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Keywords: SRv6, source-routing, network-programming

Internet Engineering Task Force (IETF) C. Filsfils, Ed.
Request for Comments: 8754 D. Dukes, Ed.
Category: Standards Track Cisco Systems, Inc.
ISSN: 2070-1721 S. Previdi
Huawei
J. Leddy
Individual
S. Matsushima
SoftBank
D. Voyer
Bell Canada
March 2020
IPv6 Segment Routing Header (SRH)
Abstract
Segment Routing can be applied to the IPv6 data plane using a new
type of Routing Extension Header called the Segment Routing Header
(SRH). This document describes the SRH and how it is used by nodes
that are Segment Routing (SR) capable.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8754.
Copyright Notice
Copyright (c) 2020 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. Introduction
1.1. Terminology
1.2. Requirements Language
2. Segment Routing Header
2.1. SRH TLVs
2.1.1. Padding TLVs
2.1.2. HMAC TLV
3. SR Nodes
3.1. SR Source Node
3.2. Transit Node
3.3. SR Segment Endpoint Node
4. Packet Processing
4.1. SR Source Node
4.1.1. Reduced SRH
4.2. Transit Node
4.3. SR Segment Endpoint Node
4.3.1. FIB Entry Is a Locally Instantiated SRv6 SID
4.3.2. FIB Entry Is a Local Interface
4.3.3. FIB Entry Is a Nonlocal Route
4.3.4. FIB Entry Is a No Match
5. Intra-SR-Domain Deployment Model
5.1. Securing the SR Domain
5.2. SR Domain as a Single System with Delegation among
Components
5.3. MTU Considerations
5.4. ICMP Error Processing
5.5. Load Balancing and ECMP
5.6. Other Deployments
6. Illustrations
6.1. Abstract Representation of an SRH
6.2. Example Topology
6.3. SR Source Node
6.3.1. Intra-SR-Domain Packet
6.3.2. Inter-SR-Domain Packet -- Transit
6.3.3. Inter-SR-Domain Packet -- Internal to External
6.4. Transit Node
6.5. SR Segment Endpoint Node
6.6. Delegation of Function with HMAC Verification
6.6.1. SID List Verification
7. Security Considerations
7.1. SR Attacks
7.2. Service Theft
7.3. Topology Disclosure
7.4. ICMP Generation
7.5. Applicability of AH
8. IANA Considerations
8.1. Segment Routing Header Flags Registry
8.2. Segment Routing Header TLVs Registry
9. References
9.1. Normative References
9.2. Informative References
Acknowledgements
Contributors
Authors' Addresses
1. Introduction
Segment Routing (SR) can be applied to the IPv6 data plane using a
new type of routing header called the Segment Routing Header (SRH).
This document describes the SRH and how it is used by nodes that are
SR capable.
"Segment Routing Architecture" [RFC8402] describes Segment Routing
and its instantiation in two data planes: MPLS and IPv6.
The encoding of IPv6 segments in the SRH is defined in this document.
1.1. Terminology
This document uses the terms Segment Routing (SR), SR domain, SR over
IPv6 (SRv6), Segment Identifier (SID), SRv6 SID, Active Segment, and
SR Policy as defined in [RFC8402].
1.2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Segment Routing Header
Routing headers are defined in [RFC8200]. The Segment Routing Header
(SRH) has a new Routing Type (4).
The SRH is defined as follows:
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-bit IPv6 address) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
...
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Segment List[n] (128-bit IPv6 address) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// //
// Optional Type Length Value objects (variable) //
// //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Next Header: Defined in [RFC8200], Section 4.4.
Hdr Ext Len: Defined in [RFC8200], Section 4.4.
Routing Type: 4.
Segments Left: Defined in [RFC8200], Section 4.4.
Last Entry: contains the index (zero based), in the Segment List, of
the last element of the Segment List.
Flags: 8 bits of flags. Section 8.1 creates an IANA registry for
new flags to be defined. The following flags are defined:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|U U U U U U U U|
+-+-+-+-+-+-+-+-+
U: Unused and for future use. MUST be 0 on transmission and
ignored on receipt.
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 the source, it MUST be set to zero on transmission. When Tag
is not used during SRH processing, it SHOULD be ignored. Tag is
not used when processing the SID defined in Section 4.3.1. It may
be used when processing other SIDs that are not defined in this
document. The allocation and use of tag is outside the scope of
this document.
Segment List[0..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 SR Policy. That is, the first
element of the Segment List (Segment List[0]) contains the last
segment of the SR Policy, the second element contains the
penultimate segment of the SR Policy, and so on.
TLV: Type Length Value (TLV) is described in Section 2.1.
In the SRH, the Next Header, Hdr Ext Len, Routing Type, and Segments
Left fields are defined in Section 4.4 of [RFC8200]. Based on the
constraints in that section, Next Header, Header Ext Len, and Routing
Type are not mutable while Segments Left is mutable.
The mutability of the TLV value is defined by the most significant
bit in the type, as specified in Section 2.1.
