All language subtitles for 3. TCP Header and Applications

TCP or Transmission Control Protocol is a transport layer protocol
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residing at layer 4 in OSI model and is connection orientated
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it once again allows high layer protocols access to the network layer or IP layer
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but in this case providing reliability.
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It is connection orientated before transmission
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a session is establish between 2 devices.
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TCP generally implements a full duplex mode of operation.
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There are some exceptions but we won’t go into them here
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In other words a TCP connection is a pair of virtual circuits
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one in each direction which operate in full duplex mode.
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The transmitter can receive data at the same time its transmitting.
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Both host in a conversation can transmit and receive at the same time.
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there is error checking in TCP
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because there is a checksum in the datagram to verify that there’s no corruption.
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TCP segments are also numbered in sequence
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so that the destination can re-order segments and determine if data is missing.
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There is also acknowledgement of receipt of data, so all data is acknowledge by the receiver.
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the transmitter or sender can re-transmit the segment or terminate the connection
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if it determines that the receiver is no longer involve in the conversation.
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TCP implements data recovery features
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in other words there can be re-transmission of lost data.
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So if there’s no acknowledgement of a segment, the segment will be re-transmitted.
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TCP segment are sent using IP packets.
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The TCP header will follow the IP header supplying information specific to the TCP protocol.
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As you can see here the TCP header has many more options than the UDP header.
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So firstly you have a 16 bit source port number which identifies the sending port.
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We have a 16 bit destination port, which identifies the receiving port.
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There’s a 32 bit sequence number, if the SYN bit is set
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then this is the initial sequence number.
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The sequence number of the actual first data byte are then this sequence number plus 1.
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If the SYN bit is not set then the sequence number, is the accumulated sequence number
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of the first data byte of this packet for the current session.
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It then has a 32 bit acknowledgement number.
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If the ACK flag is set or bit is set
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then the value of the acknowledgement number
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is the next sequence number that the receiver is expecting to receive.
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This field acknowledges receipt of all prior bytes.
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The fist ACK or acknowledgement sent by each end
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acknowledges the other ends initial sequence number but no data.
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The header length or data offset specifies the size of the TCP header in 32 bit words.
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The minimum size of the header is 5 words and the maximum is 15 words.
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The minimum size of the header is 20 bytes
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and the maximum size of the header is 60 bytes in IPv4
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which allows for up to 40 bytes of options in the header.
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The reserve fill the set to 0 and is reserve for future use.
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Now there are lot of flags or control bits available in the TCP header
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and we won’t go through all of them.
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The Congestion Window Reduced Flag is part of a congestion notification mechanism
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used in conjunction with the ECE bit or flag
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or echo congestion notification echo field or flag.
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Once again used in congestion notification.
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This can be used in quality of service where the network
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and the host communicate to indicate congestion
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therefore letting the transmitter know that it needs to slowdown.
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The URG flag can indicate that this segment is urgent
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and should be process as soon as possible.
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The ACK flag as mentioned is used for acknowledgement of data.
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PSH is the flag set by TCP sender
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to cause the TCP receiver to immediately pass
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that segment's data to the receivers application socket
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along with all other in order data that the receiver is yet to give to that application.
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Reset; resets the connection in other words, the connection is turned down.
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SYN is used to synchronize sequence numbers.
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Only the first packet sent from each end will have this flag set.
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FIN means that there is no more data from the sender.
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The window size which is 16 bits in length
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specifies the size of the received window
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which is the number of bytes that the receiver is currently willing to receive
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we'll be talking more about flow control and window sizes in a moment.
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TCP also includes a 16 bits TCP checksum
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which is used for error checking of the header and the data.
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The 16 bit urgent pointer is used with the URG flag
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which when set on means that the 16 bit urgent pointer is used.
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This indicates an offset from the sequence number indicating the last urgent data byte.
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There are also various options available in the TCP
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but this are out of the scope of this course.
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And lastly we have the data, which is the data from higher layer protocols
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encapsulated within the TCP header.
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There are some examples of applications that rely on either TCP or UDP.
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Examples are file transfer protocols include; FTP or File Transfer Protocol
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TFTP or Trivial File Transfer Protocol, NFS or Network File System.
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In email we tend to use POP3 or Post Office Protocol to receive mail
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Simple Mail Transfer Protocol or SMTP to send mail
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or IMAP or Internet Message Access Protocol
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which is another protocol used for email retrieval
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For remote log in to devices, we could use telnet
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which sends traffic in clear text
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and is therefore in secure or secure SHELL or SSH
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which allows for a secure connection to remote devices.
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For network management we can use Simple Network Management Protocol or SNMP
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and for name management we can use Domain Name System
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which allows for the use of names rather than IP addresses
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and translates those meaningful domain names, into IP addresses
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so for instance cisco.com will be converted to an IP address
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when a user browses the internet.
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Before continuing I wanna mention again how the mappings work
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between the different layers of the OSI model.
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At layer 2 in an Ethernet 2 frame, there’s a field called the type number
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which allows a host to differentiate between multiple layer 3 protocols
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at layer 3, remember you could be using a protocol like IPv4 or IPv6
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or in the old days IPX or apple talk.
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So at layer 2 the NEC needs to know which layer 3 protocol to send this traffic to
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and the type number is used to differentiate the different layer 3 protocols.
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At layer 3 a protocol number is used to differentiate
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the different protocols running at layer 4, so in an IP header
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the protocol field will denote where the TCP or UDP is being used at layer 4
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At layer 4 a port number is used to differentiate multiple applications being use at layer 7.
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So it’s important to note that at layer 4 the way TCP or UDP
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know which application this traffic is destined to is by the port number.