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Communication Systems6th lecture

Chair of Communication SystemsDepartment of Applied Sciences

University of Freiburg2008

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Communication SystemsLast lecture, next lecture, practical courses

Exercise sheet #3 is due to the 30th May If any questions: please contact our Hiwi Mohannad Zalloom

regarding lecture recordings and exercise sheets (via email: zalloom@informatik.uni-freiburg.de)

Changes of dates for first practical course block (moved ahead) now: 6th and 10th June (13th is lecture)

Last lecture – introduction to dynamic routing different routing strategies: Static vs. dynamic routing

concept and theory of dynamic routing

discussed distant vector routing algorithm and its problems

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Communication Systemsplan for this lecture

Example for distance vector algorithms: RIP (II) Discuss link state algorithms Example: OSPF Extendend taxonomy of routing protocols

IGP / EGP

RIP (II) and OSPF

BGP as example for EGP

specialized algorithms e.g. in WLANs

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Communication Systemsrouting information protocol (RIP)

RIP - simplest dynamic routing protocol (try quagga, zebra, ... on Linux machines)

IGP using distance vector algorithm Distance metric:

Hop count (maximum of 15)

No other metrics available

Routers broadcast their entire routing table Carried in data field of UDP datagram Easy to implement and configure RIP II includes (very simple) authentication mechanisms Only advisable for small (homogeneous) networks

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Communication Systemsrouting information protocol – example

Initial routing table for router A:

A

B D

C

10.1.0.0

10.2.0.0 10.3.0.0

10.4.0.0 10.5.0.0

10.6.0.0 10.7.0.0

E

1

2 3

Destination Next Hop Interface Hops 10.1.0.0 0 1 1 10.2.0.0 0 2 1 10.3.0.0 0 3 1

After router B’s advertisement received:

Destination Hops 10.2.0.0 1 10.4.0.0 1 10.6.0.0 2

Destination Next Hop Interface Hops 10.1.0.0 0 1 1 10.2.0.0 0 2 1 10.3.0.0 0 3 1 10.4.0.0 B 2 2 10.6.0.0 B 2 3

Router ARouter ARoutingRoutingTable:Table:

Router ARouter ARoutingRoutingTable:Table:

Router B only knewRouter B only knewof its direct networksof its direct networksand router C’sand router C’s

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Communication Systemsrouting information protocol – example

Final routing table for router A:

Destination Next Hop Interface Hops 10.1.0.0 0 1 1 10.2.0.0 0 2 1 10.3.0.0 0 3 1 10.4.0.0 B 2 2 10.5.0.0 D 3 2 10.6.0.0 B 2 3 10.7.0.0 D 3 3

A

B D

C

10.1.0.0

10.2.0.0 10.3.0.0

10.4.0.0 10.5.0.0

10.6.0.0 10.7.0.0

E

1

2 3

Router A only receives direct advertisementsfrom routers B and D. Router C and E’s routesare learned from router B and D.

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Communication Systemsrouting information protocol – split horizon

This “hack” adds a rule to the RIP

Router never advertises the cost of a destination to neighbor N if N is the current next hop for destination

solves trivial Count-2-Infinity problems, but see example network with 4 nodes

If path 3 – 4 is going down ...

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Communication Systemsrouting information protocol – split horizon

If path 3 – 4 is going down: With split horizon both 1 and 2 tell 3 that they cannot reach

number 4

Thus 3 concludes that 4 is unreachable

But: 1 hears from 2 that it can reach 4 within two hops

So 2 concludes it can reach 4 via 1 within three hops

On the next exchange the distance is incremented, same problem as discussed in the beginning (“split horizon” cannot solve that scenario)

Same idea: poison reverse – instead of no advertisement use infinity cost instead

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Communication SystemsRIP – problems

Limitations: Unsuitable for large networks: Interrupts routers frequently, slowing them down Farther networks than 15 hops are ignored

UDP is unreliable, RIP is very insecure Only one metric - RIP doesn’t consider bandwidth RIP converges very slowly (minutes)

The potential range for other metrics as bandwidth, throughput, delay, and reliability can be large

The value for INFINITY should be large; but this can result in slow convergence of RIP due to count-to-infinity problem

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Communication Systemslink state routing

Distance vector was used within ARPANET until 1979 and much longer within LANs

Two problems caused its demise: can not not scale

convergence of algorithm often took too long (even with split horizon or similar hacks)

line bandwidth was not taken into account – no problem in the beginning with just all lines having 56kbit/s (or in LANs with just only 10Mbit/s Ethernet)

Therefore a completely new algorithm “link state routing” was introduced

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Communication Systemslink state – topology dissemination

In distance vector the path or cost to destination is partially determined by its neighbors

