IGP Flexible Algorithms (Flex-Algorithm) In IP Networks
draft-ietf-lsr-ip-flexalgo-12
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 9502.
|
|
|---|---|---|---|
| Authors | William Britto , Shraddha Hegde , Parag Kaneriya , Rajesh Shetty , Ron Bonica , Peter Psenak | ||
| Last updated | 2023-05-22 (Latest revision 2023-05-21) | ||
| Replaces | draft-bonica-lsr-ip-flexalgo | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
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by Paul Kyzivat
Ready w/nits
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| Additional resources | Mailing list discussion | ||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Acee Lindem | ||
| Shepherd write-up | Show Last changed 2022-10-01 | ||
| IESG | IESG state | Became RFC 9502 (Proposed Standard) | |
| Consensus boilerplate | Yes | ||
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Needs 2 more YES or NO OBJECTION positions to pass. |
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| Responsible AD | John Scudder | ||
| Send notices to | acee.ietf@gmail.com | ||
| IANA | IANA review state | IANA OK - Actions Needed |
draft-ietf-lsr-ip-flexalgo-12
LSR Working Group W. Britto
Internet-Draft S. Hegde
Intended status: Standards Track P. Kaneriya
Expires: 22 November 2023 R. Shetty
R. Bonica
Juniper Networks
P. Psenak
Cisco Systems
21 May 2023
IGP Flexible Algorithms (Flex-Algorithm) In IP Networks
draft-ietf-lsr-ip-flexalgo-12
Abstract
This document extends IGP Flex-Algorithm, so that it can be used with
regular IPv4 and IPv6 forwarding.
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 https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 22 November 2023.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://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 Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Use Case Example . . . . . . . . . . . . . . . . . . . . . . 3
4. Advertising Flex-Algorithm Definitions (FAD) . . . . . . . . 3
5. Advertising IP Flex-Algorithm Participation . . . . . . . . . 4
5.1. The IS-IS IP Algorithm Sub-TLV . . . . . . . . . . . . . 4
5.2. The OSPF IP Algorithm TLV . . . . . . . . . . . . . . . . 5
6. Advertising IP Flex-Algorithm Reachability . . . . . . . . . 6
6.1. The IS-IS IPv4 Algorithm Prefix Reachability TLV . . . . 6
6.2. The IS-IS IPv6 Algorithm Prefix Reachability TLV . . . . 8
6.3. The OSPFv2 IP Algorithm Prefix Reachability Sub-TLV . . . 9
6.3.1. The OSPFv2 IP Forwarding Address Sub-TLV . . . . . . 11
6.4. The OSPFv3 IP Algorithm Prefix Reachability Sub-TLV . . . 12
6.5. The OSPF IP Flexible Algorithm ASBR Metric Sub-TLV . . . 13
7. Calculating of IP Flex-Algorithm Paths . . . . . . . . . . . 15
8. IP Flex-Algorithm Forwarding . . . . . . . . . . . . . . . . 15
9. Deployment Considerations . . . . . . . . . . . . . . . . . . 15
10. Protection . . . . . . . . . . . . . . . . . . . . . . . . . 16
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
12. Security Considerations . . . . . . . . . . . . . . . . . . . 19
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
14.1. Normative References . . . . . . . . . . . . . . . . . . 20
14.2. Informative References . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
An IGP Flexible Algorithm (Flex-Algorithm) allows IGPs to compute
constraint-based paths. The base IGP Flex-Algorithm specification
describes how it is used with Segment Routing (SR) data planes - SR
MPLS and SRv6.
An IGP Flex-Algorithm as specified in [RFC9350] computes a
constraint-based path to:
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* All Flex-Algorithm specific Prefix Segment Identifiers (SIDs)
[RFC8402].
* All Flex-Algorithm specific SRv6 Locators [RFC8986].
Therefore, Flex-Algorithm cannot be deployed in the absence of SR or
SRv6.
This document extends Flex-Algorithm, allowing it to compute paths to
IPv4 and IPv6 prefixes.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Use Case Example
The System Architecture for the 5G System [TS.23.501-3GPP] describes
the N3 interface between gNodeB and UPF (User Plane Function).
Mobile networks are becoming more and more IP centric. Each end-user
session from a gNodeB can be destined to a specific UPFs (User Plane
Function) based on the session requirements. For example, some
sessions require high bandwidth, others need to be routed along the
lowest latency path. Each UPF is assigned a unique IP address. As a
result, traffic for different sessions is destined to a different
destination IP address.
The IP address allocated to the UPF can be associated with an
algorithm. The mobile user traffic is then forwarded along the path
based on the algorithm-specific metric and constraints. As a result,
traffic can be sent over a path that is optimized for minimal latency
or highest bandwidth. This mechanism is used to achieve SLA (Service
Level Agreement) appropriate for a user session.
