Microsoft PowerPoint Presentation: 03_1_Network_Layer

prunelimitNetworking and Communications

Oct 23, 2013 (3 years and 9 months ago)

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Communication Systems

3
rd

lecture

Chair of Communication Systems

Department of Applied Sciences

University of Freiburg

2006

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Communication Systems

Last lecture


Message segmentation in packet switched networks advantage over
message switching


Different types of packet forwarding/routing in


Datagram networks (Internet, ...)


Virtual circuit networks (ISDN, ATM, ...)


Got taxonomy of different network types (circuit and packet switching
with respective subtypes)


Requirements for communication between so called end systems


Network access, different types (home, company, mobile)


depending on requirements of end users

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Communication Systems

Last lecture


Physical representation of digital bit streams all kinds of
electromagnetic waves (electromagnetic spectrum)


Encoding, decoding of data for transport over different types of
media


Physical parameters: frequency, wavelength, (effective) bandwidth,
Nyquest formula for max. bandwidth of given medium


Copper wire (single twisted pair for telephone, higher quality for
ethernet, fiber optics), “air” (mobile phones, satellite links, WLAN, ...)


Guided and unguided media, propagation delay (speed of light)

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plan for this lecture


Standards and network layering models


OSI and IP


Need of an universal service


IP as layer 3 network protocol


Start with look at IP header

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network layer models


Talked of some base concepts of data communication and
bit transportation


But how to do that in an ordered and general way?


A structured composition of networks is needed for data
communication of very different machines and operating
systems


There are several of these models, the ISO/OSI layering
model is one of them


ISO: International Standards Organization


OSI: Open Systems Interconnect


Reference model for implementation of network
architectures

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Communication Systems

network layer models


OSI: “Academic model” which shows seven layers


It helps to illustrate and implement the core function of networks,
but no real networking architecture is modeled after it


More practical is the TCP/IP layering model with fewer layers


In general:


Layering breaks down very complex tasks into simpler ones


Implementation details in one layer are abstracted away from
the others


But: Can introduce overhead and need for intentional violation
of layering concepts

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network layer models
-

“academic” OSI model

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comparison of OSI and TCP/IP layers

OSI Layers


Application Layer


Presentation Layer


Session Layer


Transportation Layer


Network Layer


Data Link Layer


Physical Layer

TCP/IP Layers


Application Layer


Transportation Layer


Internet Layer


Physical Layer


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comparison of OSI and TCP/IP layers

OSI in comparison to TCP/IP (developed by ARPA)

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why talking about layer models?


There are quite a few layering models with different levels of
abstraction


Some models reduce the OSI to five layers and move session and
presentation into application (Tanenbaum)


Some real live employment of networks will show that some layers
have to be split up


Tunneling of protocols

and protocol stacks through other layers
or protocols would introduce rather complex models, tunneling can
occur on various layers


ethernet in ATM


IP and others over PPP


IP over DNS


useful for many hotspots with blocked general IP but
open DNS, IP over HTTP/WAP ...

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Communication Systems

why talking about layer models? Cont.


Network layering is not a strictly defined issue, you will find
sublayers, e.g. in Mobile networks like GSM or 802.11


Much combinations of layers and protocols are possible (and
used
-

“tunneling of stacks within layers”)

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Communication Systems

why talking about layer models? Cont.


Further on the lecture will embroider some of the presented layers
and ignore others


But layering will help to understand complex problems and split
them into manageable units


general concept

of computer
science


The next part of this lecture will deal with the network layer (present
in nearly every network model)


The most important representative of this layer is the
internet
protocol (IP)


IP used in every host
-
to
-
host connection


Many physical layer implementations


Many applications operating over IP


So we will start with IP now ...

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Communication Systems

universal service connecting networks


We introduced some different kinds of physical networks


Modem, ISDN, DSL for connection of individuals


Technologies like Ethernet (over copper and fiber optics)


Wireless networks, ...