Section 4.3 defines the mutability of the remaining fields in the SRH
(Flags, Tag, Segment List) in the context of the SID defined in this
document.
New SIDs defined in the future MUST specify the mutability properties
of the Flags, Tag, and Segment List and indicate how the Hashed
Message Authentication Code (HMAC) TLV (Section 2.1.2) verification
works. Note that, in effect, these fields are mutable.
Consistent with the SR model, the source of the SRH always knows how
to set the Segment List, Flags, Tag, and TLVs of the SRH for use
within the SR domain. How it achieves this is outside the scope of
this document but may be based on topology, available SIDs and their
mutability properties, the SRH mutability requirements of the
destination, or any other information.
2.1. SRH TLVs
This section defines TLVs of the Segment Routing Header.
A TLV provides metadata for segment processing. The only TLVs
defined in this document are the HMAC (Section 2.1.2) and padding
TLVs (Section 2.1.1). While processing the SID defined in
Section 4.3.1, all TLVs are ignored unless local configuration
indicates otherwise (Section 4.3.1.1.1). Thus, TLV and HMAC support
is optional for any implementation; however, an implementation adding
or parsing TLVs MUST support PAD TLVs. Other documents may define
additional TLVs and processing rules for them.
TLVs are present when the Hdr Ext Len is greater than (Last
Entry+1)*2.
While processing TLVs at a segment endpoint, TLVs MUST be fully
contained within the SRH as determined by the Hdr Ext Len. Detection
of TLVs exceeding the boundary of the SRH Hdr Ext Len results in an
ICMP Parameter Problem, Code 0, message to the Source Address,
pointing to the Hdr Ext Len field of the SRH, and the packet being
discarded.
An implementation MAY limit the number and/or length of TLVs it
processes based on local configuration. It MAY limit:
* the number of consecutive Pad1 (Section 2.1.1.1) options to 1. If
padding of more than one byte is required, then PadN
(Section 2.1.1.2) should be used.
* The length in PadN to 5.
* The maximum number of non-Pad TLVs to be processed.
* The maximum length of all TLVs to be processed.
The implementation MAY stop processing additional TLVs in the SRH
when these configured limits are exceeded.
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 codepoint from the "Segment Routing Header TLVs"
[IANA-SRHTLV]. Unrecognized Types MUST be ignored on receipt.
Length: The length of the variable-length data field in bytes.
Variable-length data: data that is specific to the Type.
Type Length Value (TLV) entries contain OPTIONAL information that may
be used by the node identified in the Destination Address (DA) of the
packet.
Each TLV has its own length, format, and semantic. The codepoint
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.
The highest-order bit of the TLV type (bit 0) specifies whether or
not the TLV data of that type can change en route to the packet's
final destination:
0: TLV data does not change en route
1: TLV data does change en route
All TLVs specify their alignment requirements using an xn+y format.
The xn+y format is defined as per [RFC8200]. The SR source nodes use
the xn+y alignment requirements of TLVs and Padding TLVs when
constructing an SRH.
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 TLVs
HMAC TLV
Additional TLVs may be defined in the future.
2.1.1. Padding TLVs
There are two types of Padding TLVs, Pad1 and PadN, and the following
applies to both:
Padding TLVs are used for meeting the alignment requirement of the
subsequent TLVs.
Padding TLVs are used to pad the SRH to a multiple of 8 octets.
Padding TLVs are ignored by a node processing the SRH TLV.
Multiple Padding TLVs MAY be used in one SRH.
2.1.1.1. Pad1
Alignment requirement: none
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Type |
+-+-+-+-+-+-+-+-+
Type: 0
A single Pad1 TLV MUST be used when a single byte of padding is
required. A Pad1 TLV MUST NOT be used if more than one consecutive
byte of padding is required.
2.1.1.2. PadN
Alignment requirement: none
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: 4
Length: 0 to 5. The length of the Padding field in bytes.
Padding: Padding bits have no semantic. They MUST be set to 0 on
transmission and ignored on receipt.
The PadN TLV MUST be used when more than one byte of padding is
required.
2.1.2. HMAC TLV
Alignment requirement: 8n
The keyed Hashed Message Authentication Code (HMAC) TLV is OPTIONAL
and has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |D| RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HMAC Key ID (4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| //
| HMAC (variable) //
| //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 5.
Length: The length of the variable-length data in bytes.
D: 1 bit. 1 indicates that the Destination Address verification is
disabled due to use of a reduced Segment List (see Section 4.1.1).
RESERVED: 15 bits. MUST be 0 on transmission.
HMAC Key ID: A 4-octet opaque number that uniquely identifies the
pre-shared key and algorithm used to generate the HMAC.
HMAC: Keyed HMAC, in multiples of 8 octets, at most 32 octets.
The HMAC TLV is used to verify that the SRH applied to a packet was
selected by an authorized party and to ensure that the segment list
is not modified after generation. This also allows for verification
that the current segment (by virtue of being in the authorized
Segment List) is authorized for use. The SR domain ensures that the
source node is permitted to use the source address in the packet via
ingress filtering mechanisms as defined in BCP 84 [RFC3704] or other
strategies as appropriate.
