Transcript Review for midterm

Midterm Review
In class, 1:00-2:30 pm, Tu. 2/6
Closed Book
One 8.5” by 11” sheet of paper
permitted (single side)
Lecture 1
• Internet Architecture
• Network Protocols
• Network Edge
• A taxonomy of communication networks
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
Switched
Communication
Network
Circuit-Switched
Communication
Network
TDM
FDM
Broadcast
Communication
Network
Packet-Switched
Communication
Network
Datagram
Network
Virtual Circuit Network
Packet Switching: Statistical Multiplexing
10 Mbs
Ethernet
A
B
statistical multiplexing
C
1.5 Mbs
queue of packets
waiting for output
link
D
E
Sequence of A & B packets does not have fixed
pattern  statistical multiplexing.
In TDM each host gets same slot in revolving TDM
frame.
Packet Switching versus Circuit Switching
• Packet Switching
– Network resources (e.g., bandwidth) divided into
“pieces” for allocation
– Resource piece idle if not used by owning call (no
sharing)
– Dividing link bandwidth into “pieces”
– Great for bursty data
– NOT efficient !
• Circuit Switching:
– Excessive congestion: packet delay and loss
– protocols needed for reliable data transfer,
congestion control
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
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
• Example: ATM (Asynchronous Transfer Mode)
networks
Lecture 2
• Network access and physical media
• Internet structure and ISPs
• Delay & loss in packet-switched
networks
• Protocol layers, service models
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
local
ISP
Local and tier3 ISPs are
customers of
higher tier
ISPs
connecting
them to rest
of Internet
Tier 3
local
local
ISP
Tier-2 ISP
ISP
ISP
ISP
Tier-2 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
local
local
ISP
ISP
local
NAP
Tier 1 ISP
Tier-2 ISP
local
ISP
Tier-2 ISP
local
ISP
Four sources of packet delay
• 1. processing:
• 2. queueing
– check bit errors
– time waiting at output
link for transmission
– determine output link
– depends on congestion
level of router
transmission
A
propagation
B
processing
queueing
Delay in packet-switched networks
3. Transmission delay:
4. Propagation delay:
• R=link bandwidth (bps)
• d = length of physical link
• L=packet length (bits)
• s = propagation speed in
medium (~2x108 m/sec)
• time to send bits into
link = L/R
transmission
A
• propagation delay = d/s
Note: s and R are very
different quantities!
propagation
B
processing
queueing
Internet protocol stack
• 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
Application Layer
• Principles of app layer protocols
• Web and HTTP
• FTP
• Electronic Mail: SMTP, POP3, IMAP
• DNS
• Socket Programming
• Web Caching
HTTP connections
Nonpersistent HTTP
Persistent HTTP
• At most one object is
sent over a TCP
connection.
• Multiple objects can be
sent over single TCP
connection between
client and server.
• HTTP/1.0 uses
nonpersistent HTTP
• HTTP/1.1 uses
persistent connections
in default mode
• HTTP Message, Format, Response, Methods
• HTTP cookies
Response Time of HTTP
Nonpersistent HTTP issues:
Persistent without pipelining:
• requires 2 RTTs per object
• client issues new request
only when previous
response has been received
• 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
• 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
FTP: separate control, data connections
• FTP client contacts FTP server
at port 21, specifying TCP as
transport protocol
• Client obtains authorization over FTP
control connection
client
TCP control connection
port 21
TCP data connection
port 20
FTP
server
• Client browses remote directory
• Server opens a second TCP
by sending commands over
data connection to transfer
control connection.
another file.
• When server receives a
• Control connection: “out of
command for a file transfer, the
band”
server opens a TCP data
connection to client
• FTP server maintains “state”:
current directory, earlier
• After transferring one file,
authentication
server closes connection.
