Network Architecture (R02) - L2 14/10/13

Network Architecture (R02) - L214/10/13 Jon Crowcroft, http://www.cl.cam.ac.uk/~jac22 http://www.cl.cam.ac.uk/teaching/1314/R02/

Design Philosophy of the DARPA Internet Protocols - DDC/Sigcomm 1988 • Fundamental Goal – “effective technique for multiplexed utilization of existing interconnected networks” • Sub-Goals: • Survivability • Types of service • Diversity • Distributed management • Cost effective • Low barrier to host entry • accountable

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

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 should be “where” something is • an interface of a host (can have lots) • IP has several roles, conflated • Routing Hint, Interface Id, Part of Flow State… … • Route is “how to get there” • Computed seperately, continuously and asynchronously • Names (see later) are “what” something is

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

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

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

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

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

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

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

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

But this is all going wrong! Why? • Not enough bits -> NATs… • NAT Traversal, Stateful browser/server • end is URL + Persistent HTTP state + cookie! • Four M’s (historical order) • Multicast • Mobility • Multihoming • Multipath • 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

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

Eventually, converged on IPv6 • Committee design (SIP/PIP/Novell) • Overtaken by reality ? • Four M’s (current order) • Multihoming - killing aggregation • Mobility (smart phones roaming and receiving IP) • Multipath (load balance, but how to id sub-flow) • Multicast - sidelined? • New requirements • Receiver control of input • New kinds of bad guys • Authentic addresses (HIP) • New content type (video “interest”)

For thursday- oct §7 • you talk about • Haggle! http://www.cl.cam.ac.uk/teaching/1314/R02/materials.html

Network Architecture (R02) - L2 14/10/13