Chapter4 Routing

7/28/2019 Chapter4 Routing

1/71

Network Layer 4-1

Chapter 4 Network

Layer (4b - Routing)

Computer Networking:A Top Down ApproachFeaturing the Internet,5th edition.Jim Kurose, Keith RossAddison-Wesley, July2009.

A note on the use of these ppt slides:Were making these slides freely available to all (faculty, students, readers).

Theyre in PowerPoint form so you can add, modify, and delete slides

(including this one) and slide content to suit your needs. They obviously

represent a lotof work on our part. In return for use, we only ask the

following:

If you use these slides (e.g., in a class) in substantially unaltered form,that you mention their source (after all, wed like people to use our book!)

If you post any slides in substantially unaltered form on a www site, that

you note that they are adapted from (or perhaps identical to) our slides, and

note our copyright of this material.

Thanks and enjoy! JFK/KWR

All material copyright 1996-2006

J.F Kurose and K.W. Ross, All Rights Reserved JAC 10-25-2012

Modified by John CopelandGeorgia Tech

for use in ECE3076


2/71

Network Layer 4-2

Chapter 4: Network Layer

4. 1 Introduction

4.2 Virtual circuit anddatagram networks

4.3 What

s inside arouter

4.4 IP: InternetProtocol Datagram format

IPv4 addressing

ICMP

IPv6

4.5 Routing algorithms Link state (OSPF) Distance Vector (RIP) Hierarchical routing (BGP)

4.6 Routing in theInternet RIP OSPF BGP

4.7 Broadcast andmulticast routing


3/71

Network Layer 4-3

IP Addressing: introduction

IP address: 32-bitidentifier for host, androuter interface

interface:connectionbetween host/router and

physical link (sometimescalled a "port"). routers typically have

multiple interfaces

host typically has oneinterface

IP addresses associatedwith each interface

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.1 = 11011111 00000001 00000001 00000001

223 1 11


4/71

Network Layer 4-4

Subnets

IP address: subnet part (high

order bits)

host part (low orderbits)

Whats a subnet ? device interfaces with

same subnet part of IPaddress

can physically reacheach other withoutintervening router

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

network consisting of 3 subnets

subnet


5/71

Network Layer 4-5

Subnets 223.1.1.0/24 223.1.2.0/24

223.1.3.0/24

Recipe

To determine thesubnets, detach eachinterface from its

host or router,creating islands ofisolated networks.Each isolated network

is called a subnet.Subnet mask: /24


6/71

Network Layer 4-6

Subnets

How many? 223.1.1.1

223.1.1.3

223.1.1.4

223.1.2.2223.1.2.1

223.1.2.6

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.2

223.1.7.0

223.1.7.1223.1.8.0223.1.8.1

223.1.9.1

223.1.9.2

Stub Subnet ->223.1.2.0/24

Transit Subnet ->223.1.9.0/28

Stub Subnet ->

223.1.3.0/24

223.1.1.0/24

223.1.8.0/28


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Network Layer 4-7

Stub Subnet223.1.2.0/24

Transit Subnet223.1.9.0/28





A

C

B

Routers ("Nodes") designated by a letter: A, B, C, ...All subnets are either:Transit Subnets ("Links" between nodes: A-B, B-C, A-C)

orStub Subnets (connected to a single "gateway" router)designated by the same letter: A, B, C, ...

Simplified Network

A C

B

A-B

A-C

B-C


8/71

Network Layer 4-8

1

23

223.1.3.123

Destination address (IPd) in arriving

packets IP header

routing algorithm

Interplay between routing, forwarding

AC

B

D

A-CA-D

A-B

Routing Table for Node A

Network Address Network Mask Port (A- )

223.1.1.0 255.255.255.0 B

223.1.2.0 255.255.255.0 Local

223.1.3.0 255.255.255.0 C

Match row "i" if:IPd & Maski = NetAddri

Use match with largest Maski.


9/71

Network Layer 4-9

u

yx

wv

z2

2

13

1

1

2

53

5

Graph: G = (N,E)

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

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

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

"Cost" of Link


10/71

Network Layer 4-10

Graph abstraction: costs

u

yx

wv

z2

2

1

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 tocongestion

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

Question: Whats the least-cost path between u and z ?

