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6.1.4. Map-Reply Message Format

6.1.4. Map-Reply Message Format​

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Type=2 |P|E|S|          Reserved               | Record Count  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Nonce . . .                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         . . . Nonce                           |
   +-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   |                          Record TTL                           |
   |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

R | Locator Count | EID mask-len | ACT |A| Reserved |

   e   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

c | Rsvd | Map-Version Number | EID-Prefix-AFI |

   o   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

r | EID-Prefix |

   d   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  /|    Priority   |    Weight     |  M Priority   |   M Weight    |
   | L +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | o |        Unused Flags     |L|p|R|           Loc-AFI             |
   | c +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  \|                             Locator                           |
   +-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Packet field descriptions:

Type: 2 (Map-Reply)

P: This is the probe-bit, which indicates that the Map-Reply is in response to a Locator reachability probe Map-Request. The 'Nonce' field MUST contain a copy of the nonce value from the original Map-Request. See Section 6.3.2 for more details.

E: This bit indicates that the ETR that sends this Map-Reply message is advertising that the site is enabled for the Echo-Nonce Locator reachability algorithm. See Section 6.3.1 for more details.

S: This is the Security bit. When set to 1, the following authentication information will be appended to the end of the Map-Reply. The detailed format of the Authentication Data Content is for further study.

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    AD Type    |       Authentication Data Content . . .       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Reserved: This field MUST be set to 0 on transmit and MUST be ignored on receipt.

Record Count: This is the number of records in this reply message. A record is comprised of that portion of the packet labeled 'Record' above and occurs the number of times equal to Record Count.

Nonce: This is a 24-bit value set in a Data-Probe packet, or a 64-bit value from the Map-Request is echoed in this 'Nonce' field of the Map-Reply. When a 24-bit value is supplied, it resides in the low-order 64 bits of the 'Nonce' field.

Record TTL: This is the time in minutes the recipient of the Map-Reply will store the mapping. If the TTL is 0, the entry SHOULD be removed from the cache immediately. If the value is 0xffffffff, the recipient can decide locally how long to store the mapping.

Locator Count: This is the number of Locator entries. A Locator entry comprises what is labeled above as 'Loc'. The Locator count can be 0, indicating that there are no Locators for the EID-Prefix.

EID mask-len: This is the mask length for the EID-Prefix.

ACT: This 3-bit field describes Negative Map-Reply actions. In any other message type, these bits are set to 0 and ignored on receipt. These bits are used only when the 'Locator Count' field is set to 0. The action bits are encoded only in Map-Reply messages. The actions defined are used by an ITR or PITR when a destination EID matches a negative Map-Cache entry. Unassigned values should cause a Map-Cache entry to be created, and when packets match this negative cache entry, they will be dropped. The current assigned values are:

(0) No-Action: The map-cache is kept alive, and no packet encapsulation occurs.

(1) Natively-Forward: The packet is not encapsulated or dropped but natively forwarded.

(2) Send-Map-Request: The packet invokes sending a Map-Request.

(3) Drop: A packet that matches this map-cache entry is dropped. An ICMP Destination Unreachable message SHOULD be sent.

A: The Authoritative bit, when sent, is always set to 1 by an ETR. When a Map-Server is proxy Map-Replying [RFC6833] for a LISP site, the Authoritative bit is set to 0. This indicates to requesting ITRs that the Map-Reply was not originated by a LISP node managed at the site that owns the EID-Prefix.

Map-Version Number: When this 12-bit value is non-zero, the Map-Reply sender is informing the ITR what the version number is for the EID record contained in the Map-Reply. The ETR can allocate this number internally but MUST coordinate this value with other ETRs for the site. When this value is 0, there is no versioning information conveyed. The Map-Version Number can be included in Map-Request and Map-Register messages. See Section 6.6.3 for more details.

EID-Prefix-AFI: Address family of the EID-Prefix according to [AFI].

EID-Prefix: This prefix is 4 octets for an IPv4 address family and 16 octets for an IPv6 address family.

Priority: Each RLOC is assigned a unicast Priority. Lower values are more preferable. When multiple RLOCs have the same Priority, they MAY be used in a load-split fashion. A value of 255 means the RLOC MUST NOT be used for unicast forwarding.

Weight: When priorities are the same for multiple RLOCs, the Weight indicates how to balance unicast traffic between them. Weight is encoded as a relative weight of total unicast packets that match the mapping entry. For example, if there are 4 Locators in a Locator-Set, where the Weights assigned are 30, 20, 20, and 10, the first Locator will get 37.5% of the traffic, the 2nd and 3rd Locators will get 25% of the traffic, and the 4th Locator will get 12.5% of the traffic. If all Weights for a Locator-Set are equal, the receiver of the Map-Reply will decide how to load-split the traffic. See Section 6.5 for a suggested hash algorithm to distribute the load across Locators with the same Priority and equal Weight values.

M Priority: Each RLOC is assigned a multicast Priority used by an ETR in a receiver multicast site to select an ITR in a source multicast site for building multicast distribution trees. A value of 255 means the RLOC MUST NOT be used for joining a multicast distribution tree. For more details, see [RFC6831].

M Weight: When priorities are the same for multiple RLOCs, the Weight indicates how to balance building multicast distribution trees across multiple ITRs. The Weight is encoded as a relative weight (similar to the unicast Weights) of the total number of trees built to the source site identified by the EID-Prefix. If all Weights for a Locator-Set are equal, the receiver of the Map-Reply will decide how to distribute multicast state across ITRs. For more details, see [RFC6831].

Unused Flags: These are set to 0 when sending and ignored on receipt.

L: When this bit is set, the Locator is flagged as a local Locator to the ETR that is sending the Map-Reply. When a Map-Server is doing proxy Map-Replying [RFC6833] for a LISP site, the L-bit is set to 0 for all Locators in this Locator-Set.

p: When this bit is set, an ETR informs the RLOC-Probing ITR that the locator address for which this bit is set is the one being RLOC-probed and MAY be different from the source address of the Map-Reply. An ITR that RLOC-probes a particular Locator MUST use this Locator for retrieving the data structure used to store the fact that the Locator is reachable. The p-bit is set for a single Locator in the same Locator-Set. If an implementation sets more than one p-bit erroneously, the receiver of the Map-Reply MUST select the first Locator. The p-bit MUST NOT be set for Locator-Set records sent in Map-Request and Map-Register messages.

R: This is set when the sender of a Map-Reply has a route to the Locator in the Locator data record. This receiver may find this useful to know if the Locator is up but not necessarily reachable from the receiver's point of view. See also Section 6.4 for another way the R-bit may be used.

Locator: This is an IPv4 or IPv6 address (as encoded by the 'Loc-AFI' field) assigned to an ETR. Note that the destination RLOC address MAY be an anycast address. A source RLOC can be an anycast address as well. The source or destination RLOC MUST NOT be the broadcast address (255.255.255.255 or any subnet broadcast address known to the router) and MUST NOT be a link-local multicast address. The source RLOC MUST NOT be a multicast address. The destination RLOC SHOULD be a multicast address if it is being mapped from a multicast destination EID.

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