Midterm%20Review

1
Midterm Review

• In class, 930-11 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
Packet Switching Statistical Multiplexing
10 Mbs Ethernet
C
A
statistical multiplexing
1.5 Mbs
B
queue of packets waiting for output link

• Sequence of A B packets does not have fixed
  pattern ? statistical multiplexing.
• In TDM each host gets same slot in revolving TDM
  frame.

5
Packet Switching versus Circuit Switching

• 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

6
Packet Switching versus Circuit Switching

• Packet switching allows more users to use network!

• 1 Mbit link
• Each user
• 100 kbps when active
• active 10 of time
• Circuit-switching
• 10 users
• Packet switching
• with 35 users, probability gt 10 active less than
  .0004

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
Internet structure network of networks

• Tier-3 ISPs and local ISPs
• last hop (access) network (closest to end
  systems)
• Tier-3 Turkish Telecom, Minnesota Regional
  Network

Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
11
Four sources of packet delay

• 1. processing
• check bit errors
• determine output link

• 2. queueing
• time waiting at output link for transmission
• depends on congestion level of router

12
Delay in packet-switched networks

• 4. Propagation delay
• d length of physical link
• s propagation speed in medium (2x108 m/sec)
• propagation delay d/s

• 3. Transmission delay
• Rlink bandwidth (bps)
• Lpacket length (bits)
• time to send bits into link L/R

Note s and R are very different quantities!
13
Internet protocol stack

• 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

14
Application Layer

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

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
Internet transport protocols services

• 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?

• TCP service
• connection-oriented setup required between
  client and server processes
• reliable transport between sending and receiving
  process
• flow control sender wont overwhelm receiver
• congestion control throttle sender when network
  overloaded
• does not provide timing, minimum bandwidth
  guarantees

20
HTTP connections

• 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 Message, Format, Response, Methods
• HTTP cookies

21
Response Time of HTTP

• 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

• 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

22
Web caches (proxy server)
Goal satisfy client request without involving
origin server

• 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?

origin server
Proxy server
HTTP request
HTTP request
client
HTTP response
HTTP response
HTTP request
HTTP response
client
origin server
23
Caching example (3)

• 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
• .62 sec .6.01 secs milliseconds lt 1.3
  secs

origin servers
public Internet
1.5 Mbps access link
institutional network
10 Mbps LAN
institutional cache
24
FTP separate control, data connections

• 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.

• 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
Electronic Mail mail servers

• Mail Servers
• mailbox contains incoming messages for user
• message queue 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

26
DNS

• 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_at_hotmail.com
• Load distribution
• Replicated Web servers set of IP addresses for
  one canonical name
• E.g., cnn.com

• Why not centralize DNS?
• single point of failure
• traffic volume
• distant centralized database
• maintenance
• doesnt scale!

27
DNS example
root name server

• Root name server
• may not know authoritative name server
• may know intermediate name server who to contact
  to find authoritative 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
DNS iterated queries
root name server

• 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 dont know this name, but ask this 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

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

30
Demultiplexing

• 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

• 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
Rdt1.0 reliable transfer over a reliable channel

• 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

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
Rdt2.0 channel with bit errors

• 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-gtsender

33
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(A
CK)
34
rdt2.0 has a fatal flaw!

• What happens if ACK/NAK corrupted?
• sender doesnt know what happened at receiver!
• cant just retransmit possible duplicate

• Handling duplicates
• sender adds sequence number to each pkt
• sender retransmits current pkt if ACK/NAK garbled
• receiver discards (doesnt deliver up) duplicate
  pkt

Sender sends one packet, then waits for receiver
response
35
rdt2.2 a NAK-free protocol

• 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

36
rdt3.0 channels with errors and loss

• 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

37
Go-Back-N

• Sender
• k-bit seq in pkt header
• window of up to N, consecutive unacked pkts
  allowed

• 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
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
Fast Retransmit

• 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

42
TCP Flow control how it works

• Rcvr advertises spare room by including value of
  RcvWindow in segments
• Sender limits unACKed data to RcvWindow
• guarantees receive buffer doesnt overflow

• (Suppose TCP receiver discards out-of-order
  segments)
• spare room in buffer
• RcvWindow
• RcvBuffer-LastByteRcvd - LastByteRead

43
TCP Congestion Control

• 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

44
TCP Phases
45
Refinement
Philosophy

• 3 dup ACKs indicates network capable of
  delivering some segments
• timeout before 3 dup ACKs is more alarming

• 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

• In exponential backoff
• RTO doubled after each successive timeout

46
Why is TCP fair?

• Two competing sessions
• Additive increase gives slope of 1, as throughout
  increases
• multiplicative decrease decreases throughput
  proportionally

R
equal bandwidth share
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
Delay modeling - homework

• 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

• 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

48
Fixed congestion window (1)

• First case
• WS/R gt RTT S/R ACK for first segment in window
  returns before windows worth of data sent

delay 2RTT O/R
49
Fixed congestion window (2)

• Second case
• WS/R lt RTT S/R wait for ACK after sending
  windows worth of data sent

delay 2RTT O/R (K-1)S/R RTT - WS/R
Where KO/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.
51
TCP Delay Modeling Slow Start (2)

• Delay components
• 2 RTT for connection estab and request
• O/R to transmit object
• time server idles due to slow start
• Server idles P minK-1,Q times

• Example
• O/S 15 segments
• K 4 windows
• Q 2
• P minK-1,Q 2
• Server idles P2 times

52
TCP Delay Modeling (3)
53
TCP Delay Modeling (4)
Recall K number of windows that cover
object How do we calculate K ?
Calculation of Q, number of idles for
infinite-size object, is similar (see HW).