Routing algorithm: algorithm that finds least-cost path


11/71

Network Layer 4-11

Routing Algorithm classification

Global or decentralizedinformation?Global (e.g., OSPF): all routers have complete

topology, link cost info

link state algorithms

Decentralized (e.g., RIP): router knows physically-

connected neighbors, linkcosts to neighbors

iterative process ofcomputation, exchange ofinfo with neighbors

distance vectoralgorithms

Static or dynamic?Static (Manual updates):

routes change slowlyover time

Dynamic (RIP, OSPF):

routes change morequickly

periodic update

in response to linkcost changes


12/71

Network Layer 4-12

A Link-State Routing Algorithm(OSPF)

Dijkstras algorithm net topology, link costs

known to all nodes

accomplished via link

state broadcast all nodes have same info

computes least cost pathsfrom one node (source)to all other nodes

gives forwarding tablefor that node

iterative: after kiterations, know least costpath to k dest.s

Notation:

c(x,y): link cost from nodex to y; = if not directneighbors

D(v): current value of costof path from source todest. v

p(v): predecessor node

along path from source to v N': set of nodes whose

least cost path definitivelyknown


13/71

Network Layer 4-13

Dijsktras Algorithm

1 Init ial ization: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 un t i l al l nodes in N'


14/71

Network Layer 4-14

Dijkstras algorithm: example (for "u") - 1

u

yx

wv

z2

2

13

1

1

2

53

5

Permanent Nodes: u (start with home node)Temporary Nodes: v(u,2), x(u,1), w(u,5)

(linked to a permanent node, path cost in ()s)New Permanent Node: x(u,2) (lowest-cost path to u)

New Permanent Link: u-xDelete Links: (from new permanent node to anypermanent node, other than the New Permanent Link)


15/71

Network Layer 4-15


u

yx

wv

z2

2

13

1

1

2

53

5

Permanent Nodes: u(0), x(2)Temporary Nodes: v(u,2 or x,3), y(x,2), w(x,4 or u,5)

New Permanent Node: v(u,2)New Permanent Link: v-uDelete Link: v-x

Note: You can wait to delete (all non-permanent) linksafter the tree is complete.


16/71

Network Layer 4-16


u

yx

wv

z2

2

13

1

1

2

53

5

Permanent Nodes: u, x(1), v(2)Temporary Nodes: y(x,2), w(y,3 or x,4 or v,5)

New Permanent Node: y(x,2)New Permanent Link: x-yDelete Link: none


17/71

Network Layer 4-17


u

yx

wv

z2

2

13

1

1

2

53

5

Permanent Nodes: u, x(1), v(2), y(2)Temporary Nodes: w(y,3 or x,4 or v,5 or u,5), z(y,4)

New Permanent Node: w(y,3)New Permanent Link: w-y

Delete Links: w-x, w-v, w-u


18/71

Network Layer 4-18


u

yx

wv

z2

2

13

1

1

2

53

5

Permanent Nodes: u, x(1), v(2), y(2), w(3)Temporary Nodes: z(y,4 or w,8)

New Permanent Node: z(y,4)

New Permanent Link: y-zDelete Link: z-w

This is called the "shortest-path tree", or"sink tree," for node u.


19/71

Network Layer 4-19

Dijkstras algorithm: example (2)

u

yx

wv

z

Resulting shortest-path tree from u:

vx

y

w

z

(u,v)(u,x)

(u,x)

(u,x)

(u,x)

destination link

Resulting forwarding table in u:

Two-step process based oninformation received bybroadcast OSPF messagesfrom every router.

1. Construct a table of all

advertised blocks andthe edge router whichconnects to them.

2. Add link to forward onfor each edge router,based on the routing

algorithm.


20/71

Network Layer 4-20

u

yx

wv

z

2

2

1

3

1

1

2

5

3

5 (link cost)

Graphical Method - Sink Treefor Node "U"

(animated - keep clicking)

2

1

3

5 (total cost)

4

2

5

4

83

Next Slide


21/71

Network Layer 4-21


4. 1 Introduction


4.3 What

s inside arouter


IPv4 addressing

ICMP

IPv6

4.5 Routing algorithms Link state (OSPF) Distance Vector (RIP) Hierarchical routing

4.6 Routing in theInternet RIP OSPF BGP

4.7 Broadcast andmulticast routing


22/71

Network Layer 4-22

Distance Vector Algorithm (RIP)

Bellman-Ford Equation (dynamic programming)

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.

