Support for Path MTU (PMTU) in the Path Computation Element Communication Protocol (PCEP).
draft-dhody-pce-pcep-pmtu-00

Abstract

The Path Computation Element (PCE) provides path computation functions in support of traffic engineering in Multiprotocol Label Switching (MPLS) and Generalized MPLS (GMPLS) networks.

The Source Packet Routing in Networking (SPRING) architecture describes how Segment Routing (SR) can be used to steer packets through an IPv6 or MPLS network using the source routing paradigm. A Segment Routed Path can be derived from a variety of mechanisms, including an IGP Shortest Path Tree (SPT), explicit configuration, or a Path Computation Element (PCE).

Since the SR does not require signaling, the path maximum transmission unit (MTU) information for SR path is not available. This document specify the extension to PCE communication protocol (PCEP) to carry path (MTU) in the PCEP messages.

Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on January 3, 2019.

Copyright Notice

Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

• 1. Introduction
• 2. PCEP Extention
• 2.1. Extensions to METRIC Object
• 2.2. Stateful PCE and PCE Initiated LSPs
• 2.3. Segement Routing
• 3. Security Considerations
• 4. IANA Considerations
• 4.1. METRIC Types
• 5. Acknowledgments
• 6. References
• 6.1. Normative References
• 6.2. Informative References
• Authors' Addresses

1. Introduction

[RFC5440] describes the Path Computation Element (PCE) Communication Protocol (PCEP). PCEP enables the communication between a Path Computation Client (PCC) and a PCE, or between PCE and PCE, for the purpose of computation of Multiprotocol Label Switching (MPLS) as well as Generalzied MPLS (GMPLS) Traffic Engineering Label Switched Path (TE LSP) characteristics.

[RFC8231] specifies a set of extensions to PCEP to enable stateful control of TE LSPs within and across PCEP sessions in compliance with [RFC4657]. It includes mechanisms to effect LSP State Synchronization between PCCs and PCEs, delegation of control over LSPs to PCEs, and PCE control of timing and sequence of path computations within and across PCEP sessions. The model of operation where LSPs are initiated from the PCE is described in [RFC8281].

As per [I-D.ietf-spring-segment-routing], with Segment Routing (SR), a node steers a packet through an ordered list of instructions, called segments. A segment can represent any instruction, topological or service-based. A segment can have a semantic local to an SR node or global within an SR domain. SR allows to enforce a flow through any path and service chain while maintaining per-flow state only at the ingress node of the SR domain. Segments can be derived from different components: IGP, BGP, Services, Contexts, Locators, etc. The SR architecture can be applied to the MPLS forwarding plane without any change, in which case an SR path corresponds to an MPLS Label Switching Path (LSP). The SR is applied to IPV6 forwarding plane using SRH. A SR path can be derived from an IGP Shortest Path Tree (SPT), but SR-TE paths may not follow IGP SPT. Such paths may be chosen by a suitable network planning tool, or a PCE and provisioned on the ingress node.

As per [I-D.ietf-pce-segment-routing], it is possible to use a stateful PCE for computing one or more SR-TE paths taking into account various constraints and objective functions. Once a path is chosen, the stateful PCE can initiate an SR-TE path on a PCC using PCEP extensions specified in [RFC8281] using the SR specific PCEP extensions specified in [I-D.ietf-pce-segment-routing]. [I-D.ietf-pce-segment-routing] specifies PCEP extensions for supporting a SR-TE LSP for MPLS data plane. [I-D.negi-pce-segment-routing-ipv6] extend PCEP to support SR for IPv6 data plane.

The maximum transmission unit (MTU) is the largest size packet or frame, in bytes, that can be sent in a network. An MTU that is too large might cause retransmissions. Too small an MTU might cause the router to send and handle relatively more header overhead and acknowledgments. When an LSP is created across a set of links with different MTU sizes, the ingress router need to know what the smallest MTU is on the LSP path. If this MTU is larger than the MTU of one of the intermediate links, traffic might be dropped, because MPLS packets cannot be fragmented. Also, the ingress router may not be aware of this type of traffic loss, because the control plane for the LSP would still function normally. [RFC3209] specify the mechanism of MTU signaling in RSVP.

Since the SR does not require signaling, the path MTU information for SR path is not available. This document specify the extension to PCEP to carry path MTU in the PCEP messages. It is assumed that the PCE is aware of the link MTU as part of the Traffic Engineering Database (TED) population. This could be done via IGP, BGP-LS or some other means. Thus the PCE can find the path MTU at the time of path computation and include this information as part of the PCEP messages.

Though the key use case for path MTU is SR, the PCEP extension (as specified in this document) creates a new metric type for path MTU, making this a generic extension that can be used independent of SR.

