E E Module 13 © Wayne D. Grover 2002, p-Cycles Jens Myrup Pedersen Aalborg University © Wayne D. Grover 2002, 2003 E E Module 13 ( Version for book website Dec. 2003)

E E Module 13 © Wayne D. Grover 2002, Background and Motivation “ Ring “ A. 50 msec restoration times B. Complex network planning and growth C. High installed capacity for demand-served D. Simple, low-cost ADMs E. Hard to accommodate multiple service classes F. Ring-constrained routing “Mesh” G. Up to 1.5 sec restoration times H. Simple, exact capacity planning solutions I. well under 100% redundancy J. Relatively expensive DCS/OXC K. Easy / efficient to design for multiple service classes L. Shortest-path routing “ Shopping list” : A, D, H, I, L (and K) please...keep the rest

E E Module 13 © Wayne D. Grover 2002, Relative Characteristics of Known Schemes Restoration Times Capacity Redundancy 1+1 APS, Rings p-Cycles Mesh Span Restoration Shared Backup Path Protection (SBPP) True Mesh Path Restoration 100% redundancy: “the dividing line”

E E Module 13 © Wayne D. Grover 2002, p-Cycles

E E Module 13 © Wayne D. Grover 2002, p-Cycles - an “on-cycle” failure Reaction to an “on-cycle” failure is logically identical to a unit-capacity BLSR loopback reaction loopback “on-cycle” spans have both working and spare capacity like a BLSR

E E Module 13 © Wayne D. Grover 2002, p- Cycles - a “straddling span” failure Reaction to a straddling span failure is to switch failed signals onto two protection paths formed from the related p-cycle Break-in Straddling spans have two protected working signal units and have no spare capacity

E E Module 13 © Wayne D. Grover 2002, A lot ! Re-consider the example: It consumes 13 unit-hops of spare capacity It protects one working signal on 13 spans and two working on 9 spans i.e., spare / working ratio = 13 / (13*1 + 9*2 ) = 42% How much difference can this make ? A fully-loaded Hamiltonian p-cycle reaches the redundancy limit, 1/(d-1) x2

E E Module 13 © Wayne D. Grover 2002, Working span capacities arising from one unit of demand on each node-pair: Example of a whole p-cycle network design Total working capacity: 158 units

E E Module 13 © Wayne D. Grover 2002, Design Solution: 53.8 % overall redundancy A1A1 B1B1 C1C1 D2D2 E2E2 Total protection capacity:85 units Redundancy:53.8% Optimal configuration dynamically computable or self-organized p-Cycle Copies Total:7

E E Module 13 © Wayne D. Grover 2002, Summary: Important Features of p-Cycles Working paths go via shortest routes over the graph p-Cycles are formed only in the spare capacity Can be either OXC-based or based on ADM-like nodal devices a unit-capacity p-cycle protects: –one unit of working capacity for “on cycle” failures –two units of working capacity for “straddling” span failures Straddling spans: –there may be up to N(N-1)/2 -N straddling span relationships –straddling spans each bear two working channels and zero spare –-> mesh capacity efficiency Only two nodes do any real-time switching for restoration –protection capacity is fully pre-connected –switching actions are known prior to failure –-> BLSR speed “pre-configured protection cycles”  p - cycles

E E Module 13 © Wayne D. Grover 2002, p-Cycle Capacity Design If span i fails, p-cycle j provides one unit of restoration capacity If span i fails, p-cycle j provides two units of restoration capacity i j i j

E E Module 13 © Wayne D. Grover 2002, Optimal Spare capacity design with p-cycles

E E Module 13 © Wayne D. Grover 2002, Basic (non-joint) p-cycle Design Model “p-cycle SCP” Min cost of spare capacity =1 if span k in cycle j (parameter) P=set of candidate cycles, index j Number of unit copies of cycle j to build as a p-cycle (solution variable). x ij =2 if span i straddles cycle j. x ij =1 if span i on-cycle to cycle j. x ij =0 if neither of above.

