Publish/Subscribe Functionality for LISP
draft-ietf-lisp-pubsub-04

Abstract

This document specifies an extension to the use of Map-Request to enable Publish/Subscribe (PubSub) operation for LISP.

Status of This Memo

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Table of Contents

• 1. Introduction
• 2. Requirements Language
• 3. Deployment Assumptions
• 4. Map-Request PubSub Additions
• 5. Mapping Request Subscribe Procedures
• 6. Mapping Notification Publish Procedures
• 7. Security Considerations
• 8. Acknowledgments
• 9. IANA Considerations
• 10. Normative References
• Authors' Addresses

1. Introduction

The Locator/ID Separation Protocol (LISP) [I-D.ietf-lisp-rfc6833bis] splits current IP addresses in two different namespaces, Endpoint Identifiers (EIDs) and Routing Locators (RLOCs). LISP uses 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) / Proxy Ingress Tunnel Routers (PITRs) pull EID-to-RLOC mapping information from the Mapping System by means of an explicit request message. [I-D.ietf-lisp-rfc6833bis] 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 steps occur:

(1)

The ITR/RTR/PITR sends a Map-Request for that EID-prefix.

(2)

The ITR/RTR/PITR sets the Notification-Requested bit (N-bit) on the Map-Request and includes its xTR-ID and Site-ID.

(3)

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

(4)

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

(5)

The Map-Server sends a Map-Notify to the ITR/RTR/PITR to acknowledge the successful subscription.

(6)

When there is an RLOC-set change for 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 ITR/RTR/PITR want to be notified. The ITR/RTR/PITR can set the N-bit for several EID-prefixes within a single Map-Request.

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.

3. Deployment Assumptions

The specification described in this document makes the following deployment assumptions:

(1)

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

(2)

Map-Servers are configured in proxy-reply mode, i.e., they are solicited to generate and send Map-Reply messages for the mappings they are serving.

(3)

There can be either a soft-state or hard-state security association between the xTRs and the Map-Servers.

The distribution of xTR-IDs (and Site-IDs) as well as the management of security associations are out of the scope of this document and are for further study (see Section 7).

4. Map-Request PubSub Additions

Figure 1 shows the format of the updated Map-Request to support the PubSub functionality.

     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 [I-D.ietf-lisp-rfc6833bis]:

xTR-ID bit (I-bit): The I-bit of a Map-Request message is set to 1 to indicate that a 128 bit xTR-ID and a 64 bit Site-ID fields are present at the end of the Map-Request message. If an xTR is configured with an xTR-ID or Site-ID, it MUST set the I-bit to 1 and include its xTR-ID and Site-ID in the Map-Request messages it generates. If either the xTR-ID or Site-ID is not configured, an unspecified value is encoded for whichever ID that is not configured.

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 mapping record.

xTR-ID field: xTR-ID is a 128 bit field at the end of the Map-Request message, starting after the final Record in the message (or the Map-Reply Record, if present). The xTR-ID is used to uniquely identify the sender of a Map-Request message. The xTR-ID is defined in [I-D.ietf-lisp-rfc6833bis]

Site-ID field: Site-ID is a 64 bit field at the end of the Map-Request message, following the xTR-ID. Site-ID is used by the Map-Server receiving the Map-Request message to identify which xTRs belong to the same site. The Site-ID is defined in [I-D.ietf-lisp-rfc6833bis]

5. Mapping Request Subscribe Procedures

The xTR subscribes for RLOC-set changes for 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 [I-D.ietf-lisp-rfc6833bis] but 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. The xTR-ID uniquely identifies the xTR.

(2)

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

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 to notify 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 [I-D.ietf-lisp-rfc6833bis]. Furthermore, upon processing, for each EID-Record that has the N-bit set to 1, the Map-Server proceeds adding the xTR-ID contained in the Map-Request to the list of xTR that have requested to be subscribed to that mapping record.

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 MUST follow the specification in Section 6.1.7 of [I-D.ietf-lisp-rfc6833bis] to build the Map-Notify 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 (see Section 3) to compute the authentication data of the Map-Notify.

(3)

The Map-Server MUST send the Map-Notify to one of the ITR-RLOCs received in the Map-Request.

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 [I-D.ietf-lisp-rfc6833bis] with the following considerations. The xTR MUST use its security association with the Map-Server (see Section 3) to validate the authentication data on the Map-Notify. The xTR MUST use the Map-Notify to populate its map-cache with the returned EID-prefix and RLOC-set.

The subscription of an xTR-ID to the list of subscribers for the EID-Record may fail for a number of reasons. For example, because of local configuration policies (such as accept and drop lists of subscribers), or because the Map-Server has exhausted the resources to dedicate to the subscription of that EID-Record (e.g., the number of subscribers excess the capacity of the Map-Server).