This is the distance to y advertised by x.

x will forward datagrams for y to v.

v


23/71

Network Layer 4-23

Bellman-Ford algorithm example

u

yx

wv

z2

2

13

1

1

2

53

5

Known:, 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 ( -> x)

B-F equation says:

Find forwarding link for u to z when the costto neighbors is known, c(u,?), and the

cost from neighbors to z, d?(z) is known.

u

x

w

v z2

13

5

3

5

The way u sees the network.

Node that provides minimum distance (node x) is next

hop in shortest path to z, forwarding table


24/71

Network Layer 4-24

Distance Vector Algorithm

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 N ]

Node x also maintains its neighbors

distance vectors For each neighbor v, x maintains

Dv = [Dv(y): y N ]


25/71

Network Layer 4-25

Distance vector algorithm (4)

Basic idea:

Each node periodically sends its own distancevector estimate to neighbors

When a node x receives new DV estimate fromneighbor, it updates its own DV using B-F equation:

Dx(y) minv{c(x,v) + Dv(y)} for each node yN

Under minor changes, natural conditions, theestimated Dx(y) converges to the actual least costdx(y)


26/71

Network Layer 4-26

Distance Vector Algorithm (5)

Iterative, asynchronous:each local iteration causedby:

local link cost change

DV update message from

neighborDistributed: each node notifies

neighbors onlywhen its DVchanges neighbors then notify

their neighbors ifnecessary

waitfor (change in local linkcost or msg from neighbor)

recompute estimates

if DV to any dest haschanged, notifyneighbors

Each node:

D ( ) i { ( )


27/71

Network Layer 4-27

x y z

xyz

0 2 7

from

cost to

from

from

x y zx

yz

0 2 3

from

cost tox y z

xyz

0 2 3

from

cost to

x y zx

yz

cost to

x y zx

yz

0 2 7

from

cost to

x y z

xyz

0 2 3

from

cost to

x y z

xyz

0 2 3

from

cost tox y z

xyz

0 2 7

from

cost to

x 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 0

2 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

+


28/71

Network Layer 4-28

Distance Vector: link cost changes

Link cost changes: node detects local link cost change

updates routing info, recalculatesdistance vector

if DV changes, notify neighbors

goodnews

travelsfast

x z

14

50

y1

At time t0, ydetects the link-cost change, updates its DV,and informs its neighbors.

At time t1, zreceives the update from yand updates its table.It computes a new least cost to x and sends its neighbors its DV

At time t2, yreceives zs update and updates its distance table.ys least costs do not change and hence y does notsend anymessage to z.


29/71

Network Layer 4-29

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: seetext

Poisoned reverse: If Z routes through Y to

get to X : Z tells Y its (Zs) distance

to X is infinite (so Y wontroute to X via Z)

will this completely solvecount to infinity problem?

x z

14

50

y60

Y advertises X in 4 hops

Z sends datagrams for X to YZ advertises "X in 5 hops".

Y-X link cost goes to 60

Y thinks Z can route in 5 hops,

so Y advertises "X in 6", sends

datagrams back to Z.Z sends datagrams back to Y,

advertises "X in 7".

Y sends datagrams back to Z,

advertises "X in 8".

( l h )


30/71

Network Layer 4-30

RIP (Distance-Vector Algorithm)

A

B

CX

M

Y Z

Router A Table

Prefix Distance Port

128.230. 2 X

130.207. 6 N

209.196. 7 X

24.56. 9 X

Router B Table


128.230. 2 X

130.207. 6 X

209.196. 5 M

24.56. 11 X

Router C Table


128.230. 2 X

130.207. 4 X

209.196. 7 X

24.56. 11 P

Construct the Routing Table for Router X. Use "L" for the port to the local LAN.

Using Poison Reverse*, construct the Updates sent from Router X to A, B, and C. (infinity -> 15).

