1. Chapter 5 – Big Picture Dr. V.T. Raja Oregon State University

2. Chapter 5 – Big Picture Minimum Information Needed by a Computer in a TCP/IP Network Identify minimum information needed by any computer that is installed on (or dials into) a TCP/IP network Some of the minimum information can be provided by ___________ file, or via a server such as _____ server.

3. Example illustrating Big Picture Example: Assume the following: All computers have been provided minimum info specified in 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 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 has the IP of D in its address table. Assume each device has the DL address of its immediate neighbor(s). 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 illustrating Big Picture (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. Description of data flow at Client A User’s request is translated to HTTP format; passed to TL. Transport Layer: TCP is protocol used in this layer TCP performs ‘Packetizing’ (i.e., breaks message into smaller sized packets); Since DL protocol is Ethernet each frame has a maximum of 1492 bytes. Since message is a web page request, it probably fits within one packet.

7. Transport Layer at Client A (Continued) TCP prepares TCP header which includes: Source and Destination Port ID indicating software used at source and to be used at destination. Being a web application – Default Source/Dest. Port ID would be 80. Packet numbers (here there is probably only one packet) TCP passes [TCP|HTTP|User Data] to NL of client A.

8. Network Layer at Client A (Continued) IP is the protocol used in this layer. IP performs addressing and routing. IP determines NIC address 00-E0-B0-C0-E4-45- 6E of next hop device “R” (from address table) and IP address (16.32.64.129) of final destination (Web Server D) from address table. If addresses are unavailable in address table, ARP is used for determining NIC address of R, and DNS is used for determining IP of D.

9. Network Layer at Client A (Continued) NIC address of R is passed to DLL of client A. IP prepares IP header, sample contents of IP header include: Source (Client A’s) IP (16.32.64.33) and Destination (Web Server D’s) IP (16.32.64.129) addresses Routing (e.g., next hop device) information is decided using subnet mask addresses, subnet mask algorithm, and routing software (where appropriate). Note that client A and router R are not on same subnet. IP passes [IP|TCP|HTTP|User Data] to data link layer of client A.

10. Data Link Layer at Client A (Continued) Ethernet is protocol used at Data Link Layer. Ethernet performs Media Access Control (MAC), Message Delineation (MD), and Error Detection (ED)/Error Correction (EC)

11. Data Link Layer at Client A (Continued) Ethernet creates DL Header and DL Trailer packets, which include: Source (Client A’s) DL address (00-A0-B0-C0-A4-54-6A) Receiver’s (Router R’s) DL address (00-E0-B0-C0-E4- 45-6E) MAC is specified (i.e., CSMA/CD) Value (i.e., 32-bit remainder obtained using CRC-32) that helps R determine if there were any errors during transmission Info on how R should perform EC (i.e., Stop-and-wait ARQ) MD info (i.e., length of frame is specified in header)

12. Data Link Layer and Physical Layers at Client A (Continued) DL passes [DLH|IP|TCP|HTTP|User Data|DLT] to DLL of R via PL. Physical Layer: Transmission media passes packets to DL layer of R; if there is a collision during transmission, MAC is performed as per CSMA/CD rules. (Complete this answer by describing data flow across different layers at router R and at final destination).