Network Working Group A. Ayyangar, Ed.
Internet-Draft Juniper Networks
Expires: October 3, 2005 JP. Vasseur
Cisco Systems, Inc.
April 2005
Label Switched Path Stitching with Generalized MPLS Traffic
Engineering
draft-ietf-ccamp-lsp-stitching-00
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
In certain scenarios, there may be a need to combine together two
different Generalized Multi-Protocol Label Switching (GMPLS) Label
Switched Paths (LSPs) such that in the data plane, a single
end-to-end (e2e) LSP is achieved and all traffic from one LSP is
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switched onto the other LSP. We will refer to this as "LSP
stitching". This document covers cases where: a) the node performing
the stitching does not require configuration of every LSP pair to be
stitched together b) the node performing the stitching is not the
egress of any of the LSPs c) LSP stitching not only results in an
end-to-end LSP in the data plane, but there is also a corresponding
end-to-end LSP (RSVP session) in the control plane. It might be
possible to configure a GMPLS node to switch the traffic from an LSP
for which it is the egress, to another LSP for which it is the
ingress, without requiring any signaling or routing extensions
whatsoever, completely transparent to other nodes. This will also
result in LSP stitching in the data plane. However, this document
does not cover this scenario of LSP stitching.
This document describes the mechanisms to accomplish LSP stitching in
the scenarios described above.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Conventions used in this document . . . . . . . . . . . . 5
2. Routing aspects . . . . . . . . . . . . . . . . . . . . . . . 6
3. Signaling aspects . . . . . . . . . . . . . . . . . . . . . . 7
3.1 RSVP-TE signaling extensions . . . . . . . . . . . . . . . 7
4. Summary of LSP Stitching procedures . . . . . . . . . . . . . 10
4.1 LSP segment setup . . . . . . . . . . . . . . . . . . . . 10
4.2 Setup of e2e LSP . . . . . . . . . . . . . . . . . . . . . 10
4.3 Stitching of e2e LSP into an LSP segment . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 12
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
6.1 Attribute Flags for LSP_ATTRIBUTES object . . . . . . . . 13
6.2 New Error Codes . . . . . . . . . . . . . . . . . . . . . 13
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
8.1 Normative References . . . . . . . . . . . . . . . . . . . 15
8.2 Informative References . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 16
Intellectual Property and Copyright Statements . . . . . . . . 17
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1. Introduction
LSP hierarchy ([2]) provides signaling and routing procedures so
that:
a. a GMPLS node can form a forwarding adjacency (FA) over the FA LSP
b. other Label Switching Routers (LSRs) can see this FA LSP as a
Traffic Engineering (TE) link
c. the GMPLS node can nest one or more LSPs over the FA LSP. This
covers intra-domain LSPs only.
d. RSVP signaling for LSP setup can occur between nodes that do not
have routing adjacency.
LSP stitching is a special case of LSP hierarchy. In case of LSP
stitching, instead of an FA LSP, we will create an "LSP segment"
between two GMPLS nodes. So an LSP segment for stitching is
considered to be the moral equivalent of an FA LSP for nesting.
While LSP hierarchy allows more that one LSP to be admitted into the
FA-LSP, in case of LSP stitching, the desired switching type of the
LSP and the switching capability of the LSP segment are such that at
most one LSP may be admitted into an LSP segment. E.g. if LSP-AB is
an FA-LSP between nodes A and B, then multiple LSPs, say LSP1, LSP2,
LSP3 could potentially be 'nested into' LSP-AB. This is achieved by
exchanging a unique label for each of LSP1..3 over the LSP-AB hop
thereby permitting LSP1..3 to share the FA-LSP LSP-AB. Each of
LSP1..3 may reserve some bandwidth on LSP-AB. On the other hand, if
LSP-AB is an LSP segment, then at most one LSP, say LSP1 may be
'stitched to' the LSP segment LSP-AB. No labels are exchanged for
LSP1 over the LSP-AB hop (i.e. between A and B directly).
