RFC 4638
Accommodating a Maximum Transit Unit/Maximum Receive Unit (MTU/MRU) Greater Than 1492 in the Point-to-Point Protocol over Ethernet (PPPoE)
Pages: 9
Informational
Informational
Network Working Group P. Arberg Request for Comments: 4638 D. Kourkouzelis Category: Informational Redback Networks M. Duckett T. Anschutz BellSouth J. Moisand Juniper Networks September 2006 Accommodating a Maximum Transit Unit/Maximum Receive Unit (MTU/MRU) Greater Than 1492 in the Point-to-Point Protocol over Ethernet (PPPoE) 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 Internet Society (2006). IESG Note As of this writing, no current IEEE standard supports the use of "jumbo frames" (MTU greater than 1500). Although this document contains recommended mechanisms to detect problems in the path, interoperability and reliability of non-standard extensions cannot be assured. Both implementors and users of the protocol described here should exercise caution in its use.Abstract
The Point-to-Point Protocol over Ethernet (PPPoE), as described in RFC 2516, mandates a maximum negotiated Maximum Receive Unit (MRU) of 1492. This document outlines a solution that relaxes this restriction and allows a maximum negotiated MRU greater than 1492 to minimize fragmentation in next-generation broadband networks.
Table of Contents
1. Introduction ....................................................2 2. Terminology .....................................................4 3. Proposed Solution ...............................................4 4. PPPoE Discovery Stage ...........................................5 5. LCP Considerations ..............................................5 5.1. MRU Negotiations ...........................................5 5.2. MRU Test and Troubleshooting ...............................6 6. Security Considerations .........................................7 7. IANA Considerations .............................................7 8. Acknowledgements ................................................7 9. Normative References ............................................7 10. Informative References .........................................81. Introduction
As broadband network designs are changing from PC-initiated PPPoE [1] sessions in a combined Ethernet/Asynchronous Transfer Mode (ATM) setup, as shown in Figure 1, to more intelligent PPPoE-capable Residential Gateway (RG) and Gigabit Ethernet/ATM broadband network designs, as shown in Figures 2 and 3, the need to increase the maximum transmit and receive unit in the PPPoE protocol is becoming more important in order to reduce fragmentation in the network. <------------------ PPPoE session ------------------> +-----+ +-----+ +--+ +---+ | | | | |PC|--------------|CPE|-----------|DSLAM|-----------| BRAS| +--+ <Ethernet> +---+ <ATM> | | <ATM> | | +-----+ +-----+ Figure 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. Therefore, a PPP-negotiated MRU of 1492 octets is fully acceptable, as it makes the largest PPPoE frame adhere to the standard Ethernet MTU of 1500 octets.
<----- IPoE -----> <--------- PPPoE session ---------> +-----+ +-----+ +--+ +---+ | | | | |PC|--------------| RG|-----------|DSLAM|------------| BRAS| +--+ <Ethernet> +---+ <ATM> | | <GigE> | | +-----+ +-----+ Figure 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 uses a Maximum Transit Unit (MTU) of 1500 octets. However, when the Residential Gateway and the Broadband Remote Access Server (BRAS) are the PPPoE session endpoints and therefore negotiate an MTU/MRU of 1492 octets, the result is 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> | | +-----+ +-----+ Figure 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 Point-to-Point Protocol over AAL5 (PPPoA) and PPPoE sessions. The subscriber session is a PPP session running over a combination of PPPoA and PPPoE. The PPP/PPPoA host typically negotiates a 1500- octet MRU. Widely deployed PPP/PPPoA hosts in Customer Premises Equipment (CPE) do not support a 1492-octet MRU, which creates an issue in turn for the BRAS (PPPoE server) if strict compliance to RFC
2516 [1] is mandated. For PPP/PPPoA hosts capable of negotiating a 1492-octet MRU size, then we are back to a fragmentation issue.2. 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 RFC 2119 [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 PPPoE - client PC, RG, or CPE that initiates a PPPoE session PPPoE - server BRAS terminating PPPoE sessions initiated by client PADI - PPPoE Active Discovery Initiation PADO - PPPoE Active Discovery Offer PADR - PPPoE Active Discovery Request PADS - PPPoE Active Discovery Session-confirmation3. Proposed Solution