Electronic Mail: SMTP [RFC 2821]
• uses TCP to reliably transfer email message from client to server,
port 25
• direct transfer: sending server to receiving server
• three phases of transfer
– handshaking (greeting)
– transfer of messages
– closure
• command/response interaction
– commands: ASCII text
– response: status code and phrase
• messages must be in 7-bit ASCII
DNS name servers
• no server has all name-to-IP
Why not centralize DNS?
address mappings
• single point of failure
• traffic volume
• distant centralized
database
• maintenance
doesn’t scale!
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
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
7
local name server
dns.eurecom.fr
1
8
3
intermediate name server
dns.nwu.edu
4
5
authoritative name server
dns.cs.nwu.edu
requesting host
surf.eurecom.fr
www.cs.nwu.edu
DNS: iterated queries
recursive query:
root name server
2
• 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”
iterated query
3
4
7
local name server
dns.eurecom.fr
1
8
intermediate name server
dns.umass.edu
5
6
authoritative name server
dns.cs.umass.edu
requesting host
surf.eurecom.fr
gaia.cs.umass.edu
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
origin
server
client
Proxy
server
– object in cache: cache
returns object
– else cache requests object
from origin server, then
returns object to client
• Why web caching?
client
origin
server
Caching example (3)
Install cache
origin
servers
• suppose hit rate is .4
Consequence
• 40% requests will be satisfied
almost immediately
public
Internet
• 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
1.5 Mbps
access link
institutional
network
10 Mbps LAN
institutional
cache
Transport Layer
• Transport-layer services
• Multiplexing and demultiplexing
• Connectionless transport: UDP
• Principles of reliable data transfer
• TCP
– Segment structures
– Flow control
– Congestion control
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
UDP: User Datagram Protocol [RFC 768]
32 bits
source port #
dest port #
length
checksum
Application
data
(message)
UDP segment format
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 checksum
Goal: detect “errors” (e.g., flipped bits) in transmitted
segment
Receiver:
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
Addition:
1’s complement sum:
0110
0101
1011
0100
• 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
….
1’s complement sum:
Addition:
0110
0101
0100
1111
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
Wait for
call from
above
rdt_send(data)
packet = make_pkt(data)
udt_send(packet)
sender
Wait for
call from
below
rdt_rcv(packet)
extract (packet,data)
deliver_data(data)
receiver
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->sender
rdt2.0: FSM specification
rdt_send(data)
snkpkt = make_pkt(data, checksum)
udt_send(sndpkt)
rdt_rcv(rcvpkt) &&
isNAK(rcvpkt)
Wait for
Wait for
call from
ACK or
udt_send(sndpkt)
above
NAK
rdt_rcv(rcvpkt) && isACK(rcvpkt)
L
sender
receiver
rdt_rcv(rcvpkt) &&
corrupt(rcvpkt)
udt_send(NAK)
Wait for
call from
below
rdt_rcv(rcvpkt) &&
notcorrupt(rcvpkt)
extract(rcvpkt,data)
deliver_data(data)
udt_send(ACK)
rdt2.0 has a fatal flaw!
What happens if ACK/NAK
corrupted?
• sender doesn’t know what
happened at receiver!
• can’t just retransmit:
possible duplicate
What to do?
• sender ACKs/NAKs
receiver’s ACK/NAK? What
if sender ACK/NAK lost?
• retransmit, but this might
cause retransmission of
correctly received pkt!
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
Sender sends one packet,
then waits for receiver
response
rdt2.1: sender, handles garbled ACK/NAKs
rdt_send(data)
sndpkt = make_pkt(0, data, checksum)
udt_send(sndpkt)
rdt_rcv(rcvpkt) &&
( corrupt(rcvpkt) ||
Wait
for
Wait for
isNAK(rcvpkt) )
ACK or
call 0 from
udt_send(sndpkt)
NAK 0
above
rdt_rcv(rcvpkt)
&& notcorrupt(rcvpkt)
&& isACK(rcvpkt)
rdt_rcv(rcvpkt)
&& notcorrupt(rcvpkt)
&& isACK(rcvpkt)
L
rdt_rcv(rcvpkt) &&
( corrupt(rcvpkt) ||
isNAK(rcvpkt) )
udt_send(sndpkt)
L
Wait for
ACK or
NAK 1
Wait for
call 1 from
above
rdt_send(data)
sndpkt = make_pkt(1, data, checksum)
udt_send(sndpkt)
rdt2.2: a NAK-free protocol
• same functionality as rdt2.1, using NAKs 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
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
Q: how to deal with loss?
