1 | 44 communication systems 4 th lecture chair of communication systems department of applied...

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Communication Systems4th lecture

Chair of Communication SystemsDepartment of Applied Sciences

University of Freiburg2008

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Communication SystemsLast lecture and practical course

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 Fragmentation of packets

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Communication Systemslast lecture – addressing scheme

Address is split into two virtual parts: network and host part address could be split at every bit

network and host parts add up to 32 bit in every case

important for routing decisions

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Communication Systemsplan for this lecture

IP sub- and supernetting Datagram delivery Address mapping in broadcast nets Packet routing in IP networks

Discussed address adaptation in broadcast nets for local delivery

Talked of routing principles (matching destination addresses against network address of an interface)

Mostly involves static routing (addresses, netmasks, ...) assigned by administrator directly or via DHCP

How does routing LAN-wide or globally work?

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Communication Systemsip – new subnetting scheme

The number of class B networks was much to small (Germany has around 100(?) universities and colleges and therefore would need for them at least 100 class B networks out of 16,384)

There is no real need for class A networks (imagine a big company connecting all there machines to the Internet directly – e.g. IBM or HP had class A networks or a provider with over million customers in a given area)

There is great need for bigger networks than class C but much smaller then B

The waste of addresses with the old scheme was enormous and the need for IP v6 seemed very urgent :-)

Concept of subnetting and supernetting was introduced

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Communication Systemsip – new subnetting scheme

Introduction of netmasks (were implicit with old addressing scheme)

Supernetting means the combining of address ranges into larger ones with just one common network and broadcast address

The IP addresses arn't self explanatory any more For the information of the span of subnetworks netmasks where

introduced: “1” marks prefix part of IP (network) “0” marks suffix part of IP (host)

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Communication Systemsip – new subnetting scheme cont.

The netmask of 255.255.0.0 just marks an old class B network 255.0.0.0 depicts class A and 255.255.255.0 class C The netmask may be abbreviated with the numbers of “1” in the

netmask (e.g. class A: 8, B: 16, C: 24) If you combine two class C networks into a larger one, e.g.

network 192.168.10.0 with broadcast 192.168.10.255 and

network 192.168.11.0 with broadcast 192.168.11.255

The result is: network 192.168.10.0 with broadcast 192.168.11.255 and netmask

255.255.254.0

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Communication Systemsip – new subnetting scheme cont.

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Communication Systemsip – new subnetting scheme – principles

Split of netmasks into prefix and suffix is done on the boundary between the “1” and “0”

e.g. 1111 1111.1111 1111.1 000 0000.0000 0000 is 255.255.128.0 (some commands use abbreviation 17, first practical course)

We would split that way the network 132.230.0.0/255.255.0.0 into two subnets: 132.230.0.0 – 132.230.127.255 and 132.230.128.0 – 132.230.255.255

But we could split that network another way: e.g. 1111 1111.1111 1111.0000 0000.0000 0001 is 255.255.0.1

and get two subnets, one with the even (in the last octet) IP addresses and one with the odd IP addresses in it

Managing networks that way implements a lot of risks :-)

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Communication Systemsip – new subnetting scheme – conclusion

Networks may combined into larger ones, large networks may be split

Splitting networks means adding a “1” to the netmask (increasing prefix and decreasing suffix)

Combining networks via removing “1” from netmask and adding “0”

Therefore at the moment are enough blocks of class C networks still available for assignment (the need for IP v6 declined)

Additional information is needed, routers need more memory to store netmasks in combination with net names

Routing tables could be simplified through aggregation of routes

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Communication Systemsdatagram delivery

Why the long introduction on addressing schemes, network names and netmasks?

