October 16, 18, 20

Server

after fork

parent closes sd2(slave)

child closes sd(master)

void sig_chld(int signo)

{

    pid_t pid;

    int stat;

    while((pid=waitpid(-1,&stat,WNOHANG)))

    {

        printf("child exited\n");

        signal(SIGCHLD, &sigchld);

    }

}

main()

{

....

struct sigactionact;

act.sa_handler=sigchld;

sigempty(&act.sa_mask);

}

The while loop for the server should look something like that:

#include <errno.h>

...

while(1)

{

   ss=accpet(s1,....);

   if (errono==EINTR)

        continue;

   else

        perror();

}

Stage 3

Your server has to build a list of files in the directory with hyperlinks.

int fstat (int fileds, struct stat *buf) obtains info about an open file descriptor.

struct stat {

   mode_t st_mode;

   off_t     st_size;     //size

   time_t   st_mtime; //time last modified

}

Use S_ISDIR to check if a file descriptor is a directory or a file.

Stage 4

http://...cgi-bin/hello?a=b%c=d

1) putenv("name=value");

2)"QUERY_STRING=a=b%c=d"

3)"REQUEST_METHOD=GET"

4) The header must be changed so that the server stops sending information before the content type is sent. The CGI script will tell the client what the content type is.

5)Redirect the output to socket using dup2(ss,1);

6)execvp("http_root_dor/cgi-bin/exe");

Local Loop

- Standard  connection between home/business and central office

• Analog services -POTS

• Digital services

  •  ISDN - Voice data

  • DSL - Digital Subscribers line

Frequency Spectrum in Phone Lines

- Frequency response of a phone line for high frequency is irregular

- A DSL modem looks for places in the frequency spectrum where the response is food and use those channels

- ADSL:

• Asymmetric Digital Subscribers Line. This is a DSL variant that is very popular.

• has higher capacity downstream than upstream

  • downstream 6.4 Mbps

  • upstream 640 Kbps

Cable Modems

- use cable TV

- uses FDM

- group of subscribers in neighborhood share the same bandwidth

- neighborhoods are connected to central office using optical fiber

All computers look like they have the same IP address. This allows multiple computers to share internet connection. HUB/Address Translation/Firewall

Wide Area Networks

- Span large distance

- Connect multiple LANs and MANs using routers (also called packet switches)

- The router connects different networks and forwards packets using destination

- Different between bridge and router

• bridge extends one Ethernet network

• routers links two or more separate networks

• each Ethernet is still autonomous

A router:

• receives packet

• stores in memo

• looks at destination

• forwards packet to destination

A host address is divided into two parts (Network #, Host #)

The router uses the Network # for routing the packet. The router has information on how to route a packet to its destination.

Routing Algorithms

Next Hop Forwarding - most popular

- each router has a routing table that gives the next router necessary to reach destination

Next Hop Forwarding

- Routing table gives next router necessary to reach  destination network

- Forward table can be abbreviated:

- protocol use in groups of LAN to exchange route information is called RIP (Routing Internet Protocol)

- to get routing table from a router you can use the command /usr/local/bin/ripquery

- The source of routing info can be:

• Manual - fixed routes, small networks

• Automatic:

  • needed for large networks

  • changes the routes if there is failure

  • neighbor routers exchange routing information

  • RIP

- We can represent routers/nets using graphs

- Edges represent distances or how slow/fast the connection is.

-What is the best route?

- The shortest path is not always the one that has the less number of router

- To find/build routing table

Link State Routing

- All Routers exchange routing information

- Every single router has the whole picture of the network

- Each router runs Dijkstra's shortest path algorithm

- Routers build their own table using this information

- Optimal

- Distavantages:

• need large storage for large networks

• overhead communicating all routes in all routers

Distance Vector Algorithm

- Each router sends information to neighbor routers with the networks they can reach

- A router will change a route if the new information contains a better route considering the distance between the two routers.

Example:

- Current routing table:

Routing update from R3 to R4

- R4 receives update and check if it is better to update route entries

- >From R4 reaching N1 currently costs 3. If N3 is used the cost is 5+2=7. Do not change rout to N1.

- >From R4 reaching N2 currently costs 6. If R3 is used the cost is 3+2 =5. Change route to R3.

- >From R4 reaching N3 currently costs 2. If R3 is used the cost is 2+2=4. Do not change route. 

- Advantage:

• less memory

• less routing information is transmitted

- Disadvantage:

• not optimal

• gives good routes but not necessarily the best routes

- RIP uses Distance Vector Routing

• each router broadcasts routing info every 30 seconds.

Examples of WAN:

- Arpanet:

• 30 years old

• initial internet

- X.25

• old

• used leased lines

- Frame Relay

• blocks of 8K bytes of data

• high data rates 4-100 Mbps

- ATM

• Asynchronous Transfer Mode

• Sponsored by the telephone companies

• used for voice, video, data

• 5 octets header

• 48 octets data (payload)

• expensive

• still used for permanent connections

• overhead when establishing a connection