IETF Internet Draft                             Arthi Ayyangar(Editor)
Proposed Status: Standards Track                      Juniper Networks
Expires: August 2005
                                                 Jean-Philippe Vasseur
                                                   Cisco Systems, Inc.

                                                          February 2005

                 LSP Stitching with Generalized MPLS TE


Status of this Memo

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Copyright (C) The Internet Society (2005).  All Rights Reserved.


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 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

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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.

1. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2. Introduction

   LSP hierarchy ([LSP-HIERARCHY]) 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,

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   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

   Signaling and routing procedures for LSP stitching are basically
   similar to that described in [LSP-HIERARCHY]. The LSP segment will be
   seen seen as a TE link by all nodes in the network. Also, non-
   adjacent RSVP signaling defined in [LSP-HIERARCHY] 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
   described in [LSP-HIERARCHY] 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 ([P2MP-RSVP]) 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 ([INTER-DOMAIN-RSVP]).

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3. 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
   [RSVP-UNNUM]. 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 [LSP-HIERARCHY] 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 ([BUNDLING]) 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.

4. 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
   [RSVP-GMPLS] 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.

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   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 [RSVP-TE].

   Additional signaling extensions for stitching are described in the
   next section.

4.1. 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 used for

   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 [LSP-ATTRIBUTES]:

   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:

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   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

   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
   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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5. Summary of LSP Stitching procedures

5.1. 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.

5.2. 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
   ([INTER-DOMAIN-PD-PATH-COMP]) 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.

5.3. 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

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   identified in the IF_ID RSVP_HOP (PHOP) object of the Path 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.

6. Security Considerations

   Similar to [LSP-HIERARCHY], 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  [RFC2747] 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, [RFC2747] should
   be revisited to check if IPSec AH is now a viable means of securing
   RSVP-TE messages.

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7. IANA Considerations

   The following values have to be defined by IANA for this document.
   The registry is,

7.1. 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.

7.2. 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.

8. Acknowledgements

   The authors would like to thank Adrian Farrel and Kireeti Kompella
   for their comments and suggestions.

9. References

9.1. Normative References

   [LSP-HIERARCHY] Kompella K., Rekhter Y., "LSP Hierarchy with
   Generalized MPLS TE", (work in progress).

   [LSP-ATTRIBUTES] Farrel A. et al, "Encoding of Attributes for
   Multiprotocol Label Switching (MPLS) Label Switched Path (LSP)
   Establishment Using RSVP-TE", (work in progress).

   [RSVP-GMPLS] L. Berger, et al, "Generalized Multi-Protocol Label

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   Switching (GMPLS) Signaling Resource ReserVation Protocol-Traffic
   Engineering (RSVP-TE) Extensions", RFC 3473, January 2003.

   [RSVP-TE] Awduche, et al, "Extensions to RSVP for LSP Tunnels", RFC
   3209, December 2001.

   [RFC2747] Baker, F., Lindell, B., and M. Talwar, "RSVP Cryptographic
   Authentication", RFC 2747, January 2000.

9.2. Informative References

   [P2MP-RSVP] R. Agarwal, et al, "Extensions to RSVP-TE for Point to
   Multipoint TE LSPs", (work in progress).

   [INTER-DOMAIN-RSVP] Ayyangar A., Vasseur JP, "Inter domain GMPLS
   Traffic Engineering - RSVP-TE extensions", (work in progress).

   [RSVP-UNNUM] Kompella K., Rekhter Y., "Signalling Unnumbered Links in
   Resource ReSerVation Protocol - Traffic Engineering (RSVP-TE)", RFC
   3477, January 2003.

   [BUNDLING] Kompella K., Rekhter Y., "Link Bundling in MPLS Traffic
   Engineering", (work in progress).

   [INTER-DOMAIN-PD-PATH-COMP] Vasseur JP, Ayyangar A.,Zhang R., "A Per-
   domain path computation method for computing Inter-domain Traffic
   Engineering Label Switched Path", (work in progress).

Author's addresses

Arthi Ayyangar
Juniper Networks, Inc.
1194 N.Mathilda Ave
Sunnyvale, CA 94089

Jean Philippe Vasseur
Cisco Systems, Inc.
300 Beaver Brook Road
Boxborough , MA - 01719

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Ayyangar and Vasseur                                           [Page 12]