Network Working Group Kireeti Kompella
Internet Draft Juniper Networks
Expiration Date: March 2001 Yakov Rekhter
Cisco Systems
LSP Hierarchy with MPLS TE
draft-ietf-mpls-lsp-hierarchy-01.txt
1. Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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2. Abstract
To improve scalability of MPLS TE it may be useful to aggregate TE
LSPs. The aggregation is accomplished by (a) an LSR creating a TE
LSP, (b) the LSR forming a forwarding adjacency out of that LSP
(advertising this LSP as a link into ISIS/OSPF), (c) allowing other
LSRs to use forwarding adjacencies for their path computation, and
(d) nesting of LSPs originated by other LSRs into that LSP (by using
the label stack construct).
This document describes the mechanisms to accomplish this.
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3. Overview
An LSR uses MPLS TE procedures to create and maintain an LSP. The
LSR then may (under its local configuration control) announce this
LSP as a link into ISIS/OSPF. When this link is advertised into the
same instance of ISIS/OSPF as the one that determines the route taken
by the LSP, we call such a link a "forwarding adjacency". We refer
to the LSP as the "forwarding adjacency LSP", or just FA-LSP. Note
that since the advertised entity is a link in ISIS/OSPF, both the end
point LSRs of the FA-LSP must belong to the same ISIS level/OSPF
area.
In general, creation/termination of a forwarding adjacency and its
FA-LSP could be driven either by mechanisms outside of MPLS (e.g.,
via configuration control on the LSR at the head-end of the
adjacency), or by mechanisms within MPLS (e.g., as a result of the
LSR at the head-end of the adjacency receiving LSP setup requests
originated by some other LSRs).
ISIS/OSPF floods the information about forwarding adjacencies just as
it floods the information about any other links. As a result of this
flooding, an LSR has in its link state database the information about
not just conventional links, but forwarding adjacencies as well.
An LSR, when performing path computation, uses not just conventional
links, but forwarding adjacencies as well. Once a path is computed,
the LSR uses RSVP/CR-LDP for establishing label binding along the
path.
In this document we define mechanisms/procedures to accomplish the
above. These mechanisms/procedures cover both the routing
(ISIS/OSPF) and the signalling (RSVP/CR-LDP) aspects.
4. Routing aspects
In this section we describe procedures for constructing forwarding
adjacencies out of LSPs, and handling of forwarding adjacencies by
ISIS/OSPF. Specifically, this section describes how to construct the
information needed to advertise LSPs as links into ISIS/OSPF.
Procedures for creation/termination of such LSPs are defined in
Section 6.
Forwarding adjacencies may be represented as either unnumbered or
numbered links.
If there are multiple LSPs that all originate on one LSR and all
terminate on another LSR, then at one end of the spectrum all these
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LSPs could be merged (under control of the head-end LSR) into a
single forwarding adjacency using the concept of Link Bundling (see
[BUNDLE], while at the other end of the spectrum each such LSP could
be advertised as its own adjacency.
When a forwarding adjacency is created under administrative control
(static provisioning), the attributes of this adjacency have to be
provided via configuration. Specifically, the following attributes
MAY be configured for the FA-LSP: the head-end address (if left
unconfigured, this defaults to the head-end LSR's Router ID); the
tail-end address (if left unconfigured, the FA will be unnumbered);
bandwidth and resource colors constraints. The path taken by the
FA-LSP may be either computed by the LSR at the head-end of the FA-
LSP, or specified by explicit configuration; this choice is
determined by configuration.
When a forwarding adjacency is created dynamically, its attributes
are inherited from the LSP which induced its creation. Note that the
bandwidth of the FA-LSP must be at least as big as the LSP that
induced it, but may be bigger if only discrete bandwidths are
available for the FA-LSP. In general, for dynamically provisioned
forwarding adjacencies, a policy-based mechanism may be needed to
associate attributes to forwarding adjacencies.
4.1. Traffic Engineering parameters
In this section, the Traffic Engineering parameters (see [OSPF-TE]
and [ISIS-TE]) for forwarding adjacencies are described.
4.1.1. Link type (OSPF only)
The Link Type of a forwarding adjacency is set to "point-to-point".
4.1.2. Link ID (OSPF only)
The Link ID is set to the Router ID of the tail end of FA-LSP.
4.1.3. Local and remote interface IP address
If the FA is to be numbered, the local interface IP address (OSPF) or
IPv4 interface address (ISIS) is set to the head-end address of the
FA-LSP. The remote interface IP address (OSPF) or IPv4 neighbor
address (ISIS) is set to the tail-end address of the FA-LSP.
