Internet Engineering Task Force Q. Zhao
Internet-Draft Huawei Technology
Intended status: Standards Track Z. Ali
Expires: February 14, 2012 T. Saad
S. Sivabalan
Cisco Systems
D. King
Old Dog Consulting
R. Casellas
CTTC - Centre Tecnologic de
Telecomunicacions de Catalunya
September 14, 2011
PCE-based Computation Procedure To Compute Shortest Constrained P2MP
Inter-domain Traffic Engineering Label Switched Paths
draft-ietf-pce-pcep-inter-domain-p2mp-procedures-00
Abstract
The ability to compute paths for constrained point-to-multipoint
(P2MP) Traffic Engineering Label Switched Paths (TE LSPs) across
multiple domains has been identified as a key requirement for the
deployment of P2MP services in MPLS and GMPLS networks. The Path
Computation Element (PCE) has been recognized as an appropriate
technology for the determination of inter-domain paths of P2MP TE
LSPs.
This document describes the procedures and extensions to the PCE
communication Protocol (PCEP) to handle requests and responses for
the computation of inter-domain paths for P2MP TE LSPs.
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 14, 2012.
Zhao, et al. [Page 1]
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Copyright Notice
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . .3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . .3
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . .4
4. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . .7
5. Requirements . . . . . . . . . . . . . . . . . . . . . . . . .8
6. Objective Functions . . . . . . . . . . . . . . . . . . . . .9
7. P2MP Path Computation Procedures . . . . . . . . . . . . . . .9
7.1. Core Trees . . . . . . . . . . . . . . . . . . . . . . . .9
7.2. Core Tree Computation Procedures . . . . . . . . . . . . .11
7.3. Sub Tree Computation Procedures . . . . . . . . . . . . .12
7.4. PCEP Protocol Extensions . . . . . . . . . . . . . . . . .12
7.4.1. The Extension of RP Object . . . . . . . . . . . . . .12
7.4.2. The PCE Sequence Object . . . . . . . . . . . . . . .13
7.5. Relationship with Hierarchical PCE . . . . . . . . . . . .14
7.6. Parallelism . . . . . . . . . . . . . . . . . . . . . . .15
8. Protection . . . . . . . . . . . . . . . . . . . . . . . . . .15
8.1 End-to-end Protection . . . . . . . . . . . . . . . . . . .15
8.2 Domain Protection . . . . . . . . . . . . . . . . . . . . .15
9. Manageability Considerations . . . . . . . . . . . . . . . . .15
9.1 Control of Function and Policy . . . . . . . . . . . . . .16
9.2. Information and Data Models . . . . . . . . . . . . . . .16
9.3. Liveness Detection and Monitoring . . . . . . . . . . . .16
9.4. Verifying Correct Operation . . . . . . . . . . . . . . .16
9.5. Requirements on Other Protocols and Functional Component.17
9.6. Impact on Network Operation . . . . . . . . . . . . . . .17
9.7. Policy Control . . . . . . . . . . . . . . . . . . . . .17
10. Security Considerations . . . . . . . . . . . . . . . . . . .17
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .18
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .18
13. References . . . . . . . . . . . . . . . . . . . . . . . . . .18
13.1. Normative References . . . . . . . . . . . . . . . . . .18
13.2. Informative References . . . . . . . . . . . . . . . . .19
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1. Introduction
Multicast services are increasingly in demand for high-capacity
applications such as multicast Virtual Private Networks (VPNs), IP-
television (IPTV) which may be on-demand or streamed, and content-
rich media distribution (for example, software distribution,
financial streaming, or data-sharing). The ability to compute
constrained Traffic Engineering Label Switched Paths (TE LSPs) for
point-to-multipoint (P2MP) LSPs in Multiprotocol Label Switching
(MPLS) and Generalized MPLS (GMPLS) networks across multiple domains.
A domain can be defined as a collection of network elements within a
common sphere of address management or path computational
responsibility such as an IGP area or an Autonomous Systems.
The applicability of the Path Computation Element (PCE) [RFC4655] for
the computation of such paths is discussed in [RFC5671], and the
requirements placed on the PCE communications Protocol (PCEP) for
this are given in [RFC5862].