2.1.2.1. HMAC Generation and Verification
Local configuration determines when to check for an HMAC. This local
configuration is outside the scope of this document. It may be based
on the active segment at an SR Segment endpoint node, the result of
an Access Control List (ACL) that considers incoming interface, HMAC
Key ID, or other packet fields.
An implementation that supports the generation and verification of
the HMAC supports the following default behavior, as defined in the
remainder of this section.
The HMAC verification begins by checking that the current segment is
equal to the destination address of the IPv6 header. The check is
successful when either:
* HMAC D bit is 1 and Segments Left is greater than Last Entry, or
* HMAC Segments Left is less than or equal to Last Entry, and the
destination address is equal to Segment List[Segments Left].
The HMAC field is the output of the HMAC computation as defined in
[RFC2104], using:
* key: The pre-shared key identified by HMAC Key ID
* HMAC algorithm: Identified by the HMAC Key ID
* Text: A concatenation of the following fields from the IPv6 header
and the SRH, as it would be received at the node verifying the
HMAC:
- IPv6 header: Source address (16 octets)
- SRH: Last Entry (1 octet)
- SRH: Flags (1 octet)
- SRH: HMAC 16 bits following Length
- SRH: HMAC Key ID (4 octets)
- SRH: All addresses in the Segment List (variable octets)
The HMAC digest is truncated to 32 octets and placed in the HMAC
field of the HMAC TLV.
For HMAC algorithms producing digests less than 32 octets long, the
digest is placed in the lowest-order octets of the HMAC field.
Subsequent octets MUST be set to zero such that the HMAC length is a
multiple of 8 octets.
If HMAC verification is successful, processing proceeds as normal.
If HMAC verification fails, an ICMP error message (parameter problem,
error code 0, pointing to the HMAC TLV) SHOULD be generated (but rate
limited) and logged, and the packet SHOULD be discarded.
2.1.2.2. HMAC Pre-shared Key Algorithm
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 identifier of the right combination of pre-
shared key and hash algorithm.
At the HMAC TLV generating and verification nodes, the Key ID
uniquely identifies the pre-shared key and HMAC algorithm.
At the HMAC TLV generating node, the Text for the HMAC computation is
set to the IPv6 header fields and SRH fields as they would appear at
the verification node(s), not necessarily the same as the source node
sending a packet with the HMAC TLV.
Pre-Shared key rollover is supported by having two key IDs in use
while the HMAC TLV generating node and verifying node converge to a
new key.
The HMAC TLV generating node may need to revoke an SRH for which it
previously generated an HMAC. Revocation is achieved by allocating a
new key and key ID, then rolling over the key ID associated with the
SRH to be revoked. The HMAC TLV verifying node drops packets with
the revoked SRH.
An implementation supporting HMAC can support multiple hash
functions. An implementation supporting HMAC MUST implement SHA-2
[FIPS180-4] in its SHA-256 variant.
The selection of pre-shared key and algorithm and their distribution
is outside the scope of this document. Some options may include:
* setting these items in the configuration of the HMAC generating or
verifying nodes, either by static configuration or any SDN-
oriented approach
* dynamically using a trusted key distribution protocol such as
[RFC6407]
While key management is outside the scope of this document, the
recommendations of BCP 107 [RFC4107] should be considered when
choosing the key management system.
3. SR Nodes
There are different types of nodes that may be involved in segment
routing networks: SR source nodes that originate packets with a
segment in the destination address of the IPv6 header, transit nodes
that forward packets destined to a remote segment, and SR segment
endpoint nodes that process a local segment in the destination
address of an IPv6 header.
3.1. SR Source Node
A SR source node is any node that originates an IPv6 packet with a
segment (i.e., SRv6 SID) in the destination address of the IPv6
header. The packet leaving the SR source node may or may not contain
an SRH. This includes either:
* A host originating an IPv6 packet, or
* An SR domain ingress router encapsulating a received packet in an
outer IPv6 header, followed by an optional SRH.
It is out of the scope of this document to describe the mechanism
through which a segment in the destination address of the IPv6 header
and the Segment List in the SRH are derived.
3.2. Transit Node
A transit node is any node forwarding an IPv6 packet where the
destination address of that packet is not locally configured as a
segment or a local interface. A transit node is not required to be
capable of processing a segment or SRH.
3.3. SR Segment Endpoint Node
An SR segment endpoint node is any node receiving an IPv6 packet
where the destination address of that packet is locally configured as
a segment or local interface.
4. Packet Processing
This section describes SRv6 packet processing at the SR source,
Transit, and SR segment endpoint nodes.
4.1. SR Source Node
A source node steers a packet into an SR Policy. If the SR Policy
results in a Segment List containing a single segment, and there is
no need to add information to the SRH flag or add TLV; the DA is set
to the single Segment List entry, and the SRH MAY be omitted.