With link state every router gets information on complete topology of a given network All routers use same algorithm – so they will compute the

same paths (avoiding of loops) Two components

Topology dissemination and shortest path algorithm Topology dissemination should establish a consistent link state

database in each router Once established each router computes shortest paths to all

destinations from itself as origin

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Communication Systemsrouting protocols

The idea behind this algorithm can be stated as five parts:

1. Each router must discover its neighbors and learn their network addresses

2. ... measure the delay or cost to each of its neighbors

3. ... construct a packet telling all it has just learned

4. ... send this packet to all other routers

5. ... compute the shortest path to every other router So the complete topology and all the possible delays are

measured Information is distributed to every router

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Communication Systemsrouting protocols

During boot procedure of a router it has to learn its neighbors first This is done by sending a special discovery packet (HELLO) on

every point-to-point line The router on the other end should send a response packet telling

who it is Router names must be unique, so if one router hears of routes to

a router N it should know that the same machine is meant Point-to-point connections are normal within WAN, but within LAN

more than one router may be present An artificial router is introduced presenting the LAN, so every

router has point-to-point connections again

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Communication Systemslink state – measuring line costs

Each router should know (or an estimate of it) of the delay to each of its neighbors

Direct way to getting this information is via a special ECHO packet that other routers should send back immediately – round trip time divided by two gives an estimate (or for better approx. send several packets and compute the average)

Question is if the load of a line should be taken into account Load is interfering with the delay

But it could lead to problems in special scenarios

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Communication Systemsrouting protocols – general considerations

If you have a routing scenario as shown in the picture (examples are typical over sea connections or similar ...)

Two parts of the net are connected through the lines 2 – 3 and 6 -7

If most traffic is using 6 – 7 the line is heavily loaded as a result (and the delay increased)

If delay is taken into account, 2 – 3 is the shortest path instead of 6 - 7

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Communication Systemsrouting protocols – pitfalls

In the next round a new computation would find 6 – 7 as shortest path again

As an result the routing tables may oscillate wildly, leading to erratic routing and many difficulties finding errors

If load is ignored then the aggregated bandwidth of the two lines is not used optimally

If traffic is spread over the two lines bandwidth may better utilized but routing may be suboptimal

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Communication Systemsrouting protocols – theory

After collecting needed information the router should build a packet containing all data

The packet starts with identity of sender, followed by a sequence number, age and a list of neighbors

For each neighbor the delay to it is given Building the packets is relatively easy, the hard part is when

to distribute them This may be done in regular intervals or when a significant

change occurs (line going down or up, changing properties)

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Communication Systemsrouting protocols – theory

How to distribute packets reliably? As packets distributed and installed the router getting the

first ones will use it and change it routes Consequently different routers may have a different view of

the topology, which may lead to inconsistencies, loops, unreachable machines, ...

Fundamental idea is to use flooding (described last lecture) for distribution of those packets

Therefore a sequence number is included

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Communication Systemsrouting – theory cont.

Routers keep track of all pairs (source router, sequence number) they see

When new packet arrives it is checked against the packets already seen – if new it is forwarded to all lines except the one it came from (duplicates are discarded)

If packet with lower sequence number arrives it is rejected as obsolete (already seen)

Some problems may occur Wrapping around of sequence number counter

Corrupted packets with wrong numbers which confuse the algorithm

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Communication Systemslink state

Once a router has accumulated a full set of state packets it can construct the entire graph

Every link is presented (even twice – for each direction!) Now Dijkstras algorithm can be used on every router for computing

the shortest paths to every destination For a subnet of N routers with K neighbors the memory required to

store is proportional to K*N (and the computing power required is growing too)

For larger networks hierarchical routing is introduced

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Communication Systemslink state versus distant vector – comparison

Principle: Periodic advertisement of the routes in their routing tables Example: RIP (II) Advantages

Simpler - Easy to configure Disadvantages

Large routing tables

High network traffic overhead

Does not scale (very well), maximum of 15 hops

High convergence time

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Communication Systemslink state versus distant vector – comparison

Principle: exchange link state advertisements (LSAs) LSAs are advertised upon startup and when changes in the

internetwork topology Advantages

Smaller routing tables

Low network overhead

Ability to scale

Lower convergence time Disadvantages

Complex

More difficult to configure

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Communication Systemsrouting protocols – IGP / EGP

After theoretical introduction to routing concepts the practical implementations laid out Different implementations for different kind of networks

Taxonomy – dynamic routing could be divided into Interior Gateway Protocols (IGP)

Exterior Gateway Protocols (EGP) Autonomous system (AS definition) - unit of routing policy, either

a single network or a group of networks that is controlled by a common network administrator on behalf of a single administrative entity (such as a university, a business enterprise, or a business division)

AS is also sometimes referred to as a routing domain

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Communication Systemsrouting – interior gateway protocols

