CEENET Workshop Budapest 16-26 August 1999 1 TCP/IP Introduction George Macri ROMTELECOM S.A. Romania 5 th Network Technologies Workshop

CEENET Workshop Budapest 16-26 August 1999

1

TCP/IP Introduction

George Macri

<[email protected]>

ROMTELECOM S.A.

Romania

5th Network Technologies Workshop.


2

Technological Prerequisites

• Internetworks

• Internet Protocols

• Internet Addresses

• Routing

• Subneting

• CIDR


3

What internetworks are

• Start with lots of little networks• Many different types

– ethernet, dedicated leased lines, dialup, ATM, Frame Relay, FDDI

• Each type has its own idea of addressing and protocols

• Want to connect them all together and provide a unified view of the whole lot


4

The unifying effect of the network layer

• Define a protocol that works in the same way with any underlying network

• Call it the network layer

• routers operate at the network layer

• There are defined ways of using:• protocol over ethernet, ATM, FDDI

• protocol over serial lines (PPP)

• protocol over almost anything


5

The 7 Layer OSI ModelApplication

Presentation

Session

Transport

Network

Datalink

Physical


6

Protocol Stacks

• Layers:

ethernet token ring atm dialup frame relayx.25 hdlc

IP

TCP / UDP

Applications

Network layer

Transport layer


7

Layer Functions

Physical

Data Link

Network

Transport

IP

TCP End to end reliability

Forwardingbest-effort

Packet delivery

Raw signal

Application Mail, Web etc.

Session

Presentation


8

ISO seven layer model

• 1: Physical layer– moves bits using voltage, current, light, etc.

• 2: Data Link layer– bundles bits into frames and moves frames

between hosts on the same link


9


• 3: Network layer (e.g. IP)– Makes routing decisions

• uses destination address in packet

– Forwards packet hop by hop

• encapsulates network layer packet inside data link layer frame

• different framing on different underlying network types

– Unreliable

– Single address space for the entire internetwork


10


• 4: Transport layer (e.g. TCP)– end to end transport of datagrams– encapsulates datagrams in network layer

packets– adds reliability by detecting and retransmitting

lost packets• uses acknowledgements and sequence

numbers to keep track


11


• 5: Session layer– not used in the TCP/IP network model

• 6: Presentation layer– not used in the TCP/IP network model

• 7: Application layer– Uses the underlying layers to carry out work


12

Layer interaction

Presentation

Session

Transport

Network

LinkPhysical

Link

Network

Physical Physical

Link Link

Network

Transport

Session

Presentation

Application Application

Network


13

INTERNET PROTOCOLS

• Internet protocols – can be used for communications between heterogeneous systems;

– can be used for communications between systems connected in a LAN;

– can be used for communications between systems connected in a WAN;

– can be used for communications between a set of interconnected networks;

• Documents called RFCs (Requests For Comments), which are reviewed and analyzed by the IETF community; improvements, additions and refinements of protocols are published in new RFCs (see ftp://ftp.rs.internic.net., ftp://ftp.ripe.net/).

• Looking at all RFCs, you can see the history of the development of Internet protocols, people and companies that have contributed to this

• TCP and IP are the best known of the Internet protocols and very often the term TCP/IP refers to the whole family of protocols.

TCP/IP ModelApplication

UDP TCP

ICMP IP

ARP RARP

Datalink

Physical

Message

Segment

Datagram

Frame

Bit

5

4

3

2

1


15

TCP/IP is a 5 Layered model

• Layers 1 and 2 are not actually defined by TCP/IP , as TCP/IP was defined to be independent of physical media .

• Layer 3 is the Internet Protocol (IP) layerThis provides a basic datagram service – ICMP (Internet Control Message Protocol) is

normally provided in this layerICMP reports problems in transmission of datagrams

– ARP (Adress Resolution Protocol)– RARP (Reverse Address Resolution Protocol)

• In layer 4 are 2 possible protocols : TCP (Transport Control Protocol) and UDP (User Datagram Protocol) .– TCP provides a reliable service with error correction

and flow control .The cost of providing a reliable service is more overhead in connection setup and closedown, processing power for correcting errors and data transmission, but some applications need reliability irrespective of cost.

– UDP just extends IP’s connectionless datagram service to applications that do not require reliability .UDP datagrams can be sent to a network without the overhead of creating and maintaining a connection


18

• Layer 5 is the Application layerThis layer provides services suitable for the different types of application that might wish to use the network .It does not provide the application itself .For example : SMTP , FTP , Telnet ...


19

TCP/IP


20

Internet Protocols

PPP HDLC SLIP LAPB

Public telephone networkLAN

X.25

Ethernet/IEEE 802.3

ARP RFC 826

IP RFC 791

TelnetRFC 854

FTP RFC 959

SMTPRFC 821

SNMP

TCP RFC 793 UDP RFC 768

DNSRFC 1035

NFS RPC

RIPRFC 1058

ICMPRFC 792

Routing protocols BGP OSPF IGRP EIGRP


21

• There is a protocol for mail that defines a set of commands and messages that one machine sends to the other, for example, a conversation between machines linkguide.ici.ro and mail.iob.ro:

Linkguide: HELO linkguide.ici.ro

Mail.iob.ro: 250 mail.iob.ro - HELO Linkguide.ici.ro

Linkguide: MAIL From:<[email protected]>

Mail.iob.ro: 250 MAIL accepted

Linkguide: RCPT To:<[email protected]>

Mail.iob.ro: 250 Recipient accepted

Linkguide: DATA

Mail.iob.ro: 354 Start mail input; end with <CTRL>,<CRLF>

Linkguide: Date: Sat, 26 Jul 96 14:23:34 +02

Linkguide: From: [email protected]

Linkguide: To: [email protected]

Linkguide: Subject: helo

Linkguide: text of the message

Linkguide: .

