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Versions: 00 01 02 03 04 05 06 RFC 4938

INTERNET-DRAFT                                                 B. Berry
Category: Informational                                      H. Holgate
Expires:  May 18, 2007                                Cisco Systems,Inc.
                                                          10 March 2004


                      PPP Over Ethernet (PPPoE) Extensions
                        for Credit Flow and Link Metrics

                         draft-bberry-pppoe-credit-06.txt

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Copyright Notice

    Copyright (C) The Internet Society (2006).

Abstract

    This document extends the Point-to-Point Over Ethernet (PPPoE)
    Protocol [2] with a credit-based flow control mechanism and
    Link Quality Metric report.  This optional extension should
    improve the performance of PPPoE over media with variable
    bandwidth and limited buffering, such as mobile radio links.


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Table of Contents

    1. Introduction..................................................3
    2. Payload.......................................................4
    3. Overview of Protocol Extensions...............................4
    4. Discovery Stage...............................................4
       4.1 PPPoE Active Discovery Request (PADR).....................4
       4.2 PPPoE Active Discovery Session-confirmation (PADS)........5
       4.3 PPPoE Active Discovery Session-Grant (PADG)...............5
       4.4 PPPoE Active Discovery Session-Credit Response (PADC).....6
       4.5 PPPoE Active Discovery Quality (PADQ).....................7
    5. PPP Session Stage.............................................8
    6. Credit Flow Considerations....................................8
    7. Other Considerations..........................................9
    8. IANA Considerations...........................................9
    9. Security Considerations......................................10
    10. Appendix A: Tag Values......................................10
    11. Appendix B: Example Message Formats.........................12
    12. Acknowledgements............................................19
    13. Normative References........................................19
    Authors' Addresses..............................................19
    Intellectual Property and Copyright Statements..................20





























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1. Introduction

    PPP over Ethernet (PPPoE) [2] is a protocol for establishing
    and encapsulating sessions between hosts and traffic
    aggregators (Access Concentrators) for PPP transport over real
    or emulated Ethernet.  PPPoE works well when both session
    endpoints have similar bandwidth, forwarding, and buffering
    capabilities that do not vary over time.  However, it is
    insufficient for applications with variable bandwidth and
    limited buffering, for example, mobile radio links.  This
    document addresses this problem by suggesting an extension to
    PPPoE to support credit-based session flow control and
    session-based link metric exchanges.

    The diagram below illustrates the problem that this extension
    is intended to solve, for the case of a radio link.  Here PPPoE
    sessions are used between access concentrators (routers) and
    radio transmission systems that are shown as radio neighbors.
    Each radio transmission system establishes point-to-point Radio
    Link Protocol (RLP) sessions with its neighbors and establishes
    a corresponding PPPoE session for each neighbor with the
    transmission system's associated access concentrator (router).
    The radio logically associates the PPPoE session with the
    corresponding RLP session.

     +--------+       +-------+       +-------+       +--------+
     | Access |       | Host  |       | Host  |       | Access |
     | Conc.  |=======| Radio |~~~~~~~| Radio |=======| Conc.  |
     +--------+       +-------+       +-------+       +--------+
              |       |       |       |       |       |
              |-PPPoE-|       |--RLP--|       |-PPPoE-|
              |                                       |
              |-------------PPP Session---------------|


    Figure 1: PPPoE Network


    The capabilities of the RF links between RLP neighbors may vary
    over time due to mobility and environmental conditions.  In many
    instances, the Host Radio has limited buffering capability to
    handle capacity changes in the RLP sessions.  To limit buffering
    in the Host Radio, the PPPoE credit flow control mechanism
    provides dynamic buffering feedback to the access concentrator.







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    In the diagram above, from the access concentrator's perspective,
    each PPPoE session between it and the Host Radio represent a
    connection to a remote routable peer.  For efficient routing,
    the local Host Radio uses the link metric mechanism to dynamically
    update the access concentrator route cost of the associated link.

    While the example shows an RF based application, the extensions
    are applicable to other media.


2. Payload

    The Ethernet payload version field retains its value of 0x01.  The
    extensions for credit flow control and link quality metrics are
    optional and backward compatible.


