draft-ietf-mpls-te-feed-03.txt   draft-ietf-mpls-te-feed-04.txt 
MPLS Working Group Peter Ashwood-Smith MPLS Working Group Peter Ashwood-Smith
Internet Draft Bilel Jamoussi Internet Draft Bilel Jamoussi
Expiration Date: May 2002 Don Fedyk Expiration Date: Nov 2002 Don Fedyk
Darek Skalecki Darek Skalecki
Nortel Networks Nortel Networks
November 2001 May 2002
Improving Topology Data Base Accuracy with LSP Feedback Improving Topology Data Base Accuracy with LSP Feedback in CR-LDP
draft-ietf-mpls-te-feed-03.txt draft-ietf-mpls-te-feed-04.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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One key component of traffic engineering is a concept known as One key component of traffic engineering is a concept known as
constraint based routing. In constraint based routing a topology constraint based routing. In constraint based routing a topology
database is maintained on all participating nodes. This database database is maintained on all participating nodes. This database
contains a complete list of all the links in the network that contains a complete list of all the links in the network that
participate in traffic engineering and for each of these links a set participate in traffic engineering and for each of these links a set
of constraints, which those links can meet. Bandwidth, for example, of constraints, which those links can meet. Bandwidth, for example,
is one essential constraint. Since the bandwidth available changes is one essential constraint. Since the bandwidth available changes
as new LSPs are established and terminated the topology database as new LSPs are established and terminated the topology database
will develop inconsistencies with respect to the real network. It is will develop inconsistencies with respect to the real network. It is
not possible to increase the flooding rates arbitrarily to keep the not possible to increase the flooding rates arbitrarily to keep the
database discrepancies from growing. We propose a new mechanism database discrepancies from growing. A new mechanism is proposed
whereby a source node can learn about the successes or failures of whereby a source node can learn about the successes or failures of
its path selections by receiving feedback from the paths it is its path selections by receiving feedback from the paths it is
attempting. This fed-back information can be incorporated into attempting. This information is most valuable in failure scenarious
subsequent route computations, which greatly improves the accuracy but is benificial during other path setup functions as well. This
of the overall routing solution by significantly reducing the fed-back information can be incorporated into subsequent route
database discrepancies. computations, which greatly improves the accuracy of the overall
routing solution by significantly reducing the database
discrepancies.
1. Introduction 1. Introduction
Because the network is a distributed system, it is necessary to have Because the network is a distributed system, it is necessary to have
a mechanism to advertise information about links to all nodes in the 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 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 the network. This information is required to be as up-to-date as
possible for accurate traffic engineered paths. Information about possible for accurate traffic engineered paths. Information about
link or node failures must be rapidly propagated through the network link or node failures must be rapidly propagated through the network
so that recovery can be initiated. Other information about links so that recovery can be initiated. Other information about links
that may be useful for reasons of quality of service includes that may be useful for reasons of quality of service includes
parameters such as available bandwidth, and delay. The information parameters such as available bandwidth, and delay. The information
in this topology database is often out of date with respect to the in this topology database is often out of date with respect to the
real network. Available bandwidth is the most critical of these real network. Available bandwidth is the most critical of these
attributes and it can drift substantially with respect to reality attributes and it can drift substantially with respect to reality
due to the low frequency of link state updates that can be sustained due to the low frequency of link state updates that can be sustained
in a very large topology. We refer to the deviation in the topology in a very large topology. The deviation in the topology database
database available bandwidth as being optimistic if the database available bandwidth is refered to as being optimistic if the
shows more available bandwidth than there really is, or pessimistic database shows more available bandwidth than there really is, or
if the topology database shows less bandwidth than there really is. pessimistic if the topology database shows less bandwidth than there
This distinction is important because we shall propose an efficient really is. This distinction is important to enable an efficient
algorithm to deal with optimistic databases without resorting to algorithm to deal with optimistic databases without resorting to
shorter flooding intervals. shorter flooding intervals.
