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  |U|B|I|R|   Reserved        | Result      | Filter Len  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Nonce . . .                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         . . . Nonce                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            Key ID             |  Authentication Data Length   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                     Authentication Data                       ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Expiry Timer                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Length        |                                               |
       +-+-+-+-+-+-+-+-+                                               :
       :                             Filter                            :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                      ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Length        |                                               |
       +-+-+-+-+-+-+-+-+                                               :
       :                             Filter                            :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                  Redirect Map-Resolver                        |
       |                  IP Address (128 bits)                        |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2: Map-Subscribe-Ack Message Format

• Type is to be defined (see Section 11).
• U (unsolicited-map-reply bit): When set, this flag indicates that the Map-Resolver can issue unsolicited Map-Reply messages.
• B (bulk-support bit): When set, this flag indicates that the Map-Resolver supports mapping bulk retrieval method (e.g., [I-D.boucadair-lisp-bulk]).
• I (immediate-retrieval bit): When set, this flag indicates that the Map-Resolver will initiate an immediate retrieval cycle of the portion of the mapping table that matches the filters included in the request.
• R (Redirect bit): When set, this flag indicates that a redirect Map-Resolver is enclosed in the message.
• Reserved: reserved bits, MUST be set to 0 and MUST be ignored when received.
• Result: indicates the result code of the processing of the Map-Subscribe request. The following codes are defined:

  0:

  SUCCESS. This code is used to indicate the request is successfully processed.

  1:

  PARTIAL-FILTERS-INSTALLED-LIMIT. This code is used to indicate a request is successfully processed but some filters were not installed because the number of filters carried in the initial Map-Subscribe message exceeds the filter limit.

  2:

  PARTIAL-FILTERS-INSTALLED-BAD. This code is used to indicate a request is successfully processed but some filters were not installed because they were malformed.

  3:

  PARTIAL-FILTERS-INSTALLED-LOCAL. This code is used to indicate a request is successfully processed but some filters were not installed because of local reasons. The ITR SHOULD, after a certain timer expires, send a Map-Subscribe request message for the set of filters that are not included in the Map-Subscribe-Ack message received by the ITR in response to its initial Map-Subscribe request message.

  4:

  FILTERS-PROHIBITED. This code is used to indicate a request is successfully processed but the installation of filters is prohibited.

• Filter Len: This field indicates in bytes, the length of the filters included in the request. It is equal to (sum of "8+Length of Filter"), where Filter represents the filters included in the message.
• Nonce, Key ID, Authentication Data Length, and Authentication Data are similar to those of a LISP Map-Request message ([RFC6830]).
• Expiry Timer: This field indicates, in seconds, the validity timer for the subscription.
• Length: This field indicates, in octets, the length of the filter that is encoded in the "Filter" field.
• Filter: This field carries a destination EID (or a set thereof, if several filters are carried in the message) that is encoded as an UTF-8 string. This specification allows to convey IP prefix literals, Names and/or AS numbers. One or multiple filters may be present in a request. IPv4 prefixes are encoded as IPv4-mapped IPv6 prefixes [RFC4291] (i.e., starting with ::ffff:0:0/96). A mix of names, IP prefixes and AS numbers may be enclosed in the same request.
• Redirect Map-Resolver IP Address (128 bits): When the R-bit is set, this field carries the IP address of the Map-Resolver where mapping requests should be redirected. An IPv4 address is encoded as an IPv4-mapped IPv6 address.

4. Generating a Map-Subscribe Message

An ITR uses the U-bit to inform a Map-Resolver whether it is ready to handle unsolicited Map-Reply messages or not. The ITR MUST set the U-bit when it supports such capability.

An ITR uses the B-bit to inform a Map-Resolver whether it supports the mapping bulk transfer method or not. The ITR MUST set to the B-bit when it supports such method (e.g., [I-D.boucadair-lisp-bulk]).

An ITR that joins the LISP network is likely to delete all notifications that are bound to its RLOCs. It does so by including a Null filter prior to any filter that it wishes the Map-Resolver to record.

An ITR that loses its mapping cache for some reason SHOULD generate a Map-Subscribe message towards its Map-Resolver(s) with the I-bit set.

