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Computer Networks Network layer (Part 4) 1 Network layer (so far) • Network layer functions • Network layer implementation (IP) • Today – Network layer devices (routers) • • • • Network processors Input/output port functions Forwarding functions Switching fabric – Advanced network layer topics • Routing problems • Routing metric selection • Overlay networks 2 NL: Router Architecture Overview Key router functions: – Run routing algorithms/protocol (RIP, OSPF, BGP) and construct routing table – Switch/forward datagrams from incoming to outgoing link based on route 3 NL: Routing vs. Forwarding • Routing: process by which the forwarding table is built and maintained – One or more routing protocols – Procedures (algorithms) to convert routing info to forwarding table. • Forwarding: the process of moving packets from input to output – The forwarding table – Information in the packet 4 NL: What Does a Router Look Like? • Network processor/controller – Handles routing protocols, error conditions • Line cards – Network interface cards • Forwarding engine – Fast path routing (hardware vs. software) • Backplane – Switch or bus interconnect 5 NL: Network Processor • Runs routing protocol and downloads forwarding table to forwarding engines – Use two forwarding tables per engine to allow easy switchover (double buffering) • Typically performs “slow” path processing – – – – ICMP error messages IP option processing IP fragmentation IP multicast packets 6 NL: Fast-path router processing • • • • • Packet arrives arrives at inbound line card Header transferred to forwarding engine Forwarding engine determines output interface Forwarding engine signals result to line card Packet copied to outbound line card 7 NL: Input Port Functions Physical layer: bit-level reception Data link layer: e.g., Ethernet see chapter 5 Decentralized switching: • given datagram dest., lookup output port using routing table in input port memory • goal: complete input port processing at ‘line speed’ • queuing: if datagrams arrive faster than forwarding rate into switch fabric 8 NL: Input Port Queuing • Fabric slower than input ports combined => queuing may occur at input queues • Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward • queueing delay and loss due to input buffer overflow! 9 NL: Input Port Queuing • Possible solution – Virtual output buffering • Maintain per output buffer at input • Solves head of line blocking problem • Each of MxN input buffer places bid for output – Crossbar connect – Challenge: map of bids to schedule for crossbar 10 NL: Forwarding Engine • General purpose processor + software • Packet trains help route hit rate – Packet train = sequence of packets for same/similar flows – Similar to idea behind IP switching (ATM/MPLS) where long-lived flows map into single label • Example – Partridge, et. al. “A 50-Gb/s IP Router”, IEEE Trans. On Networking, Vol 6, No 3, June 1998. – 8KB L1 Icache • Holds full forwarding code – 96KB L2 cache • Forwarding table cache – 16MB L3 cache • Full forwarding table x 2 - double buffered for updates 11 NL: Binary trie Route A B C D E F G H I Prefixes 0* 01000* 011* 1* 100* 1100* 1101* 1110* 1111* 0 1 A D 1 0 0 1 0 C 0 1 0 1 E 0 1 0 1 F G H I 0 B 12 NL: Path-compressed binary trie • Eliminate single branch point nodes • Variants include PATRICIA and BSD tries Route A B C D E F G H I Prefixes 0* 01000* 011* 1* 100* 1100* 1101* B 1110* 1111* Bit=1 0 1 Bit=3 A 0 Bit=2 D 1 0 C 1 E Bit=3 0 Bit=4 0 F 1 1 Bit=4 0 1 G H I 13 NL: Patricia tries and variable prefix match • Patricia Tree • Arrange route entries into a series of bit tests • Worst case = 32 bit tests • Problem: memory speed is a bottleneck • Used in older BSD Unix routing implementations 0 Bit to test – 0 = left child,1 = right child 10 default 0/0 128.2/16 16 128.32/16 19 128.32.130/240 128.32.150/24 14 NL: Multi-bit tries • Compare multiple bits at a time – – – – Reduces memory accesses Forces table expansion for prefixes falling in between strides Variable-length multi-bit tries Fixed-length multi-bit tries • Most route entries are Class C • Cut prefix tree at 16 bit depth – – – – – Many prefixes 8, 16, 24 bits in length 64K bit mask Bit = 1 if tree continues below cut (root head) Bit = 1 if leaf at depth 16 or less (genuine head) Bit = 0 if part of range covered by leaf 15 NL: Variable stride multi-bit trie • Single level has variable stride lengths Route A B C D E F G H I Prefixes 0* 01000* 011* 1* 100* A 1100* 1101* 1110* 1111* 00 01 10 11 A 00 01 D 0 10 11 C C E D 1 00 01 F G 10 11 H I 0 1 B 16 NL: Fixed stride multi-bit trie • Single level has equal strides