Internet Engineering Task Force (IETF) X. Xu

Internet Engineering Task Force (IETF) X. Xu
Request for Comments: 8663 Alibaba, Inc
Category: Standards Track S. Bryant
ISSN: 2070-1721 Futurewei Technologies
A. Farrel
Old Dog Consulting
S. Hassan
Cisco
W. Henderickx
Nokia
Z. Li
Huawei
December 2019

MPLS Segment Routing over IP

Abstract

MPLS Segment Routing (SR-MPLS) is a method of source routing a packet
through an MPLS data plane by imposing a stack of MPLS labels on the
packet to specify the path together with any packet-specific
instructions to be executed on it. SR-MPLS can be leveraged to
realize a source-routing mechanism across MPLS, IPv4, and IPv6 data
planes by using an MPLS label stack as a source-routing instruction
set while making no changes to SR-MPLS specifications and
interworking with SR-MPLS implementations.

This document describes how SR-MPLS-capable routers and IP-only
routers can seamlessly coexist and interoperate through the use of
SR-MPLS label stacks and IP encapsulation/tunneling such as MPLS-
over-UDP as defined in RFC 7510.

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/rfc8663.

Copyright Notice

Copyright (c) 2019 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
2. Use Cases
3. Procedures of SR-MPLS-over-IP
3.1. Forwarding Entry Construction
3.1.1. FIB Construction Example
3.2. Packet-Forwarding Procedures
3.2.1. Packet Forwarding with Penultimate Hop Popping
3.2.2. Packet Forwarding without Penultimate Hop Popping
3.2.3. Additional Forwarding Procedures
4. IANA Considerations
5. Security Considerations
6. References
6.1. Normative References
6.2. Informative References
Acknowledgements
Contributors
Authors' Addresses

1. Introduction

MPLS Segment Routing (SR-MPLS) [RFC8660] is a method of source
routing a packet through an MPLS data plane. This is achieved by the
sender imposing a stack of MPLS labels that partially or completely
specify the path that the packet is to take and any instructions to
be executed on the packet as it passes through the network. SR-MPLS
uses an MPLS label stack to encode a sequence of source-routing
instructions. This can be used to realize a source-routing mechanism
that can operate across MPLS, IPv4, and IPv6 data planes. This
approach makes no changes to SR-MPLS specifications and allows
interworking with SR-MPLS implementations. More specifically, the
source-routing instructions in a source-routed packet could be
uniformly encoded as an MPLS label stack regardless of whether the
underlay is IPv4, IPv6 (including Segment Routing for IPv6 (SRv6)
[RFC8354]), or MPLS.

This document describes how SR-MPLS-capable routers and IP-only
routers can seamlessly coexist and interoperate through the use of
SR-MPLS label stacks and IP encapsulation/tunneling such as MPLS-
over-UDP [RFC7510].

Section 2 describes various use cases for tunneling SR-MPLS over IP.
Section 3 describes a typical application scenario and how the packet
forwarding happens.

1.1. Terminology

This memo makes use of the terms defined in [RFC3031] and [RFC8660].

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. Use Cases

Tunneling SR-MPLS using IPv4 and/or IPv6 (including SRv6) tunnels is
useful at least in the use cases listed below. In all cases, this
can be enabled using an IP tunneling mechanism such as MPLS-over-UDP
as described in [RFC7510]. The tunnel selected MUST have its remote
endpoint (destination) address equal to the address of the next node
capable of SR-MPLS identified as being on the SR path (i.e., the
egress of the active segment). The local endpoint (source) address
is set to an address of the encapsulating node. [RFC7510] gives
further advice on how to set the source address if the UDP zero-
checksum mode is used with MPLS-over-UDP. Using UDP as the
encapsulation may be particularly beneficial because it is agnostic
of the underlying transport.

* Incremental deployment of the SR-MPLS technology may be
facilitated by tunneling SR-MPLS packets across parts of a network
that are not SR-MPLS as shown in Figure 1. This demonstrates how
islands of SR-MPLS may be connected across a legacy network. It
may be particularly useful for joining sites (such as data
centers).

