Putting it all together

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1 Putting it all together What happens when a user shows up to a new network and wants to access a web site? (These are new slides. Please stop and ask questions if anything is unclear!)

2 Scenario

3 Scenario Network: /24 24 bits: network 8 bits: host Network: /16 16 bits: network 16 bits: host AS 1 AS 2

4 Step 0: Routing Protocol Before anyone starts sending data, we ll assume the routers have run a routing protocol (BGP) to learn about each other. Send to me to get to /24! AS 1 AS 2

5 Step 0: Routing Protocol Before anyone starts sending data, we ll assume the routers have run a routing protocol (BGP) to learn about each other. Send to me to get to /16! AS 1 AS 2

6 Step 1: User Joins Network User arrives and needs an IP address. They bring MAC address with them (built in to hardware).

7 Step 1: User Joins Network User broadcasts DISCOVER message to acquire IP address. (Alternative, they manually enter IP config details.)

8 Step 1: User Joins Network responds with: IP address (5), subnet mask ( ), gateway (), and server (). 5

9 Step 2: User Resolves Name Suppose user tries to access website: Must resolve name using. Query local resolver. 5

10 Step 2: User Resolves Name User s PC must answer: is the resolver () I was given by on my subnet? (Local vs. ) my address subnet mask 5: : ANDed together: my network prefix target address Match! It s local. Send directly, no need to go through gateway (router).

11 Step 2: User Resolves Name User s PC does NOT know server s MAC address! Broadcast request looking for! 5 IP Address MAC Address ?

12 Step 2: User Resolves Name server responds with MAC address. 5 IP Address MAC Address :AA:BB:CC:DD:FF

13 Step 2: User Resolves Name User queries local resolver for Resolver runs necessary queries (root, TLD, etc.) Eth Header Dest MAC: 00:AA:BB:CC:DD:FF IP Header Dest IP: UDP Header Dest port: 53 Query 5 IP Address MAC Address :AA:BB:CC:DD:FF

14 Step 2: User Resolves Name reply says that is

15 Step 3: Establish a TCP Connection User s PC must answer: is the destination ( ) on my subnet? (Local vs. ) my address subnet mask 5: : ANDed together: my network prefix target address No Match! Send it to the default gateway (router that connects to the ) that gave us ().

16 Step 3: Establish a TCP Connection User s PC does NOT know router s MAC address! Broadcast request looking for! 5 IP Address MAC Address :AA:BB:CC:DD:FF ?

17 Step 3: Establish a TCP Connection Router responds with MAC address. 5 IP Address MAC Address :AA:BB:CC:DD:FF :00:11:11:22:22

18 Step 3: Establish a TCP Connection Send TCP SYN to the destination, start 3-way handshake. No data in SYN Eth Header Dest MAC: 00:00:11:11:22:22 IP Header Dest IP: TCP Header Dest port: 80 SYN 5 IP Address MAC Address :AA:BB:CC:DD:FF :00:11:11:22:22 Link layer header: next hop destination! Network layer header: final destination!

19 Step 3: Establish a TCP Connection Send SYN to router. NOTE: while the switch moves the frame to router, it is not ever addressed directly. Eth Header Dest MAC: 00:00:11:11:22:22 IP Header Dest IP: TCP Header Dest port: 80 SYN 5

20 Step 3: Establish a TCP Connection Router removes Ethernet header. Eth Header Dest MAC: 00:00:11:11:22:22 IP Header Dest IP: TCP Header Dest port: 80 SYN 5

21 Step 3: Establish a TCP Connection Router R 1 compares destination IP with its forwarding table, looks for longest prefix match. A B C IP Header Dest IP: R 1 R 2 TCP Header Dest port: 80 SYN Prefix Output Port Next Router s Link Layer Addr /24 A (N/A - no router there) /8 B Some router s address /16 C R 2 s Address: 55:44:33:22:11:00 Best match: /16 -> Output port C Destination MAC: 55:44:33:22:11:00

22 Step 3: Establish a TCP Connection Router R 1 constructs frame and forwards it to R 2. Link Layer Header Dest MAC: 55:44:33:22:11:00 IP Header Dest IP: TCP Header Dest port: 80 SYN 5

23 Step 3: Establish a TCP Connection Router R 2 compares destination IP with its forwarding table, looks for longest prefix match. IP Header Dest IP: A B R 1 R 2 TCP Header Dest port: 80 SYN C Prefix Output Port Next Router s Link Layer Addr /24 A R 1 s Address /8 B Some router s address /16 C (N/A - no router there) Best match: /16 -> Output port C Destination MAC:?

24 Step 3: Establish a TCP Connection R 2 does NOT know destination s MAC address! Broadcast request looking for ! Data packet is queued while waiting for to resolve. IP Address MAC Address ?

25 Step 3: Establish a TCP Connection Host replies with MAC address. IP Address MAC Address AA:BB:CC:DD:EE:FF

26 Step 3: Establish a TCP Connection R 2 constructs frame, forwards it to destination. Ethernet Header Dest MAC: AA:BB:CC:DD:EE:FF IP Header Dest IP: TCP Header Dest port: 80 SYN 5

27 Mission Accomplished! Destination peels off headers, generates reply (SYN+ACK). 5 Ethernet Header Dest MAC: AA:BB:CC:DD:EE:FF IP Header Dest IP: TCP Header Dest port: 80 SYN

28 Mission Accomplished! Process repeats in the opposite direction, without the s this time. (MAC addresses were recently used, thus cached.) 5

29 Steady State With cached and entries cached, host encapsulates data in TCP, IP, Eth headers and sends to router. Router forwards. Even with all the /, all that stuff happens in < 1 second (besides step 0: routing protocol)

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