# Call Headers

*Call Headers* can be used to pass custom data from a Sinch SDK client to another, or specified in an *ICE* response to be made available to the receiving client. Further, if *Call Headers* is specified they will be available in *ICE* and *DICE* events.

Example use cases:

- Specify call headers at call initiation in end-user application (via Sinch SDK client) to make them available to *callee* client.
- Specify call headers in ICE response to make them available to *callee* client.


## Specifying Call Headers in Sinch SDK Clients (iOS & Android)

If the custom data is available with the initiating client and you want to make it available to the receiving client and in ICE and DICE events, pass call headers when initiating the calls using the following Sinch SDK APIs:

- Android: `CallClient.call(String userId, Map<String,String> headers)`
- iOS: `-[SINCallClient callUserWithId:headers:]`


This will make the call headers available in *ICE* and *DICE* events (as `callHeaders`), and on the call objects in the Sinch SDKs (`Call.headers` on Android, `-[SINCall headers]` on iOS).

## Specifying Call Headers in ICE response

You can provide call headers in the ICE response as part of an *action*, example `connectMXP`. This will make the call headers available to the callee client and in the *DICE* event.

Example of ICE response:


```json
{
  "action": {
    "name": "connectMXP",
    "callHeaders": [
      { "key": "foo", "value": "bar" },
      { "key": "baz", "value": "qux" }
    ]
  }
}
```

The maximum size for the value of call headers is 1024 bytes (counted as the UTF-8 encoded size of each header key/value pair).

Important!
When call headers are specified in an ICE response, they will be made available to the callee, but not to the caller. If the call headers were specified first by caller client, but then overriden via ICE response, the caller won't be updated of the changed call headers.

## Call Headers and the `custom` Field

WARNING
This is a deprecated feature.

The *ICE* and *DICE* events have a field named `custom`. If call headers are specified at call initation on SDK client or via ICE response, the call headers will be available in `custom` in a JSON encoded format. example if a call is initiated with headers `{"foo": "x"}` then the value of `custom` will be `"{\"foo\":\"x\"}"`.

Note that call headers are only mapped to `custom` for Sinch SDK client calls and it's considered a legacy feature. It's *strongly recommended* to use `callHeaders` as specify/override call headers in *ICE* and *DICE*.

Important!
The value type of `custom` is always a *string* and the JSON-encoded representation of *headers* will be escaped when the string value is part of the larger ICE event structure. From the perspective of the structure of the ICE-event, the value of `custom` is just an opaque string.

The maximum size for the value of `custom` is 1024 bytes.