4. Advertising Flex-Algorithm Definitions (FAD)
To guarantee loop-free forwarding, all routers that participate in a
Flex-Algorithm MUST agree on the Flex-Algorithm Definition (FAD).
Selected nodes within the IGP domain MUST advertise FADs as described
in Sections 5, 6, and 7 of [RFC9350].
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5. Advertising IP Flex-Algorithm Participation
A node may use various algorithms when calculating paths to nodes and
prefixes. Algorithm values are defined in the IGP Algorithm Type
Registry [IANA-ALG].
A node MUST participate in a Flex-Algorithm to be:
* Able to compute path for such Flex-Algorithm
* Part of the topology for such Flex-Algorithm
Flex-Algorithm participation MUST be advertised for each Flex-
Algorithm data-plane independently, as specified in [RFC9350]. Using
Flex-Algorithm for regular IPv4 and IPv6 prefixes represents an
independent Flex-Algorithm data-plane, and as such, the Flex-
Algorithm participation for the IP Flex-Algorithm data-plane MUST be
signalled independently of any other Flex-Algorithm data-plane (e.g.,
SR).
All routers in an IGP domain participate in default algorithm 0.
Advertisement of participation in IP Flex-Algorithm does not impact
the router participation in default algorithm 0.
Advertisement of participation in IP Flex-Algorithm does not impact
the router participation signaled for other data-planes. For
example, it is possible that a router participates in a particular
flex-algo for the IP data-plane but does not participate in the same
flex-algo for the SR data-plane.
The following sections describe how the IP Flex-Algorithm
participation is advertised in IGP protocols.
5.1. The IS-IS IP Algorithm Sub-TLV
The IS-IS [ISO10589] IP Algorithm Sub-TLV is a sub-TLV of the IS-IS
Router Capability TLV [RFC7981] and has the following format:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm 1 | Algorithm 2 | Algorithm ... | Algorithm n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: IS-IS IP Algorithm Sub-TLV
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* Type: IP Algorithm Sub-TLV (Value 29)
* Length: Variable
* Algorithm (1 octet): Value from 128 to 255.
The IP Algorithm Sub-TLV MUST be propagated throughout the level and
MUST NOT be advertised across level boundaries. Therefore, the S bit
in the Router Capability TLV, in which the IP Algorithm Sub-TLV is
advertised, MUST NOT be set.
The IP Algorithm Sub-TLV is optional. It MUST NOT be advertised more
than once at a given level. A router receiving multiple IP Algorithm
sub-TLVs from the same originator MUST select the first advertisement
in the lowest-numbered LSP and subsequent instances of the IP
Algorithm Sub-TLV MUST be ignored.
The use of the IP Algorithm Sub-TLV to advertise support for
algorithms outside the Flex-Algorithm range (128-255) is outside the
scope of this document.
The IP Flex-Algorithm participation advertised in the IS-IS IP
Algorithm Sub-TLV is topology independent. When a router advertises
participation in the IS-IS IP Algorithm Sub-TLV, the participation
applies to all topologies in which the advertising node participates.
5.2. The OSPF IP Algorithm TLV
The OSPF [RFC2328] IP Algorithm TLV is a top-level TLV of the Router
Information Opaque LSA [RFC7770] and has the following format:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm 1 | Algorithm... | Algorithm n | |
+- -+
| |
+ +
Figure 2: OSPF IP Algorithm TLV
* Type: IP Algorithm TLV (Value TBD1 by IANA)
* Length: Variable
* Algorithm (1 octet): Value from 128 to 255.
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The IP Algorithm TLV is optional. It MUST only be advertised once in
the Router Information LSA.
When multiple IP Algorithm TLVs are received from a given router, the
receiver MUST use the first occurrence of the TLV in the Router
Information LSA. If the IP Algorithm TLV appears in multiple Router
Information LSAs that have different flooding scopes, the IP
Algorithm TLV in the Router Information LSA with the area-scoped
flooding scope MUST be used. If the IP Algorithm TLV appears in
multiple Router Information LSAs that have the same flooding scope,
the IP Algorithm TLV in the Router Information LSA with the
numerically smallest Instance ID (Opaque ID for OSPFv2 or Link State
ID for OSPFv3) MUST be used and subsequent instances of the IP
Algorithm TLV MUST be ignored.
The Router Information LSA can be advertised at any of the defined
flooding scopes (link, area, or Autonomous System (AS)). For the
purpose of IP Algorithm TLV advertisement, area or Autonomous System
scoped flooding is REQUIRED. The AS flooding scope SHOULD NOT be
used unless local configuration policy on the originating router
indicates domain-wide flooding.
The IP Flex-Algorithm participation advertised in the OSPF IP
Algorithm TLV is topology independent. When a router advertises
participation in OSPF IP Algorithm TLV, the participation applies to
all topologies in which the advertising node participates.
