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.
IPv4 Addressing 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 Address Structure IPv4 Addresses: 32 bit Human readable form: a.b.c.d example: 130.238.8.176 network/prefix x bits (where a,b,c,d are 8bit values) 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 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)
Routing Table Example Forwarding Policy Routing Table at router R2 (simplified) 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 3 2 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. 1 * this subnet is directly connected to the router. Default Router Address Resolution Protocol (ARP) 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. 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 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 Address: network/prefix, host > identifyer Network mask > recognise prefix (network) Default router > router for traffic not on same netw. DNS server 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 Example without guarantee Network Address Allocation sunet.se 130.242.88.17/11 242: 11110010 uu.se 130.238.0.0/16 238: 11101110 it.uu.se 130.238.8.0/24 student.uu.se 130.238.5.0/24
Network Address Allocation 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. How does an IPS get a block of addresses? ICANN: Internet Corporation for Assigned Names and Numbers 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.