MS-ASCMD § 2.2.3.88.2 HeartbeatInterval Sync correct usage unclear

Question

MS-ASCMD § 2.2.3.88.2 HeartbeatInterval Sync correct usage unclear

Noah Bergbauer 0

According to the specification, if the specified HeartbeatInterval is within the range allowed by the server and there is no a Wait element, a Sync containing no other errors should return successfully.

I am testing this against outlook.com with a very simple request:

<Sync>
    <Collections>
        <Collection>
            <SyncKey>2xxxxxx37</SyncKey>
            <CollectionId><id></CollectionId>
        </Collection>
    </Collections>
    <HeartbeatInterval>120</HeartbeatInterval>
</Sync>

The server appears to understand the HeartbeatInterval: the request blocks and a response is only received when I either make a change in the given folder or the given timeout of 120 seconds expires.

However, the server then always replies this:

<Sync>
    <Status>102</Status>
</Sync>

This is puzzling because error code 102 is specified in MS-ASCMD § 2.2.2 as "InvalidWBXML" which doesn't make any sense because the server evidently must have decoded every element in the request correctly. Without correctly decoding the HeartbeatInterval it could not have known to block the request for the requested amount of time.

Is there some other requirement that a Sync request with a HeartbeatInterval must fulfil in order to be considered valid? Is there any scenario where error 102 can have a meaning other than that the request body is not valid WBXML (which it definitely is in this case)?

KristianSmith-MSFT 451 Reputation points Microsoft Employee Moderator

2025-11-20T16:57:58.0433333+00:00

Hi Noah,

Thanks for reaching out with your question regarding MS-ASCMD. One of our Open Specs engineers will investigate this question and reach out in this thread. In the meantime, please have a look at the response from Vergil-V and respond accordingly.

Regards,
Kristian S
Microsoft Open Specifications

2 answers

Your answer

KristianSmith-MSFT 451 Reputation points Microsoft Employee Moderator

2025-11-20T16:57:58.0433333+00:00

Hi Noah,

Thanks for reaching out with your question regarding MS-ASCMD. One of our Open Specs engineers will investigate this question and reach out in this thread. In the meantime, please have a look at the response from Vergil-V and respond accordingly.

Regards,
Kristian S
Microsoft Open Specifications

Answer 1

The error code 102, which corresponds to "InvalidWBXML," indicates that the server has encountered an issue with the WBXML format of the request. Even though the server seems to understand the HeartbeatInterval and blocks the request as expected, there may be other elements in the request that are not formatted correctly or are not compliant with the expected WBXML structure.

Here are a few considerations that might help clarify the situation:

Request Structure: Ensure that the entire request is properly formatted according to the WBXML specifications. Any malformed tags or incorrect nesting can lead to the server interpreting the request as invalid.
Required Elements: Check if there are any mandatory elements that must be included in the Sync request when using HeartbeatInterval. If any required elements are missing, it could trigger an invalid response.
Server-Side Constraints: Sometimes, server implementations may have additional constraints or requirements that are not explicitly documented in the specification. It may be useful to consult with the server's documentation or support to see if there are known issues or additional requirements for the Sync request.
Error Handling: The server may return an error code for reasons other than the request body being invalid. It is worth investigating if there are specific conditions under which the server might return an error code despite the request being well-formed.

If you continue to encounter issues, consider reaching out to the support channels for the server or checking community forums for similar experiences.

Answer 2

Vergil-V 7,555 Microsoft External Staff Moderator

Please note that our forum is a public platform, and we will modify your question to hide your personal information in the description. Kindly ensure that you hide any personal or organizational information the next time you post an error or other details to protect personal data.
Hi @Noah Bergbauer

Thank you for reaching out to Microsoft Learn Q&A!

Based on my research, error 102 does not indicate anything beyond suggesting that the WBXML is not valid.

Additionally, I noticed that your XML snippet does not include HeartbeatInterval. May I ask where you placed the HeartbeatInterval? It should appear before the Collections element and be at the same node level.
If possible, please share your XML that includes HeartbeatInterval so we can better understand your situation.

Regarding the 120-second timeout, may I clarify that this corresponds to the HeartbeatInterval? To check whether the server honors the HeartbeatInterval, you could try adjusting its value and observe if the timeout changes from 120 seconds.

Furthermore, please double-check that your MS-ASProtocolVersion is supported according to the reference table.

undefined Reference: [MS-ASCMD]: HeartbeatInterval (Sync) | Microsoft Learn

I look forward to your updates.

Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.

Vergil-V 7,555 Reputation points Microsoft External Staff Moderator

2025-11-20T23:11:45.0066667+00:00

Hi @Noah Bergbauer

I just wanted to kindly follow up to see if you’ve had a chance to review my previous response.   
I would greatly appreciate any updates you may be able to provide, particularly regarding the usefulness of the information we shared. Your feedback is essential to our ongoing efforts to enhance the quality of our support.   

Thanks again for your time, and we look forward to hearing from you.
Noah Bergbauer 0 Reputation points

2025-11-21T09:03:53.5133333+00:00

Thank you. The tag was missing because of a problem with the editor here which would not let me paste any XML. I had to type it in by hand and simply missed the tag when doing so. I have edited the question to fix this.

I can confirm that the server blocks for as many seconds as given in the HeartbeatInterval if I vary the value. If I give a value below 60 (with no other changes to the request!), I get a Limit error telling me that the interval needs to be at least 60 seconds. So I am very confident that the request is decoded correctly. If you would like to double-check, here is a hexdump of the request that I am sending:

MS-ASProtocolVersion is set to 16.1 and if I go below 14.0 (using Wait on 12.1) then the code 102 changes into a HTTP 400 as documented in §2.2.2.
Vergil-V 7,555 Reputation points Microsoft External Staff Moderator

2025-11-24T14:29:37.56+00:00

Please note that our forum is a public platform, and we will modify your response to hide your personal information in the description. Kindly ensure that you hide any personal or organizational information the next time you post an error or other details to protect personal data.

Hi @Noah Bergbauer

Thank you for your follow-up. To protect your privacy, I’d like to move the hexdump to a private message.

To rule out expired sync keys as the cause, you could try generating a new sync key by running SyncKey = 0 and check if the 102 error persists.

Additionally, could you confirm the Content-Type header?

It should be: application/vnd.ms-sync.wbxml.

From my research, it’s also recommended to add a WindowSize parameter. However, as a forum moderator without access to a dedicated testing environment, I’m unable to replicate and confirm whether this resolves the issue. In the meantime, I recommend reaching out to the Microsoft Tech Community, where experts with the right tools and experience can provide deeper insights.

I apologize for any inconvenience caused by suggesting another forum, but I hope this gives you additional support for your situation.

Please let me know if you have any updates.
Noah Bergbauer 0 Reputation points

2025-11-24T22:47:30.31+00:00

Hello Vergil-V,

the process you describe is exactly what I am doing. I am starting with SyncKey=0 and calling Sync until I have downloaded all available changes. At this point a Sync request without HeartbeatInterval returns an empty response, and a request with HeartbeatInterval blocks, and eventually returns the 102 error. I can confirm that the Content-Type is application/vnd.ms-sync.wbxml. The presence or absence of an additional WindowSize parameter does not appear to make any difference.

It is perhaps worth noting that if the server has pending changes (e.g. when calling Sync with the initial sync key returned from SyncKey=0) then adding a HeartbeatInterval does not make any difference and the server still returns an instant successful response with no errors. The 102 error only happens when the request actually blocks, i.e., when the HeartbeatInterval "does something".

I also tested all of this with Wait instead of HeartbeatInterval and the behavior is exactly the same.

I understand that you are just a forum moderator, but I posted in this forum because I am trying to get a response on this from Microsoft. As the error code indicates an invalid request, but I am at this point quite certain that my request complies with the specification, my best guess right now is that perhaps my request is violating some requirement that is missing in the specification. So I hope that I can get some insight on this question from a Microsoft engineer, as indicated by @KristianSmith-MSFT above.
Mariusz Kaszewiak 95 Reputation points Microsoft Employee Moderator

2025-11-24T23:45:02.47+00:00

Hi @Noah Bergbauer

My name is Mariusz Kaszewiak from the Microsoft Office Open Specifications team.

You are correct that error code 102 should be generated when there is a WBXML decoding issue, but that is not the only possible cause. I am still investigating your case; however, I noticed in the MS-ASWBXML specification (section 2.1.2.1.1, Code Page 0: AirSync) that the HeartbeatInterval tag is only supported in some protocol versions. This might be contributing to the error.

It could be worth testing without the HeartbeatInterval tag to rule this out while I continue my internal investigation. I will update you as soon as I have more information.

