Ch.2: Link Layer &LAN Chapter 5 Network Layer: Routing 1

Ch.2: Link Layer &LAN

Chapter 5Network Layer:

Routing

1

Chapter 5: Network Layer: Routing

#2

Network Layer: Protocols

forwarding

Network layer functions:

Routing protocols•path selection•e.g., RIP, OSPF, BGP

Network layer protocol (e.g., IP)•addressing conventions•packet format•packet handling conventions

Control protocols•error reporting e.g. ICMP

Transport layer

Link layer

physical layer

Networklayer

Control protocols- router “signaling” e.g. RSVP


#3

Control: ROUTING algorithms


#4

Control Plane: Routing

Graph abstraction for the routing problem:

graph nodes are routers graph edges are

physical links links have properties:

delay, capacity, $ cost, policy

Goal: determine “good” paths(sequences of routers) thru

network from sources to dest.

Routing

A

ED

CB

F2

21

3

1

12

53

5


#5

Key Desired Properties of a Routing Algorithm

Robustness

Optimalityfind good path

(for user/provider)

Simplicity


#6

Routing Design Space Routing has a large design space

who decides routing?• source routing: end hosts make decision• network routing: networks make decision

– centralized control : one node builds tables for all routers– distributed control : each router builds own routing table

how many paths from source s to destination d?• multi-path routing (several alternatives used)• single path routing

will routing adapt to network traffic demand or to topology changes?

• dynamic routing : tables adapt to state changes• static routing : tables configured manually

…

- Robustness- Optimality- Simplicity


#7

Routing Algorithm classification(distributed control network)

Global or decentralized information?

Global information: all routers have complete

topology, link-cost info “link state” algorithms

Decentralized information: router knows physically-

connected neighbors, link costs to neighbors

iterative process of computation, exchange of info with neighbors

“distance vector” algorithms

Centralized of distributed control?

Centralized control: one central node in the

network builds tables for all routers and sends to them

collects data from routers

Distributed control: every router builds its

own table based on information

exchange with other routers


#8

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 routing table for

that node iterative: after k iterations,

know least cost path to k destinations

Notation: 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 dest. V

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

N: set of nodes whose least cost path definitively known


#9

Dijsktra’s Algorithm

1 Initialization: 2 N = {A} 3 for all nodes v 4 if v adjacent to A 5 then D(v) = c(A,v) , p(v)=A6 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) ) , adjust p(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


#10

Dijkstra’s algorithm: example

Step012345

start NA

ADADE

ADEBADEBC

ADEBCF

D(B),p(B)2,A2,A2,A

D(C),p(C)5,A4,D3,E3,E

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

D(E),p(E)infinity

2,D

D(F),p(F)infinityinfinity

4,E4,E4,E

A

ED

CB

F

2

2

13

1

1

2

53

5


#11

Dijkstra’s algorithm, discussion

Algorithm 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(nlogn)


#12

Distance Vector Routing Algorithm

iterative: continues until no

nodes exchange info. self-terminating: no

“signal” to stop

asynchronous: nodes need not

exchange info/iterate in lock step!

distributed: each node

communicates only with directly-attached neighbors

Distance Table data structure each node has its own table row for each possible destination column for each directly-

attached neighbor to node example: in node X, for dest. Y

via neighbor Z:

D (Y,Z)X

distance from X toY, via Z as next hop

c(X,Z) + min {D (Y,w)}Z

w

=

=


#13

Distance Vector Routing

Basis of RIP, IGRP, EIGRP routing protocols

Based on the Bellman-Ford algorithm (BFA)

Conceptually, runs for each destination separately


#14

Distance Vector Routing: Basic Idea At node i, the basic update rule

where - di denotes the distance

estimation from i to the destination,

- N(i) is set of neighbors of node i, and

- dij is the distance of the direct link from i to j;assumed positive

)(min )( jijiNji ddd

i

jid

jdijd

destination

reported by j


#15

Distance Table: ExampleA

E D

CB7

8

10

2

1

2

d ()

A

B

C

D

E

dist. tables recv’d from neighbors

dest

inat

ions

computation at E

E’s computeddistance

tabledistance

table E sends to its neighbors

A: 10

B: 8

C: 4

D: 2

E: 0

Table below shows just one step done by EThe algorithm repeats forever!

