Layer 3 Layers in the Network Two Fundamental Philosophies

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Lehrstuhl für Informatik 4
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Layer 3
Chapter 3: Protocols and Services
Layer 2 is responsible only for the transmission of data
•
•
•
•
•
between adjacent computers.
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
OSI Reference Model
Layer 3: Network (Internet) Layer
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Internet Protocols
• Global addressing
• (Global flow control)
Physical Layer
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Chapter 3.1: Layer 3 – The Internet Protocol IP
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Two Fundamental Philosophies
Application
Process
Application
Process
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.
Transport
Layer
Application
Layer
Presentation
Layer
Session
Layer
Transport
Layer
Network
Layer
Network
Layer
Network
Layer
Network
Layer
Data Link
Layer
Data Link
Layer
Data Link
Layer
Data Link
Layer
Physical
Layer
Physical
Layer
Physical
Layer
Physical
Layer
Host A
Router A
Router B
Host B
Chapter 3.1: Layer 3 – The Internet Protocol IP
Chapter 3.1: Layer 3 – The Internet Protocol IP
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Layers in the Network
Session
Layer
• Coupling of sub-networks
Data Link Layer
Chapter 2: Computer Networks
Presentation
Layer
• Control of global traffic:
• Routing of data packets
Chapter 4: Application Protocols
Application
Layer
• Boundary between network carrier and customer
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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
Chapter 3.1: Layer 3 – The Internet Protocol IP
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Connectionless Communication
Connection-Oriented Communication
Computer A
Computer B
Computer A
Computer B
1
1
2
3
3
virtual connection
2
Computer C
Computer C
• 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-Forward
network
• Simple communication method
Keyword: Packet Switching
Chapter 3.1: Layer 3 – The Internet Protocol IP
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Keyword: Circuit Switching
• 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
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On the Way to Today's Internet
ARPANET
Implemented in the Internet in connectionless communication. Why?
Design objective for ARPANET
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 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
• The OSI specification was still in developing phase
ARPA
Advanced Research
Projects Agency
ARPANET
Subnet
1969
Result: ARPANET (predecessor of today's Internet)
Chapter 3.1: Layer 3 – The Internet Protocol IP
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Chapter 3.1: Layer 3 – The Internet Protocol IP
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ARPANET
The Beginning of 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)
XDS
940
Host-host Protocol
Stanford Research
Institutes (SRI)
Host
Host-IMP
Protocol
IMP
IMP
Source IMP to
destination IMP protocol
IMP
IBM
360/75
P
-I M
IMP tocol
Pro
A node consists of
• an IMP (predecessor of a router)
• a host
IMP
University of
California Santa
Barbara (UCSB)
Subnet
University
of Utah
IMP
XDS
1-7
Several protocols for the
communication between
IMP-IMP, host-IMP,…
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DEK
PDP-10
California
University of California
Los Angeles (UCLA)
ARPANET
(December 1969)
Chapter 3.1: Layer 3 – The Internet Protocol IP
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Evolution by ARPANET
Interworking
Very fast evolution of ARPANET within shortest time:
Problem: Interworking!
SRI
Utah
Illinois
WITH
Simultaneously to the ARPANET further (smaller) networks were developed.
All the LANs, MANs, WANs
SRI
Utah
Illinois
USC
Stanford
• had different protocols, media,…
Harvard
Aberdeen
• could not be interconnected at first and were not be able to communicate
with each another.
WITH
UCSB
Therefore:
UCSB
UCLA
Harvard
ARPANET in April 1972
Chapter 3.1: Layer 3 – The Internet Protocol IP
UCLA
USC
CMU
ARPANET in September 1972
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Development of uniform protocols on the transport- and network level
(without a too accurate definition of these levels, in particular without
exact coordination with the respective OSI levels).
Result: TCP/IP networks.
Chapter 3.1: Layer 3 – The Internet Protocol IP
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TCP/IP
Internet
What does it mean: “a computer is in the Internet”?
