Link layer ARQ paper
Jean Tourrilhes <jt@bougret.hpl.hp.com> Fri, 09 March 2001 19:17 UTC
Date: Fri, 09 Mar 2001 11:17:10 -0800
To: "gorry@erg.abdn.ac.uk" <GorryFairhurst@bougret.hpl.hp.com>, "lwood@cisco.com" <LloydWood@bougret.hpl.hp.com>
Cc: pilc@grc.nasa.gov
Subject: Link layer ARQ paper
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From: Jean Tourrilhes <jt@bougret.hpl.hp.com>
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Hi, I just came across you IETF draft : http://www.ietf.org/internet-drafts/draft-ietf-pilc-link-arq-issues-01.txt First, I must congratulate you on the very high quality of your work. Your analysis is very complete and accurate, and echo what I've been thinking for quite a while. It dispell efficiently of the old myth present in the TCP/IP community (Wireless link == loss, let's fix TCP). It's one of the best thing on the subject I've read for years ;-) The second thing, I would like to know if there exist a permanent URL for this work. I would like to link to it from my web page where I have article related to a similar topic, and it would be a welcome addition (I still have to dispell the myth). Now, I have a few suggestions of topic to further enhance this work ;-) Hey, when you are on a roll... Sorry if these topics have already been beaten to death, but I just thought you might be interested... You'll probably need to pick up the papers on my web page wich give a few clues about what I'm talking about : http://www.hpl.hp.com/personal/Jean_Tourrilhes/Papers/papers.html In one of my paper, I discuss limits on ARQ. 802.11 has a maximum number of retries, whereas I advocate a maximum time to live for the Tx packet. As I'm not a specialist of TCP/IP, I've no idea of what would be an ideal MAC TTL, but maybe you could give us clues based on link characteristics... It would be even better if the MAC had an way to determine this optimal TTL based on some simple metric of the current TCP/IP traffic (i.e. : should the MAC calculate the TCP RTO ?)... <Paper : SWAP and TCP/IP> Cause of packet losses : you've forgotten the number 1 cause of packet loss over 802.11, which is MAC contention. From my similations of 802.11, it's not unusual to have 10% losses or more due to 2 nodes picking up the same slot in the contention window. Of course, this applies only to CSMA/CA protocols. <Paper : Robust Broadcast> Problem for broadcast packets : ARQ applies only to Unicast, so expect losses for Broadcast. Just provide a small warning that this discussion applies only to Unicast stream, and the multicast and broadcast are a different matter (maybe another paper for you there ;-). <Paper : Robust Broadcast> Not everything is TCP. The classical argument for removing ARQ from the link layer is that TCP will take care of that. However, not everything is TCP traffic, and you can have UDP and ICMP traffic. For example, VoIP won't sustain a 10% frame erasure rate, so the link has better to ARQ. <Aironet mailing list discussion -> ping losses> ARQ latency. I believe that for Wireless LAN (very small local RTO), stop-and-go is better that a sliding window mechanism, because it keeps the delay jitter minimal and keep packets in order. Keeping packet in order is especially important because of fast retransmit. However, I could not seem much difference in practive (IrDA use a 7 frames window, 802.11 use stop-and-go). Comments ? <Aironet mailing list discussion> Half duplex links : I thought half duplex would be a problem. IrDA use a half duplex link and turnaround the link on a regular basis. With no load, ping RTT is 10ms, but climb up to ~100ms when link is loaded (due to increased window size). On the other hand, TCP throughput is around 3.2 Mb/s (over 4 Mb/s link). One of my early concern with BlueTooth is the short CRC (16 bits), especially with regards to DM5/DH5 packets (802.11 and SWAP have 32 bits). BlueTooth has FEC in DM5, but that's worthless against deep fading and interference. With IrDA that has a 16 bits CRC at the link layer, I had occurence of garbage going up the stack in very noisy environments. On the other hand, TCP/IP is checksumed as well, so we should be safe ? Differential latency : there is some case of 802.11FH where latency of large and small packet are very different (factor 10). If the latency is small enough, can I assume that it doesn't matter ? <Dwell Adaptive Reduction> Link adaptation and variation. Most 802.11 products have automatic bit rate selection, changing the modulation based on the current SNR. Also, base on the BER, you can vary the amount of fragmentation, which changes the overhead. For wide area, protocol like GPRS use the spare slots of a cell, so data capacity vary in function of voice calls and handovers. So far, it seems that TCP/IP adapt pretty well to that. Not related to this paper, but... <Fragment adaptive reductions> Interferer, roaming, fading, ... The impact of those stuff on Wireless LANs tend to be a long period where nothing go through (seconds) and packet are dropped despite ARQ (hit max retries). That's the classical link handover problem. With BlueTooth, a link handover can take up to 20s (because of inquiry+scan). I guess that Fast recovery will deal with that... <Fragment adaptive reduction> Sorry for the length, I hope you will find my random blurb usefull, and have fun... Jean
- Link layer ARQ paper Jean Tourrilhes
- Re: Link layer ARQ paper Jean Tourrilhes