CSE 561 – Scalable Routing David Wetherall Spring 2000

djw // CS 561, Spring 2000 L5.2 This Lecture First, Clark on Internet design philosophy Making routing scale –Landmark paper –Other techniques Mobile IP

djw // CS 561, Spring 2000 L5.3 Clark, 1988 Design philosophy in retrospect; important themes Survivability and impact on where to store state Division into IP + TCP / UDP for QOS

djw // CS 561, Spring 2000 L5.4 Network Service Models Datagram delivery: postal service –Also connectionless, best-effort or unreliable service –Network can’t guarantee delivery of the packet –Each packet from a host is routed independently –Example: IP Virtual circuit models: telephone –Also connection-oriented service –Signaling: connection establishment, data transfer, teardown –All packets from a host are routed the same way –Example: ATM, Frame Relay, X.25

djw // CS 561, Spring 2000 L5.5 “Survivability” What state goes where? –Router versus end system What does survivability entail? –Application versus network perspective Problem: loss of state held at routers could disrupt application even though a path existed Solution: fate sharing Follow-on: flows and soft-state

djw // CS 561, Spring 2000 L5.6 “Types of Service” Application requirements –Impact of reliability –The TCP/IP split, with UDP Quality of service –What is provided by the TCP/UDP distinction? –What is provided by other mechanisms?

djw // CS 561, Spring 2000 L5.7 IPv4 Header Fields … Type of Service Abstract notion of kind of service, never really worked out –Routers ignore Field will be reused for Diffserv VersionHLen TOSLength Identifier for FragmentsFlagsFragment Offset TTLProtocolChecksum Source Address Destination Address Options (variable) Pad (variable) Data

djw // CS 561, Spring 2000 L5.8 Shortcomings Some have become increasingly apparent today … Accounting End host control Distributed management

djw // CS 561, Spring 2000 L5.9 Scalability Concerns Routing burden grows with size of an internetwork –Size of routing tables –Volume or routing messages –Amount of routing computation RIP/OSPF do not scale to the size of the Internet We must apply further techniques: –Careful address allocation –Use of hierarchy –Route aggregation

djw // CS 561, Spring 2000 L5.10 Landmark Routing Area versus landmark hierarchy Pros and cons of hierarchy –Routing table size, number of messages, computation –Path length –Forwarding algorithm

djw // CS 561, Spring 2000 L5.11 Backbone service provider Peering point Peering point Large corporation Small corporation “ Consumer ” ISP “Consumer” ISP “ Consumer” ISP You at home You at work Structure of the Internet

djw // CS 561, Spring 2000 L5.12 IP Addresses Reflect location in topology; used for scalable routing –Unlike “flat” Ethernet addresses Interfaces on same network share prefix –Prefix administratively assigned (IANA or ISP) –Addresses globally unique Routing only advertises entire networks –Local delivery within a single “network” doesn’t involve router –(will make “network” precise later on)

djw // CS 561, Spring 2000 L5.13 NetworkHost NetworkHost NetworkHost IPv4 Address Formats 32 bits written in “dotted quad” notation, e.g., Class A Class B Class C

djw // CS 561, Spring 2000 L5.14 IPv6 Address Format 128 bits written in 16 bit hexadecimal chunks Still hierarchical, just more levels SubscriberIDProviderIDRegistryID001InterfaceIDSubnetID

djw // CS 561, Spring 2000 L5.15 Network numberHost number Class B address Subnet mask ( ) Subnetted address Network numberHost IDSubnet ID Subnetting Split up one network number into multiple physical networks Internal structure isn’t propagated Helps allocation efficiency

djw // CS 561, Spring 2000 L5.16 Subnet mask: Subnet number: H1 R Subnet mask: Subnet number: R2 H Subnet mask: Subnet number: H3 Subnet Example

djw // CS 561, Spring 2000 L5.17 Inter-Domain Routing Network comprised of many Autonomous Systems (ASes) or domains To scale, use hierarchy: separate inter-domain and intra-domain routing Also called interior vs exterior gateway protocols (IGP/EGP) –IGP = RIP, OSPF –EGP = EGP, BGP

