Computer Laboratory

Technical reports

Multi-layer network monitoring and analysis

James Hall

July 2003, 230 pages

This technical report is based on a dissertation submitted April 2003 by the author for the degree of Doctor of Philosophy to the University of Cambridge, King’s College.

Some figures in this document are best viewed in colour. If you received a black-and-white copy, please consult the online version if necessary.


Passive network monitoring offers the possibility of gathering a wealth of data about the traffic traversing the network and the communicating processes generating that traffic. Significant advantages include the non-intrusive nature of data capture and the range and diversity of the traffic and driving applications which may be observed. Conversely there are also associated practical difficulties which have restricted the usefulness of the technique: increasing network bandwidths can challenge the capacity of monitors to keep pace with passing traffic without data loss, and the bulk of data recorded may become unmanageable.

Much research based upon passive monitoring has in consequence been limited to that using a sub-set of the data potentially available, typically TCP/IP packet headers gathered using Tcpdump or similar monitoring tools. The bulk of data collected is thereby minimised, and with the possible exception of packet filtering, the monitor’s available processing power is available for the task of collection and storage. As the data available for analysis is drawn from only a small section of the network protocol stack, detailed study is largely confined to the associated functionality and dynamics in isolation from activity at other levels. Such lack of context severely restricts examination of the interaction between protocols which may in turn lead to inaccurate or erroneous conclusions.

The work described in this report attempts to address some of these limitations. A new passive monitoring architecture — Nprobe — is presented, based upon ‘off the shelf’ components and which, by using clusters of probes, is scalable to keep pace with current high bandwidth networks without data loss. Monitored packets are fully captured, but are subject to the minimum processing in real time needed to identify and associate data of interest across the target set of protocols. Only this data is extracted and stored. The data reduction ratio thus achieved allows examination of a wider range of encapsulated protocols without straining the probe’s storage capacity.

Full analysis of the data harvested from the network is performed off-line. The activity of interest within each protocol is examined and is integrated across the range of protocols, allowing their interaction to be studied. The activity at higher levels informs study of the lower levels, and that at lower levels infers detail of the higher. A technique for dynamically modelling TCP connections is presented, which, by using data from both the transport and higher levels of the protocol stack, differentiates between the effects of network and end-process activity.

The balance of the report presents a study of Web traffic using Nprobe. Data collected from the IP, TCP, HTTP and HTML levels of the stack is integrated to identify the patterns of network activity involved in downloading whole Web pages: by using the links contained in HTML documents observed by the monitor, together with data extracted from the HTML headers of downloaded contained objects, the set of TCP connections used, and the way in which browsers use them, are studied as a whole. An analysis of the degree and distribution of delay is presented and contributes to the understanding of performance as perceived by the user. The effects of packet loss on whole page download times are examined, particularly those losses occurring early in the lifetime of connections before reliable estimations of round trip times are established. The implications of such early packet losses for pages downloads using persistent connections are also examined by simulations using the detailed data available.

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BibTeX record

  author =	 {Hall, James},
  title = 	 {{Multi-layer network monitoring and analysis}},
  year = 	 2003,
  month = 	 jul,
  url = 	 {},
  institution =  {University of Cambridge, Computer Laboratory},
  number = 	 {UCAM-CL-TR-571}