eec-484/584 computer networks lecture 7 wenbing zhao [email protected] (part of the slides are...
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EEC-484/584EEC-484/584Computer NetworksComputer Networks
Lecture 7Lecture 7
Wenbing ZhaoWenbing Zhao
[email protected] [email protected] (Part of the slides are based on Drs. Kurose & (Part of the slides are based on Drs. Kurose &
RossRoss’’s slides for their s slides for their Computer Networking Computer Networking book)book)
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OutlineOutline
• Reliable data tranfer (part II)
• UDP
• TCP– Segment header structure– Connection management
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rdt3.0: channels with errors rdt3.0: channels with errors andand loss 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
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rdt3.0 senderrdt3.0 sender
sndpkt = make_pkt(0, data, checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) ||isACK(rcvpkt,1) )
Wait for call 1 from
above
sndpkt = make_pkt(1, data, checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt,0)
rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) ||isACK(rcvpkt,0) )
rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt,1)
stop_timer
stop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
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rdt3.0 in actionrdt3.0 in action
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rdt3.0 in actionrdt3.0 in action
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Performance of rdt3.0Performance of rdt3.0
• rdt3.0 works, but performance stinks• ex: 1 Gbps link, 15 ms prop. delay, 8000-bit packet:
U sender: utilization – fraction of time sender busy sending
U sender
= .008
30.008 = 0.00027
microseconds
L / R
RTT + L / R =
1KB pkt every 30 msec -> 33kB/sec thruput over 1 Gbps link network protocol limits use of physical resources!
dsmicrosecon8bps10
bits80009
R
Ldtrans
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rdt3.0: stop-and-wait operationrdt3.0: stop-and-wait operation
first packet bit transmitted, t = 0
sender receiver
RTT
last packet bit transmitted, t = L / R
first packet bit arriveslast packet bit arrives, send ACK
ACK arrives, send next packet, t = RTT + L / R
U sender
= .008
30.008 = 0.00027
microseconds
L / R
RTT + L / R =
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Pipelined ProtocolsPipelined ProtocolsPipelining: sender allows multiple, “in-flight”, yet-to-be-
acknowledged pkts– range of sequence numbers must be increased– buffering at sender and/or receiver
• Two generic forms of pipelined protocols: go-back-N, selective repeat
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Pipelining: Increased UtilizationPipelining: Increased Utilization
first packet bit transmitted, t = 0
sender receiver
RTT
last bit transmitted, t = L / R
first packet bit arriveslast packet bit arrives, send ACK
ACK arrives, send next packet, t = RTT + L / R
last bit of 2nd packet arrives, send ACKlast bit of 3rd packet arrives, send ACK
U sender
= .024
30.008 = 0.0008
microseconds
3 * L / R
RTT + L / R =
Increase utilizationby a factor of 3!
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Pipelining ProtocolsPipelining Protocols
Go-back-N: big picture:• Sender can have up to N
unacked packets in pipeline
• Rcvr only sends cumulative acks– Doesn’t ack packet if
there’s a gap
• Sender has timer for oldest unacked packet– If timer expires, retransmit
all unacked packets
Selective Repeat: big pic• Sender can have up to N
unacked packets in pipeline
• Rcvr acks individual packets
• Sender maintains timer for each unacked packet– When timer expires,
retransmit only unack packet
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Go-Back-NGo-Back-NSender:• 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 receive duplicate ACKs (see receiver)
timer for oldest in-flight pkt timeout(n): retransmit pkt n and all higher seq # pkts in window
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GBN: Sender Extended FSMGBN: Sender Extended FSM
Waitstart_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])…udt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum < base+N) { sndpkt[nextseqnum] = make_pkt(nextseqnum,data,chksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) // start timer if first unacked pkt start_timer nextseqnum++ }else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) // no more unacked pkts stop_timer else start_timer
rdt_rcv(rcvpkt) && notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) && corrupt(rcvpkt)
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GBN: Receiver Extended FSMGBN: Receiver Extended FSM
ACK-only: always send ACK for correctly-received pkt with highest in-order seq #– may generate duplicate ACKs– need only remember expectedseqnum
• out-of-order pkt: – discard (don’t buffer) -> no receiver buffering!– Re-ACK pkt with highest in-order seq #
