DNS, Web, TCP Sequence Numbers

EE122 Discussion10/19/2011

DNS

• Mapping between names (e.g., www.cnn.com) and addresses (e.g., 157.166.255.18)

• Hierarchy of DNS servers– Root servers– Top-level domain (TLD) servers– Authoritative DNS servers– Default Name Server

HTTP

• Request-response protocol

• Based on a global naming system– …which uses the DNS for resolution

• HTTP runs on top of TCP

TCP Sequence Numbers

• Initial sequence number for the connection is not zero

• Sequence number refers to the first byte in the TCP packet payload

Example (from class)

• Packet arrives:– Seq: 2323– Ack: 4001– W=3000– [no payload]

• Appropriate response?Seq: 4001, payload: 4001-8000Seq: 2001, payload: 2001-5000Seq: 4001, payload: 4001-5000Seq: 5001, payload: 5001-6001Seq: 8001, payload: 8001-9000