Transport Layer Study Guide Networking

The transport layer is responsible for

logical communications between applications running on different hosts such as establishing a temporary session between two hosts and the reliable transmission of information for an application.

The transport layer has no

knowledge of 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.

What two protocols does the transport layer use?

TCP and UDP (Transmission Control Protocol and User Datagram Protocol)

At the transport layer, each set of data flowing between a source application and a destination application is known as?

a conversation and is tracked separately.

True or False: Most networks have a limitation on the amount of data that can be included in a single packet. Therefore, data must be divided into manageable pieces.

True

It is the transport layer's responsibility to divide the application data into

appropriately sized blocks.

Depending on the transport layer protocol used, the transport layer blocks are called

either segments or datagrams.

The transport layer divides the data into smaller blocks (i.e., segments or datagrams) so that

it's easier to manage and transport.

The transport layer protocol also adds header information containing binary data organized into several fields to each block of data. It is the values in these fields that enable

various transport layer protocols to perform different functions in managing data communication.

True or False: The transport layer ensures that even with multiple applications running on a device, all applications receive the correct data.

True

The transport layer must be able to separate and manage multiple communications with different transport requirement needs. To pass data streams to the proper applications, the transport layer identifies the target application using an identifier called

a port number.

each software process that needs to access the

network is assigned a port number unique to that host.

Sending some types of data (e.g., a streaming video) across a network, as one complete communication stream, can consume

all the available bandwidth.

This would prevent other communication conversations from occurring at the same time.

It would also make error recovery and retransmission of damaged data difficult.

The transport layer uses segmentation and multiplexing to enable

different communication conversations to be interleaved on the same network.

Error checking can be performed on the data in the segment

to determine if the segment was altered during transmission.

Transport layer protocols specify how to transfer messages between hosts, and are responsible for

managing reliability requirements of a conversation.

TCP is considered

a reliable, full-featured transport layer protocol, which ensures that all of the data arrives at the destination.

TCP includes fields which ensure the delivery of the application data. These fields require additional

processing by the sending and receiving hosts.

TCP transport is analogous to

sending packages that are tracked from source to destination.

TCP provides reliability and flow control using these basic operations

Number and track data segments transmitted to a specific host from a specific application Acknowledge received data, Retransmit any unacknowledged data after a certain amount of time, Sequence data that might arrive in wrong order Send data at an efficient rate that is acceptable by the receiver

In order to maintain the state of a conversation and track the information, TCP must first establish a connection between the sender and the receiver. This is why TCP is known as a

connection-oriented protocol.

UDP is a simpler transport layer protocol than TCP. It does not

provide reliability and flow control, which means it requires fewer header fields.

Because the sender and the receiver UDP processes do not have to manage reliability and flow control, this means UDP datagrams can be

processed faster than TCP segments.

UDP provides the basic functions for delivering datagrams between the appropriate applications, with

very little overhead and data checking.

True or False: UDP is a connectionless protocol. Because UDP does not provide reliability or flow control, it does not require an established connection.

True

True or False: Because UDP does not track information sent or received between the client and server, UDP is also known as a state protocol.

False

True or False: UDP is also known as a best-effort delivery protocol because there is no acknowledgment that the data is received at the destination. With UDP, there are no transport layer processes that inform the sender of a successful delivery.

True

True or False: UDP is like placing a regular, non registered, letter in the mail. The sender of the letter is not aware of the availability of the receiver to receive the letter. Nor is the post office responsible for tracking the letter or informing the sender if the letter does not arrive at the final destination.

True

True or False: Some applications can tolerate some data loss during transmission over the network, but delays in transmission are unacceptable. For these applications, TCP is the better choice because it requires less network overhead.

False

True or False: For other applications it is important that all the data arrives and that it can be processed in its proper sequence. For these types of applications, TCP is used as the transport protocol

True

The application uses TCP to perform

buffering, bandwidth probing, and congestion control, in order to better control the user experience.

What features does TCP have?

Establishes a Session, Ensures Reliable Delivery, Provides Same-Order Delivery, Supports Flow Control.

TCP is a stateful protocol which means

it keeps track of the state of the communication session.

To track the state of a session,

TCP records which information it has sent and which information has been acknowledged.

The stateful session begins with

the session establishment and ends with the session termination.

True or False: TCP handles all tasks associated with dividing the data stream into segments, providing reliability, controlling data flow, and reordering segments.TCP blocks the application from having to manage any of these tasks

True

True or False: UDP is such a complex protocol that it is usually described in terms of what it does not do compared to TCP.

False

What features do UDP have?

Data is reconstructed in the order that it is received; Any segments that are lost are not resent., There is no session establishment., The sending is not informed about resource availability.

True or False: If reliability is required when using UDP as the transport protocol, it must be handled by the application.

