chapter 4 network layer network layer.pdf · 2011. 10. 22. · 23/10/2011 2 chapter 4: network...

23/10/2011

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Fundamentals of Data Communication and Networking

Chapter 4Network Layer

Computer Networking: A Top Down Approach F t i th I t t

A note on the use of these ppt slides:We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following:

Network Layer 4-1

Featuring the Internet

Jim Kurose, Keith RossAddison-Wesley

following: If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!) If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.

Thanks and enjoy! JFK/KWR

All material copyright 1996-2011J.F Kurose and K.W. Ross, All Rights Reserved

Chapter 4: Network Layer

Chapter goals:d d l b h d k l understand principles behind network layer

services: routing (path selection) dealing with scale how a router works

d d t i IP 6 bilit

Network Layer 4-2

advanced topics: IPv6, mobility instantiation and implementation in the

Internet

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4. 1 Introduction 4 2 Virtual circuit and

4.5 Routing algorithms Hierarchical routing 4.2 Virtual circuit and

datagram networks 4.3 What’s inside a

router 4.4 IP: Internet

ProtocolD t f t

Hierarchical routing 4.6 Routing in the

Internet RIP OSPF BGP

Network Layer 4-3

Datagram format IPv4 addressing ICMP IPv6

Network layer transport segment from

sending to receiving host on sending side network

d l k

applicationtransportnetworkdata linkphysical

encapsulates segments into datagrams

on rcving side, delivers segments to transport layer

network layer protocols

networkdata linkphysical


data linkphysical




networkd t li k


physical

application

Network Layer 4-4

y pin every host, router

Router examines header fields in all IP datagrams passing through it

data linkphysical

applicationtransportnetworkdata linkphysical

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Key Network-Layer Functions

forwarding: move packets from router’s

analogy:packets from router s input to appropriate router output

routing: determine route taken by

routing: process of planning trip from source to dest

forwarding: process of getting through

Network Layer 4-5

packets from source to dest.

Routing algorithms

of getting through single interchange

routing algorithm

local forwarding table

Interplay between routing and forwarding

value in arrivingpacket’s header

local forwarding tableheader value output link

0100010101111001

3221

Network Layer 4-6

1

23

0111

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Network service modelQ: What service model for “channel” transporting datagrams from sender to rcvr?

Example services for individual datagrams:

guaranteed delivery Guaranteed delivery

with less than 40 msec d l

Example services for a flow of datagrams:

In-order datagram delivery

Guaranteed minimum b d idth t fl

Network Layer 4-7

delay bandwidth to flow Restrictions on

changes in inter-packet spacing

Network layer service models:

NetworkArchitecture

ServiceModel Bandwidth Loss Order Timing

Congestionfeedback

Guarantees ?

Internet

ATM

ATM

ATM

best effort

CBR

VBR

ABR

none

constantrateguaranteedrateguaranteed

no

yes

yes

no

no

yes

yes

yes

no

yes

yes

no

no (inferredvia loss)nocongestionnocongestionyes

Network Layer 4-8

ATM UBR

gminimumnone no

y

yes no

y

no

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ProtocolD t f t



Network Layer 4-9


Network layer connection and connection-less serviceDatagram network provides network-layer

connectionless serviceconnectionless service VC network provides network-layer

connection serviceAnalogous to the transport-layer services,

but:S i : h st t h st

Network Layer 4-10

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

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Virtual circuits“source-to-dest path behaves much like telephone

circuit”

call setup, teardown for each call before data can flow each packet carries VC identifier (not destination host

address)

performance-wise network actions along source-to-dest path

Network Layer 4-11

every router on source-dest path maintains “state” for each passing connection

link, router resources (bandwidth, buffers) may be allocated to VC

VC implementation

A VC consists of:1 Path from source to destination1. Path from source to destination2. VC numbers, one number for each link along

path3. Entries in forwarding tables in routers along

path Packet belonging to VC carries a VC

Network Layer 4-12

g gnumber.

VC number must be changed on each link. New VC number comes from forwarding table

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Forwarding table12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC # Outgoing interface Outgoing VC #

1 12 3 222 63 1 18 3 7 2 17

Forwarding table innorthwest router:

Network Layer 4-13

3 7 2 171 97 3 87… … … …

Routers maintain connection state information!

