Communication Systems
3rd lecture
Chair of Communication Systems
Department of Applied Sciences
University of Freiburg
2006
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Communication Systems
Last lecture
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Message segmentation in packet switched networks advantage over
message switching
Different types of packet forwarding/routing in
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Datagram networks (Internet, ...)
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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
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Network access, different types (home, company, mobile)
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depending on requirements of end users
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Communication Systems
Last lecture
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Physical representation of digital bit streams all kinds of
electromagnetic waves (electromagnetic spectrum)
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Encoding, decoding of data for transport over different types of
media
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Physical parameters: frequency, wavelength, (effective) bandwidth,
Nyquest formula for max. bandwidth of given medium
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Copper wire (single twisted pair for telephone, higher quality for
ethernet, fiber optics), “air” (mobile phones, satellite links, WLAN, ...)
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Guided and unguided media, propagation delay (speed of light)
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Communication Systems
plan for this lecture
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Standards and network layering models
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OSI and IP
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Need of an universal service
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IP as layer 3 network protocol
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Start with look at IP header
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Communication Systems
network layer models
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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
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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
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More practical is the TCP/IP layering model with fewer layers
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In general:
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Layering breaks down very complex tasks into simpler ones
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Implementation details in one layer are abstracted away from
the others
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But: Can introduce overhead and need for intentional violation
of layering concepts
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Communication Systems
network layer models - “academic” OSI model
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Communication Systems
comparison of OSI and TCP/IP layers
OSI Layers
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Application Layer
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Presentation Layer
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Session Layer
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Transportation Layer
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Network Layer
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Data Link Layer
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Physical Layer
TCP/IP Layers
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Application Layer
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Transportation Layer
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Internet Layer
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Physical Layer
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Communication Systems
comparison of OSI and TCP/IP layers
OSI in comparison to TCP/IP (developed by ARPA)
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Communication Systems
why talking about layer models?
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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
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ethernet in ATM
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IP and others over PPP
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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.
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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.
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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)
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IP used in every host-to-host connection
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Many physical layer implementations
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Many applications operating over IP
So we will start with IP now ...
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Communication Systems
universal service connecting networks
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We introduced some different kinds of physical networks
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Modem, ISDN, DSL for connection of individuals
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Technologies like Ethernet (over copper and fiber optics)
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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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Communication Systems
universal service connecting networks
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Each type of network structure may require or be
implemented with different low level protocols
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Communication Systems
connecting networks
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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
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Multilink (broadcast) network with no dedicated point-to-point
connections
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Different speeds over copper wire, coaxial cable and fiber
optics
ATM – connection orientated LAN and WAN protocol
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Virtual ptp connection through virtual channels and pathes
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Modem, ISDN for WAN, FDDI, TokenRing, ... for LAN
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Wireless links on GPRS, HSCSD, UMTS, WLAN, ...
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Communication Systems
connecting networks cont.
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Intermediate protocol with unified addressing scheme is needed
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Communication Systems
connecting networks
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Universal protocol needed is implemented in the network layer
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Network layer is third layer in OSI model
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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
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Needed for packet handling and routing
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Communication Systems
connecting networks
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Helper protocols needed during packet processing
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Inform senders on congestion or lost packets
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Map different layers addresses on each other
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Communication Systems
functions of an universal service
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Define unified addressing scheme which is hardware/software
independent
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Realize datagram delivery between networks
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Hosts – end systems as defined in first lecture
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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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Communication Systems
issues of an universal service
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Network layer provides end-to-end delivery (routing)
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Provides consistent datagram abstraction:
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best-effort delivery
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no error detection on data
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consistent maximum datagram size
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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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Communication Systems
internet protocol details
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IP version 4 is current
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IPv6 forthcoming
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Solution to the address space exhaustion of IPv4
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Predicted for a while, but limit not reached yet
Solutions for preserving numbers in IPv4 (masquerading,
private networks ...)
