Download : Integrated Measurement and Analysis In-House Environment B. Rathke

Snue : Integrated Measurement and Analysis
Tool for Internet and its Use in Wireless
In-House Environment
B. Rathke1 , Th. Assimakopoulos1, R. Morich1, G. Schulte1 , and A. Wolisz1 2
;
1
Telecommunication Networks Group, Dept. of Electrical Engineering,
Technical University Berlin, Germany
frathke,thass,morich,schulte,[email protected],
WWW home page: http://www-tkn.tu-berlin.de
2
also with GMD-Fokus
Abstract. In this paper we describe Snue , a new measurement tool
for capturing, displaying and analyzing the operation of the Internet
protocol stack within end-systems. Snue is a set of modules operating
in a distributed fashion and supporting an on-line analysis of network and
protocol performance. This kind of tool is especially suited for wireless
networks.
1 Introduction
The popular network monitoring tools (e.g. tcpdump [2], snoop [4], ethernd,
Remote Network Monitoring (RMON) [5] etc.) follow the principle of observing packets traversing the network. Thus, there is only a limited possibility to
infer about the internal Internet protocol variables [6]. The accuracy of this inference depends on the completeness of the trace, i.e. each lost sample falsies
the conclusions about the protocol variables and makes performance evaluations
inaccurate. Therefore it is necessary to measure directly within the end-systems
for precise investigation of Internet protocols and their internal state variables .
Especially in an error prone wireless environment it cannot be guaranteed,
that an observer placed outside of the communicating end-systems records the
same packets as transmitted or received by the observed nodes. It is known
that in wireless networks operating around 1GHz or higher, small displacements
of mobile systems may result in dramatically degradations of the physical link
quality. In the worst case, the observed end-systems can still communicate, while
the observing host has lost contact to them.
In this paper we present Snue , a tool suitable for measurements in Internet and a case study in a wireless LAN environment. Snue is currently implemented on Linux V2.0.27 and available at http://www-tkn.tu-berlin.de.
2 Design and Implementation Approach of Snue
Snue consists of interacting modules providing a set of measurement functionalities. The modules can be placed on dierent end-systems. We have dened
four types of modules: Snue Information Base (SIB), Tracer, Synchronizer, and
Analyzer (depicted in Fig.1).
Host A
Host B
SNUFFLE
Application
Application
Analyzer
TCP/UDP
SIB
Tracer
Tracer
TCP/UDP
IP
IP
Synchronizer
Network Interface
Network Interface
Module interface
Data flow
Measurement point
INTERNET
Fig. 1. Snue architecture
The Snue Information Base contains objects holding traced protocol data
(protocol variables and header information) and derived information. A timestamp is attached to every object dening its sampling time. We have currently
implemented object classes for TCP, UDP and IP. Each object holds information
about:
{ Packet Header Information, e.g. TCP segment and ACK sequence numbers,
window size, port numbers, IP identier.
{ Protocol Variables, e.g. TCP windows sizes (congestion window, sending and
receiving windows), round trip time values, retransmission timer values.
{ Derived Values, e.g. delays, cause of retransmissions, throughput.
Other modules (e.g. the Analyzer) can access objects of a SIB through an interface. This interface supports currently three functions: initialization of SIBs,
reading objects from a SIB and setting lter functions.
The Tracer module is the interface between the measurement points and a
SIB. The measurement points operate as samplers gathering internal protocol
information and support lters for selective capturing of data. A measurement
point may be placed arbitrary in the Internet protocol stack. Its implementation
has to be adapted to specic Internet protocol implementations.
The Synchronizer is needed to perform accurate delay measurements between
Tracers (see Fig.1), if they are located on dierent communication systems. To
ensure correct delay measurements, clocks used by Tracers have to be synchronized. Well known approaches for clock synchronization, e.g. global positioning
systems (GPS), the network time protocol (NTP), DCF or (radio) broadcasts
can be integrated into the Synchronizer. Another possibility is to estimate the
time skewness of the clocks. The Analyzer takes the time skewness into consideration.
The Analyzer processes data captured by the Tracer and stored in a SIB.
