Chapter 4 Network Layer - UNSW School of Computer...

Network Layer 4-1

Chapter 4Network Layer

Computer Networking: A Top Down Approach 4th edition. Jim Kurose, Keith RossAddison-Wesley, July 2007.

A note on the use of these ppt slides:The notes used in this course are substantially based on powerpoint slides developed and copyrighted by J.F. Kurose and K.W. Ross, 1996-2007

Network Layer 4-2

Chapter 4: Network Layer

Chapter goals:Understand principles behind network layer services:

Network layer service modelsForwarding versus routingHow a router worksRouting (path selection)Dealing with scaleAdvanced topics: IPv6, mobility

Instantiation, implementation in the Internet

Network Layer 4-3


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

Network Layer 4-4

Network LayerTransport segment from sending to receiving host On sending side encapsulates segments into datagramsOn rcving side, delivers segments to transport layerNetwork layer protocols in every host, routerRouter examines header fields in all IP datagrams passing through it

networkdata linkphysical








applicationtransportnetworkdata linkphysical


Network Layer 4-5

Two Key Network-Layer Functions

Forwarding: move packets from router’s input to appropriate router output

Routing: determine route taken by packets from source to dest

Routing algorithms

Analogy:

Routing: process of planning trip from source to destForwarding: process of getting through single interchange

Network Layer 4-6

Interplay between Routing and Forwarding

1

23

0111

value in arrivingpacket’s header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Network Layer 4-7

Connection Setup

The 3rd important function in some network architectures:

ATM, frame relay, X.25Before datagrams flow, two end hosts and intervening routers establish virtual connection

Routers get involvedNetwork vs. transport layer connection service:

Network: between two hosts (may also involve intervening routers in case of VCs)Transport: between two processes

Network Layer 4-8

Network Service Model

Q: What service model for “channel” transporting packets from sender to receiver?Guaranteed bandwidth?Preservation of inter-packet timing (no jitter)?Loss-free delivery?In-order delivery?Congestion feedback to sender?

service abstraction? ??virtual circuit

or datagram?

The most importantabstraction provided

by network layer:

Network Layer 4-9

Network Layer Service Models:

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Loss

no

yes

yes

no

no

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effortCBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Guarantees ?

Network Layer 4-10






RIPOSPFBGP

Network Layer 4-11

Network Layer Connection and Connection-less Service

Datagram network provides network-layer connectionless serviceVC network provides network-layer connection serviceAnalogous to the transport-layer services, but:

Service: host-to-hostNo choice: network provides one or the otherImplementation: in network core

Network Layer 4-12

Virtual Circuits

“source-to-dest path behaves much like telephone circuit”

Performance-wiseNetwork actions along source-to-dest path

Call setup, teardown for each call before data can flowEach packet carries VC identifier (not destination host address)Every router on source-dest path maintains “state” for each passing connectionLink, router resources (bandwidth, buffers) may be allocated to VC (dedicated resources = predictable service)

Network Layer 4-13

VC Implementation

A VC consists of:1. Path from source to destination2. VC numbers, one number for each link along

path3. Entries in forwarding tables in routers along

pathPacket belonging to VC carries VC number (rather than dest. address)VC number can be changed on each link

New VC number comes from forwarding table

Network Layer 4-14

Forwarding Table12 22 32

1 23

VC number

interfacenumber

Forwarding table innorthwest router:

Incoming interface Incoming VC # Outgoing interface Outgoing VC #

1 12 3 222 63 1 18 3 7 2 171 97 3 87… … … …

Routers maintain connection state information!

Network Layer 4-15

Virtual Circuits: Signaling Protocols

Used to setup, maintain teardown VCUsed in ATM, frame-relay, X.25Not used in today’s Internet



1. Initiate call 2. incoming call3. Accept call4. Call connected

5. Data flow begins 6. Receive data

Network Layer 4-16

Datagram NetworksNo call setup at network layerRouters: no state about end-to-end connections

No network-level concept of “connection”Packets forwarded using destination host address

Packets between same source-dest pair may take different paths



1. Send data 2. Receive data

Network Layer 4-17

4 billion possible entriesForwarding Table

Destination Address Range Link Interface

11001000 00010111 00010000 00000000through 0

11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through 1

11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through 2

11001000 00010111 00011111 11111111

otherwise 3

Network Layer 4-18

Longest Prefix Matching

Prefix Match Link Interface11001000 00010111 00010 0 11001000 00010111 00011000 111001000 00010111 00011 2

otherwise 3

Examples

Which interface?