E E Module 13 © Wayne D. Grover 2002, AMPL Model to Implement p-Cycle SCP (1) # p-cycle SCP IP Model for AMPL - Version December-2001 by John Doucette # Copyright (C) 2001 TRLabs, Inc. All Rights Reserved. # This is an AMPL model for determining the minimum-cost p-cycle network design. # This model optimizes p-cycles only... working capacity is provided as inputs. # SETS #**************************** set SPANS; # Set of all spans. set CYCLES; # Set of all candidate cycles. # PARAMETERS #**************************** param Cost{j in SPANS}; # Cost of each unit of capacity on span j. param Work{j in SPANS}; # Number of working links placed on span j. param Xpi{p in CYCLES, i in SPANS} default 0; # Number of paths a single copy of cycle p provides for restoration of failure of span i (2 if straddling span, 1 if on-cycle span, 0 otherwise). param pCrossesj{p in CYCLES, j in SPANS} := sum{i in SPANS: i = j and Xpi[p,j] = 1} 1; # Equal to 1 if cycle p passes over span j, 0 otherwise. i.e. if Xpi[p,j] = 1, then cycle p crosses span j.

E E Module 13 © Wayne D. Grover 2002, AMPL Model to Implement p-Cycle SCP (2) Continued….. # VARIABLES #**************************** var p_cycle_usage{p in PCYCLES} >=0 integer, <=10000; # Number of copies of p-cycle p used. var spare{j in SPANS} >=0 integer, <=10000; # Number of spare links placed on span j. # OBJECTIVE FUNCTION #**************************** minimize sparecost: sum{j in SPANS} Cost[j] * spare[j]; # CONSTRAINTS #**************************** subject to full_restoration{i in SPANS}: Work[i] <= sum{p in PCYCLES} Xpi[p,i] * p_cycle_usage[p]; subject to spare_capacity_placement{j in SPANS}: spare[j] = sum{p in PCYCLES} pCrossesj[p,j] * p_cycle_usage[p];

E E Module 13 © Wayne D. Grover 2002, Optimal Spare capacity design - Typical Results “Excess Sparing” = Spare Capacity compared to Optimal Span- Restorable Mesh i.e., “mesh-like” capacity

E E Module 13 © Wayne D. Grover 2002, Understanding why p-cycles are so efficient... 9 Spares cover 9 Workers 9 Spares cover 29 working channels on 19 spans Spare Working Coverage UPSR or BLSR p-Cycle …with same spare capacity “the clam-shell diagram”

E E Module 13 © Wayne D. Grover 2002, Further comparing p-cycles to rings

E E Module 13 © Wayne D. Grover 2002, A Priori p-Cycle Efficiency: AE(p) AE (p) measures a cycle’s potential to provide protection relationships for working channels X p,i = 1 if on cycle X p,i = 2 if straddler S S,p = 3 S C,p = 9 AE(p) = 1.67 S S,p = 4 S C,p = 10 AE(p) = 1.80 S C,p = # on cycle S S,p = # straddlers c i = unit cost of i

E E Module 13 © Wayne D. Grover 2002, Demand-weighted p-Cycle Efficiency: E w (p) Ew(p) measures a cycle’s actual efficiency in providing protection relationships for uncovered working channels AE(p) = 1.67 E w (p) = AE(p) = 1.67 E w (p) = 3.67 X p,i = 1 if on cycle X p,i = 2 if straddler w i = working on i c i = unit cost of i

E E Module 13 © Wayne D. Grover 2002, Self-organization of the p-cycles... p-cycles certainly could be centrally computed and configured. –based on the preceding formulation However, an interesting option is to consider if the network can adaptively and continually self- organize - a near-optimal set of p-cycles within itself, - for whatever demand pattern and capacity configuration it currently finds.