Route A B C D E F G H I Prefixes 0* 01000* 011* 1* 100* 000 1100* 1101* A 1110* 1111* 001 A 010 011 A C B 00 01 10 11 100 101 E 110 D 111 D D F F G G H H I I 00 01 10 11 00 01 10 11 17 NL: Other data structures • Ruiz-Sanchez, Biersack, Dabbous, “Survey and Taxonomy of IP address Lookup Algorithms”, IEEE Network, Vol. 15, No. 2, March 2001 – – – – – – – – LC trie Lulea trie Full expansion/compression Binary search on prefix lengths Binary range search Multiway range search Multiway range trees Binary search on hash tables (Waldvogel – SIGCOMM 97) 18 NL: Prefix Match issues • Scaling – IPv6 • Stride choice – Tuning stride to route table – Bit shuffling 19 NL: Speeding up Prefix Match - Alternatives • Route caches – Temporal locality – Many packets to same destination • Protocol acceleration – Add clue (5 bits) to IP header – Indicate where IP lookup ended on previous node (Bremler-Barr SIGCOMM 99) • Content addressable memory (CAM) – Hardware based route lookup – Input = tag, output = value associated with tag – Requires exact match with tag • Multiple cycles (1 per prefix searched) with single CAM • Multiple CAMs (1 per prefix) searched in parallel – Ternary CAM • 0,1,don’t care values in tag match • Priority (i.e. longest prefix) by order of entries in CAM 20 NL: Types of network switching fabrics Memory Multistage interconnection Crossbar interconnection Bus 21 NL: Types of network switching fabrics • Issues – Switch contention • Packets arrive faster than switching fabric can switch • Speed of switching fabric versus line card speed determines input queuing vs. output queuing 22 NL: Switching Via Memory First generation routers: • packet copied by system’s (single) CPU • 2 bus crossings per datagram • speed limited by memory bandwidth Modern routers: • input port processor performs lookup, copy into memory • Cisco Catalyst 8500 Input Port Memory System Bus Output Port 23 NL: Switching Via Bus • Datagram from input port memory to output port memory via a shared bus • Bus contention: switching speed limited by bus bandwidth • 1 Gbps bus, Cisco 1900: sufficient speed for access and enterprise routers (not regional or backbone) 24 NL: Switching Via An Interconnection Network • Overcome bus bandwidth limitations • Crossbar networks – Fully connected (n2 elements) – All one-to-one, invertible permutations supported 25 NL: Switching Via An Interconnection Network • Crossbar with N2 elements hard to scale • Multi-stage interconnection networks (Banyan) – – – – – – Initially developed to connect processors in multiprocessor Typically (n log n) elements Datagram fragmented fixed length cells Cells switched through the fabric Cisco 12000: Gbps through an interconnection network Blocking possible (not all one-to-one, invertible permutations supported) A W B X C Y D Z 26 NL: Output Ports • Output contention – Datagrams arrive from fabric faster than output port’s transmission rate – Buffering required – Scheduling discipline chooses among queued datagrams for transmission 27 NL: Output port queueing • buffering when arrival rate via switch exceeds ouput line speed • queueing (delay) and loss due to output port buffer overflow! 28 NL: Advanced topics • • • • Routing synchronization Routing instability Routing metrics Overlay networks 29 NL: Routing Update Synchronization • Another interesting robustness issue to consider... • Even apparently independent processes can eventually synchronize – Intuitive assumption that independent streams will not synchronize is not always valid – Periodic routing protocol messages from different routers – Abrupt transition from unsynchronized to synchronized system states 30 NL: Examples/Sources of Synchronization • TCP congestion windows – Cyclical behavior shared by flows through gateway • Periodic transmission by audio/video applications • Periodic downloads • Synchronized client restart – After a catastrophic failure • Periodic routing messages – Manifests itself as periodic packet loss on pings • Pendulum clocks on same wall • Automobile traffic patterns 31 NL: How Synchronization Occurs T A Message from B T Weak Coupling when A’s behavior is triggered off of B’s message arrival! A Weak coupling can result in eventual synchronization 32 NL: Routing Source of Synchronization • Router resets timer after processing its own and incoming updates • Creates weak coupling among routers • Solutions – Set timer based on clock event that is not a function of processing other routers’ updates, or – Add randomization, or reset timer before processing update • With increasing randomization, abrupt transition from predominantly synchronized to predominantly unsynchronized • Most