________________________
_______ ( ) _______
( ) ( IP Network ) ( )
( SR-MPLS ) ( ) ( SR-MPLS )
( Network ) ( ) ( Network )
( -------- -------- )
( | Border | SR-in-UDP Tunnel | Border | )
( | Router |========================| Router | )
( | R1 | | R2 | )
( -------- -------- )
( ) ( ) ( )
( ) ( ) ( )
(_______) ( ) (_______)
(________________________)

Figure 1: SR-MPLS-over-UDP to Tunnel between SR-MPLS Sites

* If the encoding of entropy [RFC6790] is desired, IP-tunneling
mechanisms that allow the encoding of entropy, such as MPLS-over-
UDP encapsulation [RFC7510] where the source port of the UDP
header is used as an entropy field, may be used to maximize the
utilization of Equal-Cost Multipath (ECMP) and/or Link Aggregation
Groups (LAGs), especially when it is difficult to make use of the
entropy-label mechanism. This is to be contrasted with [RFC4023]
where MPLS-over-IP does not provide for an entropy mechanism.
Refer to [RFC8662]) for more discussion about using entropy labels
in SR-MPLS.

* Tunneling MPLS over IP provides a technology that enables Segment
Routing (SR) in an IPv4 and/or IPv6 network where the routers do
not support SRv6 capabilities [IPv6-SRH] and where MPLS forwarding
is not an option. This is shown in Figure 2.

__________________________________
__( IP Network )__
__( )__
( -- -- -- )
-------- -- -- |SR| -- |SR| -- |SR| -- --------
| Ingress| |IR| |IR| | | |IR| | | |IR| | | |IR| | Egress|
-->| Router |===========| |======| |======| |======| Router|-->
| SR | | | | | | | | | | | | | | | | | | SR |
-------- -- -- | | -- | | -- | | -- --------
(__ -- -- -- __)
(__ __)
(__________________________________)

Key:
IR : IP-only Router
SR : SR-MPLS-capable Router
== : SR-MPLS-over-UDP Tunnel

Figure 2: SR-MPLS Enabled within an IP Network

3. Procedures of SR-MPLS-over-IP

This section describes the construction of forwarding information
base (FIB) entries and the forwarding behavior that allow the
deployment of SR-MPLS when some routers in the network are IP only
(i.e., do not support SR-MPLS). Note that the examples in Sections
3.1.1 and 3.2 assume that OSPF or IS-IS is enabled; in fact, other
mechanisms of discovery and advertisement could be used including
other routing protocols (such as BGP) or a central controller.

3.1. Forwarding Entry Construction

This subsection describes how to construct the forwarding information
base (FIB) entry on an SR-MPLS-capable router when some or all of the
next hops along the shortest path towards a prefix Segment Identifier
(Prefix-SID) are IP-only routers. Section 3.1.1 provides a concrete
example of how the process applies when using OSPF or IS-IS.

Consider router A that receives a labeled packet with top label L(E)
that corresponds to the Prefix-SID SID(E) of prefix P(E) advertised
by router E. Suppose the i-th next-hop router (termed NHi) along the
shortest path from router A toward SID(E) is not SR-MPLS capable
while both routers A and E are SR-MPLS capable. The following
processing steps apply:

* Router E is SR-MPLS capable, so it advertises a Segment Routing
Global Block (SRGB). The SRGB is defined in [RFC8402]. There are
a number of ways that the advertisement can be achieved including
IGPs, BGP, and configuration/management protocols. For example,
see [DC-GATEWAY].

* When Router E advertises the Prefix-SID SID(E) of prefix P(E), it
MUST also advertise the egress endpoint address and the
encapsulation type of any tunnel used to reach E. This
information is flooded domain wide.

* If A and E are in different routing domains, then the information
MUST be flooded into both domains. How this is achieved depends
on the advertisement mechanism being used. The objective is that
router A knows the characteristics of router E that originated the
advertisement of SID(E).

* Router A programs the FIB entry for prefix P(E) corresponding to
the SID(E) according to whether a pop or swap action is advertised
for the prefix. The resulting action may be:

- pop the top label

- swap the top label to a value equal to SID(E) plus the lower
bound of the SRGB of E

Once constructed, the FIB can be used by a router to tell it how to
process packets. It encapsulates the packets according to the
appropriate encapsulation advertised for the segment and then sends
the packets towards the next hop NHi.

3.1.1. FIB Construction Example

This section is non-normative and provides a worked example of how a
FIB might be constructed using OSPF and IS-IS extensions. It is
based on the process described in Section 3.1.

