The Allow header field lists the set of methods supported by the User Agent
generating the message. All methods, including ACK and CANCEL, understood
by the User Agent MUST be included in the list of methods in the Allow header
field, when present.
The absence of an Allow header field MUST NOT be interpreted to mean that
the User Agent sending the message supports no methods. Rather, it implies
that the User Agent is not providing any information on what methods it
supports. Supplying an Allow header field in responses to methods other than
OPTIONS reduces the number of messages needed.
Allow: INVITE, ACK, OPTIONS, CANCEL, BYE
- BEA Systems, NIST