Update X to A Table

Prefix Distance

128.230. 1

130.207. 5

209.196. 6

24.56. 15

Update X to B Table

Prefix Distance

128.230. 1

130.207. 5

209.196. 15

24.56. 10

Update X to C Table

Prefix Distance

128.230. 1

130.207. 15

209.196. 6

24.56. 10

128.230.0.0/16

Router X Table


128.230. 1 L

130.207. 5 C

209.196. 6 B

24.56. 10 A

111

0

Router X Calculation

Prefix Distance - A Distance - B Distance - C Min + 1 Closest Rtr

128.23 1 1 1 0* *Local

130.207. 6 6 4 5 C

209.196 7 5 7 6 B

24.96. 9 11 11 10 A

000

*For "Poison Reverse" a distance of 15 is sent to destination router

This is the format for a quiz question.


31/71

Network Layer 4-31

Comparison of LS (OSPF) and DV (RIP) algorithmsLS = Link State, DV = Distance Vector)

Message complexity LS: with n nodes, E links,

O(nE) msgs sent

DV: exchange betweenneighbors only

convergence time varies

Speed of Convergence LS: O(n2) algorithm requires

O(nE) msgs

may have oscillations DV: convergence time varies

may be routing loops

count-to-infinity problem

Robustness: what happens ifrouter malfunctions?

LS: node can advertise incorrect

linkcost each node computes only its

owntableDV:

DV node can advertiseincorrectpathcost

each nodes table used byothers

errors propagate thru thenetwork


32/71

Network Layer 4-32


4. 1 Introduction


4.3 What

s inside arouter


IPv4 addressing

ICMP

IPv6

4.5 Routing algorithms Link state

Distance Vector

Hierarchical routing

4.6 Routing in theInternet RIP

OSPF

BGP 4.7 Broadcast and

multicast routing


33/71

Network Layer 4-33

Hierarchical Routing

scale: with 200 milliondestinations:

cant store all dests in

routing tables! routing table exchange

would swamp links!

administrative autonomy internet = network of

networks

each network admin maywant to control routing in itsown network

Our routing study thus far - idealization all routers identical

network flat

nottrue in practice


34/71

Network Layer 4-34

Hierarchical RoutingBGP - Border Gateway Protocol

aggregate routers intoregions,autonomoussystems (AS)

routers in same AS runsame routing protocol [no,

hierarchical architecturesare possible]

intra-AS routingprotocol

routers in different AS

can run different intra-AS routing protocol

Gateway router

Direct link to router inanother AS


35/71

Network Layer 4-35

3b

1d

3a

1c2aAS3

AS1

AS21a

2c2b

1b

Intra-AS

Routing

algorithm

Inter-AS

Routing

algorithm

Forwarding

table

3c

Interconnected ASes

Forwarding table isconfigured by bothintra- and inter-ASrouting algorithm

Intra-AS sets entriesfor internal dests

Inter-AS & Intra-Assets entries forexternal dests


36/71

Network Layer 4-36

3b

1d

3a

1c2aAS3

AS1

AS21a

2c2b

1b

3c

Inter-AS tasks Suppose router in AS1

receives datagram forwhich destination isoutside of AS1 Router should forward

packet towards one of

the gateway routers,but which one?

AS1 needs:

1. to learn which dests

are reachable throughAS2 and whichthrough AS3

2. to propagate this

reachability info to allrouters in AS1

Job of inter-AS routing!


37/71

Network Layer 4-37

Example: Setting forwarding table in router 1d

Suppose AS1 learns (via inter-AS protocol) that subnetxis reachable via AS3 (gateway 1c) but not via AS2.

Inter-AS protocol propagates reachability info to allinternal routers.

Router 1d determines from intra-AS routing info thatits interface I is on the least cost path to 1c.

Puts in forwarding table entry (x,I).

3b

1d

3a

1c2aAS3

AS1

AS21a

2c2b

1b

3c

X


38/71

Network Layer 4-38

Example: Choosing among multiple ASes

Now suppose AS1 learns from the inter-AS protocolthat subnet xis reachable from AS3 andfrom AS2.

To configure forwarding table, router 1d mustdetermine towards which gateway it should forwardpackets for dest x.

This is also the job on inter-AS routing protocol!

