Understanding Network Layer Addressing & Routing

1. Announcements • List hy535-list@csd.uoc.gr • Lab is still under construction • Next session we will have paper discussion, assign papers, 3x15 min presentation • first homework will be given next class

2. Review • IP protocol • Address formats IPv4, IPv6 • Address per interface • Subnets, network mask, prefix len • IP is connectionless, packet forwarding • Packets follow independent paths to destination • Based on destination address • DNS to allow me to use symbolic names

3. Network layer vs. Link Layer • Each link has also a link layer address that is technology specific • Ethernet MAC • ATM etc … • Network layer address (IP) is independent of the networking technology

4. Sending to the subnet • Nodes are “close” can reach them directly in layer 2 - same “broadcast domain” ! • Need to know the MAC address • Need to map IP to MAC address • ARP, cache response, broadcast • No need for any other mechanism, this is not routing • But I can not have everything in a single subnet

5. What is routing/router • Device with multiple ports of different networking technologies • Forward a packet between subnets • Forwarding table • Contains prefixes (and not addresses) • Device • Gateway • Longest prefix match (LPM) • Default route 0.0.0.0/0

6. Concept of ASes • A set of systems under the same administration • Forth-net, EDET k.o.k • Same rules, policies • Different protocols inside and among the domains • Inside is relatively small • Intra-domain • Among is massively huge… • Inter-domain

7. The Big picture • Multiple Ases talking to each other • PoPs • We will revisit later… • Some numbers • ASes: 23,400 • Prefixes: 214,000 • Average AS Path lengths: 3,6 • Average Prefix length: 22,3

8. Hierarchy, what makes it all work • Can aggregate multiple routing table entries in large ones (less specific, larger prefix) • Is convenient to allocate addresses hierarchically • Global provider, local ISPs, customers • There are some problems though: multi-homing

9. What is important • Scale: can have tons of prefixes • Speed: need to forward fast • Resilience to faults: some link somewhere is bound to fail • Management and misconfigurations: there are 23,400 entities that collaborate to make all this work

10. Some generalities about routing • Attempt to find a “good” path for the packet • In reality, I just find the best “next-hop” • Routing is packet-packet • EXAMPLE

11. Cost in Routing • Good can have multiple definitions • Small delay • Do not send the packet to athens through the US • Less loaded (related to delay) • Less expensive (real money) • Less cost (administrative cost) • Less hops • In practice it is least cost routing today • See example SPF: cost is set according to some recipies/rules of thumb

12. ECMP • May have multiple next-hops with the same cost • Why not use them all • Router will load balance • But have to be done carefully to avoid out-of-order packets • ECMP, 8 or 16 in today;s routers • EXAMPLE

13. Standards • The role of standards • Necessary if different boxes are to work together • Standards bodies, IETF, ISO • The role of IETF • Democratic, collaborative • Working groups • Rough consensus and working code • Requests for Comments • Standards • Proposed standards • Informational • Historical

14. The local view – Intra-domain routing • Link state routing • The most commonly used today • Basic concept: • Each router has a complete view of the topology of the network • Pros: simple and fast convergence • Cons: expensive to maintain reliably • Flooding • Compute SPF routes • Link state routing allows me to do much-much more

15. What is important • The view of each router about the network has to agree • Else routing loops • TTL will catch it • EXAMPLE

16. Basic Structures • Each router has • A list of neighbors • The topology database that describes the network • And the routing table

17. Basic Operations • Join the network • Discover neighbors • Forming adjacency • Database exchange • Monitor for faults and handle changes • Monitor neighbor’s up status • Reliable Flooding • Route Computation • Scaling • Multiple areas

18. OSPF • Open SPF, standard protocol today • Not the only one though IS-IS is also strong • Has all the elements: • HELLO protocol for neighbor discovery and health monitoring • Database exchange for database syn on start • Reliable flooding for propagating changes

19. Details • Packets sent as an IP protocol (OSPF protocol 89) • Does not use TCP/UDP etc… • 5 packet types: hello, LS-req, LS-upd, LS-ack and DD-desc • LS-* packets carry link states

20. What is a LS • Describes an object in the network • Router, network, external prefix • Is originated by a specific router, has an id and a sequence number • Each OSPF router has a unique router-id • Routers exchange LS through flooding, build their LS database and then compute routes • EXAMPLE • EXAMPLE for link failure

21. Flooding • When receiving an update send it to all your adjacencies except the one it came from • It is reliable, each LS sent must be acknowledged (with an LS-ack packet) • Can receive duplicates • Discard • It is a bit expensive

22. Joining an OSPF network • EXAMPLE

23. Route computation • Build the shortest path tree rooted at the computing node and derive the next hop information for each destination • EXAMPLE