Internet DRAFT - draft-ietf-spring-sr-replication-segment
draft-ietf-spring-sr-replication-segment
Network Working Group D. Voyer, Ed.
Internet-Draft Bell Canada
Intended status: Standards Track C. Filsfils
Expires: 29 February 2024 R. Parekh
Cisco Systems, Inc.
H. Bidgoli
Nokia
Z. Zhang
Juniper Networks
28 August 2023
SR Replication segment for Multi-point Service Delivery
draft-ietf-spring-sr-replication-segment-19
Abstract
This document describes the Segment Routing Replication segment for
Multi-point service delivery. A Replication segment allows a packet
to be replicated from a Replication node to Downstream nodes.
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.
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 29 February 2024.
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Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (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
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Replication Segment . . . . . . . . . . . . . . . . . . . . . 4
2.1. SR-MPLS data plane . . . . . . . . . . . . . . . . . . . 6
2.2. SRv6 data plane . . . . . . . . . . . . . . . . . . . . . 7
2.2.1. End.Replicate: Replicate and/or Decapsulate . . . . . 9
2.2.2. OAM Operations . . . . . . . . . . . . . . . . . . . 13
2.2.3. ICMPv6 Error Messages . . . . . . . . . . . . . . . . 13
3. Implementation Status . . . . . . . . . . . . . . . . . . . . 13
3.1. Cisco implementation . . . . . . . . . . . . . . . . . . 14
3.2. Nokia implementation . . . . . . . . . . . . . . . . . . 14
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
5. Security Considerations . . . . . . . . . . . . . . . . . . . 15
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 17
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
8.1. Normative References . . . . . . . . . . . . . . . . . . 18
8.2. Informative References . . . . . . . . . . . . . . . . . 19
Appendix A. Illustration of a Replication Segment . . . . . . . 20
A.1. SR-MPLS . . . . . . . . . . . . . . . . . . . . . . . . . 21
A.2. SRv6 . . . . . . . . . . . . . . . . . . . . . . . . . . 22
A.2.1. Pinging Replication SID . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
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1. Introduction
Replication segment is a new type of segment for Segment Routing (SR)
[RFC8402], which allows a node (henceforth called a Replication node)
to replicate packets to a set of other nodes (called Downstream
nodes) in a Segment Routing Domain. A Replication segment can
replicate packets to directly connected nodes or to downstream nodes
(without need for state on the transit routers). This document
focuses on specifying behavior of a Replication segment for both
Segment Routing with Multiprotocol Label Switching (SR-MPLS)
[RFC8660] and Segment Routing with IPv6 (SRv6) [RFC8986]. The
examples in the Appendix illustrate the behavior of a Replication
Segment in SR domain. The use of two or more Replication segments
stitched together to form a tree using a control plane is left to be
specified in other documents. The management of IP multicast groups,
building IP multicast trees, and performing multicast congestion
control are out of scope of this document.
1.1. Terminology
This section defines terms introduced and used frequently in this
document. Refer to Terminology sections of [RFC8402], [RFC8754] and
[RFC8986] for other terms used in Segment Routing.
* Replication segment: A segment in SR domain that replicates
packets. See Section 2 for details.
* Replication node: A node in SR domain which replicates packets
based on Replication segment.
* Downstream nodes: A Replication segment replicates packets to a
set of nodes. These nodes are Downstream nodes.
* Replication state: State held for a Replication segment at a
Replication node. It is conceptually a list of replication
branches to Downstream nodes. The list can be empty.
* Replication SID: Data plane identifier of a Replication segment.
This is a SR-MPLS label or SRv6 Segment Identifier (SID).
* SRH: IPv6 Segment Routing Header [RFC8754].
* Point-to-Multipoint Service: A service that has one ingress node
and one or more egress nodes. A packet is delivered to all the
egress nodes
* Root node: An ingress node of a P2MP service,
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* Leaf node: An egress node of a P2MP service.
* Bud node: A node that is both a Replication node and a Leaf node.
1.2. Use Cases
In the simplest use case, a single Replication segment includes the
ingress node of a multi-point service and the egress nodes of the
service as all the Downstream nodes. This achieves Ingress
Replication [RFC7988] that has been widely used for Multicast VPN
(MVPN) [RFC6513] and Ethernet VPN (EVPN)[RFC7432] bridging of
Broadcast, Unknown Unicast, and Multicast (BUM) traffic. This
Replication segment can be either provisioned locally on ingress and
egress nodes, or using dynamic auto-discovery procedures for MVPN and
EVPN. Note SRv6 [RFC8986] has End.DT2M replication behavior for EVPN
BUM traffic.
Replication segments can also be used to form trees by stitching
Replication segments on a Root node, intermediate Replication nodes
and Leaf nodes for efficient delivery of MVPN and EVPN BUM traffic.
2. Replication Segment
In a Segment Routing Domain, a Replication segment is a logical
construct which connects a Replication node to a set of Downstream
nodes. A Replication segment is a local segment instantiated at a
Replication node. It can be either provisioned locally on a node or
programmed by a control plane.
Replication segments can be stitched together to form a tree by
either local provisioning on nodes or using a control plane. The
procedures for doing this are out of scope of this document. One
such control plane using a PCE with SR P2MP policy is specified in
[I-D.ietf-pim-sr-p2mp-policy]. However, if local provisioning is
used to stitch Replication segments, then a chain of Replication
segments SHOULD NOT form a loop. If a control plane is used to
stitch Replication segments, the control plane specification MUST
prevent loops, or to detect and mitigate loops in steady state.
