1

Optical IP Switching: A Flow-Based Approachto Distributed Cross-Layer Provisioning

Optical IP Switching: A Flow-Based Approach to Distributed Cross-Layer Provisioning

By Urooj Fatima31-10-2012

AuthorsMarco Ruffini, Donal O’Mahony, and Linda Doyle

Other References: M. Ruffini, D. O’Mahony, and L. Doyle, “Optical IP switching for dynamic traffic engineering in next-generation optical networks,” in Proc. of the Optical Network Design and Modeling Conf., 2007, pp. 309–318.

TM8106-Optical Networking

2

Overview• Why we need hybrid architectures?• Contribution of the paper• Previous work• Optical IP Switching

– Architectures– Mechanisms

• Simulation results– Technical (architecture simulations)– Economical (cost evaluation)

• Testbed implementation– Effects on TCP and UDP– Summary

3

Why hybrid architectures?

Problem• Revolution in optical transmission due to technological

improvements – WDM and EDFAs– Increase in bandwidth availability– Decrease in cost of data transfer– Boost in development and deployment of Internet

• IP routing technology struggles to deliver the necessary bandwidth at competitive costs - network bottleneck

Solution• Hybrid electro-optical architectures – bridging the gap between

optical transport and electronic routing– Solution to reduce costs at the IP layer– Deliver new revenue generating services and applications

4

Contribution• Most of the hybrid architectures have focused on end-to-end

lightpath provisioning– Centralized management plane

• This paper proposes Optical IP Switching (OIS) – a hybrid electro-optical network architecture that combines IP routing and wavelength switching using distributed decision-making process.

• Technical and economic analysis is reported based on simulations

• Test-bed results – Effects on TCP and UDP

5

Hybrid Electro-Optical Solutions-I• Optical Circuit Switching (OCS)

– Dynamic setup of optical lightpaths to transport data transparently– Group and switch all the packets sharing a common route into

dedicated all-optical channels, bypassing some of the intermediate IP hops

• Optical Packet Switching (OPS)– Packet by packet routing– Electronic processing of packet header and determination of next

hop– Activation of optical switch to route payload in optical domain– Not cost effective for large scale deployments

6

Hybrid Electro-Optical Solutions-IIFocus of the paper

• Optical IP Switching (OIS)– Relates to OCS– Creation of shortcut (cut-through connection) directly at optical level

i.e. Without optical-to-electrical conversion process– Distributed approach - Optical paths are automatically engineered

by analyzing local traffic at each node.– OIS nodes classify IP packets based on routing destination prefixes

instead of considering distinct IP flows.– Aggregates the classified packets into dynamically created optical

paths

7

Previous work-I• Standardizing optical control plane

– Developing one integrated control suite operating at different layers• International bodies involved:

– ITU-T worked on ASON architecture• basic functional requirements of ON• Principles for UNI and NNI

– IEFT worked on GMPLS architecuture– OIF (Optical Internet-working Forum) focuses on actual

implementation of the control interfaces (UNI and NNI)• These standards facilitate optical paths provisioning by

automating tasks– Topology discovery and connection provisioning

8

Previous work-IIExamples of Optical Control Plane Implementation

• Hybrid architecture from NTT (Nipon Telegraph and Telephone) – GMPLS is used by electronic routers to create new optical paths

during congestion• Optical Flow switching (OFS) – in process by MIT

– Creation of highly dynamic end-to-end lightpaths (requested by users) for traffic flow from source-to-destination LANs.

– Transparent to MANs and WANs• Grid network architectures

Research is focusing on increasing dynamic capability of wavelength-switched networks, although the physical layer is not fully flexible

9

The need for automatic lightpath provisioning• Global IP forecast – growth of cunsumer traffic will totally

dominate over business traffic

10

• To accomodate variable traffic demands from different users (large customers from banks, small customers from residential areas) automatic lightpath provisioning is required for optical networking reconfiguration.

• Current traffic engineering operations – mainly human driven– suitable for reconfiguration over large time scales (weeks – months)– Not suitable to address traffic dynamics and unpredictability

• OIS – continuous adaptation of the wavelength topology by distributed decision making– Suitable, scalable and allows traffic variations for short time scales

(seconds to minutes)

The need for automatic lightpath provisioning-II

11

Optical IP Switching• The idea is inherited from IP switching

– Bypassing the IP layer by creating switched connection i.e. all packets flowing to the same destination proceeds directly without being analyzed one after the other.

