The PDU at the network layer is a ________
Packet
Frame
Segment
Bit
Answer Posted / pachicoo
Explanation:
A segment is the unit of end-to-end transmission in the TCP
protocol. In general discussion about transmitting
information from one node to another, the term "packet" is
used loosely to refer to a piece of data. However, the
specific packet of data formed by TCP in the transport
layer is called a "segment."
When an application needs to send information to another
network-attached computer, it forms a message. The message
is passed to TCP (assuming a reliable protocol is needed;
otherwise, it is sent to UDP), where it is encapsulated
into a segment. The segment is sent to IP where it is
encapsulated into a datagram. The datagram is sent to the
data link layer where it is encapsulated into frames.
Message: The actual application data, command, or
instruction specified by the user or application. A message
is encapsulated within a TCP segment assuming TCP is used.
Segment: The packet of information exchanged between two
connected systems (peers) that contains transport layer
protocol information. TCP exchanges segments. Segments
encapsulate upper-layer messages.
Datagram: The packet of information that is exchanged
between two connected systems that contains network layer
protocol information. IP exchanges datagrams. Datagrams
encapsulate segments.
Frame: The packet of information at the data link layer
with a layout that is specific to the protocol in use.
Frames encapsulate datagrams.
More: In general, the term packet applies to any message
formatted as a packet, while the term datagram is generally
reserved for packets of an "unreliable" service.
A "reliable" service is one that notifies the user if
delivery fails, while an "unreliable" one does not notify
the user if delivery fails. For example, IP provides an
unreliable service. Together, TCP and IP provide a reliable
service, whereas UDP and IP provide an unreliable one. All
these protocols use packets, but UDP packets are generally
called datagrams.
Transmission Control Protocol (TCP) accepts data from a
data stream, 'segments' it into chunks, and adds a TCP
header creating a TCP segment. The TCP segment is then
encapsulated into an IP packet. A TCP segment is "the
packet of information that TCP uses to exchange data with
its peers."
Note that the term TCP packet is now used interchangeably
with the term TCP segment. Although in the original RFC
segment usually referred to the TCP unit of data, datagram
to the IP unit and packet to the data communications
network unit:
Processes transmit data by calling on the TCP and passing
buffers of data as arguments. The TCP packages the data
from these buffers into segments and calls on the internet
module [e.g. IP] to transmit each segment to the
destination TCP.
http://tools.ietf.org/html/rfc791#section-2.1
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