Routing within Autonomous System (AS) Always finds shortest path within AS Most common IGPs:

RIP (II) (Routing Information Protocol version 1 & 2)

OSPF (Open Shortest Path First)

ISIS (Intermediate System to Intermediate System)

Others, like OLSF

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Communication Systemsrouting – exterior gateway protocol

Routing between different AS Routing protocols and tables may differ between different AS Most common EGP: BGP(4) (Border Gateway Protocol) Example: A,B,C autonomous systems - C.b, A.a, A.c and B.a EGP

routers – small letters IGP routers

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Communication Systemsinterior gateway protocols

Interior Gateway Protocol (IGP) specified in RFC 2178 discovers the topology of a network through the operation of a

distributed routing protocol

describe the current network topology

routing protocols distribute how to reach address prefix groups

routing protocols function through either

distributed computing model (distance vector, discussed last lecture)

parallel computing model (link state, will be introduced now)

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Communication Systemsopen shortest path first (OSPF)

OSPF Link State Routing algorithm Router contains a routing directory (called a "routing database"). Exchange Link state advertisements Route computation using Dijkstra’s algorithm Advertisements disseminated to entire AS (via flooding explained

before) Utilizes the Hello protocol for advertising state information

between neighbors Neighbors exchange Hello packets periodically

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Communication SystemsOSPF – hierarchy

OSPF can cope with large networks (no restrictions as with RIP) Two-level hierarchy: local area, backbone

local area: aggregation of routers, hosts backbone: distributes routing information between different

areas area border routers: advertise distances in own area to other

area border routers boundary routers: connect to other AS

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Communication SystemsOSPF – hierarchy (example)

Routers 1 – 4 and 8 – 11 are within the same area Routers 5,6,7 are backbone routers and form an additional area

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Communication SystemsOSPF – protocol operation

Hello packets used to find adjacencies Adding neighbours to the local list

Flood network with LSA (Link State Advertisement introduced beginnig of this lecure) to propagate information

Each router forwards only new information

Hello packets check state of neighbours

No response → router down → LSA flood → update of routing tables → fast convergence

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Communication SystemsOSPF – protocol operation

Hello packets (sent to special multicast address) in ethereal

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Communication SystemsOSPF – routing tables

A

B D

C

10.1.0.0 (3)

10.2.0.0 (1) 10.3.0.0 (2)

10.4.0.0 (1) 10.5.0.0 (8)

10.6.0.0 (2) 10.7.0.0 (2)

E

Step Confirmed Tentative1 A (0) 10.1.0.0 [1] (3)

10.2.0.0 [2] (1) *10.3.0.0 [3] (2)

2 A (0) 10.1.0.0 [1] (3)10.3.0.0 [3] (2) *

10.2.0.0 [2] (1) 10.4.0.0 [2, B] (1 + 1 = 2) *

3 A (0) 10.1.0.0 [1] (3)10.2.0.0 [2] (1) 10.4.0.0 [2, B] (1 + 1 = 2) *10.3.0.0 [3] (2) 10.5.0.0 [3, D] (2 + 8 = 10)

4 A (0) 10.1.0.0 [1] (3) *10.2.0.0 [2] (1)10.3.0.0 [3] (2) 10.5.0.0 [3, D] (2 + 8 = 10)10.4.0.0 [2, B] (2) 10.6.0.0 [2, B, C] (1 + 1 + 2 = 4)

[1]

[2] [3]

** represents tentative entries that are lowest cost; one of these is represents tentative entries that are lowest cost; one of these ismoved to confirmed list at end of each step.moved to confirmed list at end of each step.

Steps for building routing table for Router A:

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Communication Systemsrouting algorithms – routing mechanisms

A (0)10.2.0.0 [2] (1)10.3.0.0 [3] (2)10.4.0.0 [2, B] (2)10.1.0.0 [1] (3)10.6.0.0 [2, B, C] (4)10.7.0.0 [2, B, C, E] (6)10.5.0.0 [3, D] (10)

Router A’s final routing table

A

B D

C

10.1.0.0 (3)

10.2.0.0 (1) 10.3.0.0 (2)

10.4.0.0 (1) 10.5.0.0 (8)

10.6.0.0 (2) 10.7.0.0 (2)

E

[1]

[2] [3]

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Communication Systemsexterior gateway protocol – principles

You tell me all the address prefixes you can reach, but don’t tell me the path you use to get there

I’ll tell you the same

If anything changes, please let me know If you tell me an address I’ll send you traffic destined to that

address. If I tell you an address I will accept traffic destined to that address

Beside that: Hide network internal topologies

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Communication Systemsexterior gateway protocol – BGP

Protocol to connect different AS Exterior Gateway Protocol Specified in RFC 1771 extension mBGP (multiprotocol BGP)