Many implementations for different networking purposes


Wide Area Networks (
WAN
) which may span countries (e.g.
DFN

for Germany or
GEANT

for Europe) or even continents;
connecting infrastructure components like routers not end user
machinery


Local Area Networks (
LAN
) which implemented within buildings,
mostly bridging only short distances with many hosts connected


In the past distinction of networks by bandwidth offered

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universal service connecting networks


Each type of network structure may require or be
implemented with different low level protocols

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connecting networks


DSL as an example for dedicated point
-
to
-
point connection over a
two wire copper cable with a length up to 6km


Ethernet


packet orientated LAN protocol


Multilink (broadcast) network with no dedicated point
-
to
-
point
connections


Different speeds over copper wire, coaxial cable and fiber
optics


ATM


connection orientated LAN and WAN protocol


Virtual ptp connection through virtual channels and pathes


Modem, ISDN for WAN, FDDI, TokenRing, ... for LAN


Wireless links on GPRS, HSCSD, UMTS, WLAN, ...

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connecting networks cont.


Intermediate protocol with unified addressing scheme is needed

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connecting networks


Universal protocol needed is implemented in the network layer


Network layer is
third layer

in OSI model


Physical layer (first layer) implements

the real bit connection
over different media (e.g. Twisted pair or optical fiber with
Ethernet or “air” for UMTS or GSM)


Data Link Layer contains higher level protocols of Ethernet, GSM,
UMTS, ... not discussed in depth in this lecture


Each layer adds its header to a packet


Needed for packet handling and routing

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connecting networks


Helper protocols needed during packet processing


Inform senders on congestion or lost packets


Map different layers addresses on each other

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functions of an universal service


Define unified addressing scheme which is hardware/software
independent


Realize datagram delivery between networks


Hosts



end systems as defined in first lecture


Routers

touch two or more networks, forward network
-
layer
datagrams between them (routers use layer 3)


Routers

execute routing protocols to learn how to reach
destinations


Universal service protocol should be an open standard without
fees and licenses to be paid to get acceptance


May implement/use helper protocols (
ARP

and
ICMP

for IP,
introduced in practical or theoretical exercises)

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issues of an universal service


Network layer provides end
-
to
-
end delivery (routing)


Provides consistent datagram abstraction:


best
-
effort delivery


no error detection on data


consistent maximum datagram size


consistent global addressing scheme


Link layer (second in OSI) networks provides delivery within the
same network


Typically includes its own addressing format (e.g. Ethernet), and
maximum frame size (MTU)

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internet protocol details


IP version 4 is current


IPv6 forthcoming


Solution to the address space exhaustion of IPv4


Predicted for a while, but limit not reached yet


Solutions for preserving numbers in IPv4 (masquerading,
private networks ...)


At the moment nobody knows when it will be used other then
in backbone structures


3G/UMTS mobile telephone market may push IPv6


Defined for a while


we will spend a dedicated lecture on it

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internet protocol details


Protocol header includes:


Version field


Source and Destination addresses


Lengths (header, options, data)


Header checksum


Fragmentation control


TTL, and TOS info



But TOS info often ignored


Easy changeable along the path (so what for?)

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internet protocol header details


Version field (4 standard, 5 STII, 6 next gen IP) and IP header
length are of 4 byte


IP header normally consists of 20Byte, with options more


Length needed to compute where next header starts

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internet protocol header details cont.


IP options may sum up to 40Byte


Total length field is 16bit, maximum packet length therefore may
not exceed
64kByte


Minimum is 20Byte (just the IP header)


MTU of standard physical networks much smaller (e.g.
1,5kByte)


16bit identification field for fragments


Set for every packet by original sender of datagram


Sender can not know if fragmentation may occur


Initial message segmentation may not small enough


Copied into each datagram during fragmentation

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internet protocol header


fragmentation


Content of 16bit identification field is computed by sender


Different OS use different computing schemes (tool “nmap” in
practical course)


Might give away information on OS, internal network structure


Masqueraded machines could be identified by their
fragmentation IDs


Counter on every machine will have different values (amount
of traffic generated, computing scheme ...)


A private network may give more information away than
intended

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internet protocol header


fragmentation


Flags for fragmentation control


MF
: more fragments (follow)


DF
: dont fragment (some protocol implementation like DHCP
in Boot
-
ROMs are not able to reassemble fragmented
packets), feature may be used for MTU path discovery
(increment packet size until ICMP error message is generated
because auf fragmentation need)


Fragmentation offset


Offset of this fragment into the original datagram


Zero if no fragmentation used


why offset and not fragment number?
-

if further fragmentation
is needed

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ip header


protocol field


Protocol field


the payload with headers removed is passed
to a higher layer in the networking stack
-
> where?


