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CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

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CS 4284 Systems Capstone Godmar Back Networking
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Page 1: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284Systems Capstone

Godmar Back

Networking

Page 2: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

IPV4

CS 4284 Spring 2013

Page 3: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

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

Page 4: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

IP Datagram Format

ver length

32 bits

data (variable length,typically a TCP

or UDP segment)

16-bit identifier

Internet checksum

time tolive

32 bit source IP address

IP protocol versionnumber

header length (bytes)

max numberremaining hops

(decremented at each router)

forfragmentation/reassembly

total datagramlength (bytes)

upper layer protocolto deliver payload to

head.len

type ofservice

“type” of data flgsfragment

offsetupper layer

32 bit destination IP address

Options (if any) E.g. timestamp,record routetaken, specifylist of routers to visit.

Page 5: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

IP Fragmentation & Reassembly• network links have MTU

(max.transfer 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 destination– IP header bits used to

identify, order related fragments

fragmentation: in: one large datagramout: 3 smaller datagrams

reassembly

Page 6: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

IP Fragmentation and Reassembly

ID=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

Example• 4000 byte datagram• MTU = 1500 bytes

1480 bytes in data field

offset =1480/8

Page 7: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

IP Addressing: Introduction• IP address: 32-bit identifier

for host or router interface • interface: connection

between host/router and physical link– routers typically have

multiple interfaces– host may have multiple

interfaces– IP addresses are

associated with each interface

– Link can be multipoint-link, e.g. LAN – or even entire network, e.g., ATM

• Key point: no routing table lookup is necessary to get to destination within subnet

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

223.1.1.1 = 11011111 00000001 00000001 00000001

223 1 11

Page 8: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

Subnets• IP address:

– subnet part (high order bits)

– host part (low order bits)

• What’s a subnet ?– (a set of) device

interfaces with a common subnet part of IP address

– can physically reach each other without intervening router

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

network consisting of 3 subnets

LAN

Page 9: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

Subnets 223.1.1.0/24223.1.2.0/24

223.1.3.0/24

Recipe• To determine the

subnets, detach each interface from its host or router, creating islands of isolated networks. Each isolated network is called a subnet– And needs its

own subnet address!

Subnet mask: /24

255.255.255.0

Page 10: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

SubnetsHow many? 223.1.1.1

223.1.1.3

223.1.1.4

223.1.2.2223.1.2.1

223.1.2.6

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.2

223.1.7.1

223.1.7.2223.1.8.2223.1.8.1

223.1.9.1

223.1.9.2

Page 11: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

Addressing in IP• IP addresses denote

interfaces, not hosts• Sets of interfaces form

subnets– Subnets share

common prefix• Route to CIDR-ized

subnet addresses– a.b.c.d/x

• Within subnet, reach destination directly

223.1.1.1

223.1.1.3

223.1.1.4

223.1.2.2223.1.2.1

223.1.2.6

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.2

223.1.7.1

223.1.7.2223.1.8.2223.1.8.1

223.1.9.1

223.1.9.2

Page 12: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

IP Addressing: CIDRCIDR: Classless InterDomain Routing

– subnet portion of address of arbitrary length– address 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

Page 13: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

Before CIDR: Classful Routing

• A, B, C: Pretty much only of historical interest today

Page 14: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

Special IP Addresses

Page 15: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

R2

R1

R3

Internet

__________

EthernetLAN 160 Machines

EthernetLAN 2120 Machines

Subnet address:______________Default gateway:______________

Subnet address:______________Default gateway:______________

__________

__________

__________

__________

__________PPP Link 1

PPP Link 2

Page 16: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

R2

R1

R3

Internet

191.23.25.198

EthernetLAN 160 Machines

EthernetLAN 2120 Machines

Subnet address:191.23.25.128/26Default gateway:191.23.25.129

Subnet address:191.23.25.0/25Default gateway:191.23.25.1

191.23.25.1

191.23.25.193

191.23.25.197

191.23.25.194

191.23.25.129PPP Link 1

PPP Link 2191.23.25.192/30

191.23.25.196/30

Page 17: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

Routing Tables in End Systems• Typical: local subnets + default gateway (“first-

hop router”)• Example: “route print” on Windows XP

– 128.173.55.90 FastEthernet– 192.82.175.230 802.11g wireless

Active Routes:Network Destination Netmask Gateway Interface Metric 0.0.0.0 0.0.0.0 128.173.48.1 128.173.55.90 20 0.0.0.0 0.0.0.0 198.82.174.1 198.82.175.230 25 127.0.0.0 255.0.0.0 127.0.0.1 127.0.0.1 1 128.173.48.0 255.255.248.0 128.173.55.90 128.173.55.90 20 198.82.174.0 255.255.254.0 198.82.175.230 198.82.175.230 25 …Default Gateway: 128.173.48.1

Page 18: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

ICMP: Internet Control Message Protocol

• used by hosts & routers to communicate network-level information– error reporting:

unreachable host, network, port, protocol

– echo 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

Page 19: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

Traceroute and ICMP

• Source sends series of UDP segments to dest– 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

ICMP message (type 11, code 0)

– Message includes name of router& IP address

• When ICMP message arrives, source calculates RTT

• Traceroute does this 3 times

Stopping criterion• UDP segment eventually

arrives at destination host• Destination returns ICMP

“port unreachable” packet (type 3, code 3)

• When source gets this ICMP, stops.

