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Internet Engineering Task Force (IETF) T. Beckhaus, Ed.
Request for Comments: 7032 Deutsche Telekom AG
Category: Standards Track B. Decraene
ISSN: 2070-1721 Orange
K. Tiruveedhula
Juniper Networks
M. Konstantynowicz, Ed.
L. Martini
Cisco Systems, Inc.
October 2013
LDP Downstream-on-Demand in Seamless MPLS
Abstract
Seamless MPLS design enables a single IP/MPLS network to scale over
core, metro, and access parts of a large packet network
infrastructure using standardized IP/MPLS protocols. One of the key
goals of Seamless MPLS is to meet requirements specific to access
networks including high number of devices, device position in network
topology, and compute and memory constraints that limit the amount of
state access devices can hold. This can be achieved with LDP
Downstream-on-Demand (DoD) label advertisement. This document
describes LDP DoD use cases and lists required LDP DoD procedures in
the context of Seamless MPLS design.
In addition, a new optional TLV type in the LDP Label Request message
is defined for fast-up convergence.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7032.
Beckhaus, et al. Standards Track [Page 1]
RFC 7032 LDP DoD October 2013
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Beckhaus, et al. Standards Track [Page 2]
RFC 7032 LDP DoD October 2013
Table of Contents
1. Introduction ....................................................4
2. Reference Topologies ............................................6
2.1. Access Topologies with Static Routing ......................6
2.2. Access Topologies with Access IGP .........................10
3. LDP DoD Use Cases ..............................................11
3.1. Initial Network Setup .....................................12
3.1.1. AN with Static Routing .............................12
3.1.2. AN with Access IGP .................................13
3.2. Service Provisioning and Activation .......................14
3.3. Service Changes and Decommissioning .......................16
3.4. Service Failure ...........................................17
3.5. Network Transport Failure .................................17
3.5.1. General Notes ......................................17
3.5.2. AN Failure .........................................18
3.5.3. AN/AGN Link Failure ................................19
3.5.4. AGN Failure ........................................20
3.5.5. AGN Network-Side Reachability Failure ..............20
4. LDP DoD Procedures .............................................20
4.1. LDP Label Distribution Control and Retention Modes ........21
4.2. LDP DoD Session Negotiation ...............................23
4.3. Label Request Procedures ..................................23
4.3.1. Access LSR/ABR Label Request .......................23
4.3.2. Label Request Retry ................................24
4.4. Label Withdraw ............................................25
4.5. Label Release .............................................26
4.6. Local-Repair ..............................................27
5. LDP Extension for LDP DoD Fast-Up Convergence ..................27
6. IANA Considerations ............................................29
6.1. LDP TLV Type ..............................................29
7. Security Considerations ........................................29
7.1. LDP DoD Native Security Properties ........................30
7.2. Data-Plane Security .......................................31
7.3. Control-Plane Security ....................................31
8. Acknowledgements ...............................................32
9. References .....................................................33
9.1. Normative References ......................................33
9.2. Informative References ....................................33
Beckhaus, et al. Standards Track [Page 3]
RFC 7032 LDP DoD October 2013
1. Introduction
Seamless MPLS design [SEAMLESS-MPLS] enables a single IP/MPLS network
to scale over core, metro, and access parts of a large packet network
infrastructure using standardized IP/MPLS protocols. One of the key
goals of Seamless MPLS is to meet requirements specific to access
including high number of devices, device position in network
topology, and compute and memory constraints that limit the amount of
state access devices can hold.
In general, MPLS Label Switching Routers (LSRs) implement either LDP
or RSVP for MPLS label distribution.
The focus of this document is on LDP, as Seamless MPLS design does
not include a requirement for general-purpose explicit traffic
engineering and bandwidth reservation. This document concentrates on
the unicast connectivity only. Multicast connectivity is a subject
for further study.
In Seamless MPLS design [SEAMLESS-MPLS], IP/MPLS protocol
optimization is possible due to relatively simple access network
topologies. Examples of such topologies involving access nodes (ANs)
and aggregation nodes (AGNs) include:
a. A single AN homed to a single AGN.
b. A single AN dual-homed to two AGNs.
c. Multiple ANs daisy-chained via a hub-AN to a single AGN.
d. Multiple ANs daisy-chained via a hub-AN to two AGNs.
e. Two ANs dual-homed to two AGNs.
f. Multiple ANs chained in a ring and dual-homed to two AGNs.
The amount of IP Routing Information Base (RIB) and Forwarding
Information Base (FIB) state on ANs can be easily controlled in the
listed access topologies by using simple IP routing configuration
with either static routes or dedicated access IGP. Note that in all
of the above topologies, AGNs act as the access area border routers
(access ABRs) connecting the access topology to the rest of the
network. Hence, in many cases, it is sufficient for ANs to have a
default route pointing towards AGNs in order to achieve complete
network connectivity from ANs to the network.
Beckhaus, et al. Standards Track [Page 4]
RFC 7032 LDP DoD October 2013
However, the amount of MPLS forwarding state requires additional
consideration. In general, MPLS routers implement LDP Downstream
Unsolicited (LDP DU) label advertisements [RFC5036] and advertise
MPLS labels for all valid routes in their RIB tables. This is seen
as an inadequate approach for ANs, which require a small subset of
the total routes (and associated labels) based on the required
connectivity for the provisioned services. Although filters can be
applied to those LDP DU label advertisements, it is not seen as a
suitable tool to facilitate any-to-any AN-driven connectivity between
access and the rest of the MPLS network.
This document describes an AN-driven "subscription model" for label
distribution in the access network. The approach relies on the
standard LDP DoD label advertisements as specified in [RFC5036]. LDP
DoD enables on-demand label distribution ensuring that only required
labels are requested, provided, and installed. Procedures described
in this document are equally applicable to LDP IPv4 and IPv6 address
families. For simplicity, the document provides examples based on
the LDP IPv4 address family.
The following sections describe a set of reference access topologies
considered for LDP DoD usage and their associated IP routing
configurations, followed by LDP DoD use cases and LDP DoD procedures
in the context of Seamless MPLS design.
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 [RFC2119].
Beckhaus, et al. Standards Track [Page 5]
RFC 7032 LDP DoD October 2013
2. Reference Topologies
LDP DoD use cases are described in the context of a generic reference
end-to-end network topology based on Seamless MPLS design
[SEAMLESS-MPLS] as shown in Figure 1.
+-------+ +-------+ +------+ +------+
---+ AGN11 +--+ AGN21 +--+ ABR1 +--+ LSR1 +--> to LSR/AGN
+--------+/ +-------+ +-------+ +------+ +------+
| Access | \/ \/
| Network| /\ /\
+--------+ +-------+ +-------+ +------+ +------+
\---+ AGN12 +--+ AGN22 +--+ ABR2 +--+ LSR2 +--> to LSR/AGN
+-------+ +-------+ +------+ +------+
static routes
or access IGP IGP area IGP area
<----Access----><--Aggregation Domain--><----Core----->
<------------------------- MPLS ---------------------->
Figure 1: Seamless MPLS End-to-End Reference Network Topology
The access network is either single- or dual-homed to AGN1x, with
either a single parallel link or multiple parallel links to AGN1x.
Seamless MPLS access network topologies can range from a single- or
dual-homed access node to a chain or ring of access nodes, and it can
use either static routing or access IGP (IS-IS or OSPF). The
following sections describe reference access topologies in more
detail.
