Network Working Group S. Previdi
Internet-Draft Individual
Intended status: Standards Track C. Filsfils, Ed.
Expires: October 21, 2018 Cisco Systems, Inc.
J. Leddy
Comcast
S. Matsushima
Softbank
D. Voyer, Ed.
Bell Canada
April 19, 2018
IPv6 Segment Routing Header (SRH)
draft-ietf-6man-segment-routing-header-12
Abstract
Segment Routing (SR) allows a node to steer a packet through a
controlled set of instructions, called segments, by prepending an SR
header to the packet. A segment can represent any instruction,
topological or service-based. SR allows a node to steer a flow
through any path (topological, or application/service based) while
maintaining per-flow state only at the ingress node to the SR domain.
Segment Routing can be applied to the IPv6 data plane with the
addition of a new type of Routing Extension Header. This document
describes the Segment Routing Extension Header Type and how it is
used by SR capable nodes.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at 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
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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 October 21, 2018.
Copyright Notice
Copyright (c) 2018 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 extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Segment Routing Documents . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Data Planes supporting Segment Routing . . . . . . . . . 4
2.2. SRv6 Segment . . . . . . . . . . . . . . . . . . . . . . 4
2.3. Segment Routing (SR) Domain . . . . . . . . . . . . . . . 5
3. Segment Routing Extension Header (SRH) . . . . . . . . . . . 5
3.1. SRH TLVs . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1.1. Padding TLVs . . . . . . . . . . . . . . . . . . . . 8
3.1.2. HMAC TLV . . . . . . . . . . . . . . . . . . . . . . 10
3.2. SRH Processing . . . . . . . . . . . . . . . . . . . . . 11
4. SRH Functions . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1. Endpoint Function (End) . . . . . . . . . . . . . . . . . 11
4.2. End.X: Endpoint with Layer-3 cross-connect . . . . . . . 12
5. SR Nodes . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.1. Source SR Node . . . . . . . . . . . . . . . . . . . . . 13
5.2. Transit Node . . . . . . . . . . . . . . . . . . . . . . 14
5.3. SR Segment Endpoint Node . . . . . . . . . . . . . . . . 14
5.4. Load Balancing and ECMP . . . . . . . . . . . . . . . . . 14
6. Security Considerations . . . . . . . . . . . . . . . . . . . 15
6.1. Threat model . . . . . . . . . . . . . . . . . . . . . . 15
6.1.1. Source routing threats . . . . . . . . . . . . . . . 15
6.1.2. Applicability of RFC 5095 to SRH . . . . . . . . . . 16
6.1.3. Service stealing threat . . . . . . . . . . . . . . . 17
6.1.4. Topology disclosure . . . . . . . . . . . . . . . . . 17
6.1.5. ICMP Generation . . . . . . . . . . . . . . . . . . . 17
6.2. Security fields in SRH . . . . . . . . . . . . . . . . . 18
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6.2.1. Selecting a hash algorithm . . . . . . . . . . . . . 19
6.2.2. Performance impact of HMAC . . . . . . . . . . . . . 19
6.2.3. Pre-shared key management . . . . . . . . . . . . . . 20
6.3. Deployment Models . . . . . . . . . . . . . . . . . . . . 20
6.3.1. Nodes within the SR domain . . . . . . . . . . . . . 20
6.3.2. Nodes outside of the SR domain . . . . . . . . . . . 21
6.3.3. SR path exposure . . . . . . . . . . . . . . . . . . 21
6.3.4. Impact of BCP-38 . . . . . . . . . . . . . . . . . . 22
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
7.1. Segment Routing Header Flags Register . . . . . . . . . . 22
7.2. Segment Routing Header TLVs Register . . . . . . . . . . 23
8. Implementation Status . . . . . . . . . . . . . . . . . . . . 23
8.1. Linux . . . . . . . . . . . . . . . . . . . . . . . . . . 23
8.2. Cisco Systems . . . . . . . . . . . . . . . . . . . . . . 23
8.3. FD.io . . . . . . . . . . . . . . . . . . . . . . . . . . 24
8.4. Barefoot . . . . . . . . . . . . . . . . . . . . . . . . 24
8.5. Juniper . . . . . . . . . . . . . . . . . . . . . . . . . 24
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 24
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 24
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
11.1. Normative References . . . . . . . . . . . . . . . . . . 25
11.2. Informative References . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
1. Segment Routing Documents
Segment Routing terminology is defined in
[I-D.ietf-spring-segment-routing].
The network programming paradigm
[I-D.filsfils-spring-srv6-network-programming] defines additional
functions associated to a Segment Routing for IPv6 (SRv6) Segment ID
(SID).
Segment Routing use cases are described in [RFC7855] and [RFC8354].
Segment Routing protocol extensions are defined in
[I-D.ietf-isis-segment-routing-extensions], and
[I-D.ietf-ospf-ospfv3-segment-routing-extensions].
2. Introduction
Segment Routing (SR), defined in [I-D.ietf-spring-segment-routing],
allows a node to steer a packet through a controlled set of
instructions, called segments, by prepending an SR header to the
packet. A segment can represent any instruction, topological or
service-based. SR allows a node to steer a flow through any path
(topological or service/application based) while maintaining per-flow
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state only at the ingress node to the SR domain. Segments can be
derived from different components: IGP, BGP, Services, Contexts,
Locators, etc. The list of segment forming the path is called the
Segment List and is encoded in the packet header.
