Open Shortest Path First Z. Zhang
Internet-Draft L. Wang
Updates: 2328, 5340 Juniper Networks, Inc.
(if approved) October 21, 2013
Intended status: Standards Track
Expires: April 24, 2014
OSPF Two-part Metric
draft-zzhang-ospf-two-part-metric-00.txt
Abstract
This document specifies an optional extension to the OSPF protocol,
to represent the metric on a multi-access network as two parts: the
metric from a router to the network, and the metric from the network
to the router. The router to router metric would be the sum of the
two.
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 RFC2119.
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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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 April 24, 2014.
Copyright Notice
Copyright (c) 2013 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 . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Proposed Enhancement . . . . . . . . . . . . . . . . . . . . . 3
3. Speficications . . . . . . . . . . . . . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6.1. Normative References . . . . . . . . . . . . . . . . . . . 6
6.2. Informative References . . . . . . . . . . . . . . . . . . 6
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1. Introduction
For a broadcast network, a Network LSA is advertised to list all
routers on the network, and each router on the network includes a
link in its Router LSA to describe its connection to the network.
The link in the Router LSA includes a metric but the listed routers
in the Network LSA does not include a metric. This is based on the
assumption that from a particular router, all others on the same
network can be reached with the same metric.
With some broadcast networks, different routers can be reached with
different metrics. RFC 6845 extends the OSPF protocol with a hybrid
interface type for that kind of broadcast networks, with which no
Network LSA is used and routers simply includes p2p links to all
routers on the same network with individual metrics. Broadcast
capability is still utilized to optimize database synchronization and
adjacency maintenance.
That works well for broadcast networks on which metric between
different pair of routers are really independent. For example, VPLS
networks.
With certain types of broadcast networks, further optimization can be
made to reduce the size of the Router LSAs and number of updates.
Consider a satellite radio network, with the satellite being the
central controller and some ground terminals being mobile. All
communication go through the satellite. When the mobile terminals
move about, their communication capability may change. When OSPF
runs over the radio network (routers being or in tandem with the
terminals), RFC 6845 hybrid interface can be used, but with the
following drawbacks.
Consider that one terminal/router moves into an area where
communication capability degrades significantly. Through the radio
control protocol all others determine that the metric to this
particular one changed and they all need to update their Router LSAs
accordingly. This router also determines that its metric to reach
all others also changed and it also need to update its Router LSA.
Consider that there could be many terminals and many of them can be
moving fast and frequently, the number/frequency of updates of those
large Router LSAs could become inhibiting.
2. Proposed Enhancement
Notice that in the above scenario, when one terminal's communication
capability changes, its metric to all others and the metric from all
others to it will all change in a similar fashion. Given this, the
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above problem can be easily addressed by breaking the metric into two
parts: the metric to the controller and the metric from the
controller. The metric from terminal R1 to R2 would be the sum of
the metric from R1 to the controller and the metric from the
controller to R2.
Now instead of using the RFC6845 hybrid interface type, the network
is just treated as a regular broadcast one. A router on the network
no longer needs to list individual metrics to each neighbors in its
Router LSA. The transit link's metric in the Router LSA either
represents the router's symmetric metric to/from the network, or in
case of asymetric metric, for OSPFv3 an additional TLV can be added
to represent the metric from the network to the router, following
draft-acee-ospfv3-lsa-extend (details TBD), and for OSPFv2, an MT
metric field can be used.
With this, the size of Router LSA will be significantly reduced.
When a router's communication capability changes, only itself need to
update its Router LSA and nobody else does.
Note that while the example uses the satellite as the relay point at
radio level (layer 2), at layer 3 the satellite does not play any
role. It does not need to be running layer 3 protocol at all.
Rather, the metric is abstracted as to/from the "network".
