RSVP-TE P2MP Tunnels on RMR
draft-zzhang-teas-rmr-rsvp-p2mp-00

Abstract

This document specifies the optimization in RSVP-TE P2MP tunnel signaling over Resilient MPLS Rings (RMR).

Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC2119.

Status of This Memo

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This Internet-Draft will expire on January 25, 2019.

Copyright Notice

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Table of Contents

• 1. Introduction
• 2. Specification
• 2.1. RMR Object
• 2.2. Procedures
• 2.2.1. PATH Message/State
• 2.2.2. RESV Message/State
• 3. Security Considerations
• 4. Acknowledgements
• 5. Normative References
• Authors' Addresses

1. Introduction

Traditional RSVP-TE P2MP tunnel signaling could be quite involving. With RMR, this could be significantly simplifed:

There is no need for ERO/RRO/SERO/SRRO or hop by hop rouing. The tunnel ingress simply sends PATH messages in one or both directions of the ring, depending on how leaves are best reached. The <S2L Sub-LSP Descriptor List> only needs to list the tunnel leaves, and a transit router does not need to "branch" a PATH message into multiple ones Therefore, unless there are many tunnel leaves on a huge ring, a single PATH message is enough. In the rare situation of a large tunnel with many leaves to list, a small number of PATH messages should suffice. Additionally, there is no need to signal and maintain individual sub-LSPs (one for each leaf) any more. As a result, corresponding PATH/RESV state is also reduced. Each node only needs to maintain a single PATH state and a single RESV state for each P2MP tunnel, and the RESV state does not need to track individual leaves - it just need to track if a RESV is received from downstream and/or if this node itself is a leaf.

A RESV message is triggered to the PHOP when the RESV state is first created (either because the node is a leaf or because a RESV message is received from downstream) and it is refreshed periodically. A RESV Tear is sent when the RESV state is deleted (when the node is no longer a Leaf and the RESV from downstream has timed out or a RESV Tear is received).

Optionally, the tunnel ingress may not need to list any/all leaves. It could simply send the PATH message around the ring, with the <S2L Sub-LSP Descriptor List> listing the root itself. Through methods outside the scope of this document, a node determines if it is a leaf of the tunnel, and if yes, it will send back a RESV message. With this, a single PATH message is surely enough.

In this document, leaves in <S2L Sub-LSP Descriptor List> are referred to as explicit leaves, and leaves not listed there but self-determined by ring nodes are referred to as implicit leaves. There could be both explicit and implicit leaves for a tunnel. The ingress allows implicit leaves by including itself as the last one in the <S2L Sub-LSP Descriptor List>.

Optionally, the RESV message could also include a <S2L Sub-LSP Descriptor List> to list all the leaves on the established tunnel so that the each node knows its downstream leaves. In that case, when the set of downstream leaves changes, a RESV message with the new <S2L Sub-LSP Descriptor List> is triggered.

Adding/removing explicit leaves is straighforward. The ingress simply sends a triggered PATH message with new <S2L Sub-LSP Descriptor List>. As it passes around the ring, each node determines if it is an explicit leaf and updates its state accordingly. The triggered PATH message does not have to go all the way to the last leaf - if on a node the <S2L Sub-LSP Descriptor List> in the to-be-sent PATH message is the same as what was sent before, the triggered PATH message will not be sent further.

To indicate that the tunnel signaling is with above mentioned RMR optimizations, a new object is included in the PATH message to specify the Ring ID and direction.

Link/Node protection is achieved by tunneling packets to the next node using the Ring LSP to that node in the other direction. This does not need any additional signaling but is based on a reasonable premise that unicast Ring LSPs are always in place. Once the ingress learns the failure (through IGP discovery or through other error detection/notification mechanisms), global repair kicks in to reach some leaves via PATH message sent in the other direction. Before global repair is finished, traffic continues to flow in the original path except that at the failure point it is tunneled to the next node.

If an RMR is just part of a general RSVP network the optimization can also be applied on the ring nodes. If the tunnel ingress knows the leaves that are on the ring, it could put all those leaves in the single PATH message and construct the ERO/SERO only towards the entry points on the ring. The entry points then includes the RMR object in the PATH messages that they send. For leaves beyond the ring, the ingress may include the exit points on the ring as loose hops in the ERO/SERO, and when a ring node needs to send the PATH message off the ring, it removes the RMR object. Details will be provided in future revisions of this document.

2. Specification

2.1. RMR Object

The RMR object is a new object of the following:

• Class Name: RMR
• Class-Num: TBA1 (to be assigned by IANA)
• C-Type: TBA2 (to be assigned by IANA)

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Ring ID (4 octets)                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |D| Flags       |      Reserved                                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	

The format of the object content following the common object header is the folowing:

Following the 4-octect Ring ID, there is an 8-bit Flags field. The first bit of the Flags field indicates the direction. If it is set, it is clockwise direction. Otherwise, it is anti-clockwise.

2.2. Procedures

This section describes the differces in the procedures for ring nodes to set up RSVP-TE P2MP tunnels across the ring, compared to the conventional non-RMR-aware case. For now it is assumed that all nodes (ingress, tranist, and leaves) on the tunnel are on the ring.

More details will be provided in future revisions.

2.2.1. PATH Message/State

The tunnel ingress includes the RMR object with the Ring ID and the direction flag bit set accordingly. The explicit tunnel leaves are encoded in the <S2L Sub-LSP Descriptor List>, and no ERO/SERO is included. If the tunnel allows implicit leaves, the descriptor list encodes the ingress itself as the last element. The message is sent to the next node on the ring in the direction specified in the RMR object, w/o using ERO/SERO or hop-by-hop routing.

When a node recevies a PATH message with the RMR object, it checks if itself is listed in the <S2L Sub-LSP Descriptor List>, or if the <S2L Sub-LSP Descriptor List> encodes the tunnel ingress as the last element and this node itself is an implicit leaf. If yes, it creates corresponding RESV state and sends a RESV message to the PHOP.

The receiving node removes itself from the <S2L Sub-LSP Descriptor List> in the PATH message, and saves the list locally. The PATH message is sent to the next node on the ring in the specified direction if one of the following conditions is met:

• The <S2L Sub-LSP Descriptor List> encodes the tunnel ingress itself as the last element.
• The <S2L Sub-LSP Descriptor List> is not empty and either the PATH state is newly created or the <S2L Sub-LSP Descriptor List> is different from the previously saved one.

If <S2L Sub-LSP Descriptor List> is empty and different from the previously saved one, a PATH Teardown is sent instead with the saved <S2L Sub-LSP Descriptor List>.

2.2.2. RESV Message/State

A ring node may know that it is a leaf when the PATH message is first processed as described in the previous section. In case of implicit leaves, it may become a leaf after the PATH messages has been processed. A non-leaf node may also receive a RESV message from its NHOP. In all cases, the node creates RESV state and sends a RESV message to the PHOP, w/o encoding RRO/SRRO.

If a ring node was a leaf but stops being a leaf, either because it is no longer listed in the <S2L Sub-LSP Descriptor List> or it is no longer an implicit leaf, it removes/updates corresponding local state. A RESV Teardown is sent to the PHOP if there is no RESV received from its downstream.

3. Security Considerations

This document does not introduce new security risks?

4. Acknowledgements

5. Normative References

Authors' Addresses