* Router E is SR-MPLS capable, so it advertises a Segment Routing
Global Block (SRGB) using [RFC8665] or [RFC8667].

* When Router E advertises the Prefix-SID SID(E) of prefix P(E), it
also advertises the encapsulation endpoint address and the tunnel
type of any tunnel used to reach E using [ISIS-ENCAP] or
[OSPF-ENCAP].

* If A and E are in different domains, then the information is
flooded into both domains and any intervening domains.

- The OSPF Tunnel Encapsulations TLV [OSPF-ENCAP] or the IS-IS
Tunnel Encapsulation Type sub-TLV [ISIS-ENCAP] is flooded
domain wide.

- The OSPF SID/Label Range TLV [RFC8665] or the IS-IS SR-
Capabilities sub-TLV [RFC8667] is advertised domain wide so
that router A knows the characteristics of router E.

- When router E advertises the prefix P(E):

o If router E is running IS-IS, it uses the extended
reachability TLV (TLVs 135, 235, 236, 237) and associates
the IPv4/IPv6 or IPv4/IPv6 Source Router ID sub-TLV(s)
[RFC7794].

o If router E is running OSPF, it uses the OSPFv2 Extended
Prefix Opaque Link-State Advertisement (LSA) [RFC7684] and
sets the flooding scope to Autonomous System (AS) wide.

- If router E is running IS-IS and advertises the IS-IS Router
CAPABILITY TLV (TLV 242) [RFC7981], it sets the "Router ID"
field to a valid value or includes an IPv6 TE Router ID sub-TLV
(TLV 12), or it does both. The "S" bit (flooding scope) of the
IS-IS Router CAPABILITY TLV (TLV 242) is set to "1".

* Router A programs the FIB entry for prefix P(E) corresponding to
the SID(E) according to whether a pop or swap action is advertised
for the prefix as follows:

- If the No-PHP (NP) Flag in OSPF or the Persistent (P) Flag in
IS-IS is clear:

pop the top label

- If the No-PHP (NP) Flag in OSPF or the Persistent (P) Flag in
IS-IS is set:

swap the top label to a value equal to SID(E) plus the lower
bound of the SRGB of E

When forwarding the packet according to the constructed FIB entry,
the router encapsulates the packet according to the encapsulation as
advertised using the mechanisms described in [ISIS-ENCAP] or
[OSPF-ENCAP]. It then sends the packets towards the next hop NHi.

Note that [RFC7510] specifies the use of port number 6635 to indicate
that the payload of a UDP packet is MPLS, and port number 6636 for
MPLS-over-UDP utilizing DTLS. However, [ISIS-ENCAP] and [OSPF-ENCAP]
provide dynamic protocol mechanisms to configure the use of any
Dynamic Port for a tunnel that uses UDP encapsulation. Nothing in
this document prevents the use of an IGP or any other mechanism to
negotiate the use of a Dynamic Port when UDP encapsulation is used
for SR-MPLS, but if no such mechanism is used, then the port numbers
specified in [RFC7510] are used.

3.2. Packet-Forwarding Procedures

[RFC7510] specifies an IP-based encapsulation for MPLS, i.e., MPLS-
over-UDP. This approach is applicable where IP-based encapsulation
for MPLS is required and further fine-grained load balancing of MPLS
packets over IP networks over ECMP and/or LAGs is also required.
This section provides details about the forwarding procedure when UDP
encapsulation is adopted for SR-MPLS-over-IP. Other encapsulation
and tunneling mechanisms can be applied using similar techniques, but
for clarity, this section uses UDP encapsulation as the exemplar.

Nodes that are SR-MPLS capable can process SR-MPLS packets. Not all
of the nodes in an SR-MPLS domain are SR-MPLS capable. Some nodes
may be "legacy routers" that cannot handle SR-MPLS packets but can
forward IP packets. A node capable of SR-MPLS MAY advertise its
capabilities using the IGP as described in Section 3. There are six
types of nodes in an SR-MPLS domain:

* Domain ingress nodes that receive packets and encapsulate them for
transmission across the domain. Those packets may be any payload
protocol including native IP packets or packets that are already
MPLS encapsulated.

* Legacy transit nodes that are IP routers but that are not SR-MPLS
capable (i.e., are not able to perform Segment Routing).

* Transit nodes that are SR-MPLS capable but that are not identified
by a SID in the SID stack.

