Chuyên đề mạng thế hệ mới mạng 6a bgp interdomain

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Interdomain Routing

BGP

Prométhée Spathis promethee.spathis@lip6.fr

Thème NPA, LIP6 Paris, FRANCE

Goals of Today’s Lecture

• Challenges of interdomain routing – Scale, privacy, and policy – Limitations of link-state and distance-vector routing

• Path-vector routing – Faster loop detection than distance-vector routing – More flexibility than shortest-path routing

• Border Gateway Protocol (BGP) – Incremental, prefix-based, path-vector protocol – Programmable import and export policies – Multi-step decision process for selecting “best” route

• Multiple routers within an AS

• BGP convergence delay

• IP addressing

– Address allocation blocks

– Packet forwarding

• Routing protocols

– Autonomous Systems

– Interdomain routing

– Intradomain routing

Background

IP Address : 12.4.0.0 IP Mask: 255.254.0.0

Address

Mask

for hosts Network Prefix

Use two 32-bit numbers to represent a network

Network number = IP address + Mask

Usually written as 12.4.0.0/15

Classless Inter-Domain Routing (CIDR)

Scalability: Address Aggregation

Provider is given 201.10.0.0/21

201.10.0.0/22 201.10.4.0/24 201.10.5.0/24 201.10.6.0/23

Provider

Routers in the rest of the Internet just need to know how to

reach 201.10.0.0/21 The provider can direct the IP packets

to the appropriate customer

8

Hierarchical Addressing: more specific routes

201.10.0.0/21

201.10.0.0/22 201.10.4.0/24 201.10.5.0/24 201.10.6.0/23

Provider 1 Provider 2

Multi-homed customer with 201.10.6.0/23 has two providers

Other parts of the Internet need to know how to reach these

destinations through both providers.

9

Scalability Through Hierarchy

• Hierarchical addressing

– Critical for scalable system

– Don’t require everyone to know everyone else

– Reduces amount of updating when something changes

• Non-uniform hierarchy

– Useful for heterogeneous networks of different sizes

– Classless InterDomain Routing (CIDR) helps

• Destination-based forwarding

– Packet has a destination address

– Router identifies longest-matching prefix

– Cute algorithmic problem: very fast lookups

16

R

R R

A

B

C

D

R1 R2 R3 R4 R5

E

Net Nxt Hop

R4 R3 R3 R4 Direct

Net Nxt Hop

A B C D E default

R2 R2 Direct R5 R5 R2

Net Nxt Hop

A B C D E default

R1 Direct R3 R1 R3 R1

Default to upstream router

A B C D E

Forwarding: determine next hop Routing: establish end-to-end paths

Forwarding always works Routing can be badly broken Routing vs Forwarding

Routers exchange network reachability information using ROUTING PROTOCOLS

Routers use this to compute best routes

Administrator

manually configures

forwarding table entries

In practice : a mix of these.

Static routing mostly at the “edge”

+ More control

+ Not restricted to

destination-based

forwarding

-Doesn’t scale

- Slow to adapt to

network failures

+ Can rapidly adapt to changes

in network topology + Can be made to scale well

- Complex distributed algorithms

- Consume CPU, Bandwidth, Memory

- Debugging can be difficult

- Current protocols are destination-based

How Are Forwarding Tables Populated to

• Forwarding: data plane – Directing a data packet to an outgoing link – Individual router using a forwarding table

• Routing: control plane – Computing the paths the packets will follow – Routers talking amongst themselves – Individual router creating a forwarding table

What is Routing?

• A famous quotation from RFC 791

“A name indicates what we seek.

An address indicates where it is.

A route indicates how we get there ”

Jon Postel

Internet Routing Architecture

• Divided into Autonomous Systems – Distinct regions of administrative control – Routers/links managed by a single “institution” – Service provider, company, university, …

• Hierarchy of Autonomous Systems – Large, tier-1 provider with a nationwide backbone – Medium-sized regional provider with smaller backbone – Small network run by a single company or university

• Interaction between Autonomous Systems – Internal topology is not shared between ASes – … but, neighboring ASes interact to coordinate routing

Autonomy: network of networks

LIP6

network

DT

AS 1

AS 3

AS 2

• Internet = interconnection of Autonomous Systems (AS)

– Distinct regions of administrative control

– Routers/links managed by a single “institution”

– Service provider, company, university, etc

Autonomous Systems (ASes)

An autonomous system is an autonomous routing domain that has been assigned an Autonomous System Number (ASN).

RFC 1930: Guidelines for creation, selection, and registration of an Autonomous System

… the administration of an AS appears to other ASes to have a single coherent interior routing plan and presents a consistent picture of what networks are reachable through it.

