Segment Routing is gaining steam among the service provider community for its simplicity, especially as they adopt SDN for centralized control. In this two-part blog series, I’ll examine the current and future methods of MPLS traffic engineering. In this post, I explain what is MPLS Traffic Engineering (MPLS-TE) and Resource Reservation Protocol-TE (RSVP-TE), the technology widely in use today to implement traffic engineering. In part two, I’ll cover Segment Routing.
Let us start with covering what is MPLS Traffic Engineering. As we all know, networks exist to deliver data packets between different endpoints. In a traditional IP-based network, data packets are forwarded on a per-hop basis. Each router between the source and destination does a route look-up and selects the lowest cost path upon which to forward packets. The disadvantage of this is that if a path is found to be optimal due to its low cost, every router in the network will tend to use that path to forward packets. This holds even when other underused but higher cost paths are available. This approach to data transmission can cause performance issues such as packet drops or latency on the chosen path.
With Traffic Engineering (TE), rather than routing decisions being made at each hop, the network operator’s headend1 ingress router determines the source to destination path for specific traffic. This way, traffic that would have taken an optimal but congested path may be directed through underused paths in the network, helping distribute bandwidth load across different links.
For TE to work, TE tunnels that use separate paths from a source to destination edge router are configured by the network operator. Interior Gateway Protocols (IGP) such as OSPF and IS-IS collect information about the network topology and the availability of the resources. They update information about the links in the IGP domain to all the other routers within the network. This data helps the headend ingress router in the IP/MPLS network analyze the traffic patterns and availability of resources across the links and compute the best hop-by-hop path for the TE tunnels between different endpoints.
A TE tunnel, in addition to the bandwidth requirements, can also include Class of Service (CoS) requirements of the data to be forwarded using the tunnel. Once the TE tunnels are created and the bandwidth requirements of the traffic are understood, data is forwarded across the TE tunnel to its destination using MPLS label switching. In addition to helping with congestion avoidance on the primary link, Traffic Engineering also allows for failover when the primary path or tunnel between two endpoints in the network fails, by providing Fast Reroute (FRR) on the TE tunnels.
Resource Reservation Protocol (RSVP) reserves resources along the end-to-end path of a traffic flow in an IP network. An RSVP request consists of a FlowSpec that specifies the Quality of Service (QoS) requirement for the traffic flow and a FilterSpec that defines which flow must receive the QoS priority. Once the necessary bandwidth is reserved along the path with RSVP, the application that made the request begins to transmit the traffic. RSVP is often used by real-time and multimedia applications to set up bandwidth reservations.
The RSVP signaling protocol was extended with MPLS features to support MPLS TE. This enabled RSVP to set up label switched paths2 (LSP) in an MPLS TE network. With RSVP-TE, the headend router sends an RSVP PATH message that checks the availability of requested resources on all the label switched routers (LSR) in the path on which the TE tunnel is to be created. Upon receiving the PATH message, the tailend3 router in the path then confirms the reservation with an RSVP RESERVATION message, which confirms the assignment of an LSP to a TE tunnel. This message is then propagated upstream to the headend router through all the LSRs along the future TE tunnel path.
After all the LSRs in the path accept and confirm the LSP, the MPLS TE LSP is operational. With this, the headend router can then direct traffic through new tunnels based on requirements, and your traffic engineered MPLS network is ready.
In the next part, we will look at the new buzz in Traffic Engineering: Segment Routing.
Detailed steps in RSVP path reservation:
Instructions on configuring RSVP-TE on a Cisco ASR 9000:
Glossary of additional terms: http://www.cisco.com/c/en/us/td/docs/ios/12_0s/feature/guide/fs_areat.html#wp1033446