Soluciones 3COM

As discussed in the previous section, the implementation o f OPS architectures appears to be difficult at the moment. Due to this fact, optical burst switched (OBS) network architectures emerged (for instance, see [Qial]), as a trade-off between wavelength-level and packet-level switching granularities. The average burst sizes in OBS are in the range o f microseconds at core speeds o f 40 Gb/s, hence ultra-fast wavelength re-allocation can be achieved in response to changes in traffic pattern.

In the conventional OBS approach, packets, arriving from access layer, are electronically aggregated to bursts in the network edge-routers. In order for a burst to be routed hop-by-hop over the optical core to its destination, a control packet is signalled over the corresponding route ahead of the data. Using the information extracted from the control packet and checking it against the routing tables, switches carry out wavelength channel allocation and cross-connect configuration, needed to forward the incoming data burst (see Figure 2.5).

C H ALLEN G ES IN Q O S PROVISIONIN G IN INTERNET-OVER-OPTIC AL N E T W O R K A R C H ITEC T U RES E n d-to-end IP connectivity IP layer acknow l. p a c k et data burst (R^-^R*) data burst (R^-^RJ WDM layer Control channels

Electronic IP router with burst aggregation functionality Dynamically re-configurable OXC

Figure 2.5. IP over OBS networks. Control packet configures resources ahead of the data

burst (R2^ R i) , such that it will be routed via Xi between nodes Ri and Rj, and via X-2

between nodes R 3 and R 4 . At the same time, data burst (Rj-^Ri) is routed via Xj between

nodes R 3 and R 4 , and via X, between nodes R 4 and R|. For simplicity, signalling for the

burst (R2—>R4) only is depicted, and the case of uni directional connectivity is shown.

QoS provisioning in OBS is based on a combination o f the following two general techniques, aimed at minimisation of burst blocking in the optical core:

A. Signalling protocols that ensure end-to-end reservation of resources, ahead of the arriving burst, such that its QoS requirements are taken into account. One-way reservation is widely adopted in OBS, to minimise burst edge delays and simplify signalling functionality. (See Table 2.6 for the review of the currently proposed signalling protocols.)

B. Channel scheduling algorithms, carrying out actual allocation of a wavelength to the incoming burst on an outgoing link, whilst minimising bandwidth utilisation. (The review of channel scheduling techniques is presented in Table 2.7.)

CHALLENGES IN QOS PROVISIONING IN INTERNET-OVER-OPTICAL NETWORK

ARCHITECTURES 49

Table 2.6 Signalling protocols in OBS. The typical burst sizes are given wherever specified in the literature. T’omet - the offset time between the transmissions of control packet and the corresponding data burst, Tdeiay.min - burst minimum end-to-end delay (excluding burst aggregation time), Tprop - end-to-end propagation time, H - the number of hops in the end-to-end path, /node - control packet processing delay per node, itma ~

burst transmission delay, Q - high-priority traffic class, - low-priority traffic class. Protocol and references Algorithm Performance In-band- terminator (IBT) [Qial], [Qia2]

Header and delimiter (terminator) packets are attached to each data burst such that core switches forward bursts according to the header data, and stop transmission once the terminator has been detected.

Since bursts and control packets are not separated, a burst can be routed straight after processing of its header.

Tdelay.min ~ T’prop + T T/nodc ^trans

Typical burst sizes: 15 kB

Optical buffering: Yes

QoS support: No

Tell-and-go (TAG) [Qial], [Qia2]

Each data burst is sent along with the control packet. In intermediate nodes, the burst is buffered, whilst the control packet is being processed. Once the burst has been transmitted, a packet, releasing the involved resources, is issued.

Tdelay.min ~ T^jrop T T /node ^trans

Typical burst sizes: 15 kB

Optical buffering: Yes

QoS support: No

Reserve-a-fixed- duration (RFD) [Qial], [Qia2], [Turl]

The transmission of data burst starts in

an offset time period Toffset after the

control packet transmission. However, control packet always contains the burst size. Similar to TAG, the use of optical buffers is assumed to increase the offset between control packets and data bursts when it becomes too short after a high number of hops, traversed by the control packets.

Since the burst size is known by the control packet, RFD is capable of efficient resource scheduling.

