Internet DRAFT - draft-koren-pppext-pppmux-over-multilink
draft-koren-pppext-pppmux-over-multilink
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
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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
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except as set forth therein, the authors retain all their rights.
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