Minimizing Packet Loss

Transcription

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2 Minimizing Packet Loss Eric Osborne Russ White

3 genda Intro What Is Convergence? Brief History Talk Talk Faster Precompute Precompute and Tunnel Current State of the rt 3

4 Minimizing Packet Loss with IGPs and MPLS What is this session about? The development and deployment of techniques to minimize packet loss Which one should you use? Hint: The answer is it depends! But answering why is the fun part What this session isn t about Deep level details about each fast convergence mechanism Choosing a winner 4

5 WHT IS FST CONVERGENCE?

6 What is Convergence? Two similar, but different, definitions Subtle difference is important! Convergence is the amount of time (and thus packet loss) after a failure in the network and before the network settles into a steady state Convergence is the amount of time (and thus packet loss) after a failure in the network and before the network responds to the failure Both assume zero loss after convergence congestion loss is out of scope What is the subtlety? Steady state vs. just responding. 6

7 What is Interesting? Only three numbers are interesting in the context of network convergence What is the longest the network could take to converge? What is the average amount of time the network could take to converge? How long before applications running on the network lose their state? If you know these three numbers you can tell How well the network is going to perform Whether or not the network is meeting application requirements Whether or not to look for some way to make the network faster (or slower!) 7

8 HOW ROUTING PROTOCOLS CONVERGE

9 Convergence Theory Convergence (definition 1) has four parts How long does it take to detect the failure? How long does it take to describe the failure? How long does it take to find another route? How long does it take to switch to the alternate route? Detect Each of these must be attacked individually Leaving any one on the table means slow convergence Describe Find Switch 9

10 RIP Convergence Time Detect <1 second best,105 seconds average, 119 seconds worst Describe 15 seconds average, 30 seconds worst Find lternate 15 seconds average, 30 seconds worst Total Time Best verage Case: 31 seconds verage Case: 135 seconds Worst Case: 179 seconds Detect Describe Find Switch 10

11 OSPF Convergence Time Detect <1 second best, 20 seconds average, 40 seconds worst Describe <1 second best, about 5 seconds average Find lternate <1 second average Total Time 2 to 3 seconds best 25 seconds average 45 seconds worst Detect Describe Find Switch 11

12 EIGRP Convergence Time Detect <1 second best, 15 seconds average, 30 seconds worst Find lternate (lternate found before describe in EIGRP) <500ms per query hop average ssume 2 seconds average Describe ssume 2 seconds average Total Time <1 second best 20 seconds average 35 seconds worst Detect Describe Find Switch 12

13 Is this Fast Enough? RIP OSPF EIGRP 50 msec 500 msec 0 500ms 1 sec 1s 5 secs 20s 30 secs 1 Minute 30s 60s+ 13

14 Is this Fast Enough? RIP OSPF EIGRP Video VOIP Not any more TCP 50 msec 500 msec 0 500ms 1 sec 1s 5 secs 20s 30 secs 1 Minute 30s 60s+ 14

15 Brief Timeline Talk Talk Faster Precompute Tunnel 15

16 TLK

17 In The Beginning In the beginning, convergence was the amount of packet loss before the network settled into a steady state This is how all routing protocols work Routing protocol convergence requires some number of nodes (n>=1) to agree on the post-failure behavior Packet loss comes when (n>1) and not all nodes have agreed upon the correct behavior (pedantic note: n>1 for link failures, per direction. Doesn t matter. Important point is that nodes don t need to communicate the failure to anyone else in order for things to start converging) 17

18 TLK FSTER

19 Talking Faster Q: This network convergence stuff is slow. How do we fix it? : Just do the same things but faster! This gives us Fast Convergence. 19

20 Fast Convergence What s in fast convergence? OSPF and ISIS: Decreased various timers (hold, flooding, SPF, etc.) Incremental and partial SPFs to make overall recomputation faster EIGRP Improving query speed and logic Filtering to reduce queries 20

21 Convergence Meltdown Why do networks fail? Networks react to local changes in topology with global modifications in the control plane If the speed of local changes is faster than the ability of the control plane to react This can develop into a positive feedback loop nd cause the control plane to fail to converge If the control plane is desynchronized Not all nodes have the same paths Loops can form during convergence (microloops) 21

22 The Problem of Stability Network stability is diametrically opposed to fast convergence The goal is to design a network and protocol to lower the stability point in the network Stability Point Faster Network and Protocol Design More Stable 22

23 The Mistake in Fast Convergence Fast Convergence is doing the same things but faster Makes the routing protocol quicker to converge The mistake: we should not have been thinking about routing protocol convergence, but of network convergence What can the network do to minimize loss in the event of a failure? What can we do outside the routing protocol? 23

24 PRECOMPUTE

25 nd Then There Were LFs Loop Free lternate: a routing protocol calculates the next-best hop should a link fail (per-link LF) or a particular prefix become unreachable (per-prefix LF) EIGRP s concept of Feasible Successor is per-prefix LF Recent LF technologies simply apply FS logic to link-state protocol 25