SR allows the use of strict and loose source based routing paradigms
without requiring any additional signaling protocols in the
infrastructure hence delivering an excellent scalability property.
The source based routing model described in
[I-D.ietf-spring-segment-routing] inherits from the ones proposed by
[RFC1940] and [RFC8200]. The source based routing model offers the
support for explicit routing capability.
2.1. Data Planes supporting Segment Routing
Segment Routing (SR), can be instantiated over MPLS
(SRMPLS)([I-D.ietf-spring-segment-routing-mpls]) and IPv6 (SRv6).
This document defines its instantiation over the IPv6 data-plane
based on the use-cases defined in [RFC8354].
This document defines a new type of Routing Header (originally
defined in [RFC8200]) called the Segment Routing Header (SRH) in
order to convey the Segment List in the packet header as defined in
[I-D.ietf-spring-segment-routing]. Mechanisms through which segments
are known and advertised are outside the scope of this document.
2.2. SRv6 Segment
An SRv6 Segment is a 128-bit value. "SRv6 SID" or simply "SID" are
often used as a shorter reference for "SRv6 Segment".
An SRv6-capable node N maintains a "My Local SID Table". This table
contains all the local SRv6 segments explicitly instantiated at node
N. N is the parent node for these SIDs.
A local SID of N could be an IPv6 address of a local interface of N
but it does not have to be. Most often, a local SID is not an
address of a local interface of N. The My Local SID Table is thus
not populated by default with all the addresses of a node.
Every SRv6 local SID instantiated has a specific instruction bounded
to it.
This information is stored in the My Local SID Table. The My Local
SID Table has three main purposes:
o Define which local SIDs are explicitly instantiated.
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o Specify which instruction is bound to each of the instantiated
SIDs.
o Store the parameters associated to such instruction (i.e. OIF,
NextHop,...).
A Segment ID (SID) in the destination address of an IPv6 header, is
routed through an IPv6 network as an IPv6 address. SIDs, or the
prefix(es) covering SIDs in the My Local SID Table, and their
reachability may be distributed by means outside the scope of this
document. For example, [RFC5308] or [RFC5340] may be used to
advertise a prefix covering the My Local SID Table.
A node may receive a packet with an SRv6 SID in the DA without an
SRH. In such case the packet should still be processed by the
Segment Routing engine.
2.3. Segment Routing (SR) Domain
The Segment Routing Domain (SR Domain) is defined as the set of nodes
participating in the source based routing model. These nodes may be
connected to the same physical infrastructure (e.g.: a Service
Provider's network).
The SRH is added to the packet by its source, consistent with the
source routing model defined in [RFC8200]. For example:
o At the node originating the packet (host, server).
o At the ingress node of an SR domain where the ingress node
receives a packet and encapsulates it in an outer IPv6 header
followed by a Segment Routing header.
3. Segment Routing Extension Header (SRH)
A new type of the Routing Header (originally defined in [RFC8200]) is
defined: the Segment Routing Header (SRH) which has a new Routing
Type, (suggested value 4) to be assigned by IANA.
The Segment Routing Header (SRH) is defined as follows:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len | Routing Type | Segments Left |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Last Entry | Flags | Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Segment List[0] (128 bits IPv6 address) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
...
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Segment List[n] (128 bits IPv6 address) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// //
// Optional Type Length Value objects (variable) //
// //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
o Next Header: Defined in [RFC8200]
o Hdr Ext Len: Defined in [RFC8200]
o Routing Type: TBD, to be assigned by IANA (suggested value: 4).
o Segments Left: Defined in [RFC8200]. See Section 3.2 for detailed
usage during SRH processing.
o Last Entry: contains the index (zero based), in the Segment List,
of the last element of the Segment List.
o Flags: 8 bits of flags. Following flags are defined:
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0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| U |H| U |
+-+-+-+-+-+-+-+-+
U: Unused and for future use. SHOULD be 0 on transmission and
MUST be ignored on receipt.
H-flag: HMAC flag. If set, the HMAC TLV is present and is
encoded as the last TLV of the SRH. In other words, the last
36 octets of the SRH represent the HMAC information. See
Section 3.1.2 for details on the HMAC TLV.
o Tag: tag a packet as part of a class or group of packets, e.g.,
packets sharing the same set of properties. When tag is not used
at source it SHOULD be set to zero on transmission. When tag is
not used during SRH Processing it MUST be ignored. The allocation
and use of tag is outside the scope of this document.
o Segment List[n]: 128 bit IPv6 addresses representing the nth
segment in the Segment List. The Segment List is encoded starting
from the last segment of the path. I.e., the first element of the
segment list (Segment List [0]) contains the last segment of the
path, the second element contains the penultimate segment of the
path and so on.
o Type Length Value (TLV) are described in Section 3.1.
3.1. SRH TLVs
This section defines TLVs of the Segment Routing Header.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-----------------------
| Type | Length | Variable length data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-----------------------
Type: An 8 bit value. Unrecognized Types MUST be ignored on receipt.
Length: The length of the Variable length data. It is RECOMMENDED
that the total length of new TLVs be multiple of 8 bytes to avoid the
use of Padding TLVs.
Variable length data: Length bytes of data that is specific to the
Type.
