Internet DRAFT - draft-dugeon-pce-stateful-interdomain
draft-dugeon-pce-stateful-interdomain
Path Computation Element Working Group O. Dugeon
Internet-Draft J. Meuric
Intended status: Standards Track Orange Labs
Expires: January 11, 2021 Y. Lee
Huawei Technologies
D. Ceccarelli
Ericsson
July 10, 2020
PCEP Extension for Stateful Inter-Domain Tunnels
draft-dugeon-pce-stateful-interdomain-04
Abstract
This document specifies how to combine a Backward Recursive or
Hierarchical method with inter-domain paths in the context of
stateful Path Computation Element (PCE). It relies on the PCInitiate
message to set up independent paths per domain. Combining these
different paths together enables to operate them as end-to-end inter-
domain paths without the need for a signaling session between inter-
domain border routers. A new Stitching Label is defined, new Path
Setup Types, a new Association Type and a new PCEP communication
Protocol (PCEP) Capability are considered for that purpose.
Requirements Language
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 [RFC2119].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on January 11, 2021.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. General Assumptions . . . . . . . . . . . . . . . . . . . 5
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
2. Stitching Label . . . . . . . . . . . . . . . . . . . . . . . 8
2.1. Definition . . . . . . . . . . . . . . . . . . . . . . . 8
2.2. Inter-domain LSP-TYPE . . . . . . . . . . . . . . . . . . 9
3. Backward Recursive PCInitiate Procedure . . . . . . . . . . . 9
3.1. Mode of Operation . . . . . . . . . . . . . . . . . . . . 10
3.2. Example . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.3. Completion Failure of Inter-domain Path Setup Procedure . 14
4. Hierarchical PCInitiate Procedure . . . . . . . . . . . . . . 14
4.1. Mode of Operation . . . . . . . . . . . . . . . . . . . . 14
4.2. Completion Failure of Inter-domain Path Setup Procedure . 17
4.3. Example for Stateful H-PCE Stiching Procedure . . . . . . 17
5. Inter-domain Path Management . . . . . . . . . . . . . . . . 21
5.1. Stitching Label PCE Capabilities . . . . . . . . . . . . 21
5.2. Identification of Inter-domain Paths . . . . . . . . . . 22
5.3. Inter-domain Association Group . . . . . . . . . . . . . 23
5.4. Modification of Inter-domain Paths . . . . . . . . . . . 24
5.5. Modification of Inter-domain Paths . . . . . . . . . . . 25
5.6. Tear-Down of Inter-domain Paths . . . . . . . . . . . . . 25
6. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 25
6.1. RSVP-TE . . . . . . . . . . . . . . . . . . . . . . . . . 25
6.2. Segment Routing . . . . . . . . . . . . . . . . . . . . . 26
6.3. Mixing Technologies . . . . . . . . . . . . . . . . . . . 27
6.4. Inter-Area . . . . . . . . . . . . . . . . . . . . . . . 27
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
7.1. Path Setup Type Values . . . . . . . . . . . . . . . . . 28
7.2. Association Type Value . . . . . . . . . . . . . . . . . 28
7.3. PCEP Error Values . . . . . . . . . . . . . . . . . . . . 29
7.4. PCEP TLV Type Indicators . . . . . . . . . . . . . . . . 29
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7.5. Stitching Label PCE Capability . . . . . . . . . . . . . 29
8. Security Considerations . . . . . . . . . . . . . . . . . . . 30
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 30
10. Disclaimer . . . . . . . . . . . . . . . . . . . . . . . . . 30
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
11.1. Normative References . . . . . . . . . . . . . . . . . . 30
11.2. Informative References . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33
1. Introduction
The PCE working group has produced a set of RFCs to standardize the
behavior of the Path Computation Element as a tool to help
MultiProtocol Label Switching - Traffic Engineering (MPLS-
TE)/Generalized MPLS (GMPLS) Label Switched Paths (LSPs) and Segment
Routing paths placement. This also includes the ability to compute
inter-domain LSPs or Segment Routing paths following a distributed or
hierarchical approach. To complement the original stateless mode, a
stateful mode has been added and supports both passive and active
control models. In particular, the new PCInitiate message allows a
PCE to directly ask a PCC to set up an MPLS-TE/GMPLS LSP or a Segment
Routing path. However, once computed, the inter-domain LSPs or
Segment Routing paths are hard to set up in the underlying network.
Especially, in operational networks, RSVP-TE signaling is usually not
enabled between AS border routers. But, such RSVP-TE signaling is
mandatory to set up contiguous LSP tunnels or to stitch or nest
independent LSP tunnels to form the end-to-end inter-domain paths.
Looking at the different RFCs that describe the PCE architecture and
in particular the PCE-based architecture [RFC4655], the PCE
communication Protocol [RFC5440], BRPC [RFC5441] and H-PCE [RFC6805],
the PCE is able to compute inter-domain paths, thus complementing the
intra-domain computation. Such inter-domain paths could then serve
as an Explicit Route Object (ERO) input for the RSVP-TE signaling to
set up the tunnels within the underlying network. Three kinds of
inter-domain paths could be established:
o Contiguous tunnel ([RFC3209] and [RFC3473]): The RSVP-TE signaling
crosses the boundary between two domains, e.g. between two AS
Border Routers (ASBRs) as if they were two routers of the same
domain. This kind of tunnel is not recommended mostly for
security and scalability purpose. In addition, the initiating
domain imposes huge constraints on subsequent domains, because
they undergo the tunnel request without being able to control it.
o Stitching tunnel ([RFC5150]): Each domain establishes in its own
network the corresponding part of the inter-domain path
independently. Then, a second end-to-end RSVP-TE Path message is
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sent by the initiating domain to stitch the different tunnel parts
to form the inter-domain path. In fact, this second RSVP-TE Path
message is used by border nodes to request the label that must be
used by the previous domain to send the traffic in order that the
MPLS packets follow the next LSP in the downstream domain. These
labels are conveyed in the RSVP-TE Resv message.
o Nesting tunnel ([RFC4206]): This is similar to the stitching mode
but, this time, with the possibility to set up tunnel hierarchy.
For example, an LSP between two edge domains crossing a transit
domain could be carried over a tunnel of a higher level in the
transit domain. Again, a second end-to-end RSVP-TE Path message
is sent from the source to the destination. Labels that must be
used by the ASBRs of transit domains to identify flows to be
nested are carried by the RSVP-TE Resv message.
In all cases, RSVP-TE signaling must be exchanged between the
different domains. However, from an operational point of view,
looking to different networks under the responsibility of different
administrative entities, typically only BGP sessions are set up and
configured between ASBRs. Technologically speaking, this is possible
and many RFCs describe how to use RSVP-TE for inter-domain. But, due
to security, scalability, management and contract constraints, RSVP-
TE is not exposed at the network boundary. To address some of the
security concerns, RSVP-TE can be carried inside an IPsec tunnel
between ASBRs, but, this does not eliminate the scalability aspect
nor the constraints imposed by setting up inter-domain paths.
For Segment Routing, issues are different as there is no signaling
between routers. Here, the main problem comes from label stacking.
The first issue concerns the size of the labels stack which is
limited due to hardware constraint. The PCEP Extensions for Segment
Routing [RFC8664] takes into account this limitation within the PCEP
Capability when the PCEP session is established. Thus, taking into
account Maximum Stack Depth (MSD), a PCE may be unable to find a
solution when it computes an end-to-end inter-domain path. The
second issue is related to the path confidentiality. With SR-TE, to
express an explicit path, all Node-SID must be stacked by the head
end router while some of the Node-SIDs are associated to routers of
the next domains. It is clear that operators would not disclose
details of their network, which includes Node-SIDs. Thus, it is not
possible to stack remote labels for an end-to-end inter-domain path
even if MSD constraint is respected.
The purpose of this memo is to take the benefit of active stateful
PCE [RFC8231] and PCE-Initiated [RFC8281] modes to stitch or nest
inter-domain paths directly using PCEP between domains' PCEs. This
avoids using another signaling (e.g. RSVP-TE) at the inter-domain
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border nodes, while keeping each operator free to independently set
up their respective part of the inter-domain paths. The PCInitiate
message is used in a Backward Recursive way like the PCReq message in
BRPC [RFC5441], to recursively set up the end-to-end tunnel. PCRep
message is used to automatically stitch or nest the different local
LSPs. And, PCRep in conjunction with PCUpd messages are used to
report, maintain, modify and remove inter-domain paths. This method
is also applicable to Segment Routing to build inter-domain segment
paths.