Interface 0

DA: 11001000 00010111 00010110 10100001

Which interface?DA: 11001000 00010111 00011000 10101010

Interface 1

Network Layer 4-19

Datagram or VC Network: Why?

Internet (datagram)Data exchange among computers

“Elastic” service, no strict timing req.

“Smart” end systems (computers)

Can adapt, perform control, error recoverySimple inside network, complexity at “edge”

Many link types Different characteristicsUniform service difficult

ATM (VC)Evolved from telephonyHuman conversation:

Strict timing, reliability requirementsNeed for guaranteed service

“Dumb” end systemsTelephonesComplexity inside network

Network Layer 4-20






RIPOSPFBGP

Network Layer 4-21

Router Architecture Overview

Two key router functions:Run routing algorithms/protocol (RIP, OSPF, BGP)Forwarding datagrams from incoming to outgoing link

Network Layer 4-22

Input Port Functions

Decentralized switching:Given datagram dest., lookup output port using forwarding table in input port memoryGoal: complete input port processing at ‘line speed’Queuing: if datagrams arrive faster than forwarding rate into switch fabric

Physical layer:bit-level reception

Data link layer:e.g., Ethernetsee chapter 5

Network Layer 4-23

Three Types of Switching Fabrics

Network Layer 4-24

Output Ports

Buffering required when datagrams arrive from fabric faster than the transmission rateScheduling discipline chooses among queued datagrams for transmission

Network Layer 4-25

Output Port Queuing

Buffering when arrival rate via switch exceeds output line speedQueuing (delay) and loss due to output port buffer overflow!

Network Layer 4-26

Input Port QueuingFabric slower than input ports combined -> queueing may occur at input queues Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forwardQueuing delay and loss due to input buffer overflow!

Network Layer 4-27






RIPOSPFBGP

Network Layer 4-28

The Internet Network Layer

forwardingtable

Host, router network layer functions:

Routing protocols•Path selection•RIP, OSPF, BGP

IP protocol•Addressing conventions•Datagram format•Packet handling conventions

ICMP protocol•Error reporting•Router “signaling”

Transport layer: TCP, UDP

Link layer

physical layer

Networklayer

Network Layer 4-29






RIPOSPFBGP

Network Layer 4-30

IP Datagram Format

ver length

32 bits

data (variable length,typically a TCP

or UDP segment)

16-bit identifierheader

checksumtime to

live

32 bit source IP address

head.len

type ofservice

flgs fragmentoffset

upperlayer

32 bit destination IP address

Options (if any)

IP protocol versionnumber

header length(bytes)

max numberremaining hops

(decremented at each router)

forfragmentation/reassembly

total datagramlength (bytes)

upper layer protocolto deliver payload to

“type” of data (QoS)

E.g. timestamp,record routetaken, specifylist of routers to visit.

How much overhead with TCP?20 bytes of TCP20 bytes of IP= 40 bytes + app layer overhead

Network Layer 4-31

IP Fragmentation & ReassemblyNetwork links have MTU (max. transfer unit/size) - largest possible link-level frame

Different link types, different MTUs

Large IP datagram divided (“fragmented”) within net

One datagram becomes several datagrams“Reassembled” only at final destinationIP header bits used to identify, order related fragments

fragmentation: in: one large datagramout: 3 smaller

datagrams

reassembly

Network Layer 4-32

IP Fragmentation & ReassemblyID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example4000 byte datagramMTU = 1500 bytes

1480 bytes in data field

offset =1480/8

Network Layer 4-33






RIPOSPFBGP

Network Layer 4-34

IP Addressing: Introduction223.1.1.1

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

IP address: 32-bit identifier for host, router interfaceInterface: connection between host/router and physical link

Router’s typically have multiple interfacesHost typically has one interface (may have multiple today)IP addresses associated with each interface, not with host or router

223.1.1.1 = 11011111 00000001 00000001 00000001

223 1 11

Network Layer 4-35

Subnets223.1.1.1

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

subnet

IP address:Subnet part (high order bits)Host part (low order bits)