The procedure described in this document does not strictly conform to IEEE standards for Ethernet packet size but relies on a widely deployed behavior of supporting frames with Ethernet packet format, but exceeding the maximum packet lengths defined by [4]. 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 octets in order to limit the amount of fragmented packets in networks similar to those described in Section 1. By default, the Maximum-Receive-Unit (MRU) option MUST follow the rules set forward in RFC 1661 [2] but MUST NOT be negotiated to a size larger than 1492 to guarantee compatibility with Ethernet network segments limited to 1500-octet frames. In such a case, as the PPPoE header is 6 octets and the PPP Protocol ID is 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 the PPPoE server, the server MAY allow the negotiation of an MRU larger than 1492 and the use of an MTU larger than 1492, subject to limitations of its local configuration and according to the rules set forward in RFC 1661 [2], within the limits of the maximum payload size indicated by the PPPoE client.4. PPPoE Discovery Stage
If a PPPoE client wants to use an MTU/MRU higher than 1492 octets, then it MUST include an optional PPP-Max-Payload Tag in the PADI and PADR packets. If the PPPoE server can support an MTU/MRU higher than 1492 octets, it MUST respond with an echo of the clients tag in the PADO and PADS packets when the PPP-Max-Payload 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 a given maximum PPP payload greater than 1492 octets for both the sending and receiving directions. Note that this value represents the PPP payload; therefore it is directly comparable with the value used in the PPP MRU negotiation.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 size larger 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 {
PPP_MRU_Max = 1492
If (PPP-Max-Payload-Tag) AND (PPP-Max-Payload-Tag > 1492)
Then 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 and greater than 1492, it MUST
be considered along with the server's interface MTU settings when the
maximum value is selected for the normal RFC 1661 [2] MRU negotiation
which decides the actual MRU to use.
If the PPP-Max-Payload tag isn't present or is present but below
1492, then the existing MRU constraint of 1492 octets MUST stay
applicable, thus preserving backward compatibility.
This, in summary, indicates the following behavior:
1. When a "PPP-Max-Payload" tag is received,
a. the value in this tag will indicate the maximum MRU allowed to
be accepted or suggested in an MRU negotiation; and
b. if MRU is not negotiated, then RFC 1661 [2] will set the
default MRU at 1500. This will say that the "PPP-Max-Payload"
tag can have a value greater than 1500, but in this case RFC
1661 [2] sets the default MRU to 1500, and only if MRU is
negotiated higher (up to maximum payload) will the "PPP-Max-
Payload" tag value be used.
2. When a "PPP-Max-Payload" tag is not received by either end, then
RFC 2516 [1] sets the rule.
5.2. MRU Test and Troubleshooting
If the MRU is negotiated to a value larger than 1492 octets, the
sending side SHOULD have the option of sending 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 Ethernet segments
and equipment can handle such a packet size.
If no Echo-Replies are received, the sending side MAY choose to
repeat the test with 1492 octets 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 enabled by default. It SHOULD be configurable and MAY be disabled on networks where there is some prior knowledge indicating that the test is not necessary.6. Security Considerations
This document does not introduce new security issues. The security considerations pertaining to the original PPPoE protocol [1] remain relevant.7. IANA Considerations
This document defines a new value in a space that currently has no IANA registry. There is work in progress to define a registry [5] and that document already contains the value assigned here. No IANA action is required for this document.8. Acknowledgements
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.9. Normative References
[1] 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. [2] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, July 1994. [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [4] Institute of Electrical and Electronic Engineers, IEEE Std 802.3-2005, "IEEE Standard for Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications - Draft amendment to - Information technology - Telecommunications and information exchange between systems - Local and metropolitan area networks - Specific requirements - Part 3: Carrier sense multiple access with collision detection (CSMA/CD) access method and physical layer specifications - Media Access Control Parameters, Physical Layers and Management Parameters", December 2005.
10. Informative References
[5] Arberg, P. and V. Mammoliti, "IANA Considerations for PPP over Ethernet (PPPoE), Work in Progress, June 2006.Authors' Addresses
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 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
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