Network Architecture (R02) - L1

1. Network Architecture (R02) - L1 Jon Crowcroft, http://www.cl.cam.ac.uk/~jac22 http://www.cl.cam.ac.uk/teaching/1010/R02/

2. Course Structure • 16 Lectures • several guest slots - • Andrew Moore - router h/w algorithmics • Cecilia Mascolo - sensor/mobile net arch • Dirk Trossen - pub/sub in the net • Hamed Haddadi - topology over time • Participation by you • Reading & • Critique-ing papers • See “How to read a paper” by Keshav • http://www.sigcomm.org/ccr/drupal/files/p83-keshavA.pdf

3. Course Assessment • Your involvement each week • Alternate me present & you all contribute • 3 essays - compare/contrast • Due dates • oct 29, • nov 26, • start of next term • An annotated bibligraphy • At start of next term, but update each week

4. Workload • Read 1-2 papers per week • Plus scan related material • Keep notes • Feel free to ask me more • Essays can be • 2-4 pages • Note form is fine • References/citing source material essential

5. Review of Internet Architecture • Packet switching • No circuit, virtual or otherwise • Datagram Network • No set up - fast for transactions • Work Conserving • (video download can be faster than viewing) • “Stateless” • (end and router don’t share state) • (max pkt size unchanged for 30 yrs!)

6. Parsimony • End to end model • (clark et al) • Cautious Sender • Forgiving Receiver • (postel principle) Many different kinds of applications and higher-level protocols IP Many different kinds of networks The “Hourglass Model”, Steve Deering

7. Vers H. Len TOS Total Length Identification Flags Fragment Offset Time to live Protocol Header Checksum Source IP Address Destination IP Address IP Options (if any) Padding Data IP packet

8. IP Address & Forwarding • Based on destination address (32 bits!) • Not source (why is it there?) • Forwarding is “hop by hop” • May change (or fail) somewhere along path • Address is “where” something is • an interface of a host (can have lots) • Route is “how to get there” • Computed seperately, continuously and asynchronously • Names (see later) are “what” something is

9. Two components of routing • Control component • Decides where the packets will go • Use a set of routing protocols (e.g. OSPF, BGP) to collect information and produce a “forwarding table” • “Control plane” • Forwarding component • Moving packets from input to output ports according to forwarding table and packet header • “Forwarding plane” Routing “daemon” collect routing info and maintain routing DB routes kernel Forwarding table Forwarding algorithm and mechanism packets

10. Address Matching • Packet forwarding requires • Address matching Followed by table lookup of output port • Moving the packet through the router (from input port to output port) This involves scheduling, queueing, design of switch fabric etc, conventional aspects of switch design • Address matching • Exact matching e.g. bridge forwarding, DECnet, OSI/CLNP… • Longest prefix match – “best matching” IP networks

11. Exact match • Easier • Software approach: • Binary search • Hash function • Hardware: Content Addressable Memory (CAM)

12. Longest prefix match • IP addresses are assigned in a manner that reflect network topology • Address aggregation: group destinations with the same prefix together if they exit the same output port • Therefore, longer prefixes tend to be announced by customers ISPs who are closer to the destination, whereas provider ISPs tend to announce aggregated addresses • Hence a route to the longest prefix match is preferred

13. Example to show why “longest prefix match” is better BGP route advertisement for 1.2.3/24 Forwarding table Forwarding table ISP B (provider of ISP A) ISP C (provider of ISP A) Peer relationship 1.2.3/24 1.2.3/24 1.2.3.123/26 BGP route advertisement for 1.2.3.123/26 BGP route advertisement for 1.2.3.123/26 Longer prefix is a better route! ISP A Subnet 1.2.3.123/26

14. Example • Each entry in forwarding table has address + prefix e.g. address: 11001111 01011100 00000000 10000111 mask: 11111111 11111111 11111111 11111111 address: 11001111 01011100 00000000 00000000 mask: 11111111 11111111 00000000 00000000 address: 11001111 01011100 00000000 00000000 mask: 11111111 11111111 11100000 00000000 Longest match 11001111 01011100 00000000 10000111 matches with all three entries

15. How to do Longest Prefix Match • Not as easy as exact match • Approaches: • Create a data structure for doing LPM • Convert the problem into a form so that we can do binary search • Reduce the problem to a sequence of exact match problems which we can apply hashing • Optimization based on distribution of prefix lengths • Combine software and hardware techniques

16. Algorithms There is an entire industry of algorithms: • Binary search among all prefixes in forwarding table Perlman’s book, 13.4 Lampson et al “IP Lookups using Multiway and Multicolumn Search”, IEEE Infocom 1998 • Trie: bit-by-bit match Perlman’s book, 13.3 • Binary search based on prefix length Perlman’s book, 13.3.3 Waldvogel et al “Scalable High Speed IP Routing Lookups”, Sigcomm 1997

17. But this is all going wrong! Why? • Not enough bits -> NATs… • NAT Traversal, Stateful browser/server • end is URL + Persistent HTTP state + cookie! • Three M’s (historical order) • Multicast • Mobility • Multihoming • Security and Social Scale • Unsolicited traffic • Byzantine (v. selfish or rational or altruistic) • Despite original ARPANET packet radio • And multicast since 1988, • Hierarchy is wrong

18. So Ipng effort started in 1992 • See course web site for papers! • Specification of desiderata • Led to a set of competing efforts • Look at SIP & PIP • Represent extremes of • CS (SIP) & Telco (PIP) • SIP from PARC looks XNS • Just ip with more address bits • PIP looks VC/ATM ish… • QoS, fancy routing options

19. Eventually, converged on IPv6 • Committee design • Overtaken by reality ? • Three M’s (current order) • Multihoming - killing aggregation • Mobility (smart phones roaming and receiving IP) • Multicast - sidelined? • New requirements • Receiver control of input • New kinds of bad guys • Authentic addresses (HIP) • New content type (video “interest”)

20. For next week (Tuesday 12th oct) • I want each of you to read the papers • And come up with • 1 good feature of IPv6 • 1 bad feature of IPv6 • And email me 1 slide with that on! • Which YOU will present! • And we will discuss how the desiderata (requirements) changed!