2.1. Access Topologies with Static Routing
In most cases, access nodes connect to the rest of the network using
very simple topologies. Here, static routing is sufficient to
provide the required IP connectivity. The following topologies are
considered for use with static routing and LDP DoD:
a. [I1] topology - a single AN homed to a single AGN.
b. [I] topology - multiple ANs daisy-chained to a single AGN.
c. [V] topology - a single AN dual-homed to two AGNs.
d. [U2] topology - two ANs dual-homed to two AGNs.
e. [Y] topology - multiple ANs daisy-chained to two AGNs.
Beckhaus, et al. Standards Track [Page 6]
RFC 7032 LDP DoD October 2013
The reference static routing and LDP configuration for [V] access
topology is shown in Figure 2. The same static routing and LDP
configuration also applies to the [I1] topology.
+----+ +-------+
|AN1 +------------------------+ AGN11 +-------
| +-------\ /-----------+ +-\ /
+----+ \ / +-------+ \ /
\/ \/
/\ /\
+----+ / \ +-------+ / \
|AN2 +-------/ \-----------+ AGN12 +-/ \
| +------------------------+ +-------
+----+ +-------+
--(u)-> <-(d)--
<----- static routing -------> <------ IGP ------>
<---- LDP DU ----->
<--------- LDP DoD ----------> <-- labeled BGP -->
(u) static routes: 0/0 default, (optional) /32 routes
(d) static routes: AN loopbacks
Figure 2: [V] Access Topology with Static Routes
In line with the Seamless MPLS design, static routes configured on
AGN1x and pointing towards the access network are redistributed in
either IGP or BGP labeled IP routes [RFC3107].
Beckhaus, et al. Standards Track [Page 7]
RFC 7032 LDP DoD October 2013
The reference static routing and LDP configuration for [U2] access
topology is shown in Figure 3.
+----+ +-------+
(d1) |AN1 +------------------------+ AGN11 +-------
| | + + +-\ /
v +-+--+ +-------+ \ /
| \/
| /\
^ +-+--+ +-------+ / \
| |AN2 + + AGN12 +-/ \
(d2) | +------------------------+ +-------
+----+ +-------+
--(u)-> <-(d)--
<----- static routing -------> <------ IGP ------>
<---- LDP DU ----->
<--------- LDP DoD ----------> <-- labeled BGP -->
(u) static route 0/0 default, (optional) /32 routes
(d) static route for AN loopbacks
(d1) static route for AN2 loopback and 0/0 default with
lower preference
(d2) static route for AN1 loopback and 0/0 default with
lower preference
Figure 3: [U2] Access Topology with Static Routes
Beckhaus, et al. Standards Track [Page 8]
RFC 7032 LDP DoD October 2013
The reference static routing and LDP configuration for [Y] access
topology is shown in Figure 4. The same static routing and LDP
configuration also applies to the [I] topology.
+-------+
| |---/
/----+ AGN11 |
+----+ +----+ +----+ / | |---\
| | | | | +----/ +-------+
|ANn +...|AN2 +---+AN1 |
| | | | | +----\ +-------+
+----+ +----+ +----+ \ | |---/
\----+ AGN12 |
<-(d2)-- <-(d1)-- | |---\
--(u)-> --(u)-> --(u)-> +-------+
<-(d)--
<------- static routing --------> <------ IGP ------>
<---- LDP DU ----->
<----------- LDP DoD -----------> <-- labeled BGP -->
(u) static routes: 0/0 default, (optional) /32 routes
(d) static routes: AN loopbacks [1..n]
(d1) static routes: AN loopbacks [2..n]
(d2) static routes: AN loopbacks [3..n]
Figure 4: [Y] Access Topology with Static Routes
Note that in all of the above topologies, parallel Equal-Cost
Multipath (ECMP) (or Layer 2 Link Aggregation Group (L2 LAG)) links
can be used between the nodes.
ANs support Inter-area LDP [RFC5283] in order to use the IP default
route to match the LDP Forwarding Equivalence Class (FEC) advertised
by AGN1x and other ANs.
Beckhaus, et al. Standards Track [Page 9]
RFC 7032 LDP DoD October 2013
2.2. Access Topologies with Access IGP
A dedicated access IGP instance is used in the access network to
perform the internal routing between AGN1x and connected AN devices.
Examples of such an IGP could be IS-IS, OSPFv2 and v3, or RIPv2 and
RIPng. This access IGP instance is distinct from the IGP of the
aggregation domain.
The following topologies are considered for use with access IGP
routing and LDP DoD:
a. [U] topology - multiple ANs chained in an open ring and dual-
homed to two AGNs.
b. [Y] topology - multiple ANs daisy-chained via a hub-AN to two
AGNs.
The reference access IGP and LDP configuration for [U] access
topology is shown in Figure 5.
+-------+
+-----+ +-----+ +----+ | +---/
| AN3 |---| AN2 |---|AN1 +-----+ AGN11 |
+-----+ +-----+ +----+ | +---\
. +-------+
.
. +-------+
+-----+ +-----+ +----+ | +---/
|ANn-2|---|ANn-1|---|ANn +-----+ AGN12 |
+-----+ +-----+ +----+ | +---\
+-------+
<---------- access IGP ------------> <------ IGP ------>
<---- LDP DU ----->
<------------ LDP DoD -------------> <-- labeled BGP -->
Figure 5: [U] Access Topology with Access IGP
Beckhaus, et al. Standards Track [Page 10]
RFC 7032 LDP DoD October 2013
The reference access IGP and LDP configuration for [Y] access
topology is shown in Figure 6.
+-------+
| |---/
/----+ AGN11 |2
+----+ +----+ +----+ / | |---\
| | | | | +----/ +-------+
|ANn +...|AN2 +---+AN1 |
| | | | | +----\ +-------+
+----+ +----+ +----+ \ | |---/
\----+ AGN12 |
| |---\
+-------+
<---------- access IGP ------------> <------ IGP ------>
<---- LDP DU ----->
<------------ LDP DoD -------------> <-- labeled BGP -->
Figure 6: [Y] Access Topology with Access IGP
Note that in all of the above topologies, parallel ECMP (or L2 LAG)
links can be used between the nodes.
In both of the above topologies, ANs (ANn ... AN1) and AGN1x share
the access IGP and advertise their IPv4 and IPv6 loopbacks and link
addresses. AGN1x advertises a default route into the access IGP.
ANs support Inter-area LDP [RFC5283] in order to use the IP default
route for matching the LDP FECs advertised by AGN1x or other ANs.
3. LDP DoD Use Cases
LDP DoD use cases described in this document are based on the
Seamless MPLS scenarios listed in Seamless MPLS design
[SEAMLESS-MPLS]. This section illustrates these use cases focusing
on services provisioned on the access nodes and clarifies expected
LDP DoD operation on the AN and AGN1x devices. Two representative
service types are used to illustrate the service use cases: MPLS
Pseudowire Edge-to-Edge (PWE3) [RFC4447] and BGP/MPLS IP VPN
[RFC4364].
Described LDP DoD operations apply equally to all reference access
topologies described in Section 2. Operations that are specific to
certain access topologies are called out explicitly.
References to upstream and downstream nodes are made in line with the
definition of upstream and downstream LSRs [RFC3031].
