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Ospf1

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    Ospf1 Ospf1 Presentation Transcript

    • Open Shortest Path First (OSPF) Presenter: Lilian Atieno Course: ECE 697C (Fall 2002) Instructor: Professor Linix Gao
    • Outline Introduction OSPF routing Protocol Overview – Features – Processing of LSAs and data – OSPF packet Format Black-box OSPF delay measurements – Testbed setup & Procedure – Actual experimental results Conclusions
    • Introduction OSPF is an internal Gateway Protocol – Determines how routing is done in an AS Primary Characteristics – I)Protocol is open • Specifications published in RFC – ii)It’s a Link-state routing protocol • router broadcasts routing information to other routers and so each router has complete view of topology
    • OSPF overview
    • Protocol overview Internal details of an AS is invisible to other ASs Each OSPF router maintains database of its AS topology. Router builds shortest path tree. Routers obtain forwarding table (FIB) from the tree. OSPF is a dynamic routing protocol
    • AS Splitting OSPF allows AS to be configured into areas – Each area runs its own OSPF link-state routing algorithm. • Router broadcasts its link state only to area routers Topology of an area invisible to other areas Adv. of splitting: –
    • An OSPF AS Consists of Multiple Areas
    • Classification of routers I)Internal routers • only perform intra AS routing II)Area Border routers • Belong to both area and backbone III)Backbone routers • Perform routing within the backbone IV)Boundary routers • Exchange routing info. With routers in other Ass.
    • Backbone area One area in the AS is configured to be backbone area. • Consists of Area boarder routers (ABRs) and other non-ABRs. • It must be contiguous otherwise virtual links have to be configured to restore connectivity. • It routes traffic between areas in the AS. Backbone topology invisible to intraarea routers.
    • Stub area It’s one of the enhancements in OSPF protocol. These are areas configured not be flooded by external advertisements. All data traffic to external destinations will follow default route
    • Types of OSPF routing I)Inter-area routing • Source & destination in different areas ii)Intra-area routing • Source & destination in same area OSPF also receives and transmits routes to other Ass through boundary routes.
    • Inter-area routing Its like a star configuration • backbone-hub • areas-spokes How does routers choose ABR for packet exit? • Each ABR in an area advertise location of AS boundary router. • Also advertise cost to ABRs in other areas
    • Router Model LSA Processing Route Processor (CPU) OSPF Process LSA Flooding Topology View SPF Calculation SPF Calculation FIB Update FIB LSA LS Ack Data packet Forwarding Forwarding Interface card LSA Switching Fabric Interface card Data packet
    • LSA Processing Receive an LS Update packet Get (next) LSA new New/duplicate? duplicate Update link state database Schedule SPF calc. if reqd. Acknowledge LSA immed. Send LS Ack packet back Determine which interfaces LAS needs to be flooded out Next LSA exist? LS Update processing over Bundle LSAs into packet & flood it out SPF calculation FIB is updated
    • Types of Link-state Advertisements Router link adv. • Originated by all routers (flooded per area). • Describes states of the router’s interfaces Network link adv • Originated by Designated router for multiaccess networks (flooded per area) • Contain list of all routers connected to the network
    • Cont’ Summary link adv • Originated by ABRs (flooded per area) • Describe route to dest. Outside the area but in AS • 2 types: – Type 3 – describe routes to networks – Type 4 - describe routes to AS boundary routers AS external adv • Originated by AS boundary routers (flooded in AS) • Describe routes to a dest in another AS
    • Packet format Version Number - OSPF used Type - Hello, Database Description, Link-state request etc Packet length - Specifies packet length including the header Router ID - Identifies source of packet Area ID - Identifies area to which the packet belongs Checksum - Checks packet for damage suffered during transit Authentication type - Contains Auth. Type configured in the area Authentication - Contains authentication information
    • Routing protocol packets Are given preference over regular data packets Include:• • • • • Hello Database Description Link-state Request Link-state Update Link-state Acknowledgement
    • Hello packet Used to acquire neighbors – After a router is assured by the lower-layer protocols that it’s interfaces are functional, it uses hello packet to acquire neighbors It acts as keepalives – Sent periodically on all interfaces – Let routers know that other routers are functional
    • Database Description packet Type 2 OSPF packet Describe contents of topological database These packets are exchanged when adjacency is being initialized
    • Link-state Request Are type 3 Packets Requests pieces of topological dbase from neighbors Are sent when router discovers that part of its topological database is outdated
    • Link-state Update Type 4 Responds to link-state request – Implement flooding of LSAs Several LSAs may be included in a packet
    • Link-state Acknowledgement Are type 5 It ensures reliability – It acknowledges the link-state update packets
    • Packet Authentication Authentication differs for each area – Autype 0 -No authentication – Autype 1 -Simple password – All others - Reserved for assignment by IANA Additional authentication on perinterface basis. • Example: Simple password for area and additional password configured for each network in the area
    • OSPF Delay Measurements Why measure OSPF ? – Internal OSPF processing delays impact:• speed updates propagate through network • Amount of load on routers • time needed for routing to converge incase of topology/configuration change
