Chapter 12 Notes

        Layer 1:  The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating,
        maintaining, and deactivating the physical link between end systems.

        Allows bit streams to be created and to travel.

        Layer 2:  The data link layer is concerned with physical (as opposed to logical) addressing, network topology, network
        access, error notification, ordered delivery of frames, and flow control.

        Packages those data packeges into framesto be converted to bit streamsand makes LAN delivery possible.

        Layer 3:  The network layer is a complex layer that provides connectivity and path selection between two host systems
        that may be located on geographically separated networks.

        Layer three packes data from upper layers in packets and makes routing and WAN delivery possible.

        Layer 4 supplies "quality of service" or ensures delivery of data.
        The Layer four of the OSI protocols are TCP and UDP.
        Transmission Control Protocol (TCP)- A connection-oriented Layer 4 (transport layer) protocol that provides reliable
        full-duplex data transmission, TCP is part of the TCP/IP protocol stack
        TCP supplies a virtual circuit between end-user applications.  These are its characteristics.

            connection orriented

            devides outgoing messages into segments

            reassembles messages at the destination station

            re-sends anything not received

            reassemples messages from incoming segments.

        User Datagram Protocol (UDP)- The connectionless transport protocol in the TCP/IP protocol stack. UDP is a simple
        protocol that exchanges datagrams, without acknowledgments or guaranteed delivery. Error processing and
        retransmission must be handled by other protocols.

        UDP transports data unreliably between between hosts.


            transmit messages (called user datagrams)

            provides no software checking for message delivery (unreliable)

            does not reassmble incoming messages

            uses no acknowledgment

            provides no flow control

TCP Segment Fields

            source port- number of calling port

            destination port- number of called port

            sequence number- number used to ensure correct sequencing of arriving data

            acknowledgment number- next expected TCP octet

            window- number of octets that the sender is willing to accept


        FTP- 21

        TELNET- 23

        SMTP- 25

        DNS- 53

        TFTP- 69

        SNMP- 161

        Both TCP and UDP use port (or socket) numbers to pass information to the upper layers

        Numbers below 255 - for public applications

        Numbers from 255-1023 - assigned to companies for marketable applications

        Numbers above 1023 - are unregulated

        TCP hosts establish a connection-oriented session with one another using a three-way handshake. A three-way
        handshake/open connection sequence synchronizes a connection at both ends before data is transferred. This exchange of
        introductory sequence numbers, during the connection sequence is important. It ensures that any data that is lost, due to
        transmission problems, can be recovered.

        TCP hosts establish a connection-oriented session with one another using a three-way handshake

        A three-way handshake synchronises the connection on both ends before any user data is sent

        Windowing is a flow control mechanism requiring that the source device receive and acknowledgement from the
        destination after transmitting a certain amount of data

        Window size determines the amount of data you can transmit at one time before receiving an acknowledgement from the