TCP-DNS

• Keeping your phone number is recent, but still may need to change
• Home, cell numbers services to balance between them
• Ultimately, we get a phone number from the person
• But we can trust other people who have it to share

• The first subdomain can be registered by domain registrars
• Further subdomains can be generated by domain owners (eg. www.google.com, people.ischool.berkeley.edu
• Subdomains can be arbitrary, but typically used for department (eg ischool), service (eg. www), or area (en)
• com is not a hostname because it does not have an IP

• Caching can happen at many levels
• Typically your computer will ask the ISP name server
• The ISP nameserver will recursively lookup the result and cache it for the next customer

• Root server addresses are lettered a through m, we'll use ICANN's
• Provide authority information about who to query next
• Down to DNS provider for people

• http://en.wikipedia.org/wiki/List_of_DNS_record_types
• TXT originally started as human notes, but has evolved to store metadata about a domain, for example encryption keys

• Flexibility wins again

• Computers on a network likely to ask for the same stuff
• eg. all computers in this classroom probably have now asked for github.com
• Why have all of us do the full recursion?
• Instead have a nameserver on campus do the recursive lookup for us, cache

the result

• If it is in cache, and still valid just return answer
• DNS returns "time to live" that specifies how long result can be cached

• Operating System typically breaks up a stream of data into packets

• http://www.domaining.com/topsales/

• How to build stateful applications on stateless protocols?
• Keep state on the host

• How do you know if the data you receive is the same as was sent? Checksum
• How does the sender know if a packet was delivered? Ack
• How do we know the ordering of the data? Packet sequence numbering
• TCP also adds "ports": ability to connect to a particular application on the destination host

• Take the bits in a message
• Add them up*
• Append the sum
• Example: "jim" = 106 + 105 + 109 = 320

• Many other ways of calculating besides addition, eg xor or polynomial formulas
• Difference between checksum for errors and checksum against attackers

• When a machine receives a packet, it acknowledges it
• If a machine does not receive an ack, it resends
• To keep track of messages, uses a sliding window

• If the sender waits for every ack, things get pretty slow
• So instead we keep sending up to a certain point, but then if we don't hear anything for a while we stop and wait for ack
• Like a good phone conversation

• If we can resend, or packets can take different routes, they can be delivered in a different order
• How do we know the right order?
• Agree on a starting number, send numbered packets
• Client will keep track of packets received, deliver data in order to app

• We can also use this as an optimization for acks: Send ack for all packets < current one
• Agreement = "TCP Handshake"

SYN

Synchronize using a random starting number

SYN-ACK

Acknowledge syn packet with an ack number and its own random starting number

ACK

Acknowledge the SYN-ACK packet

• Client sends request
• Server responds
• Client responds

• Why are packets delayed?
• Often, it is because the system is overwhelmed
• So back off! If it is still overwhelmed, back off more!

• Wait 1 second, 2 seconds, 4, 8, 16…
• Randomize wait, so not everyone tries again at the same time
• Then ramp up again: larger packets, bigger window

• That's why we must send Host header in HTTP

• Each component evolved, and we're not stuck with any one particular technology
• But all parts must be working correctly for users to get a great experience

• What do these arrows represent? TCP/IP connections
• What kind of requests are being made to the DNS server?
• What is in the "payload" of the first TCP/IP packets sent to the web

server? (GET … HTTP/1.1)