10 15

A B D

0 7

A B D

10 8 2

7 0

1 2

0

A: 10

B: 8

C: 4

D: 2

17 8

9 4

2

E


#16

Distance Table: example

A

E D

CB7

8

1

2

1

2

D ()

A

B

C

D

A

1

7

6

4

B

14

8

9

11

D

5

5

4

2

Ecost to destination via

dest

inat

ion

D (C,D)E

c(E,D) + min {D (C,w)}D

w== 2+2 = 4

D (A,D)E

c(E,D) + min {D (A,w)}D

w== 2+3 = 5

D (A,B)E

c(E,B) + min {D (A,w)}B

w== 8+6 = 14

loop!

(why not 15?)


#17

Distance table gives routing table

D ()

A

B

C

D

A

1

7

6

4

B

14

8

9

11

D

5

5

4

2

Ecost to destination via

dest

inat

ion

A

B

C

D

A,1

D,5

D,4

D,2

Outgoing link to use, cost to dest

dest

inat

ion

Distance table Routing table


#18

Distance Vector Routing: overview

Iterative, asynchronous: each local iteration caused by:

local link cost change message from neighbor:

its least cost path change from neighbor

Distributed: 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 all neighbors

Each node:


#19

Distance Vector Algorithm:

1 Initialization: 2 for all adjacent nodes v: 3 DX(*,v) = ∞ /* “*” means "for all rows" */ 4 DX(v,v) = c(X,v) 5 for all destinations, y 6 send minw DX(y,w) to each neighbor /* w over all X's neighbors */

At all nodes, X:


#20

Distance Vector Algorithm (cont.):8 loop 9 wait (until a link cost C(X,V) change to neighbor V 10 or until receive update from neighbor V) 11 12 if (c(X,V) changes by d) 13 /* change cost to all dest's via neighbor v by adding d */ 14 /* note: d could be positive or negative */ 15 for all destinations y: DX(y,V) = DX(y,V) + d 16 17 else if (update received from V wrt destination Y) 18 /* shortest path from V to some Y has changed */ 19 /* V has sent a new value for its minw DV(Y,w) */ 20 /* call this received new value is "newval" */ 21 for the single destination Y, D (Y,V) = c(X,V) + newval 22 23 if minw DX(Y,w) for any destination Y changed,24 send new value of minw DX(Y,w) to all neighbors 25 26 forever

X


#21

Distance Vector Algorithm: example

X Z12

7

Y

D (Y,Z)X

c(X,Z) + min {D (Y,w)}w=

= 7+1 = 8

Z

D (Z,Y)X

c(X,Y) + min {D (Z,w)}w=

= 2+1 = 3

Y


#22

Distance Vector Algorithm: example

X Z12

7

Y


#23

Distance Vector: link cost changes

Link cost changes: node detects local link cost

change updates distance table (line 15) if cost change in least cost path,

notify neighbors (lines 23,24)

we show distance to X only

X Z14

50

Y1

algorithmterminates“good

news travelsfast”

report 1 report 2


#24

Distance Vector: link cost changes

Link cost changes: good news travels fast bad news travels slow -

“count to infinity” problem!

we show distance to X only

X Z14

50

Y60

algorithmcontinues

on!

Y Y Y

report 6report 7


#25

Distance Vector: poisoned reverseIf Z routes through Y to get to X

(= Z learned its best dist. to X from Y):

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?

X Z14

50

Y60

algorithmterminatesY Y Y Y


#26

Comparison of LS and DV algorithms

Message complexity LS: with n nodes, E links,

O(nE) msgs sent DV: exchange between

neighbors only but larger msgs

Speed of Convergence LS: requires O(nE) msgs

may have oscillations

DV: convergence time varies may have 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 propagates thru

network

Broadcast and Multicast Routing


#27

R1

R2

R3 R4

sourceduplication

R1

R2

R3 R4

in-networkduplication

duplicatecreation/transmissionduplicate

duplicate

Broadcast Routing Deliver packets from source to all other nodes Source duplication is inefficient:

