Ethernet ring networks are known, for example from ITU-T recommendation G.8032. Referring to FIG. 1, an Ethernet ring is a network comprising Ethernet nodes and links connected together in a ring topology. Within the ring topology, each node is connected to a pair of links, each of which can normally support a plurality of connections. As may be seen in FIG. 1, each ring node includes a set of East ring ports connected to a corresponding East link (or segment) of the ring, a set of West ring ports connected to a corresponding West link (or segment) of the ring, a UNI port through which subscriber traffic can access the ring, a switch fabric for forwarding traffic between the ring ports and the UNI port, and a controller for controlling the ring node. It should be noted that the designation of East and West ring ports (and links) us for ease of reference only, and have no physical meaning. In FIG. 1, the East ports are indicated by the letter “E” at each node, and the West ports are indicated by the letter “W”.
In order to prevent loops within the ring network, a “seam” is implemented that prevents forwarding of packets past the seam point in the ring. In ITU-T recommendation G.8032, this is accomplished by designating one of the links of the ring as a Ring Protection Link (RPL), which is disabled during normal operation of the ring by placing a channel block on that link. In some cases, the channel block can be imposed on a per-channel basis, in which case it may be permissible to designate a different RPL for each channel. However, for ease of implementation, it is common practice to designate the same PRL for all of the channels of the ring. Typically, a channel block is imposed at a node at one end of the RPL, which node may then be referred to as the RPL Owner. In some cases, the channel block may comprise a policy that prevents packets of the ring from being forwarded (or received) through a ring port connected to the RPL. In such cases, the seam that ensures loop-freeness in the ring can be considered to be localized in the ring ports in which the channel block policy is installed. In the example of FIG. 1, the channel block is imposed on link A-D of the ring by installing appropriate policies at the West ports of node “A”, which is therefore the RPL Owner. With the channel block (seam) in place, the ring is guaranteed to be loop free, and conventional Ethernet MAC-learning and path computation can be used to compute and install appropriate forwarding information in each node of the ring.
An advantage of ITU-T recommendation G.8032 is that it utilizes conventional Ethernet path labelling and packet forwarding techniques to implement a ring network. However, ITU-T recommendation G.8032 contemplates that the location of the seam is selected by the network service provider as part of the process of provisioning the ring network. A limitation of this approach is that it offers no means of ensuring that the seam is optimally located to enable a desirable distribution of traffic in the ring network.
For example, FIGS. 2A and 2B show three connection mapped through the network, including: connection AD between nodes A and D via nodes B and C; connection BD, between nodes B and D via node C; and connection BC between nodes B and C. Consider a scenario in which the bandwidth allocated to each connection is: 100 for connection AD; 40 for connection BD; and 10 for connection BC. In this scenario, it may be seen that high-bandwidth connection AD transits nodes B and C solely because of the location of the seam on link A-D.
On the other hand, it may be seen that transit bandwidth resources on nodes B and C could be released, and the distribution of traffic in the network improved, if connection AD were to be routed directly between nodes A and D, as shown in FIGS. 3A and 3B. As may be seen in FIG. 3A, this improved traffic distribution could be obtained by relocating the seam to link A-B of the ring. However, without advance knowledge of the bandwidth allocation to each connection, the network service provider cannot know the optimal location for the seam. Since connections can be set up, changed, and torn down at various times, the optimal location of the seam may also change from time to time. Thus it may not even be possible to anticipate the optimal seam location during provisioning of the network.
Techniques that enable optimization of the RPL remain highly desirable.