I stand corrected. Thank you. RFC1122 also provides guidelines here.
I see a bit clearer the interaction between the application, TCP and IP
layers in this situation. Live and Learn!
Thank you again,
Harold Grovesteen
Holger Kruse wrote:
> On Tue, Apr 10, 2001 at 10:36:08PM -0500, Harold Grovesteen wrote:
>
>> Not true.
>
>
> Yes, it is :-). Check the sources (BSD e.g.) or just use tcpdump to watch what
> happens.
>
>
>> The ICMP port unreachable message relates to whether and
>> application is accepting connections or not from the port. A TCP RST is
>> sent when an non SYN message is received for a non-existent TCP
>> connection on the receiving system.
>
>
> A TCP RST is sent in lots of different situations. In the sources of the
> (inofficial reference implementation) BSD 4.x this is triggered by a jump to
> "dropwithreset" within tcp_input(). "dropwithreset" is reached in many
> situations, one of which is when the PCB search comes up empty, i.e. if no
> listening socket on the specified address/port combination exists. Pretty
> much all TCP/IP-enabled operating systems with the exception of Linux and
> some embedded systems are based on BSD and inherit this behavior.
>
>
>> Refer to the TCP RFC for details.
>
>
> Yes, please do. RFC 793, 3.4, "Reset Generation" state 1: "If the connection
> does not exist (CLOSED) then a reset is sent in response to any incoming
> segment except another reset." ("CLOSED" in TCP terminology includes the case
> of not having a listener on an address/port combination.)
>
> Or check Stevens' TCP/IP Illustrated Vol 2, 28.2: "If the PCB was not found,
> the input segment is dropped and an RST is sent as a reply. This is how TCP
> handles SYNs that arrive for a server that doesn't exist, for example."
>
> You are probably confusing TCP with UDP. UDP does not have connection semantics
> or control packets of its own, and therefore falls back to ICMP for reporting
> unreachable ports. TCP never causes "ICMP port unreachable" messages, except as
> part of firewalling.
>
>
> In any case, we are getting off topic here...
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