The Transport Layer:The Transport Layer:TCP andTCP and UDPUDP

Chap 2

Basic Philosophy of TCP/IPBasic Philosophy of TCP/IP

Simple core, complex edge Edge can be hosts, edge routers, network boundaries, etc. Why? Scalable, flexibility for different complexities at edge

Internet ArchitectureInternet Architecture

Mesh of separate networks connected at exchange points Tier 1 providers carry full Internet routing tables no defaults Tier 2+ providers carry subset and point to upstream default

Overview of TCP/IP ProtocolsOverview of TCP/IP Protocols

Transport services and protocolsTransport 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 Internet: TCP and UDP

application

transportnetworkdata linkphysical

application

transportnetworkdata linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysicalnetwork

data linkphysical

logical end-end transport

Transport vs. network layerTransport 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

Internet transport-layer protocolsInternet 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 bandwidth guarantees

application

transportnetworkdata linkphysical

application

transportnetworkdata linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysicalnetwork

data linkphysical

logical end-end transport

UDP: User Datagram ProtocolUDP: User Datagram Protocol

“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

UDP: moreUDP: 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

Applicationdata

(message)

UDP segment format

length checksumLength, in

bytes of UDPsegment,including

header

TCP OverviewTCP Overview

full duplex data: bi-directional data flow in

same connection MSS: maximum segment

size

connection-oriented: handshaking (exchange of

control msgs) init’s sender, receiver state before data exchange

flow controlled: sender will not overwhelm

receiver

point-to-point: one sender, one receiver

reliable, in-order byte steam: no “message boundaries”

pipelined: TCP congestion and flow

control set window size

send & receive buffers

socketdoor

T C Psend buffer

T C Preceive buffer

socketdoor

segm ent

applicationwrites data

applicationreads data

TCP segment structureTCP segment structure

source port # dest port #

32 bits

applicationdata

(variable length)

sequence number

acknowledgement numberReceive window

Urg data pnterchecksum

FSRPAUheadlen

notused

Options (variable length)

URG: urgent data (generally not used)

ACK: ACK #valid

PSH: push data now(generally not used)

RST, SYN, FIN:connection estab(setup, teardown

commands)

# bytes rcvr willingto accept

countingby bytes of data(not segments!)

Internetchecksum

(as in UDP)

Reliable TrReliable Transfer in TCPansfer in TCP

Host A Host B

Seq=42, ACK=79, data = ‘C’

Seq=79, ACK=43, data = ‘C’

Seq=43, ACK=80

Usertypes

‘C’

host ACKsreceipt

of echoed‘C’

host ACKsreceipt of

‘C’, echoesback ‘C’

timesimple telnet scenario

Sliding Window

TCP Connection Setup and TeardownTCP Connection Setup and Teardown Establishment: Three-Way Handshake

Termination

Sliding WindowSliding Window

Each byte has a sequence number ACKs are cumulative Sending side

LastByteAcked LastByteSent LastByteWritten Bytes between LastByteAcked and LastByteWritten must be buffered

Receiving side

NextByteRead < NextByteExpected LastByteRcvd + 1 Bytes between NextByteRead and LastByteRcvd must be buffered

NextByteRead

TCP State Transition DiagramTCP State Transition Diagram

TIME_WAIT StateTIME_WAIT State

MSL: maximum segment lifetime 30 sec in BSD-derived implementation 2 min in RFC 1122

Reason for the TIME-WAIT state(waiting for 2MSL) to implement TCP’s full-duplex connection termination

reliably termination 중에 lost, duplicated packet 문제를 해결하기

위해 to allow old duplicate segments to expire in the

network 같은 host 의 같은 port 로 연결되는 next TCP connection 에

그전 session 의 packet 을 제거하기 위해

Multiplexing/demultiplexingMultiplexing/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:

How demultiplexing worksHow 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 & port numbers to direct segment to appropriate socket

source port # dest port #

32 bits

applicationdata

(message)

other header fields

TCP/UDP segment format

Connectionless demultiplexingConnectionless 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

Connectionless demux (cont)Connectionless demux (cont)

DatagramSocket serverSocket = new DatagramSocket(6428);

ClientIP:B

P2

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”

Connection-oriented demuxConnection-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

Connection-oriented demux (cont)Connection-oriented demux (cont)

