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rfc:rfc2516

Network Working Group L. Mamakos Request for Comments: 2516 K. Lidl Category: Informational J. Evarts

                                             UUNET Technologies, Inc.
                                                            D. Carrel
                                                            D. Simone
                                               RedBack Networks, Inc.
                                                           R. Wheeler
                                                     RouterWare, Inc.
                                                        February 1999
        A Method for Transmitting PPP Over Ethernet (PPPoE)

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

 The Point-to-Point Protocol (PPP) [1] provides a standard method for
 transporting multi-protocol datagrams over point-to-point links.
 This document describes how to build PPP sessions and encapsulate PPP
 packets over Ethernet.

Applicability

 This specification is intended to provide the facilities which are
 defined for PPP, such as the Link Control Protocol, Network-layer
 Control Protocols, authentication, and more.  These capabilities
 require a point-to-point relationship between the peers, and are not
 designed for the multi-point relationships which are available in
 Ethernet and other multi-access environments.
 This specification can be used by multiple hosts on a shared,
 Ethernet to open PPP sessions to multiple destinations via one or
 more bridging modems.  It is intended to be used with broadband
 remote access technologies that provide a bridged Ethernet topology,
 when access providers wish to maintain the session abstraction
 associated with PPP.

Mamakos, et. al. Informational [Page 1] RFC 2516 Transmitting PPP Over Ethernet February 1999

 This document describes the PPP Over Ethernet encapsulation that is
 being deployed by RedBack Networks, RouterWare, UUNET and others.

1. Introduction

 Modern access technologies are faced with several conflicting goals.
 It is desirable to connect multiple hosts at a remote site through
 the same customer premise access device.  It is also a goal to
 provide access control and billing functionality in a manner similar
 to dial-up services using PPP.  In many access technologies, the most
 cost effective method to attach multiple hosts to the customer
 premise access device, is via Ethernet.  In addition, it is desirable
 to keep the cost of this device as low as possible while requiring
 little or no configuration.
 PPP over Ethernet (PPPoE) provides the ability to connect a network
 of hosts over a simple bridging access device to a remote Access
 Concentrator.  With this model, each host utilizes it's own PPP stack
 and the user is presented with a familiar user interface.  Access
 control, billing and type of service can be done on a per-user,
 rather than a per-site, basis.
 To provide a point-to-point connection over Ethernet, each PPP
 session must learn the Ethernet address of the remote peer, as well
 as establish a unique session identifier.  PPPoE includes a discovery
 protocol that provides this.

2. Conventions

 The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
 SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
 document, are to be interpreted as described in [2].

3. Protocol Overview

 PPPoE has two distinct stages.  There is a Discovery stage and a PPP
 Session stage.  When a Host wishes to initiate a PPPoE session, it
 must first perform Discovery to identify the Ethernet MAC address of
 the peer and establish a PPPoE SESSION_ID.  While PPP defines a
 peer-to-peer relationship, Discovery is inherently a client-server
 relationship.  In the Discovery process, a Host (the client)
 discovers an Access Concentrator (the server).  Based on the network
 topology, there may be more than one Access Concentrator that the
 Host can communicate with.  The Discovery stage allows the Host to
 discover all Access Concentrators and then select one.  When
 Discovery completes successfully, both the Host and the selected
 Access Concentrator have the information they will use to build their
 point-to-point connection over Ethernet.

Mamakos, et. al. Informational [Page 2] RFC 2516 Transmitting PPP Over Ethernet February 1999

 The Discovery stage remains stateless until a PPP session is
 established.  Once a PPP session is established, both the Host and
 the Access Concentrator MUST allocate the resources for a PPP virtual
 interface.