26 EIGRP Feasible Successor In some topologies, EIGRP can figure out that the second-best route is guaranteed to not be a loop This route is flagged as a Feasible Successor (FS) When the prefix in question goes away, the FS-based route is installed in the routing and forwarding tables 26

27 LF in Link-State Protocols WWLSPD (What Would Link-State Protocols Do?) OSPF and ISIS can apply the same concept as EIGRP s FS Calculate the second-best NH in the event of a failure variants can handle link or node failure Cannot work in all topologies Same as EIGRP; not all topos have a FS 27

28 What LFs Can and Can t Do With LFs can be made to use B as a backup towards C if the ->C link fails B can be made to use as a backup towards C if the B->C link fails You can actually force and B to act as backups for each other, so if either link fails the network continues forwarding traffic Note routing protocols won t let you do this for reasons that will become obvious in a moment! What happens if /24 is disconnected from the network? B B B B C C C C / /24 28

29 What LFs Can and Can t Do If /24 is disconnected from the network will forward traffic to B because this is the alternate path B will forward traffic to because this is the alternate path We have a routing loop! This loop will last until the routing protocol reconverges (if the looping traffic doesn t keep the routing protocol from reconverging) B B C C /24 29

30 What LFs Can and Can t Do This is true of any ring of routers t four routers, it s impossible not to have a routing loop or temporary traffic drop while the routing protocol converges If you want fast convergence, don t design in squares, rings, etc. lways put in cross links to make triangles lways consider fast convergence along with QoS and other factors when setting the metrics for links How do we get beyond the topology restrictions of LFs? 30

31 PRECOMPUTE ND TUNNEL

32 Tunnel? LFs can t solve all topologies Some topologies will always need (n>1) nodes to agree in order to be loop-free 32

33 What LFs Can and Can t Do The answer is to tunnel Get the traffic past the point where it will loop Turns a ring into a mesh In this network Router s best path is through E Router B s best path is through Router C s best path is through D Router must forward traffic directly to C in order prevent packets from looping back B B C C E E D D /24 33

34 What LFs Can and Can t Do Tunnel from to D How do you tunnel? Need a tunnel that isn t subject to looping Some systems use IP only IP fast reroute Such as NotVI None of these are implemented today Need MPLS If you use LDP, it s called Remote LF If you use TE, it s called TE Fast ReRoute There s also a hybrid model that combines both B B C C E E D D /24 34

35 Remote LFs Remote LFs simply extend the LF space to tunneled neighbors First crack uses LDP, later enhancements can use TE tunnels. How s it go? Router runs a constrained SPF to determine the nearest router that will provide a loop free alternate This constrained SPF can be run to avoid a particular loop (link protection), a particular node or neighbor (node protection) The protection can be computed to all destinations reachable across the protected element, or per prefix Thus, you can have per-prefix link protection, per-prefix node protection, link protection, and node protection 35

36 Remote LF Router runs a constrained SPF and finds C is a valid LF Since C is not directly connected, Router must tunnel to C Router uses LDP to configure an MPLS path to C Installs this alternate path as an LF in the CEF table If the ->E link fails Router begins forwarding traffic along the LDP path B B C C E E D D /24 36

37 Remote LF Convergence Detect Depends on routing protocol or underlying trigger Describe 0 (no need to describe anything to anyone!) Find lternate Fast (precomputed). Msec. Switch Fast (prepopulated in HW). Msec. Total Time <50ms, often <10ms Detect Describe Find Switch 37

38 Remote LF Remote LFs do have topology constraints Cannot be calculated across a flooding domain boundary (an BR or L1/L2 boundary) Cannot be calculated in some non-planar topologies Topologies where links crisscross There are tools that can calculate the coverage in a given network topology Work in about 80% of all possible topologies Work in 90%+ of real deployed networks 38

39 LDP Tunneling: Remote LF Remote LF with LDP handles most failure cases draft-shand-remote-lfa sec. 7.3: 78%-100% depending on topology mean 96%, median 99% Has topological restrictions Need to either be OK with <100% coverage or ensure your topology can deliver 100% coverage Need to think not just about today s topology, but tomorrow s 39

40 TE Tunneling: TE-FRR has a TE tunnel pre-established to E (-B-C-D-E) Link protection tunnel When link E fails, puts all E-bound packets into this tunnel ll packets are delivered to E The nature of a TE tunnel prevents microloops Can also terminate the tunnel at D Node protection tunnel B B C C E E D D /24 40

41 TE Tunneling: TE-FRR Benefit of TE-FRR: Guarantees 100% coverage as long as there is a physical path In technical terms, so long as the network is two connected Cost: requires you to deploy TE in your network There are features to build the necessary tunnels automatically but it s still a new protocol you have to roll out 41

42 Hybrid Model: Remote-LF + TE-FRR Use remote-lf everywhere you can Figure out where you don t have coverage Build TE tunnels as necessary to close the gap Pro: minimize use of TE, may make it easier to track Con: still need a new protocol in the network 42

43 MPLS/TE FRR FRR calculates a backup path per protected resource (link or node: here, link) tunnel is signaled across that backup path The tunnel is used if the protected resource fails Cost: extra tunnels in the network Benefit: protection without regard to topology B D PE F G E PE 43