An ITR MAY generate several Map-Subscribe messages to make the Map-Resolver install the required filters. Nevertheless, an ITR MUST expect that the Map-Resolver may limit the number of filters that may be installed. Filters that are not accepted or not processed by the Map-Resolvers are not included in a Map-Subscribe-Ack message.

An ITR that wants to delete one or a set of filters MUST generate a Map-Subscribe message which carries those filters with an Expiry Timer set to 0.

5. Processing a Map-Subscribe Message

A Map-Resolver that does not support the Map-Subscribe message MUST silently ignore any Map-Subscribe message it receives.

Map-Resolvers MUST support a configurable parameter to enable/disable the processing of Map-Subscribe messages. The default value is set to "enabled".

A Map-Resolver SHOULD support a configuration parameter to limit the number of filters per leaf LISP network, per ITR, etc.

If a Map-Resolver receives a Map-Subscribe message and is enabled to process it, a Map-Resolver MUST reply with a Map-Subscribe-Ack message to acknowledge the receipt of the corresponding Map-Subscribe message.

When building a Map-Subscribe-Ack message, the Map-Resolver MUST:

• Set the U-bit if it supports the unsolicited Map-Reply capability, except if a redirect Map-Resolver is to be returned.
• Set the B-bit if it supports a method for mapping bulk transfer, except if a redirect Map-Resolver is to be returned.
• Set the R-bit if it wants to inform the requesting ITR about another Map-Resolver it should contact. The Map-Resolver MAY return a set of filters that are to be applied by the ITR to select the Map-Resolver (i.e., destination EID Map-Resolver address selection).
• Echo the I-bit if the Map-Resolver accepts to initiate unsolicited mapping retrievals, except if a redirect Map-Resolver is to be returned.
• If no redirect is enabled and the request includes one or several filters, the Map-Resolver MUST echo the filters it succeeds to install, and in the same order of appearance, in the Map-Subscribe-Ack message.
• If the Map-Resolver is configured with maximum and minimum values for the expiry timer, the Map-Resolver MUST adjust the Expiry Timer enclosed in the request so that it does not exceed these boundary values.

If the I-bit is set in the Map-Subscribe request and the Map-Resolver supports the unsolicited mapping retrieval capability, the Map-Resolver SHOULD generate unsolicited Map-Reply messages or dedicated bulk transfer messages that carry the EID-RLOC mapping entries that match the filters already present in the Mapping System for that ITR or that match those carried by the Map-Subscribe message.

If filters are included in the request, the Map-Resolver MUST extract those filters and update its mapping system subscription for that ITR accordingly. In particular, the Map-Resolver MUST delete all filters that are active for that ITR if a Null filter is included in the Map-Subscribe request or if the expiry timer is null.

If filters are not allowed for a given ITR, the 'Result' field MUST be set to FILTERS-PROHIBITED.

If all filters are successfully installed, the 'Result' field MUST be set to SUCCESS.

If the Map-Resolver fails to install some of the filters included in a request because the filter limits for that ITR are exceeded, it MUST NOT echo the corresponding filters in the Map-Subscribe-Ack message. The 'Result' field MUST be set to PARTIAL-FILTERS-INSTALLED-LIMIT.

If the Map-Resolver fails to install some of the filters included in a request because these filters were malformed, it MUST NOT echo the corresponding filters in the Map-Subscribe-Ack message; only successfully installed filters MUST be included. The 'Result' field MUST be set to PARTIAL-FILTERS-INSTALLED-BAD.

If, for some other reasons, the Map-Resolver fails to apply the filters included in a request, it MUST NOT echo the said filters in the Map-Subscribe-Ack message; only the successfully installed filters MUST be included. The 'Result' field MUST be set to PARTIAL-FILTERS-INSTALLED-LOCAL.

If a filter that is included in the request is more specific than a filter that is already present in the mapping system for the same ITR, the Map-Resolver MUST NOT add a new filter but MUST include the old filter in the response to the requesting ITR.

If a more specific filter exists in the mapping system for the same ITR, the Map-Resolver MUST replace the old filter (i.e., the one already stored in the system) with the new filter (i.e., the one included in the Map-Subscribe message).