ClientIP:B

P1

client IP: A

P1P2P4

serverIP: C

SP: 9157

DP: 80

SP: 9157

DP: 80

P5 P6 P3

D-IP:CS-IP: A

D-IP:C

S-IP: B

SP: 5775

DP: 80

D-IP:CS-IP: B

Connection-oriented demux: Connection-oriented demux: Threaded Web ServerThreaded Web Server

ClientIP:B

P1

client IP: A

P1P2

serverIP: C

SP: 9157

DP: 80

SP: 9157

DP: 80

P4 P3

D-IP:CS-IP: A

D-IP:C

S-IP: B

SP: 5775

DP: 80

D-IP:CS-IP: B

Socket programmingSocket programming

Socket API introduced in BSD4.1 UNIX,

1981 explicitly created, used,

released by apps client/server paradigm two types of transport service

via socket API: unreliable datagram reliable, byte stream-

oriented

a host-local, application-created,

OS-controlled interface (a “door”) into which

application process can both send and

receive messages to/from another

application process

socket

Goal: learn how to build client/server application that communicate using sockets

Socket-programming using TCPSocket-programming using TCP

Socket: a door between application process and end-end-transport protocol (UCP or TCP)

TCP service: reliable transfer of bytes from one process to another

process

TCP withbuffers,

variables

socket

controlled byapplicationdeveloper

controlled byoperating

system

host orserver

process

TCP withbuffers,

variables

socket

controlled byapplicationdeveloper

controlled byoperatingsystem

host orserver

internet

Socket programming Socket programming with TCPwith TCP

Client must contact server server process must first

be running server must have created

socket (door) that welcomes client’s contact

Client contacts server by: creating client-local TCP

socket specifying IP address, port

number of server process When client creates

socket: client TCP establishes connection to server TCP

When contacted by client, server TCP creates new socket for server process to communicate with client allows server to talk with

multiple clients source port numbers

used to distinguish clients (more in Chap 3)

TCP provides reliable, in-order transfer of bytes (“pipe”) between client and server

application viewpoint

Port NumberPort Number

TCP Port Numbers and Concurrent Servers (1)TCP Port Numbers and Concurrent Servers (1)

foreignlocal

?21

?0

foreignlocal

150021

206.168.112.21912.106.32.254

foreignlocal

?21

?0

foreignlocal

?21

?0

foreignlocal

211500

12.106.32.254206.168.112.219

foreignlocal

211500

12.106.32.254206.168.112.219

TCP Port Numbers and Concurrent Servers (2)TCP Port Numbers and Concurrent Servers (2)

foreignlocal

150021

206.168.112.21912.106.32.254

foreignlocal

021

00

foreignlocal

150121

206.168.112.21912.106.32.254

foreignlocal

211500

12.106.32.254206.168.112.219

foreignlocal

211501

12.106.32.254206.168.112.219

TCP OutputTCP Output

Successful return from write means that we can reuse application buffer

TCP must keep a copy of our data in the socket send buffer until ACK is received.

UDP OutputUDP Output

Successful return from write means that the datagram or fragments of the datagram have been added to the datalink output queue

Buffer Size and LimitationBuffer Size and Limitation Max. size of IP datagram

IPv4: 65535 bytes including header(20 bytes) IPv6: 65535 bytes(payload) + 40 bytes(header)

MTU (Max. Transmission Unit) Network 이 전달해 줄 수 있는 최대 payload 크기 (Ethernet 에서 1500

bytes) path MTU: the smallest MTU in the path between two hosts

Fragmentation is performed if datagram size > link MTU

In IPv4: by host or router, IPv6: only by host DF bit in IPv4 header

may be used for path MTU discovery Min. reassembly buffer size (guaranteed by any implementation)

576 bytes in IPv4, 1500 bytes in IPv6 MSS (Max. Segment Size): max TCP payload size to avoid IP

fragmentation In Ethernet, MSS = MTU(1500) – IP header(20 or 40) – TCP header(20)

= 1460 B (IPv4) or 1440 B (IPv6)

Standard Internet ServicesStandard Internet Services

See /etc/services Services running on TCP or UDP

Distinguished by protocol and port number A Service is a symbolic representation of port number

Protocol Usage by Common Internet Protocol Usage by Common Internet ApplicationsApplications