>Да, в прикладном уровне (http) всё расписано.
>А меня интересует именно транспорт - tcp.
Там имеется ввиду именно TCP транспорт. Читайте и не ленитесь.
8.1 Persistent Connections
Prior to persistent connections, a separate TCP connection was
established to fetch each URL, increasing the load on HTTP servers
and causing congestion on the Internet. The use of inline images and
other associated data often require a client to make multiple
requests of the same server in a short amount of time. Analysis of
these performance problems and results from a prototype
implementation are available  . Implementation experience and
measurements of actual HTTP/1.1 (RFC 2068) implementations show good
results . Alternatives have also been explored, for example,
Persistent HTTP connections have a number of advantages:
- By opening and closing fewer TCP connections, CPU time is saved
in routers and hosts (clients, servers, proxies, gateways,
tunnels, or caches), and memory used for TCP protocol control
blocks can be saved in hosts.
- HTTP requests and responses can be pipelined on a connection.
Pipelining allows a client to make multiple requests without
waiting for each response, allowing a single TCP connection to
be used much more efficiently, with much lower elapsed time.
- Network congestion is reduced by reducing the number of packets
caused by TCP opens, and by allowing TCP sufficient time to
determine the congestion state of the network.
- Latency on subsequent requests is reduced since there is no time
spent in TCP's connection opening handshake.
- HTTP can evolve more gracefully, since errors can be reported
without the penalty of closing the TCP connection. Clients using
future versions of HTTP might optimistically try a new feature,
but if communicating with an older server, retry with old
semantics after an error is reported.
HTTP implementations SHOULD implement persistent connections.