Packet switched networks depend on routing decision for every packet (network taxonomy)

How datagrams sent through (global) network to end systems? Two types of delivery in IP networks:

local delivery (no router involved)

non-local delivery (router needed)

determined by common prefix

Routers may or may not additionally switch packets between different LAN or WAN protocols

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We need a rule to decide how to deliver packets in IP networks every router and host maintains a routing table

read destination address of given packet

get the netmask of the smallest network (we will see why we start with the biggest netmask and descend to smallest)

compute: netmask AND destination address

compare the result against the network address connected with the used netmask

match: deliver packet that route

not matched: start the algorithm with the netmask of next bigger network

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When you got the route the packet should take if no gateway is given -> deliver locally (we will see how later on)

see if gateway is given -> deliver the packet to the router (use locally specific mechanism for delivery to the router)

Example: network address: 10.8.4.0

“class C” netmask (255.255.255.0)

broadcast 10.8.4.255

network and broadcast addresses special IPs which could not be assigned to host machines (last lecture)

Host machine: 10.8.4.202, router: 10.8.4.254

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Described simple example Ethernet network typical LAN situation like the setup in the several computer pools (as

seen in first practical exercise)

nearly same setup in a typical home installation (wired Ethernet could be exchanged with wireless LAN connections)

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Routing table of a standard host machine in a subnet (LAN) normally consists of three entries (you should have seen that in the practical course):

route to the local LAN

loopback route

default route

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Now lets see how a packet to the host 10.8.4.204 would be routed take routing entry with the smallest netmask (here: 255.255.255.0)

10.8.4.204 & 255.255.255 -> 10.8.4.0 (match!!)

local delivery

Packet to 132.230.1.204 take routing entry with the smallest netmask (here: 255.255.255.0)

132.230.1.204 & 255.255.255 -> 132.230.1.0 (miss!)

try next entry: 132.230.1.204 & 255.0.0.0 -> 132.0.0.0 (miss!)

try next: 132.230.1.204 & 0.0.0.0 -> 0.0.0.0 (match!)

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local delivery to the router

Default route matches every packet, therefore its to be tested last Local delivery takes place in every case

directly to the destination machine

directly to the router

router/gateway IP has to be part of the subnet

For packet delivery only the destination address is checked! security hazards because of possible IP spoofing

most of modern routers do source address checking (but that is not part of the protocol definition)

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Communication Systemsuniversal service – address and size adaptation

Seldom one single network spans between two end systems IP runnable on top of many different hardware types and software

protocols Address and size adaptation needed

mapping from Internet standard addresses (IP addresses) to link-specific addresses

datagram size adaptation

Internet datagram has universal common size (64KByte for IP)

mapping from common size to link-specific MTU requires fragmentation

fragmentation allows the splitting of packets into smaller units with reassembling at the receiving station

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Communication Systemsaddressing schemas

IP addresses are topologically sensitive interfaces on same network share prefix

prefix is assigned via ISP/local network administrator

32bit globally unique

address is implemented in software

e.g. 802.x addresses are vendor-specific interfaces made by same vendor share prefix 48bit globally unique

networks may have ethernet adaptors from a wide range of distributors with completely different prefixes

prefix is put in hardware

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Communication Systemsdatagram delivery cont.

Local delivery with point-to-point connections is easy, just send the packet to the other end of the connection

modem, (GPRS, UMTS) – addressing is done other ways:

device number of serial port, telephone number of the telephone system, ...

PPP point-to-point route (network consisting of just two IP addresses)

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Communication Systemsdatagram delivery cont.

Routing table looks a little bit different (compared to LAN e.g. Ethernet connection)

netmask is 255.255.255.255 (just one address in network)

Addresses do not have to share same prefix

e.g. 80.43.112.34 for the local machine and 217.67.12.33 for the providers gateway

Seen with modem, ISDN, GPRS/UMTS, PPPoE (ADSL) connections for individuals toward end user ISPs

default gateway is just the machine at the other end of connection

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Communication Systemsaddress mapping in broadcast nets

But what to do in broadcast nets with many connected hosts? in broadcast nets every host gets every packet sent out in the

segment (switching may reduce traffic, but for some services packets to all are inevitable)

For local delivery, need to map network-layer address to link-layer address:

consider the machines 132.230.15.6 and 132.230.15.18 (netmask e.g. 255.255.255.0) ... [on same network]

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Communication Systemsaddress mapping cont.

