be acknowledged before sending the next segment.
TCP utilizes a sliding window when determining transmission
size. A sliding window allows for devices to negotiate a window
size to allow for more than one byte to be sent during a single
transmission. This sliding window also allows the destination
device to indicate to the source a need to decrease or increase
the amount of data being sent because it is incapable at that
time of dealing with that much data. Interactive Media
Activity Matching: Windowing After completing this
activity, the student will be able to understand windowing.
Web Links Windowing Packets http://howto.lycos.com/l
ycos/step/ 1,,5+30+34556+34645+34650,00.html
Content
10.1 TCP Operation 10.1.5
Sequencing numbers TCP breaks data into segments. The data
segments are then transported from sender to receiver,
following the synchronization process and the negotiation of a
window size that dictates the number of bytes that can be
transmitted at any one time. The data segments being
transmitted must be reassembled once all the data is received.
There is no guarantee that the data will arrive in the order it
was transmitted. TCP applies sequence numbers to the data
segments it is transmitting so that the receiver will be able
to properly reassemble the bytes in their original order. If
TCP segments arrive out of order, the segments may be
reassembled incorrectly. Sequencing numbers indicate to the
destination device the correct order in which to put the bytes
when they are received. These sequencing numbers also act as
reference numbers so that the receiver will know if it has
received all of the data. They also identify the missing data
pieces to the sender so it can retransmit the missing data.
This offers increased efficiency since the sender only needs to
re-transmit the missing segments instead of the entire set of
data. Each TCP segment is numbered before transmission. Notice
that following the destination port in the segment format is
the sequence number portion. At the receiving station, TCP uses
the sequence numbers to reassemble the segments into a complete
message. If a sequence number is missing in the series, that
segment is re-transmitted.
Content 10.1
TCP Operation 10.1.6 Positive ACK
Acknowledgement is a common step in the synchronization process
which includes sliding windows and data sequencing. In a TCP
segment, the sequence number field is followed by the
acknowledgment number field, also referred to as the code
field. This field is where acknowledgments, or ACKs, (as well
as SYN) are indicated.One problem with the unreliable IP
protocol is that there is no verification method for
determining that data segments actually reached their
destination. So data segments may be constantly forwarded with
no knowledge as to whether or not they were actually received.
TCP utilizes positive acknowledgment and retransmission to
control data flow and confirm data delivery. Positive
acknowledgment and retransmission (PAR) is a common technique
many protocols use to provide reliability. With PAR, the source
sends a packet, starts a timer, and waits for an acknowledgment
before sending the next packet. If the timer expires before the
source receives an acknowledgment, the source retransmits the
packet and starts the timer over again. TCP uses expectational
acknowledgments in which the acknowledgment number refers to
the next octet that is expected. Windowing is a flow control
mechanism requiring that the source device receive an
acknowledgment from the destination after transmitting a
certain amount of data. With a window size of three, the source
device can send three octets to the destination. It must then
wait for an acknowledgment. If the destination receives the
three octets, it sends an acknowledgment to the source device,
which can now transmit three more octets. If, for some reason,
the destination does not receive the three octets, possibly due
to overflowing buffers, it does not send an acknowledgment.
Because the source does not receive an acknowledgment, it knows
that the octets should be retransmitted and that the
transmission rate should be slowed. Lab Activity Lab
Exercise: Multiple Active Host Sessions This lab will show port
usage on a single host attached to a router. Interactive
Media Activity Interactivity: TCP Sliding Windows After
completing this activity, the student will be able to
understand window size. Web Links TCP - Positive
Acknowledgment and Re-transmission
http://www.it.iitb.ac.in/~jaju/ tutorials/net/tcpip/
node22.html
Content 10.1 TCP
Operation 10.1.7 UDP operation The TCP/IP
protocol stack contains many different protocols, each designed
to perform a certain task. IP provides Layer 3 connectionless
transport through an internetwork. TCP enables
connection-oriented, reliable transmission of packets at Layer
4 of the OSI model. UDP provides connectionless, non-guaranteed
transmission of packets at Layer 4 of the OSI model.Both TCP
and UDP use IP as their underlying Layer 3 protocol. In
addition, TCP and UDP are used by various application layer
protocols. TCP provides services for applications, such as FTP,
HTTP, SMTP, and DNS. UDP is the transport layer protocol used
by DNS, TFTP, SNMP, and DHCP. TCP must be used when
applications need to guarantee that a packet arrives intact, in
sequence, and unduplicated. The overhead associated with
ensuring delivery of the packet is sometimes a problem when
using TCP. Not all applications need to guarantee delivery of
the data packet, so they use the faster, connectionless
delivery mechanism afforded by UDP. The UDP protocol standard,
described in RFC 768, is a simple protocol that exchanges
segments without acknowledgments or guaranteed delivery. UDP
does not use windowing or acknowledgments so application layer
protocols must provide error detection. The Source Port field
is an optional field used only if information needs to return
to the sending host. When a destination router receives a
routing update, the source router is not requesting anything so
nothing needs to return to the source. There is no exchange of
information or data. The Destination Port field specifies the
application to which UDP needs to pass the protocol. A DNS
request from a host to a DNS server would have a Destination
Port field of 53, the UDP port number for DNS. The Length field
identifies the number of octets in the UDP segment. The UDP
checksum is optional but should be used to ensure that the data
has not been damaged during transmission. For transport across
the network, UDP is encapsulated within the IP packet. Once a
UDP segment arrives at the destination IP address, a mechanism
must exist which allows the receiving host to determine the
exact destination application. Destination ports are used for
this purpose. If a host is running both TFTP and DNS services,
it must be able to determine what service the arriving UDP
segments need. The Destination Port field in the UDP header
determines the application to which a UDP segment will be
delivered. Web Links UDP - Internet User Datagram
Protocol http://www.networksorcery.com/ enp/ protocol/ udp.htm
Content 10.2 Overview of Transport Layer
Ports 10.2.1 Multiple conversations between
hosts At any given moment, thousands of packets providing
hundreds of different services traverse a modern network. In
many cases, servers provide for a multitude of services which
causes unique problems for the addressing of packets. If a
server is running both SMTP and WWW, it uses the destination
port field to determine what service the source is requesting.
The source cannot construct a packet destined for just the
server IP address because the destination would not know what
service was being requested. A port number must be associated
with the conversation between hosts to ensure that the packet
reaches the appropriate service on the server. Without a way to
distinguish between different conversations, the client would