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 . 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. 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. 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