routing protocol review

1

Routing Protocol Review

(some slides borrowed from Eugene Ng @ Rice Univ.)

2

Internet Routing

AS 73AS 9

AS 5050

AS 4554AS 101

AS 3999

Internet

3


Part 1: Intra-domain routing

4

Intra-domain Routing Protocols

• Two intra-domain routing protocols• Both try to achieve the “shortest path” forwarding• Quite commonly used

• OSPF: Based on Link-State routing algorithm• RIP: Based on Distance-Vector routing algorithm

• In Project 2, you implemented and played around with OSPF!

5

Link State Routing (OSPF): Flooding

• Each node knows its connectivity and cost to a direct neighbor

• Every node tells every other node this local connectivity/cost information– Via flooding

• In the end, every node learns the complete topology of the network

• E.g. A floods messageA

ED

CB

F

2

2

13

1

1

2

53

5

A connected to B cost 2A connected to D cost 1A connected to C cost 5

6

Flooding Details• Each node periodically generates Link State Packet (LSP)

contains– ID of node created LSP

– List of direct neighbors and costs

– Sequence number (64 bit, assume to never wrap around)

– Time to live

• Flood is reliable– Use acknowledgement and retransmission

• Sequence number used to identify *newer* LSP– An older LSP is discarded

– What if a router crash and sequence number reset to 0?

• Receiving node flood LSP to all its neighbors except the neighbor where the LSP came from

• LSP is also generated when a link’s state changes (failed or restored)

7

Link State Flooding Example

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7

8

5

4

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2

12

10

13

11

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6

7

8

5

4

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2

12

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6

7

8

5

4

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2

12

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7

8

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4

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11

A Link State Routing Algorithm

Dijkstra’s algorithm• Net topology, link costs known

to all nodes– Accomplished via “link state

flooding” – All nodes have same info

• Compute least cost paths from one node (‘source”) to all other nodes

• Repeat for all sources

Notations• c(i,j): link cost from node i to

j; cost infinite if not direct neighbors

• D(v): current value of cost of path from source to node v

• p(v): predecessor node along path from source to v, that is next to v

• S: set of nodes whose least cost path definitively known

12

Dijsktra’s Algorithm (A “Greedy” Algorithm)

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

13

Example: Dijkstra’s Algorithm

Step012345

start SA

D(B),p(B)2,A

D(C),p(C)5,A

D(D),p(D)1,A

D(E),p(E) D(F),p(F)

A

ED

CB

F

2

2

13

1

1

2

53

5

1 Initialization: 2 S = {A};3 for all nodes v 4 if v adjacent to A 5 then D(v) = c(A,v); 6 else D(v) = ;…

14


Step012345

start SA

AD

D(B),p(B)2,A

D(C),p(C)5,A4,D

D(D),p(D)1,A

D(E),p(E)

2,D

D(F),p(F)

A

ED

CB

F

2

2

13

1

1

2

53

5

…8 Loop 9 find w not in S s.t. D(w) is a minimum; 10 add w to S; 11 update D(v) for all v adjacent to w and not in S: 12 D(v) = min( D(v), D(w) + c(w,v) );13 until all nodes in S;

15


Step012345

start SA

ADADE

D(B),p(B)2,A

D(C),p(C)5,A4,D3,E

D(D),p(D)1,A

D(E),p(E)

2,D

D(F),p(F)

4,E

A

ED

CB

F

2

2

13

1

1

2

53

5…8 Loop 9 find w not in S s.t. D(w) is a minimum; 10 add w to S; 11 update D(v) for all v adjacent to w and not in S: 12 D(v) = min( D(v), D(w) + c(w,v) );13 until all nodes in S;

16


Step012345

start SA

ADADE

ADEB

D(B),p(B)2,A

D(C),p(C)5,A4,D3,E

D(D),p(D)1,A

D(E),p(E)

2,D

D(F),p(F)

4,E

A

ED

CB

F

2

2

13

1

1

2

53


17


Step012345

start SA

ADADE

ADEBADEBC

D(B),p(B)2,A

D(C),p(C)5,A4,D3,E

D(D),p(D)1,A

D(E),p(E)

2,D

D(F),p(F)

