routing and networking

Lecture 1

Introduction to the courseRouting protocols

Lecturer: Alexander Pyattaev

[email protected]

2

Outline Info about the TLT-2336 course

Overview of the course content Arrangements and grading

Routing algorithms Link state Distance Vector Hierarchical routing

Routing in the Internet – IGP protocols RIP OSPF

Slides have been derived from the support material from “Computer Networking” by Kurose and Ross

Purpose of this course(1/2) Computer networking 1

gives you an understanding of how networks are made So you can use the same

approach to design similar networks.

But is that always the good solution?

Sometimes the classic solutions simply do not work: they may not fit the conditions, they may not scale to the load that has to be carried, they may lack speed, may be hard to set up/administrate, And many more issues may arise

Computer networking II aims to illustrate those special needs that are typical in modern networks and to prepare you for the challenges they present

Purpose of this course (2/2) In our example, to handle the new conditions you may need a

different car For instance:

Routing may be done with RIP, but there are many better ways – we will study them

IP routing is scalable, but it is no match for the efficiency of label switching with MPLS

What if we want to guarantee service? Quality of Service protocols come in handy

TCP and UDP are not the only two transport layer protocols, and for a good reason

This course will teach you to think ”outside of the box” To consider different approaches to networking tasks To understand why and where a certain architecture is

useful To combine the technologies together To design your own network architectures

5

Course Info Computer Networking I is highly recommended Lectures

Third & fourth period (09.01.2013 – 13.05.2013): Wednesdays, 16:15 - 17:45

Tietotalo, TB222

Web page in Moodle2 To be announced soon

Class text and general references Lecture slides (MOST IMPORTANT) Necessary IEEE, IETF and ITU documents + the Internet resources William Stallings "Data & Computer Communications", 7th edition,

Prentice-Hall International Inc., 2004 Andrew S. Tanenbaum "Computer Networks", 4th edition, Prentice-

Hall International Inc., 2003 Kurose, Ross “Computer Networking. A top-down approach

featuring the Internet”, 3rd edition, 2005

11/01/2012 TLT-2336, spring 2012, Lecture #1 6

Course Syllabus Course persons responsible

Dmitri Moltchanov and Yevgeni Koucheryavy Email: [email protected], [email protected] Office: Tietotalo, TH208

Instructors Alexander Pyattaev, Vitaly Petrov Email: [email protected], [email protected] Office: Tietotalo, TH209 and TC240

Course structure, no exceptions will be granted Lectures (weekly, NOT compulsory) Homeworks with a set of problems to solve (compulsory) Simulation assignment done with OPNET (compulsory)

Supplementary courses: TLT-2156 Laboratory excercises (basic level, recommended) TLT-2506 Communications laboratory (advanced, recommended)

mailto:[email protected]




7

Course Syllabus - grading5 credit units, grading scale 0-5 Written exam (see dates in POP)

3h, multiple choice test, will also include basic calculus tasks to solve (similar to Computer Networking I)

Exam admission requires successful completion of all the following: Homeworks (2 homeworks will be published in Moodle2) Assignment (will be published in Moodle2)

The final grade (max 100 pts) consists of: Examination 70 points

Must be passed to graduate from the course

Homework Assignments 20 pts OPNET or NS2 Assignment 10 pts (tba)

More info will be posted in Moodle2

8

Outline Info about the TLT-2336 course Routing algorithms

Link state Distance Vector Hierarchical routing


The following slides have been derived from the support material from “Computer Networking” by Kurose and Ross

Routing algorithms

Routing algorithms Purpose Link state Distance Vector Hierarchical routing

9

Consider revising lecture from Computer Networking I course as well

1

23

0111

value in arrivingpacket’s header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Reminder - what is routing?

So what is routing? A set of proceures/algorithms that allows to set up the

forwarding tables automatically and intelligently It is not a network layer protocol – it is an application, just like

any other service like DNS

Routing is a process that allows to construct routes, it is not responsible for the actual forwarding In fact, routing protocol is not needed after the network is

configured But the forwarding part is of course needed as long as the

network works

What happens if the network changes? We would need to recompute the routing tables

How do we do all that?

u

yx

wv

z2

21

3

1

1

2

53

5

Graph: G = (N,E)

N = set of routers = { u, v, w, x, y, z }

E = set of links ={ (u,v), (u,x), (u,w), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) }

Graph abstraction

Remark: Graph abstraction is useful in other network contexts

Example: P2P, where N is set of peers and E is set of TCP connections

Graph abstraction: costs

u

yx

wv

z2

21

3

1

1

2

53

5 • c(x,x’) = cost of link (x,x’)

- e.g., c(w,z) = 5

• cost could always be 1, or inversely related to bandwidth,or inversely related to congestion

Cost of path (x1, x2, x3,…, xp) = c(x1,x2) + c(x2,x3) + … + c(xp-1,xp)

Question: What’s the least-cost path between u and z ?

