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TDTS41 Computer Networks

Lecture 4: Network layer I

Claudiu Duma, cladu@ida.liu.seIISLAB/IDA

Linköpings universitet

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Network Layer

Goals: understand principles behind routing routing in the Internet

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Outline

Introduction Routing algorithms

Link state shortest path first Distance vector Hierarchical routing

Routing in the Internet RIP OSPF BGP

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Network layer Transport data from sending to

receiving host

IP datagram/packet

Network layer protocols in every host, router

H1

H2

application

transportnetworkdata linkphysical

application

transportnetworkdata linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysical

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Key Network-Layer Functions

1st: routing: determine route taken by packets from source to dest.

2nd: forwarding: move packets from router’s input to appropriate router output

analogy:

process of planning trip from source to dest

process of getting through single interchange

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1

23

0111

value in arrivingpacket’s header

routing algorithm

local forwarding tableheader value output link

0100010101111001

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Interplay between routing and forwarding

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Routing

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Routing Principles

Minimize the cost of the routing path Scalability

Local changes should not affect globally Administrative issues

Different networks belong to different organizations

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Routing Algorithms

Simple Flooding

Those that minimize costs Link-state shortest-path-first Distance vector

Those that scale and meet administrative needs Intra-/Inter-autonomous systems routing

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Routing algorithms that minimize cost

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Graph abstraction

u

yx

wv

z2

2

13

1

1

2

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5 • Set of nodes {u,v, w, ..}• Set of edges {(u,v), …}• Cost of links, e.g. c(u,v) = 2

• inverse to bandwidth• proportional to congestion• …

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

Question: What’s the shortest-path between u and w?

Note: “shortest-path” vs. “least-cost-path”

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Link-state shortest-path-first

Routers find, by broadcasts, about all links in the net (costs).

Each router computes locally the shortest-paths from itself to all other routers. Dijkstra algorithm

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Dijkstra’s Algorithm

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'

N': set of nodes whose least cost path is known D(v): current value of cost of path from source to dest. v

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Distance Vector Routing

Bellman-Ford Equation (dynamic programming)

Definedx(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

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Bellman-Ford Example

u

yx

wv

z2

2

13

1

1

2

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5dv(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 achieves minimum is nexthop in shortest path ➜ forwarding table

B-F equation says:

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Distance Vector Algorithm (1)

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

Each node x knows: Its distance vector (DV): Dx = [Dx(y): y є N ] Cost to each neighbor v: c(x,v) Its neighbors’ distance vectors: Dv

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Distance Vector Algorithm (2)

Basic idea: Each node periodically sends its own distance

vector estimate to neighbors When node x receives new DV estimate from

neighbor, it updates its own DV using B-F equation:

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

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

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x y z

xyz

0 2 7

∞ ∞ ∞∞ ∞ ∞

from

cost to

x y z

xyz

∞ ∞

∞ ∞ ∞

x y z

xyz

∞ ∞ ∞7 1 0

∞2 0 1

∞ ∞ ∞

x y z

xyz

0 2 3

x y z

xyz

0 2 3

x y z

xyz

0 2 3

2 0 13 1 0

2 0 1

3 1 0

2 0 1

3 1 0

x z12

7

y

node x

node y

node z

x y z

xyz

0 2 7

2 0 17 1 0

x y z

xyz

0 2 7

x y z

xyz

0 2 7

2 0 17 1 0

2 0 17 1 0

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

3

3

Next hop: x y z

x y z

x y z

x y z

x y z

x y z

x y y

x y z

y y z

y

y

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Count to infinity problem

Solution: Poisoned reverse If Z routes through Y to get to X, Z tells Y that Dz(x) =

∞.

A variant of split horizon Do not advertise a route back to the interface from

which you have learned about it!

x z14

50

y60y z

Dz(x) = 5Dy(x) = 6

Dy(x) = 6Dz(x) = 7

Dz(x) = 7Dy(x) = 8

.

.

.Dz(x) = 49Dy(x) = 50

Dy(x) = 8Dz(x) = 7

.

.

. ...

Dy(x) = 50Dz(x) = 50But not through y!

After the cost change occurs …

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Comparison of LS-SPF and DV algorithms

Message complexity LS-SPF: with n nodes, E

links, O(nE) msgs sent DV: exchange between

neighbors only convergence time varies

Speed of Convergence LS-SPF: O(n2) algorithm

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-SPF: 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 thru

network

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Hierarchical routing

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Hierarchical Routing

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

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3b

1d

3a

1c2aAS3

AS1

AS21a

2c2b

1b

3c

Autonomous systems (AS)

Internal and gateway routers

Two routing protocols: intra-AS routing inter-AS routing

Routers in same AS run same intra-AS routing protocol

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

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3b

1d

3a

1c2aAS3

AS1

AS21a

2c2b

1b

3c

Inter-AS tasks Suppose a router in

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

packet towards one 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!

