Does a domain's Glue Records get transferred when I transfer the domain to another registrar?

It depends on the registrar and on who's providing the DNS servers for DOMAIN.NET

In your example .NET is the top-level domain. Omitting some details, when a computer on the Internet is trying to get to a computer in DOMAIN.NET

• it already knows how to find the nameservers for .NET (and the other top-level domains like .COM .ORG .CA and so on), so it goes to one of those and asks for the address of the nameservers for DOMAIN.NET
• the nameservers for .NET know the IP addresses of the nameservers for DOMAIN.NET (those are the glue records)
• and then the nameservers for DOMAIN.NET know the IP addresses of computers within your domain

In principle, if you are running your own DNS servers and simply change registrars, the glue records don't change and you could probably switch by doing nothing.

The way your question is worded, it sounds like the registrar is also providing the DNS servers for your domain, in which case the glue records don't need to be transferred, they need to be changed, and that's not going to happen automatically.

If you're relying on your registrar's name servers, you need to talk to the new registrar and find out when they'll change the glue records after you transfer the domain to them. Then you need to talk to the old registrar and make sure they continue to provide services (with the old glue records) until the change takes place.

Yes, glue records are transferred.

For the following reasons:

• at registries that use this model (most but not all) nameservers are objects in their database, as are domain names
• you (more precisely: your registrar) need to create those host objects before being able to use them (associate them) with any domain name
• you need to create those host objects for both internal and external hosts: internal hosts are those with a name ending in one of the TLD managed by the registry
• external hosts are either shared by all registrars or are owned by the first registrar creating them
• internal hosts are always only possible to create by the registrar sponsoring the underlying internal domain; which has this consequence: if you want to register example.com and have ns1.example.com be authoritative on it, a registrar will need to follow this list of steps: 1) create example.com without nameservers attached to it, 2) create host object ns1.example.com - as only he can do it by being the sponsor of the underlying domain and 3) associate ns1.example.com to example.com domain
• host objects, if internal, and if used, will have the consequence of publishing glue records by the registry; but the fact that there are glue records at the DNS level is just a consequence of the above, the important point is their existence as host object.
• host objects, contrary to domain objects, can not be transfered explicitely
• however, internal host objects are transfered implicitely when the internal underlying domain name is transfered.

This is summarily outlined as such in RFC 5732 §3.2.4:

Transfer semantics do not directly apply to host objects, so there is no mapping defined for the EPP command. Host objects are subordinate to an existing superordinate domain object and, as such, they are subject to transfer when a domain object is transferred.

And also in RFC 5731 §3.2.4:

Transfer of a domain object MUST implicitly transfer all host objects that are subordinate to the domain object. For example, if domain object "example.com" is transferred and host object "ns1.example.com" exists, the host object MUST be transferred as part of the "example.com" transfer process. Host objects that are subject to transfer when transferring a domain object are listed in the response to an EPP command performed on the domain object.

About this:

and send the Glue IPs directly to the Internet root servers for .Net, so how would the new registrar know about it during a domain transfer?

This is either wrong or misworded, the above should clear things out.

Registrars send commands to registries and registry publish data on their authoritative nameservers. First, here there is nothing about "root servers". Root servers are the authoritative nameservers for root (!) aka . and they basically just have the list of all TLDs; whatever you do with your second level domain like example.net has 0 consequences on root servers, so you were probably thinking about registry (of .net) authoritative nameservers.

Second, as described above, glues are really host objects in the registry database. As any other objects there, they can be queried and updated by registrars, under some conditions.

Third, the registrar can both know all "subordinate" nameservers existing under a specific domain name, through the appropriate EPP domain:info command. If output is restricted by registry it does not matter, because as explained above, the host objects will be transfered by the registry implicitely if the underlying (superordinate) domain is transfered. As such the glues continue to be published at all times, and the host objects will be sponsored at the end by the new registrar, which will then be able to update them or delete them, as needed.