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MPLS and RSVP Overview


MPLS provides a mechanism for engineering network traffic patterns that is independent of routing tables. MPLS assigns short labels to network packets that describe how to forward them through the network. MPLS is independent of any routing protocol and can be used for unicast packets.

In the traditional Level 3 forwarding paradigm, as a packet travels from one router to the next, an independent forwarding decision is made at each hop. The IP network layer header is analyzed, and the next hop is chosen based on this analysis and on the information in the routing table. In an MPLS environment, the analysis of the packet header is performed just once, when a packet enters the MPLS cloud. The packet is then assigned to a stream, which is identified by a label, which is a short (20-bit), fixed-length value at the front of the packet. Labels are used as lookup indexes for the label forwarding table. For each label, this table stores forwarding information. You can associate additional information with a label—such as class-of-service (CoS) values—that can be used to prioritize packet forwarding.

Packets traveling along an LSP are identified by a label—a 20-bit, unsigned integer in the range 0 through 1,048,575. For push labels on ingress routers, no labels in this range are restricted. For incoming labels on the transit static LSP, the label value is restricted to 1,000,000 through 1,048,575.

On MX Series, PTX Series, and T Series routers, the value for entropy and flow labels is restricted to 16 through 1,048,575.

In the Junos OS, label values are allocated per router. The display output shows only the label (for example, 01024). Labels for multicast packets are independent of those for unicast packets. Currently, the Junos OS does not support multicast labels.

Labels are assigned by downstream routers relative to the flow of packets. A router receiving labeled packets (the next-hop router) is responsible for assigning incoming labels. A received packet containing a label that is unrecognized (unassigned) is dropped. For unrecognized labels, the router does not attempt to unwrap the label to analyze the network layer header, nor does it generate an Internet Control Message Protocol (ICMP) destination unreachable message.

A packet can carry a number of labels, organized as a last-in, first-out stack. This is referred to as a label stack. At a particular router, the decision about how to forward a labeled packet is based exclusively on the label at the top of the stack.

Figure 1 shows the encoding of a single label. The encoding appears after data link layer headers, but before any network layer header.

Figure 1: Label Encoding
Label Encoding

Figure 2 illustrates the purpose of the class-of-service bits (also known as the EXP or experimental bits). Bits 20 and 21 specify the queue number. Bit 22 is the packet loss priority (PLP) bit used to specify the random early detection (RED) drop profile.

Figure 2: Class-of-Service Bits
Class-of-Service Bits

RSVP Overview

RSVP is a resource reservation setup protocol that is used by both network hosts and routers. Hosts use RSVP to request a specific class of service (CoS) from the network for particular application flows. Routers use RSVP to deliver CoS requests to all routers along the data path. RSVP also can maintain and refresh states for a requested CoS application flow.

RSVP treats an application flow as a simplex connection. That is, the CoS request travels only in one direction—from the sender to the receiver. RSVP is a transport layer protocol that uses IP as its network layer. However, RSVP does not transport application flows. Rather, it is more of an Internet control protocol, similar to the Internet Control Message Protocol (ICMP) and Internet Group Management Protocol (IGMP). RSVP runs as a separate software process in the Junos OS and is not in the packet forwarding path.

RSVP is not a routing protocol, but rather is designed to operate with current and future unicast and multicast routing protocols. The routing protocols are responsible for choosing the routes to use to forward packets, and RSVP consults local routing tables to obtain routes. RSVP only ensures the CoS of packets traveling along a data path.

The receiver in an application flow requests the preferred CoS from the sender. To do this, the receiver issues an RSVP CoS request on behalf of the local application. The request propagates to all routers in reverse direction of the data paths toward the sender. In this process, RSVP requests might be merged, resulting in a protocol that scales well when there are a large number of receivers.

Because the number of receivers in an application flow is likely to change and the flow of delivery paths might change during the life of an application flow, RSVP takes a soft-state approach in its design, creating and removing the protocol states in routers and hosts incrementally over time. RSVP sends periodic refresh messages to maintain its state and to recover from occasional lost messages. In the absence of refresh messages, RSVP states automatically time out and are deleted.