protocols now incorporate some form of randomization 33 NL: Routing Instability • References – C. Labovitz, R. Malan, F. Jahanian, ``Internet Routing Stability'', SIGCOMM 1997. • Record of BGP messages at major exchanges • Discovered orders of magnitude larger than expected updates – Bulk were duplicate withdrawals • Stateless implementation of BGP – did not keep track of information passed to peers • Impact of few implementations – Strong frequency (30/60 sec) components • Interaction with other local routing/links etc. 34 NL: Route Flap Storm • Overloaded routers fail to send Keep_Alive message and marked as down • BGP peers find alternate paths • Overloaded router re-establishes peering session • Must send large updates • Increased load causes more routers to fail! 35 NL: Route Flap Dampening • Routers now give higher priority to BGP/Keep_Alive to avoid problem • Associate a penalty with each route – Increase when route flaps – Exponentially decay penalty with time • When penalty reaches threshold, suppress route 36 NL: BGP Oscillations • Can possible explore every possible path through network (n-1)! Combinations • Limit between update messages (MinRouteAdver) reduces exploration – Forces router to process all outstanding messages • Typical Internet failover times – New/shorter link 60 seconds • Results in simple replacement at nodes – Down link 180 seconds • Results in search of possible options – Longer link 120 seconds • Results in replacement or search based on length 37 NL: Routing Metrics • Choice of link cost defines traffic load – Low cost = high probability link belongs to SPT and will attract traffic, which increases cost • Main problem: convergence – Avoid oscillations – Achieve good network utilization 38 NL: Metric Choices • Static metrics (e.g., hop count) – Good only if links are homogeneous – Definitely not the case in the Internet • Static metrics do not take into account – Link delay – Link capacity – Link load (hard to measure) 39 NL: Original ARPANET Metric • Cost proportional to queue size – Instantaneous queue length as delay estimator • Problems – Did not take into account link speed – Poor indicator of expected delay due to rapid fluctuations – Delay may be longer even if queue size is small due to contention for other resources 40 NL: Metric 2 - Delay Shortest Path Tree • Delay = (depart time - arrival time) + transmission time + link propagation delay – (Depart time - arrival time) captures queuing – Transmission time captures link capacity – Link propagation delay captures the physical length of the link • Measurements averaged over 10 seconds – Update sent if difference > threshold, or every 50 seconds 41 NL: Performance of Metric 2 • Works well for light to moderate load – Static values dominate • Oscillates under heavy load – Queuing dominates 42 NL: Specific Problems • Range is too wide – 9.6 Kbps highly loaded link can appear 127 times costlier than 56 Kbps lightly loaded link – Can make a 127-hop path look better than 1-hop • No limit to change between reports • All nodes calculate routes simultaneously – Triggered by link update 43 NL: Example A Net X Net Y B 44 NL: Example After everyone re-calculates routes: A Net X Net Y B .. Oscillations! 45 NL: Consequences • • • • Low network utilization (50% in example) Congestion can spread elsewhere Routes could oscillate between short and long paths Large swings lead to frequent route updates – More messages – Frequent SPT re-calculation 46 NL: Revised Link Metric • Better metric: packet delay = f(queueing, transmission, propagation) • When lightly loaded, transmission and propagation are good predictors • When heavily loaded queueing delay is dominant and so transmission and propagation are bad predictors 47 NL: Normalized Metric • If a loaded link looks very bad then everyone will move off of it • Want some to stay on to load balance and avoid oscillations • It is still an OK path for some • Hop normalized metric diverts routes that have an alternate that is not too much longer • Also limited relative values and range of values advertised gradual change 48 NL: Revised Metric • Limits on relative change – Measured link delay is taken over 10sec period – Link utilization is computed as .5*current sample + .5*last average – Max change limited to slightly more than ½ hop – Min change limited to slightly less than ½ hop – Bounds oscillations • Normalized according to link type – Satellite should look good when queueing on other links increases 49 NL: Routing Metric vs. Link Utilization 225 140 9.6 satellite New metric (routing units) 90 75 60 30 0 9.6 terrestrial 56 satellite 56 terrestrial 25% 50% 75% 100% Utilization 