4,E

A

ED

CB

F

2

2

13

1

1

2

53


18


Step012345

start SA

ADADE

ADEBADEBC

ADEBCF

D(B),p(B)2,A

D(C),p(C)5,A4,D3,E

D(D),p(D)1,A

D(E),p(E)

2,D

D(F),p(F)

4,E

A

ED

CB

F

2

2

13

1

1

2

53


19

Distance Vector Routing (RIP)

• What is a distance vector?– Current best known cost to get to a destination

• Idea: Exchange distance vectors among neighbors to learn about lowest cost paths

Dest. Cost

A 7

B 1

D 2

E 5

F 1

G 3

Node C

Note no vector entry for C itself

At the beginning, distance vector only has information about directly attached neighbors, all other dests

have cost

Eventually the vector is filled

20

Distance Vector Routing Algorithm

• Iterative: continues until no nodes exchange info• Asynchronous: nodes need not exchange info/iterate in lock

steps• Distributed: each node communicates only with directly-

attached neighbors• Each router maintains

– Row for each possible destination– Column for each directly-attached neighbor to node– Entry in row Y and column Z of node X best known distance from X

to Y, via Z as next hop

• Note: for simplicity in this lecture examples we show only the shortest distances to each destination

21

Distance Vector Routing

• Each local iteration caused by: – Local link cost change – Message from neighbor: its least cost

path change from neighbor to destination

• Each node notifies neighbors only when its least cost path to any destination changes

– Neighbors then notify their neighbors if necessary

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

recompute distance table

if least cost path to any dest

has changed, notify neighbors

Each node:

22

Distance Vector Algorithm (cont’d)

1 Initialization: 2 for all neighbors V do3 if V adjacent to A 4 D(A, V) = c(A,V); 5 else • D(A, V) = ∞; • loop: 8 wait (until A sees a link cost change to neighbor V 9 or until A receives update from neighbor V) 10 if (D(A,V) changes by d) 11 for all destinations Y through V do 12 D(A,Y) = D(A,Y) + d 13 else if (update D(V, Y) received from V) /* shortest path from V to some Y has changed */ 14 D(A,Y) = D(A,V) + D(V, Y);15 if (there is a new minimum for destination Y)16 send D(A, Y) to all neighbors 17 forever

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Example: Distance Vector Algorithm

A C12

7

B D3

1

Dest. Cost NextHop

B 2 B

C 7 C

D ∞ -

Node A

Dest. Cost NextHop

A 2 A

C 1 C

D 3 D

Node B

Dest. Cost NextHop

A 7 A

B 1 B

D 1 D

Node C

Dest. Cost NextHop

A ∞ -

B 3 B

C 1 C

Node D1 Initialization: 2 for all neighbors V do3 if V adjacent to A 4 D(A, V) = c(A,V); 5 else 6 D(A, V) = ∞; …

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Dest. Cost NextHop

B 2 B

C 7 C

D 8 C

Node A

Example: 1st Iteration (C A)

A C12

7

B D3

1Dest. Cost NextHop

A 2 A

C 1 C

D 3 D

Node B

Dest. Cost NextHop

A 7 A

B 1 B

D 1 D

Node C

Dest. Cost NextHop

A ∞ -

B 3 B

C 1 C

Node D

D(A, D) = D(A, C) + D(C,D) = 7 + 1 = 8

(D(C,A), D(C,B), D(C,D))

7 loop: …13 else if (update D(V, Y) received from V) 14 D(A,Y) = D(A,V) + D(V, Y);15 if (there is a new min. for destination Y)16 send D(A, Y) to all neighbors 17 forever

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Dest. Cost NextHop

B 2 B

C 3 B

D 5 B

Node A

Example: 1st Iteration (BA, CA)

A C12

7

B D3

1

Dest. Cost NextHop

A 2 A

C 1 C

D 3 D

Node B

Dest. Cost NextHop

A 7 A

B 1 B

D 1 D

Node C

Dest. Cost NextHop

A ∞ -

B 3 B

C 1 C

Node D

D(A,D) = D(A,B) + D(B,D) = 2 + 3 = 5 D(A,C) = D(A,B) + D(B,C) = 2 + 1 = 3

7 loop: …13 else if (update D(V, Y) received from V) 14 D(A,Y) = min(D(A,V), D(A,V) + D(V, Y))15 if (there is a new min. for destination Y)16 send D(A, Y) to all neighbors 17 forever

