[e2e] TCP Loss Differentiation

Fahad Dogar fahad.dogar at gmail.com
Mon Feb 9 09:43:37 PST 2009

On Mon, Feb 9, 2009 at 11:24 AM, David P. Reed <dpreed at reed.com> wrote:

> Before going too far in this direction, one should note that unicast
> traffic on layer 2 transports commonly used in practice for Internet
> transport has negligible loss rates, even on wireless networks such as
> 802.11.

I guess you are restricting yourself to 'well behaved'  802.11 settings.
Multi-hop networks (with outdoor links) and mobility scenarios (such as wifi
from moving cars) do experience losses even with link layer reliability and
no loss of connection.

> The problem of differentiation arises when attempting to elide layer 2
> functionality and run "TCP/IP on bare PHY".  Otherwise "link loss rate" is a
> concept without much reality at layer 3.  We don't run TCP/IP on bare PHY
> layers.  We run it on layer 2 protocol, over PHY layers, which protocols
> always have high reliability today.  Some  multicast layer 3 protocols run
> on unreliable layer 2 multicast protocols (such as 802.11 multicast), but
> TCP/IP never uses multicast.

> Layer 3 losses are nearly always the result of *only* 2 very different
> phenomena: 1) buffer overflow drops due to router/switch congestion queue
> management or 2) layer 2 breaks in connectivity.
> Thinking about "link loss rates" is a nice academic math modeling exercise
> for a world that doesn't exist, but perhaps the practical modeling
> differentiation should focus on  these two phenomena, rather than focusing
> on "link loss rates".  The "connectivity break" case (which shows up in
> 802.11 when the NIC retransmits some number of times - 255?) doesn't have
> very good statistical models, certainly not the kind of models that can be
> baked into TCP's congestion/rate control algorithms.  And that model is not
> likely to be poisson, or any distribution easily characterized by a "rate
> parameter"


> Detlef Bosau wrote:
>> Hi.
>> Some years ago, the issue of packet loss differentation in TCP was a big
>> issue. Does somebody happen to know the state of the art in this area?
>> I'm particularly interested in those cases were we do _not_ have a
>> reliable knowledge about the loss rate on a link. (So, particularly the
>> approach by Allman and Eddy cannot be easily applied.)
>> Thanks.
>> Detlef
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