A Replication segment is identified by the tuple <Replication-ID,
Node-ID>, where:
* Replication-ID: An identifier for a Replication segment that is
unique in context of the Replication node.
* Node-ID: The address of the Replication node that the Replication
segment is for. Note that the Root of a multi-point service is
also a Replication node.
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Replication-ID is a variable length field. In simplest case, it can
be a 32-bit number, but it can be extended or modified as required
based on specific use of a Replication segment. This is out of scope
for this document. The length of Replication-ID is specified in the
signaling mechanism used for Replication segment. Examples of such
signaling and extensions are described in
[I-D.ietf-pim-sr-p2mp-policy]. When the PCE signals a Replication
segment to its node, the <Replication-ID, Node-ID> tuple identifies
the segment.
A Replication segment includes the following elements:
* Replication SID: The Segment Identifier of a Replication segment.
This is a SR-MPLS label or a SRv6 SID [RFC8402].
* Downstream nodes: Set of nodes in Segment Routing domain to which
a packet is replicated by the Replication segment.
* Replication state: See below.
The Downstream nodes and Replication state of a Replication segment
can change over time, depending on the network state and Leaf nodes
of a multi-point service that the segment is part of.
Replication SID identifies the Replication segment in the forwarding
plane. At a Replication node, the Replication SID operates on
Replication state of the Replication segment.
Replication state is a list of replication branches to the Downstream
nodes. In this document, each branch is abstracted to a <Downstream
node, Downstream Replication SID> tuple. <Downstream node> represents
the reachability from the Replication node to the Downstream node.
In its simplest form, this MAY be specified as an interface or next-
hop if downstream node is adjacent to the Replication node. The
reachability may be specified in terms of Flexible Algorithm path
(including the default algorithm) [RFC9350], or specified by an SR
explicit path represented either by a SID-list (of one or more SIDs)
or by a Segment Routing Policy [RFC9256]. Downstream Replication SID
is the Replication SID of the Replication segment at the Downstream
node.
A packet is steered into a Replication segment at a Replication node
in two ways:
* When the Active Segment [RFC8402] is a locally instantiated
Replication SID
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* By the Root of a multi-point service based on local configuration
outside the scope of this document.
In either case, the packet is replicated to each Downstream node in
the associated Replication state.
If a Downstream node is an egress (Leaf) of the multi-point service,
no further replication is needed. The Leaf node's Replication
segment has an indicator for Leaf role and it does not have any
Replication state i.e. the list of Replication branches is empty.
The Replication SID at a Leaf node MAY be used to identify the multi-
point service. Notice that the segment on the Leaf node is still
referred to as a Replication segment for the purpose of
generalization.
A node can be a Bud node, i.e. it is a Replication node and a Leaf
node of a multi-point service [I-D.ietf-pim-sr-p2mp-policy].
Replication segment of a Bud node has a list of Replication Branches
as well as Leaf role indicator.
In principle it is possible for different Replication segments to
replicate packets to the same Replication segment on a Downstream
node. However, such usage is intentionally left out of scope of this
document.
2.1. SR-MPLS data plane
When the Active Segment is a Replication SID, the processing results
in a POP [RFC8402] operation and lookup of the associated Replication
state. For each replication in the Replication state, the operation
is a PUSH [RFC8402] of the downstream Replication SID and an optional
segment list on to the packet to steer the packet to the Downstream
node.
The operation performed on incoming Replication SID is NEXT [RFC8402]
at Leaf/Bud nodes where delivery of payload off tree is per local
configuration. For some usages, this may involve looking at the next
SID for example to get the necessary context.
When the Root of a multi-point service steers a packet to a
Replication segment, it results in a replication to each Downstream
node in the associated replication state. The operation is a PUSH of
the replication SID and an optional segment list on to the packet
which is forwarded to the downstream node.
The following applies to Replication SID in MPLS encapsulation:
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* SIDs MAY be inserted before the downstream SR-MPLS Replication SID
in order to guide a packet from a non-adjacent SR node to a
Replication node.
* A Replication node MAY replicate a packet to a non-adjacent
Downstream node using SIDs it inserts in the copy preceding the
downstream Replication SID. The Downstream node may be a Leaf
node of the Replication segment, or another Replication node, or
both in case of Bud node.
* A Replication node MAY use an Anycast SID or Border Gateway
Protocol (BGP) PeerSet SID in segment list to send a replicated
packet to one downstream Replication node in an Anycast set if and
only if all nodes in the set have an identical Replication SID and
reach the same set of receivers.
* For some use cases, there MAY be SIDs after the Replication SID in
the segment list of a packet. These SIDs are used only by the
Leaf/Bud nodes to forward a packet off the tree independent of the
Replication SID. Coordination regarding the absence or presence
and value of context information for Leaf/Bud nodes is outside the
scope of this document.
2.2. SRv6 data plane
For SRv6 [RFC8986], this document specifies “Endpoint with
replication” behavior (End.Replicate for short) to replicate a packet
and forward the replicas according to a Replication state.
