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<pre>Independent Submission B. Berry, Ed.
Request for Comments: 5578 S. Ratliff
Obsoletes: <a href="./rfc4938">4938</a> E. Paradise
Category: Informational Cisco
ISSN: 2070-1721 T. Kaiser
Harris Corporation
M. Adams
L3 Communications
February 2010
<span class="h1">PPP over Ethernet (PPPoE) Extensions for Credit Flow and Link Metrics</span>
Abstract
This document extends the Point-to-Point Protocol over Ethernet
(PPPoE) with an optional credit-based flow control mechanism and an
optional Link Quality Metric report. These optional extensions
improve the performance of PPPoE over media with variable bandwidth
and limited buffering, such as mobile point-to-point radio links.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not a candidate for any level of Internet
Standard; see <a href="./rfc5741#section-2">Section 2 of RFC 5741</a>.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
<a href="http://www.rfc-editor.org/info/rfc5578">http://www.rfc-editor.org/info/rfc5578</a>.
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
<span class="grey">Berry, et al. Informational [Page 1]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-2" ></span>
<span class="grey"><a href="./rfc5578">RFC 5578</a> PPPoE with Credit Flow and Metrics February 2010</span>
nodes. Implementors are better advised to avoid split termination
with inter-media protocol translation, and use standard Internet
Protocol routing instead.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http:trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.
Table of Contents
<a href="#section-1">1</a>. Introduction ....................................................<a href="#page-3">3</a>
<a href="#section-2">2</a>. Terminology .....................................................<a href="#page-4">4</a>
<a href="#section-3">3</a>. Overview of Protocol Extensions .................................<a href="#page-5">5</a>
<a href="#section-3.1">3.1</a>. TLVs .......................................................<a href="#page-5">5</a>
<a href="#section-3.1.1">3.1.1</a>. Credits TLV .........................................<a href="#page-5">5</a>
<a href="#section-3.1.2">3.1.2</a>. Metrics TLV .........................................<a href="#page-6">6</a>
<a href="#section-3.1.3">3.1.3</a>. Sequence Number TLV .................................<a href="#page-7">7</a>
<a href="#section-3.1.4">3.1.4</a>. Credit Scale Factor TLV .............................<a href="#page-8">8</a>
<a href="#section-3.2">3.2</a>. Discovery Stage Extensions .................................<a href="#page-8">8</a>
<a href="#section-3.2.1">3.2.1</a>. PPPoE Active Discovery Request (PADR) ...............<a href="#page-9">9</a>
3.2.2. PPPoE Active Discovery Session-Confirmation
(PADS) .............................................<a href="#page-11">11</a>
<a href="#section-3.2.3">3.2.3</a>. PPPoE Active Discovery Session-Grant (PADG) ........<a href="#page-13">13</a>
3.2.4. PPPoE Active Discovery Session-Credit
Response (PADC) ....................................<a href="#page-15">15</a>
<a href="#section-3.2.5">3.2.5</a>. PPPoE Active Discovery Quality (PADQ) ..............<a href="#page-16">16</a>
<a href="#section-3.3">3.3</a>. PPP Session Stage Extensions ..............................<a href="#page-18">18</a>
<a href="#section-4">4</a>. Credit Flow Considerations .....................................<a href="#page-19">19</a>
<a href="#section-5">5</a>. PADG and PADC Retransmission ...................................<a href="#page-20">20</a>
<a href="#section-6">6</a>. Other Considerations ...........................................<a href="#page-20">20</a>
<a href="#section-7">7</a>. IANA Considerations ............................................<a href="#page-21">21</a>
<a href="#section-8">8</a>. Security Considerations ........................................<a href="#page-21">21</a>
<a href="#section-9">9</a>. References .....................................................<a href="#page-21">21</a>
<a href="#section-9.1">9.1</a>. Normative References ......................................<a href="#page-21">21</a>
<a href="#section-9.2">9.2</a>. Informative References ....................................<a href="#page-21">21</a>
<a href="#appendix-A">Appendix A</a>. Examples of Session Credit Flows ......................<a href="#page-22">22</a>
<span class="grey">Berry, et al. Informational [Page 2]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-3" ></span>
<span class="grey"><a href="./rfc5578">RFC 5578</a> PPPoE with Credit Flow and Metrics February 2010</span>
<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Introduction</span>
PPP over Ethernet (PPPoE) [<a href="#ref-2" title=""A Method for Transmitting PPP Over Ethernet (PPPoE)"">2</a>] is a protocol for establishing and
encapsulating sessions between Hosts (Clients) and traffic-access
aggregators (Servers) for PPP [<a href="#ref-1" title=""The Point-to-Point Protocol (PPP)"">1</a>] 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, improvements can be made for applications with
variable bandwidth and limited buffering. This document addresses
these improvements with optional extensions to PPPoE that support
credit-based session flow control and session-based link metric
quality reports.