E E Module 13 © Wayne D. Grover 2002, Self-organization of the p-cycles Based on an extension / adaptation of SHN™ distributed mesh restoration algorithm –“DCPC” = distributed cycle pre-configuration protocol Operates continually in background –Non-real time phase self-organizes p-cycles –Real time phase is essentially BLSR switching –p-cycles in continual self-test while in “storage” Centralized “oversight” but not low-level control –Method is autonomous, adaptive Networks actual state on the ground is the database

E E Module 13 © Wayne D. Grover 2002, Key concepts of DCPC protocol Node roles: –Cycler node state, Tandem node state DCPC implemented as event-driven Finite State Machine (FSM) Nodal interactions are (directly) only between adjacent nodes –Indirectly between all nodes (organic self-organization) –via “statelets” on carrier / optical signal overheads Three main steps / time-scales / processes –Each nodes act individually, “exploring” network from its standpoint as cycler node. –All nodes indirectly compare results –Globally best p-cycle is created

E E Module 13 © Wayne D. Grover 2002, Overview of DCPC protocol

E E Module 13 © Wayne D. Grover 2002, How DCPC discovers “best p-cycles” (2)

E E Module 13 © Wayne D. Grover 2002, How DCPC discovers “best p-cycles” (1)

E E Module 13 © Wayne D. Grover 2002, DCPC Performance studies

E E Module 13 © Wayne D. Grover 2002, Illustrating the Real time phase

E E Module 13 © Wayne D. Grover 2002, Adapting p-cycles to the IP-layer …

E E Module 13 © Wayne D. Grover 2002, IP Network Restoration IP Networks are already “Restorable” Restoration occurs when the Routing protocol updates the Routing Tables This update can take a Minute or more - Packets are lost until this happens Speed-up of IP Restoration is needed Not losing packets would be great too Also some control over capacity / congestion impacts needed p-cycles proposed as “fast” part of a fast + slow strategy that retains normal OSPF-type routing table re-convergence

E E Module 13 © Wayne D. Grover 2002, Span versus Node Restoration Restoration Layer-Strategy Choice Unavailability of Spans/Nodes in the Different Layers Feasibility of IP Restoration in the Different Layers

E E Module 13 © Wayne D. Grover 2002, IP p-Cycle Properties p-Cycles are Virtual Circuits –Consume Zero Capacity until used –Well suited to MPLS-like Emerging Standards p-Cycles are Pre-planned –Centrally or Distributed (~DCPC) –Designed prior to Failure –At Failure, a p-Cycle requires no time to Setup or Use p-Cycles can restore Node and Span Failures

E E Module 13 © Wayne D. Grover 2002, (1)Network setup: logical p-cycle establishment in routing tables. –p-cycles are established as “virtual circuits” using MPLS or a small number of “reserved” IP addresses (2) Real-time phase: nodal p-cycle behavior: –encapsulation, –deflection, –re-introduction IP-layer p-cycles

E E Module 13 © Wayne D. Grover 2002, Insertion of an IP packet into a p-cycle –If the packet’s normal routing table entry indicates forwarding into a now-dead port, encapsulate the packet with the “p-cycle address” for dead neighbour router, route encapsulated packet (I.e, into p-cycle) –The IP packet is encapsulated in a p-cycle packet and routed along the p-cycle Operation of IP-layer p-cycles

E E Module 13 © Wayne D. Grover 2002, IP Packet p-Cycle Packet Encapsulation Routing Table p-cycle Packet Operation of IP-layer p-cycles

E E Module 13 © Wayne D. Grover 2002, Insertion of an IP packet into a p-cycle… –The p-cycle packet packet contains the IP packet two new fields: –The ID of the p-cycle on which the packet belongs –The cost of the original pre-failure path for the IP packet Operation of IP-layer p-cycles

E E Module 13 © Wayne D. Grover 2002, Router Processing of a p-cycle packet arrival –(a) the router checks if it has a routing entry (with a functional port) for the encapsulated IP packet’s destination; if no, continue the packet along the p-cycle –(b) If yes to (a), test ; is cost of local “continuing route” option >= cost in p-cycle packet (I.e., from the encapsulation point); If yes, continue along the p-cycle. If no, remove the IP packet from the p-cycle packet and route it “normally” from this node. Operation of IP-layer p-cycles

E E Module 13 © Wayne D. Grover 2002, Operation of IP-layer p-cycles Failed Link Router Data De-Encapsulation Data Encapsulation Router p-cycle (a) On-Cycle Failure (1 restoration Path) (b) Straddling Failure (2 Restoration paths)