6. Advertising IP Flex-Algorithm Reachability
To be able to associate the prefix with the Flex-Algorithm, the
existing prefix reachability advertisements cannot be used, because
they advertise the prefix reachability in default algorithm 0.
Instead, new IP Flex-Algorithm reachability advertisements are
defined in IS-IS and OSPF.
The M-flag in the FAD is not applicable to IP Algorithm Prefixes.
Any IP Algorithm Prefix advertisement includes the Algorithm and
Metric fields. When an IP Algorithm Prefix is advertised between
areas or domains, the metric field in the IP Algorithm Prefix
advertisement MUST be used irrespective of the M-flag in the FAD
advertisement.
6.1. The IS-IS IPv4 Algorithm Prefix Reachability TLV
The IPv4 Algorithm Prefix Reachability top-level TLV is defined for
advertising IPv4 Flex-Algorithm Prefix Reachability in IS-IS.
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This new TLV shares the sub-TLV space defined for TLVs Advertising
Prefix Reachability.
The IS-IS IPv4 Algorithm Prefix Reachability TLV has the following
format:
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 | Rsvd | MTID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: IS-IS IPv4 Algorithm Prefix Reachability TLV
* Type: IPv4 Algorithm Prefix Reachability TLV (Value 126).
* Length: Variable based on number of prefix entries encoded
* Rsvd (4 bits): Reserved for future use. They MUST be set to zero
on transmission and MUST be ignored on receipt.
* MTID (12 bits): Multitopology Identifier as defined in [RFC5120].
Note that the value 0 is legal.
Followed by one or more prefix entries of the form:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Algorithm |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pfx Length | Prefix (variable)...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-tlv-len | Sub-TLVs (variable) . . . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: IS-IS IPv4 Algorithm Prefix Reachability TLV
* Metric (4 octets): Metric information as defined in [RFC5305].
* Flags (1 octet):
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0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|D| Reserved |
+-+-+-+-+-+-+-+-+
D-flag: When the Prefix is leaked from level-2 to level-1, the
D bit MUST be set. Otherwise, this bit MUST be clear.
Prefixes with the D bit set MUST NOT be leaked from level-1 to
level-2. This is to prevent looping.
* Algorithm (1 octet): Associated Algorithm from 128 to 255.
* Prefix Len (1 octet): Prefix length measured in bits.
* Prefix (variable length): Prefix mapped to Flex-Algorithm.
* Optional Sub-TLV-length (1 octet): Number of octets used by sub-
TLVs
* Optional sub-TLVs (variable length).
If a router receives multiple IPv4 Algorithm Prefix Reachability
advertisements for the same prefix from the same originator, it MUST
select the first advertisement in the lowest-numbered LSP and ignore
any subsequent IPv4 Algorithm Prefix Reachability advertisements for
the same prefix.
If a router receives multiple IPv4 Algorithm Prefix Reachability
advertisements for the same prefix, from different originators, where
all of them do not advertise the same algorithm, it MUST ignore all
of them and MUST NOT install any forwarding entries based on these
advertisements. This situation SHOULD be logged as an error.
In cases where a prefix advertisement is received in both a IPv4
Prefix Reachability TLV ([RFC5305], [RFC5120]) and an IPv4 Algorithm
Prefix Reachability TLV, the IPv4 Prefix Reachability advertisement
MUST be preferred when installing entries in the forwarding plane.
6.2. The IS-IS IPv6 Algorithm Prefix Reachability TLV
The IS-IS IPv6 Algorithm Prefix Reachability TLV is identical to the
IS-IS IPv4 Algorithm Prefix Reachability TLV, except that it has a
distinct type. The type is 127.
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If a router receives multiple IPv6 Algorithm Prefix Reachability
advertisements for the same prefix from the same originator, it MUST
select the first advertisement in the lowest-numbered LSP and ignore
any subsequent IPv6 Algorithm Prefix Reachability advertisements for
the same prefix.
If a router receives multiple IPv6 Algorithm Prefix Reachability
advertisements for the same prefix, from different originators, where
all of them do not advertise the same algorithm, it MUST ignore all
of them and MUST NOT install any forwarding entries based on these
advertisements. This situation SHOULD be logged as an error.
In cases where a prefix advertisement is received in both an IPv6
Prefix Reachability TLV ([RFC5308], [RFC5120]) and an IPv6 Algorithm
Prefix Reachability TLV, the IPv6 Prefix Reachability advertisement
MUST be preferred when installing entries in the forwarding plane.
In cases where a prefix advertisement is received in both an IS-IS
SRv6 Locator TLV [RFC9352] and in IS-IS IPv6 Algorithm Prefix
Reachability TLV, the receiver MUST ignore both of them and MUST NOT
install any forwarding entries based on these advertisements. This
situation SHOULD be logged as an error.
