Understanding the network.pdf - Back to Home

5. The network packet is handed to the data link layer.
6. At the data link layer, the network packet is wrapped in a
header and footer that are appropriate to the data-link protocol.
After the destination host receives the network packet, the process is
reversed and the data is handed back to the ULP. Remember that the
transport protocol is independent from the ULP and the LLP (Lower
Layer Protocols). Its job is only to provide data transport. The actual
delivery is in part a joint effort between the ULP and the LLP.
TCP
TCP provides connection-oriented data transport over dedicated
process owned by a VC. It works over almost any type of network and
is designed to provide reliable data transport between nodes over
unreliable network delivery services. For TCP, reliable transport
means in-sequence, adjustable-rate, and error-free transmission of
ULP data.
TCP provides the following services to facilitate these transport goals:
• Full duplex—TCP connections are bidirectional. Continuous data
flow is provided to communicating with the ULPs.
• Data sequencing—Data is delivered to the ULP in the byte order
in which it was sent. TCP provides for transport level and below
data fragmentation. TCP assigns sequence numbers for each
ULP data segment. These sequence numbers are used at the
peer end of the TCP session to reassemble the ULP data back
into its original form. If a sequence is lost, the peer requests it
to be resent, and the data is not sent to the ULP until all the data
is reassembled.
• Flow control—TCP uses a sliding window strategy to transmit
data. TCP establishes the speed of the entire path at the start of
the session. It then creates a window whose size is based on the
speed of the path between the source and destination. TCP uses
the window to frame the amount of data it will send at one time
and uses the sequencing numbers to keep track of each byte it
transmits. TCP sends a burst of data and waits for confirmation
from the peer that all the data has been received. If no response
is received, TCP will retransmit the data until it gets an
acknowledgment that the peer has received all the data. TCP
uses timers (to measure between when the data was sent and
the acknowledgment was received) to adjust the size of the