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Reading a couple of AD Replication posts from TechNet/ MSDN. Below are my notes on them. 

What is the AD Replication Model?

(source)

• AD makes use of pull replication and not push. That is, each DC pulls in changes from other DCs (as opposed to the DC that has changes pushing these to targets). Reason for pulling instead of pushing is that only the destination DC knows what changes it needs. It is possible it has got some changes from another DC – remember AD is multimaster – so no point pushing all changes to a destination DC.
• Initial replication to a new DC in the domain is via LDAP. Subsequent replications are via RPC.
• All DCs do not replicate with all other DCs. They only replicate with a subset of DCs, as determined by the replication topology. (I am not sure, but I think this is only in the case of InterSite replication …). This is known as store and forward replication. DCs store updates they get from replication partners ams forward these to other DCs.
• There are two sorts of replication – state-based and log-based.
  • State-based means DCs apply updates to their replicas (their copies of the partitions) as and when they arrive.
  • In log-based replication each DC maintains a log of the changes and sends this log to all other DCs. Once a destination DC receives this log it applies it to its replica and becomes up-to-date. This also means the destination DC will receive a list of all changes (from the source DC perspective), not just the changes it wants. 
  • Active Directory (actually called Active Directory Domain Services since Server 2008) uses state-based replication. 
  • Each DC maintains a list of objects by last modification. This way it can easily examine the last few objects in the list to identify how much of the replication source’s changes have already been processed. 

Linked & Multivalued attributes

Before the next post it’s worth going into Linked attributes and Multivalued attributes.

• Linked attributes are pairs of attributes. AD calculates the value of one of the attributes (called the “back link”) based on the value of the other attribute (called the “forward link”). For example: group membership. Every user object has an attribute called memberOf. You can’t see this by default in ADUC in the “Attribute Editor” tab. But if you click “Filter” and select to show “Backlinks” too then you can view this attribute. The memberOf attribute is an example of a back link. And it’s generated by AD, which is why it’s a read-only attribute. It’s counterpart, the forward link, is the member attribute of a group object. Linked attributes are linked together in the schema. In the schema definitions of those attributes – which are instances of an attributeSchema class – there is a linkID attribute that defines the back and forward links. The attribute with an even linkID is the forward link (the write-able attribute that we can set) while the attribute with a linkID that’s one more than the linkID of the forward link is the back link (the read-only, AD generated attribute). For instance, using ADSI Edit I found the CN=Member,CN=Schema,CN=Configuration,DC=mydomain object in the Schema partition. This is the schema definition of the member attribute. Being a forward link it has an even linkID. Its counterpart can be easily found using ldp.exe. Search the Schema partition for all objects with linkID of 3. The result is CN=Is-Member-Of-DL,CN=Schema,CN=Configuration,DC=mydomain, which is the attributeSchema object defining the memberOf attribute (notice the lDAPDisplayName attribute of this object in the above screenshot).
  • It’s worth taking a moment to understand the above example. When one uses ADUC to change a user/ group’s group membership, all one does is go to the user/ group object, the “Member Of” tab, and add a group. This gives the impression that the actual action happens in the user/ group object. But as we see above that’s not the case. Although ADUC may give such an impression, when we add a user/ group to a group, it is the group that gets modified with the new members and this reflects in the user/ group object that we see. ADUC makes the change on the user/ group, but the DSA makes the real change behind the scenes. The cause and effect is backwards here …
  • Another obvious but not so obvious point is that when you add a user/ group to a group, it is the group’s whenChanged attribute that gets changed and it is the change to the group that is replicated throughout the domain. The user object has no change and nothing regarding it is replicated. Obvious again, because the change really happens on the group and the effect we see on the user/ group is what AD calculates and shows us. 
• Multivalued attributes, as the name suggests, are attributes that can hold multiple values. Again, a good example would be group membership. The member and memberOf attributes mentioned above are also multivalued attributes. Obvious, because a group can contain multiple members, and a user/ group can be a member of multiple groups. Multivalued attributes can be identified by an attribute isSingleValued in their attributeSchema definition. If this value is TRUE it’s a single valued attribute, else its a multivalued attribute.

