Network Working Group C. Filsfils, Ed.
Internet-Draft S. Previdi, Ed.
Intended status: Standards Track A. Bashandy
Expires: August 12, 2017 Cisco Systems, Inc.
B. Decraene
S. Litkowski
Orange
February 8, 2017
Segment Routing interworking with LDP
draft-ietf-spring-segment-routing-ldp-interop-06
Abstract
A Segment Routing (SR) node steers a packet through a controlled set
of instructions, called segments, by prepending the packet with an SR
header. A segment can represent any instruction, topological or
service-based. SR allows to enforce a flow through any topological
path and service chain while maintaining per-flow state only at the
ingress node to the SR domain.
The Segment Routing architecture can be directly applied to the MPLS
data plane with no change in the forwarding plane. This drafts
describes how Segment Routing operates in a network where LDP is
deployed and in the case where SR-capable and non-SR-capable nodes
coexist.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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."
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This Internet-Draft will expire on August 12, 2017.
Copyright Notice
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. SR/LDP Ship-in-the-night coexistence . . . . . . . . . . . . 3
2.1. MPLS2MPLS co-existence . . . . . . . . . . . . . . . . . 5
2.2. IP2MPLS co-existence . . . . . . . . . . . . . . . . . . 6
3. Migration from LDP to SR . . . . . . . . . . . . . . . . . . 6
4. SR and LDP Interworking . . . . . . . . . . . . . . . . . . . 7
4.1. LDP to SR . . . . . . . . . . . . . . . . . . . . . . . . 8
4.1.1. LDP to SR Behavior . . . . . . . . . . . . . . . . . 8
4.2. SR to LDP . . . . . . . . . . . . . . . . . . . . . . . . 8
4.2.1. SR to LDP Behavior . . . . . . . . . . . . . . . . . 10
5. SR/LDP Interworking Use Cases . . . . . . . . . . . . . . . . 10
5.1. SR Protection of LDP-based Traffic . . . . . . . . . . . 10
5.2. Eliminating Targeted LDP Session . . . . . . . . . . . . 12
5.3. Guaranteed FRR coverage . . . . . . . . . . . . . . . . . 13
5.4. Inter-AS Option C, Carrier's Carrier . . . . . . . . . . 15
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
7. Manageability Considerations . . . . . . . . . . . . . . . . 15
7.1. SR and LDP co-existence . . . . . . . . . . . . . . . . . 15
7.2. SRMS Management . . . . . . . . . . . . . . . . . . . . . 16
7.3. Dataplane Verification . . . . . . . . . . . . . . . . . 16
8. Security Considerations . . . . . . . . . . . . . . . . . . . 16
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17
10. Contributors' Addresses . . . . . . . . . . . . . . . . . . . 17
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
11.1. Normative References . . . . . . . . . . . . . . . . . . 17
11.2. Informative References . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
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1. Introduction
Segment Routing, as described in [I-D.ietf-spring-segment-routing],
can be used on top of the MPLS data plane without any modification as
described in [I-D.ietf-spring-segment-routing-mpls].
Segment Routing control plane can co-exist with current label
distribution protocols such as LDP ([RFC5036]).
This draft outlines the mechanisms through which SR interworks with
LDP in cases where a mix of SR-capable and non-SR-capable routers co-
exist within the same network and more precisely in the same routing
domain.
Section 2 describes the co-existence of SR with other MPLS Control
Plane. Section 3 documents a method to migrate from LDP to SR-based
MPLS tunneling. Section 4 documents the interworking between SR and
LDP in the case of non-homogeneous deployment. Section 5 describes
how a partial SR deployment can be used to provide SR benefits to
LDP-based traffic including a possible application of SR in the
context of inter-domain MPLS use-cases.
Typically, an implementation will allow an operator to select
(through configuration) which of the described modes of SR and LDP
co-existence to use.
2. SR/LDP Ship-in-the-night coexistence
We call "MPLS Control Plane Client (MCC)" any control plane protocol
installing forwarding entries in the MPLS data plane. SR, LDP, RSVP-
TE, BGP 3107, VPNv4, etc are examples of MCCs.