These extensions are designed to support radio systems that exhibit
point-to-point waveforms. The diagram below is used to illustrate
the improvement that these extensions address. When the local Client
(Radio) detects the presence of a remote Radio neighbor, it initiates
a PPPoE session with its local Server (router). The Radio also
establishes a radio link connection with the remote Radio over the
point-to-point RF (radio frequency) link (which is beyond the scope
of this document). The remote Radio is also establishing a PPPoE
session with its local Server (router). The Radios associate the two
PPPoE sessions and the point-to-point radio link protocol (RLP),
creating a complete data path. Now a PPP session is established via
the PPP IP Control Protocol (IPCP) as described in <a href="./rfc1661">RFC 1661</a>.
Included in this IPCP exchange is the router IP address. With the
exchange of the IPCP IP addresses, each router inserts the remote IP
address into its local routing tables. Note that the radio IP
information is not inserted into the routing tables.
|-----Local Neighbor-----| |-----Remote Neighbor----|
+--------+ +-------+ +-------+ +--------+
| Router |=======| Radio |{~~~~~~~~}| Radio |=======| Router |
| Server | | Client| | Client| | Server |
+--------+ +-------+ +-------+ +--------+
| | | | | |
|-PPPoE-| |----RLP---| |-PPPoE-|
| |
|-----------PPP IPCP (IP Address)---------|
| |
|-------------PPP Data Session-------------|
Figure 1: PPPoE Network
<span class="grey">Berry, et al. Informational [Page 3]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-4" ></span>
<span class="grey"><a href="./rfc5578">RFC 5578</a> PPPoE with Credit Flow and Metrics February 2010</span>
The capabilities of the RF links between RLP neighbors will vary over
time due to mobility and environmental conditions as well as changes
in the RF waveforms and encoding. To reflect these dynamic changes,
the Radio may periodically generate Link Quality Metrics to the
router. The router uses the link metric to update route costs and
influence route selection. The influence upon the routing protocols
is beyond the scope of this document.
A PPPoE Client implementation can be found at [<a href="#ref-3" title=" PPP Over Ethernet (PPPoE) Extensions for Credit Flow and Link Metrics">3</a>]. It is open source
(GNU GPLv2 -- General Public License).
<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. Terminology</span>
Access Concentrator
The <a href="./rfc2516">RFC 2516</a> term used to describe the Server. This
document uses the terms Router or Server instead.
BCN Backward Credit Notification. The BCN represents the number
of remaining credits at the local node that were granted by
peer node. The local node uses these credits to transmit
payload back to the peer node. BCN ranges from 0-65535.
CDR The Current Datarate.
FCN Forward Credit Notification. The FCN represents the credits
that the local node is granting to the peer node. The peer
node uses these granted credits to transmit payload back to
the local node. FCN ranges from 0-65535.
Gbit/s gigabits (1,000,000,000) per second.
Host The <a href="./rfc2516">RFC 2516</a> term used to describe the Server. This
document uses the terms Radio or Client.
kbit/s kilobits (1,000) per second.
LCP PPP Link Control Protocol, <a href="./rfc1661">RFC 1661</a>.
Mbit/s megabits (1,000,000) per second.
MDR The Maximum Datarate.
NCP PPP Network Control Protocol, <a href="./rfc1661">RFC 1661</a>.
RLP Radio Link Protocol.
