[Cisco Networking Academy] Del Mar College
ITNW-Beginning Router Configuration
 Instructor: Michael P. Harris
Sem2Les3

Semester 2, Lesson 3 Notes:

The Host Layers:

7-Application Layer, 6-Presentation Layer, and 5-Session

1. Give three examples of both computer and network Applications. Explain the difference between computer and network applications.

The "Application layer" (layer 7) supports the communicating component of an application.

"Computer applications" include word processing, presentation graphics, spreadsheets, and web browsers.

"Network applications" include e-mail, file transfer, remote access, and the World Wide Web. Computer applications can only require information that resides on the computer; network applications have a communicating component. The two typically work together.

2. Name five file formats associated with the "Presentation layer" (layer 6); classify each as text/data format; sound/video format; or graphics/visual image format.

The "Presentation layer" (layer 6) formats and converts network application data to represent text, graphics, images, video, and audio. Later 6 provides code formatting and conversion. Code formatting is used to make sure that applications have meaningful information to process. If it is necessary, this layer can translate between different data formats.

The "Presentation layer" (layer 6) is not only concerned with the format and representation of data. It is also concerned with the data structure that is used by programs. Thus, Layer 6 arranges for Layer 7 how data will be organized when it is transferred.

Examples of text/data formats are ASCII (American Standard Code for Information Interchange), EBCDIC (Extended Binary Coded Decimal Interchange Code), and encrypted.

Examples of sound/video formats are MIDI (Musical Instrument Digital Interface), MPEG (Motion Picture Expert's Group), and Quicktime.

Examples of Graphics/Visual Image formats are PICT, TIFF, JPEG, and GIF.

What does the "Session layer" (layer 5) do?

The "Session layer" (layer 5) establishes, manages, and terminates sessions between applications. Essentially, the sessions layer functions coordinates service requests and responses and communication interactions that occur when applications communicate between hosts (between applications).

4. Name and briefly describe four tasks accomplished by the "Transport layer" (layer 4).

The "Transport layer" (layer 4) data stream is a logical connection between the endpoints of a network. It...

  1. "segments" upper layer applications, allowing multiple applications to share a transport connection.
  2. establishes an "end-to-end connection", through a 'synchronization', 'negotiation', and 'acknowledgment' process.
  3. sends segments with "flow control", so as not to overwhelm the receiving computer's buffer and ensure the integrity of the data.
  4. help assure "data reliability" or reliable data transmission through a sending, receiving, and acknowledgment process called 'windowing'.

Reliable Transport-layer functions include:

  1. Connection synchronization
  2. Flow control
  3. Error recovery
  4. Reliability through windowing

The "Transport layer" (layer 4) can provide a connection-oriented relationship between the communicating end systems. Reliable transport can accomplish the following:

  • Ensure that segments delivered will be acknowledged back to the sender.
  • Provide for retransmission of any segments that are not acknowledged.
  • Put segments back into their correct sequence at the destination.
  • Provide congestion avoidance and control.
To establish a (layer 4) connection:
--------synchronize------->
<--negotiate connection-->
<-------synchronize------->
-------acknowledge-------->
[connection established]
<------data transfer------>
(send segments)

TCP "Three-Way Handshake"
===========================
1. Send SYN (seq=x)        --------> Receive SYN (seq=x)
2. Receive SYN (seq=x)               Send SYN
ACK (x+1)                       <-------- (seq=y, ack=x+1)
3. Send ACK (ack=y+1)     --------> Receive ACK (ack=y+1)

5. How does the "Transport layer" (layer 4) 'view' the "Network layer" (layer 3)?

The "Transport layer" (layer 4) views the 'Network layer" (layer 3) as a cloud to send data packets from source to destination.

Each layer in the source uses its own "protocol" to communicate with its peer layer in the destination.

... segment -> network (packet) -> frame   (headers)

Multiple applications can share a transport connection using a technique called "segmentation".

The number of data packets the sender is allowed to have outstanding - without yet receiving an acknowledgment - is known as the "window". Sending a packet with window size=0 means stop; sending a packet with a window size>0 means ready to receive. (a layer 4 function)

"Windowing" is a method to control the amount of information transferred end-to-end. Some protocols measure information in terms of the number of packets; TCP/IP measures information in terms of the number of bytes.

PAR - Positive Acknowledgment and Retransmission

All four upper layers - application (Layer 7), presentation (Layer 6), session (Layer 5), and transport (Layer 4) - can encapsulate data in end-to- end "segments". The Transport layer assumes it can use the network as a "cloud" to send data packets across from sender source to receiver destination.

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