One document matched: draft-gredler-isis-label-advertisement-00.xml
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<rfc category="std" docName="draft-gredler-isis-label-advertisement-00" ipr="trust200902">
<!-- ***** FRONT MATTER ***** -->
<front>
<title>Advertising MPLS labels in IS-IS</title>
<!-- add 'role="editor"' below for the editors if appropriate -->
<author fullname="Hannes Gredler" initials="H." surname="Gredler" role="editor">
<organization>Juniper Networks, Inc.</organization>
<address>
<postal>
<street>1194 N. Mathilda Ave.</street>
<city>Sunnyvale</city>
<region>CA</region>
<code>94089</code>
<country>US</country>
</postal>
<email>hannes@juniper.net</email>
</address>
</author>
<author fullname="Shane Amante" initials="S." surname="Amante">
<organization>Level 3 Communications, Inc.</organization>
<address>
<postal>
<street>1025 Eldorado Blvd</street>
<city>Broomfield</city>
<region>CO</region>
<code>80021</code>
<country>US</country>
</postal>
<email>shane@level3.net</email>
</address>
</author>
<author fullname="Tom Scholl" initials="T." surname="Scholl">
<organization>Amazon</organization>
<address>
<postal>
<street></street>
<city>Seattle</city>
<region>WN</region>
<code></code>
<country>US</country>
</postal>
<email>tscholl@amazon.com</email>
</address>
</author>
<author fullname="Luay Jalil" initials="L." surname="Jalil">
<organization>Verizon</organization>
<address>
<postal>
<street>1201 E Arapaho Rd.</street>
<city>Richardson</city>
<region>TX</region>
<code>75081</code>
<country>US</country>
</postal>
<email>luay.jalil@verizon.com</email>
</address>
</author>
<date day="5" month="April" year="2013"/>
<area>Routing</area>
<workgroup>IS-IS for IP Internets</workgroup>
<keyword>MPLS</keyword>
<keyword>IGP</keyword>
<keyword>IS-IS</keyword>
<keyword>Label advertisement</keyword>
<abstract>
<t>Historically MPLS label distribution was driven by
protocols like LDP, RSVP and LBGP. All of those protocols are session
oriented. In order to obtain label binding for a given destination FEC from
a given router one needs first to establish an LDP/RSVP/LBGP session
with that router.
</t>
<t><xref target="I-D.gredler-rtgwg-igp-label-advertisement">
Advertising MPLS labels in IGPs advertisement</xref>
describes several use cases where utilizing the flooding machinery
of link-state protocols for MPLS label distribution allows
to obtain the binding without requiring to establish an LDP/RSVP/LBGP
session with that router.
</t>
<t>This document describes the protocol extension to distribute
MPLS labels by the IS-IS protocol.</t>
</abstract>
<note title="Requirements Language">
<t>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 <xref
target="RFC2119">RFC 2119</xref>.</t>
</note>
</front>
<middle>
<section title="Introduction">
<t>MPLS label allocations are predominantly distributed by using
the LDP <xref target="RFC5036"/>, RSVP <xref target="RFC5151"/> or
labeled BGP <xref target="RFC3107"/> protocol. All of those protocols
have in common that they are session oriented, which means that in
order to obtain label binding for a given destination FEC from
a given router one needs first to establish a direct
control plane (LDP/RSVP/LBGP) session with that router.
</t>
<t>There are a couple of
<xref target="I-D.gredler-rtgwg-igp-label-advertisement">use cases</xref>
where the consumer of a MPLS label binding may not
be adjacent to the router that performs the binding. Bringing up an
explicit session using the existing label distribution protocols between
the non-adjacent router that bind the label and the router that
acts as a consumer of this binding is the existing remedy for
this dilemma. </t>
<t>This document describes a single IS-IS protocol extension which allows
routers to advertise MPLS label bindings within and beyond an IGP domain, and
controlling inter-area distribution.
</t>
</section>
<section title="Motivation, Rationale and Applicability">
<t>One possible way of distributing MPLS labels using IS-IS has been described in
<xref target="I-D.previdi-filsfils-isis-segment-routing"> Segment
Routing</xref>. The authors propose to re-use the IS-Reach TLVs (22,
23, 222) and Extended IP Prefix TLVs (135, 236) for carrying the label
information. While retrofitting existing protocol machinery for new
purposes is generally a good thing, <xref
target="I-D.previdi-filsfils-isis-segment-routing">Segment
Routing</xref> falls short of addressing some use-cases defined in
<xref target="I-D.gredler-rtgwg-igp-label-advertisement"/>.
