Homework 1 solution
and some questions
Ying Zhang
Homework 1

total score: 90
Max score: 90
Mean: 85.8858
Std: 6.17
tin_setup_conn()
struct sockaddr_in server_addr;
int sock, sockopt_on=1;
struct hostent * he;
//get a socket
if ((sock = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP)) == -1)
{
perror("socket acquisition failed");
exit(1);
}
// make socket reusable
if (setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, & sockopt_on, sizeof (int)) == -1)
{
perror ("socket reusable option failed");
exit (1);
}
// look up host
he = gethostbyname (servname);
if (he == NULL)
{
perror ("name lookup failed ");
exit (1);
}
// prepare server address datastructure
memset (( char *) &server_addr, 0, sizeof (struct sockaddr_in));
server_addr.sin_family = PF_INET;
server_addr.sin_port = htons (port);
server_addr.sin_addr = *((struct in_addr *)he->h_addr);
// connect to server
if (connect (sock, (struct sockaddr *) &server_addr, sizeof (struct sockaddr_in)) == -1)
{
perror ("connection failed");
exit(1);
}
return sock;
tin_send_hdr()
struct _tin_head header;
int bytes_to_send= sizeof (struct _tin_head);
int bytes_sent=0;
//make sure asgn has TIN_ALEN-1 chars.
if (strlen(asgn) != (TIN_ALEN-1))
{
perror ("assignment name length");
exit(1);
}
header.tih_vers = htons(TIN_VERS);
header.tih_op = htons(op);
memcpy(&header.tih_asgn,asgn,TIN_ALEN-1);
header.tih_asgn[TIN_ALEN-1]='\0';
// 0 for TIN_LIST taken care in parse
header.tih_nfiles=htons((unsigned short)nfiles);
// now need to send the header
while (bytes_sent < bytes_to_send)
{
int sent=send (sd, ((char *)(& header)+bytes_sent), bytes_to_send-bytes_sent,0);
if (sent==-1)
{
perror ("tin_send_hdr");
exit(1);
}
bytes_sent+=sent;
}
tin_send_fhdr()
struct _tif_head header;
int bytes_to_send=sizeof (struct _tif_head);
int bytes_sent=0;
int file;
// open the file for reading
file=open(fname,O_RDONLY);
if (file==-1)
{
perror ("cannot open file");
exit(1);
}
// get file size and rewind
*fsize=(int)lseek (file, 0, SEEK_END);
lseek (file, 0, SEEK_SET);
// prepare packet
memset(&header,0,bytes_to_send);
memcpy(&header.tif_fname,fname,strlen(fname));
header.tif_fsize=htonl(*fsize);
// send header
while (bytes_sent < bytes_to_send)
{
int sent=send (sd, ((char *)(&header)+bytes_sent), bytes_to_send-bytes_sent,0);
if (sent==-1)
{
perror ("tin_send_fhdr");
exit(1);
}
bytes_sent+=sent;
}
return file;
tin_send_file()
int total_to_send = (int)lseek (fd, 0, SEEK_END);
int bytes_to_send;
int bytes_sent=0;
char buff [TIN_DLEN];
lseek (fd, 0, SEEK_SET); // rewind
while(read(fd, buff, TIN_DLEN)!=0)
{
bytes_sent=0;
bytes_to_send = total_to_send < TIN_DLEN ? total_to_send : TIN_DLEN;
// send chunk
while (bytes_sent < bytes_to_send)
{
int sent=send (sd, ((char *)(& buff)+bytes_sent), bytes_to_send-bytes_sent,0);
if (sent==-1)
{
perror ("tin_send_file");
exit(1);
}
bytes_sent+=sent;
}
total_to_send -= bytes_sent;
}
tin_recv_ack()
int bytes_received=0;
char buff_curr, buff_prev=0;
int recvd;
do {
recvd=recv (sd, & (buff_curr), 1,0);
if (recvd==-1)
{
perror ("tin_recv_ack");
exit(1);
}
bytes_received +=recvd;
fprintf(stderr,"%c",buff_curr);
buff_prev=buff_curr;
} while (recvd!=0);
return bytes_received;
tind_recv()
while (try < TIND_MAXTRIES)
{
FD_ZERO (&rfds); //init
FD_ZERO (&efds); //init
FD_SET(td,&rfds); //add td to listening set
FD_SET(td,&efds); //add td to error set
to.tv_sec = TIND_SLEEPUSEC / 1000000;
to.tv_usec = TIND_SLEEPUSEC % 1000000;
// blocking call to select
rval=select(td+1,&rfds,NULL,&efds,&to);
if (rval<0)
{
perror ("select");
exit(1);
}
if (FD_ISSET (td, &efds)) //some error with client connection
{
//client closes connection too early
close (td);
return tind_errmsg ("Connection closed\n",0);
}
if (FD_ISSET (td, &rfds)) //check if it is there
{
received=recv(td, buf + total_received, *blen - total_received,0
if (received==-1)
{
//client closes connection too early
close (td);
return tind_errmsg ("Connection closed\n",0);
}
else if (received != 0)
{
total_received += received;
if (total_received == * blen)
{return NULL;} // correct termination
}
else
{
try++; //increment try only if nothing is received
}
}
else
{
try++; //increment try only if nothing is received
}
}
// if you are here then a client error had occured
*blen=total_received;
close(td);
return tind_errmsg("Connection time-out",0);
Questions
Q1

