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Protocol implementation

Protocol implementation. Next-hop resolution Reliability and graceful restart. What is a next-hop. The destination of the packets I am sending Not the same as the interface An ethernet interface will have many nodes behind it Directly connected next hop is 1 hop away

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Protocol implementation

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  1. Protocol implementation • Next-hop resolution • Reliability and graceful restart

  2. What is a next-hop • The destination of the packets I am sending • Not the same as the interface • An ethernet interface will have many nodes behind it • Directly connected next hop is 1 hop away • E.g. RSVP sends a PATH message to the next downstream node • Next hop may be directly connected (strict ERO) • Or not (loose ERO) • OSPF sends an LS update to the other end of a link or a neighbor on an eithernet • Always directly connected • BGP has an iBGP-next hop for each of its paths • Not directly connected

  3. Next-hop • If the next hop is not directly connected the way to reach it depends on the IGP • May change when IGP routing changes • Will have to use a different interface to reach it • Need to keep track of these changes • Next hop resolution

  4. Next hop resolution • Periodic resolution • may take a bit more time • But next-hops will not be too many • Or will they? Tunnels, VLANs … • Quagga uses this approach • Through the IPV4_LOOKUP_NEXTHOP command • Registration/notification • RSVP would tell zebra which nexthops it is interested in • Zebra will notify RSVP when something changes in the IGP path to it • Better scaling for RSVP • Difficult to ensure good scaling inside zebra • Various protocols may register 1000s of next hops • More complex code in zebra

  5. Network Reliability • Availability: How many nines? • 99.999% is 5.26 min down time/year • 99.9999% is 31.5 sec down time/year • Telephone networks are between 5 and 6 nines • Internet will have to get there • Currently at 4 nines? (vendors claim 5) • Very important with the new types of traffic • Voip, Ipvt • What can go wrong (% of failures for US telephone network ca. 1992): • Hardware failures (19%) • Software failures (14%) • Human errors (49%) • Vandalism/Terrorism • Acts of nature (11%) • Overload (6% but had the largest impact on customers)

  6. Hardware failures • Link failures • Protocols can cope with that • Re-route, may be slow • More aggressive repair methods • we will see them later • Router failures • Can not do much just add redundancy • Power supplies, fans, disks, etc • Line-card failure is similar to a link failure • Control processor failure is more serious • Always have two of them • Primary and backup

  7. Modern Router architectures • Dual controllers • For running the control plane • Multiple line-cards • Can operate without the controllers • Router can forward traffic even when the control plane crashes • Called non-stop forwarding or head-less operation

  8. Software failures • When primary fails start using backup • Switchover • Must be as fast as possible • Things in the network change in the meanwhile • Need to minimize this window • What happens with the control software • Need to keep primary and backup instance in sync • How tight is this synchronization?

  9. Tight synchronization • Both primary and backup are active, keep them in sync by: • Send them both the same input (I.e. duplicate control packets) • Fastest possible switchover • Expensive, may need to duplicate packets • Does not work for TCP based protocols • The primary keeps sending state updates to the backup • May need to send too many messages • Being totally in-sync is not easy • Needs transactional communication

  10. Loose synchronization • Backup is idle • But we keep configuration up to date • Each configuration change on the primary is mirrored on the backup • Backup instance is started when the primary fails • Switchover will take longer • Much-much simpler • Configuration changes are much less • Variation: • Keep only the RIB process in sync in both primary and backup

  11. Non-stop forwarding • Key concept • forwarding happens in the line cards • Even if control processor fails forwarding can continue • Non stop forwarding, head-less operation • Old Common sense: when router s/w crashes do not use the router • But with head-less operation it is ok to continue using routers that their s/w crashed • Assuming their s/w will be operational again soon

  12. Special Case • Planned restart • For s/w upgrade • These are a significant percentage of downtime • For refresh • Memory is leaking but s/w still operational • Restart to get a clean start • I can use graceful restart

  13. Graceful Restart • Other routers in the network will keep using a neighbor router • Even if is looks like its control plane has failed • Assuming it will come back soon • Needs coordination • The failed router needs to do some special processing when it comes back • It has to tell its neighbors first that it supports graceful restart • Zero impact on the network • The failed router will have the chance to restart its s/w and come back • Nobody in the rest of the network will know that something happened

  14. How does it work • Used for all protocols by now • OSPF, BGP, RSVP-TE… • The neighbor will discover that the router is dead or it has restarted • HELLO timeout, different information in the HELLOs etc… • But will ignore it for a certain time period • If the failed router comes back within this period • It will re-sync its state (database exchange for OSPF, resend all the LSPs for RSVP, …) • And all is back to normal

  15. Example RSVP • Use HELLOs • Special recovery label messages • Restarting router needs to remember the labels it allocated before the crash • Where? • Shared memory • recover them from the forwarding plane • Why? • Must use the same labels again • Must make sure it does not use an allocated label for some other LSP

  16. Example OSPF • Trick is to re-establish the adjacencies after a failure • Remember the set of neighbors • Shared memory or in the backup controller • After restart do not originate any LSAs • Just re-establish adjacencies and re-sync database

  17. Graceful restart catches • All routers in the network should implement this to work • Mostly for planned restarts: • S/w upgrades • Refreshes (if a router runs low on memory) • But it is possible to use for crashes too! • It can not work if something changes in the network while the restart is going on • There may be routing loops

  18. Router self-monitoring • Automatically restart failed or stuck processes • A separate monitor process • Keeps an eye on other processes • If there is a failure the failed process is restarted • Of course it may fail again • Heart-beats to determine liveness • Failure may not necessarily be a crash • Could be a software bug that causes an infinite loop or very-very slow processing

  19. Why is it important • Remember the PoP structure • Need dual routers for reliability • If I had a single router that was extra-reliable I could save a lot of money

  20. Issues • Strict Isolation • VMs • Other methods • Global resource coordination • For example memory

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