AARNet Copyright 2011 Network Operations SDP Deep Dive Bill Efthimiou APAN33 SIP workshop February 2012

AARNet Copyright 2011 Agenda Overview 2. Protocol Structure 3. Media Negotiation

AARNet Copyright 2011 Overview 3 RFC When initiating multimedia sessions, there is a requirement to convey media details; transport addresses, and other session description metadata to the participants. The purpose of SDP is to provide a structure to convey information about media streams in multimedia sessions to allow the recipients of a session description to participate in the session. SDP is purely a format for session description. SDP must be used in conjuction with other protocols such as SIP. SIP carries SDP information within the SIP message body.

AARNet Copyright 2011 Overview - why bother with SDP? 4 4 Why is it important for us to understand SDP? To understand the media negotiation mechanism. Because of media (codec) interoperability. An understanding of SDP helps us understand interoperability issues and often where and what to look for when troubleshooting. When negotiating video codecs the process gets a little more complicated. To understand common NAT Traversal issues with multimedia signalling.

AARNet Copyright 2011 NAPT – SIP REGISTER SIP REGISTER message across a NAPT firewall

AARNet Copyright 2011 NAPT – SIP INVITE SIP INVITE message across a NAPT firewall

AARNet Copyright 2011 Protocol Structure 7 Similar to SIP, SDP is a text based protocol. An SDP session description is denoted by the message body content media type “application/sdp”.

AARNet Copyright 2011 Protocol Structure 8 Similar to SIP, SDP is a text based protocol. An SDP session description is denoted by the message body content media type “application/sdp”. An SDP session description consists of a number of lines of text of the form: = where MUST be exactly one case-significant character and is structured text whose format depends on.

AARNet Copyright 2011 Protocol Structure (cont-2) 9 In general, is either a number of fields delimited by a single space character or a free format string, and is case-sensitive unless a specific field defines otherwise. SDP consists of a session- level section followed by zero or more media-level sections. The session-level section continues until the first occurrence of the media description field (m=). Each subsequent occurrence of the media description field marks the beginning of data related to another media stream in the session. Generally, session-level values are the default for all media unless overridden by an equivalent media-level value.

AARNet Copyright 2011 Protocol Structure (cont-3) 10 Some lines in each description are REQUIED and some are OPTIONAL. Ordering of Fields: Session description v= (protocol version) o= (originator and session identifier) s= (session name) i=* (session information) u=* (URI of description) e=* ( address) p=* (phone number) c=* (connection information -- not required if included in all media) b=* (zero or more bandwidth information lines) One or more time descriptions ("t=" and "r=" lines; see below) z=* (time zone adjustments) k=* (encryption key) a=* (zero or more session attribute lines) Zero or more media descriptions Time description t= (time the session is active) r=* (zero or more repeat times) Media description, if present m= (media name and transport address) i=* (media title) c=* (connection information -- optional if included at session level) b=* (zero or more bandwidth information lines) k=* (encryption key) a=* (zero or more media attribute lines) The attribute mechanism “a=“ is the primary means for extending SDP and tailoring it to particular applications or media. An example SDP description is: v=0 o=jdoe IN IP s=SDP Seminar i=A Seminar on the session description protocol u= (Jane Doe) c=IN IP /127 t= a=recvonly m=audio RTP/AVP 0 m=video RTP/AVP 99 a=rtpmap:99 h /90000

AARNet Copyright 2011 Protocol Structure (cont-5) 11

AARNet Copyright 2011 Media Negotiation (Offer/Answer Model) 12 SDP lacks the negotiation process to enable signalling endpoints to reach agreement. RFC 3264: SIP using SDP in an Answer/Offer mechanism. RFC 4317 SDP Offer/Answer examples. The initiator of the session offers a selection of multimedia formats to be used in the session. The receiver of the offer either rejects the offer, or selects those media formats that have been offered and which the responder is willing/able to accept. The offer will contain zero or more media streams. The answerer will respond with one or more media streams or reject the offer.

AARNet Copyright 2011 Media Negotiation (Offer- example) 13

AARNet Copyright 2011 Media Negotiation (Answer- example) 14

AARNet Copyright Thank you Questions ?