How the AD Replication Model Works

(source)

A must read post if you are interested in the details! 

On replication

• AD objects have attributes. AD replicates data at the attribute level – i.e. only changes to the attributes are replicated, not the entire object itself. 
  • Attributes that cannot be changed (for e.g. back links, administrative attributes) are never replicated. 
• Updates from the same directory partition are replicated as a unit to the destination DC – i.e. over the same network connection – to optimize network traffic.
• Updates are always specific a single directory partition. 
• Replication of data between DCs consist of replication cycles (i.e. the DCs keep replicating until all the data has replicated). 
  • There’s no limit to the number of values that can be transferred in a replication cycle. 
  • The transfer of data happens via replication packets. Approximately 100 values can be transferred per packet. 
    • Replication packets encode the data using ASN.1 (Abstract Syntax Notation One) BER (Basic Encoding Rules). Check out Wikipedia and this post for more information on ASN.1 BER (note: ASN.1 defines the syntax; BER defines the encoding. There are many other encodings such as CER, DER, XER but BER is what LDAP uses). See this blog post for an example LDAP message (scroll down to the end). 
  • Once a replication cycle has begun, all updates on the source DC – including updates that occur during the replication cycle – are send to the destination DC. 
  • If an attribute changes multiple times between replication cycles only the latest value is replicated. 
  • As soon as an attribute is written to the AD database of a DC, it is available for replication. 
• The number of values that can be written in a single database transaction (i.e. the destination DC has all the updates and needs to commit them to the AD database) is 5000. 
  • All updates to a single object are guaranteed to be written as a single database transaction. This way the state of an object is always consistent with the DC and there are no partially updated objects. 
    • However, for updates to a large number of values to multivalued attributes (e.g. the member attribute from the examples above) the update may not happen in the same transaction. In such cases the values are still guaranteed to be written in the same replication cycle (i.e. all updates from the source DC to that object will be written before the DC performs another replication cycle). (This exception for multivalued attributes is from Server 2003 and above domains, I think).
  • Prior to Windows Server 2003, when a multivalued attribute had a change the entire attribute was replicated (i.e. the smallest change that could be replicated was an attribute). 
    • For example, if a multivalued attribute such as member (referring to the members of a group) had 300 entries and now an additional entry was added, all 301 entries were replicated rather than just the new entry. 
    • So if a group had 5000 members and you added an extra member, 5001 entries were replicated as updates to its member attribute. Since that is larger than the number of values that can be committed in a single transaction, it would fail. Hence the maximum number of members in a group in a Windows 2000 forest functional level domain was 5000. 
  • Starting from Windows Server 2003, when a multivalued attribute has a change only the changed value is replicated (i.e. the smallest change that can be replicated is a value). Thus groups in Windows Server 2003 or Windows Server 2003 interim functional level domains don’t have the limitation of 5000 members. 
    • This feature is known as Linked Value Replication (LVR). I mentioned this in an earlier post of mine in the context of having multiple domains vs a single domain. Not only does LVR remove limitations such as the above, it also leads to efficient replication by reducing the network traffic.
    • When a Windows 2000 forest level domain is raised to Windows 2003/ 2003 interim forest level, existing multivalued attributes are not affected in any way. (If they were, there’d be huge network traffic as these changes propagate through the domain). Only when the multivalued attribute is modified does it convert to a replicate as single values. 
• Earlier I mentioned how all attributes are actually objects of the class attributeSchema.  The AD schema is what defines attributes and the relations between them.
  • Before replication between two DCs can happen, the replication system (DRS – see below) checks whether the schema of both DCs match. If the schemas do not match, replication is rescheduled until the schemas match (makes sense – if replication happens between DCs with different schemas, the attributes may have different meanings and relationships, and replication could mess things up). 
  • If the forest schema is changed, schema replication takes precedence over all other replication. In fact, all other replication is blocked until the schema replicates. 

