Internet Engineering Task Force (IETF) A. Rodriguez-Natal

Internet Engineering Task Force (IETF) A. Rodriguez-Natal
Request for Comments: 9437 Cisco
Category: Standards Track V. Ermagan
ISSN: 2070-1721 Google
A. Cabellos
UPC/BarcelonaTech
S. Barkai
Nexar
M. Boucadair
Orange
August 2023

Publish/Subscribe Functionality for the Locator/ID Separation Protocol
(LISP)

Abstract

This document specifies an extension to the Locator/ID Separation
Protocol (LISP) control plane to enable Publish/Subscribe (PubSub)
operation.

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/rfc9437.

Copyright Notice

Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents

1. Introduction
1.1. Scope of Applicability
2. Terminology and Requirements Notation
3. Deployment Requirements
4. Map-Request PubSub Additions
5. Mapping Request Subscribe Procedures
6. Mapping Notification Publish Procedures
7. Security Considerations
7.1. Security Association between ITR and Map-Server
7.2. DDoS Attack Mitigation
8. IANA Considerations
9. References
9.1. Normative References
9.2. Informative References
Appendix A. Sample PubSub Deployment Experiences
A.1. PubSub as a Monitoring Tool
A.2. Mitigating Negative Map-Cache Entries
A.3. Improved Mobility Latency
A.4. Enhanced Reachability with Dynamic Redistribution of
Prefixes
A.5. Better Serviceability
Acknowledgments
Contributors
Authors' Addresses

1. Introduction

The Locator/ID Separation Protocol (LISP) [RFC9300] [RFC9301] splits
IP addresses into two different namespaces: Endpoint Identifiers
(EIDs) and Routing Locators (RLOCs). LISP uses a map and encapsulate
(a.k.a., map-and-encap) approach that relies on (1) a Mapping System
(basically a distributed database) that stores and disseminates EID-
RLOC mappings and on (2) LISP Tunnel Routers (xTRs) that encapsulate
and decapsulate data packets based on the content of those mappings.

Ingress Tunnel Routers (ITRs), Re-encapsulating Tunnel Routers
(RTRs), and Proxy Ingress Tunnel Routers (PITRs) pull EID-to-RLOC
mapping information from the Mapping System by means of an explicit
request message. Section 6.1 of [RFC9301] indicates how Egress
Tunnel Routers (ETRs) can tell ITRs/RTRs/PITRs about mapping changes.
This document presents a Publish/Subscribe (PubSub) extension in
which the Mapping System can notify ITRs/RTRs/PITRs about mapping
changes. When this mechanism is used, mapping changes can be
notified faster and can be managed in the Mapping System versus the
LISP sites.

In general, when an ITR/RTR/PITR wants to be notified for mapping
changes for a given EID-Prefix, the following main steps occur:

1. The ITR/RTR/PITR builds a Map-Request for that EID-Prefix with
the Notification-Requested bit (N-bit) set and that also includes
its xTR-ID and Site-ID.

2. The Map-Request is forwarded to one of the Map-Servers that the
EID-Prefix is registered to.

3. The Map-Server creates subscription state for the ITR/RTR/PITR on
the EID-Prefix.

4. The Map-Server sends a Map-Notify to the ITR/RTR/PITR to confirm
that the subscription has been created and then waits for an
acknowledgement of the notification.

5. The ITR/RTR/PITR sends back a Map-Notify-Ack to acknowledge the
successful receipt of the Map-Notify.

6. When there is a change in the mapping of the EID-Prefix, the Map-
Server sends a Map-Notify message to each ITR/RTR/PITR in the
subscription list.

7. Each ITR/RTR/PITR sends a Map-Notify-Ack to acknowledge the
received Map-Notify.

This operation is repeated for all EID-Prefixes for which ITRs/RTRs/
PITRs want to be notified. An ITR/RTR/PITR can set the N-bit for
several EID-Prefixes within a single Map-Request. Please note that
the steps above illustrate only the simplest scenario and that
details for this and other scenarios are described later in the
document.

The reader may refer to [FLOW-EXAMPLES] for sample flows to
illustrate the use of the PubSub specification.

1.1. Scope of Applicability

The PubSub procedure specified in this document is intended for use
in contexts with controlled access to the Map-Server. How a
deployment controls access to a Map-Server is deployment specific and
therefore out of the scope of this document. However, the Map-
Resolvers and Map-Servers need to be configured with the required
information to ensure at least the following:

1. Map-Resolvers MUST verify that an xTR is allowed to (1) set the
N-bit to 1 and (2) use the xTR-ID, Site-ID, and ITR-RLOCs
included in a Map-Request.

2. Map-Servers MUST only accept subscription requests from Map-
Resolvers that verify Map-Requests as previously described.

2. Terminology and Requirements Notation

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.

