- where the application runs and where the Java APIs are. The application layer only talks to the Transport Layer. Applicaiton Layer
- Responsible for ensuring that packets (datagrams) are received in the order sent and that no data is lost or corrupt. In the event of lost or corrupted data, the transport layer requests a copy of the packet be retransmitted. Two protocols dominate the transport layer. Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). The transport layer only talks to the application layer and the internet layer. Transport Layer
- 4-bit Version - indicates the IP version IPv4 is 4 and IPv6 is 6
- 4-bit Header Length - The length of the IP header including options, fields and and padding in 32 bit words.
- 1-byte Type of Service - Originally for things like priority but not used as intended. Now used for Diffentiated Service
- 2-byte Total Length - the total length of the datagram. Since this number is 16 bits wide, the max total length is 65,535 bytes.
- 2-byte Identification Number - used for multi-part or fragmented messages
- 3-bit Flags - Handles fragmentation...
- 1-bit DF - indicates that this message should not be fragmented.
- 1-bit MF - More Fragments indicates that there are more fragments. The last is set to 0.
- 13-bit fragnent offset - When fragmented this field indicates the order of the message
- 1-byte time-to-live (TTL) - Specifies how long a message should live based on router hops.
- 1-byte Protocol -
- 1 - ICMP
- 2 - IGMP
- 3 - GGP
- 4 - IP in IP Encapsulation
- 6 - TCP
- 8 - EGP
- 17 - UDP
- 50 - ESP - Encapsulating Security Payload
- 51 - Authentication Header
- 2-byte Header Checksum - simple checksum. Devides the header into 4 words then adds them together. Only for the header.
- 4-byte Source Address - the IP addres the message came from. Routers don't change this address.
- 4-byte destination address - the IP address the message is going to.
- options - mainly used to timestamp, routing, recording the route, etc.
- padding - pads out the header to a multiple of 32 bits
- Remander of the 65,535 bytes is payload (data)
- or Network Layer. This protocol determines how byte level data is organized into packets before being shiped to the data link layer. The most widley used protocol on the network layer is Internet Protocol (IP). It is also the only network layer Java understands. The network layer only talks to the transport layer and the host-to-network layer. Internet Layer
- or network layer, link layer, data link layer, network interface layer or the physical layer. This layer does the actual coding/decoding of the anolog signals, converting them to and from digital bytes, then passes the IP datagram off to the network layer. By far the most complex layer. The data link layer has to account for noise, error correction, redundancy, etc. This layer is vendor specific because the vendor makes the physical card that does all this. Host-to-Network Layer
An individual network must use the same kind of host-to-network layer, Ethernet being the most popular. However different network types can communicate using a Gateway server.
Java doesn't deal with the physical layer. Although the data link layer is transparent to the application layer, data can be optimized to run accross a particular layer type. Host-to-network layer only talks to the internet layer.
