Network Working Group J. Ash
Request for Comments: 3214 AT&T
Category: Standards Track Y. Lee
Ceterus Networks
P. Ashwood-Smith
B. Jamoussi
D. Fedyk
D. Skalecki
Nortel Networks
L. Li
SS8 Networks
January 2002
LSP Modification Using CR-LDP
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document presents an approach to modify the bandwidth and
possibly other parameters of an established CR-LSP (Constraint-based
Routed Label Switched Paths) using CR-LDP (Constraint-based Routed
Label Distribution Protocol) without service interruption. After a
CR-LSP is set up, its bandwidth reservation may need to be changed by
the network operator, due to the new requirements for the traffic
carried on that CR-LSP. The LSP modification feature can be
supported by CR-LDP by use of the _modify_value for the _action
indicator flag_ in the LSPID TLV. This feature has application in
dynamic network resources management where traffic of different
priorities and service classes is involved.
Table of Contents
1. Conventions Used in This Document ............................ 2
2. IntrodUCtion ................................................. 2
3. LSP Modification Using CR-LDP ................................ 3
3.1 Basic Procedure for Resource Modification .................. 3
3.2 Rerouting LSPs ............................................. 5
3.3 Priority Handling .......................................... 6
3.4 Modification Failure Case Handling ......................... 6
4. Application of LSP Bandwidth Modification in Dynamic Resource
Management ................................................... 7
5. Acknowledgments .............................................. 8
6. Intellectual Property Considerations ......................... 8
7. Security Considerations ...................................... 8
8. References ................................................... 8
9. Authors' Addresses ........................................... 9
10. Full Copyright Statement ..................................... 11
1. Conventions Used in This Document
L: LSP (Label Switched Path)
L-id: LSPID (LSP Identifier)
T: Traffic Parameters
R: LSR (Label Switching Router)
FEC: Forwarding Equivalence Class
NHLFE: Next Hop Label Forwarding Entry
FTN: FEC To NHLFE
TLV: Type Length Value
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 [4].
2. Introduction
Consider an LSP L1 that has been established with its set of traffic
parameters T0. A certain amount of bandwidth is reserved along the
path of L1. Consider then that some changes are required on L1. For
example, the bandwidth of L1 needs to be increased to accommodate the
increased traffic on L1. Or the SLA associated with L1 needs to be
modified because a different service class is desired. The network
operator, in these cases, would like to modify the characteristics of
L1, for example, to change its traffic parameter set from T0 to T1,
without releasing the LSP L1 to interrupt the service. In some other
cases, network operators may want to reroute a CR-LSP to a different
path for either improved performance or better network resource
utilization. In all these cases, LSP modification is required. In
section 3 below, a method to modify an active LSP using CR-LDP is
presented. The concept of LSPID in CR-LDP is used to achieve the LSP
modification, without releasing the LSP and interrupting the service
and, without double booking the bandwidth. In Section 4, an example
is described to demonstrate an application of the presented method in
dynamically managing network bandwidth requirements without
interrupting service. In CR-LDP, an action indicator flag of
_modify_ is used in order to eXPlicitly specify the behavior, and
allow the existing LSPID to support other networking capabilities in
the future. Reference [3], RFCXXXX, specifies the action indicator
flag of _modify_ for CR-LDP.
3. LSP Modification Using CR-LDP
3.1 Basic Procedure for Resource Modification
LSP modification can only be allowed when the LSP is already set up
and active. That is, modification is not defined nor allowed during
the LSP establishment or label release/withdraw phases. Only
modification requested by the ingress LSR of the LSP is considered in
this document for CR-LSP. The Ingress LSR cannot modify an LSP
before a previous modification procedure is completed.
Assume that CR-LSP L1 is set up with LSPID L-id1, which is unique in
the MPLS network. The ingress LSR R1 of L1 has in its FTN (FEC To
NHLFE) table FEC1 -> Label A mapping where A is the outgoing label
for LSP L1. To modify the characteristics of L1, R1 sends a Label
Request Message. In the message, the TLVs will have the new
requested values, and the LSPID TLV is included which indicates the
value of L-id1. The Traffic Parameters TLV, the ER-TLV, the Resource
Class (color) TLV and the Preemption TLV can have values different
from those in the original Label Request Message, which has been
used to set up L1 earlier. Thus, L1 can be changed in its bandwidth
request (traffic parameter TLV), its traffic service class (traffic
parameter TLV), the route it traverses (ER TLV) and its setup and
holding (Preemption TLV) priorities. The ingress LSR R1 now still has
the entry in its FTN as FEC1 -> Label A. R1 is waiting to establish
another entry for FEC1.