H-PCE [RFC6805] describes a Hierarchical PCE architecture which can
be used for computing end-to-end paths for inter-domain MPLS-TE and
GMPLS LSPs. Within this architecture, the Parent PCE (P-PCE) is used
to compute a multi-domain path based on the domain connectivity
information. A Child PCE (C-PCE) may be responsible for a single
domain or multiple domains, it is used to compute the intra-domain
path based on its domain topology information.
Stateful H-PCE [RFC8751] presents general considerations for stateful
PCE(s) in the hierarchical PCE architecture. In particular, the
behavior extends the existing stateful PCE mechanisms (including PCE-
initiated LSP setup and active PCE usage) in the context of networks
using the H-PCE architecture. Section 3.3.1 [RFC8751] describes the
per-domain stitched LSP mode, where the individual per-domain LSPs
are stitched together. PCInitiate message is also used to stitch the
end-to-end tunnel. See section 4 for details.
1.1. General Assumptions
In the remainder of this document, the same references as per BRPC
[RFC5441] are used and the following set of assumptions are made (see
figure below):
o Domain refers to administrative partitions, i.e. an IGP area or an
Autonomous System (AS).
o Inter-domain path is used to refer to a path that crosses two or
more different domains as defined previously,
o At least one PCE is deployed in each domain. These PCEs are all
active stateful-capable and can request to enforce LSPs in their
respective domain by means of PCInitiate messages.
o LSRs, including border nodes, are PCC-enabled and support active
stateful mode. PCEP sessions are established between these
routers and their domains' PCE.
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o Each PCE establishes a PCEP session with its respective neighbor
domains' PCEs. The way a PCE discovers its neighboring PCEs is
out of the scope of this document. This information could be
administratively configured or automatically discovered through,
for example, [I-D.dong-pce-discovery-proto-bgp].
o PCEs are able to compute an end-o-end path as per BRPC procedure
[RFC5441] or as per H-PCE procedure (stateless [RFC6805] or
stateful [RFC8751]).
o "Path" is a generic term to refer to both LSP setup by mean of
RSVP-TE or Segment Path in a Segment Routing network.
+----------------+ +----------------+
| Domain (B) | | Domain (C) |
| | | |
| /-------|---PCEP---|--------\ |
| / | | \ |
| (PCE) | | (PCE) |
| / (BN)<------>(BN) |
| / | Inter | |
+---|--(BN)------+ Domain +----------------+
| ^ Link
PCEP |
| | Inter-domain Link
| v
+---|--(BN)------+
| | |
| | Domain (A) |
| \ |
| (PCE) |
| |
| |
+----------------+
Example of the representation of 3 domains with 3 PCEs
1.2. Terminology
ABR: Area Border Routers. Routers used to connect two IGP areas
(areas in OSPF or levels in IS-IS).
AS: Autonomous System
ASBR: Autonomous System Border Router. Router used to connect
together ASes (of the same or different service providers) via one or
more inter-AS links.
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Border Node (BN): a boundary node is either an ABR in the context of
inter-area TE or an ASBR in the context of inter-AS TE.
BN-en(i): Entry BN of domain(i) connecting domain(i-1) to domain(i)
along a determined sequence of domains. Multiple entry BN-en(i)
could be used to connect domain(i-1) to domain(i).
BN-ex(i): Exit BN of domain(i) connecting domain(i) to domain(i+1)
along a determined sequence of domains. Multiple exit BN-ex(i) could
be used to connect domain(i) to domain(i+1).
Domains: Autonomous System (AS) or IGP Area. An Autonomous System is
composed by one or more IGP area.
ERO(i): The Explicit Route Object scoped to domain(i)
IGP-TE: Interior Gateway Protocol with Traffic Engineering support.
Both OSPF-TE and IS-IS-TE are identified in this category.
Inter-domain path: A path that crosses two or more domains through a
pair of Border Node (BN-ex, BN-en).
LK(i): A Link that connect BN-ex(i-1) to BN-en(i). Note that BN-
ex(i-1) could be connected to BN-en(i) by more than one link. LK(i)
identifies which of the multiple links will be used for the inter-
domain path setup. For inter-AS scenario, LK(i) represents the link
between ASBR of domain i to the ASBR of domain i-1. For inter-area
scenario, LK(i) is present only in IS-IS networks and represents the
link between ABR of region L1, reciprocally L2, to the ABR of region
L2, reciprocally L1.
Local path: A path that does not cross a domain border. It is set up
either from entry BN-en, to output BN-ex or between both. This path
could be enforce by means of RSVP-TE signaling or Segment Routing
labels stack.
Local path(i): A Local path of domain(i)
PLSP-ID(i): A PLSP-ID that identifies, in the domain(i), the local
part of an inter-domain path.
PCE: Path Computation Element. An entity (component, application, or
network node) that is capable of computing a network path or route
based on a network graph and applying computational constraints.
PCE(i) is a PCE within the scope of domain(i).
PST: Path Setup Type
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R(i,j): The router j of domain i
Stitching Label (SL): A dedicated label that is used to stitch two
RSVP-TE LSPs or two Segment Routing paths.
SL(i): A Stitching Label that links domain(i-1) to domain(i).
2. Stitching Label
This section introduces the concept of Stitching Label that allows
stitching and nesting of local paths in order to form an inter-domain
path that cross several different domains.
2.1. Definition
The operation of stitch or nest a local path(i) to a local path(i+1)
in order to form and inter-domain path mainly consists in defining
the label that the output BN-ex(i) will use to send its traffic to
the entry BN-en(i+1). Indeed, the entry BN-en(i+1) needs to identify
the incoming traffic (e.g. IP packets), in order to know if this
traffic must follow the local path(i+1) or not. Forwarding
Equivalent Class (FEC) could be used for that purpose. But, when
stitching or nesting tunnels, the FEC is reduced to the incoming
label that the entry BN-en(i+1) has chosen for the local path(i+1).
In this memo, we introduce the term of "Stitching Label (SL)" to
refer to this label. Such label is usually exchanged between output
BN-ex(i) and entry BN-en(i+1) with the RSVP-TE signaling. But, as we
want to avoid to use RSVP-TE signaling due to operational
constraints, and allow compatibility support for Segment Routing,
this Stitching Label is here conveyed by PCEP. In fact, the Explicit
Route Object (ERO) and the Record Route Object (RRO) are already
defined in order to transport (G)MPLS labels (for RSVP-TE or Segment
Routing) in the PCEP signaling. Thus, the Stitching Label could be
conveyed in the ERO and RRO without any modification of PCEP nor PCEP
Objects.
As per RFC4003 [RFC4003], the Stitching Label will be conveyed as a
companion of a link identifier (e.g. an IP address for numbered
links). In our case, this is one of the endpoint IDs of the link
LK(i) which connects BN-ex(i) to BN-en(i+1) and carries the traffic
from the domain(i) to domain(i+1). It is left to implementation to
select which of the two endpoint IDs of the link LK(i) is used.
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2.2. Inter-domain LSP-TYPE
Even if PCEP could convey the Stitching Label, a PCC is not aware
that a PCE requests or provides such a label. For that purpose, this
specification relies on the use of the PST as defined in [RFC8408]
with new values (See IANA section of this memo) defined as follow:
o TBD1: Inter-Domain TE end-to-end path is set up using Backward
Recursive or Hierarchical method. This new PST value MUST be set
in a PCInitiate messages sends by a PCE(i-1) to its neighbor
PCE(i) in the Backward Recursive method or by the Parent PCE to
the Child PCE(i) to initiate a new inter-domain path. In its
response, the neighbor PCE(1) or Child PCE(i) MUST return a
Stitching Label SL with an identifier of the associated link in
the RRO of the PCRpt message to PCE(i-1) or Parent PCE.
o TBD2: Inter-Domain TE local path is set up using RSVP-TE. This
new PST value MUST be set in the PCInitiate message sends by a
PCE(i) requesting to a PCC of domain(i) to initiate a new local
path(i) which is part of an inter-domain path. This PST value
MUST be used by the PCE(i) only after receiving a PCInitiate
message with an PST equal to TBD1 from a neighbor PCE(i-1) in the
Backward Recursive method or Parent PCE in the Hierarchical
method. In its response, the PCC of domain(i) MUST return a
Stitching Label SL with the an identifier of associated link in
the RRO of the PCRpt message.
o TBD3: Inter-Domain TE local path is set up using Segment Routing.
