Useful Definitions 1-5
Introduction
Maximum transmission unit (MTU)
The largest amount of user-data (e.g. the largest size of an IP datagram) that can
be sent in a single frame on a particular medium.
Passive open
A sequence of events occurring when an entity using an application protocol of
the Internet suite (e.g., SMTP, FTP, SNMP, or Telnet) informs the Transmission
Control Protocol that it is willing to accept a connection to another user of the
application’s particular service. See Transmission Control Protocol (TCP),
page 1-6, for more information.
Ports
Integer quantities which identify to a transport protocol (UDP or TCP) the
particular application entity (e.g., SMTP, FTP, SNMP, or Telnet) used in the
transmission/reception of the data (e.g., UDP uses a port value of 161 decimal to
identify SNMP data).
Protocol Data Unit (PDU)
A unit of information, which uses a protocol to offer a service, that is exchanged
by protocol machines, A PDU usually contains protocol control information (a
header identifying data to be transferred) and user data.
Reassembly
The process of recombining fragments, at the final destination, into the original
datagram.
Retransmission
The process of a source TCP entity resending a unit of data while waiting for an
acknowledgment of receipt by the destination TCP entity. Each time a source TCP
entity transmits a segment, it starts a retransmission timer. If this timer expires
before an acknowledgment from the destination, the segment will be transmitted
and the timer will be restarted. Retransmission can occur only a certain number of
times until the transmitting entity aborts the connection.
Segment
The unit used for data exchange between two entities using TCP.
Socket
A pairing of an IP address (destination or source) and a TCP port number. The
pairing of two internet sockets (destination and source IP addresses and TCP
ports) forms a connection.
Simple Network Management Protocol (SNMP)
The application protocol which offers network management services in the
Internet suite of protocols. SNMP provides four operations for network
management via a device’s MIB (its manageable objects): get (retrieval of specific
management information), get-next (retrieval of management information in
series by traversing the MIB), set (manipulation of management information), and
trap (reports on extraordinary events at the device).