Network Working Group Y. Zhu
Internet-Draft China Telecom
Intended status: Standards Track Z. Hu
Expires: January 1, 2019 G. Yan
J. Yao
Huawei Technologies
June 30, 2018
BGP-LS Extensions for Advertising Path MTU
draft-hu-idr-bgp-ls-path-mtu-00
Abstract
BGP Link State (BGP-LS) describes a mechanism by which link-state and
TE information can be collected from networks and shared with
external components using the BGP routing protocol. The centralized
controller (PCE/SDN) completes the service path calculation based on
the information transmitted by the BGP-LS and delivers the result to
the Path Computation Client (PCC) through the PCEP protocol.
Segment Routing (SR) leverages the source routing paradigm, which can
be directly applied to the MPLS architecture with no change on the
forwarding plane and applied to the IPv6 architecture, with a new
type of routing header, called SRH. The SR uses the IGP protocol as
the control protocol. Compared to the MPLS tunneling technology, the
SR does not require signaling. Therefore, the SR does not support
the negotiation of the Path MTU. Since Multiple labels or SRv6 SIDs
are pushed in the packets, it is more likely that the MTU exceeds the
limit in the SR tunneling technology.
This document specify the extension to BGP Link State (BGP-LS) to
carry maximum transmission unit (MTU) messages. The PCE/SDN
calculates the Path MTU while completing the service path calculation
based on the information transmitted by the BGP-LS.
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 RFC 2119 [RFC2119].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Internet-Draft BGP-LS Extensions for Advertising Path MTU June 2018
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Deploying scenarios . . . . . . . . . . . . . . . . . . . . . 4
3. BGP_LS Extensions for Path MTU . . . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Normative References . . . . . . . . . . . . . . . . . . 6
7.2. Informative References . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
[RFC7752]describes the implementation mechanism of BGP-LS by which
link-state and TE information can be collected from networks and
shared with external components using the BGP routing protocol
[RFC4271]. BGP-LS allows the necessary Link-State Database (LSDB)
and Traffic Engineering Database (TED) information to be collected
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from the IGP within the network, filtered according to configurable
policy, and distributed to the PCE as necessary.
The appropriate MTU size guarantees efficient data transmission. If
the MTU size is too small and the packet size is large, fragmentation
may occur too much and packets are discarded by the QoS queue. If
the MTU configuration is too large, packet transmission may be slow.
PathMTU is the maximum length of a packet that can pass through a
path without fragmentation. [RFC1191] describes a technique for
dynamically discovering the maximum transmission unit (MTU) of an
arbitrary internet path.
The traditional MPLS tunneling technology has signaling for
establishing a path. [RFC3988] defines the mechanism for
automatically discovering the Path MTU of LSPs of LDP tunnels. For a
certain FEC, the LSR compares the MTU advertised by all downstream
devices with the MTU of the FEC output interface in the local device,
and calculates the minimum value for the upstream device.
[RFC3209] specify the mechanism of MTU signaling in RSVP-TE. The
ingress node of the RSVP-TE tunnel sends a Path message to the
downstream device. The Adspec object in the Path message carries the
MTU. Each node along the tunnel receives a Path message, compares
the MTU value in the Adspec object with the interface MTU value and
MPLS MTU configured on the physical output interface of the local
tunnel , obtains the minimum MTU value, and puts it into the newly
constructed Path message and continues to send it to the downstream
equipment. Thus, the MTU carried in the Path message received by the
Egress node is the minimum value of the path MTU. The Egress node
brings the negotiated Path MTU back to the Ingress node through the
Resv message.
Segment Routing (SR) described in [I-D.ietf-spring-segment-routing]
leverages the source routing paradigm. Segment Routing can be
directly applied to the MPLS architecture with no change on the
forwarding plane [I-D.ietf-spring-segment-routing-mpls] and applied
to the IPv6 architecture with a new type of routing header called the
SR header (SRH) [I-D.ietf-6man-segment-routing-header].
[I-D.ietf-idr-bgp-ls-segment-routing-ext] defines SR extensions to
BGP-LS and specifies the TLVs and sub-TLVs for advertising SR
information. Based on the SR information reported by the BGP-LS, the
SDN can calculate the end-to-end explicit SR-TE paths or SR Policies.
Nevertheless, Segment Routing is a tunneling technology based on the
IGP protocol as the control protocol, and there is no signaling for
establishing the path. so the Segment Routing tunnel cannot currently
support the negotiation mechanism of the MTU. Multiple labels or
SRv6 SIDs are pushed in the packets. This causes the length of the
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packets encapsulated in the Segment Routing tunnel to increase during
packet forwarding. This is more likely to cause MTU overrun than
traditional MPLS.
This document specify the extension to BGP Link State (BGP-LS) to
carry maximum transmission unit (MTU) messages.