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Example: End of 1st Iteration

A C12

7

B D3

1

Dest. Cost NextHop

B 2 B

C 3 B

D 5 B

Node A

Dest. Cost NextHop

A 2 A

C 1 C

D 2 C

Node B

Dest. Cost NextHop

A 3 B

B 1 B

D 1 D

Node C

Dest. Cost NextHop

A 5 B

B 2 C

C 1 C

Node D


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Example: End of 2nd Iteration

A C12

7

B D3

1

Dest. Cost NextHop

B 2 B

C 3 B

D 4 B

Node A

Dest. Cost NextHop

A 2 A

C 1 C

D 2 C

Node B

Dest. Cost NextHop

A 3 B

B 1 B

D 1 D

Node C

Dest. Cost NextHop

A 4 C

B 2 C

C 1 C

Node D


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Example: End of 3rd Iteration

A C12

7

B D3

1

Dest. Cost NextHop

B 2 B

C 3 B

D 4 B

Node A

Dest. Cost NextHop

A 2 A

C 1 C

D 2 C

Node B

Dest. Cost NextHop

A 3 B

B 1 B

D 1 D

Node C

Dest. Cost NextHop

A 4 C

B 2 C

C 1 C

Node D

Nothing changes algorithm terminates


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Distance Vector: Link Cost Changes

A C

14

50

B1

“goodnews travelsfast”

D C N

A 4 A

C 1 B

Node B

D C N

A 5 B

B 1 B

Node C

D C N

A 1 A

C 1 B

D C N

A 5 B

B 1 B

D C N

A 1 A

C 1 B

D C N

A 2 B

B 1 B

D C N

A 1 A

C 1 B

D C N

A 2 B

B 1 B

Link cost changes heretime

7 loop:8 wait (until A sees a link cost change to neighbor V 9 or until A receives update from neighbor V) 10 if (D(A,V) changes by d) 11 for all destinations Y through V do 12 D(A,Y) = D(A,Y) + d 13 else if (update D(V, Y) received from V) 14 D(A,Y) = D(A,V) + D(V, Y);15 if (there is a new minimum for destination Y)16 send D(A, Y) to all neighbors 17 forever

Algorithm terminates

30

Distance Vector: Count to Infinity Problem

A C

14

50

B60

“badnews travelsslowly”

D C N

A 4 A

C 1 B

Node B

D C N

A 5 B

B 1 B

Node C

D C N

A 6 C

C 1 B

D C N

A 5 B

B 1 B

D C N

A 6 C

C 1 B

D C N

A 7 B

B 1 B

D C N

A 8 C

C 1 B

D C N

A 2 B

B 1 B

Link cost changes here; recall that B also maintains shortest distance to A through C, which is 6. Thus D(B, A) becomes 6 !

time

7 loop:8 wait (until A sees a link cost change to neighbor V 9 or until A receives update from neighbor V) 10 if (D(A,V) changes by d) 11 for all destinations Y through V do 12 D(A,Y) = D(A,Y) + d ;13 else if (update D(V, Y) received from V) 14 D(A,Y) = D(A,V) + D(V, Y);15 if (there is a new minimum for destination Y)16 send D(A, Y) to all neighbors 17 forever

…

31

Distance Vector: Poisoned Reverse

A C

14

50

B60 If C routes through B to get to A:

- C tells B its (C’s) distance to A is infinite (so B won’t route to A via C)

D C N

A 4 A

C 1 B

Node B

D C N

A 5 B

B 1 B

Node C

D C N

A 60 A

C 1 B

D C N

A 5 B

B 1 B

D C N

A 50 A

B 1 B

Link cost changes here; B updates D(B, A) = 60 as

C has advertised D(C, A) = ∞

time

D C N

A 60 A

C 1 B

D C N

A 50 A

B 1 B

D C N

A 51 C

C 1 B

D C N

A 50 A

B 1 B

D C N

A 51 C

C 1 B

Algorithm terminates

32

Intra-domain Routing Protocols

• Based on unreliable datagram delivery• Distance vector

– Routing Information Protocol (RIP), based on Bellman-Ford algorithm

– Each neighbor periodically exchange reachability information to its neighbors

– Minimal communication overhead, but it takes long to converge, i.e., in proportion to the maximum path length