Therefore, LSP-AB is dedicated to LSP1 and no other LSPs can be
associated with LSP-AB. The LSPs LSP1..3 which are either nested or
stitched into another LSP are termed as end-to-end (e2e) LSPs in the
rest of this document.
Signaling and routing procedures for LSP stitching are basically
similar to that described in [2]). The LSP segment will be seen seen
as a TE link by all nodes in the network. Also, non-adjacent RSVP
signaling defined in [2]) will still be required to stitch an LSP to
an LSP segment. So, in the control plane, there is one RSVP session
corresponding to the e2e LSP as well as one for each LSP segment that
the e2e LSP is being stitched to. An LSP segment may be created
either via a configuration trigger or dynamically due to an incoming
LSP request. In this document we will highlight, where applicable,
similarities and differences in the routing and signaling procedures
between LSP hierarchy and LSP stitching. Additional signaling
extensions required for LSP stitching are also described here.
LSP stitching SHOULD be used when the switching types (capabilities)
of the LSP and the LSP segment are such that LSP hierarchy as
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described in [2]) is not possible. E.g. if the e2e LSP is a lambda
LSP and the LSP segment is also a lambda LSP, then in this case LSP
hierarchy is not possible. LSP stitching could also be useful in
networks to bypass legacy nodes which may not have certain new
capabilities in the control plane and/or data plane. E.g. one
suggested usage in case of P2MP RSVP LSPs ([7]) is the use of LSP
stitching to stitch a P2MP RSVP LSP to an LSP segment between P2MP
capable LSRs in the network. The LSP segment would traverse legacy
LSRs that may be incapable of acting as P2MP branch points, thereby
shielding them from the P2MP control and data path. LSP stitching
procedures could also be used for inter-domain TE LSP signaling to
stitch an inter-domain LSP to a local intra-domain TE LSP segment
([8]).
1.1 Conventions used in this document
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 RFC 2119 [1].
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2. Routing aspects
An LSP segment is similar to an FA-LSP in that, an LSP segment like
an FA-LSP is created and treated like a TE link between two GMPLS
nodes whose path transits zero or more GMPLS nodes in the same
instance of the GMPLS control plane. These TE links may be numbered
or unnumbered. For an unnumbered LSP segment, the assignment and
handling of the local and remote link identifiers is specified in
[9]. Unlike an FA-LSP, a GMPLS node does not have a data plane
adjacency with the end point of the LSP segment. This implies that
the traffic that arrives at the GMPLS node will be switched into the
LSP segment contiguously with a label swap and no label is exchanged
directly between the end nodes of the LSP segment itself. Also, a
routing adjacency will not be established over an LSP segment.
ISIS/OSPF may, however, flood the TE information associated with an
LSP segment, which will exist in the TE database (TED) and can then
be used for path computation by other GMPLS nodes in the network.
The TE parameters defined for an FA in [2]) are also applicable to an
LSP segment TE link.
Note that, while an FA-LSP TE link can admit zero or more LSPs over
it, an LSP segment can admit at most one LSP over it. So, once an
LSP is stitched into an LSP segment, the unreserved bandwidth on the
LSP segment is set to zero. This prevents any more LSPs from being
computed and admitted over the LSP segment TE link. Multiple LSP
segments between the same pair of nodes may be bundled using the
concept of Link Bundling ([10]) into a single TE link. When any
component LSP segment is allocated for an LSP, the component's
unreserved bandwidth MUST be set to zero and the Minimum and Maximum
LSP bandwidth of the TE link SHOULD be recalculated.
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3. Signaling aspects
In general, the trigger for the creation or termination of an LSP
segment may be either mechanisms which are outside of GMPLS
(configuration of LSP segment on the stitching node) or mechanisms
within GMPLS (arrival of an LSP setup request with appropriate
switching type on the stitching node).
Although not adjacent in routing, the end nodes of the LSP segment
will have a signaling adjacency and will exchange RSVP messages
directly between each other. When an RSVP-TE LSP is signaled over an
LSP segment, the Path message MUST contain an IF_ID RSVP_HOP object
[4] and the data interface identification MUST identify the LSP
segment. For the purpose of ERO and RRO as well, an LSP segment is
treated exactly like an FA.
