From application message consumer viewpoint, one of the big difference btw FIX and FIXP is that FIXP appl msg does not carry header/trail any more?
I assume that you mean the FIX application layer when you say “FIX”. You cannot compare FIX and FIXP as they are for different layers of the technical stack. FIXP does not have application messages of its own, it is a session protocol and is able to transport FIX application messages between two counterparties that can include header and trailer, e.g. when you use tag=value encoding for them. You are always using FIX application messages but you have a choice for the encoding (tag=value, SBE, FIXML,…) and the session layer (FIX4, FIXT, FIXP). The need for a header/trailer and its content depends on your encoding choice.
However, FIXP is a high performance session layer and it is recommended to also use a high performance encoding such as SBE and not tag=value. Please see FIX SBE with Simple Open Framing Header on top of a FIX Performance Session for a related question.
So for most typical FIX systems, they are FIXML over FIXT for example for FIX 4.4 protocol version systems. Take CME iLink3 and MDP 3 for example. Both are SBE over FIXP although CME’s MDP 3 webpage ( CME MDP 3.0 Market Data - Electronic Platform Information Console - Confluence (cmegroup.com)) does not make is very clear as for iLink 3 Order Entry ( iLink 3 Binary Order Entry - Electronic Platform Information Console - Confluence (cmegroup.com)). Or MDP 3 does not use FIXP?
CME uses UDP for market data and you simply subscribe/listen to channels (see https://www.cmegroup.com/confluence/display/EPICSANDBOX/CME+Market+Data+Platform+Connectivity). FIXP is not needed.
Correct, CME Futures and Options Market Data uses UDP.
However, for BTEC and EBS markets CME have introduced MDP3 on TCP, with some elements of FIXP similar to Ilink3. MDP3 session management specifications are available here:
https://www.cmegroup.com/confluence/display/EPICSANDBOX/CME+Globex+EBS+Market+-+Market+Data+Message+Specification#CMEGlobexEBSMarketMarketDataMessageSpecification-CMEGroupMDPTCPSessionManagementSpecification