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Internet Engineering Task Force Olivier Bonaventure
INTERNET DRAFT Pierre Francois
UCLouvain
Mike Shand
Stefano Previdi
cisco
February, 2006
Expires August, 2006
ISIS extensions for ordered FIB updates
<draft-bonaventure-isis-ordered-00.txt>
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document proposes extensions to allow IS-IS to support the
ordered convergence defined in [RTG-OFIB] that allows to avoid
transient forwarding loops during the FIB updates that follow a non-
urgent topology change.
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1 Introduction
In large ISP networks, topology changes are frequent events. When a
link metric changes, the router responsible for the changing link
floods a new LSP and forces all routers to recompute their routing
table and update their FIB. This routing convergence is that it can
lead to packet losses due to transient forwarding loops [RTG-OFIB].
There are various types of topology changes in IP networks. Sudden
failures of unprotected links are urgent and should be handled by
using a normal, fast, convergence of the link state routing protocol.
However, there are frequent topology changes that are non-urgent. For
example, operators often need to enable or disable links for short
periods of time during maintenance operations. Another example are
the IP networks where traffic engineering is performed by changing
the ISIS link metrics. These events are non-urgent and they should
not cause packet losses or transient forwarding loops. Transient
forwarding loops can be avoided by forcing the routers to orderly
update their FIB as proposed in [RTG-OFIB]
In this document, we propose IS-IS extensions allowing IS-IS routers
to implement the ordered FIB update described in [RTG-OFIB]. We first
describe the considered topology changes in section 2. Section 3
defines the new IS-IS TLVs that are used to support [RTG-OFIB].
2. Graceful topology changes
In this section, we describe two types of graceful topology changes.
The first one is the change of a link metric (increase or decrease).
The second one is the change of the settings of the overload bit (Set
to Unset or Unset to Set).
2.1 Change in link metric
The first change that we consider is a modification of the metric
associated to a link. This change can be a metric increase or a
metric decrease. A non-urgent link shutdown can be converted into a
weight change by doing the topology change in two steps. First, the
metric of the link is set to the highest possible value. This
triggers an ordered FIB update. After the FIB update, the link can
be safely removed without risking transient loops. Similarly, a
failure of a protected link [IPFRR] [MPLSFRR] can be handled as a
non-urgent failure.
2.2 Overload status change
The second change that we consider is the transition of the overload
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from Set to Unset or from Unset to Set.
3. Extensions to IS-IS
In this section, we describe the IS-IS extensions that are required
to support ordered FIB updates that allow to avoid transient loops.
There are two types of new TLVs. First, we define a capability sub-
TLV to allow a router to advertise its ability to support ordered FIB
updates. Second, we define a TLVs to be used inside the IS-IS Hello
PDUs to implement the completion messages defined in [RTG-OFIB].
3.1 Ordered FIB capability sub-TLV
We define a FIB capability sub-TLV (advertised in the IS-IS
CAPABILITY TLV defined in [ISIS-CAP] ) to allow routers to determine
which routers in the network support the ordered FIB updates. The FIB
capability sub-TLV is composed of 1 octet for the type, 1 octet
specifying the TLV length and a value field. The FIB capability TLV
is used to advertise the type of ordered FIB updates supported by
this router. The FIB capability sub-TLV shall only be flooded inside
the current area.
TYPE: TBD (Ordered FIB capability sub-TLV)
LENGTH: 1 byte
VALUE: Indicates the version of the ordered FIB update supported
by the router. This document defines version 1.
This ordered FIB capability sub-TLV must be sent in a router
capability TLV where the S bit is set to 0x0 and the D bit is set to
0x0 according to the leaking rules defined in [ISISCAP].
3.2 Completion message TLV
We define a new type of TLV to encode inside the IS-IS Hello PDUs the
completion messages proposed in [RTG-OFIB]. A IS-IS Hello PDU may
carry several completion messages. Each completion message is encoded
as a sub-TLV inside the completion message TLV.
Type TBD (Completion TLV)
Length # of bytes in the value field (variable)
Value : one or several sub-TLVs defined below
This document defines three types of sub-TLVs for the completion
messages.
3.2.1 Metric change sub-TLV
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The metric change sub-TLV shall be used as a completion message when
the metric of a link changes. This happens when a metric is changed
manually. It is also possible to use the mechanisms defined in [RTG-
OFIB] and this document for link failures. For example, when a link
is shutdown manually, the router could first advertise the link with
metric maxmetric-1 as a non-urgent change to let all routers apply
the ordered FIB update. Later, the link can be safely removed without
risking transient loops. A router could use the same technique when
one of its protected links fails.
We define two types of metric changes sub-TLV that are used inside
the completion TLV. The first one is used with wide metrics and the
second one with normal metrics. We expect that most deployments will
use wide metrics.
Type TBD (Wide Metric change sub-TLV)
Length # of bytes in the value field (21)
Value
No. of bytes
+-----------------------+
| 0 0 0 0 0 0 0 |Ack | 1
+-----------------------+
| Upstream Node Id | 7
+-----------------------+
| Downstream Node Id | 7
+-----------------------+
| Old metric | 3
+-----------------------+
| New metric | 3
+-----------------------+
Figure 1: New sub-TLV for (wide) metric change completion message
In the Wide metric change sub-TLV, the rightmost bit of the first
byte is used to indicate whether it corresponds to a completion
message (value 0) or an acknowledgment (value 1). The upstream and
downstream node Ids are the system identifiers of the link whose
metric changes. "Old metric" is the previous value of the wide metric
for this link and "New metric" the current value.