3. Speficications
The following protocol specifications are added to or modified from
the base OSPF protocol. If an area contains one or more two-part
metric networks, then all routers in the area must support the
extensions specified here. This document does not currently specify
a way to detect a router's capability of supporting this, and relies
on operator's due diligence in provisioning. A protocol mechanism
may be developer in the future.
The "Router interface parameters" has the following additions:
o Two-part metric: TRUE if the interface connects to a multi-access
network that uses two-part metric.
o Interface input cost: Link state metric from the network to this
router. Defaulted to "Interface output cost". May be configured
or dynamically adjusted to a value different from the "Interface
output cost", and in which case, it MUST be advertised in addition
to the link (output) cost for this interface in the router's
Router LSA, (see details below).
To signal that a network is using Two-part Metric, a new Link Type X
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(value TBD) is added. It is similar to Type 2, except that the
network uses Two-part Metric, and there may be an optional network-
to-router metric (interface input cost).
Link type Description Link ID
--------------------------------------------
X Link to transit Interface of
network that uses the Designated
Two-part Metric Router
For the network-to-router metric (interface input cost) that is
different from the router-to-network metric (interface output cost),
with OSPFv2 it is carried in an MT-ID field ([RFC4915]):
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | # MT-ID | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MT-ID | |1| MT-ID metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
As illustrated above, the lowest bit of the currently reserved field
between MT-ID and MT-ID metric fields can be set to indicate that
this is the network-to-router metric for the corresponding topology.
It is clear that this scheme is compatible with multi-topology.
For OSPFv3, a TLV can be added to represent the network-to-router
metric, per draft-acee-ospfv3-lsa-extend (details TBD).
During intra-area SPF calculation, when a vertex W corresponding to a
Network LSA is added to the candidate list because of a Type X link
in a Router LSA for vertex V (that was just added to the shortest-
path tree), W is marked that it uses Two-part Metric. Later, when a
vertex V marked with Two-part Metric (which must correspond to a
Network LSA) is added to the shortest-path tree, for the vertex W
that is reached via a link in V's corresponding LSA, the exact
reverse link (of Type X) from W to V is located from W's
corresponding Router LSA. If the reverse link does not exist, W is
not considered and the next link in V is checked. If the reverse
link has a network-to-router metric, that metric is used as the link
cost between V and W. Otherwise, that reverse link's (default) metric
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is used as the link cost beteen V and W.
4. IANA Considerations
This document makes no request of IANA (unless the Link Type values
in Router LSAs are assigned by IANA).
Note to RFC Editor: this section may be removed on publication as an
RFC.
5. Security Considerations
This document does not introduce new security risks.
6. References
6.1. Normative References
[I-D.acee-ospfv3-lsa-extend] Lindem, A., Mirtorabi, S., Roy, A., and
F. Baker, "OSPFv3 LSA Extendibility",
draft-acee-ospfv3-lsa-extend-01 (work
in progress), July 2013.
[RFC2119] Bradner, S., "Key words for use in RFCs
to Indicate Requirement Levels",
BCP 14, RFC 2119, March 1997.
[RFC2328] Moy, J., "OSPF Version 2", STD 54,
RFC 2328, April 1998.
[RFC4915] Psenak, P., Mirtorabi, S., Roy, A.,
Nguyen, L., and P. Pillay-Esnault,
"Multi-Topology (MT) Routing in OSPF",
RFC 4915, June 2007.
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and
A. Lindem, "OSPF for IPv6", RFC 5340,
July 2008.
6.2. Informative References
[RFC6845] Sheth, N., Wang, L., and J. Zhang,
"OSPF Hybrid Broadcast and Point-to-
Multipoint Interface Type", RFC 6845,
January 2013.
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Authors' Addresses
Jeffrey Zhang
Juniper Networks, Inc.
10 Technology Park Drive
Westford, MA 01886
EMail: zzhang@juniper.net
Lili Wang
Juniper Networks, Inc.
10 Technology Park Drive
Westford, MA 01886
EMail: liliw@juniper.net
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