CIS 553: Networked Systemscis553/slides/11-Interdomain.pdf · Inter-domain routing: between ASes n ASes want freedom in picking routes n “My traffic can’t be carried over my competitor’s

CIS 553: Networked Systems

Interdomain Routing

February 24, 2020

Agenda

n Intradomain Routingn Link State

n Layer-3 Checkpointn Interdomain Routing

n Structure of the Internetn Policyn BGPn Issues with BGP

2University of Pennsylvania

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The Internet: A Network of Networks

n No central authority that manages Internet interconnection

n Overall system arises because of many bilateral and multilateral decisions

ComcastAbilene

AT&T Cogent

Penn

The Internet


Context and terminology“End hosts”

“Clients”, “Users”“End points”

“Interior Routers”

“Autonomous System (AS)” or “Domain”Region of a network under a single administrative entity

“Route” or “Path”

“Border Routers”

Topology of the Internet is the graph of ASes and connections between them4

University of Pennsylvania

Interconnection Pre-1995Network interconnection in the U.S. has evolved significantly since the early days of the Internet.

GovernmentBackbone

RegionalAccess

Networks

LocalArea

Networks

ARPANET

WIDEBAND NET

SATNETPacket Radio NetworksSatellite Networks

Stanford Utah … NTARE

UCL RSRE DFVLR

BBN-NET

EDN

BBN-FIBER

BBN-ETHER

TELENET/IPSS/PSS

Berkeley Wash ISI


Regional ISP Regional ISP Regional ISP

ISP 1 ISP 3 ISP 4 …ISP 2

Consumers and Business Customers

NationalBackboneOperators

RegionalAccess

Providers

LocalAccess

Providers

Customer IPNetworks

PeeringTransit

Backbone ProviderBackbone Provider

Interconnection Circa 1995The backbone eventually transitioned from a single government-operated backbone to a federated backbone model comprised of multiple commercial network operators.


Tier 1-3 networksn Tier 1 – Global ISPs. A network that peers with every other

network to reach the Internet.n AT&Tn CenturyLinkn Verizonn NTT Communicationsn GTT Communicationsn Zayo Groupn Sprint

n Tier 2 – Peers with some networks, but still purchases IP transit to reach at least some portion of the Internet.

n Tier 3 - Solely purchases transit from other networks to reach the Internet.


Interconnection TodayInterconnection today has evolved into a complex amalgam of models incorporating new connectivity options, delivery options, traffic management requirements and business practices. (“Flattening” of the hierarchy.)

National ISP

Regional ISP

Consumers and Business Customers

NationalBackboneOperators

RegionalAccess

Providers

Customer IPNetworks

PeeringTransit

Backbone ProviderBackbone Provider

Regional ISP Regional ISP

Regional ISPCDN

CDN

CDNCDN

CDN

Large Content, C

onsumer, H

osting CD

N


Inter-domain routing: between ASes

n ASes want freedom in picking routesn “My traffic can’t be carried over my competitor’s network”n “I don’t want to carry A’s traffic through my network”n Not expressible as Internet-wide “least cost”

n ASes want autonomyn Want to choose their own internal routing protocoln Want to choose their own policy

n ASes want privacyn Choice of network topology, routing policies, etc.


Choice of routing algorithm

n Link-staten Scaling limitationsn No privacy – broadcasts all network information n Limited autonomy – needs agreement on metric, algo

n Distance-vector is what they chosen Per-destination updates give some controln BUT wasn’t designed to implement policy n AND is vulnerable to loops

n The “Border Gateway Protocol” (BGP) extends distance-vector to accommodate policy


Agenda





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Policy dictates how routes are “selected” and “exported”

n Selection: Which path to use?n Controls whether/how traffic leaves the network

n Export: Which path to advertise?n Controls whether/how traffic enters the network

Can reach128.3/16blah blah

Route selection

A

P

C

B

Q

Route export


Topology & policy shaped by inter-AS business relationship

peer peerprovider customer

Business implicationsn Customers pay providern Peers don’t pay each other


Why peer?

A

B C

D E

D and E communicate a lot

Peering savesB and C money


Routing follows the money!

n ASes provide “transit” between their customersn Peers do not provide transit between other peers

A B C

D E F

Q Pr CuPeer Peer

traffic allowed traffic not allowed



A B C

D E F

Q Pr CuPeer Peer

1. An AS only carries traffic to/from its own customers over a peering link

???



A B

E

Pr CuPeer Peer

2. Routes are “valley” free (more details later)

???


In short

n AS topology reflects business relationships between ASes

n Business relationships between ASes impact which routes are acceptable


Typical selection policy

n In decreasing order of priorityn Make/save money (send to customer > peer > provider)n Maximize performance (smallest AS path length) n Minimize use of my network bandwidth (“hot potato”)n …


Typical export policy

Destination prefix advertised by… Export route to…

Customer Everyone (providers, peers, other customers)

Peer Customers

Provider Customers

We’ll refer to these as the “Gao-Rexford” rules(capture common – but not required! – practice)


Gao-Rexford Rules

Peers

Providers

Customers

With Gao-Rexford, the AS policy graph is a DAG (directed acyclic graph) and routes are “valley free”


Agenda





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BGP: Basic idea

Each AS selects the “best” route it hears

advertised for a prefix

An AS advertises (“exports”) its best routes to one or more IP prefixes

n Like DV:n Per-destination route advertisements n No global sharing of network topology informationn Iterative and distributed convergence on paths

n With four crucial differences!23

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BGP & DV differences:

(1) Not picking shortest-path routesn BGP selects the best route based on policy, not

shortest distance (i.e., least-cost) n AS A may prefer “A,B,C” over “A,C”

A B

C



(2) Path-Vector routing n Key idea: advertise the entire path

n Distance vector: send distance metric per destinationn Path vector: send the entire path for each destination

C B A

dest d

“d: path (B,A)” “d: path (A)”

data traffic data traffic



(2) Path-Vector routing n Key idea: advertise the entire path

n Distance vector: send distance metric per destinationn Path vector: send the entire path for each destination

n Benefitsn Loop avoidance is straightforward (simply discard paths with loops)n Flexible and expressive policies based on entire path



(3) Selective route advertisement n For policy reasons, an AS may choose not to

advertise a route to a destination n Hence, reachability is not guaranteed even if graph is

physically connected

A B

C

AS-C does not want to carry traffic to AS-B


BGP & DV differences:(4) BGP may aggregate routes n For scalability, BGP may aggregate routes for

different prefixesn Again, not common today

AT&Ta.0.0.0/8

France Telecom

baz.coma.b.0.0/16

bar.coma.c.0.0/16

a.*.*.* is this way

foo.coma.d.0.0/16


BGP sessions: External

Border routers in an AS speaks BGP with border routers in other ASes using eBGP sessions


BGP sessions: Internal

A border routers speaks BGP with other routers in the same AS using iBGP sessions


eBGP, iBGP, and IGP

n eBGP: BGP sessions between border routers in different ASesn Learn routes to external destinations

n iBGP: BGP sessions between border routers and other routers within the same ASn Distribute externally learned routes internally

n IGP: “Interior Gateway Protocol” = Intra-domain routing protocoln Provide internal reachability n E.g., OSPF, RIP


Documents

CIS 553: Networked Systemscis553/slides/11-Interdomain.pdf · Inter-domain routing: between ASes n ASes want freedom in picking routes n “My traffic can’t be carried over my competitor’s