To expose network characteristics by active/passive measurements, measuring some timing issues such as one-way delay, one-way queuing delay, and inter-packet time is essential, and is conducted by time-stamping for packets passing through an observation point. However, emerging high-speed networks require very high precision of time-stamping, far beyond the precision of conventional software-based time-stamping systems such as 'tcpdump'. For example, the inter-packet time of two consecutive 64-byte length packets on a giga-bit link can be less than 0.001 msec. In this paper, to demonstrate the usefulness and strong necessity of precise time-stamping on high-speed links, experiments of network measurements over a nation-wide IPv6 testbed in Japan have been performed, using a hardware-based time-stamping system that can synchronize to GPS with a high resolution of 0.0001 msec and within a small error of 0.0003 msec. In our experiments, several interesting results are seen, e.g., i) the distribution of one-way queuing delay exhibits a considerable difference depending on the size and the type (UDP/ICMP) of packets; ii) the minimal one-way delays for various sizes of UDP/ICMP packets give an accurate estimate of the transmission delay and the propagation delay; iii) the correlation between interpacket times at the sender and the receiver sides in a sequence of TCP ACK packets clearly shows the degree of ACK compression; iv) the inter-packet time in a UDP stream generated by a DV streaming application shows three dominant sending rates and a very rare peak rate, which might provide crucial information to bandwidth dimensioning; all of which would indicate the usefulness of precise time-stamping.
Emerging delay-sensitive applications on the Internet increase awareness of the Quality of Service (QoS) parameters of a path for Internet Service Providers (ISPs) as well as users, However, it is costly to frequently monitor delays along individual paths among every edge-router in the ISP. The most widely used way of estimating such statistics is by actively sending probe packets along each path, despite the increased transmission of wasteful traffic introduced by the probe packets itself for frequent and accurate estimations. On the other hand, each router can passively observe local queuing delays experienced at the router. However, while the mean delays can
always be concatenated concentrating simply by summing those at tandem routers along a path, the statistics (other than the mean) such as the 90-percentile cannot be estimated accurately by such a simple-sum scheme because of dependence among delays at such routers on the Internet. In this work, a novel scheme to estimate the QoS parameters of a path is proposed, which combines statistics
gatherd at each router and data obtained from a small number of sampling along the path. For delays, considering an unknown joint discrete distribution of quantized queuing delays on routers along a path, we find the maximum likelihood estimator for the unknown distribution (under the constraints of the marginal distributions measured at each router) from the samples. Theoretical analysis and numerical simulations indicate that this scheme effectively estimates the delay statistics along a path even with a small number of samples, which allows continual measurements capturing statistics with a broad range of time-scales.
We have started a long-term experiment of end-to-end active measurements along a number of Internet paths, while such kinds of distributed measurement infrastructures have been developed on the Internet and a number of experiments on them have already been reported. Our objective is to explore correlations among various properties of an individual path measured within a period in which the path state does not change, which have not yet been clearly covered. A PC-based measurement system has been developed to
measure a set of path properties in sequence or in parallel for this purpose.
In our preliminary experiment over several Internet paths in Japan,
loss (rate and pattern) and delay (RTT and queuing delay) statistics;
bottleneck bandwidths (the capacity and available bandwidth); and TCP throughput as well as the end-to-end route (to validate no changes of itself) are measured. Some interesting correlations (and no-correlation) among those properties are shown, which indicate the potential of efficient and/or reliable measurement of some path property utilizing the multiple properties measured on the path.
The recent evolution on the network tomography have successfully provided
principles and methodologies of inferring network-internal (local)
from only end-to-end measurements,
which should be followed by deployment in practical use.
In this paper, we propose two types of
user-oriented tools for inferring one-way
packet losses on paths from/to an user-host (a client) to/from a specified target
host (a server or router)
without any measurement on the target,
which utilize a method based on the network tomography.
One is a stand-alone tool running on the client, and
the other is a client-server style tool running on both the client
and proxy measurement server(s) distributed in the Internet.
Both of them can infer one-way packet loss rates not only on a
path between the client and an application server, but on
a path segment (a portion of the path)
between the client and any router residing in the path, and thus
can find the congested area along the path.
We have developed prototypes of the tools
and have evaluated them in experiments in the Internet environment,
which showed that
the tools could infer one-way packet loss rates
within 1% errors in various network conditions.