Mail.iob.ro: 250 OK

Linkguide: QUIT

Mail.iob.ro: 221 mail.iob.ro Service closing transmission channel

• The protocol assumes that we have a reliable way of command and message communication

SMTP mail exchange as an example


22

TCP/IP Architecture Terms

FTP server

TCP

IP

Token Ring Driver

FTP client

TCP

IP

Host A Host B

Ethernet Driver

router

IP

ethdrv

t.r.drv


23

Encapsulation

• Lower layers add headers (and sometimes trailers) to data from higher layers

Data

Data

Data

Data

Header

HeaderHeader

HeaderHeader

Header

Application

Transport

Internet

Network Access


24

IP Addresses

• Purpose

• Basic Structure

• Network mask

• Special addresses


25

Purpose of an IP address

• Unique Identification of – Source

Sometimes used for security or policy-based filtering of data

– DestinationSo the networks know where to send the data

• Network Independent Format– IP over anything


26

Basic Structure of an IP Address

• 32 bit / 4 byte number:(e.g. 204.152.8.1)

• Decimal Representation:

• Binary Representation:

152 8 1204

1100110010011000 00001000 00000001


27

Address Structure Revisited

• Hierarchical Division in IP Address:– Network Part (Prefix)

• describes which physical network

– Host Part (Host Address)• describes which host on that network

– Boundary can be anywhere• not necessarily at a multiple of 8 bits

Network Host

205 . 154 . 8 1

11001101 10011010 00001000 00000001


28

Network Masks

• Define which bits are used to describe the Network Part

• Different Representations:– decimal dot notation: 255.255.248.0– number of network bits: /19

• Binary AND of 32 bit IP address with 32 bit netmask yields network part of address


29

Subnetting

• One class address (either B or C) space could be too large for a given organization, or for a certain site of the organization.

• Subnetting divides a single network address into many subnet addresses, so that each subnetwork can have its own unique address.

• A subnet is defined by applying a bit mask (the subnet mask) to the IP address.

• If a bit is 1 in the mask, the equivalent bit in the address is interpreted as a network bit.

• If a bit in the mask is 0, the bit belongs to the host part of the address.

• Ex: mask to divide the 193.226.2.0 address into 4 subnets:

11111111 11111111 11111111 11000000


30

Example Prefixes

• 137.158.128.0/17 (netmask 255.255.128.0)

• 198.134.0.0/16 (netmask 255.255.0.0)

• 205.37.193.128/26 (netmask 255.255.255.192)

10001001 10011110 1 0000000 00000000

11000110 10000110 00000000 00000000

11001101 00100101 11000111 10 000000

11111111 11111111 1 0000000 00000000

11111111 11111111 11111111 11 000000

11111111 11111111 00000000 00000000


31

Old-Style Classes of Address

• Different classes used to represent different sizes of network (small, medium, large)

• Class A networks: x.0.0.0 - 16.777.215 host addresses– 8 bits network, 24 bits host (/8, 255.0.0.0)

– First byte in range x=1-127

• Class B networks: x.y.0.0 - 65.536 host addresses – 16 bits network, 16 bits host (/16 ,255.255.0.0)

– First byte in range x=128-191 y=0-254

• Class C networks: x.y.z.0 - 256 host address– 24 bits network, 8 bits host (/24, 255.255.255.0)

– First byte in range x=192-223 y,z=0-254


32

IP Address Structure - Class-full

Network address Host addressAddress format32 bits

Class Anetwork=8 bits

Class Bnetwork=16 bits

Class Cnetwork=24 bits

Class D (multicast)

Class E(reserved)

0

1 0

1 1 0

1 1 1 0

1 1 1 1


33

Special Addresses

• All 0’s in host part: Represents Network– e.g. 193.0.0.0/24

– e.g. 138.37.128.0/17

• All 1’s in host part: Broadcast– e.g. 137.156.255.255 (137.156.0.0/16)

– e.g. 134.132.100.255 (134.132.100.0/24)

– e.g. 190.0.127.255 (190.0.0.0/17)

• 127.0.0.0/8: Loopback address (127.0.0.1)• 0.0.0.0: Various special purposes


34

TCP/IP Basics: Physical & Datalink


35

The Physical and Datalink layer

• Ethernet

• IEEE and ISO

• Token Ring

• FDDI

• SLIP

• PPP

• ISDN


36

Ehernet

• Network access protocol

– The medium for communication between two machines directly connected can be: coax, twisted cable, telephone link, radio link, satellite link, etc. The lowest layer of protocols provides functions that manage the data transmission specific to a certain physical medium.

• Classes of links

– Point to point

– Broadcast

– Non-broadcast multi-access

• Ethernet/IEEE 802.3 is a coaxial based bus cabling system developed by Digital Equipment Corporation, Intel, Xerox (DIX)

• Ethernet was the technological basis for the IEEE 802.3 specification

• Both of them specify the CSMA/CD (Carrier Sense Multiple Access with Collision Detection), also referred as “listen while talk” (LWT)

• Both are broadcast networks

Ethernet Topologies

Thick Wire10 Base 5

Transceivers

Thin Wire10 Base 2

Transceivers onboards incomputers

Twisted Pairconcentrator

On BoardTransceivers

10/100/1000 Base T

Fiberconcentrator

Transceivers

10/100/1000 Base F


38

The Ethernet frame

• This Ethernet frame encapsulates the TCP/IP protocol and is responsible for transporting it across the cabling system to layer 2 of the destination device , whether it’s a Router , Gateway or end node .

8 Octets 6 Octets 6 Octets 2 Octets 46-1500 Octets 4 OctetsPreamble Destination address Source address Type Data CRC


39

MAC addressing

• The ethernet frame uses addresses referred to as MAC (Medium Access Control)

• MAC addresses identify the specific network cards

• These are 48 bits long

• Each network card has a unique address configured by its manufacturer

• The LAN card will accept only 3 types of MAC address .– Unicast - Frames with destination to the exact MAC

address .– Broadcast - Has all 48 bits set to binary 1

(or Hex FF FF FF FF FF FF) .This type of frame is used when the sender does not know the destination MAC address it tries to communicate , so we broadcast to all .

– Multicast - Addressing to groups of LAN cards that are related in some way .The LAN cards have to be configured to know they are part of a multicast group .


41

The type field

Type Protocol

0x0800 IP

0x0806 ARP

0x8035 RARP

• The Type field identifies different protocols .

• A computer running multiple protocols can easily differentiate between them , and path the contents to the relevant layer .