What’s a subnet ?Device interfaces with same subnet part of IP addressCan physically reach each other without intervening router network consisting of 3 subnets

(for IP addresses starting with 223, first 24 bits are network address)

Network Layer 4-36

IP AddressesGiven notion of “network”, let’s re-examine IP addresses:

“class-full” addressing:

class1.0.0.0 to127.255.255.255A 0network host

128.0.0.0 to191.255.255.255

B 10 network host

192.0.0.0 to223.255.255.255C 110 network host

224.0.0.0 to239.255.255.255D 1110 multicast address

32 bits

Network Layer 4-37

IP Addressing: CIDRClassful addressing:

Inefficient use of address space, address space exhaustionE.g., class B net allocated enough addresses for 65K hosts, even if only 2K hosts in that network

CIDR: Classless InterDomain RoutingSubnet portion of address of arbitrary lengthAddress format: a.b.c.d/x, where x is # bits in subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

200.23.16.0/23

Network Layer 4-38

IP Addresses: How to Get One?

Q: How does host get IP address?

Hard-coded by system admin in a fileWintel: control-panel->network->configuration->tcp/ip->propertiesUNIX: /etc/rc.config

DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server

“plug-and-play” (more later)

Network Layer 4-39

DHCP: Dynamic Host Configuration Protocol

Goal: allow host to dynamically obtain its IP address from network server when it joins network

Can renew its lease on address in useAllows reuse of addresses (only hold address while connected an “on”Support for mobile users who want to join network (more shortly)

DHCP overview:Host broadcasts “DHCP discover” msgDHCP server responds with “DHCP offer” msgHost requests IP address: “DHCP request” msgDHCP server sends address: “DHCP ack” msg

Network Layer 4-40

DHCP Client-Server Scenario

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

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-41

DHCP Client-Server ScenarioDHCP server: 223.1.2.5 arriving

client

time

DHCP discoversrc : 0.0.0.0, 68 dest.: 255.255.255.255,67yiaddr: 0.0.0.0transaction ID: 654

DHCP offersrc: 223.1.2.5, 67 dest: 255.255.255.255, 68yiaddrr: 223.1.2.4transaction ID: 654Lifetime: 3600 secs

DHCP requestsrc: 0.0.0.0, 68 dest:: 255.255.255.255, 67yiaddrr: 223.1.2.4transaction ID: 655Lifetime: 3600 secs

DHCP ACKsrc: 223.1.2.5, 67 dest: 255.255.255.255, 68yiaddrr: 223.1.2.4transaction ID: 655Lifetime: 3600 secs

Network Layer 4-42

IP Addresses: How to Get One?

Q: How does network get subnet part of IP addr?

A: Gets allocated portion of its provider ISP’s address space

ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20

Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23 Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23 Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23

... ….. …. ….Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23

Network Layer 4-43

Hierarchical Addressing: Route Aggregation

Hierarchical addressing allows efficient advertisement of routing information:

“Send me anythingwith addresses beginning 200.23.16.0/20”

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us “Send me anythingwith addresses beginning 199.31.0.0/16”

200.23.20.0/23Organization 2

...

...

Network Layer 4-44

Hierarchical Addressing: More Specific Routes

ISPs-R-Us has a more specific route to Organization 1

“Send me anythingwith addresses beginning 200.23.16.0/20”

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us “Send me anythingwith addresses beginning 199.31.0.0/16or 200.23.18.0/23”

200.23.20.0/23Organization 2

...

...

Network Layer 4-45

IP Addressing: the Last Word...

Q: How does an ISP get block of addresses?

A: ICANN: Internet Corporation for Assigned Names and Numbers

Allocates addressesManages DNSAssigns domain names, resolves disputes

Network Layer 4-46

NAT: Network Address Translation

10.0.0.1

10.0.0.2

10.0.0.3

10.0.0.4

138.76.29.7

local network(e.g., home network)

10.0.0/24

rest ofInternet

Datagrams with source or destination in this networkhave 10.0.0/24 address for source, destination (as usual)

All datagrams leaving localnetwork have same single source

NAT IP address: 138.76.29.7,different source port numbers

Network Layer 4-47


Motivation: local network uses just one IP address as far as outside world is concerned:

Range of addresses not needed from ISP: just one IP address for all devicesCan change addresses of devices in local network without notifying outside worldCan change ISP without changing addresses of devices in local networkDevices inside local net not explicitly addressable, visible by outside world (a security plus)

Network Layer 4-48

NAT: Network Address TranslationImplementation: NAT router must:

Outgoing datagrams: replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #)

. . . remote clients/servers will respond using (NAT IP address, new port #) as destination addr.

Remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair

Incoming datagrams: replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table

Network Layer 4-49


10.0.0.1

10.0.0.2

10.0.0.3

S: 10.0.0.1, 3345D: 128.119.40.186, 80

110.0.0.4

138.76.29.7

1: host 10.0.0.1 sends datagram to 128.119.40.186, 80

NAT translation tableWAN side addr LAN side addr138.76.29.7, 5001 10.0.0.1, 3345…… ……

S: 128.119.40.186, 80 D: 10.0.0.1, 3345 4

S: 138.76.29.7, 5001D: 128.119.40.186, 802

2: NAT routerchanges datagramsource addr from10.0.0.1, 3345 to138.76.29.7, 5001,updates table

S: 128.119.40.186, 80 D: 138.76.29.7, 5001 3

3: Reply arrivesdest. address:138.76.29.7, 5001

4: NAT routerchanges datagramdest addr from138.76.29.7, 5001 to 10.0.0.1, 3345

Network Layer 4-50


Outside node cannot initiate the communication

Reserved addresses:10.0.0.0 - 10.255.255.255/8172.16.0.0 – 172.31.255.255/12192.168.0.0 – 192.168.255.255/16

Network Layer 4-51


16-bit port-number field: 60,000 simultaneous connections with a single LAN-side address!

NAT is controversial:Routers should only process up to layer 3Violates end-to-end argument

• NAT possibility must be taken into account by app designers, eg, P2P applications

Address shortage should instead be solved by IPv6

Network Layer 4-52

NAT Traversal ProblemClient want to connect to server with address 10.0.0.1

Server address 10.0.0.1 local to LAN (client can’t use it as destination addr)Only one externally visible NATted address: 138.76.29.7

10.0.0.1

10.0.0.4

NAT router

138.76.29.7

Client ?

Solution 1: statically configure NAT to forward incoming connection requests at given port to server

e.g., (123.76.29.7, port 2500) always forwarded to 10.0.0.1 port 25000

Network Layer 4-53

NAT Traversal ProblemSolution 2: Universal Plug and Play (UPnP) Internet Gateway Device (IGD) Protocol. Allows NATtedhost to:

Learn public IP address (138.76.29.7)Enumerate existing port mappingsAdd/remove port mappings (with lease times)

i.e., automate static NAT port map configuration

10.0.0.1

10.0.0.4

NAT router

138.76.29.7

IGD

Network Layer 4-54

NAT Traversal ProblemSolution 3: relaying (used in Skype)

NATed server establishes connection to relayExternal client connects to relayRelay bridges packets between to connections

10.0.0.1

NAT router

138.76.29.7Client

1. connection torelay initiatedby NATted host

2. connection torelay initiatedby client

3. relaying established

Network Layer 4-55






RIPOSPFBGP

Network Layer 4-56

ICMP: Internet Control Message Protocol

Used by hosts & routers to communicate network-level information

Error reporting: unreachable host, network, port, protocolEcho request/reply (used by ping)

Network-layer “above” IP:ICMP msgs carried in IP datagrams

ICMP message: type, code plus first 8 bytes of IP datagram causing error

Type Code Description0 0 echo reply (ping)3 0 dest. network unreachable3 1 dest host unreachable3 2 dest protocol unreachable3 3 dest port unreachable3 6 dest network unknown3 7 dest host unknown4 0 source quench (congestion

control - not used)8 0 echo request (ping)9 0 route advertisement10 0 router discovery11 0 TTL expired12 0 bad IP header

Network Layer 4-57

Traceroute and ICMP

Source sends series of UDP segments to dest

First has TTL =1Second has TTL=2, etc.Unlikely port number

When nth datagram arrives to nth router:

Router discards datagramAnd sends to source an ICMP message (type 11, code 0)Message includes name of router& IP address

