 Authenticated Identity Body (AIB) Format," IETF Internet-Draft, Work in Progress, February 2003. Sparks, R., "The SIP Referred-By Mechanism," IETF Internet-Draft, Work in Progress, February 2003. Anttalainen, T., Introduction to Telecommunications Network Engineering, Norwood, MA: Artech House, 1999. Rosenberg, J., and H. Schulzrinne, "Session Initiation Protocol (SIP): Locating SIP Serv- ers," RFC 3263, 2002. SIP Header Fields This chapter describes the header fields present in SIP messages. In RFC 2543, there were four categories of SIP header fields: general, request, response, and entity. RFC 3261 removed this distinction since it was not used by the pro- tocol. However, the header fields discussed in this chapter are categorized as request and response, request only, response only, and message body header fields. Except as noted, header fields are defined in the SIP specification RFC 3261 [1]. SIP header fields in most cases follow the same rules as HTTP header fields [2]. Header fields are defined as Header field, where Header is the case-insensitive token (but conventionally lower case with some capitaliza- tion) used to represent the header field name, and field is the case-insensitive set of tokens that contain the information. Except when otherwise noted, their order in a message is not important. Header fields can continue over multiple lines as long as the line begins with at least one space or horizontal tab character. Unrecognized header fields are ignored by proxies. Many common SIP header fields have a compact form, where the header field name is denoted by a single lower-case character. These header fields are shown in Table 6.1. Header fields can be either end-to-end or hop-by-hop. Hop-by-hop header fields are the only ones that a proxy may insert or, with a few exceptions, modify. Because SIP typically involves end-to-end control, most header fields are end-to-end. The hop-by-hop header fields that may be inserted by a proxy are shown in Table 6.2. SIP: Understanding the Session Initiation Pro