What do packet dispersion techniques measure?

Internet Systems and Technologies - Monitoring

Contents Introduction Packet pair dispersion Cross traffic interference Packet train dispersion The size of probing packets The “pathrate” methodology Heuristic evaluation Conclusion

Introduction Why we need bandwidth monitoring? Metrics

Capacity (no cross traffic) Available bandwidth

Packet-pair technique – compute the transmission delay

What about cross-traffic?

Packet pair dispersion The dispersion of the packet pair is the

interval from the instant the last bit of the first packet is received at a certain path point to the instant the last bit of the second packet is received at that point.

When there is no cross traffic, all the bandwidth estimates are equal to the capacity

Post-narrow link - when the first probing packet is delayed more than the second

In heavy load conditions, the probability of CT packets interfering with the probing packets is large

Cross traffic interference

Capacity Mode (CM) Sub-Capacity Dispersion Range (SCDR) Post-Narrow Capacity Modes (PNCMs)

Packet train dispersion 1/2 As the length N of the train increases, the CM

and PCNMs become weaker and the SCDR prevails.

As N increases, the dispersion becomes unimodal.

The range of the distribution, which is related to the measurement variance, decreases as N increases.

When N is sufficiently large, the center of the (unique) mode (a.k.a. Asymptotic Dispersion Rate) is independent on N.

Packet train dispersion 2/2

The size of probing packets A large packet size leads to a wide time

interval in which a CT packet can interfere with the packet pair.

When it is small, the formation of PNCMs becomes more likely and the CM becomes weaker.

The empirical conclusion from Internet experiments is that a packet size around 800 bytes leads to the stronger CM in heavily loaded paths.

Small vs large packet size

The “pathrate” methodology Phase I: Packet pair probing.

use a large number of packet pair experiments to ‘uncover’ all the local modes of the bandwidth distribution.

Denote the sequence M of local modes in increasing order (one of these modes is the CM)

Phase II: Packet train probing. Find Asymptotic Dispersion Rate and use heuristic

rule to estimate the capacity mode

Heuristic evaluation

the resolution (bin width) has to be chosen based on a rough estimate of the path’s capacity.

the specified bin width is 1 Mbps for capacity paths < 40 Mbps, and 2 Mbps for capacity paths > 40 Mbps.

Conclusion We studied the dispersion of packet pairs and

packet trains, focusing on the effects of the cross traffic.

The insight gained can be applied to congestion control mechanisms, server selection algorithms and quality of service monitoring.

References Constantinos Dovrolis, Parameswaran

Ramanathan, David Moore: What do packet dispersion techniques measure? In INFOCOM 01.