TCP/IP Transport and Application Layer

Services that are located in the transport layer enable users to segment several upper-layer applications onto the same transport layer data stream. These services also allow for the reassembly of the same upper-layer application segments at the receiving end.

The transport layer data stream provides transport services from the host to the destination. Services such as these are sometimes referred to as end-to-end services. The transport layer data stream is a logical connection between the endpoints of a network.

As the transport layer sends its data segments, it can also ensure the integrity of the data. One method of doing this is called flow control. Flow control avoids the problem of a host at one side of the connection overflowing the buffers in the host at the other side. Overflows can present serious problems because they can result in data loss.

Transport-layer services also allow users to request reliable data transport between hosts and destinations. To obtain such reliable transport of data, a connection-oriented relationship is used between the communicating end systems. Reliable transport can accomplish the following:

It ensures that delivered segments will be acknowledged back to the sender.

It provides for retransmission of any segments that are not acknowledged.

It puts segments back into their correct sequence at the destination.

It provides congestion avoidance and control.

The application layer supports the communicating component of an application. A computer application can require only information that resides on its computer. However, a network application might have a communicating component from one or more network applications.

Concept Questions

Demonstrate your knowledge of these concepts by answering the following questions in the space provided.

For data transfer to begin, both the sending and receiving application programs inform their respective operating systems that a connection will be initiated. How is this accomplished?

One user of the transport layer must establish a connection-oriented session with its peer system. For data transfer to begin, both the sending and the receiving applications inform their respective operating systems that a connection will be initiated. One machine initiates a connection that must be accepted by the other. Protocol software modules in the two operating systems communicate by sending messages across the network to verify that the transfer is authorized and that both sides are ready.

In concept, one machine places a call that the other must accept. If the receiving machine does not accept the call, what happens?

Without an established session, data transmission will not be allowed.

Protocol software modules in the two operating systems communicate by sending messages. Messages are sent across the network to verify that the transfer is authorized and that both sides are ready. How is this accomplished?

Protocol software modules in the two operating systems communicate by sending messages across the network to verify that the transfer is authorized and that both sides are ready.

After all synchronization has occurred, a connection is said to be established and the transfer of data begins. During transfer, the two machines continue to communicate with their protocol software to verify that data is received correctly.

After all synchronization occurs, a connection is established and data transfer begins. How do both machines know that the data is flowing correctly?

There are three methods that allows both machines to know that the data is flowing correctly. These methods are various forms of flow control and include windowing, source quench, and buffering. Both windowing and source quench require the sending and recieving devices to communicate to acknowledge the data being transmitted and recieved. Buffering, uses a different method where information can be stored until the information can be reprocessed. The only problem with buffering is that if the buffers overfill you may lose data

The application layer provides services to application processes. What services are provided?

T        Provides services to application processes

T        Identifies and establishes the availability of intended communication partners

T        Synchronizes cooperating applications

T        Establishes agreement on procedures for error recovery and control of data integrity.

The application layer identifies and establishes the availability of intended communication partners and the resources that are required to connect with them. What are these resources?

The application layer is the layer closest to the end user. This determines whether sufficient resources exist for communication between systems (such as electronic mail, file transfer, and terminal emulation). The application layer provides a direct interface for network applications (such as browser, e-mail, FTP, and Telnet) or an indirect interface for standalone applications (such as word processors, spreadsheets, and presentation managers) with a network redirector.

Vocabulary Exercise

Define the following terms as completely as you can. Use the online curriculum or Chapter 9 from the Cisco Networking Academy Program CCNA 1 and 2 Companion Guide, Third Edition, for help.

Application layer This layer provides services to application processes (such as electronic mail, file transfer, and terminal emulation). The application layer identifies and establishes the availability of intended communication partners (and the resources required to connect with them), synchronizes cooperating applications, and establishes agreement on procedures for error recovery and control of data integrity.

Best-effort delivery Describes a network system that does not use a sophisticated acknowledgment system to guarantee reliable delivery of information.

ClientNode or software program (front-end device) that requests services from a server

Client/server computing Term used to describe distributed computing (processing) network systems in which transaction responsibilities are divided into two parts: client (front end) and server (back end). Both terms (client and server) can be applied to software programs or actual computing devices. Also called distributed computing (processing).