</t>
<t>The dominant issue around re-using IS-Reach TLVs and the extended IP Prefix TLVs is that
both family of TLVs have existing protocol semantics, which might not be well suitable
to advertising MPLS label switched paths in a generic fashion. These are specifically:</t>
<t><list style="symbols">
<t>Bi-directionality semantics</t>
<t>IP path semantics</t>
<t>Lack of 'path' notion</t>
</list>
</t>
<section title="Issue: Bi-directionality semantics">
<t>
'Bi-directionality semantics', affects the complexity around advertisement of unidirectional LSPs.
Label advertisement of per-link labels or <xref
target="I-D.previdi-filsfils-isis-segment-routing">'Adj-SIDs'</xref>
is done using IS-reach TLVs. Usually implementations need to have
an adjacency in 'Up' state prior to advertising this adjacency
as IS-reach TLV in its Link State PDUs (LSPs).
In order to advertise a per-link LSP an implementation first needs to have
an adjacency, which only transitions to 'Up' state after passing the 3-way
check. This implies bi-directionality. If an implementation wants to advertise
per-link LSPs to e.g. outside the IGP domain then it would need
to fake-up an adjacency. Changing existing IGP Adjacency code to support
such cases defeats the purpose of re-using existing functionality as there is
not much common functionality to be shared.
</t>
</section>
<section title="Issue: IP path semantics">
<t>
LSPs pointing to a Node are advertised as <xref
target="I-D.previdi-filsfils-isis-segment-routing">'Node-SIDs'</xref>
using the family of extended IP Reach TLVs. That means that in order
to advertise a LSP, one is inheriting the semantics of advertising an
IP path. Consider router A has got existing LSPs to its entire one-hop
neighborhood and is re-advertising those LSPs using IP reachability semantics.
Now we have two exact matching IP advertisements. One from the owning router
(router B) which advertises its stable transport loopback address and another one
from router A re-advertising a LSP path to router B. Existing routing software
may get confused now as the 'stable transport' address shows up from multiple places
in the network and more worse the IP forwarding path for control-plane protocols may get
mingled with the MPLS data plane.
</t>
</section>
<section title="Issue: Lack of 'path' notion">
<t>
Both IS-Reach TLVs and IP Prefix Reachability TLVs have a
limited semantics describing MPLS label-switched paths in the sense of
a 'path'. Both encoding formats allow to specify a pointer to some
specific router, but not to describe a MPLS label switched path
containing all of its path segments. <xref
target="I-D.previdi-filsfils-isis-segment-routing"/> allows to define
'Forwarding Adjacencies' as per <xref target="RFC4206"/>. The way to
describe a path of a given forwarding adjacency is to carry a list of
"Segment IDs". That implies that nodes which do not yet participate in
'Segment routing' or are outside of a 'Segment routing' domain can not
be expressed using those path semantics.
</t>
<t>
A protocol for advertising MPLS label switched paths, should
be generic enough to express paths sourced by existing MPLS LSPs,
such that ingress routers can flexibly combine them according to
application needs.
</t>
</section>
<section title="Motivation">
<t>
IGP advertisement of MPLS label switched paths requires a new set of
protocol semantics (path paradigm), which hardly can be expressed using
the existing IS-IS protocol. This document describes IS-IS protocol extensions
which allows generic advertisement of MPLS label switched paths in IS-IS.
</t>
</section>
</section>
<section title="MPLS label TLV">
<t>The MPLS Label TLV may be originated by any <xref target="RFC5305">Traffic Engineering</xref>
capable router in an IS-IS domain. A router may advertise MPLS labels along with so called 'ERO'
path segments describing the label switched path. Since ERO style path notation allows to
express pointers to link and node IP addresses any label switched path, sourced by any protocol
can be described.
</t>
<t>Due to the limited size of subTLV space (See <xref
target="RFC5311"/> section 4.5 for details), The MPLS Label TLV has
cumulative rather than canceling semantics. If a router originates
more than one MPLS Label TLV with the same Label value, then the
subTLVs of the second, third, etc. TLV are accumulated. Since some
subTLVs represent an ordered set (e.g. ERO subTLVs) and in order to
not complicate the cross-fragment tracking logic, all subTLVs for a
given label value MUST be present in a single IS-IS fragment.
