1. TCP/IP Discussion Related to Essay Question on Final Dr. V.T. Raja Oregon State University

2. Minimum Information Needed by a Computer in a TCP/IP Network The computer’s IP address The minimum information needed is either provided by DHCP Server or is obtained from the computer’s configuration file A subnet mask address With smask, the computer can determine which IP addresses are part of its subnet IP address of DNS Server DNS can help translate between AL addresses and IP addresses IP address of Router/Gateway that connects the computer outside its subnet (for static routing) If dynamic routing is used, then appropriate routing software/ access to routing tables would be needed

3. Example for Essay Question Example: Assume the following: All computers have been provided minimum info specified in the previous slide Client A requests a web page from Web Server D. Client A is connected to Router R, which is connected to Server D. TCP/IP are the transport and network layer protocols, and Ethernet is the data link layer protocol.

4. Example (Continued) Client A knows IP address of Server D, and also AL address of D (www.D.edu). This implies that client A has already requested IP of D from DNS in the past, and now client A has the IP of D in its address table. Assume A and D know the DL address of R, and that R knows DL addresses of A and D. This implies that both A and D have already issued ARP messages to determine DL of R in the past, and now have the DL address of R in their respective address tables. Similarly, R has the DL address of A and D in its address tables.

5. Example for Essay Question (Continued) IP of A is 16.32.64.33; Subnet mask of A is 255.255.255.128 IP of R is 16.32.64.128; IP of D is 16.32.64.129; Assume NIC address of A is 00-A0-B0-C0-A4-54-6A; NIC address of R is 00-E0-B0-C0-E4-45-6E; NIC address of D is 00-D0-B0-C0-D4-75-7D Assume there are no errors in transmission Question: Describe the data flow (across the different layers in the 5-layer model) for the web request made by Client A to Web Server D.

6. For Description of Data Flow at Client A See part 1 ‘big picture’ handout

7. Description of Data Flow (Continued) At R Ethernet performs error detection (i.e., validates CRC-32), based on info provided in DLH/DLT packets obtained from Client A. Ethernet performs error correction (Stop-and-Wait- ARQ); Since there are no errors in transmission, R sends ACK for each packet it receives from A. After sending ACK, DL removes DLH and DLT; passes [IP|TCP|HTTP|User Data] to NL layer of R.

8. Description of Data Flow (Continued) At R Network Layer of R: IP looks at destination IP (16.32.64.129) specified in IP header Compares destination IP with its own IP (16.32.64.128) Concludes that it needs to re-route packet(s). Since destination has not been reached, IP does not invoke TCP.

9. Description of Data Flow (Continued) At R IP modifies routing info and creates modified IP header that reflects this change. IP determines and passes NIC address of next hop device (D) to DLL of R. IP passes [Modified-IP|TCP| HTTP| UserData] to R’s DLL.

10. Description of Data Flow (Continued) At R Data Link Layer creates new DL Header and Trailer, which delineate each frame received from IP. Sample contents of DLH/DLT include: Source DL address (00-E0-B0-C0-E4-45-6E) Destination DL address (00-D0-B0-C0-D4-75-7D) MAC specified (i.e., CSMA/CD) CRC-32 bit value that would help D detect transmission errors Info on how D should perform EC (i.e., Stop-and-wait ARQ) MD info (i.e., length of frame is specified in header) DL passes [New DLH|Modified IP|TCP|HTTP|User Data| NewDLT] to DL layer of R via physical layer.

11. Description of Data Flow (Continued) At D Physical layer follows MAC as per CSMA/CD rules while passing packets to DL layer of D. DL layer performs error detection (e.g., CRC-32), based on info provided in ‘New DLH/DLT’ obtained from R. DL layer performs error correction/ ACK/NAK (e.g., Stop- and-Wait-ARQ), based on info provided in ‘New DLH/DLT’ obtained from R. After sending ACK, DL removes ‘New DLH/DLT’, and passes [IP|TCP|HTTP|User Data] to NL layer of D.

12. Description of Data Flow (Continued) At D Network Layer of D: IP looks at destination IP (16.32.64.129) specified in ‘modified IP header’ Compares destination IP with its own (16.32.64.129) IP Concludes that final destination (D) has been reached IP removes ‘modified IP header’ Passes [TCP|HTTP|User Data] to D’s TL

13. Description of Data Flow (Continued) At D Transport Layer: TCP reassembles packets based on packet # specified in TCP Header. In this example, there is no necessity for this due to the short size of message. TCP looks at destination port address (80), and invokes the correct web application software in D TCP removes TCP header Passes [HTTP|User Data] to Application Layer of D

14. Description of Data Flow (Continued) At D Application Layer: Application layer removes HTTP Header and presents user data in expected user- friendly format (Now the Web Server will have to pass its web page to Client A via R!!)