PCE. Path Computation Element (PCE)

Transcription

1 PCE Path Computation Element (PCE)

2 Index 1. What is QoS? 2. Why it s necessary in networks? 3. Qualities of traffic 1. An example of IGP Cost, OSPF 4. Adding QoS in OSPF 1. Problem: Adding QoS in IGP(OSPF) 5. Solution: PCE 6. How PCE works? 7. PCE and TED 8. PCE and Multi-Domains 9. PCE Architecture 10. PCECP Protocol 1. PCRequest & PCReply 11. Multi-Domain Path computation 1. Per-Domain Path Computation 2. Cooperating PCEs 3. Backward Recursive Path Computation 4. BRPC: Mesh Domain Problems 12. Conclusions 2

3 What is QoS? QoS(Quality of Service): is the collective effect of service performance which determines the degree of satisfaction of a user of the service 3

4 Why it s necessary in networks? Sometimes there is stricter service demands that can t be provided by the default systems For example: o Video Streaming o VoIP o Cloud Computing 4

5 Qualities of traffic Low throughput Dropped Packets Errors Latency Jitter Out-of-order delivery Green Routing 5

6 An example of IGP Cost, OSPF The cost of an interface in OSPF is an indication of the overhead required to send packets across a certain interface The cost of an interface is inversely proportional to the bandwidth of that interface Cost= 10 8 /bandwidth in bps 6

7 Adding QoS in OSPF In practice, this OSPF routing cost is not enough It s necessary improve a new next-hope calculation Cost = α BW + β Delay + χ Jitter + + ζ Losses 7

8 Problem: Adding QoS in IGP(OSPF) Adding new parameters, seriously complicates the dynamics of flooding because the flooding convergence time can be longer than the change rate of some metrics like available bandwidth Adding new parameters increments the network overhead Creates inaccuracy 8

9 Solution: PCE Definition: An entity (component, application, or network node) that is capable of computing a network path or route based on a network graph and applying computational constrains (RFC4655) 9

10 How PCE works? PCC-PCE communication protocol (PCEP) PCE The remote server that provides path computation Path computation capabilities PCE Responds with a path (or failure) PCC The network element that request computation services PCC Requests a computation From where to where Type of path? (Constraints) 10

11 PCE and TED Path computation requires knowledge of available network resources TED: Traffic Engineering Database The TED may be built from Information distributed by a routing protocol (OSPF-TE) Information configured directly 11

12 How PCE works? A single PCE cannot compute a multi-domain path PCE 12

13 PCE and Multi-Domains A PCE only see inside its domain then Computation of a multi-domain path may use cooperating PCEs Communication between PCEs This communication is exactly as PCE-PCC communication 13

14 How PCE works? A single PC cannot compute a multi-domain path PCE PCE Domain 4 PCE PCE Domain 1 14 Domain 2 Domain 3

15 PCE Architecture 1. Composite PCE: PCC and PCE are located in the same node 2. External PCE: PCC and PCE are located in different nodes 3. Management-based PCE: PCC is a NMS 4. Multiple PCE Path Computation: Multiple PCEs are used to compute the path No inter PCE communication 5. Multiple PCE Path Computation with Inter-PCE communication: Multiple PCEs collaborate to compute the path Inter PCE communication 15

16 PCE Architecture NMS TED LSR TED Server TED Server TED PCE PCE PCE PCE LSR LSR LSR LSR LSR LSR LSR Signalling Engine Signalling Engine Signalling Engine Signalling Engine Signalling Engine Signalling Engine Signalling Engine Signalling Engine Type 3 Type 1 Type 2 & 5 16

17 PCEP Protocol Operates over TCP Session-based protocol Seven messages Open Keepalive Request Response Notify Error Close 17

18 PCRequest & PCReply PCReq message asks for a path computation From and To Constraints (bandwidth, etc ) Metric to optimize IGP metric (ospf) TE metric Hop count PCRep reports the computed path (or failure) Explicit route Failure 18

19 Multi-Domain Path computation Three main methods have already been defined 1. Per-domain path computation 2. Simple cooperating PCEs 3. Backward Recursive Path Computation 19

20 Per-Domain Path Computation PCE calculates the path for it s own domain. Pro: Simple Cont: It s not the optimal path route, because you can t see all the domains. 1 PCE1 2 3 PCE2 4 20

21 Cooperating PCEs 1. I want to reach the Egress 2. Thinks A looks best PCE 3. How should I reach the Egress? 5. Route thru B PCE 4. Thinks D would be best 7. I want to reach the Egress 6. Route thru X and B 8. Route thru Y Ingress A C X Y B D Egress 21

22 Backward Recursive Path Computation PCE calculates the path for all domains. Pass the set of paths to the neighboring PCEs Pro: Compute the optimal global path Cont: It s slow and expensive. It s a point of failures. The destination must be known. PCE1 B C D E G I PCE2 J L M Q S T V PCE3 A 22

23 BRPC: Mesh Domain Problems BRPC uses recursive backward algorithm The root it s the destination That means every PCE-AS calculates it s own domain part of path and then delegates route tree to the backward crossed AS to find the optimum path This algorithm generates death loops A B C E L F G N PCE2 H I P J R S T U 23 PCE1 D K M Q PCE3 23 PCE4

24 Conclusions PCE is a logical functional component May be centralized within a domain or distributed It is not an all-seeing oracle PCEs may cooperate to determine end-to-end multidomain path It may too complex for large multidomain paths Loops, erased by knowing sequences of domains 24

25 Bibliography configuration/guide/rc37ospf.html

26 PCE Path Computation Element (PCE)