SAP and SDP

Session announcements are handled by two protocols, Session Announcement Protocol (SAP) and Session Description Protocol (SDP). These two protocols display multicast session names and correlate the names with multicast traffic. SDP is a session directory protocol that is used for multimedia sessions. As such, it assists in advertising multimedia conference sessions and in communicating setup information to participants who want to join the session. SDP simply formats the session description; it does not incorporate a transport protocol. SDP is commonly used by a client to announce a conference session by periodically multicasting an announcement packet to a well-known multicast address and port using SAP. SAP is a session directory announcement protocol that is used by SDP as its transport protocol. Table 9.10 lists the SAP and SDP standards supported by the JUNOS software.

Table 9.10. SAP and SDP Standards Supported by JUNOS Software

Standard	Title
RFC 2327	SDP: Session Description Protocol
RFC 2974	Session Announcement Protocol

SAP and SDP Packets

SAP data packets are UDP packets that consist of a SAP header and a text payload. The packets contain the following fields:

• Version number ”SAP version number, which is 1

• Address type ”Indicates whether the originating source field contains a 32-bit IPv4 address or a 128-bit IPv6 address

• Message type ”Indicates whether the packet is a session announcement or a session deletion packet

• Encryption bit ”Indicates whether the payload is encrypted

• Compressed bit ”Indicates whether the payload is compressed using the zlib compression algorithm

• Authentication length ”Length of authentication data in the packet

• Authentication data ”Authentication data including a digital signature of the packet

• Message identifier hash ”Used in combination with the originating source to provide a globally unique identifier indicating the precise version of this announcement

• Originating source ”IP address of the message's original source

• Payload type field ”Type of data in the packet

• Payload ”Packet data

Configuring SAP and SDP

You must explicitly enable SDP and SAP to enable the receipt of session announcements. By default, SAP always listens to the address and port 224.2.127.254:9875 for session advertisements. To add other addresses and ports, specify other address and port numbers . Sessions learned by SDP time out after 60 minutes.

 [edit protocols] sap {   disable;   listen <  address  > <port  port  >; }