E E Module 13 © Wayne D. Grover 2002, Capacity Planning for Span Restoration with p-cycles Integer Program Formulation –chooses a set of p-cycles to restore all Span failures –Failed working demands may be re-routed over all available p-cycles –User-defined input constraint on number of p-cycles in the design –Objective: Minimize the Max Restoration-induced total flow on any span (i.e., min (max (oversubscription)) –subject to: The maximum over-subscription is over all Span Failures Every span failure is “covered” Hop count limit is respected Design limiting number of p-cycles

E E Module 13 © Wayne D. Grover 2002, /12 3/6 Pre-Failure flow 4.5/6 10.5/12 4.5/6 “Straddling” Failure Flow lost = 3 Split 1.5 units each way around p-cycle Max subscription = 10.5/12 = /6 “On-Span” Failure Flow lost = 9 9 units around p-cycle Max (over-) subscription = 12/6 = 2 Installed capacity Illustrating restoration-induced “over-subscription” of capacity

E E Module 13 © Wayne D. Grover 2002, Integer Program design results Run in “Bellcore” network –Minimally provisioned to be fully mesh- span restorable Some sample results

E E Module 13 © Wayne D. Grover 2002, This set of five p-cycles producing  20% over- subscription for any IP span restoration Allowed 15 design p-cycles,  max over-subscription goes to  2% Sample Result

E E Module 13 © Wayne D. Grover 2002, Node Encircling p-Cycles. Each Node has a p-Cycle dedicated to its failure For each Node, a p-Cycle is chosen which includes all logically “Adjacent” Nodes but not the Protected Node Router Failure Restoration using “Node-Encircling” p-Cycles Node- Encircling p- cycle Other Nodes Encircled Node

E E Module 13 © Wayne D. Grover 2002, p-Cycles are Virtual Circuits/Protection Structures which can redirect Packets around Failures –Plain IP is Connectionless but p-Cycles can be realized with MPLS, IP Tunneling/Static Routes Router Restoration using “Node-Encircling” p-Cycles Node Failure

E E Module 13 © Wayne D. Grover 2002, An “encircling” p-cycle for node k includes all nodes that are logically adjacent (directly connected) to node k but not node k itself. “Encircling” p-cycles may be visually (graphically) apparent as such, may require a Figure 8, and / or may be non-apparent, I.e., logically, but not graphically encircling. –A special case of “Figure-8’ing” is when the Figure 8 loop is logically required to go down and back the same span, to include one or more degree 2 sites. The important property is that the encircling structure “intercepts” all transiting flows through the subject node. –Examples of each case follow... Key concept / extension of basic p- cycles: “node encircling” p-cycles

E E Module 13 © Wayne D. Grover 2002, Router Restoration using p-Cycles p-Cycle Examples Simple, Apparent Simple, Non-Apparent Non-Simple (Segment) Non-Simple (Figure “8”) final demo.....

E E Module 13 © Wayne D. Grover 2002, Packet Routing (Pre-Failure) Path Cost = 3 But … Failure IP Encapsulate P-Packet Operation of node-encircling p-cycles

E E Module 13 © Wayne D. Grover 2002, (Infinite) > 3 (3) == 3 (2) < 3 ! P-packet continues along p-cycle until the local path cost is less than at entry P-Packet At which point the IP packet is “unencapsulated” and routed as usual P-Packet IP 1 unit of cost per link Operation of Node-encircling p-cycles

E E Module 13 © Wayne D. Grover 2002, Concluding Comments p-cycles offer new approaches to both WDM and IP-layer transport –“ mesh-like efficiency with ring-like speed ” Capacity-planning theory –for 100% span restoration in WDM / Sonet with mesh sparing –for controlled worst-case over-subscription in IP-layer “Node-encircling” p-cycles –fast integrated restoration against either router or link-failures Nortel has implemented span-restoration via IP p-cycles –~ 10 msec restoration time, no packet loss in their experiments Ongoing studies: Integrated planning of composite node / link restoration p-cycles Availability analysis of p-cycles