The main difference between signaling an LSP over an LSP segment
instead of over an FA-LSP is that no Labels are allocated and
exchanged for the e2e LSP over the LSP segment hop. So, at most one
e2e LSP is associated with one LSP segment. If a node at the
head-end of an LSP segment receives a Path Msg for an LSP that
identifies the LSP segment in the ERO and the LSP segment bandwidth
has already been allocated to some other LSP, then regular rules of
RSVP-TE pre-emption apply. If the LSP request over the LSP segment
cannot be satisfied, then the node SHOULD send back a PathErr with
the error codes as described in [5].
Additional signaling extensions for stitching are described in the
next section.
3.1 RSVP-TE signaling extensions
The signaling extensions described here MUST be used if the LSP
segment is a packet LSP and an e2e packet LSP may be stitched to it.
These extensions are optional for non-packet LSPs and SHOULD be used
if no other local mechanisms exist to automatically detect a
requirement for stitching at both the ingress and egress nodes of the
LSP segment.
If a GMPLS node desires to perform LSP stitching, then it MUST
indicate this in the Path message for the LSP segment that it plans
to use for stitching. This signaling explicitly informs the egress
node for the LSP segment that the ingress node is planning to perform
stitching over the LSP segment. This will allow the egress of the
LSP segment to allocate the correct label(s) as explained below.
Also, so that the head-end node can ensure that correct stitching
actions were carried out at the egress node, a new flag is defined
below in the RRO subobject to indicate that the LSP segment can be
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used for stitching.
In order to request LSP stitching on the LSP segment, we define a new
flag bit in the Attributes Flags TLV of the LSP_ATTRIBUTES object
defined in [3]:
0x02 (TBD): LSP stitching desired bit - This flag will be set in the
Attributes Flags TLV of the LSP_ATTRIBUTES object in the Path message
for the LSP segment by the head-end of the LSP segment, that desires
LSP stitching. This flag MUST not be modified by any other nodes in
the network.
An LSP segment can only be used for stitching if appropriate label
actions were carried out at the egress node of the LSP segment. In
order to indicate this to the head-end node of the LSP segment, the
following new flag bit is defined in the RRO Attributes sub-object:
0x02 (TBD): LSP segment stitching ready.
If an egress node receiving a Path message, supports the
LSP_ATTRIBUTES object and the Attributes Flags TLV, and also
recognizes the "LSP stitching desired" flag bit, but cannot support
the requested stitching behavior, then it MUST send back a PathErr
message with an error code of "Routing Problem" and an error
sub-code=16 (TBD) "Stitching unsupported" to the head-end node of the
LSP segment.
If an egress node receiving a Path message with the "LSP stitching
desired" flag set, recognizes the object, the TLV and the flag and
also supports the desired stitching behavior, then it MUST allocate a
non-NULL label for that LSP segment in the corresponding Resv
message. Now, so that the head-end node can ensure that the correct
label actions will be carried out by the egress node and that the LSP
segment can be used for stitching, the egress node MUST set the "LSP
segment stitching ready" bit defined in the RRO Attribute sub-object.
Also, when the egress node for the LSP segment receives a Path
message for an e2e LSP using this LSP segment, it SHOULD first check
if it is also the egress for the e2e LSP. If the egress node is the
egress for both the LSP segment as well as the e2e TE LSP, and this
is a packet LSP which requires Penultimate Hop Popping (PHP), then
the node MUST send back a Resv refresh for the LSP segment with a new
label corresponding to the NULL label.
Finally, if the egress node for the LSP segment supports the
LSP_ATTRIBUTES object but does not recognize the Attributes Flags
TLV, or supports the TLV as well but does not recognize this
particular flag bit, then it SHOULD simply ignore the above request.
An ingress node requesting LSP stitching MUST examine the RRO
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Attributes sub-object flag corresponding to the egress node for the
LSP segment, to make sure that stitching actions were carried out at
the egress node. It MUST NOT use the LSP segment for stitching if
the "LSP segment stitching ready" flag is cleared.