50 NL: Observations • Utilization effects – High load never increases cost more than 3*cost of idle link – Cost = f(link utilization) only at moderate to high loads • Link types – Most expensive link is 7 * least expensive link – High-speed satellite link is more attractive than low-speed terrestrial link • Allows routes to be gradually shed from link 51 NL: Idealized Network Response Maps •Load of “average” link as a function of that link’s cost •Created empirically 1.0 0.8 Mean load on link Increasing applied network load 0.6 0.4 0.2 0.0 0.5 1.0 1.5 2.0 2.5 Link cost 3.0 3.5 4.0 52 NL: Equilibrium Calculation •Combine utilization to cost and cost to utilization maps •Equilibrium points at intersections 1.0 Increasing applied network load HN-SPF 0.8 D-SPF Mean load on link 0.6 0.4 0.2 0.0 0.5 1.0 1.5 2.0 2.5 Link cost 3.0 3.5 4.0 53 NL: Routing Dynamics •Limiting maximum metric change bounds oscillation Bounded oscillation 1.0 Metric map 0.75 Utilization 0.5 0.25 0 Network response 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Link reported cost 54 NL: Routing Dynamics Metric map 1.0 0.75 Easing in a new link Utilization 0.5 Network response 0.25 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Reported cost 55 NL: Overlay Routing • Basic idea: – Treat multiple hops through IP network as one hop in an overlay network – Run routing protocol on overlay nodes • Why? – For performance – can run more clever protocol on overlay – For efficiency – can make core routers very simple – For functionality – can provide new features such as multicast, active processing, IPv6 56 NL: Overlay for Performance • References – Savage et. al. “The End-to-End Effects of Internet Path Selection”, SIGCOMM 99 – Anderson et. al. “Resilient Overlay Networks”, SOSP 2001 • Why would IP routing not give good performance? – Policy routing – limits selection/advertisement of routes – Early exit/hot-potato routing – local not global incentives – Lack of performance based metrics – AS hop count is the wide area metric • How bad is it really? – Look at performance gain an overlay provides 57 NL: Quantifying Performance Loss • Measure round trip time (RTT) and loss rate between pairs of hosts – ICMP rate limiting • Alternate path characteristics – 30-55% of hosts had lower latency – 10% of alternate routes have 50% lower latency – 75-85% have lower loss rates 58 NL: Bandwidth Estimation • RTT & loss for multi-hop path – RTT by addition – Loss either worst or combine of hops – why? • Large number of flows combination of probabilities • Small number of flows worst hop • Bandwidth calculation – TCP bandwidth is based primarily on loss and RTT • 70-80% paths have better bandwidth • 10-20% of paths have 3x improvement 59 NL: Overlay for Efficiency • Multi-path routing – More efficient use of links or QOS – Need to be able to direct packets based on more than just destination address can be computationally expensive – What granularity? Per source? Per connection? Per packet? • Per packet re-ordering • Per source, per flow coarse grain vs. fine grain – Take advantage of relative duration of flows • Most bytes on long flows 60 NL: Overlay for Features • How do we add new features to the network? – Does every router need to support new feature? – Choices • Reprogram all routers active networks • Support new feature within an overlay – Basic technique: tunnel packets • Tunnels – IP-in-IP encapsulation – Poor interaction with firewalls, multi-path routers, etc. 61 NL: Examples • IP V6 & IP Multicast – Tunnels between routers supporting feature • Mobile IP – Home agent tunnels packets to mobile host’s location – http://www.rfc-editor.org/rfc/rfc2002.txt • QOS – Needs some support from intermediate routers 62 NL: Overlay Challenges • How do you build efficient overlay – Probably don’t want all N2 links – which links to create? – Without direct knowledge of underlying topology how to know what’s nearby and what is efficient? 63 NL: Future of Overlay • Application specific overlays – Why should overlay nodes only do routing? • Caching – Intercept requests and create responses • Transcoding – Changing content of packets to match available bandwidth • Peer-to-peer applications 64 NL: Network layer summary • • • • Network layer functions Specific network layers (IPv4, IPv6) Specific network layer devices (routers) Advanced network layer topics 65 NL: Network trace • http://www.cse.ogi.edu/class/cse524/trace.txt 66 NL: End of material for midterm • Midterm next Monday 10/29/2001 covering…. – Technical material in lectures – Chapters 1, 4, and 5 • Chapter 1 • Chapter 4.1-4.7 • Chapter 5 – Review questions at end of chapters 67