Developed 1974:
Transmission Control Protocol/Internet Protocol (TCP/IP)
- Use of the TCP/IP protocol suite
Requirements:
- Accessibility over an IP address
• Fault tolerance
- Ability to send IP packets
• Maximal possible reliability and availability
In its early period, the Internet was limited to the following
applications:
• 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:
E-mail
electronic mail (partly because the US post was not very reliable
and the different time zones made telephone accessibility of the
telephone partner more difficult)
Remote login
running jobs on external computers
File transfer
exchange of data between computers
• Network protocol IP; (Internet Protocol; connectionless)
• End-to-end protocols TCP (Transmission Control Protocol; connection-oriented)
and UDP (User Datagram Protocol; connectionless)
The ARPA network evolved by being connected to lots of other networks to a worldwide available network (called “Internet”) and gradually lost its early militarily
dominated character.
Chapter 3.1: Layer 3 – The Internet Protocol IP
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Chapter 3.1: Layer 3 – The Internet Protocol IP
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Internet and Intranet
The TCP/IP Protocol Suite
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
HTTP
Protocols
FTP
Telnet
SMTP
DNS
SNMP
TFTP
Transport
Layer
UDP
TCP
Application
Layer
Intranet
•
Enterprise-internal communication with the same protocols and applications
as in the Internet.
Computers are sealed off from the global Internet (data security)
IGMP
Networks
Heterogeneous network structures from different branches can be
integrated with TCP/IP easily
Ethernet
ICMP
Token Ring
IP
Token Bus
ARP
RARP
Wireless LAN
Internet
Layer
Host-tonetwork Layer
Use of applications like in the WWW for internal data exchange
Chapter 3.1: Layer 3 – The Internet Protocol IP
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Chapter 3.1: Layer 3 – The Internet Protocol IP
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Sandglass Model
Internet Layer
E-Mail, File Transfer,
Video Conferencing, …
HTTP, SMTP, FTP, …
Because of the small number of central
protocols but the large number of
applications and communication
networks, the TCP/IP protocol stack
(and applications/networks) can be
represented like a sandglass.
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)
Routing Protocols
Transfer Protocols:
IPv4, IPv6
Routing Tables
“Control” Protocols:
ICMP, ARP, RARP, IGMP
Twisted Pair, Optical Fiber,
Radio, …
Chapter 3.1: Layer 3 – The Internet Protocol IP
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Chapter 3.1: Layer 3 – The Internet Protocol IP
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IP – Internet Protocol
IP Packet
32 bits (4 bytes)
IP: connectionless, unreliable transmission of datagrams/packets (“Best Effort”)
• Transparent end-to-end communication between the hosts
Version IHL
• Routing, interoperability between different network types
Type of
Service
Identification
• IP addressing (IPv4):
Time to Live
→ Uses 32-bit addresses
→ Hierarchical addressing
Protocol
Total length
DM
FF
Fragment Offset
Header Checksum
Source Address
→ 3 network classes
IP header,
normally 20 bytes
Destination Address
→ 4 address formats (including multicast)
• Fragmenting and reassembling of packets
Options (variable, 0-40 byte)
Padding
• Maximum packet size: 64 kByte (in practice: 1500 byte)
Data (variable)
• At present commonly used: Version 4 of IP protocol: IPv4
Chapter 3.1: Layer 3 – The Internet Protocol IP
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Chapter 3.1: Layer 3 – The Internet Protocol IP
Header
Data
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The IP Header (1)
The IP Header (2)
• Version: IP version number (for simultaneous use of several IP versions)
• 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.
• 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)
Precedence
3 bit priority
(0 = normal data,
7 = control packet)
• 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.
D T R unused
Delay
Reliability
Throughput
• Protocol: Which transport protocol is used in the data part (UDP, TCP,…)? To
which transport process the packet is to be passed on?
• 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.
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• Header Checksum: 1’s complement of the sum of the 16-bit half words of the
header. 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.
Chapter 3.1: Layer 3 – The Internet Protocol IP
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Fragmentation
The IP Header (3)
• 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.
Ident. Flags Offset
777
x00
Data
0
0
x01
• Security: How secret is the transported information? (Application e.g. in
military: Avoidance of crossing of certain countries/networks.)