The egress node MUST not allocate any Label in the Resv message for
the e2e TE LSP. Similarly, in case of bidirectional e2e TE LSP, no
Upstream Label is allocated over the LSP segment in the corresponding
Path message. An ingress node stitching an e2e TE LSP to an LSP
segment MUST ignore any Label object received in the Resv for the e2e
TE LSP.
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4. Summary of LSP Stitching procedures
4.1 LSP segment setup
A GMPLS node that originates an LSP segment may decide to use this
LSP segment for stitching. The creation of this LSP segment and its
use for stitching may be dictated either by configuration or
dynamically on arrival of an LSP setup request at the GMPLS node.
Successful completion of signaling procedures for the LSP segment as
described in Section 3.1 will allow the GMPLS node to : a) advertise
this LSP segment as a TE link with the bandwidth of the LSP as the
unreserved bandwidth in the IGP and b) carry out stitching procedures
to actually stitch an e2e LSP to the LSP segment. Similar to setup,
tearing down the LSP segment may also be decided either via local
configuration or due to the fact that there is no longer an e2e LSP
stitched to the LSP segment. E.g. Let us consider an LSP segment
LSP-AB being setup between two nodes A and B. A sends a Path message
for the LSP-AB with "LSP stitching desired". If on the egress node
B, stitching procedures are successfully carried out, then B will set
the "LSP segment stitching ready" in the RRO sent in the Resv. Once
A receives the Resv for LSP-AB and sees this bit set in the RRO, it
can then use LSP-AB for stitching.
4.2 Setup of e2e LSP
Other nodes in the network (in the same domain) trying to setup an
e2e LSP across the network may see the LSP segment as a TE link in
their TE databases and may compute a path over this TE link. In case
of an inter-domain e2e LSP, however, the LSP segment TE link, like
any other basic TE link in the domain will probably not be advertised
outside the domain. In this case, either per-domain path computation
([11]) or PCE based computation will permit setting up e2e LSPs over
LSP segments in other domains. The LSP segment TE link may be
identified as an ERO hop in the Path message of the e2e LSP message.
E.g. Let us consider an e2e LSP LSP1-2 starting one hop before A on
R1 and ending on node R2. R1 may compute a path for LSP1-2 over the
LSP segment LSP-AB and identify the LSP-AB hop in the ERO.
4.3 Stitching of e2e LSP into an LSP segment
When the Path message for an e2e LSP arrives at the GMPLS stitching
node, the LSP segment to stitch the e2e LSP to is determined. The
Path message for the e2e LSP is then sent directly to the LSP segment
end node with the destination IP address of the Path message set to
the address of the LSP segment end node. The Router Alert option
MUST not be set in this case. Furthermore, when the message arrives
at the end node, RSVP TTL checks MUST be disabled. The LSP segment
MUST be identified in the IF_ID RSVP_HOP (PHOP) object of the Path
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message. It is assumed that the receiver of this Path message can
identify the LSP segment based on the data interface identification
in the IF_ID RSVP_HOP. When the Resv is sent back for the e2e LSP,
no Label is allocated on the LSP segment hop. E.g. When the Path
message for the e2e LSP LSP1-2 arrives at node A, and the LSP segment
LSP-AB to stitch LSP1-2 to has been identified (either based on
explicit hop in ERO or due to local decision), then Path message for
LSP1-2 is sent directly to node B with the IF_ID RSVP_HOP identifying
the LSP segment LSP-AB. When B receives this Path message for
LSP1-2, if B is also the egress for LSP1-2, and if this is a packet
LSP requiring PHP, then B will send a Resv refresh for LSP-AB with
the NULL Label. If B is not the egress, then Path message for LSP1-2
is propagated to R2. The Resv for LSP1-2 is sent back from B
directly to A with no Label in it. Node A then propagates the Resv
to R1. This stitches an e2e LSP LSP1-2 to an LSP segment LSP-AB
between nodes A and B. In the data plane, this yields a series of
label swaps from R1 to R2 along LSP LSP1-2.
