Internet Packet Loss – some research results on internet packet loss (2004-05-30)

According to /7/ systematic measurements of packet delay and loss were carried out on the ARPANET as early as 1971. They examined the variations of packet delay for different paths, different time of day and days of weeks.
Some of the results of experiments and measurements on Internet packet loss taken a bit later in time (around 1993 – 2000) are summarized below.

Vern Paxson /1/ /2/ analysed 20.000 TCP 100Kbyte bulk transfers between 35 Internet sides packet loss along Internet paths in Dec. 1994 (N1) and Nov. - Dec. 1995 (N2) using a synthetic Poisson sampling methodology.
He distinguished between three types of losses, “loaded” data packet, meaning those whoch are queued behind a predecessor at the bottleneck; “unloaded” data packet where no queueing occurred and “ACK” packets.
He finds that basically two types of network paths exist: “quiescent”, on which no losses occurs, and “busy”, in which one of more losses occurred.
Quiescent accounted for 50% of connections, but for busy connections packet losses increased significantly.
He further finds that loss rates vary dramatically between different regions:

•	Europe had higher packet loss rates than North America then.
•	North America were fairly low
•	Umont (Univ. of Montreal/Canada) had such a high loss rate that it didn’t make it into the statistics for North America
•	Trans-Atlantic links from Europe to North America had fairly high loss rates
•	Trans-Atlantic links from North America to Europe are disastrous.

The aggregated loss statistics over the course of the day showed a Packet Loss rate increase at noon.
Another focus of his work was on loss symmetry, finding that loss rates along the two directions of an Internet path are nearly uncorrelated. No periodic packet loss pattern, indicating e.g. global synchronisation problems, was found.
Some evidence is given, that isolated packet losses (not bursts) occurs at or before the first bottleneck link. Some 25% of packet loss occurs late in the network path.
Considering loss “outages” he finds that outage durations exhibit clear Pareto distributions, indicating they span a large range of time scale. To some degree observing no loss along a path is a good predictor that, even far into the, future connections will not suffer from packet loss. On the other hand, while packet loss may be predictable but then only for time scales of minutes to hours.
Paxson’s work didn’t consider UDP nor VoIP or other real time streaming protocols.
Due to the centralized control of his experiment the packet loss statistics underestimate Internet packet loss rate somewhat.

Handley /3/ and Moon, Kurose, Skelly and D. Towsley /4/ examined mbone performance from the point of view of a single end-system to derive a valuable insight into how packet loss affects traffic flows. He used variable-rate video test traffic sources and evaluated the RTP and RTCP packets and data provided by mtrace.
The loss rate by site shows that 50% of receivers had a mean loss rate of <=10% and around 80% of sides had some interval during the day when no loss was observed. However, 80% of sites reported some interval with loss rate >20%, and 95% of site reported at least one interval with loss rate >95% at some time during the day.
Loss rate distribution with time showed that although 25% of the receivers are suffering no loss for much of the day, the median loss rate is 5% - 10%. Also 25% suffer from loss rate >15% between 12:00 – 4:30 UTC.
Although single packet loss dominate it was shown that an excess of bursts of 2 – 5 consecutive packets losses is statistically significant and there also exists another bursts of 5 – 15 consecutive packet losses.

Since the loss rate statistics where derived from RTCP, some inaccuracy out of losing RTCP packets may occur.

In /5/ Yajnik, Kurose and Towsley experimentally and quantitatively examine the spatial and temporal correlation of packet loss in multicast sessions of the “Worldwide Radio Network” http://wwrn.net on July 1995. Their experiment run on 14 multicast hosts at 11 geographically distinct locations in Europe and US, connected via the MBONE, for 2 hours during the day and 1 hour during the night.
They show that there was a significant spatial correlation in loss among the multicast sites, specifically the found that in one two hour period almost 70% of transmitted packets were not received by one or more sides, and that nearly 30% of the packets transmitted were lost at two or more sites. Furthermore it was found, that the “backbone” loss between two main routers to be significantly smaller than that near the “edges”, and only a negligible number of packets were lost at the receiving hosts.

They also found a significant amount of burst loss (consecutive losses) at each site. For example the found, that the length of the longest number of consecutive packet losses is more than 100 packets at each of the sites, the average number of consecutive losses varies from approximately 2 to 8 among the sites.

The experiments were performed only one-way, due to the use of the “Worldwide Radio Network” and since the experiment was done back in 1995 it maybe outdated.

In /6/ Yajnik, Moon, Kurose and Towsley carefully analysed end-to-end loss as seen by packet probes sent at regular intervals (20ms, 40ms, 80ms and 160ms) sent on both unicast and multicast Internet connections in 1997.
They showed that the correlation timescale, the time over which what happens to one packet is connected to what happens to another packet, is approximately 1 second or less. Beyond this timescale, packet losses are independent of each other.
The distributions of the good run lengths and the loss run lengths were found to be approximately geometrically distributed for the 160ms traces. For the 20ms and 40ms traces, the good run lengths were found to be not geometrically distributed, while the loss run length were geometrically distributed.
For on-line loss estimation it was found, that using the sliding window average for average loss rate estimation has advantages over exponential smoothing since it varies approximately half as much when the same effective number of past observations where used.