AS Numbers (ASNs)

ASNs are 16 bit values

64512 through 65535 are “private”

• Level 3: 1

• MIT: 3

• Harvard: 11

• Yale: 29

• Princeton: 88

• AT&T: 7018, 6341, 5074, …

• UUNET: 701, 702, 284, 12199, …

• Sprint: 1239, 1240, 6211, 6242, …

• …

ASNs represent units of routing policy

Currently around 20,000 in use

AS ≠ Institution

• Not equivalent to an AS – Many institutions span multiple autonomous systems – Some institutions do not have their own AS number – Ownership of an AS may be hard to pinpoint (whois)

• Not equivalent to a block of IP addresses (prefix) – Many institutions have multiple (non-contiguous) prefixes – Some institutions are a small part of a larger address block – Ownership of a prefix may be hard to pinpoint (whois)

• Not equivalent to a domain name (att.com) – Some sites may be hosted by other institutions – Some institutions have multiple domain names (att.net)

The AS graph

may look like this Reality may be closer to this…

BGP was designed to

throw away information!

AS Graph != Internet Topology

• Tier-1: small number of tier-1 ASes – A near-clique of ~15 ASes with no providers – AT&T, Sprint, UUNET, …

• Transit core: peer with tier-1s and each other – Around 100-200 large ASes

– UUNET Europe, KDDI, and Singapore Telecom

• Regional ISPs: non-stubs near the edge – Around 2000 medium-sized ASes – Minnesota Regional Network, US West

• Stub ASes: no peer or customer neighbors – Princeton, Rutgers, MIT, AT&T Research, …

Characterizations of AS Topology

Hierarchical routing

LIP6

network

DT

AS 1

AS 3

AS 2

Intra-AS routing (Interior Gateway Protocol) Most common: OSPF,IS-IS determines path from ingress

to egress

Inter-AS routing

(Border Gateway Protocol)

determines AS path and

egress point

• Goal: distributed management of resources

– Internetworking of multiple networks – Networks under separate administrative control

• Solution: two-tiered routing architecture

– Intradomain: inside a region of control

 Okay for routers to share topology information

 Routers configured to achieve a common goal – Interdomain: between regions of control

 Not okay to share complete information

 Networks may have different/conflicting goals

• Led to the use of different protocols…

Two-Tiered Internet Routing Architecture

Interconnected ASes

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

– Intra-AS sets entries for internal dests

– Inter-AS & Intra-As sets entries for external dests

3a

3c

3b

1c

1b 1a

1d

2a 2c 2b

AS 1

AS 2

AS 3

Intra-AS

Routing

algorithm

Inter-AS Routing algorithm

Forwarding table

– Routing policies based on business relationships

– No common metrics, and limited cooperation – BGP: policy-based, path-vector routing protocol

– Shortest-path routing based on link metrics

– Routers all managed by a single institution – OSPF and IS-IS: link-state routing protocol – RIP and EIGRP: distance-vector routing protocol

Two-Tiered Internet Routing System

AS 1

AS 2

BGP

EGP = Exterior Gateway Protocol

IGP = Interior Gateway Protocol

Metric based: OSPF, IS-IS, RIP,

EIGRP (cisco) Policy based: BGP

The Routing Domain of BGP is the entire Internet

OSPF

EIGRP

Architecture of Dynamic Routing

• Topology information is flooded within the routing domain

• Best end-to-end paths are computed locally at each router

• Best end-to-end paths determine next-hops

• Based on minimizing some notion of distance

• Works only if policy is shared and uniform

• Examples: OSPF, IS-IS

• Each router knows little about network topology

• Only best next-hops are chosen by each router for each destination network

• Best end-to-end paths result from composition of all next-hop choices

• Does not require any notion

of distance

• Does not require uniform policies at all routers

• Examples: RIP, BGP

Technology of Distributed Routing

Routers Talking to Routers

Routing info

Routing info

• Routing computation is distributed among routers within a routing

domain

• Computation of best next hop based on routing information is the

most CPU/memory intensive task on a router

• Routing messages are usually not routed, but exchanged via layer 2

between physically adjacent routers (internal BGP and multi-hop

external BGP are exceptions)

• Link-state routing with static link weights – Static weights: avoid stability problems – Link state: faster reaction to topology changes

• Most common protocols in backbones – OSPF: Open Shortest Path First – IS-IS: Intermediate System–Intermediate System

• Some use of distance vector in enterprises – RIP: Routing Information Protocol – EIGRP: Enhanced Interior Gateway Routing Protocol

• Growing use of Multi-Protocol Label Switching

Intradomain Routing Today

Link-State Routing is Problematic

• Topology information is flooded

– High bandwidth and storage overhead

– Forces nodes to divulge sensitive information

• Entire path computed locally per node

– High processing overhead in a large network

• Minimizes some notion of total distance

– Works only if policy is shared and uniform

• Typically used only inside an AS

– E.g., OSPF and IS-IS

Challenges for Interdomain Routing

• Scale – Prefixes: 150,000-200,000, and growing – ASes: 20,000 visible ones, and growing – AS paths and routers: at least in the millions…

• Privacy – ASes don’t want to divulge internal topologies – … or their business relationships with neighbors

• Policy – No Internet-wide notion of a link cost metric – Need control over where you send traffic – … and who can send traffic through you