Architecture

• On each DC, replication operates within the Directory System Agent (DSA) component – ntdsa.dll. 
  • DSA is the interface through which clients and other DCs gain access to the AD database of that DC. 
  • DSA is also the LDAP server. Directory-aware applications (basically, LDAP-aware applications) access DSA via the LDAP protocol or LDAP C API, through the wldap32.dll component  (am not very clear about this bit). LDAPv3 is used. 
• A sub-component of the DSA is DRS (Directory Replication System) using the DRS RPC protocol (Microsoft’s specification of this protocol can be found at this MSDN link). Client & server components of the DRS interact with each other to transfer and apply updates between DCs. 
  • Updates happen in two ways: via RPC or SMTP (the latter is only for non-domain updates, which I infer to mean Schema or Configuration partition updates) 
  • Domain joined computers have a component called ntdsapi.dll which lets them communicate with DCs over RPC.
    • This is what domain joined computers use to communicate with the DC.
    • This is also what tools such as repadmin.exe or AD Sites and Services (dssites.msc) use to communicate with the DC (even if these tools are running on the DC). 
  • DCs additionally have a private version of ntdsapi.dll which lets them replicate with other DCs over RPC. (I infer the word “private” here to mean it’s meant only for DCs).
    • As mentioned earlier, DC to DC replication can also bypass RPC and use SMTP instead. Then the ntdsapi.dll component is not used. Other components, such as ISMServ.exe and the CDO library are used in that case. Remember: this is only for non-domain updates. 
  • The RPC interface used by DRS is called drsuapi. It provides the functions (also see this link) for replication and management. Check this section of the TechNet post for an architecture diagram and description of the components. 
    • The TechNet post also mentions DRS.idl. This is a file that contains the interface and type library definitions. 
• The DSA also has a sub-component for performing low-level operations on the AD database. 
• The DSA also has a sub-component that provides an API for applications to access the AD database. 
  • The AD database is called ntds.dit (DIT == Directory Information Tree; see this link for more info). The database engine is Extensible Storage Engine (ESE; esent.dll)

Characteristics of AD replication

• Multimaster
  • Changes can be made to any DCs (as long as they are authoritative for the objects).
• Pull
  • When an update occurs on a DC it notifies its partners. The partners then requests (pulls) changes from the source DC. 
• Store-and-forward
  • When a DC receives a change from its partners, it stores the change and in-turn forwards on to others (i.e. informs others so they can issue pull requests from itself). Thus, the DC where a change is made does not have to update every other DCs in the domain. 
  • Because of the store-and-forward mechanism replication must be thought of as sequentially moving through the domain. 
• State-based
  • Each DC has metadata to know the “state” of its replicas. This state is compared with that of its partner to identify the changes required. (This is in contrast to log-based replication where each DC keeps a log of changes it made and sends that log to its partners so they can replay the log and update themselves). 
  • This makes uses of metadata such as Update Sequence Number (USN), Up-to-dateness vector, and High-watermark vector. Synchronized time is not primarily used or required to keep track of this (it is used, but in addition to other mechanisms). 

Updates

LDAP/ AD supports the following four types of update requests:

• Add an object to the directory.
• Delete an object from the directory. 
• Modify (add, delete, remove) attribute values of an existing object in the directory. 
• Move an object by changing the name or parent of the object. 

Each of the above update requests generates a separate write transaction (because remember, all updates to a single object happen in a single transaction). Updates are an all-or-nothing event. If multiple attributes of an object are updated, and even one of those updates fail when writing to the database, all attributes fail and are not updated. 

Once an update request is committed to the database it is called an originating update. When a DC receives an originating update, writes it to its database, and sends out updates to other DCs these are called replication updates. There is no one-to-one relation between originating updates and replication updates. For instance, a DC could receive multiple originating updates to an object – possible even from different DCs – and then send a replication update with the new state of that object. (Remember: AD is state-based, not log-based. It is the state that matters, not the steps taken to reach that state). 