3b

1d

3a

1c2aAS3

AS1

AS21a

2c 2b

1b

3c

X


39/71

Network Layer 4-39

Learn from inter-ASprotocol that subnet

x is reachable via

multiple gateways

Use routing info

from intra-ASprotocol to determine

costs of least-cost

paths to each

of the gateways

Hot potato routing:Choose the gateway

that has the

smallest least cost

Determine from

forwarding table theinterface I that leads

to least-cost gateway.

Enter (x,I) in

forwarding table

Example: Choosing among multiple ASes

Now suppose AS1 learns from the inter-AS protocolthat subnet xis reachable from AS3 andfrom AS2.

To configure forwarding table, router 1d mustdetermine towards which gateway it should forwardpackets for dest x.

This is also the job on inter-AS routing protocol! Hot potato routing: send packet towards closest oftwo routers.


40/71

Network Layer 4-40


4. 1 Introduction


4.3 Whats inside arouter


IPv4 addressing

ICMP

IPv6


Distance Vector



OSPF


multicast routing


41/71

Network Layer 4-41

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 (Ciscoproprietary)


42/71

Network Layer 4-42


4. 1 Introduction




IPv4 addressing

ICMP

IPv6


Distance Vector


4.6 Routing in theInternet RIP (Distance Vector)

OSPF (Link State)

BGP (Hierarchical) 4.7 Broadcast and

multicast routing


43/71

Network Layer 4-43

RIP ( Routing Information Protocol)

Distance vector algorithm

Included in BSD-UNIX Distribution in 1982

Distance metric: # of hops (max = 15 hops)

DC

BA

u v

w

x

yz

destination hopsu 1v 2

w 2x 3y 3z 2

From router A to subsets:


44/71

Network Layer 4-44

RIP advertisements

Distance vectors*: exchanged amongneighbors every 30 sec via ResponseMessage (also called advertisement)

Each advertisement: list of up to 25destination nets within AS

* List of all subnets and their "distance" (cost: delay, hops, ).


45/71

Network Layer 4-45

RIP: Example

Destination Network Next Router Num. of hops to dest.

w A 2y B 2

z B 7x -- 1. . ....

w x yz

A

C

D B

Routing table in D

E l


46/71

Network Layer 4-46

RIP: Example

Destination Network Next Router Num. of hops to dest.

w A 2y B 2z B A 7 5x -- 1. . ....

Routing table in D

w x y

z

A

C

D B

Dest Next hopsw - 1

x - 1z C 4. ...

Advertisement

from A to D


47/71

Network Layer 4-47

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 (iftables changed)

link failure info quickly (?) propagates to entire net

poison reverse used to prevent ping-pong loops

(infinite distance = 15 hops)


48/71

Network Layer 4-48

RIP Table processing

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

advertisements sent in UDP packets, periodicallyrepeated

physical

link

network forwarding(IP) table

Transprt(UDP)

routed

physical

link

network(IP)

Transprt(UDP)

routed

forwardingtable


49/71

Network Layer 4-49


4. 1 Introduction




IPv4 addressing ICMP

IPv6


Distance Vector



OSPF


multicast routing


50/71

Network Layer 4-50

OSPF (Open Shortest Path First)

open: publicly available

Uses Link State algorithm Link State packet dissemination

Topology map at each node

Route computation using Dijkstras algorithm

OSPF advertisement carries one entry per neighborrouter

Advertisements disseminated to entire AS (viaflooding) [exception Hierarchical Routing] Carried in OSPF messages directly over IP (rather than TCP

or UDP


51/71

Network Layer 4-51

OSPF advanced features (not in RIP)

Security: all OSPF messages authenticated (toprevent malicious intrusion)

Multiple same-cost paths allowed (only one path inRIP)

For each link, multiple cost metrics for differentTOS (e.g., satellite link cost set low for besteffort; high for real time)

Integrated uni- and multicast support:

Multicast OSPF (MOSPF) uses same topology database as OSPF

Hierarchical OSPF in large domains.


52/71

Network Layer 4-52

Hierarchical OSPF

Boundary routers canaggregate internalroutes.


53/71

Network Layer 4-53

Hierarchical OSPF

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

each nodes has detailed area topology; only knowdirection (shortest path) to nets in other areas.

