PPP Extensions Working Group
Internet Draft Peter Arberg
Diamantis Kourkouzelis
Intended status: Informational Redback Networks
Expiration date: April 2006
Mike Duckett
Tom Anschutz
BellSouth
Jerome Moisand
Juniper Networks
October 2005
Accommodating an MTU/MRU greater than 1492 in PPPoE
<draft-arberg-pppoe-mtu-gt1492-01.txt>
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Abstract
Point-to-Point Protocol Over Ethernet (PPPoE), as described in RFC
2516 [1], mandates a maximum negotiated MRU of 1492. This document
outlines a solution to relax that restriction and allow a maximum
negotiated MRU greater than 1492 to minimize fragmentation in next
generation broadband networks.
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1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [3].
ATM - Asynchronous Transfer Mode .
PPP - Point-to-Point Protocol.
PPPoA - PPP over AAL5.
PPPoE - PPP over Ethernet.
MTU - Maximum Transmit Unit
MRU - Maximum Receive Unit
PC - Personal Computer.
CPE - Customer Premises Equipment.
RG - Residential Gateway.
BRAS - Broadband Remote Access Server.
DSLAM Digital Subscriber Line Access Multiplexer
2. Motivation
With broadband network designs changing from PC initiated PPPoE [1]
sessions in a combined Ethernet/ATM setup as shown in figure 1, to
more intelligent PPPoE capable Residential Gateway (RG) and
Gigabit Ethernet/ATM broadband network designs as show in figure 2
and 3, the need to increase the maximum transmit and receive unit in
the PPPoE protocol is becoming more important to reduce fragmentation
in the network.
<------------------ PPPoE session ------------------>
+-----+ +-----+
+--+ +---+ | | | |
|PC|--------------|CPE|-----------|DSLAM|-----------| BRAS|
+--+ <Ethernet> +---+ <ATM> | | <ATM> | |
+-----+ +-----+
Fig. 1: Initial broadband network designs with PPPoE.
In the network design shown in figure 1, fragmentation is typically
not a problem since the subscriber session is PPPoE end-to-end from
the PC to the BRAS, so a PPP negotiated MRU of 1492 bytes is
fully acceptable as it makes the largest PPPoE frame adhere to
the standard Ethernet MTU of 1500 bytes.
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<----- IPoE -----> <--------- PPPoE session --------->
+-----+ +-----+
+--+ +---+ | | | |
|PC|--------------| RG|-----------|DSLAM|------------| BRAS|
+--+ <Ethernet> +---+ <ATM> | | <GigE> | |
+-----+ +-----+
Fig. 2: Next generation broadband network designs with PPPoE.
In the network design shown in figure 2, fragmentation becomes a
major problem since the subscriber session is a combination of
IPoE and PPPoE. The IPoE typically negotiates a MTU of 1500 bytes.
However, when the Residential Gateway and the Edge Aggregation
Router are the PPPoE session endpoints, and therefore negotiate
a MTU/MRU of 1492 bytes resulting in a large number of fragmented
packets in the network.
<----- IPoE -----> <---- PPPoA ----> <- PPPoE session ->
+-----+ +-----+
+--+ +---+ | | | |
|PC|--------------| RG|------------|DSLAM|------------| BRAS|
+--+ <Ethernet> +---+ <ATM> | | <GigE> | |
+-----+ +-----+
<-------------- PPPoA -------------> <- PPPoE session ->
+-----+ +-----+
+--+ +---+ | | | |
|PC|--------------|CPE|------------|DSLAM|------------| BRAS|
+--+ <ATM> +---+ <ATM> | | <GigE> | |
+-----+ +-----+
Fig. 3: Broadband network designs with PPPoA to PPPoE conversion.
In the network design shown in figure 3, which is studied by the
DSL-Forum in the context of the migration to Ethernet for broadband
aggregation networks, fragmentation is not the only problem when
MRU differences exist in PPPoA and PPPoE sessions.
The subscriber session is a PPP session running over a combination
of PPPoA and PPPoE. The PPP/PPPoA client typically negotiates a
1500 bytes MRU. Widely deployed PPP/PPPoA clients in DSL Routers do
not support an 1492 bytes MRU, which creates an issue in turn for
the PPPoE server if strict compliance to RFC 2516 is mandated.
For PPP/PPPoA clients capable of negotiating a 1492 bytes MRU size,
then we are back to a fragmentation issue.
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3. Proposed solution
The procedure described in this document do not strictly conform
to IEEE standards for Ethernet packet size, but rely on a widely
deployed behavior of supporting jumbo frames on Ethernet segments.
Since next generation broadband networks are built around Ethernet
systems supporting baby-giants and jumbo frames with payload sizes
larger than the normal Ethernet MTU of 1500 octets, a BRAS acting
as a PPPoE server MUST support PPPoE MRU negotiations larger than
1492 bytes in order to limit the amount of fragmented packets in
network designs shown in section 1.
By default, the Maximum-Receive-Unit (MRU) option MUST follow the
rules set forward in RFC 1661, but MUST NOT be negotiated to a
larger size than 1492 to guarantee compatibility with Ethernet
network segments limited to 1500 octets frames. In such a case,
the PPPoE header being 6 octets and the PPP Protocol ID being
2 octets, the PPP MRU MUST NOT be greater than 1492.