• See also [Heideman 2008]

Page 20: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

IP addresses: how to get one?• Host gets IP address either hardcoded or via

DHCP (Dynamic Host Configuration Protocol)• Network gets subnet part of IP address allocated

from ISP’s address space• ISP gets address space assigned by ICANN

(Internet Corporation for Assigned Names and Numbers)

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

Page 21: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

IPv6

Page 22: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

IPv6• Initial motivation: 32-bit address space soon to be

completely allocated. • Additional motivation:

– header format helps speed processing/forwarding– header changes to facilitate QoS– easier configuration of both hosts & backbone routers

IPv6 datagram format: – fixed-length 40 byte header– no fragmentation allowed

Page 23: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

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

Page 24: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

Other Changes from IPv4

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

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

Big”– multicast group management functions

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

Page 25: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

Extension Headers

• Grouped in six types:– Hop-by-hop options, e.g. Jumbograms– Destination options– Routing, e.g. source routing– Fragment – can be done, but end hosts only!– Authentication– Encapsulation

• Routers quickly know which headers they must examine and which they can skip

Page 26: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

IPv6 Addresses

• Written as eight 16bit values– e.g. fe80::020e:7bff:fe32:d716 (made from 00:0E:7B:32:D7:16)

0000 0000 Reserved

0000 0001 Unassigned

0000 001 Reserved for NSAP (non-IP addresses used by ISO)

0000 010 Reserved for IPX (non-IP addresses used by IPX)

0000 011 - 0001 Unassigned

001 Unicast Address Space

010 - 110 Unassigned

1110 - 1111 1110 0 Unassigned

1111 110 Unique Local Addresses (ULA)

1111 1110 10 Link Local Use addresses

1111 1110 11 Site Local Use addresses (Deprecated)

1111 1111 Multicast addresses

Page 27: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

IPv6 autoconf

• stateless autoconfiguration see [Donzé 2004]– Plug in and interface creates link-local address based

on adapter MAC– Interface can have link-local (fe80::…), site-local &

global (2001::…) addresses• VT’s campus has had IPv6 testbed since 1998,

now connected to public IPv6 network• Try it out yourself!

– MacOS, Linux: enabled by default of recent installations

– Windows XP: “ipv6 install” at command prompt– Tools add 6: ping6, traceroute6, etc..

Page 28: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

Transition From IPv4 To IPv6

• Not all routers can be upgraded simultaneously– no “flag days”– How will the network operate with mixed IPv4

and IPv6 routers? • Tunneling: IPv6 carried as payload in IPv4

datagram among IPv4 routers

Page 29: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

Tunneling

A B E F

IPv6 IPv6 IPv6 IPv6

tunnelLogical view:

Physical view:A B E F

IPv6 IPv6 IPv6 IPv6

C D

IPv4 IPv4

Flow: XSrc: ADest: F

data

Flow: XSrc: ADest: F

data

Flow: XSrc: ADest: F

data

Src:BDest: E

Flow: XSrc: ADest: F

data

Src:BDest: E

A-to-B:IPv6

E-to-F:IPv6

B-to-C:IPv6 inside

IPv4

B-to-C:IPv6 inside

IPv4

Page 30: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

IPv6 – Opposing View

• Bernstein points out some hindrances [The IPv6 mess]– Lack of interoperability b/c no embedding of addresses– Transition path (comparison to MX records)

• IPv6 – the next OSI?• DoD requirement by 2008

– What happened to it?• Federal 2012 deadline that all public-facing

websites talk IPv6• Asian countries are pushing for transition

Page 31: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

ROUTING IN THE INTERNET

CS 4284 Spring 2013

Page 32: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

Hierarchical Addressing: Route Aggregation

“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

...

...

Hierarchical addressing allows efficient advertisement of routing information:

Page 33: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

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

...

...