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At the moment nobody knows when it will be used other then
in backbone structures
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3G/UMTS mobile telephone market may push IPv6
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Defined for a while – we will spend a dedicated lecture on it
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Communication Systems
internet protocol details
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Protocol header includes:
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Version field
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Source and Destination addresses
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Lengths (header, options, data)
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Header checksum
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Fragmentation control
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TTL, and TOS info
But TOS info often ignored
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Easy changeable along the path (so what for?)
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Communication Systems
internet protocol header details
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Version field (4 standard, 5 STII, 6 next gen IP) and IP header
length are of 4 byte
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IP header normally consists of 20Byte, with options more
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Length needed to compute where next header starts
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Communication Systems
internet protocol header details cont.
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IP options may sum up to 40Byte
Total length field is 16bit, maximum packet length therefore may
not exceed 64kByte
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Minimum is 20Byte (just the IP header)
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MTU of standard physical networks much smaller (e.g.
1,5kByte)
16bit identification field for fragments
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Set for every packet by original sender of datagram
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Sender can not know if fragmentation may occur
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Initial message segmentation may not small enough
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Copied into each datagram during fragmentation
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Communication Systems
internet protocol header – fragmentation
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Content of 16bit identification field is computed by sender
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Different OS use different computing schemes (tool “nmap” in
practical course)
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Might give away information on OS, internal network structure
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Masqueraded machines could be identified by their
fragmentation IDs
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Counter on every machine will have different values (amount
of traffic generated, computing scheme ...)
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A private network may give more information away than
intended
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Communication Systems
internet protocol header – fragmentation
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Flags for fragmentation control
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MF: more fragments (follow)
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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
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Offset of this fragment into the original datagram
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Zero if no fragmentation used
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why offset and not fragment number? - if further fragmentation
is needed
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Communication Systems
ip header – protocol field
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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
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1: ICMP – discussed later this lecture
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6: TCP – Transmission Control Protocol
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17: UDP - User Datagram Protocol
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50: ESP – Encapsulating Security Payload
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51: AH - Authentication Header
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All protocol names and corresponding numbers are listed in
(/etc/)protocols file (linux operating system – see practical course)
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Communication Systems
ip header – protocol field
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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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Communication Systems
ip header – address fields
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Source address
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32 bit length defines the IP address space
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Should never be changed through ordinary routing (there are
some exceptions like network address translation (NAT))
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Protocol does not force authentication of source (often
enforced by modern routers now)
Destination address
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32 bit length defines the IP address space
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Should never be changed through ordinary routing
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Changes when source routing used (realised through IP
option header)
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Communication Systems
internet protocol – header details
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Source routing
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Special handling for particular datagrams, sometimes don't
take router's "fast path"
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Rarely used, but the more common are: Loose Source
Routing, Strict Source Routing and Record Route
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Timestamp
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Must copied on fragmentation
More on routing little bit later on
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Communication Systems
ip – fragmentation of packets
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Gave short introduction already, but ...
Adapting datagram size one of the most important tasks of the
internetworking protocol:
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IP datagrams itself cannot exceed 64kbyte
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Lower protocol levels report MTU (max. transfer unit)
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Linux loopback 16384byte
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Ethernet frames offer max. payload of 1500byte
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ATM offers 48byte
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slow modem-ppp connections 296byte packet length
The tool ifconfig or ip (first practical course) reports MTU of each
interface
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Communication Systems
ip – fragmentation of packets
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Fragmentation & Reassembly
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divide network-layer datagram into multiple link-layer units, all
have to be equal or smaller then link MTU size
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Further fragmentation may be needed if MTU is decreased
along the path again
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Sometimes it is more clever to set MTU smaller at source to
avoid later fragmentation
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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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Communication Systems
ip – fragmentation of packets cont.
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If fragmented, identification triple is copied into each resulting
packet
Also contains (offset, length, more) triple
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more - boolean indicates is last fragment
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offset - relative to original datagram
Relating fragments to original datagram provides:
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Tolerance to re-ordering and duplication
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Ability to fragment fragments (!)
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Communication Systems
ip – fragmentation of packets cont.