4. Payloads

 The following packet formats are defined here.  The payload contents
 will be defined in the Discovery and PPP sections.
 An Ethernet frame is as follows:
                                     1
                 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                |       DESTINATION_ADDR        |
                |          (6 octets)           |
                |                               |
                +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                |         SOURCE_ADDR           |
                |          (6 octets)           |
                |                               |
                +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                |    ETHER_TYPE  (2 octets)     |
                +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                ~                               ~
                ~           payload             ~
                ~                               ~
                +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                |           CHECKSUM            |
                +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The DESTINATION_ADDR field contains either a unicast Ethernet
 destination address, or the Ethernet broadcast address (0xffffffff).
 For Discovery packets, the value is either a unicast or broadcast
 address as defined in the Discovery section.  For PPP session
 traffic, this field MUST contain the peer's unicast address as
 determined from the Discovery stage.
 The SOURCE_ADDR field MUST contains the Ethernet MAC address of the
 source device.
 The ETHER_TYPE is set to either 0x8863 (Discovery Stage) or 0x8864
 (PPP Session Stage).

Mamakos, et. al. Informational [Page 3] RFC 2516 Transmitting PPP Over Ethernet February 1999

 The Ethernet payload for PPPoE is as follows:
                      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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  VER  | TYPE  |      CODE     |          SESSION_ID           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            LENGTH             |           payload             ~
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The VER field is four bits and MUST be set to 0x1 for this version of
 the PPPoE specification.
 The TYPE field is four bits and MUST be set to 0x1 for this version
 of the PPPoE specification.
 The CODE field is eight bits and is defined below for the Discovery
 and PPP Session stages.
 The SESSION_ID field is sixteen bits.  It is an unsigned value in
 network byte order.  It's value is defined below for Discovery
 packets.  The value is fixed for a given PPP session and, in fact,
 defines a PPP session along with the Ethernet SOURCE_ADDR and
 DESTINATION_ADDR.  A value of 0xffff is reserved for future use and
 MUST NOT be used
 The LENGTH field is sixteen bits.  The value, in network byte order,
 indicates the length of the PPPoE payload.  It does not include the
 length of the Ethernet or PPPoE headers.

5. Discovery Stage

 There are four steps to the Discovery stage.  When it completes, both
 peers know the PPPoE SESSION_ID and the peer's Ethernet address,
 which together define the PPPoE session uniquely.  The steps consist
 of the Host broadcasting an Initiation packet, one or more Access
 Concentrators sending Offer packets, the Host sending a unicast
 Session Request packet and the selected Access Concentrator sending a
 Confirmation packet.  When the Host receives the Confirmation packet,
 it may proceed to the PPP Session Stage.  When the Access
 Concentrator sends the Confirmation packet, it may proceed to the PPP
 Session Stage.
 All Discovery Ethernet frames have the ETHER_TYPE field set to the
 value 0x8863.

Mamakos, et. al. Informational [Page 4] RFC 2516 Transmitting PPP Over Ethernet February 1999

 The PPPoE payload contains zero or more TAGs.  A TAG is a TLV (type-
 length-value) construct and is defined as follows:
                      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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |          TAG_TYPE             |        TAG_LENGTH             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |          TAG_VALUE ...                                        ~
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 TAG_TYPE is a sixteen bit field in network byte order.  Appendix A
 contains a list of all TAG_TYPEs and their TAG_VALUEs.
 TAG_LENGTH is a sixteen bit field.  It is an unsigned number in
 network byte order, indicating the length in octets of the TAG_VALUE.
 If a discovery packet is received with a TAG of unknown TAG_TYPE, the
 TAG MUST be ignored unless otherwise specified in this document.
 This provides for backwards compatibility if/when new TAGs are added.
 If new mandatory TAGs are added, the version number will be
 incremented.
 Some example Discovery packets are shown in Appendix B.

5.1 The PPPoE Active Discovery Initiation (PADI) packet

 The Host sends the PADI packet with the DESTINATION_ADDR set to the
 broadcast address.  The CODE field is set to 0x09 and the SESSION_ID
 MUST be set to 0x0000.
 The PADI packet MUST contain exactly one TAG of TAG_TYPE Service-
 Name, indicating the service the Host is requesting, and any number
 of other TAG types.  An entire PADI packet (including the PPPoE
 header) MUST NOT exceed 1484 octets so as to leave sufficient room
 for a relay agent to add a Relay-Session-Id TAG.

