Chapter 3: Protocols and Services€¦ · Presentation Layer Application Layer Application Process...

Lehrstuhl für Informatik 4

Kommunikation und verteilte Systeme

Chapter 3.1: Layer 3 – The Internet Protocol IP

Data Link Layer

Chapter 3: Protocols and Services• 3.1: Layer 3 - The Internet Protocol IP• 3.2: Routing in the Internet• 3.3: Auxiliary Protocols• 3.4: Quality of Service• 3.5: Layer 4 - Transport Protocols TCP and UDP

Presentation Layer

Session Layer

Chapter 2: Computer Networks Physical Layer

Network Layer

Transport Layer

Application Layer

Internet Protocols

Chapter 4: Application Protocols

OSI Reference Model




Layer 3

Layer 3: Network (Internet) Layer

• Boundary between network carrier and customer

• Control of global traffic:

• Coupling of sub-networks

• Global addressing

• Routing of data packets

• (Global flow control)

Layer 2 is responsible only for the transmission of data

between adjacent computers.




Layers in the Network

PhysicalLayer

Data LinkLayer

NetworkLayer

TransportLayer

SessionLayer

PresentationLayer

ApplicationLayer

ApplicationProcess

PhysicalLayer

Data LinkLayer

NetworkLayer

TransportLayer

SessionLayer

PresentationLayer

ApplicationLayer

ApplicationProcess

PhysicalLayer

Data LinkLayer

NetworkLayer

PhysicalLayer

Data LinkLayer

NetworkLayer

Host A Host BRouter A Router B

Routers in the network receive frames from layer 2, extract the layer 3 content (packet) and decide due to the global address, to which outgoing connection the packet must be passed on. Accordingly the packet becomes payload of a new frame and is sent.




Two Fundamental Philosophies

Connectionless communication• Data are transferred as packets of variable length

• Source and destination address are being indicated• Sending is made spontaneously without reservations

• Very easy to implement • But: packets can take different ways to the receiver (wrong order of packets at the

receiver, differences in transmission delay, unreliability)

Connection-oriented communication• Connection establishment:

- Selection of the communication partner resp. of the terminal,- Examination of the communication readiness,- Establishment of a connection

• Data transmission: Information exchange between the partners• Connection termination: Release of the terminals and channels

• Advantages: no change of sequence, reservation of capacity, flow control




Connectionless Communication

Computer A

Computer C

Computer B

• Message is divided into packets

• Access is always possible, small susceptibility to faults

• Alternative paths for the packets

• Additional effort in the nodes: Store-and-Forwardnetwork

Keyword: Packet Switching

1

3

2




Connection-Oriented Communication

Computer A

Computer C

Computer B

• Simple communication method

• Defined way between the participants

• Switching nodes connect the lines• Exclusive use of the line (telephone) or virtual connection:

Establishment of a connection over a (possibly even packet switching) network

Keyword: Circuit Switching

virtual connection

1

23




Result: ARPANET (predecessor of today's Internet)

Origin of the Internet:• First interconnection of computers in the 1960ies

• A particularly important initiative was initiated by the ARPA(Advanced Research Project Agency, with military interests)

• The participation of the military was the only sensible way to implement such an ambitious and extremely expensive project

• The OSI specification was still in developing phase

On the Way to Today's Internet

Implemented in the Internet in connectionless communication. Why?




Design objective for ARPANET

• The operability of the network should remain intact even after a largest disaster possible, e.g. a nuclear war, thus high connectivity and packet switching

• Network computer and host computer are separated

ARPA

Advanced Research Projects Agency

ARPANET

1969Subnet

ARPANET




A subnet consists of: • Interface Message Processors

(IMPs), which are connected by rented transmission circuits.

• High connectivity (in order to guarantee the demanded reliability)

Host

IMP

Subnet

IMP-IMP

Protocol

Source IMP todestination IMP protocol

Host-host Protocol

Host-IMPProtocol

A node consists of• an IMP (predecessor of a router)

• a host

Several protocols for the communication between IMP-IMP, host-IMP,…

ARPANET




XDS940

DEKPDP-10

XDS1-7

Stanford ResearchInstitutes (SRI)

Universityof Utah

University of CaliforniaLos Angeles (UCLA)

University ofCalifornia SantaBarbara (UCSB)

ARPANET(December 1969)

IMP

IMPIBM

360/75 IMP

IMP California

The Beginning of ARPANET




SRI

UCSB

UCLA

Utah

WITH

Harvard

Illinois

USC

SRI Utah Illinois WITH

USCUCLA

UCSB

Stanford

HarvardAberdeen

CMU

ARPANET in April 1972 ARPANET in September 1972

Very fast evolution of ARPANET within shortest time:

Evolution by ARPANET




Problem: Interworking!Simultaneously to the ARPANET further (smaller) networks were developed.