This new PST value MUST be set in the PCInitiate message sends by
a PCE(i) requesting to a PCC of domain(i) to initiate a new
Segment Routing path which is part of and inter-domain Segment
Routing path. This PST value MUST be used by the PCE(i) only
after receiving a PCInitiate message with an PST equal to TBD1
from a neighbor PCE(i-1). In its response, the PCC MUST return a
Stitching Label SL with an identifier of the associated link in
the RRO of the PCRpt message.
3. Backward Recursive PCInitiate Procedure
This section describes how to set up inter-domain paths that cross
different domains by using a Backward Recursive method. It is
compatible with the inter-domain path computation by means of the
BRPC procedure as describe in RFC5441 [RFC5441].
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3.1. Mode of Operation
This section describes how PCInitiate and PCRpt messages are combined
between PCE in order to set up inter-domain paths between a source
domain(1) to a destination domain(n). S and D are respectively the
source and destination of the inter-domain path. Domain(1) and
domain(n) are different and connected through 0 (i.e. direct
connection when n = 2) or more intermediate domains denoted domain(i)
with i = [2, n-1].
First, the PCE(1) runs standard BRPC algorithm as per RFC5441
[RFC5441] with its neighbor PCEs in order to compute the inter-domain
path from S to D, where S and D are respectively a node in the
domain(1) and domain(n). Path Key confidentiality as per RFC5520
[RFC5520] SHOULD be used to obfuscate the detailed ERO(i) of the
different domains(i). The resulting ERO is in the form {S, PKS(1),
BN-ex(1), ..., BN-en(i), PKS(i), BN-ex(i), ..., BN-en(n), PKS(n), D}
when Path Key is used and of the form {S, R(1,1), ..., R(1,k), BN-
ex(1), ..., BN-en(i), R(i,1), ..., R(i,l), BN-ex(i), ..., BN-en(n),
R(n,1), ..., R(n,m), D} otherwise . As subsequent domains are not
aware about the computed end-to-end ERO in case of Virtual Source
Path trees (VSPTs), the final ERO selected by the PCE(1) MUST be sent
in the PCInitiate message to indicate to the subsequent PCEs which
path has been finally chosen. PCE(1) MUST ensure that this ERO is
self comprehensive by subsequent PCEs. Indeed, when a PCE(i)
receives the ERO, it MUST be able to verify that this ERO matches its
own scope and to determine the PCE(i+1). When Path Key is used, PCEs
MUST encode the Path Key with a reachable IP address so that previous
PCEs in the AS chain are able to join them. When Path Key is not
used, the PCEs MUST be able to retrieve an IP address of the next PCE
corresponding to the ERO (e.g., relying on a per prefix table).
The complete procedure with Path Key follows the different steps
described below:
Steps 1: Initialization
Once ERO(S, D) is computed, PCE(1) sends a PCInitiate message to
PCE(2) containing an ERO equal to {S, PKS(2), ..., PKS(i), ...,
PKS(n), D}, PST = TBD1 and End-Points Object = (S, D). The ERO
corresponds to the one PCE(1) has received from PCE(2) during the
BRPC process in which only Path Key are kept. In case of multiple
EROs, i.e. VSPT, PCE(1) has chosen one of them and used the selected
one for the PCInitiate message. PKS(i) could be replaced by the full
ERO description if Path Key is not used by PCE(i).
When PCE(i) receives a PCInitiate message from domain(i-1) with PST =
TBD1 and ERO = {PKS(i), PKS(i+1), ..., PKS(n), D)}, it sends a
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PCInitiate message to PCE(i+1) with a popped ERO and records its
received PKS(i) part. All PCE(i)s generate the appropriate
PCInitiate message to PCE(i+1) up to PCE(n), i.e. to the destination
domain(n).
Steps 2: Actions taken at the destination domain(n) by PCE(n)
1. When a PCInitiate message reaches the destination domain(n),
PCE(n) retrieves the ERO from the PKS(n) if necessary and sends
to BN-en(n) a PCInitiate message with the ERO(n) = {BN-en(n),
..., D}, PST = TBD2 and End-Points Object = {BN(n), D} in order
to inform the PCC BN-en(n) that this local path(n) is part of an
inter-domain path.
2. When the PCC BN-en(n) receives the PCInitiate message from its
PCE(n), it sets up the local path from entry BN-en(n) to D by
means of RSVP-TE signaling with the given ERO(n).
3. Once the tunnel is set up, BN-en(n) chooses a free label for the
Stitching Label SL(n) and adds a new entry in its MPLS L(F)IB
with this SL(n) label. Then, it sends a PCRpt message to its
PCE(n) with an RRO equal to {[LK(n), SL(n)], RRO(n)} and PLSP-
ID(n).
4. Once PCE(n) receives the PCRpt from the PCC BN-en(n) with the
RRO, PLSP-ID and PST = TBD2, it sends to the PCE(n-1) a PCRpt
containing the RRO equal to {[LK(n), SL(n)]} and PLSP-ID(n).
PCE(n) MAY add {PKS(n), D} in the RRO.
Steps i: Actions performed by all intermediate domains(i), for i = 2
to n-1
1. When the PCE(i) receives a PCRpt message from domain(i+1) with
PST = TBD1, RRO = {[LK(i+1), SL(i+1)]} and PLSP-ID(i+1), it
retrieves the ERO(i) from the PKS(i), recorded in step 1, and
sends to the PCC BN-en(i) a PCInitiate message with ERO =
{ERO(i), [LK(i+1), SL(i+1)]}, PST = TBD2 and End-Points Object =
{BN-en(i), BN-ex(i)} in order to inform the PCC BN-en(i) that
this local path(i) is part of an inter-domain path.
2. When the PCC BN-en(i) receives the PCInitiate message from its
PCE(i), it sets up the local path from BN-en(i) to BN-ex(i) by
means of RSVP-TE signaling with the given ERO(i).
3. Egress Control mechanism, as per RFC4003 section 2.1 [RFC4003],
is used to instruct the egress node of domain(i), i.e. BN-ex(i),
to forward packets belonging to this tunnel with the Stitching
Label. Both the Stitching Label and the identifier of the
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interface are carried in the ERO = {..., [LK(i+1), SL(i+1)]} as
the last SubObject in conformance to [RFC4003]. As a result, BN-
ex(i) installs in its MPLS L(F)IB the SWAP instruction to label
SL(i+1) with forward to LK(i+1).
4. Once the tunnel is set up, PCC BN-en(i) chooses a free label for
the Stitching Label SL(i) and adds a new entry in its MPLS L(F)IB
with this SL(i) label. Then, it sends a PCRpt message to its
PCE(i) with an RRO equal to {[LK(i), SL(i)], RRO(i)} and PLSP-
ID(i).
5. Once PCE(i) receives the PCRpt from the PCC BN-en(i) with the RRO
and PST = TBD2, it sends to PCE(i-1) a PCRpt message containing
the RRO equal to {[LK(i), SL(i)]} and the PLSP-ID(i). PCE(i) MAY
add {PKS(i), ..., PKS(n)} in the RRO.
Steps n: Actions performed at the source domain(1) by PCE(1)
Once PCE(1) receives the PCRpt message from PCE(2) with the RRO
containing the label SL(2), it sends a PCInitiate message to PCC node
S with ERO equal to {ERO(1), [LK(2), SL(2)]}, PST = 0 and End-Points
Object = {S, BN-ex(1)}. This time, the PST is equal to 0 as the PCC S
does not need to return a Stitching Label SL, because it is the head-
end of the inter-domain path. A usual PCRpt message is sent back to
PCE(1) by the PCC node S.