3. Overview of Protocol Extensions

    PPPoE has two distinct stages.  There is a Discovery Stage and a PPP
    Session Stage.  During the Discovery Stage, the Host can optionally
    request a flow controlled PPP Session Stage.  Once the Access
    Concentrator acknowledges the Host flow control request, all PPP
    Session Stage traffic must be flow controlled.


4. Discovery Stage

    The packet exchange of the Discovery Stage is unchanged by this
    specification.  The specifications of the Session Request (PADR) and
    the Session Confirmation (PADS) packets are extended to include the
    optional Credit Tag Type-Length-Value (TLV).

    In addition, the optional Credit Grant (PADG) packet, the Credit
    Response (PADC) packet and the Link Quality Metric (PADQ) packets
    are introduced.


4.1 PPPoE Active Discovery Request (PADR)

    The PADR packet is extended to optionally contain a single Credit
    Tag TLV, indicating that the Host requests credit flow control for
    this session.  The Credit Tag contains the Forward Credit
    Notification (FCN) and the Backward Credit Notification (BCN) to be
    applied to the PPP Session Stage.  The FCN provides the initial
    credits granted to the Access Concentrator by the Host.  The BCN
    value is set to 0.

    An example packet is shown in Appendix B.


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4.2 PPPoE Active Discovery Session-confirmation (PADS)

    The PADS packet is extended to optionally contain a single Credit
    Tag TLV, indicating the Forward Credit Notification (FCN) and the
    Backward Credit Notification (BCN) of the PPP Session Stage.

    If the PADR contained a Credit Tag, then the Access Concentrator
    PADS packet indicates support for credit flow control by including a
    Credit Tag.  The PADS Credit Tag FCN represents the number of
    credits being initially granted to the Host.  The Credit Tag BCN is
    an echo of the number of credits that the Host had granted to the
    Access Concentrator in the previous PADR packet.

    Exchange of the Credit Tag TLV in the PADR and PADS indicates that
    credit flow control is supported by both the Access Concentrator and
    the Host for the designated PPP Session Stage.  This is binding and
    must be followed for the entire duration of the PPP Session Stage.
    A session’s credit binding must be established prior to any other
    credit indications can be exchanged.

    The Access Concentrator PADS should only carry the Credit Tag in
    response to a Host PADR with Credits. If the Access Concentrator
    does not support credit flow, it should not include the Credit Tag
    in its PADS response.  The Host must terminate a credit based
    session that can not be supported by the Access Concentrator.
    Credit Tags transmitted outside an established credit based session
    must be ignored.

    An example packet is shown in Appendix B.


4.3 PPPoE Active Discovery Session-Grant (PADG)

    The PPPoE Active Discovery Session-Grant (PADG) is a new packet
    defined in this specification.  An Access Concentrator or Host MAY
    send a PADG at any time after the PADR/PADS exchange to grant
    incremental flow control credits.  The CODE field is set to 0x0A and
    the SESSION_ID must be set to the unique value generated for this
    PPPoE Session.

    The peer may then transmit data until the credits are exhausted.

    When the peer receives a PADG packet, it adds the incremental
    credits to its working credit count and responds with a PPPoE Active
    Discovery Session-Credit (PADC) packet indicating the accumulated
    credits.

    The PADG packet must contain a single Credit Tag TLV, indicating the
    Forward Credit Notification (FCN) and the Backward Credit
    Notification (BCN) of the PPP Session.

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    The Credit Tag FCN indicates the number of incremental credits
    being granted to the peer by the node.  A value between 1 and
    0xffff represent an incremental credit grant.  The peer must
    add these credits to its accumulated transmit credit count.
    A value of 0x0000 represents a NULL grant, meaning that there
    are no additional credits being granted.

    The Credit Tag BCN indicates the remaining absolute credits that
    have been granted by the peer to the node.

    Once a credit has been granted, it must be honored.  The largest
    number of outstanding credits at any time is 0xffff.


    The PADG packet must contain a single Sequence Number Tag TLV.  This
    tag is used to carry a unique 16-bit sequence number to uniquely
    identify each request.  The sequence number should be initialized to
    zero and incremented by one for each new PADG.  For re-transmitted
    PADGs, the same sequence number that was used in the previous packet
    transmission is repeated.

    An example packet is shown in Appendix B.