Adding request:

• Creates a new object with a unique objectGUID attribute. Values of all replicated attributes are set to Version = 1. (The Version attribute will be discussed later, below). 
• The add request fails if the parent object does not exist or the DC does not contain a writeable replica of the parent object’s directory partition. 

Modify request:

• If an attribute is deleted then its value is set with NULL and Version is incremented. 
• If values are to be added/ removed from an attribute, it is done so and the attribute Version is incremented. The Modify request compares the existing and new values. If there are no changes then nothing happens – the request is ignored, the Version does not change. 

Move request:

• This is a special case of the Modify request in that only the name attribute is changed. 

Delete request:

• The isDeleted attribute is set to TRUE. This marks the object as tombstoned (i.e. deleted but not removed from AD). 
• The DN of the object is set to a value that cannot be set by an LDAP application (i.e. an impossible value). 
• Except a few important attributes, the rest are stripped from the object (“stripped”, not removed as one would do above). 
• The object itself is moved to the Deleted Objects container. 

Certain objects are protected from deletion:

• Cross-references (class crossRef) and RID Object for the DC. Any delete request for these objects are rejected. And any originating update deletion of these objects is rejected, and all attributes of the object are updated (Version incremented) so the object replicates to other DCs and is reanimated wherever it was deleted.  
  • Cross-references are present in the Configuration partition: CN=Partitions,CN=Configuration,DC=(domain)
  • RID Objects are present under each DC at CN=RID Set,CN=(DC),OU=Domain Controllers,DC=(domain)
• The NTDS Settings (class nTDSDSA) object.
  • This object represents the DSA (Directory System Agent). It is present in the Configuration partition under the site and DC: CN=NTDS Settings,CN=(DC),CN=Servers,CN=(SITE),CN=Sites,CN=Configuration,DC=(domain)
  • Remember, the DSA is the LDAP server within the DC. The DSA’s GUID is what we see when DCs replicate or in their DNS names (DSAGUID._msdcs.domain is a CNAME to the DC name).
  • The objectGUID of the DC object is not the same as the objectGUID of its DSA object. The latter is what matters. When a DC computer object is opened in ADUC, it is possible to view the corresponding DSA object by clicking on the “NTDS Settings” button in the first tab. 
  • Trivia: to find all the DCs in a forest one can query for objects of class nTDSDSA. Below query, for instance, finds all objects of that class and returns their DN and objectGUID. 

    

  • When a DC is demoted, its DSA object is deleted but not really deleted. It is protected and disabled from receiving replication requests. Thus, a query such as the above will return DSA objects of DCs that may no longer exist in the forest.

    A better way to structure the above query then would be to also filter out objects whose isDeleted attribute is not TRUE.

    

Conflicts

AD does not depend on timestamps to resolve conflicts. In case of conflicts the “volatility” of changes is what primarily matters. That is, if an attribute was updated twice on one DC and thrice on another DC, even if the timestamps of changes from the first DC are later than the second DC, the change from the second DC will win because the attribute was updated thrice there. More on this in my next post. 

Database GUID

I mentioned above that each DSA has its own objectGUID. This GUID is created with the server is promoted to a DC and deleted (sort of) when the DC is demoted. Thus the GUID is present for the lifetime of the DC and doesn’t change even if the DC name changes. 

The AD database (ntds.dit) on each DC has its own GUID. This GUID is stored in the invocationId attribute of the DSA. Unlike the DSA GUID, the database GUID can change. This happens when (1) the DC is demoted and re-promoted (so the database changes), (2) when an application NC is added/ removed to the DC, (3) when the DC/ database is restored from a backup, or (4) (only in Server 2012 and above) whenever a DC running in a VM is snapshotted or copied.

The invocationId attribute can be viewed via ldp.exe as above, or in the “NTDS Settings” of the DC computer object in AD Users & Computers, or in the “NTDS Settings” in AD Sites & Services. It can also be viewed in the output of repadmin.exe.

