Midterm Review

1. Midterm Review In class, 9:30-11 am, Tu. 2/8 Closed Book One 8.5” by 11” sheet of paper permitted (single side)

2. Lecture 1 • Internet Architecture • Network Protocols • Network Edge • A taxonomy of communication networks

3. A Taxonomy of Communication Networks • The fundamental question: how is data transferred through net (including edge & core)? • Communication networks can be classified based on how the nodes exchange information: Communication Networks SwitchedCommunication Network BroadcastCommunication Network Packet-SwitchedCommunication Network Circuit-SwitchedCommunication Network TDM FDM Datagram Network Virtual Circuit Network

4. Sequence of A & B packets does not have fixed pattern  statistical multiplexing. In TDM each host gets same slot in revolving TDM frame. D E Packet Switching: Statistical Multiplexing 10 Mbs Ethernet C A statistical multiplexing 1.5 Mbs B queue of packets waiting for output link

5. Circuit Switching Network resources (e.g., bandwidth) divided into “pieces” for allocation Resource piece idle if not used by owning call (no sharing) NOT efficient ! Packet Switching: Great for bursty data Excessive congestion: packet delay and loss protocols needed for reliable data transfer, congestion control Packet Switching versus Circuit Switching

6. 1 Mbit link Each user: 100 kbps when “active” active 10% of time Circuit-switching: 10 users Packet switching: with 35 users, probability > 10 active less than .0004 Packet switching allows more users to use network! Packet Switching versus Circuit Switching N users 1 Mbps link

7. Datagram Packet Switching • Each packet is independently switched • Each packet header contains destination address which determines next hop • Routes may change during session • No resources are pre-allocated (reserved) in advance • Example: IP networks

8. Virtual-Circuit Packet Switching • Hybrid of circuit switching and packet switching • All packets from one packet stream are sent along a pre-established path (= virtual circuit) • Each packet carries tag (virtual circuit ID), tag determines next hop • Guarantees in-sequence delivery of packets • However, packets from different virtual circuits may be interleaved • Q. What is the difference btw. circuit switching and virtual-circuit packet switching?

9. Lecture 2 • Network access and physical media • Internet structure and ISPs • Delay & loss in packet-switched networks • Protocol layers, service models

10. “Tier-3” ISPs and local ISPs last hop (“access”) network (closest to end systems) Tier-3: Turkish Telecom, Minnesota Regional Network Tier 3 ISP local ISP local ISP local ISP local ISP local ISP local ISP local ISP local ISP NAP Local and tier- 3 ISPs are customers of higher tier ISPs connecting them to rest of Internet Tier-2 ISP Tier-2 ISP Tier-2 ISP Tier-2 ISP Tier-2 ISP Internet structure: network of networks Tier 1 ISP Tier 1 ISP Tier 1 ISP

11. 1. processing: check bit errors determine output link transmission A propagation B processing queueing Four sources of packet delay • 2. queueing • time waiting at output link for transmission • depends on congestion level of router

12. 3. Transmission delay: R=link bandwidth (bps) L=packet length (bits) time to send bits into link = L/R 4. Propagation delay: d = length of physical link s = propagation speed in medium (~2x108 m/sec) propagation delay = d/s transmission A propagation B processing queueing Delay in packet-switched networks Note: s and R are very different quantities!

13. application: supporting network applications FTP, SMTP, HTTP transport: host-host data transfer TCP, UDP network: routing of datagrams from source to destination IP, routing protocols link: data transfer between neighboring network elements PPP, Ethernet physical: bits “on the wire” application transport network link physical Internet protocol stack

14. Principles of app layer protocols Application architectures and requirements Web and HTTP FTP Electronic Mail: SMTP, POP3, IMAP DNS Socket Programming Application Layer

15. Application architectures • Client-server • Peer-to-peer (P2P) • Hybrid of client-server and P2P

16. Client-server archicture server: • always-on host • permanent IP address • server farms for scaling clients: • communicate with server • may be intermittently connected • may have dynamic IP addresses • do not communicate directly with each other

17. Pure P2P architecture • no always on server • arbitrary end systems directly communicate • peers are intermittently connected and change IP addresses • example: Gnutella Highly scalable But difficult to manage

18. Hybrid of client-server and P2P Napster • File transfer P2P • File search centralized: • Peers register content at central server • Peers query same central server to locate content Instant messaging • Chatting between two users is P2P • Presence detection/location centralized: • User registers its IP address with central server when it comes online

19. TCP service: connection-oriented: setup required between client and server processes reliable transport between sending and receiving process flow control: sender won’t overwhelm receiver congestion control: throttle sender when network overloaded does not provide: timing, minimum bandwidth guarantees UDP service: unreliable data transfer between sending and receiving process does not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee Q: why bother? Why is there a UDP? Internet transport protocols services

20. Nonpersistent HTTP At most one object is sent over a TCP connection. HTTP/1.0 uses nonpersistent HTTP Persistent HTTP Multiple objects can be sent over single TCP connection between client and server. HTTP/1.1 uses persistent connections in default mode HTTP connections • HTTP Message, Format, Response, Methods • HTTP cookies