It produces time series of inspected variables as well their distributions (e.g.
conditional probability distributions, cumulative distributions, etc.). It should
be pointed out that the analysis can be done o-line or on-line. On-line analysis
heavily depends on the available processing power of the computing systems. The
o-line method is used for time intensive analysis which can not be performed in
real time. It is also possible to export measurement results to simulation tools.
3
Snue in a Wireless LAN (WLAN) Environment
We present results of measured TCP and IP throughput and TCP behavior in
a WLAN environment using ARLAN1 [1] as WLAN technology. In this scenario
two hosts reside at xed positions in a single radio cell communicating undisturbed from other hosts. Although many researches have faced the problems of
TCP in WLANs by simulations (for an overview see [8]) we present results of
measurements in a real environment.
Fig.2(a) shows the throughput calculated by Snue on TCP and on IP level.
The measurement results show that although the IP throughput is high the TCP
throughput is very low. This indicates that TCP is not suitable for this position
in the radio cell. The loss rate on IP level does not exceed 10;6 , which is comparable to the loss rate of twisted pair cabling. Therefore losses are not responsible
for the disappointing throughput of TCP. In spite of the low loss rate the number of TCP retransmissions is very high, as shown in Fig.2(b). This corresponds
to Fig.2(c), showing the congestion window. Whenever a retransmission timeout
occurs TCP drops its congestion window down to one segment [9]. The delay
variability of packets on IP level causes retransmissions. To give an impression
about the delay variability Fig.2(d) shows the delay distribution on IP level. A
deeper analysis can be found in [7].
References
1. Arlan ISA/PCMCIA Adapter Cards, Aironet, http://www.aironet.com
1
ARLAN operates at 2.4GHz, uses Direct Sequence Spread Spectrum and CSMA/CA
with immediate ACKs.
TCP and IP throughput
160
no. of retransmissions
throughput [kbyte/s]
100
120
100
80
60
40
20
0
0
1
2
3
4
5
6
no. of transfer
7
8
80
60
40
20
0
9
(a)
2000
4000
6000 8000 10000 12000 14000
no. of packet
(b)
rel. frequency
0.1
60
40
20
0
Delay density distribution on IP level
1
80
CWND
0
TCP congestion window (CWND)
100
(c)
TCP retransmissions
120
"IP"
"TCP"
140
0.01
1e-03
1e-04
0
2000
4000
6000 8000 10000 12000 14000
no. of packet
1e-05
(d)
1
10
100
delay [ms]
1000
10000
Fig. 2. Measurement results: (a) TCP and IP throughput over 10 le transfers, (b)
TCP retransmissions and (c) corresponding TCP Congestion Window for the rst
14000 packets. (d) Delay distribution on IP level
2. Jacobson, V., Leres, C., McCanne,S.: Tcpdump Manual Page. Lawrence Berkeley
National Laboratory, University of California, Berkeley, CA, June 1997.
3. Transmission Control Protocol. IETF, RFC 793, Sep. 1981
4. Callaghan, B., Gilligan, R.: Snoop Version 2 Packet Capture File Format. Request
for Comments, RFC 1761, Internet Engineering Task Force, Feb. 1995.
5. Waldbusser, S.: Remote Network Monitoring Management Information Base Version
2. Request for Comments, RFC 2021, Internet Engineering Task Force, Jan. 1997.
6. Paxson, V.: Automated Packet Trace Analysis of TCP Implementations. Proceedings of SIGCOMM `97.
7. Rathke, B., Assimakopoulos, T.,Wolisz,A.: Internet Protocol Performance Measurements for Wireless Access using Snue. Technical Report, Telecommunication Networks Group, Technical University Berlin, Apr. 1998.
8. Balakrishnan, H., Padmanabhan, V., Seshan, S., Katz, R.-H.: A Comparison of
Mechanisms for Improving TCP Performance over Wireless Links. IEEE/ACM
Transactions on Networking, Dec. 1997.
9. Jacobson, V.: Congestion Avoidance and Control. Proceedings of SIGCOMM '88,
pp. 314-329, August 1988.