Difference between Pager mode and session mode

Pager-Mode through SIP can be provided through the SIP MESSAGE method ([RFC3428]), which may be more appropriate in some contexts. The SIP MESSAGE method [RFC3428] is an extension to SIP that allows the transfer of messages to the client. Since the SIP MESSAGE
request is an extension to SIP, it inherits all the request routing and security features of that protocol. The SIP MESSAGE request carries the content or content reference (for content indirection) in the form of MIME body parts. Because SIP MESSAGE is a
SIP signalling message, the ability to embed content is also limited by the maximum size of SIP signalling messages. The SIP MESSAGE requests do not themselves initiate a SIP dialog; under normal usage each SIP MESSAGE is a stands alone transaction, much like
pager messages. Thus, each SIP MESSAGE request is independent and no session states are stored in the system. The Push Content is carried in a SIP message body.

Session-mode (also referred to as the SIP INVITE and MSRP procedure avoids the limitations on embedded content imposed by SIP MESSAGE. This limitation is especially acute if the Push Content is multimedia in nature. To allow arbitrarily large messages, the
content is carried by MSRP [RFC4975]. A SIP session is established between the interested parties (Push Sender Agent and Push Receiver Agent) with MSRP as the media component. The SIP session can be used to transmit exactly one large message or a number of
large messages either parallel or sequentially. The file selection mechanism allows for the Push Receiver Agent to understand ahead of delivery on what is going to be transferred as a number of additional attributes are supported [[RFC5547]].This gives the
Push Receiver Agent a possibility to take decisions ahead of delivery and by that save network and terminal capacity.