One of the major problems for a constraint based routing system is One of the major problems for a constraint based routing system is
dealing with changing constraints. Obviously, since bandwidth is one dealing with changing constraints. Obviously, since bandwidth is one
of the essential constraints, dealing with the rapid changes in of the essential constraints, dealing with the rapid changes in
reserved bandwidth poses some interesting challenges. In smaller reserved bandwidth poses some interesting challenges. In smaller
networks, one can resort to higher frequency flooding but this networks, one can resort to higher frequency flooding but this
obviously does not scale. obviously does not scale. The feedback mechanism is particularily
useful in the case of link or node failures where the rapid change
and notification of resource change is crutuail 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 proposal is to add to the signaling protocol the ability The basic proposal is to add to the signaling protocol the ability
to piggyback actual link bandwidth availability information at every to piggyback actual link bandwidth availability information at every
link that the signaling traverses. This is done as part of the link that the signaling traverses. This is done as part of the
reverse messaging on success or failure (mapping, release, withdraw reverse messaging on success or failure (mapping, release, withdraw
or notification). What this means is that every time signaling or notification). What this means is that every time signaling
messages flow backwards toward a source to tell it of the success, messages flow backwards toward a source to tell it of the success,
failure or termination of a request, that message contains a failure or termination of a request, that message contains a
detailed slice of bandwidth availability information for the exact detailed slice of bandwidth availability information for the exact
path that the message has followed. This slice of reservation path that the message has followed. This slice of reservation
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source node's topology database will tend to stay synchronized with source node's topology database will tend to stay synchronized with
the slices of the network through which it is establishing paths. the slices of the network through which it is establishing paths.
This is nothing more than learning from successes and failures and This is nothing more than learning from successes and failures and
represents an intelligent alternative to either waiting for floods represents an intelligent alternative to either waiting for floods
or introducing non-determinism (guessing) into the source or introducing non-determinism (guessing) into the source
algorithms. It is important to note that the fed-back data is never algorithms. It is important to note that the fed-back data is never
re-flooded. It simply overrides flooded information for the purpose re-flooded. It simply overrides flooded information for the purpose
of route computation until a superceding flood or fed-back value of route computation until a superceding flood or fed-back value
arrives. As such, it is not actually inserted into a topology arrives. As such, it is not actually inserted into a topology
database, most likely it simply is linked to that database as an database, most likely it simply is linked to that database as an
override used only by source route computations. 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 Operating a constraint based routing system without such feedback is
inefficient at best since a source node will continue to give out inefficient at best since a source node will continue to give out
incorrect route over and over again until it gets an IGP update. 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 This could be minutes away and as a result the worst case blocking
time for a new route is the minimum repeatable flooding interval time for a new route is the minimum repeatable flooding interval
(often several minutes in big networks). Alternatives to feedback (often several minutes in big networks). Alternatives to feedback
mechanisms involve adding some non-determinism (randomness) to the mechanisms involve adding some non-determinism (randomness) to the
routing algorithm in the hopes that it will stumble onto a path that routing algorithm in the hopes that it will stumble onto a path that
works. These sorts of approaches are seen in ATM dynamic routing works. These sorts of approaches are seen in ATM dynamic routing
systems, which do not have these forms of feedback. systems, which do not have these forms of feedback.
In order to get a good understanding of how the feedback works, In order to get a good understanding of how the feedback works,
imagine a network with precisely one path (with sufficient imagine a network with precisely one path (with sufficient
unreserved bandwidth) available from the source to the destination. unreserved bandwidth) available from the source to the destination.
Further, imagine that the topology database at the source is Further, imagine that the topology database at the source is
significantly out of date with respect to the real network in that significantly out of date with respect to the real network in that
the source topology database sees sufficient bandwidth available on the source topology database sees sufficient bandwidth available on
many different routes to the destination. We call this being many different routes to the destination. This is termed being
optimistic with respect to the network since the source thinks that optimistic with respect to the network since the source thinks that
more bandwidth is available than there really is. more bandwidth is available than there really is.
