Advanced Computer Networks: Lecture 24. This lecture will cover the following: supernetting/CIDR; routing in large scale networks; autonomous system (AS); intra-domain (interior GW) protocols; routing information protocol (RIP); open shortest path first (OSPF);...
CS716 Advanced Computer Networks By Dr. Amir Qayyum Lecture No. 24 Supernetting/CIDR • CIDR: Classless InterDomain Routing • Compromise in address utilization vs scalability • Eliminate class notion; generalize subnet notion • All routers must understand CIDR addressing – Longest match in the table Supernetting/CIDR • Assign block of contiguous network numbers to nearby networks – Restrict block sizes to powers of 2 – Use bit mask(CIDR mask) to identify block size CIDR • Specify network with (network#, mask bits) – Equivalent to (network#, # of hosts) • Block of 8 class C networks may be treated as one • Organizations can still use subnetting internally ! • Routing table entries look like: subnet # 131.126.141.0 131.126.142.0 131.126.142.128 25 131.126.0.0 default 24 25 16 mask length next hop Interface 0 Interface 1 R1 R2 R3 CIDR Growth • CIDR/supernetting allows hierarchical development • Assign block of addresses to regional provider (e.g., 128.0.0.0/9 to BARRNET) • Regional provider subdivides addresses • Can hand out to subregional providers (e.g., 128.32.0.0/16 to Berkeley) • Who in turn hand out to smaller organization (e.g., 128.32.32.0/21 to Berkeley CS Dept) Routing in Large Scale Networks Route Propagation • Know a smarter router – Hosts know local router – Local routers know site routers – Site routers know core router – Core routers know everything • Autonomous System (AS) – Corresponds to an administrative domain – Examples: university, company, backbone network – Assign each AS a 16bit number Route Propagation • Twolevel route propagation hierarchy – Interior gateway protocol (each AS selects its own) – Exterior gateway protocol (Internetwide standard) • AS’s represent a third hierarchy – Define routing domains – Based on notion of autonomy of control Notion of Autonomous Systems (AS) • Intradomain routing (within an AS) – Performed using domainspecific algorithm – Selected by domain administrator (autonomously) – Allows heterogeneous interior gateway protocols • Interdomain routing (between AS’s) – Performed using standard global algorithm – Homogeneous exterior gateway protocol Intradomain (Interior GW) Protocols • Routing Information Protocol (RIP) – From the early Internet, developed for XNS – Part of Berkeley Software Distribution (BSD) Unix – Distancevector algorithm – Based on hop count (infinity set to 16 hops) Intradomain (Interior GW) Protocols • Open Shortest Path First (OSPF) – Internet standard (RFC 2328), “open” means public – Based on linkstate algorithm – Authenticates messages – Load balances across links Interdomain (Exterior GW) Protocols • Very complex and difficult – Different metrics, security, large scale: 140K prefixes! • Focuses on reachability rather than optimality – Support for very flexible policies Interdomain (Exterior GW) Protocols • Exterior Gateway Protocol (EGP) – Defined on Internet with tree structure – Embodied (and enforced) tree structure – Had to be replaced eventually – Used distancevector updates – Replaced by Border Gateway Protocol (BGP) EGP Messages • Neighbor acquisition – One router requests that another be its peer – Peers exchange reachability information • Neighbor reachability – One router periodically tests if the another is still reachable – Exchange HELLO/ACK messages – Uses a koutofn rule • Routing updates – Peers periodically exchange their routing tables (distancevector) BGP4: Border Gateway Protocol • Internet is an arbitrarily interconnected set of AS’s • Two types of traffic – Local: begins or ends within an AS – Transit: moves through an AS BGP4: Border Gateway Protocol • Three types of AS’s – Stub: one single connection to one other AS; carries local traffic only – Multihomed: connections to multiple other AS’s, but refuses to carry transit traffic – Transit: connections to multiple other AS’s and designed to carry both transit and local traffic BGP4: Borger Gateway Protocol • Each AS has: – One or more border gateways (routers) to handle interAS traffic – One or more BGP speakers that participate in routing protocol: establish BGP sessions to exchange messages BGP4: Borger Gateway Protocol • BGP speaker advertises: – Names of networks within the AS – Names of other reachable networks through the AS (transit AS only) – Full path information (intradomain protocols use heterogeneous metrics); pathvector routing – Withdrawn routes/negative advertisements (cancel previously advertised route) BGP PathVector Routing Example • AS4 advertises 128.96 and 192.4.153 as local networks • Speaker for AS2 advertises reachability to these networks – Network 128.96 and 192.4.153 can be reached via AS4, and network 192.4.32 and 192.4.3 via AS5 Regional provider A (AS 2) Backbone network (AS 1) Regional provider B (AS 3) Customer P (AS 4) 128.96 192.4.153 Customer Q (AS 5) 192.4.32 192.4.3 Customer R (AS 6) 192.12.69 Customer S (AS 7) 192.4.54 192.4.23 • Speaker for backbone advertises – Networks 128.96, 192.4.153, 192.4.32, and 192.4.3 can be reached BGP4 Details • Full path in BGP messages to avoid loops – Best route according to local policies is advertised – No obligation to advertise route to known destinations • 16 bit AS numbers are uniquely assigned – Stub ASs do not need a unique AS number BGP4 Details • BGP4 designed to support classless addresses – Update messages contain prefix & its length (10.1/16) • Update messages are reliably sent using TCP – Occasional “keepalive” messages if nothing changes Building Scalable Networks … • Subdivided the routing problem into manageable parts – New level of hierarchy is introduced • Complexity of interdomain routing: – Order of number of ASs • Complexity of interdomain routing: – Ooder of networks in an AS Integrating Interdomain and Intradomain Routing • How routers in a domain get routing information ? • In a stub AS with single border router – Inject a default route in intradomain routing protocol Integrating Interdomain and Intradomain Routing • In a domain with multiple border routers (any AS) – Border routers inject specific routes learned from outside, with some cost • In backbone networks, too costly to inject too many outside routes in intradomain protocol – Use Interior BGP (IBGP) to redistribute outside routes .. .Lecture? ?No.? ?24 Supernetting/CIDR • CIDR: Classless InterDomain Routing • Compromise in address utilization vs ... Routing table entries look like: subnet # 131.126.141.0 131.126.142.0 131.126.142.128 25 131.126.0.0 default 24 25 16 mask length next hop Interface 0 Interface 1 R1 R2 R3 CIDR Growth • CIDR/supernetting allows hierarchical