Reliable Data Transfer Protocol

Reliable Data Transfer ProtocolIMPLEMENTATION TIPS

Transport Layer – TCP2

B

Recall Reliable Data Transfer Mechanisms:

Checksum

Timer

Sequence number

ACK NAK

Window, pipelining

socketdoor

TCPsend buffer

TCPreceive buffer

socketdoor

Packet ->

applicationwrites data

applicationreads data

- Verification of integrity of packet

- Signals necessary re-transmission is required

- Keeps track of which packet has been sent and received

- Indicates receipt of packet in good or bad form

- Allows for the sending of multiple yet-to-be-acknowledged packets

Client - ServerCLIENT

0 ABCDEF1 ABCDEF2 ABCDEF3 ABCDEF...

File1_Windows.txt

SERVER

Empty...

File1_Saved.txt

Rclient_UDP.cpp Rserver_UDP.cpp

socket socket

UDP (User Datagram Protocol)– has no connection establishment- No connection state at servers- less packet overhead than TCP- light error-checking (checksum)- server doesn’t use the listen() function- server doesn’t use the accept() function

• See Lecture-2012-6-Socket Programming-Part2 (Slide #4)

UDP Basics



File1_Windows.txt

SERVER

Empty...

File1_Saved.txt


socket socket

Rserver 1235 0 0Rclient 127.0.0.1 1235 0 0

You should run the server first, before running the client. You can test your client and server using the same machine by using the example run above.

Sample run:

Bits can be corrupted Packets can be lost

Port: 1235



File1_Windows.txt

SERVER

Empty...

File1_Saved.txt


socket socket

Rserver 1235 0 0Rclient 127.0.0.1 1235 0 0

The client is the sender, while the server is the receiver.

The filenames used for sending and saving have been fixed in the start-up codes (i.e. File1_Windows.txt, File1_Saved.txt).



File1_Windows.txt

SERVER

Empty...

File1_Saved.txt


socket socket

Rserver 1235 0 0Rclient 127.0.0.1 1235 0 0

The client sends the contents of the file line by line. One packet contains exactly one line. In order to implement reliable data transfer, you will have to modify the packet header to add more details.



File1_Windows.txt

SERVER

Empty...

File1_Saved.txt


socket socket

Rserver 1235 0 0Rclient 127.0.0.1 1235 0 0

The objective is for you to implement a reliable data transfer protocol. You can choose to implement a simple stop-and-wait protocol or any of the pipe-lining protocols (i.e. Go Back-N, Selective Repeat).



File1_Windows.txt

SERVER


File1_Saved.txt


socket socket

Receives file contentsline-by-line, then stores everythinginto a file

Reads file, then sends the contents1 line at a time

UNRELIABLE CHANNELNETWORK LAYER

TRANSPORT LAYER

APPLICATION LAYER



File1_Windows.txt

SERVER


File1_Saved.txt


socket socket

Receives file contentsline-by-line, then stores everythinginto a file


UNRELIABLE CHANNELNETWORK LAYER

TRANSPORT LAYER

APPLICATION LAYER

Simulated by a function named send_unreliably()

Unreliable Channel Simulationint send_unreliably( int s, char * send_buffer, struct sockaddr_in remoteaddress) {

int fate=packets_fate(); //random number generator: 0, 1, 2 if (fate==0){ //no problem will be introduced

bytes = sendto(s, send_buffer, ...)printf("<-- SEND: %s \n",send_buffer);

} else if (fate== 1){ // introduce corrupted bitssend_buffer[damage_bit()]=random_char();send_buffer[damage_bit()]=random_char();bytes = sendto(s, send_buffer, ...)printf("<-- DAMAGED %s \n",send_buffer);…

} else if(fate==2){ // lose the packetprintf("X-- LOST %s \n",send_buffer);

}}

You are not allowed to modify this function in the

assignment.

UDP segment structure

• The optional parameters are not even included in the sendto() function.

Optional in IPv4

int sendto( SOCKET s, char *buf, int msglen, int flags, struct sockaddr *to, int tolen);

• The O/S will automatically provide the IP address of the sender of the segment.