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Type Length Value (TLV) contain optional information that may be used
by the node identified in the DA of the packet. The information
carried in the TLVs is not intended to be used by the routing layer.
Typically, TLVs carry information that is consumed by other
components (e.g.: OAM) than the routing function.
Each TLV has its own length, format and semantic. The code-point
allocated (by IANA) to each TLV Type defines both the format and the
semantic of the information carried in the TLV. Multiple TLVs may be
encoded in the same SRH.
TLVs may change en route at each segment. To identify when a TLV
type may change en route the most significant bit of the Type has the
following significance:
0: TLV data does not change en route
1: TLV data does change en route
Identifying which TLVs change en route, without having to understand
the Type, is required for Authentication Header Integrity Check Value
(ICV) computation. Any TLV that changes en route is considered
mutable for the purpose of ICV computation, the Type Length and
Variable Length Data is ignored for the purpose of ICV Computation as
defined in [RFC4302].
The "Length" field of the TLV is used to skip the TLV while
inspecting the SRH in case the node doesn't support or recognize the
Type. The "Length" defines the TLV length in octets, not including
the "Type" and "Length" fields.
The following TLVs are defined in this document:
Padding TLV
HMAC TLV
Additional TLVs may be defined in the future.
3.1.1. Padding TLVs
There are two types of padding TLVs, pad0 and padN, the following
applies to both:
Padding TLVs are optional and more than one Padding TLV MUST NOT
appear in the SRH.
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The Padding TLVs are used to align the SRH total length on the 8
octet boundary.
When present, a single Pad0 or PadN TLV MUST appear as the last
TLV before the HMAC TLV (if HMAC TLV is present).
When present, a PadN TLV MUST have a length from 0 to 5 in order
to align the SRH total length on a 8-octet boundary.
Padding TLVs are ignored by a node processing the SRH TLV, even if
more than one is present.
Padding TLVs are ignored during ICV calculation.
3.1.1.1. PAD0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Type |
+-+-+-+-+-+-+-+-+
Type: to be assigned by IANA (Suggested value 128)
A single Pad0 TLV MUST be used when a single byte of padding is
required. If more than one byte of padding is required a Pad0 TLV
MUST NOT be used, the PadN TLV MUST be used.
3.1.1.2. PADN
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 | Padding (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Padding (variable) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: to be assigned by IANA (suggested value 129).
Length: 0 to 5
Padding: Length octets of padding. Padding bits have no
semantics. They SHOULD be set to 0 on transmission and MUST be
ignored on receipt.
The PadN TLV MUST be used when more than one byte of padding is
required.
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3.1.2. HMAC TLV
HMAC TLV is optional and contains the HMAC information. The HMAC 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 | RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HMAC Key ID (4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| //
| HMAC (32 octets) //
| //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
o Type: to be assigned by IANA (suggested value 5).
o Length: 38.
o RESERVED: 2 octets. SHOULD be unset on transmission and MUST be
ignored on receipt.
o HMAC Key ID: 4 octets.
o HMAC: 32 octets.
o HMAC and HMAC Key ID usage is described in Section 6
The Following applies to the HMAC TLV:
o When present, the HMAC TLV MUST be encoded as the last TLV of the
SRH.
o If the HMAC TLV is present, the SRH H-Flag (Figure 2) MUST be set.
Nodes processing SRH SHOULD process the HMAC TLV only when the
H-Flag is set, and local policy.
o When the H-flag is set in the SRH, the router inspecting the SRH
MUST find the HMAC TLV in the last 38 octets of the SRH.
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3.2. SRH Processing
Extension header processing is as defined in RFC8200 for packets
destined to a local IPv6 address. If SRH is present, it is processed
as follows (DA represents the destination address in the IPv6
header):
1. IF the DA is in My Local SID Table
2. The function associated with the DA is processed.
(Examples are as described in section 4).
3. ELSE
4. Treat the extension header as unrecognized
and process as defined in RFC8200 section 4.4.
4. SRH Functions
Segment Routing architecture, as defined in
[I-D.ietf-spring-segment-routing] defines a segment as an instruction
or, more generally, a set of instructions (function).
In this section we review two functions that may be associated to a
segment:
o End: the endpoint (End) function is the base of the source routing
paradigm. It consists of updating the DA with the next segment
and forward the packet accordingly.
o End.X: The endpoint layer-3 cross-connect function.
Other functions may be defined in other documents.
4.1. Endpoint Function (End)
The Endpoint function (End) is the most basic function.
When the endpoint node receives a packet destined to DA "S" and S is
an entry in the My Local SID Table and the function associated with S
in that entry is "End" then the endpoint does:
1. IF SegmentsLeft > 0 THEN
2. decrement SL
3. update the IPv6 DA with SRH[SL]
4. FIB lookup on updated DA
5. forward accordingly to the matched entry
6. ELSE IF SID is assigned to an interface
7. proceed to process the next header
8. ELSE
9. drop the packet
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It has to be noted that:
o The End function performs the FIB lookup in the forwarding table
of the ingress interface.
o An SRv6-capable node MUST include the FlowLabel of the IPv6 header
in its hash computation for ECMP load-balancing as described in
Section 5.4.