Virtual circuits: signaling protocols

used to setup, maintain teardown VC d i ATM f l X 25 used in ATM, frame-relay, X.25 not used in today’s Internet

applicationtransport

t kapplicationtransport3 A t ll4 C ll t d

5. Data flow begins 6. Receive data

Network Layer 4-14


transportnetworkdata linkphysical

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

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Datagram networks no call setup at network layer routers: 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

applicationt nsp t application

Network Layer 4-15

transportnetworkdata linkphysical

appl cat ontransportnetworkdata linkphysical

1. Send data 2. Receive data

Forwarding table

Destination Address Range Link Interface

4 billion possible entries

11001000 00010111 00010000 00000000through 0

11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through 1

11001000 00010111 00011000 11111111

Network Layer 4-16

11001000 00010111 00011001 00000000through 2

11001000 00010111 00011111 11111111

otherwise 3

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Longest prefix matching

Prefix Match Link Interface11001000 00010111 00010 011001000 00010111 00010 011001000 00010111 00011000 111001000 00010111 00011 2

otherwise 3

Examples

DA: 11001000 00010111 00010110 10100001 Which interface?

Network Layer 4-17

DA: 11001000 00010111 00011000 10101010 Which interface?

Datagram or VC network: why?

Internet data exchange among

ATM evolved from telephony

computers “elastic” service, no strict

timing req. “smart” end systems

(computers) can adapt, perform

control, error recovery

f p y human conversation:

strict timing, reliability requirements

need for guaranteed service

“dumb” end systems t l ph n s

Network Layer 4-18

y simple inside network,

complexity at “edge” many link types

different characteristics uniform service difficult

telephones complexity inside

network

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ProtocolD t f t



Network Layer 4-19


The Internet Network layerHost, router network layer functions:

Transport layer: TCP, UDP

forwardingtable

Routing protocols•path selection•RIP, OSPF, BGP

IP protocol•addressing conventions•datagram format•packet handling conventions

ICMP protocol•error reporting

t “si li ”

p y ,

Networklayer

Network Layer 4-20

•router “signaling”

Link layer

physical layer

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ProtocolD t f t



Network Layer 4-21


IP datagram format

ver length

32 bits

16 bit identifier

IP protocol versionnumber

header length(bytes) for

fragmentation/

total datagramlength (bytes)head.

lentype ofservice

“type” of data flgsfragment

d t

16-bit identifierInternetchecksum

time tolive

32 bit source IP address

max numberremaining hops

(decremented at each router)

fragmentation/reassembly

upper layer protocolto deliver payload to

flgs offsetupperlayer

32 bit destination IP address

Options (if any) E.g. timestamp,record route

h h h d

Network Layer 4-22

data (variable length,typically a TCP

or UDP segment)

taken, specifylist of routers to visit.

how much overhead with TCP?

20 bytes of TCP 20 bytes of IP = 40 bytes + app

layer overhead

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IP Fragmentation & Reassembly network links have MTU

(max.transfer size) - largest possible link-level frame.

diff li k different link types, different MTUs

large IP datagram divided (“fragmented”) within net one datagram becomes

several datagrams “reassembled” only at final

destination

fragmentation: in: one large datagramout: 3 smaller datagrams

reassembly

Network Layer 4-23

IP header bits used to identify, order related fragments

IP Fragmentation and ReassemblyID=x

offset=0

fragflag=0

length=4000Example

4000 b t

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

One large datagram becomesseveral smaller datagrams

4000 byte datagram

MTU = 1500 bytes

1480 bytes in data field

Network Layer 4-24

x =185=11500

ID=x

offset=370

fragflag=0

length=1040

offset =1480/8

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ProtocolD t f t



Network Layer 4-25


IP Addressing: introduction IP address: 32-bit

identifier for host, router interface

223.1.1.1

223.1.1.2223.1.2.1

interface IP address:

subnet part (high order bits) host part (low order bits)

interface: connection between host/router and h si l li k

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.3.2223.1.3.1

223.1.3.27

Network Layer 4-26

physical link router’s typically have

multiple interfaces host typically has one

interface IP addresses associated with

each interface

223.1.1.1 = 11011111 00000001 00000001 00000001

223 1 11

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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->properties UNIX: /etc/rc.config

DHCP: Dynamic Host Configuration Protocol:

Network Layer 4-27

DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server “plug-and-play”

(more in next chapter)

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

Network Layer 4-28

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

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IP addressing: the last word...