5.2 The PPPoE Active Discovery Offer (PADO) packet

 When the Access Concentrator receives a PADI that it can serve, it
 replies by sending a PADO packet.  The DESTINATION_ADDR is the
 unicast address of the Host that sent the PADI.  The CODE field is
 set to 0x07 and the SESSION_ID MUST be set to 0x0000.

Mamakos, et. al. Informational [Page 5] RFC 2516 Transmitting PPP Over Ethernet February 1999

 The PADO packet MUST contain one AC-Name TAG containing the Access
 Concentrator's name, a Service-Name TAG identical to the one in the
 PADI, and any number of other Service-Name TAGs indicating other
 services that the Access Concentrator offers.  If the Access
 Concentrator can not serve the PADI it MUST NOT respond with a PADO.

5.3 The PPPoE Active Discovery Request (PADR) packet

 Since the PADI was broadcast, the Host may receive more than one
 PADO.  The Host looks through the PADO packets it receives and
 chooses one.  The choice can be based on the AC-Name or the Services
 offered.  The Host then sends one PADR packet to the Access
 Concentrator that it has chosen.  The DESTINATION_ADDR field is set
 to the unicast Ethernet address of the Access Concentrator that sent
 the PADO.  The CODE field is set to 0x19 and the SESSION_ID MUST be
 set to 0x0000.
 The PADR packet MUST contain exactly one TAG of TAG_TYPE Service-
 Name, indicating the service the Host is requesting, and any number
 of other TAG types.

5.4 The PPPoE Active Discovery Session-confirmation (PADS) packet

 When the Access Concentrator receives a PADR packet, it prepares to
 begin a PPP session.  It generates a unique SESSION_ID for the PPPoE
 session and replies to the Host with a PADS packet.  The
 DESTINATION_ADDR field is the unicast Ethernet address of the Host
 that sent the PADR.  The CODE field is set to 0x65 and the SESSION_ID
 MUST be set to the unique value generated for this PPPoE session.
 The PADS packet contains exactly one TAG of TAG_TYPE Service-Name,
 indicating the service under which Access Concentrator has accepted
 the PPPoE session, and any number of other TAG types.
 If the Access Concentrator does not like the Service-Name in the
 PADR, then it MUST reply with a PADS containing a TAG of TAG_TYPE
 Service-Name-Error (and any number of other TAG types).  In this case
 the SESSION_ID MUST be set to 0x0000.

5.5 The PPPoE Active Discovery Terminate (PADT) packet

 This packet may be sent anytime after a session is established to
 indicate that a PPPoE session has been terminated.  It may be sent by
 either the Host or the Access Concentrator.  The DESTINATION_ADDR
 field is a unicast Ethernet address, the CODE field is set to 0xa7
 and the SESSION_ID MUST be set to indicate which session is to be
 terminated.  No TAGs are required.

Mamakos, et. al. Informational [Page 6] RFC 2516 Transmitting PPP Over Ethernet February 1999

 When a PADT is received, no further PPP traffic is allowed to be sent
 using that session.  Even normal PPP termination packets MUST NOT be
 sent after sending or receiving a PADT.  A PPP peer SHOULD use the
 PPP protocol itself to bring down a PPPoE session, but the PADT MAY
 be used when PPP can not be used.

6. PPP Session Stage

 Once the PPPoE session begins, PPP data is sent as in any other PPP
 encapsulation.  All Ethernet packets are unicast.  The ETHER_TYPE
 field is set to 0x8864.  The PPPoE CODE MUST be set to 0x00.  The
 SESSION_ID MUST NOT change for that PPPoE session and MUST be the
 value assigned in the Discovery stage.  The PPPoE payload contains a
 PPP frame.  The frame begins with the PPP Protocol-ID.
 An example packet is shown in Appendix B.

