9/30/2003-10/2/2003 the transport layer september 30-october 2, 2003

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The Transport Layer

September 30-October 2, 2003

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Assignments

• Homework 3– Chapter 3 review

• Finish 3.1-3.4

• Read 3.5-3.7 for next week

http://www.mtholyoke.edu/courses/srollins/cs325/web/assignments/hw3.html

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Transport Layer

• Goals– Understand transport layer services

• Multiplexing/demultiplexing• Reliable data transfer• Flow control• Congestion control

– Learn about Internet transport protocols• TCP – connection-oriented• UDP – connectionless

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Transport Services

• Provide logical communication between app processes running on different hosts

• Transport protocols run in end systems – send side: breaks app

messages into segments, passes to network layer

– rcv side: reassembles segments into messages, passes to app layer

• More than one transport protocol available to apps– Internet: TCP and UDP

application

transportnetworkdata linkphysical

application


networkdata linkphysical



networkdata linkphysicalnetwork

data linkphysical

logical end-end transport

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Transport Services

• Network layer– logical communication

between hosts

• Transport layer– logical communication

between processes – relies on, enhances,

network layer services

• No transport layer in the routers

application


application






data linkphysical


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Internet Transport Protocols

• TCP – reliable, in-order – congestion control – flow control– connection setup

• UDP – unreliable, unordered– error checking

• No delay or bw guarantees

application


application






data linkphysical


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Multiplexing/Demultiplexing

application

transport

network

link

physical

P1 application

transport

network

link

physical

application

transport

network

link

physical

P2P3 P4P1

host 1 host 2 host 3

= process= socket

delivering received segmentsto correct socket

Demultiplexing at rcv host:gathering data from multiplesockets, enveloping data with header (later used for demultiplexing)

Multiplexing at send host:

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Multiplexing

• Transport layer takes the application message and creates a segment

• Why do I need both src and dest ports?

• What else might go in header?

source port # dest port #

32 bits

applicationdata

(message)

other header fields

TCP/UDP segment format

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Demultiplexing

• Transport layer receives segment

• Uses port numbers (and sometimes IP addresses) to pass to appropriate socket


32 bits

applicationdata

(message)

other header fields

TCP/UDP segment format

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Connectionless Demultiplexing

• Create sockets with port numbers

DatagramSocket mySocket1 = new DatagramSocket(99111);– This is port for client – how do I

choose the port number?

• 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

• All datagrams with same dest IP and port are directed to same socket

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Connectionless Demux

DatagramSocket serverSocket = new DatagramSocket(6428);

ClientIP:B

P3

client IP: A

P1P1P3

serverIP: C

SP: 6428

DP: 9157

SP: 9157

DP: 6428

SP: 6428

DP: 5775

SP: 5775

DP: 6428

SP provides “return address”

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Connection-Oriented Demux

• 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

• Server host may support many simultaneous TCP sockets:– each socket identified

by its own 4-tuple

• Web servers have different sockets for each connecting client

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Connection-Oriented Demux

ClientIP:B

P3

client IP: A

P1P1P3

serverIP: C

SP: 80

DP: 9157

SP: 9157

DP: 80

SP: 80

DP: 5775

SP: 5775

DP: 80

P4

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UDP: User Datagram Protocol

• Mux/Demux• Light error checking

• DNS uses UDP – why?

• Characteristics of UDP applications?

Why is there a UDP?

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UDP: User Datagram Protocol

• Mux/Demux• Light error checking

• DNS uses UDP – why?

• Characteristics of UDP applications?

• 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

• Reliable UDP?

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UDP Header

• Length – length of UDP segment

• Checksum – error detection – Send some extra

information – rcvr uses to determine whether a bit has been flipped


32 bits

Applicationdata

(message)

UDP segment format

length checksum

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UDP 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?

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Checksum Example

0110011001100110

0101010101010101

----------------------------

1011101110111011

0000111100001111

----------------------------

1100101011001010

0011010100110101 (Checksum)

0110011001100110

0101010101010101

----------------------------

1011101110111011

0000111100001111

----------------------------

1100101011001010

0011010100110101

----------------------------

1111111111111111

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Reliable Data Transfer

• No bits Corrupted• No bits lost• Bits delivered in-order

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Getting Started• Develop sender, receiver sides of reliable

data transfer protocol (rdt)• Consider only unidirectional data transfer

– but control info will flow on both directions!

• Use finite state machines (FSM) to specify sender, receiver

state1

state2

event causing state transitionactions taken on state transition

state: when in this “state” next state

uniquely determined by next event

eventactions

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rdt1.0: Reliable Transfer over Reliable Channel

Wait for call from above packet = make_pkt(data)

udt_send(packet)

rdt_send(data)

extract (packet,data)deliver_data(data)

Wait for call from

below

rdt_rcv(packet)

sender receiver

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rdt2.0: Channel with Bit Errors

• Underlying channel may flip bits in packet

• What does the protocol need to do?

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rdt2.0: Channel with Bit Errors

• Underlying channel may flip bits in packet

• What does the protocol need to do?– Error detection– Receiver feedback– Retransmission

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Techniques

• Error detection– Remember UDP

• Receiver feedback– ACK – “Yeah, that came through OK”– NAK – “Can you repeat that?”– ACK/NAK tradeoffs?