This document describes how multiple PCE techniques can be combined
to address the requirements. These mechanisms include the use of the
per-domain path computation technique specified in [RFC5152],
extensions to the backward recursive path computation (BRPC)
technique specified in [RFC5441] for P2MP LSP path computation in an
inter-domain environment, and a new procedure for core-tree based
path computation defined in this document. These three mechanisms
are suitable for different environments (topologies, administrative
domains, policies, service requirements, etc.) and can also be
effectively combined.
1.1. 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].
2. Terminology
Terminology used in this document is consistent with the related
MPLS/GMPLS and PCE documents [RFC4461], [RFC4655], [RFC4875],
[RFC5376], [RFC5440], [RFC5441]. [RFC5671], and [RFC5862].
ABR: Area Border Router. Router used to connect two IGP domains
(areas in OSPF or levels in IS-IS).
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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Boundary Node (BN): a boundary node is either an ABR in the context
of inter-area Traffic Engineering or an ASBR in the context of
inter-AS Traffic Engineering.
Core Tree: the core tree is a P2MP tree where the root is the ingress
LSR, the transit nodes and branch nodes are the BNs of the transit
domains and the leaf nodes are the leaf BNs of the leaf domains.
Destination: The lead Nodes can be in Root Domain, Transit Domain and
Leaf Domain.
Entry BN of domain(n): a BN connecting domain(n-1) to domain(n) along
a determined sequence of domains.
Exit BN of domain(n): a BN connecting domain(n) to domain(n+1) along
a determined sequence of domains.
Inter-AS TE LSP: a TE LSP that crosses an AS boundary.
Inter-area TE LSP: a TE LSP that crosses an IGP area boundary.
Leaf Domain: a domain that does not have a downstream neighbor
domain. Note that, with this definition, a domain with one or more
leaf nodes is not necessarily a leaf domain.
Leaf Boundary Nodes: the entry boundary node in the leaf domain.
Leaf Nodes: the LSR which is the P2MP LSP's final.
LSR: Label Switching Router.
LSP: Label Switched Path.
OF: Objective Function. A set of one or more optimization criterion
(criteria) used for the computation of paths either for single or for
synchronized requests (e.g. path cost minimization), or the
synchronized computation of a set of paths (e.g. aggregate bandwidth
consumption minimization, etc.). See [RFC4655] and [RFC5441].
P2MP LSP Path Tree: A set of LSRs and TE links that comprise the path
of a P2MP TE LSP from its ingress LSR to all of its egress LSRs.
Path Domain Sequence: The known sequence of domains for a path
between root and leaf.
Path Domain Tree: The tree formed by the domains that the P2MP path
crosses, where the source (ingress) domain is the root domain.
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PCC: Path Computation Client. Any client application requesting a
path computation to be performed by the Path Computation Element.
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.
P2MP LSP Path Tree: A set of LSRs and TE links that comprise the path
of a P2MP TE LSP from its ingress LSR to all of its egress LSRs.
Path Domain Sequence: the known sequence of domains for a path
between the root node and a leaf node.
PCE Sequence: the known sequence of PCEs for calculating a path
between the root node and a leaf node.
PCE Topology Tree: a list of PCE Sequences which has all the PCE
Sequence for each path of the P2MP LSP path tree.
PCE(i): a PCE that performs path computations for domain(i).
Root Boundary Node: the egress LSR from the root domain on the path
of the P2MP LSP.
Root Domain: the domain that includes the ingress (root) LSR.
TED: Traffic Engineering Database.
Transit/branch Domain: a domain that has an upstream and one or more
downstream neighbour domain.
VSPT: Virtual Shortest Path Tree [RFC5441].
3. Problem Statement
The Path Computation Element (PCE) defined in [RFC4655] is an entity
that is capable of computing a network path or route based on a
network graph, and applying computational constraints. A Path
Computation Client (PCC) may make requests to a PCE for paths to be
computed.
[RFC4875] describes how to set up P2MP TE LSPs for use in MPLS and
GMPLS networks. The PCE is identified as a suitable application for
the computation of paths for P2MP TE LSPs [RFC5671].
