unit 16 global internetworkinginst.eecs.berkeley.edu/~ee122/sp07/lec16.pdf · 2007. 3. 16. ·...

Introduction to Communication Networks Spring 2007

EECS 122 SPRING 2007

Unit 16Global Internetworking

2 of 54Prof. Adam WoliszEECS 122 SPRING 2007

Acknowledgements – slides coming from:

• Based almost completely on slides from the earlier issues of the EECS 122 taught recently by Prof Jean Walrand, Prof David Tse, Prof Abhay Parekh, Prof Shyam Parekh, Prof Ion Stoica etc.

• Some slides borrowed from Timothy G. Griffins SIGCOM 2001 tutorial; August 2001, griffin@research.att.com

ISPs... [Steenkiste]

ISPs, ISPs,

• But – the hierarchy is not so strict – there is a lot of choice...

• How many ISPs in my vicinity? in the 510 area code?

www.thelist.com

How Are ISPs related? Peering & Transit• Peering

– The business relationship whereby ISPs reciprocally provide to each other connectivity to each others’ local or “inherited” customers

• Transit– The business relationship whereby one ISP provides (usually sells)

access to all destinations in it’s routing table

William B. Norton, “Internet Service Providers and Peering”

Peering & Transit :Peering

Figure fromWilliam B. Norton, “Internet Service Providers and Peering”

West and East Peer with USNet but they can’t reach each other

Peering & Transit :TransitFigure fromWilliam B. Norton, “Internet Service Providers and Peering”

William B. Norton, “Internet Service Providers and Peering”

Peering & Transit: Benefits of Transit v/s Peering

Nontransit vs. Transit ASes [griffin]

ISP 1ISP 2

NET A Nontransit ASmight be a corporateor campus network.Could be a “content provider”

Traffic NEVER flows from ISP 1through NET A to ISP 2(At least not intentionally!)

IP traffic

Internet Serviceproviders (often)have transit networks

Hierarchical Routing

The internet has many Administrative Domains

Hierarchical Routing … Border Routers

Hierarchical Routing: Interdomain & Intradomain

InterDomainInterDomain

IntraDomain

- IntraDomain routing protocols:Local issue, different

- InterDomain routing protocol:One common!!!

Why Hierarchical Routing?

• Is a natural way for routing to scale

– Size

– Network Administration

– Governance

• Allows multiple metrics at different levels of the hierarchy

• Exploits address aggregation and allocation

Inter DomainRouting OSPF

Internet Routing• Internet organized as a two level hierarchy

– Some intra-domain routing approaches create their own hierarchy (see OSPF)

• Autonomous systems (AS’s)– AS – region of network under a single administrative domain

• AS’s run an intra-domain routing protocols– Distance Vector, Communicate current distance estimates of node to

every other node, uses Bellman_Ford, . e.g., RIP, IGRP,EIGRP– Link State, Communicate the names and costs of neighbors. Each node

maintains the entire topology, uses Dijkstra, e.g., OSPF, ISIS

• Between AS’s runs inter-domain routing protocols, e.g., Border Gateway Routing (BGP)

– De facto standard today, BGP-4, uses Path Vector: Communicate current estimates of preferred paths from node to every other node, source routing!

Overview

A: [B, 2], [C, 1]B: [A, 2], [D, 1]C: [A, 1], [D, 3]D: [B, 1], [C, 3]

1) Exchange Link States 2) Each node computesthe shortest paths tothe others

LINK STATE

DISTANCE VECTOR

PATH VECTOR

“Don’t like B”

Link state vs. Distance Vector

Routing Sub-Functions• Topology Update: Characterize and maintain connectivity

– Discover neighbors

– Measure “distance” (one or more metric)

– Disseminate

• Route Computation:– Kind of path: Multicast, Unicast

– Centralized or Distributed Algorithm

– Policy

– Hierarchy

• Switching: Forward the packets at each node

OSPF: Topology update – Flooding!!! [steenkiste]

Cost of the link might be split according to different Type of Service metrics: e.g. high for delay, low for bit error rate...

