A friend of mine mentioned that I could send TLS application data (HTTP messages) in batch fashion inside a one TCP packet instead sending one by one.
I've researched his claim using Google but couldn't find any.
Is this possible? Wouldn't this cause parsing issues for browsers?
HTTP is a request-response protocol, i.e. each request expects a response. The traditional way with HTTP/1 was to first send a request, then wait for the response, then send another request (if HTTP keep-alive is supported and the connection stayed open) and expect the response etc. With this procedure it could not be possible that multiple requests end up in the same packet since one had to wait for the response of the current request before sending the next one. Similar multiple responses could usually not end up in the same packet since there was also a single response outstanding. A special case here is a preliminary response (status code 100) which is followed by the "real" response. These could be sent together.
It is possible with HTTP pipelining to send multiple requests at once and then wait for all the responses to be sent back in the same order as the requests were sent. In this case multiple requests could end up in the same packet and multiple responses too. But the server is actually allowed to close the connection after a response is done. In this case requests might be left unanswered and one might need to resent the requests. But it might actually happen that the server has processed the request but that the connection was closed before sending the response, so replaying should only be done if it does not change the semantics (i.e. idempotent requests).
HTTP/2 then supports parallel sending of requests, i.e. requests can actually be interleaved in each others and same with responses. This way also multiple requests or responses might end up in the same packet.
Apart from that a clarification might be needed: a normal application does not send TCP packets but it sends data. How these data are packetized for transport is up to the kernel and depend for example on MTU. It might be that a single send results in multiple packets. It might also be that multiple send shortly after each other are combined together into a single packet.