documentas

Intra-AS and Inter-AS routingGateways:

•perform inter-AS routing among themselves•perform intra-AS routers with other routers in their AS

inter-AS, intra-AS routing in

gateway A.c

network layerlink layer

physical layer

a

b

b

aaC

A

Bd

A.aA.c

C.bB.a

cb

c

Intra-AS and Inter-AS routing

Host h2

a

b

b

aaC

A

Bd c

A.aA.c

C.bB.a

cb

Hosth1

Intra-AS routingwithin AS A

Inter-ASrouting

between A and B

Intra-AS routingwithin AS B

Internet Structure

Today

Backbone service provider

Peeringpoint

Peeringpoint

Large corporation

Large corporation

Smallcorporation

“Consumer” ISP

“Consumer” ISP

“Consumer” ISP

local traffic: traffic that originates at or terminates on nodes within the autonomous system;transit traffic: traffic that passes through an AS

EGP: Exterior Gateway Protocol• Overview

– designed for tree-structured Internet– concerned with reachability, not optimal routes

• Protocol messages– neighbor acquisition: one router requests that another

be its peer; peers exchange reachability information– neighbor reachability: one router periodically tests if

the another is still reachable; exchange HELLO/ACK messages; uses a k-out-of-n rule

– routing updates: peers periodically exchange their routing tables (distance-vector)

BGP-4: Border Gateway Protocol

• AS Types– stub AS: has a single connection to one other AS

• carries local traffic only– multihomed AS: has connections to more than one AS

• refuses to carry transit traffic– transit AS: has connections to more than one AS

• carries both transit and local traffic

Why interdomain routing is an hard problem

• Scalability problem: an Internet backbone router must be able to forward any packet destined anywhere in the Internet. CIDR has helped to control the number of distinct prefixes but they are of the order of 105

• Autonomous nature of the domains. Each domain may run its own interior routing protocols and can uses any scheme to assign metrics to paths

• Interdomain routing advertises only “reachability”• Issue of trust: provider A migth be unwilling to

believe certain advertisements from provider B for fear that provider B will advertise erroneous routing information.

The issue of policies

• In interdomain routing there is the need to support very flexible policies.

Examples• Use provider B only to reach these addresses• Use the path that crosses the fewest number of ASs• Use AS x in preference of AS y

1

Border Routers and BGP Speakers

•Each AS has:– one or more border routers– one BGP speaker (not necessary a border

router) that advertises:• local networks• other reachable networks (transit AS only)• gives path information

BGP and border router

R1 R3

R2

R4

R5 R6

Autonomous System 1

Autonomous System 2

Border Router

Complete path advertisements

•BGP does not belong to either of the two main classes of routing protocols (distance-vector and link-state protocols)

•Unlike these protocols BGP advertises complete paths as an enumerated list of ASs to reach a particular network.

•This is also necessary to enable policy decisions• It also enable routing loops to be readily detected

BGP Example• Speaker for AS2 advertises reachability to P and Q

– network 128.96, 192.4.153, 192.4.32, and 192.4.3, can be reacheddirectly from AS2

• Speaker for backbone advertises– networks 128.96, 192.4.153, 192.4.32, and 192.4.3 can be reached

along the path (AS1, AS2).• Speaker can cancel previously advertised paths

Backbone network(AS 1)

Regional provider A(AS 2)

Regional provider B(AS 3)

Customer P(AS 4)

Customer Q(AS 5)

Customer R(AS 6)

Customer S(AS 7)

128.96192.4.153

192.4.32192.4.3

192.12.69

192.4.54192.4.23transit networks

stub networks

networks 192.12.69, 192.4.54, 192.4.23 can be reached along the path (AS1, AS3).

MultiProtocol Label Switching

Combine some of the properties of Virtual Circuits with the flexibility and robusteness of Datagrams.It relies on IP addresses and IP routing protocols to do its job.

MPLS-enabled router forward packets by examining relatively short,fixed-length labels, and these labels have local scope, just like in a virtual circuit network.

MPLS: what is it good for?

To enable IP capabilities on devices that do not have the capability to forward IP datagrams in the normal mannerTo forward IP packets along “explicit routes” that do not necessarily match those that normal IP routing protocol would selectTo support certain types of virtual private network servicesTo improve performance

Destination-based forwarding

Prefix Interface10.1.1 0 10.3.3 0

…

Prefix Interface 10.1.1 1 10.3.3 0

…

0

0

1R1 R2

R3

R4

10.1.1/24

10.3.3/24

For sake of simplicity /24 is omitted in the pictures

Prefix Interface10.1.1 0 10.3.3 0

…

Label Prefix Interface

15 10.1.1 1 16 10.3.3 0 …

0

0

1R1 R2

R3

R4

10.1.1/24

10.3.3/24

When MPLS is enabled on a router the router allocates a label for each prefix in its routing table and advertise both the label and the predix that it represent to its neighboring routers.The advertisement is carried in the “Label Distribution Protocol”

Label=15, Prefix=10.1.1


The labels can be chosen at the convenience of the allocating router

Advertise the label and

their bindings

“Please attach the label 15 to all packets sent to me that are destined to prefix 10.1.1”

