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REDES - Coggle Diagram

- - - - TCP Session Termination
        
        To close a connection, the Finish (FIN) control flag must be set in the segment header.
        
        To end each one-way TCP session, a two-way handshake, consisting of a FIN segment and an Acknowledgment (ACK) segment, is used.
        
        Step 1 - When the client has no more data to send in the stream, it sends a segment with the FIN flag set.
        
        Step 2 - The server sends an ACK to acknowledge the receipt of the FIN to terminate the session from client to server.
        
        Step 3 - The server sends a FIN to the client to terminate the server-to-client session.
        
        Step 4 - The client responds with an ACK to acknowledge the FIN from the server.
- - - - Transport layer protocols have services that segment the application data into blocks that are an appropriate size
        
        This service includes the encapsulation required on each piece of data. A header, used for reassembly, is added to each block of data.
        
        At the destination, the transport layer must be able to reconstruct the pieces of data into a complete data stream that is useful to the application layer.
  - - - o accomplish this, the transport layer assigns each application an identifier called a port number.
        
        Each software process that needs to access the network is assigned a port number unique to that host.
  - - - Each of these applications communicates with one or more applications on one or more remote hosts
        
        It is the responsibility of the transport layer to maintain and track these multiple conversations.
- - - - It is the values in these fields that enable various transport layer protocols to perform different functions in managing data communication.
- - - - Adding overhead to ensure reliability for some applications could reduce the usefulness of the application and can even be detrimental.
        
        UDP is known as a best-effort delivery protocol. In the context of networking, best-effort delivery is referred to as unreliable because there is no acknowledgment that the data is received at the destination.
        
        UDP is similar to placing a regular, non-registered, letter in the mail.
        
        The sender of the letter is not aware of the availability of the receiver to receive the letter.
    - - It has much lower overhead than TCP because it is not connection-oriented and does not offer the sophisticated retransmission, sequencing, and flow control mechanisms that provide reliability.
        
        It simply means that these functions are not provided by the transport layer protocol and must be implemented elsewhere if required.
        
        The low overhead of UDP makes it very desirable for protocols that make simple request and reply transactions.
- - - - A connection-oriented protocol is one that negotiates and establishes a permanent connection (or session) between source and destination devices prior to forwarding any traffic.
    - - n networking terms, reliability means ensuring that each segment that the source sends arrives at the destination.
    - - Because networks may provide multiple routes that can have different transmission rates, data can arrive in the wrong order.
    - - Network hosts have limited resources, such as memory and processing power. When TCP is aware that these resources are overtaxed, it can request that the sending application reduce the rate of data flow.
- - - - Source Port (16 bits) and Destination Port (16 bits) - Used to identify the application.
        
        Header length (4 bits) - Known as ʺdata offsetʺ. Indicates the length of the TCP segment header.
      - Sequence number (32 bits) - Used for data reassembly purposes.
        
        Urgent (16 bits) - Indicates if data is urgent.
      - Acknowledgment number (32 bits) - Indicates data has been received and the next byte expected from the source.
      - Control bits (6 bits) - Includes bit codes, or flags, which indicate the purpose and function of the TCP segment.
        
        Reserved (6 bits) - This field is reserved for the future.
- - - - TCP and UDP manage these multiple simultaneous conversations by using header fields that can uniquely identify these applications.
      - TCP Three-way Handshake Analysis
        
        Establishes that the destination device is present on the network
        
        Verifies that the destination device has an active service and is accepting requests on the destination port number that the initiating client intends to use
        
        Informs the destination device that the source client intends to establish a communication session on that port number
- - - - The source port number is dynamically generated by the sending device to identify a conversation between two devices.
        
        It is common for a device to send multiple HTTP service requests to a web server at the same time.
    - - The client places a destination port number in the segment to tell the destination server what service is being requested,
        
        server can offer more than one service simultaneously such as web services on port 80 at the same time that it offers File Transfer Protocol (FTP)
- - - - The source port number acts as a return address for the requesting application.
        
        The transport layer keeps track of this port and the application that initiated the request so that when a response is returned
- - - - Any incoming client request addressed to the correct socket is accepted, and the data is passed to the server application.
        
        TCP Connection Establishment
        
        Step 1 - The initiating client requests a client-to-server communication session with the server.
        
        Step 2 - The server acknowledges the client-to-server communication session and requests a server-to-client communication session.
        
        Step 3 - The initiating client acknowledges the server-to-client communication session.
        
        TCP Flow Control - Congestion Avoidance
        
        When congestion occurs on a network, it results in packets being discarded by the overloaded router. When packets containing TCP segments don’t reach their destination,
        
        By determining the rate at which TCP segments are sent but not acknowledged, the source can assume a certain level of network congestion.
        
        Not only are new packets with TCP segments introduced into the network, but the feedback effect of the retransmitted TCP segments that were lost will also add to the congestion.
        
        If the source determines that the TCP segments are either not being acknowledged or not acknowledged in a timely manner, then it can reduce the number of bytes it sends before receiving an acknowledgment.
- - - - During session setup, an initial sequence number (ISN) is set. This ISN represents the starting value of the bytes for this session that is transmitted to the receiving application.
  - - - To accomplish this, the TCP header includes a 16-bit field called the window size.
        
        The initial window size is agreed upon when the TCP session is established during the three-way handshake.
        
        The source device must limit the number of bytes sent to the destination device based on the destination’s window size.
        
        If the availability of the destination’s buffer space decreases, it may reduce its window size to inform the source to reduce the number of bytes it should send without receiving an acknowledgment
- - - - When UDP receives a datagram destined for one of these ports, it forwards the application data to the appropriate application based on its port number.
  - - - The UDP client process dynamically selects a port number from the range of port numbers and uses this as the source port for the conversation.
        
        The destination port is usually the well-known or registered port number assigned to the server process.