IGP Extensions for Link MTU
draft-hu-lsr-igp-link-mtu-03
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Zhibo Hu, Shuping Peng , Xing Xi | ||
| Last updated | 2024-07-30 | ||
| Replaces | draft-hu-lsr-igp-path-mtu | ||
| RFC stream | (None) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-hu-lsr-igp-link-mtu-03
Network Working Group Z. Hu
Internet-Draft S. Peng
Intended status: Standards Track X. Xi
Expires: 1 February 2025 Huawei
31 July 2024
IGP Extensions for Link MTU
draft-hu-lsr-igp-link-mtu-03
Abstract
Segment routing (SR) leverages the source routing mechanism. It
allows for a flexible definition of end-to-end paths within IGP
topologies by encoding paths as sequences of topological sub-paths
which are called segments. These segments are advertised by the
link-state routing protocols (IS-IS and OSPF). Unlike the MPLS, SR
does not have the specific path construction signaling so that it
cannot support the Path MTU. This draft provides the necessary IS-IS
and OSPF extensions about the Path MTU that need to be used on SR.
Here, the term "OSPF" means both OSPFv2 and OSPFv3.
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 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 1 February 2025.
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Copyright Notice
Copyright (c) 2024 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
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Link MTU Application Examples . . . . . . . . . . . . . . . . 4
3.1. Link MTU for SR Policy Scenario . . . . . . . . . . . . . 4
3.2. Link MTU for SR Best Effort Scenario . . . . . . . . . . 5
4. IGP Extensions . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. IS-IS Extensions . . . . . . . . . . . . . . . . . . . . 5
4.2. OSPF Extensions . . . . . . . . . . . . . . . . . . . . . 6
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
Segment routing (SR) leverages the source routing mechanism. SR
allows for a flexible definition of end-to-end paths within IGP
topologies by encoding paths as sequences of topological sub-paths
which are called segments. These segments are advertised by the
link-state routing protocols (IS-IS and OSPF). The SR architecture
as well as the routing policy is proposed in [RFC8402] and
[I-D.ietf-spring-segment-routing-policy]. Two types of segments are
defined, Prefix segments and Adjacency segments. Prefix segments
represent an ECMP-aware shortest path to a prefix (or a node), as per
the state of the IGP topology. Adjacency segments represent a hop
over a specific adjacency between two nodes in the IGP. A prefix
segment is typically a multi-hop path while an adjacency segment, in
most of the cases, is a one-hop path. SR can compute the paths from
end to end and without requiring any LDP or RSVP-TE signaling. SR
supports per-flow explicit routing while just maintaining per-flow
state only at the source node.
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SR architecture supports the distributed scenario and the centralized
scenario. In the distributed scenario, the segments are allocated
and signaled by IGP or BGP and a node needs to compute the source-
routed policy. Some necessary IS-IS and OSPF extensions for SR are
proposed in [RFC8665] [RFC8666] [RFC8667]. In a centralized
scenario, the SR controller decides which nodes need to steer which
packets on which source-routed policies. However, in both
conditions, the MTU is not included in the SR policy. As the SR may
push more MPLS labels or SRv6 SIDs in the packet header, the packets
are more likely to be larger than the minimum MTU in the path
compared to the traditional MPLS forwarding process. Unfortunately,
with the current mechanisms in SR, the path MTU information cannot be
obtained in advance. Therefore it cannot be ensured that the packet
size is less than the path MTU which is the minimum link MTU of all
the links in a path between a source node and a destination node.
The definition of the path MTU is discussed in [RFC1191] [RFC8201].
This draft describes the necessary IS-IS and OSPF extensions for
obtaining the path MTU to be used on SR. New sub-TLVs are introduced
for both the IS-IS and OSPF protocols. With the IGP flooding process
in the distributed scenario or the BGP transmission to the
controller, the ingress node or the controller is able to compute the
path MTU for the SR policy.
2. Terminology
Router: A node that forwards IP packets not explicitly addressed to
itself.
Interface: A node's attachment to a link.
Segment: An instruction a node executes on the incoming packet. For
example, forward packet according to shortest path to destination or
a specific interface, etc..
SR Policy: An ordered list of segments.
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.
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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: The minimum link MTU of all the links in a path between a
source node and a destination node.