When such an optimistic source selects its first path it will likely When such an optimistic source selects its first path it will likely
contain links that do not in reality have sufficient unreserved contain links that do not in reality have sufficient unreserved
bandwidth. Therefore, the path is only established up to the link bandwidth. Therefore, the path is only established up to the link
that does not have sufficient bandwidth. A notification message is that does not have sufficient bandwidth. A notification message is
formatted that contains the actual unreserved bandwidth for this formatted that contains the actual unreserved bandwidth for this
blocking link which flows back toward the source, collapsing the blocking link which flows back toward the source, collapsing the
partially created path as it goes. In addition, at every link that partially created path as it goes. In addition, at every link that
this notification traverses, the current unreserved bandwidth this notification traverses, the current unreserved bandwidth
information for each corresponding link is appended to the vector of information for each corresponding link is appended to the vector of
unreserved bandwidth along the path. In this manner, an accurate unreserved bandwidth along the path. In this manner, an accurate
view of the slice through the network we are traversing is view of the slice through the network traversed is constructed.
constructed. Eventually this message arrives back at the source Eventually this message arrives back at the source node, where the
node, where the vector is taken and used to temporarily override the vector is taken and used to temporarily override the topology
topology database for route computations. This node has just learned database for route computations. This node has just learned from its
from its mistake and is now slightly less optimistic with respect to mistake and is now slightly less optimistic with respect to the real
the real network conditions. network conditions.
Path selection can be attempted again but this time the node will 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 not make the same mistake it made the previous time. The link in
question, at which rejection occurred the first time, will not even question, at which rejection occurred the first time, will not even
be eligible this time around, so a source route computation is be eligible this time around, so a source route computation is
guaranteed to produce a different path (or none). The same procedure guaranteed to produce a different path (or none). The same procedure
may be repeated as many times as is necessary, each time learning may be repeated as many times as is necessary, each time learning
from its mistakes, until eventually no paths remain in the source from its mistakes, until eventually no paths remain in the source
topology to the destination, or we actually find a path that works. topology to the destination, or a path is found that works. This
This tendency to converge either to a solution or determine that tendency to converge either to a solution or determine that there is
there is no solution is an important property of a routing system no solution is an important property of a routing system (it
(it actually behaves a lot like a depth first search). This property actually behaves a lot like a depth first search). This property is
is not present with flooding mechanisms alone since the source node not present with flooding mechanisms alone since the source node
must randomly hunt, or continually make the same mistakes, or abort must randomly hunt, or continually make the same mistakes, or abort
until the next flood arrives. until the next flood arrives.
In addition to feeding back bandwidth on failure, we also recommend In addition to feeding back bandwidth on failure, feedback on
feedback on success. This has important consequences on our ability success is recommended. This has important consequences on our
to spread load or to spill over to new links as existing links fill. ability to spread load or to spill over to new links as existing
It is true that spilling over to new links does not require feedback links fill. It is true that spilling over to new links does not
on success since we could simply wait for a feedback on failure, but require feedback on success since a node could simply wait for a
we can achieve better load spreading earlier. feedback on failure, but better load spreading can be achieved
earlier.
Finally, when a path is torn down the release/withdraw messages also Finally, when a path is torn down the release/withdraw messages also
contain bandwidth information that can be fed back to override the contain bandwidth information that can be fed back to override the
source topology database. This is very important during failure source topology database. This is very important during failure
scenarios where the links we need to use to reroute the path share scenarios where the links required for rerouting the path share
common sub-segments with the failed path. Without the feedback, the common sub-segments with the failed path. Without the feedback, the
common sub-segments may not indicate sufficient available bandwidth common sub-segments may not indicate sufficient available bandwidth
until we get a flood that may mean many seconds without a until an LSA flood is received which may mean many seconds without a
connection. With feedback at least we will be up to date with connection. With feedback at least the database is up to date with
respect to available bandwidth up to the point of failure in the respect to available bandwidth up to the point of failure in the
path. Also since failure involves many paths tearing down and re- path. Also since failure involves many paths tearing down and re-
establishing this is the time that it is most critical to have an establishing this is the time that it is most critical to have an
accurate view. accurate view.
