Topology of the Internet

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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 Why are there different Protocols?
Copyright © 2005 Pearson Addison-Wesley. All rights reserved.
Intra-AS Routing (RIP/DV, OSPF/LS, IGRP/DV) Policy:
• •
- administrator responsible for the choice of routing protocol. - Inter-AS: control over how traffic is routed, and who routes
through the network.
Inter-AS Routing (BGP)

- Intra-AS: single admin, so no policy decisions needed.

- hierarchical routing saves table size, reduced update traffic.

- Intra-AS: can focus on performance
- Inter-AS: scalability and policy dominate over performance.
Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 4-IPv4 Addressing
An IP address is an identifier for a host/router

- Interface: connection between host/router and physical link
- Routers have several interfaces, hosts can have several
IPv4 Address Structure Interface Addresses and Subnets
Copyright © 2005 Pearson Addison-Wesley. All rights reserved.
IPv4 Addresses: 32 bit

A Router (layer 3)
Human readable form: a.b.c.d (where a,b,c,d are 8bit values)

connects layer 2 networks.
These networks are also
called Subnet and have
network/prefix host
their own network id.
x bits 32-x bits
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., (=/24)
Copyright © 2005 Pearson Addison-Wesley. All rights reserved.Routing Table Example Forwarding Policy
Routing Table at router R2 (simplified)
check if destination address matches the prefix of the

subnet next hop L2 if
incoming network interface: 3
- if it does: pass packet to transport layer (node is destination) -* 1
- else drop packet (the destination is on same network, no 2
forwarding required) 2 -* 2
else, choose longest matching prefix in routing table. -* 3
forward packet based on next hop information.

* this subnet is directly connected to the router.
Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 4-
Default Router Address Resolution Protocol (ARP)
Entry in the routing table of a host or router, Translation between network-layer addresses and link-
• •
specifying to which router a message that does not layer addresses. > 49-BD-D2-C7-56-2A
match any prefix should be forwarded to.
Resolution on same local link only (not-end-to end):
Usually a gateway to other networks, e.g., the Internet.

“who has, tell”
“reply 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 Hierarchy - a Key to Scalability
Hierarchical Naming

domain names:,
network/prefix host
fully qualified domain names:,
size: x bits
Domain Name System
Hierarchical Addressing

Address: network/prefix, host > identifyer

- use of prefixes:,
Network mask > recognise prefix (network)

- IPv4 Addresses
Default router > router for traffic not on same netw.

DNS server Hierarchical Routing
• •
- tightly related to addressing
- Autonomous Systems (intra-AS and inter-AS routing)
Hierarchical Addresses Network Address Allocation
Example without guarantee
242: 11110010
238: 11101110
Copyright © 2005 Pearson Addison-Wesley. All rights reserved.Network Address Allocation More Addresses...
Allocation of prefixes is necessary for routing

efficiency but inefficient in terms of address usage.

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
- , (“non routable” addresses)
Typical home/student network.
Note: It is not the goal to improve address usage efficiency.

How does an IPS get a block of addresses?
ICANN: Internet Corporation for Assigned Names and Numbers
Alternative Routing Approaches
Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 4-
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
Content Routing

Finding information rather than a specific address.