– sender waits until it is
certain that data or ACK
lost, then retransmits
– yuck: drawbacks?
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 sender
rdt_send(data)
sndpkt = make_pkt(0, data, checksum)
udt_send(sndpkt)
start_timer
rdt_rcv(rcvpkt)
L
rdt_rcv(rcvpkt)
&& notcorrupt(rcvpkt)
&& isACK(rcvpkt,1)
rdt_rcv(rcvpkt) &&
( corrupt(rcvpkt) ||
isACK(rcvpkt,0) )
timeout
udt_send(sndpkt)
start_timer
rdt_rcv(rcvpkt)
&& notcorrupt(rcvpkt)
&& isACK(rcvpkt,0)
stop_timer
stop_timer
timeout
udt_send(sndpkt)
start_timer
L
Wait
for
ACK0
Wait for
call 0from
above
L
rdt_rcv(rcvpkt) &&
( corrupt(rcvpkt) ||
isACK(rcvpkt,1) )
Wait
for
ACK1
Wait for
call 1 from
above
rdt_send(data)
sndpkt = make_pkt(1, data, checksum)
udt_send(sndpkt)
start_timer
rdt_rcv(rcvpkt)
L
Go-Back-N
Sender:
• k-bit seq # in pkt header
• “window” of up to N, consecutive unack’ed 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
Selective Repeat
• 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: sender, receiver windows
TCP segment structure
URG: urgent data
(generally not used)
ACK: ACK #
valid
PSH: push data now
(generally not used)
RST, SYN, FIN:
connection estab
(setup, teardown
commands)
Internet
checksum
(as in UDP)
32 bits
source port #
dest port #
sequence number
acknowledgement number
head not
UA P R S F
len used
checksum
Receive window
Urg data pnter
Options (variable length)
application
data
(variable length)
counting
by bytes
of data
(not segments!)
# bytes
rcvr willing
to accept
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
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
TCP Flow control: how it works
• Rcvr advertises spare
room by including value
of RcvWindow in
segments
(Suppose TCP receiver
discards out-of-order
segments)
• spare room in buffer
= RcvWindow
= RcvBuffer-[LastByteRcvd LastByteRead]
• Sender limits unACKed
data to RcvWindow
– guarantees receive
buffer doesn’t overflow
TCP Congestion Control
• end-end control (no network
assistance)
How does sender
perceive congestion?
• sender limits transmission:
• loss event = timeout or
3 duplicate acks
LastByteSent-LastByteAcked
 CongWin
• Roughly,
rate =
CongWin
Bytes/sec
RTT
• CongWin is dynamic, function
of perceived network
congestion
• TCP sender reduces
rate (CongWin) after
loss event
three mechanisms:
– AIMD
– slow start
– conservative after
timeout events
TCP Slow Start
• When connection begins, • When connection begins,
increase rate
CongWin = 1 MSS
exponentially fast until
– Example: MSS = 500
first loss event
bytes & RTT = 200 msec
– initial rate = 20 kbps
• available bandwidth may
be >> MSS/RTT
– desirable to quickly ramp
up to respectable rate
Refinement
• 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
Philosophy:
• 3 dup ACKs indicates
network capable of
delivering some segments
• timeout before 3 dup
ACKs is “more alarming”
Why is TCP fair?
Two competing sessions:
• Additive increase gives slope of 1, as throughout increases
• multiplicative decrease decreases throughput proportionally
equal bandwidth share
R
loss: decrease window by factor of 2
congestion avoidance: additive increase
loss: decrease window by factor of 2
congestion avoidance: additive increase
Connection 1 throughput R