7. LCP Considerations

 The Magic Number LCP configuration option is RECOMMENDED, and the
 Protocol Field Compression (PFC) option is NOT RECOMMENDED.  An
 implementation MUST NOT request any of the following options, and
 MUST reject a request for such an option:
    Field Check Sequence (FCS) Alternatives,
    Address-and-Control-Field-Compression (ACFC),
    Asynchronous-Control-Character-Map (ACCM)
 The Maximum-Receive-Unit (MRU) option MUST NOT be negotiated to a
 larger size than 1492.  Since Ethernet has a maximum payload size of
 1500 octets, the PPPoE header is 6 octets and the PPP Protocol ID is
 2 octets, the PPP MTU MUST NOT be greater than 1492.
 It is RECOMMENDED that the Access Concentrator ocassionally send
 Echo-Request packets to the Host to determine the state of the
 session.  Otherwise, if the Host terminates a session without sending
 a Terminate-Request packet, the Access Concentrator will not be able
 to determine that the session has gone away.
 When LCP terminates, the Host and Access concentrator MUST stop using
 that PPPoE session.  If the Host wishes to start another PPP session,
 it MUST return to the PPPoE Discovery stage.

Mamakos, et. al. Informational [Page 7] RFC 2516 Transmitting PPP Over Ethernet February 1999

8. Other Considerations

 When a host does not receive a PADO packet within a specified amount
 of time, it SHOULD resend it's PADI packet and double the waiting
 period. This is repeated as many times as desired.  If the Host is
 waiting to receive a PADS packet, a similar timeout mechanism SHOULD
 be used, with the Host re-sending the PADR.  After a specified number
 of retries, the Host SHOULD then resend a PADI packet.
 The ETHER_TYPEs used in this document (0x8863 and 0x8864) have been
 assigned by the IEEE for use by PPP Over Ethernet (PPPoE).  Use of
 these values and the PPPoE VER (version) field uniquely identify this
 protocol.
 UTF-8 [5] is used throughout this document instead of ASCII.  UTF-8
 supports the entire ASCII character set while allowing for
 international character sets as well.  See [5] for more details.

9. Security Considerations

 To help protect against Denial of Service (DOS) attacks, the Access
 Concentrator can employ the AC-Cookie TAG.  The Access Concentrator
 SHOULD be able to uniquely regenerate the TAG_VALUE based on the PADR
 SOURCE_ADDR.  Using this, the Access Concentrator can ensure that the
 PADI SOURCE_ADDR is indeed reachable and can then limit concurrent
 sessions for that address.  What algorithm to use is not defined and
 left as an implementation detail.  An example is HMAC [3] over the
 Host MAC address using a key known only to the Access > Concentrator.
 While the AC-Cookie is useful against some DOS attacks, it can not
 protect against all DOS attacks and an Access Concentrator MAY employ
 other means to protect resources.
 While the AC-Cookie is useful against some DOS attacks, it can not
 protect against all DOS attacks and an Access Concentrator MAY employ
 other means to protect resources.
 Many Access Concentrators will not wish to offer information
 regarding what services they offer to an unauthenticated entity.  In
 that case the Access Concentrator should employ one of two policies.
 It SHOULD never refuse a request based on the Service-Name TAG, and
 always return the TAG_VALUE that was sent to it.  Or it SHOULD only
 accept requests with a Service-Name TAG with a zero TAG_LENGTH
 (indicating any service).  The former solution is RECOMMENDED.

10. Acknowledgments

 This document is based on concepts discussed in several forums,
 including the ADSL forum.

Mamakos, et. al. Informational [Page 8] RFC 2516 Transmitting PPP Over Ethernet February 1999

 Copious amounts of text have been stolen from RFC 1661, RFC 1662 and
 RFC 2364.

11. References

 [1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD 51,
     RFC 1661, July 1994
 [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
     Levels", BCP 14, RFC 2119, March 1997.
 [3] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing
     for Message Authentication", RFC 2104, February 1998.
 [4] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC 1700,
     October 1994.  See also: http://www.iana.org/numbers.html
 [5] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
     2279, January 1998.