21. Nonpersistent HTTP issues: requires 2 RTTs per object OS must work and allocate host resources for each TCP connection but browsers often open parallel TCP connections to fetch referenced objects Persistent HTTP server leaves connection open after sending response subsequent HTTP messages between same client/server are sent over connection Persistent without pipelining: client issues new request only when previous response has been received one RTT for each referenced object Persistent with pipelining: default in HTTP/1.1 client sends requests as soon as it encounters a referenced object as little as one RTT for all the referenced objects Response Time of HTTP

22. user sets browser: Web accesses via cache browser sends all HTTP requests to cache object in cache: cache returns object else cache requests object from origin server, then returns object to client Consistency problem Why web caching? Web caches (proxy server) Goal: satisfy client request without involving origin server origin server Proxy server HTTP request HTTP request client HTTP response HTTP response HTTP request HTTP response client origin server

23. Install cache suppose hit rate is .4 Consequence 40% requests will be satisfied almost immediately 60% requests satisfied by origin server utilization of access link reduced to 60%, resulting in negligible delays (say 10 msec) total delay = Internet delay + access delay + LAN delay = .6*2 sec + .6*.01 secs + milliseconds < 1.3 secs Caching example (3) origin servers public Internet 1.5 Mbps access link institutional network 10 Mbps LAN institutional cache

24. FTP client contacts FTP server at port 21, specifying TCP as transport protocol Client obtains authorization over control connection Client browses remote directory by sending commands over control connection. When server receives a command for a file transfer, the server opens a TCP data connection to client After transferring one file, server closes connection. TCP control connection port 21 TCP data connection port 20 FTP client FTP server FTP: separate control, data connections • Server opens a second TCP data connection to transfer another file. • Control connection: “out of band” • FTP server maintains “state”: current directory, earlier authentication

25. Mail Servers mailbox contains incoming messages for user messagequeue of outgoing (to be sent) mail messages SMTP protocol between mail servers to send email messages client: sending mail server “server”: receiving mail server Example  If the sending mail server cannot deliver the message, it is queued user agent user agent user agent user agent user agent user agent SMTP SMTP SMTP mail server mail server mail server Electronic Mail: mail servers

26. Why not centralize DNS? single point of failure traffic volume distant centralized database maintenance doesn’t scale! DNS services Hostname to IP address translation E.g., www.northwestern.edu Host aliasing Canonical and alias names E.g., dell.com www.dell.com Mail server aliasing E.g., bob@hotmail.com Load distribution Replicated Web servers: set of IP addresses for one canonical name E.g., cnn.com DNS

27. Root name server: may not know authoritative name server may know intermediate name server: who to contact to find authoritative name server local name server dns.eurecom.fr intermediate name server dns.nwu.edu DNS example root name server 6 2 3 7 5 4 1 8 authoritative name server dns.cs.nwu.edu requesting host surf.eurecom.fr www.cs.nwu.edu

28. recursive query: puts burden of name resolution on contacted name server heavy load? iterated query: contacted server replies with name of server to contact “I don’t know this name, but ask this server” local name server dns.eurecom.fr intermediate name server dns.umass.edu DNS: iterated queries root name server iterated query 2 3 4 7 5 6 1 8 authoritative name server dns.cs.umass.edu requesting host surf.eurecom.fr gaia.cs.umass.edu

29. Transport-layer services Multiplexing and demultiplexing Connectionless transport: UDP Principles of reliable data transfer TCP Segment structures Flow control Congestion control Transport Layer

30. UDP socket identified by two-tuple: (dest IP address, dest port number) When host receives UDP segment: checks destination port number in segment directs UDP segment to socket with that port number Demultiplexing • TCP socket identified by 4-tuple: • source IP address • source port number • dest IP address • dest port number • recv host uses all four values to direct segment to appropriate socket

31. underlying channel perfectly reliable no bit errors no loss of packets separate FSMs for sender, receiver: sender sends data into underlying channel receiver read data from underlying channel Rdt1.0: reliable transfer over a reliable channel rdt_send(data) rdt_rcv(packet) Wait for call from below Wait for call from above extract (packet,data) deliver_data(data) packet = make_pkt(data) udt_send(packet) sender receiver

32. underlying channel may flip bits in packet recall: UDP checksum to detect bit errors the question: how to recover from errors: acknowledgements (ACKs): receiver explicitly tells sender that pkt received OK negative acknowledgements (NAKs): receiver explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK new mechanisms in rdt2.0 (beyond rdt1.0): error detection receiver feedback: control msgs (ACK,NAK) rcvr->sender Rdt2.0: channel with bit errors

33. Wait for ACK or NAK rdt_rcv(rcvpkt) && corrupt(rcvpkt) udt_send(NAK) Wait for call from below rdt2.0: FSM specification rdt_send(data) receiver snkpkt = make_pkt(data, checksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && isNAK(rcvpkt) Wait for call from above udt_send(sndpkt) rdt_rcv(rcvpkt) && isACK(rcvpkt) L sender rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) extract(rcvpkt,data) deliver_data(data) udt_send(ACK)