2. Deploying scenarios
This document suggests a solution to extension to BGP Link State
(BGP-LS) to carry maximum transmission unit (MTU) messages. The MTU
information of the link is acquired through the process of collecting
link state and TE information by BGP-LS. Concretely, a router
maintains one or more databases for storing link-state information
about nodes and links in any given area. The router's BGP process
can retrieve topology from these LSDBs and distribute it to a
consumer, either directly or via a peer BGP speaker (typically a
dedicated Route Reflector). As for how IGP collects link MTU
information and stores it in LSDB, which is beyond the scope of this
article.
As per [RFC7752], the collection of link-state and TE information and
its distribution to consumers is shown in the following figure.
+-----------+
| Consumer |
+-----------+
^
|
+-----------+
| BGP | +-----------+
| Speaker | | Consumer |
+-----------+ +-----------+
^ ^ ^ ^
| | | |
+---------------+ | +-------------------+ |
| | | |
+-----------+ +-----------+ +-----------+
| BGP | | BGP | | BGP |
| Speaker | | Speaker | . . . | Speaker |
+-----------+ +-----------+ +-----------+
^ ^ ^
| | |
IGP IGP IGP
Figure 1: Collection of Link-State and TE Information
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3. BGP_LS Extensions for Path MTU
[RFC7752] defines the BGP-LS NLRI that can be a Node NLRI, a Link
NLRI or a Prefix NLRI. The corresponding BGP-LS attribute is a Node
Attribute, a Link Attribute or a Prefix Attribute. [RFC7752] defines
the TLVs that map link-state information to BGP-LS NLRI and the BGP-
LS attribute. Therefore, according to this document, a new sub TLV
is added to the Link Attribute TLV.
The format of the sub-TLV is as shown below.
x TYPE - TBD
x LENGTH - Total length of the value field, it should be 2
x VALUE - 2-byte MTU value of the link
No. of Octets
+-----------------+
| MTU value | 2
+-----------------+
Figure 2. Sub-TLV Format for MTU
Whenever there is a change in MTU value represented by Link Attribute
TLV, BGP-LS should re-originate the respective TLV with the new MTU
value.
4. IANA Considerations
This document requests assigning a new code-points from the BGP-LS
Link Descriptor and Attribute TLVs registry as specified in sections
3.
5. Security Considerations
This document does not introduce security issues beyond those
discussed in RFC7752.
6. Acknowledgements
The authors of this document would like to thank Gang Yan, Peng Wu,
Zhenbin Li and Gang Zhao for their comments.
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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>.
7.2. Informative References
[]
Filsfils, C., Previdi, S., Leddy, J., Matsushima, S., and
d. daniel.voyer@bell.ca, "IPv6 Segment Routing Header
(SRH)", draft-ietf-6man-segment-routing-header-14 (work in
progress), June 2018.
[I-D.ietf-idr-bgp-ls-segment-routing-ext]
Previdi, S., Talaulikar, K., Filsfils, C., Gredler, H.,
and M. Chen, "BGP Link-State extensions for Segment
Routing", draft-ietf-idr-bgp-ls-segment-routing-ext-08
(work in progress), May 2018.
[I-D.ietf-spring-segment-routing]
Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
Litkowski, S., and R. Shakir, "Segment Routing
Architecture", draft-ietf-spring-segment-routing-15 (work
in progress), January 2018.
[I-D.ietf-spring-segment-routing-mpls]
Bashandy, A., Filsfils, C., Previdi, S., Decraene, B.,
Litkowski, S., and R. Shakir, "Segment Routing with MPLS
data plane", draft-ietf-spring-segment-routing-mpls-14
(work in progress), June 2018.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<https://www.rfc-editor.org/info/rfc1191>.
[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>.
[RFC3988] Black, B. and K. Kompella, "Maximum Transmission Unit
Signalling Extensions for the Label Distribution
Protocol", RFC 3988, DOI 10.17487/RFC3988, January 2005,
<https://www.rfc-editor.org/info/rfc3988>.
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[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
[RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and
Traffic Engineering (TE) Information Using BGP", RFC 7752,
DOI 10.17487/RFC7752, March 2016,
<https://www.rfc-editor.org/info/rfc7752>.
Authors' Addresses
Yongqing Zhu
China Telecom
109, West Zhongshan Road, Tianhe District.
Guangzhou 510000
China
Email: zhuyq@gsta.com
Zhibo Hu
Huawei Technologies
Huawei Bld., No.156 Beiqing Rd.
Beijing 100095
China
Email: huzhibo@huawei.com
Gang Yan
Huawei Technologies
Huawei Bld., No.156 Beiqing Rd.
Beijing 100095
China
Email: yangang@huawei.com
Junda Yao
Huawei Technologies
Huawei Bld., No.156 Beiqing Rd.
Beijing 100095
China
Email: yaojunda@huawei.com
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