• Link state– Open Shortest Path First (OSPF), based on Dijkstra’s

algorithm– Each router periodically floods immediate reachability

information to other routers– Fast convergence, but high communication and computation

overhead

33


Part 2: Inter-domain routing

34

Internet Structure

AS-1

AS-2

AS-3

Interior router

BGP router

35

Intra-Domain

AS-1

AS-2

AS-3

Interior router

BGP router

Intra-domain routing protocol aka Interior Gateway Protocol (IGP), e.g. OSPF, RIP

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Inter-Domain

AS-2

Interior router

BGP router

Inter-domain routing protocol aka Exterior Gateway Protocol (EGP), e.g. BGP

AS-1

AS-3

37

BGP: Path Vector Protocol

• Distance vector algorithm with extra information– For each route, store the complete path (ASs)– No extra computation, just extra storage

• Advantages:– can make policy choices based on set of ASs in path– can easily avoid loops

38

BGP Operations (Simplified)

Establish session on TCP port 179

Exchange all active routes

Exchange incremental updates

AS1

AS2

While connection is ALIVE exchangeroute UPDATE messages

BGP session

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Four Types of BGP Messages

• Open : Establish a peering session.

• Keep Alive : Handshake at regular intervals.

• Notification : Shuts down a peering session.

• Update : Announcing new routes or withdrawing previously

announced routes.

Announcement=

prefix + attributes values

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Attributes are Used to Select Best Routes

192.0.2.0/24pick me!

192.0.2.0/24pick me!

192.0.2.0/24pick me!

192.0.2.0/24pick me!

Given multipleroutes to the sameprefix, a BGP speakermust pick at mostone best route

(Note: it could reject them all!)

41

Example: Multiple AS Paths

AS701

AS73

AS7018

AS1239

AS9 128.2/16

128.2/169 701

128.2/169 7018 1239

42


Part 3: Multicast Routing

43

Example Uses of Multicast

• Internet TV radio• Stock price update• Video conference• Spam?!

44

This approach does not scale…

BackboneISP

BroadcastCenter

45

Use routers in distribution tree

BackboneISP

BroadcastCenter

Copy data at routersAt most one copy of a data packet per link

LANs implement link layer multicast by broadcasting

Routers compute trees and forward packets along them

46

Multicast Service Model

• Receivers join a multicast group which is identified by a multicast address (e.g. G)

• Sender(s) send data to address G• Network routes data to each of the receivers

R0 joins G

R1 joins G

Rn-1 joins G

S

R0

R1

.

.

.

[G, data]

[G, data]

[G, data]

[G, data]

Rn-1

Net

47

Supporting Multicast on the Internet

IP

Application

Internet architecture

Network

?

?

At which layer should multicast be implemented?

48

IP Multicast

CMU

BerkeleyMIT

UCSD

routersend systemsmulticast flow

• Highly efficient• Good delay

49

Failure of IP Multicast

• Not widely deployed even after 15 years!– Use carefully – e.g., on LAN or campus, rarely over WAN

• Various failings– Scalability of routing protocols– Hard to manage– Hard to implement TCP equivalent– Hard to get applications to use IP Multicast without existing

wide deployment– Hard to get router vendors to support functionality and hard

to get ISPs to configure routers to enable

50

End System Multicast

MIT1

MIT2

CMU1

CMU2

UCSD

MIT1

MIT2

CMU2

Overlay Tree

Berkeley

CMU1

CMU

BerkeleyMIT

UCSD

51

• Quick deployment• All multicast state in end systems• Computation at forwarding points simplifies support

for higher level functionality

Potential Benefits Over IP Multicast

MIT1

MIT2

CMU1

CMU2

CMU

BerkeleyMIT

UCSD

52

Concerns with End System Multicast

• Self-organize recipients into multicast delivery overlay tree– Must be closely matched to real network topology to be efficient

• Performance concerns compared to IP Multicast– Increase in delay– Bandwidth waste (packet duplication)– Penalty can be kept small in practice

MIT2

Berkeley MIT1

UCSD

CMU2

CMU1

IP Multicast

MIT2

Berkeley MIT1

CMU1

CMU2

UCSD

End System Multicast

53

Multicast: Take-away

• Multicast provides support for efficient data delivery to multiple recipients

• Difficult to deploy IP multicast• End system-based techniques can provide similar

efficiency and it is easier to deploy

55

Routing on a Graph

• Goal: determine a “good” path through the network from source to destination

– Good often means the shortest path

• Network modeled as a graph– Routers nodes– Link edges

• Edge cost: delay, congestion level,…

A

ED

CB

F

2

2

13

1

1

2

53

5

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Link State vs. Distance Vector