A Map-Resolver that is overloaded or configured by means of a specific policy to redirect requests sent by a set of ITRs to other Map-Resolvers, the Map-Resolver MUST reply with a Map-Subscribe-Ack message with the R-bit set and 'Redirect Map-Resolver IP Address' field set to the redirect Map-Resolver'address. All other bit flags MUST be returned unset. Moreover, the Expiry Timer MUST be set to 0. No Filter MUST be included in the message.

If an event matches one of the filters that have been installed by an ITR, the Map-Resolvers MUST generate the corresponding unsolicited mapping update message (e.g., Map-Reply, mapping bulk method).

Upon expiry of the validity timer associated with a filter, the Map-Resolver MUST delete that filter from its mapping subscription system.

6. Processing a Map-Subscribe-Ack Message

Upon receipt of a Map-Subscribe-Ack message, the ITR MUST check whether the message matches a Map-Subscribe message it sent to the same Map-Resolver. If no matching state is found, the message MUST be silently dropped. If a state is found, in addition to authentication checks, the ITR MUST proceed as follows:

• If the U-bit is set, it should expect that unsolicited Map-Reply messages will be received from this Map-Resolver. Appropriate security mechanisms (e.g., Access Control Lists) SHOULD be activated to allow the processing of these incoming unsolicited Map-Reply messages.
• If the B-bit is set, it should expect that (unsolicited) mapping bulk transfer messages are supported by this Map-Resolver. Appropriate security mechanisms (e.g., Access Control Lists) SHOULD be activated to allow the processing of these incoming unsolicited bulk transfer messages.
• If the R-bit is set and the 'Redirect Map-Resolver IP Address' field embeds a valid IP address, the ITR MUST update its Map-Resolver contact information with the new Map-Resolver's IP address. The ITR MUST use that IP address for subsequent exchanges. Optionally, if filters were included in the reply sent by the Map-Resolver, these filters are used for the destination EID Map-Resolver address selection.
• If the message includes one or several filters, the ITR MUST check whether the same set of filters as those included in the Map-Subscribe request are carried in the Map-Subscribe-Ack message. Filters that are not returned in the Map-Subscribe-Ack message may not be valid or have exceeded a limit. The "Result" code indicates the reason for not installing these filters. In particular:
  • An ITR that receives the result code set to PARTIAL-FILTERS-INSTALLED-LIMIT MUST NOT try to install new filters unless it clears all the filters maintained by the Map-Resolver or it removes some of them.
  • An ITR that receives the result code set to PARTIAL-FILTERS-INSTALLED-BAD MUST NOT resend the same filters that were not returned in the Map-Subscribe-Ack message, in subsequent Map-Subscribe requests.
  • An ITR that receives the result code set to FILTERS-PROHIBITED MUST NOT include the filters that were not returned in the Map-Subscribe-Ack message, in a Map-Subscribe message sent to that Map-Resolver.
  • An ITR that receives the result code set to PARTIAL-FILTERS-INSTALLED-LOCAL SHOULD wait for at least 60 seconds before issuing another Map-Subscribe message to install the filters that were not returned in the Map-Subscribe-Ack message.
• The ITR MUST adjust the Expiry Timer carried in the Map-Subscribe-Ack. Subscription should be renewed before the expiry of that timer.

7. Subscription to Multiple Resolvers

In order to subscribe to multiple Map-Resolvers, an ITR sends Map-Subscribe messages to a list of Map-Resolvers. Each of these requests is handled as specified in Section 4.

8. Mapping Policy Table

Section 2 and Section 3 specifies a solution for a Map-Resolver to redirect an ITR to another Map-Resolver. This section focuses on the dynamic provisioning of advanced mapping resolution forwarding policy tables.

This section assumes that the entity managing a leaf LISP network has subscribed to a Mapping Service using a variety of means, including static or dynamic such as [I-D.boucadair-connectivity-provisioning-protocol]. For the sake of network automation, we assume that the outcome of such negotiation is translated into appropriate provisioning actions, which include in particular the identity of Map-Resolvers that will be solicited by the ITRs of the leaf LISP network.

The provisioning information may be as simple as a list of IP addresses or names, but it may also be enriched with traffic engineering rules, such as priority information, geolocation information of the a Map-Resolver, the set of destination EIDs that are serviced by a given Map-Resolver, etc. A leaf LISP network can be provisioned with a list of Map-Resolvers together with policies to instruct the ITR how to contact these Map-Resolvers. These policies are enclosed in a Mapping Policy Table. This table is similar to the one defined in [RFC6724].