</t>
<t>The MPLS Label TLV has type 149 and has the following format:</t>
<figure anchor="MPLS-TLV-figure" title="MPLS TLV format">
<artwork>
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|R|R|R| MPLS Label |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
</figure>
<t><list style="symbols">
<t>4 bits of flags, consisting of:
<list style="symbols">
<t>1 bit of up/down information (U bit)</t>
<t>3 bits are reserved for future use</t>
</list></t>
<t>20 bits of MPLS label information</t>
<t>0-252 octets of sub-TLVs, where each sub-TLV consists of a
sequence of:
<list style="symbols">
<t>1 octet of sub-TLV type</t>
<t>1 octet of length of the value field of the sub-TLV</t>
<t>0-250 octets of value</t>
</list></t>
</list></t>
<section title="Flags">
<t>Flags
<list style="hanging">
<t>Up/Down Bit: A router may flood MPLS label information across level boundaries.
In order to prevent flooding loops, a router will Set the Up/Down (U-Bit)
when propagating from Level 2 down to Level 1. This is done as per the
procedures for IP Prefixes lined out in <xref target="RFC5302"/>.
</t>
</list></t>
</section>
<section title="subTLV support">
<t>
An originating router MAY want to attach one or more
subTLVs to the MPLS label TLV. SubTLVs presence is inferred from the
length of the MPLS Label TLV. If the MPLS Label TLV Length field is >
3 octets then one or more subTLVs may be present.
</t>
</section>
<section title="IPv4 Prefix ERO subTLV">
<t>The IPv4 ERO subTLV (Type 1) describes a path segment using IPv4 Prefix style of encoding.
Its appearance and semantics have been borrowed from <xref target="RFC3209">
Section 4.3.3.2</xref>.
</t>
<t>The 'Prefix Length' field contains the length of the prefix in bits.
Only the most significant octets of the prefix are encoded. I.e. 1
octet for prefix length 1 up to 8, 2 octets for prefix length 9 to
16, 3 octets for prefix length 17 up to 24 and 4 octets for prefix
length 25 up to 32, etc.</t>
<t>
The 'L' bit in the subTLV is a one-bit attribute. If the L bit is
set, then the value of the attribute is 'loose.' Otherwise, the
value of the attribute is 'strict.'
</t>
<figure anchor="IPv4-Prefix-ERO-subTLV-figure" title="IPv4 Prefix ERO subTLV format">
<artwork>
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|L| Type | Length | Prefix Length | IPv4 Prefix |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Prefix (continued, variable-length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
</figure>
</section> <!-- end IPv4 Prefix ERO subtLV -->
<section title="IPv6 Prefix ERO subTLV">
<t>The IPv6 ERO subTLV (Type 2) describes a path segment using IPv6 Prefix style of encoding.
Its appearance and semantics have been borrowed from <xref target="RFC3209">
Section 4.3.3.3</xref>.
</t>
<t>The 'Prefix Length' field contains the length of the prefix in bits.
Only the most significant octets of the prefix are encoded. I.e. 1
octet for prefix length 1 up to 8, 2 octets for prefix length 9 to
16, 3 octets for prefix length 17 up to 24 and 4 octets for prefix
length 25 up to 32, ...., 16 octets for prefix length 113 up to 128.
</t>
<t>
The 'L' bit in the subTLV is a one-bit attribute. If the L bit is
set, then the value of the attribute is 'loose.' Otherwise, the
value of the attribute is 'strict.'
</t>
<figure anchor="IPv6-Prefix-ERO-subTLV-figure" title="IPv6 Prefix ERO subTLV format">
<artwork>
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|L| Type | Length | Prefix Length | IPv6 Prefix |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 Prefix (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 Prefix (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 Prefix (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 Prefix (continued, variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
</figure>
</section> <!-- end IPv6 Prefix ERO subtLV -->
<section title="Prefix ERO subTLV path semantics">
<t>
All 'Prefix ERO' information represents an ordered set
which describes the segments of a label-switched path. The last Prefix
ERO subTLV describes the segment closest to the egress point of the
LSP. Contrary the first Prefix ERO subTLV describes the first segment
of a label switched path. If a router extends or stitches a label
switched path it MUST prepend the new segments path information to the
Prefix ERO list.
</t>
</section>
</section>
<section title="Advertising Label Examples">
<section title="Sample Topology">
<t>
The following <xref target="sample-topology">topology</xref> and IP addresses shall be used throughout
the Label advertisement examples.