TAG The <a href="./rfc2516">RFC 2516</a> PPPoE synonym for TLV. This document uses TLV.
<span class="grey">Berry, et al. Informational [Page 4]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-5" ></span>
<span class="grey"><a href="./rfc5578">RFC 5578</a> PPPoE with Credit Flow and Metrics February 2010</span>
Tbit/s terabits (1,000,000,000,000) per second.
TLV Type-Length-Value.
<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. Overview of Protocol Extensions</span>
The extensions consist of optional TLVs as well as enhanced and newly
defined Discovery packets. The enhancements are applied to the
Discovery Stage and the PPP Session Stage.
<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>. TLVs</span>
The new TLVs are listed in the table below:
TLV TLV
Value Description
=========================================
0x0106 Credits TLV
0x0107 Metrics TLV
0x0108 Sequence Number TLV
0x0109 Credit Scale Factor TLV
<span class="h4"><a class="selflink" id="section-3.1.1" href="#section-3.1.1">3.1.1</a>. Credits TLV</span>
This TLV contains the Forward Credit Notification (FCN) and the
Backward Credit Notification (BCN). The Credits TLV is optional with
the PADR (PPPoE Active Discovery Request) and PADS (PPPoE Active
Discovery Session-Confirmation) and required in the PADG (PPPoE
Active Discovery Session Grant) and PADC (PPPoE Active Discovery
Session-Credit) Discovery packets (ETHER_TYPE=8863).
The Credits TLV is optionally carried in the PPPoE data payload
packet of the PPP Stage (ETHER_TYPE=8864).
The FCN represents the number of credits being granted by the local
node to the peer node. The BCN represents the number of credits
remaining at the local node that were granted by the peer node.
The Credits TLV is shown below:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Credits TLV = 0x0106 | TLV Length = 0x04 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FCN | BCN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
<span class="grey">Berry, et al. Informational [Page 5]</span></pre>
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These fields are transmitted in network byte order. The same byte
order is used throughout the document in other structures as well.
<span class="h4"><a class="selflink" id="section-3.1.2" href="#section-3.1.2">3.1.2</a>. Metrics TLV</span>
The Metrics TLV is used to report the link-quality parameters. The
Metrics TLV is required with the PADQ (PPPoE Discovery Quality)
Discovery packet.
The Metrics TLV contains the following fields:
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.
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.
Reserved - reserved fields are zeroed unless otherwise specified.
CD - two bits that designate the units of the Current Datarate.
CD Scale: 00 == kbit/s (default)
01 == Mbit/s
10 == Gbit/s
11 == Tbit/s
MD - two bits that designate the units of the Maximum Datarate.
MD Scale: 00 == kbit/s (default)
01 == Mbit/s
10 == Gbit/s
11 == Tbit/s
Current Datarate - the Current Datarate, in un-scaled units per
second, achieved on the RLP link. If the Radio makes no
distinction between Maximum Datarate and Current Datarate, Current
Datarate should equal the Maximum Datarate. When metrics are
reported, the Current Datarate must be reported. The Current
Datarate must be less than or equal to the Maximum Datarate.
<span class="grey">Berry, et al. Informational [Page 6]</span></pre>
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Latency - the transmission delay that a packet encounters as it is
transmitted over the link. This is reported in absolute delay,
milliseconds. If latency cannot be calculated, a value of 0
should be reported. The calculation of latency is Radio
dependent. For example, the latency may be a running average
calculated from the internal queuing.
Maximum Datarate - the maximum theoretical data rate, in un-scaled
units per second, that the RLP link is capable of providing. When
metrics are reported, the Maximum Datarate must be reported.
The Metrics TLV is shown below:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metrics TLV = 0x0107 | TLV Length = 0x0A |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | MD| CD|R| RLQ | Resources |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Latency (MS) | Current Datarate |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Maximum Datarate |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
<span class="h4"><a class="selflink" id="section-3.1.3" href="#section-3.1.3">3.1.3</a>. Sequence Number TLV</span>
This TLV 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.