34. What happens if ACK/NAK corrupted? sender doesn’t know what happened at receiver! can’t just retransmit: possible duplicate Handling duplicates: sender adds sequence number to each pkt sender retransmits current pkt if ACK/NAK garbled receiver discards (doesn’t deliver up) duplicate pkt stop and wait rdt2.0 has a fatal flaw! Sender sends one packet, then waits for receiver response

35. same functionality as rdt2.1, using ACKs only instead of NAK, receiver sends ACK for last pkt received OK receiver must explicitly include seq # of pkt being ACKed duplicate ACK at sender results in same action as NAK: retransmit current pkt rdt2.2: a NAK-free protocol

36. New assumption: underlying channel can also lose packets (data or ACKs) checksum, seq. #, ACKs, retransmissions will be of help, but not enough Approach: sender waits “reasonable” amount of time for ACK retransmits if no ACK received in this time if pkt (or ACK) just delayed (not lost): retransmission will be duplicate, but use of seq. #’s already handles this receiver must specify seq # of pkt being ACKed requires countdown timer rdt3.0: channels with errors and loss

37. Sender: k-bit seq # in pkt header “window” of up to N, consecutive unack’ed pkts allowed Go-Back-N • ACK(n): ACKs all pkts up to, including seq # n - “cumulative ACK” • may deceive duplicate ACKs (see receiver) • Single timer for all in-flight pkts • timeout(n): retransmit pkt n and all higher seq # pkts in window

38. Selective repeat: sender, receiver windows

39. 32 bits source port # dest port # sequence number acknowledgement number head len not used Receive window U A P R S F checksum Urg data pnter Options (variable length) application data (variable length) TCP segment structure URG: urgent data (generally not used) counting by bytes of data (not segments!) ACK: ACK # valid PSH: push data now (generally not used) # bytes rcvr willing to accept RST, SYN, FIN: connection estab (setup, teardown commands) Internet checksum (as in UDP)

40. TCP Round Trip Time and Timeout EstimatedRTT = (1- )*EstimatedRTT + *SampleRTT • Exponential weighted moving average • influence of past sample decreases exponentially fast • typical value:  = 0.125

41. Time-out period often relatively long: long delay before resending lost packet Detect lost segments via duplicate ACKs. Sender often sends many segments back-to-back If segment is lost, there will likely be many duplicate ACKs. If sender receives 3 ACKs for the same data, it supposes that segment after ACKed data was lost: fast retransmit:resend segment before timer expires Fast Retransmit

42. (Suppose TCP receiver discards out-of-order segments) spare room in buffer = RcvWindow = RcvBuffer-[LastByteRcvd - LastByteRead] Rcvr advertises spare room by including value of RcvWindow in segments Sender limits unACKed data to RcvWindow guarantees receive buffer doesn’t overflow TCP Flow control: how it works

43. end-end control (no network assistance) sender limits transmission: LastByteSent-LastByteAcked  CongWin Roughly, CongWin is dynamic, function of perceived network congestion How does sender perceive congestion? loss event = timeout or 3 duplicate acks TCP sender reduces rate (CongWin) after loss event three mechanisms: Slow start AIMD Exponential backoff CongWin rate = Bytes/sec RTT TCP Congestion Control

44. TCP Phases

45. After 3 dup ACKs: CongWin is cut in half window then grows linearly But after timeout event: Enter “slow start” CongWin instead set to 1 MSS; window then grows exponentially to a threshold, then grows linearly Refinement Philosophy: • 3 dup ACKs indicates network capable of delivering some segments • timeout before 3 dup ACKs is “more alarming” • In exponential backoff: • RTO doubled after each successive timeout

46. Two competing sessions: Additive increase gives slope of 1, as throughout increases multiplicative decrease decreases throughput proportionally Why is TCP fair? equal bandwidth share R loss: decrease window by factor of 2 congestion avoidance: additive increase Connection 2 throughput loss: decrease window by factor of 2 congestion avoidance: additive increase Connection 1 throughput R

47. Q:How long does it take to receive an object from a Web server after sending a request? Ignoring congestion, delay is influenced by: TCP connection establishment data transmission delay slow start Notation, assumptions: Assume one link between client and server of rate R S: MSS (bits) O: object size (bits) no retransmissions (no loss, no corruption) Window size: First assume: fixed congestion window, W segments Then dynamic window, modeling slow start Delay modeling - homework

48. First case: WS/R > RTT + S/R: ACK for first segment in window returns before window’s worth of data sent Fixed congestion window (1) delay = 2RTT + O/R

49. Second case: WS/R < RTT + S/R: wait for ACK after sending window’s worth of data sent Fixed congestion window (2) delay = 2RTT + O/R + (K-1)[S/R + RTT - WS/R] Where K=O/WS

50. TCP Delay Modeling: Slow Start (1) Now suppose window grows according to slow start Will show that the delay for one object is: where P is the number of times TCP idles at server: - where Q is the number of times the server idles if the object were of infinite size. - and K is the number of windows that cover the object.