2. Network Performance
The main network performance parameters are those affecting the
speed with which individual messages can be transferred
between two interconnected computers.
Latency is the delay that occurs after a send operation is executed
and before data starts to arrive at the destination computer. It
can be measured as the time required to transfer an empty
message.
Data transfer rate is the speed at which data can be transferred
between two computers in the network once transmission has
begun, usually quoted in bits per second.
Message transmission time = latency + length ⁄ data transfer
rate
The total system bandwidth of a network is a measure of
throughput – the total volume of traffic that can be transferred
across the network in a given time. 2
18. IP Addressing
Assigning host addresses to networks and the computers connected
to them had to satisfy the following requirements:
• It must be universal – any host must be able to send packets to any
other host in the Internet.
• It must be efficient in its use of the address space – it is impossible to
predict the ultimate size of the Internet and the number of network and
host addresses likely to be required.
TCP/IP provision for 232 or approximately 4 billion addressable hosts.
Short-sighted, for two reasons:
– The rate of growth of the Internet has far outstripped all predictions.
– The address space has been allocated and used much less efficiently than
expected.
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22. Unregistered addresses and Network Address Translation (NAT)
Not all of the computers and devices that access
the Internet need to be assigned globally unique IP
addresses.
Computers that are attached to a local network and
access to the Internet through a NAT-enabled
router can rely upon the router to redirect incoming
UDP and TCP packets for them.
The network includes Internet-enabled computers
that are connected to the router by a wired
Ethernet connection as well as others that are
connected through a WiFi access point.
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27. Firewall
IP packet filtering: This is a filter process examining
individual IP packets. It may make decisions based on the
destination and source addresses.
It may also examine the service type field of IP packets and
interpret the contents of the packets based on the type.
For example, it may filter TCP packets based on the port
number to which they are addressed, and since services are
generally located at well-known ports, this enables packets to
be filtered based on the service requested. For example,
many sites prohibit the use of NFS servers by external clients.
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28. Firewall
TCP gateway: A TCP gateway process checks all TCP
connection requests and segment transmissions.
When a TCP gateway process is installed, the setting up of
TCP connections can be controlled and TCP segments can
be checked for correctness (some denial of service attacks
use malformed TCP segments to disrupt client operating
systems).
When desired, they can be routed through an application-
level gateway for content checking.
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29. Firewall
Application-level gateway: An application-level gateway process acts
as a proxy for an application process.
For example, a policy may be desired that allows certain internal users to
make Telnet connections to certain external hosts.
When a user runs a Telnet program on their local computer, it attempts to
establish a TCP connection with a remote host.
The request is intercepted by the TCP gateway. The TCP gateway starts
a Telnet proxy process and the original TCP connection is routed to it. If
the proxy approves the Telnet operation (i.e., if the user is authorized to
use the requested host) it establishes another connection to the
requested host and relays all of the TCP packets in both directions.
A similar proxy process would run on behalf of each Telnet client, and
similar proxies might be employed for FTP and other services.
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33. Issues
Hidden stations: Carrier sensing may fail to detect that
another station on the network is transmitting.
If tablet D is transmitting to the base station E, laptop A may not
be able to sense D’s signal because of the radio obstruction
shown. A might then start transmitting, causing a collision at E
unless steps are taken to prevent this.
Fading: Due to the inverse square law of electromagnetic
wave propagation, the strength of radio signals diminishes
rapidly with the distance from the transmitter. Stations within a
wireless LAN may be out of range of other stations in the
same LAN.
Thus laptop A may not be able to detect a transmission by C,
although each of them can transmit successfully to B or E.
Fading defeats both carrier sensing and collision detection.
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34. Issues
Collision masking: The ‘listening’ technique used
in the Ethernet to detect collisions is not very
effective in radio networks.
Because of the inverse square law the locally
generated signal will always be much stronger than
any signal originating elsewhere, effectively
drowning out the remote transmission.
So, laptops A and C might both transmit
simultaneously to E and neither would detect that
collision, but E would receive only a garbled
transmission.
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35. Carrier Sensing, Multiple Access with Collision Avoidance (CSMA/CA).
When a station is ready to transmit, it senses the medium. If it detects no
carrier signal it may assume that one of the following conditions is true:
1. The medium is available.
2. An out-of-range station is in the process of requesting a slot.
3. An out-of-range station is using a slot that it had previously reserved.
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