Routing Basic principles Karst Koymans Informatics Institute University of Amsterdam (version 16.4, 2017/02/23 10:58:58) Friday, February 24, 2017 Karst Koymans (UvA) Routing Friday, February 24, 2017 1 / 30
1 Basic setup Directly connected Not directly connected Addressing and routing commands Route selection 2 Mathematical representation 3 Routing in the Internet 4 Different routing mechanisms Karst Koymans (UvA) Routing Friday, February 24, 2017 2 / 30
Basic setup Outline 1 Basic setup Directly connected Not directly connected Addressing and routing commands Route selection 2 Mathematical representation 3 Routing in the Internet 4 Different routing mechanisms Karst Koymans (UvA) Routing Friday, February 24, 2017 3 / 30
Basic setup Directly connected Outline 1 Basic setup Directly connected Not directly connected Addressing and routing commands Route selection 2 Mathematical representation 3 Routing in the Internet 4 Different routing mechanisms Karst Koymans (UvA) Routing Friday, February 24, 2017 4 / 30
Basic setup Directly connected Directly Connected IP Networks Traditional configuration under Unix with ifconfig command Packets can be delivered immediately by the data link (layer 2 software) No explicit route commands needed under Linux, because ifconfig sets a route automatically ifconfig command ifconfig <intf> <ipv4_addr> netmask <mask> [broadcast <bcast>] ifconfig <intf> inet6 add <ipv6_addr>/<prefix> Karst Koymans (UvA) Routing Friday, February 24, 2017 5 / 30
Basic setup Directly connected Data link layer routing Only outgoing interface and layer 2 address is needed How is the layer 2 address obtained in IPv4/IPv6? Packet is (selectively) flooded by layer 2 Bridges and switches use a lookup table to map layer 2 addresses to outgoing interfaces Spanning tree protocol takes care of loops Newer protocols like TRILL and SPB can handle topologies with loops Karst Koymans (UvA) Routing Friday, February 24, 2017 6 / 30
Basic setup Not directly connected Outline 1 Basic setup Directly connected Not directly connected Addressing and routing commands Route selection 2 Mathematical representation 3 Routing in the Internet 4 Different routing mechanisms Karst Koymans (UvA) Routing Friday, February 24, 2017 7 / 30
Basic setup Not directly connected Global routing Not directly connected networks need a gateway (next hop) Recursive lookup In practice this is only one level deep Hence the gateway needs to be directly connected In theory a deeper recursion level would be possible and maybe even useful Karst Koymans (UvA) Routing Friday, February 24, 2017 8 / 30
Basic setup Not directly connected Routing Table netstat -r is the traditional command to show the routing table Route flags Flag Set Unset G route needs gateway route is directly connected H route to host route to network S route added statically route added dynamically (mostly by admin) (by a protocol) Karst Koymans (UvA) Routing Friday, February 24, 2017 9 / 30
Basic setup Addressing and routing commands Outline 1 Basic setup Directly connected Not directly connected Addressing and routing commands Route selection 2 Mathematical representation 3 Routing in the Internet 4 Different routing mechanisms Karst Koymans (UvA) Routing Friday, February 24, 2017 10 / 30
Basic setup Addressing and routing commands route command Syntax varies between systems On Linux systems 1 Adding a static default route route add default <gateway> Adding a static host route route add -host <host> gw <gateway> Adding a static network route route add -net <network> netmask <netmask> gw <gateway> 1 Exercise: Rewrite the first two entries as instances of the last Karst Koymans (UvA) Routing Friday, February 24, 2017 11 / 30
Basic setup Addressing and routing commands arp command Used to interact with the arp table Read the arp table arp Delete an entry from the arp table arp -d <address> Add a static entry to the arp table arp -s <address> <hardware_address> arp only works for IPv4 For IPv6 use the ip [-6] command Karst Koymans (UvA) Routing Friday, February 24, 2017 12 / 30
Basic setup Addressing and routing commands ip command The command ip was introduced in the Linux iproute2 package General interface to kernel addressing and routing Replaces ifconfig, route and arp (almost) completely Has support for IPv6, routing policies and multiple routing tables Karst Koymans (UvA) Routing Friday, February 24, 2017 13 / 30
Basic setup Addressing and routing commands ip subcommands ip <subcommand>... Subcommand link address neighbor route rule Meaning layer 2 interface settings (MAC) layer 3 interface settings (IP) arp cache data routing table data routing table selection Karst Koymans (UvA) Routing Friday, February 24, 2017 14 / 30