When preemption is being employed it is also extremely important When preemption is being employed it is also extremely important
that the topology database inconsistencies be small. If not, high that the topology database inconsistencies be small. If not, high
setup priority LSPs may unnecessarily preempt lower holding priority setup priority LSPs may unnecessarily preempt lower holding priority
LSPs to obtain bandwidth that, had they had a more up to date view LSPs to obtain bandwidth that, had they had a more up to date view
of unreserved bandwidth, they would have been able to find of unreserved bandwidth, they would have been able to find
elsewhere. Since preempted LSPs may in turn preempt other LSPs in a elsewhere. Since preempted LSPs may in turn preempt other LSPs in a
domino like effect, the results of such database inconsistencies can domino like effect, the results of such database inconsistencies can
have wide reaching ripple like impacts. These feedback mechanisms have wide reaching ripple like impacts. These feedback mechanisms
help reduce these occurrences significantly. help reduce these occurrences significantly.
There are a number of network conditions where feedback shows its There are a number of network conditions where feedback shows its
value. One can think of a constraint-based network as being in one 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 of three conditions. The first is called ramp-up, this is when the
rate of arriving reservations exceeds the rate of departing rate of arriving reservations exceeds the rate of departing
reservations. The second is called steady-state, this is when the reservations. The second is called steady state, this is when the
rate of arriving reservations is about the same as the rate of rate of arriving reservations is about the same as the rate of
departing reservations. Finally, the ramp-down condition is that departing reservations. Finally, the ramp-down condition is that
which has a greater rate of departing reservations than arriving which has a greater rate of departing reservations than arriving
reservations. reservations.
These three network conditions show distinctly different types of These three network conditions show distinctly different types of
error in the topology databases. In particular an optimistic view of error in the topology databases. In particular an optimistic view of
available bandwidth by a source node is characteristic of the ramp- available bandwidth by a source node is characteristic of the ramp-
up condition of a network. A pessimistic view of available bandwidth up condition of a network. A pessimistic view of available bandwidth
by a source node is characteristic of the ramp-down condition of a by a source node is characteristic of the ramp-down condition of a
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periodically snap the error back to 0 at flooding intervals. The periodically snap the error back to 0 at flooding intervals. The
effect of the feedback algorithm is to bring an optimistic error effect of the feedback algorithm is to bring an optimistic error
back to zero without having to wait for the flood interval. On back to zero without having to wait for the flood interval. On
average then, the feedback algorithm tends to halve the absolute average then, the feedback algorithm tends to halve the absolute
error, keeping it mostly negative or pessimistic. This makes sense error, keeping it mostly negative or pessimistic. This makes sense
since a routing system will never give paths to links that it thinks 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 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 world stays that way until it gets a flood. This relieves the IGP
updates of the most urgent requirement of flooding when bandwidth is updates of the most urgent requirement of flooding when bandwidth is
consumed. Availability of new bandwidth occurs when paths are consumed. Availability of new bandwidth occurs when paths are
released or new links become available. New links are already released or new links become available. Floods already accompany
accompanied by floods. Significant releases of bandwidth can be new links. Significant releases of bandwidth can be broadcast at
broadcast at relatively low frequencies in the order of several relatively low frequencies in the order of several minutes with
minutes with little operational impact. little operational impact.
Extensive operational experience with this feedback protocol in Extensive operational experience with this feedback protocol in
proprietary Nortel Networks (pre-standard CR-LDP) products has shown proprietary Nortel Networks (pre-standard CR-LDP) products has shown
it to work very well for networks up to 1000 nodes with significant it to work very well for networks up to 1000 nodes with significant
flooding intervals damped to several minutes. Without this protocol, flooding intervals damped to several minutes. Without this protocol,
these networks would block setups for up to several minutes. With these networks would block setups for up to several minutes. With
this protocol, the blocking in most cases is reduced to a small this protocol, the blocking in most cases is reduced to a small
number of retry attempts which is usually sub-second depending number of retry attempts which is usually sub-second depending
mostly on the propagation delays in the network. mostly on the propagation delays in the network.
