Virtual Topology

"A virtual topology is a set of of all-optical connections or a group of light-paths that are established over the physical topology of a network in order to accommodate the traffic demand of network users." 1

The limitations of number of wavelengths available in each fiber link, and other network resource constraints, the virtual topology design becomes an important problem which objective is to maximize network performance by combining benefits of optical transmission and electronic processing of data.


Since traffic demand and network topology do no remain constant forever in a large transport network, reconfiguration of the physical topology is important to maintain the optimal network performance.


Network performance metrics can be: 

• number of hop between pair of nodes
• offered traffic on a light-path
• end-to-end delay from one node to another
• number of wavelength conversion required throughout a route

1 Zheng, J. & Moutfah, H.T. (2004) "Optical Networks. Concepts and Design Principles". Wiley-Interscience. NJ. pp 89-91

LSP setup delay

Label Switched Path (LSP) Dynamic Provisioning Performance Metrics in Generalized MPLS Networks RFC 5814

An LSP setup delay is an important metric to assess the quality of provisioning of a GMPLS control plane on a network. A longer LSP setup delay may incur in higher overhead for he requesting application and the impact is worsen the shorter the LSP lasting time is.

The delay itself consists of link propagation delay and nodal processing delay, and this metric may be used as testing and diagnostic of the control channel performance. The metrics unit used is either a real number of milliseconds or undefined. The setup delay metric is expressed in milliseconds.

RSVP-TE preemption policy

The different preemption priorities are defined in the RSVP-TE protocol (Awduche, 2001). A higher priority LSP may preempt a lower priority LSP that is then re-routed though a new path as if it was setup after the higher priority LSP.

The setup priority value determines the level of capability of that LSP to preempt another and a holding priority value determines the level of capability of a LSP to resit to a preemption. Both values range from 0 (highest priority) to 7 (lowest priority).


D. Awduche, L. Berger, D. Gan, T. Li, V. Srinivasan, and G. Swallow, “RSVP-TE: Extensions to RSVP for LSP tunnels,” RFC 3209, December 2001.

Types of connection in a ASON

From Hu, 2011

In a Automatic Switched Network 3 types of connections are possible, they vary in terms of the lasting time of the connection, and they are:

Permanent connection –usually a static connection that last for months or years. It is setup manually or by management system.

Switched connection: established on demand though routing and signaling capabilities of the control plane. Requires a user-network signaling interface. Are setup within seconds, enabling bandwidth on demand. Promotes optimization of resource utilization and traffic engineering mechanisms.

Soft permanent connection: two permanent connections are specified at the edge of the network, a switched connection is setup within the network. Soft permanent connections are triggered by management plane, but are setup within the network by the control plane. They may support TE, dynamic re-establishment of failed connections and time varying bandwidths.
Hu, W.; Sun, W.; Jin, Y.; Guo, W.; He, H. & Yi, L. (2011), 'Sub-Wavelength Switching for Future Internet'' 13th International Conference on Transparent Optical Networks (ICTON), 2011'

Issues in preemption policies

Even though preemption mechanisms may be used to reduce the total number of rejected tunnels in case of failure in a MPLS network, they can also have a negative impact on the convergence time of a network. N Chaieb et al., 2007, two approaches are proposed:

• Least preempted tunnels are prioritized for next preemption. 
• No tunnel can be preempted more than N times in a given interval of time. 

Chaieb et al., 2007c] Chaieb, I., Le Roux, J., et Cousin, B. A New Pre-emption Policy for MPLS-TE Networks. 15th IEEE International Conference on Networks (ICON) (2007c).

Preemption in MPLS network

In a MPLS network LSPs preemption is based on their set-up and holding priority values. According to (de Oliveira, 2004), this preemption strategy can lead to excessive re-routing decisions. A preemption optimization is based on three criteria:

•  Number of LPSs to be preempted 
•  Priority of the LPSs 
•  Preempted bandwidth 

In (de Oliveira, 2004), an adaptive scheme is proposed to select lower priority LSPs that can afford having their rate reduced.
Besides prioritizing access policies preemption can also be used for restoration policies in event of a network failure.
In order to establish, maintain and tear-down a LSP the Label Distribution Protocol (LDP) and the Resource Reservation Protocol (RSVP) are use (Awduche, 2002).

de Oliveira, J.; Scoglio, C.; Akyildiz, I. & Uhl, G. (2004), 'New preemption policies for DiffServ-aware traffic engineering to minimize rerouting in MPLS networks', IEEE/ACM Transactions on Networking (TON) 12(4), 733—745.

D. O. Awduche and B. Jabbari, “Internet traffic engineering using multiprotocol label switching (MPLS),” Comput. Netw., vol. 40, pp. 111–129, 2002.

Traffic Engineering Database

As defined in (Katz et al., 2003), the Traffic Engineering Database (TED) was developed from a proposal of adding more link attributes in the Opens Shortest Path First (OSPF) advertisements. The Traffic Engineering Database is in fact an extended Link Attributes Database that are built by exchange of Link State Advertising messages between routers. Some of the uses of the TED are Monitoring the extended link attributes, local constraint-based routing and global traffic engineering.