Packets in MPLS are routed according to preconfigured label switched paths (LSPs). When packets are received, label lookup is performed from which it is identified to which LSP packets belong to and hence it identifies the next hop of the packet.
GMPLS generalizes the concept of MPLS. Instead of the concept of labels, GMPLS uses any other property like time slot in TDM, Wavelength in DWDM or fiber port number. Thus GMPLS is generic and extends the concept of MPLS to transport networks.
The existing protocols for MPLS have been extended to support GMPLS control plane i.e.
- OSPF-TE ( Extensions to Open Short Path First- Traffic Engineering to support GMPLS)
- Resource reservation protocol-Traffic Engineering ( RSVP-TE) extensions for GMPLS
- CR-LDP-TE ( GMPLS extensions for Constraint-based Label Distribution Protocol)
- LMP ( GMPL extensions for Link Management Protocol)
In this post, let’s have a look at OSPF-TE and RSVP-TE:
What are the extensions to OSPF-TE?
OSPF-TE added the capability of advertising bandwidth information (available and used) in the standard advertisement used by OSPF. Further additions in advertisements were done for GMPLS. These additions are meant to make routing protocol more suitable for transport world. Those include link switching capability ( e.g. different switch capabilities of node along with bandwidth supported), Node protection capability (e.g. more than one protection route to a node) and SRLG (Shared Risk link Group). Shared Risk Link Group is information about logical routes sharing same physical routes that can be single point of failure.
What are the extensions to RSVP-TE?
First a word about RSVP and RSVP-TE. RSVP is used in MPLS as signaling protocol to set up, maintain and tear down connection. RSVP-TE is the traffic engineering extension of RSVP that allows establishment of LSPs taking into account additional constraints like available bandwidth and optimal hops/routes. RSVP-TE uses PATH message from ingress router to downstream router to request reservation of path/bandwidth. This message reaches hop by hop and the egress router responds by RESERVE message. The RESERVE message reaches back ingress router reserving resources hop by hop on the way. Labels are always suggested by downstream router to upstream router.
There are some extensions done to RSVP-TE in GMPLS
- Use of Hierarchal LSP Setup. RSVP-TE enables now to establish hierarchal LSPs at different layers e.g. Packet LSP, OTN LSP, Lambda LSP.
- Bidirectional LSP Setup. While traditional LSPs in MPLS were unidirectional. GMPLS allows the setup of bidirectional LSPs (LSPs in which the upstream and downstream traffic follow same route). This is more suitable for traditional transport like communication which relies on bidirectional opposite paths.
- Label Suggestion by Upstream- GMPLS allows upstream node to suggest label to downstream node. This is very suitable for transport world e.g. it may not be always possible to change a wavelength if a suggestion comes from downstream node.
- Notify Message- A node passing transit connections should have the capability to NOTIFY the node responsible for traffic restoration to restore