Source duplication: how does source determine recipient addresses


#28

In-network duplication

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

Controlled flooding: node only brdcsts pkt if it hasn’t brdcst same packet before Node keeps track of pckt ids already brdcsted Or reverse path forwarding (RPF): only forward

pckt if it arrived on shortest path between node and source

Spanning tree No redundant packets received by any node


#29

A

B

G

DE

c

F

A

B

G

DE

c

F

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

Spanning Tree

First construct a spanning tree Nodes forward copies only along

spanning tree


#30

A

B

G

DE

c

F1

2

3

4

5

(a) Stepwise construction of spanning tree

A

B

G

DE

c

F

(b) Constructed spanning tree

Spanning Tree: Creation Center node Each node sends unicast join message to

center node Message forwarded until it arrives at a node already

belonging to spanning tree


#31

Multicast Routing: Problem Statement Goal: find a tree (or trees) connecting

routers having local mcast 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 treesChapter 5: Network Layer:

Routing#32

Approaches for building mcast treesApproaches: source-based tree: one tree per source

shortest path trees reverse path forwarding

group-shared tree: group uses one tree minimal spanning (Steiner) center-based trees

…we first look at the basic approaches


#33

Shortest Path Tree

mcast forwarding tree: tree of shortest path routes from source to all receivers Dijkstra’s algorithm

R1

R2

R3

R4

R5

R6 R7

21

6

3 4

5

i

router with attachedgroup member

router with no attachedgroup member

link used for forwarding,i indicates order linkadded by algorithm

LEGENDS: source


#34

Reverse Path Forwarding

if (mcast datagram received on incoming link on shortest path back to center)

then flood datagram onto all outgoing links else ignore datagram

rely on router’s knowledge of unicast shortest path from it to sender

each router has simple forwarding behavior:


#35

Reverse Path Forwarding: example

• result is a source-specific reverse SPT– may be a bad choice with asymmetric links

R1

R2

R3

R4

R5

R6 R7



datagram will be forwarded

LEGENDS: source

datagram will not be forwarded


#36

Reverse Path Forwarding: pruning forwarding tree contains subtrees with no mcast

group members no need to forward datagrams down subtree “prune” msgs sent upstream by router with

no downstream group members

R1

R2

R3

R4

R5

R6 R7



prune message

LEGENDS: source

links with multicastforwarding

P

P

P


#37

Shared-Tree: Steiner Tree

Steiner Tree: minimum cost tree connecting all routers with attached group members

problem is NP-complete excellent heuristics exists not used in practice:

computational complexity information about entire network needed monolithic: rerun whenever a router needs

to join/leave


#38

Center-based trees

single delivery tree shared by all one router identified as “center” of tree to join:

edge router sends unicast join-msg addressed to center router

join-msg “processed” by intermediate routers and forwarded towards center

join-msg either hits existing tree branch for this center, or arrives at center

path taken by join-msg becomes new branch of tree for this router


#39

Center-based trees: an example

Suppose R6 chosen as center:

R1

R2

R3

R4

R5

R6 R7



path order in which join messages generated

LEGEND

21

3

1


#40

Hierarchical Routing


#41


scale: with 200 million destinations:

can’t store all dest’s in routing tables!

routing table exchange would swamp links!

administrative autonomy

internet = network of networks

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

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


#42


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


#43

3b

1d

3a

1c2aAS3

AS1

AS21a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

Forwarding table is configured by both intra- and inter-AS routing algorithm Intra-AS sets entries

for internal dests Inter-AS & Intra-As

sets entries for external dests


#44

3b

1d

3a

1c2aAS3

AS1

AS21a

2c2b

1b

3c

Inter-AS tasks Suppose router in

AS1 receives datagram for which dest is outside of AS1 Router should forward

packet towards on of the gateway routers, but which one?

AS1 needs:1. to learn which dests

are reachable through AS2 and which through AS3

2. to propagate this reachability info to all routers in AS1

Job of inter-AS routing!


#45

Example: Setting forwarding table in router 1d

Suppose AS1 learns from the inter-AS protocol that subnet x is reachable from AS3 (gateway 1c) but not from AS2.

Inter-AS protocol propagates reachability info to all internal routers.

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

Puts in forwarding table entry (x,I).


#46

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routing:Choose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface 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 protocol that subnet x is reachable from AS3 and from AS2.

To configure forwarding table, router 1d must determine towards which gateway it should forward packets for dest x.

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

of two routers.


