SAP – Session Announcement Protocol

Session Announcement Protocol as an experimental protocol designed for the purpose of multicasting a session’s information. IETF issued it as RFC 2974. SDP (Session Description Protocol) is being used by SAP as real-time transport protocol’s session depiction arrangement. With SAP use, correspondent can transmit SDP descriptions from time to time to an acknowledged multicast address and also to port.

The announcement interval can be considerately altered in a way that each and everyone SAP announcement within a multiple transmission deliverance scope may use 4000 bits/second. Announcements will by design terminate after every 10 times (announcement interval) but greater value for termination can be an hour.

Cache Timeout for SAP

A command SAP cache-timeout can be used to restrict how long a SAP cache entry should be hanged as active within cache by means of the global configuration mode.

Ways to Authenticate, Encrypt and Compress

SAP offered methods for authentication and encryption of announcements but encryption use is not always recommended. Anyway, process of authentication is necessary to prevent the not permitted changes and also to stop DoS (denial-of-service attack) attacks. But this way of authentication is not obligatory. But anyone, from two suggested schemes for authentication, can use anyone. These schemes are named as Pretty Good Privacy (specified within RFC 2440) and Cryptographic Message Syntax (specified within RFC 5652)

SAP (v2) as an announcement protocol can be utilized by the session directory clients. But SAP announcer need to multicast the announcement packets periodically to a renowned multicast port and address. The announcement can be multicast (sending to cluster of hosts) with the identical scope. Following are some information relating to SAP (for IPv4) data packet format.

The SAP packet (for IPv4) is consisted on version type, MT (message type field that is pointed to whether packet is broadcasting a session, or canceling an announcement), E (payload encrypted of one bit that pointed out, in any case payload should be encrypted),  C (payload compressed of one bit that pointed out, in any case payload should be compressed). Auth length (words relating authentication statistics where word = 32 bits but data can be padded if it is required). Message id hash field of packet is used as a unique identifier intended for the message.

The grouping of message ID hash field and original source field can be made-up to proffer an exclusive announcement ID so to recognize a particular version of current specific session. This is practical approach for both caching and ignoring packets in case of decryption, etc. SAP announcement can be validated by incorporating a payload’s digital signature in the field “optional authentication header”. SAP comprises method can make sure the reliability of session announcement by means of message ID hash plus encrypting announcement.

In short, SAP protocol can be use to publicize multicast conferencing session over the internet. But a conference needs to be announced by sporadically multicasting the packet with UDP announcement towards a multicast address (assigned to system) and to port. That is because this specific protocol SAP is intended for multicasting for example, well for conference calls, but not for IP telephone calls between two.



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