TRILL: Parent node Shifts in Tree Construction, Mitigation.
draft-ietf-trill-parent-selection-00
TRILL WG R. Parameswaran,
INTERNET-DRAFT Brocade Communications, Inc.
Intended Status:Informational October 6, 2017
Expires: April 09, 2018
TRILL: Parent node Shifts in Tree Construction, Mitigation.
<draft-ietf-trill-parent-selection-00.txt>
Abstract
This draft documents a known problem in the TRILL tree construction
mechanism and offers an approach requiring no change to the TRILL
protocol in order to solve the problem.
Status of This Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Distribution of this document is unlimited. Comments should be sent
to the authors or the TRILL working group mailing list:
trill@ietf.org.
Internet-Drafts are working documents of the Internet Engineering
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Terminology and Acronyms.
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].
Table of Contents
1. Introduction...............................................1
2. Tree construction in TRILL.................................2
3. Issues with the TRILL tree construction algorithm..........2
4. Solution using the Affinity sub-TLV........................4
5. Network wide selection of computation algorithm............7
6. Relationship to draft-ietf-trill-resilient-trees...........7
7. Security Considerations....................................9
8. IANA Considerations........................................9
9. Informative References.....................................9
1. Introduction.
TRILL is a data center technology that uses link-state routing
mechanisms in a layer 2 setting, and serves as a replacement
for spanning-tree. TRILL uses trees rooted at pre-determined nodes
as a way to distribute multi-destination traffic. Multi-destination
traffic includes traffic such as layer-2 broadcast frames, unknown
unicast flood frames, and layer 2 traffic with multicast MAC
addresses (collectively referred to as BUM traffic). Multi-destination
traffic is typically hashed onto one of the available trees and sent
over the tree, potentially reaching all nodes in the network (hosts
behind which may own/need the packet in question).
2. Tree construction in TRILL.
Tree construction in TRILL is defined by [RFC6325], with additional
corrections defined in [RFC7780].
The tree construction mechanism used in TRILL codifies
certain tree construction steps which make the resultant trees
very brittle. Specifically, the parent selection mechanism in TRILL
causes problems in case of node failures. TRILL uses the following rule
- when constructing an SPF tree, if there are multiple possible
parents for a given node (i.e. if multiple upstream nodes can
potentially pull in a given node during SPF, all at the same
cumulative cost, then the parent selection is imposed in the
following manner):
[RFC6325]:
"When building the tree number j, remember all possible
equal cost parents for node N. After calculating the entire 'tree'
(actually, directed graph), for each node N, if N has 'p' parents,
then order the parents in ascending order according to the
7-octet IS-IS ID considered as an unsigned integer, and number them
starting at zero. For tree j, choose N's parent as choice j mod p."
There is an additional correction posted to this in [RFC7780]:
[RFC7780], Section 3.4:
"Section 4.5.1 of [RFC6325] specifies that, when building
distribution tree number j, node (RBridge) N that has multiple
possible parents in the tree is attached to possible parent
number j mod p. Trees are numbered starting with 1, but possible
parents are numbered starting with 0. As a result, if there are
two trees and two possible parents, then in tree 1 parent 1 will
be selected, and in tree 2 parent 0 will be selected.
This is changed so that the selected parent MUST be (j-1) mod p. As
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