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) && notcurrupt(rcvpkt) && hasseqnum(rcvpkt,expectedseqnum)
extract(rcvpkt,data)deliver_data(data)sndpkt = make_pkt(expectedseqnum,ACK,chksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnum,ACK,chksum)
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GBN inGBN inactionaction
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Selective RepeatSelective 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
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Selective Repeat: Sender, Receiver WindowsSelective Repeat: Sender, Receiver Windows
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Selective RepeatSelective Repeat
data from above :• if next available seq # in
window, send pkt
timeout(n):• resend pkt n, restart timer
ACK(n) in [sendbase,sendbase+N-1]:
• mark pkt n as received• if n smallest unACKed pkt,
advance window base to next unACKed seq #
Senderpkt n in [rcvbase, rcvbase+N-1]
send ACK(n) out-of-order: buffer in-order: deliver (also deliver
buffered, in-order pkts), advance window to next not-yet-received pkt
pkt n in [rcvbase-N,rcvbase-1]
ACK(n)
otherwise: ignore
Receiver
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Selective Repeat In ActionSelective Repeat In Action
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Selective Repeat:Selective Repeat: Dilemma Dilemma
Example: • seq #’s: 0, 1, 2, 3• window size=3
• receiver sees no difference in two scenarios!
• incorrectly passes duplicate data as new in (a)
Q: what relationship between seq # size and window size?
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Non-Sequential Receive ProblemNon-Sequential Receive Problem
• The problem is caused by the overlap of sequence number between the new receiving window and the old receiving window
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
0 1 2 3 4 5 6
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
Overlap Overlap
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Non-Sequential Receive ProblemNon-Sequential Receive Problem
• Solution: – make sure no overlap when receiver advances its
window– Make window size w =1/2 range of seq numbers
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
0 1 2 3
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
No Overlap
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UDP: User Datagram ProtocolUDP: User Datagram Protocol
• “No frills,” “bare bones” Internet transport protocol• “Best effort” service, UDP segments may be:
– Lost– Delivered out of order to app
• Connectionless:– No handshaking between UDP sender, receiver– Each UDP segment handled independently of
others
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Why is There a UDP?Why is There a UDP?
• No connection establishment (which can add delay)
• Simple: no connection state at sender and receiver
• Small segment header
• No congestion control: UDP can blast away as fast as desired
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UDPUDP• Often used for streaming
multimedia apps– Loss tolerant– Rate sensitive
• Other UDP uses– DNS– SNMP
• Reliable transfer over UDP: add reliability at application layer
source port # dest port #
32 bits
Applicationdata
(message)
UDP segment format
length checksumLength, in
bytes of UDPsegment,including
header
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UDP ChecksumUDP Checksum
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:• compute checksum of
received segment• check if computed checksum
equals checksum field value:– NO - error detected– YES - no error detected. But
maybe errors nonetheless?
Goal: detect “errors” (e.g., flipped bits) in transmitted segment
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TCP: OverviewTCP: Overview• Full duplex data:
– Bi-directional data flow in same connection
– MSS: maximum segment size
• Connection-oriented: – Handshaking (exchange
of control msgs) init’s sender, receiver state before data exchange
• Flow controlled:– Sender will not
overwhelm receiver
• Point-to-point:– One sender, one receiver
• Reliable, in-order byte steam:– No “message boundaries”
• Pipelined:– TCP congestion and flow
control set window size
• Send & receive buffers
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TCP: OverviewTCP: Overview
• TCP connection is byte stream, not message stream, no message boundaries
• TCP may send immediately or buffer before sending• Receiver stores the received bytes in a buffer
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TCP Segment StructureTCP Segment Structure
source port # dest port #
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG: urgent data (generally not used)
ACK: ACK #valid
PSH: push data now(generally not used)
RST, SYN, FIN:connection estab(setup, teardown
commands)
# bytes rcvr willingto accept
countingby bytes of data(not segments!)