Per node message complexity• LS: O(n*d) messages; n –

number of nodes; d – degree of node

• DV: O(d) messages; where d is node’s degree

Complexity• LS: O(n2) with O(n*d)

messages• 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:– node can advertise incorrect

path cost

– each node’s table used by others; error propagate through network

57

Routing between ISPs

• Routing protocol (BGP) contains reachability information (no metrics)– Not about optimizing anything– All about policy (business and politics)

• Why?– Metrics optimize for a particular criteria– AT&T’s idea of a good route is not the same as UUnet’s– Scale

• What a BGP speaker announces or not announces to a peer determines what routes may get used by whom

58

Nontransit vs. Transit ASes

ISP 1ISP 2

Nontransit ASmight be a corporateor campus network.

NET ATraffic NEVER flows from ISP 1through NET A to ISP 2(At least not intentionally!)

IP traffic

Internet Serviceproviders (often)have transit networks

59

Selective Transit

NET BNET C

NET A provides transitbetween NET B and NET Cand between NET D and NET C NET A

NET D

NET A DOES NOTprovide transitBetween NET D and NET B

Most transit networks transit in a selective manner…

IP traffic

60

Customers and Providers

Customer pays provider for access to the Internet

provider

customerIP traffic

provider customer

61

Customers Don’t Always Need BGP

provider

customer

Default route 0.0.0.0/0pointing to provider.

Configured route 192.0.2.0/24pointing to customer

192.0.2.0/24

Static routing is the most common way of connecting anautonomous routing domain to the Internet. This helps explain why BGP is a mystery to many …

62

Customer-Provider Hierarchy

IP trafficprovider customer

63

The Peering Relationship

peer peer

customerprovider

Peers provide transit between their respective customers

Peers do not provide transit between peers

Peers (often) do not exchange $$$trafficallowed

traffic NOTallowed

64

Peering Provides Shortcuts

Peering also allows connectivity betweenthe customers of “Tier 1” providers.

peer peer

customerprovider

65

Implementing Customer/Provider and Peer/Peer relationships

• What you announce determines what route can be used by whom

• Enforce transit relationships – Outbound route filtering

• Enforce order of route preference– provider < peer < customer

66

Import Routes

Frompeer

Frompeer

Fromprovider

Fromprovider

From customer

From customer

provider route customer routepeer route ISP route

67

Export Routes

Topeer

Topeer

Tocustomer

Tocustomer

Toprovider

From provider

provider route customer routepeer route ISP route

filtersblock

68

How Can Routes be Colored?BGP Communities!

A community value is 32 bits

By convention, first 16 bits is ASN indicating who is giving itan interpretation

communitynumber

Very powerful BECAUSE it has no predefinedmeaning

Community Attribute = a list of community values.(So one route can belong to multiple communities)

Used for signalingwithin and betweenASes

69

IPv4 Multicast

• Class D addresses– These are group identifiers– Not specific to an end host– Flat address space– In practice, pick a group address at random, hope no

collision– No security in the network layer

• Will use “G” to designate an IP multicast group address

1110 Multicast Group Address

28

First octet: 224 - 239

70

Multicast Implementation Issues

• How is join implemented?• How is send implemented?• How much information about trees is kept and who

keeps it?

71

IP Multicast Routing

• Intra-domain– Distance-vector multicast– Link-state multicast

• Inter-domain– Protocol Independent Multicast, Sparse Mode

• Key idea: Core-Based Tree

72

Distance Vector Multicast Routing Protocol (DVRMP)

• An elegant extension to DV routing• Use shortest path DV routes to determine if link is on

the source-rooted spanning tree• Three steps in developing DVRMP

– Reverse Path Flooding– Reverse Path Broadcasting– Truncated Reverse Path Broadcasting

73

Reverse Path Flooding (RPF)

