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Midterm Review

Midterm Review. In class Closed Book One 8.5 ” by 11 ” sheet of paper permitted (single side) Calculators Recommended. 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.

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Midterm Review

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  1. Midterm Review In class Closed Book One 8.5” by 11” sheet of paper permitted (single side) Calculators Recommended

  2. 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

  3. 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

  4. 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

  5. “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

  6. 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

  7. 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!

  8. application: supporting network applications FTP, SMTP, STTP 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

  9. 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

  10. 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

  11. 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

  12. uses TCP to reliably transfer email message from client to server, port 25 direct transfer: sending server to receiving server user agent user agent user agent user agent user agent user agent SMTP SMTP mail server mail server mail server outgoing message queue user mailbox Electronic Mail: SMTP [RFC 2821]

  13. no server has all name-to-IP address mappings local name servers: each ISP, company has local (default) name server host DNS query first goes to local name server authoritative name server: for a host: stores that host’s IP address, name can perform name/address translation for that host’s name Why not centralize DNS? single point of failure traffic volume distant centralized database maintenance doesn’t scale! DNS name servers

  14. 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

  15. 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

  16. 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 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

  17. 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

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

  19. 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

  20. Why is there a UDP? no connection establishment (which can add delay) simple: no connection state at sender, receiver small segment header no congestion control: UDP can blast away as fast as desired UDP: User Datagram Protocol [RFC 768] 32 bits source port # dest port # checksum length Application data (message) UDP segment format

  21. Sender: treat segment contents as sequence of 16-bit integers checksum: addition (1’s complement sum) of segment contents sender puts checksum value into UDP checksum field Receiver: addition of all segment contents + checksum check if all bits are 1: NO - error detected YES - no error detected. But maybe errors nonetheless? More later …. UDP checksum Goal: detect “errors” (e.g., flipped bits) in transmitted segment 0110 0101 1011 0100 0110 0101 0100 1111 1’s complement sum: Addition: Addition: 1’s complement sum:

  22. 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

  23. receiver individually acknowledges all correctly received pkts buffers pkts, as needed, for eventual in-order delivery to upper layer sender only resends pkts for which ACK not received sender timer for each unACKed pkt sender window N consecutive seq #’s again limits seq #s of sent, unACKed pkts Selective Repeat

  24. Selective repeat: sender, receiver windows

  25. 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)

  26. End-end congestion control: no explicit feedback from network congestion inferred from end-system observed loss, delay approach taken by TCP Network-assisted congestion control: routers provide feedback to end systems single bit indicating congestion (SNA, DECbit, TCP/IP ECN, ATM) explicit rate sender should send at Approaches towards congestion control Two broad approaches towards congestion control:

  27. When connection begins, increase rate exponentially until first loss event: double CongWin every RTT done by incrementing CongWin for every ACK received Summary: initial rate is slow but ramps up exponentially fast time TCP Slow Start (more) Host A Host B one segment RTT two segments four segments

  28. TCP sender congestion control

  29. 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

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