6

SIP Header Fields

This chapter describes the header fields present in SIP messages. The header fields discussed in this chapter are categorized as request and response, request only, response only, and message body header fields, depending on their usage in SIP. Except as noted, header fields are defined in the SIP specification RFC 3261 [1]. Chapter 7 also lists some special header fields defined for 3GPP IMS and OMA.

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 caseinsensitive token (but conventionally lowercase with some capitalization) 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.

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