DNS (Domain Name System) System used in the Internet for translating names of network nodes into addresses.

Domain server Server that exists on a local-area network responsible for sharing services and authenticating network access.

ES Generally, an end-user device on a network. 2) Non-routing host or node on an network.

Flow control Technique for ensuring that a transmitting entity, such as a modem, does not overwhelm a receiving entity with data. When the buffers on the receiving device are full, a message is sent to the sending device to suspend the transmission until the data in the buffers has been processed. In IBM networks, this technique is called pacing.

FTP (File Transfer Protocol) Application protocol, part of the TCP/IP protocol stack, used for transferring files between network nodes.

Full duplex Capability for simultaneous data transmission between a sending station and a receiving station.

HTML (Hypertext Markup Language) Simple hypertext document formatting language that uses tags to indicate how a given part of a document should be interpreted by a viewing application, such as a WWW browser.

Hypertext Electronically stored text that allows direct access to other texts by way of encoded links. Hypertext documents can be created using HTML and often integrate images, sound, and other media that are commonly viewed using a WWW browser.

IP address 1) A 32-bit address assigned to hosts using TCP/IP. An IP address belongs to one of five classes (A, B, C, D, or E) and is written as 4 octets separated with periods (dotted-decimal format). Each address consists of a network number, an optional subnetwork number, and a host number. The network and subnetwork numbers together are used for routing; whereas, the host number is used to address an individual host within the network or subnetwork. A subnet mask is used to extract network and subnetwork information from the IP address. Also called an Internet address. 2) Command used to establish the logical network address of this interface.

Redirector Software that intercepts requests for resources within a computer and analyzes them for remote access requirements. If remote access is required to satisfy the request, the redirector forms an RPC and sends the RPC to lower-layer protocol software for transmission through the network to the node that can satisfy the request.

ServerNode or software program that provides services to clients.

Telnet A standard terminal emulation protocol in the TCP/IP protocol stack.

Telnet is used for remote terminal connection, enabling users to log in to

remote systems and use resources as if they were connected to a local system.

TCP (Transmission Control Protocol)Connection-oriented transport layer protocol that provides reliable full-duplex data transmission. TCP is part of the TCP/IP protocol stack.

Transport layer This layer is responsible for reliable network communication between end nodes. The transport layer provides mechanisms for the establishment, maintenance, and termination of virtual circuits, transport fault detection and recovery, and information flow control.

UDP (User Datagram Protocol)Connectionless transport layer protocol in the TCP/IP protocol stack. UDP is a simple protocol that exchanges datagrams without acknowledgments or guaranteed delivery, requiring that error processing and retransmission be handled by other protocols. UDP is defined in RFC 768.

URL (Universal Resource Locator) Standardized addressing scheme for accessing hypertext documents and other services using a WWW browser.

Window sizeRefers to the number of messages that can be transmitted while awaiting an acknowledgment.

Focus Questions

What type of numbers is used to keep track of different conversations that cross the network at the same time?

Both TCP and UDP use port numbers to pass information to the upper layers. Port numbers are used to keep track of different conversations that cross the network at the same time.

What is the name of a protocol that combines connectionless and connection-oriented service?

The Internet Protocol (IP) includes both TCP (connection-oriented) and UDP (connectionless) protocols.

What is the difference between TCP and UDP?

User Datagram Protocol (UDP) is 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.

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 because it ensures that any data that is lost due to transmission problems can be recovered.

What is the field in a TCP segment that ensures correct sequencing of the arriving data?

TCP provides sequencing of bytes with a forward reference acknowledgment. The sequence number in the TCP header is associated with the first byte in the user data area. At the receiving station, TCP reassembles the segments into a complete message

Which protocols use UDP?

Trivial File Transfer Protocol (TFTP)

Simple Network Management Protocol (SNMP)

Dynamic Host Control Protocol (DHCP)

Domain Name System (DNS)

BOOTP

What range of port numbers is reserved for public applications?

Numbers below 255 are used for public applications.

Which window type has a window size that is negotiated dynamically during the TCP session?

The sliding part of sliding window refers to the fact that the window size is negotiated dynamically during the TCP session.

Which network application uses the direct interface that the application layer provides?