Area border routers:summarize distances tonets in own area, advertise to other Area Borderrouters.

Backbone routers: run OSPF routing limited to

backbone. Boundary routers: connect to other ASs.


54/71

Network Layer 4-54


4. 1 Introduction





IPv6


Distance Vector



OSPF


multicast routing


55/71

Network Layer 4-55

Internet inter-AS routing: BGP

BGP (Border Gateway Protocol):thedefacto standard

BGP provides each AS a means to:1. Obtain subnet reachability information from

neighboring ASs.2. Propagate reachability information to all AS-

internal routers.3. Determine good routes to subnets based on

reachability information and policy. allows subnet to advertise its existence to

rest of Internet: I am here

BGP b i


56/71

Network Layer 4-56

BGP basics Pairs of routers (BGP peers) exchange routing info

over semi-permanent TCP connections: BGP sessions BGP sessions need not correspond to physical links.

When AS2 advertises a prefix to AS1, AS2 ispromisingit will forward any datagrams destined tothat prefix towards the prefix. AS2 can aggregate prefixes in its advertisement

3b

1d

3a

1c

2aAS3

AS1

AS21a

2c

2b

1b

3c

eBGP session

iBGP session

Dist ib ti h bilit i f


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Network Layer 4-57

Distributing reachability info With eBGP session between 3a and 1c, AS3 sends prefix

reachability info to AS1. 1c can then use iBGP do distribute this new prefix reach info

to all routers in AS1 1b can then re-advertise new reachability info to AS2 over

1b-to-2a eBGP session When router learns of new prefix, creates entry for prefix

in its forwarding table.

3b

1d

3a

1c

2a

AS3

AS1

AS21a

2c

2b

1b

3c

eBGP session

iBGP session


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Network Layer 4-58

Path attributes & BGP routes

When advertising a prefix, advert includes BGPattributes. prefix + attributes = route

Two important attributes: AS-PATH: contains ASs through which prefix advertisement

has passed: AS 67 AS 17

NEXT-HOP: Indicates specific internal-AS router to next-hop AS. (There may be multiple links from current AS tonext-hop-AS.)

When gateway router receives route advertisement,uses import policy to accept/decline.


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Network Layer 4-59

BGP route selection

Router may learn about more than 1 routeto some prefix. Router must select route.

Elimination rules:

1. Local preference value attribute: policydecision

2. Shortest AS-PATH

3. Closest NEXT-HOP router: hot potato routing

4. Additional criteria


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Network Layer 4-60

BGP messages

BGP messages exchanged using TCP. BGP messages:

OPEN: opens TCP connection to peer andauthenticates 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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Network Layer 4-61

BGP routing policy

A,B,C are provider networks

X,W,Y are customer (of provider networks)

X is dual-homed: attached to two networks X does not want to route from B via X to C

.. so X will not advertise to B a route to C


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Network Layer 4-62

BGP routing policy (2)

A advertises to B the path AW

B advertises to X the path BAW

Should B advertise to C the path BAW? No way! B gets no revenue for routing CBAW since neitherW nor C are Bs customers

B wants to force C to route to w via A

B wants to route onlyto/from its customers!


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Network Layer 4-63

Why different Intra- and Inter-AS routing ?

Policy: 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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4-64

Area: Lab, Home Intra-AS (Ga.Tech)

Inter-AS

Routing Type: Distance Vector Link State Manual + Others

Example Protocol: RIP OSPF, IGMP BGP

RoutingAlgorithm

Bellman-Ford, wPoison Reverse

Dijkstra Mixed

Cost Unit: Links Delay Dollars, Policy,Rules

Messaging: UDP/IP unicast OSPF/IPbroadcast (flood)

TCP/IP unicast

Adjust tocongestion:

No Yes Some places

Maximum Path: path:


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Network Layer 4-65


4. 1 Introduction 4.2 Virtual circuit and

datagram networks




IPv6


Distance Vector



OSPF


multicast routing

Broadcast Routing


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Network Layer 4-66

R1

R2

R3 R4

source

duplication

R1

R2

R3 R4

in-network

duplication

duplicate

creation/transmissionduplicate

duplicate

Broadcast Routing

Deliver packets from source to all other nodes

Source duplication is inefficient:

Source duplication: how does sourcedetermine recipient addresses?