When an LSR Ri along the path of L1 receives the Label Request
message, its behavior is the same as that of receiving any Label
request message. The only extension is that Ri examines the LSPID
carried in the Label Request Message, L-id1, and identifies if it
already has L-id1. If Ri does not have L-id1, Ri behaves the same as
receiving a new Label Request message. If Ri already has L-id1, Ri
takes the newly received Traffic Parameter TLV and computes the new
bandwidth required and derives the new service class. Compared with
the already reserved bandwidth for L-id1, Ri now reserves only the
difference of the bandwidth requirements. This prevents Ri from doing
bandwidth double booking. If a new service class is requested, Ri
also prepares to receive the traffic on L1 in just the same way as
handling it for a Label Request Message, perhaps using a different
type of queue. Ri assigns a new label for the Label Request Message.
When the Label Mapping message is received, two sets of labels exist
for the same LSPID. Then the ingress LSR R1 will have two outgoing
labels, A and B, associated with the same FEC, where B is the new
outgoing label received for LSP L1. The ingress LSR R1 can now
activate the new entry in its FTN, FEC1 - > Label B. This means that
R1 swaps traffic on L1 to the new label _B_ (_new_ path) for L1. The
packets can now be sent with the new label B, with the new set of
traffic parameters if any, on a new path, that is, if a new path is
requested in the Label Request Message for the modification. All the
other LSRs along the path will start to receive the incoming packets
with the new label. For the incoming new label, the LSR has already
established its mapping to the new outgoing label. Thus, the packets
will be sent out with the new outgoing label. The LSRs do not have
to implement new procedures to track the new and old characteristics
of the LSP.
The ingress LSR R1 then starts to release the original label A for
LSP L1. The Label Release Message is sent by R1 towards the down
stream LSRs. The Release message carries the LSPID of L-id1 and the
Label TLV to indicate which label is to be released. The Release
Message is propagated to the egress LSR to release the original
labels previously used for L1. Upon receiving the Label Release
Message, LSR Ri examines the LSPID, L-id1, and finds out that the L-
id1 has still another set of labels (incoming/outgoing) under it.
Thus, the old label is released without releasing the resource in
use. That is, if the bandwidth has been decreased for L1, the delta
bandwidth is released. Otherwise, no bandwidth is released. This
modification procedure can not only be applied to modify the traffic
parameters and/or service class of an active LSP, but also to reroute
an existing LSP (as described in Section 3.2 below), and/or change
its setup/holding priority if desired. After the release procedure,
the modification of the LSP is completed.
The method described above follows the normal behavior of Label
Request / Mapping / Notification / Release / Withdraw procedure of a
CR-LDP operated LSR with a specific action taken on an LSPID. If a
Label Withdraw Message is used to withdraw a label associated with an
LSPID, the Label TLV should be included to specify which label to
withdraw. Since the LSPID can also be used for other feature
support, an action indication flag of _modify_ assigned to the LSPID
would explicitly explain the action/semantics that should be
associated with the messaging procedure. The details of this flag
are addressed in the CR-LDP document, Reference [3].
3.2 Rerouting LSPs
LSP modification can also be used to reroute an existing LSP. Only
modification requested by the ingress LSR of the LSP is considered in
this document for CR-LSP. The Ingress LSR cannot modify an LSP before
a previous modification procedure is completed.
As in the previous section, consider a CR-LSP L1 with LSPID L-id1.
To modify the route of the LSP, the ingress LSR R1 sends a Label
Request Message. In the message, the LSPID TLV indicates L-id1 and
the Explicit Route TLV is specified with some different hops from the
explicit route specified in the original Label Request Message. The
action indication flag has the value _modify_.
At this point, the ingress LSR R1 still has an entry in FTN as
FEC1 -> Label A. R1 is waiting to establish another entry for FEC1.
When an LSR Ri along the path of L1 receives the Label Request
message, its behavior is the same as that of receiving a Label
Request Message that modifies some other parameters of the LSP. Ri
assigns a new label for the Label Request Message and forwards the
message along the explicit route. It does not allocate any more
resources except as described in section 3.1.