6.3. The OSPFv2 IP Algorithm Prefix Reachability Sub-TLV
A new Sub-TLV of the OSPFv2 Extended Prefix TLV is defined for
advertising IP Algorithm Prefix Reachability in OSPFv2, the OSPFv2 IP
Algorithm Prefix Reachability Sub-TLV.
The OSPFv2 IP Algorithm Prefix Reachability Sub-TLV has the following
format:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MT-ID | Algorithm | Flags | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: OSPFv2 IP Algorithm Prefix Reachability Sub-TLV
* Type (2 octets) : The value is TBD2.
* Length (2 octet): 8
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* MT-ID (1 octet): Multi-Topology ID as defined in [RFC4915]
* Algorithm (1 octet): Associated Algorithm from 128 to 255.
* Flags: (1 octet): The following flags are defined:
0 1 2 3 4 5 6 7 8
+-+-+-+-+-+-+-+-+-+
|E| Reserved |
+-+-+-+-+-+-+-+-+-+
where:
- bit E: Same as bit E defined in section A.4.5 of [RFC2328].
- The remaining bits, are reserved for future use. They MUST be
set to zero on transmission and MUST be ignored on receipt.
* Reserved: (1 octets). SHOULD be set to 0 on transmission and MUST
be ignored on reception.
* Metric (4 octets): The algorithm-specific metric value. The
metric value of 0XFFFFFFFF MUST be considered as unreachable.
An OSPFv2 router receiving multiple OSPFv2 IP Algorithm Prefix
Reachability Sub-TLVs in the same OSPFv2 Extended Prefix TLV, MUST
select the first advertisement of this Sub-TLV and MUST ignore all
remaining occurences of this Sub-TLV in the OSPFv2 Extended Prefix
TLV.
An OSPFv2 router receiving multiple OSPFv2 IP Algorithm Prefix
Reachability TLVs for the same prefix, from different originators,
where all of them do not advertise the same algorithm, MUST ignore
all of them and MUST NOT install any forwarding entries based on
these advertisements. This situation SHOULD be logged as an error.
In cases where a prefix advertisement is received in any of the LSAs
advertising the prefix reachability for algorithm 0 and in an OSPFv2
IP Algorithm Prefix Reachability Sub-TLV, only the prefix
reachability advertisement for algorithm 0 MUST be used and all
occurences of the OSPFv2 IP Algorithm Prefix Reachability Sub-TLV
MUST be ignored.
When computing the IP Algorithm Prefix reachability in OSPFv2, only
information present in the OSPFv2 Extended Prefix TLV MUST be used.
There will not be any information advertised for the IP Algorithm
Prefix in any of the OSPFv2 LSAs that advertise prefix reachability
for algorithm 0. For the IP Algorithm Prefix the OSPFv2 Extended
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Prefix TLV is used to advertise the prefix reachability, unlike for
algorithm 0 prefixes, where the OSPFv2 Extended Prefix TLV is only
used to advertise additional attributes, but not the reachability
itself.
6.3.1. The OSPFv2 IP Forwarding Address Sub-TLV
A new Sub-TLV of the OSPFv2 Extended Prefix TLV is defined for
advertising IP Forwarding Address, the OSPFv2 IP Forwarding Address
Sub-TLV.
The OSPFv2 IP Forwarding Address Sub-TLV has the following format:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Forwarding Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: OSPFv2 IP Forwarding Address Sub-TLV
* Type (2 octets) : The value is TBD4.
* Length (2 octet): 4
* Forwarding Address: Same as defined in section A.4.5 of [RFC2328].
The OSPFv2 IP Forwarding Address Sub-TLV MUST NOT be used for
computing algorithm 0 prefix reachability and MUST be ignored for
algorithm 0 prefixes.
The OSPFv2 IP Forwarding Address Sub-TLV is optional. If it is not
present, the forwarding address for computing the IP Algorithm Prefix
reachability is assumed to be equal to 0.0.0.0.
The OSPFv2 IP Forwarding Address Sub-TLV is only applicable to
Autonomous System (AS) External and Not-So-Stubby Area (NSSA)
External route types. If the OSPFv2 IP Forwarding Address Sub-TLV is
advertised in the OSPFv2 Extended Prefix TLV that has the Route Type
field set to any other type, the OSPFv2 IP Forwarding Address Sub-TLV
MUST be ignored.
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6.4. The OSPFv3 IP Algorithm Prefix Reachability Sub-TLV
The OSPFv3 [RFC5340] IP Algorithm Prefix Reachability Sub-TLV is
defined for advertisement of the IP Algorithm Prefix Reachability in
OSPFv3.