Mamakos, et. al. Informational [Page 9] RFC 2516 Transmitting PPP Over Ethernet February 1999

Appendix A

 TAG_TYPES and TAG_VALUES
 0x0000 End-Of-List
    This TAG indicates that there are no further TAGs in the list. The
    TAG_LENGTH of this TAG MUST always be zero.  Use of this TAG is
    not required, but remains for backwards compatibility.
 0x0101 Service-Name
    This TAG indicates that a service name follows.  The TAG_VALUE is
    an UTF-8 string that is NOT NULL terminated. When the TAG_LENGTH
    is zero this TAG is used to indicate that any service is
    acceptable.  Examples of the use of the Service-Name TAG are to
    indicate an ISP name or a class or quality of service.
 0x0102 AC-Name
    This TAG indicates that a string follows which uniquely identifies
    this particular Access Concentrator unit from all others. It may
    be a combination of trademark, model, and serial id information,
    or simply an UTF-8 rendition of the MAC address of the box.  It is
    not NULL terminated.
 0x0103 Host-Uniq
    This TAG is used by a Host to uniquely associate an Access
    Concentrator response (PADO or PADS) to a particular Host request
    (PADI or PADR).  The TAG_VALUE is binary data of any value and
    length that the Host chooses.  It is not interpreted by the Access
    Concentrator.  The Host MAY include a Host-Uniq TAG in a PADI or
    PADR.  If the Access Concentrator receives this TAG, it MUST
    include the TAG unmodified in the associated PADO or PADS
    response.
 0x0104 AC-Cookie
    This TAG is used by the Access Concentrator to aid in protecting
    against denial of service attacks (see the Security Considerations
    section for an explanation of how this works).  The Access
    Concentrator MAY include this TAG in a PADO packet.  If a Host
    receives this TAG, it MUST return the TAG unmodified in the
    following PADR.  The TAG_VALUE is binary data of any value and
    length and is not interpreted by the Host.

Mamakos, et. al. Informational [Page 10] RFC 2516 Transmitting PPP Over Ethernet February 1999

 0x0105 Vendor-Specific
    This TAG is used to pass vendor proprietary information.  The
    first four octets of the TAG_VALUE contain the vendor id and the
    remainder is unspecified.  The high-order octet of the vendor id
    is 0 and the low-order 3 octets are the SMI Network Management
    Private Enterprise Code of the Vendor in network byte order, as
    defined in the Assigned Numbers RFC [4].
    Use of this TAG is NOT RECOMMENDED.  To ensure inter-operability,
    an implementation MAY silently ignore a Vendor-Specific TAG.
 0x0110 Relay-Session-Id
    This TAG MAY be added to any discovery packet by an intermediate
    agent that is relaying traffic.  The TAG_VALUE is opaque to both
    the Host and the Access Concentrator.  If either the Host or
    Access Concentrator receives this TAG they MUST include it
    unmodified in any discovery packet they send as a response.  All
    PADI packets MUST guarantee sufficient room for the addition of a
    Relay-Session-Id TAG with a TAG_VALUE length of 12 octets.
    A Relay-Session-Id TAG MUST NOT be added if the discovery packet
    already contains one.  In that case the intermediate agent SHOULD
    use the existing Relay-Session-Id TAG.  If it can not use the
    existing TAG or there is insufficient room to add a Relay-
    Session-Id TAG, then it SHOULD return a Generic-Error TAG to the
    sender.
 0x0201 Service-Name-Error
    This TAG (typically with a zero-length data section) indicates
    that for one reason or another, the requested Service-Name request
    could not be honored.
    If there is data, and the first octet of the data is nonzero, then
    it MUST be a printable UTF-8 string which explains why the request
    was denied.  This string MAY NOT be NULL terminated.
 0x0202 AC-System-Error
    This TAG indicates that the Access Concentrator experienced some
    error in performing the Host request.  (For example insufficient
    resources to create a virtual circuit.)  It MAY be included in
    PADS packets.

Mamakos, et. al. Informational [Page 11] RFC 2516 Transmitting PPP Over Ethernet February 1999

    If there is data, and the first octet of the data is nonzero, then
    it MUST be a printable UTF-8 string which explains the nature of
    the error.  This string MAY NOT be NULL terminated.
 0x0203 Generic-Error
    This TAG indicates an error.  It can be added to PADO, PADR or
    PADS packets when an unrecoverable error occurs and no other error
    TAG is appropriate.  If there is data then it MUST be an UTF-8
    string which explains the nature of the error.  This string MUST
    NOT be NULL terminated.

