MPLS Working Group Peter Ashwood-Smith
Internet Draft Bilel Jamoussi
Expiration Date: MAY 2003 Don Fedyk
Standards Track Darek Skalecki
Nortel Networks
Nov 2002
Improving Topology Data Base Accuracy with Label Switched Path
Feedback in Constraint Based Label Distribution Protocol
draft-ietf-mpls-te-feed-05.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
One key component of traffic engineering is a concept known as
constraint based routing. In constraint based routing a topology
database is maintained on all participating nodes. This database
contains a complete list of all the links in the network that
participate in traffic engineering and for each of these links a set
of constraints, which those links can meet. Bandwidth, for example,
is one essential constraint. Since the bandwidth available changes
as new LSPs are established and terminated the topology database
will develop inconsistencies with respect to the real network. It is
not possible to increase the flooding rates arbitrarily to keep the
database discrepancies from growing. A new mechanism is proposed
whereby a source node can learn about the successes or failures of
its path selections by receiving feedback from the paths it is
attempting. This information is most valuable in failure scenario's
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but is beneficial during other path setup functions as well. This
fed back information can be incorporated into subsequent route
computations, which greatly improves the accuracy of the overall
routing solution by significantly reducing the database
discrepancies.
Table of Contents
1.0 Introduction and Description . . . . . . . . . . . . . . . 2
2.0 Adding feedback TLVs to CR-LDP . . . . . . . . . . . . . . 6
2.1 Bandwidth directionality considerations. . . . . . . . 6
3.0 Link Feedback TLV. . . . . . . . . . . . . . . . . . . . . 7
3.1 Link feedback TLV Description . . . . . . . . . . . . . 7
3.2 Local Interface IP Address Subtypes . . . . . . . . . . 8
3.3 Remote Interface IP Address Subtypes . . . . . . . . .. 8
3.4 Unreserved Bandwidth Sub Type. . . . . . . . . . . . . 8
4.0 Detailed Procedures. . . . . . . . . . . . . . . . . . . . 9
5.0 IGP Considerations . . . . . . . . . . . . . . . . . . . .10
6.0 Future Considerations . . . . . . . . . . . . . . . . . .10
7.0 RSVP-TE Considerations . . . . . . . . . . . . . . . . . .11
8.0 Intellectual Property Considerations . . . . . . . . . . .11
9.0 Security Considerations. . . . . . . . . . . . . . . . . .11
10.0 IANA Considerations . . . . . . . . . . . . . . . . . . .11
11.0 Acknowledgements . . . . . . . . . . .. . . . . . . . . .11
12.0 Normative References. . . . . . . . . . . . . . . . . . .11
13.0 Informative References. . . . . . . . . . . . . . . . . .12
14.0 Authors Addresses . . . . . . . . . . . . . . . . . . . .12
1.0 Introduction and Description
Because the network is a distributed system, it is necessary to have
a mechanism to advertise information about links to all nodes in the
network [IS-IS], [OSPF]. A node can then build a topology map of
the network. This information is required to be as up-to-date as
possible for accurate traffic engineered paths. Information about
link or node failures must be rapidly propagated through the network
so that recovery can be initiated. Other information about links
that may be useful for reasons of quality of service includes
parameters such as available bandwidth, and delay. The information
in this topology database is often out of date with respect to the
real network. Available bandwidth is the most critical of these
attributes and it can drift substantially with respect to reality
due to the low frequency of link state updates that can be sustained
in a very large topology. The deviation in the topology database
available bandwidth is referred to as being optimistic if the
database shows more available bandwidth than there really is, or
pessimistic if the topology database shows less bandwidth than there
really is. This distinction is important to enable an efficient
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algorithm to deal with optimistic databases without resorting to
shorter flooding intervals.
One of the major problems for a constraint based routing system is
dealing with changing constraints. Obviously, since bandwidth is one
of the essential constraints, dealing with the rapid changes in
reserved bandwidth poses some interesting challenges. In smaller
networks, one can resort to higher frequency flooding but this
obviously does not scale. The feedback mechanism is particularly
useful in the case of link or node failures where the rapid change
and notification of resource change is crucial to the restoration
time. Feedback is work conserving in this case since the
availability of feedback information minimizes the extra burden of
dealing with out of date topology and resource information.