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If the FA is to be unnumbered, the advertising LSR must assign an
interface identifier to the FA, just like to any other unnumbered
link (see [UNNUM]). The interface identifier MUST be unique within
the scope of the advertising LSR. The local interface IP address
(OSPF) or IPv4 interface address (ISIS) is set to the router ID of
the head end of the FA-LSP. The first two octets of the remote
interface IP address (OSPF) or IPv4 neighbor address (ISIS) are set
to zero; the remaining two octets are set to the assigned interface
identifier.
If an FA-LSP does not have a tail-end address, the advertised FA is
unnumbered. However, one may configure an FA to be unnumbered even
if the corresponding FA-LSP has a tail-end address, for example in
the case that there are multiple FA-LSPs to the same tail-end
address.
4.1.4. Traffic Engineering metric
By default the TE metric on the forwarding adjacency is set to max(1,
(the TE metric of the FA-LSP path) - 1) so that it attracts traffic
in preference to setting up a new LSP. This may be overridden via
configuration at the head-end of the forwarding adjacency.
4.1.5. Maximum bandwidth
By default the maximum reservable bandwidth and the initial maximum
LSP bandwidth for all priorities of the forwarding adjacency is set
to the bandwidth of the FA-LSP. These may be overridden via
configuration at the head-end of the forwarding adjacency (note that
the maximum LSP bandwidth at any one priority should be no more than
the bandwidth of the FA-LSP).
4.1.6. Unreserved bandwidth
By default, the initial unreserved bandwidth for all priority levels
of the forwarding adjacency is set to the bandwidth of the FA-LSP.
4.1.7. Resource class/color
By default, a forwarding adjacency does not have resource colors
(administrative groups). This may be overridden by configuration at
the head-end of the forwarding adjacency.
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4.1.8. Link Mux Capability
The Link Mux Capability (see Section 7.1) associated with the
forwarding adjacency is the Link Mux Capability of the last link in
the FA-LSP.
4.1.9. Path information
A forwarding adjacency advertisement could contain the information
about the path taken by the FA-LSP associated with that forwarding
adjacency. This information may be used for path calculation by other
LSRs. This information is carried in the Path TLV. In both IS-IS and
OSPF, this TLV is encoded as follows: the type is TBD, the length is
4 times the path length, and the value is a list of 4 octet IPv4
addresses identifying the links in the order that they form the path
of the forwarding adjacency.
It is possible that the underlying Path information might change over
time, via configuration updates, or dynamic route modifications,
resulting in the change of the Path TLV.
If forwarding adjacencies are bundled (via link bundling), and if the
resulting bundled link carries a Path TLV, it MUST be the case that
the underlying path followed by each of the FA-LSPs that form the
component links is the same.
5. Other considerations
It is expected that forwarding adjacencies will not be used for
establishing ISIS/OSPF peering relation between the routers at the
ends of the adjacency.
It may be desired in some cases that forwarding adjacencies only be
used in Traffic Engineering path computations. In IS-IS, this can be
accomplished by setting the default metric of the extended IS
reachability TLV for the FA to the maximum link metric (2^24 - 1).
In OSPF, this can be accomplished by not advertising the link as a
regular LSA, but only as a TE opaque LSA.
Since LSPs are in general unidirectional, it follows that forwarding
adjacencies are (by definition) unidirectional links. Therefore, the
TE path computation procedures should not perform two-way
connectivity check on the links used by the procedures.
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6. Controlling FA-LSPs boundaries
To facilitate controlling the boundaries of FA-LSPs this document
introduces two new mechanisms: Link Mux Capability, and "LSP region"
(or just "region").
6.1. Link Mux Capability sub-TLV
Associated with each link (including forwarding adjacencies) is a new
attribute - Link Mux Capability. In this section we define the Link
Mux Capability sub-TLV and describe the various values it can take.
A network may have links with different multiplexing/demultiplexing
capabilities. For example, a node may not be able to demultiplex
individual packets on a given link, but it may be able to multiplex/
demultiplex channels within a SONET payload. The Link Mux Capability
sub-TLV identifies the associated multiplexing/demultiplexing
capability of a link. If there is no Link Mux Capability attribute
for a link, the link is assumed to be packet-switch capable (PSC-1).
