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RFC 4938

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

Pages: 17
Obsoleted by:  5578

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Network Working Group                                           B. Berry
Request for Comments: 4938                                    H. Holgate
Category: Informational                               Cisco Systems,Inc.
                                                               June 2007


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

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 IETF Trust (2007).

IESG Note

   The PPP Extensions Working Group (PPPEXT) has reservations about the
   desirability of the feature described in this document.  In
   particular, it solves a general problem at an inappropriate layer and
   it may have unpredictable interactions with higher and lower level
   protocols.  The techniques described in this document are intended
   for use with a particular deployment technique that uses a PPP
   termination separated from a radio termination by an Ethernet, and
   that has radio-side flow control for a slower PPP-only link to remote
   nodes.  Implementors are better advised to avoid split termination
   with inter-media protocol translation, and use standard Internet
   Protocol routing instead.

Abstract

This document extends the Point-to-Point over Ethernet (PPPoE) Protocol 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 ....................................................2 2. Payload .........................................................3 3. Overview of Protocol Extensions .................................3 4. Discovery Stage .................................................3 4.1. PPPoE Active Discovery Request (PADR) ......................4 4.2. PPPoE Active Discovery Session-confirmation (PADS) .........4 4.3. PPPoE Active Discovery Session-Grant (PADG) ................5 4.4. PPPoE Active Discovery Session-Credit Response (PADC) ......5 4.5. PPPoE Active Discovery Quality (PADQ) ......................6 5. PPP Session Stage ...............................................7 6. Credit Flow Considerations ......................................7 7. PADG and PADC Retransmission ....................................8 8. Other Considerations ............................................9 9. IANA Considerations .............................................9 10. Security Considerations ........................................9 Appendix A: Tag Values.............................................10 Appendix B: Example Message Formats................................11 Acknowledgements...................................................15 Normative References...............................................15

1. Introduction

PPP over Ethernet (PPPoE) [2] is a protocol for establishing and encapsulating sessions between hosts and traffic aggregators (Access Concentrators) for PPP [1] 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.
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   +--------+       +-------+       +-------+       +--------+
   | 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.

   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).
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   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.

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 cannot 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.
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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. The Credit Tag FCN indicates the number of incremental credits being granted to the peer by the node. A value between 1 and 0xffff represents 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 retransmitted 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
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   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 been 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.

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 [1] 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 - a bit that indicates whether the link is bi- directional or receive only. A value of -1- indicates that the link is receive-only. Maximum data rate - 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 - 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 the maximum data rate.
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       Latency - the transmission delay that a packet encounters as it
       is transmitted over the Host link.  This is reported in absolute
       delay, milliseconds.  If latency cannot be calculated, a value of
       0 should be reported.

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

       Relative Link Quality (RLQ) - 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 cannot 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.

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.
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   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.  However, they are
   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.

   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 that 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 whether or not the previously granted credits were accumulated. If they were 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.
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   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 generated PADQ metric packets may need to be throttled so as 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 receiving node to set the credit values of its peer.

9. IANA Considerations

IANA has assigned the following PPPoE TAG Values as noted in [3]: TAG Value TAG Name Tag Description Reference ----------- ------------------- --------------------- --------- 262 0x0106 Credits See the reference [RFC4938] 263 0x0107 Metrics See the reference [RFC4938] 264 0x0108 Sequence Number See the reference [RFC4938] IANA has assigned the following PPPoE Code fields as noted in [3]: Code PPPoE Packet Name Description Reference -------- ----------------------------- ----------------- --------- 10 0x0a PADG, Session-Grant See the reference [RFC4938] 11 0x0b PADC, Session-Credit Response See the reference [RFC4938] 12 0x0c PADQ, Quality See the reference [RFC4938]

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.
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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). 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Tag Type = 0x0106 | Tag Length=0x04 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | FCN | BCN | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 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. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 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 retransmitted 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.
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   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.

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

Appendix B: Example Message Formats

A PADR packet with OPTIONAL Credit Tag Type 0x0106: 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 = 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:

    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 = 0x65  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     SESSION_ID = 0x1234       |      LENGTH = 0x0C            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Tag Type = 0x0101       |        Tag Length=0x00        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Tag Type = 0x0106       |        Tag Length=0x04        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              FCN              |              BCN              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   A PADG packet with Credit Tag Type 0x0106:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     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:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     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              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   A PADQ packet to query for the link metrics: This is indicated by
   the Metric Tag Length=0.

    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:

    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 = 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.

    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 = 0x8864        | v = 1 | t = 1 |  CODE = 0x00  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     SESSION_ID = 0x1234       |      LENGTH = (payload)       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Tag Type = 0x0106       |        Tag Length=0x04        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              FCN              |              BCN              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    PPP PROTOCOL = 0xc021      |        PPP payload           ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Acknowledgements

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

Normative References

[1] Simpson, W., Ed., "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, July 1994. [2] Mamakos, L., Lidl, K., Evarts, J., Carrel, D., Simone, D., and R. Wheeler, "A Method for Transmitting PPP Over Ethernet (PPPoE)", RFC 2516, February 1999. [3] Arberg, P. and V. Mammoliti, "IANA Considerations for PPP over Ethernet (PPPoE)", RFC 4937, June 2007.
ToP   noToC   RFC4938 - Page 16

Authors' Addresses

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
ToP   noToC   RFC4938 - Page 17
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