The basic approach is to add to the signaling protocol the ability
to piggyback actual link bandwidth availability information at every
link that the signaling traverses. This is done as part of the
reverse messaging on success or failure (mapping, release, withdraw
or notification). What this means is that every time signaling
messages flow backwards toward a source to tell it of the success,
failure or termination of a request, that message contains a
detailed slice of bandwidth availability information for the exact
path that the message has followed. This slice of reservation
information, which is very up to date, is received by the source
node and attached to the source node's topology database prior to
making any further source route computations. The result is that the
source node's topology database will tend to stay synchronized with
the slices of the network through which it is establishing paths.
This is nothing more than learning from successes and failures and
represents an intelligent alternative to either waiting for floods
or introducing non-determinism (guessing) into the source
algorithms. It is important to note that the fed-back data is never
re-flooded. It simply overrides flooded information for the purpose
of route computation until a superceding flood or fed-back value
arrives. As such, it is not actually inserted into a topology
database, most likely it simply is linked to that database as an
override used only by source route computations. Also the inclusion
of feedback information is optional. At a minimum the blocked or
failed link is required but if processing resources are scarce the
additional feedback at other hops is optional.
Operating a constraint based routing system without such feedback is
inefficient at best since a source node will continue to give out
incorrect route over and over again until it gets an IGP update.
This could be minutes away and as a result the worst case blocking
time for a new route is the minimum repeatable flooding interval
(often several minutes in big networks). Alternatives to feedback
mechanisms involve adding some non-determinism (randomness) to the
routing algorithm in the hopes that it will stumble onto a path that
works. These sorts of approaches are seen in ATM dynamic routing
systems, which do not have these forms of feedback.
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In order to get a good understanding of how the feedback works,
imagine a network with precisely one path (with sufficient
unreserved bandwidth) available from the source to the destination.
Further, imagine that the topology database at the source is
significantly out of date with respect to the real network in that
the source topology database sees sufficient bandwidth available on
many different routes to the destination. This is termed being
optimistic with respect to the network since the source thinks that
more bandwidth is available than there really is.
When such an optimistic source selects its first path it will likely
contain links that do not in reality have sufficient unreserved
bandwidth. Therefore, the path is only established up to the link
that does not have sufficient bandwidth. A notification message is
formatted that contains the actual unreserved bandwidth for this
blocking link which flows back toward the source, collapsing the
partially created path as it goes. In addition, at every link that
this notification traverses, the current unreserved bandwidth
information for each corresponding link is appended to the vector of
unreserved bandwidth along the path. In this manner, an accurate
view of the slice through the network traversed is constructed.
Eventually this message arrives back at the source node, where the
vector is taken and used to temporarily override the topology
database for route computations. This node has just learned from its
mistake and is now slightly less optimistic with respect to the real
network conditions.
Path selection can be attempted again but this time the node will
not make the same mistake it made the previous time. The link in
question, at which rejection occurred the first time, will not even
be eligible this time around, so a source route computation is
guaranteed to produce a different path (or none). The same procedure
may be repeated as many times as is necessary, each time learning
from its mistakes, until eventually no paths remain in the source
topology to the destination, or a path is found that works. This
tendency to converge either to a solution or determine that there is
no solution is an important property of a routing system (it
actually behaves a lot like a depth first search). This property is
not present with flooding mechanisms alone since the source node
must randomly hunt, or continually make the same mistakes, or abort
until the next flood arrives.
In addition to feeding back bandwidth on failure, feedback on
success is recommended. This has important consequences on our
ability to spread load or to spill over to new links as existing
links fill. It is true that spilling over to new links does not
require feedback on success since a node could simply wait for a
feedback on failure, but better load spreading can be achieved
earlier.
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Finally, when a path is torn down the release/withdraw messages also
contain bandwidth information that can be fed back to override the
source topology database. This is very important during failure
scenarios where the links required for rerouting the path share
common sub-segments with the failed path. Without the feedback, the
common sub-segments may not indicate sufficient available bandwidth
until an LSA flood is received which may mean many seconds without a
connection. With feedback at least the database is up to date with
respect to available bandwidth up to the point of failure in the
path. Also since failure involves many paths tearing down and re-
establishing this is the time that it is most critical to have an
accurate view.
When preemption is being employed it is also extremely important
that the topology database inconsistencies be small. If not, high
setup priority LSPs may unnecessarily preempt lower holding priority
LSPs to obtain bandwidth that, had they had a more up to date view
of unreserved bandwidth, they would have been able to find
elsewhere. Since preempted LSPs may in turn preempt other LSPs in a
domino like effect, the results of such database inconsistencies can
have wide reaching ripple like impacts. These feedback mechanisms
help reduce these occurrences significantly.
