Transport Layer

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Chapter 3 Transport Layer

Computer Networking:

A Top Down Approach Featuring the Internet, 3^rd edition.

Jim Kurose, Keith Ross Addison-Wesley, July 2004.

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Chapter 3: Transport Layer

Our goals:

understand principles behind transport

layer services:

multiplexing/demultipl exing

reliable data transfer

flow control

congestion control

learn about transport layer protocols in the Internet:

UDP: connectionless transport

TCP: connection-oriented transport

TCP congestion control

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Chapter 3 outline

3.1 Transport-layer services

3.2 Multiplexing and demultiplexing

3.3 Connectionless transport: UDP

3.4 Principles of

reliable data transfer

3.5 Connection-oriented transport: TCP

segment structure

flow control

connection management

3.6 Principles of congestion control

3.7 TCP congestion

control

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Transport services and protocols

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

application transport

network data link physical

network data link physical network

data link physical

data link physical network data link physical network

data link physical

logical end -end t

ransport

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Transport vs. network layer

network layer: logical communication

between hosts

transport layer: logical communication

between processes

relies on, enhances, network layer services

Household analogy:

12 kids sending letters to 12 kids

processes = kids

app messages = letters in envelopes

hosts = houses

transport protocol = Ann and Bill

network-layer protocol

= postal service

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Internet transport-layer protocols

reliable, in-order delivery (TCP)

congestion control

flow control

connection setup

unreliable, unordered delivery: UDP

no-frills extension of

“best-effort” IP

services not available:

delay guarantees

network data link physical

data link physical

data link physical network data link physical network

data link physical

logical end -end t

ransport

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Chapter 3 outline

3.1 Transport-layer services

3.2 Multiplexing and demultiplexing

3.3 Connectionless transport: UDP

3.4 Principles of

reliable data transfer

3.5 Connection-oriented transport: TCP

segment structure

flow control

3.6 Principles of congestion control

3.7 TCP congestion

control

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Multiplexing/demultiplexing

application transport network link

P1 application

transport network link physical application

transport network

link

P3 P1 P2 P4

= process

= socket

delivering received segments to correct socket

Demultiplexing at rcv host:

gathering data from multiple sockets, enveloping data with header (later used for

demultiplexing)

Multiplexing at send host:

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How demultiplexing works

host receives IP datagrams

each datagram has source IP address, destination IP address

each datagram carries 1 transport-layer segment

each segment has source, destination port number (recall: well-known port numbers for specific applications)

host uses IP addresses and port numbers to direct

segment to appropriate socket

source port # dest port # 32 bits

application data (message)

other header fields

TCP/UDP segment format

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Connectionless demultiplexing

Create sockets with port numbers:

DatagramSocket mySocket1 = new DatagramSocket(99111);

DatagramSocket mySocket2 = new DatagramSocket(99222);

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

IP datagrams with different source IP

addresses and/or source

port numbers directed

to same socket

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Connectionless demux (cont)

DatagramSocket serverSocket = new DatagramSocket(6428);

Client IP:B

P2

client IP: A

P3 P1P1

server IP: 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

non-persistent HTTP will have different socket for each request

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Connection-oriented demux (cont)

Client

IP:B P1

client IP: A

P2 P1 P4

server IP: C

SP: 9157 DP: 80

P5 P6 P3

D-IP:C S-IP: A

D-IP:C

S-IP: B SP: 5775

DP: 80 D-IP:C S-IP: B

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Chapter 3 outline

3.1 Transport-layer services

3.2 Multiplexing and demultiplexing

3.3 Connectionless transport: UDP

3.4 Principles of

reliable data transfer

3.5 Connection-oriented transport: TCP

segment structure

flow control

3.6 Principles of congestion control

3.7 TCP congestion

control

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UDP: User Datagram Protocol ^{[RFC 768]}

“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

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

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UDP: more

often used for streaming multimedia apps

loss tolerant

rate sensitive

other UDP uses

DNS

SNMP

reliable transfer over UDP:

add reliability at application layer

application-specific error recovery!

source port # dest port # 32 bits

Application data (message)

length checksum Length, in

bytes of UDP segment, including header

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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

Goal: detect “errors” (e.g., flipped bits) in transmitted

segment

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Internet Checksum Example

Note

When adding numbers, a carryout from the most significant bit needs to be added to the result

Example: add two 16-bit integers

1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 1 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1 wraparound

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Chapter 3 outline

3.1 Transport-layer services

3.2 Multiplexing and demultiplexing

3.3 Connectionless transport: UDP

3.4 Principles of

reliable data transfer

3.5 Connection-oriented transport: TCP

segment structure

flow control

3.6 Principles of congestion control

3.7 TCP congestion

control

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Principles of Reliable Data Transfer

important in app., transport, link layers

top-10 list of important networking topics!

