The Migration Toward the Optical Internet

Transcription

1 The Migration Toward the Optical Internet Lesson 9 Luca Valcarenghi

2 Resilience in IP over WDM Networks A network is defined as resilient or survivable if it provides some ability to recover ongoing connections disrupted by the failure of a network component, such as a line interruption or node failure Resiliency in survivable networks is obtained by exploiting restoration and/or protection schemes 2

3 Restoration and Protection Restoration upon network failure a restoration scheme dynamically looks for backup paths of spare capacity in the network Protection a protection scheme reserves, in advance, dedicated backup paths and wavelengths in the network Restoration schemes are commonly available at higher layers (e.g., the IP layer) Protection schemes are commonly used at the physical transport layer (e.g., WDM) 3

4 Resilient Schemes in IP over WDM Networks In the two-layer IP over WDM architecture each layer can provide its own independent resilient scheme Resilience concept is naturally embedded in the IP layer the actual path used to route a packet from source to destination is dynamically found and maintained by the routers Protection techniques at the optical layer are just emerging driven by the need for coarse and fast protection schemes 4

5 IP Layer Resilient Schemes Resilient schemes available at the network layer (e.g., IP/MPLS) have the capability to recover faults and operate at fine traffic granularity Granularity is determined by the protocol traffic unit at that particular layer Drawbacks generally slow require online processing upon failure occurrence Network layer resilient schemes IP dynamic routing MPLS protection switching 5

6 IP Dynamic Routing With IP dynamic routing reachable active routers are are found dynamically, thus adapting IP routing to possible network faults The task is accomplished by exchanging between adjacent routers control messages used to update routers routing tables (e.g., LSAs) IP packet get therefore dynamically rerouted around link and node failures IP dynamic rerouting guarantees networkwide survivability, independent of the underlying physical network 6

7 IP Dynamic Routing Fault Detection Faults can be detected by the routers either explicitely or implicitely Explicit fault detection faults are detected at local level and signaled to neighboring routers through regular exchange of routing protocol control messages (ICMP) Implicit fault detection based on expiration of timers such as KEEPALIVE (TCP) and HELLO (IP) messages 7

8 IP Dynamic Routing Fault Recovery Once a router detects a line fault it recalculates the affected routes and updates its routing tables Occurred changes are propagated through UPDATE messages such as OSPF LSA and Border Gateway Protocol-4 (BGP-4) 8

9 IP Dynamic Routing Advantages and Drawbacks Advantages efficient use of network spare resources flexible to topological changes Drawbacks usually slow (from tens of seconds to minutes) unpredictable behavior 9

10 IP Dynamic Routing Enhancements Equal Cost Multipath Forwarding (ECMF) router relies on more than one path for transmitting packets sharing a common destination in case of failure, a fraction of packets are guaranteed to flow to the destination until the router routing table is update with the recalculated routes Partitioning the network into multiple areas as defined in hierarchical link state routing protocols update are confined to the affected area minimizing the network reconfiguration convergence time Increase frequency of HELLO messages or implementing rapid rate pinging through ICMP ECHO request it permits to decrease the failure detection time 10

11 MPLS Protection Switching MPLS protection switching is an alternative approach to circumvent the latency drawback of dynamic rerouting MPLS protection switching is enabled through a hierarchy of LSPs Protection entities can be set up either dynamically or on in a pre-negotiated way 11

12 Dynamic MPLS Protection Protection entities dynamically set up, restore traffic based on failure information bandwidth allocation optimized reroute assignment LSP crossing a failed line or Label Switch Router (LSR) are reestablished using reservation signaling 12

13 Prenegotiated MPLS Protection Working LSPs have pre-established protection paths The pre-established protection path is node and link disjoint from the working path Network resources of the protection path can be reserved beforehand (unused unless failure occurs) dynamically allocated to low-priority traffic that is allowed to use them in absence of network failure 13

14 MPLS Protection Granularity Both MPLS protection switching schemes can be performed on a line basis link rerouting only the portion of the LSPs around the failed line is rerouted on a path basis edge-to-edge rerouting the entire failed LSPs are independently rerouted 14

15 MPLS Protection Switching Scheme Comparison Dynamic protection vs. pre-established protection increases resource utilization requires longer restoration time Link rerouting vs. end-to-end rerouting faster because in end-to-end rerouting the failure notification must reach the head-end of all the LSPs link rerouting not well suited for handling node failure 15

16 Ethernet Advanced Features During Ethernet evolution some advanced features were introduced: Automatic learning of MAC address to allow plug and play 802.1d Spanning Tree (ST) To avoid loops and provide a slow fault tolerance 802.1q Virtual LAN (VLAN) To separate one physical network in many logical networks 802.1s Multiple Spanning Tree (MST) To allow separate spanning tree for each VLAN 802.1p Priority Tagged Frame To provide CoS features 802.3ad Link aggregation To increase bandwidth (multiple physical links joined into one logical link) 16

