TCP/IP. 2 경북대학교 이동통신망 연구실 Layering in a TCP/IP Internet Environment Application Transport Network Interface Internet Application Transport Network Interface

TCP/IPTCP/IP

경북대학교 이동통신망 연구실2

Layering in a TCP/IP Internet Environment

Application

Transport

NetworkInterface

Internet

Application

Transport

NetworkInterface

Internet

Host AHost A Host BHost B

Internet

NetworkInterface

Physical Net 1

Router RRouter R

identicalmessage

identicalpacket

identicaldatagram

identicalframe

identicaldatagram

identicalframe

Physical Net 1


Two Boundaries in the TCP/IP Model High-level protocol address boundary

Application programs as well as all protocol s/w from the Internet layer upward use only IP addresses; the network interface layer handles physical addresses

Operating system boundary

Software outside the operating systemApplication

Transport

Internet

Network Interface

Conceptual LayerConceptual Layer

Hardware

BoundaryBoundary

Software inside the operating system

Only IP addresses used

Physical addresses usedHigh-level protocol address boundary

Operating Systemboundary


TCP/IP LayeringTCP/IP LayeringUser

ProcessUser

ProcessUser

ProcessUser

Process

TCP UDP

ICMP IP

ARP HardwareInterface

IGMP

RARP

application

transport

network

link

media


EncapsulationEncapsulation

Application

TCP

Application

Ethernetdriver

IP

Ethernet

Application

TCP

Application

Ethernetdriver

IP

User data

User dataApplheader

Application dataTCP

header

Application dataTCP

headerIP

header

Application dataTCP

headerIP

headerEthernet

trailerEthernetheader Application data

TCPheader

IPheader

Ethernettrailer

Ethernetheader

Application data

Application dataTCP

header

User dataAppl

header User data

TCP (Transmission Control Protocol)

TCP (Transmission Control Protocol)


TCP three-way handshakeTCP three-way handshake

Connection Request (SYN=1, seq=client_isn)

ACK(SYN=0, seq=client_isn+1, ack=server_isn+1)

Connection Response (SYN=1, seq=server_isn,

ack=client_isn)

Connection Establish


TCP three-way handshakeTCP three-way handshake Connection Termination

FIN

ACK

FIN

ACK

Close

Close

Closed

Timed

wait


TCP Segment FormatTCP Segment Format

SOURCE PORT DESTINATION PORT

SEQUENCE NUMBER

ACKNOWLEDGEMENT NUMBER

WINDOWHLEN RESERVEDCODE BITS

URGENT POINTERCHECKSUM

OPTIONS (IF ANY) PADDING

DATA

. . .

0 4 10 16 24 31

SOURCE PORT DESTINATION PORTSOURCE PORT

Identify the sending and receiving application

SEQUENCE NUMBERSEQUENCE NUMBER

ACKNOWLEDGEMENT NUMBERACKNOWLEDGEMENT NUMBER

HLEN

The length of the header in 32-bit words

CODE BITS

URG The urgent pointer is validACK The acknowledgment number is validPSH The receiver should pass this data to the application as soon as possibleRST Reset the connectionSYN Synchronize sequence numbers to initiate a connection FIN The sender is finished sending data

WINDOW

The number of bytes, starting with the one specified by the acknowledgmnet number field, that the receiver is willing to accept

CHECKSUM

This is a mandatory filed that must be calculated and stored by the sender and than verified by the reciever

URGENT POINTER

Valid only if the URG flag is set

Contains the next sequence number that the sender of the acknowledgment expects to receive

Identifies the byte in the stream of data from the sending TCP to the receiving TCP that the first byte of data in this segment represents


Sliding windowsSliding windows

1 2 3 4 5 6 7 8 9 10 11 …

Offered window

(advertised by receiver)

Usable window

Sent andacknowledged

Sent, not ACKedCan send ASAP Window moves

Can’t send until

1 1024 1025 20482049 30723073 40964097 51205121 61446145 71687169 8192

Window advertised by segment2

Data sent inSegments 4, 5, 6

Window advertised by segment7ACKed bySegments 7

Data sent inSegments 11,12,13

Window advertised by segment10ACKed by

Segment 10Data sent

inSegments 9

Window ∼ segment8ACKed by

Segment 8ACKed by

Segments 14Window ∼ segment14

Data sent inSegments 15


Congestion ControlCongestion Control

Transmissionrate adjustment

Transmissionnetwork

Small-capacityreceiver

Large-capacityreceiver

Internalcongestion

Congestion: a condition of severe delay caused by an overload of datagrams at one or more switching point (e.g., at routers) When the sum of the connection rates over a link is higher than the link’s rate segments can be dropped