The Sequence Number TLV is shown below:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number TLV = 0x0108 | TLV Length = 0x02 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
<span class="grey">Berry, et al. Informational [Page 7]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-8" ></span>
<span class="grey"><a href="./rfc5578">RFC 5578</a> PPPoE with Credit Flow and Metrics February 2010</span>
<span class="h4"><a class="selflink" id="section-3.1.4" href="#section-3.1.4">3.1.4</a>. Credit Scale Factor TLV</span>
This TLV contains the scale factor value that is to be applied to the
session credit calculations. The Credit Scale Factor TLV is optional
with the PADR and PADS packets. Once the session is established with
specified scale factors, the scale factors are set for the entire
session. The scale factor value represents the units that the local
node grants to the remote node. The remote node is responsible for
maintaining the credit accounting relative to the data flow back to
the local node.
The Credit Scale Factor TLV can be used to change from the default
64-byte credit unit during the PADR-PADS exchange. The credit scale
factor value can range from 1 byte to 65535 bytes. A zero value is
ignored and the default 64-byte unit remains set. The scale factor
is set per each payload flow: peer-to-local and local-to-peer.
The Credit Scale Factor TLV is shown below:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Credit Scale Factor = 0x0109 | TLV Length = 0x02 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Scale Factor Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>. Discovery Stage Extensions</span>
The specifications of the PPPoE Active Discovery Request (PADR) and
the PPPoE Active Discovery Session-confirmation (PADS) packets are
extended to include the optional Credits TLV and the Credit Scale
Factor TLV. The PPPoE Active Discovery Session-Grant (PADG) packet,
the PPPoE Active Discovery Session-Credit Response (PADC), and the
Quality packets are newly defined Discovery Stage packets.
<span class="grey">Berry, et al. Informational [Page 8]</span></pre>
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Discovery
Packet Status
=======================================================
PADR Enhanced Optionally includes the Credits
TLV and the Credit Scale Factor
TLV
PADS Enhanced Optionally includes the Credits
TLV and the Credit Scale Factor
TLV
PADG New Includes the Credits TLV and the
Sequence Number TLV
PADC New Includes the Credits TLV and the
Sequence Number TLV
PADQ New Includes the Metrics TLV
<span class="h4"><a class="selflink" id="section-3.2.1" href="#section-3.2.1">3.2.1</a>. PPPoE Active Discovery Request (PADR)</span>
The PADR packet is extended to optionally contain a single Credits
TLV, indicating that the Client requests credit flow control for this
session. The Credits TLV 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 Server by the Client. The BCN value is set to 0, as the Client
has not yet been granted credits from the Server.
The PADR packet is enhanced to optionally contain a single Credit
Scale Factor TLV. The Credit Scale Factor TLV defines the credit
scale factor value. If the Credit Scale Factor TLV is omitted, the
default 64-byte value is used for the session. When the Client
includes the optional Credit Scale Factor TLV in the PADR, this
credit scale factor value is applied to all credit grants associated
with the Client credits that are granted to the Server.
The Server must echo the Credit Scale Factor TLV in the PADS response
to confirm the credit scaling session and to designate the Server
credit scaling factor. This PADS Credit Scaling Factor TLV
represents the scale factor value that is applied to all credits
granted from the Server to the Client.
Once the session is established during the PADR-PADS exchange, the
credit scale factor value cannot be changed.
<span class="grey">Berry, et al. Informational [Page 9]</span></pre>
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A Discovery PADR packet with the optional Credits TLV is shown below:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLV Type = 0x0101 | Metrics TLV Length = 0x00 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Credits TLV = 0x0106 | TLV Length = 0x04 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FCN | BCN = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The credit units are expressed in the default 64-byte units.
<span class="grey">Berry, et al. Informational [Page 10]</span></pre>
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A Discovery PADR packet with the optional Credits TLV and the
optional Credit Scale Factor TLV is shown below:
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 = 0x12 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLV Type = 0x0101 | TLV Length = 0x00 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Credits TLV = 0x0106 | TLV Length = 0x04 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FCN | BCN = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Credit Scale Factor = 0x0109 | TLV Length = 0x02 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| scale factor value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Credits TLV FCN value is expressed in units of the session's
credit scale factor value.