When needed, the SRH is created as follows:
The Next Header and Hdr Ext Len fields are set as specified in
[RFC8200].
The Routing Type field is set to 4.
The DA of the packet is set with the value of the first segment.
The first element of the SRH Segment List is the ultimate segment.
The second element is the penultimate segment, and so on.
The Segments Left field is set to n-1, where n is the number of
elements in the SR Policy.
The Last Entry field is set to n-1, where n is the number of
elements in the SR Policy.
TLVs (including HMAC) may be set according to their specification.
The packet is forwarded toward the packet's Destination Address
(the first segment).
4.1.1. Reduced SRH
When a source does not require the entire SID list to be preserved in
the SRH, a reduced SRH may be used.
A reduced SRH does not contain the first segment of the related SR
Policy (the first segment is the one already in the DA of the IPv6
header), and the Last Entry field is set to n-2, where n is the
number of elements in the SR Policy.
4.2. Transit Node
As specified in [RFC8200], the only node 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
toward the DA according to its IPv6 routing table.
When a SID is in the destination address of an IPv6 header of a
packet, it's routed through an IPv6 network as an IPv6 address.
SIDs, or the prefix(es) covering SIDs, 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 SIDs on a node.
4.3. SR Segment Endpoint Node
Without constraining the details of an implementation, the SR segment
endpoint node creates Forwarding Information Base (FIB) entries for
its local SIDs.
When an SRv6-capable node receives an IPv6 packet, it performs a
longest-prefix-match lookup on the packet's destination address.
This lookup can return any of the following:
* A FIB entry that represents a locally instantiated SRv6 SID
* A FIB entry that represents a local interface, not locally
instantiated as an SRv6 SID
* A FIB entry that represents a nonlocal route
* No Match
4.3.1. FIB Entry Is a Locally Instantiated SRv6 SID
This document and section define a single SRv6 SID. Future documents
may define additional SRv6 SIDs. In such a case, the entire content
of this section will be defined in that document.
If the FIB entry represents a locally instantiated SRv6 SID, process
the next header chain of the IPv6 header as defined in Section 4 of
[RFC8200]. Section 4.3.1.1 describes how to process an SRH;
Section 4.3.1.2 describes how to process an upper-layer header or the
absence of a Next Header.
Processing this SID modifies the Segments Left and, if configured to
process TLVs, it may modify the "variable-length data" of TLV types
that change en route. Therefore, Segments Left is mutable, and TLVs
that change en route are mutable. The remainder of the SRH (Flags,
Tag, Segment List, and TLVs that do not change en route) are
immutable while processing this SID.
4.3.1.1. SRH Processing
S01. When an SRH is processed {
S02. If Segments Left is equal to zero {
S03. Proceed to process the next header in the packet,
whose type is identified by the Next Header field in
the routing header.
S04. }
S05. Else {
S06. If local configuration requires TLV processing {
S07. Perform TLV processing (see TLV Processing)
S08. }
S09. max_last_entry = ( Hdr Ext Len / 2 ) - 1
S10. If ((Last Entry > max_last_entry) or
S11. (Segments Left is greater than (Last Entry+1)) {
S12. Send an ICMP Parameter Problem, Code 0, message to
the Source Address, pointing to the Segments Left
field, and discard the packet.
S13. }
S14. Else {
S15. Decrement Segments Left by 1.
S16. Copy Segment List[Segments Left] from the SRH to the
destination address of the IPv6 header.
S17. If the IPv6 Hop Limit is less than or equal to 1 {
S18. Send an ICMP Time Exceeded -- Hop Limit Exceeded in
Transit message to the Source Address and discard
the packet.
S19. }
S20. Else {
S21. Decrement the Hop Limit by 1
S22. Resubmit the packet to the IPv6 module for transmission
to the new destination.
S23. }
S24. }
S25. }
S26. }
4.3.1.1.1. TLV Processing
Local configuration determines how TLVs are to be processed when the
Active Segment is a local SID defined in this document. The
definition of local configuration is outside the scope of this
document.
For illustration purposes only, two example local configurations that
may be associated with a SID are provided below.
Example 1:
For any packet received from interface I2
Skip TLV processing
Example 2:
For any packet received from interface I1
If first TLV is HMAC {
Process the HMAC TLV
}
Else {
Discard the packet
}
4.3.1.2. Upper-Layer Header or No Next Header
When processing the upper-layer header of a packet matching a FIB
entry locally instantiated as an SRv6 SID defined in this document:
IF (Upper-layer Header is IPv4 or IPv6) and
local configuration permits {
Perform IPv6 decapsulation
Resubmit the decapsulated packet to the IPv4 or IPv6 module
}
ELSE {
Send an ICMP parameter problem message to the Source Address and
discard the packet. Error code (4) "SR Upper-layer
Header Error", pointer set to the offset of the upper-layer
header.
}
A unique error code allows an SR source node to recognize an error in
SID processing at an endpoint.
