Thanks Paul. This is really good explanation. On Tue, Apr 21, 2009 at 5:19 PM, Paul Kyzivat <pkyzi...@cisco.com> wrote: > I disagree with the notion that you should include C-T:application/sdp and > C-L:0 to indicate no offer. In fact, I think it is probably invalid to do > so. > > There are at least a couple of issues with that: > > - If you specify a C-T, then the body needs to conform to the specification > for that type. In the case of SDP, a valid SDP body always has a few lines. > For instance it must have an o-line. So your zero length body will be > ill-formed, resulting in an error. > > - If that were not the case, then the empty sdp body would be an offer. In > that regard, 3261 differs from 2543. In 2543 you could have an SDP body, and > if it didn't have any m-lines, then it was considered not to be an offer. In > 3261, the presence of an sdp body (with C-D session) constitutes an offer > (or answer) assuming it is in a place where an offer or answer is valid. > This is true even if it has no m-lines. If an entirely empty sdp body were > valid, then it too would be an offer if in an initial INVITE. > > If you want to omit the offer, then you need to omit the C-T: > application/sdp as well as the body itself. > > Thanks, > Paul > > Vikram Chhibber wrote: >> >> On Mon, Apr 20, 2009 at 3:29 PM, Kevin Spencer >> <spenk.l...@googlemail.com> wrote: >>> >>> Dave, >>> Thanks, thats clear from the point of view of the arbitrary binary body. >>> Vikram, >>> But in the case where no offer is made in the initial INVITE surely there >>> is >>> no need to indicate a content-type at that stage (it may not be known at >>> that point?), for example the "Session via Redirect and Proxy Servers >>> with >>> SDP in ACK" scenario in 3665 shows the initial INVITE with a >>> content-length >>> 0 but no content-type header. >> >> As such, there is no defined behavior in case Content-Length 0 is >> defined and Content-Type is missing. >> I have seen few UA which rejects initial INVITE without SDP if it has >> missing type. On the safe side, you might >> want to include the type even though length is 0. >>> >>> In this context would it be correct to say that while the type isn't >>> necessary you should still be prepared to receive it while not >>> necessarily >>> acting upon it? >> >> For the above context, yes, we should be lenient enough to recieve or >> not receive the type if length is 0. >> But we can not generalize as it very much depends on the type and context. >>> >>> Or, in the case of an SDP body does 0 length have no >>> significance and the content-type shouldn't be used, and rejected if >>> received - I don't see that in 4566 or 3264 though. >> >> I do not think RFCs should be that harsh and define the behavior for >> unexpected scenarios where presence >> or absence of something does not violate anything. >>> >>> Thanks for the responses, >>> /K >>> >>> On Mon, Apr 20, 2009 at 10:15 PM, Dale Worley <dwor...@nortel.com> wrote: >>> >>>> On Mon, 2009-04-20 at 16:21 +0100, Kevin Spencer wrote: >>>>> >>>>> Does anyone have any suggestions as to under what circumstances you >>>>> would >>>>> consider including a Content-Type: header when your Content-Length: is >>>> >>>> zero? >>>>> >>>>> 3261 gives the following example in section 20.15: >>>> >>>> I don't know of any current application, but SIP is careful to allow the >>>> body to be an arbitrary (binary) object of any type. As such, we must >>>> provide for the possibility that for a particular media type, a content >>>> of zero bytes has a useful meaning. >>>> >>>> Dale >>>> >>> On Mon, Apr 20, 2009 at 6:4 PM, Vikram Chhibber >>> <vikram.chhib...@gmail.com> >>> wrote: >>> >>>> You will carry Conent-Type with 0 Content-Length when you say "I am >>>> carrying 0 length XYZ Content-Type", which is different then saying "I >>>> am carrying no body with an type". Typical scenario would be INVITE >>>> without offer sdp. You still need to have Conent-Type as >>>> "application/sdp". If you remove Content-Type itself, you can receive >>>> 488 response. >>>> Example of 0 Content-Length and no type would be BYE message where you >>>> typically do not send any body. >>> >>> _______________________________________________ >>> Sip-implementors mailing list >>> Sip-implementors@lists.cs.columbia.edu >>> https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors >>> >> >> _______________________________________________ >> Sip-implementors mailing list >> Sip-implementors@lists.cs.columbia.edu >> https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors >> >
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