<span class="h4"><a class="selflink" id="section-3.2.2" href="#section-3.2.2">3.2.2</a>. PPPoE Active Discovery Session-Confirmation (PADS)</span>
The Server PADS is extended to optionally contain a single Credits
TLV, indicating the Forward Credit Notification (FCN) and the
Backward Credit Notification (BCN) of the PPP Session Stage.
If the Client PADR contained a Credits TLV, then the Server PADS must
indicate support for credit flow control by including a Credits TLV.
The PADS Credits TLV FCN represents the number of credits initially
granted to the Client. The Credits TLV BCN is an echo of the number
of credits that the Client had granted to the Server in the
originating PADR packet.
Exchange of the Credits TLV in the PADR and PADS indicates that
credit flow control is supported by both the Server and the Client
for the designated PPP Session Stage. This is binding and must be
<span class="grey">Berry, et al. Informational [Page 11]</span></pre>
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<span class="grey"><a href="./rfc5578">RFC 5578</a> PPPoE with Credit Flow and Metrics February 2010</span>
followed for the entire duration of the PPP Session Stage. A
session's credit binding must be established prior to any other
credit indications being exchanged.
The Server PADS should only include the Credits TLV in response to a
Client PADR that included the Credits TLV. If the Server does not
support credit flow, it should not include the Credits TLV in its
PADS response. The Client must terminate a credit-based session that
cannot be supported by the Server. A Credits TLV transmitted outside
an established credit-based session must be ignored.
The Server PADS is enhanced to optionally contain a single Credit
Scale Factor TLV. The Credit Scale Factor TLV defines the credit
scale unit value. The Credit Scale Factor TLV must be included if it
was included in the Client PADR. If the Credits TLV was not included
in the originating PADR, it must be omitted, indicating that the
64-byte default is used for the directional flow. This credit scale
factor is applied to Server grants to the Client.
A Discovery PADS packet with the optional Credits TLV is shown below:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLV Type = 0x0101 | TLV Length = 0x00 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Credits TLV = 0x0106 | TLV Length = 0x04 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FCN | BCN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The BCN is expressed in the default 64-byte units.
<span class="grey">Berry, et al. Informational [Page 12]</span></pre>
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A Discovery PADS packet with the optional Credits TLV and the
optional Credit Scale Factor TLV is shown below:
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 = 0x12 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLV Type = 0x0101 | TLV Length = 0x00 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Credits TLV = 0x0106 | TLV Length = 0x04 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FCN | BCN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Credit Scale Factor = 0x0109 | TLV Length = 0x02 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| scale factor value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Credits TLV BCN value is expressed in units of the session scale
factor value received in the PADR.
<span class="h4"><a class="selflink" id="section-3.2.3" href="#section-3.2.3">3.2.3</a>. PPPoE Active Discovery Session-Grant (PADG)</span>
The PPPoE Active Discovery Session-Grant (PADG) is a new packet
defined in this specification. The local node (Server or Client) may
send a PADG at any time after the PADR/PADS exchange to grant
incremental flow control credits to a peer. The CODE field is set to
0x0A and the SESSION_ID must be set to the unique value generated for
this PPPoE Session.
Each flow control credit corresponds to the amount of PPP payload
bytes that can be sent or received. For example, if the default
credit scale factor of 64 bytes is used, and 128 bytes of PPP payload
data are sent, then 2 credits would be consumed. When calculating
credits to consume, all credit calculations must be rounded up. If,
in the previous example, 130 bytes of PPP payload data were sent, 3
credits would have been consumed.
<span class="grey">Berry, et al. Informational [Page 13]</span></pre>
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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 Response (PADC) packet, indicating the
accumulation of the credits. The FCN and BCN values must be scaled
by the value established during session establishment in the Credit
Scale Factor TLV or by the default 64-byte value prior to processing.
The PADG packet must contain a single Credits TLV, indicating the
Forward Credit Notification (FCN) and the Backward Credit
Notification (BCN) of the PPPoE Session.
The Credits TLV 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 multiply the
credit units by the credit scale factor and 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 Credits TLV BCN indicates the remaining absolute credits that
have been granted by the peer to the local node. When the local node
exhausts the BCN, it must stop transmitting payload packets.