Beckhaus, et al. Standards Track [Page 11]
RFC 7032 LDP DoD October 2013
3.1. Initial Network Setup
An access node is commissioned without any services provisioned on
it. The AN can request labels for loopback addresses of any AN, AGN,
or other nodes within the Seamless MPLS network for operational and
management purposes. It is assumed that AGN1x has the required
IP/MPLS configuration for network-side connectivity in line with
Seamless MPLS design [SEAMLESS-MPLS].
LDP sessions are configured between adjacent ANs and AGN1x using
their respective loopback addresses.
3.1.1. AN with Static Routing
If access static routing is used, ANs are provisioned with the
following static IP routing entries (topology references from
Section 2 are listed in square brackets):
a. [I1, V, U2] - Static default route 0/0 pointing to links
connected to AGN1x. Requires support for Inter-area LDP
[RFC5283].
b. [U2] - Static /32 routes pointing to the other AN. Lower
preference static default route 0/0 pointing to links connected
to the other AN. Requires support for Inter-area LDP [RFC5283].
c. [I, Y] - Static default route 0/0 pointing to links leading
towards AGN1x. Requires support for Inter-area LDP [RFC5283].
d. [I, Y] - Static /32 routes to all ANs in the daisy-chain pointing
to links towards those ANs.
e. [I1, V, U2] - Optional - Static /32 routes for specific nodes
within the Seamless MPLS network, pointing to links connected to
AGN1x.
f. [I, Y] - Optional - Static /32 routes for specific nodes within
the Seamless MPLS network, pointing to links leading towards
AGN1x.
The upstream AN/AGN1x requests labels over an LDP DoD session(s) from
downstream AN/AGN1x for configured static routes if those static
routes are configured with an LDP DoD request policy and if they are
pointing to a next hop selected by routing. It is expected that all
configured /32 static routes to be used for LDP DoD are configured
with such a policy on an AN/AGN1x.
Beckhaus, et al. Standards Track [Page 12]
RFC 7032 LDP DoD October 2013
The downstream AN/AGN1x responds to the Label Request from the
upstream AN/AGN1x with a label mapping if the requested route is
present in its RIB and there is a valid label binding from its
downstream neighbor or if it is the egress node. In such a case, the
downstream AN/AGN1x installs the advertised label as an incoming
label in its label information base (LIB) and its label forwarding
information base (LFIB). The upstream AN/AGN1x also installs the
received label as an outgoing label in its LIB and LFIB. If the
downstream AN/AGN1x does have the route present in its RIB, but does
not have a valid label binding from its downstream neighbor, it
forwards the request to its downstream neighbor.
In order to facilitate ECMP and IP Fast Reroute (IPFRR) Loop-Free
Alternate (LFA) local-repair [RFC5286], the upstream AN/AGN1x also
sends LDP DoD Label Requests to alternate next hops per its RIB, and
installs received labels as alternate entries in its LIB and LFIB.
The AGN1x on the network side can use BGP labeled IP routes [RFC3107]
in line with the Seamless MPLS design [SEAMLESS-MPLS]. In such a
case, AGN1x will redistribute its static routes pointing to local ANs
into BGP labeled IP routes to facilitate network-to-access traffic
flows. Likewise, to facilitate access-to-network traffic flows,
AGN1x will respond to access-originated LDP DoD Label Requests with
label mappings based on its BGP labeled IP routes reachability for
requested FECs.
3.1.2. AN with Access IGP
If access IGP is used, an AN(s) advertises its loopbacks over the
access IGP with configured metrics. The AGN1x advertises a default
route over the access IGP.
Routers request labels over LDP DoD session(s) according to their
needs for MPLS connectivity (via Label Switching Paths (LSPs)). In
particular, if AGNs, as per Seamless MPLS design [SEAMLESS-MPLS],
redistribute routes from the IGP into BGP labeled IP routes
[RFC3107], they request labels over LDP DoD session(s) for those
routes.
Identical to the static route case, the downstream AN/AGN1x responds
to the Label Request from the upstream AN/AGN1x with a label mapping
(if the requested route is present in its RIB and there is a valid
label binding from its downstream neighbor), and installs the
advertised label as an incoming label in its LIB and LFIB. The
upstream AN/AGN1x also installs the received label as an outgoing
label in its LIB and LFIB.
Beckhaus, et al. Standards Track [Page 13]
RFC 7032 LDP DoD October 2013
Identical to the static route case, in order to facilitate ECMP and
IPFRR LFA local-repair, the upstream AN/AGN1x also sends LDP DoD
Label Requests to alternate next hops per its RIB, and it installs
received labels as alternate entries in its LIB and LFIB.
The AGN1x on the network side can use labeled BGP [RFC3107] in line
with Seamless MPLS design [SEAMLESS-MPLS]. In such a case, AGN1x
will redistribute routes received over the access IGP (and pointing
to local ANs), into BGP labeled IP routes to facilitate network-to-
access traffic flows. Likewise, to facilitate access-to-network
traffic flows, the AGN1x will respond to access-originated LDP DoD
Label Requests with label mappings based on its BGP labeled IP routes
reachability for requested FECs.
3.2. Service Provisioning and Activation
Following the initial setup phase described in Section 3.1, a
specific access node, referred to as AN*, is provisioned with a
network service. AN* relies on LDP DoD to request the required MPLS
LSP(s) label(s) from the downstream AN/AGN1x node(s). Note that LDP
DoD operations are service agnostic; that is, they are the same
independently of the services provisioned on the AN*.
For illustration purposes, two service types are described: MPLS PWE3
[RFC4447] service and BGP/MPLS IPVPN [RFC4364].
MPLS PWE3 service: For description simplicity, it is assumed that a
single segment pseudowire is signaled using targeted LDP (tLDP)
FEC128 (0x80), and it is provisioned with the pseudowire ID and the
loopback IPv4 address of the destination node. The following IP/MPLS
operations need to be completed on the AN* to successfully establish
such PWE3 service:
a. LSP labels for destination /32 FEC (outgoing label) and the local
/32 loopback (incoming label) need to be signaled using LDP DoD.
b. A tLDP session over an associated TCP/IP connection needs to be
established to the PWE3 destination Provider Edge (PE). This is
triggered either by an explicit tLDP session configuration on the
AN* or automatically at the time of provisioning the PWE3
instance.
c. Local and remote PWE3 labels for specific FEC128 PW ID need to be
signaled using tLDP and PWE3 signaling procedures [RFC4447].
d. Upon successful completion of the above operations, AN* programs
its RIB/LIB and LFIB tables and activates the MPLS PWE3 service.
Beckhaus, et al. Standards Track [Page 14]
RFC 7032 LDP DoD October 2013
Note: Only minimum operations applicable to service connectivity have
been listed. Other non-IP/non-MPLS connectivity operations that are
required for successful service provisioning and activation are out
of scope in this document.