Mamakos, et. al. Informational [Page 12] RFC 2516 Transmitting PPP Over Ethernet February 1999

Appendix B

 The following are some example packets:
 A PADI packet:
                         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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         0xffffffff                            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |           0xffff              |        Host_mac_addr          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    Host_mac_addr (cont)                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    ETHER_TYPE = 0x8863        | v = 1 | t = 1 |  CODE = 0x09  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     SESSION_ID = 0x0000       |      LENGTH = 0x0004          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      TAG_TYPE = 0x0101        |    TAG_LENGTH = 0x0000        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Mamakos, et. al. Informational [Page 13] RFC 2516 Transmitting PPP Over Ethernet February 1999

 A PADO packet:
                         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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Host_mac_addr                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      Host_mac_addr (cont)     | Access_Concentrator_mac_addr  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             Access_Concentrator_mac_addr (cont)               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    ETHER_TYPE = 0x8863        | v = 1 | t = 1 |  CODE = 0x07  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     SESSION_ID = 0x0000       |      LENGTH = 0x0020          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      TAG_TYPE = 0x0101        |    TAG_LENGTH = 0x0000        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      TAG_TYPE = 0x0102        |    TAG_LENGTH = 0x0018        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     0x47      |     0x6f      |     0x20      |     0x52      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     0x65      |     0x64      |     0x42      |     0x61      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     0x63      |     0x6b      |     0x20      |     0x2d      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     0x20      |     0x65      |     0x73      |     0x68      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     0x73      |     0x68      |     0x65      |     0x73      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     0x68      |     0x6f      |     0x6f      |     0x74      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Mamakos, et. al. Informational [Page 14] RFC 2516 Transmitting PPP Over Ethernet February 1999

 A PPP LCP packet:  The PPP protocol value is shown (0xc021) but the
 PPP payload is left to the reader.  This is a packet from the Host to
 the Access Concentrator.
                         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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                  Access_Concentrator_mac_addr                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Access_Concentrator_mac_addr(c)|        Host_mac_addr          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Host_mac_addr (cont)                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    ETHER_TYPE = 0x8864        | v = 1 | t = 1 |  CODE = 0x00  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     SESSION_ID = 0x1234       |      LENGTH = 0x????          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    PPP PROTOCOL = 0xc021      |        PPP payload            ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Authors' Addresses

 Louis Mamakos
 UUNET Technologies, Inc.
 3060 Williams Drive
 Fairfax, VA  22031-4648
 United States of America
 EMail: louie@uu.net
 Kurt Lidl
 UUNET Technologies, Inc.
 3060 Williams Drive
 Fairfax, VA  22031-4648
 United States of America
 EMail: lidl@uu.net
 Jeff Evarts
 UUNET Technologies, Inc.
 3060 Williams Drive
 Fairfax, VA  22031-4648
 United States of America
 EMail: jde@uu.net

Mamakos, et. al. Informational [Page 15] RFC 2516 Transmitting PPP Over Ethernet February 1999

 David Carrel
 RedBack Networks, Inc.
 1389 Moffett Park Drive
 Sunnyvale, CA  94089-1134
 United States of America
 EMail: carrel@RedBack.net
 Dan Simone
 RedBack Networks, Inc.
 1389 Moffett Park Drive
 Sunnyvale, CA  94089-1134
 United States of America
 EMail:dan@RedBack.net
 Ross Wheeler
 RouterWare, Inc.
 3961 MacArthur Blvd., Suite 212
 Newport Beach, CA  92660
 United States of America
 EMail: ross@routerware.com

Mamakos, et. al. Informational [Page 16] RFC 2516 Transmitting PPP Over Ethernet February 1999

Full Copyright Statement

 Copyright (C) The Internet Society (1999).  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
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Mamakos, et. al. Informational [Page 17]

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