3.2. Example
In the figure below, two different domains S and D are interconnected
through BN respectively BN-S and BN-D. PE-S and PE-D are edge
routers. All routers in the figure are connected to their respective
PCE through PCEP. In this example, we consider that PCE(S) needs to
set up an inter-domain path between PE-S and PE-D acting as source
and destination of the path. To simplify the figure, neither
intermediate routers between (PE-S, BN-S), (BN-D and PE-D), nor RSVP-
TE messages are represented, but they are all presents. The
following notation is used (in this example, we use the PKS for the
sake of simplicity):
o PKS(D) = Path Key corresponding to the path from BN(D) to PE-D
o ERO(D) = Explicit Route Object corresponding to the path from
BN(D) to PE-D, retrieved from PKS(D)
o RRO(D) = Record Route Object of the local path(D) from BN(D) to
PE-D
o SL(D) = Stitching Label for the local path from BN(D) to PE-D
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o ERO(S) = Explicit Route Object corresponding to the path from PE-S
to BN(S)
o RRO(S) = Record Route Object of local path(S) from PE-S to BN(S)
PE-S PCE-S BN-D PCE-D
| | | |
| [ -------- Standard BRPC exchange ------------]
| | | |
| | PCInitiate(ERO={PKS(D)}, PST = TBD1)
| | --------------------------------------> |
| | | |
| | PCInitiate(ERO = ERO(D), PST = TBD2)
| | | <------- |
| | | |
| | PCRpt(RRO = {SL(D), RRO(D)}, PST = TBD2)
| | | ------> |
| | | |
| PCRpt(RRO = {SL(D), PKS(D)}, PST = TBD1, PLSP-ID(D))
| | <-------------------------------------- |
| | | |
| PCInitiate(ERO={ERO(S), SL(D), BN(D)}, PST = 0)
| <------- | | |
| | | |
| PCRpt(RRO={RRO(S)}, PST = 0) | |
| -------> | | |
| | | |
+----------------------+ +----------------------+
| | | |
| +------+ | PCEP | +------+ |
| +---->|PCE(S)|<-------------------------------->|PCE(D)| |
| | +------+ | | +------+ |
| | ^ | | ^ ^ |
| | | | | | | |
| |PCEP | | | | | |
| | |PCEP | | PCEP | | PCEP |
| v | | | | | |
(PE-S) +------> (BN-S) <---------> (BN-D)<----+ +----> (PE-D)
| | Inter-Domain | |
| Domain (S) | Link | Domain (D) |
+----------------------+ +----------------------+
[--- LSP Tunnel (S) ---][---- SL label ----][--- LSP Tunnel (D) ---]
Example of inter-domain path setup between two domains
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3.3. Completion Failure of Inter-domain Path Setup Procedure
In case of error during path setup, PCRpt and or PCErr messages MUST
be used to signal the problem to the neighbor PCE domain backward.
In particular, if the new PST values defined in this memo are not
supported by the neighbor PCE or the PCC, the PCE, respectively the
PCC, MUST return a PCErr message with Error-Type = 21 (TE path setup
error) and Error-Value = 1 (Unsupported path setup type) to its
neighbor PCE. If a PCE(i) receives a PCInitiate message from its
peer PCE(i-1) without PST set to TBD1 or PST set to a value different
from TBD1, it MUST return a PCErr message with Error-Type = 21 (TE
path setup error) and Error-Value = 1 (Unsupported path setup type)
to its peer PCE(i-1).
Following a PCInitiate message with PST set to TBD1, if a PCC or a
PCE returns no RRO, or an RRO without the Stitching Label SL and an
identifier of the associated link, the PCE MUST return a PCErr
message with Error-Type = 21 (TE path setup error) and Error-Value =
TBD5 (Mandatory Stitching Label missing in the RRO).
In case of completion failure, the PCE(i) MUST propagate the PCErr
message up to the PCE(1). In turn, PCE(1) MUST send a PCInitate
message (R flag set in the SRP Object as per [RFC8281]) to tear down
this inter-domain path from its neighbor PCEs. PCE(i) MUST propagate
the PCInitiate message and remove its local path by means of
PCInitiate message to its PCC BN-en(i) and send back PCRpt message to
PCE(i-1).
In case of error in domain(i+1), PCE(i) MAY add the AS number of
domain(i+1) in the RRO to identify the faulty domain.
4. Hierarchical PCInitiate Procedure
This section describes how to set up inter-domain paths that cross
different domains by using a hierarchical method. It is compatible
with inter-domain path computation as described in [RFC6805].
4.1. Mode of Operation
This section describes how PCInitiate and PCRpt messages are combined
between PCEs in order to set up inter-domain paths between a source
domain(1) to a destination domain(n). S and D are respectively the
source and destination of the inter-domain path. Domain(1) and
domain(n) are different and connected through 0 or more intermediate
domains denoted domain(i) with i = (2, n-1). Domains are directly
connected when n = 2.
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First, the Parent PCE contacts its Child PCE as per [RFC6805] in
order to compute the inter-domain path from S to D, where S and D are
respectively a node in the domain(1) and domain(n). Path Key
confidentiality as per RFC5520 [RFC5520] SHOULD be used to obfuscate
the detailed ERO(i) of the different domains(i). The resulting ERO
is of the form (S, PKS(1), BN-ex(1), ..., BN-en(i), PKS(i), BN-ex(i),
..., BN-en(n), PKS(n), D) when Path Key is used and of the form {S,
R(1,1), ..., R(1,k), BN-ex(1), ..., BN-en(i), R(i,1), ..., R(i,l),
BN-ex(i), ..., BN-en(n), R(n,1), ..., R(n,m), D} otherwise.
The complete procedure with Path Key follow the different steps
described below:
Step 1: Initialization
1. The Parent PCE sends a PCInitiate message to Child PCE(n) with an
ERO = {PKS(n)} and End-Points = {BN-en(n), D}. Then, PCE(n)
retrieves the ERO from the PKS(n) (if necessary) and sends to BN-
en(n) a PCInitiate message with the ERO(n) = {BN-en(n), ..., D},
PST = TBD2 and End-Points Object = {BN-en(n), D} in order to
inform the PCC BN-en(n) that this local path(n) is part of an
inter-domain path.
2. When the PCC BN-en(n) receives the PCInitiate message from its
PCE(n), it sets up the local path from the entry BN-en(n) to D by
means of RSVP-TE signaling with the given ERO(n).
3. Once the path is set up, it chooses a free label for the
Stitching Label SL(n) and adds a new entry in its MPLS L(F)IB
with this SL(n) label. Then, it sends a PCRpt message to its
PCE(n) with an RRO equal to {[LK(n), SL(n)], RRO(n)} and PLSP-
ID(n).
4. Once PCE(n) receives the PCRpt from the PCC BN-en(n) with the
RRO, PLSP-ID and PST = TBD2, it sends to its Parent PCE a PCRpt
containing the RRO equal to {[LK(n), SL(n)]} and PLSP-ID(n).
PCE(n) MAY add PKS(n) in the RRO.
Steps i: Actions performed for all intermediate domains(i), for i =
n-1 to 2
1. The Parent PCE sends a PCInitiate message to Child PCE(i) with
PST = TBD1, ERO = {PKS(i), [LK(i+1), SL(i+1)]} and End-Points =
{BN-en(i), BN-ex(i)}
2. Then, PCE(i) retrieves the ERO from the PKS(i) if necessary and
sends to the PCC BN-en(i) a PCInitiate message with ERO =
{ERO(i), [LK(i+1), SL(i+1)]}, PST = TBD2 and End-Points Object =
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{BN-en(i), BN-ex(i)} in order to inform the PCC BN-en(i) that
this local path(i) is part of an inter-domain path.
3. When the PCC BN-en(i) receives the PCInitiate message from its
PCE(i), it sets up the local path from BN-en(i) to BN-ex(i) by
means of RSVP-TE signaling with the given ERO(i).
4. Egress Control mechanism, as per RFC4003 section 2.1 [RFC4003],
is used to instruct the egress node of domain(i), i.e. BN-ex(i)
to forward packets belonging to this tunnel with the Stitching
Label. Both the Label Stitching and an identifier of the
outgoing interface are carried in the ERO = {..., [LK(i+1),
SL(i+1)]} as the last SubObject in conformance to [RFC4003]. So
that, BN-ex(i) installs in its MPLS L(F)IB the SWAP instruction
to label SL(i+1) with forward to LK(i+1) instead of the usual POP
instruction.
5. Once the tunnel is set up, PCC BN-en(i) chooses a free label for
the Stitching Label SL(i) and adds a new entry in its MPLS L(F)IB
with this SL(i) label. Then, it sends a PCRpt message to its
PCE(i) with an RRO equal to {[LK(i), SL(i)], RRO(i)} and PLSP-
ID(i).
6. Once PCE(i) receives the PCRpt from the PCC BN-en(i) with the RRO
and PST = TBD2, it sends to its Parent PCE a PCRpt message
containing the RRO equal to {[LK(i), SL(i)]} and the PLSP-ID(i).
PCE(i) MAY add PKS(i) in the RRO.
7. Once the Parent PCE receives the PCRpt from the Child PCE(i), it
stores the corresponding PLSP-ID for this inter-domain path part.
Steps n: Actions performed to the source domain(1)
Finally, the Parent PCE sends a last PCInitiate message to its Child
PCE(1) with PST = TBD1, ERO = {PKS(1), [LK(2), SL(2)]} and End-Points
= {S, BN-ex(1)}. In turn, Child PCE(1) sends a PCInitiate message to
PCC node S with ERO equal to {ERO(1), [LK(2), SL(2)]}, PST = 0 and
End-Points Object = {S, BN-ex(1)}. This time, the PST is equal to 0
as the PCC S does not need to return a Stitching Label SL, because it
is the head-end of the inter-domain path. A usual PCRpt message is
sent back to PCE(1) by the PCC node S. In turn, Child PCE(1) sends a
final PCRpt message to the Parent PCE with the PSLP-ID(1). PCE(1)
MAY add {S, BN-ex(1)} in the RRO as a loose path.