When processing a packet destined to a local Replication SID, the
packet is replicated according to the associated Replication state to
Downstream nodes and/or locally delivered off tree when this is a
Leaf/Bud node.For replication, the outer header is re-used, and the
Downstream Replication SID, from Replication state, is written into
the outer IPv6 header destination address. If required, an optional
segment list may be used on some branches using H.Encaps.Red
[RFC8986] (while some other branches may not need that). Note that
this H.Encaps.Red is independent of the replication segment – it is
just used to steer the replicated packet on a traffic engineered path
to a Downstream node. The penultimate segment in encapsulating IPv6
header will execute Ultimate Segment Decapsulation (USD) flavor
[RFC8986] of End/End.X behavior and forward the inner (replicated)
packet to the Downstream node. If H.Encaps.Red is used to steer a
replicated packet to a Downstream node, the operator must ensure the
MTU on path to the Downstream node is sufficient to account for
additional SRv6 encapsulation. This also applies when the
Replication segment is for the Root node, whose upstream node has
placed the Replication-SID in the header.
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A local application on Root, for e.g. MVPN [RFC6513] or EVPN
[RFC7432], may also apply H.Encaps.Red and then steer the resulting
traffic into the Replication segment. Again, note that the
H.Encaps.Red is independent of the Replication segment – it is the
action of the application (e.g. MVPN/EVPN service). If the service
is on a Root node, the two H.Encaps mentioned, one for the service
and other in the previous paragraph for replication to Downstream
node SHOULD be combined for optimization (to avoid extra IPv6
encapsulation).
When processing a packet destined to a local Replication SID, IPv6
Hop Limit MUST be decremented and MUST be non-zero to replicate the
packet. A Root node that encapsulates a payload can set the IPv6 Hop
Limit based on a local policy. This local policy SHOULD set the IPv6
Hop Limit so that a replicated packet can reach the furthest Leaf
node. A Root node can also have a local policy to set the IPv6 Hop
Limit from the payload. In this case, IPv6 Hop Limit may not be
sufficient to get the replicated packet to all the Leaf nodes; non-
replication nodes i.e. nodes which forward replicated packets based
on IPv6 locator unicast prefix can decrement IPv6 Hop Limit to zero
and originate ICMPv6 Error packets to the Root node. This can result
in a storm of ICMPv6 packets (see Section 2.2.3) to the Root node.
To avoid this, a Replication Segment has an optional IPv6 Hop Limit
threshold. If this threshold is set, a Replication node MUST discard
an incoming packet with local Replication SID if the IPv6 Hop Limit
in the packet is less than the threshold and log this in a rate
limited manner. The IPv6 Hop Limit Threshold SHOULD be set so that
incoming packet can be replicated to furthest Leaf node.
For Leaf/Bud nodes local delivery off the tree is per Replication SID
or next SID (if present in SRH). For some usages, this may involve
getting the necessary context either from the next SID (e.g., MVPN
with shared tree) or from the replication SID itself (e.g., MVPN with
non-shared tree). In both cases, the context association is achieved
with signaling and is out of scope of this document.
The following applies to Replication SID in SRv6 encapsulation:
* There MAY be SIDs preceding the SRv6 Replication SID in order to
guide a packet from a non-adjacent SR node to a Replication node
via an explicit path.
* A Replication node MAY steer a replicated packet on an explicit
path to a non-adjacent Downstream node using SIDs it inserts in
the copy preceding the downstream Replication SID. The Downstream
node may be a Leaf node of the Replication segment, or another
Replication node, or both in case of Bud node.
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* For SRv6, as described in above paragraphs, the insertion of SIDs
prior to Replication SID entails a new IPv6 encapsulation with
SRH, but this can be optimized on Root node or for compressed SRv6
SIDs.
* The locator of Replication SID is sufficient to guide a packet on
shortest path, for default or Flexible algorithm, between non-
adjacent nodes.
* A Replication node MAY use an Anycast SID or BGP PeerSet SID in
segment list to send a replicated packet to one downstream
Replication node in an Anycast set if and only if all nodes in the
set have an identical Replication SID and reach the same set of
receivers.
* There MAY be SIDs after the Replication SID in the SRH of a
packet. These SIDs are used to provide additional context for
processing a packet locally at the node where the Replication SID
is the Active Segment. Coordination regarding the absence or
presence and value of context information for Leaf/Bud nodes is
outside the scope of this document.
2.2.1. End.Replicate: Replicate and/or Decapsulate
The "Endpoint with replication and/or decapsulate behavior
(End.Replicate for short) is variant of End behavior. The pseudo-
code in this section follows the convention introduced in RFC 8986
[RFC8986].
A Replication state conceptually contains the following elements:
Replication state:
{
Node-Role: {Head, Transit, Leaf, Bud};
IPv6 Hop Limit Threshold; # default is zero
# On Leaf, replication list is zero length
Replication-List:
{
Downstream node: <Node-Identifier>;
Downstream Replication SID: R-SID;
# Segment-List may be empty
Segment-List: [SID-1, .... SID-N];
}
}
Below is the Replicate function on a packet for Replication state
(RS).