There are different transportation layer protocols for different
purposes


1: ICMP


discussed later this lecture


6: TCP


Transmission Control Protocol


17: UDP
-

User Datagram Protocol


50: ESP


Encapsulating Security Payload


51: AH
-

Authentication Header


All protocol names and corresponding numbers are listed in
(/etc/)protocols file (linux operating system


see practical course)

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ip header


protocol field


In general: each layer has to provide the information which
upper layer should process a given type of packets


Each protocol adds its own header to the packet

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ip header


address fields


Source address


32 bit length defines the IP address space


Should never be changed through ordinary routing (there are
some exceptions like network address translation (NAT))


Protocol does not force authentication of source (often
enforced by modern routers now)


Destination address


32 bit length defines the IP address space


Should never be changed through ordinary routing


Changes when source routing used (realised through IP
option header)

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internet protocol


header details


Source routing


Special handling for particular datagrams, sometimes don't
take router's "fast path"


Rarely used, but the more common are: Loose Source
Routing, Strict Source Routing and Record Route


Timestamp


Must copied on fragmentation


More on routing little bit later on

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ip


fragmentation of packets


Gave short introduction already, but ...


Adapting datagram size one of the most important tasks of the
internetworking protocol:


IP datagrams itself cannot exceed 64kbyte


Lower protocol levels report MTU (max. transfer unit)


Linux loopback 16384byte


Ethernet frames offer max. payload of
1500byte


ATM offers 48byte


slow modem
-
ppp connections 296byte packet length


The tool
ifconfig or ip

(first practical course) reports MTU of each
interface

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ip


fragmentation of packets


Fragmentation & Reassembly


divide network
-
layer datagram into multiple link
-
layer units, all
have to be equal or smaller then link MTU size


Further fragmentation may be needed if MTU is decreased
along the path again


Sometimes it is more clever to set MTU smaller at source to
avoid later fragmentation


Reconstruct datagram at final station


Each fragment otherwise acts as a complete, routeable datagram


Datagrams are identified by the (source, destination,
identification) triple


Concept of fragmentation changes with IP v6

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ip


fragmentation of packets cont.


If fragmented, identification triple is copied into each resulting
packet


Also contains (offset, length, more) triple


more
-

boolean indicates is last fragment


offset
-

relative to original datagram


Relating fragments to original datagram provides:


Tolerance to re
-
ordering and duplication


Ability to fragment fragments (!)

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ip


fragmentation of packets cont.


IP fragments are re
-
assembled at final destination before
datagram is passed up to transport layer


Routers do not reassemble fragmented datagrams


Allows for independent routing of fragments


Reduces complexity (need for CPU and memory) in routers


Problems with fragmenting:


Loss of 1 or more fragments implies loss of datagram at the IP
layer


IP is best effort, provides no retransmission, will time
-
out if
frag(s) appear to be lost


May be interesting for DoS attacks

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ip


fragmentation of packets cont.


Avoid fragmentation through computing path MTU


Problems if path changes (dynamic routing) and new path has
smaller MTU along its way


Adapting size of packets in the source machine according to the
“minimum MTU”:
Path MTU Discovery


IP v6 uses MTU discovery and assumes standard
minimum MTU


If datagram size is smaller then MTU, no fragmentation needed


How to do this?


Probe network for largest size that will fit


If possible, have network tell us this size


Operates through ICMP messaging (presented later on)

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ip


addressing scheme


We saw that IP packet header reserved two 32 bit fields for
source and destination address


For computation for delivery decisions the binary form is used
only


Programs and operating systems implementing IP automatically
convert the addresses between the two representations


IP addresses are topologically sensitive


Interfaces on same network share prefix


Prefix is assigned via ISP/local network administrator


32
-
bit globally unique

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ip


addressing scheme cont.


Address is split into two virtual parts: network and host part


See later how division is done


For better reading the binary representation could be split into
four octets, which are transferred into the decimal system

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ip


addressing scheme cont.


The early IP standard defined five address classes: A, B, C, D
and E


An IP address should be
selfexplanatory, it should countain
information on the networking sub structures


History by now


In this view the address consists of a pattern of high order
bits, which shows their class, the network and the host
component


Machines in the same network share a common prefix (the
class definition and network component of IP) and must have
unique suffix (the host component of IP)

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ip


(historic) address classes

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ip


address classes cont.


