[e2e] TCP Loss Differentiation

Detlef Bosau detlef.bosau at web.de
Mon Feb 9 19:30:26 PST 2009

David P. Reed wrote:
> I was (perhaps not very clearly) including multihop and mobility in 
> "loss of connection" cases.   I meant loss of PHY or layer 2 
> connectivity - not "session connectivity." Those situations do, as you 
> suggest, drop packets when "link layer reliability" fails - but I 
> would call the cause of that loss process a "loss of connectivity" 
> however transient or healable.

So, the question is: What is "loss of connection"?

In cellular networks, this could mean a mobile is detached from its base 
station.  It could mean as well: A mobile is not served by its base 
Allegedly, in HSDPA a transport block (L1, L2) is not scheduled if a 
channel's quality is too bad.

So, a "bad channel" can introduce an unpredictable delay, because in 
general we cannot predict when a channel's quality will become "good" 
again, if at all.
In case a block is scheduled despite the "bad channel", the block may be 
eventually lost.

In both cases, the underlying problem is the "bad channel" which we can 
neither predict nor prevent.

> My main point was that these loss processes are not characterizable by 
> a "link loss rate".  They are not like Poisson losses at all, which 
> are statistically a single parameter (called "rate"), memoryless 
> distribution.  They are causal, correlated, memory-full processes.

Differently put, perhpas more simple: Our knowledge about a wireless 
channel is extremely small. And from what I've seen so far even in
scientific papers, we tend to use extremely simplified channel models. 
E.g. _pure_ Rayleigh channels. Or we _only_ consider distance based 
loss. (of signal strength).
> And more  importantly, one end or the other of the relevant link 
> experiences a directly sensed "loss of connectivity" event.
In cellular networks, this may cause a mobile to attach to another base 
station. This process itself may cause random loss. This loss is not due
to packet (block) corruption but due to roaming, if the technology in 
use does not provide a handover process.

Hence, there is actually more than one reason for packet loss in mobile 
networks which is _not_ caused by congestion.

> Thus my point: one SHOULD NOT model practical TCP/IP congestion/flow 
> control based on an assumption of "links" with "loss rates" as if they 
> were Poisson loss processes.  One should instead focus on modeling 
> loss processes that come from congestion and from loss of link 
> connectivity or route changes arising from responses to connectivity 
> loss in the appropriate ways that reflect practical reality.

Where a "route change" (which covers my comment on roaming) may result 
not only in packet loss but in a sudden change of path capacity as well.

BTW: If HSDPA does not support handover, HSDPA is not very well suited 
for media streaming. This is off topic in this post, but I just think 
about it because a huge number of papers talks about media streaming in 
HSDPA. This may work - as long as the mobile is not mobile ;-)

However, what are the consequences from an end to end point of view?

Do you agree that in cellular mobile networks, there is significant 
packet loss which is not congestion related? And which may lead to 
throughput degradation?



Detlef Bosau                          Mail:  detlef.bosau at web.de
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