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S01. Replicate(RS, packet)
S02. {
S03. For each Replication R in RS.Replication-List {
S04. Make a copy of the packet
S05. Set IPv6 DA = RS.R-SID
S06. If RS.Segment-List is not empty {
S07. # Head node may optimize below encapsulation and
S08. # the encapsulation of packet in a single encapsulation
S09. Execute H.Encaps or H.Encaps.Red with RS.Segment-List
on packet copy #RFC 8986 Section 5.1, 5.2
S10. }
S11. Submit the packet to the egress IPv6 FIB lookup and
transmission to the new destination
S12. }
S13. }
Notes:
* The IPv6 destination address in the copy of a packet is set from
local state and not from SRH
When N receives a packet whose IPv6 DA is S and S is a local
End.Replicate SID, N does:
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S01. Lookup FUNCT portion of S to get Replication state RS
S02. If (IPv6 Hop Limit <= 1) {
S03. Discard the packet
S04. # ICMPv6 Time Exceeded is not permitted (ICMPv6 section below)
S05. }
S06. If RS is not found {
S07. Discard the packet
S08. }
S09. If (IPv6 Hop Limit < RS.IPv6 Hop Limit Threshold) {
S10. Discard the packet
S11. # Rate-limited logging
S12. }
S13. Decrement IPv6 Hop Limit by 1
S14. If (IPv6 NH == SRH and SRH TLVs present) {
S15. Process SRH TLVs if allowed by local configuration
S16. }
S17. Call Replicate(RS, packet)
S18. If (RS.Node-Role == Leaf OR RS.Node-Role == Bud) {
S19. If (IPv6 NH == SRH and Segments Left > 0) {
S20. Derive packet processing context(PPC) from Segment List
S21. If (Segments Left != 0) {
S22. Discard the packet
S23. # ICMPv6 Parameter Problem with Code 0
S24. # (Erroneous header field encountered)
S25. # is not permitted (ICMPv6 section below)
S26. }
S27. } Else {
S28. Derive packet processing context(PPC)
from FUNCT of Replication SID
S29. }
S30. Process the next header
S31. }
The processing of Upper-Layer header of a packet matching
End.Replicate SID at Leaf/Bud node is as follows:
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S01. If (Upper-Layer header type == 4(IPv4) OR
Upper-Layer header type == 41(IPv6) ) {
S02. Remove the outer IPv6 header with all its extension headers
S03. Process the packet in context of PPC
S04. } Else If (Upper-Layer header type == 143(Ethernet) ) {
S05. Remove the outer IPv6 header with all its extension headers
S06. Process the Ethernet Frame in context of PPC
S07. } Else If (Upper-Layer header type is allowed
by local configuration) {
S08. Proceed to process the Upper-Layer header
S09. } Else {
S10. Discard the packet
S11. # ICMPv6 Parameter Problem with Code 4
S12. # (SR Upper-layer Header Error)
S13. # is not permitted (ICMPv6 section below)
S14. }
Notes:
* The behavior above MAY result in a packet with partially processed
segment list in SRH under some circumstances. Fox example a head
node may encode a context SID in an SRH. As per pseudo-code
above, a Replication node that receives a packet with local
Replication SID will not process the SRH segment list and just
forward a copy with unmodified SRH to Downstream nodes.
* The packet processing context usually is a FIB table T
Processing the Replication SID may modify, if configured to process
TLVs, the "variable-length data" of TLV types that change en route.
Therefore, TLVs that change en route are mutable. The remainder of
the SRH (Segments Left, Flags, Tag, Segment List, and TLVs that do
not change en route) are immutable while processing this SID.
2.2.1.1. Hashed Message Authentication Code (HMAC) SRH TLV
If a Root node encodes a context SID in SRH with an optional HMAC SRH
TLV [RFC8754], it MUST set the 'D' bit as defined in Section 2.1.2
because the Replication SID is not part of the segment list in SRH.
HMAC generation and verification is as specified in RFC 8754.
Verification of HMAC TLV is determined by local configuration. If
verification fails, an implementation of Replication SID MUST NOT
originate an ICMPv6 error message (parameter problem, code 0). The
failure SHOULD be logged (rate limited) and the packet SHOULD be
discarded.
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2.2.2. OAM Operations
RFC 9259 [RFC9259] specifies procedures for OAM operations like ping
and traceroute on SRv6 SIDs.
It is possible to ping a Replication SID of a Leaf/Bud node, assuming
the source node knows the Replication SID a priori, directly by
putting it in the IPv6 destination address without a SRH or in a SRH
as the last segment. While it is not possible to ping a Replication
SID of a transit node because transit nodes do not process upper
layer headers, it is still possible to ping a Replication SID of
Leaf/Bud node of a tree via the Replication SID of intermediate
transit nodes. The source of ping MUST compute the ICMPv6 Echo
Request checksum using the Replication SID of Leaf/Bud as destination
address. The source can then send the Echo Request packet to a
transit node's Replication SID. The transit nodes replicate the
packet by replacing the IPv6 destination address till the packet
reaches the Leaf/Bud node which responds with an ICMPv6 Echo Reply.
Note that a transit Replication node may replicate Echo Request
packets to other Leaf/Bud nodes. These nodes will drop the Echo
Request due to incorrect checksum. Procedures to prevent the mis-
delivery of Echo Request may be addressed in a future document.
Appendix A.2.1 illustrates examples of ping to a Replication SID.
Traceroute to a Leaf/Bud node Replication SID is not possible due to
restriction prohibiting origination of ICMPv6 Time Exceeded error
message for a Replication SID as described in the section below.
2.2.3. ICMPv6 Error Messages
ICMPv6 RFC [RFC4443] Section 2.4 states an ICMPv6 error message MUST
NOT be originated as a result of receiving a packet destined to an
IPv6 multicast address. This is to prevent a storm of ICMPv6 error
messages resulting from replicated IPv6 packets from overwhelming a
source node. There are two exceptions (1) the Packet Too Big message
for Path MTU discovery, and (2) Parameter Problem Message, Code 2
reporting an unrecognized IPv6 option. An implementation of
Replication segment for SRv6 MUST enforce these same restrictions and
exceptions.
3. Implementation Status
Note to the RFC Editor: Please remove this section and reference to
RFC 7942 before publication.