Mostly used by ISPs not in local LAN/MAN TCP for delivery (less bandwidth needed) Distance vector approach Allows policy-based routing

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Communication Systemsshortest path routing

Inter-AS routing with BGP:

AS2 (OSPF

intra-AS routing)

AS1 (RI P intra-AS

routing) BGP

AS3 (OSPF intra-AS

routing)

BGP

R1 R2

R3

R4

R5

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Communication SystemsBGP – vector protocol

Each Gateway broadcasts entire path (sequence of AS identified by a 16-bit number) to destination to peers

Operates on a path vector protocol: Similar to Distance Vector protocol

Each Border Gateway broadcast to neighbors (peers) entire path (i.e., sequence of AS’s) to destination

BGP routes to networks (ASs), not individual hosts

E.g., Gateway X may send its path to dest. Z:

Path (X,Z) = X,Y1,Y2,Y3,…,Z Initially whole routing table exchanged

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Communication SystemsBGP – vector protocol

Incremental updates exchange Keepalive messages to neighbours Four basic components in a BGP system

Speakers

Peers

Links

Border routers Receiving and filtering route advertisements from directly

attached neighbors

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Communication Systemsshortest path routing cont.

Do a route selection Send route advertisements to neighbors BGP uses TCP for message exchange Messages

Open: opens TCP connection to peer and authenticates sender

Update: advertises new path (or withdraws old)

Keepalive: keeps connection alive in absence of UPDATES; also ACKs OPEN request

Notification: reports errors in previous msg; also used to close connection

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Communication Systemsshortest path routing cont.

BGP reduces transit traffic Types of traffic for BGP routers

Local traffic: origin or destination in AS Transit traffic: all other traffic

AS classification: Stub - only single connection to another AS

Multihomed - multiple connections, no transit traffic

Transit - connections to several other AS, designed to carry both local and transit traffic

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Communication Systemsshortest path routing cont.

Implications You have control over policy settings

what is advertised to your immediate peers

what you accept from your immediate peers

what transits you will accept (send traffic)

But you cannot control transit path of received traffic

symmetry of transit policy

Thus “peering” between different providers is a complicated issue (estimation of traffic (asymmetries), contracts and handling)

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Communication Systemsdecision on routing protocols

Different routing policies depending on type of network Inter-AS: admin wants control over how its traffic routed, who routes

through its net. Intra-AS: single admin, so no policy decisions needed

Scale hierarchical routing saves table size, reduced update traffic

Performance Intra-AS: can focus on performance Inter-AS: policy may dominate over performance

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Communication Systemsother routing protocols

Special routing scenarios e.g. for distributed WLANs (z.B. Berlin freifunk.net)

OLSF (Optimized Link State Routing Protocol, RFC3536)

ISIS - Intermediate system to intermediate system link-state routing protocol invented by DEC, standardized in 1992

operates by reliably flooding topology information throughout a network of routers

each router then independently builds a picture of the network's topology

IS-IS uses Dijkstra's algorithm

Enhanced Interior Gateway Routing Protocol (EIGRP) - Cisco proprietary routing protocol deploying multiple metrics

Diffusing Update Algorithm (DUAL) for guaranteed loop-free operation and a mechanism for fast convergence

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Communication Systemsnext lecture / literature

Next lecture is on Friday, then again on Tuesday Practical course will be on 6, 10th June, starting at 1pm (!) in the

Computer Center (H.-Herder-Str.) Routing Algorithms

Kurose & Ross: Computer Networking, 3rd edition: Section 4.5 Routing Algorithms

Link State Routing Tanenbaum: Computer Networks, 4th edition: Section 5.2.5 Link

State Routing

RIP Kurose & Ross: Computer Networking, 3rd edition: Section 4.6.1

Intra-AS Routing in the Internet:RIP

Stevens, TCP/IP Illustrated Vol. 1: Section 10.4 RIP, Section 10.5 RIP Version 2

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Communication Systemsliterature

OSPF Kurose & Ross: Computer Networking, 3rd edition: Section 4.6.2

Intra-AS Routing in the Internet: OSPF

Stevens, TCP/IP Illustrated Vol. 1: Section 10.6 OSPF: Open Shorest Path First

Tanenbaum: Computer Networks, 4th edition: Section 5.6.4 OSPF -- The Interior Gateway Routing Protocol

BGP Kurose & Ross: Computer Networking, 3rd edition: Section 4.6.3

Inter-Autonomous System Routing: BGP

Stevens, TCP/IP Illustrated Vol. 1: Section 10.7 BGP: Border Gateway Protocol

Tanenbaum: Computer Networks, 4th edition: Section 5.6.5 BGP - The Exterior Gateway Routing Protocol