Congestion ControlCongestion Control To avoid congestion collapse, TCP must reduce transmission rates when congestion occurs Routers watch queue lengths and use techniques ICMP source quench to inform host that congestion has occurred TCP uses packet drops and timeout as congestion indication To avoid congestion in advance, the sender must adapt its transmission window to the available link bandwidth TCP connection’s rate is determined as transmission window/round trip time


Flow ControlFlow Control TCP views the data stream as a sequence of octets that it divides into segments for transmission TCP uses a sliding window mechanism to adjust the sender’s transmission speed to that of the receiver The sliding window permits the sending of multiple segments before waiting for an ACK -> efficient transmission ACK segments indicate the last correctly received byte and the number of bytes the receiver is still willing to accept A sender keeps three pointers associated with every connection

1 2 3 4 5 6 7 8 9 10 11 . . .

current window


Flow ControlFlow Control TCP allows the window size to vary over time ACK contains a window advertisement that specifies how many additional octets of data the receiver is prepared to accept (receiver’s buffer size) In response to an increased(decreased) window advertisement, the sender increases(decreases) the size of its sliding window Variable size window provides flow control as well as reliable transfer Flow control mechanism is essential in Internet environment, where machines of various speeds and sizes communicate through networks and routers of various speed and capacities

End-to-end flow control: sliding window scheme Congestion control: no explicit mechanism, implementation dependent


A B

cwnd=1

cwnd=2

cwnd=3cwnd=4

cwnd=5cwnd=6

cwnd=8cwnd=7

cwnd=9

cwnd=16

Slow StartSlow Start

A B

cwnd=1

cwnd=2

cwnd=3cwnd=4

cwnd=5cwnd=6

cwnd=8cwnd=7

cwnd=9

cwnd=16

20

15

0

10

1 2 3 4

5

...

.

.

CWND/rtt

If CWND is less than or equal to SSTHRESTH : Slow start Slow start dictates that CWND start at one segment, and be incremented by one segment every time an ACK

is received

A B

cwnd=1

cwnd=2

cwnd=3cwnd=4

cwnd=5cwnd=6

cwnd=8cwnd=7

cwnd=9

cwnd=16

A B

cwnd=1

cwnd=2

cwnd=3cwnd=4

cwnd=5cwnd=6

cwnd=8cwnd=7

cwnd=9

cwnd=16


CWNDCWND

If (cwnd >= ssthreshod) When Ack is received, Cwnd = cwnd+(1/cwnd)

The linear increase of CWND

Round-trip time

Cw

nd s

eg

men

t20

15

0

10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

5

...

.

.

. .

.

.ssthreshold

.

. . . . . . .Time out occurs

Congestion avoidance


Congestion control & AvoidanceCongestion control & Avoidance

Con

gesti

on

win

dow Slow startSlow start

ssthresh

time

Cwnd = ssthres + 3 * segment

Next ack arrivesthat acknowledgesnew segment

Fast recoveryFast recovery

Cwnd/2

[additive increase & multiplicative decrease]

Fast retransmit !

Congestion Congestion avoidanceavoidance



Con

gesti

on

win

dow

Slow startSlow start

ssthresh


Cwnd = 1, Ssthresh = current window size / 2

timeoutSlow startSlow start

time

[Fast retransmit & fast recovery]

Timeout occurBy congestion!




