Term
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Definition
The management of data flow between devices in a network. It is used to avoid too much data arriving before a device can handle it, causing data overflow.
Flow control can prevent the loss of segments on the network and avoid the need for retransmission. |
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Term
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Definition
Data that directs a process. A flag in a data-link frame is an example of control data.
To support these reliability operations, more control data is exchanged between the sending and receiving hosts. This control information is contained in the Layer 4 header. |
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Term
| Internet Assigned Numbers Authority (IANA) |
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Definition
| An organization that assigns the numbers important to the proper operation of the TCP/IP protocol and the Internet, including assigning globally unique IP addresses. |
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Term
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Definition
| Used by TCP and UDP, with values between 0 and 1023, these ports are allocated by high-privilege processes. They are used so that all clients know the correct port number to connect to. |
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Term
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Definition
| Using values between 1024 and 49,151, these numbers are equivalent to well-known ports in concept, but they are specifically used for nonprivileged application processes. |
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Term
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Definition
| TCP and UDP ports that range from 49152 to 65535 and are not used by any defined server applications. |
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Term
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Definition
| A 1-bit flag in the TCP header used to indicate that the receiving host should notify the destination process to do urgent processing. |
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Term
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Definition
| A 1-bit flag in the TCP header that indicates the acknowledgment field is valid. |
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Term
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Definition
| A 1-bit flag in the TCP header that is used to request tot he higher layers for immediate delivery of the packet. |
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Term
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Definition
| A 1-bit flag in the TCP header that is used to request that a connection be re-established. |
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Term
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Definition
| A 1-bit flag int he TCP header used to indicate the initial value of the sequence number. The SYN flag is only set in the first two segments of the three-way TCP connection establishment sequence. |
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Term
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Definition
| A 1-bit field int he TCP header that is used by a device that wants to terminate its session with the other device. This is done by inserting the FIN flag in the flag field found in the TCP segment. |
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Term
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Definition
| A notification sent from one network device to another to confirm that some event (for example, receipt of a message) has occurred. |
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Term
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Definition
| As filed in the TCP header that is set in a sent segment, signifies the maximum amount of unacknowledged data the host is willing to receive before the other sending host must wait for an acknowledgment. used for flow control. |
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Term
| The Transport layer encompasses these functions: |
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Definition
Enables multiple applications to communicate over the network at the same time on a single device
Ensures that, if requried, all the data is received reliably and in order by the correct application
Employs error-handling mechanisms |
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Term
| Roles of the Transport Layer |
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Definition
provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers.
The transport layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control.
Some protocols are state and connection oriented. |
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Term
| Purpose of the Transport Layer |
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Definition
Tracking the individual communications between applications on the source and destination hosts
Segmenting data and managing each piece
Reassembling the segments into streams of application data
Identifying the different applications
Performing flow control between end users
Enabling error recovery
Initiating a session |
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Term
| Tracking Individual Conversations |
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Definition
It is the responsibility of the transport layer to maintain the multiple communication streams between these applications.
Each of these applications [instant messaging, video stream, VoIP] are sending and receiving data over the network at the same time. Data is not sent to the wrong application. |
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Term
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Definition
The Transport layer breaks the data into smaller pieces, which are better suited for transmission. These pieces are called segments.
This process includes the encapsulation required on each piece of data. Each piece of application data requires headers to be added at the transport layer to indicate to which communication it is associated. |
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Term
| Reassembling the segments |
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Definition
By numbering and sequencing the segments, the transport layer can ensure that these segments are reassembled into the proper order.
At the receiving host, each segment of data must be reassembled in the correct order and then directed to the appropriate application.