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in RFC 7942
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[RFC7942]. The description of implementations in this section is
intended to assist the IETF in its decision processes in progressing
drafts to RFCs. Please note that the listing of any individual
implementation here does not imply endorsement by the IETF.
Furthermore, no effort has been spent to verify the information
presented here that was supplied by IETF contributors. This is not
intended as, and must not be construed to be, a catalog of available
implementations or their features. Readers are advised to note that
other implementations may exist. According to RFC 7942 [RFC7942],
"this will allow reviewers and working groups to assign due
consideration to documents that have the benefit of running code,
which may serve as evidence of valuable experimentation and feedback
that have made the implemented protocols more mature. It is up to
the individual working groups to use this information as they see
fit".
There are two known implementations of this draft by Cisco and Nokia.
Interoperability reports for the implementations are not applicable
since this draft does not specify inter-operable elements of
Replication segments.
3.1. Cisco implementation
Cisco Implementation uses Replication segments defined in this draft
as a basis for PCE to compute and establish P2MP trees in SR domain
to provide multi-point services. The implementation, based on latest
version of this draft, is in production and supports all MUST and
SHOULD clauses for SR-MPLS Replication segments. The documentation
is available at Cisco documentation
(https://www.cisco.com/c/en/us/td/docs/routers/asr9000/software/
asr9k-r7-3/segment-routing/configuration/guide/b-segment-routing-cg-
asr9000-73x/b-segment-routing-cg-asr9000-71x_chapter_01001.html) and
the point of contact is Rishabh Parekh (riparekh@cisco.com).
3.2. Nokia implementation
Nokia has implemented replication SID as defined in this draft to
establish P2MP tree in segment routing domain. The implementation
supports SR-MPLS encapsulation and has all the MUST and SHOULD clause
in this draft. The implementation is at general availability
maturity and is compliant with the latest version of the draft. The
documentation for implementation can be found at Nokia help
(https://infocenter.nokia.com/public/7750SR207R1A/
index.jsp?topic=%2Fcom.sr.multicast%2Fhtml%2Ftreesid.html) and the
point of contact is hooman.bidgoli@nokia.com.
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4. IANA Considerations
IANA has assigned the following codepoint for End.Replicate behavior
in the "SRv6 Endpoint Behaviors" registry in the "Segment Routing"
registry group.
+=======+========+===================+===========+
| Value | Hex | Endpoint behavior | Reference |
+=======+========+===================+===========+
| 75 | 0x004B | End.Replicate | [This.ID] |
+-------+--------+-------------------+-----------+
Table 1: IETF - SRv6 Endpoint Behaviors
5. Security Considerations
The SID behaviors defined in this document are deployed within an SR
domain [RFC8402]. An SR domain needs protection from outside
attackers as described in [RFC8754] and following is a brief reminder
of the same:
* For SR-MPLS deployments:
- By disabling MPLS on external interfaces of each edge node or
any other technique to filter labeled traffic ingress on these
interfaces.
* For SRv6 deployments:
- Allocate all the SIDs from an IPv6 prefix block S/s and
configure each external interface of each edge node of the
domain with an inbound infrastructure access list (IACL) that
drops any incoming packet with a destination address in S/s.
- Additionally, an iACL may be applied to all nodes (k)
provisioning SIDs as defined in this specification:
o Assign all interface addresses from within IPv6 prefix A/a.
At node k, all SIDs local to k are assigned from prefix Sk/
sk. Configure each internal interface of each SR node k in
the SR domain with an inbound IACL that drops any incoming
packet with a destination address in Sk/sk if the source
address is not in A/a.
- Denying traffic with spoofed source addresses by implementing
recommendations in BCP 84 [RFC3704].
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- Additionally the block S/s from which SIDs are allocated may be
a non-globally-routable address such as ULA or the prefix
defined in [I-D.ietf-6man-sids].
Failure to protect the SR MPLS domain by correctly provisioning MPLS
support per interface permits attackers from outside the domain to
send packets that use the replication services provisioned within the
domain.
Failure to protect the SRv6 domain with IACLs on external interfaces,
combined with failure to implement BCP 38 [RFC2827]or apply IACLs on
nodes provisioning SIDs, permits attackers from outside the SR domain
to send packets that use the replication services provisioned within
the domain.
Given the definition of the Replication segment in this document, an
attacker subverting ingress filter above cannot take advantage of a
stack of replication segments to perform amplification attacks nor
link exhaustion attacks. Replication segment trees always terminate
at a Leaf or Bud node resulting in a decapsulation. This however
does allow an attacker to inject traffic to the receivers within a
P2MP service.
This document introduces a SR segment endpoint behavior that
replicates and decapsulates an inner payload for both the MPLS and
IPv6 data planes. Similar to any MPLS end of stack label, or SRv6
END.D* behavior, if the protections described above are not
implemented an attacker can perform an attack via the decapsulating
segment (including the one described in this document).
Incorrect provisioning of Replication segments can result in a chain
of Replication segments forming a loop. This can happen if
Replication segments are provisioned on SR nodes without using a
control plane. In this case, replicated packets can create a storm
till MPLS TTL (for SR-MPLS) or IPv6 Hop Limit (for SRv6) decrements
to zero. A control plane, for example PCE, can be used to prevent
loops. The control plane protocols (like PCEP, BGP, etc.) used to
instantiate Replication segments can leverage their own security
mechanisms such as encryption, authentication filtering etc.
