TCP/UDP/IP

TCP/UDP/IP

Courtesy of Kevin Fall at UC Berkeley& Raghupathy Sivakumar at

GATECH

TCP/IP Protocol SuitePhysical layerData-link layer – ARP, RARP, Network layer – IP, ICMP, IGMPTransport layer – TCP, UDP, RTPApplication layer – http, smtp, ftp

ApplicationTransport

IPDataLinkPhysical

TCP/IP Protocol SuiteIP is used for each network node (or router)


IPDataLinkPhysical


IPDataLinkPhysical

IPDataLinkPhysical

IPDataLinkPhysical

Source Dest

Router Router

Internet Protocol (IP) service model

best-effort datagram model error detection in header only addressing, routing signaling (ICMP) Fragmentation and reassembly Multiplexing and Demultiplexing

AddressingNeed a unique identifier for every host in the Internet (analogous to postal address)IP addresses are 32 bits longHierarchical addressing schemeConceptually … IPaddress

=(NetworkAddress,HostAddress)

Address ClassesClass A

Class B

Class C

0 netId hostId7 bits 24 bits

1 0 netId hostId14 bits 16 bits

1 1 0 netId hostId21 bits 8 bits

Addresses and HostsSince netId is encoded into IP address, each host will have a unique IP address for each of its network connectionsHence, IP addresses refer to network connections and not hostsWhy will hosts have multiple network connections?

Special AddressesPrefix(netID)

Suffix(hostID)

Type of Address

Purpose

All 0s All 0s This computer Used during bootstrap

Network All 0s Network Identifies a network

Network All 1s Direct broadcast Broadcast on a specified net

All 1s All 1s Limited broadcast

Broadcast on a local net

127 any loop back testing

Exceptions to AddressingSubnetting Splitting hostId into subnetId and hostId Achieved using subnet masks

Supernetting (Classless Inter-domain Routing or CIDR) Combining multiple lower class address

ranges into one range Achieved using 32 bit masks and max prefix

routing

ExamplesSubnetting (B class)

Network Host

Network HostSubnet

IP RoutingDirect If source and destination hosts are

connected directly Still need to perform IP address to physical

address translationIndirect Table driven routing Each entry: (NetId, RouterId)

Default router Host-specific routes

IP FragmentationThe physical network layers of different networks in the Internet might have different maximum transmission units (MTUs)The IP layer performs fragmentation when the next network has a smaller MTU than the current network

MTU = 1500 MTU=500

IP fragmentation

IP ReassemblyFragmented packets need to be put together

Where does reassembly occur? The router at the other end of the smaller

MTU network Router overhead: complexity, buffering More than one path

The final destination Many fragments on the path

more chance of missing packets Utilization inefficiency (many headers)

IP HeaderUsed for conveying information to peer IP layers


IPDataLinkPhysical


IPDataLinkPhysical

IPDataLinkPhysical

IPDataLinkPhysical

Source Dest

Router Router

IP Header (contd.)16 bit total length4 bit

version4 bit hdrlength

16 bit identification

8 bit TTL 8 bit protocol 16 bit header checksum

3 bitflags

32 bit source IP address

32 bit destination IP address

13 bit fragment offset

Options (if any) (maximum 40 bytes)

data

8 bitTOS

Multiplexing

Web Email MP3

TCP UDP

IP

Web Email MP3

TCP UDP

IP

IP datagrams IP datagrams

Endpoint identification how to identify a remote application/service on the Internet? [IP_address, port number, protocol] expect to find a process listening for incoming packets

Port numbers port numbers are in range [0..64K-1] ports below 1024 are known as well-known ports and reserved by IANA ports in range [1024..64K-1] may be registered but are not enforced

User datagram protocol (UDP)

UDPprovides a datagram service model Additional intelligence built at the

application layer if needed Error detection header (8bytes)

Sending a UDP datagram application requires that dest IP address, port number to send application chooses message size, requests send using API (e.g. sockets) API allocates OS-level buffer, leaving for some headers, copies data from user-level buffer to OS-level buffer, gives to UDP module

Sending a UDP datagram UDP module receives data and prepends IP and UDP headers fills in IP header info proto, len, src, dst,… fills in UDP header src_port, dst_port, len,… sets TTL and TOS sends UDP/IP packet to IP module

UDPheaderIP headerEthernet

header Application dataEthernet trailer

Sending a UDP datagram IP module receives packet insert options if enabled sets IP vers, IHL, offset, ID fields determines an interface/MTU fragments if needed and sends to link layer

Receiving a UDP datagram network adapter receives a frame, interrupts processor device driver determines frame contains IP type data, strips link layer header and gives to IP module IP checks IP header, processes options IP checks IP address (unicast, multicast, …) IP reassembles if necessary, give the whole packet to UDP based on protocol field

Receiving a UDP datagram UDP receives IP/UDP packet checks length and checksum locates OS PCB based on dest port, providing receiving process’ ID; generates ICMP unreachable if nobody there copies to receiving process’ buffer makes receiving process get to this

*PCB: protocol control block

Why use UDP? downsides no error correction No flow control No congestion control App picks packet size

upsides No connection establishment

stateless Broadcast/multicast more straight forward App picks packet size

Transmission Control Protocol (TCP)

TCPEnd-to-end transport protocolResponsible for reliability, congestion control, flow control, and sequenced deliveryApplications that use TCP: http (web), telnet, ftp (file transfer), smtp (email), chatApplications that don’t: multimedia (typically) – use UDP instead

Ports, End-points, & Connections

Thus, an end-point is represented by (IP address,Port)Ports can be re-used between transport protocolsA connection is (SRC IP address, SRC port, DST IP address, DST port)Same end-point can be used in multiple connections

IP LayerTCP UDP

http ftp smtptelnet

IP address

Protocol ID

A1 A2 A3

Transport

Port

TCPConnection EstablishmentConnection Maintenance Reliability

by acknowledgement packet (ACK) Congestion control Flow control SequencingConnection Termination

Fundamental MechanismSimple stop and go protocolTimeout based reliability (loss recovery)Multiple unacknowledged packets (W)

data

retxack

data

ack

data

Sliding Window Protocol: 1 2 3 4 5 6 7 8 9 10 11 12 ….