#47


#48

Intra-AS and Inter-AS routing

Gateways:•perform inter-AS routing amongst themselves•perform intra-AS routers with other routers in their AS

inter-AS, intra-AS routing in

gateway A.c

network layer

link layer

physical layer

a

b

b

aaC

A

Bd

A.a

A.c

C.bB.a

cb

c


#49

Intra-AS and Inter-AS routing

Host h2

a

b

b

aaC

A

Bd c

A.a

A.c

C.bB.a

cb

Hosth1

Intra-AS routingwithin AS A

Inter-AS routingbetween A and B

Intra-AS routingwithin AS B

We’ll examine specific inter-AS and intra-AS Internet routing protocols shortly


#50

Routing: Example

AS A(OSPF)

AS B(OSPF intra routing)

AS D

AS Ci

b

b->i: I can reach hosts

in D; my path: BCD

a1

a2

d

d->a2: I

can re

ach

hosts in

D; m

y path: D

a1->i: I can reach hosts

in D; my path: AD

E

F

Export to E: i->e: I can

reach hosts in D; path: IBCD

AS I

a2->a1: I can reach

hosts in D; path: D

choose BCD

using i2

b->i2: I can reach hosts

in D; my path: BCD

i2

i2->i: I can reach

hosts in D; path: BCD

No Exportto F


#51

Routing: Example

AS A(OSPF)

AS B(OSPF intra routing)

AS D

AS C

i

b

How to specify?

a1

a2

d

a1->i: I can

reachhosts in D; my

path: AD

E

F

AS I

d1

d2


#52

Routing in the Internet

The Global Internet consists of Autonomous Systems (AS) interconnected with each other: Stub AS: small corporation Multihomed AS: large corporation (no transit) Transit AS: provider

Two-level routing: Intra-AS: administrator is responsible for choice Inter-AS: unique standard


#53

Internet AS HierarchyInter-AS border (exterior gateway) routers

Intra-AS interior (gateway) routers


#54

Intra-AS Routing

Also known as Interior Gateway Protocols (IGP) Most common IGPs:

RIP: Routing Information Protocol

OSPF: Open Shortest Path First

IGRP: Interior Gateway Routing Protocol (Cisco propr.)


#55

RIP ( Routing Information Protocol)

Distance vector algorithm RIP messages sent using UDP Included in BSD-UNIX Distribution in 1982 Distance metric: # of hops (max = 15 hops)

why?

Distance vectors: exchanged every 30 sec via Response Message (also called advertisement)

Each advertisement: route to up to 25 destination nets


#56

RIP (Routing Information Protocol)

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 table in D


#57

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)


#58

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

OSPF advertisement carries one entry per neighbor router

Advertisements disseminated to entire AS (via flooding)


#59

OSPF “advanced” features (not in RIP)

Security: all OSPF messages authenticated (to prevent malicious intrusion); TCP connections used

Multiple same-cost paths allowed only one path in RIP

For each link, multiple cost metrics for different ToS (eg, satellite link cost set “low” for best effort; high for real time)

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

as OSPF

Hierarchical OSPF in large domains.


#60

Hierarchical OSPF


#61

Hierarchical OSPF

Two-level hierarchy: local area, backbone. Link-state advertisements only in area each nodes has detailed area topology; only know

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 ASs.


#62

IGRP (Interior Gateway Routing Protocol) CISCO proprietary; successor of RIP (mid 80s) Distance Vector, like RIP several cost metrics (delay, bandwidth,

reliability, load etc) uses TCP to exchange routing updates Loop-free routing via Distributed Updating Alg.

(DUAL) based on diffused computation


#63

Inter-AS routing


#64

Internet inter-AS routing: BGP

BGP (Border Gateway Protocol): the de facto standard

Path Vector protocol: similar to Distance Vector protocol each Border Gateway broadcast to

neighbors (peers) entire path (i.e., sequence of ASs) to destination

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

Path (X,Z) = X,Y1,Y2,Y3,…,Z


#65


Suppose: gateway X sends its path to peer gateway W

W may or may not select path offered by X cost, policy (don’t route via competitor’s AS),

loop prevention reasons.

If W selects path advertised by X, then:Path (W,Z) = W, Path (X,Z)

Note: X can control incoming traffic by controlling its route advertisements to peers: e.g., don’t want to route traffic to Z -> don’t

advertise any routes to Z


#66


BGP messages exchanged using TCP. BGP 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 received msg; also used to close connection


#67

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

Documents

Ch.2: Link Layer &LAN Chapter 5 Network Layer: Routing 1