The protocols at the transport layer describe how the transport layer header information is used to reassemble the data pieces into in-order data streams to be passed to the application layer. |
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Term
| Identifying the Applications |
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Definition
| To pass data streams to the proper applications, the transport layer must identify the target application. To accomplish this, the transport layer assigns an identifier to an application, called a port number, which is unique to the application and host. |
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Term
Conversation
At the Transport Layer |
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Definition
| At the transport layer, each particular set pieces of flowing between a source application and a destination application is known as a conversation. Dividing data into small parts, and sending these parts from the source to the destination, enables many different communications to be interleaved (multiplexed) on the same network. |
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Term
| The transport layer is the link between the application layer and the lower layers that are responsible for network transmission. |
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Definition
This layer accepts data from different conversations and passes it down to the lower layers as manageable pieces that can be eventually multiplexed over the media.
Applications do not need to know the operation details of the network in use (without regard to the destination host type, the type of media over which the data must travel, the path taken by the data, the congestion on a link, or the size of the network).
Additionally, the lower layers are not aware that multiple applications are sending data on the network. Their responsibility is to deliver data to the appropriate device. The transport layer then sorts these pieces before delivering them to the appropriate application. |
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Term
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Definition
| Network hosts have limited resources, such as memory or bandwidth. When the transport layer is aware that these resources are overtaxed, some protocols can request that the sending application reduce the rate of data flow. This is done at the transport layer by regulating the amount of data the source transmits as a group. Flow control can prevent the loss of segments on the network and avoid the need for retransmission. |
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Term
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Definition
| The transport layer can provide connection orientation by creating a session between the applications. These connections prepare the applications to communicate with each other before any data is transmitted. Within these sessions, the data for a communication between the two applications can be closely managed. |
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Term
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Definition
| The transport layer can provide connection orientation by creating a session between the applications. These connections prepare the applications to communicate with each other before any data is retransmitted. Within these sessions, the data for a communication between the two applications can be closely managed. |
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Term
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Definition
Multiple transport layer protocols exist to meet the requirements of different applications.
Some prtocols, such as UDP (User Datagram Protocol), provide just the basic functions for efficiently delivering the data pieces between the appropriate applications. These types of protocols are useful for applications whose data is sensitivie to delays.
Other transport layer protocols, such as TCP (Transmission Control Protocol), describe processes that provide additional features, such as ensuring reliable delivery between the applications. While these additional functions provide more robust communication at the transport layer between applications, they have additional overhead and make larger demands on the network. |
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Term
| Supporting Reliable Communication |
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Definition
TCP is a transport layer protocol that can be implemented to ensure reliable delivery of the data. In networking terms, reliability means ensuring that each piece of data that the source sends arrives at the destination. At the transport layer, the three basic operations of reliability are:
Tracking tansmitted data
Acknowleging received data
Retransmitting any unacknowledged data |
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Term
| Tracking all data pieces for a conversation |
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Definition
| The transport layer of the sending host tracks all the data pieces for each conversation and retransmits any data that the receiving host did not acknowledge. These reliability processes place additional overhead on the network resources because of the acknowledgement, tracking, and retransmission. |
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Term
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Definition
| In the context of networking, best-effort delivery is referred to as unreliable, because the destination does not acknowledge whether it received the data. |
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Term
| Choosing between reliable and unreliable transmission |
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Definition
Web pages, databases, and e-mail require accurate transmission of data, reliable transmission is chosen
Video and VoIP can tolerate small amounts of data loss, but having 100% accurate transmission would degrade the performance of such services, unreliable transmission is chosen |
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Term
| User Datagram Protocol (UDP) |
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Definition
UDP is a simple, connectionless protocol, described in RFC 768. It has the advantage of providing low-overhead data delivery. The segments of communication in UDP are called datagrams. UDP sends datagrams as "best effort".
Applications that use UDP include:
Domain Name System (DNS)
Video streaming
Voice over IP (VoIP) |
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Term
| Transmission Control Protocol (TCP) |
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Definition
| TCP is a connection-oriented protocol, described in RFC 793. TCP incurs additional overhead to gain functions. Additional functions specified by TCP are same-order delivery, reliable delivery, and flow control. Each TCP segment has 20 bytes of overhead in the header encapsulating the application layer data, whereas each UDP segment has only 8 bytes of overhead. |
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