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Reliable data transfer: getting started

send side receive

side

rdt_send(): called from above, (e.g., by app.). Passed data to deliver to receiver upper layer

udt_send(): called by rdt, to transfer packet over unreliable channel to receiver

rdt_rcv(): called when packet arrives on rcv-side of channel deliver_data(): called by

rdt to deliver data to upper

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Reliable data transfer: getting started

We’ll:

incrementally 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

state

1 state

event causing state transition actions taken on state transition

event

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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 reads data from underlying 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

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

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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

below

sender

receiver

rdt_send(data)

Λ

Sender sends one packet, then waits for receiver response

stop and wait

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rdt2.0: operation with no errors

Wait for call from

above

extract(rcvpkt,data) rdt_rcv(rcvpkt) &&

isNAK(rcvpkt)

ACK or NAK

Wait for call from

below rdt_send(data)

Λ

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rdt2.0: error scenario

Wait for call from

above

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

isNAK(rcvpkt)

ACK or NAK

Wait for call from

below rdt_send(data)

Λ

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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

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

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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)

isNAK(rcvpkt) ) rdt_rcv(rcvpkt)

&& isACK(rcvpkt)

above Wait for

ACK or NAK 1

Λ Λ

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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)

Wait for 1 from

below

&& has_seq0(rcvpkt) extract(rcvpkt,data) deliver_data(data)

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

rdt_rcv(rcvpkt) && (corrupt(rcvpkt)

rdt_rcv(rcvpkt) &&

not corrupt(rcvpkt) &&

has_seq1(rcvpkt)

rdt_rcv(rcvpkt) && (corrupt(rcvpkt)

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

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

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rdt2.2: sender, receiver fragments

above

rdt_send(data)

udt_send(sndpkt) rdt_rcv(rcvpkt) &&

isACK(rcvpkt,1) )

rdt_rcv(rcvpkt)

&& isACK(rcvpkt,0)

Wait for ACK

sender FSM 0

fragment

Wait for 0 from

below

&& has_seq1(rcvpkt) extract(rcvpkt,data) rdt_rcv(rcvpkt) &&

(corrupt(rcvpkt) ||

has_seq1(rcvpkt)) udt_send(sndpkt)

receiver FSM fragment

Λ

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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

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rdt3.0 sender

start_timer rdt_send(data)

Wait for ACK0

rdt_rcv(rcvpkt) &&

isACK(rcvpkt,1) )

above

start_timer rdt_send(data)

rdt_rcv(rcvpkt)

&& isACK(rcvpkt,0)

rdt_rcv(rcvpkt) &&

isACK(rcvpkt,0) ) rdt_rcv(rcvpkt)

&& isACK(rcvpkt,1)

stop_timer stop_timer

udt_send(sndpkt) start_timer

timeout

udt_send(sndpkt) start_timer

timeout

Wait for call 0from

above

Wait for ACK1

Λ

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rdt3.0 in action

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rdt3.0 in action

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Performance of rdt3.0

rdt3.0 works, but performance stinks

example: 1 Gbps link, 15 ms e-e prop. delay, 1KB packet:

T_transmit = 8kb/pkt

10**9 b/sec = 8 microsec

U _sender: utilization – fraction of time sender busy sending

1KB pkt every 30 msec -> 33kB/sec thruput over 1 Gbps link

U _sender= .008

30.008 = ^0.00027 L / R

RTT + L / R = L (packet length in bits)

R (transmission rate, bps) =

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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 arrives

last packet bit arrives, send ACK

ACK arrives, send next packet, t = RTT + L / R

U sender = .008

30.008 = ^0.00027 L / R

RTT + L / R =

Transport Layer

Chapter 3

Transport Layer

Chapter 3: Transport Layer

Our goals:

understand principles behind transport

layer services:

learn about transport layer protocols in the Internet:

Chapter 3 outline

3.1 Transport-layer services

3.2 Multiplexing and demultiplexing

3.3 Connectionless transport: UDP

3.4 Principles of

reliable data transfer

3.5 Connection-oriented transport: TCP

3.6 Principles of congestion control

3.7 TCP congestion

control

Transport services and protocols

Transport vs. network layer

network layer: logical communication

between hosts

transport layer: logical communication

between processes

Household analogy:

12 kids sending letters to 12 kids

processes = kids

app messages = letters in envelopes

hosts = houses

transport protocol = Ann and Bill

network-layer protocol

= postal service

Internet transport-layer protocols

reliable, in-order delivery (TCP)

unreliable, unordered delivery: UDP

services not available:

Chapter 3 outline

3.1 Transport-layer services

3.2 Multiplexing and demultiplexing

3.3 Connectionless transport: UDP

3.4 Principles of

reliable data transfer

3.5 Connection-oriented transport: TCP

3.6 Principles of congestion control

3.7 TCP congestion

control

Multiplexing/demultiplexing

How demultiplexing works

Connectionless demultiplexing

Create sockets with port numbers:

UDP socket identified by two-tuple:

(

When host receives UDP segment:

IP datagrams with different source IP

addresses and/or source

port numbers directed

to same socket

Connectionless demux (cont)

Connection-oriented demux

TCP socket identified by 4-tuple:

recv host uses all four values to direct

segment to appropriate socket

Server host may support many simultaneous TCP sockets:

Web servers have

different sockets for each connecting client

Connection-oriented demux (cont)

Chapter 3 outline

3.1 Transport-layer services

3.2 Multiplexing and demultiplexing

3.3 Connectionless transport: UDP

3.4 Principles of

reliable data transfer

3.5 Connection-oriented transport: TCP

3.6 Principles of congestion control

3.7 TCP congestion

control

UDP: User Datagram Protocol [RFC 768]

Why is there a UDP?

UDP: more

other UDP uses

UDP: User Datagram Protocol ^{[RFC 768]}

rdt2.1: receiver, handles garbled ^ACK/NAKs