(Link ID and metric repeated for each link)

OSPF link-state advertisement [Peterson]

LS Age Options Type=1

0 Flags 0 Number of links

Link type Num_TOS Metric

Link-state IDAdvertising router

LS sequence number

Link IDLink data

Optional TOS informationMore links

LS checksum Length

Packet used to advertise links...

OSPF: Topology update – processing [steenkiste]

Flooding- More issues [steenkiste]

Some Issues• What happens if some routers are much faster at

transmitting LSPs?

• What happens if sequence numbers wrap?

• What happens when a partitioned network is reconstituted?

• What about security?

• Etc., etc.

• Many lines of code

OSPF allows for some local hierarchy!

This hierarchy simplifies routing (complexity of Dijkstra!!), but might introduceLonger routes...

Remember CIDR?

Hierarchical Adressing helps in Routing structuring

ISPs divide the available addresses...

ISPs... [Steenkiste]

How can I learn who owns an IP address?Go to the interactive ARIN WHOIS data base:http://www.arin.net/whois/

Search Sample (line borders edited for readability!)

• Search results for: 216.35.221.77• OrgName: Savvis OrgID: SAVVI-2• Address: 3300 Regency Parkway_ City: Cary_StateProv: NC_PostalCode:

27511_Country: US • ReferralServer: rwhois://rwhois.savvis.net:4321/ • NetRange: 216.32.0.0 - 216.35.255.255• CIDR: 216.32.0.0/14 • NetName: SAVVIS NetHandle: NET-216-32-0-0-1 Parent: NET-216-0-0-0-0• NetType: Direct Allocation • NameServer: DNS01.SAVVIS.NET _NameServer: DNS04.SAVVIS.NET• Comment: RegDate: 1998-07-30 _Updated: 2004-10-07 • OrgAbuseHandle: ABUSE11-ARIN _OrgAbuseName: Abuse Org• AbusePhone: +1-877-393-7878 _OrgAbuseEmail: abuse@savvis.net• OrgNOCHandle: NOC99-ARIN _OrgNOCName: SAVVIS Support Center

OrgNOCPhone: + 1-888-638-6771 _OrgNOCEmail: ipnoc@savvis.net OrgTechHandle: UIAA-ARINOrgTechName: US IP Address Administration

• OrgTechPhone: + 1-888-638-6771 _OrgTechEmail: ipadmin@savvis.net

The hierarchy again...

This hierarchy simplifies routing (complexity of Dijkstra!!), but might introduceLonger routes...

What about the Forwarding Table?

1c2aAS3

AS1AS2

Inter-AS tasks

• Suppose router in AS1 receives datagram for dest outside of AS1

– Router should forward packet towards an AS-border router, but which one?

AS1 needs:

1. to learn which dests are reachable through AS2 and which through AS3

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

Job of inter-AS routing!

R border router

internal router

you can reachnet A via me

traffic to A

table at R1:dest next hopA R2

Share connectivity information across ASes

Concept

TOC – IP – Routing – Types – Interdomain – BGP – Concept

Advertising a route means readiness to carry traffic!!!

Border Routers have to communicate

Forwarding Table

Join EGP with IGP For Connectivity [griffin]

AS 1 AS 2192.0.2.1

135.207.0.0/16

10.10.10.10

192.0.2.1135.207.0.0/16

destination next hop

10.10.10.10192.0.2.0/30

135.207.0.0/16Next Hop = 192.0.2.1

192.0.2.0/30

135.207.0.0/16

10.10.10.10

192.0.2.0/30 10.10.10.10

192.0.2.0/24

192.0.2.0/24Accidental or maliciousannouncement of your prefixcan blackhole your destinations in large part of the Internet

peer peer

customerprovider

Need Filter Here!

legitimate

not legitimate

Danger: Blackholes

Internet inter-AS routing: BGP

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

• BGP provides each AS a means to:1. Obtain subnet reachability information from neighboring ASs2. Propagate reachability information to all routers in the AS3. Determine “good” routes to subnets based on reachability

information and routing policy.