When ICMP message arrives, source calculates RTTTraceroute does this 3 times

Stopping criterionUDP segment eventually arrives at destination hostDestination returns ICMP “host unreachable” packet (type 3, code 3)When source gets this ICMP, stops

Network Layer 4-58

ICMP: Brief Summary

ICMP is the control sibling of IP ICMP is used by IP and uses IP as network layer protocol ICMP is used for ping, traceroute, and path MTU discovery

Ping: Uses ICMP Echo request/reply messagesPath MTU Discovery

• Send a large IP datagram with “No fragment” bit set • Reduce size until success (No ICMP message received)

Network Layer 4-59






RIPOSPFBGP

Network Layer 4-60

IPv6Initial motivation: 32-bit address space soon to be completely allocatedAdditional motivation:

Header format helps speed processing/forwardingHeader changes to facilitate QoS

IPv6 datagram format:Fixed-length 40 byte headerNo fragmentation allowed

Network Layer 4-61

IPv6 Header (Cont.)Priority: identify priority among datagrams in flowFlow Label: identify datagrams in same “flow”

(concept of“flow” not well defined)Next header: identify upper layer protocol for data

Network Layer 4-62

Other Changes from IPv4

Checksum: removed entirely to reduce processing time at each hopOptions: allowed, but outside of header, indicated by “Next Header”fieldICMPv6: new version of ICMP

Additional message types, e.g. “Packet Too Big”Multicast group management functions

Network Layer 4-63

Transition From IPv4 To IPv6

Not all routers can be upgraded simultaneousno “flag days”How will the network operate with mixed IPv4 and IPv6 routers?

Tunneling: IPv6 carried as payload in IPv4 datagram among IPv4 routers

Network Layer 4-64

TunnelingA B E F

IPv6 IPv6 IPv6 IPv6

tunnelLogical view:

Physical view:A B E F

IPv6 IPv6 IPv6 IPv6IPv4 IPv4

Network Layer 4-65

TunnelingA B E F

IPv6 IPv6 IPv6 IPv6

tunnelLogical view:

A B E F

IPv6 IPv6 IPv6 IPv6

C D

IPv4 IPv4

Flow: XSrc: ADest: F

data


data


data

Src:BDest: E


data

Src:BDest: E

A-to-B:IPv6

E-to-F:IPv6

Physical view:

B-to-C:IPv6 inside

IPv4

B-to-C:IPv6 inside

IPv4

Network Layer 4-66






RIPOSPFBGP

Network Layer 4-67

Interplay between Routing and Forwarding

1

23

0111

value in arrivingpacket’s header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Network Layer 4-68

Graph Abstraction

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) }

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 4-69

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 inversely 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 4-70

Routing Algorithm ClassificationGlobal or decentralizedinformation?

Static or dynamic?