Best regards,
Mariusz Kaszewiak
Vergil-V 7,555 Reputation points Microsoft External Staff Moderator

2025-11-25T05:18:51.1266667+00:00

Hi @Noah Bergbauer

Thank you for sharing your insights. I truly appreciate your understanding and cooperation.

As your case is currently being reviewed by Mariusz Kaszewiak, we would greatly appreciate it if you could kindly provide your response to his follow-up message. Your input will be invaluable in helping us clarify the situation and move forward effectively.

Thank you once again for your time and support.
Noah Bergbauer 0 Reputation points

2025-11-25T08:24:32.7+00:00

Hi @Mariusz Kaszewiak ,

can you clarify what you mean by that? When I look at the table you referenced, I see that HeartbeatInterval is supported in versions 14.0 through 16.1. To my understanding, 16.1 is the latest version.
Mariusz Kaszewiak 95 Reputation points Microsoft Employee Moderator

2025-11-25T09:40:49.2666667+00:00

Hi @Noah Bergbauer
Appoligises my mistake, it ment to be in some versions (not all ) it was queit late for me when I wrote the comment and didn't notice it. As I understand you are on the 16.1 so this doesn't affect you.

I am currently troublshooting your issue and will update as soon I will have an update.

Best regards

Mariusz Kaszewiak
Mariusz Kaszewiak 95 Reputation points Microsoft Employee Moderator

2025-11-26T14:58:03.5166667+00:00

Hi @Noah Bergbauer Thank you for your patience. I just wanted to give you an update on the issue: I’ve reached out to the Exchange Dev team internally for insights and I’m also working on reproducing the issue in my test environment. I’ll share any updates as soon as I have them.
Best Regards
Mariusz K

Mariusz Kaszewiak 95 Microsoft Employee Moderator

Hi @Noah Bergbauer
Thank you for your patience. I did tried to reproduce your issue on my test environment but I am not able to get exactly same error as you do with the below request

!--Initial Request -->
<?xml version="1.0" encoding="utf-8"?>
<Sync xmlns="AirSync:">
	<Collections>
		<Collection>
			<SyncKey>0</SyncKey>
			<CollectionId>9</CollectionId>
	</Collection>
	</Collections>
	<HeartbeatInterval>120</HeartbeatInterval>
</Sync>


!--Subsequential Request -->
<?xml version="1.0" encoding="utf-8"?>
<Sync xmlns="AirSync:">
    <Collections>
        <Collection>
            <SyncKey>686393510</SyncKey>
            <CollectionId>9</CollectionId>
            <GetChanges>1</GetChanges>
        </Collection>
    </Collections>
    <HeartbeatInterval>60</HeartbeatInterval>
</Sync>

but I get some decoding issue on client site.  
I want to highlight that I am running my tests against the on-premises version of Exchange Server 2019 CU15, whereas, as I understand, you are using Exchange Online, which may behave differently.

As part of the Open Specifications Team, we focus on on-premises versions, so I have reached out internally to the Exchange Online team to gather insights on your issue. Once I receive an update, I will share it here.

Best regards,

Mariusz Kaszewiak

Mariusz Kaszewiak 95 Reputation points Microsoft Employee Moderator

2025-12-03T09:35:13.9433333+00:00

Hi @Noah Bergbauer To investigate your issue further, we need logs from the Outlook mailbox. Could you please capture the EAS mailbox log by following this guide:

Collect ActiveSync device logs to troubleshoot sync issues for mobile devices - Exchange | Microsoft Learn

For security reasons, please do not share the logs here. Instead, send them by email to dochelp@microsoft.com, referencing my name: Mariusz Kaszewiak.

If you need any assistance capturing the logs, we can continue the conversation over email.

Best regards, Mariusz Kaszewiak Office Open Specification Team

Noah Bergbauer 0

Hello @Mariusz Kaszewiak thank you for your help. I noticed that your request has the SyncKey set to 0. However as I explained above, the initial sync with key 0 also works for me. The issue appears once I have downloaded all the initial changes and the requests start blocking. This might explain why you were not able to reproduce the problem.

