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Transcript
Intradomain Routing Protocols By Behzad Akbari These slides are based in part upon slides of Prof. Shivkumar (Rpi university) and Sanjay Rao (Purdue university) Outline Intradomain routing protocols Distance Vector Link State RIP, RIPv2, EIGRP OSPF, IS-IS Intradomain Traffic Engineering RIP: Routing Information Protocol Uses hop count as metric (max: 16 is infinity) Tables (vectors) “advertised” to neighbors every 30 s. Each advertisement: up to 25 entries No advertisement for 180 sec: neighbor/link declared dead routes via neighbor invalidated new advertisements sent to neighbors (Triggered updates) neighbors in turn send out new advertisements (if tables changed) link failure info quickly propagates to entire net poison reverse used to prevent ping-pong loops (infinite distance = 16 hops) If Z routes through Y to get to X : Z tells Y its (Z’s) distance to X is infinite (so Y won’t route to X via Z) 60 will this completely solve count to infinity problem? x 4 y 50 1 z RIPv1 Problems (Continued) Split horizon/poison reverse does not guarantee to solve count-to-infinity problem Broadcasts consume non-router resources 16 = infinity => RIP for small networks only! Slow convergence It sends updates as broadcasts on 255.255.255.255 RIPv1 does not support subnet masks (VLSMs) It does not send subnet mask information in its updates. It does not support authentication RIPv2 Why ? Installed base of RIP routers Provides: VLSM support Authentication Multicasting Uses reserved fields in RIPv1 header. First route entry replaced by authentication info. EIGRP (Interior Gateway Routing Protocol) CISCO proprietary; successor of RIP (late 80s) Several metrics (delay, bandwidth, reliability, load etc) Uses TCP to exchange routing updates Loop-free routing via Distributed Updating Alg. (DUAL) based on diffused computation Freeze entry to particular destination Diffuse a request for updates Other nodes may freeze/propagate the diffusing computation (tree formation) Unfreeze when updates received. Tradeoff: temporary un-reachability for some destinations Link State vs. Distance Vector Link State (LS) advantages: More stable (aka fewer routing loops) Faster convergence than distance vector Easier to discover network topology, troubleshoot network. Can do better source-routing with link-state Type & Quality-of-service routing (multiple route tables) possible Link State Protocols Key: Create a network “map” at each node. 1. Node collects the state of its connected links and forms a “Link State Packet” (LSP) 2. Flood LSP => reaches every other node in the network and everyone now has a network map. 3. Given map, run Dijkstra’s shortest path algorithm (SPF) => get paths to all destinations 4. Routing table = next-hops of these paths. 5. Hierarchical routing: organization of areas, and filtered control plane information flooded. Link State Issues Reliable Flooding: sequence #s, age LSA types, Neighbor discovery and maintenance (hello) Efficiency in Broadcast LANs, NBMA, Pt-Mpt subnets: designated router (DR) concept Areas and Hierarchy Area types: Normal, Stub, NSSA: filtering External Routes (from other ASs), interaction with inter-domain routing. Sending Link States by Flooding X Wants to Send Information C B X A D C B (a) D (b) Send on all links other than Z Naïve Approach: A Sends on all outgoing links When Node Y Receives Information from Z X Floods indefinitely. Prevent through sequence numbers X A C B (c) D X A C B (d) D OSPF Reliable Flooding Transmit Link State Advertisements Originating Router List of directly connected neighbors of that node with the cost of the link to each one Sequence Number Incremented each time sending new link information Link State Age Packet expires when a threshold is reached, OSPF Flooding Operation Node X Receives LSA from Node Y With Sequence Number q Looks for entry with same origin/link ID Cases No entry present Entry present with sequence number p < q Update entry, propagate to all neighbors other than Y Entry present with sequence number p > q Add entry, propagate to all neighbors other than Y Send entry back to Y To tell Y that it has out-of-date information Entry present with sequence number p = q Ignore it Flooding Issues When Should it be Performed Periodically When status of link changes Detected by connected node What Happens when Router Goes Down & Back Up Sequence number reset to 0 Other routers may have entries with higher sequence numbers Router will send out LSAs with number 0 Will get back LSAs with last valid sequence number p Router sets sequence number to p+1 & resends Flooding Issues (Cont.) What if Sequence Number Wraps Around Use circular comparison a OSPF v1 b a Max 0 a<b b Max 0 a<b Force sequence number back to 0 OSPF v2 With 32-bit counter, doesn’t happen very often OSPF Load Balancing E Table for B Dst Cst 3 C 1 Hop A 3 A B 0 B C 2 F D 3 D,F D E 4 A,F F F 1 F 1 F 1 6 1 A 3 3 B Modification to Dijkstra’s algorithm Keep track of all links giving optimum cost d(v) Only get multiple routes when exactly same cost Routing Alternate link used Tends to cause packets to arrive out of order D Type of Service (TOS) Metrics Link Characteristic Vary in Multiple Dimensions Latency Throughput Cost Reliability Example Satellite link Fiber optic link High throughput, long latency High throughput, low latency Routing Requirements Vary Typing at terminal: minimize latency for short packet Sending video data: maximize throughput Proposed OSPF Support for TOS Support up to Five Different Routing Metrics Normal service Minimize cost Don’t do anything extreme For networks that charge for traffic Maximize reliability Maximize throughput Minimize delay Link Can Have Different LSA for each TOS Expressed in units where lower value is better Path cost either sum or maximum of link costs Designated Router (DR) Dijkstra algo view Encoding of LSAs, Flooding/DB sync model