# Introduction
`CVR` is a command introduced with [MSNP2](../versions/msnp2.md).

It is a Notification Server and as of [MSNP8](../versions/msnp8.md) a Dispatch Server command, without either a request or response payload.

Sends the current client information and retrieves the latest avaliable version of the client.
For the version of this command that is sent in [CVR0](../versions/cvr0.md), read [CVQ](cvq.md).

# Client/Request
`CVR TrID locale system-type system-ver system-arch requesting-library client-version {client-identification} {user-handle}`

Where `locale` is a 16-bit hexadecimally encoded LCID. `0x0409` is the LCID for English, United States.

Where `system-type` is a string that defines the operating system you are using, such as `winnt`.

Where `system-ver` is the major and minor version of the operating system, such as `5.1`.

Where `system-arch` is the architecture of the processor that is used to run the client, usually `i386`.

Where `requesting-library` is the name of the library that requested this CVR, usually `MSMSGS` or `MSNMSGR`.

Where `client-version` is the current client's major, minor and patch version, should be in the format of "{M}M.m.pppp".

Where `client-identification` is the internal name of the codebase used to create `requesting-library`, usually `MSMSGS` or an empty parameter.
Added since [MSNP4](../versions/msnp4.md).

Where `user-handle` is the user's handle. Added since [MSNP8](../versions/msnp8.md).

# Server/Response
`CVR TrID recommended-version recommended-version-2 minimum-allowed-version download-url {fallback-url}`

Where `recommended-version` is the current version of the client for this system.

Where `recommended-version-2` is (usually) the same as `recommended-version`,
Could possibly be a maximum allowed version? (forced downgrade)

Where `minimum-allowed-version` is the lowest version the server considers "safe" to connect with.
If the client's `client-version` is lower than the server's `minimum-allowed-version`,
the client should automatically disconnect from the server,
and request an forced upgrade using the binary provided in `download-url`.

Where `download-url` is the file to download and open to update this client to `recommended-version`.

Where `fallback-url` is the URL the client opens if it failed to download `download-url` for any reason. Added since [MSNP2](../versions/msnp2.md).

# Examples
*NOTE: This has been line-breaked.
Lines beginning with `..` are continuations of the previous line."

## Post-login
*Only used in [MSNP2](../versions/msnp2.md) to [MSNP7](../versions/msnp2.md).*
```
C: CVR 1 0x0409 win 4.10 i386 MSMSGS 1.0.0863 MSMSGS
S: CVR 1 5.0.0537 5.0.0537 1.0.0863
.. http://download.microsoft.com/download/msnmessenger/install/5.0/w98nt42kmexp/en-us/setupdl.exe
.. http://messenger.microsoft.com
```

Client asks if user would like to update, and stays connected.

## Pre-login
*Since [MSNP8](../versions/msnp8.md).*
```
C: CVR 2 0x0409 winnt 5.1 i386 MSNMSGR 6.0.0602 MSMSGS example@hotmail.com
S: CVR 2 7.0.0813 7.0.0813 6.2.0205
.. http://msgr.dlservice.microsoft.com/download/5/d/9/5d9bb5b2-49c7-4890-94ab-d1d5e44a0e6d/Install_MSN_Messenger.exe
.. http://messenger.msn.com
```

Client disconnects from server, since it sees that `minimum-allowed` is above it's `client-version`.

## Invalid client identification or version
*This response may or may not disconnect you.*
```
C: CVR 3 0x0409 winnt 5.0 i386 MSNMSGR 99.9.9999 MSMSGS
S: 420 3
C: CVR 4 0x0409 winnt 5.0 i386 YCOMM 0.1.0023 MSMSGS
S: 420 4
```

## Invalid language ID
```
C: CVR 5 0x1111 winnt 5.1 MSNMSGR 6.0.0602 MSMSGS example@hotmail.com
S: 710 5
```

## Invalid CVR
```
C: CVR 6 spaghetti
S: 731 6
```

Server disconnects client.

# Known changes
* [MSNP2](../versions/msnp2.md): Added a fallback URL parameter to the response (parameter 5). 
  In [CVR0](../versions/cvr0.md) Only: Changed [CVR](cvr.md) to [CVQ](cvq.md) carrying the previous change.
* [MSNP4](../versions/msnp4.md): Added a client codebase identification parameter (parameter 7).
* [MSNP8](../versions/msnp8.md): Added a current user parameter (parameter 8),
  and replaced [INF](inf.md) in the login process.
* Removed in MSNP24.