These feedback algorithms have been particularly beneficial in cases These feedback algorithms have been particularly beneficial in cases
of failure recovery during which the network is in a sudden of failure recovery during which the network is in a sudden
condition of ramp-up. Since a large number of reservations must be condition of ramp-up. Since a large number of reservations must be
remade, it is highly likely that we will exceed the limits of remade, it is highly likely that the bandwidth reservation limits of
certain key links in the network. Without feedback, the rerouting certain key links in the network will be exceeded. Without feedback,
must block until a flood arrives telling us of the situation at the rerouting must block until a flood arrives telling us of the
those key links at which time rerouting can continue. With feedback, situation at those key links at which time rerouting can continue.
the rerouting simply continues until a feedback indicates that a With feedback, the rerouting simply continues until a feedback
link is full. In addition since reservation-balancing algorithms are indicates that a link is full. In addition since reservation-
also often used, feedback allows the balancing algorithms to make balancing algorithms are also often used, feedback allows the
better distribution decisions based on immediate feedback. balancing algorithms to make better distribution decisions based on
immediate feedback.
We have also explored through simulation and implementation a We have also explored through simulation and implementation a
variety of mechanisms to deal with the pessimistic error in the variety of mechanisms to deal with the pessimistic error in the
database. One simple proposal is to use selective forgetting. In database. One simple proposal is to use selective forgetting. In
this algorithm, a reserved bandwidth value slowly drops back to zero this algorithm, a reserved bandwidth value slowly drops back to zero
over a relatively short time interval. The theory being that you over a relatively short time interval. The theory being that you
shift the network back to an optimistic state (by forgetting your shift the network back to an optimistic state (by forgetting your
pessimism) where the feedback algorithm will again correctly pessimism) where the feedback algorithm will again correctly
operate. These algorithms have not shown any great advantage and are operate. These algorithms have not shown any great advantage and are
actually non-optimal when the error is purely optimistic. actually non-optimal when the error is purely optimistic.
Other algorithmic permutations we have explored include such Other algorithmic permutations explored include such variations as:
variations as:
Feeding-back to all intermediate nodes, information learned from Feeding-back to all intermediate nodes, information learned from
control messages upstream of that intermediate node. control messages upstream of that intermediate node.
Feeding back in both directions so that both the source and Feeding back in both directions so that both the source and
destination node's databases stay synchronized. destination node's databases stay synchronized.
Allowing a request to continue to its destination despite there Allowing a request to continue to its destination despite there
being insufficient bandwidth at some intermediate hop. Then, being insufficient bandwidth at some intermediate hop. Then,
rejecting the request with a full bandwidth vector slice all the way rejecting the request with a full bandwidth vector slice all the way
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2. Adding feedback TLVs to CR -LDP 2. Adding feedback TLVs to CR -LDP
Two new TLVs are optionally added to the CR-LDP mapping, Two new TLVs are optionally added to the CR-LDP mapping,
notification, and withdraw messages. There may be an arbitrary notification, and withdraw messages. There may be an arbitrary
number of these TLV in any order or position in the message. It is 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 recommended that they be placed such that they can be read and
applied to override the topology database by scanning the message applied to override the topology database by scanning the message
forwards and walking the topology database from the point where the forwards and walking the topology database from the point where the
last link feedback TLV left off. last link feedback TLV left off.
Each TLV consists of the 8 unreserved bandwidth values for each Each TLV consists of the eightunreserved bandwidth values for each
holding priority 0 through 7 as IEEE floating point numbers (the holding priority 0 through 7 as IEEE floating-point numbers (the
units are unidirectional bytes per second). Following this are the units are unidirectional bytes per second). Following this are the
IP addresses of the two ends of the interface. Two TLVs are IP addresses of the two ends of the interface. Two TLVs are
possible, one for IPV4 and one for IPV6 addressing of the link. possible, one for IPV4 and one for IPV6 addressing of the link.