djw // CS 561, Spring 2000 L5.18 Inter-Domain Routing Border routers summarize and advertise internal routes to external neighbors and vice- versa Border routers apply policy Internal routers can use notion of default routes Core is “default-free”; routers must have a route to all networks in the world AS1 AS2 Border router

djw // CS 561, Spring 2000 L5.19 NSFNET backbone Stanford BARRNET regional Berkeley PARC NCAR UA UNM Westnet regional UNL KU ISU MidNet regional … Exterior Gateway Protocol (EGP) First major inter-domain routing protocol Constrained Internet to tree structure; no longer in use

djw // CS 561, Spring 2000 L5.20 Border Gateway Protocol (BGP-4) EGP used in the Internet backbone today Features: –Path vector routing –Application of policy –Operates over reliable transport (TCP) –Works with CIDR

djw // CS 561, Spring 2000 L5.21 Path Vectors Similar to distance vector, except send entire paths –e.g. 321 hears [7,12,44] –stronger avoidance of loops –supports policies (later) Modulo policy, shorter paths are chosen in preference to longer ones Reachability only – no metrics

djw // CS 561, Spring 2000 L5.22 Policies Choice of routes may depend on owner, cost, AUP, … –Business considerations Local policy dictates what route will be chosen and what routes will be advertised! –e.g., X doesn’t provide transit for B, or A prefers not to use X A BX

djw // CS 561, Spring 2000 L5.23 Impact of Policies – Example Early Exit / Hot Potato –“if it’s not for you, bail” Combination of best local policies not globally best Side-effect: asymmetry B A

djw // CS 561, Spring 2000 L5.24 Operation over TCP Most routing protocols operate over UDP/IP BGP uses TCP –TCP handles error control; reacts to congestion –Allows for incremental updates Issue: Data vs. Control plane –Shouldn’t routing messages be higher priority than data?

djw // CS 561, Spring 2000 L5.25 CIDR (Supernetting) CIDR = Classless Inter-Domain Routing Aggregate advertised network routes –e.g., ISP has class C addresses through –Really like one larger 20 bit address class … –Advertise as such (network number, prefix length) –Reduces size of routing tables But IP forwarding is more involved –Based on Longest Matching Prefix operation

djw // CS 561, Spring 2000 L5.26 Border gateway (advertises path to ) Regional network Corporation X ( ) Corporation Y ( ) CIDR Example X and Y routes can be aggregated

djw // CS 561, Spring 2000 L5.27 Mobile IP Problem: addresses tied to topology and so change as a node moves; this disrupts communications Approaches: –Change DNS –Mobile IP (level of indirection at network level) –Link-layer mobility Mobile IP Solution

djw // CS 561, Spring 2000 L5.28 Getting an IP address Old fashioned way: sysadmin configured each machine Dynamic Host Configuration Protocol (DHCP) –One DHCP server with the bootstrap info Host address, gateway address, subnet mask, … Find it using broadcast –Addresses may be leased; renew periodically “Stateless” Autoconfiguration (in IPv6) –Get rid of server – reuse Ethernet addresses for lower portion of address (uniqueness) and learn higher portion from routers

djw // CS 561, Spring 2000 L5.29 Address Resolution Protocol (ARP) On a single link, need Ethernet addresses to send a frame … source is a given, but what about destination? –Requires mapping from IP to MAC addresses ARP is a dynamic approach to learn mapping –Node A sends broadcast query for IP address X –Node B with IP address X replies with its MAC address M –A caches (X, M); old information is timed out (~15 mins) –Also: B caches A’s MAC and IP addresses, other nodes refresh

djw // CS 561, Spring 2000 L5.30 Mobile IP Issues in Practice Scalable, and backwards compatible Slow takeoff: quasi-mobile and DHCP Handoff latencies Security

djw // CS 561, Spring 2000 L5.31 ARP Example To send a message … common case doesn’t require ARP time A B Who-is X? I-am X