The invocationID is a part of replication requests (more later). When the invocationID changes other know that they have to replicate changes to this DC so it can catch up. The first DC in a domain will have its invocationID and objectGUID as same (until the invocationID changes). All other DCs will have different values for these both. 

Update Sequence Numbers (USNs)

(I was planning on covering this separately as part of my AD Troubleshooting WorkshopPLUS posts, but USNs are mentioned in this TechNet post so I might as well cover them now). 

USNs are 64-bit counters maintained on each DC. The number is different for each DC and is stored in the highestCommittedUsn attribute of the rootDSE object.

Each time a DC commits an update (originating update/ replication update), it updates the highestCommittedUSN attribute. Thus the USN is associated with each successful transaction to the AD database. Remember: all updates to a single object are written in a single transaction, so a USN is essentially associated with each successful update to an object – be it a single attribute, or multiple updates to the same/ multiple attributes (but all written in the same transaction). 

USNs are update incrementally for each write transaction. So, for example, when I took the above screenshot the USN for WIN-DC02 was 77102. When I am writing this paragraph (the next day) the USN is 77230. This means between yesterday and today WIN-DC02 has written 128 transactions (77230-77102) to its database.  

Every attribute of an object has a USN associated with it. This is a part of the replication metadata for that object, and can be viewed through the repadmin.exe command. For instance, using the /showobjmeta switch (note that here we specify DC first and then object):

Notice how the attribute USNs vary between the two DCs. Also notice the metadata stored – the Local USN, the originating DSA, the Originating DSA's USN, the timestamp of the update, and Version. If you are running the above command it is best to set the command prompt window width to 160, else the output doesn’t make much sense. I will talk about Local USN and Originating USN in a moment. 

Another switch is /showmeta (here we specify the object first and then the DC):

Both switches seem to produce the same output. The important items are the Local USN and Originating USN, and the Version. 

• Version starts from 1 and is incremented for each update. Version will be same on all DCs – when an DC commits an update request (originating update or replication update) it will increment the Version. Since all attributes start with the same Version = 1, the current value will be the same on all DCs. 
• Local USN is the USN value for that attribute on the DC we are looking at. It is the value of the highestCommittedUSN for the transaction that committed the update to this attribute/ set of attributes. 
• Originating USN is the USN for that attribute on the DSA where the originating update was sent from.

For instance: the attribute description. WIN-DC01 has local USN 46335, WIN-DC02 has 19340, and WIN-DC03 has 8371. The latter two DCs got their update for this attribute from WIN-DC01 so they show the originating DSA as WIN-DC01 and the Originating USN as 46335.

Every object has an attribute called uSNChanged. This is the highest Local USN among all the attributes of that object. (What this means is that from the USN of an object we can see which of its attributes have the same local USN and so easily determine the attributes changed last). Unlike the attribute USNs which are metadata, uSNChanged is an object attribute and thus can be viewed via ADSI Edit or ADUC. From the command line, repadmin.exe with the /showattr switch can be used to view all attributes of an object (this switch takes the DC name first, then the object). 

Above output tells me that on WIN-DC01 the uSNChanged for this object is 46335.  From the earlier /showobjmeta output I know that only the description attribute has that local USN, so now I know this was the last attribute changed for this object. 

So, to recap: DCs have a USN counter stored in highestCommitedUSN attribute. This counter is updated for each successful write transaction to the database. Since writes happen per object, it means this USN is updated every time an object is updated in the database. Each object attribute has its own USN – this too is per DC, but is not an attribute, it is metadata. Finally, each object has a uSNChanged attribute which is simply the highest attribute USN of that object. This too is per DC as the attribute USN is per DC. The DC’s highestCommittedUSN attribute and an object’s uSNChanged attribute are related thus: 

• When an attribute update is committed to the database the DC’s highestCommittedUSN is incremented by 1.
• The Local USN of the attribute/ attributes is set to this new highestCommittedUSN. 
• This in turns updates the object’s uSNChanged to be the new highestCommittedUSN (because that is the highest attribute Local USN now)

Thus, the highestCommittedUSN is the highest uSNChanged attribute among all the replicas held by the DC.  