• Retransmission– ARQ – sender retransmits packet(s) in error

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rdt2.0: FSM specification

Wait for call from above

snkpkt = make_pkt(data, checksum)udt_send(sndpkt)

extract(rcvpkt,data)deliver_data(data)udt_send(ACK)

rdt_rcv(rcvpkt) && notcorrupt(rcvpkt)

rdt_rcv(rcvpkt) && isACK(rcvpkt)

udt_send(sndpkt)

rdt_rcv(rcvpkt) && isNAK(rcvpkt)

udt_send(NAK)

rdt_rcv(rcvpkt) && corrupt(rcvpkt)

Wait for ACK or

NAK

Wait for call from

belowsender

receiverrdt_send(data)

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rdt2.0

• Stop-and-Wait– Sender sends one packet, then waits for

receiver response

• Flaw – what if ACK/NAK is corrupt?– ACK/NAK the ACK/NAK– Error correction– Resend data packet

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Handling Duplicate Packets

• Sequence Number– Lets the receiver know if the packet is a

duplicate

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rdt2.1: Sender Handles Garbled ACK/NAKs

Wait for call 0 from

above

sndpkt = make_pkt(0, data, checksum)udt_send(sndpkt)

rdt_send(data)

Wait for ACK or NAK 0 udt_send(sndpkt)

rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) ||isNAK(rcvpkt) )


rdt_send(data)

rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt)

udt_send(sndpkt)

rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) ||isNAK(rcvpkt) )

rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt)


above

Wait for ACK or NAK 1

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rdt2.1: Receiver Handles Garbled ACK/NAKs

Wait for 0 from below

sndpkt = make_pkt(NAK, chksum)udt_send(sndpkt)

rdt_rcv(rcvpkt) && not corrupt(rcvpkt) && has_seq0(rcvpkt)

rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq1(rcvpkt)

extract(rcvpkt,data)deliver_data(data)sndpkt = make_pkt(ACK, chksum)udt_send(sndpkt)



extract(rcvpkt,data)deliver_data(data)sndpkt = make_pkt(ACK, chksum)udt_send(sndpkt)

rdt_rcv(rcvpkt) && (corrupt(rcvpkt)

sndpkt = make_pkt(ACK, chksum)udt_send(sndpkt)

rdt_rcv(rcvpkt) && not corrupt(rcvpkt) && has_seq1(rcvpkt)

rdt_rcv(rcvpkt) && (corrupt(rcvpkt)

sndpkt = make_pkt(ACK, chksum)udt_send(sndpkt)

sndpkt = make_pkt(NAK, chksum)udt_send(sndpkt)

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rdt2.1: Discussion

• Sender– seq # added to pkt– two seq. #’s (0,1) will

suffice. Why?– must check if received

ACK/NAK corrupted – twice as many states

• state must “remember” whether “current” pkt has 0 or 1 seq. #

• Receiver– must check if received

packet is duplicate• state indicates whether

0 or 1 is expected pkt seq #

– note: receiver can not know if its last ACK/NAK received OK at sender

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rdt2.2: A NAK-free Protocol

• Why go NAK-free?• Instead of NAK, send ACK for last packet

received• When sender receives duplicate ACK –

retransmit

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rdt2.2: Sender, Receiver Fragments


above


rdt_send(data)

udt_send(sndpkt)

rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) || isACK(rcvpkt,1) )

rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt,0)

Wait for ACK

0

sender FSMfragment



extract(rcvpkt,data)deliver_data(data)sndpkt = make_pkt(ACK1, chksum)udt_send(sndpkt)

rdt_rcv(rcvpkt) && (corrupt(rcvpkt) || has_seq1(rcvpkt))

udt_send(sndpkt)

receiver FSMfragment

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rdt3.0: Channels with Errors and Loss

• Channel can lose packets• 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 in Action

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Performance of rdt3.0

• Alternating Bit Protocol

• Poor utilization

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rdt3.0: 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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Pipelining• Pipelining – sender allows multiple, “in-

flight”, yet-to-be-acknowledged pkts– range of sequence numbers must be increased– buffering at sender and/or receiver

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Increased Utilizationfirst 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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Pipelined Protocols

• Go-Back-N– If timeout occurs, resend ALL outstanding

packets

• Selective Repeat– Retransmit only packets that were lost or

received in error

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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

• timer for each in-flight pkt• timeout(n): retransmit pkt n and all higher seq # pkts in

window

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GBN

Receiver

• 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) – isn’t this bad?– Re-ACK pkt with highest in-order seq #

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GBN Example

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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

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Selective repeat: sender, receiver windows

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Selective Repeat

Sender• Receive data from app

– if next available seq # in window, send pkt

• timeout(n)– resend pkt n, restart timer

• ACK(n) in [sendbase,sendbase+N]:– mark pkt n as received– if n smallest unACKed pkt,

advance window base to next unACKed seq #

Receiver• Received pkt 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

• Received pkt n in [rcvbase-N,rcvbase-1]

– ACK(n)– Why???

• Otherwise – ignore

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Selective repeat in action

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Window size vs Sequence Number Space

9/30/2003-10/2/2003 the transport layer september 30-october 2, 2003

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