17 Optical Ethernet 1GE and 10GE over fiber optic media (Optical Ethernet, OE) allow connections between 10/100/1000 Ethernet LAN to OAN, MAN and WAN, combining lowest cost of equipments, simplicity and scalability with high and flexible bandwidth capability and large distances. Ethernet goal: to be used as the unique end-to-end Layer 2 transport of IP packets in every region of the network, thus replacing SONET/SDH equipments (more expensive and less efficient for data transport) WAN MAN Mesh MAN Ring OAN Star Costumer premises OAN Ring Costumer premises 17

18 Ethernet Fault Tolerance Enhancements Fault-recovery Port-based Fast Spanning Tree (FRP-FAST) algorithm [S01FRPFAST] Switched ethernet Measured failure detection and recovery time < 2s EtheReal [V99Eth] fault detection and recovery mechanism Fasr spanning tree reconfiguration algorithm Delayed link inactivaiton to allow real-time connections to continue to exist even though some links are marked as blocked in the new spanning tree topology Fault detection + recovery time ~ 250ms in 10 hop network 18

19 Resilient Packet Ring (RPR) New MAC protocol (Layer 2 technology) Requires 2 counter-rotating fiber ringlets Increases Bandwidth efficiency using Spatial Reuse QoS (High, Medium and Low service classes) Restoration within 50 ms Steering (all stations are notified of the entire topology and of the failure location: the transmitting stations choose the ringlet that does not contain the failure) Wrapping (station adjacent the failure wraps traffic to other ringlet) Protection hierarchy (for simultaneous failures) New MAC not supported by most of equipments Requires specific network topology (ring) Current RPR interfaces are generally included into more expensive layer 3 equipments, where other efficient resilient solutions are possible without the topology constraint. 19

20 WDM Layer Resilient Schemes Both OCh and OMS sublayers feature dynamic restoration preplanned protection Main differences between OCh and OMS resilient schemes is represented by the granularity at which they operate OCh schemes protect individual lightpaths this allows selective recovery of optical line terminal (OLT) failures OMS resilient schemes work at the aggregate signal level all the lightpaths present on the failed line are concurrently recovered 20

21 WDM Resilient Schemes (2) Like in higher layers restoration schemes more efficient from a capacity viewpoint relatively slow (recovery time on the order of seconds or minutes) protection schemes less capacity efficient fast (recovery time on the order of seconds or minutes) 21

22 Terminology working lightpath default path of light established between a source-destination pair absent a network failure protection wavelength spare channel that may be used in case of network failure protection lightpath concatenation of protection wavelengths along a path 22

23 Optical Layer Protection and Restoration Schemes 23

24 Optical Layer Restoration OCh Path restoration upon network failure each affected working lightpath is dynamically replaced by a protection lightpath the protection lightpath is computed using either a centralized or distributed approach OMS line restoration an alternative route is found to locally divert all affected working lightpaths around the faulty line this scheme is triggered by the failed line end nodes which make use of a distributed algorithm to dynamically discover the alternative route 24

25 WDM Restoration Advantages Adaptable to network (traffic and topology) changes Small spare bandwidth required (< 50%) Drawbacks Usually slow (recovery time > 50ms) Coordination required upon failure 25

26 Restoration Scheme Characteristics Centralized Simplicity of a central controller + possible optimal solution Need for reliable controller + reliable controller communication network Distributed High restorability + capacity efficiency Difficult protocol implementation + high message contention degree Real-time High restorability because up-to-date information Slow recovery time + high resource contention Preplanned Fast recovery time Low restorability because out-of-date information 26

27 Optical Layer Protection OCh protection scheme is also called path protection OMS protection scheme is also called line protection In both schemes protection (spare) resources can be dedicated the spare resource is dedicated to a single working lightpath shared the same spare resource may be used to provide protection to multiple working lightpaths OCh and OMS protection schemes are available in mesh and ring network topologies 27

28 Dedicated Protection 1+1 the source node transmits on both the working and protection lightpaths simultaneously destination node keeps monitoring both lightpaths dynamically choosing the signal with the best performance (e.g., signal-to-noise ratio) 1:1 transmission occurs on the working lightpath only the protection lightpath may be used to transmit lowpriority traffic upon failure of the working lightpath both the source and destination nodes switch over the protection lightpath low-priority traffic is preempted 28

29 1+1/1:1 Protection Working link Splitter Protection link 1+1 Switch Working link Switch Protection link 1:1 Switch 29

33 Shared Protection Also referred as 1:N Allows spare wavelengths to be shared by a number of working lightpaths In case of a fault each disrupted transmission is switched over the protection wavelengths The operation requires signaling to notify network nodes of the new transmission paths to make sure that the protection wavelengths on the various fibers are correctly interconnected to form the required protection lightpaths once spare resource is used to protect a working lightpath it will not be available to protect the other working lightpaths until the original working lightpath is established 33