All the LANs, MANs, WANs

• had different protocols, media,…

• could not be interconnected at first and were not be able to communicate

with each another.

Therefore:

Development of uniform protocols on the transport- and network level (without a too accurate definition of these levels, in particular withoutexact coordination with the respective OSI levels).Result: TCP/IP networks.

Interworking




Developed 1974:

Transmission Control Protocol/Internet Protocol (TCP/IP)

Requirements:

• Fault tolerance

• Maximal possible reliability and availability

• Flexibility (i.e. suitability for applications with very different requirements); however, less or not suitable for real-time applications such as audio or video.

The result:

• Network protocol IP; (Internet Protocol; connectionless)

• End-to-end protocols TCP (Transmission Control Protocol; connection-oriented) and UDP (User Datagram Protocol; connectionless)

TCP/IP

The ARPA network evolved by being connected to lots of other networks to a world-wide available network (called “Internet”) and gradually lost its early militarily dominated character.




What does it mean: “a computer is in the Internet”?

- Use of the TCP/IP protocol suite

- Accessibility over an IP address

- Ability to send IP packets

In its early period, the Internet was limited to the following applications:

E-mail electronic mail (partly because the US post was not very reliable and the different time zones made telephone accessibility of thetelephone partner more difficult)

Remote login running jobs on external computers

File transfer exchange of data between computers

Internet




Internet and Intranet

Internet

• Communication via the TCP/IP protocols

• Local operators control and finance

• Global coordination by some organizations

• Internet Providers provide access points for private individuals

Intranet

• Enterprise-internal communication with the same protocols and applications as in the Internet.

� Computers are sealed off from the global Internet (data security)

� Heterogeneous network structures from different branches can be integrated with TCP/IP easily

� Use of applications like in the WWW for internal data exchange




The TCP/IP Protocol Suite

Networks

Protocols

ApplicationLayer

TransportLayer

InternetLayer

Host-to-

network LayerWireless LANEthernet Token Ring Token Bus

FTP Telnet SMTP DNS SNMP TFTPHTTP

UDPTCP

ICMP IP RARPARPIGMP




Sandglass Model

HTTP, SMTP, FTP, …

E-Mail, File Transfer, Video Conferencing, …

Twisted Pair, Optical Fiber, Radio, …

Because of the small number of centralprotocols but the large number of applications and communicationnetworks, the TCP/IP protocol stack(and applications/networks) can berepresented like a sandglass.




Internet Layer

Routing Protocols Transfer Protocols:IPv4, IPv6

“Control” Protocols:ICMP, ARP, RARP, IGMP

Routing Tables

Raw division into three tasks:• Data transfer over a global network

• Route decision at the sub-nodes (chapter 3.2)• Control of the network- or transmission status (chapter 3.3)




IP: connectionless, unreliable transmission of datagrams/packets (“Best Effort”)

• Transparent end-to-end communication between the hosts

• Routing, interoperability between different network types

• IP addressing (IPv4):

→ Uses 32-bit addresses

→ Hierarchical addressing

→ 3 network classes

→ 4 address formats (including multicast)

• Fragmenting and reassembling of packets

• Maximum packet size: 64 kByte (in practice: 1500 byte)

• At present commonly used: Version 4 of IP protocol: IPv4

IP – Internet Protocol




IP Packet

IP header,normally 20 bytesSource Address

VersionType ofService

Total lengthIHL

Identification Fragment Offset

ProtocolTime to Live Header Checksum

Destination Address

PaddingOptions (variable, 0-40 byte)

32 bits (4 bytes)

Data (variable)

DF

MF

Header Data




The IP Header (1)

• Version: IP version number (for simultaneous use of several IP versions)

• IHL: IP Header Length (in words of 32 bit; between 5 and 15, depending upon options)

• Type of Service: Indication of the desired service: Combination of reliability (e.g. file transfer) and speed (e.g. audio)

• Total Length: Length of the entire packet (in byte, ≤ 216-1 = 65535 bytes)

• Identification: definite marking of a packet

• Time to Live (TTL): Lifetime of packets is limited to maximal 255 Hops (endless circling of packets in the network is prevented). In principle, also the processing time in routers is to be considered, which does not happen in practice. The counter is reduced with each hop, with 0 the packet is discarded.