The OSPFv3 IP Algorithm Prefix Reachability Sub-TLV is a sub-TLV of
the following OSPFv3 TLVs defined in [RFC8362]:
* Intra-Area-Prefix TLV
* Inter-Area-Prefix TLV
* External-Prefix TLV
The format of OSPFv3 IP Algorithm Prefix Reachability Sub-TLV is
shown 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: OSPFv3 IP Algorithm Prefix Reachability Sub-TLV
Where:
Type (2 octets): The value is TBD3.
Length (2 octets): 8.
Algorithm (1 octet): Associated Algorithm from 128 to 255.
Reserved: (3 octets). SHOULD be set to 0 on transmission and MUST
be ignored on reception.
Metric (4 octets): The algorithm-specific metric value. The
metric value of 0XFFFFFFFF MUST be considered as unreachable.
When the OSPFv3 IP Algorithm Prefix Reachability Sub-TLV is present,
the NU-bit in the PrefixOptions field of the parent TLV MUST be set.
This is needed to prevent the OSPFv3 IP Algorithm Prefix Reachability
advertisement from contributing to the base algorithm reachability.
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If the NU-bit in the PrefixOptions field of the parent TLV is not
set, the OSPFv3 IP Algorithm Prefix Sub-TLV MUST be ignored by the
receiver.
The metric value in the parent TLV is RECOMMENDED to be set to
LSInfinity [RFC2328]. This recommendation is provided as a network
troubleshooting convenience; if it is not followed the protocol will
still function correctly.
An OSPFv3 router receiving multiple OSPFv3 IP Algorithm Prefix
Reachability Sub-TLVs in the same parent TLV, MUST select the first
advertisement of this Sub-TLV and MUST ignore all remaining
occurences of this Sub-TLV in the parent TLV.
An OSPFv3 router receiving multiple OSPFv3 IP Algorithm Prefix
Reachability TLVs for the same prefix, from different originators,
where all of them do not advertise the same algorithm, MUST ignore
all of them and MUST NOT install any forwarding entries based on
these advertisements. This situation SHOULD be logged as an error.
In cases where a prefix advertisement is received in any of the LSAs
advertising the prefix reachability for algorithm 0 and in an OSPFv3
OSPFv3 IP Algorithm Prefix Reachability Sub-TLV, only the prefix
reachability advertisement for algorithm 0 MUST be used and all
occurences of the OSPFv3 IP Algorithm Prefix Reachability Sub-TLV
MUST be ignored.
In cases where a prefix advertisement is received in both an OSPFv3
SRv6 Locator TLV and in an OSPFv3 IP Algorithm Prefix Reachability
Sub-TLV, the receiver MUST ignore both of them and MUST NOT install
any forwarding entries based on these advertisements. This situation
SHOULD be logged as an error.
6.5. The OSPF IP Flexible Algorithm ASBR Metric Sub-TLV
[RFC9350] defines the OSPF Flexible Algorithm ASBR Metric Sub-TLV
(FAAM) that is used by an OSPFv2 or an OSPFv3 ABR to advertise a
Flex-Algorithm specific metric associated with the corresponding ASBR
LSA.
As described in [RFC9350] each data-plane signals its participation
independently. IP Flex-Algorithm participation is signaled
independent of Segment Routing (SR) Flex-Algorithm participation. As
a result, the calculated topologies for SR and IP Flex-Algorithm
could be different. Such difference prevents the usage of FAAM for
the purpose of the IP Flex-Algorithm.
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The OSPF IP Flexible Algorithm ASBR Metric (IPFAAM) Sub-TLV is
defined for the advertisement of the IP Flex-Algorithm specific
metric associated with an ASBR by the ABR.
The IPFAAM Sub-TLV is a Sub-TLV of the:
- OSPFv2 Extended Inter-Area ASBR TLV as defined in [RFC9350]
- OSPFv3 Inter-Area-Router TLV defined in [RFC8362]
The OSPF IPFAAM Sub-TLV has the following format:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Figure 8: OSPF IP Flexible Algorithm ASBR Metric Sub-TLV
Type (2 octets): 2 (allocated by IANA) for OSPFv2, TBD5 for
OSPFv3.
Length (2 octets): 8.
Algorithm (1 octet): Associated Algorithm from 128 to 255.
Reserved: (3 octets). SHOULD be set to 0 on transmission and MUST
be ignored on reception.
Metric (4 octets): The algorithm-specific metric value.
The usage of the IPFAAM Sub-TLV is similar to the usage of the FAAM
Sub-TLV defined in [RFC9350], but it is used to advertise IP Flex-
Algorithm metric.
An OSPF ABR MUST include the OSPF IPFAAM Sub-TLVs as part of the ASBR
reachability advertisement between areas for every IP Flex-Algorithm
in which it participates and the ASBR is reachable in.
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The FAAM Sub-TLV as defined in [RFC9350] MUST NOT be used during IP
Flex-Algorithm path calculation, the IPFAAM Sub-TLV MUST be used
instead.