• TCP/IP Generally uses 3 Ethernet types registered in IEEE .


42

CRC - Cyclic Redundancy Check

• At the end of the frame is a CRC .• This is a 32 bit value that is calculated from all the

bits of the Ethernet frame and its contents , but ignoring the preamble and the CRC itself .

• The remote node does the same calculation and compares the CRC .If the value is different , the LAN card will not pass the Frame to the network layer .


43

The service provided by Ethernet

• The medium access mechanism used by Ethernet is CSMA/CD (Carrier Sense Multiple Access with Collision Detection) .– This allows nodes on the network to manage

shared access to the cable , but it restricts the length of the cabling , and the number of nodes that use it .

– They are not specific to Protocol , therefore for TCP/IP .


44

Ethernet Packet size

• Minimum packet size - 64 octets

• Maximum packet size - 1518 octets

• The sizes above include all the frame apart from the preamble .

• Because of the frame header fields , the CRC and the overhead of the IP and TCP or UDP higher layer protocols , the amount left for useful application data is less then 1518 .

• To give an example :The Ethernet frame overhead consists of 18 octets and the higher layer protocols often need 40 octets .That leaves 1460 (1518-40-18=1460) octets for application data .


46

IEEE and ISO systems

• IEEE 802.3 uses CSMA/CD .

• IEEE 802.4 uses a token mechanism on a bus .

• IEEE 802.5 and FDDI (IS9314) use a token passing mechanism on a ring .


47

LLC (Logical Link Layer)

• For LAN’s , layer 2 is split to 2 sublayers .

• The lower is MAC and above we have the LLC , which has the standard number IEEE 802.2 .

• One of the major functions of LLC is to differentiate between the different types of network layer protocols , in a similar way to the type field of Ethernet .


48

Ethernet


Presentation Presentation

Session Session

Transport Transport

Network Network

IEEE 802.2 IEEE 802.2

IEEE 802.3 IEEE 802.3


49

Token Ring



Session Session

Transport Transport

Network Network

IEEE 802.2 IEEE 802.2

IEEE 802.5 IEEE 802.5


50

FDDIApplication Application


Session Session

Transport Transport

Network Network

IEEE 802.2 IEEE 802.2

IEEE 802.5 IEEE 802.5

IS 9314 IS 9314


51

Encapsulation • The type field specifies the upper-layer protocol to receive the data after Ethernet processing is

complete

• The CRC (Cyclic Redundancy check) is created by the sender and recalculated by the receiver

• The frame length (header, data, and CRC) 64-1518 bytes

Application

TCP

IP

Ethernet E I T Data C

I T Data

T Data

Data

E I T Data C

I T Data

T Data

Ethernet

Ethernet

IP

TCP

Application


52

The IEEE 802.3 frame• The IEEE 802.3 frame has the same general format as DIX

Ethernet (Ethernet_II) frame .

• The Type field in Ethernet DIX is the Length field in IEEE 802.3

• THE FCS (Frame Check Sequence) is instead of CRC

• As there is no Type field , it is not possible to detect which network layer protocol is carried in the MAC layerThe MAC frame consists of only addresses , length and FCS.It is the function of LLC to separate the different network layer protocols .


53

IEEE 802.3 frame

7 octets 1octet

6 octets 6 octets 2 octets 4octets

Preamble Destinationaddress

Sourceaddress

Length LLC Data FCS

46-1500Octets


54

Bridging TCP/IP• Bridging between IEEE LAN’s is often promoted as

transparent to any protocol above the MAC layer .This will bring expectations that there are no particular problems with TCP/IP .

• There are 4 issues that need consideration :– The length field for the 802.3 bus.

– Encapsulation on bus networks.

– The maximum frame sizes.

– The representation of MAC addresses.


55

Length fields

• The IEEE 802.3 CSMA/CD network has a length field immediately before the LLC .Other IEEE networks do not .

• Bridging will at least involve changing the content of the frame and recalculating the FCS .This action will be totally transparent to the network planners .


56

Frame size• For TCP/IP , the transmitted frame size is determined by

the Maximum Transfer Unit (MTU) set in the driver software for the LAN interface .

• It is possible on most TCP/IP implementations to modify the MTU to match the number of data octets carried by the Link Layer protocol .Setting the MTU’s of each interface on a Token Ring to 1492 will prevent its frames from being to large for bridging to IEEE 802.3 .This reduction will limit Token Ring efficiency .


57

Representation of MAC addresses• The IEEE 802.1 committee defined how LAN’s should

represent 48 bit MAC addresses as a bit stream on the cable .IEEE 802.3 and 802.5 committee chose to represent these addresses higher in the protocol .

• IEEE 802.3 and 802.5 represent differently the MAC address .

• Bridges now have to be wise and not only reverse the address but also to calculate the FCS .


58

Example of vendor-dependant Ethernet addresses

Prefix Manufacturer00:00:0C Cisco

00:00:95 Proteon

00:00:A2 Wellfleet

00:00:C0 Western Digital

00:AA:00 Intel

02:60:8C 3Comm

08:00:09 Hewlett-Packard

08:00:10 AT&T

08:00:0B Unisys

08:00:20 Sun

08:00:2B DEC

08:00:46 Sony

08:00:5A IBM

AA:00:03 DEC

AA:00:04 DEC


59

TCP/IP Basics: Serial Connections


60

SLIP - Serial Line Internet Protocol

• In some situations , it is advantageous to use asynchronous Serial lines to carry TCP/IP protocols , either by :– Dialup modems– Modems on private wires– through an asynchronous network– Direct connection between 2 computers


61

SLIP functionality

LAN

Host

AsynchronousconnectionsV.24/RS232C

Dialupmodemlink

Modemlink

Directconnection

PC’swithSLIP


62

SLIP frame format

• SLIP defines 2 special characters :– SLIP END - 0xC0– SLIP ESC - 0xDB

• Datagrams sent using SLIP are framed SLIP END characters .


63

SLIP frame format0xC0 IP

datagram0xC0

Data beforeSLIP

21 31 32 C0 5F

SLIP detectsC0 andinserts DB

21 31 32 DB C0 5F


64

PPP - Point to Point Protocol

• PPP came to overcome a number of limitations of SLIP .