The logs seem confirm that there is no problem on my end - here is the relevant part (with any potentially sensitive information redacted):

RequestBody : 
<?xml version="1.0" encoding="utf-8" ?>
<Sync xmlns="AirSync:">
   <Collections>
           <Collection>
                   <SyncKey>1xxxxxxx</SyncKey>
                   <CollectionId>x</CollectionId>
                   <GetChanges>1</GetChanges>
                   <WindowSize>1</WindowSize>
                   <Options>
                           <Class>Calendar</Class>
                           <BodyPreference xmlns="AirSyncBase:">
                                   <Type>1</Type>
                           </BodyPreference>
                   </Options>
           </Collection>
   </Collections>
   <HeartbeatInterval>120</HeartbeatInterval>
</Sync>
                                     
WasPending : 
[Response was pending]
                                     
ResponseHeader : 
HTTP/1.1 200 OK
X-MS-BackOffDuration: L/-470
                                     
                                     
ResponseBody : 
<?xml version="1.0" encoding="utf-8" ?>
<Sync xmlns="AirSync:">
   <Status>102</Status>
</Sync>

So the server has no problems decoding my wbxml and still replies with the 102 error. I also sent the full unredacted log by email as you asked.

Mariusz Kaszewiak 95 Reputation points Microsoft Employee Moderator

2025-12-04T11:02:12.5933333+00:00
Hi @Noah Bergbauer Apologies for that — I didn’t copy the full two samples I had prepared for you. I’ve now added the subsequent request as well. In this particular example you shared, the HeartbeatInterval is not the issue. Here is the revised version of your sample:

<?xml version="1.0" encoding="utf-8"?> <Sync xmlns="AirSync:" xmlns:airsyncbase="AirSyncBase:"> <Collections> <Collection> <SyncKey>1014044839</SyncKey> <CollectionId>1</CollectionId> <GetChanges>1</GetChanges> <WindowSize>512</WindowSize> <Options> <Class>Calendar</Class> <airsyncbase:BodyPreference> <airsyncbase:Type>1</airsyncbase:Type> </airsyncbase:BodyPreference> </Options> </Collection> </Collections> <HeartbeatInterval>120</HeartbeatInterval> </Sync>

Inside BodyPreference, you change the default namespace to AirSyncBase and omit the prefix.

While this is legal XML, many ActiveSync WBXML encoders (and some server-side decoders) do not properly switch the WBXML code page when you rely on a local default namespace change. They expect explicit, prefix-qualified elements for non-default namespaces.
ActiveSync uses WBXML code pages. Elements from different specs (AirSync vs AirSyncBase, Email, Email2, Contacts, etc.) live in different code pages.

A typical WBXML encoder maps XML prefixes to those code pages. If you:

Use airsyncbase:BodyPreference the encoder switches to the AirSyncBase code page and encodes the right tokens.

Change the default namespace within a subtree and omit the prefix some encoders stay on the AirSync code page (or fail to switch), producing wrong tokens (e.g., encoding BodyPreference as if it were an AirSync element). The server then rejects or misinterprets the request.

So for this particular request please try the sample I shared.
For the other request you had the problem with could you please collect logs using the method I shared in my previous comment and send them via email I shared so I can escalate your issue.

Best regards,

Mariusz Kaszewiak
Noah Bergbauer 0 Reputation points

2025-12-04T15:38:45.2733333+00:00

Hi @Mariusz Kaszewiak there must be a misunderstanding because the XML I shared does not come from my end. It comes from the logs I collected using the commands you pointed me to. You should have also received them by email as I sent them yesterday - if not please let me know.
Mariusz Kaszewiak 95 Reputation points Microsoft Employee Moderator

2025-12-04T16:54:38.5166667+00:00

Hi @Noah Bergbauer thanks for the clarification. I shared this link with you https://v4.hkg1.meaqua.org/en-us/troubleshoot/exchange/mobile-devices/issues-for-mobile-devices to collect logs I don't see this xml you are referencing there. Can you confirm that 102 happens with the last sample xml request I sent you ?
I didn't receive the logs unfortunatelly. Can you please try to send them again. Possibly send email without attached logs referencing my name so I can reply and we can continue conversation via email.
Thanks
Noah Bergbauer 0 Reputation points

2025-12-04T21:13:53.68+00:00
Hi @Mariusz Kaszewiak here is what I did:

I completed the steps from the link you shared to collect the logs

I received the logs in my inbox

I looked into the log file to find the request with the 102 error

I saw that the logfile contains exactly the same request and response as I see on my end (as I posted above)

The local default namespace that you pointed out is how the O365 server happened to decode and log the wbxml wbxml. It is not how I constructed the request on my end.

I sent another email to dochelp@microsoft.com requesting a reply from you.

Thanks

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MS-ASCMD § 2.2.3.88.2 HeartbeatInterval Sync correct usage unclear

2 answers

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