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[RFC5441] specifies a procedure relying on the use of multiple PCEs
to compute (P2P) inter-domain constrained shortest paths across a
predetermined sequence of domains, using a backward recursive path
computation technique. The technique can be combined with the use of
path keys [RFC5520] to preserve confidentiality across domains, which
is sometimes required when domains are managed by different Service
Providers.
The PCE communication Protocol (PCEP) [RFC5440] is extended for
point-to-multipoint(P2MP) path computation requests and in [RFC6006].
However, that specification does not provide all the necessary
mechanisms to request the computation of inter-domain P2MP TE LSPs.
As discussed in [RFC4461], a P2MP tree is a graphical representation
of all TE links that are committed for a particular P2MP LSP. In
other words, a P2MP tree is a representation of the corresponding
P2MP tunnel on the TE network topology. A sub-tree is a part of the
P2MP tree describing how the root or an intermediate P2MP LSPs
minimizes packet duplication when P2P TE sub-LSPs traverse common
links. As described in [RFC5671] the computation of a P2MP tree
requires three major pieces of information. The first is the path
from the ingress LSR of a P2MP LSP to each of the egress LSRs, the
second is the traffic engineering related parameters, and the third
is the branch capability information.
Generally, an inter-domain P2MP tree (i.e., a P2MP tree with source
and at least one destination residing in different domains) is
particularly difficult to compute even for a distributed PCE
architecture. For instance, while the BRPC recursive path
computation may be well-suited for P2P paths, P2MP path computation
involves multiple branching path segments from the source to the
multiple destinations. As such, inter-domain P2MP path computation
may result in a plurality of per-domain path options that may be
difficult to coordinate efficiently and effectively between domains.
That is, when one or more domains have multiple ingress and/or egress
border nodes, there is currently no known technique for one domain to
determine which border routers another domain will utilize for the
inter-domain P2MP tree, and no way to limit the computation of the
P2MP tree to those utilized border nodes.
A trivial solution to the computation of inter-domain P2MP tree would
be to compute shortest inter-domain P2P paths from source to each
destination and then combine them to generate an inter-domain,
shortest-path-to-destination P2MP tree. This solution, however,
cannot be used to trade cost to destination for overall tree cost
(i.e., it cannot produce a MCT tree) and in the context of inter-
domain P2MP LSPs it cannot be used to reduce the number of domain
border nodes that are transited.
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Computing P2P LSPs individually is not an acceptable solution for
computing a P2MP tree. Even per domain path computation [RFC5152]
can be used to compute P2P multi-domain paths, but it does not
guarantee to find the optimal path which crosses multiple domains.
Furthermore, constructing a P2MP tree from individual source to leaf
P2P LSPs does not guarantee to produce a least-cost tree. This
approach may also be considered to have scaling issues during LSP
setup. That is, the LSP to each leaf is signaled separately, and
each border node must perform path computation for each leaf.
P2MP Minimum Cost Tree (MCT), i.e. one which guarantees the least
cost resulting tree, is an NP-complete problem. Moreover, adding
and/or removing a single destination to/from the tree may result in
an entirely different tree. In this case, frequent MCT path
computation requests may prove computationally intensive, and the
resulting frequent tunnel reconfiguration may even cause network
destabilization. There are several heuristic algorithms presented in
the literature that approximate the result within polynomial time
that are applicable within the context of a single-domain.
This document presents a solution, and procedures and extensions to
PCEP to support P2MP inter-domain path computation.
4. Assumptions
It is assumed that, due to deployment and commercial limitations
(e.g., inter-AS peering agreements), the sequence of domains for a
path (the path domain tree) will be known in advance.
In the figure below, the P2MP tree spans 6 domains, with D1 being the
root domain. The corresponding domain sequences which are assumed
known would be: D1-D3-D6, D1-D3-D5 and D1-D2-D4.
