Electronic Mail. DNS. P2P file sharing презентация

Review of Previous Lecture Electronic Mail DNS P2P file sharing

Overview P2P file sharing (cont.) Socket programming with TCP Socket programming with UDP

P2P file sharing Example Alice runs P2P client application on her notebook computer Intermittently connects to Internet; gets new IP address for each connection Asks for “Hey Jude” Application displays other peers that have copy of Hey Jude.

P2P: centralized directory original “Napster” design 1) when peer connects, it informs central server: IP address content 2) Alice queries for “Hey Jude” 3) Alice requests file from Bob

P2P: problems with centralized directory Single point of failure if the directory server crashes, then the entire p2p application crashes Performance bottleneck a centralized server must maintain a huge database Copyright infringement Easy to shut down the directory servers by legal actions

Query flooding: Gnutella fully distributed no central server public domain protocol many Gnutella clients implementing protocol

Gnutella: protocol

Gnutella: Peer joining Joining peer X must find some other peer in Gnutella network: use list of candidate peers X sequentially attempts to make TCP with peers on list until connection setup with Y X sends Ping message to Y; Y forwards Ping message. All peers receiving Ping message respond with Pong message X receives many Pong messages. It can then setup additional TCP connections

Exploiting heterogeneity: KaZaA Napster fully centralized Gnutella floods in limited area KaZaA: Each peer is either a group leader or assigned to a group leader. TCP connection between peer and its group leader. TCP connections between some pairs of group leaders. Group leader tracks the content in all its children.

KaZaA: Querying Each file has a hash and a descriptor Client sends keyword query to its group leader Group leader responds with matches: For each match: filename, hash, IP address If group leader forwards query to other group leaders, they respond with matches Client then selects files for downloading HTTP requests using hash as identifier sent to peers holding desired file

DoS resilience in p2p file-sharing systems P2p networks – highly replicated content not enough to protect against DoS attacks Music industry places false content on p2p networks (e.g., KaZaA) companies such as “Overpeer” and “Ratsnap” publicly publicly offer their pollution-based services My dilemma…

DoS resilience in p2p file-sharing systems (cont.) Modeling the propagation of polluted files in the system User-behavior factors Willingness to share files Persistence in downloading files Negligence in cleansing the infected hosts Designed and evaluated attacks against p2p networks % of nodes needed to collapse the system Hierarchical vs. structured p2p networks Counter-measures Reputations systems, randomization

Summary P2P file sharing (cont.) Socket programming with TCP Socket programming with UDP

Socket programming Socket API introduced in BSD4.1 UNIX, 1981 explicitly created, used, released by apps client/server paradigm two types of transport service via socket API: unreliable datagram reliable, byte stream-oriented

Socket-programming using TCP Socket: a door between application process and end-end-transport protocol (UDP or TCP) TCP service: reliable transfer of bytes from one process to another

Socket programming with TCP Client must contact server server process must first be running server must have created socket (door) that welcomes client’s contact Client contacts server by: creating client-local TCP socket specifying IP address, port number of server process When client creates socket: client TCP establishes connection to server TCP

Stream jargon A stream is a sequence of characters that flow into or out of a process. An input stream is attached to some input source for the process, eg, keyboard or socket. An output stream is attached to an output source, eg, monitor or socket.

Socket programming with TCP Example client-server app: 1) client reads line from standard input (inFromUser stream) , sends to server via socket (outToServer stream) 2) server reads line from socket 3) server converts line to uppercase, sends back to client 4) client reads, prints modified line from socket (inFromServer stream)

Client/server socket interaction: TCP

Example: Java client (TCP)

Example: Java client (TCP), cont.

Example: Java server (TCP)

Example: Java server (TCP), cont

Outline P2P file sharing (cont.) Socket programming with TCP Socket programming with UDP

Socket programming with UDP UDP: no “connection” between client and server no handshaking sender explicitly attaches IP address and port of destination to each packet server must extract IP address, port of sender from received packet UDP: transmitted data may be received out of order, or lost

Client/server socket interaction: UDP

Example: Java client (UDP)

Example: Java client (UDP)

Example: Java client (UDP), cont.

Example: Java server (UDP)

Example: Java server (UDP), cont

Summary P2P file sharing (cont.) Socket programming with TCP Socket programming with UDP

Application Layer: Summary Application architectures client-server P2P hybrid application service requirements: reliability, bandwidth, delay Internet transport service model connection-oriented, reliable: TCP unreliable, datagrams: UDP

Application Layer: Summary typical request/reply message exchange: client requests info or service server responds with data, status code message formats: headers: fields giving info about data data: info being communicated

Quiz (Application Layer) Q1. List four Internet apps and the application layer protocols

Quiz Q2. What is the difference between network architecture and application architecture?

Quiz Q3. In what way is instant messaging a hybrid of client-server and P2P architectures?

Quiz Q4. For a communication session between a pair of processes, which process is the client and which is the server?

Quiz Q5. Do you agree with the statement: “In P2p file sharing, there is no notion of client and server sides of a communication session”? Why or why not?

Quiz Q6. What information is used by a process running on one host to identify a process running on another host?

Quiz Q9. What is meant by a handshaking protocol?

Quiz Q10. Why HTTP, FTP, SMTP, POP3, and IMAP run on top of TCP rather than UDP?

Quiz Q12. What is the difference between persistent HTTP with pipelining and persistent HTTP without pipelining? Which of the two is used by HTTP/1.1?

Quiz Q15. Why is it said that FTP sends control information “out-of-band”?

Quiz Q19. Is it possible for an organization’s Web server and mail server to have exactly the same alias for a hostname? What would be the type for the RR that contains the hostname of the mail server?

Quiz Q22. A UDP-based server needs only one socket, whereas the TCP server needs two sockets. Why? If the TCP server were to support n simultaneous connections, each from a different client host, how many sockets would the TCP server need?

Quiz (Chapter 1) Q3. What is a client program? What is a server program? Does a server program request and receive services from a client program?

Quiz Q4. What are the two types of transport services that the Internet provides to its applications?

Quiz Q5. What is the difference between flow and congestion control?

Quiz Q7. What advantage does a circuit-switched network has over a packet-switched network?

Quiz Q8. Why is it said that packet switching employs statistical multiplexing?

Quiz Q12. List five Internet access technologies. Classify each one as residential, company access, or mobile access.

Quiz Q15. Is cable-modem transmission rate dedicated or shared among users? Are collisions possible in the downstream channel? Why or why not?

Quiz Q19. Consider sending packet from a sending host to a receiving host over a fixed route. List the delay components in the end-to-end delay. Which of these delays are constant and which are variable?

Quiz Q21. What are the five layers in the Internet protocol stack?

Quiz Q23. Which layers in the Internet protocol stack does a router process? Which layers does a link-layer switch process? Which layers does a host process?