• PPP has been designed to operate over both : asynchronous (start/stop) connections , and bit oriented synchronous systems .


65

• PPP provides more then just a simple connection between hosts .It also defines several management and testing functions to deal with line quality , option negotiation and the setup of IP addresses .


66

The service provided by PPP

• PPP provides a Point to Point connection between 2 TCP/IP systems for the transfer of IP datagrams .

• PPP can operate over virtually any serial link interface .

• The only limitation is that it requires a full duplex connection .


67

• It does not need serial interface control signals , but the standard recommends it for performance improvements .

• There is no restriction for the speed used for PPP .


68

The PPP frame

Flag01111110

Address11111111

Control00000011

Protocol16 bits

Information FCS16 bits

Flag01111110

• The address field is all 1’s.

• The control octet contains the value 0x03.

• The protocol field defines the protocol carried by this frame :

– Link Control Protocol - 0xC021

– Network Control Protocol - 0x8021

– Internet Protocol - 0x0021


69

• PPP can multiplex data from many sources, which makes it practical for high speed connections between bridges or routers.


70

TCP/IP Basics: Network Layer


71

Why do we need IP protocol layer?• Although the services provided by TCP protocol are needed by many

applications, there are still some kind of applications that don’t need them;

• However, there are some services that every application needs.

• The services that every application needs are put together into the IP protocol layer;

• IP protocol provides the basic service for the transmission of a datagram from one machine to another machine which do not need to be connected directly;

• As a result, TCP calls on the services of IP;

• Like TCP, IP protocol layer can be viewed as a library of routines that TCP calls on, but which is also available to applications that don’t use TCP


72

IP - Internet Protocol• IP is described as a “connectionless datagram service” .

• Datagrams are packets of information that can be destined for one , many or all stations (unique , multicast or broadcast) - provide addressing.

• There is no requirement for the intended recipient/s to acknowledge whether the datagram was received (no flow control, no end-to-end data reliability).

• As IP is connectionless , no specific route is defined between 2 communicating nodes , so datagrams traveling can travel through different routes and reach destination in a different order (no sequencing and allow for fragmentation).

• One of the major roles of IP layer is to make it unnecessary for higher layer protocols to understand anything about the physical capabilities of the media supporting them .Note : This is important for application developers writing programs on top of the transport layer with no variations because of the different kind of media used .

The IP ArchitectureApplication

UDP TCP

ICMP IP

ARP RARP

Datalink

Physical

Message

Segment

Datagram

Frame

Bit

5

4

3

2

1

( ) (

)( )

( )

1 0800

8035 0806


74

Encapsulation

• Both the header and data of the IP datagram become the datalink frame of whichever network they happen to be on.This is called encapsulation .

• Protocol number identifies the protocol in the layer above IP to which the data is passed (/etc/protocols)– 0 IP pseudo protocol number

– 1 ICMP

– 6 TCP– 17 UDP


75

Fragmentation and Reassemble• IEEE 802.3 and Ethernet systems have maximum data

sizes of 1492 and 1500 octets respectively .IEEE 802.5 frames is not defined , but in practice it is usually no greater then 8192 octets .

• This size limit seen by IP is known as the Maximum Transfer Unit (MTU) .

• The MTU can be adjusted for each interface , but it’s not necessary unless bridging different LAN technologies .


76

IP datagram FormatVersion IHL TOS Total length

Identification Flags Fragment Offset

TTL Protocol Header Checksum

Source IP address

Destination IP address

Options Padding

Data

• Version - 4 bitsVersion of the IP protocolCurrent version is 4

• Internet Header Length - 4 bitsFor easy finding of beginning of data .Normally the value is 5 indicated no options are used .

• Type Of Service - 8 bitsThe first of 3 bits are used to indicate 1 of 8 levels of priority .Some Routers Ignore these flags .

• Total length - 16 bitsThe total length of the IP datagramThe size of data is computed from the total length field and IHL .

• Identification - 16 bitsThis is an integer value used to help identify all fragments of a datagram .This field is unique for each new datagram .

• Flags - 3 bitsThe 2 low order bits are used as flags to control fragmentation .The low order bit , if 0 , indicates the last fragment of a datagram - MF (More Flag) .The middle bit is used to indicate that the datagram should not be fragmented - DF (Do not Fragment) .

• Fragment Offset - 13 bitsUsed in a fragmented datagram to indicate the position that the fragment occupies .

• Time To Live (TTL) - 8 bitsThis prevents datagrams to get routed in a loop .If it’s set to 0 , a router should discard the datagram .The recommended value is 32 , but it can be set to a maximum of 255 too .

• Protocol - 8 bitsThe transport layer protocol carried by this datagram .It tells the IP layer where to path the datagram .17 - UDP6 - TCP1 - ICMP

• Header checksum - 16 bitsIt protects only the header and not the data .The reason is because the checksum must be recalculated every time it passes through a router .Other parameters change too .

• Source IP address - 32 bits

• Destination IP address - 32 bits

• Data variableThis includes the headers of higher layer protocols and user’s data .


80

Routing IP Datagrams

Target Internet

H

H

G

G

N

N

N

G

Source

Where do I sendthat datagram?


81

IP Routing

SubNet

DirectConnection

•local host•same subnet•default gateway

•local host•default gateway •local host

•same subnet•next-hop

Subnet

DefaultGateway


82

IP algorithm

1. Search the routing table for an entry that matches the complete destination IP address (network ID or host ID). If found, send the packet to the indicated next-hop router or to the directly connected interface. (second interface or ppp)

2. Search the routing table for an entry that matches just the destination network ID. If found, send the packet to the indicated next-hop router or to the directly connected interface. (local networks)

3. Search the routing table for an entry labeled “default”. If found, send the packet to the indicated next-hop router


83

ARP - Address Resolution Protocol

• If we wish to connect to a remote computer we must know it’s IP address , but we do not need to know it’s MAC address .

• ARP was invented for this reason .It relates IP’s to MAC addresses only on media that supports broadcasts .

• Each node maintains a cache called the ARP cache , which holds a table of IP’s against MAC addresses .