Encapsulate IP datagram within link-layer frame What lower level destination (MAC) address to use? Helper protocol is needed

IP has no feature to do mapping itself

such mapping is not needed in PPP environments

this protocol is specific to the underlying hardware / software protocol

ARP is for address mapping in Ethernets and TokenRings More on ARP in practical/theoretical exercises

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Communication SystemsIP routing

By now simple point-to-point routes and local routing What happens in bigger networks of connected networks? Machines are connected over continents and/or different media

introduction: BelWue, DFN, GEANT(2), ...

Next topic is IP routing in general and dynamic routing and algorithms

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Communication Systemsdefinition of routers

Somehow magically an IP packet travels long distances and finds its way between two end systems (from source machine to destination)

As we introduced: IP is a packet switched network so on every intermediate system a

routing decision is to be made

These intermediate systems normally have more than one IP interfaces (each interface with its own IP number matching to the net the machine is member of)

Formally: each machine with interfaces in two different IP subnets (and the ability to forward packets from one interface to the other) is called a router

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Communication Systemsdefinition of routers cont.

Every router maintains a routing table In the simplest case the router has three entries in that table

route to local subnet #1

route to local subnet #2

default route with the router in one of the subnets

the routing table grows with the number of interfaces and nets connected to each

Routing tables in Internet routers grew huge because of non-consecutive IP ranges (aggregation of networks is impossible then)

IPv6 should solve this issue and simplify the routing tables again

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Communication Systemsrouting example

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Communication Systemsrouting example

The routing tables of the two routers #1, #2 are longer then routing table of end system

For each interface a routing entry is present

We find a default route on both of them (most routers have default entry, we will see why later)

Maintaining this routing information manually is the standard mechanism used for relatively static and very small LAN environments

Routing tables on a larger scale are not as fixed as local ones Remember the networking structure graphs of BelWue, DFN and

GEANT(2), many network nodes are connected one more than one path with each others

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Communication Systemsrouting cont.

Reasons for multilink IP connections Every ISP must have more than one uplink connection to get the

permission to operate (fox hole principle)

Links are of differing bandwidth, quality, latency and price

These variables may differ over time periods (different rates for daily or night use, failing lines, congested paths, ...)

You will need mechanisms to consider these information and compute an optimal way to every destination network

Routing techniques and protocols working over IP are to be introduced ...

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Communication Systemsrouting protocols

In general: routing protocols are not IP specific Routing protocols may be needed on different network layers It depends on the type of underlying networking infrastructure and

concept of connection We can make some general assumptions on routing algorithms

independently of the type of network Within connection orientated networks like ATM infrastructure we

find virtual channel switching ATM packets follow a previous installed route

Route is active during the whole session

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IP – packet orientated network Routing decision is renewed for every packet (introduction to static

IP routing last lecture)

No state of previous routing decisions is kept (!)

Static routing (manual setup) is acceptable in small networks Routing setup for end systems often by DHCP These mechanisms not suitable for routing on larger scale, e.g.

campus-wide inter LAN routing

DFN-wide, inter-provider-routing, ...

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Routing tables grow relatively fast, e.g. simple subnetting in university LAN of roughly 256 class-C subnets in 132.230.X.Y IP domain produces long tables in core routers

IP subnet aggregation is often impossible

routers may have several links

network should have redundant links ...

Routing could be defined: Algorithms to establish routing table to make widely distributed endpoints appear to be directly connected

So mechanisms for automated setup of router tables desired Different routing protocols run on routers implement several

routing algorithms

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Communication Systemsrouting protocols – general considerations

In general: forwarding is local made decision, requiring only next hop information

But: computation of best route requires global information This information is challenging:

hard to collect, often outdated, huge amounts of data

no single network owner

General needs for routing compute optimal paths for each destination (we need a

definition of term “optimal”)

minimize control message exchanges

minimize routing table space

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Communication Systemsrouting protocols – pitfalls

While considering automatic setup of routing tables some risks may show up

Loops: should local forwarding information be inconsistent with global topology – it can form loops (black holes in which packets “disappear” - you may have observed this phenomenon with traceroute when a route oscillated between two routers ...)