The document uses the terms defined in Section 3 of [RFC9300].

3. Deployment Requirements

In addition to the general assumptions and expectations that
[RFC9301] makes for LISP deployments, this document imposes the
following deployment requirements:

1. A unique 128-bit xTR-ID (plus a 64-bit Site-ID) identifier is
assigned to each xTR.

2. Map-Servers are configured to proxy Map-Replying (i.e., they are
solicited to generate and send Map-Reply messages) for the
mappings they are serving.

3. A security association (e.g., a PubSubKey) is required between
the ITRs and the Map-Servers (see Section 7.1).

If a requirement is not met, a subscription cannot be established,
and the network will continue operating without this enhancement.
The configuration of xTR-IDs and Site-IDs is out of the scope of this
document. The reader may refer to [LISP-YANG] for an example of how
these identifiers can be provisioned to LISP nodes.

4. Map-Request PubSub Additions

Figure 1 shows the format of the updated Map-Request to support the
PubSub functionality. In particular, this document associates a
meaning with one of the reserved bits (see Section 8).

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=1 |A|M|P|S|p|s|R|I| Rsvd |L|D| IRC | Record Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Nonce . . . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . . . Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source-EID-AFI | Source EID Address ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ITR-RLOC-AFI 1 | ITR-RLOC Address 1 ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ITR-RLOC-AFI n | ITR-RLOC Address n ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ |N| Reserved | EID mask-len | EID-Prefix-AFI |
Rec +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ | EID-Prefix ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Map-Reply Record ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ xTR-ID +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Site-ID +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1: Map-Request with I-bit, N-bit, xTR-ID, and Site-ID

The following is added to the Map-Request message defined in
Section 5.2 of [RFC9301]:

xTR-ID bit (I-bit): This bit is set to 1 to indicate that 128-bit
xTR-ID and 64-bit Site-ID fields are present in the Map-Request
message. For PubSub operation, an xTR MUST be configured with an
xTR-ID and Site-ID, and it MUST set the I-bit to 1 and include its
xTR-ID and Site-ID in the Map-Request messages it generates. If
the I-bit is set but the Site-ID and/or xTR-ID are not included, a
receiver can detect the error because, after processing that last
EID-Record, there are no bytes left from processing the message.
In this case, the receiver SHOULD log a malformed Map-Request and
MUST drop the message.

Notification-Requested bit (N-bit): The N-bit of an EID-Record is
set to 1 to specify that the xTR wants to be notified of updates
for that EID-Prefix.

xTR-ID field: If the I-bit is set, this field is added to the Map-
Request message as shown in Figure 1, starting right after the
final Record in the message (or the Map-Reply Record, if present).
The xTR-ID is specified in Section 5.6 of [RFC9301].

Site-ID field: If the I-bit is set, this field is added to the Map-
Request message as shown in Figure 1, following the xTR-ID field.
The Site-ID is defined in Section 5.6 of [RFC9301].

5. Mapping Request Subscribe Procedures

The xTR subscribes for changes to a given EID-Prefix by sending a
Map-Request to the Mapping System with the N-bit set on the EID-
Record. The xTR builds a Map-Request according to Section 5.3 of
[RFC9301] and also does the following:

1. The xTR MUST set the I-bit to 1 and append its xTR-ID and Site-ID
to the Map-Request.

2. The xTR MUST set the N-bit to 1 for the EID-Record to which the
xTR wants to subscribe.

3. If the xTR has a nonce associated with the EID-Prefix, it MUST
use this nonce increased by one in the Map-Request. Otherwise,
it generates a nonce as described in Section 5.2 of [RFC9301].
It is RECOMMENDED that the xTR use persistent storage to keep
nonce state. If the xTR does not have persistent storage and
does not have a nonce associated with the EID-Prefix, it MUST
reset the nonce by using the procedure described in Section 7.1
to successfully create a new security association with the Map-
Server.

The Map-Request is forwarded to the appropriate Map-Server through
the Mapping System. This document does not assume that a Map-Server
is pre-assigned to handle the subscription state for a given xTR.
The Map-Server that receives the Map-Request will be the Map-Server
responsible for notifying that specific xTR about future mapping
changes for the subscribed mapping records.

Upon receipt of the Map-Request, the Map-Server processes it as
described in Section 8.3 of [RFC9301]. In addition, unless the xTR
is using the procedure described in Section 7.1 to create a new
security association, the Map-Server MUST verify that the nonce in
the Map-Request is greater than the stored nonce (if any) associated
with the xTR-ID (and EID-Prefix, when applicable). Otherwise, the
Map-Server MUST silently drop the Map-Request message and SHOULD log
the event to record that a replay attack could have occurred.
Furthermore, upon processing, for the EID-Record that has the N-bit
set to 1, the Map-Server proceeds to add the xTR-ID contained in the
Map-Request to the list of xTRs that have requested to be subscribed
to that EID-Prefix.