An MCC, operating at node N, must ensure that the incoming label it
installs in the MPLS data plane of Node N has been uniquely allocated
to himself.
Thanks to the defined segment allocation rule and specifically the
notion of the Segment Routing Global Block (SRGB, as defined in
[I-D.ietf-spring-segment-routing]), SR can co-exist with any other
MCC.
This is clearly the case for the adjacency segment: it is a local
label allocated by the label manager, as for any MCC.
This is clearly the case for the prefix segment: the label manager
allocates the SRGB set of labels to the SR MCC client and the
operator ensures the unique allocation of each global prefix segment/
label within the allocated SRGB set.
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Note that this static label allocation capability of the label
manager exists for many years across several vendors and hence is not
new. Furthermore, note that the label-manager ability to statically
allocate a range of labels to a specific application is not new
either. This is required for MPLS-TP operation. In this case, the
range is reserved by the label manager and it is the MPLS-TP
([RFC5960]) NMS (acting as an MCC) that ensures the unique allocation
of any label within the allocated range and the creation of the
related MPLS forwarding entry.
Let us illustrate an example of ship-in-the-night (SIN) coexistence.
PE2 PE4
\ /
PE1----A----B---C---PE3
Figure 1: SIN coexistence
The EVEN VPN service is supported by PE2 and PE4 while the ODD VPN
service is supported by PE1 and PE3. The operator wants to tunnel
the ODD service via LDP and the EVEN service via SR.
This can be achieved in the following manner:
The operator configures PE1, PE2, PE3, PE4 with respective
loopbacks 192.0.2.201/32, 192.0.2.202/32, 192.0.2.203/32,
192.0.2.204/32. These PE's advertised their VPN routes with next-
hop set on their respective loopback address.
The operator configures A, B, C with respective loopbacks
192.0.2.1/32, 192.0.2.2/32, 192.0.2.3/32.
The operator configures PE2, A, B, C and PE4 with SRGB [100, 300].
The operator attach the respective Node Segment Identifiers (Node-
SID's, as defined in [I-D.ietf-spring-segment-routing]): 202, 101,
102, 103 and 204 to the loopbacks of nodes PE2, A, B, C and PE4.
The Node-SID's are configured to request penultimate-hop-popping.
PE1, A, B, C and PE3 are LDP capable.
PE1 and PE3 are not SR capable.
PE3 sends an ODD VPN route to PE1 with next-hop 192.0.2.203 and VPN
label 10001.
From an LDP viewpoint: PE1 received an LDP label binding (1037) for
FEC 192.0.2.203/32 from its nhop A. A received an LDP label binding
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(2048) for that FEC from its nhop B. B received an LDP label binding
(3059) for that FEC from its nhop C. C received implicit-null LDP
binding from its next-hop PE3.
As a result, PE1 sends its traffic to the ODD service route
advertised by PE3 to next-hop A with two labels: the top label is
1037 and the bottom label is 10001. A swaps 1037 with 2048 and
forwards to B. B swaps 2048 with 3059 and forwards to C. C pops
3059 and forwards to PE3.
PE4 sends an EVEN VPN route to PE2 with next-hop 192.0.2.204 and VPN
label 10002.
From an SR viewpoint: PE2 maps the IGP route 192.0.2.204/32 onto
Node-SID 204; A swaps 204 with 204 and forwards to B; B swaps 204
with 204 and forwards to C; C pops 204 and forwards to PE4.
As a result, PE2 sends its traffic to the VPN service route
advertised by PE4 to next-hop A with two labels: the top label is 204
and the bottom label is 10002. A swaps 204 with 204 and forwards to
B. B swaps 204 with 204 and forwards to C. C pops 204 and forwards
to PE4.
The two modes of MPLS tunneling co-exist.
The ODD service is tunneled from PE1 to PE3 through a continuous
LDP LSP traversing A, B and C.