Their results should be validated in today’s Internet.

Bolot 1993 /7/ measured round trip delay of small UDP probe packets send at regular intervals (a few milliseconds to a few minutes) with the echo tool to characterize end-to-end packet loss (and delay too). Only one specific connection was examined in detail (INRIA in France to University of Maryland), but some experiments with connections between Europe and US and between INRIA and Universities in Europe showed that the oberservations made hold for the other connections too.
He found that the losses of probe packets were essentially random when the probe traffic uses a small fraction of the available bandwidth (<10%).
Specifically Bolot showed that the loss gap stays close to 1 even for relatively small values of delta – with important consequences for the design of audio applications, which send audio packets at regular intervals over the Internet. His experiments suggest therefore that an open loop error control mechanism based on FEC would be usefill to compensate for lost audio packets.
In Bolot and García 1997 /8/ they designed and build an “Internet telphone” application according to the findings in /7/ that proves to provide good audio quality even across fairly congested links.

Their results should be validated in today’s Internet and with todays Internt telephones.

In /9/ Salza, Draoli, Gaibisso, Palma and Puccinelli present the Netlab Quality Management Framework (NQMF), a set of tools especially designed to support experimental VoIP analysis, e.g. packet loss.
Measurement where mainly performed according to the Internet Protocol Performance Metrics (IPPM) WG. They used test flows of fixed 32 bytes UDP packets with an emission rate of 50 packets per second. Three sides where involved, one in England, another in Rome and Padova.
They found that the unconditioned loss probability in peak period is much greater than in the normal period.

Hsiung, Fischer, Masi, Cuffie and Scheurich /10/ published in their paper the network access performance of VoIP in a simple testbed consisting of only a few PCs, four analog  phones for a government environment. At this time the packet loss probability is not available in their testbed derived analytic approximation although initial analyses showed that there is sensitivity to QoS when VoIP access systems are sized.

Since packet loss statistics isn’t available at this time this study isn’t very useful yet.

Marsh, Li and Karlsson /12/ made some VoIP measurements in 2003 with a small test bed of nine fully inter-connected world-wide distributed hosts and synthetic bi-directional VoIP packet generation.
Specifically the showed that 75% of the Internet VoIP conversations had zero packet loss.
Over 96% had a loss rate of less than 10%.
Connections to and from Europe/USA and South America had an average loss rate of just over 3%.
The majority of consecutive losses are one  to four consecutive losses. Some very long loss sequences where observed almost inevitably caused by temporary outages at either the sender or the receiver and they were nearly always at the end of the session.

Still the most comprehensive study on packet loss, in respect to VoIP, I found.

Literature:

/1/  Vern Paxson
Measurements and Analysis of End-to-End Internet Dynamics (1997)
Ph.D. dissertation, University of California, Berkeley, 1997

/2/  Vern Paxson
End-to-End Internet Packet Dynamics (1997)
IEEE \ ACM Transactions on Networking

/3/  Mark Handley
An Examination of MBone Performance (1997)
UCL and ISI January 10, 1997

/4/  S. B. Moon, J. Kurose, P. Skelly and D. Towsley
Correlation of Packet Delay and Loss in the Internet
UM-CS-1998-011, March, 1998

/5/  Maya Yajnik, Jim Kurose, Don Towsley
Packet Loss Correlation in the MBone Multicast Network: Experimental Measurements and Markov Chain Models (1996)

/6/  Maya Yajnik, Sue Moon, Jim Kurose and Don Towsley
Measurement and Modelling of the Temporal Dependence in Packet Loss
Department of Computer Science, Universoty of Massachusetts, Amherst, 1998.

/7/  Jean-Chrysostome Bolot
Characterizing End-to-End Packet Delay and Loss in the Internet
(1993)

/8/  Jean-Chrysostome Bolot, Andrés Vega García
Control Mechanisms for Packet Audio in the Internet
INFOCOM 1996

/9/  Silvio SALZA, Mauro DRAOLI, Carlo GAIBISSO, Antonio Laureti PALMA, Roberto PUCCINELLI
Methods and Tools for the Objective Evaluation of Voice-over-IP Communications
INET 2000 <http://www.isoc.org/inet2000/cdproceedings/1i/1i_2.htm>

/10/ Hsiaosu HSIUNG, Martin J. FISCHER, Denise M. MASI, Darren CUFFIE, Scott SCHEURICH
An Approach to IP Telephony Performance Measurement and Modeling in Government Environments 
Mitretek Systems, Inc., USA
INET 1999 Proceedings < http://www.isoc.org/isoc/conferences/inet/99/proceedings/4p/4p_2.htm>

/12/ Ian Marsh and Fengyi Li and Gunnar Karlsson
Wide Area Measurements of VoIP Quality
Quality of Future Internet Services (QoFIS), Stockholm, Sweden 2003
< http://www.sics.se/~ianm/Papers/rvk02.pdf>