If an xTR-ID is successfully added to the list of subscribers for an
EID-Prefix, the Map-Server MUST extract the nonce and ITR-RLOCs
present in the Map-Request and store the association between the EID-
Prefix, xTR-ID, ITR-RLOCs, and nonce. Any state that is already
present regarding ITR-RLOCs and/or nonce for the same xTR-ID MUST be
overwritten. When the LISP deployment has a single Map-Server, the
Map-Server can be configured to keep a single nonce per xTR-ID for
all EID-Prefixes (when used, this option MUST be enabled at the Map-
Server and all xTRs).

If the xTR-ID is added to the list, the Map-Server MUST send a Map-
Notify message back to the xTR to acknowledge the successful
subscription. The Map-Server builds the Map-Notify according to
Sections 5.5 and 5.7 of [RFC9301] with the following considerations:

1. The Map-Server MUST use the nonce from the Map-Request as the
nonce for the Map-Notify.

2. The Map-Server MUST use its security association with the xTR
(Section 7.1) to sign the authentication data of the Map-Notify.
The xTR MUST use the security association to verify the received
authentication data.

3. The Map-Server MUST send the Map-Notify to one of the ITR-RLOCs
received in the Map-Request (which one is implementation
specific).

As a reminder, the initial transmission and retransmission of Map-
Notify messages by a Map-Server follow the procedure specified in
Section 5.7 of [RFC9301]. Some state changes may trigger an overload
that would impact, e.g., the outbound capacity of a Map-Server. A
similar problem may be experienced when a large number of state
entries are simultaneously updated. To prevent such phenomena, Map-
Servers SHOULD be configured with policies to control the maximum
number of subscriptions and also the pace of Map-Notify messages.
For example, the Map-Server may be instructed to limit the resources
that are dedicated to unsolicited Map-Notify messages to a small
fraction (e.g., less than 10%) of its overall processing and
forwarding capacity. The exact details to characterize such policies
are deployment and implementation specific. Likewise, this document
does not specify which notifications take precedence when these
policies are enforced.

When the xTR receives a Map-Notify with a nonce that matches one in
the list of outstanding Map-Request messages sent with an N-bit set,
it knows that the Map-Notify is to acknowledge a successful
subscription. The xTR processes this Map-Notify, as described in
Section 5.7 of [RFC9301] and MUST use the Map-Notify to populate its
Map-Cache with the returned EID-Prefix and RLOC-set. As a reminder,
following Section 5.7 of [RFC9301], the xTR has to send a Map-Notify-
Ack back to the Map-Server. If the Map-Server does not receive the
Map-Notify-Ack after exhausting the Map-Notify retransmissions
described in Section 5.7 of [RFC9301], the Map-Server can remove the
subscription state. If the Map-Server removes the subscription
state, and absent explicit policy, it SHOULD notify the xTR by
sending a single Map-Notify with the same nonce but with Loc-Count =
0 (and Loc-AFI = 0) and ACT bits set to 5 "Drop/Auth-Failure". It is
OPTIONAL for the xTR to update its Map-Cache entry for the EID-Prefix
(if any) based on this Map-Notify. This message is specifically
useful for cases where Map-Notifies are successfully received by an
xTR, but the corresponding Map-Notify-Acks are lost when forwarded to
the Map-Server. xTR implementations can use this signal to try to
reinstall their subscription state instead of maintaining stale
mappings.

The subscription of an xTR-ID may fail for a number of reasons. For
example, it fails because of local configuration policies (such as
accept and drop lists of subscribers), because the Map-Server has
exhausted the resources to dedicate to the subscription of that EID-
Prefix (e.g., the number of subscribers excess the capacity of the
Map-Server), or because the xTR was not successful tried but was not
successful in establishing a new security association (Section 7.1).

If the subscription request fails, the Map-Server sends a Map-Reply
to the originator of the Map-Request, as described in Section 8.3 of
[RFC9301], with the following considerations:

* If the subscription request fails due to policy (e.g., for
explicitly configured subscriptions, as described later in this
section), the Map-Server MUST respond to the Map-Request with a
Negative Map-Reply (Loc-Count = 0 and Loc-AFI = 0) with ACT bits
set to 4 "Drop/Policy-Denied".

* If the subscription request fails due to authentication (e.g.,
when a new security association is being established, as described
in Section 7.1), the Map-Server MUST respond to the Map-Request
with a Negative Map-Reply (Loc-Count = 0 and Loc-AFI = 0) with ACT
bits set to 5 "Drop/Auth-Failure".

* If the subscription request fails due to any other reason, the
Map-Server MUST follow Section 8.3 of [RFC9301] with no changes.