The application layer provides a direct interface, by using network applications (for instance, WWW, e-mail, FTP, Telnet), or an indirect interface, by using standalone applications (for instance, word processors, spreadsheets, presentation managers, network redirector).

Where is the server side of a client/server application located?

These applications (such as FTP, web browsers, and e-mail) all have two components that allow them to function: the client side and the server side. The client side is located on the local computer and is the requestor of the services. The server side is located on a remote computer and provides services in response to the clients requests.

What is the looped routine that a client/server application constantly repeats?

A client/server application works by constantly repeating the following looped routine: client-request, server-response; client-request, server-response; and so on.

What does DNS do?

Domain name system (DNS) is a service or application running on a network that manages domain names and responds to requests from clients to translate domain names into the associated IP addresses. --

Which protocol do file utility programs use to copy and move files between remote sites?

File utility programs use the FTP protocol for copying and moving files between remote sites.

Which protocol do remote-access programs use to directly connect to remote resources?

Remote-access programs use the Telnet protocol for directly connecting to remote resources.

CCNA Exam Review Questions

The following questions help you prepare for the CCNA Exam. Answers appear in Appendix B, 'CCNA Exam Review Questions Answer Key.'

Which of the following layers provides transport services from the host to the destination?

A. Application

B. Presentation

C. Session

D. Transport

Which of the following best describes the function of the transport layer?

A. It establishes, manages, and terminates applications.

B. It provides transport services from the host to the destination.

C. It supports communication among programs such as electronic mail, file transfer, and web browsers.

D. It translates between different data formats such as ASCII and EBCDIC.

Which of the following methods best describes flow control?

A. A method to manage limited bandwidth

B. A method of connecting two hosts synchronously

C. A method to ensure data integrity

D. A method to check data for viruses prior to transmission

Which of the following functions best describes flow control?

A. It checks data packets for integrity and legitimacy prior to transmission.

B. It avoids traffic backup by cycling hosts quickly through alternate send and receive modes during peak traffic periods.

C. It connects two hosts over an exclusive high-speed link for critical data transfer.

D. It avoids the problem of a host at one side of the connection, overflowing the buffers in the host at the other side.

Which of the following occurs in the transport layer when a connection is first established between computers in a network?

A. Acknowledgment and retransmission

B. Encapsulation and broadcasting

C. Synchronization and acknowledgment

D. Recovery and flow control

Which of the following occurs in the transport layer when data congestion occurs?

A. Broadcasting

B. Windowing

C. Error recovery

D. Flow control

Which of the following layers handles flow control and error recovery?

A. Application

B. Presentation

C. Transport

D. Network

Which of the following techniques allows multiple applications to share a transport connection?

A. Broadcasting

B. Synchronicity

C. Encapsulation

D. Segmentation

Which of the following best describes segmentation?

A. It breaks data into smaller packets for faster transmission.

B. It switches hosts from send to receive mode continuously during peak traffic periods.

C. It allows multiple applications to share a transport connection.

D. It transfers data from the presentation layer to the network layer for encoding and encapsulation.

Which of the following methods controls the amount of information transferred end-to-end and helps to enable TCP reliability?

A. Broadcasting

B. Windowing

C. Error recovery

D. Flow control

If the window size were set to 1, when would an acknowledgment of data packet receipt be sent back to the source?

A. After one packet

B. After two packets

C. After three packets

D. After four packets

If the window size were set to 3, when would an acknowledgment of data packet receipt be sent back to the source?

A. After one packet

B. After three packets

C. After six packets

D. After nine packets

Which of the following layers supports communication between programs, such as e-mail, file transfer, and web browsers?

A. Application

B. Presentation

C. Session

D. Transport

Which of the following best describes the function of the application layer?

A. It establishes, manages, and terminates applications.

B. It supports communication between programs such as e- mail, file transfer, and web browsers.

C. It provides transport services from the host to the destination.

D. It translates between different data formats such as ASCII and EBCDIC.

Which of the following is a network application?

A. E-mail

B. Word processor

C. Web browser

D. Spreadsheet

Which of the following is a computer application?

A. Remote access

B. File transfer

C. Web browser

D. E-mail

E-mail and file transfer are typical functions of which layer?

A. Transport

B. Network

C. Application

D. Presentation