At another LSR Rj further along the path, the explicit route diverges
from the previous route. Rj acts as Ri, but forwards the Label
Request message along the new route. From this point onwards the
Label Request Message is treated as setting up a new LSP by each LSR
until the paths converge at later LSR Rk. The _modify_ value of the
action indication flag is ignored.
At Rk and subsequent LSRs, the Label Request Message is handled as at
Ri.
On the return path, when the Label Mapping message is received, two
sets of labels for the LSPID exist where the new route coincide with
the old. Only one set of labels will exist at LSRs where the routes
diverge.
When the Label Mapping message is received at the ingress LSR R1 it
has two outgoing labels, A and B, associated with the same FEC, where
B is the new outgoing label received for LSP L1. R1 can now activate
the new entry in the FTN, FEC1 - > Label B and de-activate the old
entry FEC1 - > Label A. This means that R1 swaps traffic on L1 to the
new label B. The packets are now sent with the new label B, on the
new path.
The ingress LSR R1 then starts to release the original label A for
LSP L1. The Label Release Message is sent by R1 towards the down
stream LSRs following the original route. The Release message carries
the LSPID of L-id1 and the Label TLV to indicate which label is to be
released. At each LSR the old label is released - no further action
is required to change the path of the data packets which are already
following the new route programmed by the Label Mapping message.
At some LSRs, where the routes diverged, there is only one label for
the LSPID. For example, between Rj and Rk, the Label Release Message
will follow the old route. At LSRs between Rj and Rk only the labels
from the original route will exist for LSPID L-id1. At these LSRs
the LSPID TLV does not need to be examined to release the correct
label, but it must still be updated and passed on to the next LSR as
the Label Release message is propagated. In this way, at Rk where the
routes converge, the downstream LSR will know which label to release
and can continue to forward the Label Release Message along the old
route.
3.3 Priority Handling
When sending a Label Request Message for an active LSP L1 to request
changes, the setup priority used in the label Request Message can be
different from the one used in the previous Label Request Message,
effectively indicating the priority of this _modification_ request.
Network operators can use this feature to decide what priority is to
be assigned to a modification request, based on their
policies/algorithms and other traffic situations in the network. For
example, the priority for modification can be determined by the
priority of the customer/LSP. If a customer has exceeded the
reserved bandwidth of its VPN LSP tunnel by too much, the
modification request's priority may be given as a higher value. The
Label Request message for the modification of an active LSP can also
be sent with a holding priority different from its previous one.
This effectively changes the holding priority of the LSP. Upon
receiving a Label Request Message that requests a new holding
priority, the LSR assigns the new holding priority to the bandwidth.
That is, the new holding priority is assigned to both the existing
incoming / outgoing labels and the new labels to be established for
the LSPID in question. In this way self-bumping is prevented.
3.4 Modification Failure Case Handling
A modification attempt may fail due to insufficient resource or other
situations. A Notification message is sent back to the ingress LSR
R1 to indicate the failure of Label Request Message that intended to
modify the LSP. A retry may be attempted if desired by the network
operator. If the LSP on the original path failed when a modification
attempt is in progress, the attempt should be aborted by using the
Label Abort Request message as specified in the LDP document [5].
In the event of a modification failure, all modifications to the LSP
including the holding priority must be restored to their original
values.
4. Application of LSP Bandwidth Modification in Dynamic Resource
Management
In this section, we gave an example of dynamic network resource
management using the LSP bandwidth modification capability. The
details of this example can be found in a previous internet-draft
[2]. Assume that customers or services are assigned with given CR-
LSPs. These customers/services are assigned with one of three
priorities: key, normal or best effort. The network operator does
not want to bump any LSPs during an LSP setup, so after these CR-LSPs
are set up, their holding priorities are all assigned as the highest
value.
The network operator wants to control the resource on the links of
the LSRs, so each LSR keeps the usage status of its links. Based on
the usage history, each link is assigned a current threshold priority
Pi, which means that the link has no bandwidth available for a Label
Request with a setup priority lower than Pi. When an LSP's bandwidth
needs to be modified, the operator uses a policy-based algorithm to
assign a priority for its modification request, say Mp for LSP L2.