What are the two types of transport
services that the Internet provides to its
applications? What are some
characteristics of each of these services?
A1:
The Internet provides its applications a
connection-oriented service (TCP) and a
connectionless service (UDP). Each Internet
application makes use of one these two
services.

A1 (cont.)
Some of the
principle characteristics of the connection-oriented service are:

Two end-systems first “handshake” before either starts to send application data to
the other.

Provides reliable data transfer, i.e., all application data sent by one side of the
connection arrives at the other side of the connection in order and without any
gaps.

Provides flow control, i.e., it makes sure that neither end of a connection
overwhelms the buffers in the other end of the connection by sending to many
packets to fast.

Provides congestion control, i.e., regulates the amount of data that an application
can send into the network, helping to prevent the Internet from entering a state of
grid lock.
A1 (cont.)
The principle characteristics of
connectionless service are:
 No handshaking
 No guarantees of reliable data transfer
 No flow control or congestion control
Q2:
Flow control and congestion control are
equivalent.
 Is it true for the Internet’s connectionoriented service? Are the objectives of flow
control and congestion control the same?

A2:


Flow control and congestion control are two
distinct control mechanisms with distinct
objectives. Flow control makes sure that neither
end of a connection overwhelms the buffers in
the other end of the connection by sending to
many packets to fast.
Congestion control regulates the amount of data
that an application can send into the network,
helping to prevent congestion in the network
core (i.e., in the buffers in the network routers).
Q3:

What is the key distinguishing difference
between a tier-1 ISP and a tier-2 ISP?
A3
A tier-1 ISP connects to all other tier-1
ISPs; a tier-2 ISP connects to only a few of
the tier-1 ISPs. Also, a tier-2 ISP is a
customer of one or more tier-1

Q4:




Design and describe an application-level protocol to be used
between an automatic teller machine and a bank’s centralized
computer.
Your protocol should allow a user’s card and password to be verified,
the account balance (which is maintained at the centralized
computer) to be queried, and an account withdrawal to be
made( that is, money disbursed to the user).
Your protocol entities should be able to handle the all-too-common
cases in which there is not enough money in the account to cover
the withdrawal.
Specify your protocol by listing the messages exchanged and the
action taken by the automatic teller machine or the bank’s
centralized computer on transmission and receipt of messages.
A4:
Messages from ATM machine to Server
Msg name
purpose
-------------HELO <userid>
Let server know that there is a card in the
ATM machine
ATM card transmits user ID to Server
PASSWD <passwd>
User enters PIN, which is sent to server
BALANCE
User requests balance
WITHDRAWL <amount> User asks to withdraw money
BYE
user all done

A4 (Cont.)

Messages from Server to ATM machine (display)
Msg name
-------PASSWD
OK
ERR
AMOUNT <amt>
BYE
purpose
------Ask user for PIN (password)
last requested operation
(PASSWD, WITHDRAWL) OK
last requested operation
(PASSWD, WITHDRAWL) in ERROR
sent in response to BALANCE request
user done, display welcome screen at
ATM
Correct operation:
client
HELO (userid)
server
--------------> (check if valid userid)
<------------- PASSWD
PASSWD <passwd> --------------> (check password)
<------------- OK (password is OK)
BALANCE
-------------->
<------------- AMOUNT <amt>
WITHDRAWL <amt> --------------> check if enough $ to cover
withdrawl
<------------OK
ATM dispenses $
BYE
-------------->
<------------BYE
In situation when there's not
enough money
client
server
HELO (userid) --------------> (check if valid userid)
<------------- PASSWD
PASSWD <passwd> --------------> (check password)
<------------- OK (password is OK)
BALANCE -------------->
<------------- AMOUNT <amt>
WITHDRAWL <amt> --------------> check if enough $ to cover withdrawl
<------------- ERR (not enough funds)
error msg displayed
no $ given out
BYE
-------------->
<------------- BYE