RESV: Sent by the Tail end router in response to the PATH message received confirming the resource reservation and travels from Tail end towards the Headend router.PATH: Sent by the Headend router requesting the resources and is forwarded through the network from Headend towards the Tail end router.RSVP-TE rides over IP (Protocol 46) and consists of following main messages: If you want to dig deeper on various Label allocation modes, take a peek here: RSVP uses Downstream on Demand (DoD) with ordered control and conservative retention label allocation method. Other mechanisms for establishing transport LSPs are IGP in the case of SR and BGP-LU. Similar to LDP, RSVP-TE provides transport LSP to carry IP. As you may already know that RSVP-TE is used for Traffic Engineering (Strategic/Tactical) and fast-reroute. Later it was extended to signal MPLS LSPs. It was used to signal traffic characteristics and requirements of the traffic flows and establish sessions. RSVP was originally designed to serve IntServ QOS model. The way we need to look at it is that both RSVP-TE and SR-TE are tools and it’s our job to pick the right tool for the right job. It’s possible that you may think why the heck I am writing about RSVP-TE when we already have SR-TE, and the reason is that RSVP-TE is already deployed in networks and is probably going to stay with us for sometime. So it’s time to fix that and we will try to take a deeper look into the RSVP-TE from a protocol perspective and my assumption here is that the reader already has some familiarity with the RSVP-TE Protocol. A while back, I wrote few articles on MPLS-TE primarily focused from a design perspective but I didn’t really covered much from a protocol perspective.
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