Day16 Protocols

TCP “Transmission Control Protocol” –Connection oriented Very like a phone call, an actual connection is made between the 2 parties. –One part initiates the “call” –Other party must accept it. –At the end, the connection must be dismantled. –Robust & Reliable Error Checking Timeouts / Retransmission

Creating the connection step 1 Computer 1 sends a packet to computer 2 –Contains: Initial Sequence Number (Should be random) –For example 1001 Source Address Destination Address Port Number Information

Creating the connection step 2 Computer 2 sends a packet to computer 1 –Contains: Acknowledgement of receipt of packet 1001 Its Initial Sequence Number (Should also be random) –For example 5001 Source Address Destination Address Port Number Information

Creating the connection step 3 Computer 1 sends a packet to computer 2 –Contains: Acknowledgement of receipt of packet 5001 The next sequence number (1002) Actual Data

Now they are connected Every packet has a sequence number in it In theory every packet must be acknowledged by the other side –If not, the sender will realize some packets have never been acknowledged, and will retransmit them (Timeout) If a packet is received which is corrupted, the receiver can either specifically report it got a bad packet, or just pretend it never got it at all –Eventually the sender will timeout and re-send it

Many common applications use TCP: Web browsers (HTTP) FTP Clients (FTP) Mail Clients (POP, IMAP) Mail Servers (SMTP) Lotus Notes Telnet Databases etc.

UDP “User Datagram Protocol” –Packet by packet No connection is created –Unreliable No error checking No retransmission Packets often get lost. –Faster Without all the overhead of TCP, the packets are able to flow faster

Uses: Used for most Audio/Video/Games –Real Audio/Video –Telephony –DNS lookups –Time synchronization programs –VOIP Usually used where some data loss is acceptable: –In Video if you miss one frame, it makes more sense to just forget about it, rather than try to retransmit it. –In audio, often the sample rate is high enough that you can’t hear the difference if a lot is lost.

TCP & UDP use port numbers. Need for ports: –Since a given workstation could be performing many networking related tasks at once there has to be a way for the OS to determine where to send return data. –Standardized ports make it simple to find servers on machines. For example, how does your web browser know which port to connect to on? –HTTP servers are usually set up on port 80

Some common port numbers: HTTP80/TCP telnet23/TCP FTP21/TCP (For the control connection) POP110/TCP SMTP25/TCP DNS53/UDP (And some TCP) NTP123 UDP *Notice it’s not sufficient to specify a port number, you should also specify which protocol.

Some applications use multiple ports For example FTP –Port 21/TCP is used to create a connection between the 2 machines –You login in that connection. –When you request a listing of documents in a directory, or when you request a file be transferred a new connection is negotiated The client and server, must negotiate a free TCP port which the client can accept a connection on from the server

Using Protocols Lets manually get a web page… Open a telnet connection: Start – Run – cmd telnet 80 Once connected type: GET / HTTP/1.0 Hit enter twice

telnet student.claytonstate.net 25 Helo bla Mail from: Rcpt to: Data Hi, This is a test.