4.4 PPPoE Active Discovery Session-Credit Response (PADC)

    The PPPoE Active Discovery Session-Credit Response (PADC) is a new
    packet defined in this specification.  An Access Concentrator or
    Host must send a PADC in response to a PADG.  The CODE field is set
    to 0x0B and the SESSION_ID must be set to the unique value generated
    for this PPPoE session.

    The PADC packet must contain a single Credit Tag TLV, indicating the
    Forward Credit Notification (FCN) and the Backward Credit
    Notification (BCN) of the PPPoE session, and any number of other Tag
    types.

    The Credit Tag FCN represents the absolute credits remaining that
    have granted to the peer by the node.  The Credit Tag BCN represents
    the remaining absolute credits that have been granted to the node
    from the peer.

    The PADC packet must contain a single Sequence Number Tag.  The
    sequence number must be the sequence number associated with the
    PADG.

    An example packet is shown in Appendix B.




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4.5 PPPoE Active Discovery Quality (PADQ)

    The PPPoE Active Discovery Quality (PADQ) is a new packet defined in
    this specification.  An Access Concentrator or Host may send an
    optional PADQ at any time to query or report link quality metrics.

    When transmitting PPP streams over wireless links through radio
    modems, the quality of the RF link directly affects the throughput.
    The PPPoE Active Discovery Quality (PADQ) packet can be used by the
    radio modem to report RF link metrics. The CODE field is set to 0x0C
    and the SESSION_ID must be set to the unique value generated for
    this PPPoE session.

    The PADQ must carry a single Metric Tag TYPE, which contains the
    following fields:

        Receive only - is a bit that indicates if the link is bi-
        directional or receive only.  A value of -1- indicates that the
        link is receive-only.

        Maximum data rate - is the maximum theoretical data rate, in
        kilobits per second (kbps), that the Host link is capable of
        providing.  When metrics are reported, the maximum data rate
        must be reported.

        Current data rate - is the current data rate, in kilobits per
        second (kbps), achieved on the Host link.  If there is no
        distinction between maximum data rate and current data rate,
        current data rate should equal to maximum data rate.

        Latency - is the transmission delay that a packet encounters as
        it is transmitted over the Host link.  This is reported in
        absolute delay, milliseconds.  If latency can not be calculated,
        a value of 0 should be reported.

        Resources - is a percentage, 0-100, representing the amount of
        remaining or available resources, such as battery power.  If
        resources can not be calculated, a value of 100 should be
        reported.

        Relative Link Quality (RLQ) - is a non-dimensional number, 0-100,
        representing the relative link quality.  A value of 100
        represents a link of the highest quality.  If the RLQ can not be
        calculated, a value of 100 should be reported.

    The PPPoE Active Discovery Quality (PADQ) packet can be used to
    query link metrics by setting the PADQ Metric Tag Length to zero.

    An example packet is shown in Appendix B.


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5. PPP Session Stage

    This specification defines the optional use of TLV Tags in the PPP
    Session Stage  The first field following the PPP Session Stage
    LENGTH must be checked.  If the value is equal to the PPP Protocol
    identifier (0xc021), then normal packet (payload) processing occurs.
    When the field following the PPP Session Stage LENGTH is not the PPP
    Protocol identifier (0xc021), a TLV is assumed.  In this case, the
    Tag length is subtracted from the overall payload length.

    The Credit Tag is the only optional TLV permitted in the PPP Session
    Stage.  The Credit Tag TLV is used to support in-band flow control.

    A PPP Session Stage packet with Credits is shown in Appendix B.


6. Credit Flow Considerations

    For a given session, credit grants exchanged in the Discovery Stage,
    PADG-PADC, are referred to as out-of-band.  Credit grants exchanged
    in the PPP Session Stage are referred to as in-band.  Credit
    processing is only applied to the packets transmitted in the PPP
    Session Stage.

    Out-of-band credit management is handled by periodic exchange of the
    PPPoE Active Discovery Grant (PADG) and PPPoE Active Discovery Credit
    (PADC) packets.

    In-band credit management allows credits to be incrementally granted
    with each PPP Session Stage packet.  These in-band incremental credit
    grants are not explicitly unacknowledged.  They are however reflected
    in the in-band credit flow from the peer node.  This offers the
    greatest credit granting efficiency when traffic rates are high.