The EVEN service is tunneled from PE2 to PE4 through a continuous
SR node segment traversing A, B and C.
2.1. MPLS2MPLS co-existence
We want to highlight that several MPLS2MPLS entries can be installed
in the data plane for the same prefix.
Let us examine A's MPLS forwarding table as an example:
Incoming label: 1037
- outgoing label: 2048
- outgoing nhop: B
Note: this entry is programmed by LDP for 192.0.2.203/32
Incoming label: 203
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- outgoing label: 203
- outgoing nhop: B
Note: this entry is programmed by SR for 192.0.2.203/32
These two entries can co-exist because their incoming label is
unique. The uniqueness is guaranteed by the label manager allocation
rules.
The same applies for the MPLS2IP forwarding entries.
2.2. IP2MPLS co-existence
By default, if both LDP and SR propose an IP to MPLS entry (IP2MPLS)
for the same IP prefix, then the LDP route SHOULD be selected.
A local policy on a router MUST allow to prefer the SR-provided
IP2MPLS entry.
Note that this policy may be locally defined. There is no
requirement that all routers use the same policy.
3. Migration from LDP to SR
PE2 PE4
\ /
PE1----P5--P6--P7---PE3
Figure 2: Migration
Several migration techniques are possible. We describe one technique
inspired by the commonly used method to migrate from one IGP to
another.
At time T0, all the routers run LDP. Any service is tunneled from an
ingress PE to an egress PE over a continuous LDP LSP.
At time T1, all the routers are upgraded to SR. They are configured
with the SRGB range [100, 300]. PE1, PE2, PE3, PE4, P5, P6 and P7
are respectively configured with the node segments 101, 102, 103,
104, 105, 106 and 107 (attached to their service-recursing loopback).
At this time, the service traffic is still tunneled over LDP LSP.
For example, PE1 has an SR node segment to PE3 and an LDP LSP to
PE3 but by default, as seen earlier, the LDP IP2MPLS encapsulation
is preferred. However, it has to be noted that the SR
infrastructure is usable, e.g. for Fast Reroute (FRR) or IGP Loop
Free Convergence to protect existing IP and LDP traffic. FRR
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mechanisms are described in
[I-D.francois-rtgwg-segment-routing-ti-lfa].
At time T2, the operator enables the local policy at PE1 to prefer SR
IP2MPLS encapsulation over LDP IP2MPLS.
The service from PE1 to any other PE is now riding over SR. All
other service traffic is still transported over LDP LSP.
At time T3, gradually, the operator enables the preference for SR
IP2MPLS encapsulation across all the edge routers.
All the service traffic is now transported over SR. LDP is still
operational and services could be reverted to LDP.
However, any traffic switched through LDP entries will still
suffer from LDP-IGP synchronization.
At time T4, LDP is unconfigured from all routers.
4. SR and LDP Interworking
In this section, we analyze the case where SR is available in one
part of the network and LDP is available in another part. We
describe how a continuous MPLS tunnel can be built throughout the
network.
PE2 PE4
\ /
PE1----P5--P6--P7--P8---PE3
Figure 3: SR and LDP Interworking
Let us analyze the following example:
P6, P7, P8, PE4 and PE3 are LDP capable.
PE1, PE2, P5 and P6 are SR capable. PE1, PE2, P5 and P6 are
configured with SRGB (100, 200) and respectively with node
segments 101, 102, 105 and 106.
A service flow must be tunneled from PE1 to PE3 over a continuous
MPLS tunnel encapsulation. We need SR and LDP to interwork.
If the SR/LDP node operates in LDP ordered label distribution control
mode (as defined in [RFC5036]), then the SR/LDP node MUST consider SR
learned labels as if they were learned through an LDP neighbor and
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create LDP bindings for each Prefix-SID and Node-SID learned in the
SR domain.
4.1. LDP to SR
In this section, we analyze a right-to-left traffic flow.
PE3 has learned a service route whose nhop is PE1. PE3 has an LDP
label binding from the nhop P8 for the FEC "PE1". Hence PE3 sends
its service packet to P8 as per classic LDP behavior.
