Hi all,
we gave it a first try to rewrite the component section for TCP. The insight I
took from the discussion on the list is that components probably are much more
linked to the implementation (choices) a certain protocol made while features
are probably more high-level having the question in mind what does an
application potentially want/need to know.
So for the components in TCP we now have the following list:
- Connection-oriented bidirectional communication using three-way handshake
connection setup with
feature negotiation and an explicit distinction between passive and
active open:
This implies both unicast addressing and a guarantee of return
routability.
- Single stream-oriented transmission:
The stream abstraction atop the datagram service provided by IP is implemented
by dividing
the stream into segments.
- Limited control over segment transmission scheduling (Nagle's algorithm):
This allows for delay minimization in interactive applications.
- Port multiplexing, with application-to-port mapping during connection setup:
Note that in the presence of network address and port translation (NAPT), TCP
ports are
in effect part of the endpoint address for forwarding purposes.
- Full reliability based on ack-based loss detection and retransmission:
Loss is sensed using duplicated acks ("fast retransmit"), which places a lower
bound on
the delay inherent in this approach to reliability.
- Error detection based on a checksum covering the network and transport headers
as well as payload:
Packets that are detected as corrupted are dropped, relying on the reliability
mechanism
to retransmit them.
- Window-based flow control, with receiver-side window management and signaling
of available window:
Scaling the flow control window beyond 64kB requires the use of an optional
feature,
which has performance implications in environments where this option is not
supported.
- Window-based congestion control reacting to loss, delay, retransmission
timeout, or
an explicit congestion signal (ECN):
Most commonly used is a loss signal from the reliability component's
retransmission mechanism.
TCP reacts to a congestion signal by reducing the size of the
congestion window;
retransmission timeout is generally handled with a larger reaction than other
signals.
We are currently still working on the list of features that results from thiese
components but we are not there yet. Probably we not only need the features
itself but also properties/aspects (or however you want to call this) of the
feature. We already had this discussion a bit but wanted to postpone the
decision if we really need to define an own term for this until we are sure that
we need it.
We are posting this list of (TCP) components now because we would like to get
some feedback if this goes into the right direction/is on the right level of
detail before we go on and apply this also to other protocols.
Brian and Mirja
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