4.2. End.X: Endpoint with Layer-3 cross-connect
When the endpoint node receives a packet destined to DA "S" and S is
an entry in the My Local SID Table and the function associated with S
in that entry is "End.X" then the endpoint does:
1. IF SegmentsLeft > 0 THEN
2. decrement SL
3. update the IPv6 DA with SRH[SL]
4. forward to layer-3 adjacency bound to the SID "S"
5. ELSE
6. drop the packet
It has to be noted that:
o If an array of layer-3 adjacencies is bound to the End.X SID, one
entry of the array is selected based on a hash of the packet's
header as described in Section 5.4
o An End.X function does not allow decaps. An End.X SID cannot be
the last SID of an SRH and cannot be the DA of a packet without
SRH.
o The End.X function is required to express an explicit path through
an IP network.
o The End.X function is the SRv6 instantiation of a Adjacency SID as
defined in [I-D.ietf-spring-segment-routing].
o Note that with SR-MPLS ([I-D.ietf-spring-segment-routing-mpls]),
an AdjSID is typically preceded by a PrefixSID. This is unlikely
in SRv6, most likely an End.X SID is globally routed address of N.
o If a node N has 30 outgoing interfaces to 30 neighbors, usually
the operator would explicitly instantiate 30 End.X SIDs at N: one
per layer-3 adjacency to a neighbor. Potentially, more End.X
could be explicitly defined (groups of layer-3 adjacencies to the
same neighbor or to different neighbors).
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o If node N has a bundle outgoing interface I to a neighbor Q made
of 10 member links, N may allocate up to 11 End.X local SIDs for
that bundle: one for the bundle itself and then up to one for each
member link. This is the equivalent of the L2-Link Adj SID in SR-
MPLS ([I-D.ietf-isis-l2bundles]).
5. SR Nodes
There are different types of nodes that may be involved in segment
routing networks: source nodes that originate packets with an SRH,
transit nodes that forward packets having an SRH and segment endpoint
nodes that MUST process the SRH.
5.1. Source SR Node
A Source SR Node can be any node originating an IPv6 packet with its
IPv6 and Segment Routing Headers. This include either:
A host originating an IPv6 packet.
An SR domain ingress router encapsulating a received packet in an
outer IPv6 header followed by an SRH.
The mechanism through which a Segment List is derived is outside the
scope of this document. As an example, the Segment List may be
obtained through:
Local path computation.
Local configuration.
Interaction with a centralized controller delivering the path.
Any other mechanism.
The following are the steps of the creation of the SRH:
Next Header and Hdr Ext Len fields are set as specified in
[RFC8200].
Routing Type field is set as TBD (to be allocated by IANA,
suggested value 4).
The DA of the packet is set with the value of the first segment.
The first element of the segment list is the last segment (the
final DA of the packet). The second element is the penultimate
segment and so on.
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In other words, the Segment List is encoded in the reverse order
of the path.
The Segments Left field is set to n-1 where n is the number of
elements in the Segment List.
The Last_Entry field is set to n-1 where n is the number of
elements in the Segment List.
HMAC TLV may be set according to Section 6.
If the segment list contains a single segment and there is no need
for information in flag or TLV, then the SRH MAY be omitted.
The packet is sent towards the packet's DA (the first segment).
5.2. Transit Node
As specified in [RFC8200], the only node who is allowed to inspect
the Routing Extension Header (and therefore the SRH), is the node
corresponding to the DA of the packet. Any other transit node MUST
NOT inspect the underneath routing header and MUST forward the packet
towards the DA and according to the IPv6 routing table.
As a generic security measure, any service provider will block any
packet destined to one of its internal routers, especially if these
packets have an extension header in it.
5.3. SR Segment Endpoint Node
The SR segment endpoint node is the node whose My Local SID
Table contains an entry for the DA of the packet.
The segment endpoint node executes the function bound to the SID as
per the matched My Local SID entry (Section 4).
5.4. Load Balancing and ECMP
Within an SR domain, an SR source node encapsulates a packet in an
outer IPv6 header for transport to an endpoint. The SR source node
MUST impose a Flow Label computed based on the inner packet. The
computation of the flow label is as recommended in [RFC6438] for the
sending TEP.
At any transit node within an SR domain, the flow label MUST be used
as defined in [RFC6438] to calculate the ECMP hash toward the
destination address.
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At an SR segment endpoint node, the flow label MUST be used as
defined in [RFC6438] to calculate any ECMP hash used to forward the
processed packet to the next segment.
6. Security Considerations
This section analyzes the security threat model, the security issues
and proposed solutions related to the new Segment Routing Header.
The Segment Routing Header (SRH) is simply another type of the
routing header as described in [RFC8200] and is:
o Added by an SR edge router when entering the segment routing
domain or by the originating host itself. The source host can
even be outside the SR domain;
o inspected and acted upon when reaching the destination address of
the IP header per [RFC8200].
Per [RFC8200], routers on the path that simply forward an IPv6 packet
(i.e. the IPv6 destination address is none of theirs) will never
inspect and process the content of the SRH. Routers whose one
interface IPv6 address equals the destination address field of the
IPv6 packet MUST parse the SRH and, if supported and if the local
configuration allows it, MUST act accordingly to the SRH content.
As specified in [RFC8200], the default behavior of a non SR-capable
router upon receipt of an IPv6 packet with SRH destined to an address
of its, is to:
o ignore the SRH completely if the Segment Left field is 0 and
proceed to process the next header in the IPv6 packet;
o discard the IPv6 packet if Segment Left field is greater than 0,
it MAY send a Parameter Problem ICMP message back to the Source
Address.
