Network Measurement

1
Network Measurement

• Bandwidth Analysis

2
Why measure bandwidth?

• Network congestion has increased tremendously.
• Bottlenecks are not always obvious.
• Measuring bandwidth may become more essential for
  service providers as congestion increases.
• Measuring bandwidth enables us to improve current
  systems as well as diagnosis network problems.
• Measuring bandwidth may be the key to observing
  what is wrong with current protocol standards.
  In effect, measurement is a tool for research in
  general.

3
How we measure bandwidth

• Its really more complicated than a connection
  speed.
• We might want to look at capacity or we might be
  looking at throughput or bandwidth congestion.
• We can observe packet loss, propagation delay,
  link capacity, but some of this results in
  educated guess work.
• There are many theories and applications intended
  to measure bandwidth and network statistics.
• For our purposes we will look at the three most
  common utilities used traceroute, ping, and
  pathchar.
• It really depends on what you are after!

4
traceroute

• Written by Van Jacobsen in 1988 to solve
  persistent network problems.
• Traceroute counts hops roughly tracing the path
  of an IP packet from the client to the
  destination.
• Traceroute does this by sending UDP packets with
  an extremely short TTL.
• If all routing nodes in the path are working
  properly, an ICMP (Internet Connection Message
  Protocol) Time Exceeded message is sent (RFC
  792).

5
traceroute

• Traceroute utilizes the information encapsulated
  in the ICMP message to determine the source (the
  router that sent the packet).
• We continue sending packets until we get an ICMP
  host unreachable message (this implies that we
  have reached the destination) or until the max
  number of hops has been reached.

6
traceroute disadvantages

• Traceroute is a simple tool that is based on a
  few key ideas
• 1. All packets will be sent on the same paths
  (going).
• 2. Consistent Routing (all packets will be
  routed back the same way).
• 3. TCP/IP implementations supporting ICMP.
• In reality, poor TCP/IP implmentation means that
  Traceroute is not dependable.
• Using three different Traceroute
  implementations, to the same IP address, resulted
  in three different routes.

7
ping

• One of the most widely distributed analysis
  tools. First released in 1980.
• The UNIX version of ping is slightly more robust,
  allowing us to specify the testing data and
  modify the patterns.
• ping, works by sending a single packet and
  waiting for the ICMP Echo response.

8
ping

• ping puts its own Round Trip Time value on each
  packet so we are not left at the mercies of the
  router (as in traceroute).
• ping also provides us with a diagnostic of ICMP
  messages, usually buried by the system.
• ping is clearly a much different tool from
  traceroute, but its simplicity makes it more
  reliable.
• ping is only useful for estimating bandwidth
  under certain conditions.

9
Pathchar

• Also written by Van Jacobson, in 1997.
• pathchar attempts to improve upon traceroute by
  adding mathematical analysis to the problems that
  occur in propagation.
• Working in the same basic manner as traceroute,
  pathchar sends out packets and waits for the
  response. Only instead of one set of packets, it
  sends out several.
• The difference being the analysis of the returned
  data.
• pathchar attempts to account for
• - loss rate
• - link capacity
• - propagation and queing delay
• (Grossglauser, pg40)

10
pathchar

• Taking into account the rtt from two nodes, say n
  and n 1 we generate the following formula using
  Van Jacobsons specifications
• But he assumes three things
• The error message is small enough to ignore (toss
  error_size/bandwidth out)
• The forward time is not big enough to worry
  about.
• If enough transmission groups are sent at least
  one will not have any queuing delays.
• And so, we get

11
pathchar

• In practice pathchar is not the easiest tool to
  use. It can be difficult to implement and its
  output is often chaotic.
• A better implementation of pathchar was made by
  Bruce Mah, called pchar
• Here is an example of a particular node in a
  trace.

12
What we learn from pathchar

• Pathchars focus is on the statistics of data
  loss and the analysis of delay.
• Instead of capacity, we can look at data loss and
  latency.
• Using pathchar and traceroute, one is more likely
  to track down the source of delay than to
  estimate bandwidth in the sense of capacity.
• In a commercial sense, we can utilize this
  information to see where end users are running
  into difficulty.
• In private application weak network components
  can be sorted out.
• For our purposes, bandwidth congestion allows us
  to think intelligibly about improving network
  protocols and gives us some real world metric to
  diagnosis real world problems.

13
Conclusion

• There are two things we can conclude from this
• ICMP may need to be rewritten to facilitate
  better tools.
• Bandwidth Analysis is at its heart a simple idea.

14
Sources

• Downey, Alan B. Using pathchar to estimate
  Internet length characteristics.
  http//www.acm.org/sigcomm/sigcomm99/papers/sessio
  n7-1.pdf. 1999
• Jacobson, Van. pathchar A Tool to Infer
  Characteristics of Internet Paths.
  ftp//ftp.ee.lbl.gov/pathchar/msri-talk.pdf.
  April 21, 1997.
• Postel, J. RFC 792 Internet Control Message
  Protocol. http//www.freesoft.org/CIE/RFC/792/ind
  ex.htm. September, 1981.