Imp.id

Identifier for this impression, unique within the bid request. Sellers usually number impressions from "1" upward, and every bid echoes the value in Bid.impid. Required in every version.

CurrentstringRequiredSince 2.0

Field facts

Path: imp.id
Object: Imp
Type: string
Requirement: Required
Added: OpenRTB 2.0
Spec section: 3.2.4

How it is used

Treat it as an opaque string. It is only unique inside one request, so it cannot serve as a placement key across requests; Imp.tagid does that job.

Example

Where the field sits in a payload, with everything else left out:

{
  "id": "req-1",
  "imp": [
    {
      "id": "1"
    }
  ]
}

Common mistakes

Sending the id as a number, such as 1 instead of "1", is a type mismatch for a string field.
Repeating the same id on two impressions in one request makes every bid on them ambiguous.
Bidders that answer with their own internal placement id instead of echoing this value produce bids that match no impression. When the validator checks a request and response together it reports openrtb.bid.impid_unknown.

Version history

Added in OpenRTB 2.0, and present in the latest snapshot. The table shows how each version's spec text types the field.

Versions	Type in the spec	Required	Deprecated
2.0 to 2.1	string	Yes	No
2.2	string, default - impression	Yes	No
2.3 to 2.4	string	Yes	No
2.5 to 2.6-202606	string	Yes	No

Related fields

Other Imp fields

For a walkthrough of the whole object, read Bid request anatomy.