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4.2. Completion Failure of Inter-domain Path Setup Procedure
In case of error during path set up, PCRpt and or PCError messages
MUST be used to signal the problem to the Parent PCE. In particular,
if the new PST values defined in this memo are not supported by the
Child PCE or the PCC, the Child PCE, respectively the PCC, MUST
return a PCErr message with Error-Type = 21 (TE path setup error) and
Error-Value = 1 (Unsupported path setup type) to its Parent PCE. If
Child PCE(i) receives a PCInitiate message from its Parent PCE
without PST set to TBD1 or PST set to a value different from TBD1, it
MUST return a PCErr message with Error-Type = 21 (TE path setup
error) and Error-Value = 1 (Unsupported path setup type) to its
Parent PCE.
Following a PCInitiate message with PST set to TBD1, if a Child PCE
or a PCC returns no RRO, or an RRO without the Stitching Label SL and
an identifier of the associated link, the Parent PCE, respectively
the Child PCE, MUST return a PCErr message with Error-Type = 21 (TE
path setup error) and Error-Value = TBD5 (Mandatory Stitching Label
missing in the RRO).
In case of completion failure, the Parent PCE MUST send a PCInitate
message (R flag set in the SRP Object as per [RFC8281]) to tear down
this inter-domain path from the Child PCEs that already set up their
respective part of the inter-domain path. Child PCE(i) MUST remove
its local path by means of PCInitiate message with R flag set to 1 to
its PCC BN-en(i) and send back a PCRpt message to the Parent PCE.
4.3. Example for Stateful H-PCE Stiching Procedure
Taking the sample hierarchical domain topology example from [RFC6805]
as the reference topology for the entirety of this section.
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-----------------------------------------------------------------
| Domain 5 |
| ------- |
| |P-PCE 5| |
| ------- |
| |
| ---------------- ---------------- ---------------- |
| | Domain 1 | | Domain 2 | | Domain 3 | |
| | | | | | | |
| | ------- | | ------- | | ------- | |
| | |C-PCE 1| | | |C-PCE 2| | | |C-PCE 3| | |
| | ------- | | ------- | | ------- | |
| | | | | | | |
| | ----| |---- ----| |---- | |
| | |BN11+---+BN21| |BN23+---+BN31| | |
| | - ----| |---- ----| |---- - | |
| | |S| | | | | |D| | |
| | - ----| |---- ----| |---- - | |
| | |BN12+---+BN22| |BN24+---+BN32| | |
| | ----| |---- ----| |---- | |
| | | | | | | |
| | ---- | | | | ---- | |
| | |BN13| | | | | |BN33| | |
| -----------+---- ---------------- ----+----------- |
| \ / |
| \ ---------------- / |
| \ | | / |
| \ |---- ----| / |
| ----+BN41| |BN42+---- |
| |---- ----| |
| | | |
| | ------- | |
| | |C-PCE 4| | |
| | ------- | |
| | | |
| | Domain 4 | |
| ---------------- |
| |
-----------------------------------------------------------------
Hierarchical domain topology from RFC6805
Section 3.3.1 of [RFC8751] describes the per-domain stitched LSP mode
and list all the steps needed. To support SL-based stitching, using
the reference architecture described in the figure above, the steps
are modified as follows (note that we do not use PKS in this example
for simplicity):
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Step 1: initialization
The P-PCE (PCE5) is requested to initiate a path. Steps 4 to 10 of
section 4.6.2 of [RFC6805] are executed to determine the end-to-end
path, which are split into per-domain paths, e.g. {S-BN41,
BN41-BN33, BN33-D}.
Step 2: Path (BN33-D) at C-PCE3:
1. The P-PCE (P-PCE5) sends the initiate request to the C-PCE
(C-PCE3) via PCInitiate message for path (BN33-D) with
ERO={BN33..D} and PST = TBD1.
2. C-PCE3 further propagates the initiate message to BN33 with the
ERO and PST = TBD2/TBD3 based on the setup type.
3. BN33 initiates the setup of the path and reports to the status
("GOING-UP") to C-PCE3.
4. C-PCE3 further reports the status of the path to the P-PCE
(P-PCE5)
5. The node BN33 notifies the path state to C-PCE3 when the state is
"UP"; it also sends the Stitching Label (SL33) in the RRO as
{SL33,BN33..D}.
6. C-PCE3 further reports the status of the path to the P-PCE
(P-PCE5) as well as sends the Stitching Label (SL33) in the RRO
as {LK33,SL33,BN33..D}.
Step 3: Path (BN41-BN33) at C-PCE4
1. The P-PCE (P-PCE5) sends the initiate request to the C-PCE
(C-PCE4) via PCInitiate message for path (BN41-BN33) with
ERO={BN41..BN42,LK33,SL33,BN33} and PST = TBD1.
2. C-PCE4 further propagates the initiate message to BN41 with the
ERO and PST = TBD2/TBD3 based on the setup type. In case of
RSVP_TE, the node BN41 encode the Stitching Label SL33 as part of
the ERO to make sure the node BN42 uses the label SL33 towards
node BN33. In case of SR, the label SL33 is part of the label
stack pushed at node BN41.
3. BN41 initiates the setup of the path and reports the path status
("GOING-UP") to C-PCE4.
4. C-PCE4 further reports the status of the path to the P-PCE
(P-PCE5).
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5. The node BN41 notifies the path state to C-PCE4 when the state is
"UP"; it also sends the Stitching Label (SL41) in RRO as
{LK41,SL41,BN41..BN33}.
6. C-PCE4 further reports the status of the to the P-PCE (P-PCE5) as
well as sends the Stitching Label (SL41) in the RRO as
{LK41,SL41,BN41..BN33}.
Step 3: Path (S-BN41) at C-PCE1
1. The P-PCE (P-PCE5) sends the initiate request to the C-PCE
(C-PCE1) via PCInitiate message for path (S-BN41) with
ERO={S..BN13,LK41,SL41,BN41}.
2. C-PCE1 further propagates the initiate message to node S with the
ERO. In case of RSVP-TE, node S encodes the Stitching Label SL41
as part of the ERO to make sure the node BN13 uses the label SL41
towards node BN41. In case of SR, the label SL41 is part of the
label stack pushed at node S.
3. S initiates the setup of the path and reports the path status
("GOING-UP") to C-PCE1.
4. C-PCE1 further reports the status of the path to the P-PCE
(P-PCE5)
5. The node S notifies the path state to C-PCE1 when the state is
"UP".
6. C-PCE1 further reports the status of the path to the P-PCE
(P-PCE5).
In this way, per-domain paths are stitched together using the
Stitching Label (SL). The per-domain paths MUST be set up from the
destination domain towards the source domain one after the other.
Once the per-domain path is set up, the entry BN chooses a free label
for the Stitching Label SL and adds a new entry in its MPLS L(F)IB
with this SL label. The SL from the destination domain is propagated
to adjacent transit domain, towards the source domain at each step.
This happens from the entry BN to C-PCE then to the P-PCE, and vice-
versa. In case of RSVP-TE, the entry BN further propagates the SL
label to the exit BN via RSVP-TE. In case of SR, the SL label is
pushed as part of the SR label stack.
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5. Inter-domain Path Management
This section describes how inter-domain paths could be managed.
5.1. Stitching Label PCE Capabilities
A PCE needs to know if its neighbor PCEs as well as PCCs are able to
configure and provide a Stitching Label. The STITCHING-LABEL-PCE-
CAPABILITY TLV is an optional TLV for use in the OPEN object for
Stitching Label PCE capability advertisement. Its format is shown in
the following figure:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=TBD7 | Length=4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags |I|R|S|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
STITCHING-LABEL-PCE-CAPABILITY TLV Format
The Type (16 bits) of the TLV is TBD7. The Length field is 16 bits
long and has a fixed value of 4.
The value comprises a single 32 bits "Flags" field:
R (RSVP-TE-STITCHING-LABEL-CAPABILITY - 1 bit): if set to 1 by a PCC,
the R flag indicates that the PCC is able to provide Stitching
Labels, for RSVP-TE inter-domain paths, when requested by a PCE. If
set to 1 by a PCE, the R flag indicates that the domain controlled by
this PCE is able to set up inter-domain paths by means of RSVP-TE
signaling.