* Transit nodes that are SR-MPLS capable and need to perform SR-MPLS
routing because they are identified by a SID in the SID stack.

* The penultimate node capable of SR-MPLS on the path that processes
the last SID on the stack on behalf of the domain egress node.

* The domain egress node that forwards the payload packet for
ultimate delivery.

3.2.1. Packet Forwarding with Penultimate Hop Popping

The description in this section assumes that the label associated
with each Prefix-SID is advertised by the owner of the Prefix-SID as
a Penultimate Hop-Popping (PHP) label. That is, if one of the IGP
flooding mechanisms is used, the NP-Flag in OSPF or the P-Flag in IS-
IS associated with the Prefix-SID is not set.

+-----+ +-----+ +-----+ +-----+ +-----+
| A +-------+ B +-------+ C +-------+ D +-------+ H |
+-----+ +--+--+ +--+--+ +--+--+ +-----+
| | |
| | |
+--+--+ +--+--+ +--+--+
| E +-------+ F +-------+ G |
+-----+ +-----+ +-----+

+--------+
|IP(A->E)|
+--------+ +--------+ +--------+
| UDP | |IP(E->G)| |IP(G->H)|
+--------+ +--------+ +--------+
| L(G) | | UDP | | UDP |
+--------+ +--------+ +--------+
| L(H) | | L(H) | |Exp Null|
+--------+ +--------+ +--------+
| Packet | ---> | Packet | ---> | Packet |
+--------+ +--------+ +--------+

Figure 3: Packet-Forwarding Example with PHP

In the example shown in Figure 3, assume that routers A, E, G, and H
are capable of SR-MPLS while the remaining routers (B, C, D, and F)
are only capable of forwarding IP packets. Routers A, E, G, and H
advertise their Segment Routing related information, such as via IS-
IS or OSPF.

Now assume that router A (the Domain ingress) wants to send a packet
to router H (the Domain egress) via the explicit path {E->G->H}.
Router A will impose an MPLS label stack on the packet that
corresponds to that explicit path. Since the next hop toward router
E is only IP capable (B is a legacy transit node), router A replaces
the top label (that indicated router E) with a UDP-based tunnel for
MPLS (i.e., MPLS-over-UDP [RFC7510]) to router E and then sends the
packet. In other words, router A pops the top label and then
encapsulates the MPLS packet in a UDP tunnel to router E.

When the IP-encapsulated MPLS packet arrives at router E (which is a
transit node capable of SR-MPLS), router E strips the IP-based tunnel
header and then processes the decapsulated MPLS packet. The top
label indicates that the packet must be forwarded toward router G.
Since the next hop toward router G is only IP capable, router E
replaces the current top label with an MPLS-over-UDP tunnel toward
router G and sends it out. That is, router E pops the top label and
then encapsulates the MPLS packet in a UDP tunnel to router G.

When the packet arrives at router G, router G will strip the IP-based
tunnel header and then process the decapsulated MPLS packet. The top
label indicates that the packet must be forwarded toward router H.
Since the next hop toward router H is only IP capable (D is a legacy
transit router), router G would replace the current top label with an
MPLS-over-UDP tunnel toward router H and send it out. However, since
router G reaches the bottom of the label stack (G is the penultimate
node capable of SR-MPLS on the path), this would leave the original
packet that router A wanted to send to router H encapsulated in UDP
as if it was MPLS (i.e., with a UDP header and destination port
indicating MPLS) even though the original packet could have been any
protocol. That is, the final SR-MPLS has been popped exposing the
payload packet.

To handle this, when a router (here it is router G) pops the final
SR-MPLS label, it inserts an explicit NULL label [RFC3032] before
encapsulating the packet in an MPLS-over-UDP tunnel toward router H
and sending it out. That is, router G pops the top label, discovers
it has reached the bottom of stack, pushes an explicit NULL label,
and then encapsulates the MPLS packet in a UDP tunnel to router H.

3.2.2. Packet Forwarding without Penultimate Hop Popping

Figure 4 demonstrates the packet walk in the case where the label
associated with each Prefix-SID advertised by the owner of the
Prefix-SID is not a Penultimate Hop-Popping (PHP) label (e.g., the
NP-Flag in OSPF or the P-Flag in IS-IS associated with the Prefix-SID
is set). Apart from the PHP function, the roles of the routers are
unchanged from Section 3.2.1.