Q: How does an ISP get block of addresses?A ICANN I C i f A i d A: ICANN: Internet Corporation for Assigned

Names and Numbers allocates addressesmanages DNS assigns domain names, resolves disputes

Network Layer 4-29






ProtocolD t f t



Network Layer 4-30


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ICMP: Internet Control Message Protocol

used by hosts & routers to communicate network-level i f ti

Type Code description0 0 echo reply (ping)information

error reporting: unreachable host, network, port, protocol

echo request/reply (used by ping)

network-layer “above” IP: ICMP msgs carried in IP

0 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)

Network Layer 4-31

gdatagrams

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

8 0 echo request (ping)9 0 route advertisement10 0 router discovery11 0 TTL expired12 0 bad IP header

Traceroute and ICMP

Source sends series of UDP segments to dest

When ICMP message arrives, source calculates

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

When nth datagram arrives to nth router: Router discards datagram And sends to source an

RTT Traceroute does this 3

timesStopping criterion UDP segment eventually

arrives at destination host Destination returns ICMP

Network Layer 4-32

ICMP message (type 11, code 0)

Message includes name of router& IP address

Dest nat on returns I M “host unreachable” packet (type 3, code 3)

When source gets this ICMP, stops.

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ProtocolD t f t



Network Layer 4-33


IPv6 Initial motivation: 32-bit address space soon

to be completely allocated. p yAdditional motivation:

header format helps speed processing/forwarding header changes to facilitate QoS IPv6 datagram format: fixed-length 40 byte header

Network Layer 4-34

g y no fragmentation allowed

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

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

Network Layer 4-35

Other Changes from IPv4

Checksum: removed entirely to reduce processing time at each hopprocessing time at each hop

Options: allowed, but outside of header, indicated by “Next Header” field

ICMPv6: new version of ICMP additional message types, e.g. “Packet Too Big”

lti t t f ti

Network Layer 4-36

multicast group management functions

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ProtocolD t f t



Network Layer 4-37


routing algorithm

Interplay between routing and forwarding

value in arrivingpacket’s header

local forwarding tableheader value output link

0100010101111001

3221

Network Layer 4-38

1

23

0111

packet s header

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Routing Algorithm classificationGlobal or decentralized

information?Gl b l

Static or dynamic?Static:

Global: all routers have complete

topology, link cost info “link state” algorithmsDecentralized: router knows physically-

connected neighbors, link

routes change slowly over time

Dynamic: routes change more

quickly periodic update

Network Layer 4-39

gcosts to neighbors

iterative process of computation, exchange of info with neighbors

“distance vector” algorithms

periodic update in response to link

cost changes






ProtocolD t f t



Network Layer 4-40


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Hierarchical RoutingOur routing study thus far - idealization all routers identical

scale: with 200 million destinations:

can’t store all dest’s in

administrative autonomy internet = network of

networks

network “flat”… not true in practice

Network Layer 4-41

routing tables! routing table exchange

would swamp links!

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

Hierarchical Routing

aggregate routers into regions, “autonomous

Gateway router Direct link to router in systems” (AS)

routers in same AS run same routing protocol “intra-AS” routing

protocol routers in different AS

diff t i t

Direct link to router in another AS

Network Layer 4-42

can run different intra-AS routing protocol

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3a 2c3c

Interconnected ASes

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting

Inter-ASRouting

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

Network Layer 4-43

Routing algorithm

Routing algorithm

Forwardingtable

routing algorithm Intra-AS sets entries

for internal dests Inter-AS & Intra-As

sets entries for external dests

Inter-AS tasks Suppose router in AS1

receives datagram for which dest is outside of AS1

AS1 needs:1. to learn which dests

are reachable through AS2 and which thr u h AS3of AS1

Router should forward packet towards one of the gateway routers, but which one?

through AS32. to propagate this

reachability info to all routers in AS1

Job of inter-AS routing!

Network Layer 4-44

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

3c

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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 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

Network Layer 4-45

routing info that its interface I is on the least cost path to 1c.

Puts in forwarding table entry (x,I).