T'lelay.min ~ TjflFset T’prop ^trans

Typical burst sizes: 15 kB

Optical buffering: Yes

QoS support: No Just-enough-time (JET) [Qial], [Qia2], [Qia3], [Yool], [Yoo2]

Variation of RFD: the value of Toffsct

between control packets and data bursts is calculated so that at any node, there is always a sufficient delay for the control packet processing and switch configuring before the arrival of corresponding data burst. Additionally, the data burst size is included in the control packet.

JET can supports bufferless core, as data bursts arrive only after processing of their control packets. By varying the

value of Toffset for a given class of

service, QoS differentiation can be supported.

T o flfse t(^ l) — 5Z<burst((^2^ T T /node, T ^ f f s e K Q ) = TT'/node>

Tdelay,m in((^/) ~ T"jffset(C';) T’prop'^ ^trans

Typical burst sizes: 15 kB

Optical buffering: No

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ARCHITECTURES 50

Table 2.6 (continued). Protocol and

references

Algorithm Perform ance

Just-in-time (JIT) [Wei2]

As in JET, the value of Toffset is set to

ensure the delay between control packets and data bursts. However, to simplify the complexity of core switches with respect to JET, the burst size is not included in the control packet. This requires another packet to be issued once a burst has been transmitted, to release the resources (as in TAG scheme).

JIT provides bufferless core, as the

value of Toffset is always sufficient for

the control packet processing and

switch configuring. However, JIT

cannot efficiently schedule resources, since they cannot be released straight after the burst transmission.

T jffset H ^node»

Tlelay.min ~ O ffs e t ^ r o p ^trans»

Typical burst sizes: 12 kB

Optical buffering: No QoS support: No Forward resource reservation (ERR) [Liul]

RFD-based scheme, which initiates channel reservation process as soon as the burst’s first packet has arrived at burst assembler. A burst size prediction technique is used for estimating the duration of reservation.

ERR attempts at minimising Toffsci delay, as reservation is carried out in

parallel with burst aggregation.

However, an impact of traffic pattern changes on the accuracy of burst size prediction algorithms should be further investigated.

T|)rop"*"^trans*' Tlelay.m in*'node"*" T^rop"*" 1trans

Optical buffering: No

QoS support: Yes

Proactive reservation- based scheduling (PRS) - signalling protocol [Elhl]

PRS combines ERR with traffic shaping in the ingress and in the core, with periodic issuing of reservation requests, based on estimating burst aggregation. (See Table 2.7 for PRS channel scheduling.)

Burst end-to-end delays are reduced by

“hiding” the delay of T ^ s x i. At the same

time, the implications of PRS on the core switches control, as well as the use of EDLs, require further investigation. Tlelay.min ~ Tirop ^trans»

Optical buffering: No

QoS support: Yes

To carry out resource reservation with minimum burst blocking rate, the OBS signalling protocols have to ensure that at each node along the end-to-end route, the control packets are processed and the switch is configured before the arrival time of the corresponding data bursts. As it can be seen from Table 2.6, approaches to the resource reservation can be divided into two groups. The first group includes approaches based on delaying data bursts using FDLs, whilst the control packets are processed, e.g. In- Band-Terminator (IBT) or Tell-and-Go (TAG) [Qial]. The second group includes approaches that use an offset time between the control packets and the transmission of corresponding bursts, e.g. Just-Enough-Time (JET) [Yoo3], [Qial]. The comparison of timing in these two groups of protocols, using IBT and JET as examples, is shown in Figure 2.6.

C H ALLEN G ES IN Q O S PROVISIONING IN INTERN ET-OVER-OPTIC AL N E T W O R K

A R C H ITEC T U RES 51

Source 1 2 Destination Source 1 2 Destination

le a d e r burst ■prop node prop node pro| node time control p a c k et prop node offset prop burst _node prop node time

(a)

(b)

Figure 2.6. Timing diagrams of the conventional OBS protocols, (a) IBT, (b) JET. Toffset - offset time between the control packet and the data burst, tprop - propagation time per hop, t„ode - control packet processing delay per node.

The second group of protocols is of special importance to QoS provisioning, since the offset time can be adjusted for each traffic class such that the service differentiation is provided, whilst store-and-forward QoS model, associated with buffering in the optical domain, is avoided [Qial]. For instance, it was shown in [Yoo3] that JET, maintaining the offset time o f high-priority bursts higher than 51,

where L is the average size of low-priority bursts, achieves 99% isolation of blocking

probability of the high-priority traffic from that of the low-priority traffic (see Table 2.6). This way, the blocking probability of high-priority bursts becomes independent of the load of the low-priority traffic, whilst the blocking probability o f low-priority bursts depends on the load of both traffic classes.