    Once agreed upon during the Discovery Stage, credit grants are
    required to transmit packets in the PPP Session Stage.  A node must
    grant credits to its peer, before the peer can transmit packets to
    the granting node.

    Credits are granted incrementally in the forward direction.  Locally
    a node manages the credits that it has granted to a peer as well as
    the credits that a peer has granted to it.

    Grants received from a peer are added to a local running credit
    counter.  The accumulated credits are decremented with each packet
    the node transmits to the peer.  When the running counter reaches
    zero, the node stops transmitting packets to the peer.  The values
    of the PADC are not simply an echo of the PADG.  They represent
    the current internal FCN/BCN values of that node.


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    To manage the credits that a node has granted, the node maintains a
    running counter.  With each PPP Session Stage packet received from
    the peer, the running counter is decremented.  When the running
    counter reaches zero, no additional packets are expected.  The node
    incrementally grants more credits to the peer to maintain packet
    flow.  Packets received when granted credits have been exhausted are
    discarded.

    The largest possible credit limit is 0x0ffff.  If an incremental
    credit grant causes the accumulated count to exceed this value, the
    max value is used.

    One unit of credit represents 64-bytes, so a grant of 4 credits
    translates to 256-bytes.


7. PADG and PADC Retransmission

    When a node does not receive a PADC packet in response to a PADG
    within a specified amount of time, it should transmit a new PADG
    packet with zero credits,  using the same sequence number and
    double the waiting period. A PADC response with the associated
    sequence number will indicate if the previously granted credits
    were accumulated or not.  If not, a PADG with credits, with
    an incremented sequence number, should be transmitted. This
    process should be repeated until granted credits are properly
    acknowledged or as many times as desired.

    When a node does not receive a PADQ metric packet within a specified
    amount of time, it should resend the PADQ query packet and double
    the waiting period. This can be repeated as many times as desired.


8. Other Considerations

    A node may autonomously generate PADQ metric packets.  The rate of
    autonomously generate PADQ metric packets may need to be throttled
    so not to overrun the peer.

    The sending and receiving of PPPoE control packets are independent
    of credit counts.  For example, a node must always be able to
    receive a PADG and send a PADC.

    During normal operation, nodes may disagree about the number of
    credits.  Operational credit mismatches would occur due to packets
    in transit on the wire.  Much larger credit mismatches can occur
    if there are transmission errors.  To correct these larger errors,
    the BCN fields of the PADG and PADC packets and in-band credit
    grants from a peer should be used by the receiveing node to set
    the credit values of its peer.

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9. IANA Considerations

    At the time of publication, the IANA considerations were being
    covered by draft-arberg-pppoe-iana, a work in progress.


10. Security Considerations

    This memo defines a mechanism for adding flow control to the
    existing PPP Over Ethernet (PPPoE) sessions.  These extensions
    are subsequent to the existing PPPoE security mechanisms as
    described in RFC 2516 [2].  It is required that the Service
    Tag and Session ID always be validated prior to processing
    credits.


11. Appendix A: Tag Values

    Feature Tag_Types and Tag_Values

    0x0106 Credits

    This tag contains the Forward Credit Notification (FCN) and the
    Backward Credit Notification (BCN). The Credit Tag TLV is
    OPTIONAL with the PADR, PADS and the PPPoE data payload packet
    (ETHER_TYPE=8864).

                         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 = 0x0106       |        Tag Length=0x04        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |              FCN              |              BCN              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
















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    0x0107 Metrics

    This tag is used to report the link quality and performance.  The
    Metrics Tag TLV contains the Receive Only indicator, Resource
    status, Latency, Relative Link Quality (RLQ), Current data rate
    and Maximum data rate.  The Metrics TLV is required by the PADQ
    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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       Tag Type = 0x0107       |        Tag Length=0x0A        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |           Reserved          |R|      RLQ      |    Resource   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Latency (MS)          |   Current Datarate (kbps)     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    Maximum Datarate (kbps)    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


    0x0108 Sequence Number

    This tag is used to carry a unique 16-bit sequence number in
    order to identify a specific request and the associated response.
    The sequence number should be initialized to zero and incremented
    by one for each new request.  For re-transmitted packets, the
    same sequence number that was used in the previous packet
    transmission is repeated.  The PADG and PADC packets require
    the Sequence Number Tag.