84

How ARP works• When IP is requested to send a datagram to

another IP address , it first looks in the ARP cache to find the corresponding MAC address .If there is no entry it then attempts to look for it using ARP .

• In order to do this ARP sends an ARP request datagram to all LAN cards using a broadcast address .

• ARP uses its own Ethernet type 0x0806 for these requests , so they are passed to the ARP software in all nodes within the broadcast area .

• All cards on a network read this request datagram and any that discover a match between their IP and the requested IP reply with an ARP response .

• If a response is received , the answer is entered to the ARP cache for future use .If none is received , the request is repeated .

ARP datagrams are not passed through routers , as a router operates at the IP layer and will not relay MAC broadcast traffic .This makes routers a good buffer between broadcast domains and prevent flooding networks .


86

ARP commands

• arp command can be used to display the content of the ARP table;

• Formats:– arp -a ! displays all the entries in the ARP table;

– arp <hostname> ! displays the entry for <hostname> specified

– arp -d <hostname> ! deletes an entry for <hostname>

– arp -s <hastname> <ether-address> ! adds a new entry


87

RARP - Reverse ARP• RARP is intended for use with devices that cannot store

their IP address , usually diskless workstations.• RARP , like ARP , operates directly over the datalink layer

and has an Ethernet type 0x8035 .

• Nodes acting as RARP servers that find a match for the MAC address in their RARP tables will reply with the corresponding IP address in a RARP response .


88

• This system requires that at least one server is present and that the server has a table defining which IP addresses should be used by each MAC address .


89

ICMP - Internet Control Message Protocol

• Even though IP is a datagram service and there is no delivery guarantee , ICMP is provided within IP and can generate error messages regarding datagram delivery .

• ICMP uses IP datagrams to carry its messages back and forth between relevant nodes .


90

• ICMP error messages are generated by a node recognizing there is a transmission problem and they are sent back to the originating address of the datagram that caused the problem .


91

Frame header Frame data

IP header IP data

Type Code …


92

General format of ICMP message

Type (8): specifies the type of ICMP message

Code (8): used to specify parameters of the message that can be encoded in a few bits

Checksum (16): checksum of the entire ICMP message

Parameters (32): used to specify more lengthy parameters

Information (variable):provides additional information related to the message– ECHO and ECHO REPLY - mechanism for testing if communication is possible between two entities.

A host can send the ICMP ECHO message to see if a remote IP is up and operational. When a system receives an echo message, it send the same packet back to the source host in an ICMP ECHO REPLY message. The ping command uses this message.

– A TIME EXCEEDED message is sent by a gateway if the ttl value of a datagram expires (becomes zero). This facility is used by the traceroute command.

Type (8 bits) Code (8 bits) Checksum (16 bits)

Parameters (32 bits)

Information (variable)


93

Type field• 0

• 3

• 4

• 5

• 8

• 11

• 12

• 13

• 14

• 15

• 16

• 17

• 18

Echo reply

Destination unreachable

Source quench

Redirect

Echo request

Time exceeded for datagram

Parameter problem on datagram

Time stamp request

Time stamp reply

Information request

Information reply

Address mask request

Address mask response

Message Type


94

The ping command

ping

• it is a simple function, extremely useful for testing the network connection;

• it allows the network administrator to determine whether further testing should be directed toward the network (the lower layers) or the application (the upper layers)

• if ping shows that packets can travel to the destination system and back, the problem is probably in the upper layers

• If packets can’t make the round-trip, lower protocol layers are probably at fault

Basic format

ping <host> [<packetsize>] [<count>]

<host> The host name or IP address of the remote host being testyed.

<packetsize> Defines the size in bytes of the test packets. This field is only required if the count field is going to be used. Default packet size is 56 bytes.

<count> The number of packets to be sent in the test. Default number is usually 5.


95

ping example

Examples

#ping ftp.ripe.net

info.ripe.net is alive

# ping -s ftp.ripe.net 100 10

PING info.ripe.net: 100 data bytes

108 bytes from info.ripe.net (39.13.5.97): icmp_seq=0. time=1070. ms








----info.ripe.net PING Statistics----

8 packets transmitted, 8 packets received, 0% packet loss

round-trip (ms) min/avg/max = 980/998/1070


96

traceroute - Tracing routes

• is the program that can help the network administrator locate the problem when something is down between the local host and a remote destination

• traces the route of UDP packets from the local host to a remote host

• prints the name (if it can be determined) and IP address of each gateway along the route to the remote host

• uses two techniques: small ttl values and invalid port number


97

traceroute - Tracing routesOperation

• traceroute sends out 3 UDP packets with ttl value set to one

• the first gateway decrement ttl and gets the value zero.

• The first gateway will send back to the source host an ICMP TIME EXCEEDED message as error message

• traceroute displays one line of output for each gateway from which it receives an ICMP TIME EXCEEDED message

• traceroute will then increment by one the ttl value and sends again 3 UDP packets

• the flow of packets tracing to a host three hops away is illustrated below

• When the destination host receives a packet from traceroute, it returns back an ICMP “Unreachable Port” message. This happens because traceroute intentionally uses an invalid port number (33434) to force this error.

• When traceroute receives the “Unreachable Port” message, it knows that it has reached the destination host, and it terminates the trace.

• In this way, traceroute is able to develop a list of the gateways, starting at one hop away and increasing one hop at a time, until the remote host is reached.


98

traceroute example

# traceroute ftp.ripe.net

traceroute to info.ripe.net (39.13.5.97), 30 hops max, 40 byte packets

1 agsici1.ici.ro (192.162.16.25) 20 ms 10 ms 0 ms

2 Vienna-EBS1.Ebone.NET (192.121.159.97) 870 ms 870 ms 870 ms

3 Paris-EBS2.Ebone.net (192.121.156.17) 900 ms 890 ms 890 ms

4 Stockholm-ebs.ebone.net (192.121.154.21) 920 ms 930 ms 960 ms

5 Amsterdam-ebs.Ebone.NET (192.121.155.13) 970 ms 990 ms 970 ms

6 Amsterdam.ripe.net (193.0.15.130) 1000 ms 970 ms 970 ms

7 info.ripe.net (39.13.5.97) 1040 ms 970 ms 990 ms


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Flow of traceroute packetsping program First router Second router Third router

decrements ttl to 0return error “TIME EXCEEDED”

ttl=1

ttl=2

ttl=3

decrements ttl to 1forward

decrements ttl to 0return error “TIME EXCEEDED”



received at destinationport unreachableReturn error “port unreachable”

• ICMP has it’s own IP protocol number (1) so the IP layer knows when it receives them.