+-----+ +-----+ +-----+ +-----+ +-----+
| A +-------+ B +-------+ C +--------+ D +--------+ H |
+-----+ +--+--+ +--+--+ +--+--+ +-----+
| | |
| | |
+--+--+ +--+--+ +--+--+
| E +-------+ F +--------+ G |
+-----+ +-----+ +-----+

+--------+
|IP(A->E)|
+--------+ +--------+
| UDP | |IP(E->G)|
+--------+ +--------+ +--------+
| L(E) | | UDP | |IP(G->H)|
+--------+ +--------+ +--------+
| L(G) | | L(G) | | UDP |
+--------+ +--------+ +--------+
| L(H) | | L(H) | | L(H) |
+--------+ +--------+ +--------+
| Packet | ---> | Packet | ---> | Packet |
+--------+ +--------+ +--------+

Figure 4: Packet-Forwarding Example without PHP

As can be seen from the figure, the SR-MPLS label for each segment is
left in place until the end of the segment where it is popped and the
next instruction is processed.

3.2.3. Additional Forwarding Procedures

Non-MPLS Interfaces: Although the description in the previous two
sections is based on the use of Prefix-SIDs, tunneling SR-MPLS
packets is useful when the top label of a received SR-MPLS packet
indicates an Adjacency SID and the corresponding adjacent node to
that Adjacency SID is not capable of MPLS forwarding but can still
process SR-MPLS packets. In this scenario, the top label would be
replaced by an IP tunnel toward that adjacent node and then
forwarded over the corresponding link indicated by the Adjacency
SID.

When to Use IP-Based Tunnels: The description in the previous two
sections is based on the assumption that an MPLS-over-UDP tunnel
is used when the next hop towards the next segment is not MPLS
enabled. However, even in the case where the next hop towards the
next segment is MPLS capable, an MPLS-over-UDP tunnel towards the
next segment could still be used instead due to local policies.
For instance, in the example as described in Figure 4, assume F is
now a transit node capable of SR-MPLS while all the other
assumptions remain unchanged; since F is not identified by a SID
in the stack and an MPLS-over-UDP tunnel is preferred to an MPLS
LSP according to local policies, router E replaces the current top
label with an MPLS-over-UDP tunnel toward router G and sends it
out. (Note that if an MPLS LSP was preferred, the packet would be
forwarded as native SR-MPLS.)

IP Header Fields: When encapsulating an MPLS packet in UDP, the
resulting packet is further encapsulated in IP for transmission.
IPv4 or IPv6 may be used according to the capabilities of the
network. The address fields are set as described in Section 2.
The other IP header fields (such as the ECN field [RFC6040], the
Differentiated Services Code Point (DSCP) [RFC2983], or IPv6 Flow
Label) on each UDP-encapsulated segment SHOULD be configurable
according to the operator's policy; they may be copied from the
header of the incoming packet; they may be promoted from the
header of the payload packet; they may be set according to
instructions programmed to be associated with the SID; or they may
be configured dependent on the outgoing interface and payload.
The TTL field setting in the encapsulating packet header is
handled as described in [RFC7510], which refers to [RFC4023].

Entropy and ECMP: When encapsulating an MPLS packet with an IP
tunnel header that is capable of encoding entropy (such as
[RFC7510]), the corresponding entropy field (the source port in
the case of a UDP tunnel) MAY be filled with an entropy value that
is generated by the encapsulator to uniquely identify a flow.
However, what constitutes a flow is locally determined by the
encapsulator. For instance, if the MPLS label stack contains at
least one entropy label and the encapsulator is capable of reading
that entropy label, the entropy label value could be directly
copied to the source port of the UDP header. Otherwise, the
encapsulator may have to perform a hash on the whole label stack
or the five-tuple of the SR-MPLS payload if the payload is
determined as an IP packet. To avoid recalculating the hash or
hunting for the entropy label each time the packet is encapsulated
in a UDP tunnel, it MAY be desirable that the entropy value
contained in the incoming packet (i.e., the UDP source port value)
is retained when stripping the UDP header and is reused as the
entropy value of the outgoing packet.

Congestion Considerations: Section 5 of [RFC7510] provides a
detailed analysis of the implications of congestion in MPLS-over-
UDP systems and builds on Section 3.1.3 of [RFC8085], which
describes the congestion implications of UDP tunnels. All of
those considerations apply to SR-MPLS-over-UDP tunnels as
described in this document. In particular, it should be noted
that the traffic carried in SR-MPLS flows is likely to be IP
traffic.

