[PDF] - CS 457 Networking and the Internet Fall 2016 The Global Internet PDF Document

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CS 457 Networking and the Internet

Fall 2016

The Global Internet (Then)

The tree structure of the Internet in 1990

The Global Internet (And Now)

A simple multi-provider Internet

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The Global Internet

Some large corporations connect directly to one or

more of the backbone, while others connect to smaller, non-backbone service providers.

Many service providers exist mainly to provide

service to “consumers” (individuals with PCs in their homes), and these providers must connect to the backbone providers

Often many providers arrange to interconnect with

each other at a single “peering point”

Autonomous Systems

Internet is organized as autonomous systems (AS)

each of which is under the control of a single administrative entity

Autonomous System (AS)
corresponds to an administrative domain
examples: University, company, backbone network
A corporation’s internal network might be a single AS,

as may the network of a single Internet service provider

Autonomous Systems

A network with two autonomous system

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Route Propagation

Idea: Provide an additional way to hierarchically aggregate

routing information in a large internet.

– Improves scalability

Divide the routing problem in two parts:

– Routing within a single autonomous system – Routing between autonomous systems

Another name for autonomous systems in the Internet is

routing domains

– Two-level route propagation hierarchy

Inter-domain routing protocol (Internet-wide standard)
Intra-domain routing protocol (each AS selects its own)

Routing by AS

Uses an interior gateway protocol (IGP) and

common metrics to route packets within the AS (Intra-AS)

Uses an exterior gateway protocol (EGP) to

route packets to other AS’s (Inter-AS)

AS may use multiple IGPs and metrics, but

appears as single AS to other AS’s

IGP and EGP Example

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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 performance
Inter-AS: policy may dominate over performance

Inter-AS Routing - EGP and BGP

Exterior Gateway Protocol (EGP)

– Forced a tree-like topology onto the Internet – Did not allow for the topology to become general

Tree like structure: there is a single backbone and

autonomous systems are connected only as parents and children and not as peers

Border Gateway Protocol (BGP)

– Assumes that the Internet is an arbitrarily interconnected set

f ASs.

– Today’s Internet consists of an interconnection of multiple backbone networks (they are usually called service provider networks, and they are operated by private companies rather than the government)

Sites are connected to each other in arbitrary ways
The goal of Inter-domain routing is to find

any path to the intended destination that is loop free

– We are concerned with reachability than

ptimality

– Finding path anywhere close to optimal is considered to be a great achievement

Why?

BGP

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Path Vectors

Each routing update carries the entire path
Loops are detected as follows:

– When AS gets route check if AS already in path

If yes, reject route
If no, add self and (possibly) advertise route further
Advantage:

– metrics are local - AS chooses path, protocol ensures no loops

Scalability: An Internet backbone router must be able to

forward any packet destined anywhere in the Internet

– Having a routing table that will provide a match for any valid IP address

Autonomous nature of the domains

– It is impossible to calculate meaningful path costs for a path that crosses multiple ASs – A cost of 1000 across one provider might imply a great path but it might mean an unacceptable bad one from another provider

Issues of trust

– Provider A might be unwilling to believe certain advertisements from provider B

BGP Philosophy

BGP does not belong to either of the two

main classes of routing protocols (distance vectors and link-state protocols)

BGP advertises complete paths as an

enumerated lists of ASs to reach a particular network

BGP

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BGP-4: Border Gateway Protocol

Assumes the Internet is an arbitrarily interconnected set of

AS's.

Define local traffic as traffic that originates at or

terminates on nodes within an AS, and transit traffic as traffic that passes through an AS.

We can classify AS's into three types:
Stub AS: an AS that has only a single connection to one other AS;

such an AS will only carry local traffic

Multihomed AS: an AS that has connections to more than one other AS,

but refuses to carry transit traffic

Transit AS: an AS that has connections to more than one other AS, and is

designed to carry both transit and local traffic

BGP

Stub AS Multihomed AS Transit AS

BGP Example

Example of a network running BGP

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Each AS has:

One BGP speaker that advertises:

– local networks – other reachable networks (transit AS only) – gives path information

In addition to the BGP speakers, the AS has one or more

border “gateways” which need not be the same as the speakers

The border gateways are the routers through which packets

enter and leave the AS

BGP BGP Example

Speaker for AS 2

advertises reachability to P and Q

Network 128.96,

192.4.153, 192.4.32, and 192.4.3, can be reached directly from AS 2.