S (SEGMENT-ROUTING-STITCHING-LABEL-CAPABILITY - 1 bit): if set to 1
by a PCC, the S flag indicates that the PCC is able to provide
Stitching Labels, for Segment-Routing inter-domain paths, when
requested by a PCE. If set to 1 by a PCE, the R flag indicates that
the domain controlled by this PCE is able to set up inter-domain
paths by means of Segment Routing.
I (INTER-DOMAIN-STITCHING-LABEL-CAPABILITY - 1 bit): if set to 1 by a
PCE, the I flag indicates that the domain is supporting Stitching
Label to set up inter-domain paths. This flag is reserved for PCEP
session established between PCEs and MUST be kept unset by a PCC.
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Unassigned bits are considered reserved. They MUST be set to 0 on
transmission and MUST be ignored on receipt.
PCCs MUST set the R and/or S flags and MUST NOT set the I flag when
adding the Stitching Label Capability to the PCEP Open Message. The
RSVP-TE-STITCHING-LABEL-CAPABILITY, respectively SEGMENT-ROUTING-
STITCHING-LABEL-CAPABILITY, flag must be set by both the PCC and PCE
in order to enable the configuration of Stitching Labels with RSVP-
TE, respectively with Segment-Routing.
A PCE MUST set the I flag when establishing a PCEP session with a
neighbor PCE when adding Stitching Label Capability to the PCEP Open
Message. It MAY set R and/or S flags depending if the operator would
like to keep confidential the technology used to set up inter-domain
paths or not. The INTER-DOMAIN-STITCHING-LABEL-CAPABILITY flag must
be set by both PCEs in order to enable inter-domain paths
instantiation by means of Stitching Label.
5.2. Identification of Inter-domain Paths
First, in order to manage inter-domain paths composed by the
stitching or nesting of local paths, it is important to identify
them. For this purpose, the PLSP-ID managed by the PCEs are combined
to one provided by PCCs to form a global identifier as follow:
o PCE(i) in the Backward Recursive method or the Child PCE in
Hierarchical method MUST create a new unique PLSP-ID for this
inter-domain path part and MUST send it in the PCRpt message, to
the PCE(i-1), respectively the Parent PCE. In addition this new
PLSP-ID MUST be associated to the one received from the PCC that
instantiates the local path part for further reference.
o In the Hierarchical mode, the Parent PCE MUST store and associate
the different PLSP-ID(i)s received from the different Child
PCE(i)s in order to identify the different part of the inter-
domain paths.
o In the Backward Recursive method, PCE(i) MUST store and associate
its PLSP-ID(i) and the PLSP-ID(i+1) it received from the PCE(i+1).
PCE(n), i.e. the last one in the chain, does not need to perform
such association.
Further reference to the inter-domain path will use this PLSP-ID(i).
In the Backward Recursive method, PCE(i) MUST replace the PLSP-ID(i)
by PLSP-ID(i+1) in the PCUpd, PCRpt or PCinitiate message before
propagating it to PCE(i+1); and PCE(i) MUST replace the PLSP-ID(i+1)
by PLSP-ID(i) in the PCRpt message before propagating it to the
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PCE(i-1). In the Hierarchical method, the Parent PCE MUST use the
corresponding PLSP-ID(i) of the Child PCE(i).
5.3. Inter-domain Association Group
In case of failure, a PCE(i) will received PCRpt messages from its
PCCs and neighbors PCE(i+1) to synchronize the Inter-domain paths.
In addition, it may received PCInitiate messages from its previous
neighbors PCE(i-1) to re-initiate its inter-domain path part. As the
PCE(i) may loose the PLSP-ID association, a new association group
(within Association Object) is used to ease the association of the
different parts of the inter-domain path: the local part and the PCE-
to-PCE part. The use of the Association Object is MANDATORY in the
Backward Recursive method and OPTIONAL in the Hierarchical method.
For that purpose, a new Inter-Domain Association Type with value TBD4
is defined. The first PCE in the Backward Recursive chain (the one
which received the initial request) MUST send the PCInitiate message
with an Association Object as follows:
o Association Type field MUST be set to new value TBD4
o Association ID MUST be set to a unique value. In case the
Association ID field is too short or wraps, the first PCE MAY use
the Extended Association ID to increase the number of association
groups. The Association ID is managed locally by the PCE and does
not need to be coordinated with neighbor or remote PCEs.
o IPV4 or IPv6 association source MUST be set to the IP address
which identifies PCE(1) in domain(1).
o The Global Association Source TLV MUST be present and set with the
ASN number of domain(1). It allows to create a globally unique
association scope without putting constraint on operator's IP
association source. Thus the IP Association Source is associated
with the Global Association source to form a unique identifier.
o Extended Association ID MAY be present and MANDATORY if
association ID is too short or wraps.
Subsequent PCE(i), for i = 2 to n, MUST send this Association Object
as is to the local PCC and the neighbor PCE(i+1).
In case of error with the association group, a PCErr message MUST be
raised with Error = 26 (Association Error) and Error value set
accordingly. A new Error value TBD6 is defined to identify
association of inter-domain paths.
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In the Hierarchical method, the Parent PCE MAY act as the initiator
of the Association and send to the Child PCEs an Association Object
that follows the same rules as for the Backward Recursive method. In
turn, Child PCEs MUST propagate the Association Object to the local
PCCs as is.
5.4. Modification of Inter-domain Paths
For the Backward Recursive method, each domain manages their
respective local path part of an inter-domain path independently of
each other. In particular, Stitching Label(i) is managed by
domain(i) and is of interest of domain(i-1) only. Thus, Stitching
Label SL(i) is not supposed to be propagated to other domains. The
same behavior apply to PLSP-ID(i). In the Hierarchical method, the
Parent PCE MUST ensure the correct distribution of Stitching Label
SL(i) to Child PCE(i-1). The PLSP-ID(i) is kept for the usage of the
Parent PCE and thus is not propagated. Only the Association Object
defined in section 5.2 is propagated if it is present.
If PCE(i) needs to modify its local path(i) with a PCUpd message to
the PCC BN-en(i), once the PCRpt message received from the PCC BN-
en(i), it MUST sends a new PCRpt message to advertise the
modification. This message is targeted to its neighbor PCE(i-1) in
the Backward Recursive method, respectively to the Parent PCE in the
Hierarchical method. In this case PLSP-ID(i) is used to identify the
inter-domain path. PCE(i-1), respectively the Parent PCE, MUST
propagate the PCRpt message if the modification implies the upstream
domain, e.g. if the PCRpt indicates that the Stitching Label SL(i)
has changed.
PCE(1), respectively the Parent PCE, could modify the inter-domain
path. For that purpose, it MUST send a PCUpd message to its neighbor
PCEs, respectively Child PCE, using the PLSP-ID it received. Each
PCE(i) MUST process the PCUpd message the same way they process the
PCInitiate message as define in section 3.1 for the Backward
Recursive method and in section 4.1 for the Hierarchical method.
In case a failure appear in domain(i), e.g. path becoming down,
PCE(i) MUST sends a PCRpt message to its neighbor PCE(i-1),
respectively its Parent PCE to advertise the problem in its local
part of the inter-domain path. Once PCE(1), respectively the Parent
PCE, receives this PCRpt message indicating that the path is down, it
is up to the PCE(1), respectively the Parent PCE to take appropriate
correction e.g. start a new path computation to update the ERO.
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5.5. Modification of Inter-domain Paths
Modification of local path, BN-en(i) and BN-ex(i) is left for further
study.
5.6. Tear-Down of Inter-domain Paths
The tear-down of an inter-domain path is only possible by the inter-
domain path initiator i.e. PCE(1). For the Backward Recursive
method, a PCInitiate message with R flag set to 1, PLSP-ID set
accordingly to section 5.1 and the Association Object with R flag set
to 1, is sent by PCE(1) to PCE(n) through PCE(i), and processed the
same way as described in section 3.1. For the Hierarchical method, a
PCInitiate message with R flag set to 1 is sent by the Parent PCE to
each Child PCE(i) with corresponding PLSP-ID(i), and processed
according to section 4.1. Each domain PCE(i) is responsible to tear
down its part of the path and the PCC MUST release both the Stitching
label SL in its L(F)IB and the path when it receives the PCInitiate
message with the R flag set to 1 and the corresponding PLSP-ID. The
Association Group MUST also be removed by the PCC and PCE(i).