P8 has an LDP label binding from its nhop P7 for the FEC "PE1" and
hence P8 forwards to P7 as per classic LDP behavior.
P7 has an LDP label binding from its nhop P6 for the FEC "PE1" and
hence P7 forwards to P6 as per classic LDP behavior.
P6 does not have an LDP binding from its nhop P5 for the FEC "PE1".
However P6 has an SR node segment to the IGP route "PE1". Hence, P6
forwards the packet to P5 and swaps its local LDP-label for FEC "PE1"
by the equivalent node segment (i.e. 101).
P5 pops 101 (assuming PE1 advertised its node segment 101 with the
penultimate-pop flag set) and forwards to PE1.
PE1 receives the tunneled packet and processes the service label.
The end-to-end MPLS tunnel is built from an LDP LSP from PE3 to P6
and the related node segment from P6 to PE1.
4.1.1. LDP to SR Behavior
It has to be noted that no additional signaling or state is required
in order to provide interworking in the direction LDP to SR.
A SR node having LDP neighbors MUST create LDP bindings for each
Prefix-SID and Node-SID learned in the SR domain and, for each FEC,
stitch the incoming LDP label to the outgoing SR label. This has to
be done in both LDP independent and ordered label distribution
control modes as defined in [RFC5036].
4.2. SR to LDP
In this section, we analyze the left-to-right traffic flow.
We assume that the operator configures P5 to act as a Segment Routing
Mapping Server (SRMS) and advertises the following mappings: (P7,
107), (P8, 108), (PE3, 103) and (PE4, 104).
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These mappings are advertised as Remote-Binding SID as described in
[I-D.ietf-isis-segment-routing-extensions].
The mappings advertised by one or more SR mapping servers result from
local policy information configured by the operator.
If PE3 had been SR capable, the operator would have configured PE3
with node segment 103. Instead, as PE3 is not SR capable, the
operator configures that policy at the SRMS and it is the latter
which advertises the mapping.
The mapping server advertisements are only understood by the SR
capable routers. The SR capable routers install the related node
segments in the MPLS data plane exactly like if the node segments had
been advertised by the nodes themselves.
For example, PE1 installs the node segment 103 with nhop P5 exactly
as if PE3 had advertised node segment 103.
PE1 has a service route whose nhop is PE3. PE1 has a node segment
for that IGP route: 103 with nhop P5. Hence PE1 sends its service
packet to P5 with two labels: the bottom label is the service label
and the top label is 103.
P5 swaps 103 for 103 and forwards to P6.
P6's next-hop for the IGP route "PE3" is not SR capable (P7 does not
advertise the SR capability). However, P6 has an LDP label binding
from that next-hop for the same FEC (e.g. LDP label 1037). Hence,
P6 swaps 103 for 1037 and forwards to P7.
P7 swaps this label with the LDP-label received from P8 and forwards
to P8.
P8 pops the LDP label and forwards to PE3.
PE3 receives the tunneled packet and processes the service label.
The end-to-end MPLS tunnel is built from an SR node segment from PE1
to P6 and an LDP LSP from P6 to PE3.
Note: SR mappings advertisements cannot set Penultimate Hop Popping.
In the previous example, P6 requires the presence of the segment 103
such as to map it to the LDP label 1037. For that reason, the P flag
available in the Prefix-SID is not available in the Remote-Binding
SID.
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4.2.1. SR to LDP Behavior
SR to LDP interworking requires a SRMS as defined in
[I-D.ietf-isis-segment-routing-extensions].
The SRMS MUST be configured by the operator in order to advertise
Node-SIDs on behalf of non-SR nodes.
At least one SRMS MUST be present in the routing domain. Multiple
SRMSs SHOULD be present for redundancy.
Each SR capable router installs in the MPLS data plane Node-SIDs
learned from the SRMS exactly like if these SIDs had been advertised
by the nodes themselves.