• Switching data directly in the optical domain without OEO conversion

• Advantage– Cost saving – optical switch ports costs tens of times less than IP

ports• Disadvantage

– Lacks the packet granularity offered by electronic routers and switches

– Cannot be used to offer deterministic QoS for individual flows

12

OIS Architecture

13

OIS Architecture-II

• Close intergration of IP routing and Optical switching has advantages:

– Introduction of a mechanism to engineer both layers together

– Guarantees full backward compatibility with IP protocol – an important characterisitic for the practical implementation of Internet architectures

14

OIS Mechanisms-I

First Step - Traffic Analysis

• OIS node is in observation state – performs constant traffic analysis

• Information that needs to be collected includes:– Interface from which the packet arrived– Output interface – which is selected by the router through longest

prefix matching algorithm– Payload size– Arrival time

15

OIS Mechanisms-II

• Original electronic IP switching approach– Create an optical path as soon as IP flow of suitable size (dubbed

‘elephant flow’ in literature) is observed – Does not scale for optical IP switching – because average elephant

flow rate differ by 3 to 4 orders of magnitude

How to increase the wavelength utilization?

16

OIS Mechanisms-IIISecond step – Aggregation

• Aggregation of multiple flows into the same light path

• Typical flow routing techniques are not feasible– Individual TCP flows are identified by 5-tuple (transport protocol, src

port, dst port, src IP address, dst IP address)– This granularity requires routers to keep track of millions of flows at

the same time

• Solution – Aggregation Matrix

17

OIS Mechanisms-IVPrefix-based aggregation mechanism

• Basic Idea : Classification of the forwarded IP traffic using the network prefixes stored in the IP routing table

• Packet classification depends on arrival (In) interfaces (number of columns) and departure (Out) interfaces (number of rows)– The generic matrix cell (i,j) identifies traffic incoming from interface i, relayed through

interface j• Within each of the previous classes operates a finer classification by destination

prefix: in each cell of the matrix a list of destination prefixes is built up, reachable through the corresponding interface

18

OIS Mechanisms-VPrefix-based aggregation mechanism-II

• For each packet the router checks its destination address and determines the output interface, using the longest prefix matching algorithm.

• The size of the packet payload adds up to the total amount of data carried by its matching prefix within the cell (i,j).

19

OIS Mechanisms-VIPrefix-based aggregation mechanism-III

Advantages• The size of the routing at the upstream node is not unduly increased –

each prefix summarizes a large amount of IP flows• Upstream node rarely needs to add new entries – most of the prefix

entries in RTs of peering nodes are similar• Prefix summarization diminishes the signaling overhead – only prefixes

are signaled upstream• Traffic analysis phase is simplified – information is processed at the

granularity of the prefixes

Disadvantage• Information about each flow is disregarded

– Not possible to guarantee QoS to individual flows

20

OIS Mechanisms-VIIThird step - Optical Path Creation (Provisioning state)

• At decision time, the router analyzes the statistics collected in the observation state

• It sums up the amount of data brought by the different prefixes within each cell• Only cells whose cumulative data is over a pre-established “path threshold” (100

Mbps in this case) are considered for Optical IP Switching• The router signals the upstream and downstream neighbors (using interfaces i

and j) checking their capability to support a new optical cut-through path and proposing a suitable wavelength

• After both neighbors have acknowledged the request, the router passes upstream the list of prefixes to be switched through the new optical path.

21

OIS Mechanisms-VIIIOptical Path Creation-II

• The upstream neighbor updates its IP routing table and starts injecting packets into the cut-through path

• The field ‘Dynamic link’ is added to enable coexistance of the legacy IP and OIS protocols without interfering with each other

• Dynamic cut-through paths are hidden to the link discovery protocol of the IP layer to avoid stability problems caused by frequent link reconfiguration

22

OIS Mechanisms-IXPath Extension

• One of the cases where the outgoing interface of the selected cell is the source of an already existing path, the node creates an upstream extension to an already existing path.

• This algo selects a subset of the prefixes switched by the original paths

• Only this subset will be carried by the new extended path – diminishing the amount of data transported by optical channel decreasing channel efficiency

• Longer cut-through paths increase the number of transparent hops, enhancing the cost-saving potentials of optical switching

• This algorithm plays important role in the trade-off between length of the optical path and amount of data carried by the path.

23

• The cut-through paths carrying data rates below pre-established ‘path cancellation threshold’ are deleted. – Path cancellation threshold < path creation th., path extension th.

• Existing cut-through paths are deleted to free resources such as interfaces, optical ports and underexploited wavelength channels

• Only cut-through path source and destination can cancel.