6. Applicability
The newly introduce Stitching Label SL serves to stitch or nest part
of local paths to form an inter-domain path. Each domain is free to
decide if the incoming path is stitched or nested and how the path is
enforced, e.g. through RSVP-TE or Segment Routing. At the peering
point, the Border Node BN-ex(i) MUST encapsulate the packet with the
Stitching Label, i.e. the MPLS label prior to send them to the next
Border Node BN-en(i+1). Thus, only RSVP-TE and Segment Routing over
MPLS technology are detailed in the following sections.
6.1. RSVP-TE
In case of RSVP-TE, the Border Node BN-ex(i) needs to received the
Stitching Label from BN-en(i) through the RSVP-TE message and install
in its L(F)IB a SWAP instruction to the Stitching Label and forward
it to the next Border Node BN-en(i+1). For that purpose, the Egress
Control mechanism, as per RFC4003 section 2.1 [RFC4003], is
RECOMMENDED to instruct the Border Node BN-ex(i) of this action.
Other mechanisms to program the L(F)IB could be used, e.g. NETCONF.
As the Stitching Label could serves to stitch or nest tunnels, a
domain(i) may decide to nest the incoming LSPs into a higher
hierarchy of LSPs for a Traffic Engineering purpose. A PCE(i) may
also decide to group local LSPs part of inter-domain paths into a
higher hierarchical LSP to carry all these local paths from a BN-
en(i) to a BN-ex(i).
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6.2. Segment Routing
To use Segment Routing instead of RSVP-TE to set up the local LSP
tunnels as defined in [RFC8664], PCE(i) MUST send a PCInitiate
message with PST = TBD3 instead of TBD2 to advertise its respective
PCC that the local path is enforce by means of Segment Routing.
The Stitching Label SL(i+1) will be inserted into the label stack in
order to become the top label in the stack when the packet reaches
BN-en(i+1). Thus, the Stitching Label SL(i+1) serves as a FEC entry
for BN-en(i+1) to identify the packets that follow the next Segment
Path. For that purpose, BN-en(i+1) MUST install in its MPLS L(F)IB
an instruction to replace the incoming Stitching Label SL(i+1) by the
label stack given by the ERO(i+1) plus the Stitching Label SL(i+2),
if any.
When a packet reaches BN-ex(i), the last label in the stack before
the label SL(i+1) corresponds to a SID that allows to reach BN-
en(i+1). When there are multiple interfaces between Border Nodes,
BN-ex(i) needs to know how to send the packets to BN-en(i+1).
Similarly to the Egress Control mechanism used with RSVP-TE, it is
RECOMMENDED to use the inter-domain SID defined as per draft Egress
Peer Engineering [I-D.ietf-idr-bgpls-segment-routing-epe] for that
purpose. The inter-domain SID is announced by BN-ex(i) to PCE(i)
through BGP-LS for each interface that connect BN-ex(i) to neighbors
BN-en(i+1). Thus, the label stack will end with {BN-ex(i) SID,
Inter-Domain SID, SL(i+1)} and should be processed as follows:
o The penultimate router of domain(i) pops its node SID, and sends
the packet to the next node designated by the top label in the
label stack, i.e. the node SID of BN-ex(i) or the adjacency SID of
the link between the router and BN-ex(i).
o BN-ex(i) pops its node SID or its adjacency SID and looks up the
next label in the stack, i.e. the inter-domain SID which
corresponds to the interface to BN-en(i+1). BN-ex(i) pops this
inter-domain SID as well and sends the packet to BN-ex(i) through
the corresponding interface.
o BN-en(i+1) looks up the top label which is the Stitching Label
SL(i+1), pops it and replaces it by the sub-sequent label stack.
Other mechanisms, e.g. NETCONF, could be used to configure the
inter-domain SID on exit Border Nodes.
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6.3. Mixing Technologies
During the instantiation procedure, if PCE(i) decides to reuse a
local tunnel which is not yet part of an inter-domain tunnel, it
SHOULD send a PCUpd message with PST = TBD2 to the PCC BN-en(i), in
order to request a Stitching Label SL(i), and new ERO(i) to add the
Stitching Label SL(i+1) and the associated link to the previous ERO.
[RFC8453] describes framework for Abstraction and Control of TE
Networks (ACTN), where each Physical Network Controller (PNC) is
equivalent to C-PCE and the Multi-Domain Service Coordinator (MDSC)
to the P-PCE. The per-domain stitched LSP as per the Hierarchical
PCE architecture described in Section 3.3.1 and Section 4.1 of
[RFC8751] is well suited for ACTN. The Stitching Label mechanism as
described in this document is well suited for ACTN when per-domain
LSPs need to be stitched to form an E2E tunnel or a VN Member. It is
to be noted that certain VNs require isolation from other clients.
The SL mechanism described in this document can be applicable to the
VN isolation use-case by uniquely identifying the concatenated
stitching labels across multi-domain only to a certain VN member or
an E2E tunnel.
As each operator is free to enforce the tunnel with its technology
choice, it is a local policy decision for PCE(i) to instantiate the
local part of the end-to-end tunnel by either RSVP-TE or Segment
Routing. Thus, the PST value (i.e. TBD2 or TBD3) used in the
PCinitiate message sent by the PCE(i) to the local PCC is determined
by the local policy. How the local policy decision is set in the PCE
is out of the scope of this memo. This flexibility is allowed
because the SL principle allows to mix (data plane) technologies
between domains. For example, a domain(i) could use RSVP-TE while
domain(i+1) uses SR. The SL could serve to stitch indifferently
Segment Paths and RSVP-TE tunnels. Indeed, the SL will be part of
the label stack in order to become the top label in the stack when
reaching the BN-en(i+1). This SL could be swapped as usual if the
next domain uses RSVP-TE tunnels. When the upstream domain uses an
RSVP-TE tunnel, the SL will serve as a key for the BN-en(i+1) to
determine which label stack it must use on top of the packet for a
Segment Routing path.
6.4. Inter-Area
If use cases for inter-AS are easily identifiable, this is less
evident for inter-area. However, two scenarios have been identified:
o Paths between levels for IS-IS networks.
o Reduction of labels stack depth for Segment Routing.
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Thus, the SL could be used to stitch or nest independent tunnels
deployed through different IS-IS levels, even if there are controlled
by the same PCE. IS-IS levels are considered as domains but under
the control of the same PCE. In this scenario, there is no exchange
between PCEs (it remains internal and implementation matter) and new
TLVs are only applicable between the PCE and PCCs. The PCE requests
to the different PCCs it identifies (i.e. BNs of the different IS-IS
levels) to set up SLs and propagated them.
In large-scale networks, MSD could constraints the path computation
in the possibility of path selection i.e. explicit expression of a
path could exceeded the MSD. The SL could be used to split a too
long explicit path regarding the MSD constraints. In this scenario,
there is also no communications between PCEs and new TLVs are only
used between PCE and PCCs.
7. IANA Considerations
7.1. Path Setup Type Values
[RFC8408] defines the PATH-SETUP-TYPE TLV. IANA is requested to
allocate new code points in the PCEP PATH-SETUP-TYPE TLV PST field
registry, as follows:
+-------+-----------------------------------------------+-----------+
| Value | Description | Reference |
+-------+-----------------------------------------------+-----------+
| TBD1 | Inter-domain TE end-to-end path is set up | This |
| | using the Backward Recursive method | Document |
| TBD2 | Inter-domain TE local path is set up using | This |
| | RSVP-TE signaling | Document |
| TBD3 | Inter-domain TE local path is set up using | This |
| | Segment Routing | Document |
+-------+-----------------------------------------------+-----------+
7.2. Association Type Value
PCE Association Group [RFC8697] defines the ASSOCIATION Object and
requests that IANA creates a registry to manage the value of the
Association Type value. IANA is requested to allocate a new code
point in the PCEP ASSOCIATION GROUP TLV Association Type field
registry, as follows:
+------------------+--------------------------------+
| Association Type | Description |
+------------------+--------------------------------+
| TBD4 | Inter-domain Association Group |
+------------------+--------------------------------+
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7.3. PCEP Error Values
IANA is requested to allocate code-points in the PCEP-ERROR Object
Error Values registry for a new error-value of Error-Type 21 Invalid
TE path setup and new error-value of Error-Type 26 Association Error:
+------------+-------------+----------------------------------------+
| Error-Type | Error-Value | Description |
+------------+-------------+----------------------------------------+
| 21 | TBD5 | Mandatory Stitching Label missing in |
| | | the RRO |
| 26 | TBD6 | Error in association of Inter-domain |
| | | LSPs |
+------------+-------------+----------------------------------------+
7.4. PCEP TLV Type Indicators
IANA is requested to allocate a new TLV Type Indicator for the
"Stitching Label PCE Capability" within the "PCEP TLV Type
Indicators" subregistry of the "Path Computation Element Protocol
(PCEP) Numbers" registry:
+-------+--------------------------------+---------------+
| Value | Description | Reference |
+-------+--------------------------------+---------------+
| TBD7 | STITCHING-LABEL-PCE-CAPABILITY | This Document |
+-------+--------------------------------+---------------+
7.5. Stitching Label PCE Capability
IANA is requested to allocate a new subregistry, named "STITCHING-
LABEL-PCE-CAPABILITY TLV Flag Field", within the "Path Computation
Element Protocol (PCEP) Numbers" registry, to manage the Flag field
in the STITCHING-LABEL-PCE-CAPABILITY TLV of the PCEP OPEN object
(class = 1). New values are assigned by Standards Action [RFC8126].