BGP/MPLS IPVPN service: For description simplicity, it is assumed
that the AN* is provisioned with a unicast IPv4 IPVPN service (VPNv4
for short) [RFC4364]. The following IP/MPLS operations need to be
completed on the AN* to successfully establish VPNv4 service:
a. BGP peering sessions with associated TCP/IP connections need to
be established with the remote destination VPNv4 PEs or Route
Reflectors.
b. Based on configured BGP policies, VPNv4 BGP Network Layer
Reachability Information (NLRI) needs to be exchanged between AN*
and its BGP peers.
c. Based on configured BGP policies, VPNv4 routes need to be
installed in the AN* VPN Routing and Forwarding (VRF) RIB and
FIB, with corresponding BGP next hops.
d. LSP labels for destination BGP next-hop /32 FEC (outgoing label)
and the local /32 loopback (incoming label) need to be signaled
using LDP DoD.
e. Upon successful completion of above operations, AN* programs its
RIB/LIB and LFIB tables, and activates the BGP/MPLS IPVPN
service.
Note: Only minimum operations applicable to service connectivity have
been listed. Other non-IP/-MPLS connectivity operations that are
required for successful service provisioning are out of scope in this
document.
To establish an LSP for destination /32 FEC for any of the above
services, AN* looks up its local routing table for a matching route
and selects the best next hop(s) and associated outgoing link(s).
If a label for this /32 FEC is not already installed based on the
configured static route with LDP DoD request policy or access IGP RIB
entry, AN* sends an LDP DoD label mapping request. A downstream
AN/AGN1x LSR(s) checks its RIB for presence of the requested /32 and
associated valid outgoing label binding, and if both are present,
replies with its label for this FEC and installs this label as
incoming in its LIB and LFIB. Upon receiving the label mapping, the
AN* accepts this label based on the exact route match of the
advertised FEC and route entry in its RIB or based on the longest
Beckhaus, et al. Standards Track [Page 15]
RFC 7032 LDP DoD October 2013
match in line with Inter-area LDP [RFC5283]. If the AN* accepts the
label, it installs it as an outgoing label in its LIB and LFIB.
In access topologies [V] and [Y], if AN* is dual-homed to two AGN1x
and routing entries for these AGN1x's are configured as equal-cost
paths, AN* sends LDP DoD Label Requests to both AGN1x devices and
installs all received labels in its LIB and LFIB.
In order for AN* to implement IPFRR LFA local-repair, AN* also sends
LDP DoD Label Requests to alternate next hops per its RIB, and
installs received labels as alternate entries in its LIB and LFIB.
When forwarding PWE3 or VPNv4 packets, AN* chooses the LSP label
based on the locally configured static /32 or default route or
default route signaled via access IGP. If a route is reachable via
multiple interfaces to AGN1x nodes and the route has multiple equal-
cost paths, AN* implements ECMP functionality. This involves AN*
using a hash-based load-balancing mechanism and sending the PWE3 or
VPNv4 packets in a flow-aware manner with appropriate LSP labels via
all equal-cost links.
The ECMP mechanism is applicable in an equal manner to parallel links
between two network elements and multiple paths towards the
destination. The traffic demand is distributed over the available
paths.
The AGN1x on the network side can use labeled BGP [RFC3107] in line
with Seamless MPLS design [SEAMLESS-MPLS]. In such a case, the AGN1x
will redistribute its static routes (or routes received from the
access IGP) pointing to local ANs into BGP labeled IP routes to
facilitate network-to-access traffic flows. Likewise, to facilitate
access-to-network traffic flows, the AGN1x will respond to access-
originated LDP DoD Label Requests with label mappings based on its
BGP labeled IP routes reachability for requested FECs.
3.3. Service Changes and Decommissioning
Whenever the AN* service gets decommissioned or changed and
connectivity to a specific destination is no longer required, the
associated MPLS LSP label resources are to be released on AN*.
MPLS PWE3 service: If the PWE3 service gets decommissioned and it is
the last PWE3 to a specific destination node, the tLDP session is no
longer needed and is to be terminated (automatically or by
configuration). The MPLS LSP(s) to that destination is no longer
needed either.
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BGP/MPLS IPVPN service: Deletion of a specific VPNv4 (VRF) instance
via local or remote reconfiguration can result in a specific BGP next
hop(s) no longer being needed. The MPLS LSP(s) to that destination
is no longer needed either.
In all of the above cases, the following operations related to LDP
DoD apply:
o If the /32 FEC label for the aforementioned destination node was
originally requested based on either tLDP session configuration
and default route or required BGP next hop and default route, AN*
deletes the label from its LIB and LFIB, and releases it from the
downstream AN/AGN1x by using LDP DoD procedures.
o If the /32 FEC label was originally requested based on the static
/32 route configuration with LDP DoD request policy, the label is
retained by AN*.
3.4. Service Failure
A service instance can stop being operational due to a local or
remote service failure event.
In general, unless the service failure event modifies required MPLS
connectivity, there is no impact on the LDP DoD operation.
If the service failure event does modify the required MPLS
connectivity, LDP DoD operations apply as described in Sections 3.2
and 3.3.
3.5. Network Transport Failure
A number of different network events can impact services on AN*. The
following sections describe network event types that impact LDP DoD
operation on AN and AGN1x nodes.
3.5.1. General Notes
If service on any of the ANs is affected by any network failure and
there is no network redundancy, the service goes into a failure
state. Upon recovery from network failure, the service is to be
re-established automatically.
The following additional LDP-related functions need to be supported
to comply with Seamless MPLS [SEAMLESS-MPLS] fast service restoration
requirements:
Beckhaus, et al. Standards Track [Page 17]
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a. Local-repair: AN and AGN1x support local-repair for adjacent link
or node failure for access-to-network, network-to-access, and
access-to-access traffic flows. Local-repair is to be
implemented by using either IPFRR LDP LFA, simple ECMP, or
primary/backup switchover upon failure detection.
b. LDP session protection: LDP sessions are configured with LDP
session protection to avoid delay upon the recovery from link
failure. LDP session protection ensures that FEC label binding
is maintained in the control plane as long as the LDP session
stays up.
c. IGP-LDP synchronization: If access IGP is used, LDP sessions
between ANs, and between ANs and AGN1x, are configured with IGP-
LDP synchronization to avoid unnecessary traffic loss in case the
access IGP converged before LDP and there is no LDP label binding
to the best downstream next hop.
3.5.2. AN Failure
If the AN fails, adjacent AN/AGN1x nodes remove all routes pointing
to the failed node from their RIB tables (including /32 loopback
belonging to the failed AN and any other routes reachable via the
failed AN). In turn, this triggers the removal of associated
outgoing /32 FEC labels from their LIB and LFIB tables.
If access IGP is used, the AN failure will be propagated via IGP link
updates across the access topology.
If a specific /32 FEC(s) is no longer reachable from those
ANs/AGN1x's, they also send LDP Label Withdraw messages to their
upstream LSRs to notify them about the failure, and remove the
associated incoming label(s) from their LIB and LFIB tables.
Upstream LSRs, upon receiving a Label Withdraw, remove the signaled
labels from their LIB/LFIB tables, and propagate LDP Label Withdraws
across their upstream LDP DoD sessions.
In the [U] topology, there may be an alternative path to routes
previously reachable via the failed AN. In this case, adjacent
AN/AGN1x pairs invoke local-repair (IPFRR LFA, ECMP) and switch over
to an alternate next hop to reach those routes.
AGN1x is notified about the AN failure via access IGP (if used)
and/or cascaded LDP DoD Label Withdraw(s). AGN1x implements all
relevant global-repair IP/MPLS procedures to propagate the AN failure
towards the core network. This involves removing associated routes
(in the access IGP case) and labels from its LIB and LFIB tables, and
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propagating the failure on the network side using labeled BGP and/or
core IGP/LDP DU procedures.
