1. 5/12/05CS118/Spring051 A Day in the Life of an HTTP Query 1.HTTP Brower application Socket interface 3.TCP 4.IP 5.Ethernet 2.DNS query 6.IP router 7.Running routing protocol IP Ethernet Socket interface Web server application HTTP demultiplexing multiplexing client/server paradiam TCP 3.Transport protocols E I T HTTP & data

2. 5/12/05CS118/Spring052 Router-2 Router-1 Packet (carries destination address) header data Efficient resource sharing Flexible delivery Computer Networking: the overall picture r What is packet switching?  Data is cut into chunks, sent in a "store-and-forward: way  statistical multiplexing  queueing delay, potential losses r why packet switching? r What’s in a packet:  header: contains all the information,and only the information that’s needed for the protocol’s functionality

3. 5/12/05CS118/Spring053 Relative importance of transmission&propagation delay which link takes less time to get a packet to the other end? (1) 1 Mbps bandwidth, 1 ms propagation delay (2) 100 Mbps bandwidth, 100 ms propagation delay 1. Small packet (50bytes) 2. Large packet (10MB)

4. 5/12/05CS118/Spring054 propagation delay bandwidth A B Link Utilization: One Example A sends packets to B using sliding window  bandwidth: 1Mbps  Propagation delay: 100 msec  data packet size: 1000 bytes (ACK very small)  window size: 4 packets  RTT = trans-delay(8ms) + 200ms = 208ms r utilization: (A's busy-time  total-time) = (8ms  4)  208ms 8 msec

5. 5/12/05CS118/Spring055 IP Suite: End Hosts vs. Routers HTTP TCP IP Ethernet interface HTTP TCP IP Ethernet interface IP Ethernet interface Ethernet interface SONET interface SONET interface host router HTTP message TCP segment IP packet This course focuses on the routers…

6. 5/12/05CS118/Spring056 The HTTP protocol Use TCP transport service r client initiates TCP connection (creates socket) to server r server accepts TCP connection from client r http messages exchanged between browser (http client) and Web server (http server) r TCP connection closed http is “stateless” r server maintains no information about past client requests Non-persistent r http/1.0: server parses request, responds, closes TCP connection r 2 RTTs to fetch object  TCP connection  object request/transfer r many browsers open multiple parallel connections Persistent r default for htp/1.1 r on same TCP connection: server parses request, responds, parses new request,.. r client sends requests for all referenced objects as soon as it receives base HTML. r Pipelining: browser sending multiple requests before getting responses

7. 5/12/05CS118/Spring057 DNS server application IP cs.ucla.edu 131.179.96.15 TCP 131.179.96.15 DNS: Domain Name System r provides name to IP address translation  DNS name: variable length, easy for human to remember  IP address: fixed length, tied to network topology, easy for computer to process r example: send mail to john@cs.ucla.edu

8. 5/12/05CS118/Spring058 DNS usage example r user provides the DNS name of the server r Client process sends a query to a local DNS server  each host knows a local DNS server’s IP address r the local DNS server either has the info or can get it from another DNS server r Client process receives the IP address from DNS server reply, open a TCP connection to the receiving host

9. 5/12/05CS118/Spring059 name space and naming system name space r defines the set of all possible names  similar to address space  can be either flat or hierarchical Ethernet address space is flat; DNS name space is hierarchical naming system r a collection of name-to-value bindings name resolution r given a name, return the corresponding value

10. 5/12/05CS118/Spring0510 Basic DNS design a hierarchical name space r starting from the root, growing downward (and side ways too) r variable depth hierarchy  each non-leaf node in the tree is a domain  any domain can have its own sub-domains, no limit on the depth of any branch r DNS name structure is completely independent from the Internet's topological structure

11. 5/12/05CS118/Spring0511 Reliable data delivery between two nodes r error/loss recovery by retransmission:  sequence number for each piece of data  Acknowledgment  retransmission timer r sliding window: relations between seq# and window size (Max. seq. number + 1)/2  window-size r relation between window size, throughput, and round-trip time(RTT): Window-size round-trip time Throughput  Equality achieved in the absence of errors/losses

12. 5/12/05CS118/Spring0512 u a p r s F r c s s y i g k h t n n source portdestination port Data sequence number acknowledgment number Hlen unused window size checksumurgent pointer Options (viable length) 01616 3131 TCP header format data IP header

13. 5/12/05CS118/Spring0513 ON TCP r How does TCP achieve reliable data delivery?  What are the fundamental mechanisms?  How do they work? r How does TCP perform flow control? r How does TCP perform congestion control? r How does TCP estimate its RTT and set the retransmission timeout (RTO)?

14. 5/12/05CS118/Spring0514 IP Packet Structure 4-bit Version 4-bit Header Length 8-bit Type of Service (TOS) 16-bit Total Length (Bytes) 16-bit Identification 3-bit Flags 13-bit Fragment Offset 8-bit Time to Live (TTL) 8-bit Protocol 16-bit Header Checksum 32-bit Source IP Address 32-bit Destination IP Address Options (if any) Payload 20-byteHeader usually IPv4 usually 20 bytes fragments more later error check header