4.3.2. FIB Entry Is a Local Interface
If the FIB entry represents a local interface and is not locally
instantiated as an SRv6 SID, the SRH is processed as follows:
If Segments Left is zero, the node must ignore the routing header
and proceed to process the next header in the packet, whose type
is identified by the Next Header field in the routing header.
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.
4.3.3. FIB Entry Is a Nonlocal Route
Processing is not changed by this document.
4.3.4. FIB Entry Is a No Match
Processing is not changed by this document.
5. Intra-SR-Domain Deployment Model
The use of the SIDs exclusively within the SR domain and solely for
packets of the SR domain is an important deployment model.
This enables the SR domain to act as a single routing system.
This section covers:
* securing the SR domain from external attempts to use its SIDs
* using the SR domain as a single system with delegation between
components
* handling packets of the SR domain
5.1. Securing the SR Domain
Nodes outside the SR domain are not trusted: they cannot directly use
the SIDs of the domain. This is enforced by two levels of access
control lists:
1. Any packet entering the SR domain and destined to a SID within
the SR domain is dropped. This may be realized with the
following logic. Other methods with equivalent outcome are
considered compliant:
* Allocate all the SIDs from a block S/s
* 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
* Failure to implement this method of ingress filtering exposes
the SR domain to source-routing attacks, as described and
referenced in [RFC5095]
2. The distributed protection in #1 is complemented with per-node
protection, dropping packets to SIDs from source addresses
outside the SR domain. This may be realized with the following
logic. Other methods with equivalent outcome are considered
compliant:
* Assign all interface addresses from 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.
5.2. SR Domain as a Single System with Delegation among Components
All intra-SR-domain packets are of the SR domain. The IPv6 header is
originated by a node of the SR domain and is destined to a node of
the SR domain.
All interdomain packets are encapsulated for the part of the packet
journey that is within the SR domain. The outer IPv6 header is
originated by a node of the SR domain and is destined to a node of
the SR domain.
As a consequence, any packet within the SR domain is of the SR
domain.
The SR domain is a system in which the operator may want to
distribute or delegate different operations of the outermost header
to different nodes within the system.
An operator of an SR domain may choose to delegate SRH addition to a
host node within the SR domain and delegate validation of the
contents of any SRH to a more trusted router or switch attached to
the host. Consider a top-of-rack switch T connected to host H via
interface I. H receives an SRH (SRH1) with a computed HMAC via some
SDN method outside the scope of this document. H classifies traffic
it sources and adds SRH1 to traffic requiring a specific Service
Level Agreement (SLA). T is configured with an IACL on I requiring
verification of the SRH for any packet destined to the SID block of
the SR domain (S/s). T checks and verifies that SRH1 is valid and
contains an HMAC TLV; T then verifies the HMAC.
An operator of the SR domain may choose to have all segments in the
SR domain verify the HMAC. This mechanism would verify that the SRH
Segment List is not modified while traversing the SR domain.
5.3. MTU Considerations
An SR domain ingress edge node encapsulates packets traversing the SR
domain and needs to consider the MTU of the SR domain. Within the SR
domain, well-known mitigation techniques are RECOMMENDED, such as
deploying a greater MTU value within the SR domain than at the
ingress edges.
Encapsulation with an outer IPv6 header and SRH shares the same MTU
and fragmentation considerations as IPv6 tunnels described in
[RFC2473]. Further investigation on the limitation of various
tunneling methods (including IPv6 tunnels) is discussed in
[INTAREA-TUNNELS] and SHOULD be considered by operators when
considering MTU within the SR domain.
5.4. ICMP Error Processing
ICMP error packets generated within the SR domain are sent to source
nodes within the SR domain. The invoking packet in the ICMP error
message may contain an SRH. Since the destination address of a
packet with an SRH changes as each segment is processed, it may not
be the destination used by the socket or application that generated
the invoking packet.
For the source of an invoking packet to process the ICMP error
message, the ultimate destination address of the IPv6 header may be
required. The following logic is used to determine the destination
address for use by protocol-error handlers.
* Walk all extension headers of the invoking IPv6 packet to the
routing extension header preceding the upper-layer header.
- If routing header is type 4 Segment Routing Header (SRH)
o The SID at Segment List[0] may be used as the destination
address of the invoking packet.
ICMP errors are then processed by upper-layer transports as defined
in [RFC4443].
For IP packets encapsulated in an outer IPv6 header, ICMP error
handling is as defined in [RFC2473].
5.5. Load Balancing and ECMP
For any interdomain packet, 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 Tunnel End
Point.
For any intradomain packet, the SR source node SHOULD impose a flow
label computed as described in [RFC6437] to assist ECMP load
balancing at transit nodes incapable of computing a 5-tuple beyond
the SRH.
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. If a flow label is not used, the transit node
would likely hash all packets between a pair of SR Edge nodes to the
same link.
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.
5.6. Other Deployments
Other deployment models and their implications on security, MTU,
HMAC, ICMP error processing, and interaction with other extension
headers are outside the scope of this document.
6. Illustrations
This section provides illustrations of SRv6 packet processing at SR
source, transit, and SR segment endpoint nodes.