Label Prefix Interface

15 10.1.1 1 16 10.3.3 0 …

0

0

1R1 R2

R3

R4

10.1.1/24

10.3.3/24

Advertising labels

Prefix Interface Remote Label

10.1.1 0 15 10.3.3 0 16

…

Outgoing Label

Label Prefix Interface Remote Label

15 10.1.1 1 24 16 10.3.3 0 …

0

0

1R1 R2

R3

R4

10.1.1/24

10.3.3/24

Advertising labels


10.1.1 0 15 10.3.3 0 16

…

Outgoing Label


Outgoing Label


15 10.1.1 1 24 16 10.3.3 0 …

0

0

1R1 R2

R3

R4

10.1.1/24

10.3.3/24

Label switching


10.1.1 0 15 10.3.3 0 16

…

INFO IP Dest 10.1.1.5

INFO IP Dest 10.1.1.5 15

LERLabel Edge Router


15 10.1.1 1 24 16 10.3.3 0 …

0

0

1R1 R2

R3

R4

10.1.1/24

10.3.3/24

Label swapping


10.1.1 0 15 10.3.3 0 16

…

LERLabel Edge Router

INFO IP Dest 10.1.1.5

INFO IP Dest 10.1.1.5 24

There is no need to examine theIP header* at router R2:

exact Match using labels

* IP addresses are always of the same length but IP prefixes are of variable length and the IP dest. addr. look-up algorithm needs to find the longest match

MPLS is a forwarding paradigm

Note that while the forwarding algorithm has changed from longest match to exact match the routing algorithm can be any standard IP routing algorithm (such as the one implemented in OSPF) . The chosen path would be the same.

The major effect of changing the forwarding algorithm is that devices that normally don’t know how to forward IP packets can be used in an MPLS network.

In this way ATM switches equipped by MPLS software can become Label Switching Routers (LSR)

How to insert or use labels

VPI PT CLP HEC

5 ByteATM Header

Format VCI

Label LabelOption 1Option 2 Combined Label

Option 3 LabelATM VPI (Tunnel)

DLCI C/R

EA DLCI FE

CNBECN

DE

EA

Q.922Header

Generic Encap.(PPP/LAN Format) Layer 3 Header and Packet

DLCI Size = 10, 17, 23 Bits

ATM:just use VPI/VCI

as labels

Frame Relay

PPP & LAN 802.3

How to insert a label:the shim header

Label: Label Value, 20 bit (0-16 reserved)0: IPv4 explicit null1: Router alert2: IPv6 explicit null3: Implicit null

Exp.: Experimental, 3 bit (Class of Service nel Tag Switching)S: Bottom of Stack, 1 bit (1 = last entry in label stack)TTL: Time to Live, 8 bit legato al TTL di IP

Layer 2 Header(PPP, 802.3)

•••Network Layer Header

and Info (IP or L3)

MPLS ‘Shim’ Headers (1-n)1n

Label Exp. S TTL

4 Byte

Label StackEntry Format

Overlay networks

ATM Backbone

IPBackbone

Five routing adjacencies

R1

R2

R3 R4

R5

R6

L2L2

L2 L2

L2 L2

Il routing L2 (ATM o FR) implementa l’ingegneria del

trafficoA livello L3 si vedono solo collegamenti diretti tra

router

L3 L3

L3 L3

L3L3

Svantaggi:

• costo maggiore

• network management non integrato tra i due livelli

• impossibilità di routing esplicito

Overlay networks

9

R1

R3

R2

Explicit Routing

IP routing is destination-based; IP has a source routing option but limited in number of hops and processed outside the “fast path” on most routers

FISH PICTURE

R6R7

R4 R5

R8

Explicit Routing

How do all the routers in the network agree on what labels to use and how to forward packets with particular labels? A new mechanism is needed. It turns out that the protocol used for this task is the Resource Reservation Protocol (RSVP).It is possible to send an RSVP message along an explicitly specified path (e.g. R1-R3-R6-R7-R8) and use it to set up label forwarding entries all along that path.This is very similar to the process related to the opening packet which establish a virtual circuitOn of the application of explicit routing is “traffic engineering”which refers to the task of ensuring that sufficient resources are available in a network to meet the demands placed on it.Fast reroute is another relevant application of explicit routing. There are a range of algorithms that routers can use to calculate explicit route automatically. The most common is CSPF (Constrained Stortest Path First)

Virtual Private Networks and Tunnels

Head TailR1

R2 R3

R4

ATM Cells arrive

Tunneled dataarrives at tail

ATM Cells sent

Pseudowire emulation

Tunnel header consist of an MPLS header rather than an IP header

Virtual Private Networks and Tunnels

Head TailR1

R2 R3

R4

1. ATM Cells arrive

Tunneled dataarrives at tail

6. ATM Cells sent

Pseudowire emulationLabels can be stacked

101 INFO

101 INFODL

101 INFODLTL

2. Demux Label added

3. Tunnel Label added

101 INFODLTL

4. Packet is forwarded to tail

5. Demux Label examined101 INFODL

202 INFO

33©

L3 VPN

Provider Network

VPN A / Site 1

VPN A / Site 2

VPN A / Site 3

VPN B / Site 1VPN B / Site 2

VPN B / Site 3

Virtually private networks

Optical

ATM

IP/MPLS

lower complexity in control & management planes time

Optical

SDH

ATM

IP

Optical

SDH

IP/MPLS

Optical

IP/GMPLS

L2L2

L2 L2

L2 L2

L3 L3

L3 L3

L3L3

OVERLAY MODEL

PEER to PEER MODEL

L2L2

L2 L2

L2 L2

L3 L3

L3 L3

L3L3

Architectural Evolution

L3 Total mesh

documentas

Documents

routing loops

paths interdomain routing

interior routing protocols

erroneous routing information

transit traffic transit

border gateway protocol

provider b

n rule routing updates