The xTR processes any Map-Reply or Negative Map-Reply as specified in
Section 8.1 of [RFC9301], with the following considerations: if the
xTR receives a Negative Map-Reply with ACT bits set to 4 "Drop/
Policy-Denied" or 5 "Drop/Auth-Failure" as a response to a
subscription request, it is OPTIONAL for the xTR to update its Map-
Cache entry for the EID-Prefix (if any). If the subscription request
fails (for whichever reason), it is up to the implementation of the
xTR to try to subscribe again.

If the Map-Server receives a subscription request for an EID-Prefix
not present in the mapping database, it SHOULD follow the same logic
described in Section 8.4 of [RFC9301] and create a temporary
subscription state for the xTR-ID to the least specific prefix that
both matches the original query and does not match any EID-Prefix
known to exist in the LISP-capable infrastructure. Alternatively,
the Map-Server can determine that such a subscription request fails
and send a Negative Map-Reply following Section 8.3 of [RFC9301]. In
both cases, the TTL of the temporary subscription state or the
Negative Map-Reply SHOULD be configurable, with a value of 15 minutes
being RECOMMENDED.

The subscription state can also be created explicitly by
configuration at the Map-Server (possible when a pre-shared security
association exists, see Section 7) using a variety of means that are
outside the scope of this document. If there is no nonce that can be
used for the explicit subscription state at the time the explicit
subscription is configured (e.g., from a different subscription
already established with the same xTR when a single nonce is kept per
xTR-ID), then both the xTR and Map-Server MUST be configured with the
initial nonce. RECOMMENDED to have a configuration option to enable
(or disable) the xTR to accept publication information for EID-
Prefixes that the xTR did not explicitly subscribe to. By default,
the xTR is allowed to modify explicitly configured subscription state
following the procedures described in this section; however, this MAY
be disabled at the Map-Server via configuration. If the Map-Server
is instructed to not allow xTRs to modify explicitly configured
subscriptions, and an xTR tries to do so, this triggers a Negative
Map-Reply with ACT bits set to 4 "Drop/Policy-Denied" as described
earlier in this section.

The following specifies the procedure to remove a subscription:

* If a valid Map-Request with the N-bit set to 1 only has one ITR-
RLOC with AFI = 0 (i.e., Unknown Address), the Map-Server MUST
remove the subscription state for that xTR-ID (unless this is
disabled via configuration, see previous paragraph).

* If the subscription state is removed, the Map-Server MUST send a
Map-Notify to the source RLOC of the Map-Request.

* If the subscription removal fails due to configuration, this
triggers a Negative Map-Reply with ACT bits set to 4 "Drop/Policy-
Denied" as described earlier in this section; the Map-Server sends
the Negative Map-Reply to the source RLOC of the Map-Request in
this case.

* Removing subscription state at the Map-Server can lead to replay
attacks. To soften this, the Map-Server SHOULD keep the last
nonce seen per xTR-ID (and EID-Prefix, when applicable).

* If the Map-Server does not keep the last nonces seen, then the
Map-Server MUST require the xTRs to subscribe using the procedure
described in Section 7.1 to create a new security association with
the Map-Server.

If the Map-Server receives a Map-Request asking to remove a
subscription for an EID-Prefix without subscription state for that
xTR-ID and the EID-Prefix is covered by a less-specific EID-Prefix
for which subscription state exists for the xTR-ID, the Map-Server
SHOULD stop publishing updates about this more-specific EID-Prefix to
that xTR until the xTR subscribes to the more-specific EID-Prefix.
The same considerations regarding authentication, integrity
protection, and nonce checks, which are described in this section and
Section 7 for Map-Requests used to update subscription state, apply
for Map-Requests used to remove subscription state.

When an EID-Prefix is removed from the Map-Server (either when
explicitly withdrawn or when its TTL expires), the Map-Server
notifies its subscribers (if any) via a Map-Notify with TTL equal to
0.

6. Mapping Notification Publish Procedures

The publish procedure is implemented via Map-Notify messages that the
Map-Server sends to xTRs. The xTRs acknowledge the receipt of Map-
Notifies by sending Map-Notify-Ack messages back to the Map-Server.
The complete mechanism works as follows:

When a mapping stored in a Map-Server is updated (e.g., via a Map-
Register from an ETR), the Map-Server MUST notify the subscribers of
that mapping via sending Map-Notify messages with the most up to date
mapping information. If subscription state in the Map-Server exists
for a less-specific EID-Prefix and a more-specific EID-Prefix is
updated, then the Map-Notify is sent with the more-specific EID-
Prefix mapping to the subscribers of the less-specific EID-Prefix
mapping. The Map-Notify message sent to each of the subscribers as a
result of an update event follows the encoding and logic defined in
Section 5.7 of [RFC9301] for Map-Notify, except for the following:

1. The Map-Notify MUST be sent to one of the ITR-RLOCs associated
with the xTR-ID of the subscriber (which one is implementation
specific).