However, JET-based QoS provisioning implies an extra delay for the high priority traffic, which can be against its QoS requirements. Additionally, recent research in [Doll] showed that bounding the blocking probability o f the high-priority traffic to 10*^ leads to unacceptably high blocking probability o f the low-priority traffic (reaching 0.1), since the latter cannot be isolated from the load o f the high-priority traffic. This effect is especially emphasised under high traffic loads [D oll], and can also be observed in JET-based OBS, operating without service differentiation, when traffic loads exceed 0.3 [Myel]. At the same time, it was shown that burst blocking

CHALLENGES IN QOS PROVISIONING IN INTERNET-OVER-OPTICAL NETWORK ARCHITECTURES

probability, exceeding 10"^, significantly affects the performance o f higher layer network protocols, such as Transmission Control Protocol (TCP), resulting in the increase in out-of-order re-transmissions of packets, carried by the blocked bursts, which adversely affects the overall network throughput [Detl].

Furthermore, buffering in the optical domain may still be required to resolve contention between the bursts, thus adding to the complexity o f OBS switch architecture (making it similar to that of OPS), as well as further increasing burst end- to-end delays. Finally, as the offset time between control packets and bursts decreases at each consecutive hop along the source-destination route, due to the delays involved in the processing o f the control packets, the blocking rate increases by two orders of magnitude towards the end o f long routes, resulting in an unfair servicing o f routes, traversing high number o f hops (see, for instance, [Whil]). Moreover, it was shown that longer bursts can experience the blocking rate o f more than one order of magnitude higher than that in shorter ones [Dol2], [Popl].

To address the problem of fairness, a technique, referred to as 'merit-based scheduling’, based on scheduling bursts according to special metrics that take into account burst distances to their destinations in hops, has been proposed in [Whil]. Under this scheme, bursts are prioritised at each node according to their consumption o f resources and distances to their destination. Using 5x5 mesh-torus network, it has been shown that such technique can decrease burst blocking with respect to JET. However, the improvement was observed to be lower than one order of magnitude. Hence, the performance of the merit-based scheduling in combination with QoS- differentiated burst transmission requires further investigation. It is also important to note that the algorithms involved in burst pre-emption according to both the distance- related metrics and class of service priority, would add to the computational complexity.

Another approach to fair QoS provisioning in RFD-based OBS, known as the ‘JumpStart architecture’, was proposed in [Zail] and [Ball]. According to this architecture, two types of connectivity are maintained simultaneously: bursts (whose duration is less than the network round-trip time), provisioned by a JIT-based scheme, and persistent connections, provisioned by the lightpaths. The latter are proposed to be

CHALLENGES IN QOS PROVISIONING IN INTERNET-OVER-OPTICAL NETWORK ARCHITECTURES

used for high-priority traffic as well as for bursts whose duration is longer than the round-trip time. Additionally, multicast support was considered, by assuming a limited availability o f switches, capable o f splitting the optical signal. Protocols, similar to GMPLS (discussed in appendix C), are envisaged for implementation in JumpStart’s control plane. Whilst such separation o f service classes is a promising QoS provisioning technique, issues, related to the efficiency o f resource sharing and pre­ emption of connections in JumpStart are yet to be addressed.

On the other hand, signalling protocols that use FDLs in addition to the offset time (such as TAG [Qial]), to hold data bursts during the processing o f control packets, appear to be less efficient in terms of both bandwidth utilisation and FDLs utilisation with respect to JET-based OBS [Qial], [Qia2] (see Table 2.6). Additionally, in case of IBT-based protocols, detection of burst terminator is currently not a straightforward technique.