• Extension to DV unicast routing• Packet forwarding

– If incoming link is shortest path to source

– Send on all links except incoming– Packets always take shortest path

• assuming delay is symmetric• Issues

– Some links (LANs) may receive multiple copies

– Every link receives each multicast packet, even if no interested hosts

s:2s:2

ss

s:1s:1

s:3s:3

s:2s:2

s:3s:3

rr

74

Example

• Flooding can cause a given packet to be sent multiple times over the same link

• Solution: Called “Reverse Path Broadcasting”

xx yy

zz

SS

a

b

duplicate packet

75

Reverse Path Broadcasting (RPB)

• Chose parent of each link along reverse shortest path to source

• Only parent forward to a link (child link)

• Use DV routing update to identify parent

xx yy

zz

SS

a

b

5 6

child link of xfor S

forward onlyto child link

76

Don’t Really Want to Flood!

• This is still a broadcast algorithm – the traffic goes everywhere

• Need to “Prune” the tree when there are subtrees with no group members

• Solution: Truncated Reverse Path Broadcasting

77

Truncated Reverse Path Broadcasting (TRPB)

• Extend RPB to eliminate unneeded forwarding

• Explicit group joining– Members periodically send “join”

requests– If another LAN member has joined

(overheard join message), other members do not send join message

• Router with no member downstream is removed from tree

– Router sends “prune” message to upstream router when no member

r1r1

r2r2

SS

78

Distance Vector Multicast Scaling

• State requirements: – O(Sources Groups) active state

79

Core Based Trees (CBT)

• The key idea in Inter-domain PIM-SM protocol• Pick a “rendezvous point” for the group called the core

– Build a tree towards the core

– Union of the unicast paths from members to the core

– Shared tree

• To send, unicast packet to core and bounce it back to multicast group

• Reduce routing table state from O(S x G) to O(G)

80

Example

• Group members: M1, M2, M3• M1 sends data

core

M1

M2 M3

control (join) messagesdata

81

Disadvantages

• Sub-optimal delay• Single point of failure

– Core goes out and everything lost until error recovery elects a new core

• Small, local groups with non-local core– Need good core selection– Optimal choice (computing topological center) is NP hard

82

Why Need the Concept of AS or Domain?

• Routing algorithms are not efficient enough to deal with the size of the entire Internet

• Different organizations may want different internal routing policies

• Allow organizations to hide their internal network configurations from outside

• Allow organizations to choose how to route across multiple organizations (BGP)

• Basically, easier to compute routes, more flexibility, more autonomy/independence

83

RIP

From the book: Routing TCP/IP Volume I (CCIE Professional Development)

84

Split Horizon

85

Split Horizon with Poisoned Reverse

86

Counting to Infinity

87

Purpose of E-BGP

R border router

internal router

E-BGPR2

R1

R3

A

AS1

AS2

you can reachnet A via me

table at R1:dest next hopA R2

Share connectivity information across ASes

88

Part II: I-BGP, Carrying Info within an AS

R border router

internal router

R1

AS1

R4

R5

B

AS3

E-BGP

R2R3

A

AS2 announce B

I-BGP

E-BGP

89

I-BGP

I-BGP neighbors do not announce routes received via I-BGP to other I-BGPneighbors.

E-BGP update

I-BGP updates

• Problem: Injecting external routes into IGP (e.g. OSPF) does not scale and causes BGP policy information to be lost

• I-BGP can be used to disseminate BGP routes to all routers in AS

• BGP route + IGP route suffice to create forwarding table

90

Join I-BGP with IGP to Create Forwarding Table

AS 1 AS 2192.0.2.1

135.207.0.0/16

10.10.10.10

I-BGP: 135.207.0.0/16Next Hop = 192.0.2.1

192.0.2.0/30

Forwarding Table

135.207.0.0/16

destination next hop

10.10.10.10192.0.2.0/30 10.10.10.10

I-BGP

192.0.2.1135.207.0.0/16


+

IGP

10.10.10.10192.0.2.0/30


E-BGP

91

Multiple Routing Processes on a Single Router

Forwarding Table

OSPFDomain

BGP

OS kernel

OSPF Process

OSPF Routing tables

BGP Process

BGP Routing tables

Forwarding Table Manager

92

Internet Structure

Large ISP Large ISP

Dial-UpISP Stub

Small ISP

Stub Stub

Stub

93

Inter-Domain Routing

• Global connectivity is at stake• Inevitably leads to one single protocol that everyone

must speak– Unlike many choices in intra-domain routing

• What are the requirements?• Scalability• Flexibility in choosing routes• If you were to choose, link state based or distance

vector based?

