Transport Layer and Application Layer

The Transport Layer

“Allows traffic to be directed to specific network applications”

• It handles multiplexing and demultiplexing through ports

  

• Port

  A 16-bit number that’s used to direct traffic to specific services running on a networked computer

  

  

Dissection of a TCP Segment

• IP datagram encapsulate TCP segment

TCP segment

“Made up of a TCP header and a data section.”

TCP Header

• Destination port

  The port of the service the traffic is intended for.

• Source port

  A high-numbered port chosen from a special section of 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.

• Acknowledgement number

  The number of the next expected segment.

• Data offset field

  A 4-bit number that communicates how long the TCP header for this segment is.

• Control Flag (See next part)

• 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. (No real world adoption of this TCP feature)

• Options field

  It is sometimes used for more complicated flow control protocols. (rarely used in real world)

• Padding

  Just a sequence of zeros to make sure the data payload section starts at the expected location.

TCP Control Flags and the Three-way Handshake

TCP Control Flags

Not in strict order;

• 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. (No particular real world use for this flag)

• ACK (acknowledged)

  A value of one in this field means that the acknowledgement number field should be examined.

• PSH (push)

  The transmitting device wants the receiving device to push currently-buffered data to the application on the receiving end asap.

• RST (reset)

  On 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 knows 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.

The Three-way Handshake

• Handshake

  “A way for two devices to ensure that they’re speaking the same protocol and will be able to understand each other.”

  

  

The Four-way Handshake

• Not very common
• TCP connection when finishes sending data, it sends FIN to request the port closure.
• Then receiving end responds with ACK flag and connection closes

• Even though the port, on one end, can simply remain open, and the connection ends without closing it

  

TCP Socket States

Socket

“The instantiation of an end-point in a potential TCP connection.”

Instantiation

“The actual implementation of something defined elsewhere.”

Socket States

• LISTEN

  A TCP socket is ready and listening for incoming connection.

• SYN-SENT

  A synchronization request has been sent, but the connection has not been established yet.

• SYN-RECEIVED

  A socket previously in a LISTEN state has received a synchronization request and sent a SYN/ACK 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 the application that opened the socket hasn’t yet released its hold on the socket yet.

• CLOSED

  The connection has been fully terminated and that no further communication is possible.

Connection-oriented and Connectionless Protocols

Connection-oriented Protocol

“Established a connection, and uses this to ensure that all data has been properly transmitted.”

Connectionless Protocol

• The most common one is UDP
• Used where data integrity is not super important, i.e., video streaming

System Ports vs. Ephemeral Ports

• Port 0 isn’t in use for network traffic, but sometimes used in communications taking place between different programs on the same computer
• Ports 1-1024 are referred as system ports or sometimes as well-known ports. These ports represent the official ports for the most well-known network services.
  • i.e., HTTP uses port-80, FTP uses port-21
  • Admin level access is needed to listen on these port in mos OSs
• Ports 1024-49151 are known as registered ports. These ports are used for lots of other network services that might not be quite as common as the ones that are on system ports.
  • i.e., Port-3306 is used for many Databases listen on
  • Some of these ports are registered with IANA but not always
• Ports 49152-65535 are known as Private or ephemeral ports. Ephemeral ports can’t be registered with the IANA and are generally used for establishing outbound connections.
  • When a client wants to communicate with a server, the client will be assigned an ephemeral port to be used for just that one connection, while the server listen on a static system or registered port
  • Not all OSs follow the ephemeral port recommendation of the IANA

Firewalls

“A device that blocks traffic that meets certain criteria.”

The Application Layer

“Allows network applications to communicate in a way they understand.”

• Too many protocols in use at application layer, a hassle to list them all.
  • i.e., HTTP, SMTP, etc.

The Application Layer and the OSI Model

Session Layer

“Facilitating the communication between actual applications and the transport layer.”

• Takes application layer data and hands it off to the presentation layer

Presentation Layer

“Responsible for making sure that the un-encapsulated application layer data is able to understand by the application in question.”