    For example, the sequence number sent in the PADG request is
    echoed in the PADC response.  This ties a specific PADC response
    to a specific PADG request.

                         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 = 0x0108       |        Tag Length=0x02        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       Sequence Number         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+








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11. Appendix B: Example Message Formats


    A PADR packet with OPTIONAL Credit Tag Type 0x0106:

                         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 = 0x8863        | v = 1 | t = 1 |  CODE = 0x19  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     SESSION_ID = 0x1234       |      LENGTH = 0x0C            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       Tag Type = 0x0101       |        Tag Length=0x00        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       Tag Type = 0x0106       |        Tag Length=0x04        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |              FCN              |              BCN              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



























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    A PADS packet with OPTIONAL Credit Tag Type 0x0106:

                         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 = 0x8863        | v = 1 | t = 1 |  CODE = 0x65  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     SESSION_ID = 0x1234       |      LENGTH = 0x0C            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       Tag Type = 0x0101       |        Tag Length=0x00        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       Tag Type = 0x0106       |        Tag Length=0x04        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |              FCN              |              BCN              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+






























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    A PADG packet with Credit Tag Type 0x0106:

                         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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Destination_mac_addr                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    Destination_mac_addr(c)    |       Source_mac_addr         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Source mac_addr (cont)                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    ETHER_TYPE = 0x8863        | v = 1 | t = 1 |  CODE = 0x0A  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     SESSION_ID = 0x1234       |      LENGTH = 0x0E            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       Tag Type = 0x0108       |        Tag Length=0x02        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       Sequence Number         |       Tag Type = 0x0106       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |        Tag Length=0x04        |               FCN             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |              BCN              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




























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    A PADC packet with Credit Tag Type 0x0106:

                         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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Destination_mac_addr                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    Destination_mac_addr(c)    |       Source_mac_addr         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Source mac_addr (cont)                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    ETHER_TYPE = 0x8863        | v = 1 | t = 1 |  CODE = 0x0B  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     SESSION_ID = 0x1234       |      LENGTH = 0x0E            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       Tag Type = 0x0108       |        Tag Length=0x02        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       Sequence Number         |       Tag Type = 0x0106       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |        Tag Length=0x04        |               FCN             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |              BCN              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




























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    A PADQ packet to query for the link metrics: This is indicated by
    the Metric Tag Length=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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                  Access_Concentrator_mac_addr                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Access_Concentrator_mac_addr(c)|        Host_mac_addr          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Host_mac_addr (cont)                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    ETHER_TYPE = 0x8863        | v = 1 | t = 1 |  CODE = 0x0C  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     SESSION_ID = 0x1234       |      LENGTH = 0x08            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       Tag Type = 0x0101       |        Tag Length=0x00        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       Tag Type = 0x0107       |        Tag Length=0x00        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+































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    A PADQ packet with Metric Tag Type 0x0107:

                         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 = 0x8863        | v = 1 | t = 1 |  CODE = 0x0C  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     SESSION_ID = 0x1234       |      LENGTH = 0x12            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       Tag Type = 0x0101       |        Tag Length=0x00        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       Tag Type = 0x0107       |        Tag Length=0x0A        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |           Reserved          |R|      RLQ      |    Resource   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Latency (MS)          |   Current Datarate (kbps)     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    Maximum Datarate (kbps)    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


























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    A PPP LCP packet with optional Credit Tag Type 0x0106:
    While the PPP protocol value is shown (0xc021), 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????          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       Tag Type = 0x0106       |        Tag Length=0x04        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |              FCN              |              BCN              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    PPP PROTOCOL = 0xc021      |        PPP payload           ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



























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12. Acknowledgements

    The authors would like to acknowledge the influence and
    contributions from Billy Moon, Fred Baker, Stan Ratliff
    and Ed Paradise.


13. Normative References

    [1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD
        51, RFC 1661, July 1994

    [2] Mamakos L., et. al., "A Method for Transmitting PPP Over
        Ethernet (PPPoE)", RFC 2516, February 1999.



Authors' Address

    Bo Berry
    Cisco
    170 West Tasman Drive
    San Jose, CA  95134
    USA
    email: bberry@cisco.com


    Howard Holgate
    Cisco
    170 West Tasman Drive
    San Jose, CA  95134
    USA
    email: hholgate@cisco.com


















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