Transport and Application Layers

Transport Layer Allows traffic to be directed to specific network applications
Application Layer Allows these applications to communicate in a wau they understand

Processes -> Multiplexer -> IP
Processes -> Demultiplexer -> IP
Port A 16-bit number that’s used to direct traffic to specific services running on a networked computer
10.1.1.100
10.1.1.100:80 - Socket number or socket port
File Transfer Protocol
Port 21
10.1.1.100:21

Destination port
The port of the service the traffic is intended for
Source port A high-numbered port chosen from a special section oof ports known as ephemeral ports
Sequence number A 32-bit number that’s used to keep track of where in a Sequence of TCP segments this one is expected to be
Data offset field A 64-bit number that communicates how long the TCP header for this segment is
TCP Window Specifies the range of sequence numbers that might be sent before an acknowledgement is required
TCP Checksum Operates just like the checksum fields at the IP and ethernet level
Urgent pointer field Used in conjunction with one of the TCP control flags to point out particular segments that might be more important than others
Options field It is sometimes used for more complicated flow control protocols

URG(urgent) A value of one here indicates that the segment is considered urgent and that the urgent pointer field has more data about this
ACK(acknowledged) A value of one in this field means that the acknowledgement number field should be examined
PSH(push) The transmitted device wants the receiving device to push currently-buffered data to the application on the receiving end as soon as possible
RST(reset) One of the sides in a TCP connection hasn’t been able to properly recover from a series of missing or malformed segments.
SYN(synchronize) It’s used when first establishing a TCP connection and makes sure the receiving end knoes to examine the sequence number field
FIN(finish) 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
Three way Handshake Computer A -> SYN, SYN/ACK, ACK -> Computer B
Handshake A way for two devices to ensure that they’re speaking the same protocol and will be able to understand each other
Four way Handshake Computer A -> FIN, ACK,FIN, ACK -> Computer B

Socket The instantiation of an end-point in a potential TCP connection
Listen A TCP socket is ready and listening for incoming connections
SYN_SENT A synchronization request has been sent, but the connection hasn’t been established yet
SYN_RECEIVED A socket previusously in a LISTEN state has received a synchronization and sent a SYN/ACN back
ESTABLISHED The TCP connection is in working order and both sides are free to send each other data
FIN_WAIT A FIN has been sent, but the corresponding ACK from the other end hasn’t been received yet
CLOSE_WAIT 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
CLOSED The connection has been fully terminated and that no further communication is and that no further communication is possible

Connectionless Protocols Established a connection, and uses this to ensure that all data has been properly transmitted
User datagram Protocol

A device that blocks traffic that meets certain criteria

Open Systems Interconnection
Session layer Facilitating the communication between actual applications and the transport layer
Takes appllication layer data and hands it off to the presentation layer
Presentation Layer Responsible for making sure that the unencapsulated application layer data is able to understood by the application in question