D1
/ \
D2 D3
/ / \
D4 D5 D6
Figure 1: Domain Sequence Tree
The examples and scenarios used in this document are also based on
the following assumptions:
o The PCE that serves each domain in the path domain tree is known,
and the set of PCEs and their relationships is propagated to each
PCE during the first exchange of path computation requests;
[Editors note - this assumption needs to be more explicit.
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o Each PCE knows about any leaf LSRs in the domain it serves;
o The boundary nodes to use on the LSP are pre-determined and are
part of the path domain tree. [Editors Note - In this version of
the document we do not consider multi-homed domains.]
Additional assumptions are documented in [RFC5441] and will not be
repeated here.
5. Requirements
This section summarizes the requirements specific to computing inter-
domain P2MP paths. In these requirements we note that the actual
computation times by any PCE implementation are outside the scope of
this document, but we observe that reducing the complexity of the
required computations has a beneficial effect on the computation time
regardless of implementation. Additionally, reducing the number of
message exchanges and the amount of information exchanged will reduce
the overall computation time for the entire P2MP tree. We refer to
the "Complexity of the computation" as the impact on these aspects of
path computation time as various parameters of the topology and the
P2MP LSP are changed.
Its also important that the solution preserves confidentiality across
domains, which is required when domains are managed by different
Service Providers.
Other than the requirements specified in [RFC5376], a number of
requirements specific to P2MP are detailed below:
1. The computed P2MP LSP should be optimal when only considering the
paths among the BNs.
2. Grafting and pruning of multicast destinations in a domain should
have no impact on other domains and on the paths among BNs.
3. The complexity of the computation for each sub-tree within each
domain should be dependent only on the topology of the domain and
it should be independent of the domain sequence.
4. The number of PCEP request and reply messages should be
independent of the number of multicast destinations in each
domain.
5. Specifying the domain entry and exit nodes.
6. Specifying which nodes should be used as branch nodes.
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7. Reoptimization of existing sub-trees.
8. Computation of P2MP paths that need to be diverse from existing
P2MP paths.
6. Objective Functions
For the computation of a single or a set of P2MP TE LSPs, a request
to meet specific optimization criteria, called an Objective Function
(OF) may be indicated.
The computation of one or more P2MP TE-LSPs may be subject to an OF
in order to select the "best" candidate paths. A variety of
objective functions have been identified as being important during
the computation of inter-domain P2MP LSPs. These include:
1. The sub-tree within each domain should be optimized, which can be
either the Minimum cost tree [RFC5862] or Shortest path tree
[RFC5862].
2. The P2MP LSP path, formed by considering only the entry and exit
nodes of the domains (the Core Tree) shold be optimal.
3. It should be possible to limit the number of entry points to a
domain.
4. It should be possible to force the branches for all leaves within
a domain to be in that domain.
7. P2MP Path Computation Procedures
The following sections describe the Core Tree based procedures to
satisfy the requirements specified in the previous section. A core
tree based solution provides an optimal inter-domain P2MP TE LSP.
7.1. Core Trees
A Core Tree is defined as a node tree, with nodes from the domains
corresponding to the domain tree PCE topology, which satisfies the
following conditions:
o The root of the core tree is the ingress LSR in the root domain;
o The leaves of the core tree are the entry nodes in the leaf
domains;
o The transit and branch nodes of the core tree are from the entry
and exit nodes from the transit and branch domains.
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For example, consider the Domain Tree from the figure below,
representing a domain tree of 5 domains, and part of the resulting
Core Tree which satisfies the aforementioned conditions.
RN: Root Node
EN: Entry Border Node (domain, index)
XN: Exit Border Node (domain, index)
+----------------------+
| Domain1 |
| (RN) |
| |
| |
+--(XN1_1)---(XN1_2) --+
/ \
/ \
/ \
+-------(EN2_1) -----+ +----(EN3_1)-------+
| Domain2 | | Domain3 |
| | | |
| | | |
| | | |
+----(XN2_1)---------+ +-(XN3_1)-(XN3_2)--+
/ \
/ \
+--------(EN4_1)-------+ +---------(EN5_1)-----+
| Domain4 | | Domain5 |
| | | |
| | | |
| | | |
+----------------------+ +---------------------+
Figure 2: Domain Tree Example
(RN)
/ \
(XN1_1) (XN3_1)
/ \
(EN2_1) (EN3_1)
/ \
(XN2_1) (XN3_1)
/ \
(EN4_1) (EN5_1)
Figure 3: Core Tree
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7.2. Core Tree Computation Procedures
The algorithms to compute the optimal large core tree are outside
scope of this document. The following extended BRPC based procedure
can be used to compute the core tree.
