Support for Path MTU (PMTU) in the Path Computation Element (PCE) communication Protocol (PCEP).
draft-li-pce-pcep-pmtu-04
PCE Working Group S. Peng
Internet-Draft C. Li
Intended status: Standards Track Huawei Technologies
Expires: November 7, 2021 L. Han
China Mobile
L. Ndifor
MTN Cameroon
May 6, 2021
Support for Path MTU (PMTU) in the Path Computation Element (PCE)
communication Protocol (PCEP).
draft-li-pce-pcep-pmtu-04
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
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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 November 7, 2021.
Copyright Notice
Copyright (c) 2021 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. PCEP Extention . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Extensions to METRIC Object . . . . . . . . . . . . . . . 5
3.2. Stateful PCE and PCE Initiated LSPs . . . . . . . . . . . 6
3.3. Segment Routing . . . . . . . . . . . . . . . . . . . . . 7
3.4. Path MTU Adjustment . . . . . . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5.1. METRIC Type . . . . . . . . . . . . . . . . . . . . . . . 8
6. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 8
7.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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
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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 [RFC8402], 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 [RFC8664], 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
[RFC8664]. [RFC8664] specifies PCEP extensions for supporting a SR-
TE LSP for MPLS data plane. [I-D.ietf-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.
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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
[I-D.ietf-idr-bgp-ls-link-mtu] 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. Terminology
This draft refers to the terms defined in [RFC8201], [RFC4821] and
[RFC3988].
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MTU: Maximum Transmission Unit, the size in bytes of the largest IP
packet, including the IP header and payload, that can be
transmitted on a link or path. Note that this could more properly
be called the IP MTU, to be consistent with how other standards
organizations use the acronym MTU.
Link MTU: The Maximum Transmission Unit, i.e., maximum IP packet
size in bytes, that can be conveyed in one piece over a link. Be
aware that this definition is different from the definition used
by other standards organizations.
For IETF documents, link MTU is uniformly defined as the IP MTU
over the link. This includes the IP header, but excludes link
layer headers and other framing that is not part of IP or the IP
payload.
Be aware that other standards organizations generally define link
MTU to include the link layer headers.
For the MPLS data plane, this size includes the IP header and data (or
other payload) and the label stack but does not include any lower-layer
headers. A link may be an interface (such as Ethernet or Packet-over-
SONET), a tunnel (such as GRE or IPsec), or an LSP.
Path: The set of links traversed by a packet between a source node
and a destination node.
Path MTU, or PMTU: The minimum link MTU of all the links in a path
between a source node and a destination node.
For the MPLS data plane, it is the MTU of an LSP from a given LSR to
the egress(es), over each valid (forwarding) path. This size includes
the IP header and data (or other payload) and any part of the label
stack that was received by the ingress LSR before it placed the packet
into the LSP (this part of the label stack is considered part of the
payload for this LSP). The size does not include any lower-level
headers.
3. PCEP Extention
3.1. 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.
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o T = TBD: Path MTU.
o A network comprises of a set of N links {Li, (i=1...N)}.
o A path P of a LSP is a list of K links {Lpi,(i=1...K)}.
o A Link MTU of link L is denoted M(L).
o 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].
3.2. 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
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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).
3.3. Segment 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 [RFC8664]. Further,
[I-D.ietf-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.
3.4. Path MTU Adjustment
The path MTU metric can be used for both primary and protection path.
The minimal value of the link MTU along the path is collected, based
on which minor adjustment is made to cater for overhead introduced by
the protection mechanisms such as TI-LFA. The path MTU is the value
of the minimum link MTU minus the overhead. In this way, the ingress
node can use the path MTU directly.
4. 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
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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.
5. IANA Considerations
This document makes following requests to IANA for action.
5.1. METRIC Type
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
6. Acknowledgement
We would like to thank Dhruv Dhody for his contributions for this
document.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5440] Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol (PCEP)", RFC 5440,
DOI 10.17487/RFC5440, March 2009,
<https://www.rfc-editor.org/info/rfc5440>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
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[RFC8231] Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path
Computation Element Communication Protocol (PCEP)
Extensions for Stateful PCE", RFC 8231,
DOI 10.17487/RFC8231, September 2017,
<https://www.rfc-editor.org/info/rfc8231>.
[RFC8281] Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "Path
Computation Element Communication Protocol (PCEP)
Extensions for PCE-Initiated LSP Setup in a Stateful PCE
Model", RFC 8281, DOI 10.17487/RFC8281, December 2017,
<https://www.rfc-editor.org/info/rfc8281>.
7.2. Informative References
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
<https://www.rfc-editor.org/info/rfc3209>.
[RFC4657] Ash, J., Ed. and J. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol Generic
Requirements", RFC 4657, DOI 10.17487/RFC4657, September
2006, <https://www.rfc-editor.org/info/rfc4657>.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <https://www.rfc-editor.org/info/rfc5925>.
[RFC8253] Lopez, D., Gonzalez de Dios, O., Wu, Q., and D. Dhody,
"PCEPS: Usage of TLS to Provide a Secure Transport for the
Path Computation Element Communication Protocol (PCEP)",
RFC 8253, DOI 10.17487/RFC8253, October 2017,
<https://www.rfc-editor.org/info/rfc8253>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[RFC8664] Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
and J. Hardwick, "Path Computation Element Communication
Protocol (PCEP) Extensions for Segment Routing", RFC 8664,
DOI 10.17487/RFC8664, December 2019,
<https://www.rfc-editor.org/info/rfc8664>.
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[I-D.ietf-pce-segment-routing-ipv6]
Li, C., Negi, M., Sivabalan, S., Koldychev, M.,
Kaladharan, P., and Y. Zhu, "PCEP Extensions for Segment
Routing leveraging the IPv6 data plane", draft-ietf-pce-
segment-routing-ipv6-08 (work in progress), November 2020.
[I-D.ietf-idr-bgp-ls-link-mtu]
Zhu, Y., Hu, Z., Peng, S., and R. Mwehaire, "Signaling
Maximum Transmission Unit (MTU) using BGP-LS", draft-ietf-
idr-bgp-ls-link-mtu-00 (work in progress), November 2020.
Authors' Addresses
Shuping Peng
Huawei Technologies
Huawei Campus, No. 156 Beiqing Rd.
Beijing 100095
China
Email: pengshuping@huawei.com
Cheng Li
Huawei Technologies
Huawei Campus, No. 156 Beiqing Rd.
Beijing 100095
China
Email: c.l@huawei.com
Liuyan Han
China Mobile
Beijing 100053
China
Email: hanliuyan@chinamobile.com
Luc-Fabrice Ndifor
MTN Cameroon
Cameroon
Email: Luc-Fabrice.Ndifor@mtn.com
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