Shortest-Path Routing is Restrictive

• All traffic must travel on shortest paths

• All nodes need common notion of link costs

• Incompatible with commercial relationships

Regional ISP1

Regional ISP2

Regional

ISP3

Cust1

National

ISP1

National

NO

• Advantages – Hides details of the network topology – Nodes determine only “next hop” toward the dest

• Disadvantages – Minimizes some notion of total distance, which is difficult

in an interdomain setting – Slow convergence due to the counting-to-infinity problem (“bad news travels slowly”)

• Idea: extend the notion of a distance vector

Distance Vector is on the Right Track

Path-Vector Routing

• Extension of distance-vector routing

– Support flexible routing policies

– Avoid count-to-infinity problem

• Key idea: advertise the entire path

– Distance vector: send distance metric per dest d

– Path vector: send the entire path for each dest d

3

d

“d: path (2,1)” “d: path (1)”

data traffic data traffic

Faster Loop Detection

• Node can easily detect a loop – Look for its own node identifier in the path – E.g., node 1 sees itself in the path “3, 2, 1”

• Node can simply discard paths with loops – E.g., node 1 simply discards the advertisement

3

“d: path (2,1)” “d: path (1)”

“d: path (3,2,1)”

Link State Distance Vector Path Vector

Dissem-ination Flood link state advertisements to all

routers

Update distances from neighbors’ distances

Algorithm Dijsktra’s shortest

path Bellman-Ford shortest path

Converge Fast due to

flooding Slow, due to count-to-infinity

Protocols OSPF, IS-IS RIP, EIGRP

Routing Protocols

Link State Distance Vector Path Vector

Dissem-ination Flood link state advertisements to all

routers

Update distances from neighbors’ distances Update paths based on neighbors’ paths

Algorithm Dijsktra’s shortest

path Bellman-Ford shortest path Local policy to rank paths

Converge Fast due to

flooding Slow, due to count-to-infinity Slow, due to path exploration

Routing Protocols

The Gang of Four

EGP

IGP

BGP

RIP IS-IS

OSPF

Interdomain Routing (Between ASes)

1 2

3

4

5

6 7

Client

Web server

Path: 6, 5, 4, 3, 2, 1

Interdomain Routing: Border Gateway Protocol

3

12.34.158.5

“12.34.158.0/24: path (2,1)” “12.34.158.0/24: path (1)”

data traffic data traffic

• ASes exchange info about who they can reach

– IP prefix: block of destination IP addresses

– AS path: sequence of ASes along the path

• Policies configured by the AS’s operator

– Path selection: which of the paths to use?

– Path export: which neighbors to tell?

Flexible Policies

• Each node can apply local policies – Path selection: Which path to use?

– Path export: Which paths to advertise?

• Examples – Node 2 may prefer the path “2, 3, 1” over “2, 1”

– Node 1 may not let node 3 hear the path “1, 2”

1

Zooming in to AS 3

12.34.158.0/24

Border router:

• Border router

– Learns BGP route from neighbor AS

– Creates forwarding-table entry for prefix

• But, how do the other routers get there?

How do Other Routers Learn the BGP Route?

iBGP session

12.34.158.0/24

“12.34.158.0/24 through red router”

• Internal BGP – iBGP sessions between the routers – Allows other routers to get the big picture

• Simplest case: “full mesh” of iBGP sessions

How To Get to the Egress Router?

3

2

2

1 1

3 1

4

5

3

“Use Serial0/0.1

to get to the

red router”

• Interior Gateway Protocol (OSPF/IS-IS)

– Routers flood information to learn topology

– Routers determine “next hop” to other routers…

– Compute shortest paths based on the link weights

– Link weights configured by the operator

Constructing the Forwarding Table

• Three protocols – External BGP: learn the external route – Internal BGP: propagate inside the AS – IGP: learn outgoing link on path to other router

• Router joins the data – Prefix 12.34.158.0/24 reached through red router – Red router reached via link Serial0/0.1

– Forwarding entry: 12.34.158.0/24  Serial0/0.1

• Router forwards packets – Lookup destination 12.34.158.5 in table – Forward packet out link Serial0/0.1

Constructing the Forwarding Table

• Three protocols

– External BGP: learn the external route

– Internal BGP: propagate inside the AS

– IGP: learn outgoing link on path to other router

• Router joins the data

– Prefix 12.34.158.0/24 reached through red router

– Red router reached via link Serial0/0.1

– Forwarding entry: 12.34.158.0/24  Serial0/0.1

• Router forwards packets

– Lookup destination 12.34.158.5 in table

– Forward packet out link Serial0/0.1

• Interdomain routing protocol for the Internet – Prefix-based path-vector protocol

– Policy-based routing based on AS Paths – Evolved during the past 15 years

• 1989 : BGP-1 [RFC 1105]

– Replacement for EGP (1984, RFC 904)

• 1990 : BGP-2 [RFC 1163]

• 1991 : BGP-3 [RFC 1267]

• 1995 : BGP-4 [RFC 1771]

– Support for Classless Interdomain Routing (CIDR) Border Gateway Protocol