I'm sorry for running late on this review, and please don't be concerned about the length - it includes a lot of draft text as part of the comments. 

Do The Right Thing, of course. 

In this text, 

   At first, a map of endpoint properties might seem impractical,
   because it could require enumerating the property value for every
   possible endpoint.  However, in practice, it is highly unlikely that
   properties will be defined for every endpoint address.  It is much
   more likely that properties may be defined for only a subset of
   endpoint addresses, and the specification of properties uses an
   aggregation representation to allow enumeration.  This is
   particularly true if blocks of endpoint addresses with a common
   prefix (e.g., a CIDR) have the same value for a property.  Entities
   in other domains may very well allow aggregated representation and
   hence be enumerable as well.

I wonder if it’s worth saying anything about the likely effect of doing something “highly unlikely”, or perhaps something a bit more likely, like defining properties for a sufficiently large subset of endpoints to cause a problem. 

You might make an editing pass through the document looking for occurrences of “domain name” that (I think) refer to entity domain names, such as 

   *  if an entity is an endpoint with example routable IPv4 address
      "192.0.2.14", its identifier is associated with domain name "ipv4"
      and is "ipv4:192.0.2.14",

   *  if an entity is a PID named "mypid10" in network map resource
      "netmap2", its identifier is associated with domain name
      "netmap2.pid" and is "netmap2.pid:mypid10".

I understand why you have the “entity domain name” terminology, but dropping the “entity” qualifier seems likely to lead to confusion. 

In this text, 

   Thus, if a property
   "pid" is defined for entity "192.0.2.34" in two different network
   maps "netmap1" and "netmap2", the value for this property will likely
   be a different value in "netmap1" and "netmap2".

Is “likely” the right word? I think your point is that there’s no reason to expect they’d be the same, not that the reason people create another network map is to store the values for properties that are different. I think you’re saying “can be a different value”, aren’t you?

In this text,

   *  an entity domain named "netmap1.ipv4" includes the IPv4 addresses
      that appear in the "ipv4" field of the endpoint address group of
      each PID in the network map "netmap1", and that cannot be
      recognized outside "netmap1" because, for instance, these are
      local non-routable addresses,

Is “cannot be recognized” the right phrase here? My understanding is that this is more like “have no meaning outside ‘netmap1’”.

I’m confused about the use of the IPv4 literal address “192.0.2.34” in this document. I thought that https://datatracker.ietf.org/doc/html/rfc1166 reserved 192.0.2.0/24 for documentation, so when I see statements like this one: 

   *  if an entity is an endpoint with example routable IPv4 address
      "192.0.2.14", its identifier is associated with domain name "ipv4"
      and is "ipv4:192.0.2.14",

I’m not sure what “example routable IPv4 address” means - it’s not routable, is it? In general, I’m not sure what saying “routable” adds to statements like 

   *  an entity domain named "ipv4" is resource-agnostic and covers all
      the routable IPv4 addresses.

Isn’t that a convention that someone might use, rather than an invariant property of “ipv4”? It’s probably worth making an editorial pass looking for these usages. And you might also look for similar issues using “2001:db8::1/48” - isn’t that reserved for documentation as well, by https://datatracker.ietf.org/doc/html/rfc3849?

I was confused by this text: 

   Each entity property type MUST be registered with the IANA, following
   the procedure specified in Section 12.3 of this document.  The
   intended semantics of the entity property type MUST be specified at
   the same time.

   Identifiers prefixed with "priv:" are reserved for Private Use
   [RFC8126] without a need to register with IANA.  All other
   identifiers for entity property types appearing in an HTTP request or
   response with an "application/alto-*" media type MUST be registered
   in the "ALTO Entity Property Type Registry", defined in Section 12.3.

The first sentence of the first paragraph seems to be contradicted by the first sentence of the second paragraph - “each MUST be registered, except for the ones that don’t need to be registered”. 

I do see reasonable usages of SHOULD in this document (“SHOULD unless”), but I also see usages like this one - 

   For each entity in the property map:

   *  If the entity is in a resource-specific entity domain, the ALTO
      server SHOULD only return self-defined properties and resource-
      specific properties which depend on the same resource as the
      entity does.  The ALTO client SHOULD ignore the resource-specific
      property in this entity if their mapping is not registered in the
      ALTO Resource Entity Property Transfer Registry of the type of the
      corresponding resource.

Could you give an example of why the ALTO server might return properties that don’t conform to this SHOULD, or why the ALTO client might not ignore such properties? 

   *  If the entity identifier is resource-agnostic, the ALTO server
      SHOULD return the self-defined properties and all the resource-
      specific properties that are defined in the property defining
      information resources indicated, in the IRD, in the "mappings"
      capability of the property map resource.

Again, why might the ALTO server not return these properties? Or is this answered by the next paragraph? 

   For efficiency, the ALTO server SHOULD omit property values that are
   inherited rather than explicitly defined; if a client needs inherited
   values, the client SHOULD use the entity domain's inheritance rules
   to deduce those values.

And if the client needs inherited values that are omitted, is there any other option besides using inheritance rules to deduce them? 

This

   *  If there are entities covered by a requested entity but having
      different values for the requested properties, the response SHOULD
      include all those entities and the different property values for
      them.  For example, considering a request for property P of entity
      A (e.g., ipv4:192.0.2.0/31), if P has value v1 for
      A1=ipv4:192.0.2.0/32 and v2 for A2=ipv4:192.0.2.1/32, then, the
      response SHOULD include A1 and A2.

   *  If an entity identifier in the response is already covered by
      other entities identifiers in the same response, it SHOULD be
      removed from the response, for the sake of compactness.  In the
      previous example, the entity A = ipv4:192.0.2.0/31 SHOULD be
      removed because A1 and A2 cover all the addresses in A.

Is a great example of “SHOULD do something unless you SHOULD do something else”, but is it obvious why you shouldn’t remove A1 and A2 from the response, because A covers all the addresses in A1 and A2? 

These two paragraphs in the Security Considerations section

   Both Property Map and Filtered Property Map defined in this document
   fit into the architecture of the ALTO base protocol, and hence the
   Security Considerations (Section 15 of [RFC7285]) of the base
   protocol fully apply: authenticity and integrity of ALTO information
   (i.e., authenticity and integrity of Property Maps), potential
   undesirable guidance from authenticated ALTO information (e.g.,
   potentially imprecise or even wrong value of a property such as geo-
   location), confidentiality of ALTO information (e.g., exposure of a
   potentially sensitive entity property such as geo-location), privacy
   for ALTO users, and availability of ALTO services should all be
   considered.

   ALTO clients using this extension should in addition be aware that
   the entity properties they require may convey more details than the
   endpoint properties conveyed by using [RFC7285].  Client requests may
   reveal details on their activity or plans thereof, that a malicious
   user may monetize or use for attacks or undesired surveillance.
   Likewise, ALTO Servers expose entities and properties related to
   specific parts of the infrastructure that reveal details on
   capabilities, locations, or resource availability.  These details may
   be maliciously used for competition purposes, or to cause resource
   shortage or undesired publication.

Contain the only occurrences of the word “user” in the document. Is it defined in a formal way anywhere? I can imagine that the second occurrence is “ALTO server”, but I’m guessing, and the first occurrence seems to be handwaving.