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5. Security Considerations
Similar to [2]), this document permits that the control interface
over which RSVP messages are sent or received need not be the same as
the data interface which the message identifies for switching
traffic. Also, the 'sending interface' and 'receiving interface' may
change as routing changes. So, these cannot be used to establish
security association between neighbors. Mechanisms described in [6]
should be re-examined and may need to be altered to define new
security associations based on receiver's IP address instead of the
sending and receiving interfaces. Also, this document allows the IP
destination address of Path and PathTear messages to be the IP
address of a nexthop node (receiver's address) instead of the RSVP
session destination address. So, [6] should be revisited to check if
IPSec AH is now a viable means of securing RSVP-TE messages.
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6. IANA Considerations
The following values have to be defined by IANA for this document.
The registry is http://www.iana.org/assignments/rsvp-parameters.
6.1 Attribute Flags for LSP_ATTRIBUTES object
The following new flag bit is being defined for the Attributes Flags
TLV in the LSP_ATTRIBUTES object. The numeric value should be
assigned by IANA.
LSP stitching desired bit - 0x02 (Suggested value)
This flag bit is only to be used in the Attributes Flags TLV on a
Path message.
The 'LSP stitching desired bit' has a corresponding 'LSP segment
stitching ready' bit to be used in the RRO Attributes sub-object.
6.2 New Error Codes
The following new error sub-code is being defined under the RSVP
error-code "Routing Problem" (24). The numeric error sub-code value
should be assigned by IANA.
Stitching unsupported - sub-code 16 (Suggested value)
This error code is to be used only in a RSVP PathErr.
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7. Acknowledgments
The authors would like to thank Adrian Farrel and Kireeti Kompella
for their comments and suggestions.
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8. References
8.1 Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Kompella, K. and Y. Rekhter, "LSP Hierarchy with Generalized
MPLS TE", Internet-Draft draft-ietf-mpls-lsp-hierarchy-08,
September 2002.
[3] Farrel, A., Papadimitriou, D. and J. Vasseur, "Encoding of
Attributes for Multiprotocol Label Switching (MPLS) Label
Switched Path (LSP) Establishment Using RSVP-TE",
Internet-Draft draft-ietf-mpls-rsvpte-attributes-04, July 2004.
[4] Berger, L., "Generalized Multi-Protocol Label Switching (GMPLS)
Signaling Resource ReserVation Protocol-Traffic Engineering
(RSVP-TE) Extensions", RFC 3473, January 2003.
[5] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V. and G.
Swallow, "RSVP-TE: Extensions to RSVP for LSP Tunnels",
RFC 3209, December 2001.
[6] Baker, F., Lindell, B. and M. Talwar, "RSVP Cryptographic
Authentication", RFC 2747, January 2000.
8.2 Informative References
[7] Aggarwal, R., "Extensions to RSVP-TE for Point to Multipoint TE
LSPs", Internet-Draft draft-ietf-mpls-rsvp-te-p2mp-01, January
2005.
[8] Ayyangar, A. and J. Vasseur, "Inter domain GMPLS Traffic
Engineering - RSVP-TE extensions",
Internet-Draft draft-ietf-ccamp-inter-domain-rsvp-te-00,
February 2005.
[9] Kompella, K. and Y. Rekhter, "Signalling Unnumbered Links in
Resource ReSerVation Protocol - Traffic Engineering (RSVP-TE)",
RFC 3477, January 2003.
[10] Kompella, K., Rekhter, Y. and L. Berger, "Link Bundling in MPLS
Traffic Engineering", Internet-Draft draft-ietf-mpls-bundle-06,
December 2004.
[11] Vasseur, J., "A Per-domain path computation method for
computing Inter-domain Traffic Engineering (TE) Label Switched
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Path (LSP)",
Internet-Draft draft-vasseur-ccamp-inter-domain-pd-path-comp-00, February 2005
.
Authors' Addresses
Arthi Ayyangar (editor)
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
US
Email: arthi@juniper.net
Jean Philippe Vasseur
Cisco Systems, Inc.
300 Beaver Brook Road
Boxborough, MA 01719
US
Email: jpv@cisco.com
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