Chapter 4 Network Layer (Part 2) - University of Utah · 2007. 3. 5. · Network Layer 4-2 Chapter...

Network Layer 4-1

Chapter 4Network Layer(Part 2)

Computer Networking: A Top Down Approach Featuring the Internet, 3rd edition. Jim Kurose, Keith RossAddison-Wesley, July 2004.

Network Layer 4-2

Chapter 4: Network Layer

4. 1 Introduction4.2 Virtual circuit and datagram networks4.3 What’s inside a router4.4 IP: Internet Protocol

Datagram formatIPv4 addressingICMPIPv6

4.5 Routing algorithmsLink stateDistance VectorHierarchical routing

4.6 Routing in the Internet

RIPOSPFBGP

4.7 Broadcast and multicast routing

Network Layer 4-3

1

23

0111

value in arrivingpacket’s header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing, forwarding

Network Layer 4-4

u

yx

wv

z2

21

3

1

1

2

53

5

Graph: G = (N,E)

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

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

Graph abstraction

Remark: Graph abstraction is useful in other network contexts

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

Network Layer 4-5

Graph abstraction: costs

u

yx

wv

z2

21

3

1

1

2

53

5 • c(x,x’) = cost of link (x,x’)

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

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

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

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

Routing algorithm: algorithm that finds least-cost path

Network Layer 4-6

Routing Algorithm classificationGlobal or decentralized

information?Global:

all routers have complete topology, link cost info“link state” algorithms

Decentralized:router knows physically-connected neighbors, link costs to neighborsiterative process of computation, exchange of info with neighbors“distance vector” algorithms

Static or dynamic?Static:

routes change slowly over time

Dynamic:routes change more quickly

periodic updatein response to link cost changes

Network Layer 4-7






RIPOSPFBGP


Network Layer 4-8

A Link-State Routing Algorithm

Dijkstra’s algorithmnet topology, link costs known to all nodes

accomplished via “link state broadcast”all nodes have same info

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

gives forwarding tablefor that node

iterative: after k iterations, know least cost path to k dest.’s

Notation:c(x,y): link cost from node x to y; = ∞ if not direct neighborsD(v): current value of cost of path from source to dest. vp(v): predecessor node along path from source to vN': set of nodes whose least cost path definitively known

Network Layer 4-9

Dijsktra’s Algorithm1 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 Loop9 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'

Network Layer 4-10

A B

C D

Fu

Network Layer 4-11

A B

C D

Fu

Step N’ D(A),p(A) D(B),p(B) D(C),p(C) D(D),p(D) D(F),p(F)

0 U 2,U 5,U 1,U ∞ ∞

Step 0: from U, reach A, B, C

Network Layer 4-12

A B

C D

Fu


0 U 2, U 5,U 1, U ∞ ∞1 U, C 2, U 4, C 1, U 2, C ∞

Step 1: A, B, C, D, F are not in N’, D(C) = 1 is smallest

•Add C to N’

•From C, reach A, B, D

•D(C) + c(C, A)=3 > D(A) =2

•D(C) + c(C, B)=4 < D(B) =5

•D(C) + c(C, D)=2 < D(D) = ∞

•Update node B, D !

X

Network Layer 4-13

A B

C D

Fu


0 U 2, U 5, U 1, U ∞ ∞1 U, C 2, U 4, C 1, U 2, C ∞2 U, C, D 2, U 3, D 1, U 2, C 4, D

Step 2: D(A)=D(D)=2 smallest

•Randomly pick D

•Add D to N’

•From D, reach B, F

•D(D) + c(D, B) =3 < D(B) =4

•D(D) + c(D, F) =4 < D(F) = ∞

•Update node B, F !