Class
A
: (high order bits: 0)


Large Organizations, few nets (127), huge number of hosts (16.7
million)


Address range in decimal notation 0.0.0.0


127.255.255.255


Class
B
: (high order bits: 10)


Medium sized organizations and firms, e.g. University of Freiburg,
some nets (16,384) and large number of hosts (65,536)


Address range 128.0.0.0


191.255.255.255


Class
C
: (high order bits: 110)


Small organizations and firms, relatively large number of nets
(2,097,152) with a small number of hosts per net (256)


Ranging from 192.0.0.0


223.255.255.255

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ip


address classes cont.


Class
D
: (high order bits: 1110)


Multicast addresses, but service are not very often used


Address range 224.0.0.0


239.255.255.255


Class
E
: (high order bits: 1111)


Declared for experimental use only


Address range 240.0.0.0


255.255.255.255


Theoretical address space is 4,294,967,296 (seems a lot :
-
)
-

but
population on earth is higher by now)


But the address space usable for the “internet” is limited to
addresses from 1.
X.Y.Z

up to 223.X.Y.Z

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ip


addressing scheme


But you will loose some more addresses:


Special addresses like:


0.0.0.0 defines the
default route (explained later, route for
the “whole internet”) or the start address of a host
searching for a dynamically provided IP


255.255.255.255 local broadcast address (and destination
for hosts seeking an IP via DHCP)


127.0.0.0 loop back network address (you will need only
one address within this range and use typically 127.0.0.1).
This address is used by every host implementing IP
(software using IP for communication is usable without
internet connection)

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ip


“private” addresses


Addresses reserved for “private” use


many organizations,
enterprises, flat
-
sharing communities need IP communication for
their applications without or restricted internet access


10.0.0.0


10.255.255.255 (within the class A range)


172.16.0.0


172.31.255.255 (16 class B networks)


192.168.0.0


192.168.255.255 (65,536 class C networks)


University WLAN, private LAN is using 10.
X.Y.Z

addresses


Addresses within these ranges should be discarded on internet
routers


Address classifying helped in the beginning for faster network
decicion computation, routers had limited memory and cpu power

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ip addressing


For addressing whole subnets or addressing all hosts within a
given subnet (possibility depends on the underlying physical
network) special IP addresses are introduced


Network number is the smallest IP address in a given
(sub)network. it does not address a single machine and
may not assigned to a host. It is used with routing tables
(explained later in detail)


Broadcast address is the largest possible IP in a network.
It should be not assigned to a host, but is the possibility
to reach all hosts in a network with just one packet


If we use the example class B address
172.31.5.200, this
machine is a member of a network with the network number
172.31.0.0 and a broadcast address 172.31.255.255

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ip subnetting


Networks with huge number of hosts could be split into subnets
for better administration and considerations on physical topology
and global spanning net


The example class B network 172.16 with 65536 host ip numbers
in it, allows 256 subnetworks with 256 hosts in it if split on the
byte boundary


But: The resulting 256 “class C networks” have the same high
order bit like the original class B network

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literature list


Network Layering:


Kurose & Ross: Computer Networking (3rd), Section 1.7


Tanenbaum: Computer Networks (4th), Section 1.4


Internet Protocol 4


Kurose & Ross: Computer Networking (3rd), Section 4.4.1


Stevens: TCP/IP Illustrated Vol. 1, Section 3.2


Tanenbaum: Computer Networks (4th), Section 5.5.7, 5.6.1


IP Addressing


Kurose & Ross: Computer Networking (3rd): Section 4.4.2


Tanenbaum: Computer Networks (4th): Section 5.6.2


Stevens: TCP/IP Illustrated Vol.1, Section 1.4, Section 3.4

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literature list (cont.)


ARP


Kurose & Ross: Computer Networking (3rd): Section 5.4.1, Section 5.4.2


Tanenbaum: Computer Networks, 4th edition: Section 5.6.3


Stevens, TCP/IP Illustrated Vol. 1: Section 4.6, Section 5


ICMP


Stevens, TCP/IP Illustrated Vol. 1: Section 6, Section 9.3
--
9.6


Tanenbaum, Computer Networks, 4th edition: Section 5.6.3


DHCP


Kurose & Ross, Computer Networking (3rd): Section 5.4.3


http://www.ks.uni
-
freiburg.de/php_termindetails.php?id=39