Routing algorithm: algorithm that finds least-cost path

1311/01/2012

TLT-2336, spring 2012, Lecture #1

Routing Algorithm classification

Global or decentralized information?

Global: all routers have complete

topology, link cost info “link state” algorithmsDecentralized: router knows physically-

connected neighbors, link costs to neighbors

iterative process of computation, exchange of info with neighbors

“distance vector” algorithms

Static or dynamic?Static: routes change slowly

Updates periodically

Dynamic: routes change quickly

Updates in response to link cost changes

11/01/2012


Routing algorithms

Link-state routing Works by analyzing the cost of each edge of the graph

Represents the quality of the links Can be run independently on each host (but it still needs info from

each edge)

The routing protocol itself uses flooding to propagate the link states Flooding is a process of forwarding to everyone, until each host

has a copy of a certain link state In some way it works similar to how rumors spread in a crowd: ”Have you heard the news about route 666 over last 2 seconds?” ”No!” ”Guess what, the route 666 is congested for the last 2 seconds!” OR: ”Have you heard the news about route 666 over last 2 seconds?” ”Yep, I have that news” ”Okay, I'm gonna check on my other neighbors...”

A Link-State Routing Algorithm

Dijkstra’s algorithm net topology, link costs

known to all nodes accomplished via “link

state broadcast” all nodes have same

info computes least cost

paths from one node (‘source”) to all other nodes gives forwarding table

for that node iterative: after k

iterations, know least cost path to k destinations

Notation: c(x,y): link cost from

node x to y; = ∞ if not direct neighbors

D(v): current value of cost of path from source to dest. v

p(v): predecessor node along path from source to v

N': set of nodes whose least cost path definitively known

Dijsktra’s Algorithm(home study)

1 Initialization: 2 N' = {u} 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(u,v) 6 else D(v) = ∞ 7 8 Loop 9 find w not in N' such that D(w) is a minimum 10 add w to N' 11 update D(v) for all v adjacent to w and not in N' : 12 D(v) = min( D(v), D(w) + c(w,v) ) 13 /* new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v */ 15 until all nodes in N'

1811/01/2012


Dijkstra’s algorithm: example(1/2)

Step012345

N'u

uxuxy

uxyvuxyvw

uxyvwz

D(v),p(v)2,u2,u2,u

D(w),p(w)5,u4,x3,y3,y

D(x),p(x)1,u

D(y),p(y)∞

2,x

D(z),p(z)∞ ∞

4,y4,y4,y

u

yx

wv

z2

21

3

1

1

2

53

5

Dijkstra’s algorithm: example(2/2)

u

yx

wv

z

Resulting shortest-path tree from u:

vxywz

(u,v)(u,x)(u,x)(u,x)(u,x)

destination link

Resulting forwarding table in u:

2011/01/2012


Dijkstra’s algorithm, discussionAlgorithm complexity: n nodes each iteration: need to check all nodes, w, not in N’ n(n+1)/2 comparisons: O(n2) more efficient implementations possible: O(n*log(n))

Oscillations possible: e.g., link cost = amount of carried traffic

AD

CB

1 1+e

e0

e1 1

0 0

AD

CB

2+e 0

001+e 1

AD

CB

0 2+e

1+e10 0

AD

CB

2+e 0

001+e 1

initially … recomputerouting

… recompute … recompute

Solutions: Link costs not depend on the amount of trafficEnsure that not all routers run LS at the same time

11 11 11e e e

11 111 111 1111


2211/01/2012


Routing algorithms

Distance Vector Algorithm We can run an algorithm similar to Djikstra in a distributed

manner too, this way we do not need to know anything about the nodes that are not our neighbors in the graph.

For that we use the famous

Bellman-Ford Equation (dynamic programming)

For nodes x,y,and v

Define

dx(y) := cost of least-cost path from x to y

Then

dx(y) = min {c(x,v) + dv(y) }

where min is taken over all neighbors v of x

Bellman-Ford example

u

yx

wv

z2

21

3

1

1

2

53

5Clearly, dv(z) = 5, dx(z) = 3, dw(z) = 3

du(z) = min { c(u,v) + dv(z), c(u,x) + dx(z), c(u,w) + dw(z) } = min {2 + 5, 1 + 3, 5 + 3} = 4

Node that achieved minimum is nexthop in shortest path ➜ forwarding table

B-F equation says:

2411/01/2012


Distance Vector Algorithm(1/2)

