Given the network diagram below, answer the following questions. Assume that the transmission delay for HTTP GET/RESPONSE and DNS REQUEST/RESPONSE messages is so small that it can be ignored. Transmission delays are only calculated when sending the data file. Both routers, R1 and R2, use store and-forward. The transmission delays on the LANs must be calculated, but propagation delays are ignored. The round-trip time from any machine in a.com to b.com is 50 ms. The round-trip time from any machine in a.com to the Internet is 100 ms. Answer these questions:

a. Give the sequence and timing of messages sent when m1.a.com requests a 1 Mbit file from www.b.com. Assume the DNS record and the HTTP file are not cached locally. Also assume that the top-level domain is not cached.

b. Give the sequence and timing of messages sent when m2.a.com requests the same 1 Mbit file assuming the DNS records and file has been cached locally.

Authoritative DNS server for b.com www b.com 100 Mbps LAN 1 Mbps full-duplex RTT 100 ms Internet 10 Mbps LAN ml.a.com m2.a.com HTTP cache Local DNS server Shapes Message Description HTTP GET DNS REQUEST DNS REQUEST DNS RESPONSE DNS REQUEST DNS RESPONSE DNS REQUEST m1.a.com does HTTP GET for file at www.b.com Message sent to local HTTP cache. (No time based on assumptions.) Local HTTP cache does not have document. Requires DNS lookup for www.b.com Sends DNS REQUEST message to local DNS server (no time). Local DNS server does not have entry Sends DNS REQUEST to root-level DNS DNS REQUEST arrives at root-level server Sends back DNS RESPONSE containing top-level domain (TLD) Root-level DNS response received by local DNS Sends DNS REQUEST to TLD DNS TLD DNS receives request and sends response containing authoritative DNS Local DNS receives reply with authoritative DNS Sends DNS REQUEST to authoritative DNS. (25 ms time to get to b.com servers) Authoritative DNS in b.com domain receives request Sends DNS RESPONSE with A record for www.b.com. (25 ms) DNS RESPONSE received by local DNS. Forwards response to HTTP cache (no time) DNS RESPONSE DNS RESPONSE HTTP get (TCP handshake i HTTP get (sent) HTTP RESPONSE HTTP RESPONSE None HTTP cache wants to do HTTP GET to www.b.com for fle Requires 1 RTT for TCP setup before HTTP GET is sent Local HTTP cache does HTTP GET to www.b.com for file www.b.com responds with document and begins transmitting ransmit time 10 ms (b LAN)+ 1000 ms (1 Mbps link)+ 100 ms (a LAN)1110 ms Local HTTP cache sends HTTP RESPONSE to m1.a.com transmit time 100 ms Complete file arrives at m1.a.com