Intradomain Routing Protocols By Behzad Akbari These slides are based in part upon slides of Prof. Shivkumar (Rpi university) and Sanjay Rao (Purdue university )

 Intradomain routing protocols  Distance Vector  RIP, RIPv2, EIGRP  Link State  OSPF, IS-IS  Intradomain Traffic Engineering Outline

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) will this completely solve count to infinity problem? x z y 60

RIPv1 Problems (Continued) Split horizon/poison reverse does not guarantee to solve count-to-infinity problem  16 = infinity => RIP for small networks only!  Slow convergence Broadcasts consume non-router resources  It sends updates as broadcasts on 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  Sends on all outgoing links When Node Y Receives Information from Z  Send on all links other than Z Naïve Approach:  Floods indefinitely.  Prevent through sequence numbers XA CBD (a) XA CBD (b) XA CBD (c) XA CBD (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 Add entry, propagate to all neighbors other than Y  Entry present with sequence number p < q Update entry, propagate to all neighbors other than Y  Entry present with sequence number p > q 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 OSPF v1  Force sequence number back to 0 OSPF v2 With 32-bit counter, doesn’t happen very often 0Max a b a < b 0Max a b a < b

OSPF Load Balancing 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 A E F C D B Table for B DstCstHop A3A B0B C2F D3D E4F F1F Table for B DstCstHop A3A B0B C2F D3D,F E4A,F F1F

Type of Service (TOS) Metrics Link Characteristic Vary in Multiple Dimensions  Latency  Throughput  Cost  Reliability Example  Satellite link High throughput, long latency  Fiber optic link 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 Don’t do anything extreme  Minimize cost 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)  New Question: Who creates the network-LSA? Dijkstra algo view Encoding of LSAs, Flooding/DB sync model

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 /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 Intra-domain Routing Protocol Discriminator Length Indicator TLV Fields Version/Protocol ID Extension ID Length RRR PDU Type Version Reserved Maximum Area Addresses Packet-Specific Header Fields No. of Octets

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 A B C D E 2 Can not do this with OSPF A B C D E 2 Links AB and BD are overloaded A B C D E 2 Links AC and CD are overloaded

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 Distribute traffic and performance measurements Compute paths based on load, and requirements  Adaptive network-management system 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  Routers receive feedback on load and delay  Routers re-compute their forwarding tables  Fundamental problems with oscillation Load-sensitive routing at circuit (or aggregate) level  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