Static:Routes change slowly over time

Dynamic:Routes change more quickly

Periodic updateIn response to link cost changes

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

Network Layer 4-71






RIPOSPFBGP

Network Layer 4-72

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 4-73

Dijkstra’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 4-74

Dijkstra’s Algorithm: Example

Step012345

start NA

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)∞

2,A2,A 4,D

3,E3,E

2,D ∞4,E4,E4,E

ADEBCFADEBC

ADEBADE

AD

A

ED

CB

F2

21

3

1

1

2

53

5

Network Layer 4-75

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 4-76






RIPOSPFBGP

Network Layer 4-77

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 4-78

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 4-79

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 4-80

Bellman-Ford Example (1)Cost to

y

1

0

2

3

1

x

y

u

v

w

x

0

1

1

2

2

z

3

2

4

5

3

from

u

1

2

0

3

5

v

2

3

2

0

3

w

2

1

5

3

0

u

yx

wv

z2

21

3

1

1

2

53

5

Distance vectors stored at node x

Network Layer 4-81

Bellman-Ford Example (2)Cost to

y

1

0

2

3

1

x

y

u

v

w

x

0

1

1

2

2

z

3

2

4

5

3

from

u

1

2

0

3

5

v

2

3

2

0

3

w

2

1

5

3

0

u

yx

wv

z2

21

3

1

1

2

53

5

y z u v w

Routing table at node xdestination

y,1 y,1 u,1 v,2 y,2hop, cost

Network Layer 4-82

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 4-83

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 4-84

x y zxyz

0 2 7∞∞ ∞∞∞ ∞

from

cost to

from

x y zxyz

∞ ∞

∞∞ ∞

cost to

x y z

∞∞ ∞7 1 0

cost to

∞2 0 1

from

xyz

∞ ∞ ∞

node x table

node y table

node z table

x z12

7

y

time

x y zxyz

0 2 7∞∞ ∞∞∞ ∞

from

cost to

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

x

from yz

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 4-85

x y zxyz

0 2 7∞∞ ∞∞∞ ∞

from

cost to

from

x y z0 2 3

from

cost to

xyz

x y zxyz

∞ ∞

∞∞ ∞

cost tox y zcost to

0 2 7x

from y

Network Layer 4-86

from

z

x y zxyzfr

om

0 2 7

cost tox y z

∞∞ ∞7 1 0

cost to

∞2 0 1

∞ ∞ ∞

2 0 17 1 0

2 0 13 1 0

2 0 17 1 0

xyz

time

x z12

7

y

node x table

node y table

node z table



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

x y zxyz

0 2 7∞∞ ∞∞∞ ∞

from

cost to

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

Network Layer 4-87

from

xyz

x y zcost to

0 2 3xyzfr

omx y z

xyz

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



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

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 4-88

Network Layer 4-89

x y z w

wxz

1 5 1 0

from

cost to

0 4 2 12 6 0 1

node w table

xwy

cost tox y z w0 4 2 1

from 1 5 1 0

4 0 6 5

node x table

x y z w

yxz

4 0 6 5

from 0 4 2 1

2 6 0 1

cost to

x y z w

zwy

2 6 0 1

from 1 5 1 0

4 0 6 5

cost to

node y table

node z table


Initial routing table (before change)

x

yz

w1

41

7

Network Layer 4-90

x y z w

wxz

1 5 1 0

from

cost to

0 4 2 12 6 0 1

node w table

from

xwy

cost tox y z w0 4 2 11 5 1 04 0 6 5

node x table

x y z w

yxz

3 0 1 2

from 0 4 2 1

2 6 0 1

cost to

x y z w

zwy

2 1 0 1

from 1 5 1 0

4 0 6 5

cost to

x y z w

wxz

1 2 1 0

from

cost to

0 4 2 12 1 0 1

from

xwy

cost tox y z w0 4 2 11 5 1 03 0 1 2

x y z w

yxz

3 0 1 2

from 0 4 2 1

2 1 0 1

cost to

x y z w

zwy

2 1 0 1fr

om 1 5 1 03 0 1 2

cost to

cost tox y z w

wxz

1 2 1 0

from 0 4 2 1

2 1 0 1

xwy


from 1 2 1 0

3 0 1 2

x y z w

yxz

3 0 1 2

from 0 4 2 1

2 1 0 1

cost to

x y z w

zwy

2 1 0 1

from 1 2 1 0

3 0 1 2

cost to


Algorithm converges in 3 steps.

node y table

x

yz

w1

41 node z table

17

Cost of link zy changes to 1

Network Layer 4-91

cost tox y z w

wxz

1 2 1 0

from 0 3 2 1

2 1 0 1

xwy


from 1 2 1 0

3 0 1 2

x y z w

yxz

3 0 1 2

from 0 3 2 1

2 1 0 1

cost to

x y z w

zwy

2 1 0 1

from 1 2 1 0

3 0 1 2

cost to

cost tox y z w

wxz

1 2 1 0

from 0 4 2 1

2 1 0 1

xwy


from 1 2 1 0

3 0 1 2

x y z w

yxz

3 0 1 2

from 0 4 2 1

2 1 0 1

cost to

x y z w

zwy

2 1 0 1

from 1 2 1 0

3 0 1 2

cost to


node w table

node x tableAlgorithm

converges in 3 steps.