In ISIS the Link Mux Capability is a sub-TLV of the extended IS
reachability TLV (type 22) as defined in [ISIS-TE]. The type of the
Link Mux Capability sub-TLV is 19. The length of the TLV is one
octet. The value field of the sub-TLV contains the Link Mux
Capability, encoded as follows:
Value Link Mux Capabilities
1 Packet-Switch Capable-1 (PSC-1)
2 Packet-Switch Capable-2 (PSC-2)
3 Packet-Switch Capable-3 (PSC-3)
4 Packet-Switch Capable-4 (PSC-4)
51 Layer-2 Switch Capable (L2SC)
100 Time-Division-Multiplex Capable (TDM)
150 Lambda-Switch Capable (LSC)
200 Fiber-Switch Capable (FSC)
In the OSPF Traffic Engineering LSA, the Link Mux Capability is a
sub-TLV of the Link TLV as defined in [OSPF-TE], with type 11 and
length of four octets. The value field is taken from the above list.
The format of the Link Mux Capability sub-TLV is as shown in the next
figure.
If a link is of type L2SC, that means that the node receiving layer 2
frames over this link can switch the received frames based on the
layer 2 address. For example, a link terminating on an ATM switch
would be considered L2SC.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 11 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Mux Cap. | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
If a link is of type PSC-1 through PSC-4, that means that the node
receiving data over this link can demultiplex (switch) the received
data on a packet-by-packet basis. The various levels of PSC
establish a hierarchy of LSPs tunneled within LSPs.
If a link is of type TDM, that means that the node receiving data
over this link can multiplex or demultiplex channels within a
SONET/SDH payload.
If a link is of type LSC, that means that the node receiving data
over this link can recognize and switch individual lambdas within the
link (fiber).
If a link is of type FSC, that means that the node receiving data
over this link (fiber) can switch the entire contents to another link
(fiber) (without distinguishing lambdas, channels or packets).
Note that the node that is advertising a given link (i.e., the node
that is transmitting) needs to know the multiplex/demultiplex
capabilities at the other end of the link (i.e., the receiving end of
the link). One way to accomplish this is through configuration.
Other options to accomplish this are outside the scope of this
document.
6.2. LSP regions
The information carried in the Link Mux Capabilities is used to
construct LSP regions, and determine regions' boundaries as follows.
Define an ordering among link mux capabilities as follows: PSC-1 <
PSC-2 < PSC-3 < PSC-4 < TDM < LSC < FSC. Given two links link-1 and
link-2 with link mux capabilities lmc-1 and lmc-2 respectively, say
that link-1 < link-2 iff lmc-1 < lmc-2 or lmc-1 == lmc-2 == TDM, and
link-1's bandwidth is less than link-2's switching granularity.
Suppose an LSP's path is as follows: node-0, link-1, node-1, link-2,
node-2, ..., link-n, node-n. If link-i < link-(i+1), we say that the
LSP has crossed a region boundary at node-i; with respect to that LSP
path, the LSR at node-i is an edge LSR. The 'other edge' of the
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region with respect to the LSP path is node-k, where k is the
smallest number greater than i+1 such that link-k <= link-i.
Path computation may take into account region boundaries when
computing a path for an LSP. For example, path computation may
restrict the path taken by an LSP to only the links whose Link Mux
Capability is PSC-1.
7. Signalling aspects
In this section we describe procedures that an LSR at the head-end of
a forwarding adjacency uses for handling LSP setup originated by
other LSR.
As we mentioned before, establishment/termination of FA-LSPs may
triggered either by mechanisms outside of MPLS (e.g., via
administrative control), or by mechanisms within MPLS (e.g., as a
result of the LSR at the edge of an aggregate LSP receiving LSP setup
requests originated by some other LSRs beyond LSP aggregate and its
edges). Procedures described in Section 7.1 applied to both cases.
Procedures described in Section 7.2 apply only to the latter case.
7.1. Common procedures
For the purpose of processing the ERO in a Path/Request message of an
LSP that is to be tunneled over a forwarding adjacency, an LSR at the
head-end of the FA-LSP views the LSR at the tail of that FA-LSP as
adjacent (one IP hop away).
If the Link Mux Capability of the FA-LSP is PSC[1-4], the
Path/Request message for the tunneled LSP MUST be tunneled over the
FA-LSP. If the encapsulation on the carrier LSP can distinguish IP
from MPLS, the Path/Request message is sent as a plain IP packet.
Otherwise, the Path/Request message is sent with a label of 0,
meaning "pop the label and treat as IP".
If the Link Mux Capability of the FA-LSP is not PSC[1-4], the Path
message is unicast over the control plane to the tail of the carrier
LSP, without the Router Alert option. The whole Path message,
including IP header, MAY also be encapsulated in another IP header
whose destination IP address matches the tail's IP address.