Prece-dence

D T R unused

3 bit priority(0 = normal data,7 = control packet)

DelayThroughput

Reliability




The IP Header (2)

• DF: Don't Fragment. All routers must forward packets up to a size of 576 byte, everything beyond that is optional. Larger packets with set DF-bit therefore cannot take each possible way in the network.

• MF: More Fragments. “1” - further fragments follow. “0” - last fragment of a datagram)

• Fragment Offset: Sequence number of the fragments of a packet (213 = 8192 possible fragments). The offset states, at which place of a packet (counted in multiples of 8 byte) a fragment belongs. From this a maximum length of 8192 * 8 byte = 65536 byte results for a packet.

• Protocol: Which transport protocol is used in the data part (UDP, TCP,…)? To which transport process the packet is to be passed on?

• Header Checksum: 1’s complement of the sum of the 16-bit half words of theheader. Must be computed with each hop (since TTL changes)

• Source Address/Destination address: Network and host numbers of sending and receiving computer. This information is used by routers for the routing decision.




Fragmentation

• A too large or too small packet length prevents a good performance. Additionally there are often size restrictions (buffer, protocols with length specifications, standards, allowed access time to a channel,…)

• The data length must be a multiple of 8 byte. Exception: the last fragment, there only the remaining data are packed, padding to 8 byte units does not take place.

• If the “DF”-bit is set, the fragmentation is prevented.

777 x00 0 0 1200 bytes

Ident. Flags Offset Data

IP header

777 x01 0 0 511

777 x01 64 512 1023

777 x00 128 1024 1200




The IP Header (3)

Options: Prepare for future protocol extensions. Coverage: Multiple of 4 byte, therefore possibly padding is necessary. At present, 5 options are defined, however none is supported by common routers:

• Security: How secret is the transported information? (Application e.g. in military: Avoidance of crossing of certain countries/networks.)

• Strict Source Routing: Complete path defined from the source to the destination host by providing the IP addresses of all routers which are crossed. (Use by system managers e.g. in case of damaged routing tables or for time measurements)

• Loose Source Routing: The carried list of routers must be passed in indicated order. Additional routers are permitted.

• Record Route: Recording of the IP addresses of the routers passed. (Maximally 9 IP addresses possible, nowadays too few.)

• Time Stamp: Records router addresses (32 bits) as well as a time stamp for each router (32 bits). Application e.g. in fault management.




• Unique IP address for each host and router.

• IP addresses are 32 bits long and are used in the Source Address as well as in Destination Address field of IP packets.

• The IP address is structured hierarchical and refers to a certain network, i.e. machines with connection to several networks have several IP addresses.

• Structure of the address: Network address for physical network (e.g. 137.226.0.0) and host address for a machine in the addressed network (e.g. 137.226.12.221)

IP Addressing

32 bits

10 Network HostB

110 Network HostC 2097151 networks (LANs) with 256 hosts each(starting from 192.0.0.0)

0 Network Host

Class

A

1110 Multicast addressD

1111 Reserved for future useE

16383 networks with216 hosts each(starting from 128.0.0.0)

126 networks with224 host each(starting from 1.0.0.0)




IP Addresses

Binary format

Dotted Decimal Notation

10001001 11100010 00001100 00010101

137.226.12.21

137.226.12.1

137.226.112.1

137.226.112.78

Router

137.226.12.21

• Each node has (at least) one world-wide unique IP address

• Router or gateways, which link several networks with one another, have for each network an assigned IP address

137.226.12.0 137.226.112.0




Class B address

137.226.12.174

Class B addressof RWTH Aachen

10001001 11100010 00001100 10101110

Subnet(Computer science 4)

Terminal“Shadow”

The representation of the 32-bit addresses is divided into 4 sections of each 8 bits:

IP Addressing - Examples

0 0 0 .............................................................. 0 0 0

0 0 ................. 0 0 Host

Network

127 arbitrary

This host

Host in this network

1 1 1 .............................................................. 1 1 1 Broadcast in own local area network

1 1 1 ................................. 1 1 1 Broadcast in the remote network

Loop, no sending to the network

Special addresses:




Address Space

50,00%

25,00%

12,50%

6,25%

6,25%

Klasse A

Klasse B

Klasse C

Klasse D

Klasse E

ClassClassClassClassClass




Problems• Nobody had thought about such a strong growth of the Internet

(otherwise one would have defined longer addresses from the beginning).