New Question: Who creates the network-LSA? Designated Router (…) One router elected as a designated router (DR) on LAN Each router maintains flooding adjacency with the DR, I.e., sends acks of LSAs to DR DR informs each router of other routers on LAN DR generates the network-LSA on subnet’s behalf after synchronizing with all routers Primary/Backup: DR, BDR (…) Backup DR (BDR) also syncs with all routers, and takes over if DR dies (typically 5 s wait) Total: 2N – 1 adjacencies DR election: First router on net = DR, second = BDR RouterPriority: [0, 127] indicated in Hello packet=> highest priority router becomes DR If network is partitioned and healed, the two DRs are reduced to one by looking at RouterPriority Hierarchical Routing Why Hierarchy? Information hiding (filtered) => computation, bandwidth, storage saved => efficiency => scalability But filtering in control plane, not data plane Address abstraction vs. Topology Abstraction Multiple paths possible between two adj. areas Area Configured area ID A set of address prefixes Do not have to be contiguous So a prefix can be in only one area A set of router IDs Router functions may be interior, inter-area, or external Hierarchical OSPF Two-level hierarchy: local area, backbone. Link-state advertisements only in area each nodes has detailed area topology; only know direction (shortest path) to nets in other areas. Two-level restriction avoids count-to-infinity issues in backbone routing. Area border routers (ABR): “summarize” distances to nets in own area, advertise to other Area Border routers. Backbone routers: uses a DV-style routing between backbone routers Boundary routers (AS-BRs): connect to other ASs (generate “external” records) Hierarchical OSPF Sample Area Configuration 10.2.0.0/24 IS-IS Overview The Intermediate Systems to Intermediate System Routing Protocol (IS-IS) was originally designed to route the ISO Connectionless Network Protocol (CLNP) . (ISO10589 or RFC 1142) Adapted for routing IP in addition to CLNP (RFC1195) as Integrated or Dual IS-IS (1990) IS-IS is a Link State Protocol similar to the Open Shortest Path First (OSPF). OSPF supports only IP IS-IS competed neck-to-neck with OSPF. OSPF deployed in large enterprise networks IS-IS deployed in several large ISPs IS-IS Terminology Intermediate system (IS) - Router Designated Intermediate System (DIS) - Designated Router Pseudonode - Broadcast link emulated as virtual node by DIS End System (ES) - Network Host or workstation Network Service Access Point (NSAP) - Network Layer Address Subnetwork Point of attachment (SNPA) - Datalink interface Packet data Unit (PDU) - Analogous to IP Packet Link State PDU (LSP) - Routing information packet Level 1 and Level 2 – Area 0 and lower areas Functional Comparison Protocols are recognizably similar in function and mechanism (common heritage) Link state algorithms Two level hierarchies Designated Router on LANs Widely deployed (ISPs vs. enterprises) Multiple interoperable implementations OSPF more “optimized” by design (and therefore significantly more complex) Sample comparison points Encapsulation OSPF runs on top of IP=> Relies on IP fragmentation for large LSAs IS-IS runs directly over L2 (next to IP) => fragmentation done by IS-IS Media support Both protocols support LANs and point-to-point links in similar ways IS-IS supports NBMA in a manner similar to OSPF pt-mpt model: as a set of point-to-point links OSPF NBMA mode is configuration-heavy and risky (all routers must be able to reach DR; bad news if VC fails) Packet Encoding OSPF is “efficiently” encoded Positional fields, 32-bit alignment Only LSAs are extensible (not Hellos, etc.) Unrecognized types not flooded. Opaque-LSAs recently introduced. IS-IS is mostly Type-Length-Value (TLV) encoded No particular alignment Extensible from the start (unknown types ignored but still flooded) All packet types are extensible Nested TLVs provide structure for more granular extension IS-IS LS Database: Generic Packet Format No. of Octets R Intra-domain Routing Protocol Discriminator 1 Length Indicator 1 Version/Protocol ID Extension 1 ID Length 1 R R PDU Type 1 Version 1 Reserved 1 Maximum Area Addresses Packet-Specific Header Fields TLV Fields 1 Traffic Engineering: Motivation TE: “…that aspect of Internet network engineering dealing with the issue of performance evaluation and performance optimization of operational IP networks …’’ 90’s approach to TE was by changing link weights in IGP (OSPF, IS-IS) or EGP (BGP-4) Performance limited by the shortest/policy path nature Assumptions: Quasi-static traffic, knowledge of demand matrix 111 BB 14 1 A A 11 CC 22 222 D D E EE Links AB and BD are overloaded Links Can AC not and do this CD are withoverloaded OSPF Traffic Engineering What is traffic engineering? Control and optimization of routing, to steer traffic through the network in the most effective way Two fundamental approaches to adaptation Adaptive routing protocols Adaptive network-management system Distribute traffic and performance measurements Compute paths based on load, and requirements Collect measurements of traffic and topology Optimize the setting of the “static” parameters Big debates still today about the right answer QoS routing = optimization of user QoS objectives TE = optimization of user AND network QoS objectives Outline: Three Alternatives Load-sensitive routing at packet level Load-sensitive routing at circuit (or aggregate) level Routers receive feedback on load and delay Routers re-compute their forwarding tables Fundamental problems with oscillation Routers receive feedback on load and delay Router compute a path for the next circuit Less oscillation, as long as circuits last for a while Traffic engineering as a management problem Routers compute paths based on “static” values Network management system sets the parameters to influence the mapping of traffic to paths Acting on network-wide view of traffic and topology