Transport layer

What is computer A (sending) saying when it sends the SYN flag?

"Let's establish a connection and look at my sequence number field, so we know where this conversation starts."

Sequence Number

32-bit unique number in TCP segment that increases with each segment -- allows receiver to put arriving segments in order

Data Offset

5th Field of TCP Header, used to find TCP Header Length. A four-bit number that communicates how long the TCP header for this segment is.

A TCP segment is made up of what?

A TCP header and a data section

When the nodes are operating at full duplex, each segment sent in either direction should be responded to by..

A TCP segment with the ACK field set. This way, the other side always knows what has been received.

Padding

Additional data that is added to a hash to make it the correct number of bytes.

full duplex

Communication that happens in two directions at the same time.

How does computer B (receiving) respond to the SYN flag from computer A (sending)?

Computer B then responds with a TCP segment, where both the SYN and ACK flags are set. This is computer B's way of saying, "Sure, let's establish a connection and I acknowledge your sequence number."

An Ethernet frame has a payload section which is really just the

Entire contents of an IP datagram.

port 21

FTP traditionally listens to this port.

four way handshake

Four flags used when ending transmission. Node a send the Fin flag and Node b sends the ACK flag. IF node b is ready to close it will send the Fin flag. Node b will send the ACK flag.

Why is it better to use UDP with streaming video?

If some of the frames are missing the video will still be watchable. By getting rid of all the overhead of TCP, you might actually be able to send higher quality video with UDP.

Why do segments not always arrive in order?

If some of the segments had to be resend due to errors at lower layers

10.1.1.100 port 21

If you want to establish a connection to FTP server running on the same web server.

What type of access do you need for a system port?

In most operating systems, administrator-level access is needed to start a program that listens on a system port.

What are the practical applications for using a buffer?

In terms of TCP, it's used to send large chunks of data more efficiently. By keeping some amount of data in a buffer, TCP can deliver more meaningful chunks of data to the program waiting for it. But.in some cases, you might be sending a very small amount of information, that you need the listening program to respond to immediately.

Why is the source port important?

It is required to keep lots of outgoing connections separate. When the web server replies, the computer making the original request can send this data to the program that was actually requesting it. It is in this way that when it web server responds to your requests to view a webpage that this response gets received by your web browser and not your word processor.

16 bit checksum

It operates just like the checksum fields at the IP and Ethernet level. Once all of this segment has been ingested by a recipient, the checksum is calculated across the entire segment and is compared with the checksum in the header to make sure that there was no data lost or corrupted along the way.

In what situation would a RST flag be used.

It's a way for one of the partners in a TCP connection to basically say, "Wait, I can't put together what you mean, let's start over from scratch.

Servier

Or service is a program running on a computer waiting to be asked for data.

colon

Ports are usually denoted with this after the IP address.

UDP (User Datagram Protocol)

Protocol that operates instead of TCP in applications where delivery speed is important and quality can be sacrificed. You just set a destination port and send the packet.

What are some reasons traffic might not reach its destination beyond line errors?

Pure congestion might cause a router to drop your traffic in favor of forwarding more important traffic, or a construction company could cut a fiber cable connecting two ISPs.

TCP header

Source Port, Destination Port, Sequence Number, Acknowledgment Number, Header Number, Reserved, Code Bits, Window, Checksum, Urgent, Options, Data

different network services run while listening on what?

Specific ports for incoming requests

TCP Window

Specifies the range of sequence numbers that might be sent before an acknowledgement is required.

TCP segment

The connection-oriented payload of an IP packet. A TCP segment works on the Transport layer.

SYN flag

The fifth flag in the TCP header is the Synchronize flag. It's used when first establishing a TCP connection and make sure the receiving end knows to examine the sequence number field.

URG flag

The first flag in the TCP header is the Urgent flag. A value of one here indicates that the segment is considered urgent and that the urgent pointer field has more data about this.