node y table

x

yz

w1

41 node z table

17

Cost of link zy changes to 1

Network Layer 4-92

“badnews travelsslow” x

yz

cost tox y z0 4 5

from 4 0 1

5 1 0

node x table

node y table

node z table

xyz

cost tox y z0 4 5

from 4 0 1

5 1 0

xyz

cost tox y z0 4 5

from 4 0 1

5 1 0

Initial routing table

“count to infinity”problem

x z14

50

y

Network Layer 4-93

x z14

50

y60

xyz

cost tox y z0 51 50

from 4 0 1

5 1 0

node x table

node y table

node z table

xyz

cost tox y z0 4 5

from 6 0 1

5 1 0

xyz

cost tox y z0 4 5

from 4 0 1

5 1 0

xyz

cost tox y z0 51 50

from 6 0 1

5 1 0

node x table

node y table

node z table

xyz

cost tox y z0 51 50

from 6 0 1

5 1 0

xyz

cost tox y z0 51 50

from 6 0 1

7 1 0Cost of link xy changes to 60

“badnews travelsslow”

xyz

cost tox y z0 51 50

from 6 0 1

7 1 0

node x table

node y table

node z table

xyz

cost tox y z0 51 50

from 8 0 1

7 1 0

xyz

cost tox y z0 51 50

from 6 0 1

7 1 0


xyz

cost tox y z0 51 50

from 8 0 1

7 1 0

node x table

node y table

node z table

xyz

cost tox y z0 51 50

from 8 0 1

7 1 0

xyz

cost tox y z0 51 50

from 8 0 1

9 1 0

……

Network Layer 4-94

“badnews travelsslow”

……

xyz

cost tox y z0 51 50

from 48 0 1

49 1 0

node x table

node y table

node z table

xyz

cost tox y z0 51 50

from 50 0 1

49 1 0

xyz

cost tox y z0 51 50

from 48 0 1

49 1 0

xyz

cost tox y z0 51 50

from 50 0 1

49 1 0

node x table

node y table

node z table

xyz

cost tox y z0 51 50

from 50 0 1

49 1 0

xyz

cost tox y z0 51 50

from 50 0 1

50 1 0

xyz

cost tox y z0 51 50

from 50 0 1

50 1 0

node x table

node y table

node z table

xyz

cost tox y z0 51 50

from 51 0 1

50 1 0

xyz

cost tox y z0 51 50

from 50 0 1

50 1 0

xyz

cost tox y z0 51 50

from 51 0 1

50 1 0

node x table

node y table

node z table

xyz

cost tox y z0 51 50

from 51 0 1

50 1 0

xyz

cost tox y z0 51 50

from 51 0 1

50 1 0


x z14

50

y60

Cost of link xy changes to 60

Network Layer 4-95

Distance Vector: Link Cost Changes

Link cost changes:Good news travels fast Bad news travels slow -“count to infinity” problem! x z

14

50

y60

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?

Real problem is that y thinks its shortest path to x is through z, while z thinks its shortest path to x is through y. They pingpong back and forth with this information.

Network Layer 4-96

poisoned reverse

xyz

cost tox y z0 4 5

from 4 0 1

5 1 0

node x table

node y table

node z table

xyz

cost tox y z0 4 ∞

from 4 0 1∞ 1 0

xyz

cost tox y z0 4 5

from 4 0 1

5 1 0

Initial routing table

x z14

50

y

Network Layer 4-97

x z14

50

y60

xyz

cost tox y z0 51 50

from 4 0 1

5 1 0

node x table

node y table

node z table

xyz

cost tox y z0 4 ∞

from 60 0 1

∞ 1 0

xyz

cost tox y z0 4 5

from 4 0 1

5 1 0

xyz

cost tox y z0 51 50

from 60 0 1

5 1 0

node x table

node y table

node z table

xyz

cost tox y z0 51 50

from 60 0 1

∞ 1 0

xyz

cost tox y z0 ∞ 50

from 60 0 1

50 1 0

Cost of link xy changes to 60

xyz

cost tox y z0 51 50

from 60 0 1

50 1 0

node x table

node y table

node z table

xyz

cost tox y z0 51 50

from 51 0 1

50 1 0

xyz


from 60 0 1

50 1 0


xyz

cost tox y z0 51 50

from 51 0 1

50 1 0

node x table

node y table

node z table

xyz

cost tox y z0 51 50

from 51 0 1

50 1 0

xyz


from ∞ 0 1

50 1 0

Network Layer 4-98

Comparison of LS and DV algorithms

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

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

Network Layer 4-99






RIPOSPFBGP

Network Layer 4-100

Hierarchical Routing

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

Our routing study thus far - idealization All routers identicalNetwork “flat”

… not true in practice

Administrative autonomyInternet = network of networksEach network admin may want to control routing in its own network

Network Layer 4-101

Hierarchical Routing

Special routers in ASRun intra-AS routing protocol with all other routers in ASAlso responsible for routing to destinations outside AS

Run inter-AS routingprotocol with other gateway routers

gateway routers

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

Network Layer 4-102

Interconnected ASes

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

3c

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

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

Network Layer 4-103

Inter-AS Tasks AS1 needs:Suppose 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?

