Lecture 3 - Transport Layer

Transport layer

Tutor: Lưu Thanh Tràuto ưu a à

Plan

Client-Server model

Request

Response

Client Server

Introduction

Transport layer locates on the layer network3Transport layer locates on the layer network3 Considering the Internet as a black boxDeploying ports for (de)multiplexDeploying ports for (de)multiplex

Overview of TCP protocols

m routetcpdump ping traceroute applic applic traceroute pingm_routetcpdump ping traceroute applic applic traceroute ping

ICMP TCP UDP

IGMP IPv4 IPv6 ICMPv6

ARPRARP

32 bits 128 bits

RARP

BPFDLPI Data link

BSD packer filterDatalink provider interface

Protocols

IPv4, IPv6IPv4, IPv6ARP/RARP (Address Resolution protocol/Reserve ARP)p )ICMP (Internet Control Message Protocol)BPF (BSD Packet filter)BPF (BSD Packet filter)DLPI (Datalink Provider Interface)

TCP

• TCP is a true transport protocol, above IP• TCP “Features”:

• Flow control• Congestion control

TCP payload

• Congestion control• Connection setup• Connection state machine

IPhdr

TCPhdr

• Reliability, at the cost of some delay• Achieves reliability by:

• Retransmission of a segment after a timeout or duplicate ACK

• Sliding window with sequence #’sSliding window with sequence # s

TCP Format• TCP segments have a 20 byte header with >= 0 bytes of data.

20 bytes

TCP header

20 b t20 bytesSource port for TCP sending processD f TCP i iDest port for TCP receiving process32-bit Sequence # (host is sender)32-bit ACK # (host is receiver ACK’ing data sent by other endpoint)

Port Number:A port number identifies the endpoint of a connection.A pair <IP address, port number> identifies one endpoint of a connectionendpoint of a connection. Two pairs <client IP address, server port number> and <server IP address, server port

b > identify a TCP connectionnumber> identify a TCP connection. Applications Applications

TCP

23 10480Ports:

TCP

7 1680 Ports:

IP IP

TCP header fields

Sequence Number (SeqNo):Sequence Number (SeqNo):Sequence number is 32 bits long. So the range of SeqNo isSo the range of SeqNo is

0 <= SeqNo <= 232 -1 ≈ 4.3 Gbyte

Each sequence number identifies a byte in the byte streamInitial Sequence Number (ISN) of a connection is set during connection establishmentis set during connection establishment

Q: What are possible requirements for ISN ?

TCP header fields

Acknowledgement Number (AckNo):Acknowledgements are piggybacked, I.e

a segment from A -> B can contain an acknowledgement for a data sent in the B -> A direction

A hosts uses the AckNo field to send acknowledgements. (If a host sends an AckNo in a segment it sets the “ACK flag”)g g )The AckNo contains the next SeqNo that a hosts wants to receiveExample: The acknowledgement for a segment with p g g

sequence numbers 0-1500 is AckNo=1501

TCP header fieldsAcknowledge Number (cont’d)

TCP uses the sliding window flow protocol (seeTCP uses the sliding window flow protocol (see CS 457) to regulate the flow of traffic from sender to receiverTCP uses the following variation of sliding window:

no NACKs (Negative ACKnowledgement)only cumulative ACKsonly cumulative ACKs

Example:Assume: Sender sends two segments with

“1..1500” and “1501..3000”, but receiver only gets the second segment.

In this case the receiver cannot acknowledge theIn this case, the receiver cannot acknowledge the second packet. It can only send AckNo=1

TCP header fields

Header Length ( 4bits):Header Length ( 4bits):Length of header in 32-bit wordsNote that TCP header has variable lengthNote that TCP header has variable length (with minimum 20 bytes)

TCP header fields

Flag bits:Flag bits:URG: Urgent pointer is valid

If the bit is set, the following bytes contain an urgent message in the range:SeqNo <= urgent message <= SeqNo+urgent pointer

ACK: Acknowledgement Number is validACK: Acknowledgement Number is validPSH: PUSH Flag

Notification from sender to the receiver that the receiver should pass all data that it has to the application.Normally set by sender when the sender’s buffer is empty

TCP header fields

Flag bits:Flag bits:RST: Reset the connection

The flag causes the receiver to reset the connectionReceiver of a RST terminates the connection and indicates higher layer application about the reset

SYN: Synchronize sequence numbersSYN: Synchronize sequence numbersSent in the first packet when initiating a connection

FIN: Sender is finished with sendingUsed for closing a connectionBoth sides of a connection must send a FIN

TCP header fields

Window Size:Each side of the connection advertises the window size Window size is the maximum number of bytes that a

i treceiver can accept.Maximum window size is 216-1= 65535 bytes

TCP Checksum:TCP Checksum:TCP checksum covers over both TCP header and TCP data (also covers some parts of the IP header)

U t P i tUrgent Pointer:Only valid if URG flag is set

TCP header fields

Options:

End ofOptions kind=0

1 byte

NOP(no operation) kind=1

1 byte

M i maximumMaximumSegment Size kind=2

1 byte

len=4

1 byte

maximumsegment size

2 bytes

Window Scale kind=3 len=3 shift countFactor kind=3

1 byte

len=3

1 byte

shift count

1 byte

Timestamp kind=8 len=10 timestamp value timestamp echo reply

1 byte 1 byte 4 bytes 4 bytes

TCP header fieldsOptions:

NOP is used to pad TCP header to multiplesNOP is used to pad TCP header to multiples of 4 bytesMaximum Segment SizeMaximum Segment SizeWindow Scale Options

Increases the TCP window from 16 to 32 bits, I.e., the window size is interpreted differently

Q: What is the different interpretation ?This option can only be used in the SYN segment p y g(first segment) during connection establishment time

Timestamp OptionTimestamp OptionCan be used for roundtrip measurements

UDP - User Datagram Protocol

UDP extends the host-to-to-host delivery service of IP to an application process-to-application process delivery serviceIt does this by multiplexing and demultiplexing packets from multiple application-to-application communication sessions

TCP vs UDP

• The Internet supports 2 transport protocols

UDP UDP -- User Datagram ProtocolUser Datagram Protocoldatagram oriented

TCP - Transmission Control Protocolg

unreliable, connectionlesssimpleunicast and multicast

stream orientedreliable, connection-orientedcomplex

useful for multimedia applicationsused for control protocols

only unicastused for data applications:

web (http), email (smtp), file network management (SNMP), routing (RIP), naming (DNS), etc.

transfer (ftp), SecureCRT, etc.

UDP packet formatIP header UDP header UDP data (payload)

20 bytes 8 bytesy y

Source Port Number Destination Port Number

UDP message length Checksum0 15 16 31

• Port numbers identify sending and receiving applications (processes). Maximum port number is 216-1= 65,535

• Message Length is between 8 bytes (i.e., data field can be empty) and 65,535 bytes (length of UDP header and data in bytes)

• Checksum is for UDP header and UDP data

IP header

Header comparison0 15 16 31

vers hlen TOS total length

identification flags flag offset

Removed (6)• ID flags flag offset

20bytes

identification flags flag-offset

TTL protocol header checksum

source address

destination address

• ID, flags, flag offset• TOS, hlen• header checksum

Ch d (3)

IPv4

des o dd ess

options and paddingChanged (3)

• total length => payload• protocol => next header• TTL => hop limit

vers traffic class flow-label

payload length next header hop limitAdded (2)

• TTL => hop limit

• traffic class40

bytessource address

destination addressExpanded

• flow label

dd 32 128 biIPv6

• address 32 to 128 bits

Three-Way Handshake

aida.poly.edu mng.poly.edu

SYN (SeqNo = x)

SYN (SeqNo = y, AckNo = x + 1 )

(SeqNo = x+1, AckNo = y + 1 )

A Closer Look with tcpdumpaida issuesan "telnet mng"

1 aida.poly.edu.1121 > mng.poly.edu.telnet: S 1031880193:1031880193(0)aida.poly.edu mng.poly.edu

an telnet mng

1 aida.poly.edu.1121 mng.poly.edu.telnet: S 1031880193:1031880193(0) win 16384 <mss 1460,nop,wscale

0,nop,nop,timestamp>2 mng.poly.edu.telnet > aida.poly.edu.1121: S 172488586:172488586(0)2 mng.poly.edu.telnet aida.poly.edu.1121: S 172488586:172488586(0)

ack 1031880194 win 8760 <mss 1460>3 aida.poly.edu.1121 > mng.poly.edu.telnet: . ack 172488587 win 17520 4 aida poly edu 1121 > mng poly edu telnet: P 1031880194:1031880218(24)4 aida.poly.edu.1121 > mng.poly.edu.telnet: P 1031880194:1031880218(24)

ack 172488587 win 175205 mng.poly.edu.telnet > aida.poly.edu.1121: P 172488587:172488590(3)

ack 1031880218 win 8736ack 1031880218 win 87366 aida.poly.edu.1121 > mng.poly.edu.telnet: P 1031880218:1031880221(3)

ack 172488590 win 17520

Three-Way Handshake

aida.poly.edu mng.poly.edu

S 1031880193:1031880193(0)win 16384 <mss 1460mss 1460, ...>

S 172488586:172488586(0)

ack 1031880194 win 8760 <mss 1460>

ack 172488587 win 17520

Why is a Two-Way Handshake not enough?

aida.poly.edu mng.poly.eduS 1031880193:1031880193(0)win 16384

The red

S 15322112354:15322112354(win 1638

80193(0)win 16384 <mss 1460, ...>

line is adelayedduplicatepacket.322112354(0)

win 16384 <mss 1460, ...>

172488586:172488586(0)

1460>

packet.

Will be discarded as a duplicate

S 1724885

win 8760 <mss 1460

When aida initiates the data transfer (starting with SeqNo=15322112355),

SYN

When aida initiates the data transfer (starting with SeqNo 15322112355), mng will reject all data.