Note: the feedback TLVs may also optionally be included in query or Note: the feedback TLVs may also optionally be included in query or
query-reply messages in response to bandwidth update queries from an query-reply messages in response to bandwidth update queries from an
LER. Details of this mechanism are provided in [QUERY]. LER. Details of this mechanism are provided in [QUERY].
2.1 Bandwidth directionality considerations 2.1 Bandwidth directionality considerations
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addresses of the link and a different TLV with B,A as the addresses 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- of the link. Should CR-LDP evolve to be able to support bi-
directional traffic flow and reservations it is expected that bi- directional traffic flow and reservations it is expected that bi-
directional feedback would also be implemented via this mechanism. directional feedback would also be implemented via this mechanism.
3. IPV4 specified link feedback TLV 3. IPV4 specified link feedback TLV
0 1 2 3 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 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| 0x830 | Length | |U|F| TBD IANA | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BANDWIDTH UNRESERVED AT HOLDING PRIORITY 0 (IEEE float) | | BANDWIDTH UNRESERVED AT HOLDING PRIORITY 0 (IEEE float) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. . . . . . . . . . . . . . . .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BANDWIDTH UNRESERVED AT HOLDING PRIORITY 7 (IEEE float) | | BANDWIDTH UNRESERVED AT HOLDING PRIORITY 7 (IEEE float) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPV4 address of interface (near end) | | IPV4 address of interface (near end) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPV4 address of interface (far end) | | IPV4 address of interface (far end) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4. IPV6 specified link feedback TLV 4. IPV6 specified link feedback TLV
0 1 2 3 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 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| 0x831 | Length | |U|F| TBD IANA | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BANDWIDTH UNRESERVED AT HOLDING PRIORITY 0 (IEEE float) | | BANDWIDTH UNRESERVED AT HOLDING PRIORITY 0 (IEEE float) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. . . . . . . . . . . . . . . .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BANDWIDTH UNRESERVED AT HOLDING PRIORITY 7 (IEEE float) | | BANDWIDTH UNRESERVED AT HOLDING PRIORITY 7 (IEEE float) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPV6 address of interface (near end) | | IPV6 address of interface (near end) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ........ | | ........ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPV6 address of interface (far end) | | IPV6 address of interface (far end) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ........ | | ........ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5. Detailed Procedures 5. Detailed Procedures
On receipt of a withdraw, notification, query-reply, or mapping On receipt of a withdraw, notification, query-reply, or mapping
message pertaining to a request made by CR-LDP (as opposed to LDP), message pertaining to a request made by CR-LDP (as opposed to LDP),
a feedback TLV of the appropriate format for the interface over a feedback TLV of the appropriate format for the interface over
which the message was received is inserted into the message before which the message was received is inserted into the message before
forwarding it back to the source of the request. The eight bandwidth forwarding it back to the source of the request. The eight bandwidth
values are filled in with the outgoing bandwidth available on this values are filled in with the outgoing bandwidth available on this
interface for each of the 8 holding priorities in bytes per second. interface for each of the eightholding priorities in bytes per
Finally the interface's address and far end address are placed in second. Finally the interface's address and far end address are
the TLV. placed in the TLV.
On receipt of a CR-LDP request message which cannot be satisfied. A On receipt of a CR-LDP request message, which cannot be satisfied. A
notification message is formatted normally. The 8-bandwidth values notification message is formatted normally. The eight bandwidth
are filled in with the outgoing bandwidth available on this values are filled in with the outgoing bandwidth available on this
interface for each of the 8 holding priorities in bytes per second. interface for each of the eight holding priorities in bytes per
Finally, the interface's address and far end address are placed in second. Finally, the interface's address and far end address are
the TLV. placed in the TLV.
On receipt of a CR-LDP request message which has been satisfied and 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 which results in a mapping being generated. No feedback TLV is added
since the previous node will insert the proper TLV when it receives since the previous node will insert the proper TLV when it receives
the reverse flowing mapping. the reverse flowing mapping.