2. PCEP Extention

2.1. Extensions to METRIC Object

The METRIC object is defined in Section 7.8 of [RFC5440], comprising metric-value and metric-type (T field), and a flags field, comprising a number of bit flags (B bit and C bit). This document defines a new type for the METRIC object for Path MTU.

• T = TBD: Path MTU.
• A network comprises of a set of N links {Li, (i=1...N)}.
• A path P of a LSP is a list of K links {Lpi,(i=1...K)}.
• A Link MTU of link L is denoted M(L).
• A Path MTU metric for the path P = Min {M(Lpi), (i=1...K)}.

The Path MTU metric type of the METRIC object in PCEP represents the minimum of the Link MTU of all links along the path.

When PCE computes the path, it can also find the Path MTU (based on the above criteria) and include this information in the METRIC object with the above metric type in the PCEP message when replying to the PCC. In a Path Computation Reply (PCRep) message, the PCE MAY insert the METRIC object with an Explicit Route Object (ERO) so as to provide the METRIC (path MTU) for the computed path. The PCE MAY also insert the METRIC object with a NO-PATH object to indicate that the metric constraint could not be satisfied.

Further, a PCC MAY use the Path MTU metric in a Path Computation Request (PCReq) message to request a path meeting the MTU requirement of the path. In this case, the B bit MUST be set to suggest a bound (a maximum) for the Path MTU metric that must not be exceeded for the PCC to consider the computed path as acceptable. The Path MTU metric must be less than or equal to the value specified in the metric-value field.

A PCC can also use this metric to ask PCE to optimize the path MTU during path computation. In this case, the B bit MUST be cleared.

The error handling and processing of the METRIC object is as specified in [RFC5440].

2.2. Stateful PCE and PCE Initiated LSPs

[RFC8231] specifies a set of extensions to PCEP to enable stateful control of MPLS-TE and GMPLS LSPs via PCEP and the maintaining of these LSPs at the stateful PCE. It further distinguishes between an active and a passive stateful PCE. A passive stateful PCE uses LSP state information learned from PCCs to optimize path computations but does not actively update LSP state. In contrast, an active stateful PCE utilizes the LSP delegation mechanism to update LSP parameters in those PCCs that delegated control over their LSPs to the PCE. [RFC8281] describes the setup, maintenance, and teardown of PCE-initiated LSPs under the stateful PCE model. The document defines the PCInitiate message that is used by a PCE to request a PCC to set up a new LSP.

The new metric type defined in this document can also be used with the stateful PCE extensions. The format of PCEP messages described in [RFC8231] and [RFC8281] uses <intended-attribute-list> and <attribute-list>, respectively, (where the <intended-attribute-list> is the attribute-list defined in Section 6.5 of [RFC5440].

A PCE MAY include the path MTU metric in PCInitiate or PCUpd message to inform the PCC of the path MTU calculated for the path. A PCC MAY include the path MTU metric as a bound constraint or to indicate optimization criteria (similar to PCReq).

2.3. Segement Routing

A Segment Routed path (SR path) can be derived from an IGP Shortest Path Tree (SPT). Segment Routed Traffic Engineering paths (SR-TE paths) may not follow IGP SPT. Such paths may be chosen by a suitable network planning tool and provisioned on the source node of the SR-TE path.

It is possible to use a PCE for computing one or more SR-TE paths taking into account various constraints and objective functions. Once a path is chosen, the PCE can inform an SR-TE path on a PCC using PCEP extensions specified in [I-D.ietf-pce-segment-routing]. Further, [I-D.negi-pce-segment-routing-ipv6] adds the support for IPv6 data plane in SR.

The new metric type for path MTU is applicable for the SR-TE path and require no additional extensions.

3. Security Considerations

This document defines a new METRIC type that do not add any new security concerns beyond those discussed in [RFC5440] in itself. Some deployments may find the path MTU information to be extra sensitive and could be used to influence path computation and setup with adverse effect. Additionally, snooping of PCEP messages with such data or using PCEP messages for network reconnaissance may give an attacker sensitive information about the operations of the network. Thus, such deployment should employ suitable PCEP security mechanisms like TCP Authentication Option (TCP-AO) [RFC5925] or Transport Layer Security (TLS) [RFC8253]. The procedure based on TLS is considered a security enhancement and thus is much better suited for the sensitive information.

4. IANA Considerations

This document makes following requests to IANA for action.

4.1. METRIC Types

IANA maintains the "Path Computation Element Protocol (PCEP) Numbers" registry. Within this registry, IANA maintains a subregistry for "METRIC Object T Field". IANA is requested to make the following allocation:

Value                  Description                  Reference
---------------------- ---------------------------- --------------
TBD                    Path MTU.                    This document    
   

5. Acknowledgments

6. References

6.1. Normative References

6.2. Informative References

Authors' Addresses