SIP headers (also called SIP fields) is a function of SIP. These headers help convey message attributes to ensure that information packets travel along the correct path when communicating between devices existing on separate networks. 

SIP headers are present for every HTTP request made by an outbound call and can be categorized into four main types: record route headers, route headers, via headers, and contact headers. Simply put, SIP headers are similar to HTTP header fields and always have the following format:


The following rules need to be followed for header names:

  • In the form of X-PH-Yourfield.
  • Your field must start with an uppercase letter. Other characters must be in lowercase.
  • Header name can only contain alphanumeric characters(A-Z, a-z, 0-9)
  • Header name can have a maximum length of 24 characters

The following rules need to be followed for header values:

  • Header value may comprise of alphabets(A..Z, a..z), numerals(0-9), and '%' .
  • Header value should have a maximum length of 48 characters

The header name-value pairs that do not follow the above rules will not be sent by Plivo