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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. Router 1d puts in forwarding table entry (x,I).

3b

1d

3a

1c2aAS3

AS1

AS21a

2c2b

1b

3c

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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 of inter-AS routing protocol! Hot potato routing: send packet towards closest of

two routers.

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Intra-AS Routing

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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)

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RIP ( Routing Information Protocol)

Distance vector algorithm Included in BSD-UNIX Distribution in 1982 Distance metric: # of hops (max = 15

hops) RIP advertisements, containing DVs

Exchanged every 30 sec among neighbors Each advertisement list of up to 25 destination

nets within AS If no advertisement heard after 180 sec -->

neighbor/link declared dead

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RIP Table Processing

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

advertisements sent in UDP packets, periodically repeated (port 520)

physical

link

network forwarding (IP) table

Transprt (UDP)

routed

physical

link

network (IP)

Transprt (UDP)

routed

forwardingtable

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OSPF

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OSPF (Open Shortest Path First)

“open”: publicly available Uses LS-SPF algorithm

LS packet dissemination Topology map at each node Route computation using Dijkstra’s algorithm

Link weights can be configured by net admin 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)

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OSPF “advanced” features (not in RIP)

Security: all OSPF messages authenticated (to prevent malicious intrusion)

Multiple same-cost paths allowed (only one path 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)

Hierarchical OSPF in large domains.

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Hierarchical OSPF

Hierarchically structured OSPF autonomous system

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Inter-AS Routing

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Internet inter-AS routing: BGP

BGP (Border Gateway Protocol): the de facto standard

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

neighboring ASs.2. Propagate the reachability information to all

routers internal to the AS.3. Determine “good” routes to subnets based

on reachability information and policy. Allows a subnet to advertise its

existence to rest of the Internet: “I am here”

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

semi-permanent (“long-lived”) TCP connections: BGP sessions

When AS2 advertises a prefix to AS1, AS2 is promising it will forward any datagrams destined to that prefix towards the prefix. AS2 can aggregate prefixes in its advertisement

3b

1d

3a

1c2aAS3

AS1

AS21a

2c

2b

1b

3c

eBGP external session

iBGP internal session

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Distributing reachability info Example:

3a sends reach. info to 1c 1c distributes this reach. info to all routers

in AS1 1b can then re-advertise the new reach info

to AS2 over the 1b-to-2a eBGP session

3b

1d

3a

1c2aAS3

AS1

AS21a

2c

2b

1b

3c

eBGP session

iBGP session

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Path attributes & BGP routes

When advertising a prefix, advert includes BGP attributes. prefix + attributes = “route”

Two important attributes: AS-PATH: contains the ASes through which

the advert for the prefix passed: • E.g. AS 67 AS 17

NEXT-HOP: Indicates the specific internal-AS router to next-hop AS.

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ASN examples

ASN Network3356 Level33549 Global Crossing2529 Demon UK

4589 Easynet 5459 LINX

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Some AS-Paths known by PeakWebHosting's BGP routers AS path: 6453[Teleglobe], 3356[Level3], 2529[Demon UK], 5459[LINX]

AS path: 20248[NetVMG], 3356[Level3], 2529[Demon UK], 5459[LINX]

AS path: 3356[Level3], 2529[Demon UK], 5459[LINX]

AS path: 174[PSI/Cogent], 2914[Verio], 5413[GX Networks], 5459[LINX]

AS path: 2914[Verio], 5413[GX Networks], 5459[LINX]

AS path: 19151[WebUseNet], 3257[Tiscali Backbone], 5459[LINX]

AS path: 6327[Shaw Cable], 4589[Easynet], 5459[LINX]

AS path: 3549[Global Crossing], 5459[LINX]

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BGP route selection

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

Elimination rules:1. Local preference value attribute: policy

decision2. Shortest AS-PATH 3. Closest NEXT-HOP router: hot potato

routing4. Additional criteria

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BGP routing policy

Figure 4.5-BGPnew: a simple BGP scenario

A

B

C

W X

Y

legend:

customer network:

provider network

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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BGP routing policy (2)

Figure 4.5-BGPnew: a simple BGP scenario

A

B

C

W X

Y

legend:

customer network:

provider network

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 neither W nor C are B’s customers

B wants to force C to route to w via A B wants to route only to/from its customers!

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

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SummaryWhat we’ve covered: Network layer’s functions Routing principles Hierarchical routing Internet routing protocols: RIP,

OSPF, BGP