Upon the AN coming back up, adjacent AN/AGN1x nodes automatically add
routes pointing to recovered links based on the configured static
routes or access IGP adjacency and link state updates. This is then
followed by LDP DoD label signaling and subsequent binding and
installation of labels in LIB and LFIB tables.
3.5.3. AN/AGN Link Failure
Depending on the access topology and the failed link location,
different cases apply to the network operation after AN link failure
(topology references from Section 2 in square brackets):
a. [all] - link failed, but at least one ECMP parallel link remains.
Nodes on both sides of the failed link stop using the failed link
immediately (local-repair) and keep using the remaining ECMP
parallel links.
b. [I1, I, Y] - link failed, and there are no ECMP or alternative
links and paths. Nodes on both sides of the failed link remove
routes pointing to the failed link immediately from the RIB,
remove associated labels from their LIB and LFIB tables, and send
LDP Label Withdraw(s) to their upstream LSRs.
c. [U2, U, V, Y] - link failed, but at least one ECMP or alternate
path remains. The AN/AGN1x node stops using the failed link and
immediately switches over (local-repair) to the remaining ECMP
path or alternate path. The AN/AGN1x removes affected next hops
and labels. If there is an AGN1x terminating the failed link, it
immediately removes routes pointing to the failed link from the
RIB, removes any associated labels from the LIB and LFIB tables,
and propagates the failure on the network side using labeled BGP
and/or core IGP procedures.
If access IGP is used, AN/AGN1x link failure will be propagated via
IGP link updates across the access topology.
LDP DoD will also propagate the link failure by sending Label
Withdraws to upstream AN/AGN1x nodes, and Label Release messages to
downstream AN/AGN1x nodes.
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3.5.4. AGN Failure
If an AGN1x fails adjacent access then, depending on the access
topology, the following cases apply to the network operation
(topology references from Section 2 are shown in square brackets):
a. [I1, I] - ANs are isolated from the network - An AN adjacent to
the failure immediately removes routes pointing to the failed
AGN1x from the RIB, removes associated labels from the LIB and
LFIB tables, and sends LDP Label Withdraw message(s) to its
upstream neighbors. If access IGP is used, an IGP link update is
sent.
b. [U2, U, V, Y] - at least one ECMP or alternate path remains. AN
adjacent to failed AGN1x stops using the failed link and
immediately switches over (local-repair) to the remaining ECMP
path or alternate path by following LDP [RFC5036] procedures.
(Appendix A.1.7 "Detect Change in FEC Next Hop")
Network-side procedures for handling AGN1x failure have been
described in Seamless MPLS [SEAMLESS-MPLS].
3.5.5. AGN Network-Side Reachability Failure
If AGN1x loses network reachability to a specific destination or set
of network-side destinations, AGN1x sends LDP Label Withdraw messages
to its upstream ANs, withdrawing labels for all affected /32 FECs.
Upon receiving those messages, ANs remove those labels from their LIB
and LFIB tables, and use alternative LSPs instead (if available) as
part of global-repair.
If access IGP is used, and AGN1x gets completely isolated from the
core network, it stops advertising the default route 0/0 into the
access IGP.
4. LDP DoD Procedures
All LDP Downstream-on-Demand implementations follow the Label
Distribution Protocol as specified in [RFC5036]. This section does
not update [RFC5036] procedures, but illustrates LDP DoD operations
in the context of use cases identified in Section 3 in this document,
for information only.
In the MPLS architecture [RFC3031], network traffic flows from the
upstream LSR to the downstream LSR. The use cases in this document
rely on the downstream assignment of labels, where labels are
assigned by the downstream LSR and signaled to the upstream LSR as
shown in Figure 7.
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+----------+ +------------+
| upstream | | downstream |
------+ LSR +------+ LSR +----
traffic | | | | address
source +----------+ +------------+ (/32 for IPv4)
traffic
label distribution for IPv4 FEC destination
<-------------------------
traffic flow
------------------------->
Figure 7: LDP Label Assignment Direction
4.1. LDP Label Distribution Control and Retention Modes
The LDP specification [RFC5036] defines two modes for label
distribution control, following the definitions in the MPLS
architecture [RFC3031]:
o Independent mode: An LSR recognizes a particular FEC and makes a
decision to bind a label to the FEC independently from
distributing that label binding to its label distribution peers.
A new FEC is recognized whenever a new route becomes valid on the
LSR.
o Ordered mode: An LSR needs to bind a label to a particular FEC if
it knows how to forward packets for that FEC (i.e., it has a route
corresponding to that FEC) and if it has already received at least
one Label Request message from an upstream LSR.
Using independent label distribution control with LDP DoD and access
static routing would prevent the access LSRs from propagating label
binding failure along the access topology, making it impossible for
an upstream LSR to be notified about the downstream failure and for
an application using the LSP to switch over to an alternate path,
even if such a path exists.
The LDP specification [RFC5036] defines two modes for label
retention, following the definitions in the MPLS architecture
[RFC3031]:
o Conservative label retention mode: If operating in DoD mode, an
LSR will request label mappings only from the next-hop LSR
according to routing. The main advantage of the conservative
label retention mode is that only the labels that are required for
the forwarding of data are allocated and maintained. This is
particularly important in LSRs where the label space is inherently
Beckhaus, et al. Standards Track [Page 21]
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limited, such as in an ATM switch. A disadvantage of the
conservative label retention mode is that if routing changes the
next hop for a given destination, a new label must be obtained
from the new next hop before labeled packets can be forwarded.
o Liberal label retention mode: When operating in DoD mode with
liberal label retention mode, an LSR might choose to request label
mappings for all known prefixes from all peer LSRs. The main
advantage of the liberal label retention mode is that reaction to
routing changes can be quick because labels already exist. The
main disadvantage of the liberal label retention mode is that
unneeded label mappings are distributed and maintained.
Note that the conservative label retention mode would prevent LSRs
from requesting and maintaining label mappings for any backup routes
that are not used for forwarding. In turn, this would prevent the
access LSRs (AN and AGN1x nodes) from implementing any local
protection schemes that rely on using alternate next hops in case of
the primary next-hop failure. Such schemes include IPFRR LFA if
access IGP is used, or a primary and backup static route
configuration. Using LDP DoD in combination with liberal label
retention mode allows the LSR to request labels for the specific FEC
from primary next-hop LSR(s) and the alternate next-hop LSR(s) for
this FEC.
Note that even though LDP DoD operates in a liberal label retention
mode, if used with access IGP and if no LFA exists, the LDP DoD will
introduce additional delay in traffic restoration as the labels for
the new next hop will be requested only after the access IGP
convergence.
Adhering to the overall design goals of Seamless MPLS
[SEAMLESS-MPLS], specifically achieving a large network scale without
compromising fast service restoration, all access LSRs (AN and AGN1x
nodes) use LDP DoD advertisement mode with:
o Ordered label distribution control: enables propagation of label
binding failure within the access topology.
o Liberal label retention mode: enables pre-programming of alternate
next hops with associated FEC labels.
