Network Working Group B. Berry

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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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.

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

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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.

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: [email protected]

Howard Holgate
Cisco
170 West Tasman Drive
San Jose, CA 95134
USA
EMail: [email protected]

Berry & Holgate Informational [Page 16]

RFC 4938 PPPoE with Credit Flow and Metrics June 2007

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Berry & Holgate Informational [Page 17]