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IP fragments are re-assembled at final destination before
datagram is passed up to transport layer
Routers do not reassemble fragmented datagrams
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Allows for independent routing of fragments
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Reduces complexity (need for CPU and memory) in routers
Problems with fragmenting:
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Loss of 1 or more fragments implies loss of datagram at the IP
layer
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IP is best effort, provides no retransmission, will time-out if
frag(s) appear to be lost
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May be interesting for DoS attacks
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Communication Systems
ip – fragmentation of packets cont.
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Avoid fragmentation through computing path MTU
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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
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IP v6 uses MTU discovery and assumes standard
minimum MTU
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If datagram size is smaller then MTU, no fragmentation needed
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How to do this?
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Probe network for largest size that will fit
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If possible, have network tell us this size
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Operates through ICMP messaging (presented later on)
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Communication Systems
ip – addressing scheme
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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
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Interfaces on same network share prefix
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Prefix is assigned via ISP/local network administrator
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32-bit globally unique
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Communication Systems
ip – addressing scheme cont.
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Address is split into two virtual parts: network and host part
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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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Communication Systems
ip – addressing scheme cont.
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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
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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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Communication Systems
ip – (historic) address classes
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Communication Systems
ip – address classes cont.
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Class A: (high order bits: 0)
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Large Organizations, few nets (127), huge number of hosts (16.7
million)
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Address range in decimal notation 0.0.0.0 – 127.255.255.255
Class B: (high order bits: 10)
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Medium sized organizations and firms, e.g. University of Freiburg,
some nets (16,384) and large number of hosts (65,536)
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Address range 128.0.0.0 – 191.255.255.255
Class C: (high order bits: 110)
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Small organizations and firms, relatively large number of nets
(2,097,152) with a small number of hosts per net (256)
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Ranging from 192.0.0.0 – 223.255.255.255
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Communication Systems
ip – address classes cont.
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Class D: (high order bits: 1110)
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Multicast addresses, but service are not very often used
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Address range 224.0.0.0 – 239.255.255.255
Class E: (high order bits: 1111)
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Declared for experimental use only
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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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Communication Systems
ip – addressing scheme
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But you will loose some more addresses:
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Special addresses like:
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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
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255.255.255.255 local broadcast address (and destination
for hosts seeking an IP via DHCP)
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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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Communication Systems
ip – “private” addresses
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Addresses reserved for “private” use – many organizations,
enterprises, flat-sharing communities need IP communication for
their applications without or restricted internet access
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10.0.0.0 – 10.255.255.255 (within the class A range)
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172.16.0.0 – 172.31.255.255 (16 class B networks)
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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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Communication Systems
ip addressing
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For addressing whole subnets or addressing all hosts within a
given subnet (possibility depends on the underlying physical
network) special IP addresses are introduced
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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)
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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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Communication Systems
ip subnetting
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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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Communication Systems
literature list
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Network Layering:
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Kurose & Ross: Computer Networking (3rd), Section 1.7
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Tanenbaum: Computer Networks (4th), Section 1.4
Internet Protocol 4
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Kurose & Ross: Computer Networking (3rd), Section 4.4.1
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Stevens: TCP/IP Illustrated Vol. 1, Section 3.2
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Tanenbaum: Computer Networks (4th), Section 5.5.7, 5.6.1
IP Addressing
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Kurose & Ross: Computer Networking (3rd): Section 4.4.2
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Tanenbaum: Computer Networks (4th): Section 5.6.2
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Stevens: TCP/IP Illustrated Vol.1, Section 1.4, Section 3.4
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Communication Systems
literature list (cont.)
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ARP
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Kurose & Ross: Computer Networking (3rd): Section 5.4.1, Section 5.4.2
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Tanenbaum: Computer Networks, 4th edition: Section 5.6.3
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Stevens, TCP/IP Illustrated Vol. 1: Section 4.6, Section 5
ICMP
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Stevens, TCP/IP Illustrated Vol. 1: Section 6, Section 9.3--9.6
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Tanenbaum, Computer Networks, 4th edition: Section 5.6.3
DHCP
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Kurose & Ross, Computer Networking (3rd): Section 5.4.3
http://www.ks.uni-freiburg.de/php_termindetails.php?id=39
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