•Send data through a socket:sendto(SOCKET s, char *msg, int msglen, int flags,

struct sockaddr *to, int *tolen);

sendto()

Example:

sendto(s, sbuffer, strlen(sbuffer),0,(struct sockaddr*) to, &len);

s = socket (inside the socket descriptor: port and IP address...)msg = a pointer to a buffer (could be a string)msglen = the length of the bufferflags = 0 (forget about them for this exercise...)to=structure of address with the IP / port #tolen=length of the structure

PARAMETERS

14

Receive dataint recvfrom(SOCKET s, char *msg, int msglen,

int flags, struct sockaddr *from, int *fromlen)

recvfrom()

Example:

recvfrom(s, rbuffer, 1, 0,(struct sockaddr *) &from, &len);

s = socket (inside the socket descriptor: port and IP address...)msg = a pointer to a buffermsglen = the length of the bufferflags = 0from =structure of address with the IP / port #fromlen=length of the structure

PARAMETERS



File1_Windows.txt

SERVER


File1_Saved.txt


socket socket

Receives file contentsline-by-line, then stores into a file

Destination Port: 1235 DATAchecksum Length of segment


UDP SEGMENT



File1_Windows.txt

SERVER


File1_Saved.txt


socket socket


Destination Port: 1235 DATA

0 ABCDEF

checksum Length of segment


PACKET 0 ‘\r’ ‘\n’CRC_NUMUser-defined

Start-up Codes(Client-Server)

Let’s have a look at the CRC function provided as part of the start-up codes and the CRC test program.

CRC

//*******************************************************************// CREATE CLIENT'S SOCKET //******************************************************************* s = socket(AF_INET, SOCK_DGRAM, 0); if (s < 0) { printf("socket failed\n"); WSACleanup(); exit(1); }

//nonblocking option u_long iMode=1; ioctlsocket(s, FIONBIO, &iMode);

Socket (in non-blocking mode)

• We need to set our socket in non-blocking mode of operation.• This prevents the recvfrom() function from stopping and waiting for a packet.• Remember, packets could be lost in our simulation of the unreliable channel.

Start-up Codes (Client – Server)CLIENT


File1_Windows.txt

SERVER


File1_Saved.txt


socket socket


Reads file using fgets(), then sends the contents 1 line at a time

Loop: send_unreliably(data) recvfrom()

Loop: recvfrom() send_unreliably(ACK)

fopen(“file1_Windows.txt”, r) fopen(“file1_Saved”, w)

Reading the file contentsThe client reads the file contents line by line using fgets()

fgets(send_buffer, SEGMENT_SIZE, fin)• stops reading the file when it encounters either:

• a new line character (copied into send_buffer)• EOF (End-Of-File) character

• a NULL-termination character (‘\0’) is automatically appended• this is counted as one of the characters

strlen() – counts the number of characters excluding the NULL-character

CLIENT

ABC

‘\n’

‘\0’

send_buffer

strlen()=4

‘\0’

‘\0’

send_buffer

strlen()-1

0

1

2

34

0

1

2

34

ABC

Data FormatCLIENT

ABC

‘\n’

‘\0’

send_buffer

strlen()=4

‘\0’

‘\0’

send_buffer

strlen()-1

0

1

2

34

0

1

2

34

ABC

Destination Port: 1235 DATA

ABC

checksum Length of segment

PACKET 0 ‘\r’ ‘\n’CRC_NUMUser-defined

Remove the line feed character from the row of data read from the file

CLIENT


File1_Windows.txt

SERVER


File1_Saved.txt


socket socket

Loop: send_unreliably(data) recvfrom()

Loop: recvfrom() send_unreliably(ACK)


send_unreliably(”CLOSE”)closesocket() Write everything into file1_Saved.txt

fclose()closesocket()

Start-up Codes (Client – Server)

Let’s see more details...

Start-up Codes (Client – Server)CLIENT SERVER

Loop: read one line from file if(not EOF){ create packet with header fields store packet into send_buffer send_unreliably(send_buffer) Sleep(1); recvfrom(receive_buffer) trim ‘\r’, ‘\n’ from receive_buffer } else { fclose() send_unreliably(”CLOSE”) }

Loop: recvfrom(receive_buffer) trim ‘\r’, ‘\n’ from receive_buffer process receive_buffer if(receive_buffer contains DATA){ create ACK packet send_unreliably(ACK) save_line_without_header } else { fclose() }


closesocket()closesocket()



Let’s have a look at the Start-up Codes (downloadable from our website)

Start-up Codes

Recommended Way of Testing

the Codes(Client-Server)

Parameters Settings

Parameters (Client – Server)CLIENT SERVER


Rserver 1235 0 0Rclient 127.0.0.1 1235 0 0Sample run:

Bits can be corrupted

Packets can be lost

CLIENT SERVER COMMENTS

00 00 Packets can never be corrupted nor lost

01 00 Client may lose packets

00 01 Server may lose ACK packets

01 01 Both client and server may lose packets

10 00 Client may have corrupt bits

00 10 Server may have corrupt bits

10 10 Both client and server may have corrupted bits

11 11 Both client and server may have corrupted bits and may lose packets

Ultimate TestCLIENT SERVER


Rserver 1235 1 1Rclient 127.0.0.1 1235 1 1

Ultimate Test:

Bits can be corrupted

Packets can be lost

You can simply inspect the contents of File1_Windows.txt and File1_Saved.txt, to check to see if they are exactly the same.