2. The Map-Server increments the nonce by one every time it sends a
Map-Notify as publication to an xTR-ID for a particular EID-
Prefix.

3. The Map-Server MUST use its security association with the xTR to
compute the authentication data of the Map-Notify.

When the xTR receives a Map-Notify with an EID that is not local to
the xTR, the xTR knows that the Map-Notify is to update an entry on
its Map-Cache. The xTR MUST keep track of the last nonce seen in a
Map-Notify received as a publication from the Map-Server for the EID-
Prefix. When the LISP deployment has a single Map-Server, the xTR
can be configured to keep track of a single nonce for all EID-
Prefixes (when used, this option MUST be enabled at the Map-Server
and all xTRs). If a Map-Notify that is received as a publication has
a nonce value that is not greater than the saved nonce, the xTR drops
the Map-Notify message and logs the fact a replay attack could have
occurred. The same considerations discussed in Section 5.6 of
[RFC9301] regarding Map-Register nonces apply here for Map-Notify
nonces.

The xTR processes the received Map-Notify as specified in Section 5.7
of [RFC9301], with the following considerations:

* The xTR MUST use its security association with the Map-Server
(Section 7.1) to validate the authentication data on the Map-
Notify.

* The xTR MUST use the mapping information carried in the Map-Notify
to update its internal Map-Cache.

* If after following Section 5.7 of [RFC9301] regarding
retransmission of Map-Notify messages, the Map-Server has not
received the Map-Notify-Ack, it can try sending the Map-Notify to
a different ITR-RLOC for that xTR-ID.

* If the Map-Server tries all the ITR-RLOCs without receiving a
response, it may stop trying to send the Map-Notify.

7. Security Considerations

Generic security considerations related to LISP control messages are
discussed in Section 9 of [RFC9301].

In the particular case of PubSub, cache poisoning via malicious Map-
Notify messages is avoided by the use of nonce and the security
association between the ITRs and the Map-Servers.

It is RECOMMENDED to follow guidance from the last paragraph of
Section 9 of [RFC9301] to ensure integrity protection of Map-Request
messages (e.g., to prevent xTR-ID hijacking).

7.1. Security Association between ITR and Map-Server

Since Map-Notifies from the Map-Server to the ITR need to be
authenticated, there is a need for a soft-state or hard-state
security association (e.g., a PubSubKey) between the ITRs and the
Map-Servers. For some controlled deployments, it might be possible
to have a shared PubSubKey (or set of keys) between the ITRs and the
Map-Servers. However, if pre-shared keys are not used in the
deployment, LISP Security (LISP-SEC) [RFC9303] can be used as follows
to create a security association between the ITR and the Map-Server.

First, when the ITR is sending a Map-Request with the N-bit set as
described in Section 5, the ITR also performs the steps described in
Section 6.4 of [RFC9303]. The ITR can then generate a PubSubKey by
deriving a key from the One-Time Key (OTK) and Map-Request's nonce as
follows: PubSubKey = KDF(OTK + nonce), where KDF is the Key
Derivation Function indicated by the OTK Wrapping ID. If the OTK
Wrapping ID equals NULL-KEY-WRAP-128, then the PubSubKey is the OTK.
Note that, as opposed to the pre-shared PubSubKey, this generated
PubSubKey is different per EID-Prefix to which an ITR subscribes
(since the ITR will use a different OTK per Map-Request).

When the Map-Server receives the Map-Request, it follows the
procedure specified in Section 5 with the following considerations:
the Map-Server MUST verify that the OTK has not been used before. If
the Map-Server verifies the OTK and cannot determine that the OTK has
not been used before, the subscription request fails due to
authentication, which triggers a Negative Map-Reply with ACT bits set
to 5 "Drop/Auth-Failure", as described in Section 5. The xTR might
try again with a different OTK upon receipt of this Negative Map-
Reply. Note that a Map-Server implementation may decide not to keep
track of all past OTKs and instead use some form of hash. In that
case, hash collisions are handled as if the OTK has been reused.
Such an implementation needs to balance the hash length with the rate
of collisions expected for the particular deployment; this is
implementation specific. If the Map-Server has to reply with a Map-
Reply for any other reason (e.g., if PubSub is not supported or a
subscription is not accepted), then it follows the normal LISP-SEC
procedure described in Section 5.7 of [RFC9303]. No PubSubKey,
security association, or subscription state is created when the Map-
Server responds with any Map-Reply message.

Otherwise, if the Map-Server has to reply with a Map-Notify (e.g.,
due to the subscription being accepted) to a received Map-Request,
the following extra steps take place:

* The Map-Server extracts the OTK and the OTK Wrapping ID from the
LISP-SEC Encapsulated Control Message (ECM) Authentication Data.