Here’s an example. First I note the DC’s highestCommitedUSN (I have removed the other attributes from the output). 

Then I note an object’s uSNChanged:

Now I connect via ADUC and change the description field. Note the new USN. 

And note the DC’s USN:

It has increased, but due to the other changes happening on the DC it is about 10 higher than the uSNChanged of the object I updated. I searched the other replicas on my DC for changes but couldn’t find anything. (I used a filter like (uSNChanged>=125979) in ldp.exe and searched every replica but couldn’t find any other changes. Maybe I missed some replica – dunno!) This behavior is observed by others too. (From the previous linked forum post I came across this blog post. Good read).  

Finally, I must point out that even though I said the attribute metadata (Local USN, Originating USN, Originating DSA, Originating Time/ Date, Version) are not stored as attributes, that is not entirely correct. Each object has an attribute called replPropertyMetaData. This is an hexadecimal value that contains the metadata stored as a binary value. In fact, if we right click on an object in ldp.exe it is possible to view the replication metadata because ldp.exe will read the above attribute and output its contents in a readable form. 

Bear in mind this attribute is not replicated. It is an attribute that cannot be administratively changed, so can never be updated by end-users and doesn’t need replicating. It is only calculated and maintained by each DC. uSNChanged is a similar attribute – not replicated, only administratively changed. 

Note to self: I need to investigate further but it looks like uSNChanged cannot be viewed by ldp.exe for all objects. For instance, ldp.exe shows this attribute for one user object in my domain but not for the other! I think that’s because the attribute is generated by the AD tools when we view it. repadmin.exe and the GUI tools show it always. Similarly, the attribute level metadata attribute too cannot be viewed by ldp.exe for all objects. For some objects it gives an error that the replPropertyMetaData attribute cannot be found and so cannot show the replication metadata. This could also be why there was a gap between the uSNChanged and highestCommittedUSN above. 

High-watermark Vector (HWMV) & Up-to-dateness Vector (UTDV)

To recap once more, what we know so far is:

• Every attribute has replication metadata that contains its Local USN, the originating DSA's USN, the originating DSA, the timestamp, and Version. 
• The highest Local USN of an attribute is stored as the object’s uSNChanged attribute. 
• The highest uSNChanged attribute among all objects on all replicas held by the DSA is stored as its highestCommittedUSN attribute in the rootDSA. 
• All these USN counters are local to the DC (except for the Originating USN which is local to that the originating DSA). 

How can DCs replicate with each other using the above info? Two things are used for this: High-watermark vector and Up-to-dateness vector. 

The HWMV is a table maintained on each DC.

• There is one table for each directory partition the DC holds replicas for (so at minimum there will be three HWMV tables – for the domain partition, the Schema partition, and the Configuration partition). 
• The HWMV table contains the highest USN of the updates the DC has received from each of its replication partners for that replica. Note: the table contains the USN that’s local to the replication partner.
• Thus the HWMV can be thought of as a table contain the highest uSNChanged value for each directory partition of its replication partners when they last replicated.

Got it? Now whenever a DC needs to replicate with another DC, all it needs to do is ask it for each of the changes since the last uSNChanged value the destination is aware of! Neat, right! The process looks roughly like this:

1. The originating DC will have some changes. These changes will cause uSNChanged values of the affected objects to change. The highestCommittedUSN of the DC too changes. All these are local changes.
2. Now the DC will inform all its replication partners that there are changes. 
3. Each of these replication partners will send their HWMV for the partition in question.
4. From the received HWMV table, the originating DC can work what changes need to be sent to the destination DC. 
   1. The originating DC now knows what USNs it needs to look for. From this it knows what objects, and in turn which of their attributes, were actually changed. So it sends just these changed attributes to the destination DC. 