6.1. Threat model
6.1.1. Source routing threats
Using an SRH is similar to source routing, therefore it has some
well-known security issues as described in [RFC4942] section 2.1.1
and [RFC5095]:
o amplification attacks: where a packet could be forged in such a
way to cause looping among a set of SR-enabled routers causing
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unnecessary traffic, hence a Denial of Service (DoS) against
bandwidth;
o reflection attack: where a hacker could force an intermediate node
to appear as the immediate attacker, hence hiding the real
attacker from naive forensic;
o bypass attack: where an intermediate node could be used as a
stepping stone (for example in a De-Militarized Zone) to attack
another host (for example in the datacenter or any back-end
server).
6.1.2. Applicability of RFC 5095 to SRH
First of all, the reader must remember this specific part of section
1 of [RFC5095], "A side effect is that this also eliminates benign
RH0 use-cases; however, such applications may be facilitated by
future Routing Header specifications.". In short, it is not
forbidden to create new secure type of Routing Header; for example,
[RFC6554] (RPL) also creates a new Routing Header type for a specific
application confined in a single network.
In the segment routing architecture described in
[I-D.ietf-spring-segment-routing] there are basically two kinds of
nodes (routers and hosts):
o nodes within the SR domain, which is within one single
administrative domain, i.e., where all nodes are trusted anyway
else the damage caused by those nodes could be worse than
amplification attacks: traffic interception, man-in-the-middle
attacks, more server DoS by dropping packets, and so on.
o nodes outside of the SR domain, which is outside of the
administrative segment routing domain hence they cannot be trusted
because there is no physical security for those nodes, i.e., they
can be replaced by hostile nodes or can be coerced in wrong
behaviors.
The main use case for SR consists of the single administrative domain
where only trusted nodes with SR enabled and configured participate
in SR: this is the same model as in [RFC6554]. All non-trusted nodes
do not participate as either SR processing is not enabled by default
or because they only process SRH from nodes within their domain.
Moreover, all SR nodes ignore SRH created by outsiders based on
topology information (received on a peering or internal interface) or
on presence and validity of the HMAC field. Therefore, if
intermediate nodes ONLY act on valid and authorized SRH (such as
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within a single administrative domain), then there is no security
threat similar to RH-0. Hence, the [RFC5095] attacks are not
applicable.
6.1.3. Service stealing threat
Segment routing is used for added value services, there is also a
need to prevent non-participating nodes to use those services; this
is called 'service stealing prevention'.
6.1.4. Topology disclosure
The SRH may also contains IPv6 addresses of some intermediate SR-
nodes in the path towards the destination, this obviously reveals
those addresses to the potentially hostile attackers if those
attackers are able to intercept packets containing SRH. On the other
hand, if the attacker can do a traceroute whose probes will be
forwarded along the SR path, then there is little learned by
intercepting the SRH itself.
6.1.5. ICMP Generation
Per Section 4.4 of [RFC8200], when destination nodes (i.e. where the
destination address is one of theirs) receive a Routing Header with
unsupported Routing Type, the required behavior is:
o If Segments Left is zero, the node must ignore the Routing header
and proceed to process the next header in the packet.
o If Segments Left is non-zero, the node must discard the packet and
send an ICMP Parameter Problem, Code 0, message to the packet's
Source Address, pointing to the unrecognized Routing Type.
This required behavior could be used by an attacker to force the
generation of ICMP message by any node. The attacker could send
packets with SRH (with Segment Left not set to 0) destined to a node
not supporting SRH. Per [RFC8200], the destination node could
generate an ICMP message, causing a local CPU utilization and if the
source of the offending packet with SRH was spoofed could lead to a
reflection attack without any amplification.
It must be noted that this is a required behavior for any unsupported
Routing Type and not limited to SRH packets. So, it is not specific
to SRH and the usual rate limiting for ICMP generation is required
anyway for any IPv6 implementation and has been implemented and
deployed for many years.
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6.2. Security fields in SRH
This section summarizes the use of specific fields in the SRH. They
are based on a key-hashed message authentication code (HMAC).
The security-related fields in the SRH are instantiated by the HMAC
TLV, containing:
o HMAC Key-id, 32 bits wide;
o HMAC, 256 bits wide (optional, exists only if HMAC Key-id is not
0).
The HMAC field is the output of the HMAC computation (per [RFC2104])
using a pre-shared key identified by HMAC Key-id and of the text
which consists of the concatenation of:
o the source IPv6 address;
o Last Entry field;
o an octet of bit flags;
o HMAC Key-id;
o all addresses in the Segment List.
The purpose of the HMAC TLV is to verify the validity, the integrity
and the authorization of the SRH itself. If an outsider of the SR
domain does not have access to a current pre-shared secret, then it
cannot compute the right HMAC field and the first SR router on the
path processing the SRH and configured to check the validity of the
HMAC will simply reject the packet.
The HMAC TLV is located at the end of the SRH simply because only the
router on the ingress of the SR domain needs to process it, then all
other SR nodes can ignore it (based on local policy) because they
trust the upstream router. This is to speed up forwarding operations
because SR routers which do not validate the SRH do not need to parse
the SRH until the end.
