draft-ietf-roll-of0-09.txt   draft-ietf-roll-of0-10.txt 
ROLL P. Thubert, Ed. ROLL P. Thubert, Ed.
Internet-Draft Cisco Systems Internet-Draft Cisco Systems
Intended status: Standards Track April 5, 2011 Intended status: Standards Track April 11, 2011
Expires: October 7, 2011 Expires: October 13, 2011
RPL Objective Function 0 RPL Objective Function 0
draft-ietf-roll-of0-09 draft-ietf-roll-of0-10
Abstract Abstract
The Routing Protocol for Low Power and Lossy Networks defines a The Routing Protocol for Low Power and Lossy Networks (RPL) defines a
generic Distance Vector protocol for Low Power and Lossy Networks. generic Distance Vector protocol that is adapted to such networks.
That generic protocol requires a specific Objective Function to RPL requires a specific Objective Function to establish a desired
establish a desired routing topology. This specification defines a routing topology. This document specifies a basic Objective Function
basic Objective Function that operates solely with the protocol that relies only on RPL's basic Protocol Data Units; it does not use
elements defined in the generic protocol specification. extensions such as RPL metric containers.
Requirements Language Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
Status of this Memo Status of this Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 7, 2011. This Internet-Draft will expire on October 13, 2011.
Copyright Notice Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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hop count favors paths with long distance links and poor connectivity hop count favors paths with long distance links and poor connectivity
properties. Other link properties such as the expected transmission properties. Other link properties such as the expected transmission
count metric (ETX) [DeCouto03] should be used instead to compute the count metric (ETX) [DeCouto03] should be used instead to compute the
Step-of-Rank. For instance, the Minimum Rank Objective Function with Step-of-Rank. For instance, the Minimum Rank Objective Function with
Hysteresis [I-D.ietf-roll-minrank-hysteresis-of] provides guidance on Hysteresis [I-D.ietf-roll-minrank-hysteresis-of] provides guidance on
how link cost can be computed and on how hysteresis can improve Rank how link cost can be computed and on how hysteresis can improve Rank
stability. stability.
An implementation MAY allow to stretch the Step-of-Rank with a An implementation MAY allow to stretch the Step-of-Rank with a
Stretch-of-Rank up to no more than MAXIMUM_RANK_STRETCH in order to Stretch-of-Rank up to no more than MAXIMUM_RANK_STRETCH in order to
enable the selection of a feasible successor in order to maintain enable the selection of a feasible successor and maintain path
some path diversity. The use of a Stretch-of-Rank augments the diversity. The use of a Stretch-of-Rank augments the apparent
apparent distance from the node to the root and distorts the DODAG; distance from the node to the root and distorts the DODAG; it should
it should be used with care so as to avoid instabilities due to be used with care so as to avoid instabilities due to greedy
greedy behaviours. behaviours.
The Step-of-Rank is expressed in units of MINIMUM_STEP_OF_RANK. As a The Step-of-Rank is expressed in units of MINIMUM_STEP_OF_RANK. As a
result, the least significant octet in the RPL Rank is not used. The result, the least significant octet in the RPL Rank is not used. The
default Step-of-Rank is DEFAULT_STEP_OF_RANK for each hop. An default Step-of-Rank is DEFAULT_STEP_OF_RANK for each hop. An
implementation MUST maintain the stretched Step-of-Rank between implementation MUST maintain the stretched Step-of-Rank between
MINIMUM_STEP_OF_RANK and MAXIMUM_STEP_OF_RANK, which allows to MINIMUM_STEP_OF_RANK and MAXIMUM_STEP_OF_RANK, which allows to
reflect a large variation of link quality. reflect a large variation of link quality.
The gap between MINIMUM_STEP_OF_RANK and MAXIMUM_RANK_STRETCH may not The gap between MINIMUM_STEP_OF_RANK and MAXIMUM_RANK_STRETCH may not
be sufficient in every case to strongly distinguish links of be sufficient in every case to strongly distinguish links of
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