7. Calculating of IP Flex-Algorithm Paths
The IP Flex-Algorithm is considered as yet another data-plane of the
Flex-Algorithm as described in [RFC9350].
Participation in the IP Flex-Algorithm is signalled as described in
Section 5 and is specific to the IP Flex-Algorithm data-plane.
Calculation of IP Flex-Algorithm paths follows what is described in
[RFC9350]. This computation uses the IP Flex-Algorithm data-plane
participation and is independent of the Flex-Algorithm calculation
done for any other Flex-Algorithm data-plane (e.g., SR, SRv6).
The IP Flex-Algorithm data-plane only considers participating nodes
during the Flex-Algorithm calculation. When computing paths for a
given Flex-Algorithm, all nodes that do not advertise participation
for the IP Flex-Algorithm, as described in Section 5, MUST be pruned
from the topology.
8. IP Flex-Algorithm Forwarding
The IP Algorithm Prefix Reachability advertisement as described in
Section 5 includes the MTID value that associates the prefix with a
specific topology. Algorithm Prefix Reachability advertisement also
includes an Algorithm value that explicitly associates the prefix
with a specific Flex-Algorithm. The paths to the prefix MUST be
calculated using the specified Flex-Algorithm in the associated
topology.
Forwarding entries for the IP Flex-Algorithm prefixes advertised in
IGPs MUST be installed in the forwarding plane of the receiving IP
Flex-Algorithm prefix capable routers when they participate in the
associated topology and algorithm. Forwarding entries for IP Flex-
Algorithm prefixes associated with Flex-Algorithms in which the node
is not participating MUST NOT be installed in the forwarding plane.
9. Deployment Considerations
IGP Flex-Algorithm can be used by many data-planes. The original
specification was done for SR and SRv6, this specification adds IP as
another data-plane that can use IGP Flex-Algorithm. Other data-
planes may be defined in the future. This section provides some
details about the coexistence of the various data-planes of an IGP
Flex-Algorithm.
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Flex-Algorithm definition (FAD), as described in [RFC9350], is data-
plane independent and is used by all Flex-Algorithm data-planes.
Participation in the Flex-Algorithm, as described in [RFC9350], is
data-plane specific.
Calculation of the flex-algo paths is data-plane specific and uses
data-plane specific participation advertisements.
Data-plane specific participation and calculation guarantee that the
forwarding of the traffic over the Flex-Algorithm data-plane specific
paths is consistent between all nodes that apply the IGP Flex-
Algorithm to the data-plane.
Multiple data-planes can use the same Flex-Algorithm value at the
same time and, and as such, share the FAD for it. For example, SR-
MPLS and IP can both use a common Flex-Algorithm. Traffic for SR-
MPLS will be forwarded based on Flex-algorithm specific SR SIDs.
Traffic for IP Flex-Algorithm will be forwarded based on Flex-
Algorithm specific prefix reachability advertisements. Note that for
a particular Flex-Algorithm, for a particular IP prefix, there will
only be path(s) calculated and installed for a single data-plane.
10. Protection
In many networks where IGP Flexible Algorithms are deployed, IGP
restoration will be fast and additional protection mechanisms will
not be required. IGP restoration may be enhanced by Equal Cost
Multipath (ECMP).
In other networks, operators can deploy additional protection
mechanisms. The following are examples:
* Loop Free Alternates (LFA) [RFC5286]
* Remote Loop Free Alternates (R-LFA) [RFC7490]
LFA and R-LFA computations MUST be restricted to the flex-algo
topology and the computed backup nexthops should be programmed for
the IP flex-algo prefixes.
11. IANA Considerations
This specification updates the OSPF Router Information (RI) TLVs
Registry as follows:
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+=======+==================+===========================+
| Value | TLV Name | Reference |
+=======+==================+===========================+
| TBD1 | IP Algorithm TLV | This Document Section 5.2 |
+-------+------------------+---------------------------+
Table 1
This document also updates the IS-IS "Sub-TLVs for TLV 242" registry
as follows:
+=======+======================+===========================+
| Value | TLV Name | Reference |
+=======+======================+===========================+
| 29 | IP Algorithm Sub-TLV | This Document Section 5.1 |
+-------+----------------------+---------------------------+
Table 2
This document also updates the "IS-IS TLV Codepoints Registry"
registry as follows:
+=======+==================+=====+=====+=====+=======+=============+
| Value | TLV Name | IIH | LSP | SNP | Purge | Reference |
+=======+==================+=====+=====+=====+=======+=============+
| 126 | IPv4 Algorithm | N | Y | N | N | This |
| | Prefix | | | | | document, |
| | Reachability TLV | | | | | Section 6.1 |
+-------+------------------+-----+-----+-----+-------+-------------+
| 127 | IPv6 Algorithm | N | Y | N | N | This |
| | Prefix | | | | | document, |
| | Reachability TLV | | | | | Section 6.2 |
+-------+------------------+-----+-----+-----+-------+-------------+
Table 3
Since the above TLVs share the sub-TLV space managed in the "IS-IS
Sub-TLVs for TLVs Advertising Prefix Reachability" registry, IANA is
requested to add "IPv4 Algorithm Prefix Reachability TLV (126)" and
"IPv6 Algorithm Prefix Reachability TLV (127)" to the list of TLVs in
the description of that registry.
