6.1.5. EID-to-RLOC UDP Map-Reply Message

6.1.5. EID-to-RLOC UDP Map-Reply Message

A Map-Reply returns an EID-Prefix with a prefix length that is less than or equal to the EID being requested. The EID being requested is either from the destination field of an IP header of a Data-Probe or the EID record of a Map-Request. The RLOCs in the Map-Reply are globally routable IP addresses of all ETRs for the LISP site. Each RLOC conveys status reachability but does not convey path reachability from a requester's perspective. Separate testing of path reachability is required. See Section 6.3 for details.

Note that a Map-Reply may contain different EID-Prefix granularity (prefix + length) than the Map-Request that triggers it. This might occur if a Map-Request were for a prefix that had been returned by an earlier Map-Reply. In such a case, the requester updates its cache with the new prefix information and granularity. For example, a requester with two cached EID-Prefixes that are covered by a Map-Reply containing one less-specific prefix replaces the entry with the less-specific EID-Prefix. Note that the reverse, replacement of one less-specific prefix with multiple more-specific prefixes, can also occur, not by removing the less-specific prefix but rather by adding the more-specific prefixes that, during a lookup, will override the less-specific prefix.

When an ETR is configured with overlapping EID-Prefixes, a Map-Request with an EID that best matches any EID-Prefix MUST be returned in a single Map-Reply message. For instance, if an ETR had database mapping entries for EID-Prefixes:

10.0.0.0/8 10.1.0.0/16 10.1.1.0/24 10.1.2.0/24

A Map-Request for EID 10.1.1.1 would cause a Map-Reply with a record count of 1 to be returned with a mapping record EID-Prefix of 10.1.1.0/24.

A Map-Request for EID 10.1.5.5 would cause a Map-Reply with a record count of 3 to be returned with mapping records for EID-Prefixes 10.1.0.0/16, 10.1.1.0/24, and 10.1.2.0/24.

Note that not all overlapping EID-Prefixes need to be returned but only the more-specific entries (note that in the second example above 10.0.0.0/8 was not returned for requesting EID 10.1.5.5) for the matching EID-Prefix of the requesting EID. When more than one EID-Prefix is returned, all SHOULD use the same Time to Live value so they can all time out at the same time. When a more-specific EID-Prefix is received later, its Time to Live value in the Map-Reply record can be stored even when other less-specific entries exist. When a less-specific EID-Prefix is received later, its map-cache expiration time SHOULD be set to the minimum expiration time of any more-specific EID-Prefix in the map-cache. This is done so the integrity of the EID-Prefix set is wholly maintained and so no more- specific entries are removed from the map-cache while keeping less- specific entries.

Map-Replies SHOULD be sent for an EID-Prefix no more often than once per second to the same requesting router. For scalability, it is expected that aggregation of EID addresses into EID-Prefixes will allow one Map-Reply to satisfy a mapping for the EID addresses in the prefix range, thereby reducing the number of Map-Request messages.

Map-Reply records can have an empty Locator-Set. A Negative Map-Reply is a Map-Reply with an empty Locator-Set. Negative Map-Replies convey special actions by the sender to the ITR or PITR that have solicited the Map-Reply. There are two primary applications for Negative Map-Replies. The first is for a Map-Resolver to instruct an ITR or PITR when a destination is for a LISP site versus a non-LISP site, and the other is to source quench Map-Requests that are sent for non-allocated EIDs.

For each Map-Reply record, the list of Locators in a Locator-Set MUST appear in the same order for each ETR that originates a Map-Reply message. The Locator-Set MUST be sorted in order of ascending IP address where an IPv4 locator address is considered numerically 'less than' an IPv6 locator address.

When sending a Map-Reply message, the destination address is copied from one of the 'ITR-RLOC' fields from the Map-Request. The ETR can choose a locator address from one of the address families it supports. For Data-Probes, the destination address of the Map-Reply is copied from the source address of the Data-Probe message that is invoking the reply. The source address of the Map-Reply is one of the local IP addresses chosen to allow Unicast Reverse Path Forwarding (uRPF) checks to succeed in the upstream service provider. The destination port of a Map-Reply message is copied from the source port of the Map-Request or Data-Probe, and the source port of the Map-Reply message is set to the well-known UDP port 4342.

6.1.5.1. Traffic Redirection with Coarse EID-Prefixes

When an ETR is misconfigured or compromised, it could return coarse EID-Prefixes in Map-Reply messages it sends. The EID-Prefix could cover EID-Prefixes that are allocated to other sites, redirecting their traffic to the Locators of the compromised site.

To solve this problem, there are two basic solutions that could be used. The first is to have Map-Servers proxy Map-Reply on behalf of ETRs so their registered EID-Prefixes are the ones returned in Map-Replies. Since the interaction between an ETR and Map-Server is secured with shared keys, it is easier for an ETR to detect misbehavior. The second solution is to have ITRs and PITRs cache EID-Prefixes with mask lengths that are greater than or equal to a configured prefix length. This limits the damage to a specific width of any EID-Prefix advertised but needs to be coordinated with the allocation of site prefixes. These solutions can be used independently or at the same time.

At the time of this writing, other approaches are being considered and researched.