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For SRv6, Section 2.2.3 describes an exception for Parameter Problem
Message, code 2 ICMPv6 Error messages. If an attacker sends a packet
destined to Replication SID with source address of a node and with an
extension header using unknown option type marked as mandatory, then
a large number of ICMPv6 Parameter Problem messages can cause a
denial-of-service attack on the source node. Although this
specification does not specify any extension headers, any future
extension of this document doing so is susceptible to this security
concern.
If an attacker can forge an IPv6 packet with source address of a
node, Replication SID as destination address and an IPv6 Hop Limit
such that nodes which forward replicated packets on IPv6 locator
unicast prefix, decrement the Hop Limit to zero, then these nodes can
cause a storm of ICMPv6 Error packets to overwhelm the source node
under attack. The IPv6 Hop Limit Threshold check described in
Section 2.2 can help mitigate such attacks.
6. Acknowledgements
The authors would like to acknowledge Siva Sivabalan, Mike Koldychev,
Vishnu Pavan Beeram, Alexander Vainshtein, Bruno Decraene, Thierry
Couture, Joel Halpern, Ketan Talaulikar, Darren Dukes and Jingrong
Xie for their valuable inputs.
7. Contributors
Clayton Hassen Bell Canada Vancouver Canada
Email: clayton.hassen@bell.ca
Kurtis Gillis Bell Canada Halifax Canada
Email: kurtis.gillis@bell.ca
Arvind Venkateswaran Cisco Systems, Inc. San Jose US
Email: arvvenka@cisco.com
Zafar Ali Cisco Systems, Inc. US
Email: zali@cisco.com
Swadesh Agrawal Cisco Systems, Inc. San Jose US
Email: swaagraw@cisco.com
Jayant Kotalwar Nokia Mountain View US
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Email: jayant.kotalwar@nokia.com
Tanmoy Kundu Nokia Mountain View US
Email: tanmoy.kundu@nokia.com
Andrew Stone Nokia Ottawa Canada
Email: andrew.stone@nokia.com
Tarek Saad Cisco Systems Inc. Canada
Email:tsaad@cisco.com
Kamran Raza Cisco Systems, Inc. Canada
Email:skraza@cisco.com
Jingrong Xie Huawei Technologies Beijing China
Email:xiejingrong@huawei.com
8. References
8.1. Normative References
[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>.
[RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", STD 89,
RFC 4443, DOI 10.17487/RFC4443, March 2006,
<https://www.rfc-editor.org/info/rfc4443>.
[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>.
[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>.
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[RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
(SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
<https://www.rfc-editor.org/info/rfc8754>.
[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>.
[RFC9259] Ali, Z., Filsfils, C., Matsushima, S., Voyer, D., and M.
Chen, "Operations, Administration, and Maintenance (OAM)
in Segment Routing over IPv6 (SRv6)", RFC 9259,
DOI 10.17487/RFC9259, June 2022,
<https://www.rfc-editor.org/info/rfc9259>.
8.2. Informative References
[I-D.filsfils-spring-srv6-net-pgm-illustration]
Filsfils, C., Camarillo, P., Li, Z., Matsushima, S.,
Decraene, B., Steinberg, D., Lebrun, D., Raszuk, R., and
J. Leddy, "Illustrations for SRv6 Network Programming",
Work in Progress, Internet-Draft, draft-filsfils-spring-
srv6-net-pgm-illustration-04, 30 March 2021,
<https://datatracker.ietf.org/doc/html/draft-filsfils-
spring-srv6-net-pgm-illustration-04>.
[I-D.ietf-6man-sids]
Krishnan, S., "Segment Identifiers in SRv6", Work in
Progress, Internet-Draft, draft-ietf-6man-sids-03, 11
April 2023, <https://datatracker.ietf.org/doc/html/draft-
ietf-6man-sids-03>.
[I-D.ietf-pim-sr-p2mp-policy]
Voyer, D., Filsfils, C., Parekh, R., Bidgoli, H., and Z.
J. Zhang, "Segment Routing Point-to-Multipoint Policy",
Work in Progress, Internet-Draft, draft-ietf-pim-sr-p2mp-
policy-06, 13 April 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-pim-sr-
p2mp-policy-06>.
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
May 2000, <https://www.rfc-editor.org/info/rfc2827>.
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[RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March
2004, <https://www.rfc-editor.org/info/rfc3704>.
[RFC6513] Rosen, E., Ed. and R. Aggarwal, Ed., "Multicast in MPLS/
BGP IP VPNs", RFC 6513, DOI 10.17487/RFC6513, February
2012, <https://www.rfc-editor.org/info/rfc6513>.
[RFC7432] Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
2015, <https://www.rfc-editor.org/info/rfc7432>.
[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/info/rfc7942>.
[RFC7988] Rosen, E., Ed., Subramanian, K., and Z. Zhang, "Ingress
Replication Tunnels in Multicast VPN", RFC 7988,
DOI 10.17487/RFC7988, October 2016,
<https://www.rfc-editor.org/info/rfc7988>.
[RFC8660] Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing with the MPLS Data Plane", RFC 8660,
DOI 10.17487/RFC8660, December 2019,
<https://www.rfc-editor.org/info/rfc8660>.
[RFC9256] Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov,
A., and P. Mattes, "Segment Routing Policy Architecture",
RFC 9256, DOI 10.17487/RFC9256, July 2022,
<https://www.rfc-editor.org/info/rfc9256>.
[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>.
Appendix A. Illustration of a Replication Segment
This section illustrates an example of a single Replication segment.
Examples showing Replication segment stitched together to form P2MP
tree (based on SR P2MP policy) are in [I-D.ietf-pim-sr-p2mp-policy].