To make the process of replication faster, all DCs have one more table. That is the Up-to-dateness Vector (UTDV).

• Like the HWMV, this too is for every replica the DC holds. 
• Unlike the HWMV this contains the highest USN of the updates the DC has received from every other DC in the domain/ forest for that replica. 
• The table also contains the timestamp of last successful replication with each of those DCs for that replica.

The UTDV table has a different purpose to the HWMV. This table is sent by the destination DC to the originating DC when it requests for changes – i.e. in step 3 above.

When the originating DC gets the UTDV table for its destination DC, it can look at the table and note the destination DC’s highest received USNs for that partition from other DCs. Maybe the destination DC has asked for changes from USN number x upwards (the USN number being of the originating DC). But these changes were already received by the destination DC from another DC, under USN number y and upwards (the USN number being of that other DC). The destination DC does not know that the changes requested by USN x and upwards are already with it from another DC, but by looking at the UTDV table the originating DC can see that USNs y and above already contain the changes the destination DC is requesting, so it can filter out those updates when sending. (This feature is called “propagation dampening”). 

• In practice, once the originating DC compiles a list of USNs that need to be sent to the destination DC – at step 4 above – it goes through its replication metadata to check each attribute and the originating DSA and originating USN associated with that attribute.
• The originating DC then looks at the UTDV table of the destination DC, specifically at the entry for the DC that had sent it an update for the changed attribute. (This DC could be same as the originating DC). From the table it can see what USNs from this DC are already present at the destination DC. If the USN value in the table is higher than the USN value of the attribute, it means the destination DC already has the change with it. So the originating DC can filter out these and similar attributes from sending to the destination DC.  

Thus the UTDV table works along with the HWMV table to speed up replication (and also avoid loops wherein one DC updates another DC who updates the first DC and thus they keep looping). And that is how replications happen behind the scenes! 

Once a destination DC updates itself from an originating DC – i.e. the replication cycle completes – the source DC sends its UTDV table to the destination DC. The destination DC then updates its UTDV table with the info from the received UTDV table. Each entry in the received table is compared with the one it has and one of the following happens:

• If the received table has an entry that the destination DC’s UTDV table does not have – meaning there’s another DC for this replica that it isn’t aware of, this DC has replicated successfully with the originating DC and so all the info it has is now also present with the destination DC, and so it is as good as saying this new DC has replicated with the destination DC and we are aware of it the same way the originating DC is aware – so a new entry is added to the destination DC’s UTDV table with the name of this unknown DC and the corresponding info from the received UTDV table. 
• If the received table has an entry that the destination DC’s UTDV table already has, and its USN value is higher than what the destination DC’s table notes – meaning whatever changes this known DC had for this partition has already replicated with the originating DC and thus the destination DC – and so its entry in the UTDV can actually be updated, the UTDV table for that server is updated with the value from the received UTDV table.  

The UTDV table also records timestamps along with the USN value. This way DCs can quickly identify other DCs that are not replicating. These timestamps record the time the DC last replicated with the other DC – either directly or indirectly. 

Both HWMV and UTDV tables also include the invocationID (the database GUID) of the DCs. Thus, if a DC’s database is restored and its invocationID changes, other DCs can take this into account and replicate any changes they might have already replicated in the past.  

From the book I am reading side-by-side (excellent book, highly recommended!) I learnt that apart from the HWMV and UTDV tables and the naming context it wants to replicate, the destination DC also sends two other pieces of info to originating DC. These are: (1) the maximum number of object updates the destination DC wishes to receive during that replication cycle, and (2) the maximum number of values the destination DC wishes to receive during that replication cycle. I am not entirely clear what these two do. Once a replication cycle begins all object updates and values are sent to the destination DC, so the two pieces above seems to be about whether all the updates are sent in one replication packet or whether they are split and sent in multiple packets. The maximum number of values in a single packet is about 100, so I guess these two numbers are useful if you can only accept less than 100 values per packet – possibly due to network constraints.