All language subtitles for 6. Transport Layer

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OSI Layer-4 provides host-to-host communication services such as connection
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connection-oriented data stream support, reliability, flow control, and multiplexing
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for applications. TCP and UDP most common protocols used in this layer
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Let's take a look to the TCP versus UDP. TCP is suited for applications that require high reliability
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and transmission time is relatively less critical.
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UDP is suitable for applications that need fast, efficient transmission, such as games.
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UDP's stateless nature is also useful for servers that answer small queries from huge numbers
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of clients. Another difference is TCP rearranges data packets in the order specified.UDP has no inherent
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order as all packets are independent of each other.
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If ordering is required, it has to be managed by the application layer.The speed for TCP is slower
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than UDP.UDP is faster because error recovery is not attempted
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It is a "best effort" protocol.
TCP does Flow Control. TCP requires three packets to set up a socket connection,
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before any user data can be can be sent.
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TCP handles reliability and congestion control.
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UDP does not have an option for flow control
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And lastly TCP is connection oriented –
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once a connection is established, data can be sent bidirectional. UDP is a simpler, connectionless
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protocol.
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So in computer networking, port numbers are part of the addressing
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information used to identify the senders and receivers of messages.
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Port numbers allow different applications on the same computer to share network resources simultaneously.
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Let's say that we have a client in here and let's say that this client want to use different applications
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such as electronic mail html page like surfing on the web and by using the internet chat.
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Maybe. if this guy wants to use this different application it should use different port numbers to reach
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to these different applications.
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So we have a comment that we can use on our laptops our PCs our Windows machines or something like that
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on the command line.
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If we type "netstat" we can see some statistics.netstat (network statistics) is a command-line network utility tool that
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displays network connections for the Transmission Control Protocol (both incoming and outgoing),
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routing tables, and a number of network interface.
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As you can see here, here are the source IP addresses these are the source ports and these are the destination
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ports that the user wants to reach.
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For example in here this guy wants to reach the 80 port and the communication is established.
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So The port numbers in the range from 0 to 1023 are the well-known ports or system ports.
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They are used by system processes that provide widely used types of network services.
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For example a well-known port is FTP, and FTP uses TCP protocol and port number of FTP
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is 20.
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For example :Telnet.
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This is a remote access protocol and telnet uses TCP protocol and protocol number is twenty three
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and DNS
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That can use UDP or TCP and the port number is fifty three. In data communications networks, packet
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segmentation
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is the process of dividing a data packet into smaller units for transmission over the network. Packet
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segmentation happens at layer four of the OSI model or the transport layer.
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Segmentation may be required
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When The data packet is larger than the maximum transmission unit supported by the network and when
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the network is unreliable.
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and it is desirable to divide the information into smaller segments to maximize the probability
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that each one of them can be delivered correctly to the destination
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So let's talk about the TCP And UDP details.
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The client–server model is a distributed application structure that partitions tasks or workloads between
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the providers of a resource or service, called servers, and service requesters, called clients.
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For example if you try to open the networkel.comweb page your computer's web browser is the client
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client program that requested services from the networkel.com server
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which technically is called an HTTP server
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So you could read the web page of networkel.
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So in here you have.
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we have some protocols that are using TCP protocol and they are FTP,file transfer protocol and telnet
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and HTTP.
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This is the protocol we use when we are surfing on the web.
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As you know so, these clients are using some port numbers in this range.
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As you can see and they're interacting with the server to get these services from them for example this
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PC one wants to take an FTP service from the server.
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So it's using the destination port of 20 of the server.
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For example this PC, PC two wants to take an HTTP service from the server.
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So it's using the destination port 80 while it's connecting the server.
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A three-way handshake is a method used in a TCP/IP network to
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create a connection between a local host and server.
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It is a three-step method that requires both the client and server to exchange SYN and ACK (acknowledgment)
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packets before actual data communication begins.
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To establish a TCP session
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we have SYN , SYN-ACK and ACK steps.
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In the SYN step Client sends session request to server. In response, the server replies with a SYN-ACK.
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Then Finally, the client sends an ACK back to the server
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For example let's say that the client wants to
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establish a TCP session with server. what client is doing first, it's sending a SYN packet
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with a sequence number of 1000.
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The server gets the syn packet and replies back with the syn and acknowledgement message and saying
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that my sequence number of this message is 2000 and I'm acknowledging that you'll send me the sequence
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number 1000 packet.
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All right.
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And increasing the number of the sequence by 1 as you can see.
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Then this third step is the client is responding back with an ACK again and saying that this ACK has
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a sequence number of one thousand one because my syn's sq. number was 1000.
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And this is my next packet that I'm sending to you.
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And this has a sequence number of 1001 and I am ACKing the packet that you sent me
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with the sequence number of 2000 and the acknowledgement number is 2001.
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All right , when we try to terminate a TCPsession
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The steps we are using is the first fin then ack
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plus fin after this.
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The first is fin then ack then fin the receivers side.
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Then this guy is sending and ack again.
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The steps are fin - ack -fin -ack
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This is by B.
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These are by a. let's talk about the TCP re-order process right now . if you want to send a packet
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from site a to site B
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you have two different networks and they are IP Network 1 and IP Network 2. When packets are being
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sent, Segments may split to different paths ,
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some of them may prefer IP Network 1 and some of them may go over IP Network 2. TCP reorders these
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packets at the destination.
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And this process is known as TCP reorder process.
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This process is not available for UDP guys.
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Please pay attention that. The TCP windowing is an option to increase the receive window size
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allowed in Transmission Control Protocol above its maximum value.
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The TCP window size, is simply an advertisement of how much data (in bytes) the receiving device
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is willing to receive at any point in time. The receiving device can use this value to control
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the flow of data, or as a flow control mechanism.
as you can see in here.
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There is a TCP session between PCand server , PCis sending a packet to this guy.
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And this guy ACKs the first guy.
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All right.
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Then PC is sending the packet two and packet three and Server is responding back with acknowledgment
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message again.
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But in here this guy is sending four five and six to server but this time what is going on in here is the
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acknowledgement number is seven, which means I couldn't get the sequence number seven because in here
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please pay attention.
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I am sending the sequence number one and the acknowledgement is two in here
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I'm getting the sequence number three and ack is four but in here I am sending an
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acknowledgement of seven which means I got the sequence number six.
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Lastly . then the PC is sending the 7 8 9 again, again the seven and plus eight and nine.
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Then the server is acknowledging with the 10 and that means everything is OK.
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So let's take a look to the UDP protocol some now. UDP provides
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As I told you you unreliable data transfer.
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And as I told you again we don't have a reorder process .that's a connectionless protocol and loss of data may occur
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and used for application that requires fast delivery and no delay. and UDP has a client server
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model as well as TCP too. Protocols such as DNS, TFTP or SNMP are using the UDP protocol.
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So for example if PC one wants to get the DNS server from this guy that sends the UDP packets and saying
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that hey guy destination port is 53 that means I need that DNS service from you.
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For example let's take a look to the this client which is PC 2 , this is as an SNMP client and SNMP
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uses UDP also.
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So this is sending UDP messages and with the destination port of one hundred and sixty one to get the
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SNMP service from this server.