In Seamless MPLS [SEAMLESS-MPLS], an AGN1x acts as an access ABR
connecting access and metro domains. To enable failure propagation
between those domains, the access ABR implements ordered label
distribution control when redistributing routes/FECs between the
Beckhaus, et al. Standards Track [Page 22]
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access side (using LDP DoD and static or access IGP) and the network
side (using labeled BGP [RFC3107] or core IGP with LDP Downstream
Unsolicited label advertisements).
4.2. LDP DoD Session Negotiation
An access LSR/ABR proposes the DoD label advertisement by setting the
"A" value to 1 in the Common Session Parameters TLV of the
Initialization message. The rules for negotiating the label
advertisement mode are specified in the LDP specification [RFC5036].
To establish a DoD session between the two access LSR/ABRs, both
propose the DoD label advertisement mode in the Initialization
message. If the access LSR only supports LDP DoD and the access ABR
proposes the Downstream Unsolicited mode, the access LSR sends a
Notification message with status "Session Rejected/Parameters
Advertisement Mode" and then closes the LDP session as specified in
the LDP specification [RFC5036].
If an access LSR is acting in an active role, it re-attempts the LDP
session immediately. If the access LSR receives the same Downstream
Unsolicited mode again, it follows the exponential backoff algorithm
as defined in the LDP specification [RFC5036] with a delay of 15
seconds and subsequent delays growing to a maximum delay of 2
minutes.
In case a PWE3 service is required between the adjacent access
LSR/ABR, and LDP DoD has been negotiated for IPv4 and IPv6 FECs, the
same LDP session is used for PWE3 FECs. Even if the LDP DoD label
advertisement has been negotiated for IPv4 and IPv6 LDP FECs as
described earlier, the LDP session uses a Downstream Unsolicited
label advertisement for PWE3 FECs as specified in PWE3 LDP [RFC4447].
4.3. Label Request Procedures
4.3.1. Access LSR/ABR Label Request
The upstream access LSR/ABR will request label bindings from an
adjacent downstream access LSR/ABR based on the following trigger
events:
a. An access LSR/ABR is configured with /32 static route with LDP
DoD Label Request policy in line with the initial network setup
use case described in Section 3.1.
b. An access LSR/ABR is configured with a service in line with
service use cases described in Sections 3.2 and 3.3.
Beckhaus, et al. Standards Track [Page 23]
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c. Configuration with access static routes: An access LSR/ABR link
to an adjacent node comes up, and an LDP DoD session is
established. In this case, the access LSR sends Label Request
messages for all /32 static routes configured with an LDP DoD
policy and all /32 routes related to provisioned services that
are covered by the default route.
d. Configuration with access IGP: An access LSR/ABR link to an
adjacent node comes up, and an LDP DoD session is established.
In this case, the access LSR sends Label Request messages for all
/32 routes learned over the access IGP and all /32 routes related
to provisioned services that are covered by access IGP routes.
e. In all above cases, requests are sent to any next-hop LSRs and
alternate LSRs.
The downstream access LSR/ABR will respond with a Label Mapping
message with a non-null label if any of the below conditions are met:
a. Downstream access LSR/ABR: The requested FEC is an IGP or static
route, and there is an LDP label already learned from the next-
next-hop downstream LSR (by LDP DoD or LDP DU). If there is no
label for the requested FEC and there is an LDP DoD session to
the next-next-hop downstream LSR, the downstream LSR sends a
Label Request message for the same FEC to the next-next-hop
downstream LSR. In such a case, the downstream LSR will respond
back to the requesting upstream access LSR only after getting a
label from the next-next-hop downstream LSR peer.
b. Downstream access ABR only: The requested FEC is a BGP labeled IP
routes [RFC3107], and this BGP route is the best selected for
this FEC.
The downstream access LSR/ABR can respond with a label mapping with
an explicit-null or implicit-null label if it is acting as an egress
for the requested FEC, or it can respond with a "No Route"
notification if no route exists.
4.3.2. Label Request Retry
Following the LDP specification [RFC5036], if an access LSR/ABR
receives a "No Route" notification in response to its Label Request
message, it retries using an exponential backoff algorithm similar to
the backoff algorithm mentioned in the LDP session negotiation
described in Section 4.2.
Beckhaus, et al. Standards Track [Page 24]
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If there is no response to the Label Request message sent, the LDP
specification [RFC5036] (Section A.1.1) states that the LSR does not
send another request for the same label to the peer and mandates that
a duplicate Label Request be considered a protocol error and be
dropped by the receiving LSR by sending a Notification message.
Thus, if there is no response from the downstream peer, the access
LSR/ABR does not send a duplicate Label Request message.
If the static route corresponding to the FEC gets deleted or if the
DoD request policy is modified to reject the FEC before receiving the
Label Mapping message, then the access LSR/ABR sends a Label Abort
message to the downstream LSR.
To address the case of slower convergence resulting from described
LDP behavior in line with the LDP specification [RFC5036], a new LDP
TLV extension is proposed and described in Section 5.
4.4. Label Withdraw
If an MPLS label on the downstream access LSR/ABR is no longer valid,
the downstream access LSR/ABR withdraws this FEC/label binding from
the upstream access LSR/ABR with the Label Withdraw message [RFC5036]
with a specified label TLV or with an empty label TLV.
The downstream access LSR/ABR withdraws a label for a specific FEC in
the following cases:
a. If an LDP DoD ingress label is associated with an outgoing label
assigned by a labeled BGP route and this route is withdrawn.
b. If an LDP DoD ingress label is associated with an outgoing label
assigned by LDP (DoD or DU), and the IGP route is withdrawn from
the RIB or the downstream LDP session is lost.
c. If an LDP DoD ingress label is associated with an outgoing label
assigned by LDP (DoD or DU) and the outgoing label is withdrawn
by the downstream LSR.
d. If an LDP DoD ingress label is associated with an outgoing label
assigned by LDP (DoD or DU), the next hop in the route has
changed, and
* there is no LDP session to the new next hop. To minimize the
probability of this, the access LSR/ABR implements LDP-IGP
synchronization procedures as specified in [RFC5443].
Beckhaus, et al. Standards Track [Page 25]
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* there is an LDP session but no label from a downstream LSR.
See note below.
e. If an access LSR/ABR is configured with a policy to reject
exporting label mappings to an upstream LSR.
The upstream access LSR/ABR responds to the Label Withdraw message
with the Label Release message [RFC5036].
After sending the Label Release message to the downstream access
LSR/ABR, the upstream access LSR/ABR resends the Label Request
message, assuming the upstream access LSR/ABR still requires the
label.
The downstream access LSR/ABR withdraws a label if the local route
configuration (e.g., /32 loopback) is deleted.
Note: For any events inducing next-hop change, a downstream access
LSR/ABR attempts to converge the LSP locally before withdrawing the
label from an upstream access LSR/ABR. For example, if the next hop
changes for a particular FEC and if the new next hop allocates labels
by the LDP DoD session, then the downstream access LSR/ABR sends a
Label Request on the new next-hop session. If the downstream access
LSR/ABR doesn't get a label mapping for some duration, then and only
then does the downstream access LSR/ABR withdraw the upstream label.
4.5. Label Release
If an access LSR/ABR no longer needs a label for a FEC, it sends a
Label Release message [RFC5036] to the downstream access LSR/ABR with
or without the label TLV.
If an upstream access LSR/ABR receives an unsolicited label mapping
on a DoD session, it releases the label by sending a Label Release
message.