Consider the following topology:
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R3------R6
/ \
R1----R2----R5-----R7
\ /
+--R4---+
Figure 1: Topology for illustration of Replication Segment
A.1. SR-MPLS
In this example, the Node-SID of a node Rn is N-SIDn and Adjacency-
SID from node Rm to node Rn is A-SIDmn. Interface between Rm and Rn
is Lmn. The state representation uses "R-SID->Lmn" to represent a
packet replication with outgoing replication SID R-SID sent on
interface Lmn.
Assume a Replication segment identified with R-ID at Replication node
R1 and downstream nodes R2, R6 and R7. The Replication SID at node n
is R-SIDn. A packet replicated from R1 to R7 has to traverse R4.
The Replication segment state at nodes R1, R2, R6 and R7 is shown
below. Note nodes R3, R4 and R5 do not have state for the
Replication segment.
Replication segment at R1:
Replication segment <R-ID,R1>:
Replication SID: R-SID1
Replication state:
R2: <R-SID2->L12>
R6: <N-SID6, R-SID6>
R7: <N-SID4, A-SID47, R-SID7>
Replication to R2 steers the packet directly to R2 on interface L12.
Replication to R6, using N-SID6, steers the packet via shortest path
to that node. Replication to R7 is steered via R4, using N-SID4 and
then adjacency SID A-SID47 to R7.
Replication segment at R2:
Replication segment <R-ID,R2>:
Replication SID: R-SID2
Replication state:
R2: <Leaf>
Replication segment at R6:
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Replication segment <R-ID,R6>:
Replication SID: R-SID6
Replication state:
R6: <Leaf>
Replication segment at R7:
Replication segment <R-ID,R7>:
Replication SID: R-SID7
Replication state:
R7: <Leaf>
When a packet is steered into the Replication segment at R1:
* Since R1 is directly connected to R2, R1 performs PUSH operation
with just <R-SID2> label for the replicated copy and sends it to
R2 on interface L12. R2, as Leaf, performs NEXT operation, pops
R-SID2 label and delivers the payload.
* R1 performs PUSH operation with <N-SID6, R-SID6> label stack for
the replicated copy to R6 and sends it to R2, the nexthop on
shortest path to R6. R2 performs CONTINUE operation on N-SID6 and
forwards it to R3. R3 is the penultimate hop for N-SID6; it
performs penultimate hop popping, which corresponds to the NEXT
operation and the packet is then sent to R6 with <R-SID6> in the
label stack. R6, as Leaf, performs NEXT operation, pops R-SID6
label and delivers the payload.
* R1 performs PUSH operation with <N-SID4, A-SID47, R-SID7> label
stack for the replicated copy to R7 and sends it to R2, the
nexthop on shortest path to R4. R2 is the penultimate hop for
N-SID4; it performs penultimate hop popping, which corresponds to
the NEXT operation and the packet is then sent to R4 with
<A-SID47, R-SID1> in the label stack. R4 performs NEXT operation,
pops A-SID47, and delivers packet to R7 with <R-SID7> in the label
stack. R7, as Leaf, performs NEXT operation, pops R-SID7 label
and delivers the payload.
A.2. SRv6
For SRv6 , we use SID allocation scheme, reproduced below, from
Illustrations for SRv6 Network Programming
[I-D.filsfils-spring-srv6-net-pgm-illustration]
* 2001:db8::/32 is an IPv6 block allocated by a Regional Internet
Registry (RIR) to the operator
* 2001:db8:0::/48 is dedicated to the internal address space
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* 2001:db8:cccc::/48 is dedicated to the internal SRv6 SID space
* We assume a location expressed in 64 bits and a function expressed
in 16 bits
* Node k has a classic IPv6 loopback address 2001:db8::k/128 which
is advertised in the Interior Gateway Protocol (IGP)
* Node k has 2001:db8:cccc:k::/64 for its local SID space. Its SIDs
will be explicitly assigned from that block
* Node k advertises 2001:db8:cccc:k::/64 in its IGP
* Function :1:: (function 1, for short) represents the End function
with Penultimate Segment Pop of SRH (PSP) [RFC8986] and USD
support
* Function :Cn:: (function Cn, for short) represents the End.X
function from to Node n with PSP and USD support
Each node k has:
* An explicit SID instantiation 2001:db8:cccc:k:1::/128 bound to an
End function with additional support for PSP and USD
* An explicit SID instantiation 2001:db8:cccc:k:Cj::/128 bound to an
End.X function to neighbor J with additional support for PSP and
USD
* An explicit SID instantiation 2001:db8:cccc:k:Fk::/128 bound to an
End.Replicate function
Assume a Replication segment identified with R-ID at Replication node
R1 and downstream nodes R2, R6 and R7. The Replication SID at node
k, bound to an End.Replicate function, is 2001:db8:cccc:k:Fk::/128.
A packet replicated from R1 to R7 has to traverse R4.
The Replication segment state at nodes R1, R2, R6 and R7 is shown
below. Note nodes R3, R4 and R5 do not have state for the
Replication segment. The state representation uses "R-SID->Lmn" to
represent a packet replication with outgoing replication SID R-SID
sent on interface Lmn. "SL" represents and optional segment list used
to steer a replicated packet on a specific path to a Downstream node.
Replication segment at R1:
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Replication segment <R-ID,R1>:
Replication SID: 2001:db8:cccc:1:F1::0
Replication state:
R2: <2001:db8:cccc:2:F2::0->L12>
R6: <2001:db8:cccc:6:F6::0>
R7: <2001:db8:cccc:4:C7::0>, SL: <2001:db8:cccc:7:F7::0>
Replication to R2 steers the packet directly to R2 on interface L12.