BRPC Based Core Tree Path Computation Procedure:
1. Using the BRPC procedures to compute the VSPT(i) for each leaf
BN(i), i=1 to n, where n is the total number of entry nodes for
all the leaf domains. In each VSPT(i), there are a number of
P(i) paths.
2. When the root PCE has computed all the VSPT(i), i=1 to n, take
one path from each VSPT and form a set of paths, we call it a
PathSet(j), j=1 to M, where M=P(1)xP(2)...xP(n);
3. For each PathSet(j), there are n S2L (Source to Leaf BN) paths
and form these n paths into a Core Tree(j);
4. There will be M number of Core Trees computed from step3. Apply
the OF to each of these M Core Trees and find the optimal Core
Tree.
Note that the application of BRPC in the aforementioned procedure
differs from the typical one since paths returned from a downstream
PCE are not necessary pruned from the solution set by intermediate
PCEs.
The reason for this is that if the PCE in a downstream domain does
the prunning and returns the single optimal sub-path to its parent
PCE, BRPC insures that the ingress PCE will get all the best optimal
sub-paths for each LN (Leaf Border Nodes), but the combination of
these single optimal sub-paths into a P2MP tree is not necessarily
optimal even if each S2L (Source-to-Leaf) sub-path is optimal.
Without trimming, the ingress PCE will get all the possible S2L sub-
paths set for LN, and eventually by looking through all the
combinations, and taking one sub-path from each set to built one P2MP
tree it finds the optimal tree.
The proposed method may present a scalability problem for the dynamic
computation of the Core Tree (by iterative checking of all
combinations of the solution space), specially with dense/meshed
domains. Considering a domain sequence D1, D2, D3, D4, where the
Leaf border node is at domain D4, PCE(4) will return 1 path. PCE(3)
will return N paths, where N is E(3) x X(3), where E(k) x X(k)
denotes the number of entry nodes times the number of exit nodes for
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that domain. PCE(2) will return M paths, where M = E(2) x X(2) x N =
E(2) x X(2) x E(3) x X(3) x 1, etc. Generally speaking the number of
potential paths at the ingress PCE Q = \prod E(k) x X(k).
Consequently, it is expected that the Core Path will be typically
computed offline, without precluding the use of dynamic, online
mechanisms such as the one presented here, in which case it SHOULD be
possible to configure transit PCEs to control the number of paths
sent upstream during BRPC (trading trimming for optimality at the
point of trimming and downwards).
7.3. Sub Tree Computation Procedures
Once the core tree is built, the grafting of all the leaf nodes from
each domain to the core tree can be achieved by a number of
algorithms. One algorithm for doing this phase is that the root PCE
will send the request with C bit set for the path computation to the
destination(s) directly to the PCE where the destination(s) belong(s)
along with the core tree computed from the phase 1.
This approach requires that the root PCE manage a potentially large
number of adjacencies (either in persistent or non-persistent mode),
including PCEP adjacencies to PCEs that are not within neighboring
domains.
A first alternative would involve establishing PCEP adjacencies that
correspond to the PCE domain tree. This would require that branch
PCEs forward requests and responses from the root PCE towards the
leaf PCEs and vice-versa.
Finally, another alternative would use a hierarchical PCE (H-PCE)
architecture. The "hierarchically" parent would request sub tree
path computations.
The algorithms to compute the optimal large sub tree are outside
scope of this document. In the case that the number of destinations
and the number of BNs within a domain are not big, the incremental
procedure based on p2p path computation using the OSPF can be used.
