Internet Engineering Task Force Tmima Koren Point-to-Point Protocol Extensions Working Group Jagdish Sonti INTERNET-DRAFT Cisco Systems EXPIRES: February 2006 July 2005 Optimization of PPP Multiplexing over PPP Multilink draft-koren-pppext-pppmux-over-multilink-00.txt Status of this memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Copyright Notice Copyright (C) The Internet Society (2005). All Rights Reserved. Abstract PPP Multiplexing combines several packets into one larger PPP packet while PPP Multilink fragments large PPP packets into a few smaller PPP packets. This document explains how to optimize the usage of these two protocols together. 1. Introduction PPP Multiplexing (PPPMux) [PPPMUX] is used over PPP links. It combines a few PPP packets into one PPP packet. The main usage of PPPMux is to reduce the bandwidth usage of a PPP link: the muxed packet is shorter than the combined length of the individual packets. PPP Multilink (MLPPP) [MLPPP] is used to combine a few serial links into one logical link. The MLPPP protocol can also fragment PPP packets that are larger than the link MTU into smaller fragments that are sent over the link and reassembled by MLPPP at the other end of the PPP link. When both PPPMux and MLPPP are used together we have two competing protocols, one trying to enlarge packets and the other trying to reduce the packet size. It is good design practice to multiplex the packets to a size that will not have to be fragmented by MLPPP. The disadvantages of fragmenting a multiplexed packet areIf a multiplexed packet has to be fragmented, the bytes added by MLPPP cancel the savings of bytes by PPPMux, and a lot of processing power is wasted multiplexing and fragmented. 1.1. Multilink and link MTU Each link in a multilink has an MTU associated with it. The link MTU can be different from link to link. For example, one link can be a full T1 and the other only half a T1. If some of the traffic over the multilink is delay sensitive, such as voice packets, then it is likely that the MTU of the narrower link (the half T1) will be set to a smaller value than the MTU of the wider link (the full T1). When a packet that is larger than the link MTU is to be sent over that link, the packet will be fragmented by MLPPP before it is sent over the link. 1.2. PPPMux and link MTU PPPMux is performed before MLPPP. It is not desirable to fragment a multiplexed packet. The efficiency gain achieved by multiplexing is nullified by the additional multilink headers added on multiple fragments. Multiplexing and then fragmenting the same packets is a wasteful computational expense. In order to avoid multiplexing followed by fragmentation, PPPMux should limit the size of the currently multiplexed packet to the MTU of the link over which this packet is going to be transmitted. 2. IANA Considerations This document does not require any assignments from IANA. 3. Security Considerations The security issues of this document are the same as [PPPMux]. 4. References Normative References [PPPMUX] Pazhyannur, R., Ali, I., and C. Fox, "PPP Multiplexing", RFC3153, August 2001 [MLPPP] Sklower, K., Lloyd, B., McGregor, G., Carr, D., and T. Coradetti, "The PPP Multilink Protocol (MP)", RFC 1990, August 1996. 5. Authors' Addresses Tmima Koren 170 West Tasman Drive San Jose, CA 95134 United States of America Phone: +1 408 527 6169 Email: tmima@cisco.com Jagdish Sonti 170 West Tasman Drive San Jose, CA 95134 United States of America Phone: +1 408 853 6880 Email: jsonti@cisco.com 6. Copyright Notice 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. 7. 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