Congestion window

Slow start

ssthresh

time

ssthresh

Fast retransmit !Cwnd = ssthres + 3 * segment

Next ack arrivesthat acknowledgesnew segment

Fast recovery

Cwnd/2

Congestion window

Slow start

ssthreshCongesti

on avoidanc

e

Timeout occurBy congestion!Then,Cwnd = 1Ssthresh = current window size / 2

timeout

Slow startCongestion avoidance

time

[additive increase & multiplicative decrease] [Fast retransmit & fast recovery]


Fast RetransmitFast Retransmit

duplicate ACK The purpose is to let the other

end know that a segment was received out of order, and to tell it what sequence number is expected

A BF1F2

A1

A10

D u p lica te A ck 1



F3

F5

F4

F6

F7

F8

F9

A1

A1

A1

F10

F2

A1

A1

A1

A1

F11

F14

A1

F12

A11

A12

A13

to a p p l (F 1 )

to a p p l (F 2 -F 1 0 )

to a p p l

to a p p l

to a p p l

(F 1 1 )

(F 1 2 )

(F 1 3 )

sa v e to b u ffer

F13

Fast Retransmission

When the third duplicate ACK is received- ssthresh : one-half of the minimum of the cwnd and the receiver’s advertised window- retransmit the missing segment- cwnd : ssthresh +3*the segment size


Fast RecoveryFast Recovery

Fast recovery -governs the re-transmission of new data until non-duplicate ACKs are received - When 3 duplicate ACKs are received ssthresh = max (FlightSize/2,2*SMSS) - Re-transmit the lost segment and CWND = SSTHRESH + 3 * SMSS - For each duplicate ACK, CWND += SMSS - Transmit a segment if allowed by the values of CWND and RWND When a non-duplicate ACK arrives, CWND = SSTHRESH

Fast recovery

Packet loss

W

W/2

1st FastRetransmission

TimeoutCWNDInflate

W/4

2nd FastRetransmission

Wait forTimeout

Window Size

IP (Internet Protocol)

IP (Internet Protocol)


IP Service ModelIP Service Model

Datagram: each packet is independent of all others

Best effort: packet may arrive or not after some time

Internet architecture and philosophy

Application Services

Reliable Transport Service

Connectionless Packet Delivery

Service


IP Service Model A user thinks of an internet as a single

virtual network that interconnects all hosts, and through which communication is possible; its underlying architecture is both hidden and irrelevant

IP packet delivery service Unreliable: lost, duplicated, delayed, or

delivered out of order Best-effort Connectionless

IP protocol provides three important definitions Basic unit of data transfer Routing function A set of rules that embody the idea of unreliable

packet delivery


IP Datagram FormatIP Datagram Format

VERSION TOTAL LENGTH

IDENTIFICATION

SOURCE IP ADDRESS

data

0 16 31HEADERLENGTH TOS

FRAGMENT OFFSETFLAGS

HEADER CHECKSUMPROTOCOLTTL

DESTINATION IP ADDRESS

OPTIONS


Fragmentation Datagram Size, Network MTU, and Fragmentation

MTU: fixed upper bound on the amount data that can be transferred in one physical frame

e.g.) Ethernet: 1500 octets, FDDI: 4470 octets It is efficient to select a maximum datagram size such that a datagram would al

ways fit into one frame But, a datagram may travel across many types of physical networks Limiting datagrams to fit the smallest possible MTU in the internet --> inefficient

transmission Fragmentation: dividing large datagrams into smaller pieces when the datagram

needs to traverse a network that has a small MTU Fragments must be reassembled at the destination If any fragments are lost, the datagram cannot be reassembled. cf) reassembly

timer Fragmentation control

IDENTIFICATION FLAGS: Don’t Fragment bit, More Fragments bit, FRAGMENT OFFSET


Fragmentation (cont.)

Example

HostA

MTU=1500

R1Net 2

MTU=620R2

Net 3Net 1

HostB

MTU=1500


Fragmentation (cont.)Fragmentation (cont.)

DatagramHeader

data1600 octets

data2600 octets

data3200 octets

Fragment 1Header

data1 Fragment 1 (offset 0)

Fragment 3 (offset 1200)

Fragment 2 (offset 600)Fragment 2Header

data2

Fragment 3Header

data3


Internet Datagram Options Included primary for network testing or debugging Option code (1 octet)

Option length (1 octet) Data (variable) Record route option Source route option

Strict source routing Loose source routing

Time stamp option (Rarely used <-> not all routes support them)

