draft-ietf-mpls-loop-prevention-00.txt   draft-ietf-mpls-loop-prevention-01.txt 
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Eric Rosen Eric Rosen
Cisco Systems Cisco Systems
Paul Doolan Paul Doolan
Ennovate Networks Ennovate Networks
May 1999 May 1999
MPLS Loop Prevention Mechanism MPLS Loop Prevention Mechanism
<draft-ietf-mpls-loop-prevention-00.txt> <draft-ietf-mpls-loop-prevention-01.txt>
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. Drafts.
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months and may be updated, replaced, or obsoleted by other documents months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet-Drafts as at any time. It is inappropriate to use Internet-Drafts as
reference material or to cite them other than as "work in progress." reference material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
To view the list Internet-Draft Shadow Directories, see
http://www.ietf.org/shadow.html.
Abstract Abstract
This paper presents a simple mechanism, based on "threads", which This paper presents a simple mechanism, based on "threads", which
can be used to prevent MPLS from setting up label switched path can be used to prevent MPLS from setting up label switched path
(LSPs) which have loops. The mechanism is compatible with, but does (LSPs) which have loops. The mechanism is compatible with, but does
not require, VC merge. The mechanism can be used with either the not require, VC merge. The mechanism can be used with either the
ordered downstream-on-demand allocation or ordered downstream ordered downstream-on-demand allocation or ordered downstream
allocation. The amount of information that must be passed in a allocation. The amount of information that must be passed in a
protocol message is tightly bounded (i.e., no path-vector is used). protocol message is tightly bounded (i.e., no path-vector is used).
When a node needs to change its next hop, a distributed procedure is When a node needs to change its next hop, a distributed procedure is
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maintained by the node. The path setup messages that the node sends maintained by the node. The path setup messages that the node sends
downstream will contain this color. Also, when the node sends such downstream will contain this color. Also, when the node sends such
a message downstream, it will remember the color, and this color a message downstream, it will remember the color, and this color
becomes the color of the downstream link. becomes the color of the downstream link.
When a colored message is received, its color becomes the color of When a colored message is received, its color becomes the color of
the incoming link. The thread which consists of messages of a the incoming link. The thread which consists of messages of a
certain color will be known as a thread of that color. certain color will be known as a thread of that color.
A special color value "transparent"(=all 0's) is reserved. A special color value "transparent"(=all 0's) is reserved.
One possible method for unique color assignment is, starting the
event identifier from its initial value, and incrementing it by one
(modulo its maximum value) each time a color is assigned. In this
method, the initial event identifier is either selected at random or
assigned to be larger than the largest event identifier used on the
previous system incarnation.
Thread Hop Count Thread Hop Count
In order to maintain link hop counts, we need to carry hop counts in In order to maintain link hop counts, we need to carry hop counts in
the path control messages. For instance, a leaf node would assign a the path control messages. For instance, a leaf node would assign a
hop count of 1 to its downstream link, and would store that value hop count of 1 to its downstream link, and would store that value
into a path setup message it sends downstream. When a path setup into a path setup message it sends downstream. When a path setup
message is sent downstream, a node would assign a hop count which is message is sent downstream, a node would assign a hop count which is
one more than the largest of the incoming link hop counts, to its one more than the largest of the incoming link hop counts, to its
downstream link, and would store that value into a path setup downstream link, and would store that value into a path setup
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