RST flag

The fourth flag in the TCP header is the Reset flag. This means, that one of the sides in a TCP connection hasn't been able to properly recover from a series of missing or malformed segments.

Source Port

The port number of the application on the host sending the data - high numbered port chosen from a special section of ports known as ephemeral ports.

ACK flag

The second flag in the TCP header is the acknowledge flag . A value of one in this field means that the acknowledgment number field should be examined.

FIN flag

The sixth flag in the TCP header is the Finish flag. When this flag is set to one, it means the transmitting computer doesn't have any more data to send and the connection can be closed.

PSH flag

The third flag in the TCP header is the Push flag. This means, that the transmitting device wants the receiving device to push currently- buffered data to the application on the receiving end as soon as possible.

Ports 49152-65535

These are known as private or ephemeral ports.

ports

This is a 16-bit number that's used to direct traffic to specific services running on a networked computer. The transport layer handles multiplexing and demultiplexing through this.

In a small business a single server my run all applications needed for the business. Web, mail server, files, printer, network, etc. What makes this possible?

This is all possible because of multiplexing and demultiplexing, and the addition of ports to our addressing scheme.

Why is the data offset needed?

This is so that the receiving network device understands where the actual data payload begins.

FIN_WAIT

This means that a FIN has been sent, but the corresponding ACK from the other end hasn't been received yet. Server and client

SYN_RECEIVED

This means that a socket previously in a listener state, has received a synchronization request and sent a SYN_ACK back. But it hasn't received the final ACK from the client yet. This is seen only on the server side.

SYN_SENT

This means that a synchronization request has been sent, but the connection hasn't been established yet. You see this only on the client side.

ESTABLISHED.

This means that the TCP connection is in working order, and both sides are free to send each other data. you see this state on both the client and server

CLOSE_WAIT

This means that the connection has been closed at the TCP layer, but that the application that opened the socket hasn't released its hold on the socket yet. server and client

CLOSED.

This means that the connection has been fully terminated, and that no further communication is possible. server and client

How does TCP establish a connection?

Through the use of different TCP control flags, used in a very specific order.

Multiplexing/Demultiplexing

To functions that the transport layer is responsible for.

What is a connection less protocol?

UDP

With TCP there is a lot of traffic created. Why?

You have to establish the connection, you have to send a stream of constant streams of acknowledgements, you have to tear the connection down at the end.

Since both sides have now sent SYN/ACK pairs to each other, (node a and Node b)

a TCP connection in this state is operating in full duplex.

In the beginning of a transmitting and receiving process computer A (sending) will send computer B (receiving) what?

a TCP segment to with this SYN flag set.

Once one of the devices involved with the TCP connection is ready to close the connection, something known as what happens?

a four way handshake

Destination Port

a port value that uniquely identifies the destination application

you can send traffic to any port you want, but you're only going to get a response if ...

a program has opened a socket on that port.

Client

another program that is requesting this data.

Port 1024-49151

are known as registered ports.. Registered ports are sometimes officially registered and acknowledged by the IANA, but not always.

TCP Control Flags

are used to indicate a particular state of connection or to provide some additional useful information like troubleshooting purposes or to handle a control of a particular connection.

While TCP will generally send all segments in sequential order, they may not..

arrive in that order

Why will a streaming video be higher quality with UDP?

because you'll be saving more of the available bandwidth for actual data transfer instead of the overhead of establishing connections and acknowledging delivered data segments.

At the transport layer why is it important to send acknowledgements.

both ends of the connection always know which bits of data have definitely been delivered to the other side and which haven't.

Connection-oriented protocols like TCP protect against data not reaching its destination by doing what?

by forming connections and through the constant stream of acknowledgements.

Our protocols at lower levels of our network model like IP and Ethernet do use what to make sure data sent and received is the same?

checksums to ensure that all the data they received was correct.

Just like how an Ethernet frame encapsulates an IP datagram, an IP datagram does what?

encapsulates a TCP segment.

As a protocol TCP does what?

establishes connections used to send long chains of segments of data.