* The Map-Server generates a PubSubKey by deriving a key from the
OTK, as described before for the ITR. This is the same PubSubKey
derived at the ITR that is used to establish a security
association between the ITR and the Map-Server.

* The PubSubKey can now be used to sign and authenticate any Map-
Notify between the Map-Server and the ITR for the subscribed EID-
Prefix. This includes the Map-Notify sent as a confirmation to
the subscription. When the ITR wants to update the security
association for that Map-Server and EID-Prefix, it once again
follows the procedure described in this section.

Note that if the Map-Server replies with a Map-Notify, none of the
regular LISP-SEC steps regarding Map-Reply described in Section 5.7
of [RFC9303] occur.

7.2. DDoS Attack Mitigation

If PubSub is deployed under the scope of applicability defined in
Section 1.1, only known nodes can participate on the PubSub
deployment. DDoS attacks based on replayed messages by unknown nodes
are avoided by the use of nonce and the security association between
the ITRs and the Map-Servers. Misbehaving known nodes may send
massive subscription requests, which may lead to exhausting the
resources of a Map-Server. Furthermore, frequently changing the
state of a subscription may also be considered as an attack vector.
To mitigate such issues, Section 5.3 of [RFC9301] discusses rate-
limiting Map-Requests, and Section 5.7 of [RFC9301] discusses rate-
limiting Map-Notifies. Note that when the Map-Notify rate-limit
threshold is met for a particular xTR-ID, the Map-Server will discard
additional subscription requests from that xTR-ID and will fall back
to the behavior described in [RFC9301] when receiving a Map-Request
from that xTR-ID (i.e., the Map-Server will send a Map-Reply).

8. IANA Considerations

IANA has assigned the following new bit from the "LISP Control Plane
Header Bits: Map-Request" registry within the "Locator/ID Separation
Protocol (LISP) Parameters" group of registries [IANA-LISP]:

+===========+===============+==========+=============+===========+
| Spec Name | IANA Name | Bit | Description | Reference |
| | | Position | | |
+===========+===============+==========+=============+===========+
| I | Map-Request-I | 11 | xTR-ID Bit | RFC 9437 |
+-----------+---------------+----------+-------------+-----------+

Table 1: Addition to the Map-Request Header Bits Registry

IANA has also created a new registry entitled "LISP Control Plane
Header Bits: Map-Request-Record" within the "Locator/ID Separation
Protocol (LISP) Parameters" group of registries [IANA-LISP].

The initial content of this registry is shown in Table 2.

+====+===============+========+========================+===========+
|Spec| IANA Name |Bit | Description | Reference |
|Name| |Position| | |
+====+===============+========+========================+===========+
|N | Map-Request-N |1 | Notification-Requested | RFC 9437 |
| | | | Bit | |
+----+---------------+--------+------------------------+-----------+

Table 2: Initial Content of LISP Control Plane Header Bits:
Map-Request-Record Registry

The remaining bits (i.e., bit positions 2-8) are Unassigned.

The policy for allocating new bits in this registry is "Specification
Required" (Section 4.6 of [RFC8126]).

Allocation requests are evaluated on the advice of one or more
designated experts. Designated experts should consider whether the
proposed registration duplicates existing entries and whether the
registration description is sufficiently detailed and fits the
purpose of this registry. These criteria are to be considered in
addition to those provided in Section 4.6 of [RFC8126] (e.g., the
proposed registration "must be documented in a permanent and readily
available public specification"). The designated experts will either
approve or deny the registration request, and communicate their
decision to IANA. Denials should include an explanation and, if
applicable, suggestions as to how to make the request successful.

9. References

9.1. 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>.

[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>.

[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>.

[RFC9300] Farinacci, D., Fuller, V., Meyer, D., Lewis, D., and A.
Cabellos, Ed., "The Locator/ID Separation Protocol
(LISP)", RFC 9300, DOI 10.17487/RFC9300, October 2022,
<https://www.rfc-editor.org/info/rfc9300>.

[RFC9301] Farinacci, D., Maino, F., Fuller, V., and A. Cabellos,
Ed., "Locator/ID Separation Protocol (LISP) Control
Plane", RFC 9301, DOI 10.17487/RFC9301, October 2022,
<https://www.rfc-editor.org/info/rfc9301>.

[RFC9303] Maino, F., Ermagan, V., Cabellos, A., and D. Saucez,
"Locator/ID Separation Protocol Security (LISP-SEC)",
RFC 9303, DOI 10.17487/RFC9303, October 2022,
<https://www.rfc-editor.org/info/rfc9303>.

9.2. Informative References

[EID-MOBILITY]
Portoles, M., Ashtaputre, V., Maino, F., Moreno, V., and
D. Farinacci, "LISP L2/L3 EID Mobility Using a Unified
Control Plane", Work in Progress, Internet-Draft, draft-
ietf-lisp-eid-mobility-12, 4 July 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-lisp-
eid-mobility-12>.