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 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 4-46

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

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ProtocolD t f t



Network Layer 4-47


Intra-AS Routing

Also known as Interior Gateway Protocols (IGP) Most common Intra-AS routing protocols: Most common Intra-AS routing protocols:

RIP: Routing Information Protocol

OSPF: Open Shortest Path First

IGRP: Interior Gateway Routing Protocol (Cisco proprietary)

Network Layer 4-48

proprietary)

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ProtocolD t f t



Network Layer 4-49


RIP ( Routing Information Protocol)

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

BA

u vw

destination hopsu 1v 2

From router A to subsets:

Network Layer 4-50

DC

BA

x

yz

w 2x 3y 3z 2

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RIP advertisements

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

Each advertisement: list of up to 25 destination nets within AS

Network Layer 4-51

RIP: Example

w x y

z

Destination Network Next Router Num. of hops to dest.w A 2y B 2

x yA

C

D B

Network Layer 4-52

y B 2z B 7x -- 1…. …. ....

Routing table in D

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RIP: Example

z

Dest Next hopsw - 1x - 1z C 4…. … ...

Advertisementfrom A to D

Destination Network Next Router Num. of hops to dest.A 2

w x y

z

A

C

D B

Network Layer 4-53

w A 2y B 2z B A 7 5x -- 1…. …. ....

Routing table in D

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

neighbor/link declared dead hb l d d 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

Network Layer 4-54

poison reverse used to prevent ping pong loops (infinite distance = 16 hops)

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RIP Table processing

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

advertisements sent in UDP packets, periodically repeated

Transprt(UDP)

routed

Transprt(UDP)

routed

Network Layer 4-55

physicallink

network forwarding(IP) table

(UDP)

physicallink

network(IP)

(UDP)forwarding

table






ProtocolD t f t



Network Layer 4-56


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OSPF (Open Shortest Path First)

“open”: publicly available Uses Link State algorithm g

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

OSPF advertisement carries one entry per neighbor router

Network Layer 4-57

Advertisements disseminated to entire AS (via flooding) Carried in OSPF messages directly over IP (rather than TCP

or UDP

OSPF “advanced” features (not in RIP)

Security: all OSPF messages authenticated (to prevent malicious intrusion) 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)I d i d l i

Network Layer 4-58

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

base as OSPF Hierarchical OSPF in large domains.

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Hierarchical OSPF

Network Layer 4-59

Hierarchical OSPF

Two-level hierarchy: local area, backbone. Link-state advertisements only in area y 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

Network Layer 4-60

backbone. Boundary routers: connect to other AS’s.

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ProtocolD t f t



Network Layer 4-61


Internet inter-AS routing: BGP

BGP (Border Gateway Protocol): the de facto standardfacto 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

Network Layer 4-62

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 TCP conctns: BGP sessions Note that BGP sessions do not correspond to physical links. When AS2 advertises a prefix to AS1, AS2 is promising it will

f d d d i d h fi d h forward any datagrams destined to that prefix towards the prefix. AS2 can aggregate prefixes in its advertisement

3b3a

2a2c

3c

Network Layer 4-63

3b

1d

1c2aAS3

AS1

AS21a

2b

1b

eBGP session

iBGP session

Distributing reachability info With 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 AS1 1b th d ti th h i f t AS2 th 1b can then re-advertise the new reach info to AS2 over the

1b-to-2a eBGP session When router learns about a new prefix, it creates an entry

for the prefix in its forwarding table.

3b3a

22c

3c

Network Layer 4-64

3b

1d

1c2aAS3

AS1

AS21a

2b

1b

eBGP session

iBGP session

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Path attributes & BGP routes

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

Two important attributes: AS-PATH: contains the ASs through which the advert

for the prefix passed: AS 67 AS 17 NEXT-HOP: Indicates the specific internal-AS router to

next-hop AS. (There may be multiple links from current h )

Network Layer 4-65

AS to next-hop-AS.) When gateway router receives route advert, uses

import policy to accept/decline.

BGP route selection

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

Network Layer 4-66

3. Closest NEXT HOP router: hot potato routing4. Additional criteria

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BGP messages

BGP messages exchanged using TCP. BGP messages: 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 requestNOTIFICATION i i

Network Layer 4-67

NOTIFICATION: reports errors in previous msg; also used to close connection

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 neededScale: hierarchical routing saves table size, reduced update

trafficP f

Network Layer 4-68

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

chapter 4 network layer network layer.pdf · 2011. 10. 22. · 23/10/2011 2 chapter 4: network...

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