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Network Layer 4-67

In-network duplication

Flooding: when node receives brdcst pckt,sends copy to all neighbors Problems: cycles & broadcast storm

Controlled flooding: node only broadcasts pkt

if it hasnt broadcast the same packet beforeNode keeps track of pckt ids already brdcsted

Or reverse path forwarding (RPF): only forwardpckt if it arrived on shortest path between node

and sourceSpanning tree

No redundant packets received by any node


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Network Layer 4-68

A

B

G

D

E

c

F

A

B

G

D

E

c

F

(a) Broadcast initiated at A (b) Broadcast initiated at D

Spanning Tree

First construct a spanning treeNodes forward copies only along spanning

tree


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Multicast Routing: Problem Statement

Goal:find a tree (or trees) connectingrouters having local multicast group members

tree:not all paths between routers used

source-based:different tree from each sender to rcvrs

shared-tree:same tree used by all group members

Shared tree Source-based trees

mc:/Users/copeland root# netstat -r -a -l -n #MacOS 10.4

R ti t bl


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Network Layer 4-70

Routing tables

Internet:

Destination Gateway Flags Refs Use Mtu Netif

default (0/0) 199.77.146.1 UGSc 6 6716 1500 en0

17.112.152.32 199.77.146.1 UGHW3 0 6683 1500 en0

127 (local) 127.0.0.1 UCS 0 0 16384 lo0

127.0.0.1 127.0.0.1 UH 10 21599 16384 lo0

130.207.230.13 199.77.146.1 UGHW3 0 6746 1500 en0

130.207.244.240 199.77.146.1 UGHW 1 52 1500 en0

130.207.244.244 199.77.146.1 UGHW 1 1041 1500 en0

169.254 link#4 UCS 0 0 1500 en0

199.77.146/25 link#4 UCS 1 0 1500 en0199.77.146.1 0:11:bc:f4:14:0 UHLW 6 0 1500 en0

199.77.146.101 127.0.0.1 UHS 2 397306 16384 lo0

224.0.0.251 199.77.146.1 UGHW 1 270 1500 en0

127. /8 local Local links for inter-process communication

169.254 /16 blackhole Not allocated (see RFC 3330, used when no IP)

224.0.0..0/4 (1110xxxx.X.X.X) is multicast or anycast

en0 is Ethernet, lo0 is local OS port (loopback).

Flags: U=usable, G=gateway, H=host, W=setup by process, S=static

Multicast and Anycast, 224.0.0.0 - 239.255.255.255


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mc:/Users/copeland root# tcpdump -nvli en0 'ip[16]==224'tcpdump: listening on en0, link-type EN10MB (Ethernet), capture size 96 bytes

08:04:06 IP (tos 0xc0,ttl 1, id 31543, off 0, len:116)199.77.146.1 > 224.0.0.5:

OSPFv2, Hello (1), len:80 [len 44]

08:04:09 IP (tos 0xc0,ttl 1, id 31568, off 0, len:54) 199.77.146.1 >

224.0.0.13:pim v2 Hello (Hold-time 1m45s (Genid: 0xdf5b6033)(DR-Priority: 1)(State

Refresh Capable;v1)

08:04:15 IP (tos 0xc0,ttl 1, id 31584, off 0, len:28) 199.77.146.1 > 224.0.0.1:

igmp query v208:04:16 IP (tos 0xc0,ttl 1, id 31610, off 0, len:116)199.77.146.1 > 224.0.0.5:


08:04:19 IP (tos 0x0, ttl 1, id 32954, off 0, len:28) 199.77.146.99> 224.0.1.60:

igmp v1 report 224.0.1.6008:04:23 IP (tos 0x0, ttl 1, id 2665, off 0, len:32, opt:4 199.77.146.101 > 224.0.0.251:

igmp v2 report 224.0.0.251




pim v2 Hello (Hold-time 1m45s) (Genid: 0xdf5b6033)(DR-Priority: 1)(State Refresh

Capable;v1)




igmp query v2

igmp and pim are multicast protocols.

OSPF routing uses "anycast"

All 3 use "ttl 1" only reaches neighbors

Date post:	03-Apr-2018
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Chapter4 Routing

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