1 CS716 Advanced Computer Networks By Dr. Amir Qayyum

Lecture No. 24

Supernetting/CIDR CIDR: Classless Inter-Domain 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 #mask lengthnext hop Interface Interface R R2 default0R3

CIDR Growth CIDR/supernetting allows hierarchical development Assign block of addresses to regional provider (e.g., /9 to BARRNET) Regional provider subdivides addresses Can hand out to subregional providers (e.g., /16 to Berkeley) Who in turn hand out to smaller organization (e.g., /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 16-bit number

Route Propagation Two-level route propagation hierarchy –Interior gateway protocol (each AS selects its own) –Exterior gateway protocol (Internet-wide 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 domain-specific 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

Intra-domain (Interior GW) Protocols Routing Information Protocol (RIP) –From the early Internet, developed for XNS –Part of Berkeley Software Distribution (BSD) Unix –Distance-vector algorithm –Based on hop count (infinity set to 16 hops)

Intra-domain (Interior GW) Protocols Open Shortest Path First (OSPF) –Internet standard (RFC 2328), “open” means public –Based on link-state algorithm –Authenticates messages –Load balances across links

Inter-domain (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

Inter-domain (Exterior GW) Protocols Exterior Gateway Protocol (EGP) –Defined on Internet with tree structure –Embodied (and enforced) tree structure –Had to be replaced eventually –Used distance-vector 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 k-out-of-n rule Routing updates –Peers periodically exchange their routing tables (distance-vector)

BGP-4: 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

BGP-4: 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

BGP-4: Borger Gateway Protocol Each AS has: –One or more border gateways (routers) to handle inter-AS traffic –One or more BGP speakers that participate in routing protocol: establish BGP sessions to exchange messages

BGP-4: 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 (intra-domain protocols use heterogeneous metrics); path-vector routing –Withdrawn routes/negative advertisements (cancel previously advertised route)

BGP Path-Vector Routing Example AS4 advertises and as local networks Speaker for AS2 advertises reachability to these networks –Network and can be reached via AS4, and network and via AS5 Speaker for backbone advertises –Networks , , , and can be reached along the path (AS1, AS2, AS4 or AS5). Backbone network (AS 1) Regional provider A (AS 2) Regional provider B (AS 3) Customer P (AS 4) Customer Q (AS 5) Customer R (AS 6) Customer S (AS 7)

BGP-4 - 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

BGP-4 - Details BGP-4 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