[FLOW-EXAMPLES]
Boucadair, M., "LISP PubSub Flow Examples", Work in
Progress, Internet-Draft, draft-boucadair-lisp-pubsub-
flow-examples-03, 10 February 2023,
<https://datatracker.ietf.org/doc/html/draft-boucadair-
lisp-pubsub-flow-examples-03>.

[GB-ATN] Haindl, B., Lindner, M., Moreno, V., Portoles, M., Maino,
F., and B. Venkatachalapathy, "Ground-Based LISP for the
Aeronautical Telecommunications Network", Work in
Progress, Internet-Draft, draft-haindl-lisp-gb-atn-09, 27
March 2023, <https://datatracker.ietf.org/doc/html/draft-
haindl-lisp-gb-atn-09>.

[IANA-LISP]
IANA, "Locator/ID Separation Protocol (LISP) Parameters",
<https://www.iana.org/assignments/lisp-parameters/>.

[LISP-YANG]
Ermagan, V., Rodriguez-Natal, A., Coras, F., Moberg, C.,
Rahman, R., Cabellos, A., and F. Maino, "LISP YANG Model",
Work in Progress, Internet-Draft, draft-ietf-lisp-yang-19,
2 March 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-lisp-yang-19>.

[RFC6835] Farinacci, D. and D. Meyer, "The Locator/ID Separation
Protocol Internet Groper (LIG)", RFC 6835,
DOI 10.17487/RFC6835, January 2013,
<https://www.rfc-editor.org/info/rfc6835>.

[UBERLAY] Moreno, V., Farinacci, D., Rodriguez-Natal, A., Portoles-
Comeras, M., Maino, F., and S. Hooda, "Uberlay
Interconnection of Multiple LISP overlays", Work in
Progress, Internet-Draft, draft-moreno-lisp-uberlay-06, 28
September 2022, <https://datatracker.ietf.org/doc/html/
draft-moreno-lisp-uberlay-06>.

Appendix A. Sample PubSub Deployment Experiences

Some LISP production networks have been running different forms of
PubSub for some time. The following subsections provide an inventory
of some experience lessons from these deployments.

A.1. PubSub as a Monitoring Tool

Some LISP deployments are using PubSub as a way to monitor EID-
Prefixes (particularly, EID-to-RLOC mappings). To that aim, some
LISP implementations have extended the LISP Internet Groper ('lig')
[RFC6835] tool to use PubSub. Such an extension is meant to support
an interactive mode with 'lig' and to request subscription for the
EID of interest. If there are RLOC changes, the Map-Server sends a
notification, and then the 'lig' client displays that change to the
user.

A.2. Mitigating Negative Map-Cache Entries

Section 8.1 of [RFC9301] suggests two TTL values for Negative Map-
Replies: either a 15-minute TTL (if the EID-Prefix does not exist) or
a 1-minute TTL (if the prefix exists but has not been registered).
While these values are based on the original operational experience
of the LISP protocol designers, negative cache entries have two
unintended effects that were observed in production.

First, if the xTR keeps receiving traffic for a negative EID
destination (i.e., an EID-Prefix with no RLOCs associated with it),
it will try to resolve the destination again once the cached state
expires, even if the state has not changed in the Map-Server. It was
observed in production that this is happening often in networks that
have a significant amount of traffic addressed for outside of the
LISP network. This might result in excessive resolution signaling to
keep retrieving the same state due to the cache expiring. PubSub is
used to relax TTL values and cache negative mapping entries for
longer periods of time, avoiding unnecessary refreshes of these
forwarding entries and drastically reducing signaling in these
scenarios. In general, a TTL-based schema is a "polling mechanism"
that leads to more signaling where PubSub provides an "event-
triggered mechanism" at the cost of state.

Second, if the state does indeed change in the Map-Server, updates
based on TTL timeouts might prevent the cached state at the xTR from
being updated until the TTL expires. This behavior was observed
during configuration (or reconfiguration) periods on the network,
where EID-Prefixes that are no longer negative do not receive the
traffic yet, due to stale Map-Cache entries present in the network.
With the activation of PubSub, stale caches can be updated as soon as
the state changes.

A.3. Improved Mobility Latency

An improved convergence time was observed on the presence of mobility
events on LISP networks running PubSub as compared with running LISP
[RFC9301]. As described in Section 4.1.2.1 of [EID-MOBILITY], LISP
can rely on data-driven Solicit-Map-Requests (SMRs) to ensure
eventual network convergence. Generally, PubSub offers faster
convergence due to (1) no need to wait for a data-triggered event and
(2) less signaling as compared with the SMR-based flow. Note that
when a Map-Server running PubSub has to update a large number of
subscribers at once (i.e., when a popular mapping is updated), SMR-
based convergence may be faster for a small subset of the subscribers
(those receiving PubSub updates last). Deployment experience reveals
that data-driven SMRs and PubSub mechanisms complement each other and
provide a fast and resilient network infrastructure in the presence
of mobility events.