There are a number of network conditions where feedback shows its
value. One can think of a constraint-based network as being in one
of three conditions. The first is called ramp-up, this is when the
rate of arriving reservations exceeds the rate of departing
reservations. The second is called steady state, this is when the
rate of arriving reservations is about the same as the rate of
departing reservations. Finally, the ramp-down condition is that
which has a greater rate of departing reservations than arriving
reservations.
These three network conditions show distinctly different types of
error in the topology databases. In particular an optimistic view of
available bandwidth by a source node is characteristic of the ramp-
up condition of a network. A pessimistic view of available bandwidth
by a source node is characteristic of the ramp-down condition of a
network. If one plots the average error in the topology databases
with respect to the real network for the three different network
conditions, one will see the error slowly go positive during ramp
up, slowly go negative during ramp down, and drift slowly around 0
for the steady condition. The effect of flooding on this plot is to
periodically snap the error back to 0 at flooding intervals. The
effect of the feedback algorithm is to bring an optimistic error
back to zero without having to wait for the flood interval. On
average then, the feedback algorithm tends to halve the absolute
error, keeping it mostly negative or pessimistic. This makes sense
since a routing system will never give paths to links that it thinks
do not have resources and as a result its pessimistic view of the
world stays that way until it gets a flood. This relieves the IGP
updates of the most urgent requirement of flooding when bandwidth is
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consumed. Availability of new bandwidth occurs when paths are
released or new links become available. Floods already accompany
new links. Significant releases of bandwidth can be broadcast at
relatively low frequencies in the order of several minutes with
little operational impact.
2.0 Adding feedback TLVs to CR-LDP
Two new TLVs are optionally added to the CR-LDP mapping,
notification, and withdraw messages. There may be an arbitrary
number of these TLV in any order or position in the message. It is
recommended that they be placed such that they can be read and
applied to override the topology database by scanning the message
forwards and walking the topology database from the point where the
last link feedback TLV left off.
Each TLV consists of the eight unreserved bandwidth values for each
holding priority 0 through 7 as IEEE floating-point numbers (the
units are unidirectional bytes per second). Following this are the
IP addresses of the two ends of the interface. Two TLVs are
possible, one for IPV4 and one for IPV6 addressing of the link.
Note: the feedback TLVs may also optionally be included in query or
query-reply messages in response to bandwidth update queries from an
LER. Details of this mechanism are provided in [QUERY].
2.1 Bandwidth directionality considerations
The order of the two addresses in the feedback TLV implies the
direction in which the bandwidth is available. For example if the
first address is A and the second address is B the bandwidth is
unreserved in the A to B direction.
It is possible for an implementation to provide both the A to B
direction and the B to A direction as part of the same feedback
message. This is done by simply including a TLV with A, B as the
addresses of the link and a different TLV with B, A as the addresses
of the link. Should CR-LDP evolve to be able to support bi-
directional traffic flow and reservations, it is expected that bi-
directional feedback would also be implemented via this mechanism.
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3.0 Link Feedback TLV
The Feedback payload consists of one or more nested
Type/Length/Value (TLV) triplets for extensibility. The format of
each TLV is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Length field defines the length of the value portion in octets
(thus a TLV with no value portion would have a length of zero). The
TLV is padded to four-octet alignment; padding is not included in
the length field (so a three octet value would have a length of
three, but the total size of the TLV would be eight octets). Nested
TLVs are also 32-bit aligned. Unrecognized types are ignored.
3.1 Link feedback TLV Description
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|F| TBD IANA | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub TLVs |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This document introduces essentially one Feedback TLV. There may be
multiple instances of the Feedback TLV in a CR-LDP message, one for
each different links along the path. Due to the current format that
TE extension documents organize TE information the feedback TLV has
sub TLVs. This allows the information to conform to the current TE
conventions and allows options for additional future feedback
elements. The formats are derived from the TE extensions TLVs for
IS-IS [IS-IS] and OSPF [OSPF].
U bit
Unknown TLV bit must be set to 1. As with all CR-LDP messages, upon
receipt of an unknown TLV, if U is if U is set (=1), the unknown TLV
is silently ignored and the rest of the message is processed as if
the unknown TLV did not exist.
F bit
Forward unknown TLV bit must be set to 1. This bit applies only
when the U bit is set and the CR-LDP message containing the unknown
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TLV is to be forwarded. If F is set (=1), the unknown TLV is
forwarded with the containing message.
The following sub tlvs for the Feedback TLV are defined:
1 - Local interface IP address (IPv4)
2 - Remote interface IP address (IPv4)
3 - Local interface IP address (IPv6)
4 - Remote interface IP address (IPv6)
5 - Unreserved bandwidth
This document defines the sub types. The code points are to be
assigned by IANA.
