Local Area Networks and the Network Protocol Stack

Transcription

1 Local Area Networks and the CSC362, Information Security Introduction problems inherent in transmitting packets network topologies network protocol stack Packet Switching Problems packet switching can occasionally a. deliver packets out of sequence b. deliver packets that contain bad data c. lose packets altogether because the sender and receiver are not constantly connected, they cannot report these problems directly instead some sort of protocol must be established to handle these (and other) situations Protocols a protocol is an established procedure for handling some transaction in computerese, it means predefined rules for composing and sending messages that indicate the current status of the transaction 1

2 Example: ACK Protocol simple ACK protocol ACK = acknowledgement Example: ACK Protocol simple ACK protocol ACK = ACKnowledgement NAK = Negative AcKnowledgement But what about packets that have errors? NAKs signal that there was a problem in the last transmission Example: ACK Protocol an ACK/NAK protocol can be expanded with timeouts to treat problem (c), i.e., lost packets a timer represents the expected turnaround for a packet delivery Network Protocols network protocols are important to maintain consistent service between hosts they provide structure for communications and predictability for applications what is the best way to establish a messaging protocol? it will depend on the kind of service we are interested in what happens when the ACK packet is delayed rather than the original payload? 2

3 Network Topologies Local Area Networks must have some sort of physical arrangement specifically, how the nodes in the network are arranged with links this is the issue of network topology Network Topologies a. point- to- point (fully connected) b. star c. ring d. bus e. mesh f. tree g. line Network Topologies a. point- to- point b. star c. ring d. bus e. mesh f. tree g. line Star Network easy to configure insertion/deletion simple 3

4 Ring Network works like a continuous store- and- forward circuit usually associated with a specific Media Access Control (MAC) token ring Bus Topology supports a broadcast medium Mesh Network topology envisioned by Baran full mesh network is fully connected Tree Topology hierarchical used to integrate or bridge different types of equipment 4

5 Hybrid Topologies Hierarchical Abstraction full mesh between corporate centers a hub and spoke topology for centralized connections partial mesh networks for LANs on site we introduced earlier the strategy of hierarchical abstraction in the development of software for example, an OS employs this to organize and implement input- output functions layered hourglass design Hierarchical Abstraction: Advantages separating layers simplifies tasks using a divide and conquer strategy layers can be loosely- coupled so that they can redesigned and replaced without redesigning the entire system this plug and play capability makes the system more flexible and extensible networks exist to support applications the hourglass design is also effective here the network protocol stack serves as the interface between applications and the hardware that implements networks 5

6 the Internet is built on a 5- layer networking protocol stack application layer transport layer network layer data link layer physical layer application layer: application protocols prescribe messaging to support the application transport layer: general messaging services (UDP and TCP) network layer: addressing and routing (IP) data link layer: transporting messages from one link to the next (e.g., Ethernet, wireless Wi- Fi, etc.) physical layer: signals (e.g., baseband electrical signals, wireless broadcasting, etc.) client- server application: WWW application layer: HTTP (HyperText Transfer Protocol) transport layer: TCP (Transfer Control Protocol) network layer: IP (Internet Protocol) data link and physical layers: dependent on LANs 1. Client creates a GET message requesting a specific page from the server. 2. message is packaged into a segment for transmission 3. the segment is enclosed in a datagram for delivery outside the network 6

7 4. the packet is framed for transmission across LAN links 5. a switch sends it to a router for transmission outside the LAN 6. the datagram is passed across a series of Internet routers until it arrives at a router for the destination LAN 7. eventually it arrives at the server machine 8. the payload retraces the protocol stack in reverse; each layer unwraps its header and passes it upward 9. the server application receives the request and readies a response 10. the process begins again. 7