway
   San Jose, CA 95134
   e-mail: Andy.Malis@vivacenetworks.com





































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