QoS routing in DWDM Optical Packet Networks W. Cerroni CNIT Bologna Research Unit, ITALY F. Callegati, G. Muretto, C. Raffaelli, P. Zaffoni DEIS University of Bologna, ITALY
Optical Packet Switching (OPS) Statistical multiplexing of different flows on each wavelength Routing-based signalling Legacy Networks Edge Systems WDM Links Optical Packet Routers Long-term technical solution
Store & forward with optical buffers 0 D (B -1)D +D +2D +(B -1)D Realized with B Fiber Delay Lines (FDL): the delay must be chosen at packet arrival packets are delayed until the output wavelength is available available delays are consecutive multiples of the delay unit D (different choices are also possible) packets are lost when the buffer is full, i.e. the required delay is larger than the maximum delay achievable D M = (B -1)D
Wavelength and Delay Selection Problem The forwarding algorithm determines: the output fiber (from the routing table) and the output wavelength if all wavelengths are busy: packet delayed in FDL buffer or packet dropped, because the required delay is not available 1 2 3 4 D GAP Packet Loss Probability +D +2D +3D +4D D Wavelength and Delay Selection (WDS) choose the wavelength according to availability in time choose the delay in order to minimize the gaps between buffered packets and maximize the wavelength utilization
Algorithms for adaptive routing in OPS Routing algorithms can be static: routing tables change only when the topology changes adaptive: routing tables include alternatives to shortest path depending on the congestion state of the network DWDM OPS network must combine the flexibility of adaptive routing with the resources made available by WDM design routing procedures outperforming the conventional shortest path routing provide QoS differentiation at the routing level
Algorithms for adaptive routing in OPS Solutions for undifferentiated traffic presented at ONDM 04 The routing algorithm provides: a default path: shortest path used as a first chance a few alternative paths: used in case the default is congested Traffic flows are routed according to different path selection strategies (increasing complexity): SL (Single Link): only the default path is used (static routing) SA (Single Alternative): a single alternative path is used (ineffective: performance close to SL) MA (Multiple Alternative): more than one alternative paths are used
Multiple alternative routing In case the default path is congested, the best wavelength is chosen on one of the alternative path alternative path 1 WDS DESTINATION SOURCE default path alternative path 2
Multiple alternative routing In case the default path is congested, the best wavelength is chosen on one of the alternative path WDS alternative path 1 DESTINATION SOURCE congestion default path alternative path 2
Multiple alternative routing In case the default path is congested, the best wavelength is chosen on one of the alternative path alternative path 1 DESTINATION SOURCE alternative path 2 WDS congestion default path
QoS differentiation at the routing level Due to FDL buffering constraints, traditional priority queuing and scheduling techniques are not feasible QoS differentiation at the OPS node level possible through resource partitioning (cf. JSAC Jan. 2000, Comp. Net. 15/03/2004) Integration of QoS management into adaptive routing algorithms aggregate QoS classes (sort of DiffServ approach) simple set-up: 2 priority classes High-Priority (HP) traffic: always routed along the shortest path (SL) using resource partitioning limited packet loss limited delay and packet jitter Low-Priority (LP) traffic: two options always routed along the shortest path (SL) using available resources overflow traffic re-routed to alternative paths (MA)
Resource partitioning: FIX strategy H wavelengths out of W are reserved to HP traffic the remaining W H wavelengths are shared between HP and LP traffic The reserved wavelengths are fixed e.g. H=2? 1 and? 2 are reserved when? 1 and? 2 are busy, HP and LP experience the same loss LP HP λ 1 λ 2 λ 3 λ 4 λ 1 λ 2 λ 3 λ 4 +D +2D +3D +4D +D +2D +3D +4D
Resource partitioning: RES strategy H wavelengths out of W are reserved to HP traffic the remaining W H wavelengths are shared between HP and LP traffic Any H wavelengths are reserved based on the actual occupancy e.g. H=2 LP packets are allowed as long as more than 2 wavelengths are available LP HP λ 1 λ 2 λ 3 λ 4 λ 1 λ 2 λ 3 λ 4 +D +2D +3D +4D +D +2D +3D +4D