4. IANA Considerations

This document has no IANA actions.

5. Security Considerations

The security consideration of [RFC8354] (which redirects the reader
to [RFC5095]) and [RFC7510] apply. DTLS [RFC6347] SHOULD be used
where security is needed on an SR-MPLS-over-UDP segment including
when the IP segment crosses the public Internet or some other
untrusted environment. [RFC8402] provides security considerations
for Segment Routing, and Section 8.1 of [RFC8402] is particularly
applicable to SR-MPLS.

It is difficult for an attacker to pass a raw MPLS-encoded packet
into a network, and operators have considerable experience in
excluding such packets at the network boundaries, for example, by
excluding all packets that are revealed to be carrying an MPLS packet
as the payload of IP tunnels. Further discussion of MPLS security is
found in [RFC5920].

It is easy for a network ingress node to detect any attempt to
smuggle an IP packet into the network since it would see that the UDP
destination port was set to MPLS, and such filtering SHOULD be
applied. If, however, the mechanisms described in [RFC8665] or
[RFC8667] are applied, a wider variety of UDP port numbers might be
in use making port filtering harder.

SR packets not having a destination address terminating in the
network would be transparently carried and would pose no different
security risk to the network under consideration than any other
traffic.

Where control-plane techniques are used (as described in Section 3),
it is important that these protocols are adequately secured for the
environment in which they are run as discussed in [RFC6862] and
[RFC5920].

6. References

6.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031,
DOI 10.17487/RFC3031, January 2001,
<https://www.rfc-editor.org/info/rfc3031>.

[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,
<https://www.rfc-editor.org/info/rfc3032>.

[RFC4023] Worster, T., Rekhter, Y., and E. Rosen, Ed.,
"Encapsulating MPLS in IP or Generic Routing Encapsulation
(GRE)", RFC 4023, DOI 10.17487/RFC4023, March 2005,
<https://www.rfc-editor.org/info/rfc4023>.

[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>.

[RFC6040] Briscoe, B., "Tunnelling of Explicit Congestion
Notification", RFC 6040, DOI 10.17487/RFC6040, November
2010, <https://www.rfc-editor.org/info/rfc6040>.

[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.

[RFC7510] Xu, X., Sheth, N., Yong, L., Callon, R., and D. Black,
"Encapsulating MPLS in UDP", RFC 7510,
DOI 10.17487/RFC7510, April 2015,
<https://www.rfc-editor.org/info/rfc7510>.

[RFC7684] Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,
Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
Advertisement", RFC 7684, DOI 10.17487/RFC7684, November
2015, <https://www.rfc-editor.org/info/rfc7684>.

[RFC7794] Ginsberg, L., Ed., Decraene, B., Previdi, S., Xu, X., and
U. Chunduri, "IS-IS Prefix Attributes for Extended IPv4
and IPv6 Reachability", RFC 7794, DOI 10.17487/RFC7794,
March 2016, <https://www.rfc-editor.org/info/rfc7794>.

[RFC7981] Ginsberg, L., Previdi, S., and M. Chen, "IS-IS Extensions
for Advertising Router Information", RFC 7981,
DOI 10.17487/RFC7981, October 2016,
<https://www.rfc-editor.org/info/rfc7981>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[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>.

[RFC8660] Bashandy, A., Filsfils, C., Previdi, S., Decraene, B.,
Litkowski, S., and R. Shakir, "Segment Routing with the
MPLS Data Plane", RFC 8660, DOI 10.17487/RFC8660, December
2019, <https://www.rfc-editor.org/info/rfc8660>.

6.2. Informative References

[DC-GATEWAY]
Farrel, A., Drake, J., Rosen, E., Patel, K., and L. Jalil,
"Gateway Auto-Discovery and Route Advertisement for
Segment Routing Enabled Domain Interconnection", Work in
Progress, Internet-Draft, draft-ietf-bess-datacenter-
gateway-04, 21 August 2019, <https://tools.ietf.org/html/
draft-ietf-bess-datacenter-gateway-04>.

[IPv6-SRH] Filsfils, C., Dukes, D., Previdi, S., Leddy, J.,
Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
(SRH)", Work in Progress, Internet-Draft, draft-ietf-6man-
segment-routing-header-26, 22 October 2019,
<https://tools.ietf.org/html/draft-ietf-6man-segment-
routing-header-26>.