3. Link MTU Application Examples
This section uses the SR Policy and SR Best Effort scenarios as
examples to describe the applications of link MTU.
3.1. Link MTU for SR Policy Scenario
Figure 1 shows an example of the Link MTU application for the SR
Policy scenario. In the SR Policy scenario, the link MTU application
procedure is as follows:
Step 1: Collect Link MTUs on the entire network through IGP.
Step 2: BGP-LS reports the link MTU to the controller.
Step 3: After the controller calculating the SR Policy path, record
the minimum link MTU of all the links in the path as the Min Link
MTU. Consider the SRv6 encapsulation length as X. Then Path MTU =
Min Link MTU - X.
Step 4: The Path MTU is delivered to the SR Policy headend through
BGP SR policy / PCEP. Headend fragments Packets Based on the Path
MTU.
In the case of a Topology Independent Loop-Free Alternate (TI-LFA) in
P routers with encapsulation, It will re-encapsulate a new IPv6
header and fragments Packets with the Path MTU of the ti-lfa path.
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|--------------[ Controller ]-------------|
| |
| |
[ P1 ]--1500--[ P2 ]--1500--[ P3 ]--1200--[ P4 ]
| | | |
| | | |
1400 1400 1400 1400
| | | |
| | | |
[ P5 ]--1500--[ P6 ]--1500--[ P7 ]--1200--[ P8 ]
Figure 1: Figure 1: Link MTU for SR Policy Scenario
3.2. Link MTU for SR Best Effort Scenario
Figure 2 shows an example of the link MTU application for the SR Best
Effort scenario. The IGP calculates the Path MTU of per destination
address. For per destination address, record the minimum link MTU of
all the links in the IGP SPF path as the Min Link MTU. In the case
of a Topology Independent Loop-Free Alternate (TI-LFA) in P routers
with encapsulation, It will re-encapsulate a new IPv6 header and
fragments Packets with the Path MTU of the ti-lfa path.
[ P1 ]--1500--[ P2 ]--1500--[ P3 ]--1200--[ P4 ]
| | | |
| | | |
1400 1400 1400 1400
| | | |
| | | |
[ P5 ]--1500--[ P6 ]--1500--[ P7 ]--1200--[ P8 ]
Figure 2: Figure 2: Link MTU for SR Best Effort Scenario
4. IGP Extensions
This document describes IS-IS and OSPF extensions to flood the router
interface MTU to each node within an IGP domain. Then the controller
or the original node collects all the link MTUs from the routers. So
the original node can compute the minimum link MTU of all the links
in the path. The source node can limit the packet size less than the
path MTU.
4.1. IS-IS Extensions
A new sub-TLV called link MTU sub-TLV is defined for TLVs 22, 23, 25,
141, 222, 223 in the Router Information LSP to carry the MTU of the
interface associated with the link . Each sub-TLV is encoded as shown
in Figure 3.
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Type: MTU, 1 byte, TBD.
Length: # of octets in the value field, 1 byte.
Value: The value is the MTU size of a link, 2 bytes.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = MTU | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MTU-Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Figure 3: Link MTU Sub-TLV for the IS-IS extension
The use and meaning of these fields are as follows:
Type - A single octet encoding the sub-TLV type. Here the type is 1
octet.
Length - A single octet encoding the total length of the value field
of the sub-TLV in octets.
MTU-Value - Two octets encoding the MTU size of the sub-TLV. This
field identifies the size of the router interfaces.
This sub-TLV is optional.
This document defines a link MTU sub-TLV for IS-IS extension. The
codepoints need to be determined by the IANA.
4.2. OSPF Extensions
A new sub-TLV called link MTU sub-TLV is defined in the corresponding
LSA as specified for OSPFv2 and OSPFv3 to carry the MTU of the
interface associated with the link. Each sub-TLV is encoded as shown
in Figure 4.
Type: MTU, 2 bytes, TBD.
Length: # of octets in the value field, 2 bytes.
Value: The value is the MTU size of a link, 2 bytes.
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0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = MTU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MTU-Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Figure 4: Link MTU Sub-TLV for the OSPF extension
The use and meaning of these fields are as follows:
Type - Two octets encoding the TLV type. Here the type is 2 octets.
For OSPFv2, the link MTU is advertised as an optional sub-TLV of the
OSPFv2 Extended Link TLV in the OSPFv2 Extended Link Opaque LSA as
defined in [RFC7684] and the codepoints need to be determined by the
IANA.