• Too many Class A address blocks were assigned in the first Internet years.

• Inefficient use of the address space.

Example: if 500 devices in an enterprise are to be attached, a Class B address block is needed, but by this unnecessarily more than 65.000 host addresses are blocked.

Solution approach

Extension of the address space within IPv6 against the actual version IPv4

⇒ IP version 6 has 128 bit for addresses

⇒ 7 x 1023 IP addresses per square meter of the earth's surface (including the oceans!)

⇒ one address per molecule on earth's surface!

But: The success of IPv6 is not by any means safe! (The introduction of IPv6 is tremendously difficult: Interoperability, costs, migration strategies,….)

IP Addresses are scarce…




Ethernet A 128.10.1.0

128.10.1.3 128.10.1.8 128.10.1.70 128.10.1.26

Ethernet B 128.10.2.0

128.10.2.3 128.10.2.133 128.10.2.18

Restof the

Internet

All trafficfor 128.10.0.0

128.10.2.1

RouterEthernet

AHost

EthernetA

Host

EthernetA

Host

EthernetB

Host

EthernetB

Host

EthernetB

Host

Examples for subnets: subnet mask 255.255.255.0

Problem: Class C-networks are very small, Class B-networks often already too large. Therefore exists the possibility of dividing an through the IP-address identified network into so-called subnets.

IP Subnets




IP Subnets

• Within an IP network address block, several physical networks can be addressed

• Some bits of the host address part are used as network ID

• A Subnet mask identifies the „abused” bits

• All hosts of a network should use the same subnet mask

• Routers can determine through combination of an IP address and a subnet mask, into which subnet a packet must be sent.

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0SubnetMask

Network HostClass B address

10 Network HostSubnet




IP Subnets - Computation of the Destination

0111 100010001001

137. 226 . 12 . 21

11100010 00001100 00010101

255. 255. 255. 0

12. 0137. 226.

0000 0000

0000 0000

1111 1111

0000 1100

1111 1111

1110 0010

1111 1111

1000 1001

IP address

Subnet mask

Network of the addressed host

AND

The entrance router of the RWTH, which receives the IP packet, does not know, where host “12.21” is located.

The router computes the subnet “137.226.12” and sends the packet to the router, which links this subnet.




Assigned network address for the RWTH:137.226.0.0

137.226.12.0 Info IV

137.226.8.0

254 hosts per subnet:• Hosts have the addresses

1 – 254

• 0 is reserved for the subnet• 255 is reserved for broadcast

in the subnet

Subnet mask for each subnet = 255.255.255.0

Other format for writing of addresses in subnets:

137.226.12.221/24

137.226.112.0137.227.10.0

Example of a Subnet

Number of bits for “(sub)network address”




More flexible Addressing

Problem: static categorization of IP addresses - how to use the very small class C networks efficiently?

Remedy: Classless Inter-Domain Routing (CIDR)Weakening of the rigid categorization by replacing the static classes by network prefixes of variable length

• Example: 137.250.3/17: The first 17 bits of the IP address are used for the network identification

• Used together with routing: Backbone router, e.g. on transatlantic links only considers the first 13 bits; thus small routing tables, little cost of routing decision

• Routers of an ISP considers e.g. only the first 15 bits

• Routers in company networks consider e.g. the first 25 bits




NAT – Network Address Translation

• Only routers, which are attached to the “external world” need a global address• Assign only one (or a few) IP addresses to a company• Each internal computer uses an own IP address. For this purpose, “private”

address blocks are reserved, which everyone may use within its own networks and which are never routed in the Internet:

• 10.0.0.0 - 10.255.255.255

• 172.16.0.0 - 172.31.255.255• 192.168.0.0 – 192.168.255.255

• When leaving the own network, an address translation takes place• Problem: how can the re-translation into the internal IP address take place?• Solution: Network Address Port Translation – store additional port information in

your entrance router (NAT box)




NAPT – Network Address Port Translation

80207.17.4.21198.60.42.12150010.0.0.71500TCP

21137.226.12.221198.60.42.12106610.0.0.11066TCP

Port (destination)

IP (destination)IP (global)Port (global)IP (local)Port (local)Protocol




Why changing the protocol, when IPv4 works well?