1. To learn which destsare reachable through AS2 and which through AS3

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

Job of inter-AS routing!

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

3c

Network Layer 4-104

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 AS2Inter-AS protocol propagates reachability info to all internal routersRouter 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cPuts in forwarding table entry (x,I)

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

3c

Network Layer 4-105

Example: Choosing Among Multiple ASes

Now suppose AS1 learns from the inter-AS protocol that subnet x is reachable from AS3 and from AS2To configure forwarding table, router 1d must determine towards which gateway it should forward packets for dest xThis is also the job on inter-AS routing protocol!Hot potato routing: send packet towards closest of two routers

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

Network Layer 4-106






RIPOSPFBGP

Network Layer 4-107

Routing in the InternetThe Global Internet consists of Autonomous Systems (AS) interconnected with each other:

Stub AS: small corporation: one connection to other AS’sMultihomed AS: large corporation (no transit): multiple connections to other AS’sTransit AS: provider, hooking many AS’s together

Two-level routing: Intra-AS: (within AS) administrator responsible for choice of routing algorithm within networkInter-AS: (between ASes) unique standard for inter-AS routing: BGP

Network Layer 4-108

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 4-109






RIPOSPFBGP

Network Layer 4-110

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 4-111

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 4-112

RIP: Example

w x yz

A

C

D B

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

Routing table in D

Network Layer 4-113

RIP: Example

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

w x y

z

A

C

D B

Dest Next hopsw - 1x - 1z C 4…. … ...

Advertisementfrom A to D

A 5

Routing table in D

Network Layer 4-114

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 4-115

RIP Table Processing

RIP routing tables managed by application-levelprocess called route-d (daemon)Advertisements sent in UDP packets, periodically repeated

physicallink

network forwarding(IP) table

Transport(UDP)

routed

network(IP)link

physical

Transport(UDP)

routed

forwardingtable

Network Layer 4-116






RIPOSPFBGP

Network Layer 4-117

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 4-118

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 4-119

Hierarchical OSPF

Network Layer 4-120

Hierarchical OSPF

Two-level hierarchy: local area, backboneLink-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 4-121






RIPOSPFBGP

Network Layer 4-122

Inter-AS Routing in the Internet: BGP

Figure 4.5.2-new2: BGP use for inter-domain routing

AS2 (OSPF

intra-AS routing)

AS1 (RIP intra-AS

routing) BGP

AS3 (OSPF intra-AS

routing)

BGP

R1 R2

R3

R4

R5

Network Layer 4-123

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 4-124

In BGP, destination are not individual hosts, they are networks!

A network is represented by a CIDR prefix, e.g., 138.16.64/24If a gateway router broadcasts a BGP message stating that it is 138.16.64/24, it is advertisingthat it can deliver messages to any host in subnet 138.16.64/24

BGP messages between routers in same AS are called (interior) iBGP messagesBGP messages between routers in diff AS are called (exterior) eBGP messages

Network Layer 4-125

BGP BasicsPairs of routers (BGP peers) exchange routing info over semi-permanent TCP connections: BGP sessions

BGP sessions need not correspond to physical linksWhen AS2 advertises a prefix to AS1, AS2 is promisingit 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 4-126

Distributing Reachability InfoWith eBGP session between 3a and 1c, AS3 sends prefix reachability info to AS11c 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 4-127

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 4-128

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

Network Layer 4-129

BGP Messages

BGP messages exchanged using TCPBGP 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 4-130

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 4-131

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 4-132

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 4-133

Network Layer: SummaryWhat we’ve covered:

Network layer servicesRouting principles: link state and distance vectorHierarchical routingIPInternet routing protocols RIP, OSPF, BGPWhat’s inside a router?IPv6

Next stop:the Datalink layer!

Chapter 4 Network Layer - UNSW School of Computer...

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