Dx(y) = estimate of least cost from x to y

Node x knows cost to each neighbor v: c(x,v)

Node x maintains distance vector Dx = [Dx(y): y є N ]

Node x also maintains its neighbors’ distance vectors For each neighbor v, x maintains

Dv = [Dv(y): y є N ]

2511/01/2012


Distance Vector Algorithm(1/2)

Basic idea: From time-to-time, each node sends its own

distance vector estimate to neighbors Asynchronous When a node x receives new DV estimate from

neighbor, it updates its own DV using B-F equation:Dx(y) min← v{c(x,v) + Dv(y)} for each node y ∊ N

Under minor, natural conditions, the estimate Dx(y) converge to the actual least cost dx(y)

2611/01/2012


Distance Vector Algorithm

Iterative, asynchronous: each local iteration caused by:

local link cost change DV update message from

neighbor

Distributed: each node notifies

neighbors only when its DV changes neighbors then notify

their neighbors if necessary

wait for (change in local link cost or message from a neighbor)

recompute estimates

if DV to any node has changed,

notify neighbors

Each node:

2711/01/2012


x y zxyz

0 2 7∞∞ ∞∞∞ ∞

from

cost to

from

from

x y zxyz

0

from

cost to

x y zxyz

∞ ∞

∞∞ ∞

cost to

x y zxyz

∞∞ ∞7 1 0

cost to

∞2 0 1

∞ ∞ ∞

2 0 17 1 0

time

x z12

7

y

node x table

node y table

node z table

Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)} = min{2+0 , 7+1} = 2

Dx(z) = min{c(x,y) + Dy(z), c(x,z) + Dz(z)} = min{2+1 , 7+0} = 3

32

Yet another example (home study)

x y zxyz

0 2 7∞∞ ∞∞∞ ∞

from

cost to

from

from

x y zxyz

0 2 3

from

cost tox y z

xyz

0 2 3fr

om

cost to

x y zxyz

∞ ∞

∞∞ ∞

cost tox y z

xyz

0 2 7

from

cost tox y z

xyz

0 2 3

from

cost to

x y zxyz

0 2 3

from

cost tox y z

xyz

0 2 7

from

cost tox y z

xyz

∞∞ ∞7 1 0

cost to

∞2 0 1

∞ ∞ ∞

2 0 17 1 0

2 0 17 1 0

2 0 13 1 0

2 0 13 1 0

2 0 1

3 1 02 0 1

3 1 0

time

x z12

7

y

node x table

node y table

node z table

Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)} = min{2+0 , 7+1} = 2

Dx(z) = min{c(x,y) + Dy(z), c(x,z) + Dz(z)} = min{2+1 , 7+0} = 3

v*(y) = v*(z) =yForwarding table at x

11/01/2012

Yet another example (home study)

Distance Vector: link cost changes

Link cost changes: node detects local link cost change updates routing info, recalculates

distance vector if DV changes, notify neighbors

“goodnews travelsfast”

x z14

50

y1

At time t0, y detects the link-cost change, updates its DV, and informs its neighbors.At time t1, z receives the update from y and updates its table. It computes a new least cost to x and sends its neighbors its DV.

At time t2, y receives z’s update and updates its distance table. y’s least costs do not change and hence y does not send any message to z.

3011/01/2012


Distance Vector: link cost changes

Link cost changes: good news travels fast bad news travels slow - “count to

infinity” problem! 44 iterations before algorithm

stabilizes: see textPoisoned reverse: If Z routes through Y to get to X:

Z tells Y its (Z’s) distance to X is infinite (so Y won’t route to X via Z)

will this completely solve count to infinity problem?

No! It is good only for loops involving only two nodes

x z14

50

y60

3111/01/2012


Comparison of LS and DV algorithms

Message complexity LS: with n nodes, E links,

O(nE) msgs sent DV: exchange between

neighbors only convergence time

varies

Speed of Convergence LS: O(n2) algorithm

requires O(nE) messages may have oscillations

DV: convergence time varies may be routing loops count-to-infinity

problem

Robustness: what happens if router malfunctions?

LS: node can advertise

incorrect link cost each node computes

only its own table

DV: DV node can advertise

incorrect path cost each node’s table used

by others error propagate

through network

Internet routing

Routing algorithms Link state Distance Vector Hierarchical routing – Internet routing

Hierarchical Routing

scale: with 200 million destinations:

can’t store all destinations in routing tables! (~2.4 GB just to store them)

routing table exchange would take ages!

administrative autonomy

Internet = network of networks

each network administrator may want to control routing in its own network

Our routing study thus far - idealization all routers identical network “flat”… not true in practice

Hierarchical Routing

aggregate routers into regions, “autonomous systems” (AS)

routers in same AS run same routing protocol “intra-AS” routing

protocol routers in different AS

can run different intra-AS routing protocol

Gateway router Direct link to router

in another AS

3511/01/2012


3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected AS

forwarding table configured by both intra- and inter-AS routing algorithm intra-AS sets entries

for internal destinations

inter-AS & intra-AS sets entries for external destinations

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

3c

Inter-AS tasks suppose router in

AS1 receives datagram destined outside of AS1: router should

forward packet to gateway router, but which one?