Oscillations: dynamically adapting to load can shift load, lead to congestion and repeat (often with paths of small bandwidth – consider two ISDN lines with heavy load ...)

Normally these scenarios unusual under normal operation, often due to misconfiguration

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Communication Systemsrouting protocols – theory

Routing itself (discussed with IP addressing) is part of the network layer and responsible for deciding which output line an incoming packet should take Routing algorithms often implemented in applications run on

top of the underlying IP network For routing decisions hence every routing algorithm certain

properties are desirable: correctness of routes

simplicity of protocol

robustness

stability

fairness and optimality

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Correctness and simplicity are obvious requirements Robustness

once a major network is set up system wide failures and outages are not desired

should catch up with topology changes

cope with hardware failures

route changes (because of pricing changes, new infrastructure, expanding of the network, ...)

... that means, not all connected hosts shouldn't be affected

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Communication Systemsrouting – theory cont.

Stability Routing algorithms should converge towards equilibrium in a certain

amount of time

Fairness and optimality obvious but often contradictory goals

see the following picture, if the six hosts 1,1' ; 2,2' ; 3,3' communicate with each other and saturate the link the communication of X,X' should be shut off completely ...

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Communication Systemsrouting – theory cont.

Conflict between Fairness and optimality (depends of course on underlying network topology)

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Before decision on trade-off between the described problem could be done – we should see what we seek to optimize:

maximum total network throughput could be one parameter

minimum mean packet delay is an other

These two goals in conflict too: since operating any queuing system near limit implies long delays

Many networks try to compromise with minimizing the number of hops (passing a routing engine) to take from source to destination

Such the delay is reduced and the amount of bandwidth consumed minimized

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Communication Systemsrouting protocols – in packet networks

Internet doesn't have very predictable traffic flow, may have unreliable links

Routers are assumed to know address of each neighbor

cost of reaching each neighbor

Choices for Internet routing centralized vs. distributed routing

source based vs. hop-by-hop

single or multipath

dynamic vs. static

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Communication Systemsrouting strategies – (non)adaptive routing

Routing algorithms are grouped into two major classes Nonadaptive RA do not base their routing decisions on

(continuous) measurements or estimates of current bandwidth usage and topology

no need for specific measurement service run continuously or scheduled

The routes to use are computed in advance, off-line and downloaded to routers when network is coming up

That is the typical scenario for networked end systems – normally the system administrator provides the routes during machine setup

Or the routing information is transferred via DHCP (centralized setup of networking resources)

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Communication Systemsadaptive routing

Routing done that way often named static (type of routing discussed yet falls into that category)

Adaptive algorithms change their routing decisions to reflect changes in traffic/bandwidth usage and topology

Algorithms differ in where they get their information ... Locally from own measurements or from adjacent routers

Or (globally) from all routers

... and when changes are executed Every T seconds when network load changes

Or changes in topology occur

Or event driven ...

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Communication Systemsadaptive routing cont.

Measure / function needed to represent certain values Metric can be seen as a value for measuring routing costs These costs could be

physical distance between two routers

number of hops packets travel from source to destination

estimated transit time

monetary costs (cheap satellite link vs. expensive sea cable for continental crossing or vice versa)

Different routing algorithms (RA) use different metrics for their routing decisions

Different metrics have different costs of computing them

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Communication Systemsliterature list/next lecture

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

Routing Theory Tanenbaum, Computer Networks (4th): Section 5.2

Kurose & Ross, Computer Networking (3rd): Section 4.5

Next lecture pentecost break: thus next lecture is the 20th May (please hand

back your second exercise sheet at this lecture)

lecture plan/exercises are available on the lectures homepage:

http://www.ks.uni-freiburg.de/php_veranstaltungsdetail.php?id=20

1 | 44 communication systems 4 th lecture chair of communication systems department of applied...

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