IP Multicasting in MPLS networks

Abstract:

The Multiprotocol Label Switching (MPLS) is an advanced technology that enables IP networks to support traffic engineering efficiently. It speeds up packet forwarding by combining layer 3 routing with layer 2 switching. In MPLS, a label in the packet is used for making forwarding decisions and a path is pre-established to switch labeled packets at layer 2. Unfortunately, MPLS was originally designed for unicast IP traffic and there is as yet no complete definition for the support of multicast IP traffic. The key objective is to improve the network scalability by using multicast label switching to forward IP multicast packets at layer 2 with minimal forwarding at layer 3.
We are developing the new mechanism for supporting IP multicast in MPLS.

Issues in supporting IP multicast in MPLS

  • Point to multi-point LSP Establishment
    The multicast distribution tree requires to establish point to multipoint LSPs. MPLS architecture does not address point to multipoint or multipoint to point LSPs. There is no standardized mechanism for this purpose. The signalling overhead for setting of the multicast LSPs is huge due to scattered group members. The group dynamics makes these LSPs volatile.
  • Traffic Aggregation
    Like unicast traffic aggregation of the multicast traffic is not possible. In fact aggregation of unicast traffic with multicast or aggregation of multicast traffic for the same group with different senders is also difficult.
  • Co-existence of Layer 2 and Layer 3 forwarding
    In some cases LSR may need to to forward packets on both Layer 2 and Layer 3 . This situation arises when only incoming label alone can not determine the out going label. In multicasting protocols like PIM-SM in which both shared tree for group (*,G) and source specific trees (S,G) may co-exists some LSR in tree may have forwarding state for both trees, in this situation it is required to lookup the layer 3 header to avoid the forwarding of data for (*,G) from S on the interfaces which already have (S,G) state.
  • Label distribution
    MPLS architecture allows only downstream label assignments where downstream LSR assigns labels and inform the upstream LSR. In multicasting scenario the upstream router has to forward traffic to multiple downstream routers,it can't assured all the downstream routers will assign same label which results in wastage of label space Instead in upstream label allocation upstream router can assign common label to all downstream routers.

    Reference Papers and Articles