An optional PPPoE tag "PPP-Max-Payload" allows a PPPoE client to
override this default behavior by providing a maximum size for
the PPP payload it can support in both the sending and receiving
directions. When such a tag is received by a PPPoE server, the
server MAY allow the negotiation of a larger MRU than 1492 and
the use of a larger MTU than 1492 subject to limitations of its
local configuration and according to the rules set forward in
RFC 1661, and within the limits of the maximum payload size
being indicated by the PPPoE client.
4. PPPoE Discovery Stage
If a PPPoE client wants to use a higher MTU/MRU than 1492 bytes,
then it MUST include an optional PPP-Max-Payload Tag in the PADI
and PADR packets.
If the PPPoE server can support a higher MTU/MRU than 1492 bytes, it
MUST respond with a similar tag in the PADO and PADS packets when the
tag is received from the client.
Tag-name: PPP-Max-Payload
Tag-value: 0x0120
Tag-length: 2 octets
Tag-value: binary encoded value (max PPP payload in octets)
Tag-description:
This TAG indicates that the client and server are capable of
supporting the given maximum PPP payload for both the sending and
receiving directions.
Note that this value represents the PPP payload, so it is
directly comparable with the value used in the PPP MRU negotiation.
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5. LCP Considerations
5.1 MRU Negotiations
Since Ethernet (without jumbo frames) has a maximum payload size of
1500 octets, the PPPoE header is 6 octets and the PPP Protocol ID is
2 octets, the Maximum-Receive-Unit (MRU) option MUST NOT be
negotiated to a larger size than 1492, unless both the PPPoE client
and server have indicated the ability to support a larger MRU in the
PPPoE Discovery Stage.
The initial MRU negotiation for the PPP/PPPoE server MUST follow a
flow as shown below:
If PPPoE {
If (PPP-Max-Payload-Tag) Not Present
Then PPP_MRU_Max = 1492
Else PPP_MRU_Max = min (PPP-Max-Payload-Tag, Interface MTU-8)
}
Normal PPP_MRU_Negotiation (PPP_MRU_Max)
If the PPP-Max-Payload tag is present, it MUST be considered as the
maximum value for the normal MRU negotiation which is the master
and decision maker of what the actual MRU will be negotiated to,
never higher than the PPP-Max-Payload tag, but it can be negotiated
to a lower value depending on the server's interface settings and
the peer's negotiated MRU value.
If the PPP-Max-Payload tag isnt present, then the existing MRU
constraint of 1492 bytes would stay applicable, hence preserving
backward compatibility.
If a MRU option is not sent by one end of the PPP negotiation, a
maximum MRU size of 1492 bytes MUST be assumed and used, unless the
PPP-Max-Payload tag was present in the PPPoE Discovery Stage, in
which case this value MUST be used as the maximum MRU value.
5.2 MRU test and troubleshooting
If the MRU is negotiated to a larger value than 1492 bytes, the sending
Side SHOULD have the option to send one or more MRU-sized Echo-Request
packets once the session is opened. This allows it to test that the
receiving side and any intermediate equipment can handle such packet size.
If no Echo-Replies are received, the sending side MAY choose to repeat the
test with 1492 bytes Echo-Request packets. If these packets receive
replies, the sending side MUST not send packets bigger than 1492 octets
for this session.
This capability SHOULD be disabled by default, and SHOULD only be
available for debug, test purpose.
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6. Security Considerations
This document does not introduce new security issues. The security
considerations pertaining to the original PPPoE protocol [1] remain
relevant.
7. Acknowledgments
The authors would like to thank Prakash Jayaraman, Amit Cohen,
Jim Ellis, David Thorne, John Reid, Oliver Thorp, Wojciech Dec,
Jim Wilks, Mark Townsley, Bart Salaets, Tom Mistretta, Paul Howard,
Dave Bernard and Darren Nobel for their contributions and comments
to this document.
8. References
[1] Mamakos L., Lidl K., Evarts J., Carrel D., Simone D., Wheeler
R., "A Method for Transmitting PPP Over Ethernet (PPPoE)", RFC
2516, February 1999
[2] W. Simpson "The Point-to-Point Protocol (PPP)", RFC 1661,
July 1994
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
9. Author Information
Peter Arberg
Redback Networks, Inc.
300 Holger Way
San Jose, CA 95134
Email: parberg@redback.com
Diamantis Kourkouzelis
Redback Networks, Inc.
300 Holger Way
San Jose, CA 95134
Email: diamondk@redback.com
Mike Duckett
BellSouth Telecommunications, Inc.
575 Morosgo Drive
Atlanta, GA 30324
Email: mike.duckett@bellsouth.com
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Tom Anschutz
BellSouth Science and Technology
725 W. Peachtree St.
Atlanta, GA 30308
Email: tom.anschutz@bellsouth.com
Jerome Moisand
Juniper Networks, Inc.
10 Technology Park Drive
Westford, MA 01886
Email: jmoisand@juniper.net
8. Full Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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