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Practical TCP/IP and Ethernet Networking- P58
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Practical TCP/IP and Ethernet Networking- P58: The transmitter encodes the information into a suitable form to be transmitted over the
communications channel. The communications channel moves this signal as
electromagnetic energy from the source to one or more destination receivers. The
channel may convert this energy from one form to another, such as electrical to optical
signals, whilst maintaining the integrity of the information so the recipient can understand
the message sent by the transmitter....
AMBIENT/
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Nội dung Text: Practical TCP/IP and Ethernet Networking- P58
- Satellites and TCP/IP 267
immediately send a replacement packet thus avoiding the timeout condition and the
consequent lengthy recovery in TCP (which would otherwise then have reduced its
window size and then very slowly increased bandwidth utilization)
Congestion avoidance
There are two congestion avoidance techniques; but neither has been popular as yet. The
first approach, which has to be implemented in a router, is called random early
detection (RED) where the router sends an explicit notice of congestion (using the ICMP
protocol discussed in an earlier chapter) when it believes that congestion will occur
shortly if it doesn’t take corrective action.
On the other hand an algorithm can be implemented in the sender where it observes the
minimum round trip time for the packets it is transmitting to calculate the amount of data
queued in the communications channel. If the number of packets being queued is
increasing, it can reduce the congestion window. It will then increase the congestion
window when it sees the number of queued packets decreasing.
TCP for transactions (T/TCP)
As discussed earlier, the three-way handshake represents a considerable overhead for
small data transactions (often associated with HTTP transfers). An extension called
T/TCP bypasses the three-way handshake and the slow-start procedure by using the data
stored in a cache from previous transactions.
Middleware
It is also possible to effect significant improvements to the operation of TCP/IP without
actually modifying the TCP/IP protocol itself using what is called middleware where
split-TCP and TCP spoofing could be used.
Split-TCP
The end-to-end TCP connection is broken into two or three segments. This is indicated in
the figure below. Each segment is in itself a complete TCP link. This means that the outer
two links (which have minimal latency) can be setup as per usual. However the middle
TCP satellite link with significant latency would have extensions to TCP such as TCP-
LW and T/TCP. This means only minor modifications to the application software at each
end of the link.
Figure 18.8
Use of Split TCP (courtesy of Loyola University)
TCP spoofing
An intermediate router (such as at the satellite uplink) immediately acknowledges all TCP
packets coming through it to the receiver. All the receiver acknowledgment packets are
suppressed so that the originator does not get confused. If the receiver does not receive a
specific packet and the router has timed out, it will then retransmit this (missing) segment
- 268 Practical TCP/IP and Ethernet Networking
to the receiver. The resultant effect is that the originator believes that it is dealing with a
low latency network.
Figure 18.9
TCP spoofing (courtesy of Loyola University)
Application protocol approaches
There are three approaches possible here:
• Persistent TCP connections
• Caching
• Application specific proxies
Persistent TCP connections
In some client–server applications with very small amounts of data transfer, there are
considerable inefficiencies. The HTTP 1.1 standard minimizes this problem and takes a
persistent connection and combines all these transfers into one fetch. Further to this it
pipelines the individual transfers so that there is an overlap of transmission delays thus
making for an efficient implementation.
Caching
In this case, the commonly used documents (such as used with HTTP and FTP web
protocols) are broadcast to local caches. The web clients then access these local caches
rather than having to go through a satellite connection. The web clients thus have a
resultant low latency and low network utilization (meaning more bandwidth available for
higher speed requirements).
Application specific proxies
In this case, an application specific proxy can use its domain knowledge to pre-fetch web
pages so that web clients subsequently requesting these pages considerably reduce the
effects of latency.
- Satellites and TCP/IP 269
References
There are a number of excellent references (many web site-based), which have been used
in this document. It should be emphasized that due to the rapid changes in satellite
communications with respect to TCP/IP, the Web is often the best source of information
on this topic.
Montgomery, J. The Orbiting Internet: Fiber in the Sky. John Montgomery. Byte
Magazine. November 1997.
