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rfc:rfc4938

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.

Berry & Holgate Informational [Page 1] RFC 4938 PPPoE with Credit Flow and Metrics June 2007

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.

Berry & Holgate Informational [Page 2] RFC 4938 PPPoE with Credit Flow and Metrics June 2007

 +--------+       +-------+       +-------+       +--------+
 | 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).

Berry & Holgate Informational [Page 3] RFC 4938 PPPoE with Credit Flow and Metrics June 2007

 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.

Berry & Holgate Informational [Page 4] RFC 4938 PPPoE with Credit Flow and Metrics June 2007

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

Berry & Holgate Informational [Page 5] RFC 4938 PPPoE with Credit Flow and Metrics June 2007

 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.

Berry & Holgate Informational [Page 6] RFC 4938 PPPoE with Credit Flow and Metrics June 2007

     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.

Berry & Holgate Informational [Page 7] RFC 4938 PPPoE with Credit Flow and Metrics June 2007

 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.

Berry & Holgate Informational [Page 8] RFC 4938 PPPoE with Credit Flow and Metrics June 2007

 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.

Berry & Holgate Informational [Page 9] RFC 4938 PPPoE with Credit Flow and Metrics June 2007

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.

Berry & Holgate Informational [Page 10] RFC 4938 PPPoE with Credit Flow and Metrics June 2007

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

Berry & Holgate Informational [Page 11] RFC 4938 PPPoE with Credit Flow and Metrics June 2007

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

Berry & Holgate Informational [Page 12] RFC 4938 PPPoE with Credit Flow and Metrics June 2007

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

Berry & Holgate Informational [Page 13] RFC 4938 PPPoE with Credit Flow and Metrics June 2007

 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)    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Berry & Holgate Informational [Page 14] RFC 4938 PPPoE with Credit Flow and Metrics June 2007

 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.

Berry & Holgate Informational [Page 15] RFC 4938 PPPoE with Credit Flow and Metrics June 2007

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

Berry & Holgate Informational [Page 16] RFC 4938 PPPoE with Credit Flow and Metrics June 2007

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Berry & Holgate Informational [Page 17]

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