Internet DRAFT - draft-hu-lsr-isis-path-mtu
draft-hu-lsr-isis-path-mtu
Network Working Group Z. Hu
Internet-Draft Huawei
Intended status: Informational Y. Zhu
Expires: January 1, 2019 China Telecom
Z. Li
L. Dai
Huawei
June 30, 2018
IS-IS Extensions for Path MTU
draft-hu-lsr-isis-path-mtu-00
Abstract
Segment routing (SR) leverages the source routing mechanism. It
allows for a flexible definition of end-to-end paths with IGP
topologies by encoding paths as sequences of topological sub-paths
which is 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
extensions about the Path MTU that need to be used on SR.
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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This Internet-Draft will expire on January 1, 2019.
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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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Extendsion of IS-IS . . . . . . . . . . . . . . . . . . . . . 4
3.1. Protocol Extension . . . . . . . . . . . . . . . . . . . 4
4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
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 toplogical sub-paths
which is called segments. These segments are advertised by the link-
state routing protocols (IS-SI and OSPF). The SR architecture as
well as the routing policy is proposed in
[I-D.ietf-spring-segment-routing] and
[I-D.filsfils-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, 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 explict 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 extensions for SR are proposed
in [I-D.ietf-isis-segment-routing-extensions]. 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 larger than the minimum MTU in the path compared to the
traditional MPLS forwarding process. Unfortunately the paths do not
provide the path MTU informaiton so that the path can not assure 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 RFC1981
[RFC1981].
This draft describes the necessary IS-IS extensions about the path
MTU that need to be used on SR. A new TLV is introduced into the IS-
IS protocol. With the IGP flooding process in the distributed
scenario or transmission to the controller by BGP, the ingress nodes
or the controllers 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, froward 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.
link MTU: the maximum transimission unit, i.e., maximum packet size
in octets, that can be conveyed in one piece over a link.
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.
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3. Extendsion of IS-IS
This document describes an IS-IS extension to flood the router
interface MTU to each node with the IGP domain. Then the controller
or the original node collects all the link MTUs from the routers.
After the SR path is calculated, packet may be lost if the packet
size is larger than the minimum MTU along the path. 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.
3.1. Protocol Extension
A new TLV called link MTU TLV is defined to be included in the Router
Information LSP. The LSP transmitted by an interface in a router
MUST include the TLV. Each such TLV is encoded as shown in Figure 1.
Type: MTU, 1 byte
Length: # of octets in the value field (1bytes)
Value: the value is the MTU size of a link.
0 0
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Type = MTU |
+-+-+-+-+-+-+-+-+
| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MTU size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1
The use and meaning of these fields are as follows:
Type - A single octet encoding the TLV type. Here the type is 1
octet.
Length - One octet encoding the length in octet of the TLV. This
field identifies the length of the value part.
MTU size - This field identifies the size of the router interfaces.
Two octets encoding the MTU size of the TLV.
This document defines a single MTU TLV, the codepoints need to be
determined by the IANA.
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4. Acknowledgements
TBD.
5. IANA Considerations
This document requests that IANA allocate from the IS-IS TLV
Codepoints Registry a new TLV.
6. Security Considerations
This extension to IS-IS does not change the underlying security
issues iherent in the existing IGP.
7. References
[I-D.filsfils-spring-segment-routing-policy]
Filsfils, C., Sivabalan, S., Hegde, S.,
daniel.voyer@bell.ca, d., Lin, S., bogdanov@google.com,
b., Krol, P., Horneffer, M., Steinberg, D., Decraene, B.,
Litkowski, S., Mattes, P., Ali, Z., Talaulikar, K., Liste,
J., Clad, F., and K. Raza, "Segment Routing Policy
Architecture", draft-filsfils-spring-segment-routing-
policy-06 (work in progress), May 2018.
[I-D.ietf-isis-segment-routing-extensions]
Previdi, S., Ginsberg, L., Filsfils, C., Bashandy, A.,
Gredler, H., Litkowski, S., Decraene, B., and J. Tantsura,
"IS-IS Extensions for Segment Routing", draft-ietf-isis-
segment-routing-extensions-18 (work in progress), June
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.
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, DOI 10.17487/RFC1981, August
1996, <https://www.rfc-editor.org/info/rfc1981>.
[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>.
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Authors' Addresses
Zhibo Hu
Huawei
Huawei Bld., No.156 Beiqing Rd.
Beijing 100095
China
Email: huzhibo@huawei.com
Yongqing Zhu
China Telecom
109, West Zhongshan Rd.
Guangzhou 510000
China
Email: zhuyq@gsta.com
Zhenbin Li
Huawei
Huawei Bld., No.156 Beiqing Rd.
Beijing 100095
China
Email: lizhenbin@huawei.com
Longfei Dai
Huawei
Huawei Bld., No. 156 Beiqing Rd.
Beijing 100095
China
Email: larry.dai@huawei.com
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