What are the limitations for SIP headers?

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

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


Use the following rules for header names:

  • Write headers in this format: X-PH-Yourfield
  • “Yourfield” must start with an uppercase letter. Other characters must be in lowercase.
  • The header name can only contain alphanumeric characters (A-Z, a-z, 0-9)
  • The header name can have a maximum length of 24 characters

The following rules need to be followed for header values:

  • The header value may comprise of letters (A..Z, a..z), numerals (0-9), and '%' 
  • The 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. 

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