X

X

Network Layer 4-14

A B

C D

Fu

N’ D(A),p(A) D(B),p(B) D(C),p(C) D(D),p(D) D(F),p(F)

0 U 2,U 5,U 1,U ∞ ∞1 U, C 2, U 4, C 1, U 2, C ∞2 U, C, D 2, U 3, D 1, U 2, C 4, D3 U, C, D, A 2, U 3, D 1, U 2, C 4, D

Step 3: D(A)=2 smallest

•Add A to N’

•From A, reach B

•D(A) + c(A, B)=5 > D(B) =3

•No update

Network Layer 4-15

A B

C D

Fu


0 U 2,U 5,U 1,U ∞ ∞1 U, C 2, U 4, C 1, U 2, C ∞2 U, C, D 2, U 3, D 1, U 2, C 4, D3 U, C, D, A 2, U 3, D 1, U 2, C 4, D4 U, C, D, A, B 2, U 3, D 1, U 2, C 4, D

Step 4: D(B) = 3 smallest

•Add B to N’

•From B, reach F

•D(B) + c(B, F)=8 > D(F) =4

•No update

Network Layer 4-16

A B

C D

Fu


0 U 2,U 5,U 1,U ∞ ∞1 U, C 2, U 4, C 1, U 2, C ∞2 U, C, D 2, U 3, D 1, U 2, C 4, D3 U, C, D, A 2, U 3, D 1, U 2, C 4, D4 U, C, D, A, B 2, U 3, D 1, U 2, C 4, D5 U, C, D, A, B, F

Step 5: only F not in N’

•Add F to N’

•No update

Network Layer 4-17

Dijkstra’s algorithm: example (2)

u

yx

wv

z

Resulting shortest-path tree from u:

vxywz

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

destination link

Resulting forwarding table in u:

Network Layer 4-18

Dijkstra’s algorithm, discussionAlgorithm complexity: n nodes

each iteration: need to check all nodes, w, not in Nn(n+1)/2 comparisons: O(n2)more efficient implementations possible: O(nlogn)

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

AD

CB

1 1+e

e0

e1 1

0 0

AD

CB

2+e 0

001+e 1

AD

CB

0 2+e

1+e10 0

AD

CB

2+e 0

e01+e 1

initially … recomputerouting… recompute … recompute

Network Layer 4-19






RIPOSPFBGP


Network Layer 4-20

Distance Vector Algorithm

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

v

Network Layer 4-21

Bellman-Ford example

u

yx

wv

z2

21

3

1

1

2

53

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

du(z) = min { c(u,v) + dv(z),c(u,x) + dx(z),c(u,w) + dw(z) }

= min {2 + 5,1 + 3,5 + 3} = 4

B-F equation says:

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

Network Layer 4-22

Distance Vector Algorithm

Dx(y) = estimate of least cost from x to yNode x knows cost to each neighbor v: c(x,v)Node x maintains distance vector Dx = [Dx(y): y є N ]Node x also maintains its neighbors’distance vectors

For each neighbor v, x maintains Dv = [Dv(y): y є N ]

Network Layer 4-23

Distance vector algorithm (4)

Basic idea:Each node periodically sends its own distance vector estimate to neighborsWhen a 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) converge to the actual least cost dx(y)

Network Layer 4-24

Distance Vector Algorithm (5)

Iterative, asynchronous: each local iteration caused by: local link cost change DV update message from neighbor

Distributed:each node notifies neighbors only when its DV changes

neighbors then notify their neighbors if necessary

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

recompute estimates

if DV to any dest has changed, notify neighbors

Each node:

Network Layer 4-25

x y zxyz

0 2 7∞∞ ∞

∞∞ ∞

from

cost to

from

from

x y z0

from

cost to

xyz

x y zxyz

∞ ∞

∞∞ ∞

cost to

x y z

∞∞ ∞

7 1 0

cost to

∞2 0 1

∞ ∞

∞

2 0 17 1 0

xyz

time

x z12

7

y

node x table

node y table

node z table

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

Dx(z) = min{c(x,y) + Dy(z), c(x,z) + Dz(z)}

= min{2+1 , 7+0} = 3

32

Network Layer 4-26

x y zxyz

0 2 7∞∞ ∞

∞∞ ∞

from

cost to

from

from

x y zxyz

0 2 3

from

cost tox y z0 2 3

from

cost to

xyz

x y zxyz

∞ ∞

∞∞ ∞

cost tox y z0 2 7

from

cost to

xyz

x y zxyzf

rom 0 2 3

cost to

x y zxyz

0 2 3

from

cost tox y z

z

0 2 7

from

cost to

xy

x y z

∞∞ ∞

7 1 0

cost to

∞2 0 1

∞ ∞

∞

2 0 17 1 0

2 0 17 1 0

2 0 13 1 0

2 0 13 1 0

2 0 1

3 1 02 0 1

3 1 0

xyz

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

Network Layer 4-27

Distance Vector: link cost changes

Link cost changes:node detects local link cost change updates routing info, recalculates distance vectorif DV changes, notify neighbors