7.4. PCEP Protocol Extensions
7.4.1. The Extension of RP Object
The extended format of the RP object body to include the C bit is as
follows:
The C bit is added in the flag bits field of the RP object to signal
the receiver of the message that the request/reply is for inter-
domain P2MP Core Tree or not.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Flags |C|O|B|R| Pri |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request-ID-number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// Optional TLV(s) //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: RP Object Body Format
The following flag is added in this draft:
C bit ( P2MP Core Tree bit - 1 bit):
0: This indicates that this is normal PCReq/PCRrep for P2MP.
1: This indicates that this is PCReq or PCRep message for inter-
domain Core Tree P2MP. When the C bit is set, then the request
message should have the Core Tree passed along with the
destinations which and then graphed to the tree.
7.4.2. The PCE Sequence Object
The PCE Sequence Object is added to the existing PCE protocol. A
list of this objects will represent the PCE topology tree. A list of
Sequence Objects can be exchanged between PCEs during the PCE
capability exchange or on the first path computation request message
between PCEs. In this case, the request message format needs to be
changed to include the list of PCE Sequence Objects for the PCE
inter-domain P2MP calculation request.
Each PCE Sequence can be obtained from the domain sequence for a
specific path. All the PCE sequences for all the paths of P2MP
inter-domain form the PCE Topology Tree of the P2MP LSP.
The format of the new PCE Sequence Object for IPv4 (Object-Type 3) is
as follows:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Object-Class | OT |Res|P|I| Object Length (bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 address for root PCE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 address for the downstream PCE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 address for the downstream PCE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| !! |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 address for the PCE corresponding to the leafDomain |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: The New PCE Sequence Object Body Format for IPv4
The format of the new PCE Sequence Object for IPv6 (Object-Type 3) is
as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Object-Class | OT |Res|P|I| Object Length (bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 address for root PCE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 address for the downstream PCE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 address for the downstream PCE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| !! |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 address for the PCE corresponding to the leafDomain |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: The New PCE Sequence Object Body Format for IPv6
7.5. Relationship with Hierarchical PCE
The actual grafting of subtrees into the Multi-Domain tree needs to
be carried out by the source node. This means that the source node
needs to get the computed sub-trees from all the involved domains.
This requires that the source node either has a PCEP session with all
the PCEs, or PCEP messages are routed via the PCEP sessions. This
may mean an excessive number of sessions or an added complexity in
implementations.
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Alternatively, one may use an architecture based on the concept of
hierarchical PCE [H-PCE]. The parent PCE would be responsible to
request Intra-domain subtrees to the PCEs, combine them and return
the overall P2MP tree.
7.6. Parallelism
In order to minimize latency in path computation in multi-domain
networks, intra-domain path segments and intra-domain sub-trees
SHOULD be computed in parallel when possible. The proposed
procedures in this draft present opportunities for parallelism:
1. The BRPC procedure for each leaf node can be launched in parallel
by the ingress/root PCE if the dynamic computation of the Core
Tree is enabled.
2. Intra-domain P2MP paths can also be computed in parallel by the
PCEs once the entry and exit nodes within a domain are known
One of the potential issues of parallelism is that the ingress PCE
would require a potentially high number of PCEP adjacencies to
"remote" PCEs and that may not be desirable, but a given PCE would
only receive requests for the destinations that are in its domain (+
the core nodes), without PCEs forwarding requests.
8. Protection
It is envisaged that protection may be required when deploying and
using inter-domain P2MP LSPs. The procedures and mechanisms defined
in this document do not prohibit the use of existing and proposed
types of protection, including: end-to-end protection [RFC4875] and
domain protection schemes.
Segment or facility (link and node) protection is problematic in
inter-domain environment due to the limit of Fast-reroute (FRR)
[RFC4875] requiring knowledge of its next-hop across domain
boundaries whilst maintaining domain confidentiality. Although
the FRR protection might be implemented if manually provisioned if
next-hop information was known in advance.
8.1 End-to-end Protection
8.2 Domain Protection
9. Manageability Considerations
[RFC5862] describes various manageability requirements in
support of P2MP path computation when applying PCEP. This section
describes how manageability requirements mentioned in [RFC5862]
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are supported in the context of PCEP extensions specified in this
document.
