ow: Accept-Contact * ; language=en a: media=video 6.2.3 Accept-Encoding The Accept-Encoding header field, defined in HTTP [2], is used to spec- ify acceptable message body encoding schemes. Encoding can be used to ensure a SIP message with a large message body fits inside a single UDP datagram. The use of q value parameters can set preferences. If none of the listed schemes are acceptable to the UAC, a 406 Not Acceptable response is returned. If not included, the assumed encoding will be text/plain. Examples include: Accept-Encoding: text/plain Accept-Encoding: gzip 6.2.4 Accept-Language The Accept-Language header field, defined in HTTP [2], is used to spec- ify preferences of language. The languages specified can be used for reason phrases in responses, informational header fields such as Subject, or in mes- sage bodies. The HTTP definition allows the language tag to be made of a pri- mary tag and an optional subtag. This header field could also be used by a proxy to route to a human operator in the correct language. The language tags are reg- istered by IANA. The primary tag is an ISO-639 language abbreviation. The use of q values allows multiple preferences to be specified. Examples are shown in Table 6.13. SIP: Understanding the Session Initiation Protocol Table 6.13 Examples of Accept-Language Header Field Header Field Meaning Accept-Language: fr French is the only acceptable language Accept-Language: en, ea Acceptable languages include both English and Spanish Accept-Language: ea; q=0.5, Preferred languages are English, Spanish, and French, in en q=0.9, fr ;q=0.2 that order 6.2.5 Authorization The Authorization header field is used to carry the credentials of a user agent in a request to a server. It can be sent in reply to a 401 Unauthor- ized response containing challenge information, or it can be sent first without waiting for the challenge if the form of the challenge is known (e.g., if it has been cached from a previous call). The authentication mechanism for HTTP digest is described in Section 3.8. Ex