RTO

Sliding window

Sliding window for flow control

Sliding windowThe sender cannot send more data

Active and Passive OpenHow do applications initiate a connection?One end (server) registers with the TCP layer instructing it to “accept” connections at a certain portThe other end (client) initiates a “connect” request which is “accept”-ed by the server

Reliability (Loss Recovery)Sequence NumbersTCP uses cumulative Acknowledgments (ACKs)

Next expected in-sequence packet sequence number

Pros and cons? Piggybacking

Timeout calculation Rttavg = k*Rttavg + (1-k)*Rttsample RTO = Rttavg + 4*Rttdeviation

ack

data

3

1234

3

34

Retransmission (fast retransmit)

after 3 duplicate ACKs, TCP sender figures out the packet is lost

Congestion control: slow startInitial window size W = 1Each ACK will increase W by 1

(can be bottleneck!)

Congestion ControlSlow Start Start with W=1 For every ACK, W=W+1

Congestion Avoidance (linear increase) For every ACK, W = W+1/W

Congestion Control (multiplicative decrease) ssthresh = W/2 W = 1

Alternative: Fall to W/2 and startcongestion avoidance directly

Why LIMD? (fairness)• W=1

• 100 10 diff = 90• 1 1 diff = 0• Problem? – inefficient

• W=W/2• 100 10 diff = 90• 50 5 diff = 45• 51 6 diff = 45• 52 7 diff = 45• ..• 73 28 diff = 45• 37.5 14 diff = 23.5• ..• 61.75 38.25 diff = 23.5• 30.85 19.65 diff = 11.2• ..

Flow Control

Prevent sender from overwhelming the receiverReceiver in every ACK advertises the available buffer space at its endWindow calculation MIN(congestion control window, flow control window)

SequencingByte sequence numbersTCP receiver buffers out of order segments and reassembles them laterStarting sequence number randomly chosen during connection establishment Why?

3

1234

3

34

1 given to app2 given to appLoss4 buffered (not given to app)

3 & 4 given to app4 discarded

Connection Establishment & Termination

3-way handshake used for connection establishment Delay!

Randomly chosen sequence number (why?) is conveyed to the other endSimilar FIN, FIN+ACK exchange used for connection termination

SYNSYN+ACK

ACKDATA

Server does passive open

Accept connection requestSend acceptance

Start connection

Active openSend connectionrequest

TCP Segment Format

HL

16 bit SRC Port 16 bit DST Port32 bit sequence number

32 bit ACK number16 bit window sizeRsv’d flags16 bit urgent pointer16 bit TCP checksum

Options (if any)Data

Flags: URG, ACK, PSH, RST, SYN,FIN

Silly window syndrome (SWS)

TCP is a window-based protocolTCP receiver advertises a small amount of window; so TCP sender transmits only a short packet each timeInefficient utilization of network BWSo what? Save up enough to send

Nagle’s algorithmBuffer all user data if any unacknowledged data is outstandingOk to send if all is ACK’d or have a MSS size worth of dataIf small delay is wanted, Nagle’s algorithm should be disabledMSS size: maximum TCP payload size

MTU: maximum PDU size supported by link layerMTU = MSS + 20 (TCP header) + 20 (IP header)

Interactive applications: Telnet

• Remote terminal applications (e.g., Telnet) send characters to a server. The server interprets the character and sends the output at the server to the client.

• For each character typed, you see three packets:1. Client Server: Send typed character 2. Server Client: Echo of character (or user output) and

acknowledgement for first packet3. Client Server: Acknowledgement for second packet

1.send character

2.interpretcharacter

3.send echo of character

and/or output

Host withTelnet client

Host withTelnet server

Why 3 packets per character?We would expect four packets per character:

However, tcpdump shows this pattern:

What has happened? TCP has delayed the transmission of an ACK

character

ACK of character

ACK of echoed character

echo of character

character

ACK and echo of character

ACK of echoed character

Delayed ACKSProblem: In request/response programs, you send

separate ACK and Data packets for each transaction

Solution: Don’t ACK data immediately Wait 200ms (must be less than 500ms –

why?) Must ACK every other packet Must not delay duplicate ACKs

UDP-liteError-resilient CODECs appearOver wireless links, BER is not negligibleChecksumming drops corrupted packets (even 1 bit error)

UDP / UDP Lite

Socket Interface

H.263+ Encoder

RTP

IP

PPP

Packetization

RTP

UDP / UDP Lite

IP

PPP

De-packetization

H.263+ Decoder

Socket Interface

e.g. cellular networks

Fixed HostUnix BSDi 3.0

GSMBase Station

GSM Network PSTN

Mobile HostUnix BSDi 3.0

UDP-liteError-resilient CODECs means there are redundancy or FECIt may be better to use packets with some errorsIn UDP-lite, there are error-sensitive and insensitive parts

The size of the former part is called coverage E.g. application header can be error-sensitive part

Implemented in BSDi 3.0 kernel Requires MAC-lite as well

Receiver MAC should pass the data to upper layer despite errors

source port # dest port #

length / coverage checksum

0 7 8 15

Documents

TCP/UDP/IP