IPS MoMe05Davoli

7/29/2019 IPS MoMe05Davoli

1/21

Franco Davoli, Giuseppe Span,

Stefano Vignola, Sandro Zappatore

CNIT - Italian National Consortium for Telecommunications

National Multimedia Communications Laboratory, Via Diocleziano 328, Napoli, Italy

and

Department of Communications, Computer and Systems Science (DIST)

University of Genoa, Via Opera Pia 13, Genova, Italy

(franco.davoli, giuseppe.spano, stefano.vignola, sandro.zappatore)@cnit.it

Performance Measurements and Comparison of

Modified TCP Control Algorithms

over Rain-Faded Satellite Channels


2/21

Introduction and Objectives

Quite a few TCP modifications have been proposed, analyzed and

implemented, to cope with large bandwidth-delay product and error-pronenetworks, where the standard mechanisms of congestion control present

problems.

For many of such implementations, patches to the Linux kernel are

available, which can be used to effectively configure the modified TCP

and to use it in real world tests.

However, in most cases the performance has been evaluated by

simulation, and there are rather few works on measurements effected on

real satellite networks or on emulated laboratory environments.

The main goal of the paper is to compare some modified TCP congestion

control algorithms in a laboratory setting, by performing repeatableexperiments, involving traffic generators, routers, and an accurate satellite

channel emulation with recorded fading data.


3/21

Laboratory setting


4/21

Laboratory setting (contd)

We have emulated the satellite link by using the facilities offered by the CNIT

National Laboratory for Multimedia Communications in Naples, Italy.

The LAN consists of a set of hosts devoted to generate TCP traffic. According to

the specific test, up to ten connections can be running at the same time.

All packets produced feed the router # 1 (CISCO 3640), which conveys them to a

serial V.35 interface, adopting a HDLC protocol at the data link layer. Thesynchronous data flow is input to a Fairchild SM290 modem (modem # 1),

operating in quadrature phase-shift keying (QPSK), at an information rate of 2048

kbit/s, with 3/4 sequential FEC.

The outgoing modulated signal is applied to a Channel Simulator (here used as a

simple programmable attenuator) and then added to a Gaussian noise, produced bya noise generator. Both the attenuation and noise variance are suitably changed, to

reproduce the effects of real world fading as concerns the bit error probability

observed at the receiver end.


5/21

Laboratory setting (contd)

The link budget was derived according to the Italsat satellite characteristics(G/T=23 dB at the receiver, under the adopted parameter values).

After a delay of 250 ms, the corrupted signal feeds the receiver portion of asecond modem, which interfaces with the router # 2.

The return channel (i.e., the link connecting the outgoing signal from modem #

2 to the input of modem # 1) introduces again a delay of 250 ms, but it is notaffected by errors.

Fading attenuation values are taken from the results of the propagationexperiment carried out in Ka band on the Olympus satellite by the CSTS(Centro Studi sulle Telecomunicazioni Spaziali) Institute, on behalf of theItalian Space Agency (ASI). The attenuation samples considered were 1-

second averages, expressed in dB, of the signal power attenuation with respectto clear sky conditions. The adopted patterns are taken from data recorded atthe Spino dAdda (Northern Italy) station on September 21 and 23, 1992,respectively.


6/21

Fading Attenuation Patterns

4

6

8

10

12

14

16

0 20 40 60 80 100 120 140 160 180 200 220 240 260

Time [s]

Fading pattern "A"

Fading pattern "B"

The pattern indicated as B, which corresponds to a quite severe fading, causes a

temporary sync loss at the demodulator


7/21

TCP modifications

We have investigated the behaviour of 3 TCP modifications, and

compared it with that ofNew Reno:

TCP Westwood

TCP Peach

Complete Knowledge Satellite Transport Protocol (CK-STP)

The first two introduce modifications to the congestion control

mechanism; the third one only acts on the TCP New Reno

parametrization, by assuming that the characteristics of the satellite

bandwidth pipe are known at the transmitting station and losses are

only due to channel errors.


8/21

TCP New Reno

New Reno is an improved version of TCP Reno that avoids waiting for a

retransmit timer when multiple segments are lost from a window.

The change concerns the sender's behaviour duringFast Recovery, when

apartial ACKis received that acknowledges some, but not all of the

segments that were outstanding. In New Reno, partial ACKs receivedduring Fast Recovery are treated as an indication that the segment

immediately following the acknowledged one in the sequence space has

been lost, and should be retransmitted.

Thus, when multiple segments are lost from a single window of data,

New-Reno can recover without a retransmission timeout, retransmitting

one lost segment per round-trip time until all of the lost segments from

that window have been retransmitted.


9/21

TCP Westwood

End-to-end, sender-side estimate of the bandwidthB available to a TCPconnection and seen at the receiver, obtained by measuring and low-passfiltering the rate of returning ACKs.

When 3DUPACKs are received: ssthresh = (B* RTTmin) / seg_size;

cwnd = ssthresh.

