16 Transport

8/8/2019 16 Transport

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15-441 Computer Networking

Lecture 16 Transport Protocols


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Lecture 16: 03-16-2004 2

Outline

Transport introduction

Error recovery

TCP flow control


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Lecture 16: 03-16-2004 3

Transport Protocols

Lowest level end-to-

end protocol. Header generated by

sender is interpreted

only by the destination

Routers view transport

header as part of the

payload

7

7

6

6

5

5

7

7

6

6

5

5

Transport

Transport

IP

IP

Datalink

Datalink

Physical

Physical

Transport

Transport

IP

IP

Datalink

Datalink

Physical

Physical

IP

IP

router

2

2 2

2

1

1 1

1


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Lecture 16: 03-16-2004 4

Functionality Split

Network provides best-effort delivery

End-systems implement many functions Reliability

In-order delivery Demultiplexing

Message boundaries

Connection abstraction

Congestion control


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Lecture 16: 03-16-2004 5

Transport Protocols

UDP provides just integrity and demux

TCP adds Connection-oriented

Reliable Ordered Point-to-point Byte-stream

Full duplex Flow and congestion controlled


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UDP: User Datagram Protocol [RFC 768]

No frills, bare bonesInternet 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 header

No congestion control: UDPcan blast away as fast asdesired


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UDP Checksum

Sender: Treat segment contents as

sequence of 16-bit integers

Checksum: addition (1scomplement sum) of segmentcontents

Sender puts checksum valueinto 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

nonethless?

Goal: detect errors (e.g., flipped bits) in transmitted segment optional use!


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TCP Header

Source port Destination port

Sequence number

Acknowledgement

Advertised windowHdrLen Flags0

Checksum Urgent pointer

Options (variable)

Data

Flags: SYN

FINRESET

PUSH

URG

ACK


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Evolution of TCP

1975 1980 1985 1990

1982

TCP & IPRFC 793 & 791

1974

TCP described by

Vint Cerfand Bob Kahn

In IEEE Trans Comm

1983

BSD Unix 4.2supports TCP/IP

1984

Nagels algorithm

to reduce overhead

of small packets;

predicts congestion

collapse

1987

Karns algorithm

to better estimate

round-trip time

1986Congestion

collapse

observed

1988Van Jacobsons

algorithms

congestion avoidance

and congestion control

(mostimplemented in

4.3BSD Tahoe)

1990

4.3BSD Reno

fast retransmit

delayed ACKs

1975

Three-way handshake

Raymond Tomlinson

In SIGCOMM 75


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TCP Through the 1990s

1993 1994 1996

1994ECN

(Floyd)

Explicit

Congestion

Notification

1993TCP Vegas

(Brakmo et al)

real congestion

avoidance

1994

T/TCP

(Braden)

Transaction

TCP

1996

SACK TCP

(Floyd et al)

Selective

Acknowledgement

1996Hoe

Improving TCP

startup

1996FACK TCP

(Mathis et al)

extension to SACK


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Outline


Error recovery

TCP flow control


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Stop and Wait

Time

Packet

ACKTim

eou

t

ARQ Receiver sends

acknowledgement (ACK)

when it receives packet

Sender waits for ACK and

timeouts if it does not arrive

within some time period

Simplest ARQ protocol

Send a packet, stop and

wait until ACK arrives

Sender Receiver


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Recovering from Error

Packet

A

CK

Tim

eou

t

Packet

ACK

T

imeou

t

Packet

Tim

eou

t

Packet

ACK

T

imeou

t

Time

Packet

ACK

Tim

eou

t

Packet

ACK

T

imeou

t

ACK lost Packet lost Early timeout


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How to recognize a duplicate

Performance Can only send one packet per round trip

Problems with Stop and Wait


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How to Recognize Resends?

Use sequence numbers both packets and acks

Sequence # in packet is finite

-- how big should it be? For stop and wait?

