Topology of the Internet

woonsocketpoliticalNetworking and Communications

Oct 28, 2013 (4 years and 10 months ago)


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.