CSE 120 Lecture Notes - Lecture 16: Wide Area Network, Local Area Network, Packet Switching
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13 Mar 2018
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Digital Signatures
Ex: Bob gave a loan to Alice, Alice signs it to agree to pay Bob back
-Alice encrypts the document, M, using a private key KA, priv and send KA, priv (M) to Bob
-When Bob receives, decrypts using KA, pub
-can ONLY decrypt iff someone uses her public key
-if it works, that must mean that it was encrypted using Alice’s private key
-the only one who knows Alice’s private key is Alice herself!
Note: crypto system worked right, but the assumption made was wrong (human error!)
To sign AND keep private
- Alice sends KB, pub (M, KA, priv (M)) to Bob
- ONLY Bob can decrypt: KB, priv (KB, pub (M, KA,priv (M)))
- Decrypts using KA,pub proving Alice signed it
Networks - set of computing nodes
- Connected by communication links
- Allows data transfer by a sender to a receiver
Internetwork: network of networks!
- “Internet” is a global internetwork!
- Nodes communicate using IP (Internet Protocol)
Types of Networks
By “topology”: ring, star, bus, graph
Ring: to send a message to a node, it goes through its connecting nodes to reach the receiver
- MANY hops
Star: send to the Hub and the Hub forwards to the right receiver
- Note: 2 hops to send message to receiver
Bus: transmit bits to the bus (and an address to the receiver)
- Everyone can hear the message!
- Can “broadcast” if you want EVERYONE to hear your message
Graph: NO regularity
By “geographic coverage”: “how much territory do they span?”
- LAN: Local Area Network (spanning floor, building)
- WAN: Wide Area Network (spanning state, country)
- MAN: Metropolitan “” “” (spanning city)
- DAN: Desk “” “” (spanning your office)
Circuit-switching vs. Packet switching
Circuit: establish a path from A to B
- Once determined, bits flow through the path and through the path ONLY
- Still pay regardless of whether you’re on the line or not!
Packet: forward packets to a node “closer” to B, which then forwards packets
- Maybe the packets NEVER reach B
- Less certain
- Much more “efficient’ - you only pay for what you use!
What is a Protocol?
Analogy: OS -> Process as Network -> Protocol
-agreed message format AND transfer procedure
-multiparty, so NO central thread of control!
-Expectations of operation
-if you do blah, I do blah, etc.
Messages:
-header and data: | header | Data |
Data: what sender wants receiver to know
Header: info to support protocol
- Src and dest addresses
- State of protocol operation
- Error control (check integrity)
Ex: Ann sends message to Bob
Protocol:
- Message format: (from,to), message contents
- Transfer procedure: post ON refrigerator (agreed way)
Ex: Ann sengs msg to Bob on the East coast
Header Data
| Address | Message |
Header Data
| From Ann, To Bob | blah |
Layering: Separation of Functions
Ann and Bob
- Don’t have to know ANYTHING about delivery
- Aid postal system by providing addresses
Postal System
- Only has to know addresses and how to deliver
- Doesn’t care about what’s inside the message!
Real Network has 7 Layers of OSI Reference Model
8 User!
7 Application: application protocol (i.e. HTTP)
6 Presentation: syntax, network format (i.e. JPEG)
5 Session: start/stop/manage connections
4 Transport: breaks into logical pieces (segments), reliability, flow control
3 Network: logical addressing (translated to physical address appropriate to the network),
routing