TCP Connection Termination

Each end of the data flow must be shut down independently (“half-close”)If one end is done it sends a FIN segment. This

th t d t ill b tmeans that no more data will be sent

Four steps involved:Four steps involved:(1) X sends a FIN to Y (active close)(2) Y ACKs the FIN,

(at this time: Y can still send data to X)(3) and Y sends a FIN to X (passive close)(4) X ACKs the FIN(4) X ACKs the FIN.

Connection termination with tcpdump

aida issuesan "telnet mng"

aida.poly.edu mng.poly.edu

an telnet mng

1 mng.poly.edu.telnet > aida.poly.edu.1121: F 172488734:172488734(0) ack 1031880221 win 8733ack 1031880221 win 8733

2 aida.poly.edu.1121 > mng.poly.edu.telnet: . ack 172488735 win 174843 aida.poly.edu.1121 > mng.poly.edu.telnet: F 1031880221:1031880221(0)

k 172488735 i 17520ack 172488735 win 175204 mng.poly.edu.telnet > aida.poly.edu.1121: . ack 1031880222 win 8733

TCP Connection Termination

aida.poly.edu mng.poly.edu

F 172488734:172488734(0)ack 1031880221 win 8733

. ack 172488735 wi488735 win 17484

F 1031880221:1031880221(0)ack 172488735 i

. ack 1031880222 win 8733

(0)2488735 win 17520

TCP StatesState Description

CLOSED No connection is active or pendingLISTEN The server is waiting for an incoming callSYN RCVD A connection request has arrived; wait for AckSYN RCVD A connection request has arrived; wait for AckSYN SENT The client has started to open a connectionESTABLISHED Normal data transfer stateFIN WAIT 1 Client has said it is finishedFIN WAIT 1 Client has said it is finishedFIN WAIT 2 Server has agreed to releaseTIMED WAIT Wait for pending packets (“2MSL wait state”)CLOSING B th Sid h t i d t l i lt lCLOSING Both Sides have tried to close simultanesouslyCLOSE WAIT Server has initiated a releaseLAST ACK Wait for pending packets

TCP States in “Normal” Connection Lifetime

SYN (SeqNo = x)SYN_SENTLISTEN(passive open)

SYN (SeqNo = y, AckNo = x + 1 )

(AckNo = y + 1 )

(active open)

SYN_RCVD

ESTABLISHED

ESTABLISHED

FIN WAIT 1FIN (SeqNo = m)

FIN_WAIT_1(active close)

)

CLOSE_WAIT(passive close)

(AckNo = m+ 1 )

FIN (SeqNo = n )FIN_WAIT_2

(AckNo = n+1)LAST_ACK

TIME_WAIT

CLOSEDCLOSED

TCP State Transition DiagramOpening A ConnectionOpening A Connection

CLOSEDactive openi

LISTEN

active opensend: SYN

A li ti d d t

close ortimeout

passive opensend: . / .

recv: SYNsend: SYN, ACK

recv:RST

Application sends datasend: SYN

SYN RCVD SYN SENT

recv: SYN, ACKrecvd: ACK

simultaneous openrecv: SYNsend: SYN ACK

ESTABLISHED

send: ACKec d C

send: . / .send: SYN, ACK

send:FIN

recvd: FIN send: FIN

TCP State Transition DiagramClosing A ConnectionClosing A Connection

2MSL Wait State

2MSL Wait State = TIME_WAITWhen TCP does an active close, and sends the final ACK, the connection must stay in in the TIME_WAIT state for twice the maximum segment lifetime.

2MSL= 2 * Maximum Segment Lifetime

Why?Why? TCP is given a chance to resent the final ACK. (Server will timeout after sending the FIN segment and resend the FIN)The MSL is set to 2 minutes or 1 minute or 30 secondsThe MSL is set to 2 minutes or 1 minute or 30 seconds.

Resetting Connections

Resetting connections is done by setting theResetting connections is done by setting the RST flag When is the RST flag set?g

Connection request arrives and no server process is waiting on the destination portAbort (Terminate) a connection Causes the receiver to throw away buffered d t R i d t k l d th RSTdata. Receiver does not acknowledge the RST segment

TCP concurrent server

203.162.0.11

Server

203.162.0.12

3

listening

port 53

203.162.0.11

Server

203.162.0.12

listening

192.1.2.3Connection request

Clientport 53 203.162.0.11 port 53

203.162.0.11

Server

203.162.0.12

listening

192.1.2.3

Clientport 53

203.162.0.11203.162.0.12

Server

port 53Connected socket (connfd)

port 53

203.162.0.11

Server

203.162.0.12

listening Client

192.5.6.7Connection request

port 53 203.162.0.11 port 53

203 162 0 11 port 53203.162.0.11 192.1.2.3

203.162.0.11 port 53Server

port 53Connected socket (connfd)

Client

port 53

TCP outputApplication Application buffer (any size)

write User process

TCP Socket send buffer (SO-SNDBUF

write

kernel

User process

MSS size segmentMSS <= MTU-40 (IPv4) or MTU-60 (IPv6)

IP

MSS size

Output queuedatalink

UDP outputApplication Application buffer (any size)

write User process

UDP Socket send buffer (SO-SNDBUF

write

kernel

User process

UDP datagram

IP

MSS size

Output queuedatalink