What is RSVP-TE (RSVP Traffic Engineering)?
RSVP-TE refers to a resource reservation protocol that is invented in order to allocate a bandwidth for the individual flows on the network devices.
To say it another way, RSVP-TE are extensions to the RSVP protocol specified in the RFC 3209.
Although, RSVP-TE has been initially invented as a Quality Service Mechanism in an Intserv QoS architecture, it has never been used in modern networks due to scalability problems.
This is also because allocating tremendous number of flows on the networking nodes creates scalability problems.
In today’s networks, MPLS labels can be assigned in multiple ways such as LDP, RSVP, BGP, and Segment Routing.
MPLS Traffic Engineering is employed in order to allocate bandwidth on the network link. What’s more, MPLS label and label signalling require RSVP protocol extension.
These extensions are MPLS label allocation for MPLS Traffic Engineering, and many new RSVP messages are used in MPLS Traffic Engineering, MPLS Traffic Engineering, and Fast Reroute technologies.
RSVP-TE vs. LDP
RSVP-TE creates point-to-point MPLS LSP tunnels. And the MPLS LSPs, which are signalled with LDP protocol, are a multi-point-to-point tunnels.
Compared to LDP LSP, RSVP-TE LSP creates much more state in the network. That is why from the scalability point of view, it is worse than LDP.
But since RSVP-TE provides fast reroute capability, it can reroute the traffic to the alternate path (backup path/protection path) in less than 50 ms as do SONET/SDH networks.
RSVP-TE is a soft state protocol. Put simply, the path and reservation messages, which are used for bandwidth and MPLS label allocation, are sent and received every 30 seconds.
On the other hand, LDP operates based on TCP, a reliable protocol. Thus, LDP messages are not sent periodically.
In sum, RSVP-TE is a set of extensions brought into RSVP protocol for reservation capability in the MPLS Traffic Engineering.