Internet Technology: A Sampler Ramesh Johari Massachusetts Institute of Technology

Part I: Routing

A Simplified Internet My Computer Router Website

A Telephone Network My Telephone Switch Your Telephone

The Difference? Telephone NetworksCircuit Switched = Guaranteed service from the network The InternetPacket Switched = No guarantees from the network!

Packets From: My Computer To: Website Please send me your home page Data Header All files transmitted on the Internet are first broken into packets.

Routing How does a router know where to send a packet?

Routing Tables Router A Packet’s Destination Next Router WebsiteRouter B Routing Table Distance to Destination 2 Hops

Routing Table Update Router A Router C Website Router C says: “I am 1 hop away from Website.” Router B

Routing Table Update Router A Router C Website Router A’s routing table is updated.

Routing Table Update Router A Packet’s Destination Next Router WebsiteRouter C Routing Table Distance to Destination 1 Hop

Routing KEY POINTS: Routing is:Decentralized Anonymous

Part II: TCP and Denial of Service

Transmission Control Protocol Internet Protocol (routing) Transmission Control Protocol (reliable end-to-end)

Reliable Transmission Sender Receiver No acknowledgement: Sender resends packet 4 Packets Acknowledgements

TCP Examples upload/download Telnet session FTP file transfer Web page download –one web page download may mean many TCP connections!

Setting Up a TCP Connection My Computer Website …as long as you respond to this message. SYN: Can I set up a connection with you? SYN-ACK: Yes,

Denial of Service The Website can’t handle the load! What if: isn’t my real IP address? (spoofing) No response! 2.I repeat 1 million times per minute? 1 million “half-open” connections!

Countermeasures: Filtering 1.Block Incoming SYN requests 2.Check source address on outgoing packets

For More Information Vern Paxson, “How to Own the Internet in Your Spare Time:” A discussion of how “worms” gain control of vulnerable computers.

Part III: Cryptography

Cryptography Cryptography : The art and/or science of encoding data so it can only be decoded by specific individuals.

Modern Cryptography Most modern cryptographic algorithms are widely known. The “secret” is the key which drives the algorithm.

Modern Cryptography Symmetric Key Cryptography Public Key Cryptography

Symmetric Keys: Example Sender’s Message: Encryption: Add a fixed key to each number in a sequence. The Key : 3 Encrypted Message: Decryption: Subtract key from each number in sequence.

Symmetric Keys “Symmetric Keys” means: Both sender and receiver have the same key. An Internet problem: How do customers and websites share keys with each other?

Public Keys: Example Receiver says to sender: “Take this open padlock and box.”

Public Keys: Example Receiver says to sender: “Take this open padlock and box.” “Put anything you want to send me inside.”

Public Keys: Example Receiver says to sender: “Take this open padlock and box. Put anything you want to send me inside. Then close the box, and lock it.”

Public Keys: Example Receiver says to sender: “Take this open padlock and box. Put anything you want to send me inside. Then close the box, and lock it.”

Public Keys: Example Locking the box is one-way: –It is easy to do, but hard to undo But the receiver has a trap door: –He uses the key to unlock the box

On the Internet Customers use a website’s public key to encrypt data. Even though everyone knows the algorithm used, decryption is hard without the receiver’s private key.

A Final Note: Secure Sockets Layer (SSL) 1.Use public key encryption to ensure both parties share a symmetric key 2.Use (faster) symmetric key encryption for communication

Part IV: Spiders

Recursive Retrieval My Computer Web Page

Where is the Spider? My Computer Web Page

Where is the Spider? My Computer Web Page

Where is the Spider? My Computer Web Page

The Reality My Computer Web Page THE SPIDER STAYS HERE!