“goodnews travelsfast”

x z14

50

y1

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

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

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

Network Layer 4-28

Distance Vector: link cost changes

Link cost changes:good news travels fast bad news travels slow -“count to infinity” problem!44 iterations before algorithm stabilizes: see text

Poisoned reverse:If Z routes through Y to get to X :

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

will this completely solve count to infinity problem?

x z14

50

y60

Network Layer 4-29

Comparison of LS and DV algorithms

Message complexityLS: with n nodes, E links, O(nE) msgs sent DV: exchange between neighbors only

convergence time varies

Speed of ConvergenceLS: O(n2) algorithm requires O(nE) msgs

may have oscillationsDV: convergence time varies

may be routing loopscount-to-infinity problem

Robustness: what happens if router malfunctions?

LS:node can advertise incorrect link costeach node computes only its own table

DV:DV node can advertise incorrect path costeach node’s table used by others

• error propagate thru network

Network Layer 4-30






RIPOSPFBGP


Network Layer 4-31

Hierarchical Routing

scale: with 200 million destinations:can’t store all dest’s in routing tables!routing table exchange would swamp links!

administrative autonomyinternet = network of networkseach network admin may want to control routing in its own network

Our routing study thus far - idealization all routers identicalnetwork “flat”

… not true in practice

Network Layer 4-32

Hierarchical Routing

aggregate routers into regions, “autonomous systems” (AS)routers in same AS run same routing protocol

“intra-AS” routingprotocolrouters in different AS can run different intra-AS routing protocol

Gateway routerDirect link to router in another AS

Network Layer 4-33

3b

1d

3a

1c2aAS3

AS1AS2

1a

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 destsInter-AS & Intra-As sets entries for external dests

Network Layer 4-34

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

3c

Inter-AS tasksSuppose 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!

Network Layer 4-35

Example: Setting forwarding table in router 1d

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

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

3c

Network Layer 4-36

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!

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

3c

Network Layer 4-37

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

Use routing infofrom intra-AS

protocol to determinecosts of least-cost

paths to eachof the gateways

Hot potato routing:Choose the gateway

that 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.

Network Layer 4-38






RIPOSPFBGP


Network Layer 4-39

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)

Network Layer 4-40






RIPOSPFBGP


Network Layer 4-41

RIP ( Routing Information Protocol)

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

DC

BA

u vw

x

yz

destination hopsu 1v 2w 2x 3y 3z 2

From router A to subsets:

Network Layer 4-42

RIP advertisements

Distance vectors: exchanged among neighbors every 30 sec via Response Message (also called advertisement)Each advertisement: list of up to 25 destination nets within AS

Network Layer 4-43

RIP: Example

Destination Network Next Router Num. of hops to dest.w A 2y B 2z B 7x -- 1…. …. ....

w x yA

C

D

z

B

Routing table in D

Network Layer 4-44

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 - 1x - 1z C 4…. … ...

Advertisementfrom A to D

Network Layer 4-45

RIP: Link Failure and RecoveryIf no advertisement heard after 180 sec -->

neighbor/link declared deadroutes via neighbor invalidatednew advertisements sent to neighborsneighbors in turn send out new advertisements (if tables changed)link failure info quickly (?) propagates to entire netpoison reverse used to prevent ping-pong loops (infinite distance = 16 hops)

Network Layer 4-46






RIPOSPFBGP


Network Layer 4-47

OSPF (Open Shortest Path First)

“open”: publicly availableUses Link State algorithm

LS packet disseminationTopology map at each nodeRoute computation using Dijkstra’s algorithm

OSPF advertisement carries one entry per neighbor routerAdvertisements disseminated to entire AS (via flooding)

Carried in OSPF messages directly over IP (rather than TCP or UDP

Network Layer 4-48

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)Integrated uni- and multicast support:

Multicast OSPF (MOSPF) uses same topology data base as OSPF

Hierarchical OSPF in large domains.

Network Layer 4-49

Hierarchical OSPF

Network Layer 4-50

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 AS’s.