Basic setup Route selection Outline 1 Basic setup Directly connected Not directly connected Addressing and routing commands Route selection 2 Mathematical representation 3 Routing in the Internet 4 Different routing mechanisms Karst Koymans (UvA) Routing Friday, February 24, 2017 15 / 30
Basic setup Route selection Route selection Longest prefix match Host routes are preferred over network routes Default route has shortest prefix Route of last resort Cannot be used in the Internet core Why? Karst Koymans (UvA) Routing Friday, February 24, 2017 16 / 30
Mathematical representation Outline 1 Basic setup Directly connected Not directly connected Addressing and routing commands Route selection 2 Mathematical representation 3 Routing in the Internet 4 Different routing mechanisms Karst Koymans (UvA) Routing Friday, February 24, 2017 17 / 30
Mathematical representation Mathematical representation Graphs (undirected, labeled) Nodes are routers and hosts Edges are (point to point) connections Network nodes are not necessary because of this restriction Labels represent cost of using an edge in your route Undirected edges imply a restriction to two-way communication paths with the same cost in both directions Karst Koymans (UvA) Routing Friday, February 24, 2017 18 / 30
Broadcast networks (triangle) R R N R R R R
Broadcast networks (square) R R R R N R R R R
Mathematical representation Node reduction LAN with O(N) routers or hosts Full mesh of O(N 2 ) edges Reduced with only 1 extra node to O(N) edges Works for LANs, but also for NBMA networks In many situations But not always (take care) Karst Koymans (UvA) Routing Friday, February 24, 2017 21 / 30
Routing in the Internet Outline 1 Basic setup Directly connected Not directly connected Addressing and routing commands Route selection 2 Mathematical representation 3 Routing in the Internet 4 Different routing mechanisms Karst Koymans (UvA) Routing Friday, February 24, 2017 22 / 30
Routing in the Internet Internet Routing (1) Based on top level structure defined by Autonomous Systems (AS) Each AS has administrative control over a collection of Routers (and hosts) Networks Definition (AS Autonomous System (from RFC 1930)) An AS (Autonomous System) is a connected group of one or more IP prefixes run by one or more network operators which has a single and clearly defined routing policy. Karst Koymans (UvA) Routing Friday, February 24, 2017 23 / 30
Routing in the Internet Internet Routing (2) Edge routers inside an AS can be directly connected to edge routers in another AS Used for inter-as routing Using an exterior routing (or gateway) protocol (EGP) BGP4 No other protocols in use on the Internet except variants of BGP Karst Koymans (UvA) Routing Friday, February 24, 2017 24 / 30
Routing in the Internet Internet Routing (3) Routers within the boundary of a single AS communicate with each other to provide Intra-AS routing Using an interior routing (or gateway) protocol (IGP) Examples: RIP, OSPF, IS-IS Extended with an internal exterior protocol (ibgp) Karst Koymans (UvA) Routing Friday, February 24, 2017 25 / 30
Different routing mechanisms Outline 1 Basic setup Directly connected Not directly connected Addressing and routing commands Route selection 2 Mathematical representation 3 Routing in the Internet 4 Different routing mechanisms Karst Koymans (UvA) Routing Friday, February 24, 2017 26 / 30
Different routing mechanisms Routing Protocol Classification Static A human protocol Explicit commands like route add or ip route add Side effect of a script ( semi-automatic/semi-human ) Dynamic (automatically by a routing protocol) Distance Vector (for instance RIP) Path Vector (for instance BGP) Link State (for instance OSPF or IS-IS) Karst Koymans (UvA) Routing Friday, February 24, 2017 27 / 30
Different routing mechanisms Distance Vector Routing Distance vector algorithm Bellman-Ford (1957) Distributed shortest path Original ARPANET routing algorithm Decentralised Asynchronous No choice of root like in STP Karst Koymans (UvA) Routing Friday, February 24, 2017 28 / 30
Different routing mechanisms Path Vector Routing Like distance vector routing, but... Instead of the distance to the destination the complete path on AS level is specified The algorithm is still Decentralised...... but there is an informal notion of tiers Asynchronous The algorithm may depend on explicit or implicit policies Karst Koymans (UvA) Routing Friday, February 24, 2017 29 / 30
Different routing mechanisms Link State Routing Link state algorithm Dijkstra (1959) Single source shortest path Complete knowledge is distributed to all nodes in a region Knowledge about the local network topology is flooded to all participants in an area Every node executes the shortest path algorithm and draws the same conclusions Karst Koymans (UvA) Routing Friday, February 24, 2017 30 / 30