• Even though ICMP uses the IP layer, it is considered as being within IP, because it does not necessarily provide any service to the layers above.


101

ICMP types 0 and 8 - echo

• The most common ICMP messages used for diagnostics are type 0 and 8.

• These are generated by Ping.Ping sends ICMP type 8 datagrams to a node and expects an ICMP type 0 reply, returning the data sent in the request.


102

ICMP echo datagram (0 or 8)

Type Code Checksum

Identifier Sequence number

Optional data

…


103

Note :• How can Ping generate ICMP echo requests if ICMP

does not provide a service to Ping ?• A Ping implementation does not use ICMP to generate the

request.It merely mimics what ICMP would do as a program that operates over the IP layer.Ping generates an IP datagram with a data field that equates to ICMP echo request (protocol number 1 and the first octet of data is 8 - ICMP echo request).It then adds the rest of the fields including the data pattern that it expects to be echoed.


104

ICMP type 3 - destination unreachable

• If a router is unable to deliver a datagram, it can return the destination unreachable ICMP datagram to indicate why.

• The code field is used to identify the cause of failure.

• The values in the code field help to pinpoint the reason for the datagram failure to arrive its destination.


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ICMP type 3 - Destination Unreachable

Type Code Checksum

Unused (must be 0)

Internet header +64 bits of datagram prefix

…


106

Code value

• 0 Network unreachable

• 1 Host unreachable

• 2 Protocol unreachable

• 3 Port unreachable

• 4 Fragmentation needed and the do not fragment bit set

• 5 Source route failed

Meaning

• If a router is unable to deliver a datagram , it can return the destination unreachable ICMP datagram to indicate why .– Network unreachable - The network specified in the IP

address cannot be found .• The IP address and routing tables should be checked .

• This error message is only generated by a router .

• We can find where the error occurred , from the source address in IP header that carried the ICMP message .

– Host unreachable - The datagram reached the router which is directly connected to the destination network, but failed to communicate with the host.This message is generated by a router only .

– Protocol unreachable - The datagram reached the destination host , but the particular protocol carried in the datagram is not available .

– Port unreachable - A host sends the message that the particular application layer service is not available .

– Fragmentation needed and the do not fragment bit set - Normally comes from a router , indicating that it needs to fragment the datagram , but is instructed not to by the do not fragment (DF) bit in the flags field of the IP header .This fault is uncommon , DF is normally used on diskless workstations booting via TFTP .

– TFTP has only 512 octets of user data .• Check MTU size .

– Source route failed - If we specified a route and the datagram failed to complete the route , we will get this error .The point of failure will be the router that generated the ICMP message .


110

ICMP type 4 , code 0 - Source Quench

• The format of the datagram is the same as destination unreachable , but with a type of 4 and a code of 0 .

• Source quench gives a router or a host the ability to request that a source of datagrams will slow down .

• Source quench will occur if a node is running low on buffer resources and is unable to process datagrams quickly enough .

If you don’t slow down , your datagrams will be discarded .


112

ICMP type 5 - route change request• It is used only by routers .

• A router that knows that it is not the optimum router for a particular destination , uses the relevant field of a route change request to suggest a more suitable router . Type Code Checksum

Internet address of a more suitable router


…


113

ICMP type 11 - time exceeded for datagram

• The format is the same as destination unreachable .

• It can be sent in 2 situations :– From a router - Indicating that the TTL in the IP header

has been decremented to 0 .It indicates that the original Time To Live was not suitable to the number of hops needed .

– From a node - An attempt to recreate the original datagram by reassembly of fragments failed .The code value is 1 .


114

ICMP type 12 - Parameter problem message• Indicates that a wrong argument has been used with an

option field in the IP header .It can also indicate an error in the implementation of IP .

• It’s sent only if the datagram has been discarded .• The pointer field indicates the position of the octet

position of the suspect field . Type Code Checksum

Pointer Unused (must be 0)


…


115

ICMP types 13,14 - Time stamp request & reply

• This message is used to obtain the time from a clock in a distant machine .

• It is rarely used today .


116

ICMP types 15,16 - information request

• This message is used to obtain the network number of the requesting host if it’s unknown .

• It can be used in dial in systems using SLIP, as a method for allocating the appropriate network addresses for each end of the link .


117

ICMP types 17,18 - Address mask request

• Used to allow a node to discover the subnet mask of the network it is connected to .

• The node can send the request to a known address or to broadcast .


118

Transport Protocol Ports

• Port 0 - Special use• Ports 1 - 255 - Well-known ports• Ports 256 - 1023 - Reserved ports• Ports 1024 - 4999 - Dynamic client ports• Ports 5000 - 65,535 - Fixed server ports

The address of anapplication within a host Application

ApplicationApplicationApplication

HOST


119

User Datagram Protocol

• Connectionless delivery service

• Uses the IP layer service

• Does not add reliability to the IP protocol

• Enables distinguishing among multiple destinations within a host computer

End point


120

UDP Protocol Header Format

UDP Source Port UDP Destination Port

UDP Message Length UDP Checksum

Data

0 16 31

• Fragmentation– What if the packet size is larger then 1500?

• It is divided to 1500xN frames.