6.1. Abstract Representation of an SRH
For a node k, its IPv6 address is represented as Ak, and its SRv6 SID
is represented as Sk.
IPv6 headers are represented as the tuple of (source,destination).
For example, a packet with source address A1 and destination address
A2 is represented as (A1,A2). The payload of the packet is omitted.
An SR Policy is a list of segments. A list of segments is
represented as <S1,S2,S3> where S1 is the first SID to visit, S2 is
the second SID to visit, and S3 is the last SID to visit.
(SA,DA) (S3,S2,S1; SL) represents an IPv6 packet with:
* Source Address SA, Destination Addresses DA, and next header SRH.
* SRH with SID list <S1,S2,S3> with SegmentsLeft = SL.
* Note the difference between the <> and () symbols. <S1,S2,S3>
represents a SID list where the leftmost segment is the first
segment. In contrast, (S3,S2,S1; SL) represents the same SID list
but encoded in the SRH Segment List format where the leftmost
segment is the last segment. When referring to an SR Policy in a
high-level use case, it is simpler to use the <S1,S2,S3> notation.
When referring to an illustration of detailed behavior, the
(S3,S2,S1; SL) notation is more convenient.
At its SR Policy headend, the Segment List <S1,S2,S3> results in SRH
(S3,S2,S1; SL=2) represented fully as:
Segments Left=2
Last Entry=2
Flags=0
Tag=0
Segment List[0]=S3
Segment List[1]=S2
Segment List[2]=S1
6.2. Example Topology
The following topology is used in examples below:
+ * * * * * * * * * * * * * * * * * * * * +
* [8] [9] *
| |
* | | *
[1]----[3]--------[5]----------------[6]---------[4]---[2]
* | | *
| |
* | | *
+--------[7]-------+
* *
+ * * * * * * * SR domain * * * * * * * +
Figure 1
* 3 and 4 are SR domain edge routers
* 5, 6, and 7 are all SR domain routers
* 8 and 9 are hosts within the SR domain
* 1 and 2 are hosts outside the SR domain
* The SR domain implements ingress filtering as per Section 5.1 and
no external packet can enter the domain with a destination address
equal to a segment of the domain.
6.3. SR Source Node
6.3.1. Intra-SR-Domain Packet
When host 8 sends a packet to host 9 via an SR Policy <S7,A9> the
packet is
P1: (A8,S7)(A9,S7; SL=1)
6.3.1.1. Reduced Variant
When host 8 sends a packet to host 9 via an SR Policy <S7,A9> and it
wants to use a reduced SRH, the packet is
P2: (A8,S7)(A9; SL=1)
6.3.2. Inter-SR-Domain Packet -- Transit
When host 1 sends a packet to host 2, the packet is
P3: (A1,A2)
The SR domain ingress router 3 receives P3 and steers it to SR domain
egress router 4 via an SR Policy <S7,S4>. Router 3 encapsulates the
received packet P3 in an outer header with an SRH. The packet is
P4: (A3,S7)(S4,S7; SL=1)(A1,A2)
If the SR Policy contains only one segment (the egress router 4), the
ingress router 3 encapsulates P3 into an outer header (A3,S4) without
SRH. The packet is
P5: (A3,S4)(A1,A2)
6.3.2.1. Reduced Variant
The SR domain ingress router 3 receives P3 and steers it to SR domain
egress router 4 via an SR Policy <S7,S4>. If router 3 wants to use a
reduced SRH, it encapsulates the received packet P3 in an outer
header with a reduced SRH. The packet is
P6: (A3,S7)(S4; SL=1)(A1,A2)
6.3.3. Inter-SR-Domain Packet -- Internal to External
When host 8 sends a packet to host 1, the packet is encapsulated for
the portion of its journey within the SR domain. From 8 to 3 the
packet is
P7: (A8,S3)(A8,A1)
In the opposite direction, the packet generated from 1 to 8 is
P8: (A1,A8)
At node 3, P8 is encapsulated for the portion of its journey within
the SR domain, with the outer header destined to segment S8. This
results in
P9: (A3,S8)(A1,A8)
At node 8, the outer IPv6 header is removed by S8 processing, then
processed again when received by A8.
6.4. Transit Node
Node 5 acts as transit node for packet P1 and sends packet
P1: (A8,S7)(A9,S7;SL=1)
on the interface toward node 7.
6.5. SR Segment Endpoint Node
Node 7 receives packet P1 and, using the logic in Section 4.3.1,
sends packet
P7: (A8,A9)(A9,S7; SL=0)
on the interface toward router 6.
6.6. Delegation of Function with HMAC Verification
This section describes how a function may be delegated within the SR
domain. In the following sections, consider a host 8 connected to a
top of rack 5.
6.6.1. SID List Verification
An operator may prefer to apply the SRH at source 8, while 5 verifies
that the SID list is valid.