Speaker for backbone

network then advertises

Networks 128.96,

192.4.153, 192.4.32, and 192.4.3 can be reached along the path <AS 1, AS 2>.

Speaker can also cancel

previously advertised paths

BGP Issues

It should be apparent that the AS numbers

carried in BGP need to be unique

For example, AS 2 can only recognize itself

in the AS path in the example if no other AS identifies itself in the same way

AS numbers are 16-bit numbers assigned by a

central authority

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Policy With BGP

BGP provides capability for enforcing

various policies

Policies are not part of BGP: they are

provided to BGP as configuration information

BGP enforces policies by choosing paths

from multiple alternatives and controlling advertisement to other AS’s

Examples of BGP Policies

A multi-homed AS refuses to act as transit

– limit path advertisement

A multi-homed AS can become transit for some

AS’s

– only advertise paths to those AS’s

An AS can favor or disfavor certain AS’s for

traffic transit from itself

– Pick appropriate routes by examining path vectors

BGP Is NOT Needed If:

Single homed network

(stub)

AS does not provide

downstream routing

AS uses a default

route

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Routing Information Bases (RIB)

Routes are stored in RIBs
Adj-RIBs-In: routing info that has been

learned from other routers (unprocessed routing info)

Loc-RIB: local routing information selected

from Adj-RIBs-In (routes selected locally)

Adj-RIBs-Out: info to be advertised to

peers (routes to be advertised)

BGP Messages

Open

– Opens a BGP connection (establishes a TCP connection)

Update

– Withdrawn routes – New routes that include path attributes e.g., origin, path

Notification

– Used for error notification - TCP connection is closed immediately after notification

Keep alive

– Sent periodically to peers to ensure connectivity – sent in place of an update message

BGP: Controlling Who Routes To You

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

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Internet Inter-AS routing: BGP

Suppose: gateway X sends its path to peer gateway W

W may or may not select path offered by X

– cost, policy (don’t route via competitors AS), loop prevention reasons.

If W selects path advertised by X, then:

Path (W,Z) = w, Path (X,Z)

Note: X can control incoming traffic by controlling it route

advertisements to peers:

– e.g., don’t want to route traffic to Z -> don’t advertise any routes to Z

BGP: Controlling Who Routes To You

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!

BGP Operation

Q: What does a BGP router do?

Receiving and filtering route advertisements from directly

attached neighbor(s).

Route selection.

– To route to destination X, which path (of several advertised) will be taken?

Sending route advertisements to neighbors.

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Integrating Interdomain and Intradomain Routing

All routers run iBGP and an intradomain routing

protocol. Border routers (A, D, E) also run eBGP to other

ASs

Internal v.s. External BGP

R3 R4 R1 R2 BGP

BGP can be used by R3 and R4 to learn routes.
How do R1 and R2 learn routes?
Option 1: Inject routes in IGP
only works for small routing tables
Option 2: Use I-BGP

Internal BGP (I-BGP)

Same messages as E-BGP
Different rules about re-advertising

prefixes:

– prefix learned from E-BGP can be advertised to I-BGP neighbor and vice-versa, but – prefix learned from one I-BGP neighbor cannot be advertised to another I-BGP neighbor – reason: no AS PATH within the same AS and thus danger of looping

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Internal BGP (I-BGP)

R3 R4 R1 R2 E-BGP I-BGP

R3 can tell R1 and R2 prefixes from R4
R3 can tell R4 prefixes from R1 and R2
R3 cannot tell R2 prefixes from R1

R2 can only find these prefixes through a direct connection to R1 Result: I-BGP routers must be fully connected (via TCP)!

contrast with E-BGP sessions that map to physical links