Each bit should be tracked with the following qualities:
o Bit number (counting from bit 0 as the most significant bit)
o Capability description
o Defining RFC
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+-------+--------------------------------------+---------------+
| Value | Description | Reference |
+-------+--------------------------------------+---------------+
| 31 | RSVP-TE-STITCHING-CAPABILITY | This Document |
| 30 | SEGMENT-ROUTING-STITCHING-CAPABILITY | This Document |
| 29 | INTER-DOMAIN-STITCHING-CAPABILITY | This Document |
+-------+--------------------------------------+---------------+
8. Security Considerations
No modification of PCE protocol (PCEP) has been requested by this
draft which does not introduce any issue regarding security.
Concerning the PCEP session between PCEs, authors recommend to use
the secured version of PCEP as defined in PCEPS [RFC8253] or use any
other secured tunnel mechanism, e.g. IPsec tunnel to transport PCEP
session between PCEs.
9. Acknowledgements
The authors want to thanks PCE's WG members, and in particular Dhruv
Dhody who greatly contributed to the Hierarchical section of this
document and Quan Xiong for his advice.
10. Disclaimer
This work has been performed in the framework of the H2020-ICT-2014
project 5GEx (Grant Agreement no. 671636), which is partially funded
by the European Commission. This information reflects the
consortium's view, but neither the consortium nor the European
Commission are liable for any use that may be done of the information
contained therein.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5440] Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol (PCEP)", RFC 5440,
DOI 10.17487/RFC5440, March 2009,
<https://www.rfc-editor.org/info/rfc5440>.
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[RFC5441] Vasseur, JP., Ed., Zhang, R., Bitar, N., and JL. Le Roux,
"A Backward-Recursive PCE-Based Computation (BRPC)
Procedure to Compute Shortest Constrained Inter-Domain
Traffic Engineering Label Switched Paths", RFC 5441,
DOI 10.17487/RFC5441, April 2009,
<https://www.rfc-editor.org/info/rfc5441>.
[RFC8231] Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path
Computation Element Communication Protocol (PCEP)
Extensions for Stateful PCE", RFC 8231,
DOI 10.17487/RFC8231, September 2017,
<https://www.rfc-editor.org/info/rfc8231>.
[RFC8281] Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "Path
Computation Element Communication Protocol (PCEP)
Extensions for PCE-Initiated LSP Setup in a Stateful PCE
Model", RFC 8281, DOI 10.17487/RFC8281, December 2017,
<https://www.rfc-editor.org/info/rfc8281>.
[RFC8408] Sivabalan, S., Tantsura, J., Minei, I., Varga, R., and J.
Hardwick, "Conveying Path Setup Type in PCE Communication
Protocol (PCEP) Messages", RFC 8408, DOI 10.17487/RFC8408,
July 2018, <https://www.rfc-editor.org/info/rfc8408>.
[RFC8697] Minei, I., Crabbe, E., Sivabalan, S., Ananthakrishnan, H.,
Dhody, D., and Y. Tanaka, "Path Computation Element
Communication Protocol (PCEP) Extensions for Establishing
Relationships between Sets of Label Switched Paths
(LSPs)", RFC 8697, DOI 10.17487/RFC8697, January 2020,
<https://www.rfc-editor.org/info/rfc8697>.
11.2. Informative References
[I-D.dong-pce-discovery-proto-bgp]
Dong, J., Chen, M., Dhody, D., Tantsura, J., Kumaki, K.,
and T. Murai, "BGP Extensions for Path Computation Element
(PCE) Discovery", draft-dong-pce-discovery-proto-bgp-07
(work in progress), July 2017.
[I-D.ietf-idr-bgpls-segment-routing-epe]
Previdi, S., Talaulikar, K., Filsfils, C., Patel, K., Ray,
S., and J. Dong, "BGP-LS extensions for Segment Routing
BGP Egress Peer Engineering", draft-ietf-idr-bgpls-
segment-routing-epe-19 (work in progress), May 2019.
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[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
<https://www.rfc-editor.org/info/rfc3209>.
[RFC3473] Berger, L., Ed., "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling Resource ReserVation Protocol-
Traffic Engineering (RSVP-TE) Extensions", RFC 3473,
DOI 10.17487/RFC3473, January 2003,
<https://www.rfc-editor.org/info/rfc3473>.
[RFC4003] Berger, L., "GMPLS Signaling Procedure for Egress
Control", RFC 4003, DOI 10.17487/RFC4003, February 2005,
<https://www.rfc-editor.org/info/rfc4003>.
[RFC4206] Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP)
Hierarchy with Generalized Multi-Protocol Label Switching
(GMPLS) Traffic Engineering (TE)", RFC 4206,
DOI 10.17487/RFC4206, October 2005,
<https://www.rfc-editor.org/info/rfc4206>.
[RFC4655] Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
Element (PCE)-Based Architecture", RFC 4655,
DOI 10.17487/RFC4655, August 2006,
<https://www.rfc-editor.org/info/rfc4655>.
[RFC5150] Ayyangar, A., Kompella, K., Vasseur, JP., and A. Farrel,
"Label Switched Path Stitching with Generalized
Multiprotocol Label Switching Traffic Engineering (GMPLS
TE)", RFC 5150, DOI 10.17487/RFC5150, February 2008,
<https://www.rfc-editor.org/info/rfc5150>.
[RFC5520] Bradford, R., Ed., Vasseur, JP., and A. Farrel,
"Preserving Topology Confidentiality in Inter-Domain Path
Computation Using a Path-Key-Based Mechanism", RFC 5520,
DOI 10.17487/RFC5520, April 2009,
<https://www.rfc-editor.org/info/rfc5520>.
[RFC6805] King, D., Ed. and A. Farrel, Ed., "The Application of the
Path Computation Element Architecture to the Determination
of a Sequence of Domains in MPLS and GMPLS", RFC 6805,
DOI 10.17487/RFC6805, November 2012,
<https://www.rfc-editor.org/info/rfc6805>.
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[RFC8253] Lopez, D., Gonzalez de Dios, O., Wu, Q., and D. Dhody,
"PCEPS: Usage of TLS to Provide a Secure Transport for the
Path Computation Element Communication Protocol (PCEP)",
RFC 8253, DOI 10.17487/RFC8253, October 2017,
<https://www.rfc-editor.org/info/rfc8253>.
[RFC8453] Ceccarelli, D., Ed. and Y. Lee, Ed., "Framework for
Abstraction and Control of TE Networks (ACTN)", RFC 8453,
DOI 10.17487/RFC8453, August 2018,
<https://www.rfc-editor.org/info/rfc8453>.
[RFC8664] Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
and J. Hardwick, "Path Computation Element Communication
Protocol (PCEP) Extensions for Segment Routing", RFC 8664,
DOI 10.17487/RFC8664, December 2019,
<https://www.rfc-editor.org/info/rfc8664>.
[RFC8751] Dhody, D., Lee, Y., Ceccarelli, D., Shin, J., and D. King,
"Hierarchical Stateful Path Computation Element (PCE)",
RFC 8751, DOI 10.17487/RFC8751, March 2020,
<https://www.rfc-editor.org/info/rfc8751>.
Authors' Addresses
Olivier Dugeon
Orange Labs
2, Avenue Pierre Marzin
Lannion 22307
France
Email: olivier.dugeon@orange.com
Julien Meuric
Orange Labs
2, Avenue Pierre Marzin
Lannion 22307
France
Email: julien.meuric@orange.com
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Young Lee
Huawei Technologies
5340 Legacy Drive, Building 3
Plano TX 75023
USA
Email: leeyoung@huwaei.com
Daniele Ceccarelli
Ericsson
Torshamnsgatan, 48
Stockholm
Sweden
Email: daniele.ceccarelli@ericsson.com
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