3.2 Local Interface IP Address Sub Types
The Local Interface IP Address sub-TLV specifies the IP address(es)
of the interface corresponding to this link. Normally there will
only be one address. If there are multiple
local addresses on the link, they are all listed in this sub-TLV.
The Local Interface IPv4 Address sub-TLV is TLV type 1, and is 4N
octets in length, where N is the number of local addresses.
The Local Interface IPv6 Address sub-TLV is TLV type 3, and is 16N
octets in length, where N is the number of local addresses.
3.3 Remote Interface IP Address Sub Types
The Remote Interface IP Address sub-TLV specifies the IP address(es)
of the neighbor's interface corresponding to this link. This and
the local address are used to discern multiple parallel links
between systems.
The Remote Interface IPv4 Address sub-TLV is TLV type 2, and is 4N
octets in length, where N is the number of neighbor addresses.
The Remote Interface IPv6 Address sub-TLV is TLV type 4, and is 16N
octets in length, where N is the number of neighbor addresses.
3.4 Unreserved Bandwidth Sub Type
The Unreserved Bandwidth sub-TLV specifies the amount of bandwidth
not yet reserved at each of the eight priority levels, in IEEE
floating point format. The values correspond to the bandwidth that
can be reserved with a setup priority of 0 through 7, arranged in
increasing order with priority 0 occurring at the start of the sub-
TLV, and priority 7 at the end of the sub-TLV. The units are bytes
per second.
The Unreserved Bandwidth sub-TLV is TLV type 5, and is 32 octets in
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length.
4.0 Detailed Procedures
On receipt of a withdraw, notification, query-reply, or mapping
message pertaining to a request made by CR-LDP (as opposed to LDP),
a feedback TLV of the appropriate format for the interface over
which the message was received is inserted into the message before
forwarding it back to the source of the request. The interface's
local and remote interface address in the appropriate format are
placed in the TLV.
The eight bandwidth values are filled in with the outgoing bandwidth
available on this interface for each of the eight holding priorities
in bytes per second.
On receipt of a CR-LDP request message, which cannot be satisfied, a
notification message is formatted normally. The Feedback TLV with
the local and remote interface address in the appropriate format and
the eight bandwidth values are filled in with the outgoing bandwidth
available on this interface for each of the eight holding priorities
in bytes per second.
On receipt of a CR-LDP request message which has been satisfied and
which results in a mapping being generated. No feedback TLV is added
since the previous node will insert the proper TLV when it receives
the reverse flowing mapping.
When an LDP session goes down either because of a link failure,
TCP/IP timeout, keep alive timeout, adjacency timeout etc. Other LDP
sessions in the module must generate either notification, withdraw
or release messages for LSPs that traversed the LDP in question. In
the case that the LSP was created by CR-LDP and that a withdraw or
notification is about to be generated, LDP will insert a feedback
TLV for the interface which just went down that contains 0's for all
the bandwidth values and attach to it the proper interface
addresses. Where LDP FT procedures [LDP-FT] are in use, LSPs that
are protected by FT procedures should not be torn down until after
session reestablishment has failed. During LDP re-establishment
time new connections may be queued and delayed for the re-
establishment time. If signaling delay is undesirable feedback may
be used to report zero bandwidth. In this case, if LDP is
successfully re-established a Link LSA should be triggered if
sufficient amount bandwidth is available.
When the LDP session that originated a CR-LDP label request receives
a mapping that contains feedback TLV's it is recommended that these
bandwidth values supersede the corresponding values in the node's
topology database for source route computations. Doing so permits
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this node to immediately synchronize its topology with respect to
the real bandwidth reservations along the path that was just
established.
When the LDP session that originated a CR-LDP label request receives
a notification that contains feedback TLV's it is recommended that
these bandwidth values supersede the corresponding values in the
node's topology database for source route computations. Doing so
permits this node to immediately synchronize its topology with
respect to the real bandwidth reservations along the path that just
failed to establish. The source node may then re-compute a path
knowing that the computation will take into account the failure if
it was caused by the topology database being in error with respect
to the real network state.
5.0 IGP considerations
Implementations MUST NOT permit bandwidth information learned by
this feedback mechanism to be re-flooded via IS-IS, OSPF or any
other IGP. The bandwidth information learned via these feedback
mechanisms is to be used ONLY for source route computations on the
nodes that are directly on the path that fed back the bandwidth.
Normally only the source node of the LSP, or perhaps intermediate
gateway nodes will use this information. It is however permitted for
intermediate nodes that are forwarding this feedback information to
store it for their own local source route computations.