For illustration purposes, an SDN controller provides 8 an SRH
terminating at node 9, with Segment List <S5,S7,S6,A9>, and HMAC TLV
computed for the SRH. The HMAC key ID and key associated with the
HMAC TLV is shared with 5. Node 8 does not know the key. Node 5 is
configured with an IACL applied to the interface connected to 8,
requiring HMAC verification for any packet destined to S/s.
Node 8 originates packets with the received SRH, including HMAC TLV.
P15: (A8,S5)(A9,S6,S7,S5;SL=3;HMAC)
Node 5 receives and verifies the HMAC for the SRH, then forwards the
packet to the next segment
P16: (A8,S7)(A9,S6,S7,S5;SL=2;HMAC)
Node 6 receives
P17: (A8,S6)(A9,S6,S7,S5;SL=1;HMAC)
Node 9 receives
P18: (A8,A9)(A9,S6,S7,S5;SL=0;HMAC)
This use of an HMAC is particularly valuable within an enterprise-
based SR domain [SRN].
7. Security Considerations
This section reviews security considerations related to the SRH,
given the SRH processing and deployment models discussed in this
document.
As described in Section 5, it is necessary to filter packets' ingress
to the SR domain, destined to SIDs within the SR domain (i.e.,
bearing a SID in the destination address). This ingress filtering is
via an IACL at SR domain ingress border nodes. Additional protection
is applied via an IACL at each SR Segment Endpoint node, filtering
packets not from within the SR domain, destined to SIDs in the SR
domain. ACLs are easily supported for small numbers of seldom
changing prefixes, making summarization important.
Additionally, ingress filtering of IPv6 source addresses as
recommended in BCP 38 [RFC2827] SHOULD be used.
7.1. SR Attacks
An SR domain implements distributed and per-node protection as
described in Section 5.1. Additionally, domains deny traffic with
spoofed addresses by implementing the recommendations in BCP 84
[RFC3704].
Full implementation of the recommended protection blocks the attacks
documented in [RFC5095] from outside the SR domain, including
bypassing filtering devices, reaching otherwise-unreachable Internet
systems, network topology discovery, bandwidth exhaustion, and
defeating anycast.
Failure to implement distributed and per-node protection allows
attackers to bypass filtering devices and exposes the SR domain to
these attacks.
Compromised nodes within the SR domain may mount the attacks listed
above along with other known attacks on IP networks (e.g., DoS/DDoS,
topology discovery, man-in-the-middle, traffic interception/
siphoning).
7.2. Service Theft
Service theft is defined as the use of a service offered by the SR
domain by a node not authorized to use the service.
Service theft is not a concern within the SR domain, as all SR source
nodes and SR segment endpoint nodes within the domain are able to
utilize the services of the domain. If a node outside the SR domain
learns of segments or a topological service within the SR domain,
IACL filtering denies access to those segments.
7.3. Topology Disclosure
The SRH is unencrypted and may contain SIDs of some intermediate SR
nodes in the path towards the destination within the SR domain. If
packets can be snooped within the SR domain, the SRH may reveal
topology, traffic flows, and service usage.
This is applicable within an SR domain, but the disclosure is less
relevant as an attacker has other means of learning topology, flows,
and service usage.
7.4. ICMP Generation
The generation of ICMPv6 error messages may be used to attempt
denial-of-service attacks by sending an error-causing destination
address or SRH in back-to-back packets. An implementation that
correctly follows Section 2.4 of [RFC4443] would be protected by the
ICMPv6 rate-limiting mechanism.
7.5. Applicability of AH
The SR domain is a trusted domain, as defined in [RFC8402], Sections
2 and 8.2. The SR source is trusted to add an SRH (optionally
verified as having been generated by a trusted source via the HMAC
TLV in this document), and segments advertised within the domain are
trusted to be accurate and advertised by trusted sources via a secure
control plane. As such, the SR domain does not rely on the
Authentication Header (AH) as defined in [RFC4302] to secure the SRH.
The use of SRH with AH by an SR source node and its processing at an
SR segment endpoint node are not defined in this document. Future
documents may define use of SRH with AH and its processing.
8. IANA Considerations
This document makes the following registrations in the "Internet
Protocol Version 6 (IPv6) Parameters" "Routing Types" subregistry
maintained by IANA:
+-------+------------------------------+---------------+
| Value | Description | Reference |
+=======+==============================+===============+
| 4 | Segment Routing Header (SRH) | This document |
+-------+------------------------------+---------------+
Table 1: SRH Registration
This document makes the following registrations in the "Type 4 -
Parameter Problem" message of the "Internet Control Message Protocol
version 6 (ICMPv6) Parameters" registry maintained by IANA:
+------+-----------------------------+
| Code | Name |
+======+=============================+
| 4 | SR Upper-layer Header Error |
+------+-----------------------------+
Table 2: SR Upper-layer Header
Error Registration
8.1. Segment Routing Header Flags Registry
This document describes a new IANA-managed registry to identify SRH
Flags Bits. The registration procedure is "IETF Review" [RFC8126].
The registry name is "Segment Routing Header Flags". Flags are 8
bits.