Yongguang Zhang (ygz@isl.hrl.hac.com)
Dante De Lucia (dante@isl.hrl.hac.com)
Bo Ryu (ryu@isl.hrl.hac.com)
Son K. Dao (son@isl.hrl.hac.com)
Satellite Communications in the Global Internet – Issues, Pitfalls, and Potential.
Hughes Research Laboratories. Malibu, California 90265, U.S.A
Internet: http://www.wins.hrl.com/people/ygz/papers/inet97/index.html
Christoph Mahle (editor), Kul Bhasin, Charles Bostian, William Brandon, John
Evans, Alfred Mac Rae. WTEC Panel Report on Global Satellite Communications
Technology and Systems.
Internet: http://itri.loyola.edu/satcom2/04_05.htm
Suggested web sites with references:
Alcatel
Paris, France
Phone: +33 1 4058 5858
Internet: http://www.alcatel.com/our_bus/telecom/products/space
whatsnew.htm
Hughes Communications, Inc.
Long Beach, CA
Phone: 310-525-5000
Internet: http://www.spaceway.com
Lockheed
Sunnyvale, CA
Phone: 888-278-7565
Phone: 408-543-3103
Internet: http://www.astrolink.com
Loral
Palo Alto, CA
Phone: 650-852-5736
Internet: http://www.cyberstar.com
Motorola
Chandler, AZ
Phone: 602-732-4018
Internet: http://www.mot.com/
Teledesic
Kirkland, WA
Phone: 425-602-0000
Internet: http://www.teledesic.com
- Appendix A
-RUYYGX_
10Base2
IEEE 802.3 (or Ethernet) implementation on thin coaxial cable (RG58/Au).
10Base5
IEEE 802.3 (or Ethernet) implementation on thick coaxial cable.
10Base-T
IEEE 802.3 (or Ethernet) implementation on unshielded 22 AWG twisted pair cable.
'
ABM
Asynchronous Balanced Mode
Access control mechanism
The way in which the LAN manages the access to the physical transmission medium.
Address
A normally unique designator for location of data or the identity of a peripheral
device, which allows each device on a single communications line to respond to its
own message.
Address resolution protocol (ARP)
A TCP/IP process used by a router or a source host to translate the IP address into the
physical hardware address, for delivery of the message to a destination on the same
physical network.
Algorithm
Normally used as a basis for writing a computer program. This is a set of rules with a
finite number of steps for solving a problem.
- 'VVKTJO^ ' -RUYYGX_
Alias frequency
A false lower frequency component that appears in data reconstructed from original data
acquired at an insufficient sampling rate (which is less than two (2) times the maximum
frequency of the original data).
ALU
Arithmetic Logic Unit
Amplitude modulation
A modulation technique (also referred to as AM or ASK) used to allow data to be
transmitted across an analog network, such as a switched telephone network. The
amplitude of a single (carrier) frequency is varied or modulated between two levels one
for binary 0 and one for binary 1.
Analog
A continuous real time phenomenon where the information values are represented in a
variable and continuous waveform.
ANSI
American National Standards Institute. The national standards development body in the
USA.
API
Application Programming Interface.
Appletalk
A proprietary computer networking standard initiated by the Apple Computer for use in
connecting the Macintosh range of computers and peripherals. This standard operates at
230 kilobits/second.
Application layer
The highest layer of the seven-layer ISO/OSI reference model structure, which contains
all user or application programs.
Application programming interface (API)
A specification defining how an application program carries out a defined set of services.
Arithmetic logic unit
The element(s) in a processing system that perform(s) the mathematical functions such as
addition, subtraction, multiplication, division, inversion, AND, OR, NAND and NOR.
ARP
Address Resolution Protocol.
ARPANET
The packet switching network, funded by the DARPA, which has evolved into the world-
wide Internet.
ARP cache
A table of recent mappings of IP addresses to the physical addresses, maintained in each
host and router.
AS
Australian Standard
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