A similar sub-TLV is also defined for the normal metric. As current
implementations of IS-IS only support the default metric, we do not
consider changes in the delay, error or expense metrics.
Type TBD (Normal Metric change sub-TLV)
Length # of bytes in the value field (17)
Value
No. of bytes
+-----------------------+
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| 0 0 0 0 0 0 0 |Ack | 1
+-----------------------+
| Upstream Node Id | 7
+-----------------------+
| Downstream Node Id | 7
+-----------------------+
| Old Default metric | 1
+-----------------------+
| New Default metric | 1
+-----------------------+
Figure 2: New sub-TLV for (normal) metric change completion message
The Default metric shall be encoded as in normal LSPs.
3.2.2 Overload change sub-TLV
The overload change sub-TLV is used to encode the changes to the
Overload Bit.
Type TBD (Overload change sub-TLV)
Length # of octets in the value field (10)
Value
No. of bytes
+-----------------------+
|Old|New| 0 0 0 0 0 Ack | 1
+-----------------------+
| Node Id | 7
+-----------------------+
Figure 3: New sub-TLV for Overload bit change
The first octet contains three useful bits. The leftmost bit is the
old value of the overload bit, the next bit is the new value of the
overload bit and rightmost bit is the acknowledgment bit. The Node Id
is the system identifier of the node whose overload bit change
gracefully.
3.3 Utilization of the completion messages
The TLVs defined in sections 3.2 and 3.3 allow to implement the
completion messages specified in [RTG-OFIB]. Each sub-TLV corresponds
to a completion message for one topological change. The completion
TLV in one HELLO PDU may carry several sub-TLVs and thus several
completion messages. A router supporting the ordered FIB update
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defined in this document SHOULD send HELLO PDUs containing the
completion messages according to the rules defined in [RTG-OFIB].
To ensure a reliable delivery of the completion messages, an
acknowledgment scheme is used. Upon reception of a HELLO PDU
containing a completion message (rightmost bit of the first byte set
to 0 in the sub-TLV), a router MUST ensure that the next HELLO PDU
that it will send to this neighbor will contain, in the completion
TLV, the same sub-TLV but with the rightmost bit of the first byte
set to one. This sub-TLV is used to acknowledge the completion
message. A router MAY send a HELLO PDU immediately in response to a
receive completion message or wait for the next scheduled Hello
transmission. A router MAY retransmit the same completion message in
several HELLO PDUs to ensure that they are correctly received by the
neighbour.
5. Security considerations
This document raises no new security issues for IS-IS. For the
ordered FIB capability sub-TLV, the discussion in [ISISCAP] is
applicable. The authentication mechanisms defined in [RFC] can be
used to authenticate the Hello PDUs that carry the completion
messages if required.
6. IANA considerations
IANA will assign a new IS-IS sub-TLV code-point for the order FIB
capability sub-TLV that can appear inside the Router Capability TLV
defined in [ISISCAP].
IANA will assign a new IS-IS TLV code-point for the completion TLV.
This TLV can only appear inside Hello PDUs.
Name Value IIH LSP SNP
---------------------- ----- --- --- ---
Completion messages TLV TBD y n n
IANA will maintain a registry with the sub-TLVs defined for the
completion messages TLV. This document defines three sub-TLVs :
Name Value
---------------------- -----
Normal metric change sub-TLV TBD
Wide metric change sub-TLV TBD
Overload change sub-TLV TBD
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7. Conclusion
In this document, we have proposed a set of IS-IS TLVs that allows
routers using IS-IS to support the ordered FIB convergence
and the completion messages proposed in [RTG-OFIB].
References
[IS-IS] "Intermediate system to Intermediate system routeing
information exchange protocol for use in conjunction
with the Protocol for providing the Connectionless-mode
Network Service (ISO 8473)," ISO/IEC 10589:2002,
Second Edition.
[IPFRR] M. Shand, S. Bryant, IP Fast Reroute Framework,
draft-ietf-rtgwg-ipfrr-framework-04.txt, October 2005
[MPLSFRR] Pan, P. et al, "Fast Reroute Extensions to
RSVP-TE for LSP Tunnels", RFC 4090.
[PFOB05] P. Francois, O. Bonaventure. Avoiding transient loops
during IGP convergence. IEEE INFOCOM 2005, March 2005,
Miami, Fl., USA
[RTG-OFIB] P. Francois, O. Bonaventure, M. Shand, S. Previdi, S. Bryant,
Loop-free convergence using ordered FIB updates, Internet draft,
draft-francois-ordered-fib-00.txt, work in progress, February 2006
[ISISCAP] J.-P. Vasseur, S. Previdi, M. Shand, IS-IS extensions for
advertising router capabilities, Internet draft,
draft-vasseur-isis-caps-06.txt, February 2004, work in progress
[RFC3567] T. Li, R. Atkinson, Intermediate System to Intermediate
System (IS-IS), Cryptographic Authentication, July 2003, RFC3567
Authors Addresses
Olivier Bonaventure, Pierre Francois
Universite catholique de Louvain
Email: FirstName.LastName@uclouvain.be
URL : http://www.info.ucl.ac.be/people/OBO/
Stefano Previdi, Mike Shand
Cisco Systems
Email: {sprevidi,mshand}@cisco.com
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