For OSPFv3, the link MTU is advertised as an optional sub-TLV of the
Router-Link TLV in the OSPFv3 E-Router-LSA as defined in [RFC8362]
and the codepoints need to be determined by the IANA.
Length - Two octets encoding the total length of the value field of
the sub-TLV in octets.
MTU-Value - Two octets encoding the MTU size of the TLV. This field
identifies the size of the router interfaces.
If the link MTU sub-TLV is advertised for multiple times for the same
link in different OSPFv2 Extended Link Opaque LSAs or OSPFv3 E-
Router-LSAs originated by the same OSPF router, the link MTU sub-TLV
in the OSPFv2 Extended Link Opaque LSA with the smallest Opaque ID or
in the OSPFv3 E-Router-LSA with the smallest Link State ID MUST be
used by receiving OSPF routers.
5. Acknowledgements
TBD.
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6. IANA Considerations
This document requests that IANA allocates a new sub-TLV type as
defined in Section 3.1 from the "Sub-TLVs for TLVs 22, 23, 25, 141,
222, and 223 (Extended IS reachability, IS Neighbor Attribute, L2
Bundle Member Attributes, inter-AS reachability information, MT-ISN,
and MT IS Neighbor Attribute TLVs)" registry as specified.
Value Description Reference
---------------------- ---------------------------- --------------
TBD IS-IS Link MTU This document
Figure 5: Figure 3: IS-IS Link MTU
This document requests that IANA allocates a new sub-TLV type as
defined in Section 3.2 from the "OSPFv2 Extended Link TLV Sub-TLVs"
registry.
Value Description Reference
---------------------- ---------------------------- --------------
TBD OSPFv2 Link MTU This document
Figure 6: Figure 4: OSPFv2 Link MTU
This document requests that IANA allocates a new sub-TLV type as
defined in Section 3.2 from the "OSPFv3 Extended LSA Sub-TLVs"
registry.
Value Description Reference
---------------------- ---------------------------- --------------
TBD OPSFv3 Link MTU This document
Figure 7: Figure 5: OSPFv3 Link MTU
7. Security Considerations
These extensions to IS-IS and OSPF do not add any new security issues
to the existing IGP.
8. References
[I-D.ietf-spring-segment-routing-policy]
Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
P. Mattes, "Segment Routing Policy Architecture", Work in
Progress, Internet-Draft, draft-ietf-spring-segment-
routing-policy-22, 22 March 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-spring-
segment-routing-policy-22>.
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[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<https://www.rfc-editor.org/info/rfc1191>.
[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>.
[RFC7684] Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,
Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
Advertisement", RFC 7684, DOI 10.17487/RFC7684, November
2015, <https://www.rfc-editor.org/info/rfc7684>.
[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>.
[RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
"Path MTU Discovery for IP version 6", STD 87, RFC 8201,
DOI 10.17487/RFC8201, July 2017,
<https://www.rfc-editor.org/info/rfc8201>.
[RFC8362] Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and
F. Baker, "OSPFv3 Link State Advertisement (LSA)
Extensibility", RFC 8362, DOI 10.17487/RFC8362, April
2018, <https://www.rfc-editor.org/info/rfc8362>.
[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>.
[RFC8665] Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
Extensions for Segment Routing", RFC 8665,
DOI 10.17487/RFC8665, December 2019,
<https://www.rfc-editor.org/info/rfc8665>.
[RFC8666] Psenak, P., Ed. and S. Previdi, Ed., "OSPFv3 Extensions
for Segment Routing", RFC 8666, DOI 10.17487/RFC8666,
December 2019, <https://www.rfc-editor.org/info/rfc8666>.
[RFC8667] Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
Extensions for Segment Routing", RFC 8667,
DOI 10.17487/RFC8667, December 2019,
<https://www.rfc-editor.org/info/rfc8667>.
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Authors' Addresses
Zhibo Hu
Huawei
Huawei Bld., No.156 Beiqing Rd.
Beijing
100095
China
Email: [email protected]
Shuping Peng
Huawei
Huawei Bld., No. 156 Beiqing Rd.
Beijing
100095
China
Email: [email protected]
Xing Xi
Huawei
Huawei Bld., No. 156 Beiqing Rd.
Beijing
100095
China
Email: [email protected]
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