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YannickC
Guest

Posted: Tue Aug 26, 2003 12:27 pm Post subject: Who resend ?

You have a host 1 and a host 2 on two different Ethernet segments.The hosts are separated by a repeater,a bridge, 2 bridges connected by a serial line, and 2 routers. In each scenario who retransmits in the event of an error ? I can answer that the hub in not intelligent, so this equipement can detect an error (collisio, jabber, runt, giant...) but will not retransmit. Do bridges retransmit ? In my opinion, they drop the frame as hub. I would say that routers do the same, so in all cases, host will retransmit, am I wrong ?

André Franke
Guest

Posted: Tue Aug 26, 2003 12:36 pm Post subject: Re: Who resend ?

yannick@mail.opt.pf (YannickC) wrote:

Quote:

You have a host 1 and a host 2 on two different Ethernet segments.The
hosts are separated by a repeater,a bridge, 2 bridges connected by a
serial line, and 2 routers. In each scenario who retransmits in the
event of an error ?

I can answer that the hub in not intelligent, so this equipement can
detect an error (collisio, jabber, runt, giant...) but will not
retransmit. Do bridges retransmit ? In my opinion, they drop the frame
as hub. I would say that routers do the same, so in all cases, host
will retransmit, am I wrong ?

In general you are right.
There are routers which provide layer 4 functionality but that is not
a general router function.
Retransmission is a function of TCP which resides at layer 4 the
transport layer of the OSI model.

regards
André

YannickC
Guest

Posted: Wed Aug 27, 2003 5:43 am Post subject: Re: Who resend ?

André Franke <dump.it@digital-filestore.de> wrote in message news:<873mkvs1258il9grdndprmcp607v7q60e9@4ax.com>...

Quote:

yannick@mail.opt.pf (YannickC) wrote:

You have a host 1 and a host 2 on two different Ethernet segments.The
hosts are separated by a repeater,a bridge, 2 bridges connected by a
serial line, and 2 routers. In each scenario who retransmits in the
event of an error ?

I can answer that the hub in not intelligent, so this equipement can
detect an error (collisio, jabber, runt, giant...) but will not
retransmit. Do bridges retransmit ? In my opinion, they drop the frame
as hub. I would say that routers do the same, so in all cases, host
will retransmit, am I wrong ?

In general you are right.
There are routers which provide layer 4 functionality but that is not
a general router function.
Retransmission is a function of TCP which resides at layer 4 the
transport layer of the OSI model.

regards
André

I would modify my answer and say that is the responsability of the
sender to resend a bad frame to the receiver to inform the sender if
the layer 2 supports error recovery and dectection.
For ethernet (connectionless)the sender detect its own bad frame and
resends, the receiver do nothing
For serial link between router, routers detect provide error detection
and recovery (PPP, HDLC, ATM, connected protocols) the sending router
resends the frame not the host as it dectects it (time out, bad
sequence), the receiveing router informs the sending router in case of
error (time out, bad sequencing, bad crc, fcs, hec...)

Yannick.
CCNP
CCIE R/S canditate

Hansang Bae
Guest

Posted: Wed Aug 27, 2003 9:23 am Post subject: Re: Who resend ?

In article <f4d6f831.0308261643.611e618b@posting.google.com>,
yannick@mail.opt.pf says...

Quote:

The NIC does not resend anything. If it detects a bad CRC on
*receiving* the frame, it'll throw it away. But when you send, it
doesn't do any error checking a la ACKs or NACKs.

Quote:

The router cannot resend the packet. Once a packet leaves the router,
that's it. The router doesn't hold X number of packets just in case it
needs to resend the packet. It's up to the end stations to retransmit.

--

hsb

"Somehow I imagined this experience would be more rewarding" Calvin
*************** USE ROT13 TO SEE MY EMAIL ADDRESS ****************
********************************************************************
Due to the volume of email that I receive, I may not not be able to
reply to emails sent to my account. Please post a followup instead.
********************************************************************

André Franke
Guest

Posted: Wed Aug 27, 2003 3:03 pm Post subject: Re: Who resend ?

