Open Shortest Path First Z. Zhang
Internet-Draft L. Wang
Updates: 2328, 5340 (if approved) Juniper Networks, Inc.
Intended status: Standards Track D. Dubois
Expires: August 17, 2014 General Dynamics C4S
V. Julka
T. McMillan
L3 Communications, Linkabit
February 13, 2014
OSPF Two-part Metric
draft-zzhang-ospf-two-part-metric-01.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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://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 August 17, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
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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 . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Normative References . . . . . . . . . . . . . . . . . . . 7
6.2. Informative References . . . . . . . . . . . . . . . . . . 7
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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 fixed and mobile ground
terminals. 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 other routers determine that the metric to this
particular one changed and they all need to update their Router LSAs
accordingly. The router in question 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 other terminals and the metric
from all other terminals to it will all change in a similar fashion.
Given this, the above problem can be easily addressed by breaking the
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metric into two parts: the metric to the satellite and the metric
from the satellite. The metric from terminal R1 to R2 would be the
sum of the metric from R1 to the satellite and the metric from the
satellite 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. In case of symmetric metric to/from the satellite, it is
represented by the transit link's metric in the Router LSA. In case
of asymetric metric, it is rerepresented by a special MT Metric
(Section 3).
With the proposed enhancement, the size of Router LSA will be
significantly reduced. In addition, when a router's communication
capability changes, only that router needs to update its Router LSA.
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.
Therefore for generality, the metric is abstracted as to/from the
"network" rather that specifically to/from the "satellite".
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". If different from the output cost, it MUST be
advertised in addition to the link (output) cost for this
interface in the router's Router LSA.
To signal that a network is using Two-part Metric, a new Link Type X
(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-
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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 backward compatibility, if Two-Part Metric is to be used in an
area, all routers in the area must support it. To ensure this, a new
bit 0x00000004 (pending WG consensus and IANA assignment) in the LLS
EOF-TLV [RFC5613] is used:
Bit Name Reference
0x00000001 LSDB Resynchronization (LR) [RFC4811]
0x00000002 Restart Signal (RS-bit) [RFC4812]
0x00000004 Two-Part Metric (TM-bit) [this document]
Each router must be configured to include in its Hello packets an
EOF-TLV with the bit set, and any Hello packet without the EOF-TLV or
without the bit set must be ignored. With this, a router that does
not support Two-Part Metric will never receive a Router LSA
containing the new Link Type X.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
With OSPFv3, it is carried in a Router Multi-Topology sub-TLV:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 (RMT-sTLV) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MT-ID | 0 |1| MT-ID metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. sub-TLVs .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
As illustrated above for both cases, the least significant bit of the
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.
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
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
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6.1. Normative References
[I-D.ietf-ospf-mt-ospfv3] Mirtorabi, S. and A. Roy, "Multi-
topology routing in OSPFv3 (MT-
OSPFv3)",
draft-ietf-ospf-mt-ospfv3-03 (work
in progress), July 2007.
[I-D.ietf-ospf-ospfv3-lsa-extend] Lindem, A., Mirtorabi, S., Roy,
A., and F. Baker, "OSPFv3 LSA
Extendibility", draft-ietf-ospf-
ospfv3-lsa-extend-01 (work in
progress), February 2014.
[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.
[RFC5613] Zinin, A., Roy, A., Nguyen, L.,
Friedman, B., and D. Yeung, "OSPF
Link-Local Signaling", RFC 5613,
August 2009.
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
David Dubois
General Dynamics C4S
400 John Quincy Adams Road
Taunton, MA 02780
EMail: dave.dubois@gdc4s.com
Vibhor Julka
L3 Communications, Linkabit
9890 Towne Centre Drive
San Diego, CA 92121
EMail: vibhor.julka@l-3Com.com
Tom McMillan
L3 Communications, Linkabit
9890 Towne Centre Drive
San Diego, CA 92121
EMail: tom.mcmillan@l-3com.com
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