Day 29: Layered Model Protocols

The IEEE 802.3 Committee develops the standards for Ethernet technologies, here are some important ones:

• DIX: Digital Intel and Xerox for 10Mbps over coaxial cable
• IEEE 802.3 10BASE-5:   10Mbs baseband over coaxial cable (thicknet), 500m distance
• IEEE 802.3a 10BASE-2: 10Mbs baseband over coaxial cable (thinnet), 200m distance
• IEEE 802.3i 10BASE-T:  10Mbs baseband over twisted-pair copper, 100m distance
• IEEE 802.3j 10BASE-F:  10Mbs baseband over fiber
• IEEE 802.3u 100BASE-T:     100Mbs baseband over twisted pair
• IEEE 802.3z 1000BASE-X:   1 Gbps baseband over fiber
• IEEE 802.3an 10G BASE-T: 10Gbps over twisted pair

The two most common Transport layer protocols are TCP (protocol number 6) and User Datagram Protocol (UDP, protocol number 17). UDP, in contrary to TCP does not acknowledge or retransmit segments. These transport layer protocols use ports to identify a service. The client  uses a destination port, source port (16 bits and unregistered in the 1025 to 65535 range), destination IP address and source IP address to create a socket that identificies the server and service. The combination of a port and Layer 3 IP address creates a socket.

• 21 - FTP, client makes request on server on port 21 (command), server responds with data on port 20 (data transfer)
• 22 - SSH
• 23 - Telnet
• 25 - SMTP
• 53 - DNS
• 67 - DHCP, messages from a client to a server are sent to the 'DHCP server' port (67), and DHCP messages from a server to a client are sent to the 'DHCP client' port (68)
• 69 - TFTP
• 80 - HTTP
• 110 - POP3
• 143 - IMAP4
• 161 - SNMP
• 443 - HTTPS, uses Secure Socket Layer (SSL)
• 520 - RIP

Many well-known applications have assignments to well-known ports, the range for these well-known or registered ports is 0 to 1023. Applications will use these as destination but a client will dynamically select a port as source from the range 1024 to 65535.
TCP is a connection-oriented protocol, while UDP is connectionless and more a best-effort attempt. TCP uses a three-way handshake, this is summarized in sending a SYN request, SYN-ACK reply and the ACK to conmplete the connection. TCP is reliable with acknowledgment, checksums, timers, retransmission, windowing, flow control, packet sequencing. UDP is unreliable and if needed other layers should implement reliability. Off course, this means that TCP has more overhead. Reliability is measured by the mean time between failures (MTBF) and mean time to repair (MTTR), redundant hardware/connections helps a lot.

Remember that DNS uses both UDP and TCP to send messages, the larger or more important exchanges of information (for example zone transfers), TCP will be used because of its reliability and ability to handle messages of any size.

Remember that IP is a connectionless protocol, uses hierarchical addressing, delivers data on a best-effort basis and has no built-in data recovery.

Session multiplexing is provided by the transport layer. It multiplexes several sessions onto one logical link and keeps track of which messages belong to which sessions (session layer). An example is a single computer with one IP address that has several websites open at once.

Windowing (flow control) allows the sender to transmit a specified number of unacknowledged segments. The window field is a number that implies the maximum number of unacknowledged bytes allowed outstanding at any time.

Ethernet 802.3 is based on the CSMA/CD process, specifies the physical layer and the MAC portion of the data link layer.

The NIC communicates with the network through a serial connection and communicates with the computer through a parallel connection.

Four functions of ICMP are flow control, detect unreachable destinations, redirect routes and check remote hosts.