44 FRR Convergence Detect Depends on routing protocol or underlying trigger Describe 0 (no need to describe anything to anyone!) Find lternate Fast (precomputed). Msec. Switch Fast (prepopulated in HW). Msec. Total Time <50ms, often <10ms Detect Describe Find Switch 44

45 LF and FRR are Band-ids What happens once traffic is switched to the backup path? We can t leave the network forwarding traffic along a backup path forever The control plane must reconverge Hence fast reroute is a band aid for network failures The network still has to heal The key here is that control plane reconvergence has been disconnected from packet loss! B C E D /24 45

46 Is this Fast Enough? RIP OSPF EIGRP LF Rmt LF TE/ FRR Video VOIP Yes! TCP 50 msec 500 msec 1 sec 5 secs 30 secs 1 Minute 46

47 CSE STUDY

48 SUMMRY

49 Case Study In normal operation ssuming either OSPF or IS-IS with an L2 to L2 boundary or OSPF BRs configured as shown ll unmarked links have a cost of 1 If Router B fails Routers and E must recalculate SPF for traffic to begin flowing again The obvious and simple solution for this problem is to change the cost on ->D If the cost on ->D is changed to 1, rather than 2, then would have two paths through ECMP ssume this isn t possible 3 2 B C D E 2 G C D E F H 2 G Flooding Domain Boundary J K F H J 49

50 Case Study From Router s perspective Would an LF help here? Router B s cost to E is the same as Router F s cost to F Router E is a valid LF When Router C fails, can safely switch to E locally, and then recalculate SPF From Router s perspective, an LF will provide the necessary backup protection 3 2 B C D E 2 G C D E F H 2 F G Flooding Domain Boundary J H J K 50

51 Case Study From Router E s perspective Would an LF help here? ssume Router E chooses F as a local LF Router F s best path is through E ny traffic Router E sends to F will loop back to E itself Clearly this won t work! Flooding Domain Boundary 3 2 B C D E G 2 C D E F H 2 F H G J J K 51

52 Case Study How can we resolve this? Rather than use a local LF, we can use a remote LF Router E can use SPF to find another router within the local flooding domain that is downstream towards Some router that has an alternate path to that does not pass through Routers B or E 3 2 B C D E 2 G C D E F H 2 F G Flooding Domain Boundary H J J K 52

53 Case Study Remote LF from Router E s perspective Router E uses SPF to discover D has a path to that does not pass through B or E Router E uses LDP to build a tunnel to D When Router B fails, E transfers traffic destined to to this tunnel Once the traffic is transferred Router E floods a new LSP/LS ll routers recalculate the new best paths Router E releases the tunnel, and begins forwarding traffic along the E- >F->D path towards 3 2 B C D E 2 G C D E F Remote LF H 2 G Flooding Domain Boundary J K F H J 53

54 Case Study What if Router H fails? Let s examine this problem from Router G s perspective Can J use G as an LF? Router G is load sharing between G and H towards the flooding domain boundary Will remote LF work? There is a flooding domain boundary which prevents Router G from using Router F as an alternate route 3 2 B C D E 2 G C D E F H 2 G Flooding Domain Boundary J F H J K 54

55 Case Study How can we resolve this? What we need is some sort of endto-end signaling that will pass through the flooding domain boundary Some sort of overlay, or end-to-end tunnel What could we use??? 3 2 B C D E 2 G C D E F H 2 F G Flooding Domain Boundary H J J K 55

56 Case Study What about our old friend MPLS/TE? RSVP allows us to signal for paths end-to-end Constrained SPF would allow us to find two paths that do not share the same link or node (if two such paths exist) In this situation, MPLS-TE/FRR is the only solution 3 2 B C D E 2 G C D E F H 2 F G Flooding Domain Boundary H J J K 56

57 Case Study Router calculates two paths to J ->B->E->G->J ->D->F->H->J Note these paths don t really resolve failures from an intermediate node s perspective Router H failing still causes J to lose its local path to But this is okay, because we re protecting the end-to-end path 3 2 B C D E 2 G C D E F H 2 F G Flooding Domain Boundary H J J K 57

58 Summary Coverage LFs Remote LFs Complexity MPLS-TE/FRR This is all about tradeoffs There is right answer What are you requirements? What problem are you trying to solve? 58

59 Summary LFs Provide coverage for simple triangular topologies along the edge and in the core To provide coverage in both directions, the topology on both sides of the failure needs to be a triangle 59

60 Summary Remote LFs Provide coverage for ring topologies Nonplanar designs, flooding domain borders, and other complex situations make it difficult to provide coverage The topology must be a ring on both sides of the failure point to provide coverage Requires MPLS & LDP to be deployed on the network 60

61 Summary MPLS-TE/FRR Provides end-to-end coverage in any topology where there are at least two paths Called two connected in research papers (in case you re looking) Requires an additional set of protocols MPLS/TE and all that entails Remember that coverage is end-to-end, not local So carefully consider what an end is within your network environment 61

62 QUESTIONS?

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