A connection at the transport layer implies that...

every segment of data sent is acknowledged.

Why is a sequencing number important?

is It used to keep track of which segment out of many this particular segment might be.

Buffer

is a computing technique, where a certain amount of data is held somewhere, before being sent somewhere else. It helps in matching the speed of the data stream between the sender and the receiver.

After the handshake computer A is able to do what?

is free to send whatever data it wants to computer B and vice versa.

connection based protocol

is one that establishes a connection, and uses this to ensure that all data has been properly transmitted. TCP

instantiation

is the actual implementation of something defined elsewhere.

socket

is the instantiation of an endpoint in a potential TCP connection.

Urgent Pointer

is used in conjunction with one of the TCP control flags to point out particular segments that might be more important than others. (rarely used)

Port 0

isn't in use for network traffic, but it's sometimes used in communications taking place between different programs on the same computer.

Demultiplexing

it's taking traffic that's all aimed at the same node and delivering it to the proper receiving service.

It's up to TCP to determine when to resend this data since TCP expects an ACK for every bit of data it sends, it's in the best position to....

know what data successfully got delivered and it can make the decision to resend a segment if needed. This is another reason why sequence numbers are so important.

LISTEN

means that a TCP socket is ready and listening for incoming connections. You see this on the server side only.

Muliplexing

means that nodes on the network have the ability to direct traffic toward many different receiving services.

Acknowledgment number

number that indicates which TCP segment is being acknowledged. example: A sequence number of one and an acknowledgement number of two could be read as this is segment one, expect segment two next.

The data section of a TCP segment has what in it?

payload area for where the application layer places its data.

TCP sockets require

programs to instantiate them.

Application layer

provides a wide variety of applications with the ability to access the services of the lower layers - allows these applications to communicate in a way they understand.

Port 1-1023

referred to as system ports, or sometimes as "well-known ports."

How does computer A (sender) respond to computer B(receiving) after computer B sends both the SYN and ACK flags?

responds again with just the ACK flag set, which is just saying, "I acknowledge your acknowledgement.

Transport layer

responsible for providing communication with the application by acknowledging and sequencing the packets to and from the application - allows traffic to be directed to specific network applications.

Why does it not matter if the segments arrive out of order?

sequence numbers allow for all of the data to be put back together in the right order.

Hypothetically, a TCP connection can stay open in what mode?

simplex mode with only one side closing the connection.

How many TCP flags are there?

six

Where is and example of were UDP is useful?

streaming video

Why are connection oriented protocols so important?

the Internet is a vast and busy place, and lots of things could go wrong while trying to get data from point A to point B.

In the physical layer if a cyclical redundancy check fails do to cross talk what happens.

the entire frame can be discarded.

10.1.1.100:80

the full IP and port

If a single bit does not get transmitted or is discarded what can happen to the message.

the resulting data is often incomprehensible by the receiving end.

Example of specific port that network services listen for?

the traditional port for HTTP or unencrypted web traffic is port 80.

the traditional port for HTTP or unencrypted web traffic is port 80.

the traffic would be directed to port 80 on that computer.

Options

this is rarely used in the real world, but it's sometimes used for more complicated flow control protocols.

If data is discarded due to a failed checksum, what protocol is responsible for re sending data?

transport layer protocol.

socket states and their names, can vary from operating system to operating system. You need to do what to make sure you have the proper terms?

you check out the exact socket state definitions for the systems you're working with.

socket address (socket number)

A synonym for a dynamically assigned port address, used to service a temporary TCP/IP connection for data exchange.

three-way handshake

A three-step process in which Transport layer protocols establish a connection between nodes. The three steps are: Node A issues a SYN packet to node B, node B responds with SYN-ACK, and node A responds with ACK.

Handshake

A handshake is a way for two devices to ensure that they're speaking the same protocol and will be able to understand each other.

FTP (File Transfer Protocol)

A protocol used to move files and folders over a network or the Internet. (older method)