[e2e] Short Fat Networks, tcp and Policers
puddinghead_wilson007 at yahoo.co.uk
Thu May 13 12:05:16 PDT 2004
I have some queries
Please follow the >>
A) Minimum throughput
When you browse through tcp literature, you will
usually come across
the Bandwidth x Delay=Window formula. It has a very
meaning: If you want to get a full utilization of the
you better have a window that is large enough to fill
the RTT with packets.
Even a simpleton like myself knows this. But let us
ask the question in
reverse: How *low* a bandwidth can we pass through a
line with a given, fixed,
delay? Some simple thinking will show you than it is
(almost) impossible to
deliver less that a few MSS each RTT. Why? Because
even during slow start, tcp
sends a new packet once an ack is recieved.
>> excellent point, I would appreciate if this list
for one clarified the "MSS". Today we support jumbo
frames etc. Does this imply we increase the MSS
propotional to the MTU?
What is the implication of the above? That in order to
cause a tcp flow to use *less* than the
physically available link, one must *increase* the
RTT. Of course, this is exactly what shapers
do, by buffering the data. alternatively one could
buffer the acks ( and I think that "ack holding"
schemes have been proposed in the literature), but
that requires L3- L4 knowledge and treatment of
>> Shapers also provide something else. They let you
differentiate services. Assume that one wants to
really run a "converged network" which carries voice,
video and data, one also needs the flexibility to have
CBR. Shappers can do that.
Eitherways, shappers or no shappers, if everything was
"policers" or say, slotted/TDM access based, there
would still always be a point in the network where one
would have to "buffer", and that is the point of
ingress into the "policed" core. This specifically
holds true for cases where we have say a 100Mbps LAN
and a X<100Mbps WAN slotted access bandwidth in front.
B) tcp timeout Phase
So what happens when you try to limit bandwidth below
the MSS/RTT limit? A policer achieves this
by discarding all packets that are non conforming, and
this will cause the session to run in a
burst-timeout-burst phase. Typical implementations set
timeouts to as much as several hundred msec.
In this phase ( that is , when ther required bandwidth
is less that Const X MSS/RTT), the bandwidth
can range from zero to [Policer Burst Size]/[Tcp
Timeout] . This phase is not efficient, and causes
bursts in the network. For timouts around 500 msec, a
4 mb/s session ( which is not unreasonal on LANs
and WANs) needs a policer burst of 512KB. since bursts
tend to get translated into buffer sizes, we see
that a single service eats up quite a large buffer
size. The irony being, that in the timeout phase, the
burst is not buffered, because if the burst was
buffered, the RTT would increase, and we wouldn't have
in the Timoeout phase in the first place.
C) Partial window phase
A reasonable way to generate sub-physical line rates,
( without adding to the RTT), is to cause
the tcp to work at a window that is less that
[physical Rate]x[RTT]. The frame pattern would be
something like M frames every RTT, with MxMSS/RTT ~
[policed rate]. This is a much better behaviour
than the timeout phase, and a good policer design
should strive to reach this phase. As pointed out
before, this phase cannot exist when the required rate
is too low, or the RTT is too short.
>> Which is why TCP slow starts I presume (I believe
the more "literate" on the list can correct me if I am
D) Stability of the partial window phase
Eventually, because of slow start or congestion
avoidence, the number of frames in the window M
slowly creeps up, until the policer "realizes" that
the policed rate has been passed, and the policer
will discard a series of frames. If this is done
delicately enough ( suppose using a RED like
Fast retransmission will take place and the session
shall be able to slow start its way back to the target
M frames per RTT.
If the policing is too drastic, either an entire
window of M packets will be discarded, or the
fast-retransmit frame itself will be lost, and a
timeout will occur.
E) tcp defence lines and policers
tcp has three defence lines against congestion
* self clocking
* congestion window + slow start
* retransmission timeout
Not only do they protect the network, they also
control the bandwidth that the application recieves.
Policers neutralize completely the first line of
defence, since they have no effect on the RTT ( or on
more subtle inter packet gap ). The only way that
policers can indicate rate to the tcp layer is by
packet discard. Tcp responds to packet discard by
retransmission timeouts or fast retransmission, both
are considered inefficient, but compared to the huge
problems caused by timeouts, the slight ineffciency
caused by fast retransmission induced slow start, is
minor. The estimation of timeouts based on averaged
statistics are totally irrelevant when the actual
network performance is below 100msec, but the
tcp tick is 500msec.
>>Also consider the case of "how soon can that
feedback" be given to the sliding window mechanism so
as to detect that there was a "congestion". We are
talking about a system where the propogation delay or
duration of "feedback" is considerable compared to the
rate at which the input comes/packets are transmitted.
F) What is the point?!
The points are
1- policers are much easier to build that shapers, so
we should start to understand them.
>> Considering all the points you have made, it seems
shappers should be the obvious choice. They need not
be the "exact buffered" shappers as you mention them,
but they could be made such that:
a. minimal buffering is needed at the end points.
b. There is a way to differentiate CBR streams from
non CBR streams
2- acceptance tests are usually done with very short
RTT times, so that the timeout phase is
3- All these exponential smoothings and estimations of
RTT RMS are useless if the smallest timeout
is 300msec, and typical LAN's are less that 50msec.
>>Why would you say that?
4- alot of TCP improvements have been based around
"LFN"'s but it may turn out that alot of the broad
band networks are really "SFN"'s (Short Fat Networks).
>>Let me put it this way, performance is always
"relative". If you sit near an end point then it is
SFN for you, else it is a LFN. Would that not be true?
Eitherways, the idea is to get "more" out of an
existing infrastructure, not to say "this
infrastructure was not made for this".
G) The End
Thank you for you patience, comments are welcome.
Yet Another Simpleton
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