The Resv/Mapping message back to the head-end of the FA-LSP (PHOP)
cannot be sent over the same FA-LSP as it is unidirectional. The
Resv/Mapping message can either take any packet-switch capable LSP
whose end-point is the head-end of the FA-LSP, or be unicast over the
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control plane to the head-end. RSVP Resv Messages MAY be
encapsulated in another IP header whose destination IP address
matches the head-end's IP address.
When an LSP is tunneled through an FA-LSP, the LSR at the head-end of
the FA-LSP subtracts the LSP's bandwidth from the unreserved
bandwidth of the forwarding adjacency.
In the presence of link bundling (when link bundling is applied to
forwarding adjacencies), when an LSP is tunneled through an FA-LSP,
the LSR at the head-end of the FA-LSP also need to adjust Max LSP
bandwidth of the forwarding adjacency.
7.2. Specific procedures
When an LSR receives a Path/Request message, the LSR determines
whether it is at the edge of a region with respect to the ERO carried
in the message. The LSR does this by looking up the link mux
capabilities of the previous hop and the next hop in its IGP
database, and comparing them using the relation defined in Section
7.2. If the LSR is not at the edge of a region, the procedures in
this section do not apply.
If the LSR is at the edge of a region, it must then determine the
other edge of the region with respect to the ERO, again using the IGP
database. The LSR then extracts the strict hop subsequence from
itself to the other end of the region.
The LSR then compares the strict hop subsequence with all existing
FA-LSPs originated by the LSR; if a match is found, that FA-LSP has
enough unreserved bandwidth for the LSP being signaled, and the L3PID
of the FA-LSP is compatible with the L3PID of the LSP being signaled,
the LSR uses that FA-LSP as follows. The Path/Request message for
the original LSP is sent to the egress of the FA-LSP, not to the next
hop along the FA- LSP's path. The PHOP in the message is the address
of the LSR at the head-end of the FA-LSP. Before sending the
Path/Request message, the ERO in that message is adjusted by removing
the subsequence of the ERO that lies in the FA-LSP, and replacing it
with just the end point of the FA-LSP.
Otherwise (if no existing FA-LSP is found), the LSR sets up a new FA-
LSP. That is, it initiates a new LSP setup just for the FA-LSP.
After the LSR establishes the new FA-LSP, the LSR announces this LSP
into IS-IS/OSPF as a forwarding adjacency.
The unreserved bandwidth of the forwarding adjacency is computed by
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subtracting the bandwidth of sessions pending the establishment of
the FA-LSP associated from the bandwidth of the FA-LSP.
An FA-LSP could be torn down by the LSR at the head-end of the FA-LSP
as a matter of policy local to the LSR. It is expected that the FA-
LSP would be torn down once there are no more LSPs carried by the
FA-LSP. When the FA-LSP is torn down, the forwarding adjacency
associated with the FA-LSP is no longer advertised into IS-IS/OSPF.
7.3. FA-LSP Holding Priority
The value of the holding priority of an FA-LSP must be the minimum of
the configured holding priority of the FA-LSP and the holding
priorities of the LSPs tunneling through the FA-LSP (note that
smaller priority values denote higher priority). Thus, if an LSP of
higher priority than the FA-LSP tunnels through the FA-LSP, the FA-
LSP is itself promoted to the higher priority. However, if the
tunneled LSP is torn down, the FA-LSP need not drop its priority to
its old value right away; it may be advisable to apply hysteresis in
this case.
If the holding priority of an FA-LSP is configured, this document
restricts it to 0.
8. Security Considerations
Security issues are not discussed in this document.
9. Acknowledgements
Many thanks to Alan Hannan, whose early discussions with Yakov
Rekhter contributed greatly to the notion of Forwarding Adjacencies.
We would also like to thank George Swallow, Quaizar Vohra and Ayan
Banerjee.
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10. References
[BUNDLE] Kompella, K., Rekhter, Y., Berger, L., "Link Bundling in
MPLS Traffic Engineering", draft-kompella-mpls-bundle-02.txt (work in
progress)
[ISIS-TE] Smit, H., Li, T., "IS-IS extensions for Traffic
Engineering", draft-ietf-isis-traffic-01.txt (work in progress)
[OSPF-TE] Katz, D., Yeung, D., "Traffic Engineering Extensions to
OSPF", draft-katz-yeung-ospf-traffic-01.txt (work in progress)
[UNNUM] Kompella, K., Rekhter, Y., "Traffic Engineering with
Unnumbered Links", draft-kompella-mpls-unnum-01.txt (work in
progress)
11. Author Information
Kireeti Kompella
Juniper Networks, Inc.
385 Ravendale Drive
Mountain View, CA 94043
e-mail: kireeti@juniper.net
Yakov Rekhter
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
e-mail: yakov@cisco.com
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