• Allows a subnet to advertise its existence to rest of the Internet: “I am here”

• Issues: – Which routing algorithm?– How are routes advertised?– How to implement routing policies?

Some BGP principles.

Pairs of routers (BGP peers) exchange routing info over semipermanent TCP connections: BGP sessions

Note that BGP sessions do not correspond to physical links

BGP is a path vector protocol• Distance vector algorithm with extra information

– Pure distance vector does not enable policies

– Pure link state does not scale and exposes policies

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

• Two important attributes:– AS-PATH: contains all ASs along the way: AS 67 AS 17

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

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

– Can make decision based on ASes on path

– Can easily avoid loops

BGP Routing Table

Peers exchange BGP messages using TCP• OPEN:

– opens TCP conn. to peer– authenticates sender

• UPDATE:– advertises new path (or withdraws old)– Path attributes: e.g. multiple exit discriminators…

• KEEPALIVE:– keeps connections alive in absence of UPDATES, ACKs OPEN request

• NOTIFICATION:– reports errors in previous msg; closes a connection

Process:– Initialization: Open => Updates for all routes

– Ongoing: Updates for changed routes

The Philosophy: Reachability• Interdomain routing is about implementing policies of

reachability– Routing efficiency and performance is important, but not essential

• ISPs could be competitors and do not want to share internal network statistics such as load and topology

• Router learn > 1 route to some prefix

• Router must select what they believe is the best route!

• Elimination rules:1.Local preference value attribute: policy decision2.Shortest AS-PATH (the minimal number of AS on the way…)3.Best MED (multi-exit-discriminator) (announced preferred entry router)

4.Closest NEXT-HOP router: hot potato (out of my AS!) routing5.Additional criteria 6.IP address of peer router (just solving the stalemate…)

BGP Route Processing

Best RouteSelection

Apply ImportPolicies

BGP Route Table

Apply ExportPolicies

Install Best Routes

IP Forwarding Table

ReceiveBGPUpdates

Best andAlternate Routes

Apply policiesonly to Best Routes!

TransmitBGP Updates

Apply Policy =filter routes & tweak attributes

Based onAttributeValues

Routing policy

• Reflects goals of network provider– which routes to accept from other ASes

– how to manipulate the accepted routes

– how to propagate routes through network

– how to manipulate routes

before they leave the AS

– which routes to send to another AS

BGP routing policy – an example

legend:

customer network:

providernetwork

• 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

B, C = Peers

BGP routing policy, example (cont)

legend:

customer network:

providernetwork

• 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!

B, C = Peers

Prof. Adam WoliszEECS 122 SPRING 2007

Local Preference Attribute

AS 400

AS 200

160.10.0.0/16AS 100

AS 300

160.10.0.0/16 500> 160.10.0.0/16 800

500 800

• Allows providers to prefer routes within his AS -Path with highest local preference wins

– Local preference? Frequently „Hot potato“ – shortest way out of my network!!

Multi-exit discriminator...

Why different Intra- and Inter-AS routing?Policy:• Inter-AS: admin wants control over how its traffic routed, who routes

through its net. 1.Local preference value attribute: policy decision2. Shortest AS-PATH (the minimal number of AS on the way…)3. Best MED (multi-exit-discriminator) (announced preferred entry router)4. Closest NEXT-HOP router: hot potato (out of my AS!) routing5. Additional criteria 6. IP address of peer router (just solving the stalemate…)

• Intra-AS: single admin, so no policy decisions needed

Scale:• hierarchical routing saves table size, reduced update traffic

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

We need BOTH!

Midterm Scores: Avg=64, Sdev= 13.2

1 2 3 4 5 6 7 8 9

Series1

occurencies

Points*1010

The Leader: 95

unit 16 global internetworkinginst.eecs.berkeley.edu/~ee122/sp07/lec16.pdf · 2007. 3. 16. ·...

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