Note that [RFC5440] describes various manageability considerations in
PCEP, and most of manageability requirements mentioned in [PCE-P2MP
P2MP] are already covered there.
9.1 Control of Function and Policy
In addition to PCE configuration parameters listed in [RFC5440], the
following additional parameters might be required:
o The ability to enable or disable single domain P2MP path
computations on the PCE.
o The ability to enable or disable multi-domain P2MP path
computations on the PCE.
o The PCE may be configured to enable or disable the advertisement
of its single domain and multi-domain P2MP path computation
capability.
9.2. Information and Data Models
A number of MIB objects have been defined for general PCEP control
and monitoring of P2P computations in [PCEP-MIB]. [RFC5862]
specifies that MIB objects will be required to support the control
and monitoring of the protocol extensions defined in this document.
[PCEP-P2MP-MIB] describes managed objects for modeling of PCEP
communications between a PCC and PCE, and PCE to PCE, P2MP path
computation requests and responses.
Core tree management configuration. TBD.
9.3. Liveness Detection and Monitoring
No changes are necessary to the liveness detection and monitoring
requirements as already embodied in [RFC4657].
It should be noted that multi-domain P2MP computations are likely
to take longer than P2P computations, and single domain P2MP
computatoins. The liveness detection and monitoring features of
the PCECP SHOULD take this into account.
9.4. Verifying Correct Operation
There are no additional requirements beyond those expressed in
[RFC4657] for verifying the correct operation of the PCECP. Note
that verification of the correct operation of the PCE and its
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algorithms is out of scope for the protocol requirements, but a PCC
MAY send the same request to more than one PCE and compare the
results.
9.5. Requirements on Other Protocols and Functional Components
A PCE operates on a topology graph that may be built using
information distributed by TE extensions to the routing protocol
operating within the network. In order that the PCE can select a
suitable path for the signaling protocol to use to install the P2MP
LSP, the topology graph must include information about the P2MP
signaling and branching capabilities of each LSR in the network.
Mechanisms for the knowledge of other domains, the discovery of
corresponding PCEs and their capabilities should be provided and
that this information MAY be collected by other mechanisms.
Whatever means is used to collect the information to build the
topology graph, the graph MUST include the requisite information. If
the TE extensions to the routing protocol are used, these SHOULD be
as described in [RFC5073].
9.6. Impact on Network Operation
The use of a PCE to compute P2MP paths is not expected to have
significant impact on network operations. However, it should be
noted that the introduction of P2MP support to a PCE that already
provides P2P path computation might change the loading of the PCE
significantly, and that might have an impact on the network behavior,
especially during recovery periods immediately after a network
failure.
The dynamic computation of Core Trees might also have an
impact on the load of the involved PCEs as well as path computation
times.
9.7 Policy Control
TBD
10. Security Considerations
As described in [RFC5862], P2MP path computation requests are more
CPU-intensive and also utilize more link bandwidth. In the event of
an unauthorized P2MP path computation request, or a denial of service
attack, the subsequent PCEP requests and processing may be disruptive
to the network. Consequently, it is important that implementations
conform to the relevant security requirements of [RFC5440] that
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specifically help to minimize or negate unauthorized P2MP path
computation requests and denial of service attacks. These mechanisms
include:
o Securing the PCEP session requests and responses using TCP
security techniques (Section 10.2 of [RFC5440]).
o Authenticating the PCEP requests and responses to ensure the
message is intact and sent from an authorized node (Section 10.3
of [RFC5440]).
o Providing policy control by explicitly defining which PCCs, via IP
access-lists, are allowed to send P2MP path requests to the PCE
(Section 10.6 of [RFC5440]).
PCEP operates over TCP, so it is also important to secure the PCE and
PCC against TCP denial of service attacks. Section 10.7.1 of
[RFC5440] outlines a number of mechanisms for minimizing the risk of
TCP based denial of service attacks against PCEs and PCCs.
PCEP implementations SHOULD also consider the additional security
provided by the TCP Authentication Option (TCP-AO) [RFC5925].
11. IANA Considerations
A new flag of the RP object (specified in [RFC5440]) is defined in
this document.