As shown in Table 2.7, channel scheduling techniques can also be employed for decreasing the burst blocking probability through maximising channel utilisation. The efficiency of these techniques in terms of wavelength requirements, satisfying a given burst blocking probability, directly depends on the utilisation o f voids, created in a wavelength channel due to the fragmentation in previously scheduled bursts. The techniques, based on the Latest-available-unscheduled-channel with void-filling (LAUC-VF), achieve high wavelength utilisation by exploiting the unused voids in a wavelength charmel (see [Xiol]). Consequently, LAUC-VF-based techniques achieve the blocking probability that is below 10"^, which is up to two orders o f magnitude lower than that in the other channel scheduling approaches (as discussed in [Xiol], [Xio2]). However, these techniques introduce extra computational complexity in the OBS switch control, since the channel scheduler would be responsible for maintaining information on the time-slots availability in all unused wavelengths. This would also introduce limitations in terms of network scalability. Conversely, less computationally- demanding techniques, based on such heuristics as Horizon [Turl] or the First-Fit- Unscheduled-Channel (FFUC) [Xiol], would suffer from high blocking probability (about 10'^), since they do not attempt at utilising voids between the bursts (see Table 2.7).

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ARCHITECTURES 54

It should be noted, however, that the comparative investigation o f the channel scheduling techniques, to date, has been mostly limited to the case of a standalone optical node. Hence, further research on their performance in a realistic network environment is required. Additionally, the detailed comparative analysis o f the channel scheduling algorithms in terms of the architectural and computational complexity is not yet available.

Table 2.7. Channel scheduling in OBS. Unscheduled time is the instant after the latest burst reservation in a given channel. Unused time is the duration of a void time-slot between two successively scheduled bursts and after the latest burst reservation in a given channel. The performance is evaluated using a standalone OBS node [Xiol]. The considered node configuration assumes 8 ports, 15 data optical channels (wavelengths), 1 signalling channel, buffer size of 2 slots (14 ps each), and traffic loads of 0.76. Note that the blocking probability values are shown for a particular node configuration and may

vary depending on different network parameters. Heuristic

and references

Algorithm Perform ance

First-fit unscheduled channel (FFUC) [Xiol]

Once a control packet has arrived, the wavelength space is searched in a fixed order and the burst is assigned to the first channel whose unscheduled time is smaller than the time of data burst arrival time.

Although computationally simplest, i.e.

in the order of 0 (W i), FFUC leads to

blocking rate of up to 10'^, since it does not use voids between scheduled bursts.

Latest available unscheduled channel with void filling (LAUC-VF) [Xiol], [Xio2]

To each arriving burst, LAUC-VF assigns the latest unused channel, which can accommodate the data burst between previously scheduled ones.

By de-fragmenting channel payloads, LAUC-VF maintains the blocking rate of three orders of magnitude lower than that in FFUC, i.e. below 10“*. However, it implies exhaustive search through the entire wavelength space.

Generalised LAUC-VF (G-LAUC-VF) [Yan2]

G-LAUC-VF maintains separate queues that hold control bursts according to their class of service at the core routers. This way, queues are serviced according to their priority by applying LAUC-VF to each selected burst.

G-LAUC-VF carries out explicit QoS differentiation. However, the

computational process is complicated by the service differentiation policy, as well as by the synchronisation of control bursts in the queues and the corresponding data bursts. First-fit with

void filing (FF-VF) [Xiol]

FF-VF searches the ordered space of channels and selects the first unused channel, which can accommodate the data burst between previously scheduled ones.

FF-VF attempts at utilising voids similar way to LAUC-VF. However, its

blocking rate can be one order of magnitude higher than that in LAUC- FV, i.e. in the range of 10'^. This is because FF-VF does not iterate through the entire wavelength space.

Horizon [Turl]

For each of the arriving data bursts. Horizon selects the latest available unscheduled data channel, as long as its unscheduled time is earlier than the data burst arrival time.

No comparative data is available, but it is expected that the bandwidth utilisation is higher than that in FFUC, as voids between bursts are minimised.

CHALLENGES IN QOS PROVISIONING IN INTERNET-OVER-OPTICAL NETWORK ARCHITECTURES 55 Table 2.7 (continued) Heuristic and references Algorithm Performance Assured Horizon [Dol3], [Dol4]

Coarse-grained (or static) bandwidth reservation is employed for each forwarding equivalent class between the ingress and egress, whilst burst assembly mechanism carries out header processing and policing to control the load on core switches.

QoS differentiation can be achieved at the expense of buffering in the optical domain.

As a means o f decreasing PLR in the core due to burst blocking, an approach, referred to as ‘burst segmentation’, can be exploited [Vokl], [Det2]. According to this approach, a blocked burst would not be discarded completely, but would only lose packets until a time when a wavelength can be allocated to it. Whilst the burst