Once a credit has been granted, it must be honored. The largest
number of incremental credits at any time is 0xffff.
The PADG packet must contain a single Sequence Number TLV. This TLV
is used to carry a unique 16-bit sequence number to uniquely identify
each request. The sequence number should be initialized at 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.
<span class="grey">Berry, et al. Informational [Page 14]</span></pre>
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A Discovery PADG packet with the Sequence Number and Credits TLVs is
shown below:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number TLV = 0x0108 | TLV Length = 0x02 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number | Credits TLV = 0x0106 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLV Length = 0x04 | FCN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BCN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
<span class="h4"><a class="selflink" id="section-3.2.4" href="#section-3.2.4">3.2.4</a>. PPPoE Active Discovery Session-Credit Response (PADC)</span>
The PPPoE Active Discovery Session-Credit Response (PADC) is a new
packet defined in this specification. A Server or Client must send a
PADC in response to a PADG. The CODE field is set to 0x0B, and the
SESSION_ID must be set to the unique value generated for this PPPoE
session.
The PADC packet must contain a single Credits TLV, indicating the
Forward Credit Notification (FCN) and the Backward Credit
Notification (BCN) of the PPPoE session.
The Credits TLV FCN represents the absolute credits remaining that
have been granted to the peer by the node. The Credits TLV BCN
represents the remaining absolute credits that have been granted to
the local node from the peer. The FCN and BCN values must be scaled
by the value established during session establishment in the Credit
Scale Factor TLV or by the default 64-byte value prior to processing.
The PADC packet must contain a single Sequence Number TLV. The
sequence number must be the sequence number associated with the PADG.
<span class="grey">Berry, et al. Informational [Page 15]</span></pre>
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A Discovery PADC packet with the Sequence Number and Credits TLV is
shown below:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number TLV = 0x0108 | TLV Length = 0x02 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number | Credits TLV = 0x0106 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLV Length = 0x04 | FCN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BCN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The FCN and BCN values are expressed in the respective units defined
by the Credit Scale Factor TLV or the 64-byte default.
<span class="h4"><a class="selflink" id="section-3.2.5" href="#section-3.2.5">3.2.5</a>. PPPoE Active Discovery Quality (PADQ)</span>
The PPPoE Active Discovery Quality (PADQ) is a new packet defined in
this specification. An Server or Client may send an optional PADQ at
any time to query or report link-quality metrics.
When transmitting PPP [<a href="#ref-1" title=""The Point-to-Point Protocol (PPP)"">1</a>] 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 PPPoE Active Discovery Quality (PADQ) packet can be used to query
link metrics by setting the PADQ Metrics TLV Length to zero.
The PADQ must carry a single Metrics TLV. When processing the data
rates, the values must be converted using the indicated data rate
units. This document enhances the Metrics TLV as described below.
<span class="grey">Berry, et al. Informational [Page 16]</span></pre>
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A Discovery PADQ packet with the required Metrics TLV is shown below:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLV Type = 0x0101 | TLV Length = 0x00 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metrics TLV = 0x0107 | TLV Length = 0x0A |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | MD| CD|R| RLQ | Resources |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Latency (MS) | Current Datarate |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Maximum Datarate |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Maximum Datarate and the Current Datarate are expressed in units
determined by the MD and CD bits, respectively.
<span class="grey">Berry, et al. Informational [Page 17]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-18" ></span>
<span class="grey"><a href="./rfc5578">RFC 5578</a> PPPoE with Credit Flow and Metrics February 2010</span>
A Discovery PADQ packet with a Metrics TLV Length=0 to query is shown
below:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLV Type = 0x0101 | TLV Length = 0x00 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metrics TLV = 0x0107 | TLV Length = 0x00 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
<span class="h3"><a class="selflink" id="section-3.3" href="#section-3.3">3.3</a>. PPP Session Stage Extensions</span>
The PPP Session Stage Extensions define the optional use of Credits
TLV. Use of the Credits TLV in the PPP Session Stage is referred to
as an in-band credit grant.
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 TLV length
is subtracted from the overall payload length.