8.2. Segment Routing Header TLVs Registry
This document describes a new IANA-managed registry to identify SRH
TLVs. The registration procedure is "IETF Review". The registry
name is "Segment Routing Header TLVs". A TLV is identified through
an unsigned 8-bit codepoint value, with assigned values 0-127 for
TLVs that do not change en route and 128-255 for TLVs that may change
en route. The following codepoints are defined in this document:
+---------+--------------------------+---------------+
| Value | Description | Reference |
+=========+==========================+===============+
| 0 | Pad1 TLV | This document |
+---------+--------------------------+---------------+
| 1 | Reserved | This document |
+---------+--------------------------+---------------+
| 2 | Reserved | This document |
+---------+--------------------------+---------------+
| 3 | Reserved | This document |
+---------+--------------------------+---------------+
| 4 | PadN TLV | This document |
+---------+--------------------------+---------------+
| 5 | HMAC TLV | This document |
+---------+--------------------------+---------------+
| 6 | Reserved | This document |
+---------+--------------------------+---------------+
| 124-126 | Experimentation and Test | This document |
+---------+--------------------------+---------------+
| 127 | Reserved | This document |
+---------+--------------------------+---------------+
| 252-254 | Experimentation and Test | This document |
+---------+--------------------------+---------------+
| 255 | Reserved | This document |
+---------+--------------------------+---------------+
Table 3: Segment Routing Header TLVs Registry
Values 1, 2, 3, and 6 were defined in draft versions of this
specification and are Reserved for backwards compatibility with early
implementations and should not be reassigned. Values 127 and 255 are
Reserved to allow for expansion of the Type field in future
specifications, if needed.
9. References
9.1. Normative References
[FIPS180-4]
National Institute of Standards and Technology (NIST),
"Secure Hash Standard (SHS)", FIPS PUB 180-4, DOI 10.6028/
NIST.FIPS.180-4, August 2015,
<http://csrc.nist.gov/publications/fips/fips180-4/fips-
180-4.pdf>.
[IANA-SRHTLV]
IANA, "Segment Routing Header TLVs",
<https://www.iana.org/assignments/ipv6-parameters/>.
[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>.
[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>.
[RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in
IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473,
December 1998, <https://www.rfc-editor.org/info/rfc2473>.
[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>.
[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>.
[RFC4107] Bellovin, S. and R. Housley, "Guidelines for Cryptographic
Key Management", BCP 107, RFC 4107, DOI 10.17487/RFC4107,
June 2005, <https://www.rfc-editor.org/info/rfc4107>.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
DOI 10.17487/RFC4302, December 2005,
<https://www.rfc-editor.org/info/rfc4302>.
[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>.
[RFC6437] Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
"IPv6 Flow Label Specification", RFC 6437,
DOI 10.17487/RFC6437, November 2011,
<https://www.rfc-editor.org/info/rfc6437>.
[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>.
[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>.
[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>.
[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>.
9.2. Informative References
[INTAREA-TUNNELS]
Touch, J. and M. Townsley, "IP Tunnels in the Internet
Architecture", Work in Progress, Internet-Draft, draft-
ietf-intarea-tunnels-10, 12 September 2019,
<https://tools.ietf.org/html/draft-ietf-intarea-tunnels-
10>.
[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>.
[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>.
[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>.
[SRN] Lebrun, D., Jadin, M., Clad, F., Filsfils, C., and O.
Bonaventure, "Software Resolved Networks: Rethinking
Enterprise Networks with IPv6 Segment Routing", 2018,
<https://inl.info.ucl.ac.be/system/files/
sosr18-final15-embedfonts.pdf>.
Acknowledgements
The authors would like to thank Ole Troan, Bob Hinden, Ron Bonica,
Fred Baker, Brian Carpenter, Alexandru Petrescu, Punit Kumar Jaiswal,
David Lebrun, Benjamin Kaduk, Frank Xialiang, Mirja Kühlewind, Roman
Danyliw, Joe Touch, and Magnus Westerlund for their comments to this
document.
Contributors
Kamran Raza, Zafar Ali, Brian Field, Daniel Bernier, Ida Leung, Jen
Linkova, Ebben Aries, Tomoya Kosugi, Éric Vyncke, David Lebrun, Dirk
Steinberg, Robert Raszuk, Dave Barach, John Brzozowski, Pierre
Francois, Nagendra Kumar, Mark Townsley, Christian Martin, Roberta
Maglione, James Connolly, and Aloys Augustin contributed to the
content of this document.
Authors' Addresses
Clarence Filsfils (editor)
Cisco Systems, Inc.
Brussels
Belgium
Email: cfilsfil@cisco.com
Darren Dukes (editor)
Cisco Systems, Inc.
Ottawa
Canada
Email: ddukes@cisco.com
Stefano Previdi
Huawei
Italy
Email: stefano@previdi.net
John Leddy
Individual
United States of America
Email: john@leddy.net
Satoru Matsushima
SoftBank
Email: satoru.matsushima@g.softbank.co.jp
Daniel Voyer
Bell Canada
Email: daniel.voyer@bell.ca