Furthermore, experience showed that not all LISP entities on the
network need to implement PubSub for the network to get the benefits.
In scenarios with significant traffic coming from outside of the LISP
network, the experience showed that enabling PubSub in the border
routers significantly improves mobility latency overall. Even if
edge xTRs do not implement PubSub, and traffic is exchanged between
EID-Prefixes at the edge, xTRs still converge based on data-driven
events and SMR-triggered updates.

A.4. Enhanced Reachability with Dynamic Redistribution of Prefixes

There is a need to interconnect LISP networks with other networks
that might or might not run LISP. Some of those scenarios are
similar to the ones described in [GB-ATN] and [UBERLAY]. When
connecting LISP to other networks, the experience revealed that in
many deployments the point of interaction with the other domains is
not the Mapping System but rather the border router of the LISP site.
For those cases, the border router needs to be aware of the LISP
prefixes to redistribute them to the other networks. Over the years,
different solutions have been used.

First, Map-Servers were collocated with the border routers, but this
was hard to scale since border routers scale at a different pace than
Map-Servers. Second, decoupled Map-Servers and border routers were
used with static configuration of LISP entries on the border, which
was problematic when modifications were made. Third, a routing
protocol (e.g., BGP) can be used to redistribute LISP prefixes from
the Map-Servers to a border router, but this comes with some
implications; in particular, the Map-Servers need to implement an
additional protocol, which consumes resources and needs to be
properly configured. Therefore, once PubSub was available,
deployments started to adapt it to enable border routers to
dynamically learn the prefixes they need to redistribute without a
need for extra protocols or extra configuration on the network.

In other words, PubSub can be used to discover EID-Prefixes so they
can be imported into other routing domains that do not use LISP.
Similarly, PubSub can also be used to discover when EID-Prefixes need
to be withdrawn from other routing domains. That is, in a typical
deployment, a border router will withdraw an EID-Prefix that it has
been announcing to external routing domains if it receives a
notification that the RLOC-set for that EID-Prefix is now empty.

A.5. Better Serviceability

EID-to-RLOC mappings can have a very long TTL, sometimes on the order
of several hours. Upon the expiry of that TTL, the xTR checks if
these entries are being used and removes any entry that is not being
used. The problem with a very long Map-Cache TTL is that (in the
absence of PubSub) if a mapping changes but is not being used, the
cache remains but is stale. This is due to no data traffic being
sent to the old location to trigger an SMR-based Map-Cache update as
described in Section 4.1.2.1 of [EID-MOBILITY]. If the network
operator runs a show command on a router to track the state of the
Map-Cache, the router will display multiple entries waiting to expire
but with stale RLOC information. This might be confusing for
operators sometimes, particularly when they are debugging problems.
With PubSub, the Map-Cache is updated with the correct RLOC
information, even when it is not being used or waiting to expire,
which helps with debugging.

Acknowledgments

We would like to thank Marc Portoles, Balaji Venkatachalapathy,
Bernhard Haindl, Luigi Iannone, and Padma Pillay-Esnault for their
great suggestions and help regarding this document.

Many thanks to Alvaro Retana for the careful AD review.

Thanks to Chris M. Lonvick for the security directorate review, Al
Morton for the OPS-DIR review, Roni Even for the Gen-ART review, Mike
McBride for the rtg-dir review, Magnus Westerlund for the tsv
directorate review, and Sheng Jiang for the int-dir review.

Thanks to John Scudder, Erik Kline, Lars Eggert, Warren Kumari,
Martin Duke, Murray Kucherawy, Éric Vyncke, Robert Wilton,
Zaheduzzaman Sarker, and Roman Danyliw for the IESG review.

This work was partly funded by the ANR LISP-Lab project #ANR-
13-INFR-009 <https://anr.fr/Projet-ANR-13-INFR-0009>.

Contributors

Dino Farinacci
lispers.net
San Jose, CA
United States of America
Email: [email protected]

Johnson Leong
Email: [email protected]

Fabio Maino
Cisco
San Jose, CA
United States of America
Email: [email protected]

Christian Jacquenet
Orange
Rennes
France
Email: [email protected]

Stefano Secci
Cnam
France
Email: [email protected]

Authors' Addresses

Alberto Rodriguez-Natal
Cisco
Barcelona
Spain
Email: [email protected]

Vina Ermagan
Google
United States of America
Email: [email protected]

Albert Cabellos
UPC/BarcelonaTech
Barcelona
Spain
Email: [email protected]

Sharon Barkai
Nexar
Email: [email protected]

Mohamed Boucadair
Orange
Rennes
France
Email: [email protected]