The HMAC Key-id field allows for the simultaneous existence of
several hash algorithms (SHA-256, SHA3-256 ... or future ones) as
well as pre-shared keys. The HMAC Key-id field is opaque, i.e., it
has neither syntax nor semantic except as an index to the right
combination of pre-shared key and hash algorithm and except that a
value of 0 means that there is no HMAC field. Having an HMAC Key-id
field allows for pre-shared key roll-over when two pre-shared keys
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are supported for a while when all SR nodes converged to a fresher
pre-shared key. It could also allow for interoperation among
different SR domains if allowed by local policy and assuming a
collision-free HMAC Key Id allocation.
When a specific SRH is linked to a time-related service (such as
turbo-QoS for a 1-hour period) where the DA, Segment ID (SID) are
identical, then it is important to refresh the shared-secret
frequently as the HMAC validity period expires only when the HMAC
Key-id and its associated shared-secret expires.
6.2.1. Selecting a hash algorithm
The HMAC field in the HMAC TLV is 256 bit wide. Therefore, the HMAC
MUST be based on a hash function whose output is at least 256 bits.
If the output of the hash function is 256, then this output is simply
inserted in the HMAC field. If the output of the hash function is
larger than 256 bits, then the output value is truncated to 256 by
taking the least-significant 256 bits and inserting them in the HMAC
field.
SRH implementations can support multiple hash functions but MUST
implement SHA-2 [FIPS180-4] in its SHA-256 variant.
NOTE: SHA-1 is currently used by some early implementations used for
quick interoperations testing, the 160-bit hash value must then be
right-hand padded with 96 bits set to 0. The authors understand that
this is not secure but is ok for limited tests.
6.2.2. Performance impact of HMAC
While adding an HMAC to each and every SR packet increases the
security, it has a performance impact. Nevertheless, it must be
noted that:
o the HMAC field is used only when SRH is added by a device (such as
a home set-up box) which is outside of the segment routing domain.
If the SRH is added by a router in the trusted segment routing
domain, then, there is no need for an HMAC field, hence no
performance impact.
o when present, the HMAC field MUST only be checked and validated by
the first router of the segment routing domain, this router is
named 'validating SR router'. Downstream routers may not inspect
the HMAC field.
o this validating router can also have a cache of <IPv6 header +
SRH, HMAC field value> to improve the performance. It is not the
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same use case as in IPsec where HMAC value was unique per packet,
in SRH, the HMAC value is unique per flow.
o Last point, hash functions such as SHA-2 have been optimized for
security and performance and there are multiple implementations
with good performance.
With the above points in mind, the performance impact of using HMAC
is minimized.
6.2.3. Pre-shared key management
The field HMAC Key-id allows for:
o key roll-over: when there is a need to change the key (the hash
pre-shared secret), then multiple pre-shared keys can be used
simultaneously. The validating routing can have a table of <HMAC
Key-id, pre-shared secret> for the currently active and future
keys.
o different algorithms: by extending the previous table to <HMAC
Key-id, hash function, pre-shared secret>, the validating router
can also support simultaneously several hash algorithms (see
section Section 6.2.1)
The pre-shared secret distribution can be done:
o in the configuration of the validating routers, either by static
configuration or any SDN oriented approach;
o dynamically using a trusted key distribution such as [RFC6407]
The intent of this document is NOT to define yet-another-key-
distribution-protocol.
6.3. Deployment Models
6.3.1. Nodes within the SR domain
An SR domain is defined as a set of interconnected routers where all
routers at the perimeter are configured to add and act on SRH. Some
routers inside the SR domain can also act on SRH or simply forward
IPv6 packets.
The routers inside an SR domain can be trusted to generate SRH and to
process SRH received on interfaces that are part of the SR domain.
These nodes MUST drop all SRH packets received on an interface that
is not part of the SR domain and containing an SRH whose HMAC field
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cannot be validated by local policies. This includes obviously
packet with an SRH generated by a non-cooperative SR domain.
If the validation fails, then these packets MUST be dropped, ICMP
error messages (parameter problem) SHOULD be generated (but rate
limited) and SHOULD be logged.
6.3.2. Nodes outside of the SR domain
Nodes outside of the SR domain cannot be trusted for physical
security; hence, they need to request by some trusted means (outside
the scope of this document) a complete SRH for each new connection
(i.e. new destination address). The received SRH MUST include an
HMAC TLV which is computed correctly (see Section 6.2).
When an outside node sends a packet with an SRH and towards an SR
domain ingress node, the packet MUST contain the HMAC TLV (with a
Key-id and HMAC fields) and the the destination address MUST be an
address of an SR domain ingress node .
The ingress SR router, i.e., the router with an interface address
equal to the destination address, MUST verify the HMAC TLV.
If the validation is successful, then the packet is simply forwarded
as usual for an SR packet. As long as the packet travels within the
SR domain, no further HMAC check needs to be done. Subsequent
routers in the SR domain MAY verify the HMAC TLV when they process
the SRH (i.e. when they are the destination).
If the validation fails, then this packet MUST be dropped, an ICMP
error message (parameter problem) SHOULD be generated (but rate
limited) and SHOULD be logged.
6.3.3. SR path exposure
As the intermediate SR nodes addresses appears in the SRH, if this
SRH is visible to an outsider then he/she could reuse this knowledge
to launch an attack on the intermediate SR nodes or get some insider
knowledge on the topology. This is especially applicable when the
path between the source node and the first SR domain ingress router
is on the public Internet.