<span class="grey">Berry, et al. Informational [Page 18]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-19" ></span>
<span class="grey"><a href="./rfc5578">RFC 5578</a> PPPoE with Credit Flow and Metrics February 2010</span>
A PPP LCP packet with optional Credits TLV is shown below:
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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Credits TLV = 0x0106 | TLV Length = 0x04 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FCN | BCN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PPP PROTOCOL = 0xc021 | PPP payload ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. Credit Flow Considerations</span>
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
accounting 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 Session-Grant (PADG) and PPPoE Active
Discovery Credit Response (PADC) packets.
In-band credit management allows credits to be incrementally granted
with each PPP Session Stage packet. These in-band incremental credit
grants are not explicitly acknowledged. 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.
<span class="grey">Berry, et al. Informational [Page 19]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-20" ></span>
<span class="grey"><a href="./rfc5578">RFC 5578</a> PPPoE with Credit Flow and Metrics February 2010</span>
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 must stop transmitting packets to the peer.
To manage the credits that a node has granted, the node maintains a
running counter. With each PPP Session Stage packet received from
the peer, the running counter is decremented. When the running
counter reaches zero, no additional packets are expected. The node
incrementally grants more credits to the peer to maintain packet
flow. Packets received when granted credits have been exhausted are
discarded.
<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>. PADG and PADC Retransmission</span>
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 doubling
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 and an
incremented sequence number should be transmitted. This process
should be repeated until granted credits are properly acknowledged or
as many times as desired.
When a node does not receive a PADQ metric packet in response to a
query 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.
<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>. Other Considerations</span>
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 Discovery 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 can be used by the receiving node to reset the credit
values of its peer.
<span class="grey">Berry, et al. Informational [Page 20]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-21" ></span>
<span class="grey"><a href="./rfc5578">RFC 5578</a> PPPoE with Credit Flow and Metrics February 2010</span>
<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>. IANA Considerations</span>
IANA has assigned the following PPPoE TLV Values for which this RFC
serves as the reference:
TLV Value TLV Name Description Reference
----------- ------------------- ----------------- ---------
262 0x0106 Credits See the reference [<a href="./rfc5578">RFC5578</a>]
263 0x0107 Metrics See the reference [<a href="./rfc5578">RFC5578</a>]
264 0x0108 Sequence Number See the reference [<a href="./rfc5578">RFC5578</a>]
265 0x0109 Credit Scale Factor See the reference [<a href="./rfc5578">RFC5578</a>]
IANA has assigned the following PPPoE Code fields for which this RFC
serves as the reference:
Code PPPoE Packet Name Description Reference
-------- ---------------------- ----------------- ---------
10 0x0a PADG, Session-Grant See the reference [<a href="./rfc5578">RFC5578</a>]
11 0x0b PADC, Session-Credit See the reference [<a href="./rfc5578">RFC5578</a>]
Response
12 0x0c PADQ, Quality See the reference [<a href="./rfc5578">RFC5578</a>]
<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>. Security Considerations</span>
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 <a href="./rfc2516">RFC 2516</a>
[<a href="#ref-2" title=""A Method for Transmitting PPP Over Ethernet (PPPoE)"">2</a>]. It is required that the Service TLV and Session ID always be
validated prior to processing credits.
<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>. References</span>
<span class="h3"><a class="selflink" id="section-9.1" href="#section-9.1">9.1</a>. Normative References</span>
[<a id="ref-1">1</a>] Simpson, W., Ed., "The Point-to-Point Protocol (PPP)", STD 51,
<a href="./rfc1661">RFC 1661</a>, July 1994.
[<a id="ref-2">2</a>] Mamakos, L., Lidl, K., Evarts, J., Carrel, D., Simone, D., and R.
Wheeler, "A Method for Transmitting PPP Over Ethernet (PPPoE)",
<a href="./rfc2516">RFC 2516</a>, February 1999.