Additional requirements for IANA will be included in future revisions
of this document.
12. Acknowledgements
The authors would like to thank Adrian Farrel, Dan Tappan and Olufemi
Komolafe for their valuable comments on this draft.
13. References
13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5440] Vasseur, JP. and JL. Le Roux, "Path Computation Element
(PCE) Communication Protocol (PCEP)", RFC 5440, March 2009.
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[RFC6006] Takeda, T., Chaitou M., Le Roux, J.L., Ali Z.,
Zhao, Q., King, D., "Extensions to the Path Computation
Element Communication Protocol (PCEP) for
Point-to-Multipoint Traffic Engineering Label Switched
Paths", RFC6006, September 2010.
13.2. Informative References
[RFC4461] Yasukawa, S., "Signaling Requirements for
Point-to-Multipoint Traffic-Engineered MPLS Label Switched
Paths (LSPs)", RFC 4461, April 2006.
[RFC4655] Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
Element (PCE)-Based Architecture", RFC 4655, August 2006.
[RFC4657] Ash, J., Ed., and J. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol Generic
Requirements", RFC 4657, September 2006.
[RFC4875] Aggarwal, R., Papadimitriou, D., and S. Yasukawa,
"Extensions to Resource Reservation Protocol - Traffic
Engineering (RSVP-TE) for Point-to-Multipoint TE Label
Switched Paths (LSPs)", RFC 4875, May 2007.
[RFC5073] Vasseur, J., Ed., and J. Le Roux, Ed., "IGP Routing
Protocol Extensions for Discovery of Traffic Engineering
Node Capabilities", RFC 5073, December 2007.
[RFC5152] Vasseur, JP., Ayyangar, A., and R. Zhang, "A Per-Domain
Path Computation Method for Establishing Inter-Domain
Traffic Engineering (TE) Label Switched Paths (LSPs)",
RFC 5152, February 2008.
[RFC5376] Bitar, N., Zhang, R., and K. Kumaki, "Inter-AS
Requirements for the Path Computation Element Communication
Protocol (PCECP)", RFC 5376, November 2008.
[RFC5441] Roux, J., Vasseur, J., and Y. Lee, "Encoding of
Objective Functions in the Path Computation Element
Communication Protocol (PCEP)", RFC 5441, June 2009.
[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,
April 2009.
[RFC5671] Yasukawa, S. and A. Farrel, "Applicability of the Path
Computation Element (PCE) to Point-to-Multipoint (P2MP)
Multiprotocol Label Switching (MPLS) and Generalized MPLS
(GMPLS) Traffic Engineering (TE)", RFC 5671, August 2009.
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[RFC5862] Yasukawa, S. and A. Farrel, "PCC-PCE Communication
Requirements for Point to Multipoint Multiprotocol Label
Switching Traffic Engineering (MPLS-TE)", RFC 5862, June 2010.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, June 2010.
[H-PCE] King, D. and A. Farrel, "The Application of the Path
Computation Element Architecture to the Determination
of a Sequence of Domains in MPLS & GMPLS", July
2010.
[PCEP-MIB] Koushik, K., Stephan, E., Zhao, Q., and D. King, "PCE
communication protocol (PCEP) Management Information
Base", Work in Progress, July 2010.
[PCEP-P2MP-MIB] Zhao, Q., Dhody, D., Palle, U., King, D.,
"Management Information Base for the PCE Communications
Protocol (PCEP) When Requesting Point-to-Multipoint
Services", Work in Progress, September 2010.
Authors' Addresses
Quintin Zhao
Huawei Technology
125 Nagog Technology Park
Acton, MA 01719
US
Email: qzhao@huawei.com
Zafar Ali
Cisco Systems
US
Email: zali@cisco.com
Tarek Saad
Cisco Systems
US
Email: tsaad@cisco.com
Siva Sivabalan
Cisco Systems
Canada
Email: msiva@cisco.com
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Daniel King
Old Dog Consulting
UK
Email: daniel@olddog.co.uk
Ramon Casellas
CTTC - Centre Tecnologic de Telecomunicacions de Catalunya
Spain
Email: ramon.casellas@cttc.es
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