</t>
<figure anchor="sample-topology" title="Sample Topology">
<artwork>
AS1 : AS 2
:
: :
Level 1 : Level 2 :
: :
+-----+ +-----+-IP3--IP4--+-----+ :
| R1 +-IP1--IP2--+ R2 | | R3 | :
+--+--+ +--+--+-IP5--IP6--+--+--+-IP15-IP16-
| | | : \
IP3 IP7 IP13 : +--+--+
| | | : | R7 |
IP4 IP8 IP14 : +--+--+
| | | : /
+--+--+ +--+--+ +--+--+-IP17-IP18-
| R4 +-IP19-IP20-+ R5 |-IP11-IP12-| R6 | :
+-----+ +-----+ +-----+ :
: :
: :
: :
</artwork>
</figure>
<section title="Transport IP addresses and router-IDs">
<t><list style="symbols">
<t>R1: 192.168.1.1</t>
<t>R2: 192.168.1.2</t>
<t>R3: 192.168.1.3</t>
<t>R4: 192.168.1.4</t>
<t>R5: 192.168.1.5</t>
<t>R6: 192.168.1.6</t>
<t>R7: 192.168.1.7</t>
</list></t>
</section>
<section title="Link IP addresses">
<t><list style="symbols">
<t>R1 to R2 link: 10.0.0.1, 10.0.0.2</t>
<t>R1 to R4 link: 10.0.0.3, 10.0.0.4</t>
<t>R2 to R3 link #1: 10.0.0.3, 10.0.0.4</t>
<t>R2 to R3 link #2: 10.0.0.5, 10.0.0.6</t>
<t>R2 to R5 link: 10.0.0.7, 10.0.0.8</t>
<t>R3 to R6 link: 10.0.0.13, 10.0.0.14</t>
<t>R3 to R7 link: 10.0.0.15, 10.0.0.16</t>
<t>R4 to R5 link: 10.0.0.19, 10.0.0.20</t>
<t>R5 to R6 link: 10.0.0.11, 10.0.0.12</t>
<t>R6 to R7 link: 10.0.0.17, 10.0.0.18</t>
</list></t>
</section>
</section>
<section title="One-hop LSP to an adjacent Router">
<t>If R1 would advertise a label <N> bound to a one-hop LSP
from R1 to R2 it would encode as follows:</t>
<t><list style="hangout">
<t>TLV 149: MPLS label N, Flags {}:
<list style="hangout">
<t>IPv4 Prefix ERO subTLV: 192.168.1.2/32, Strict</t>
</list></t>
</list></t>
</section>
<section title="One-hop LSP to an adjacent Router using a specific link">
<t>If R2 would advertise a label <N>bound to a one-hop LSP
from R2 to R3, using the link #2 it would encode as follows</t>
<t><list style="hangout">
<t>TLV 149: MPLS label <N>, Flags {}:
<list style="hangout">
<t>IPv4 Prefix ERO subTLV: 10.0.0.6/32, Strict</t>
</list></t>
</list></t>
</section>
<section title="Advertisement of an RSVP LSP">
<t>Consider a RSVP LSP name "R2-to-R6" traversing (R2 to R3 using link #1, R6):</t>
<t>If R2 would advertise a label <N> bound to the RSVP LSP named 'R2-to-R6',
it would encode as follows</t>
<t><list style="hangout">
<t>TLV 149: MPLS label <N>, Flags {}:
<list style="hangout">
<t>IPv4 Prefix ERO subTLV: 10.0.0.4/32, Strict</t>
<t>IPv4 Prefix ERO subTLV: 192.168.1.6/32, Strict</t>
</list></t>
</list></t>
</section>
<section title="Advertisement of an LDP LSP">
<t>Consider R2 that creates a LDP label binding for FEC 172.16.0.0./12
using label <N>.</t>
<t>If R2 would re-advertise this binding in IS-IS it would encode as follows</t>
<t><list style="hangout">
<t>TLV 149: MPLS label <N>, Flags {}:
<list style="hangout">
<t>IPv4 Prefix ERO subTLV: 172.16.0.0/12, Loose</t>
</list></t>
</list></t>
</section>
<section title="Interarea advertisement of diverse paths">
<t>Consider two R2->R6 paths: {R2, R3, R6} and {R2, R5, R6}</t>
<t>Consider two R5->R3 paths: {R5, R2, R3} and {R5, R6, R3}</t>
<t>R2 encodes its two paths to R6 as follows:</t>
<t><list style="hangout">
<t>TLV 149: MPLS label <N1>, Flags {}:
<list style="hangout">
<t>IPv4 Prefix ERO subTLV: 192.168.1.3, Strict</t>
<t>IPv4 Prefix ERO subTLV: 192.168.1.6, Strict</t>
</list></t>
<t>TLV 149: MPLS label <N2>, Flags {}:
<list style="hangout">
<t>IPv4 Prefix ERO subTLV: 192.168.1.5, Strict</t>
<t>IPv4 Prefix ERO subTLV: 192.168.1.6, Strict</t>
</list></t>
</list></t>
<t>R5 encodes its two paths to R3 as follows:</t>
<t><list style="hangout">
<t>TLV 149: MPLS label <N1>, Flags {}:
<list style="hangout">