COPY OPTION CLASS OPTION NUMBER

0 1 2 3 4 5 6 7


IP Record Route OptionIP Record Route Option

Source creates empty list of <= 9 IP addresses

Option: length, pointer, list of IP addresses

Routers note outgoing interface in list


IP Source Route OptionsIP Source Route Options Source determines path taken by

packets(<= 9 hops) Loose: any number of hops between Strict: every hop; if not directly connected,

discard Same format as record route option route overwrites with address of outgoing

interface Must be copied to fragments Destination should reverse route for return

address Not too popular => degrade router

performance

New TCPNew TCP


1. Tahoe1. Tahoe

Time-out 또는 duplicate Ack 가 3 개 수신된 경우 ,

ssthreshold = cwnd/2 cwnd = 1(dup ack 3 개 수신시 fast retr

ansmission 수행 ) 정상적인 Ack 인 경우

cwnd < ssthreshold, cwnd = cwnd + 1 : Slow Start 단계

그밖의 경우 , cwnd = cwnd + 1/cwnd : 혼잡회피 단계

W

W /2

S lowS ta r t

W a it for T im eout

T im eout

S lowS ta r t

C ong estionA v oid a nce

W ind ow S ize 패 킷 손 실

Fast retransmission

Duplicate ack 3 개 수신시

Fast recovery 과정이 없다 .

Slow start, Congestion avoidance, Fast retransmission


W

W /2

1 st F a stR etra n sm iss io n

T im eo u tC W N DIn fla te

W /4

2 n d F a stR etra n sm iss io n

W a it fo rT im eo u t

W in d o w S ize

Ack 도달패킷손실

Fast recovery

2. Reno2. Reno

Duplicate Ack 가 3 개 수신된 경우 , fast retransmission, cwnd=ssthr

esh+3(ssthresh = cwnd/2), fast recovery, cwnd = ssthreshol

d - slow start 를 수행하지 않음 Time-out 시

ssthreshold = cwnd/2, cwnd =1(slow start 수행 )

정상적인 Ack 인 경우 cwnd < ssthreshold, cwnd = cwnd

+ 1 : Slow Start 단계 cwnd = cwnd + 1/cwnd :

혼잡회피 단계

Tahoe + fast recovery

송신기 정지


W

W /2

1 st F a stR etra nsm iss ion

W /4

R ece iv e P a rtia l A C K .2 nd F a st R etra nsm iss ion

T im e

W ind ow S ize

R ece iv e P a rtia l A C K .3 rd F a st R etra nsm iss ion

R ece iv e N ew A C K .F a st R ecov ery

Æ Ð Å ¶ ¼ Õ ½ Ç

3. New Reno

• 한 윈도우 내에서 다수의 패킷손실이 발생할 때 , 송신기의 재전송 타이머가 종료될 때까지 , 송신기가 정지되는 문제를 해결한 알고리즘 .• Duplicate Ack 가 3 개 수신된 경우 ,

fast retransmission, cwnd=ssthresh+3(ssthresh = cwnd/2),fast recovery, duplicate ack 없이 RTT마다 손실된 패킷을 전송 , cwnd = ssthreshold - congestion avoidance 수행 .

• Time-out 시ssthreshold = cwnd/2, cwnd =1(slow start 수행 )

• Partial ack : 송신기가 손실된 패킷을 재전송하여 수신기에 수신되어 발생된 ack로 cwnd 의 크기를 변화시키지 않으며 fast recovery 를 유지한다 .

Tahoe + fast recovery


4. Vegas

Reno + new slow start

new congestion avoidance new retransmission modified timeout

( 정교한 time tick 사용 )

40-70% 의 throughput 개선 , 1/2-1/5 의 loss 감소


5. Vegas 의 주요 특징 new slow start : rtt 마다 기존의 방식을 따르되 , expected rate 가 실제 전송율에 비해 특정값보다 크면 ssthresh 와 무관하게 congestion avoidance 수행 . new retransmission : 1st duplcate 수신시 , rtt 값이 타임아웃값보다 크면 재전송 수행 . new congestion avoidance : 예측한 전송율을 기준으로 측한 전송율을 기준으로

실제 측정한 전송율이 크 면 윈도우를 증가시키고 , 실제 측정한 전송율이 작으면 윈도우를 감소시킴으로써 망의 혼잡을 피하여 패킷손실을 줄일 수 있다 .

Diff=expected rate-actual rate

Diff< : 윈도우 증가Diff> : 윈도우 감소

<Diff< : 윈도우 변화 무

여기서 , expected rate=window size/baseRTT


6. SACK (Selective Ack) 6. SACK (Selective Ack)

한 윈도우 내에서 다수의 패킷손실이 발생한 상황에서

한 RTT 내에서 손실패킷을 전부 재전송 하는 알고리즘 .