One bit wont send seq #1

until received ACK for seq #0

Pkt0

ACK

0

Pkt0

ACK1

Pkt1ACK0


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Sliding Window

Reliable, ordered delivery

Receiver has to hold onto a packet until all priorpackets have arrived

Why might this be difficult for just parallel stop & wait? Sender must prevent buffer overflow at receiver

Circular buffer at sender and receiver Packets in transit buffer size

Advance when sender and receiver agree packets atbeginning have been received


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ReceiverReceiverSenderSender

Sender/Receiver State

Sent & Acked Sent Not Acked

OK to Send Not Usable

Max acceptable

Receiver window

Max ACK received Next seqnum

Received & Acked Acceptable Packet

Not Usable

Sender window

Next expected


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Loss Recovery

On reception of out-of-order packet Send nothing (wait for source to timeout) Cumulative ACK (helps source identify loss)

Timeout (Go-Back-N recovery) Set timer upon transmission of packet Retransmit all unacknowledged packets

Performance during loss recovery

No longer have an entire window in transit Can have much more clever loss recovery


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Go-Back-N in Action


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Selective Repeat

Receiverindividuallyacknowledges all correctlyreceived pkts

Buffers packets, as needed, for eventual in-order delivery

to upper layer

Sender only resends packets for which ACK notreceived

Sender timer for each unACKed packet

Sender window

N consecutive seq #s Again limits seq #s of sent, unACKed packets


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Selective Repeat: Sender, ReceiverWindows


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Sequence Numbers

How large do sequence numbers need to be? Must be able to detect wrap-around

Depends on sender/receiver window size

E.g. Max seq = 7, send win=recv win=7

If pkts 0..6 are sent succesfully and all acks lost Receiver expects 7,0..5, sender retransmits old 0..6!!!

Max sequence must be send window + recv window


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Outline


Error recovery

TCP flow control


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Sequence Number Space

Each byte in byte stream is numbered. 32 bit value

Wraps around

Initial values selected at start up time

TCP breaks up the byte stream in packets. Packet size is limited to the Maximum Segment Size

Each packet has a sequence number. Indicates where it fits in the byte stream

packet 8 packet 9 packet 10

13450 14950 16050 17550


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Sequence Numbers

32 Bits, Unsigned Circular Comparison

Why So Big? For sliding window, must have

|Sequence Space| > |Sending Window| + |Receiving Window|

No problem

Also, want to guard against stray packets With IP, packets have maximum lifetime of 120s

Sequence number would wrap around in this time at 286MB/s

0Max

a

b

a < b

0Max

b

a

b < a


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TCP Flow Control

TCP is a sliding window protocol For window size n, can send up to n bytes without

receiving an acknowledgement When the data is acknowledged then the window

slides forward

Each packet advertises a window size Indicates number of bytes the receiver has space for

Original TCP always sent entire window Congestion control now limits this


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Window Flow Control: Send Side

Sent but not acked Not yet sent

window

Next to be sent

Sent and acked


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acknowledged sent to be sent outside window

Source PortSource Port Dest. PortDest. Port

Sequence NumberSequence Number

AcknowledgmentAcknowledgment

HL/FlagsHL/Flags WindowWindow

D. ChecksumD. Checksum Urgent PointerUrgent Pointer

OptionsOptions

Source PortSource Port Dest. PortDest. Port

Sequence NumberSequence Number

AcknowledgmentAcknowledgment

HL/FlagsHL/Flags WindowWindow

D. ChecksumD. Checksum Urgent PointerUrgent Pointer

Options...Options...

Packet Sent Packet Received

App write

Window Flow Control: Send Side


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Acked but notdelivered to user

Not yetacked

Receive buffer

window

Window Flow Control: Receive Side

New

What should receiver do?


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TCP Persist

What happens if window is 0? Receiver updates window when application reads data

What if this update is lost?

TCP Persist state Sender periodically sends 1 byte packets

Receiver responds with ACK even if it cant store the

packet


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Performance Considerations

The window size can be controlled by receivingapplication

Can change the socket buffer size from a default (e.g.8Kbytes) to a maximum value (e.g. 64 Kbytes)

The window size field in the TCP header limits thewindow that the receiver can advertise

16 bits 64 KBytes 10 msec RTT 51 Mbit/second

100 msec RTT 5 Mbit/second TCP options to get around 64KB limit


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Important Lessons

Transport service UDP mostly just IP service

TCP congestion controlled, reliable, byte stream

Types of ARQ protocols Stop-and-wait slow, simple

Go-back-n can keep link utilized (except w/ losses)

Selective repeat efficient loss recovery

TCP flow control Sliding window mapping to packet headers

32bit sequence numbers (bytes)


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Next Lecture

TCP connection setup

TCP reliability

Congestion control

16 Transport

Documents

Transcript of 16 Transport