AS1 must:1. learn which

destinations are reachable through AS2, which through AS3

2. propagate this reachability info to all routers in AS1

Job of inter-AS routing!

Internet routing

Routing in the Internet – Interior Gateway Protocols protocols RIP OSPF

3811/01/2012


Intra-AS Routing

also known as Interior Gateway Protocols (IGP) most common Intra-AS routing protocols:

RIP: Routing Information Protocol

OSPF: Open Shortest Path First

IGRP: Interior Gateway Routing Protocol (Cisco proprietary)

3911/01/2012


RIP ( Routing Information Protocol)

Not really used anymore – home study distance vector algorithm included in BSD-UNIX Distribution in 1982 distance metric: # of hops (max = 15 hops)

DC

BA

u vw

x

yz

destination hops u 1 v 2 w 2 x 3 y 3 z 2

From router A to subnets:

4011/01/2012


RIP advertisements

distance vectors: exchanged among neighbors every 30 sec via Response Message (also called advertisement)

each advertisement: list of up to 25 destination subnets within AS

4111/01/2012


RIP: Example

Destination Network Next Router Num. of hops to dest. w A 2

y B 2 z B 7

x -- 1…. …. ....

w x y

z

A

C

D B

Routing/Forwarding table in D

4211/01/2012


RIP: Example

Destination Network Next Router Num. of hops to dest. w A 2

y B 2 z B A 7 5

x -- 1…. …. ....

Routing/Forwarding table in D

w x y

z

A

C

D B

Dest Next hops w - 1 x - 1 z C 4 …. … ...

Advertisementfrom A to D

4311/01/2012


RIP: Link Failure and Recovery

If no advertisement heard after 180 sec --> neighbor/link declared dead routes via neighbor invalidated

new advertisements sent to neighbors

neighbors in turn send out new advertisements (if tables changed)

link failure info quickly (?) propagates to entire net

poison reverse used to prevent ping-pong loops (infinite distance = 16 hops)

4411/01/2012


RIP Table processing

RIP routing tables managed by application-level process called route-d (daemon)

advertisements sent in UDP packets, periodically repeated

physicallink

network forwarding (IP) table

Transprt (UDP)

routed

physicallink

network (IP)

Transprt (UDP)

routed

forwardingtable

4511/01/2012


Internet routing


4611/01/2012


OSPF (Open Shortest Path First)

“open”: publicly available uses Link State algorithm

LS packet dissemination topology map at each node route computation using Dijkstra’s algorithm link cost definition up to the administrator

OSPF advertisement carries one entry per neighbor router

advertisements disseminated to entire AS (via flooding) carried in OSPF messages directly over IP (rather than

TCP or UDP

OSPF “advanced” features (not in RIP)

security: all OSPF messages authenticated (to prevent malicious intrusion), 2 modes: simple authentication – same password for all routers,

passed in the message (plaintext – not very secure) MD5 authentication – shared secret keys used, MD5

hash computed for each OSPF message content, used with sequence numbers (against reply attacks)

multiple same-cost paths allowed (only one path in RIP) Only one chosen for carrying traffic though

4811/01/2012


OSPF “advanced” features (not in RIP)

For each link, multiple cost metrics for different TOS (e.g., satellite link cost set “low” for best effort; high for real time) Up to the network administrator

integrated uni- and multicast support: Multicast OSPF (MOSPF) uses same topology data base as

OSPF

New type of link-state advertisement used for that

hierarchical OSPF in large domains Within a single AS, different areas defined

4911/01/2012


Hierarchical OSPF

Hierarchical OSPF

two-level hierarchy: local area, backbone. Link-state advertisements only in area

each node has detailed area topology; only knows direction (shortest path) to nets in other areas.

area border routers: “summarize” distances to nets in own area, advertise to other Area Border routers.

backbone routers: run OSPF routing limited to backbone.

boundary routers: connect to other AS’s.

Checklist:Check course regulationsUnderstand why do we need routingReview routing algorithmsReview routing protocolsUnderstand what is an AS

routing and networking

Documents

ospf advanced features

convergence time varies

distance vector algorithm

area border routers

open shortest path

internet routing routing

distance vector

routing protocol