There is a possibility of a race condition between the bandwidth
information that is received via feedback and that, which is
received via a normal IGP flood. While there may be a discrepancy
between the two, both are within a few 100 milliseconds of being
correct. Solutions to allow us to determine which information is
most up to date (say by adding a sequence number) do not add any
significant benefit. Constraint based, source routed systems will
always have errors in the local topology database with respect to
the real network. These errors can be reduced through reduced
flooding intervals, path following feedback and selective flooding
but realistically the errors cannot be reduced below the second or
so range. As a result propagation delay order race conditions are
noise with respect to the average expected errors. An implementation
SHOULD therefore consider the most recently received update (IGP or
feedback) as being the most up to date.
6.0 Future considerations
Constraint based routing systems such as CR-LDP will in the future
offer other forms of constraint than simply reserved bandwidth.
Actual utilization levels, current congestion levels, number of
discrete channels/wavelengths available etc. are all possible
constraints that change rapidly and which must be taken into
consideration when computing a route. It is expected that this
mechanism will be used to feedback these and other new forms of link
constraining data.
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7.0 RSVP-TE consideration
Nothing precludes the use of such feedback mechanisms with a similar
TLV structure in the RSVP-TE Resv and other reverse flowing messages
although repeatedly applying unchanged feedback should be avoided.
This could be accomplished by a simple rule that only permits
feedback information on the original RESV, not on subsequent
refreshes. This document only covers the CR-LDP protocol.
8.0 Intellectual Property Consideration
The IETF has been notified of intellectual property rights claimed
in regard to some or all of the specification contained in this
document. For more information consult the online list of claimed
rights.
9.0 Security Considerations
This document covers an additional data structure, a TLV to an
existing LDP message. Therefore the security aspects of this are the
same as a LDP. CR-LDP inherits the same security mechanism described
in Section 4.0 of [LDP] to protect against the introduction of
spoofed TCP segments into LDP session connection streams.
10.0 IANA Considerations
The Feedback TLV as well as Types for sub-TLVs in a Feedback TLV are
to be registered with IANA.
[RFC3212] defines the CR-LDP TLV name space. This memo requires
assignment of one TLV Type from that range.
Also, sub-Types of a Feedback TLV need to be assigned by IANA. The
types from 6 to 32767 are by expert review controlled by IANA. The
types 32768 to 65535 are reserved for private use.
11.0 Acknowledgments
The authors would like to thank Keith Dysart for his guidance and
Jerzy Miernik for helping implement these concepts and bringing them
to life. The authors' would like to acknowledge Dave Allan for his
comments and suggestions.
12.0 Normative References
[RFC3212] Jamoussi, B. et al., "Constraint-Based LSP Setup using
LDP", RFC 3212, January 2002.
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[LDP] Andersson, L. et al., "LDP Specification", RFC 3032, January
2001.
[IS-IS] Li, T., Smit, H., "Extensions to IS-IS for traffic
engineering", Internet Draft, draft-ietf-isis-traffic-04.txt, August
2001.
[OSPF] Katz,D., Yeung, D., Kompella, K., " Traffic Engineering
Extensions to OSPF Version 2," draft-katz-yeung-ospf-traffic-08.txt,
September 2002.
13.0 Informative References
[QUERY] Ashwood-Smith, P., Paraschiv, A., " Multi Protocol Label
Switching Label Distribution Protocol Query Message Description
", Internet Draft, draft-anto-ldp-query-04.txt, May 2002.
[LDP-FT] Farrel, A et al., " Fault Tolerance for the Label
Distribution Protocol (LDP)", Internet Draft, draft-ietf-mpls-ldp-
ft-06.txt, September 2002
14.0 Author's Addresses
Peter Ashwood-Smith Bilel Jamoussi
Nortel Networks Corp. Nortel Networks Corp.
P.O. Box 3511 Station C, 600 Technology Park Drive
Ottawa, ON K1Y 4H7 Billerica, MA 01821
Canada USA
Phone: +1 613-763-4534 phone: +1 978-288-4506
petera@nortelnetworks.com jamoussi@nortelnetworks.com
Darek Skalecki Don Fedyk
Nortel Networks Corp. Nortel Networks Corp.
P.O. Box 3511 Station C, 600 Technology Park Drive
Ottawa, On K1Y 4H7 Billerica, MA 01821
Canada USA
Phone: +1 613-765-2252 Phone: +1 978-228-3041
dareks@nortelnetworks.com dwfedyk@nortelnetworks.com
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