<span class="h3"><a class="selflink" id="section-9.2" href="#section-9.2">9.2</a>. Informative References</span>
[<a id="ref-3">3</a>] An open source (GPLv2) PPPoE Client implementation of <a href="./rfc5578">RFC 5578</a>,
PPP Over Ethernet (PPPoE) Extensions for Credit Flow and Link
Metrics, <a href="http://rfc4938.sourceforge.net/">http://rfc4938.sourceforge.net/</a>.
<span class="grey">Berry, et al. Informational [Page 21]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-22" ></span>
<span class="grey"><a href="./rfc5578">RFC 5578</a> PPPoE with Credit Flow and Metrics February 2010</span>
<span class="h2"><a class="selflink" id="appendix-A" href="#appendix-A">Appendix A</a>. Examples of Session Credit Flows</span>
Session Credit Flow with the default 64-byte credit unit.
Server Client
====================================================================
<------------PADI-------------- Initiate
------------PADO--------------> Offer
<------------PADR-------------- Credits TLV:
FCN represents the initial
Client credit grant to the
Server in 64-byte units.
BCN is set to 0.
------------PADS--------------> Credits TLV:
FCN represents the initial
Server credit grant to the
Client in 64-byte units.
BCN represents an echo of
initial Client credits.
<==============================> Data w/ optional in-band
Credits TLV
<------------PADG-------------- Credits TLV: (out-of-band)
FCN represents an incremental
Client credit grant to the
Server, in 64-byte units.
BCN represents the remaining
Server credits that were granted
to the Client, in 64-byte units.
------------PADC--------------> Credits TLV: (out-of-band)
FCN represents an incremental
Server credit grant to the
Client, in 64-byte units.
BCN represents the remaining
Client credits that were granted
to the Server, in 64-byte units.
<==============================> Data w/ optional in-band Credits
TLV
<------------PADT--------------> Terminate
Session Credit Flow with specific credit scale factor units for the
Server and the Client.
<span class="grey">Berry, et al. Informational [Page 22]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-23" ></span>
<span class="grey"><a href="./rfc5578">RFC 5578</a> PPPoE with Credit Flow and Metrics February 2010</span>
Server Client
====================================================================
<------------PADI-------------- Initiate
------------PADO--------------> Offer
<------------PADR-------------- Credits TLV:
FCN represents the initial
Client credit grant to the
Server, in Credit Scale Factor
TLV units.
BCN is set to 0.
------------PADS--------------> Credits TLV:
FCN represents the initial
Server credit grant to the
Client, in Credit Scale Factor
TLV units.
BCN represents an echo of the
initial Client credits, in
Credit Scale Factor TLV units.
<==============================> Data w/ optional in-band Credits
TLV
<------------PADG-------------- Credits TLV: (out-of-band)
FCN represents an incremental
Client credit grant to the Server,
in Credit Scale Factor TLV units.
BCN represents the remaining
Server credits that were granted
to the Client, in Credit Scale
Factor TLV units.
------------PADC--------------> Credits TLV: (out-of-band)
FCN represents an incremental
Server credit grant to the Client,
in Credit Scale Factor TLV units.
BCN represents the remaining
Client credits that were granted
to the Server, in Credit Scale
Factor TLV units.
<==============================> Data w/ optional inband Credits
TLV
<------------PADT--------------> Terminate
<span class="grey">Berry, et al. Informational [Page 23]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-24" ></span>
<span class="grey"><a href="./rfc5578">RFC 5578</a> PPPoE with Credit Flow and Metrics February 2010</span>
Authors' Addresses
Bo Berry, Editor
Cisco
170 West Tasman Drive
San Jose, CA 95134
EMail: bberry@cisco.com
Stan Ratliff
Cisco
170 West Tasman Drive
San Jose, CA 95134
EMail: sratliff@cisco.com
Ed Paradise
Cisco
170 West Tasman Drive
San Jose, CA 95134
EMail: pdice@cisco.com
Tim Kaiser
Harris Corporation
Government Communications System Division
Mail Stop 25-11F
P.O. Box 37
Melbourne, FL 32902-0037
EMail: timothy.kaiser@harris.com
Michael D Adams
640 N 2200 W MS F1J12
Salt Lake City, Utah 84116
EMail: Michael.D.Adams@L-3com.com
Berry, et al. Informational [Page 24]
</pre>
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