The ingress LSR then sends a Label Request message with Setup
Priority = Mp. If there is sufficient bandwidth on the link for the
modification, and the Setup priority in the Label Request Message is
higher in priority (Mp numerically smaller) than the Pi threshold of
the link, the Label Request Message will be accepted by the LSR.
Otherwise, the Label Request message will be rejected with a
Notification message which indicates that there are insufficient
resources. It should also be noted that when OSPF (or IS-IS) floods
the available-link-bandwidth information, the available bandwidth
associated with a priority lower than Pi (numerical value bigger)
should be interpreted as _0_.
This example based on a priority threshold Pi is implementation
specific, and illustrates the flexibility of the modification
procedure to prioritize and control network resources. The
calculation of Mp can be network and service dependent, and is based
on the operator's routing policy. For example, the operator may
assign a higher priority (lower Mp value) to L2 bandwidth
modification if L2 belongs to a customer or service with _Key_
priority. The operator may also collect the actual usage of each LSP
and assign a lower priority (higher Mp) to L2 bandwidth-increase
modification if, for example, in the past week L2 has exceeded its
reserved bandwidth by 2 times on the average. In addition, an
operator may try to increase the bandwidth of L2 on its existing path
unsuccessfully if there is insufficient bandwidth available on L2.
In that case, the operator is willing to increase the bandwidth of
another LSP, L3, with the same ingress/egress LSRs as L2, in order to
increase the overall ingress/egress bandwidth allocation. However,
in this case the L3 bandwidth modification is performed with a lower
priority (higher Mp value) since L3 is routed on a secondary path,
which results in the higher bandwidth allocation priority being given
to the LSPs that are on their primary paths [2].
5. Acknowledgments
The authors would like to acknowledge the careful review and comments
of Adrian Farrel.
6. Intellectual Property Considerations
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.
7. Security Considerations
Protection against modification to LSPs by malign agents has to be
controlled by the MPLS domain.
8. References
[1] Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC2026, October 1996.
[2] Ash, J., "Traffic Engineering & QoS Methods for IP-, ATM-, &
TDM-Based Multiservice Networks", Work in Progress.
[3] Jamoussi, B., Editor, Andersson, L., Callon, R., Dantu, R., Wu,
L., Doolan, P., Worster, T., Feldman, N., Fredette, A., Girish,
M., Gray, E., Heinanen, J., Kilty, T. and A. Malis, "Constraint-
based LSP Setup Using LDP", RFC3212, January 2002.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.
[5] Andersson, L., Doolan, P., Feldman, N., Fredette, A. and B.
Thomas, "LDP Specification", RFC3036, January 2001.
[6] Rosen, E., Viswanathan, A. and R. Callon, "Multiprotocol Label
Switching Architecture", RFC3031, January 2001.
[7] Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M. and J. McManus,
"Requirements for Traffic Engineering Over MPLS", RFC2702,
September 1999.
[8] Ash, J., Girish, M., Gray, E., Jamoussi,B. and G. Wright,
"Applicability Statement for CR-LDP", RFC3213, January 2002.
9. Authors' Addresses
Gerald R. Ash
AT&T
Room MT D5-2A01
200 Laurel Avenue
Middletown, NJ 07748
USA
Phone: 732-420-4578
EMail: gash@att.com
Bilel Jamoussi
Nortel Networks Corp.
600 Tech Park
Billerica, MA 01821
USA
Phone: 978-288-4506
EMail: jamoussi@NortelNetworks.com
Peter Ashwood-Smith
Nortel Networks Corp.
P O Box 3511 Station C
Ottawa, ON K1Y 4H7
Canada
Phone: +1 613 763-4534
EMail: petera@NortelNetworks.com
Darek Skalecki
Nortel Networks Corp.
P O Box 3511 Station C
Ottawa, ON K1Y 4H7
Canada
Phone: +1 613 765-2252
EMail: dareks@nortelnetworks.com
Young Lee
Ceterus Networks
EMail: ylee@ceterusnetworks.com
Li Li
SS8 Networks
495 March Rd., 5th Floor
Kanata, Ontario
K2K 3G1 Canada
Phone: +1 613 592-2100 ext. 3228
EMail: lili@ss8networks.com
Don Fedyk
Nortel Networks Corp.
600 Tech Park
Billerica, MA 01821
USA
Phone: 978-288-3041
EMail: dwfedyk@nortelnetworks.com
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