[ISIS-ENCAP]
Xu, X., Decraene, B., Raszuk, R., Chunduri, U., Contreras,
L., and L. Jalil, "Advertising Tunnelling Capability in
IS-IS", Work in Progress, Internet-Draft, draft-ietf-isis-
encapsulation-cap-01, 24 April 2017,
<https://tools.ietf.org/html/draft-ietf-isis-
encapsulation-cap-01>.

[OSPF-ENCAP]
Xu, X., Decraene, B., Raszuk, R., Contreras, L., and L.
Jalil, "The Tunnel Encapsulations OSPF Router
Information", Work in Progress, Internet-Draft, draft-
ietf-ospf-encapsulation-cap-09, 10 October 2017,
<https://tools.ietf.org/html/draft-ietf-ospf-
encapsulation-cap-09>.

[RFC2983] Black, D., "Differentiated Services and Tunnels",
RFC 2983, DOI 10.17487/RFC2983, October 2000,
<https://www.rfc-editor.org/info/rfc2983>.

[RFC5920] Fang, L., Ed., "Security Framework for MPLS and GMPLS
Networks", RFC 5920, DOI 10.17487/RFC5920, July 2010,
<https://www.rfc-editor.org/info/rfc5920>.

[RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and
L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
RFC 6790, DOI 10.17487/RFC6790, November 2012,
<https://www.rfc-editor.org/info/rfc6790>.

[RFC6862] Lebovitz, G., Bhatia, M., and B. Weis, "Keying and
Authentication for Routing Protocols (KARP) Overview,
Threats, and Requirements", RFC 6862,
DOI 10.17487/RFC6862, March 2013,
<https://www.rfc-editor.org/info/rfc6862>.

[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <https://www.rfc-editor.org/info/rfc8085>.

[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>.

[RFC8662] Kini, S., Kompella, K., Sivabalan, S., Litkowski, S.,
Shakir, R., and J. Tantsura, "Entropy Label for Source
Packet Routing in Networking (SPRING) Tunnels", RFC 8662,
DOI 10.17487/RFC8662, December 2019,
<https://www.rfc-editor.org/info/rfc8662>.

[RFC8665] Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
Extensions for Segment Routing", RFC 8665,
DOI 10.17487/RFC8665, December 2019,
<https://www.rfc-editor.org/info/rfc8665>.

[RFC8667] Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
Extensions for Segment Routing", RFC 8667,
DOI 10.17487/RFC8667, December 2019,
<https://www.rfc-editor.org/info/rfc8667>.

Acknowledgements

Thanks to Joel Halpern, Bruno Decraene, Loa Andersson, Ron Bonica,
Eric Rosen, Jim Guichard, Gunter Van De Velde, Andy Malis, Robert
Sparks, and Al Morton for their insightful comments on this document.

Additional thanks to Mirja Kuehlewind, Alvaro Retana, Spencer
Dawkins, Benjamin Kaduk, Martin Vigoureux, Suresh Krishnan, and Eric
Vyncke for careful reviews and resulting comments.

Contributors

Ahmed Bashandy
Individual
Email: [email protected]

Clarence Filsfils
Cisco
Email: [email protected]

John Drake
Juniper
Email: [email protected]

Shaowen Ma
Mellanox Technologies
Email: [email protected]

Mach Chen
Huawei
Email: [email protected]

Hamid Assarpour
Broadcom
Email:[email protected]

Robert Raszuk
Bloomberg LP
Email: [email protected]

Uma Chunduri
Huawei
Email: [email protected]

Luis M. Contreras
Telefonica I+D
Email: [email protected]

Luay Jalil
Verizon
Email: [email protected]

Gunter Van De Velde
Nokia
Email: [email protected]

Tal Mizrahi
Marvell
Email: [email protected]

Jeff Tantsura
Apstra, Inc.
Email: [email protected]

Authors' Addresses

Xiaohu Xu
Alibaba, Inc

Email: [email protected]

Stewart Bryant
Futurewei Technologies

Email: [email protected]

Adrian Farrel
Old Dog Consulting

Email: [email protected]

Syed Hassan
Cisco

Email: [email protected]

Wim Henderickx
Nokia

Email: [email protected]

Zhenbin Li
Huawei

Email: [email protected]