A SR node having LDP neighbors MUST create LDP bindings for each
Prefix-SID and Node-SID learned in the SR domain and, for each FEC,
stitch the incoming SR label to the outgoing LDP label. This has to
be done in both LDP independent and ordered label distribution
control modes as defined in [RFC5036].
The encodings of the SRMS advertisements are specific to the routing
protocol. See [I-D.ietf-isis-segment-routing-extensions],
[I-D.ietf-ospf-segment-routing-extensions] and
[I-D.ietf-ospf-ospfv3-segment-routing-extensions] for details of SRMS
encodings. See also [I-D.ietf-spring-conflict-resolution] for the
specific rules on SRMS advertisements.
It has to be noted that the SR to LDP behavior does not propagate the
status of the LDP FEC which was signaled if LDP was configured to use
the ordered mode.
It has to be noted that in the case of SR to LDP, the label binding
is equivalent to the independent LDP Label Distribution Control Mode
([RFC5036]) where a label in bound to a FEC independently from the
received binding for the same FEC.
5. SR/LDP Interworking Use Cases
SR can be deployed such as to enhance LDP transport. The SR
deployment can be limited to the network region where the SR benefits
are most desired.
5.1. SR Protection of LDP-based Traffic
In Figure 4, let us assume:
All link costs are 10 except FG which is 30.
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All routers are LDP capable.
X, Y and Z are PE's participating to an important service S.
The operator requires 50msec link-based Fast Reroute (FRR) for
service S.
A, B, C, D, E, F and G are SR capable.
X, Y, Z are not SR capable, e.g. as part of a staged migration
from LDP to SR, the operator deploys SR first in a sub-part of the
network and then everywhere.
X
|
Y--A---B---E--Z
| | \
D---C--F--G
30
Figure 4: SR/LDP interworking example
The operator would like to resolve the following issues:
To protect the link BA along the shortest-path of the important
flow XY, B requires a Remote LFA (RLFA, [RFC7490]) repair tunnel
to D and hence a targeted LDP session from B to D. Typically,
network operators prefer avoiding these dynamically established
multi-hop LDP sessions in order to reduce the number of protocols
running in the network and hence simplify network operations.
There is no LFA/RLFA solution to protect the link BE along the
shortest path of the important flow XZ. The operator wants a
guaranteed link-based FRR solution.
The operator can meet these objectives by deploying SR only on A, B,
C, D, E, F and G:
The operator configures A, B, C, D, E, F and G with SRGB (100,
200) and respective node segments 101, 102, 103, 104, 105, 106 and
107.
The operator configures D as an SR Mapping Server with the
following policy mapping: (X, 201), (Y, 202), (Z, 203).
Each SR node automatically advertises local adjacency segment for
its IGP adjacencies. Specifically, F advertises adjacency segment
9001 for its adjacency FG.
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A, B, C, D, E, F and G keep their LDP capability and hence the flows
XY and XZ are transported over end-to-end LDP LSP's.
For example, LDP at B installs the following MPLS data plane entries:
Incoming label: local LDP label bound by B for FEC Y
Outgoing label: LDP label bound by A for FEC Y
Outgoing nhop: A
Incoming label: local LDP label bound by B for FEC Z
Outgoing label: LDP label bound by E for FEC Z
Outgoing nhop: E
The novelty comes from how the backup chains are computed for these
LDP-based entries. While LDP labels are used for the primary nhop
and outgoing labels, SR information is used for the FRR construction.
In steady state, the traffic is transported over LDP LSP. In
transient FRR state, the traffic is backup thanks to the SR enhanced
capabilities.
The RLFA paths are dynamically pre-computed as defined in [RFC7490].
Typically, implementations allow to enable RLFA mechanism through a
simple configuration command that triggers both the pre-computation
and installation of the repair path. The details on how RLFA
mechanisms are implemented and configured is outside the scope of
this document and not relevant to the aspects of SR/LDP interwork
explained in this document.
This helps meet the requirements of the operator:
Eliminate targeted LDP session.
Guaranteed FRR coverage.
Keep the traffic over LDP LSP in steady state.