Internetchecksum
(as in UDP)
A TCP segment must fit into an IP datagram!
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The TCP Segment HeaderThe TCP Segment Header
• Source port and destination port: identify local end points of the connection– Source and destination end points together identify the connection
• Sequence number: identify the byte in the stream of data that the first byte of data in this segment represents
• Acknowledgement number: the next sequence number that the sender of the ack expects to receive– Ack # = Last received seq num + 1– Ack is cumulative: an ack of 5 means 0-4 bytes have been
received
• TCP header length – number of 32-bit words in header
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The TCP Segment HeaderThe TCP Segment Header
• URG – indicates urgent pointer field is set• Urgent pointer – points to the seq num of the last byte in a
sequence of urgent data• ACK – acknowledgement number is valid• SYN – used to establish a connection
– Connection request: ACK = 0, SYN = 1– Connection confirm: ACK=1, SYN = 1
• FIN – release a connection, sender has no more data• RST – reset a connection that is confused• PSH – sender asked to send data immediately
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The TCP Segment HeaderThe TCP Segment Header
• Receiver window size –number of bytes that may be sent beyond the byte acked
• Checksum – add the header, the data, and the conceptual pseudoheader as 16-bit words, take 1’s complement of sum– For more info: http://www.netfor2.com/tcpsum.htm
http://www.netfor2.com/checksum.html
• Options – provides a way to add extra facilities not covered by the regular header– E.g., communicate buffer sizes during set up
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TCP Sequence Numbers and ACKsTCP Sequence Numbers and ACKs
Sequence numbers:– byte stream “number” of
first byte in segment’s data
ACKs:– seq # of next byte
expected from other side
– cumulative ACK
Host A Host B
Seq=42, ACK=79, data = ‘C’
Seq=79, ACK=43, data = ‘C’
Seq=43, ACK=80
Usertypes
‘C’
host ACKsreceipt
of echoed‘C’
host ACKsreceipt of
‘C’, echoesback ‘C’
timesimple telnet/ssh scenario
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TCP Connection ManagementTCP Connection Management
TCP sender, receiver establish “connection” before exchanging data segments
• Initialize TCP variables:– Sequence numbers– Buffers, flow control info (e.g. RcvWindow)
• Client: connection initiator Socket clientSocket = new
Socket("hostname","port number"); • Server: contacted by client Socket connectionSocket = welcomeSocket.accept();
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TCP Connection ManagementTCP Connection Management
Three way handshake:
Step 1: client host sends TCP SYN segment to server
– specifies initial sequence number
– no data
Step 2: server host receives SYN, replies with SYN/ACK segment
– server allocates buffers
– specifies server initial sequence number
Step 3: client receives SYN/ACK, replies with ACK segment, which may contain data
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TCP Connection ManagementTCP Connection Management
Three way handshake:• SYN segment is
considered as 1 byte• SYN/ACK segment is also
considered as 1 byte
client
SYN (seq=x)
server
SYN/ACK (seq=y, ACK=x+1)
ACK (seq=x+1, ACK=y+1)
connect accept
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TCP Connection ManagementTCP Connection Management
Closing a connection:
client closes socket: clientSocket.close();
Step 1: client end system sends TCP FIN control segment to
server
Step 2: server receives FIN, replies with ACK. Closes connection, sends FIN.
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
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TCP Connection ManagementTCP Connection Management
Step 3: client receives FIN, replies with ACK.
– Enters “timed wait” - will respond with ACK to received FINs
Step 4: server, receives ACK. Connection closed.
Note: with small modification, can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
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ExerciseExercise
• A process at host A wants to establish a TCP connection with another process at host B. Assuming that host A chooses to use 1628 as the initial sequence number, and host B chooses to use 3217 as the initial sequence number for this connection, show the segments involved with the connection establishment process. You must include the following information for each such segment: (1) sequence number, (2) acknowledgement number (if applicable), (3) the SYN flag bit status, and (4) the ACK flag bit status.
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