1200 bytes
• Loose Source Routing: The carried list of routers must be passed in
indicated order. Additional routers are permitted.
0
0
777
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:
• 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)
IP header
777
• MF: More Fragments. “1” - further fragments follow. “0” - last fragment of a
datagram)
x01
64
511
512
777 x00 128
Chapter 3.1: Layer 3 – The Internet Protocol IP
• Record Route: Recording of the IP addresses of the routers passed.
(Maximally 9 IP addresses possible, nowadays too few.)
1023
1024
• Time Stamp: Records router addresses (32 bits) as well as a time stamp
for each router (32 bits). Application e.g. in fault management.
1200
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Chapter 3.1: Layer 3 – The Internet Protocol IP
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IP Addressing
IP Addresses
• 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)
32 bits
A
0
B
10
C
110
Network
137.226.112.1
Router
137.226.12.21
16383 networks with
216 hosts each
(starting from 128.0.0.0)
Host
Network
137.226.12.1
126 networks with
224 host each
(starting from 1.0.0.0)
Class
Host
Network
Host
D
1110
Multicast address
E
1111
Reserved for future use
2097151 networks
(LANs) with
256 hosts each
(starting from 192.0.0.0)
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Chapter 3.1: Layer 3 – The Internet Protocol IP
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137.226.112.0
137.226.12.0
137.226.112.78
Binary format
10001001 11100010 00001100 00010101
Dotted Decimal Notation
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
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IP Addressing - Examples
Address Space
The representation of the 32-bit addresses is divided into 4 sections of each 8 bits:
6,25%
137.226.12.174
6,25%
10001001
11100010
00001100
10101110
12,50%
Class B address
of RWTH Aachen
Class B address
Subnet
(Computer science 4)
Terminal
“Shadow”
50,00%
Special addresses:
0 0 0 .............................................................. 0 0 0
This host
0 0 ................. 0 0
Host in this network
Host
1 1 1 .............................................................. 1 1 1
Network
127
1 1 1 ................................. 1 1 1
arbitrary
Chapter 3.1: Layer 3 – The Internet Protocol IP
Broadcast in own local area network
Klasse
Class A
Klasse
Class B
Class C
Klasse
Class D
Klasse
Class E
Klasse
25,00%
Broadcast in the remote network
Loop, no sending to the network
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Chapter 3.1: Layer 3 – The Internet Protocol IP
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IP Addresses are scarce…
IP Subnets
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.
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.
Examples for subnets: subnet mask 255.255.255.0
• Inefficient use of the address space.
Ethernet A 128.10.1.0
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!)
All traffic
for 128.10.0.0
Rest
of the
Internet
128.10.1.3
Router
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Ethernet
A
Host
Ethernet B 128.10.2.0
128.10.2.133 128.10.2.18
128.10.2.3
Ethernet
B
Host
Ethernet
B
Host
But: The success of IPv6 is not by any means safe! (The introduction of IPv6 is
tremendously difficult: Interoperability, costs, migration strategies,….)
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Ethernet
A
Host
128.10.2.1
⇒ one address per molecule on earth's surface!
Chapter 3.1: Layer 3 – The Internet Protocol IP
128.10.1.70 128.10.1.26
128.10.1.8
Ethernet
A
Host
Ethernet
B
Host
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IP Subnets
IP Subnets - Computation of the Destination
The entrance router of the RWTH, which receives the IP packet, does not know,
where host “12.21” is located.
.
12
.
21
137.
226
• Within an IP network address block, several physical networks can be
addressed
• Some bits of the host address part are used as network ID
0111
1000
10001001
11100010
00001100
• A Subnet mask identifies the „abused” bits
IP address
AND
Class B
address
Subnet
Mask
Network
255.
Host
255.
00010101
255.
0
1111 1111 1111 1111 1111 1111 0000 0000
11111111111111111111110000000000
Subnet mask
10
Network
Subnet
Host
137.
226.
12.
0
1000 1001 1110 0010 0000 1100 0000 0000
Network of the addressed host
• 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.