The first remark is to state that 'security by obscurity' is never
enough; in other words, the security policy of the SR domain MUST
assume that the internal topology and addressing is known by the
attacker. A simple traceroute will also give the same information
(with even more information as all intermediate nodes between SID
will also be exposed). IPsec Encapsulating Security Payload
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[RFC4303] cannot be use to protect the SRH as per RFC4303 the ESP
header must appear after any routing header (including SRH).
To prevent a user to leverage the gained knowledge by intercepting
SRH, it it recommended to apply an infrastructure Access Control List
(iACL) at the edge of the SR domain. This iACL will drop all packets
from outside the SR-domain whose destination is any address of any
router inside the domain. This security policy should be tuned for
local operations.
6.3.4. Impact of BCP-38
BCP-38 [RFC2827], also known as "Network Ingress Filtering", checks
whether the source address of packets received on an interface is
valid for this interface. The use of loose source routing such as
SRH forces packets to follow a path which differs from the expected
routing. Therefore, if BCP-38 was implemented in all routers inside
the SR domain, then SR packets could be received by an interface
which is not expected one and the packets could be dropped.
As an SR domain is usually a subset of one administrative domain, and
as BCP-38 is only deployed at the ingress routers of this
administrative domain and as packets arriving at those ingress
routers have been normally forwarded using the normal routing
information, then there is no reason why this ingress router should
drop the SRH packet based on BCP-38. Routers inside the domain
commonly do not apply BCP-38; so, this is not a problem.
7. IANA Considerations
This document makes the following registrations in the Internet
Protocol Version 6 (IPv6) Parameters "Routing Type" registry
maintained by IANA:
Suggested Description Reference
Value
----------------------------------------------------------
4 Segment Routing Header (SRH) This document
This document request IANA to create and maintain a new Registry:
"Segment Routing Header TLVs"
7.1. Segment Routing Header Flags Register
This document requests the creation of a new IANA managed registry to
identify SRH Flags Bits. The registration procedure is "Expert
Review" as defined in [RFC8126]. Suggested registry name is "Segment
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Routing Header Flags". Flags is 8 bits, the following bits are
defined in this document:
Suggested Description Reference
Bit
-----------------------------------------------------
4 HMAC This document
7.2. Segment Routing Header TLVs Register
This document requests the creation of a new IANA managed registry to
identify SRH TLVs. The registration procedure is "Expert Review" as
defined in [RFC8126]. Suggested registry name is "Segment Routing
Header TLVs". A TLV is identified through an unsigned 8 bit
codepoint value. The following codepoints are defined in this
document:
Suggested Description Reference
Value
-----------------------------------------------------
5 HMAC TLV This document
128 Pad0 TLV This document
129 PadN TLV This document
8. Implementation Status
This section is to be removed prior to publishing as an RFC.
8.1. Linux
Name: Linux Kernel v4.14
Status: Production
Implementation: adds SRH, performs END and END.X processing, supports
HMAC TLV
Details: https://irtf.org/anrw/2017/anrw17-final3.pdf and
[I-D.filsfils-spring-srv6-interop]
8.2. Cisco Systems
Name: IOS XR and IOS XE
Status: Pre-production
Implementation: adds SRH, performs END and END.X processing, no TLV
processing
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Details: [I-D.filsfils-spring-srv6-interop]
8.3. FD.io
Name: VPP/Segment Routing for IPv6
Status: Production
Implementation: adds SRH, performs END and END.X processing, no TLV
processing
Details: https://wiki.fd.io/view/VPP/Segment_Routing_for_IPv6 and
[I-D.filsfils-spring-srv6-interop]
8.4. Barefoot
Name: Barefoot Networks Tofino NPU
Status: Prototype
Implementation: performs END and END.X processing, no TLV processing
Details: [I-D.filsfils-spring-srv6-interop]
8.5. Juniper
Name: Juniper Networks Trio and vTrio NPU's
Status: Prototype & Experimental
Implementation: SRH insertion mode, Process SID where SID is an
interface address, no TLV processing
9. Contributors
Kamran Raza, Darren Dukes, Brian Field, Daniel Bernier, Ida Leung,
Jen Linkova, Ebben Aries, Tomoya Kosugi, Eric Vyncke, David Lebrun,
Dirk Steinberg, Robert Raszuk, Dave Barach, John Brzozowski, Pierre
Francois, Nagendra Kumar, Mark Townsley, Christian Martin, Roberta
Maglione, James Connolly, Aloys Augustin contributed to the content
of this document.
10. Acknowledgements
The authors would like to thank Ole Troan, Bob Hinden, Fred Baker,
Brian Carpenter, Alexandru Petrescu, Punit Kumar Jaiswal, and David
Lebrun for their comments to this document.
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11. References
11.1. Normative References
[FIPS180-4]
National Institute of Standards and Technology, "FIPS
180-4 Secure Hash Standard (SHS)", March 2012,
<http://csrc.nist.gov/publications/fips/fips180-4/
fips-180-4.pdf>.
[I-D.ietf-spring-segment-routing]
Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
Litkowski, S., and R. Shakir, "Segment Routing
Architecture", draft-ietf-spring-segment-routing-15 (work
in progress), January 2018.
[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>.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, DOI 10.17487/RFC4303, December 2005,
<https://www.rfc-editor.org/info/rfc4303>.
