Routing in Wired Nets CS 215 W 01 - Mario Gerla

Routing Principles Routing: delivering a packet to its destination on the best possible path Routing steps: (a) determine node network address (b) compute/construct the path (c) forward the packet to destination Here, we will focus on (b) - routing alg. for path computation

Routing Alg Requirements Find path with min delay, cost or other metric dynamic reconfiguration after failures/changes adaptive load balancing

Bellman Ford Alg The algorithm used to compute DVs is the Bellman Ford (B-F) Algorithm For DV computation, we have used a “decentralized” version of the B-F algorithm The B-F based DV routing algorithm is used in many network routing protocols: BGP, ISO IDRP, RIP, Novell IPX, original ARPANET, Packet Radio net, etc.

DV table example

DV convergence example

Count-to-infinity problem

Poison Reverse If node Z uses next node Y to get to X, Z will advertises D(X) = 00 to Y

Poison Reverse (cont) Note: loops with 3 or more nodes (instead of ping-pong) not detected by Poison Reverse Solution? Path Vector: advertise not only the distance to destination, but the entire path to destination Path vector used in internet BGP (interdomain routing)

Link State Routing Each router measures the “costs” (eg, delay, bdw, pkt loss etc.) of its attached links Periodically (or upon link change/failure) it packs the link costs in a Link State (LS) pkt, and broadcasts the LS pkt to its neighbors The neighbors will in turn broadcast the LS pkt to their neighbors and so on until all nodes have heard the pkt (propagation via flooding) Duplicate pkts are detected and dropped based on source ID and unique sequence number

Link State Routing (cont) Each router builds a complete network topology and link cost map (identical for all routers) Next, it computes routes from itself to all other nodes in the network (using, for example, Dijkstra’s Alg). It creates a routing table with such routes Routing tables at different nodes are all consistent since they are based on the same topology/cost data base LS routing protocol used in OSPF intradomain routing

Link State vs Distance Vector Message complexity: For each cycle, O(nE) for both LS and DV, where E = # of links However, LS propagates change to ALL nodes; DV only to nodes affected by change Speed of Convergence: LS updates propagate much faster than DV updates; this is one of the reasons why ARPANET dumped DV for LS in 1979

Link State vs Distance Vector (cont) Robustness: both LS and DV tolerant of changes/failure; LS better protected against router mulfunctions (wrong path computation); the error remains local in LS; it affects the entire network in DV QoS support: in LS, complete topology map allows router to compute paths with QoS constraints (Q-OSPF) Implementation cost: LS requires more memory and more processing

Routing in the Internet The Global Internet consists of Autonomous Systems (AS) interconnected with eachother: Stub AS: small corporation Multihomed AS: large corp. (no transit) Transit AS: provider Two level routing: Intra-AS: administrator is responsible for choice Inter-AS: unique standard

Internet AS Hierarchy

Intra-AS Routing Also known as Interior Gateway Protocol (IGP) Most common IGPs: RIP: Routing Information Protocol OSPF: Open Shortest Path First

RIP ( Routing Info Protocol) Distance vector type scheme Included in BSD-UNIX Distribution in 1982 Distance metric: # of hops (max = 15 hops) Distance vector: exchanged every 30 sec via a Response Message (also called Advertisement) Each Advertisement contains up to 25 destination nets

OSPF (Open Shortest Path First) “open”: publicly available uses the Link State algorithm (ie, LS packet dissemination; topology map at each node; route computation using Dijkstra’s alg) OSPF advertisement carries one entry per neighbor router advertisements disseminated to ENTIRE Autonomous System (via flooding)

OSPF “advanced” features (not in RIP) Security: all OSPF messages are authenticated (to prevent malicious intrusion); TCP connections used Multiple same-cost paths allowed (only one path in RIP) For each link, multiple cost metrics for different TOS (eg, satellite link cost set “low” for best effort; high for real time) Integrated uni- and multicast support: Multicast OSPF (MOSPF) uses same topology data base as OSPF Hierarchical OSPF in large domains

Hierarchical OSPF

Two level hierarchy: local area and backbone Link state advertisements do not leave respective areas Nodes in each area have detailed area topology; they only know direction (shortest path) to networks in other areas Area Border routers “summarize” distances to networks in the area and advertise them to other Area Border routers Backbone routers run an OSPF routing alg limited to the backbone Boundary routers connect to other ASs

Inter-AS routing

Inter-AS routing (cont) BGP (Border Gateway Protocol): the de facto standard Path Vector protocol: and extension of Distance Vector Each Border Gateway broadcasts to neighbors (peers) the entire path (ie, sequence of AS’s) to destination For example, Gwy X may store the following path to destination Z: Path (X,Z) = X,Y1,Y2,Y3,…,Z

Inter-AS routing (cont) Now, suppose Gwy X send its path to peer Gwy W Gwy W may or may not select the path offered by Gwy X, because of cost, policy or loop prevention reasons If Gwy W selects the path advertised by Gwy X, then: Path (W,Z) = w, Path (X,Z) Note: path selection based not only on cost (eg,# of AS hops), but also on administrative and policy issues (eg, do not route packets through competitor’s AS)

Inter-AS routing (cont) Peers exchange BGP messages using TCP OPEN msg opens TCP connection to peer and authenticates sender UPDATE msg advertises new path (or withdraws old) KEEPALIVE msg keeps connection alive in absence of UPDATES; it also serves as ACK to an OPEN request NOTIFICATION msg reports errors in previous msg; also used to close a connection

Address “Scaling” As Internet grows, we run out of addresses Solution (a): subnetting. Eg, Class B Host field (16bits) is subdivided into fields Solution (b): CIDR (Classless Inter Domain Routing): assign block of contiguous Class C addresses to the same organization; these addresses all share a common prefix repeated “aggregation” within same provider leads to shorter and shorter prefixes CIDR helps also routing table size and processing: Border Gwys keep only prefixes and find “longest prefix” match

Why Intra- and Inter-AS routing ? Policy: Inter is concerned with policies (which provider we must select/avoid, etc). Intra is contained in a single organization, so, no policy decisions necessary Scale: Inter provides an extra level of routing table size and routing update traffic reduction above the Intra layer Performance: Intra is focused on performance metrics; needs to keep costs low. In Inter it is difficult to propagate performance metrics efficiently (latency, privacy etc). Besides, policy related information is more meaningful. We need BOTH!