Download Routing

Topology of the Internet
Autonomous Systems (AS)
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The global Internet consists of Autonomous Systems
(AS) interconnected with each other:
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Collection of routers under same administrative control, all
running the same routing protocol among themselves.
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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)
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Two-Level Routing
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Intra-AS Routing (RIP/DV, OSPF/LS, IGRP/DV)
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administrator responsible for the choice of routing protocol.
Why are there different Protocols?
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Inter-AS Routing (BGP)
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Policy:
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Inter-AS: control over how traffic is routed, and who routes
through the network.
Intra-AS: single admin, so no policy decisions needed.
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hierarchical routing saves table size, reduced update traffic.
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Intra-AS: can focus on performance
Inter-AS: scalability and policy dominate over performance.
Scale:
Performance:
IPv4 Addressing
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An IP address is an identifier for a host/router
interface.
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IPv4 Address Structure
• IPv4 Addresses: 32 bit
• Human readable form: a.b.c.d
Interface Addresses and Subnets
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(where a,b,c,d are 8bit values)
example: 130.238.8.176
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network/prefix
host
x bits
32-x bits
Routing is only based on the network identifier.
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Interface: connection between host/router and physical link
Routers have several interfaces, hosts can have several
interfaces.
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)
A Router (layer 3)
connects layer 2 networks.
These networks are also
called Subnet and have
their own network id.
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Routing Table Example
Forwarding Policy
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Routing Table at router R2 (simplified)
subnet
next hop
L2 if
223.1.1.0/24
233.1.2.0/24
223.1.9.2
-*
3
1
223.1.3.0/24
223.1.7.0/24
223.1.8.0
223.1.8.0
2
2
223.1.8.0/24
223.1.9.0/24
-*
-*
2
3
prefix
check if destination address matches the prefix of the
incoming network interface:
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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.
3
2
1
* this subnet is directly connected to the router.
Default Router
Copyright © 2005 Pearson Addison-Wesley. All rights reserved.
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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.
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Address Resolution Protocol (ARP)
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Translation between network-layer addresses and linklayer 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
Hierarchy - a Key to Scalability
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network/prefix
size: x bits
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host
Address: network/prefix, host > identifyer
Network mask > recognise prefix (network)
Default router > router for traffic not on same netw.
DNS server
Hierarchical Addresses
sunet.se
130.242.88.17/11
242: 11110010
uu.se
student.uu.se
130.238.5.0/24
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domain names: uu.se, it.uu.se
fully qualified domain names: rama.it.uu.se, www.google.com
Domain Name System
Hierarchical Addressing
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use of prefixes: 220.23.16.0/20, 200.23.16.0/23
IPv4 Addresses
Hierarchical Routing
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tightly related to addressing
Autonomous Systems (intra-AS and inter-AS routing)
Network Address Allocation
Example without guarantee
130.238.0.0/16
238: 11101110
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Hierarchical Naming
it.uu.se
130.238.8.0/24
Network Address Allocation
More Addresses...
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Allocation of prefixes is necessary for routing
efficiency but inefficient in terms of address usage.
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IPv6
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Extended addressing capabilities (net|id, id unique)
Streamlined header (40 Bytes)
Flow labelling and priority
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IP addresses have only a local scope
10.0.0.0/8 , 192.168.0.0/16 (“non routable” addresses)
Typical home/student network.
Network Address Translation (NAT)
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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.
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Label Switching
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Probabilistic Routing
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Hop-by-hop addresses (labels)
Example: Multiprotocol Label Switching (MPLS)
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
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Finding information rather than a specific address.
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Note: It is not the goal to improve address usage efficiency.