[RFC5095] Abley, J., Savola, P., and G. Neville-Neil, "Deprecation
of Type 0 Routing Headers in IPv6", RFC 5095,
DOI 10.17487/RFC5095, December 2007,
<https://www.rfc-editor.org/info/rfc5095>.
[RFC6407] Weis, B., Rowles, S., and T. Hardjono, "The Group Domain
of Interpretation", RFC 6407, DOI 10.17487/RFC6407,
October 2011, <https://www.rfc-editor.org/info/rfc6407>.
[RFC7855] Previdi, S., Ed., Filsfils, C., Ed., Decraene, B.,
Litkowski, S., Horneffer, M., and R. Shakir, "Source
Packet Routing in Networking (SPRING) Problem Statement
and Requirements", RFC 7855, DOI 10.17487/RFC7855, May
2016, <https://www.rfc-editor.org/info/rfc7855>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
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[RFC8354] Brzozowski, J., Leddy, J., Filsfils, C., Maglione, R.,
Ed., and M. Townsley, "Use Cases for IPv6 Source Packet
Routing in Networking (SPRING)", RFC 8354,
DOI 10.17487/RFC8354, March 2018,
<https://www.rfc-editor.org/info/rfc8354>.
11.2. Informative References
[I-D.filsfils-spring-srv6-interop]
Filsfils, C., Clad, F., Camarillo, P., Abdelsalam, A.,
Salsano, S., Bonaventure, O., Horn, J., and J. Liste,
"SRv6 interoperability report", draft-filsfils-spring-
srv6-interop-00 (work in progress), March 2018.
[I-D.filsfils-spring-srv6-network-programming]
Filsfils, C., Li, Z., Leddy, J., daniel.voyer@bell.ca, d.,
daniel.bernier@bell.ca, d., Steinberg, D., Raszuk, R.,
Matsushima, S., Lebrun, D., Decraene, B., Peirens, B.,
Salsano, S., Naik, G., Elmalky, H., Jonnalagadda, P., and
M. Sharif, "SRv6 Network Programming", draft-filsfils-
spring-srv6-network-programming-04 (work in progress),
March 2018.
[I-D.ietf-isis-l2bundles]
Ginsberg, L., Bashandy, A., Filsfils, C., Nanduri, M., and
E. Aries, "Advertising L2 Bundle Member Link Attributes in
IS-IS", draft-ietf-isis-l2bundles-07 (work in progress),
May 2017.
[I-D.ietf-isis-segment-routing-extensions]
Previdi, S., Ginsberg, L., Filsfils, C., Bashandy, A.,
Gredler, H., Litkowski, S., Decraene, B., and J. Tantsura,
"IS-IS Extensions for Segment Routing", draft-ietf-isis-
segment-routing-extensions-15 (work in progress), December
2017.
[I-D.ietf-ospf-ospfv3-segment-routing-extensions]
Psenak, P., Filsfils, C., Previdi, S., Gredler, H.,
Shakir, R., Henderickx, W., and J. Tantsura, "OSPFv3
Extensions for Segment Routing", draft-ietf-ospf-ospfv3-
segment-routing-extensions-11 (work in progress), January
2018.
[I-D.ietf-spring-segment-routing-mpls]
Bashandy, A., Filsfils, C., Previdi, S., Decraene, B.,
Litkowski, S., and R. Shakir, "Segment Routing with MPLS
data plane", draft-ietf-spring-segment-routing-mpls-13
(work in progress), April 2018.
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[RFC1940] Estrin, D., Li, T., Rekhter, Y., Varadhan, K., and D.
Zappala, "Source Demand Routing: Packet Format and
Forwarding Specification (Version 1)", RFC 1940,
DOI 10.17487/RFC1940, May 1996,
<https://www.rfc-editor.org/info/rfc1940>.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<https://www.rfc-editor.org/info/rfc2104>.
[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>.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
DOI 10.17487/RFC4302, December 2005,
<https://www.rfc-editor.org/info/rfc4302>.
[RFC4942] Davies, E., Krishnan, S., and P. Savola, "IPv6 Transition/
Co-existence Security Considerations", RFC 4942,
DOI 10.17487/RFC4942, September 2007,
<https://www.rfc-editor.org/info/rfc4942>.
[RFC5308] Hopps, C., "Routing IPv6 with IS-IS", RFC 5308,
DOI 10.17487/RFC5308, October 2008,
<https://www.rfc-editor.org/info/rfc5308>.
[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>.
[RFC6438] Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
for Equal Cost Multipath Routing and Link Aggregation in
Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011,
<https://www.rfc-editor.org/info/rfc6438>.
[RFC6554] Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6
Routing Header for Source Routes with the Routing Protocol
for Low-Power and Lossy Networks (RPL)", RFC 6554,
DOI 10.17487/RFC6554, March 2012,
<https://www.rfc-editor.org/info/rfc6554>.
[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>.
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Authors' Addresses
Stefano Previdi
Individual
Italy
Email: stefano@previdi.net
Clarence Filsfils (editor)
Cisco Systems, Inc.
Brussels
BE
Email: cfilsfil@cisco.com
John Leddy
Comcast
4100 East Dry Creek Road
Centennial, CO 80122
US
Email: John_Leddy@comcast.com
Satoru Matsushima
Softbank
Email: satoru.matsushima@g.softbank.co.jp
Daniel Voyer (editor)
Bell Canada
Email: daniel.voyer@bell.ca
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