Replication to R6, using 2001:db8:cccc:6:F6::0, steers the packet via
shortest path to that node. Replication to R7 is steered via R4,
using H.Encaps.Red with End.X SID 2001:db8:cccc:4:C7::0 at R4 to R7.
Replication segment at R2:
Replication segment <R-ID,R2>:
Replication SID: 2001:db8:cccc:2:F2::0
Replication state:
R2: <Leaf>
Replication segment at R6:
Replication segment <R-ID,R6>:
Replication SID: 2001:db8:cccc:6:F6::0
Replication state:
R6: <Leaf>
Replication segment at R7:
Replication segment <R-ID,R7>:
Replication SID: 2001:db8:cccc:7:F7::0
Replication state:
R7: <Leaf>
When a packet, (A,B2), is steered into the Replication segment at R1:
* Since R1 is directly connected to R2, R1 creates encapsulated
replicated copy (2001:db8::1, 2001:db8:cccc:2:F2::0) (A, B2), and
sends it to R2 on interface L12. R2, as Leaf, removes outer IPv6
header and delivers the payload.
* R1 creates encapsulated replicated copy (2001:db8::1,
2001:db8:cccc:6:F6::0) (A, B2) then forwards the resulting packet
on the shortest path to 2001:db8:cccc:6::/64. R2 and R3 forward
the packet using 2001:db8:cccc:6::/64. R6, as Leaf, removes outer
IPv6 header and delivers the payload.
* R1 has to steer packet to Downstream node R7 via node R4. It can
do this in one of two ways:
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- R1 creates encapsulated replicated copy (2001:db8::1,
2001:db8:cccc:7:F7::0) (A, B2) and then performs H.Encaps.Red
using the SL to create (2001:db8::1, 2001:db8:cccc:4:C7::0)
(2001:db8::1, 2001:db8:cccc:7:F7::0) (A, B2) packet. It sends
this packet to R2, the nexthop on shortest path to
2001:db8:cccc:4::/64. R2 forwards packet to R4 using
2001:db8:cccc:4::/64. R4 executes End.X function on
2001:db8:cccc:4:C7::0, performs USD action, removes outer IPv6
encapsulation and sends resulting packet (2001:db8::1,
2001:db8:cccc:7:F7::0) (A, B2) to R7. R7, as Leaf, removes
outer IPv6 header and delivers the payload.
- R1 is Root of replication segment. Therefore, it can combine
above encapsulations to create encapsulated replicated copy
(2001:db8::1, 2001:db8:cccc:4:C7::0) (2001:db8:cccc:7:F7::0;
SL=1) (A, B2) and sends it to R2, the nexthop on shortest path
to 2001:db8:cccc:4::/64. R2 forwards packet to R4 using
2001:db8:cccc:4::/64. R4 executes End.X function on
2001:db8:cccc:4:C7::0, performs PSP action, removes SRH and
sends resulting packet (2001:db8::1, 2001:db8:cccc:7:F7::0) (A,
B2) to R7. R7, as Leaf, removes outer IPv6 header and delivers
the payload.
A.2.1. Pinging Replication SID
This section illustrates ping of a Replication SID.
Node R1 pings replication SID of node R6 directly by sending the
following packet:
1. R1 to R6: (2001:db8::1, 2001:db8:cccc:6:F6::0; NH=ICMPv6) (ICMPv6
Echo Request)
2. Node R6 as a Leaf processes upper layer ICMPv6 Echo Request and
responds with ICMPv6 Echo Reply
Node R1 pings Replication SID of R7 via R4 by sending the following
packet with SRH:
1. R1 to R4: (2001:db8::1, 2001:db8:cccc:4:C7::0)
(2001:db8:cccc:7:F7::0; SL=1; NH=ICMPV6) (ICMPv6 Echo Request)
2. R4 to R7: (2001:db8::1, 2001:db8:cccc:7:F7::0; NH=ICMPv6) (ICMPv6
Echo Request)
3. Node R7 as a Leaf processes upper layer ICMPv6 Echo Request and
responds with ICMPv6 Echo Reply
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Assume node R4 is a transit Replication node with Replication SID
2001:db8:cccc:4:F4::0 replicating to R7. Node R1 pings Replication
SID of R7 via Replication SID of R4 as follows:
1. R1 to R4: (2001:db8::1, 2001:db8:cccc:4:F4::0; NH=ICMPv6) (ICMPv6
Echo Request)
2. R4 replicates to R7 by replacing IPv6 destination address with
Replication SID of R7 from its Replication state
3. R4 to R7: (2001:db8::1, 2001:db8:cccc:7:F7::0; NH=ICMPv6) (ICMPv6
Echo Request)
4. Node R7 as a Leaf processes upper layer ICMPv6 Echo Request and
responds with ICMPv6 Echo Reply
Authors' Addresses
Daniel Voyer (editor)
Bell Canada
Montreal
Canada
Email: daniel.voyer@bell.ca
Clarence Filsfils
Cisco Systems, Inc.
Brussels
Belgium
Email: cfilsfil@cisco.com
Rishabh Parekh
Cisco Systems, Inc.
San Jose,
United States of America
Email: riparekh@cisco.com
Hooman Bidgoli
Nokia
Ottawa
Canada
Email: hooman.bidgoli@nokia.com
Zhaohui Zhang
Juniper Networks
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Email: zzhang@juniper.net
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