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Hi,
I'm an Italian student and I'm doing a Master Thesis on TCP rate
control.
TCP rate control is a new technique for transparently augmenting
end-to-end TCP performance by controlling the sending rate of a TCP
source. The sending rate of a TCP source is determined by its window
size, the round trip time and the rate of acknowledgments.
TCP rate control affects these aspects by modifying the ack number
and receiver window fields in acknowledgments and by modulating the
acknowledgment rate. From a performance viewpoint a key benefit of
TCP rate control is to avoid adverse performance effects due to
packet losses such as reduced goodput and unfairness or large spread
in per-user goodputs. Further, TCP rate control positively affects
performance even if the bottleneck is non-local and the end-host
TCP implementations are non-conforming. [see article below]
I would like to know your opinions about that, and if there is
already something similar in the Linux world. I've already see
the kernel sources but I didn't find something like that.
I'll appreciate suggestion about how to implement such ideas on
Linux, specifically in the kernel 2.4.3.
There are two algorithms in the TCP rate control technique, as
describe in the paper below, may be usefull for you to have the
algorithms:
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Algorithm 1 - Rate Allocation Algorithm
The goal of this algorithm is to allocate max-min fair rates to
competing TCP flows. Initially, equal rate allocations are given to
all competing flows. Then sending rates of flows are estimated by
maintaining an exponential average over a rate sampling interval.
When a flow does not to utilize its allocation, it is labeled as
bottlenecked. Excess allocation is stripped off from all such
bottlenecked flows and allocated to non-bottlenecked or hungry flows.
This step is repeated until there is no residual bandwidth to
allocate or all flows are bottlenecked. This algorithm is invoked
every time a new flow begins, a flow terminates or when the rate
sampling interval expires. The resulting rate allocations are
stored into a table.
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1. For each flow i, let Ri be the measured rate
and Ai be the allocated rate.
2. If N is the total number of flows and B is the bottleneck
capacity, the initial allocation for each flow i is
Ai = B / N (2)
3. If Ri < (p * Ai) for some satisfaction percentage p
(a statically chosen parameter), then mark flow i as
bottlenecked, else mark it as hungry.
4. Let U be the aggregate residual bandwidth i.e the
bandwidth which remains unutilized by the bottlenecked
flows.
5. Distribute this residual bandwidth evenly over all the
hungry flows. If H is the total number of hungry flows,
the new allocation for a hungry flow j is given by
Aj = Aj + (U / H) (3)
6. For each bottlenecked flow k the new allocation is
given by
Ak = (Ak + Rk) / 2 (4)
So for bottlenecked flows, the allocation approaches
the measured rate over successive iterations of the
algorithm.
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About this algorithm, I've a problem... how can I measure the rate
Ri for each TCP flow? I've found NeTraMet on this URL:
http://www.auckland.ac.nz/net/Accounting/ntm.Release.note.html
... and I've also read the discussion about that on linux-kernel
mailing list. Where is the best place to make a such thing?
I've thought in /net/core/dev.c in function dev_queue_xmit.
And, again, how can I associate the rate Ri to each TCP flow?
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Algorithm 2 - Rate Enforcement Algorithm
This algorithm enforces the rate allocation by converting the rate
to a window value. Additionally, it spaces out the acknowledgments
of a flow, so that they are evenly distributed over its RTT.
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1. For each flow i let
wi = the calculated window size in units of packets.
Di = the inter-ack spacing in seconds.
RTTi = the round trip time (RTT) of flow i, ideally
not including queuing delays, in seconds.
MSSi = the maximum segment size of flow i, in bytes.
Ai = the rate allocation in bytes/s.
2. Observe that for each flow i, we have:
wi * MSSi = Ai * RTTi (5)
So the window value can be calculated as,
wi = (Ai * RTTi) / MSSi (6)
3. The inter-ack spacing time (RTTi/wi) can also be
obtained from equation (5):
Di = RTTi / wi = MSSi / Ai (7)
4. For each flow i, acks (if available) are clocked out
at intervals of Di with the receiver window field set
to Wi.
5. The receiver window field of the ack of flow i is set as:
Wi = min(wi * MSSi , actual receiver window) (8)
6. The ack number field in the header is determined based
upon two variables (max and min sequence number) used to
denote the ack queue. In the common case, the ack number
would be chosen to progress by one MSSi.
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About this algorithm, where is, for you, the best place
where to do a such thing? In general I'd like to do only minor
changes possible in the kernel sources, and I would like to
implement a module for TCP rate control.
I've also found an usefull paper by Glenn Herrin, I've try the
example with the kernel v2.4.3 but I've some problem with it:
when I export a symbol from the kernel, using EXPORT_SYMBOL and I
try to use it from a module I have the error: "undefined symbol in
mymodule" even if it's listed in /boot/System.map
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Papers:
o Shrikrishna Karandikar, Shivkumar Kalyanaraman,
Prasad Bagal, Bob Packer,
"TCP Rate Control",
Computer Communication Review, a publication of ACM SIGCOMM,
volume 30, number 1, January 2000. ISSN # 0146-4833.
URL:
http://www.acm.org/sigcomm/ccr/archive/2000/jan00/ccr-200001-kara.html
o Glenn Herrin,
Linux IP Networking,
A Guide to the Implementation and
Modification of the Linux Protocol Stack
TR 00-04, May 31, 2000
URL: http://kernelnewbies.org/documents/ipnetworking/
o Others interesting papers:
URL:
http://www.ecse.rpi.edu/Homepages/shivkuma/research/papers-rpi.html#tcp
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Commercial products based on idea of TCP Rate Control:
o Packeteer - http://www.packeteer.com/ [nasdaq: PKTR]
o Allot - http://www.allot.com/
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I beg your pardon for the lenght of this message, and also for my
bad english :/ I hope this is the right place for a such discussion,
if not, please, excuse me, and if you know send me some references.
Greetings,
Paolo Castagna.
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