In addition, columns headed '126' and '127' are added to that
registry, as follows:
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Type Description 126 127
---- ---------------------------------- --- ---
1 32-bit Administrative Tag Sub-TLV y y
2 64-bit Administrative Tag Sub-TLV y y
3 Prefix Segment Identifier n n
4 Prefix Attribute Flags y y
5 SRv6 End SID n n
6 Flex-Algorithm Prefix Metric n n
11 IPv4 Source Router ID y y
12 IPv6 Source Router ID y y
32 BIER Info n n
This document updates the "OSPFv2 Extended Prefix TLV Sub-TLVs"
registry as follows:
+=======+============================+================+
| Value | TLV Name | Reference |
+=======+============================+================+
| TBD2 | OSPFv2 IP Algorithm Prefix | This Document, |
| | Reachability Sub-TLV | Section 6.3 |
+-------+----------------------------+----------------+
| TBD4 | OSPFv2 IP Forwarding | This Document, |
| | Address Sub-TLV | Section 6.3.1 |
+-------+----------------------------+----------------+
Table 4
This document creates a new registry under "Open Shortest Path First
v2 (OSPFv2) Parameters" registry, called "IP Algorithm Prefix
Reachability Sub-TLV Flags". The new registry defines the bits in
the 8-bit Flags field in the OSPFv2 IP Algorithm Prefix Reachability
Sub-TLV (Section 6.3). New bits can be allocated via IETF Review or
IESG Approval [RFC8126]
+=======+==========+============================+
| Bit # | Name | Reference |
+=======+==========+============================+
| 0 | bit E | This Document, Section 6.3 |
+-------+----------+----------------------------+
| 1-7 | Reserved | This Document, Section 6.3 |
+-------+----------+----------------------------+
Table 5
This document updates the "OSPFv3 Extended-LSA Sub-TLVs" registry as
follows:
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+=======+==============================+================+
| Value | TLV Name | Reference |
+=======+==============================+================+
| TBD3 | OSPFv3 IP Algorithm Prefix | This Document, |
| | Reachability Sub-TLV | Section 6.4 |
+-------+------------------------------+----------------+
| TBD5 | OSPFv3 IP Flexible Algorithm | This Document, |
| | ASBR Metric Sub-TLV | Section 6.5 |
+-------+------------------------------+----------------+
Table 6
This document updates the "OSPFv2 Extended Inter-Area ASBR Sub-TLVs"
registry as follows:
+=======+============================+================+
| Value | TLV Name | Reference |
+=======+============================+================+
| 2 | OSPF IP Flexible Algorithm | This Document, |
| | ASBR Metric Sub-TLV | Section 6.5 |
+-------+----------------------------+----------------+
Table 7
12. Security Considerations
This document inherits security considerations from [RFC9350].
This document adds one new way to disrupt IGP networks that are using
Flex-Algorithm: an attacker can suppress reachability for a given
prefix whose reachability is advertised by a legitimate node for a
particular IP Flex-Algorithm X, by advertising the same prefix in
Flex-Algorithm Y from another, malicious node. (To see why this is,
consider, for example, the rule given in the second-last paragraph of
Section 6.1).
This attack can be addressed by the existing security extensions, as
described in [RFC5304] and [RFC5310] for IS-IS, in [RFC2328] and
[RFC7474]for OSPFv2, and in [RFC4552] and [RFC5340] for OSPFv3.
If a node that is authenticated is taken over by an attacker, such a
rogue node can perform the attack described above. Such an attack is
not preventable through authentication, and it is not different from
advertising any other incorrect information through IS-IS or OSPF.
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13. Acknowledgements
Thanks to Bruno Decraene for his contributions to this document.
Special thanks to Petr Bonbon Adamec of Cesnet for supporting
interoperability testing.
14. References
14.1. Normative References
[ISO10589] ISO, "Intermediate system to Intermediate system routing
information exchange protocol for use in conjunction with
the Protocol for providing the Connectionless-mode Network
Service (ISO 8473)", November 2002, <ISO/IEC 10589:2002>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[RFC4552] Gupta, M. and N. Melam, "Authentication/Confidentiality
for OSPFv3", RFC 4552, DOI 10.17487/RFC4552, June 2006,
<https://www.rfc-editor.org/info/rfc4552>.