Network performance evaluation First evaluation on a simple topology 5 nodes, 12 links W = 16 wavelengths per link connectionless transfer mode Poisson arrivals at each node exponential packet size (optimal average value according to node dimensioning) traffic distribution on the network: balanced (B): each wavelength is loaded by 0.8 traffic generated accordingly unbalanced (U): each node generates the same traffic wavelengths on different links carry different loads (max. 0.8) 2 1 4 0 3
Resource partitioning: FIX vs. RES Packet loss probability 1 FIX LP RES LP 1e-01 FIX HP RES HP 1e-02 1e-03 1e-04 1e-05 1e-06 1e-07 1e-08 0 1 2 3 4 5 No. of reserved wavelengths (H) From now on, always adopt the RES strategy
Performance for undifferentiated traffic Packet loss probability 1.2E-03 1E-03 8E-04 6E-04 4E-04 2E-04 B = balanced traffic U = unbalanced traffic SL = shortest path only MA = multiple paths 0 B-SL B-MA U-SL U-MA Not very high improvement due to the limited number of paths in the test network MA proves to be more effective on larger networks (cf. ONDM 04)
The impact of resource partitioning Packet loss probability B = balanced traffic SL/MA = routing policy adopted for LP traffic (HP uses always SL) 1 1e-01 1e-02 1e-03 1e-04 1e-05 1e-06 1e-07 1e-08 Percentage of HP traffic = 20% No. of reserved wavelengths = 3 B-SL LP B-SL HP B-MA LP B-MA HP 1 2 3 4 No. of reserved wavelengths Packet loss probability 1 1e-01 1e-02 1e-03 1e-04 1e-05 1e-06 1e-07 1e-08 B-SL LP 10 B-SL HP B-MA HP B-MA LP 20 30 40 50 Percentage of HP traffic Accurate HP dimensioning gives a good degree of traffic differentiation LP routing policy does not affect HP LP performance slightly affected by HP dimensioning (within the range considered)
Resources needed for guaranteed HP loss balanced traffic SL routing policy adopted for LP traffic Percentage of HP class traffic 80 70 60 50 40 30 20 10 PLP = 10-4 PLP = loss probability for HP packets PLP = 10-5 PLP = 10-6 0 0 5 10 15 20 25 30 35 Percentage of reserved wavelengths LP class packet loss probability 1 1e-01 1e-02 1e-03 1e-04 PLP = 10-4 PLP = 10-5 PLP = 10-6 10 20 30 Percentage of HP class traffic HP dimensioning required for a given PLP
Network design for unbalanced traffic Each node generates the same amount of traffic, uniformly distributed towards the other nodes each link is subject to a different load, depending on the traffic matrix (assuming shortest paths only) no reason to waste resources on underloaded links provide the links with the resources (i.e. no. of wavelengths) required to obtain a given average load per wavelength no. of wavelengt hs = total load on the link required load per wavelength Drawback: resource partitioning is not very effective on links with a small no. of wavelengths
Performance of unbalanced network design D = network design under unbalanced traffic SL/MA = routing policy adopted for LP traffic (HP uses always SL) Network cost (in terms of total no. of wavelengths) as close as possible to the balanced case (i.e. 16*12=192) amount of traffic generated at each node accordingly 1 Percentage of HP traffic = 20% Packet loss probability 1e-01 1e-02 1e-03 1e-04 1e-05 1e-06 1e-07 1e-08 D-SL HP D-SL LP D-MA LP D-MA HP 10 15 20 25 30 35 40 Percentage of reserved wavelenghts Resulting resource distribution: from 7 to 28 wavelengths/link Resulting loss distribution: from 10-2 to 10-6 Heavy unfairness
Design with constraint on packet loss Fix a maximum acceptable value of packet loss probability Perform the design procedure and evaluate packet loss Iterate the simulation by increasing the no. of wavelengths on links with loss exceeding the acceptable value, until the loss constraint is satisfied on all links ercentage of additional wavelengths 50 45 40 35 30 25 20 15 10 5 0 PLP = 10-5 PLP = 10-3 0 1 2 3 4 5 6 7 8 9 10 11 Network links still unfair loss distribution among links at least the loss constraint is satisfied increased overall network cost increased simulation time
Conclusions QoS differentiation in DWDM OPS networks achieved by exploiting resource partitioning and adaptive routing Iterative design procedure to satisfy loss constraints Open issues: need for extensive simulations on large networks to prove the effectiveness of MA approach evaluation of the impact of adaptive routing on packet delay and sequence extension to a connection-oriented transfer mode and impact on virtual circuits routing