• BGP is sort of a hybrid: Path vector protocol

94

Border Gateway Protocol Part I: E-BGP

R border router internal router

E-BGPR2

R1

R3AS1

AS2

Two types of routers

Border router (Edge), Internal router (Core)

95

Example AS Graph

The subgraph showing all ASes that have more than 100 neighbors in fullgraph of 11,158 nodes. July 6, 2001. Point of view: AT&T route-server

Does not reflect true topology

96

BGP Issues

• BGP designed for policy not performance

• Susceptible to router misconfiguration– Blackholes: announce a route you cannot reach

• Incompatible policies – Solutions to limit the set of allowable policies

97

More Issues

• Scaling the I-BGP mesh– Confederations– Route Reflectors

• BGP Table Growth– 140K prefixes and growing – Address aggregation (CIDR)– Address allocation

• AS number allocation and use• Dynamics of BGP

– Inherent vs. accidental oscillation – Rate limiting and route flap dampening– Lots and lots of redundant info– Slow convergence time

98

Autonomous Systems (AS)

• Internet is not a single network!

• The Internet is a collection of networks, each controlled by different administrations

• An autonomous system (AS) is a network under a single administrative control

99

AS Numbers (ASNs)

ASNs are 16 bit values.64512 through 65535 are “private”

• Genuity: 1 • AT&T: 7018, 6341, 5074, … • UUNET: 701, 702, 284, 12199, …• Sprint: 1239, 1240, 6211, 6242, …• …

Currently over 11,000 in use.

100

Implications

• ASs want to choose own local routing algorithm– AS takes care of getting packets to/from their own hosts– Intradomain routing: RIP, OSPF, etc

• ASs want to choose own non-local routing policy– Interdomain routing must accommodate this– BGP is the current interdomain routing protocol– BGP: Border Gateway Protocol

101

Shorter Doesn’t Always Mean Shorter

AS 4

AS 3

AS 2

AS 1

Path 4 1 is “better” than path 3 2 1

102

Multicast Routing Approaches

• Kinds of Trees– Source Specific Trees

• Most suitable for single sender

• E.g. internet radio

– Shared Tree• Multiple senders in a group

• E.g. Teleconference

• Group

103

Source Specific Trees

6

7

8

5

4

31

2

12

10

13

11

• Each source is the root of its own tree• One tree per source• Tree can consists of shortest paths to each receiver

Members of the multicast tree Sender

104

Source Specific Trees

6

7

8

5

4

31

2

12

10

13

11

Very good performance butexpensive to construct/maintain;routers need to manage a tree per source

• Each source is the root of its own tree• One tree per source• Tree can consists of shortest paths to each receiver

105

Shared Tree

6

7

8

5

4

31

2

12

10

13

11

One tree used by all members in a group

Easier to construct/maintain but hard to pick “good” trees for everyone!

106

Terminologies• What is “unicast”?

– Cast: to send, to throw• Broadcast: to send everywhere (recall broadcast

in local area Ethernet, ARP, DHCP)

– Unicast: to send to a single receiver• Point-to-point communication

– Nearly all wide-area Internet traffic is unicast• Web traffic, SSH traffic, FTP traffic• Two unicast streams, one in each direction

• What is “multicast”?– In between unicast and broadcast– Each packet is sent to multiple specific

receivers• Point-to-Multipoint communication

– What is multicast useful for???…… ……

107

How to Send to Multiple Receivers?

• What are the simplest ways?• Ex1: Send a copy of the packet to one receiver at a

time until all receivers have it– i.e. use unicast to implement multicast

• Ex2: Flood a packet throughout the network and have non-receivers discard the packet– i.e. use broadcast to implement multicast

• Advantages? Disadvantages?

• In general: We want a distribution tree– Many ways to do it– Big research topic for a decade

108

Example: Internet Radio

• www.digitallyimported.com– Sends out 128Kb/s MP3 music streams– Peak usage ~9000 simultaneous streams– Consumes ~1.1Gb/s

• bandwidth costs are large fraction of their expenditures

– A fat and shallow tree– Does not scale!

routing protocol review

Documents

s update dv

s example

node source

c cost

new cost

b cost

cost paths

old cost