code: def _parse_certificate(cls, response):
          
          links = _parse_header_links(response)
          try:
              cert_chain_uri = links[u'up'][u'url']
          except KeyError:
              cert_chain_uri = None
          return (
              response.content()
              .addCallback(
                  lambda body: messages.CertificateResource(
                      uri=cls._maybe_location(response),
                      cert_chain_uri=cert_chain_uri,
                      body=body))
              )
docs: Parse a response containing a certificate resource.