Introduction to Computer Networks

1. Introduction to Computer Networks09/21/2010 Outline - UNIX sockets - A simple client-server program - Project 1 - LAN bridges and learning CS 640

2. Berkeley Sockets • Networking protocols are implemented as part of the OS • The networking API exported by most OS’s is the socket interface • Originally provided by BSD 4.1c ~1982. • The principal abstraction is a socket • Point at which an application attaches to the network • Defines operations for creating connections, attaching to network, sending/receiving data, closing. CS 640

3. Connection-oriented example (TCP) Server Socket() Bind() Client Listen() Socket() Accept() Connection Establishmt. Connect() Block until connect Data (request) Send() Recv() Process request Data (reply) Send() Recv() CS 640

4. Connectionless example (UDP) Server Socket() Client Bind() Socket() Recvfrom() Bind() Block until Data from client Sendto() Data (request) Process request Data (reply) Sendto() Recvfrom() CS 640

5. Socket call • Means by which an application attached to the network • int socket(int family, int type, int protocol) • Family: address family (protocol family) • AF_UNIX, AF_INET, AF_NS, AF_IMPLINK • Type: semantics of communication • SOCK_STREAM, SOCK_DGRAM, SOCK_RAW • Not all combinations of family and type are valid • Protocol: Usually set to 0 but can be set to specific value. • Family and type usually imply the protocol • Return value is a handle for new socket CS 640

6. Bind call • Binds a newly created socket to the specified address • Int bind(int socket, struct sockaddr *address, int addr_len) • Socket: newly created socket handle • Address: data structure of address of local system • IP address and port number (demux keys) • Same operation for both connection-oriented and connectionless servers • Can use well known port or unique port CS 640

7. Listen call • Used by connection-oriented servers to indicate an application is willing to receive connections • Int(int socket, int backlog) • Socket: handle of newly creates socket • Backlog: number of connection requests that can be queued by the system while waiting for server to execute accept call. CS 640

8. Accept call • After executing listen, the accept call carries out a passive open (server prepared to accept connects). • Int accept(int socket, struct sockaddr *address, int addr_len) • It blocks until a remote client carries out a connection request. • When it does return, it returns with a new socket that corresponds with new connection and the address contains the clients address CS 640

9. Connect call • Client executes an active open of a connection • Int connect(int socket, struct sockaddr *address, int addr_len) • Call does not return until the three-way handshake (TCP) is complete • Address field contains remote system’s address • Client OS usually selects random, unused port CS 640

10. Send(to), Recv(from) • After connection has been made, application uses send/recv to data • Int send(int socket, char *message, int msg_len, int flags) • Send specified message using specified socket • Int recv(int scoket, char *buffer, int buf_len, int flags) • Receive message from specified socket into specified buffer CS 640

11. Socket Implimentation • Protocol implementation • Process per protocol • Use a separate process to implement each protocol • Messages are passes between processes • Process per message • Use one process to handle each message/communication • Generally more efficient • Buffer use • Applications use buffers as do protocols • Copies are VERY expensive • Message abstraction enables pointers to be used and minimal copies CS 640

12. Practical issues – using sockets • You have to be very careful when using these calls • Specific data structures and formats • Ports cannot be less than 1024 • You can use other tools to see if things are working • Tcpdump • /proc • netstat • Client and server can be on same system • Think about error handling methods CS 640

13. Switches and Learning Outline Why bridges (old name for switches)? Example of Layer 2 forwarding How do bridges build their forwarding tables? CS 640 CS 640 13

14. LAN Properties Exploit physical proximity. Often a limitation on the physical distance E.g. to detect collisions in a contention based network Relies on single administrative control and some level of trust. Broadcasting packets to everybody and hoping everybody (other than the receiver) will ignore the packet Broadcast: nodes can send messages that can be heard by all nodes on the network. Almost essential for network administration Can also be used for applications, e.g. video conferencing But broadcast fundamentally does not scale. CS 640

15. Building Larger LANs: Bridges Hubs are physical level devices Don’t isolate collision domains  broadcast issues At layer 2, bridges connect multiple IEEE 802 LANs BRIDGE is just an old name for a switch Separate a single LAN into multiple smaller collision domains Reduce collision domain size host host host host host host Bridge host host host host host host CS 640

16. Basic Bridge Functionality Bridges are full fledged packet switches Frame comes in on an interface Switch looks at destination LAN address Determines port on which host connected Only forward packets to the right port Must run CSMA/CD with hosts connected to same LAN Also between bridge and host connected to a LAN CS 640

17. Bridges provide “transparent” functionality Design features: “Plug and play” capability Self-configuring without hardware or software changes Bridge do not impact the operation of the individual LANs Three components of transparent bridges: Forwarding of frames Learning of addresses Spanning tree algorithm CS 640

18. Address Lookup/Forwarding Example Address is a 48 bit IEEE MAC address. Next hop: output port for packet Timer is used to flush old entries Size of the table is equal to the number of hosts Flat address  no aggregation No entry  packets are broadcasted Bridge 1 2 3 Address Next Hop Info A21032C9A591 1 8:36 99A323C90842 2 8:01 8711C98900AA 2 8:15 301B2369011C 2 8:16 695519001190 3 8:11 CS 640

19. Learning Bridge tables can be filled in manually (flush out old entries etc) Time consuming, error-prone Self-configuring preferred Bridges use “learning”  crucial to their transparent functioning Keep track of source address of packet (S) and the arriving interface (I). Fill in the forwarding table based on this information Packet with destination address S must be sent to interface I! host host host host host host Bridge host host host host host host CS 640