When an LDP session goes down either because of a link failure, When an LDP session goes down either because of a link failure,
TCP/IP timeout, keep alive timeout, adjacency timeout etc. Other LDP TCP/IP timeout, keep alive timeout, adjacency timeout etc. Other LDP
sessions in the module must generate either notification, withdraw sessions in the module must generate either notification, withdraw
or release messages for LSPs that traversed the LDP in question. In or release messages for LSPs that traversed the LDP in question. In
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intermediate nodes that are forwarding this feedback information to intermediate nodes that are forwarding this feedback information to
store it for their own local source route computations. store it for their own local source route computations.
There is a possibility of a race condition between the bandwidth There is a possibility of a race condition between the bandwidth
information that is received via feedback and that, which is information that is received via feedback and that, which is
received via a normal IGP flood. While there may be a discrepancy received via a normal IGP flood. While there may be a discrepancy
between the two, both are within a few 100 milliseconds of being between the two, both are within a few 100 milliseconds of being
correct. Solutions to allow us to determine which information is correct. Solutions to allow us to determine which information is
most up to date (say by adding a sequence number) do not add any most up to date (say by adding a sequence number) do not add any
significant benefit. Constraint based, source routed systems will significant benefit. Constraint based, source routed systems will
always have errors in the local topology database with respect to always have errors in the local topology database with respect to
the real network. We can reduce these errors through reduced the real network. These errors can be reduced through reduced
flooding intervals, path following feedback and selective flooding flooding intervals, path following feedback and selective flooding
but we cannot realistically reduce the errors below the second or so but realistically the errors cannot be reduced below the second or
range. As a result propagation delay order race conditions are noise so range. As a result propagation delay order race conditions are
with respect to the average expected errors. An implementation noise with respect to the average expected errors. An implementation
SHOULD therefore consider the most recently received update (IGP or SHOULD therefore consider the most recently received update (IGP or
feedback) as being the most up to date. feedback) as being the most up to date.
7. Future considerations 7. Future considerations
Constraint based routing systems such as CR-LDP will in the future Constraint based routing systems such as CR-LDP will in the future
offer other forms of constraint than simply reserved bandwidth. offer other forms of constraint than simply reserved bandwidth.
Actual utilization levels, current congestion levels, number of Actual utilization levels, current congestion levels, number of
discrete channels/wavelengths available etc. are all possible discrete channels/wavelengths available etc. are all possible
constraints that change rapidly and which must be taken into constraints that change rapidly and which must be taken into
consideration when computing a route. It is expected that this consideration when computing a route. It is expected that this
mechanism will be used to feedback these and other new forms of link mechanism will be used to feedback these and other new forms of link
constraining data. constraining data.
8. RSVP consideration 8. RSVP-TE consideration
Nothing precludes the use of such feedback mechanisms with a similar Nothing precludes the use of such feedback mechanisms with a similar
TLV structure in the RSVP Resv and other reverse flowing messages TLV structure in the RSVP-TE Resv and other reverse flowing messages
although repeatedly applying unchanged feedback should be avoided although repeatedly applying unchanged feedback should be avoided.
since it increases the race condition window with flooded LSPs. This This could be accomplished by a simple rule that only permits
could be accomplished by a simple rule that only permits feedback feedback information on the original RESV, not on subsequent
information on the original RESV, not on subsequent refreshes. refreshes. This document only covers the CR-LDP protocol.
9. Intellectual Property Consideration 9. Intellectual Property Consideration
The IETF has been notified of intellectual property rights claimed The IETF has been notified of intellectual property rights claimed
in regard to some or all of the specification contained in this in regard to some or all of the specification contained in this
document. For more information consult the online list of claimed document. For more information consult the online list of claimed
rights. rights.
10. Security Considerations 10. Security Considerations
This document raises no new security considerations for CR-LDP, RSVP This document raises no new security considerations for CR-LDP,
or MPLS in general. RSVP-TE or MPLS in general.
11. Acknowledgments 11. Acknowledgments
The authors would like to thank Keith Dysart for his guidance, and The authors would like to thank Keith Dysart for his guidance, and
Jerzy Miernik for helping implement these concepts and bringing them Jerzy Miernik for helping implement these concepts and bringing them
to life. to life. The authors' would like to aknowledge Dave Allan for his
comments and suggestions.
