Subject: Correction for PEP document

From: Jean Tourrilhes <[email-masked]>
Date: Tue, 13 Mar 2001 14:31:50 -0800

Hi,

	I've just read the PEP document (Performance Enhancing
Proxies). The chapter 5.3.3 doesn't read well and is quite confusing
in my opinion. Most link level designers reading this chapter will
assume that the goal of PEP is to help interfacing TCP/IP directly
with the wireless modem without using a MAC protocol, which I guess is
not the intention of the authors (please correct me if I am wrong).

	Therefore, I propose to replace chapter 5.3.3 with the text
below. This is of course a first draft and is probably a bit too long,
detailed and clumsy, but people on the list will hopefully refine it ;-)
	Of course, if you have any question, please ask me...

	Thanks, and have fun...

	Jean

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5.3.3 W-LAN PEP Motivation

   Wireless LANs suffer from an error prone wireless channel. Errors
   can typically be considered bursty and channel conditions may
   change rapidly from mobility and environmental changes. Packets are
   dropped from bit errors or during handovers. Periods of link outage
   can also be experienced. All CSMA protocols (such as 802.11) also
   drop packets due to channel contention, usually with a probability
   much higher than Ethernet [802.3] due to a small contention
   window. Although the typical link is relatively high speed with low
   latency, dropped packets have serious performance implications for
   IP performance, especially TCP.

   All modern W-LAN MAC protocols include low latency MAC level
   retransmissions (802.11, HiperLan, HiperLanII, SWAP, OpenAir, IrDA,
   BlueTooth, 802.15...). Those MAC retransmissions hide all packet
   losses due to channel error and channel contention with negligeable
   latency increase, so that TCP performance is not adversly
   affected. This mechanism are similar in nature and effect to the
   retransmissions over Ethernet [802.3] that shield TCP for the
   effect of channel contention.

   However, there are some case where the MAC doesn't shield TCP from
   packet losses and where the use of PEPs is needed to alleviate
   problems caused by packet losses and to increase performance :
        o W-LAN protocols designed more than 10 years ago (for example
          a few obsolete pre-802.11 products such as the original AT&T
          Wavelan).
	o Poorly implemented or poorly tuned MAC protocols (for example
	  too few MAC retransmissions, or packet delivered out of
	  sequence).
	o Packet losses happening above the MAC layer.

   PEPs can be used to protect TCP from long link outages, and to add
   priority multiplexing.  Techniques such as Snoop are integrally
   implemented in access points, while priority and compression
   schemes are distributed across the W-LAN.
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