[RFC4915] Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
RFC 4915, DOI 10.17487/RFC4915, June 2007,
<https://www.rfc-editor.org/info/rfc4915>.
[RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
Topology (MT) Routing in Intermediate System to
Intermediate Systems (IS-ISs)", RFC 5120,
DOI 10.17487/RFC5120, February 2008,
<https://www.rfc-editor.org/info/rfc5120>.
[RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic
Authentication", RFC 5304, DOI 10.17487/RFC5304, October
2008, <https://www.rfc-editor.org/info/rfc5304>.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, DOI 10.17487/RFC5305, October
2008, <https://www.rfc-editor.org/info/rfc5305>.
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[RFC5308] Hopps, C., "Routing IPv6 with IS-IS", RFC 5308,
DOI 10.17487/RFC5308, October 2008,
<https://www.rfc-editor.org/info/rfc5308>.
[RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
and M. Fanto, "IS-IS Generic Cryptographic
Authentication", RFC 5310, DOI 10.17487/RFC5310, February
2009, <https://www.rfc-editor.org/info/rfc5310>.
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
<https://www.rfc-editor.org/info/rfc5340>.
[RFC7474] Bhatia, M., Hartman, S., Zhang, D., and A. Lindem, Ed.,
"Security Extension for OSPFv2 When Using Manual Key
Management", RFC 7474, DOI 10.17487/RFC7474, April 2015,
<https://www.rfc-editor.org/info/rfc7474>.
[RFC7770] Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and
S. Shaffer, "Extensions to OSPF for Advertising Optional
Router Capabilities", RFC 7770, DOI 10.17487/RFC7770,
February 2016, <https://www.rfc-editor.org/info/rfc7770>.
[RFC7981] Ginsberg, L., Previdi, S., and M. Chen, "IS-IS Extensions
for Advertising Router Information", RFC 7981,
DOI 10.17487/RFC7981, October 2016,
<https://www.rfc-editor.org/info/rfc7981>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8362] Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and
F. Baker, "OSPFv3 Link State Advertisement (LSA)
Extensibility", RFC 8362, DOI 10.17487/RFC8362, April
2018, <https://www.rfc-editor.org/info/rfc8362>.
[RFC9350] Psenak, P., Ed., Hegde, S., Filsfils, C., Talaulikar, K.,
and A. Gulko, "IGP Flexible Algorithm", RFC 9350,
DOI 10.17487/RFC9350, February 2023,
<https://www.rfc-editor.org/info/rfc9350>.
[RFC9352] Psenak, P., Ed., Filsfils, C., Bashandy, A., Decraene, B.,
and Z. Hu, "IS-IS Extensions to Support Segment Routing
over the IPv6 Data Plane", RFC 9352, DOI 10.17487/RFC9352,
February 2023, <https://www.rfc-editor.org/info/rfc9352>.
14.2. Informative References
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[IANA-ALG] IANA, "Sub-TLVs for TLV 242 (IS-IS Router CAPABILITY
TLV)", August 1987, <https://www.iana.org/assignments/igp-
parameters/igp-parameters.xhtml#igp-algorithm-types>.
[RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
IP Fast Reroute: Loop-Free Alternates", RFC 5286,
DOI 10.17487/RFC5286, September 2008,
<https://www.rfc-editor.org/info/rfc5286>.
[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,
<https://www.rfc-editor.org/info/rfc7490>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
(SRv6) Network Programming", RFC 8986,
DOI 10.17487/RFC8986, February 2021,
<https://www.rfc-editor.org/info/rfc8986>.
[TS.23.501-3GPP]
3rd Generation Partnership Project (3GPP), "System
Architecture for 5G System; Stage 2, 3GPP TS 23.501
v16.4.0", March 2020.
Authors' Addresses
William Britto
Juniper Networks
Elnath-Exora Business Park Survey
Bangalore 560103
Karnataka
India
Email: bwilliam@juniper.net
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Shraddha Hegde
Juniper Networks
Elnath-Exora Business Park Survey
Bangalore 560103
Karnataka
India
Email: shraddha@juniper.net
Parag Kaneriya
Juniper Networks
Elnath-Exora Business Park Survey
Bangalore 560103
Karnataka
India
Email: pkaneria@juniper.net
Rejesh Shetty
Juniper Networks
Elnath-Exora Business Park Survey
Bangalore 560103
Karnataka
India
Email: mrajesh@juniper.net
Ron Bonica
Juniper Networks
2251 Corporate Park Drive
Herndon, Virginia 20171
United States of America
Email: rbonica@juniper.net
Peter Psenak
Cisco Systems
Apollo Business Center
Mlynske nivy 43
82109 Bratislava
Slovakia
Email: ppsenak@cisco.com
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