If the subscription fails, the Map-Server MUST send a Map-Reply to the originator of the Map-Request, as described in [I-D.ietf-lisp-rfc6833bis]. This is also the case when the Map-Server does not support PubSub operation. The xTR processes the Map-Reply as specified in [I-D.ietf-lisp-rfc6833bis].

If an xTR-ID is successfully added to the list of subscribers for an EID-Record, the Map-Server MUST extract the nonce and ITR-RLOCs present in the Map-Request, and store the association between the xTR-ID, ITR-RLOCs and nonce. Any already present state regarding ITR-RLOCs and/or nonce for the same xTR-ID MUST be overwritten.

If the Map-Request only has one ITR-RLOC with AFI = 0 (i.e., Unknown Address), the Map-Server MUST remove the subscription state for that xTR-ID. In this case, the Map-Server MUST send the Map-Notify to the source RLOC of the Map-Request. When the TTL for the EID-record expires, the EID-prefix is removed from the Map-Server’s subscription cache. On EID-Record removal, the Map-Server notifies the subscribers via a Map-Notify with TTL equal 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 reception of Map-Notifies via 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 updated mapping information. The Map-Notify message sent to each of the subscribers as a result of an update event MUST follow the exact encoding and logic defined in [I-D.ietf-lisp-rfc6833bis] 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.

(2)

The Map-Server incrementes the nonce every time it sends a Map-Notify as publication to an xTR-ID. The starting nonce is set as follows, if the subscription state at the Map-Server was created by a received Map-Request with the N-bit set, the starting nonce in the Map-Notify sent as publication MUST be the one used in the Map-Request that created the subscription state. If the subscription state was created by explicit configuration at the Map-Server, the starting nonce in the Map-Notify sent as publication MUST be randomly generated by the Map-Server.

(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 not local to the xTR, the xTR knows that the Map-Notify has been received to update an entry on its map-cache. Processing of unsolicited Map-Notify messages MUST be explicitly enabled via configuration at the xTR. The xTR keeps track of the last nonce seen in a Map-Notify received as a publication from the Map-Server. If a Map-Notify 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 [I-D.ietf-lisp-rfc6833bis] regarding storing Map-Register nonces apply here for Map-Notify nonces.

The xTR processes the received Map-Notify as specified in [I-D.ietf-lisp-rfc6833bis], with the following considerations. The xTR MUST use its security association with the Map-Server (see Section 3) 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. The xTR MUST acknowledge the Map-Notify by sending back a Map-Notify-Ack (specified in [I-D.ietf-lisp-rfc6833bis]), with the nonce from the Map-Notify, to the Map-Server. If after a configurable timeout, the Map-Server has not received back the Map-Notify-Ack, it can try to send the Map-Notify to a different ITR-RLOC for that xTR-ID.

7. Security Considerations

Generic security considerations related to LISP control messages are discussed in [I-D.ietf-lisp-rfc6833bis].

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. Nevertheless, the way to provide a security association between the ITRs and the Map-Servers must be evaluated according to the size of the deployment. For small deployments, it is possible to have a shared key (or set of keys) between the ITRs and the Map-Servers. For larger and Internet-scale deployments, scalability is a concern and further study is needed. Such evaluation is one of the goals of this experimental specification.

Misbehaving nodes may send massive subscription requests which may lead to exhaust the resources of Map-Servers. Furthermore, frequently changing the state of a subscription may also be considered as an attack vector. To mitigate such issues, xTRs SHOULD rate-limit Map-Requests and Map-Servers SHOULD rate-limit Map-Notifies. Rate-limiting Map-Requests is discussed in [I-D.ietf-lisp-rfc6833bis] and the same guidelines apply here. To rate-limit Map-Notifies, a Map-Server MUST NOT send more than one Map-Notify per second to a particular xTR-ID. This parameter MUST be configurable. Note that when the Map-Notify rate-limit threshold is met for a particular xTR-ID, the Map-Server will silently discard additional subscription requests from that xTR-ID. Similarly, for pending mapping updates that need to be notified to that xTR-ID, the Map-Server will combine them into a single Map-Notify (with multiple EID-records) which it will send when the rate-limit mechanism allows it to transmit again Map-Notifies to that xTR-ID.

8. Acknowledgments

This work is partly funded by the ANR LISP-Lab project #ANR-13-INFR-009 (https://www.lisp-lab.org).

9. IANA Considerations

This document is requesting bit allocations in the Map-Request message from the "LISP Control Plane Header Bits" registry introduced in [I-D.ietf-lisp-rfc6833bis]. In particular, this document requests allocating the following two bits from the sub-registry "Map-Request Header Bits". The position of these two bits in the Map-Request message can be found in Figure 1.

10. Normative References

Authors' Addresses