• After path cancellation, the traffic that was being switched returns to the default links

OIS Mechanisms-XPath Cancellation

24

Architecture Simulations-I

• Threshold values are statically assigned – Could be dynamically adapted to the available resources(future)

• Observation time is the time interval during which each node examines the traces and is set equal to the duration of traffic traces

Nothing about Path cancellation threshold

25

Architecture Simulations-II• Reference topology used

is GÉANT– pan-European data

communication network– Provides real traffic

traces – BGP RTs – allowed

reconstruction (C-BGT simulator)

• Reconstructed topology shown includes:– 23 nodes– Average distance 797

km• Transit traffic is 36% of

the total

26

Architecture Simulations-III• Simulator is written in PERL• Each node operates independently• Initial setup:

– Nodes start from an initial blank state (contrast to real networks)– No optical cut-through path setup– Receives full traffic (from the traces recorded from GÉANT)– Default links need to be provisioned to handle entire traffic

• Network Steady State:– Nodes start operating cut-through paths– Traffic redirected from default link to dedicated optical paths– Paths created and cancelled (no details about cancellation)– Initial default IP capacitiy remains unused

• Due to simulation and does not reflect real network behaviour

27

Architecture Simulations-IVComparison between OIS and a Centralized Architecture

• Example of Centralized Architecture – Transparent Overlay network model– Network administrator centrally provisions optical paths by

analysing traffic demand matrices

• Assumption on end-to-end architecture:– Traffic demand generated by all nodes is available in a central DB– Dedicated end-to-end path is provisioned for date rate > 100 Mbps

• For comparison with OIS

• Optimization is not operated – out of scope

28

Architecture Simulations-VComparison between OIS and a Centralized Architecture(transparent switching within single domain)

Switched data• As traffic increases, both architectures reaches 36% (transit traffic)• OIS presents higher switching capability for lower levels of traffic

– Optical paths are built in distributed fashion allowing better traffic aggregationChannel Occupancy• In favour of end-to-end provisioning architecture because of lower number of

paths created

29

• Extending cut-through paths transparently to the external networks– GÉANT becomes highly transparent core network, traffic mostly switched at optical level

and routing is mainly operated by external domains• Overall transit traffic among core GÉANT nodes is over 98%• Both protocols reached switching ratios of over 93%• Increase in network performance• Network operators cannot control and filter traffic entering and leaving the domain

Architecture Simulations-VIComparison between OIS and a Centralized Architecture(transparent switching across multiple domains)

30

Cost Evaluation of Presented Architectures-I

• Focused on savings in capital expenditures – Lower cost associated with optical switching compared with electronic IP routing with a

cost difference per port

• Cost Model considers:– reduction in routing equipment allowed by optical by-pass– Increase in optical devices to implement OIS– Increase of transport costs in terms of optical regenerators, longer-reach transmitters

and links, higher capacity WDM systems

• All devices are bidirectional

31Network Equipment for cost analysis

• Cost of IP routing is considered propotional to the amount of data routed ateach node rather than to the number of ports

32

Cost Evaluation of Presented Architectures-IINetwork traces collected in 2005Observations• Negligible difference between OIS

and Overlay• For Low level traffic

– IP over WDM is advantageous– Optical paths cannot exploit optical

bandwidth, hence not cost efficient• For High level traffic

– Transparent switched models are advantageous

• 100 times traffic increase shows 20% cost advantage

• For multidomain transparent vs opaque savings reaches above 80%

Not Considered • OIS coexistance with other architecutres in

access and metro areas • Resiliency-associated costs

Single domain

Interdomain

33

Testbed Evaluation-I• Analysis of dynamic transparent switching operation effect on TCP and UDP

transport protocols

• Hardware - Off-the-shelf • OIS software - click language• Core and edge nodes

– INTEL machines running Linux– 3 GHz Pentium 4 processors

Testbed setup

• Optical Switch - 16 X 16 port MEMs-based- Switching time 25 ms

34

Testbed Evaluation-II

• Dynamic creation, extension and cancellation of optical paths might cause:– Packet loss

• caused by the switching time of the optical devices – Jitter– Out-of-order arrival

• packets traveling optical paths experience negligible transit time, and can overtake IP routed path

• Effect on TCP– Packet jitter is negligible as long as it is not above TCP time out– Packet loss in OIS leads to erroneous network congestion

35

Testbed Evaluation-IIIOut-of-order arrival situation

• ∑ (transit times of routers bypassed) > Tgap (1)– where Tgap is the time gap between two consecutive packets

Tgap = (2)

– B = packet size (bytes)– R = sending data rate Combining (1) and (2)

R > • The higher the rate above threshold, the higher the number of

packets that will arrive out of order.

36

Testbed Evaluation-IVTests on TCP protocol• Congestion is simulated by increasing the transit time at the router-

provoking significant out of order arrival during switching time

Congestion on transmission path

Congestion on transmission and ack paths

Solutions• Introducing guard time at the upstream router

delaying the sending of data on new optical path

• Creating optical bypass on ack path

37

Testbed Evaluation-VAvoidance of packet loss during path extension

• Basic idea – avoid packets crossing optical switch while switching is in operation

• Extension times measured – order of 50 ms

• Buffer required at the source node– The cost is negligible

compared to the overall router cost

• Good for TCP but jitter introduced might create disruption for UDP

38

Summary• The comparison between

overlay and OIS models is summarized

• Advantages of distributed decision making– Increased scalability– Quicker reaction times– High support for

interdomain networking• Disadvantage is

probable failure to converge to optimal solutions

39

Thank you