File1_Windows.txt0 ABCDEF1 ABCDEF2 ABCDEF3 ABCDEF...

File1_Saved.txt

Adding data transfer reliability

Extending the codes

Let’s have a look at the Go-Back N Protocol Specifications…

See Lecture-2012-7-Transport Layer-Part-1 (Slide #37)

Go-Back N

Pipelining Protocol (Go Back-N)CLIENT

(sender)

base base + (N-1)

ABCPACKET 0 ‘\r’ ‘\n’CRC_NUM ABCPACKET 1 ‘\r’ ‘\n’CRC_NUM ABCPACKET 2 ‘\r’ ‘\n’CRC_NUM

Send a Window’s worth of packets

N=Window size = 4

Sequence Number Space

Pipelining Protocol (Go Back-N)

base base + (N-1)

ABCPACKET 0 ‘\r’ ‘\n’CRC_NUM ABCPACKET 1 ‘\r’ ‘\n’CRC_NUM ABCPACKET 2 ‘\r’ ‘\n’CRC_NUM

Send a Window’s worth of packets

N = Window size = 4

nextSequenceNum-1Packets sent but not yet ACKed

...

nextSequenceNum

CLIENT (sender)


base base + (N-1)

N = Window size = 4

nextSequenceNum-1

Packets sent but not yet ACKed

nextSequenceNum

base baseMax

ACKnum

We need to keep track of the ACK number received

CLIENT (sender)


N = Window size = 4

Upon receipt of an ACK, slide the window forward

base=0 baseMax=3

ACKnum=1

base=ACKnum+1baseMax=base+(N-1)

0 1 2 3

0 1 2 3 4 5

time=0

time=1

CLIENT (sender)


N = Window size = 4

Transmit more packets (up to baseMax)base=0 baseMax=3

ACKnum=1

base=ACKnum+1baseMax=base+(N-1)

0 1 2 3

0 1 2 3 4 5

time=0

time=2

CLIENT (sender)

Extending the codes

WARNING: The following pseudo codes are not complete. They are meant just to give you an idea of how to implement a sliding Window protocol.

The statements highlighted in red corresponds to the suggested routines that need to be incorporated.

38

Calculating the Elapsed Timeclock()

void wait ( int seconds ){ clock_t endwait;

endwait = clock () + seconds * CLOCKS_PER_SEC ; while (clock() < endwait) {}}

clock_t startTime, elapsedTime;

startTime = clock();…...elapsedTime = (clock() - startTime) / CLOCKS_PER_SEC;

Extending the codes

Loop: read one line from file if(not EOF){ create packet with header fields store packet into send_buffer send_unreliably(send_buffer) Sleep(1); recvfrom(receive_buffer) trim ‘\r’, ‘\n’ from receive_buffer extract ACK number update base, baseMax } else { fclose() send_unreliably(”CLOSE”) }

Loop: recvfrom(receive_buffer) trim ‘\r’, ‘\n’ from receive_buffer extract CRC1 from receive_buffer extract data from receive_buffer calc CRC2 using data if(CRC1 == CRC2){ extract Packet Num if(PacketNum is in-order){ if(receive_buffer contains DATA){ create ACK packet send_unreliably(ACK) save_line_without_header update expectedSeqNum, base } else if(receive_buffer contains CLOSE) { fclose(); closesocket(); } } }


closesocket()



SERVERCLIENT

Other Helpful Functions

strchr• <cstring>• const char * strchr ( const char * str, int character ); char *

strchr ( char * str, int character );• Locate first occurrence of character in string• Returns a pointer to the first occurrence of character in the C

string str.

The terminating null-character is considered part of the C string. Therefore, it can also be located to retrieve a pointer to the end of a string.

Reliable Data Transfer Protocol

Documents

Transcript of Reliable Data Transfer Protocol