RFC 3261, "SIP: Session Initiation Protocol", June 2002

Errata ID: 4553
Status: Reported
Type: Technical
Publication Format(s) : TEXT
Reported By: Christer Holmberg
Date Reported: 2015-12-04

Section 20.11 says:

The handling parameter, handling-param, describes how the UAS should
react if it receives a message body whose content type or disposition
type it does not understand.  The parameter has defined values of
"optional" and "required".  If the handling parameter is missing, the
value "required" SHOULD be assumed.  The handling parameter is
described in RFC 3204 [19].

It should say:

The handling parameter, handling-param, describes how the UAS should
react if it receives a message body whose content type or disposition
type it does not understand.  The parameter has defined values of
"optional" and "required".  If the handling parameter is missing, or if
the Content-Disposition header field is missing, the value "required" 
MUST be assumed.  The handling parameter is described in RFC 3204 [19].

Notes:

SIPCORE discussion: https://mailarchive.ietf.org/arch/msg/sipcore/bn-pRgDyGL2FP_M7eYpyT35zNp4

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