Internet Protocol Packet Flow – HOP by HOP
The Transmission Control Protocol /Internet Protocol layer 2 (network Layer) defines how to deliver packets over the entire network or end to end delivery. A router is responsible for the end system to end system deliver of interconnected data. The path may traverse a single router or many routers. Router selects the next best path. The procedure or algorithm used by router to determine the best route and to share information or to deliver next hop referred to as routing protocol. In networking, we know that data is encapsulated to frames and that are formed into packets. Packet is to be delivered across the logical Path of a routed network as shown in scenario below. Notice that the logical path or route between the devices and several type of data links: an Ethernet, FDDI ring, a serial link and token ring. The figure which illustrates the scenario as frame changes from data link to data link but the packet remain same across the entire logical path :
Steps shows how frame changes from data link to data link, but the packet remains same across the entire logical path are as follows:
1.The host (George) encapsulates the data to be delivered within a packet. The packet must then be delivered across the host’s data link to the local router—that host’s default gateway—so the host encapsulates the packet within a frame.
This operation is the same as placing an envelope inside of a larger envelope, for example, inserting an envelope containing a letter into a Letter Bag. The destination data link identifier of the frame is the identifier of the interface of the local router and the source data link identifier is the host’s. Although the purpose of a router is to create pathways between data links (networks), the router must also obey the protocols of the networks to which it is attached. So a router interface connected to an Ethernet will have a MAC identifier and must obey the CSMA/CD rules, a Token Ring interface must obey Token Ring rules, and so forth. In other words, a router is not only a router, but also a station on each of its attached networks.
2.That router (router A in Figure) removes the packet from the Ethernet frame; router A knows that the next-hop router on the path is router B, out its Serial interface, so router A encapsulates the packet in an Serial frame. Now the destination identifier in the frame is the Serial interface of router B, and the source identifier is the Serial interface of router A.
- Router B removes the packet from the Serial frame, knows that the next-hop router on the path is router C across the FDDI, and sends the packet to C encapsulated in the proper frame for the FDDI.
- Router C removes the packet and recognizes that the station for which the packet is destined is on its directly connected Token Ring network; C encapsulates the packet in a Token Ring frame with the destination identifier of the destination station and the source identifier of its Token Ring interface. The packet has been delivered.
The key to understanding this entire process is to notice that the frames and their related data link identifiers, which have relevance only for each individual network, change for each network the packet traverses. The packet remains the same from end to end. The illustration of scenario as being follows: