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Appendix B: Glossary
transport mode
: A mode in which the endpoints of an IPsec VPN connection perform their own encryption. The VPN gateway
functions solely as the transport, encapsulating (thus protecting) the upper layer payload and reusing the IP header. Compare
main mode, aggressive mode, quick mode. Also see tunnel mode.
triangulation
: A method of calculating one’s position, in Cartesian coordinates x and y, by means of distances and angles from
static points whose positions are known (or by means of distances and angles from points whose positions are in motion and by
relating each position to a specific time t).
Triangulation uses the law of sines in one plane (for coordinates x and y), and requires at least one known distance and two
known angles. Collection of this information requires at least two reference points (for example, static towers for cellular wireless
networks), so that a triangle is formed by the positions of any two reference points and the position being calculated.
NOTE: GPS does not use triangulation; GPS uses trilateration. However, when GPS signals are weak, triangulation might be useful
instead of trilateration to determine position.
Compare trilateration.
trilateration
: A method of calculating one’s position, in Cartesian coordinates x, y, and z, by means of distances from static
points whose positions are known (or by means of distances from points whose positions are in motion and by relating each
position to a specific time t).
Trilateration works with three reference points (for example, satellites). In GPS, data are needed from four satellites, because one
of the computations solves for real time. (A GPS satellite includes its clock time with each set of spatial coordinates it broadcasts.
The signal delay caused by travel through the atmosphere must be resolved in terms of real time.)
Data from four satellites allow simultaneous solution for four variables (time t and coordinates x, y, and z) in four linear equations.
If the value of one variable is already known (for example, at sea level, altitude z is zero), data are sufficient from three satellites.
NOTE: For details and examples of trilateration, please see the GPS navigation literature.
Compare triangulation.
Triple Data Encryption Algorithm (TDEA) : See Triple Data Encryption Standard
Triple Data Encryption Standard (3DES) : An encryption method incorporating three iterations of DES, each with a different
key, for added security:
• Encryption
• Decryption
• Another encryption
Some versions of 3DES use two DES keys (112 bits) in each iteration. Some versions use three DES keys (168 bits) in each iteration.
Trivial File Transfer Protocol (TFTP) : A simple protocol for transferring files. TFTP is based on UDP.
tunneling : Use of encapsulation to send one protocol through a network that uses a different protocol. Also see tunnel mode,
VPN tunneling.
tunnel mode : In IKEv1, the type of tunneling used in a transmission. For example, an IPsec VPN might use main mode or
aggressive mode, then quick mode; or it might use only transport mode. Also see tunneling, VPN tunneling.
type of service (TOS, ToS)
: Use of bits in a packet’s IP header to indicate specific priority and service type for the packet
Contrast with class of service. Also see Differentiated Services, quality of service.
U (a unit of measure) : See rack unit.
UDP
: See User Datagram Protocol.
User Datagram Protocol (UDP)
: A protocol layer used in IP. UDP does not check for packet order or packet receipt. TFTP is
based on UDP. Compare Transmission Control Protocol.