network protocols handbook.pdf

131Protocols GuideVoice Over IP(VOIP) - SignallingProtocol NameSAP: Session AnnouncementProtocolProtocol DescriptionSession Announcement Protocol (SAP) is an announcementprotocol that is used to assist the advertisement of multicastmultimedia conferences and other multicast sessions, and tocommunicate the relevant session setup information to prospectiveparticipants.A SAP announcer periodically multicasts an announcementpacket to a well-known multicast address and port. The announcementis multicast with the same scope as the session itis announcing, ensuring that the recipients of the announcementcan also be potential recipients of the session the announcementdescribes (bandwidth and other such constraints permitting).This is also important for the scalability of the protocol, asit keeps local session announcements local.A SAP listener learns of the multicast scopes it is within (for example,using the Multicast-Scope Zone Announcement Protocol)and listens on the well known SAP address and port for thosescopes. In this manner, it will eventually learn of all the sessionsbeing announced, allowing those sessions to be joined.It is to be expected that sessions may be announced by a numberof different mechanisms, not only SAP. For example, a sessiondescription may be placed on a web page, sent by email orconveyed in a session initiation protocol. To increase interoperabilitywith these other mechanisms, application level security isemployed, rather than using IPsec authentication headers.Protocol Structure3 4 5 6 7 8 16 32bitV=1 A R T E C Auth len Msg ID hashOriginating source (32 or 128 bits)Optional Authentication DataOptional timeoutOptional payload type0Payload• V - Version Number field is three bits and MUST beset to 1.• A: Address Type can have a value of 0 or 1:0 The originating source field contains a 32-bitIPv4 address.1 The originating source contains a 128-bit IPv6address.• R - Reserved. SAP announcers set this to 0. SAPlisteners ignore the contents of this field.• T - Message Type can have a value of 0 or 1:0 Session announcement packet1 Session deletion packet.• E - Encryption Bit bit may be 0 or 1.1 The payload of the SAP packet is encryptedand the timeout field must be added to thepacket header.0 The packet is not encrypted and the timeoutmust not be present.• C - Compressed Bit. If the compressed bit is set to 1,the payload is compressed.• Authentication Length - An 8bits unsigned quantitygiving the number of 32 bit words, following the mainSAP header, that contain authentication data. If it iszero, no authentication header is present.• Message Identifier Hash - used in combination withthe originating source, provides a globally uniqueidentifier indicating the precise version of this announcement.• Originating Source - This field contains the IP addressof the original source of the message. This isan IPv4 address if the A field is set to zero; otherwise,it is an IPv6 address. The address is stored in networkbyte order.• Timeout - When the session payload is encrypted,the detailed timing fields in the payload are not availableto listeners not trusted with the decryption key.Under such circumstances, the header includes anadditional 32-bit timestamp field stating when the sessionshould be timed out. The value is an unsignedquantity giving the NTP time in seconds at which timethe session is timed out. It is in network byte order.• Payload Type - The payload type field is a MIMEcontent type specifier, describing the format of thepayload. This is a variable length ASCII text string,followed by a single zero byte (ASCII NUL).• Payload - The Payload field includes various subfields.Related protocolsRTP, RTSP, SIP, SDPSponsor SourceSAP is defined by IETF (www.ietf.org ) in RFC 2974.Referencehttp://www.javvin.com/protocol/rfc2974.pdfSAP Session Announcement Protocol