True

True or False: One of the most important requirements for delivering live video and voice over the network is that the data continues to flow quickly. Live video and voice applications can tolerate some data loss with minimal or no noticeable effect, and are perfectly suited to TCP.

False

What are the three types of applications that are best suited for UDP?

Live video and multimedia applications, Simple request and reply applications, Application that handles reliability themselve

DNS will use TCP if the DNS request or DNS response is more than

512 bytes, such as when a DNS response includes many name resolutions

True or False: Although DNS and SNMP use TCP by default, both can also use UDP

False

The source port number is associated with the

originating application on the local host whereas the destination port number is associated with the destination application on the remote host.

The source and destination ports are placed within the segment. The segments are then encapsulated within an IP packet. The IP packet contains the IP address of the source and destination. The combination of the source IP address and source port number, or the destination IP address and destination port number is known as

a socket.

The Internet Assigned Numbers Authority (IANA) is the standards organization responsible for assigning various addressing standards, including the 16-bit port numbers. The 16 bits used to identify

the source and destination port numbers provides a range of ports from 0 through 65535.

What examples show that some applications may use both TCP and UDP?

DNS uses UDP when clients send requests to a DNS server. However, communication between two DNS servers always uses TCP.

Netstat is an important network utility

that can be used to verify those connections.

What is shown when you enter the command netstat to list the protocols in use

the local address and port numbers, the foreign address and port numbers, and the connection state.

True or False: To close a connection, the Finish (FIN) control flag must be set in the segment header. To end each one-way TCP session, a two-way handshake, consisting of a FIN segment and an Acknowledgement (ACK) segment, is used. Therefore, to terminate a single conversation supported by TCP, four exchanges are needed to end both sessions. Either the client or the server can initiate the termination.

True

What are the functions within a three-way handshake?

It establishes that the destination device is present on the network. It verifies that the destination device has an active service and is accepting requests on the destination port number that the initiating client intends to use. It informs the destination device that the source client intends to establish a communication session on that port number.

What are the six control bits flags?

URG, ACK, PSH, RST, SYN, FIN.

URG -

Urgent pointer field significant

ACK -

Acknowledgment flag used in connection establishment and session termination

PSH -

Push function

RST -

Reset the connection when an error or timeout occurs

SYN -

Synchronize sequence numbers used in connection establishment

FIN -

No more data from sender and used in session termination

During session setup, an initial sequence number (ISN) is set. This ISN represents

the starting value of the bytes that are transmitted to the receiving application.

True or False: As data is transmitted during the session, the sequence number is incremented by the number of bytes that have been transmitted.

True

True or False: The ISN does not begin at one but is effectively a random number. This is to prevent certain types of malicious attacks.

True

The receiving TCP process places the data from a segment into a ____________. Segments are then placed in the proper sequence order and passed to the application layer when reassembled. Any segments that arrive with sequence numbers that are out of order are held for later processing. Then, when the segments with the missing bytes arrive, these segments are processed in order.

receiving buffer

The sequence (SEQ) number and acknowledgement (ACK) number are used

together to confirm receipt of the bytes of data contained in the transmitted segments

The SEQ number identifies the first byte of data in the segment being transmitted. TCP uses the ACK number sent back to the source to indicate the next byte that the receiver expects to receive. This is called

expectational acknowledgement.

True or False: Host operating systems today typically employ an optional TCP feature called selective acknowledgment (SACK), negotiated during the three-way handshake. If both hosts support SACK, the receiver can explicitly acknowledge which segments (bytes) were received including any discontinuous segments.

True

Flow control is

the amount of data that the destination can receive and process reliably

Flow control helps maintain

the reliability of TCP transmission by adjusting the rate of data flow between source and destination for a given session

The window size determines the number of bytes that can be sent before expecting an acknowledgment. The acknowledgment number is the

number of the next expected byte.

The window size is the number of bytes that the destination device

of a TCP session can accept and process at one time

True or False: The initial window size is agreed upon when the TCP session is established during the three-way handshake

True

True or False: The source device must increase the number of bytes sent to the destination device based on the window size of the destination

False

The source is transmitting 1,460 bytes of data within each TCP segment. This is typically the

Maximum Segment Size (MSS) that the destination device can receive

A common MSS is 1,460 bytes when using IPv4. A host determines the value of its MSS field by subtracting the IP and TCP headers from the Ethernet maximum transmission unit (MTU). On an Ethernet interface, the default MTU is ______ bytes

1500

True or False: If the source determines that the TCP segments are either not being acknowledged or not acknowledged in a timely manner, then it can increase the number of bytes it sends before receiving an acknowledgment

False

True or False: If the data sequence is important to the application, the application must identify the proper sequence and determine how the data should be processed.

True

True or False: When UDP receives a datagram destined for one of these ports, it forwards the application data to the wrong application based on its port number.

False