Page 34: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

Intra-AS vs Inter-AS Routing• In Internet:

– Intra-AS known as Interior Gateway Protocols (IGP)

– Most common Intra-AS routing protocols:• RIP: Routing Information Protocol (original protocol,

now rarely used)• OSPF: Open Shortest Path First• IGRP/EIGRP: (Enhanced) Interior Gateway Routing

Protocol – Inter-AS known as Border Gateway Protocols:

• BGP4: Only protocol used

Page 35: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

RIP (Routing Information Protocol)• Distance vector algorithm

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

DC

BA

u v

w

x

yz

destination hops u 1 v 2 w 2 x 3 y 3 z 2

A’s routing table

Page 36: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

RIP: Example

Destination Network Next Router Num. of hops to dest.

w A 2y B 2

z B 7x -- 1…. …. ....

w x y

z

A

C

D B

Routing table in D

Page 37: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

RIP: Example

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

y B 2 z B A 7 5

x -- 1…. …. ....

w x y

z

A

C

D B

Dest Next hops w - - x - - z C 4 …. … ...

Advertisementfrom A to D

Routing table in D

Page 38: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

RIP: Link Failure and Recovery If no advertisement heard after 180 sec →

neighbor/link declared dead– routes via neighbor invalidated– new advertisements sent to neighbors– neighbors in turn send out new advertisements

(if tables changed)– poison reverse used to prevent ping-pong loops

(infinite distance = 16 hops)

Page 39: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

RIP Table processing

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

• advertisements sent in UDP packets, periodically repeated

physical

link

network forwarding (IP) table

Transprt (UDP)

routed

physical

link

network (IP)

Transprt (UDP)

routed

forwardingtable

Page 40: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

EIGRP

• Cisco proprietary– See [Cisco Whitepaper], [Malhotra 2002]

• Distance Vector Protocol with enhancements– Explicit Signaling (HELLO packets)

• DUAL “diffusing update algorithm”– “feasible successor” concept guarantees loop freedom

• Intuition: rather than count to infinity, trigger route recomputation unless another loop-free path is known– Optimize this by keeping track of all advertised routes,

not just best one

Page 41: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

OSPF (Open Shortest Path First)• “open”: publicly available protocol (not

proprietary)• Uses Link State algorithm

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

• OSPF advertisement carries one entry per neighbor router– Advertisements have age field to allow for expiration

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

than TCP or UDP)

Page 42: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

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.

Page 43: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

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Hierarchical OSPF

Page 44: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

Hierarchical OSPF• Two-level hierarchy: local area, backbone.

– link-state advertisements only in same 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.

Page 45: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

Internet Inter-AS routing: BGP

• BGP (Border Gateway Protocol): the de facto standard

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

neighboring ASs.2. Propagate the reachability information to all routers

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

reachability information and policy.• Allows a subnet to advertise its existence to rest

of the Internet: “I am here”

Page 46: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

BGP Basics

3b

1d

3a

1c2aAS3

AS1

AS21a

2c

2b

1b

3c

eBGP session

iBGP session

• Pairs of routers (BGP peers) exchange routing info over semi-permanent TCP conctns: BGP sessions

• Note that BGP sessions do not always 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

Page 47: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

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 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.

3b

1d

3a

1c2aAS3

AS1

AS21a

2c

2b

1b

3c

eBGP session

iBGP session

Page 48: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

Path Attributes & BGP Routes

• When advertising a prefix, advert includes BGP 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 AS to next-hop-AS.)

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

Page 49: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

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 (like DV routing, except

with more information!)3. Closest NEXT-HOP router: hot potato routing4. Additional criteria

Page 50: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

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Path Vector Routing in BGP

• Accomplished via AS-PATH attributes– Each node is entire AS!

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BGP routing policy

Figure 4.5-BGPnew: a simple BGP scenario

A

B

C

W X

Y

legend:

customer network:

provider network

• A,B,C are provider networks• X,W,Y are customer (of provider networks)• X is dual-homed: attached to two networks

– X does not want to route from B via X to C– .. so X will not advertise to B a route to C

Page 52: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

BGP routing policy (2)

Figure 4.5-BGPnew: a simple BGP scenario

A

B

C

W X

Y

legend:

customer network:

provider network

• A advertises to B the path AW • B advertises to X the path BAW • Should B advertise to C the path BAW?

– No way! B gets no “revenue” for routing CBAW since neither W nor C are B’s customers

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

Page 53: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

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Relationship between OSPF&BGP

• OSPF hierarchyis intra-AS

• BGP connectsASs

Page 54: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

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Motivation for different Intra/Inter Protocols

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

Page 55: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

Usage of Routing Protocols

• Sample obtained by reverse-engineering router config files

• Source David Maltz et al:– Routing Design in Operational Networks – A

Look from the inside, [SIGCOMM 2004]

EBGPSessions

IGP

OSPF EIGRP RIP Total

Intra- 1,490 9,624 12,741 156 22,521

Inter- 13,830 1,161 1,342 161 2,664

Page 56: CS 4284 Systems Capstone Godmar Back Networking. IPV4 CS 4284 Spring 2013.

CS 4284 Spring 2013

Summary

• IP– Addressing, subnets

• ICMP• RIP• OSPF• BGP


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