When a coarse timeout expires:

ssthresh = (B* RTTmin) / seg_size;

cwnd = 1. When ACKs are successfully received, TCPW increases cwndaccording to

Reno's congestion control algorithm.

The main features of Westwood are the following:


10/21

TCP Peach

TCP Peach implements Congestion Avoidance andFast

Retransmit(the same as Reno), and two new algorithms:

Rapid Recovery and Sudden Start, which are based on the use

ofdummy segments, low priority segments generated by thesender as a copy of the last transmitted data packet.

If a router is congested, it discards dummy segments first,

due to their low priority. On the contrary, the sender interprets

the ACKs for dummy segments as the evidence that there are

unused resources on the link and, accordingly, it can increasethe transmission rate.


11/21


It follows the TCP New Reno implementation, with the adoption of the SACKoption. "Complete Knowledge" refers to the assumed knowledge about thephysical characteristics of the satellite link, in terms of propagation delay andchannel bandwidth.

It assumes that all losses over the satellite link are due exclusively to linkerrors. A constant transmission window set to the bandwidth-delay product is

employed, in order to achieve the maximum channel bandwidth. With respect to the TCP implementation, the slow start phase is no longer

entered, and for each received acknowledgement, the congestion windowkeeps its constant value. As far as the loss detection is concerned, the arrival ofonly one duplicate acknowledgement is necessary to trigger the recoveryphase, which consists of retransmitting the lost data segments. Once the

recovery phase has terminated, the protocol continues transmitting with theconstant transmission window as set previously.

The sender buffer is adjusted according to the bandwidth delay-product.

Complete Knowledge Satellite Transport Protocol

(CK-STP)


12/21


We have used the original fading patterns over a longer time window (900 s),

by replicating each sample 3 times, in order to have a longer time scale for

observing the TCP reaction. Fading pattern A is milder, whereas B is quite

severe, and produces a loss of receiver synchronization for over 100 seconds,

starting at 400 s.

All graphs are obtained by averaging the results of 5 repeated experiments(under the same fading pattern), in order to increase their statistical

significance.

In all kernel implementations adopted for the various protocols (except CK-

STP) the sender buffer dimension has been left untouched, with respect to the

native New Reno implementation for the Linux Kernel 2.4.18 (64 kbytes).

The sender bufferis set at 320 kbytes for STP (independently of the number ofconnections).

Experimental measurements


13/21

Throughput (goodput) vs. time of the different TCP

implementations with only one connection active over a noisy

satellite link (fading pattern A)

50

100

150

200

250

0 100 200 300 400 500 600 700 800 900

Time s

Throughput[kbyte/s] Westwood New Reno STP Peach


14/21

Aggregate throughput vs. time of the different TCP

implementations with 2 connections active over a noisy satellite

link (fading pattern A).

50

100

150

200

250

300

0 100 200 300 400 500 600 700 800 900

Time s

Throughput[kbyt

e/s]

Westwood New Reno ST P Peach


15/21



link (fading pattern A).

50

100

150

200

250

300

0 100 200 300 400 500 600 700 800 900

Throughput[kbyte

/s]

Westwood New Reno STP Peach


16/21

Throughput vs. time of the different TCP implementations with

only one connection active over a noisy satellite link

(fading pattern B).

50

100

150

200

250

0 100 200 300 400 500 600 700 800 900

Throughput[kbyte/s]



17/21



link (fading pattern B).

50

100

150

200

250

0 100 200 300 400 500 600 700 800 900

T

hroughput[kbyte/s

]

W e stwood Re no SDR P e a c h


18/21



link (fading pattern B).

50

100

150

200

250

0 100 200 300 400 500 600 700 800 900

Throughput[kbyte/s]



19/21

Throughput vs. time of three connections out of 5, measured

during the realization of a single experiment with STP,

under fading pattern A.

10

20

30

40

50

60

70

80

90

100

0 100 200 300 400 500 600 700 800

Time s

T

hroughput[kbyte

/s]


20/21

Throughput vs. time of three connections out of 5, measured

during the realization of a single experiment with Westwood+,

under fading pattern A.

10

20

30

40

50

60

70

80

90

100

0 100 200 300 400 500 600 700 800

Time s

Throughput[kby

te/s]


21/21

Conclusions and future work

When a single connection is active, CK-STP achieves the best

performance. In this sense, it would be fruitfully employed withinPEPs for the creation of a bandwidth pipe.

In terms of fairness and aggregate throughput in different situations,TCP Westwood (which, by the way, is becoming a standard feature ofrecent Linux kernels) appears to offer the best performance.

In the presence of multiple connections, the effect of TCPmodifications becomes less evident, and the advantage of usingmodified algorithms may be hindered by the necessity of updating theoperating systems kernel of all the machines involved in thecommunication.

The measurements are currently being repeated under the HB6 Ka

payload link budget; some measurements will also be performed on theKa/Ku band CNIT satellite network. Software tools have been alreadydeveloped to make the laboratory test-bed remotely accessible as aweb service.

IPS MoMe05Davoli

Documents

Transcript of IPS MoMe05Davoli