• fragmentation flags are set


121

Flow using Datagrams (UDP)

Server

socket()bind()

Client

socket()

sendto()/recvfrom()

closesocket()

sendto()/recvfrom()

closesocket()


122

Transmission Control Protocol

• Connection based communication

• Uses the IP layer service

• Provides reliable service

• Enables distinguishing among multiple destinations within a host computer


123

TCP - Transmission Control Protocol• TCP is the protocol layer responsible for making sure that the commands and messages

are transmitted reliably from one application program running on a machine to another one on the other machine

• A message is transmitted and then a positive acknowledgement is being waited for• If the positive acknowledgement does not arrive in a certain period of time, the message is

retransmitted• Messages are numbered in sequence so that no one is being lost or duplicated;• Messages are delivered at the destination in the same order they were sent by the source• If the text of a mail is too large, the TCP protocol will split it into several fragments called

“datagrams” and it makes sure that all the datagrams arrive correctly at the other end where they are reassembled into the original message

• The TCP protocol layer provides all the functions that are needed for many applications and it is better to put them together on a separate protocol rather than being part of each application

• TCP can be viewed as forming a library of routines that many applications can use when they need reliable network communication with an application on another computer

• TCP provides also flow control and congestion control


124

TCP Protocol Format

Source Port Destination Port

Sequence Number

Acknowledgment Number

Checksum (16) Urgent Pointer

Options(If any) Padding

Data (variable length) 0 4 10 16 24 31

Offset Reserv Flags(6) Window (16 bits)


125

Establishing and closing TCP Connections

Three-wayhandshake

Close

timeSYN

ACK

SYN+ACK

Open

FIN

ACK

ACK

FIN


126

Sliding Windows

Positiveacknowledgmentwith retransmission

Sliding windowtransmission

time

segment 1

segment 2ack1

ack2

segments

acks 1 2 3 4

1 2 3 4


127

Application Addresses: Sockets• On a network server, normally several application programs are running at the same time:

FTP server, telnet server, mail server, www server, gopher server, etc.;

• TCP must know to which program to deliver the received message;

• If you want to connect to the FTP server it is not enough to know the IP address of the server, you have to specify that you want to talk to the FTP server program;

• This is done by having “the well-known sockets” - TCP ports - (see the file /etc/services on a UNIX machine):

• In a file server session, e.g., two different applications are involved: FTP server and FTP client

– The client program gets commands from the user and passes them to the FTP server program;

– There is no need for the client FTP program to use a well know socket number, because nobody is trying to find it, as opposed to the FTP server program which have to have a well-known socket number, so that people can open connections to it and start sending commands;

– The client FTP program asks the network software to assign it a port number that is guarantee to be unique, for example 1236 if that number was free;

• A connection is identified by four numbers:

connection 1: 192.162.16.2, 1236 193.230.3.120, 21

connection 2: 192.162.16.2, 1237 193.230.3.120, 21

• Two connections are different if at least one number is different


128

Application Addresses: SocketsSocket = IP address + port #

Physical AddressIP Address

PortAddress

PortAddress

App 1 App 2

Physical AddressIP Address

PortAddress

PortAddress

App 1 App 2Message

Segment

DatagramFrame


129

Well-known TCP ports

21 - FTP server

23 - telnet server

25 - SMTP mail server

53 - domain nameserver

109 - POP2 server

110 - POP3 server


130

Flow using Streams (TCP)

Server

socket()

bind()

listen()

accept()

send()/recv()

closesocket()

Client

connect()

send()/recv()

closesocket()

socket()


131

ROUTING

The source and the destination hosts are on the same LAN• There is no decisions for routing;

• The packet is transmitted on the cable (coax, twisted cable, optical fiber);

• Every computer connected to the LAN will receive it.

• That computer which finds that the destination Ethernet address in the header is equal to his Ethernet address will get the message, the others will discard it.

• Note that the address of each computer on the LAN begins with the same network number

• Routing table for host A:

NETWORK GATEWAY INTERFACE192.162.16 none eth0


132

Example of complex configurationA .1

D.4 .1

.2

.1

G.4

.2

.1H

IJ.2

K.3

L.4

.5

.1M

.2N

192.162.16.

193.230.3.

193.230.4.

193.230.5.

193.230.6.backbonenetwork withInternetconnectivity

eth0

ec0

ec0

eth0

eth0

sl0

sl0

sl0

Routing tables net gw int.M: 193.230.5 none eth0 193.230.6.2 sl0 193.230.4 193.230.5.1 eth0 193.230.3 193.230.5.1 eth0 192.162.16 193.230.5.1 eth0 default 193.230.6.2 sl0I 193.230.5 none eth0 193.230.4.1 sl0 193.230.3 193.230.4.1 sl0 192.162.16 193.230.4.1 sl0 default 193.230.5.5 eth0H 193.230.3 none ec0 193,230.4.2 sl0 192.162.16 193.230.1 ec0 default 193.230.4.2 sl0A 192.162.16 none eth0 default 192.162.16.4 eth0

sl0

ec1


133

Routing table initialization and updating• Initialization of routing table

– Normally at startup time by executing script command files;

– Static routes • route add <network-address> <gw-address> <metric>

route add 192.162.16.0 192.162.16.4 1

route add 193.230.3.0 192.162.16.4 1

route add default 192.162.16.4 1

• netstat -rn displays the routing table on a UNIX machine

• Static routes have the disadvantage that they do not adapt to the changes in the network topology

• Dinamic routing protocols are run to update the routing table so that they reflect the changes in topology

• Router classes

– dedicated routers - special purpose equipment

• Cisco, Wellfleet, Proteon, Telebit

– cheap router sollution: - public domain software for PCs

• ka9q, PCROUTE, Linux, Free BSD, etc.


134

Routing protocols• Types of routing protocols

– Interior Gateway Protocol (IGP): RIP, IGRP, OSPF, Hello

– Exterior routing Protocol (EGP): BGP, EGP

AS1AS2

EGPIGP

IGP


135

Autonomous System Number

• An Autonomous System Number (AS) is a set of routers under a single technical administration, using an interior gateway protocol and an exterior gateway protocol to route packets to other ASs.

• An AS is a connected group of IP networks run by one or more network operators which has a single and defined routing policy.

• AS number is a 16 bit number (65535 unique AS numbers).

• It is a finite amount of address space.

• Sometimes, the term AS is misunderstood and used for grouping together a set of prefixes which belong under the same administrative umbrella.

