Topology of the Internet Autonomous Systems (AS) The global - - PowerPoint PPT Presentation

Topology of the Internet Autonomous Systems (AS)

• The global Internet consists of Autonomous Systems

(AS) interconnected with each other:

• Collection of routers under same administrative control, all

running the same routing protocol among themselves.

• Stub AS: only one connection to another AS (small company)
• Mulithomed AS: multiple connections to other AS. No transit.

(large corporation)

• Transit AS: hooking many AS together (provider)

Two-Level Routing

• Intra-AS Routing (RIP/DV, OSPF/LS, IGRP/DV)
• administrator responsible for the choice of routing protocol.
• Inter-AS Routing (BGP)

Why are there different Protocols?

• Policy:
• Inter-AS: control over how traffic is routed, and who routes

through the network.

• 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: scalability and policy dominate over performance.

• An IP address is an identifier for a host/router

interface.

• Interface: connection between host/router and physical link
• Routers have several interfaces, hosts can have several

interfaces.

IPv4 Addressing IPv4 Address Structure

• IPv4 Addresses: 32 bit
• Human readable form: a.b.c.d (where a,b,c,d are 8bit values)

example: 130.238.8.176

• Routing is only based on the network identifier.
• prefix = x MSB of the address (x: mask)
• we use the following notation for the prefix: a.b.c.d/x
• in Windows the mask has the form of e.g., 255.255.255.0 (=/24)

network/prefix

x bits

host

32-x bits

Interface Addresses and Subnets

A Router (layer 3) connects layer 2 networks. These networks are also called Subnet and have their own network id.

Routing Table Example

subnet next hop L2 if 223.1.1.0/24 223.1.9.2 3 233.1.2.0/24

• *

1 223.1.3.0/24 223.1.8.0 2 223.1.7.0/24 223.1.8.0 2 223.1.8.0/24

• *

2 223.1.9.0/24

• *

3 prefix Routing Table at router R2 (simplified)

* this subnet is directly connected to the router. 1 2 3

Forwarding Policy

• check if destination address matches the prefix of the

incoming network interface:

• if it does: pass packet to transport layer (node is destination)
• else drop packet (the destination is on same network, no

forwarding required)

• else, choose longest matching prefix in routing table.
• forward packet based on next hop information.

Default Router

• Entry in the routing table of a host or router,

specifying to which router a message that does not match any prefix should be forwarded to.

• Usually a gateway to other networks, e.g., the Internet.

Address Resolution Protocol (ARP)

• Translation between network-layer addresses and link-

layer addresses.

130.238.8.100 > 49-BD-D2-C7-56-2A

• Resolution on same local link only (not-end-to end):

“who has 130.238.8.100, tell 130.238.8.123” “reply 130.238.8.100 is at 49-BD-D2-C7-56-2A”

• Resolution at every router!
• Cache to avoid ARP request for every single packet

(expires after ca. 20 minutes)

Configuration on a Host

• Address: network/prefix, host > identifyer
• Network mask > recognise prefix (network)
• Default router > router for traffic not on same netw.
• DNS server

network/prefix

size: x bits

host

Hierarchy - a Key to Scalability

• Hierarchical Naming
• domain names: uu.se, it.uu.se
• fully qualified domain names: rama.it.uu.se, www.google.com
• Domain Name System
• Hierarchical Addressing
• use of prefixes: 220.23.16.0/20, 200.23.16.0/23
• IPv4 Addresses
• Hierarchical Routing
• tightly related to addressing
• Autonomous Systems (intra-AS and inter-AS routing)

Hierarchical Addresses

130.238.5.0/24 130.238.8.0/24 130.238.0.0/16 238: 11101110 130.242.88.17/11 242: 11110010

uu.se sunet.se student.uu.se it.uu.se Example without guarantee

Network Address Allocation

Network Address Allocation

How does an IPS get a block of addresses? ICANN: Internet Corporation for Assigned Names and Numbers

More Addresses...

• Allocation of prefixes is necessary for routing

efficiency but inefficient in terms of address usage.

• IPv6
• Extended addressing capabilities (net|id, id unique)
• Streamlined header (40 Bytes)
• Flow labelling and priority
• Network Address Translation (NAT)
• IP addresses have only a local scope

10.0.0.0/8 , 192.168.0.0/16 (“non routable” addresses)

• Typical home/student network.
• Note: It is not the goal to improve address usage efficiency.

Alternative Routing Approaches

• Label Switching
• Hop-by-hop addresses (labels)
• Example: Multiprotocol Label Switching (MPLS)
• Probabilistic Routing
• The routing table indicates the probability to deliver to the

destination based on prior experience.

• Forward a message if higher probability than previous hop.
• Example: Prophet routing protocol (Sami Network

Connectivity)

• Content Routing
• Finding information rather than a specific address.