12. References 12. References
[CR-LDP] Jamoussi, B. et al., _Constraint-Based LSP Setup using [CR-LDP] Jamoussi, B. et al., "Constraint-Based LSP Setup using
LDP_, Internet Draft, draft-ietf-mpls-cr-ldp-05.txt, February 2001. LDP", RFC 3212, January 2002.
[LDP] Andersson, L. et al., _LDP Specification_, RFC 3032, January [LDP] Andersson, L. et al., "LDP Specification", RFC 3032, January
2001. 2001.
[IS-IS] Li, T., Smit, H., _Extensions to IS-IS for traffic [IS-IS] Li, T., Smit, H., "Extensions to IS-IS for traffic
engineering_, Internet Draft, draft-ietf-isis-traffic-04.txt, August engineering", Internet Draft, draft-ietf-isis-traffic-04.txt, August
2001. 2001.
[OSPF] Katz, Dave and Yeung, Derek, "Traffic Engineering Extensions [OSPF] Katz,D., Yeung, D., Kompella, K., "Traffic Engineering
to OSPF," draft-katz-yeung-ospf-traffic-04.txt, February 2001. Extensions to OSPF," draft-katz-yeung-ospf-traffic-06.txt, February
[QUERY] Ashwood-Smith, P., Paraschiv, A., _MPLS LDP Query Message
Description_, Internet Draft, draft-anto-ldp-query-03.txt, August
2001. 2001.
[LDP-FT] Farrel, A et al., _Fault Tolerance for LDP and CR-LDP_, [QUERY] Ashwood-Smith, P., Paraschiv, A., "MPLS LDP Query Message
Description", Internet Draft, draft-anto-ldp-query-04.txt, May 2002.
[LDP-FT] Farrel, A et al., "Fault Tolerance for LDP and CR-LDP",
13. Author's Addresses 13. Author's Addresses
Peter Ashwood-Smith Bilel Jamoussi Peter Ashwood-Smith Bilel Jamoussi
Nortel Networks Corp. Nortel Networks Corp. Nortel Networks Corp. Nortel Networks Corp.
P.O. Box 3511 Station C, 600 Technology Park Drive P.O. Box 3511 Station C, 600 Technology Park Drive
Ottawa, ON K1Y 4H7 Billerica, MA 01821 Ottawa, ON K1Y 4H7 Billerica, MA 01821
Canada USA Canada USA
Phone: +1 613-763-4534 phone: +1 978-288-4506 Phone: +1 613-763-4534 phone: +1 978-288-4506
petera@nortelnetworks.com jamoussi@nortelnetworks.com petera@nortelnetworks.com jamoussi@nortelnetworks.com
skipping to change at page 10, line 31 skipping to change at page 12, line 8
Darek Skalecki Don Fedyk Darek Skalecki Don Fedyk
Nortel Networks Corp. Nortel Networks Corp. Nortel Networks Corp. Nortel Networks Corp.
P.O. Box 3511 Station C, 600 Technology Park Drive P.O. Box 3511 Station C, 600 Technology Park Drive
Ottawa, On K1Y 4H7 Billerica, MA 01821 Ottawa, On K1Y 4H7 Billerica, MA 01821
Canada USA Canada USA
Phone: +1 613-765-2252 Phone: +1 978-228-3041 Phone: +1 613-765-2252 Phone: +1 978-228-3041
dareks@nortelnetworks.com dwfedyk@nortelnetworks.com dareks@nortelnetworks.com dwfedyk@nortelnetworks.com
Full Copyright Statement Full Copyright Statement
Copyright (C) The Internet Society (date). All Rights Reserved. This Copyright (C) The Internet Society 2002. All Rights Reserved. This
document and translations of it may be copied and furnished to document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph kind, provided that the above copyright notice and this paragraph
are included on all such copies and derivative works. However, this are included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for developing Internet standards in which case the procedures for
 End of changes. 

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