10/23

Two types of Network

• Connectionless (IP, UDP)
• Connection oriented (TCP)

1. No setup is required before transmitting data
2. Each packet is handled independently
3. No clean up is necessary after sending data
4. Think of 'postcard'

1. It leaves the connection open while sending data
2. It terninates connection when there is no more data to transmit
3. Think of 'telephone calls'

1. CO has a setup overhead
2. CO is reliable
3. CO can reserve bandwidth in some networks (phone line, ATM)
4. Example for CO: TCP, Telephone system, ATM
5. CL has less overhead
6. In CL, application has more control in message retransmission error handling, etc...
7. CL allows broadcasts and multicasts
8. Example for CL: UDP, IP

1. TCP: used in applications that need high (complete) reliability: e-mail, file transfer, ...
2. UDP: (1). used in applications that do not need a high degree of reliability but require high response: voice over internet; (2). used in applications that want to have more control in the delivery of messages. This application has to implement reliability on top of UDP (Danger of using UDP: application may try to "invent" TCP); (3). used in application that need broadcast or multicast.

Network Performance Measurement

• Delay: time taken for a packet to arrive at its destination. it likes the "length of a pipe". Caused by:
• Propagation delay (intrinsic to network media, fixed)
• Switching delay (hardware related, fixed)
• Queuing delay (time a packet stay in a buffer waiting to be delivered, variable)
• Throughput: capacity in bits/sec. width of a pipe
• Relationship between delay and throughput
• when network is idle, queue is empty and queuing delay is 0;
• when load increases, queuing delay increases;
• load is defined as the ratio between throughput to capacity


    D = D₀/(1-U)
    where D is total delay; D₀ is delay when network is idle; U is the utilization (load) 0<=U<=1
 

• possible consequence: any network that operates with utilization close to 100% of capacity is doomed.

Delay Throughput Product

• Delay: time to cross the network in second
• Throughput: capacity in bits per second
• Delay X Throughput: D (sec) X T (bits/sec) =  D X T bits

Delay = 0.5 secs    Throughput = 128 Kbps
# bits in transit = 0.5 X 128 = 64 Kbits

Protocols

• Agreement about communication
• Necessary for interoperation among different hardware and softwares
• Specifies
• format of messages
• rules of exchange
• handling problems
• Family of Protocols
• Designed to work together
• each protocol in the family solves a different communication problem
• it is also called "protocol suite" or "protocol family"
• Designed by layers
• ISO: 7 layer reference model, intended for protocol designers

Application Presentation Session transport network datalink physical

---individual application ---data presentation ---login and password ---reliablility ---packet forwarding ---hardware and frame definition ---underlying hardware

• outdated
• it was designed when networking was mostly used to commect terninals to mainframes
• it does not include "internet" layer

• Heterogeneity is inevitable, no single network technology is best for all need
• Internet goal: connects physical networks and creats a software that makes the resulting system appear homogeneous

A device used in the internet to connect heterogeneous networks. It is also called
- internet router
- internet gateway

- the router eill have one interface per network
A router can connect networks that have different

• technologies
• media
• addressing schemes
• frame format

• networks
• routers that interconnect networks
• hosts connected to networks

• to hide heterogeneity to the users

• the internet is a virtual network that runs on top of other metworks and that has its own addressing and name schemes.

Debuging a Server

• Disable fork(), so that the master server is going to crash if there is a problem
ss = accept(...);
#if defined (DEBUG)
    // iterative
    processHTTPRequest(ss);
    close(ss);
#else
    // concurrent
    int pid = fork();
    if (pid == 0) {
        processHTTPRequest(ss);
        exit(1);
    }
    close(ss);
#endif
• Debugging  a "core" file
• when process creah, it writes a "core" file
• gdb myserver core
• gdb > where        /* print stacktrace */
        strcpy
        processHTTPRequest line 160 server.cc
        main                       line 1001 server.cc
• gdb> up
• gdb> down
• gdb> print <var>
• Runing gdb on a program
• gdb myserver
• gdb> run
• to stop a program, using ctrl-c
• gdb> where
• gdb> break file:line        /* stop program when it reach this line */
• gdb> cont                     /* continue */
• gdb> help
• gdb> next                    /* execute a function if any */
• gdb> step                    /* step into a function */

If a request has the form: GET /cgi-bin/<prog>?a=b&c=d
then:
        processHTTPRequest(ss)    /* in a child process */
        {
            .
            .
            .
            if /cgi-bin as prefix in a request
                get <prog>
                get <query>    /* query is a=b&c=d in this case */
                set env vars
                // buid the string using malloc and sprintf()
                s1 = QUERY_STRING = <query>
                s2 = REQUEST_METHOD = GET
                putenv(s1);
                putenv(s2);    /* set up env vars */
            .
            .
            .
        }

Write header just before content-type
Redirect stdout to slave socket ss
Execvp for http-root-dir/cgi-bin/<prog>