• AS number are assigned by RIPE in Europe


136

Example for routing

EBONE EUROPANET

Access to Internet

National Network

BGP4BGP4

IGRP static IGRP

IGRP IGRP


137

CIDR - Classless Inter-Domain Routing

Internet Service Provider

Internet

193.230.0.0193.230.1.0 193.230.02.0

193.230.3.0

00000000 000000001110011011000001

network host

1110011011000001 00000001 00000000

00000000000000101110011011000001

11000001 1110010 00000011 00000000

Prefix HostClassless representation

Class-full representation

193.230.0.0

193.230.1.0

193.230.2.0

193.230.3.0

customers


138

Example of CIDR configuration (supernetting)

• Using BGP4 routing protocol, all the 4 C class addresses (193.230.0.0, 193.230.1.0, 193.230.2.0, 193.230.3.0) can be advertised like one entry in the routing table:

router bgp 3233

agregate-address 193.230.0.0 255.255.252.0 summary-only

neighbor 192,121,159,97 remote-as 1755

neighbor 193.226.27.86 remote-as 2614

• Using BGP4 routing protocols, all the 256 C addresses of the block 193.230.0.0 - 193.230.255.255 can be advertised like one entry in the routing table:

router bgp 3233

agregate-address 193.230.0.0 255.255.0.0 summary-only

neighbor 192,121,159,97 remote-as 1755

neighbor 193.226.27.86 remote-as 2614


139

IPng Features/Functionality

• Expanded Address Space

• Autoconfiguration

• Real-time/Multimedia support

• Integrated Security support

• IPv4 IPv6 Transition Strategy


140

IP Version 6 - So what’s really changed ?!

IHL Type of Service

Options

Total Length

Identification Flags Fragment Offset

Time to Live Protocol Header Checksum

Source Address

Destination Address

Padding

Priority Flow Label

Payload Length Next Header

Version

Version

IPv4 Header:

IPv6 Header:Hop Limit

• Address space quadrupled to 16 bytes

• Fixed Length (optional headers daisy-chained)

• No Check sum (Done by Link Layer) • No hop-by-hop segmentation (Path MTU discovery)

• Flow Label/Priority (Integrated QoS support)

Source Address

Destination Address


141

IPv6 Autoconfiguration

• Stateful•DHCPng

• Addressing Lifetime• Facilitates graceful renumbering

• Addresses defined as valid, deprecated or invalid

• Stateless

Host autonomously configures its own address

Link Local Addressing

(single subnet scope, formed fromreserved prefix and link layer address)

SUBNET PREFIX

SUBNET PREFIX + MAC ADDRESS

SUBNET PREFIX + MAC ADDRESS


142

IPv6 Real Time/Premium Services support

• Flow based, defines ‘flow label’ and ‘priority’

• Can be combined with Source Routing header options

• Integration with Tag Switching/MPLS:

Insertion into IPv6 Flow Label Field:- Version Flow Label

Tag

• • •

CoS

(Reference/Draft RFC:- draft-baker-flow-label-00.txt)


143

IPv6 Security

• IPSec Architecture

• Export restrictions recently relaxed

• Authentication - MD5 based

• Confidentiality - DES – Encrypt entire datagram or IP payload

• Retain existing use of (packet filtering based) firewalls


144

IPv6 Transition Strategy - Approaches

DRIVER

IPv4 IPv6IPv4 IPv6

APPLICATION

TCP/UDP• Hosts - Dual Stack

(IPv6 API defined)

• Networks - Tunneling

More efficient than building new IPv6 topology

DATA

DATA

Transport Layer Header

Transport Layer Header

IPv6 Header

IPv6 Header IPv4 Header


145

IPv6 Tunneling

• Network Address Translation IPv4 IPv6

IPv6Driver

IPv6 IPv6 IPv6

IPv4 BackboneIPv4

IPv6

DriverIPv4

• Configured tunnels - manual point-2-point links

• Automatic tunnels - via IPv4 compatible IPv6 addresses (96 bits of zeros prefix - 0:0:0:0:0:0/96)

• Instrumental in building existing ‘6-Bone’ (http://www.6bone.net)


146

IPv6 Routing• Hierarchy is key

• Test address space allocation available:- (RFC 1897)

Registry ID Provider ID Subscriber ID Subnetwork ID Interface ID 5 bits 16 bits 24 bits 16 bits 48 bits

• Existing routing protocols extensions for IPv6 RIPv6 - Same destination/mask/metric information as RIPv2 Multiprotocol BGP4+ - Currently Draft Integrated IS-IS - 20 byte NSAP support facilitates IPv6 address/routing EIGRPv6 - Reflects Cisco’s future proofing commitment OSPFv3 - Packet formats changed to reflect 128 bits

• Neighbour Discovery - dynamic host router Combination of ES-IS, ARP and ICMP Redirect


147

IPv6 Current Status - Standardization

• Several key components now Standards/Proposed Standards

Basic Specification Neighbor Discovery

RIP/OSPF ICMPv6/IGMPv6

• Issues remaining open Addressing Registries Interoperability

DHCP IPv6 over all media

Extension Headers


148

IPv6 Current Status - Customers/Vendors• Request for IPv6 support

•Academic Community

•ISP

•Enterprise

• Vendor support:- (the usual suspects!)

BAY Networks Cisco

Digital Ipsilon

Merit Telebit

3Com

Apple FTP Software

Hitachi IBM

Linux NRL

Siemens Nixdorf Sun … etc.


149

REFERENCES• Christian Huitema, Routing in the Internet, Prentice Hall, ISBN 013-132192-7,

1996• Kevin Dowd, “Getting Connected, Internet at 56K and Up”, O’Reilly &

Associates, Inc., Bonn, 1996• Booktexts of Network Technology Workshop, National Network Management

Track, Honolulu, June 1995• Craig Hunt, “TCP/IP Network Administration, O’Reilly & Associates, Inc.,

Sebastopol, 1993• Internetworking Technology Overview, Cisco Systems, Inc., 1993• Booktexts of the 4th Network Seminar and Intensive Course for Scientists and

Network Managers from Central Europe, Feb. 1993, Vienna University Computer Center

• E. Comer, “Internetworking with TCP/IP”, Vol I, Principles, Protocols and Architecture, Prentice Hall, Englewood Cliffs, New Jersey, 1991.

• William Stallings, Data and Computer Communications, Macmillan Publishing Company, New York, 1985.