Typically, the Mapping Policy Table may be structured as follows:

• Destination EID range
• Map-Resolver IP address
• Tie-breaking rules

An ITR may use the selection algorithm defined in Section 6 of [RFC6724] to contact a Map-Resolver.

<<to be completed>>

9. Sample Examples

This section includes a set of examples to illustrate the usage of the methods defined in Section 2.

9.1. Map-Resolver Redirect

The example shown in Figure 3, illustrates an example of an ITR (ITR1) that is redirected to another Map-Resolver (MR2).

    +--------+                  +--------+ +--------+
    |  ITR1  |                  |  MR1   | |  MR2   |
    +--------+                  +--------+ +--------+
         |                           |          |    
         |Map-Subscribe              |          |    
         |-------------------------->|          |    
         | Map-Subscribe-Ack (R, MR2)|          |    
         |<--------------------------|          |    
         |Map-Subscribe              |          |    
         |------------------------------------->|    
         |                     Map-Subscribe-Ack|    
         |<-------------------------------------|    
         |                                      |
         |Map-Request                           |    
         |------------------------------------->|    
         |                             Map-Reply|    
         |<-------------------------------------|       

    +--------+                  +--------+      
    |  ITR1  |                  |   MR   |     
    +--------+                  +--------+      
         |                            |             
         |                            |                    
         |Map-Subscribe(d_EID, d_EID2,|
         |..., d_EIDn)                |                  
         |--------------------------->|                
         |   Map-Subscribe-Ack (d_EID,|
         |        d_EID2, ..., d_EIDn)|
         |<---------------------------|
         |                            |                 
                 <<ITR Reboot>>  
         |Map-Subscribe(I)            |               
         |--------------------------->|
         |       Map-Subscribe-Ack (I)|
         |<---------------------------| 
         |           Map-Reply (d_EID)|
         |<---------------------------|
         |          Map-Reply (d_EID2)|
         |<---------------------------|
                       ....                
         |          Map-Reply (d_EIDn)|
         |<---------------------------|

    +--------+                  +--------+      
    |  ITR1  |                  |   MR   |     
    +--------+                  +--------+      
         |                            |             
         |                            |                    
         |Map-Subscribe(d_EID, d_EID2,|
         |..., d_EIDn)                |                  
         |--------------------------->|                
         |   Map-Subscribe-Ack (d_EID,|
         |        d_EID2, ..., d_EIDn)|
         |<---------------------------|
         |                            |                 
                 <<ITR Reboot>>  
         |Map-Subscribe(I)            |               
         |--------------------------->|
         |       Map-Subscribe-Ack (I)|
         |<---------------------------| 
         |  Map-Reply (d_EID, d_EID2,
         |                      ..., )|
         |<---------------------------|

    +--------+                  +--------+      
    |  ITR1  |                  |   MR   |     
    +--------+                  +--------+                
         |                            |                    
         |                            |                 
                 <<ITR Reboot>>  
         |                            |  
         |Map-Subscribe(I, d_EID      |              
         | d_EID2, ..., d_EIDn)       |                  
         |--------------------------->|                
         | Map-Subscribe-Ack (I, d_EID|
         |        d_EID2, ..., d_EIDn)|
         |<---------------------------|                  
         |           Map-Reply (d_EID)|
         |<---------------------------|
         |          Map-Reply (d_EID2)|
         |<---------------------------|
                       ....                
         |          Map-Reply (d_EIDn)|
         |<---------------------------|

    +--------+                  +--------+      +--------+
    |  ITR1  |                  |   MR   |      |  ETR   |
    +--------+                  +--------+      +--------+
         |                            |               |  
         |                            |               |      
         |Map-Subscribe               |               |      
         |--------------------------->|               |      
         |   Map-Subscribe-Ack (d_EID)|               |      
         |<---------------------------|               |  
         |                            |               |      
         |           Map-Reply (d_EID)|               |      
         |<---------------------------|               |     
                                   ....
src=s_EID|                                            |
-------->|src=RLOC_itr1                   dst=RLOC_etr|src=s_EID
dst=d_EID|==============Encapsulated Packet==========>|-------->
         |                                            |dst=d_EID
                               ....