Network Layer 4-51






RIPOSPFBGP


Network Layer 4-52

Internet inter-AS routing: BGP

BGP (Border Gateway Protocol): the de facto standardBGP 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”

Network Layer 4-53

BGP basicsPairs 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 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 session

iBGP session

Network Layer 4-54

Distributing reachability infoWith 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 AS11b can then re-advertise new reachability info to AS2 over 1b-to-2a eBGP sessionWhen router learns of new prefix, creates entry for prefix in its forwarding table.

3b

1d

3a

1c2aAS3

AS1

AS21a

2c

2b

1b

3c

eBGP session

iBGP session

Network Layer 4-55

Path attributes & BGP routes

When advertising a prefix, advert includes BGP attributes.

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 to next-hop-AS.)

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

Network Layer 4-56

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 decision

2. Shortest AS-PATH 3. Closest NEXT-HOP router: hot potato routing4. Additional criteria

Network Layer 4-57

BGP messages

BGP messages exchanged using TCP.BGP messages:

OPEN: opens TCP connection to peer and authenticates senderUPDATE: advertises new path (or withdraws old)KEEPALIVE keeps connection alive in absence of UPDATES; also ACKs OPEN requestNOTIFICATION: reports errors in previous msg; also used to close connection

Network Layer 4-58

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 networksX,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

Network Layer 4-59

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 AB wants to route only to/from its customers!

Network Layer 4-60

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 performanceInter-AS: policy may dominate over performance

Network Layer 4-61






RIPOSPFBGP


Network Layer 4-62

R1

R2

R3 R4

sourceduplication

R1

R2

R3 R4

in-networkduplication

duplicatecreation/transmissionduplicate

duplicate

Broadcast RoutingDeliver packets from source to all other nodesSource duplication is inefficient:

Source duplication: how does source determine recipient addresses?

Network Layer 4-63

In-network duplication

Flooding: when node receives brdcst pckt, sends copy to all neighbors

Problems: cycles & broadcast stormControlled flooding: node only brdcsts pktif it hasn’t brdcst same packet before

Node keeps track of pckt ids already brdcstedOr reverse path forwarding (RPF): only forward pckt if it arrived on shortest path between node and source

Spanning treeNo redundant packets received by any node

Network Layer 4-64

Reverse Path Forwarding (RPF) in broadcasting

1. A B, A C

2. B C, B D: C ignore source-A package from B, since B is not on C’s shortest path to A

3. C B, C F, C E: Bignore packet from C

4. ……

Network Layer 4-65

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 treeNodes forward copies only along spanning tree

Network Layer 4-66

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: CreationCenter nodeEach node sends unicast join message to center node

Message forwarded until it arrives at a node already belonging to spanning tree

Network Layer 4-67

Chapter 4: summary





RIPOSPFBGP


Chapter 4: Network LayerInterplay between routing, forwardingGraph abstractionGraph abstraction: costsRouting Algorithm classificationChapter 4: Network LayerA Link-State Routing AlgorithmDijsktra’s AlgorithmDijkstra’s algorithm: example (2) Dijkstra’s algorithm, discussionChapter 4: Network LayerDistance Vector Algorithm Bellman-Ford example Distance Vector Algorithm Distance vector algorithm (4)Distance Vector Algorithm (5)Distance Vector: link cost changesDistance Vector: link cost changesComparison of LS and DV algorithmsChapter 4: Network LayerHierarchical RoutingHierarchical RoutingInterconnected ASesInter-AS tasksExample: Setting forwarding table in router 1dExample: Choosing among multiple ASesExample: Choosing among multiple ASesChapter 4: Network LayerIntra-AS RoutingChapter 4: Network LayerRIP ( Routing Information Protocol)RIP advertisementsRIP: Example RIP: Example RIP: Link Failure and Recovery Chapter 4: Network LayerOSPF (Open Shortest Path First)OSPF “advanced” features (not in RIP)Hierarchical OSPFHierarchical OSPFChapter 4: Network LayerInternet inter-AS routing: BGPBGP basicsDistributing reachability infoPath attributes & BGP routesBGP route selectionBGP messagesBGP routing policyBGP routing policy (2)Why different Intra- and Inter-AS routing ? Chapter 4: Network LayerBroadcast RoutingIn-network duplicationReverse Path Forwarding (RPF) in broadcastingSpanning TreeSpanning Tree: CreationChapter 4: summary

Chapter 4 Network Layer (Part 2) - University of Utah · 2007. 3. 5. · Network Layer 4-2 Chapter...

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