The access LSR/ABR sends a Label Release message to the downstream
LSR in the following cases:
a. If it receives a Label Withdraw from the downstream access
LSR/ABR.
b. If the /32 static route with LDP DoD Label Request policy is
deleted.
c. If the service gets decommissioned and there is no corresponding
/32 static route with LDP DoD Label Request policy configured.
Beckhaus, et al. Standards Track [Page 26]
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d. If the next hop in the route has changed and the label does not
point to the best or alternate next hop.
e. If it receives a Label Withdraw from a downstream DoD session.
4.6. Local-Repair
To support local-repair with ECMP and IPFRR LFA, the access LSR/ABR
requests labels on both the best next-hop and the alternate next-hop
LDP DoD sessions, as specified in the Label Request procedures in
Section 4.3. If remote LFA is enabled, the access LSR/ABR needs a
label from its alternate next hop toward the PQ node and needs a
label from the remote PQ node toward its FEC/destination [RLFA]. If
the access LSR/ABR doesn't already know those labels, it requests
them.
This will enable the access LSR/ABR to pre-program the alternate
forwarding path with the alternate label(s) and invoke the IPFRR LFA
switchover procedure if the primary next-hop link fails.
5. LDP Extension for LDP DoD Fast-Up Convergence
In some conditions, the exponential backoff algorithm usage described
in Section 4.3.2 can result in a wait time that is longer than
desired to get a successful LDP label-to-route mapping. An example
is when a specific route is unavailable on the downstream LSR when
the label mapping request from the upstream is received, but later
comes back. In such a case, using the exponential backoff algorithm
can result in a max delay wait time before the upstream LSR sends
another LDP Label Request.
This section describes an extension to the LDP DoD procedure to
address fast-up convergence, and as such is to be treated as a
normative reference. The downstream and upstream LSRs SHOULD
implement this extension if fast-up convergence is desired.
The extension consists of the upstream LSR indicating to the
downstream LSR that the Label Request SHOULD be queued on the
downstream LSR until the requested route is available.
Beckhaus, et al. Standards Track [Page 27]
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To implement this behavior, a new Optional Parameter is defined for
use in the Label Request message:
Optional Parameter Length Value
Queue Request TLV 0 see below
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0| Queue Request (0x0971) | Length (0x00) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
U-bit = 1
Unknown TLV bit. Upon receipt of an unknown TLV, due to the
U-bit being set (=1), the unknown TLV MUST be silently ignored
and the rest of the message processed as if the unknown TLV
did not exist. In case the requested route is not available,
the downstream LSR MUST ignore this unknown TLV and send a
"No Route" notification back. This ensures backward
compatibility.
F-bit = 0
Forward unknown TLV bit. This bit applies only when the U-bit is
set and the LDP message containing the unknown TLV is to be
forwarded. Due to the F-bit being clear (=0), the unknown TLV is
not forwarded with the message.
Type = 0x0971
Queue Request TLV (allocated by IANA).
Length = 0x00
Specifies the length of the Value field in octets.
The specified operation is as follows.
To benefit from the fast-up convergence improvement, the upstream LSR
sends a Label Request message with a Queue Request TLV.
If the downstream LSR supports the Queue Request TLV, it verifies if
a route is available; if so, it replies with a label mapping as per
existing LDP procedures. If the route is not available, the
downstream LSR queues the request and replies as soon as the route
becomes available. In the meantime, it does not send a "No Route"
notification back. When sending a Label Request with the Queue
Request TLV, the upstream LSR does not retry the Label Request
message if it does not receive a reply from its downstream peer.
Beckhaus, et al. Standards Track [Page 28]
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If the upstream LSR wants to abort an outstanding Label Request while
the Label Request is queued in the downstream LSR, the upstream LSR
sends a Label Abort Request message, making the downstream LSR remove
the original request from the queue and send back a Label Request
Aborted notification [RFC5036].
If the downstream LSR does not support the Queue Request TLV, and the
requested route is not available, it ignores this unknown TLV and
sends a "No Route" notification back, in line with [RFC5036]. In
this case, the upstream LSR invokes the exponential backoff algorithm
described in Section 4.3.2, following the LDP specification
[RFC5036].
This procedure ensures backward compatibility.
6. IANA Considerations
6.1. LDP TLV Type
This document uses a new Optional Parameter, Queue Request TLV, in
the Label Request message defined in Section 5. IANA already
maintains a registry of LDP parameters called the "TLV Type Name
Space" registry, as defined by RFC 5036. The following assignment
has been made:
TLV type Description
0x0971 Queue Request TLV
7. Security Considerations
MPLS LDP DoD deployment in the access network is subject to the same
security threats as any MPLS LDP deployment. It is recommended that
baseline security measures be considered, as described in "Security
Framework for MPLS and GMPLS Networks" [RFC5920] and the LDP
specification [RFC5036] including ensuring authenticity and integrity
of LDP messages, as well as protection against spoofing and denial-
of-service attacks.
Some deployments require increased measures of network security if a
subset of access nodes are placed in locations with lower levels of
physical security, e.g., street cabinets (common practice for Very
high bit-rate Digital Subscriber Line (VDSL) access). In such cases,
it is the responsibility of the system designer to take into account
the physical security measures (environmental design, mechanical or
electronic access control, intrusion detection) as well as monitoring
and auditing measures (configuration and Operating System changes,
reloads, route advertisements).
Beckhaus, et al. Standards Track [Page 29]
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But even with all this in mind, the designer still needs to consider
network security risks and adequate measures arising from the lower
level of physical security of those locations.
7.1. LDP DoD Native Security Properties
MPLS LDP DoD operation is request driven, and unsolicited label
mappings are not accepted by upstream LSRs by design. This
inherently limits the potential of an unauthorized third party
injecting unsolicited label mappings on the wire.
This native security property enables an ABR LSR to act as a gateway
to the MPLS network and to control the requests coming from any
access LSR and prevent cases when the access LSR attempts to get
access to an unauthorized FEC or remote LSR after being compromised.
In the event that an access LSR gets compromised and manages to
advertise a FEC belonging to another LSR (e.g., in order to 'steal'
third-party data flows, or breach the privacy of a VPN), such an
access LSR would also have to influence the routing decision for
affected FECs on the ABR LSR to attract the flows. The following
measures need to be considered on an ABR LSR to prevent such an event
from occurring:
a. Access with static routes: An access LSR cannot influence ABR LSR
routing decisions due to the static nature of routing
configuration, a native property of the design.
b. Access with IGP - access FEC "stealing": If the compromised
access LSR is a leaf in the access topology (leaf node in
topologies I1, I, V, Y described earlier), this will not have any
adverse effect, due to the leaf IGP metrics being configured on
the ABR LSR. If the compromised access LSR is a transit LSR in
the access topology (transit node in topologies I, Y, U), it is
only possible for this access LSR to attract traffic destined to
the nodes upstream from it. Such a 'man-in-the-middle attack'
can quickly be detected by upstream access LSRs not receiving
traffic and by the LDP TCP session being lost.
c. Access with IGP - network FEC "stealing": The compromised access
LSR can use IGP to advertise a "stolen" FEC prefix belonging to
the network side. This case can be prevented by giving a better
administrative preference to the BGP labeled IP routes versus
access IGP routes.
In summary, the native properties of MPLS in access design with LDP
DoD prevent a number of security attacks and make their detection
quick and straightforward.
