Client-Server model

Socket programming

   http://www.cse.buffalo.edu/~milun/unix.programming.html

Java: look at java.net package at http://java.sun.com/javase/7/docs/api / Interprocess communication: Interface to layer 4 protocol

3 socket types:    SOCK_DGRAM: datagram mode of communication, typical for short messages such as finger, like a message queue in UNIX domain (AF_UNIX), implemented via UDP in the Internet domain (AF_INET).

SOCK_STREAM: connection-oriented 2-way byte stream, like full-duplex pipe in UNIX domain, implement via TCP in Internet domain (what demos use) SOCK_RAW: write your own, work w/ hardware interface

Overview

Server

socket (create socket) bind (specifies IP address & port #) listen (put socket in passive mode, ready to accept requests) accept (accepts a connect request) send and receive data via socket (must be in synch w/ client) Close

client

Socket (create socket) Port# identifies an application (process) Connect (issue a connection request to a server) Receive and send data via sockets (must be in synch w/ server) Close

Commands and structs (C language)

struct sockaddr { u_short sa_family; // address type char sa_data[14] }; // address value struct sockaddr_in { u_short sin_family, //protocol and family u_short sin_port, //port number struct in-addr sin_addr, char sin_zero[8] //not used, all 0s } struct hostent { char* h_name, //text address of host computer char** h_aliases, //not important for us int h_addrtype, //address type (AF_INET) int h_length, //address length char** h_addr_list } //not important for us #define h_addr h_addr_list[0] Specifies generalized address family, may use in UNIX network. This is the more generic version of sockets Used with Internet domain sockets where struct in_addr { u_int s_addr } //IP address We’ll use this form Null terminated Alternate names for host List of host addresses Just corresponds to 1 st in h_addr_list

 gethostname(char* name, int namelength)    Put host name into name Return 0 if OK, -1 if error (errno specifies the error) namelength is length of 1 st parameter

 gethostbyname(char* name)    

name has hostname

Returns pointer to a hostent structure (make sure you have one) Maps host name to IP address If error occurs, h_errno contains the code.

   Describe htons, ntohs, htonl, ntohl (h for host, s for short, l for long) Establishes a standard form for which bit is least/most significant in a type int big endian vs. little endian (from Gulliver’s travels, politicians who started wars because they could not agree on whether to crack an egg on the big or little end)

 socket(int domain, int type, int protocol)      Create unbound socket Return socket descriptor (index into kernel descriptor table) Domain: AF_INET Type: SOCK_STREAM Protocol: TCP or UDP or 0. If 0, system selects appropriate protocol, default TCP.

 bind(int s, struct sockaddr_in * sa, int size) 

s is socket descriptor

   

sa points to sockaddr_in struct size is size of sockaddr_in struct

Return 0 if OK, -1 if error (errno has the code) Bind (associate) a socket with an address and port # specified in sa)

 connect(int s, struct sockaddr_in *sa, int size)   

s … socket identifier sa … point to sa structure containing the port#, protocol family and IP address of remote host size is length of sa structure

 listen(int s, int n)   

s … socket descriptor n … max size of unprocessed conection requests (queue length)

Puts server in listen (passive) mode making it able to accept connections.

 accept (int s, struct sockaddr_in *sa, sizeof (sa))  Accept a connection and return a new socket descriptor w/ the same properties as s.

 send(int s, address buffer, size, flags)  Send contenst of specified memory (use 0 for flags)

  recv(int s, address buffer, size, flags)  Read into specified memory, up to max of size close(s)

Peer-to-peer (P2P) networking

   Contrast with client/server (CS) using file distribution (mp3s, videos, etc.) as an example: C/S: Server transfers each file to a client upon request. P2P: any pair of devices in the P2P network is capable of exchanging files.

Devices have equivalent abilities (True P2P). Many are hybrids. [ http://en.wikipedia.org/wiki/Peer-to-peer]

[

http://www.oreilly.com/catalog/peertopeer/chapter/ch 01.html]

BitTorrent: P2P file distribution protocol.

    [http://computer.howstuffworks.com/bittorrent.htm] Torrent (sometimes swarm): group of peers involved in the distribution of a file. They exchange (download and upload) fixed sized chunks of the file.

Tracker: each peer in a torrent registers with the tracker and periodically informs the tracker of its status.

When a new client joins a torrent the tracker selects a subset of peers and sends the IP addresses to the new client (now peer).

   Neighboring Peers: peers with which a TCP connection is established.

Peer gets a list of chunks that other peers have. Request rarest chunks first. More quickly spreads rare chunks among peers.

A peer gives priorities (for sending chunks) to others who are providing data at the best rate.

  Recalculations are made periodically.

Free-riding or leeching (downloading without uploading)

 Index for tracking: An index lists what clients make what files available. Must be frequently updated. Different approaches for indexes.

 Centralized index: Similar to the original Napster. Index service provided by a large server or server farm. Hybrid of P2P and client/server. Drawbacks include copyright issues (central servers can be shut down – Napster) and performance (Napster had real problems back in 2000) and single point of failure.

    Query flooding (Gnutella [ http://en.wikipedia.org/wiki/Gnutella] LimeWire [http://en.wikipedia.org/wiki/Limewire] for example): [http://en.wikipedia.org/wiki/Query_flooding] Each peer maintains a few TCP connection with others: creates an overlay network (graph using discrete math

term)

 To locate a file: send a request to all connected peers (one hop away). If not found, do again and ask connected peers to do the same (request goes two hops). If not found, do again but next request goes three hops. And so on.

 To join an overlay network:  Peer x finds one peer in the network (either from a known list it maintains or a tracker site).

 Find a peer, y, to which it can establish a TCP connection (use the ping-pong protocol – "ping" message and "pong" response). Y can forward "pings" (up to a hop count) and any peer, z, that gets it can "pong" back to x.

 Hierarchical overlay (Kazaa, Morpheus,Gnutella) – sort of a hybrid of the previous two. No dedicated server (farm) but not complete query flooding.

   Super peer: a peer with high bandwidth and high availability. Like a peer, but more equal.

A peer connects to a super peer which acts as a tracker for other peers connected to it.

Super peers form an overlay network among themselves.