34 M:N Shared Protection 1 Switch 2 Switch MxN Switch MxN M:N N working fibers share M single protection fiber 34

35 M:N Shared Protection 1 1 Switch 2 Switch MxN 1 Switch MxN M:N N working fibers share M single protection fiber 35

36 Mesh Protection Schemes 36

37 Dedicated Line Protection DLP reserves protection wavelengths between the end nodes of each line utilized by working lightpaths DLP may require more spare capacity allocation than other scheme DLP can be faster than DPP due to shorter failure notification 37

38 Shared-Line Protection SLP Also termed 1:N line protection applies the SPP technique locally to the faulty line better spare resource utilization than DLP becuase of resource sharing recovery time generally faster than SPP because of locally limited signaling 38

39 Dedicated Path Protection (DPP) DPP fast 1+1 DPP just requires that the receiving node switches to the protection lightpath robust in the face of multiple failures the failures must not occur simultaneously in lines belonging to the working and protection lightpath of the same connection low degree of management complexity does not efficiently use spare resources 39

40 Shared-Path Protection SPP Also termed 1:N protection Protection wavelengths are shared by a number of line and node-disjoint working lightpaths More efficient utilization of spare resources than DPP more complex control signaling required between source and destination nodes longer recovery time on the order of 100ms 40

41 Ring Protection Schemes 41

42 Dedicated-Path-Switched WSHR Also called Optical Unidirectional Path-Switched Ring (OUPSR) OCh dedicated protection ring (OCh/DPRing) It is the DPP equivalent for ring networks recovery time fast order of ms or fraction of ms total protection wavelength mileage equal or larger that the one required in the other OCh and OMS ring protection schemes 42

43 Optical Unidirectional Line-Switched Ring OULSR Similar to DP-WSHR it utilized two counter rotating fibers one for working lightpaths other for protection lightpaths because line protection scheme all lightpaths passing through the failed line are jointly switched over the protection fiber OULSR vs DP-WSHR same wavelength mileage due to line switching employs less expensive devices similar recovery times 43

44 Shared-Path WSHR SP-WSHR also termed Optical Bidirectional path-switched ring (OBPSR) or OCh shared protection ring (OCh/SPRing) SPP equivalent in ring networks scheme s peculiarity is nonloopback switching each working lightpath is switched to the protection lightpath at its source node the recovered traffic reaches the destination node only along the protection lightpath SP-WSHR si the most efficient among the WSHR protection techniques in terms of spare resource utilization it requires complex control and signaling 44

45 Shared Line-Switched WSHR (SL- WSHR) physically implemented with two fibers (optical two-fiber/blsr, O-2F/BLSR) four fibers (optical four-fiber/blsr, O-4F/BLSR) in either case working lightpaths and protection wavelengths may be carried using both directions of propagation peculiarity is loopback switching upon failure the working lightpaths are switched, at one failure end, to the protection wavelengths of the counter-rotating fiber when they reach the other failure end they are looped back along their original working wavelengths to reach their destination nodes SL-WSHR characteristics simple fast (recover time on the order of tens of milliseconds) 45

46 Recovery Times [F03MIP] BGP-4: minutes OSPF: 1s to minutes MPLS fast (link) rerouting ms MPLS edge-to-edge rerouting 1-100s Spanning Tree 50s RSTP 10ms FRP-FAST < 2s EtheReal ~ 250ms SDH / SONET / DWDM: 50 ms OCh and OMS restoration 50ms Dedicated OL protection 10µs-10ms Shared OL protection 1-100ms 46

47 Coordination between IP and WDM Layers Coordination between resilient schemes si required to avoid multiple scheme concurrent activation Coordination commonly achieved through escalation strategies Escalation strategies sequentially activate the different resilient schemes starting from either the lowest or highest network layer Escalation strategies may be governed by explicit messaging between the different layers arbitrarily setting failure detection and recovery times not coordinated 47

48 No coordination 48

49 No coordination (2) 49

52 Coordination based on alarm ALARM Routing table Revision (no link) Link Rediscovered Routing table Revision (with link) Link in Traffic MPLS Link Down Link recovered through optical protection OTN Instant response to Level 1 alarms in high layer causes unnecessary routing activity, routing instability, and traffic congestion 52

53 Coordination based on hold-off time 53

54 Coordination based on hold-off time (2) 54

55 Coordination based on recovery token 55

56 Coordination based on recovery token (2) 56

57 Resilience Conclusion Different resilience schemes applicable in optical network Multi-layer restoration is a key point in current optical survivability research Joint IP/optical restoration mechanism is the trend in next generation optical network. 57