Partial SR deployment only where needed.
5.2. Eliminating Targeted LDP Session
B's MPLS entry to Y becomes:
- Incoming label: local LDP label bound by B for FEC Y
Outgoing label: LDP label bound by A for FEC Y
Backup outgoing label: SR node segment for Y {202}
Outgoing nhop: A
Backup nhop: repair tunnel: node segment to D {104}
with outgoing nhop: C
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It has to be noted that D is selected as Remote Free Alternate
(R-LFA) as defined in [RFC7490].
In steady-state, X sends its Y-destined traffic to B with a top label
which is the LDP label bound by B for FEC Y. B swaps that top label
for the LDP label bound by A for FEC Y and forwards to A. A pops the
LDP label and forwards to Y.
Upon failure of the link BA, B swaps the incoming top-label with the
node segment for Y (202) and sends the packet onto a repair tunnel to
D (node segment 104). Thus, B sends the packet to C with the label
stack {104, 202}. C pops the node segment 104 and forwards to D. D
swaps 202 for 202 and forwards to A. A's nhop to Y is not SR capable
and hence A swaps the incoming node segment 202 to the LDP label
announced by its next-hop (in this case, implicit null).
After IGP convergence, B's MPLS entry to Y will become:
- Incoming label: local LDP label bound by B for FEC Y
Outgoing label: LDP label bound by C for FEC Y
Outgoing nhop: C
And the traffic XY travels again over the LDP LSP.
Conclusion: the operator has eliminated the need for targeted LDP
sessions (no longer required) and the steady-state traffic is still
transported over LDP. The SR deployment is confined to the area
where these benefits are required.
Despite that in general, an implementation would not require a manual
configuration of LDP Targeted sessions however, it is always a gain
if the operator is able to reduce the set of protocol sessions
running on the network infrastructure.
5.3. Guaranteed FRR coverage
As mentioned in Section 5.1 above, in the example topology described
in Figure 4, there is no RLFA-based solution for protecting the
traffic flow YZ against the failure of link BE because there is no
intersection between the extended P-space and Q-space (see [RFC7490]
for details). However:
o G belongs to the Q space of Z.
o G can be reached from B via a "repair SR path" {106, 9001} that is
not affected by failure of link BE (The method by which G and the
repair tunnel to it from B are identified are out of scope of this
document.)
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B's MPLS entry to Z becomes:
- Incoming label: local LDP label bound by B for FEC Z
Outgoing label: LDP label bound by E for FEC Z
Backup outgoing label: SR node segment for Z {203}
Outgoing nhop: E
Backup nhop: repair tunnel to G: {106, 9001}
G is reachable from B via the combination of a
node segment to F {106} and an adjacency segment
FG {9001}
Note that {106, 107} would have equally work.
Indeed, in many case, P's shortest path to Q is
over the link PQ. The adjacency segment from P to
Q is required only in very rare topologies where
the shortest-path from P to Q is not via the link
PQ.
In steady-state, X sends its Z-destined traffic to B with a top label
which is the LDP label bound by B for FEC Z. B swaps that top label
for the LDP label bound by E for FEC Z and forwards to E. E pops the
LDP label and forwards to Z.
Upon failure of the link BE, B swaps the incoming top-label with the
node segment for Z (203) and sends the packet onto a repair tunnel to
G (node segment 106 followed by adjacency segment 9001). Thus, B
sends the packet to C with the label stack {106, 9001, 203}. C pops
the node segment 106 and forwards to F. F pops the adjacency segment
9001 and forwards to G. G swaps 203 for 203 and forwards to E. E's
nhop to Z is not SR capable and hence E swaps the incoming node
segment 203 for the LDP label announced by its next-hop (in this
case, implicit null).
After IGP convergence, B's MPLS entry to Z will become:
- Incoming label: local LDP label bound by B for FEC Z
Outgoing label: LDP label bound by C for FEC Z
Outgoing nhop: C
And the traffic XZ travels again over the LDP LSP.