SACK Option 으로 수신한 패킷상태를 송신기에게 알리고 , 송신기는 손실된 패킷만을 재전송하므로 타임아웃의 발생확률이 적다 .

Sack permitted( 송신기 ) Sack option( 수신기 )

W

W /2

R etra nsm it M ultip leS eq m ents

T im e

W ind ow S ize

F a stR ecov ery

패 킷 손 실

C ong estionA v oid a nce

SS, Fast recovery, multiple Retransmission, Congestion avoidance


Characteristics of Wireless MediaCharacteristics of Wireless Media Channel Losses Low Bandwidth Signal Fading Movement across Cells Channel Asymmetry Link Latency


Selective AcknowledgmentsSelective Acknowledgments receiver informs sender of contiguous and isolated

blocks of data that have been successfully received and queued

Receiver’s Buffer

300-399

200-299

300-399

ACK 200 SACK 300-400100-199

100 -199

ACK

200

400 -499

ACK 200 SACK 500-600,300-400 100-199 300-399

sen

der

recei

ver

500-599

100-199

300-399


SMART RetransmissionSMART Retransmission

Build bitmask at sender Receiver sends both cumulative ack and packet

that caused ack

(6,6) (6,8) (6,10)

Send 7 Send 9

1 2 3 4 5 6 7 8 9 10 11 12


TCP-ProbingTCP-Probing Three distinct tactics in response

to the error detected Slow Start (for congestion detect

ed by timeout) Fast Recovery (for moderated con

gestion detected by three duplicated acks)

Immediate Recovery (for congestion-free path): more effective than Reno and Tahoe when the sending window is not too small.

ESTAB

PR1_SENT PR2_SENT

PR1_RCVD

3 Dack or Timeout

Timeout

PR1_ACK

PR2_ACK

Probe Timeout

PROBE1 PROBE2

Probing State Transition Diagram


Negative AcknowledgmentsNegative Acknowledgments Negative ACKs

in the options field of TCP header Explicitly indicate which packet has been received in

error The sender can retransmit the packet without modifyi

ng the congestion window size


ACK pacingACK pacing The goal of pacing

to evenly spread the transmission of a window of packets across the entire duration of the round trip time.

A rate based approach to ACK generation at the receiver. Advantage vs drawbacks

Avoid bursty traffic Not distinguish between

congestion losses and wireless losses

Delayed congestion signal Queue situation


Explicit Bad State Notification Explicit Bad State Notification Sending EBSN messages whenever

the base station is unsuccessful in transmitting a packet over the wireless network

Not pure End-to-end

TCP ReceiverTCP Sender Base Station

EBSN

Reset restart the TCP timer

something is wrong in

wireless link~!!!

Local recovery


Explicit Loss Notification StrategiesExplicit Loss Notification Strategies Not pure End-to-end A base station

Monitoring TCP packets in either direction. Set ELN bit, to inform the sender that the packet

has been lost on the wireless link.


ACK with ELN

4 3 2 1 4 3 2 1

1 1

Retransmission loss packet.

Packet has been loss on the

wireless link.

Ok… retransmit packet butNot reduce congestion

window


Indirect TCP Indirect TCP Two separate connections

Violating the end-to-end semantics of TCP Flow/error control protocols, packet size, time-

out, may be different for each part Need Handoff mechanism


application

transport

network

link

physical

application

transport

network

link

physical


Delayed Duplicated ACKsDelayed Duplicated ACKs It is a TCP unaware scheme. TCP receiver reduces interference between TCP and link level retran

smission by delaying third and subsequent dupacks for interval d. The base station implements a link level retransmission scheme fo

r lost packets . It is preferred over snoop in IPSEC Link level ack’s are used to trigger link level retransmission Dupacks for further consecutive packets are delayed for duration d

not use general congestion control algorithm

application

transport

network

link

physical

application

transport

network

link

physical

retransmission

Waitinterval d

Documents

TCP/IP. 2 경북대학교 이동통신망 연구실 Layering in a TCP/IP Internet Environment Application Transport Network Interface Internet Application Transport Network Interface