Flow control is a pretty broad term. Some could argue that this is a transport layer
function but in actual practice and implementation the question is right.
Bridges do not provide congestive feedback or flow control, but routers certainly can.
Here is just some food for thought:
The data link layer can provide flow control. In fact some switches employ a technique
called back pressure to throttle a client. This technique works by creating artificial
collisions, causing the sender to slow down transmission. This is typically done to
avoid a switch buffer overflow condition. If employing a connection oriented data
link, such as LLC2, there is an explicit mechanism available, RNR (Receiver Not
Ready), that provides flow control. Another good one is frame relay with it's use of
the BECN bit. There are probably many other examples.
In implementation, the network layer, specifically ICMP, allows an overwhelmed router
to send source quench messages when it is becoming congested. Again there are many
more exapmles.
There are session layer examples as well.
The point I am trying to make, is that there is often a distinction between the OSI
text book answer and the actual implementations. The OSI is a good guide and point of
reference, but very few protocols fall neatly into this cookie cutter, although they
are considered to predominantly reside at a particular layer.
Tom Kager
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