Beckhaus, et al. Standards Track [Page 30]
RFC 7032 LDP DoD October 2013
The following two sections describe other security considerations
applicable to general MPLS deployments in the access network.
7.2. Data-Plane Security
Data-plane security risks applicable to the access MPLS network
include:
a. Labeled packets from a specific access LSR that are sent to an
unauthorized destination.
b. Unlabeled packets that are sent by an access LSR to remote
network nodes.
The following mechanisms apply to MPLS access design with LDP DoD
that address listed data-plane security risks:
1. addressing (a): Access and ABR LSRs do not accept labeled packets
over a particular data link, unless from the access or ABR LSR
perspective this data link is known to attach to a trusted system
based on control-plane security as described in Section 7.3 and
the top label has been distributed to the upstream neighbor by
the receiving access or ABR LSR.
2. addressing (a) - The ABR LSR restricts network reachability for
access devices to a subset of remote network LSRs, based on
control-plane security as described in Section 7.3, FEC filters,
and routing policy.
3. addressing (a): Control-plane authentication as described in
Section 7.3 is used.
4. addressing (b): The ABR LSR restricts IP network reachability to
and from the access LSR.
7.3. Control-Plane Security
Similar to Inter-AS MPLS/VPN deployments [RFC4364], control-plane
security is a prerequisite for data-plane security.
To ensure control-plane security access, LDP DoD sessions are
established only with LDP peers that are considered trusted from the
local LSR perspective, meaning they are reachable over a data link
that is known to attach to a trusted system based on employed
authentication mechanism(s) on the local LSR.
Beckhaus, et al. Standards Track [Page 31]
RFC 7032 LDP DoD October 2013
The security of LDP sessions is analyzed in the LDP specification
[RFC5036] and in [RFC6952] ("Analysis of BGP, LDP, PCEP, and MSDP
Issues According to the Keying and Authentication for Routing
Protocols (KARP) Design Guide"). Both documents state that LDP is
subject to two different types of attacks: spoofing and denial-of-
service attacks.
The threat of spoofed LDP Hello messages can be reduced by following
guidelines listed in the LDP specification [RFC5036]: accepting Basic
Hellos only on interfaces connected to trusted LSRs, ignoring Basic
Hellos that are not addressed to all routers in this subnet multicast
group, and using access lists. LDP Hello messages can also be
secured using an optional Cryptographic Authentication TLV as
specified in "LDP Hello Cryptographic Authentication" [CRYPTO-AUTH]
that further reduces the threat of spoofing during the LDP discovery
phase.
Spoofing during the LDP session communication phase can be prevented
by using the TCP Authentication Option (TCP-AO) [RFC5925], which uses
a stronger hashing algorithm, e.g., SHA1 as compared to the
traditionally used MD5 authentication. TCP-AO is recommended as
being more secure as compared to the TCP/IP MD5 authentication option
[RFC5925].
The threat of a denial-of-service attack targeting a well-known UDP
port for LDP discovery or a TCP port for LDP session establishment
can be reduced by following the guidelines listed in [RFC5036] and in
[RFC6952].
Access IGP (if used) and any routing protocols used in the access
network for signaling service routes also need to be secured
following best practices in routing protocol security. Refer to the
KARP IS-IS security analysis document [KARP-ISIS] and to [RFC6863]
("Analysis of OSPF Security According to the Keying and
Authentication for Routing Protocols (KARP) Design Guide") for
further analysis of security properties of IS-IS and OSPF IGP routing
protocols.
8. Acknowledgements
The authors would like to thank Nischal Sheth, Nitin Bahadur, Nicolai
Leymann, George Swallow, Geraldine Calvignac, Ina Minei, Eric Gray,
and Lizhong Jin for their suggestions and review. Additional thanks
go to Adrian Farrel for thorough pre-publication review, and to
Stephen Kent for review and guidance specifically for the security
section.
Beckhaus, et al. Standards Track [Page 32]
RFC 7032 LDP DoD October 2013
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, February 2006.
[RFC4447] Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G.
Heron, "Pseudowire Setup and Maintenance Using the Label
Distribution Protocol (LDP)", RFC 4447, April 2006.
[RFC5036] Andersson, L., Minei, I., and B. Thomas, "LDP
Specification", RFC 5036, October 2007.
[RFC5283] Decraene, B., Le Roux, JL., and I. Minei, "LDP Extension
for Inter-Area Label Switched Paths (LSPs)", RFC 5283,
July 2008.
9.2. Informative References
[CRYPTO-AUTH]
Zheng, L., Chen, M., and M. Bhatia, "LDP Hello
Cryptographic Authentication", Work in Progress, August
2013.
[KARP-ISIS]
Chunduri, U., Tian, A., and W. Lu, "KARP IS-IS security
analysis", Work in Progress, March 2013.
[RFC3107] Rekhter, Y. and E. Rosen, "Carrying Label Information in
BGP-4", RFC 3107, May 2001.
[RFC5286] Atlas, A. and A. Zinin, "Basic Specification for IP Fast
Reroute: Loop-Free Alternates", RFC 5286, September 2008.
[RFC5443] Jork, M., Atlas, A., and L. Fang, "LDP IGP
Synchronization", RFC 5443, March 2009.
[RFC5920] Fang, L., "Security Framework for MPLS and GMPLS
Networks", RFC 5920, July 2010.
Beckhaus, et al. Standards Track [Page 33]
RFC 7032 LDP DoD October 2013
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, June 2010.
[RFC6863] Hartman, S. and D. Zhang, "Analysis of OSPF Security
According to the Keying and Authentication for Routing
Protocols (KARP) Design Guide", RFC 6863, March 2013.
[RFC6952] Jethanandani, M., Patel, K., and L. Zheng, "Analysis of
BGP, LDP, PCEP, and MSDP Issues According to the Keying
and Authentication for Routing Protocols (KARP) Design
Guide", RFC 6952, May 2013.
[RLFA] Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
So, "Remote LFA FRR", Work in Progress, May 2013.
[SEAMLESS-MPLS]
Leymann, N., Ed., Decraene, B., Filsfils, C.,
Konstantynowicz, M., Ed., and D. Steinberg, "Seamless MPLS
Architecture", Work in Progress, July 2013.
Beckhaus, et al. Standards Track [Page 34]
RFC 7032 LDP DoD October 2013
Authors' Addresses
Thomas Beckhaus (editor)
Deutsche Telekom AG
Heinrich-Hertz-Strasse 3-7
Darmstadt 64307
Germany
Phone: +49 6151 58 12825
EMail: thomas.beckhaus@telekom.de
Bruno Decraene
Orange
38-40 rue du General Leclerc
Issy Moulineaux cedex 9 92794
France
EMail: bruno.decraene@orange.com
Kishore Tiruveedhula
Juniper Networks
10 Technology Park Drive
Westford, Massachusetts 01886
USA
Phone: 1-(978)-589-8861
EMail: kishoret@juniper.net
Maciek Konstantynowicz (editor)
Cisco Systems, Inc.
10 New Square Park, Bedfont Lakes
London
United Kingdom
EMail: maciek@cisco.com
Luca Martini
Cisco Systems, Inc.
9155 East Nichols Avenue, Suite 400
Englewood, CO 80112
USA
EMail: lmartini@cisco.com
Beckhaus, et al. Standards Track [Page 35]