Conclusions:
o the operator has eliminated its second problem: guaranteed FRR
coverage is provided. The steady-state traffic is still
transported over LDP. The SR deployment is confined to the area
where these benefits are required.
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o FRR coverage has been achieved without any signaling for setting
up the repair LSP and without setting up a targeted LDP session
between B and G.
5.4. Inter-AS Option C, Carrier's Carrier
In inter-AS Option C, two interconnected ASes sets up inter-AS MPLS
connectivity. SR may be independently deployed in each AS.
PE1---R1---B1---B2---R2---PE2
<-----------> <----------->
AS1 AS2
Figure 5: Inter-AS Option C
In Inter-AS Option C [RFC4364], B2 advertises to B1 a BGP3107 route
for PE2 and B1 reflects it to its internal peers, such as PE1. PE1
learns from a service route reflector a service route whose nhop is
PE2. PE1 resolves that service route on the BGP3107 route to PE2.
That BGP3107 route to PE2 is itself resolved on the AS1 IGP route to
B1.
If AS1 operates SR, then the tunnel from PE1 to B1 is provided by the
node segment from PE1 to B1.
PE1 sends a service packet with three labels: the top one is the node
segment to B1, the next-one is the BGP3107 label provided by B1 for
the route "PE2" and the bottom one is the service label allocated by
PE2.
6. IANA Considerations
This document does not introduce any new codepoint.
7. Manageability Considerations
7.1. SR and LDP co-existence
As illustrated in Section 2.2, when both SR and LDP co-exist, the
following applies:
o If both SR and LDP propose an IP2MPLS entry for the same IP
prefix, then by default the LDP route MUST be selected.
o A local policy on a router MUST allow to prefer the SR-provided
IP2MPLS entry.
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o Note that this policy may be locally defined. There is no
requirement that all routers use the same policy.
7.2. SRMS Management
In the case of SR/LDP interoperability through the use of a SRMS,
mappings are advertised by one or more SRMS.
SRMS function is implemented in the link-state protocol (such as IS-
IS and OSPF). Link-state protocols allow propagation of updates
across area boundaries and therefore SRMS advertisements are
propagated through the usual inter-area advertisement procedures in
link-state protocols.
Multiple SRMSs can be provisioned in a network for redundancy.
Moreover, a preference mechanism may also be used among SRMSs so to
deploy a primary/secondary SRMS scheme allowing controlled
modification or migration of SIDs.
The content of SRMS advertisement (i.e.: mappings) are a matter of
local policy determined by the operator. When multiple SRMSs are
active, it is necessary that the information (mappings) advertised by
the different SRMSs is aligned and consistent.
[I-D.ietf-spring-conflict-resolution] illustrates mechanisms through
which such consistency is achieved.
When the SRMS advertise mappings, an implementation SHOULD provide a
mechanism through which the operator determines which of the IP2MPLS
mappings are preferred among the one advertised by the SRMS and the
ones advertised by LDP.
7.3. Dataplane Verification
When Label switch paths (LSPs) are defined by stitching LDP LSPs with
SR LSPs, it is necessary to have mechanisms allowing the verification
of the LSP connectivity as well as validation of the path. These
mechanisms are described in [I-D.ietf-mpls-spring-lsp-ping].
8. Security Considerations
This document does not introduce any change to the MPLS dataplane and
therefore no additional security of the MPLS dataplane is required.
This document introduces another form of label binding
advertisements. The security associated with these advertisement is
part of the security applied to routing protocols such as IS-IS and
OSPF which both make use of cryptographic authentication mechanisms.
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9. Acknowledgements
We would like to thank Pierre Francois, Ruediger Geib and Alexander
Vainshtein for their contribution to the content of this document.