    • How to measure OSPF Methods – Black-box - based on external observation – white-box - based on internal instrumentation Key internal tasks considered in measuring delays – Processing LSAs, performing SPF calculations,updating FIB & Flooding LSAs
    • Black-box protocol measurements 3 techniques enforced in designing expt. – Use of OSPF emulator • generates OSPF patterns – Exploit features mandated by OSPF specs • duplicate LSAs must be acknowledged immediately – Set config. Parameters to ensure tasks in an order that allow for measurement
    • Methodology (Testbed setup) LSA Emulated topology PC1 Expect script TopTracker VLAN1 VLAN2 Target router PC2 Top Tracker- OSPF topology emulator Target router-router whose OSPF impl. Is under test •Load emulated topology on target router •Initiate task of interest •Measure the time for task
    • Experimental Designs Task Logical Connectivity LSA Processing LSA Flooding SPF Calculation FIB Update PCI - target router on VLAN1 PCI - target router on VLAN1 & 2 PCI - target router on VLAN1 PCI - target router on VLAN1 & PC2 acts as pin generator on VLAN1
    • Measuring Task Time 1. Use a black-box method to bracket task start and finish times 2. Subtract out intervals that precede and exceed these times top bracket event task start time time B A X task finish time bottom bracket event X = A - (B+C) C
    • LSA Processing Time Most complex – OSPF bundles LSAs into LS Update packets For expt: – Top tracker sends 2 packets back to back • 1st contain legitimate LSAs (probeLSAs) • 2nd contain duplicate LSA – Top tracker logs time it sends packet (ts) and the time ack for duplicate LSA it received (tr)
    • LSA processing calculation TopTracker Probe LSA Sent (ts) Duplicate LSA sent t1’ Target Router Probe LSA arrive (t1) Duplicate LSA arrives t2’ time X Probe LSAs processing over t2 Duplicate LSA processing over t3 Ack for duplicate LSA arrives(tr) • X = t2 - t1 =(tr-ts)-[(tr-t3)+(t3-t2)+(t1-ts)] • Estimate the overhead = [(tr-t3)+(t3-t2)+(t1-ts)] - To estimate overhead Top tracker sends one packet with duplicate and logs transmission and reception time
    • Cont’ Processing time for the packet depends on:– Number of links the LSA describes – The number of interfaces on which The LSA must later be flooded to – Size of link state database
    • LSA Flood Time This is the time target router takes to flood LSA after receiving it TopTracker Target Router Probe LSA Sent (ts) Probe LSA arrive (t1) time Probe LSAs processing over t2 Probe LSA is flooded out t3 Probe LSA is received (tr) X
    • Cont’ X = t3 - t1 =(tr-ts)-[(tr-t3)+(t1-ts)] Estimate the overhead = [(tr-t3)+(t1-ts)] The overhead is the round trip to forward a packet between the 2 Tracker interfaces via target router Pacing-timer – Controls rate at which packets are transmitted out an interface
    • SPF Calculation 2 parameters influence the scheduling of the calculation • spf-delay-specifies how long OSPF waits between receiving topology change and starting SPF comp. • spf-hold time-Enforces a lag time between consecutive comps. Expt:• Set both parameters to 0 • Send a probe LSA whose receipt immediately initiates calc on the target router • Send duplicate LSA whose role is to bracket
    • Methodology for SPF Calculation TopTracker Target Router Load desired topology Send initiator LSA B time A Initiator LSA arrives C SPF calculation starts Send duplicate LSA X E SPF calculation ends Send ack for duplicate LSA Ack for duplicate LSA arrives • X = A – (B + C + D + E) • Estimate the overhead = B + C + D + E •The overhead is therefore a round trip between tracker and router D
    • Estimating the Overhead Remove SPF calculation from bracket set spf_delay = 60 seconds (very large value) TopTracker Send initiator LSA Send duplicate LSA B Target Router Initiator LSA arrives Duplicate LSA arrives time Overhead C Initiator LSA processing done Duplicate LSA processing done; send ack Ack for duplicate LSA arrives E SPF calculation starts overhead = B + C + D + E .tracker logs sending time of probe and the reception time of ack for duplicate LSA D
    • Results for Cisco Routers Mean SPF time (Cisco GSR) Mean SPF Time (Cisco 7513) 0.025 Time (seconds) 0.03 0.025 Time (seconds) 0.03 0.02 0.015 0.01 0.02 0.015 0.01 0.005 0.005 0 0 0 20 40 60 80 100 Number of nodes (n) Similar results for two models SPF calculation time is O(n2) 0 20 40 60 80 Number of nodes (n) 100
    • Results for GateD Time (seconds) Mean SPF Time (GateD) 0.018 0.016 0.014 0.012 0.01 0.008 0.006 0.004 0.002 0 Black-box White-box 0 20 40 60 80 100 Number of nodes (n) Black-box over-estimates white-box measurement Black-box captures the characteristics very well
    • FIB Update Time It’s time it takes to update FIB after it starts SPF calculation Expt: – PC2 pings a destination addr d every . 01sec – Until FIB updated, all pings are dropped – When change is reflected in FIB, ping packet is forwarded to PC1
    • FIB timer TopTracker (PC1) Probe LSA Sent (ts) Target Router Probe LSA arrive (t1) X PC2 ping d SPF computation starts t2 time SPF computation ends t3 Route to d installed in FIB t4 First ping packet reaches(tr) Ping packet reaches target router t5 • X = t4 - t1 =(tr-ts)-[(tr-t5)+(t5-t4)+(t2-t1)+(t1-ts)]
    • OSPF Task Delays (Cisco) LSA Processing 100-800 microseconds LSA flooding 30-40 milliseconds pacing timer is the determining factor SPF calculation 100-300 milliseconds FIB update time no dependence on the size of the topology 1-40 milliseconds
    • Conclusions OSPF routing protocol proved to be effective in serving large, heterogeneous internetworks Black-box methods for estimating OSPF processing delays worked: LSA processing and flooding SPF calculation and FIB Update Applied techniques to Cisco GSR and 7513 routers as well as GateD Future work develop techniques for other protocols, in particular BGP
    • Any questions?