• Dramatically increasing need for new IP addresses

• Improved support of real time applications

• Security mechanisms (Authentication and data protection)

• Differentiation of types of service, in particular for real time applications

• Support of mobility (hosts can go on journeys without address change)

• Simplification of the protocol in order to ensure a faster processing

• Reduction of the extent of the routing tables

• Option for further development of the protocol

The new IP - IPv6

Standardized: IPv6 (December 1995, RFC 1883)




• Address size– 128-bit addresses (8 groups of each 4 hexadecimal numbers)

• Improved option mechanism– Simplifies and accelerates the processing of IPv6 packets in routers

• Auto-configuration of addresses– Dynamic allocation of IPv6-addresses

• Improvement of the address flexibility– Anycast address: Reach any one out of several

• Support of the reservation of resources– Marking of packets for special traffic

• Security mechanisms– Authentication and Privacy

• Simpler header structure: – IHL: redundant, no variable length of header by new option mechanism

– Protocol, fragmentation fields: redundant, moved into the options – Checksum: Already done by layer 2 and 4

IPv6 - Characteristics




• Version: IP version number• Priority: 4 bits for priority. 1 - News, 4 -

ftp, 6 - telnet, 8 to 15 - real time traffic• Flow label: virtual connection with certain

characteristics/requirements• PayloadLen: packet length after the 40-

byte header• Next Header: 8-bit selector. Indicates the

type of the following extension header (or the transport header)

• HopLimit: At each node decremented by one. At zero the packet is discarded

• Source Address: The address of the original sender of the packet

• Destination Address: The address of the receiver (not necessarily the final destination, if there is an optional routing header)

• Next Header/Data: if an extension header is specified, it follows after the main header. Otherwise, the data are following

1 4 8 3216 24

The prefix of an address characterizesgeographical areas, providers, local internal areas,…

IPv6 Main Header

Version (4)

Next Header (8) HopLimit (8)PayloadLen (16)

Flow label (24)

Source Address (128)

Destination Address (128)

Next Header/Data

Priority (4)




Optional data follows in extension headers. There are 6 headers defined:

• Hop by Hop (information for single links)All routers have to examine this field. Momentarily only the support of Jumbograms (packets exceeding the normal IP packet length) is defined (length specification).

• Routing (definition of a full or partly specified route)

• Fragmentation (administration of fragments)Difference to IPv4: Only the source can do fragmentation. Routers for which a packet is too large, only send an error message back to the source.

• Authentication (of the sender)

• Ciphered data

• Destination options (additional information for the destination)

IPv6 Extension Headers




IPv4 vs. IPv6: Header

En sion

Priority

Flow label

PayloadLen NEXTheaders

Hop limit

SOURCE ADDRESS

SOURCE ADDRESS

SOURCE ADDRESS

SOURCE ADDRESS

Destination ADDRESS

Destination ADDRESS

Destination ADDRESS

Destination ADDRESS

NEXT header/DATA

En sion

IHL Totally lengthType OF service

Identification Fragment offset

Time to Live

Protocol Header Checksum

SOURCE ADDRESS

Destination ADDRESS

Option (variable)/Padding

DATA

The IPv6 header is longer, but this is only caused by the longer addresses.Otherwise it is “better sorted” and thus faster to process by routers.

Ver-sion

4 8 16 32

Prio-rity

Flow Label

PayloadLenNext

HeaderHop Limit

Source Address

Source Address

Source Address

Source Address

Destination Address

Destination Address

Destination Address

Destination Address

Next Header / Data

4 8 16 32

Ver-sion

IHL Total LengthType of Service

Identification

Time to Live

Protocol

Source Address

Destination Address

Options (variable)/Padding

Data

Chapter 3: Protocols and Services€¦ · Presentation Layer Application Layer Application Process...

Documents

Transcript of Chapter 3: Protocols and Services€¦ · Presentation Layer Application Layer Application Process...