10. Contributors' Addresses
Edward Crabbe
Individual
Email: edward.crabbe@gmail.com
Igor Milojevic
Email: milojevicigor@gmail.com
Saku Ytti
TDC
Email: saku@ytti.fi
Rob Shakir
Individual
Email: rjs@rob.sh
Martin Horneffer
Deutsche Telekom
Email: Martin.Horneffer@telekom.de
Wim Henderickx
Alcatel-Lucent
Email: wim.henderickx@alcatel-lucent.com
Jeff Tantsura
Ericsson
Email: Jeff.Tantsura@ericsson.com
11. References
11.1. Normative References
[I-D.ietf-spring-conflict-resolution]
Ginsberg, L., Psenak, P., Previdi, S., and M. Pilka,
"Segment Routing Conflict Resolution", draft-ietf-spring-
conflict-resolution-02 (work in progress), October 2016.
[I-D.ietf-spring-segment-routing]
Filsfils, C., Previdi, S., Decraene, B., Litkowski, S.,
and R. Shakir, "Segment Routing Architecture", draft-ietf-
spring-segment-routing-10 (work in progress), November
2016.
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[I-D.ietf-spring-segment-routing-mpls]
Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
Litkowski, S., Horneffer, M., Shakir, R.,
jefftant@gmail.com, j., and E. Crabbe, "Segment Routing
with MPLS data plane", draft-ietf-spring-segment-routing-
mpls-07 (work in progress), February 2017.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
11.2. Informative References
[I-D.francois-rtgwg-segment-routing-ti-lfa]
Francois, P., Bashandy, A., Filsfils, C., Decraene, B.,
and S. Litkowski, "Abstract", draft-francois-rtgwg-
segment-routing-ti-lfa-04 (work in progress), December
2016.
[I-D.ietf-isis-segment-routing-extensions]
Previdi, S., Filsfils, C., Bashandy, A., Gredler, H.,
Litkowski, S., Decraene, B., and j. jefftant@gmail.com,
"IS-IS Extensions for Segment Routing", draft-ietf-isis-
segment-routing-extensions-09 (work in progress), October
2016.
[I-D.ietf-mpls-spring-lsp-ping]
Kumar, N., Swallow, G., Pignataro, C., Akiya, N., Kini,
S., Gredler, H., and M. Chen, "Label Switched Path (LSP)
Ping/Trace for Segment Routing Networks Using MPLS
Dataplane", draft-ietf-mpls-spring-lsp-ping-02 (work in
progress), December 2016.
[I-D.ietf-ospf-ospfv3-segment-routing-extensions]
Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
Shakir, R., Henderickx, W., and J. Tantsura, "OSPFv3
Extensions for Segment Routing", draft-ietf-ospf-ospfv3-
segment-routing-extensions-07 (work in progress), October
2016.
[I-D.ietf-ospf-segment-routing-extensions]
Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
Extensions for Segment Routing", draft-ietf-ospf-segment-
routing-extensions-10 (work in progress), October 2016.
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[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
2006, <http://www.rfc-editor.org/info/rfc4364>.
[RFC5036] Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
"LDP Specification", RFC 5036, DOI 10.17487/RFC5036,
October 2007, <http://www.rfc-editor.org/info/rfc5036>.
[RFC5960] Frost, D., Ed., Bryant, S., Ed., and M. Bocci, Ed., "MPLS
Transport Profile Data Plane Architecture", RFC 5960,
DOI 10.17487/RFC5960, August 2010,
<http://www.rfc-editor.org/info/rfc5960>.
[RFC7490] Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)",
RFC 7490, DOI 10.17487/RFC7490, April 2015,
<http://www.rfc-editor.org/info/rfc7490>.
Authors' Addresses
Clarence Filsfils (editor)
Cisco Systems, Inc.
Brussels
BE
Email: cfilsfil@cisco.com
Stefano Previdi (editor)
Cisco Systems, Inc.
Via Del Serafico, 200
Rome 00142
Italy
Email: sprevidi@cisco.com
Ahmed Bashandy
Cisco Systems, Inc.
170, West Tasman Drive
San Jose, CA 95134
US
Email: bashandy@cisco.com
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Bruno Decraene
Orange
FR
Email: bruno.decraene@orange.com
Stephane Litkowski
Orange
FR
Email: stephane.litkowski@orange.com
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