Chapter 3.1: Layer 3 – The Internet Protocol IP
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The router computes the subnet “137.226.12” and sends the packet to the router,
which links this subnet.
Chapter 3.1: Layer 3 – The Internet Protocol IP
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Example of a Subnet
More flexible Addressing
Assigned network address for the RWTH:
137.226.0.0
137.226.12.0 Info IV
137.226.8.0
137.226.112.0
254 hosts per subnet:
• Hosts have the addresses
1 – 254
• 0 is reserved for the subnet
• 255 is reserved for broadcast
in the subnet
137.227.10.0
Subnet mask for each subnet = 255.255.255.0
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
Other format for writing of addresses in subnets:
137.226.12.221/24
Number of bits for “(sub)network address”
Chapter 3.1: Layer 3 – The Internet Protocol IP
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NAT – Network Address Translation
NAPT – Network Address Port 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)
Chapter 3.1: Layer 3 – The Internet Protocol IP
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Chapter 3.1: Layer 3 – The Internet Protocol IP
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Protocol Port (local) IP (local) Port (global) IP (global)
IP (destination)
Port
(destination)
TCP
1066
10.0.0.1 1066
198.60.42.12 137.226.12.221
21
TCP
1500
10.0.0.7 1500
198.60.42.12 207.17.4.21
80
Chapter 3.1: Layer 3 – The Internet Protocol IP
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The new IP - IPv6
IPv6 - Characteristics
• Address size
– 128-bit addresses (8 groups of each 4 hexadecimal numbers)
Why changing the protocol, when IPv4 works well?
• Improved option mechanism
– Simplifies and accelerates the processing of IPv6 packets in routers
• Dramatically increasing need for new IP addresses
• Auto-configuration of addresses
– Dynamic allocation of IPv6-addresses
• Improved support of real time applications
• Security mechanisms (Authentication and data protection)
• Improvement of the address flexibility
– Anycast address: Reach any one out of several
• Differentiation of types of service, in particular for real time applications
• Support of mobility (hosts can go on journeys without address change)
• Support of the reservation of resources
– Marking of packets for special traffic
• Simplification of the protocol in order to ensure a faster processing
• Security mechanisms
– Authentication and Privacy
• Reduction of the extent of the routing tables
• Option for further development of the protocol
• 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
Standardized: IPv6 (December 1995, RFC 1883)
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Chapter 3.1: Layer 3 – The Internet Protocol IP
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IPv6 Main Header
• 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 40byte 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
Chapter 3.1: Layer 3 – The Internet Protocol IP
IPv6 Extension Headers
1
4
Version
(4)
16
8
Priority
(4)
24
32
Flow label (24)
PayloadLen (16)
Next Header (8)
HopLimit (8)
Source Address (128)
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)
Destination Address (128)
• 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)
Next Header/Data
• Ciphered data
• Destination options (additional information for the destination)
The prefix of an address characterizes
geographical areas, providers, local internal
areas,…
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Chapter 3.1: Layer 3 – The Internet Protocol IP
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Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
IPv4 vs. IPv6: Header
4
VerEn
sion
8
IHL
16
Type
TypeOF
of
Service
service
Identification
Time to
Live
32
Totally
Total Length
length
Fragment offset
Protocol
Header Checksum
4
8
16
VerEn PrioPrio
sion rity
32
Flow
Flow Label
label
PayloadLen
Next
NEXT
Header
headers
Hop
Hop Limit
limit
SOURCE
Source ADDRESS
Address
SOURCE
Source ADDRESS
Address
SOURCE
Source ADDRESS
Address
Destination
DestinationADDRESS
Address
SOURCE
Source ADDRESS
Address
Options
Option (variable)/Padding
(variable)/Padding
SOURCE
Source ADDRESS
Address
DATA
Data
Destination
DestinationADDRESS
Address
Destination
DestinationADDRESS
Address
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.
Chapter 3.1: Layer 3 – The Internet Protocol IP
Destination
DestinationADDRESS
Address
Destination
DestinationADDRESS
Address
NEXT
Next Header
header/DATA
/ Data
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