<t>IPv4 Prefix ERO subTLV: 192.168.1.2, Strict</t>
<t>IPv4 Prefix ERO subTLV: 192.168.1.3, Strict</t>
</list></t>
<t>TLV 149: MPLS label <N2>, Flags {}:
<list style="hangout">
<t>IPv4 Prefix ERO subTLV: 192.168.1.6, Strict</t>
<t>IPv4 Prefix ERO subTLV: 192.168.1.3, Strict</t>
</list></t>
</list></t>
<t>
A receiving L1 router does see now all 4 paths and may decide
to load-balance across all or a subset of them.
</t>
</section>
</section> <!-- end 'Advertising Label Examples' -->
<section title="Inter Area Protocol Procedures">
<section title="Applicability">
<t>
Propagation of a MPLS LSP across a level boundary is a local policy decision.
</t>
</section>
<section title="Data plane operations">
<t>
If local policy dictates that a given L1L2 router needs to
re-advertise a MPLS LSPs from one Level to another then it MUST
allocate a new label and program its label forwarding table to connect
the new label to the path in the respective other level. Depending on
how to reach the re-advertised LSP, this is typically done using
a MPLS 'SWAP' or 'SWAP/PUSH' data plane operation.
</t>
</section>
<section title="Control plane operations">
<t>
If local policy dictates that a given L1L2 router needs to
re-advertise a MPLS LSPs from one Level to another then it
must prepend its "Traffic-Engineering-ID" as a loose hop in the
Prefix ERO subTLV list.
</t>
</section>
</section>
<section anchor="Acknowledgements" title="Acknowledgements">
<t>Many thanks to Yakov Rekhter for his useful comments.</t>
</section>
<section anchor="IANA" title="IANA Considerations">
<t>
This documents request allocation for the following TLVs and subTLVs.
</t>
<texttable anchor="iana_table" title="IANA allocations">
<ttcol align="center">PDU</ttcol>
<ttcol align="left">TLV</ttcol>
<ttcol align="left">subTLV</ttcol>
<ttcol align="left">Type</ttcol>
<ttcol align="left">subType</ttcol>
<ttcol align="right">#Occurence</ttcol>
<c>LSP</c>
<c>MPLS Label</c>
<c></c>
<c>149</c>
<c></c>
<c>>=0</c>
<c></c>
<c></c>
<c>IPv4 Prefix ERO</c>
<c></c>
<c>1</c>
<c>>=0</c>
<c></c>
<c></c>
<c>IPv6 Prefix ERO</c>
<c></c>
<c>2</c>
<c>>=0</c>
</texttable>
<t>
The MPLS Label TLV requires a new sub-registry.
Type value 149 has been assigned, with a starting sub-TLV
value of 1, range from 1-127, and managed by Expert Review.
</t>
</section>
<section anchor="Security" title="Security Considerations">
<t> This document does not introduce any change in terms of IS-IS
security. It simply proposes to flood MPLS label information via the IGP.
All existing procedures to ensure message integrity do apply here.
</t>
</section>
</middle>
<!-- *****BACK MATTER ***** -->
<back>
<references title="Normative References">
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.2119.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.3107.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.3209.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.4206.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.5036.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.5151.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.5302.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.5305.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.5311.xml"?>
</references>
<references title="Informative References">
<?rfc include="http://xml.resource.org/public/rfc/bibxml3/reference.I-D.draft-gredler-rtgwg-igp-label-advertisement-03.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml3/reference.I-D.draft-previdi-filsfils-isis-segment-routing-02.xml"?>
</references>
</back>
</rfc>
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