Hansang Bae <uonr@alp.ee.pbz> wrote:

Quote:

In article <f4d6f831.0308261643.611e618b@posting.google.com>,
yannick@mail.opt.pf says...
I would modify my answer and say that is the responsability of the
sender to resend a bad frame to the receiver to inform the sender if
the layer 2 supports error recovery and dectection.
For ethernet (connectionless)the sender detect its own bad frame and
resends, the receiver do nothing

The NIC does not resend anything. If it detects a bad CRC on
*receiving* the frame, it'll throw it away. But when you send, it
doesn't do any error checking a la ACKs or NACKs.

That's correct. Within the according RFCs the term "silently discard"
is used. Since each layer participating in true encapsulation is
unable to calculate the header and trailer of the next higher layer,
it can only see if the header and trailer of its corresponding
layer(peer) is ok. Just how you would check a parcel for damaged
packing and if the weight is the same as written on it(CRC).You can't
really tell if the china vase inside doesn't have a crack, while you
are not allowed to open the parcel.
And since there is not such thing like an ID number on a bit, a frame
or a packet, neither of physical, datalink and network layer could
tell the sender which of the recieved PDUs was damaged.
The first layer numbering it's PDUs is the transport layer using
reliable mechanisms like those of TCP or SPX sequencing.

Quote:

Again this is correct. Yannic, what you are thinking of is a proxy.
But in terms of the OSI reference model a proxy is nothing else than a
host. It stores information to shorten the distance between sender and
reciever.
Routing facilities may have a proxy implemented but then they are no
longer only routers, but rather more proxy hosts providing routing
functionality. The router itself does only forwarding of the "Missing
In Action" notifications and the recovery-resends of the upper layers.
Those "Missing In Action" events are caused by lower layers silently
discarding damaged PDUs.

regards
André

John Agosta
Guest

Posted: Wed Aug 27, 2003 11:59 pm Post subject: Re: Who resend ?

I find this discussion very interesting.
Some thoughts are inserted, in-line....

"André Franke" <dump.it@digital-filestore.de> wrote in message
news:cluokvk7taiiviic5a6rml23jtqinnr0cv@4ax.com...

Quote:

Hansang Bae <uonr@alp.ee.pbz> wrote:

In article <f4d6f831.0308261643.611e618b@posting.google.com>,
yannick@mail.opt.pf says...
I would modify my answer and say that is the responsability of the
sender to resend a bad frame to the receiver to inform the sender if
the layer 2 supports error recovery and dectection.
For ethernet (connectionless)the sender detect its own bad frame and
resends, the receiver do nothing

The NIC does not resend anything. If it detects a bad CRC on
*receiving* the frame, it'll throw it away. But when you send, it
doesn't do any error checking a la ACKs or NACKs.

That's correct. Within the according RFCs the term "silently discard"
is used. Since each layer participating in true encapsulation is
unable to calculate the header and trailer of the next higher layer,
it can only see if the header and trailer of its corresponding
layer(peer) is ok. Just how you would check a parcel for damaged
packing and if the weight is the same as written on it(CRC).You can't
really tell if the china vase inside doesn't have a crack, while you
are not allowed to open the parcel.
And since there is not such thing like an ID number on a bit, a frame
or a packet, neither of physical, datalink and network layer could
tell the sender which of the recieved PDUs was damaged.
The first layer numbering it's PDUs is the transport layer using
reliable mechanisms like those of TCP or SPX sequencing.

The first layer that "numbers" its PDUs is not always layer 4.
For example, LLC Type 2 will includes NS and NR sequencing, even when
operating on an Ethernet medium.
LLC type 2 isn't used for IP transmission, but it "does" exist for other
protocols.
Also, IP itself at layer 3 identifies each datagram with a number. Although
the datagram ID is not used for
error correction purposes - it is used for re-assembling fragmented
datagrams. So within the context of
(bit) error detection and correction, IP does not use sequence numbers for
that purpose. However, IP
does indeed number its PDUs for other purposes.

Quote:

For serial link between router, routers detect provide error detection
and recovery (PPP, HDLC, ATM, connected protocols) the sending router
resends the frame not the host as it dectects it (time out, bad
sequence), the receiveing router informs the sending router in case of
error (time out, bad sequencing, bad crc, fcs, hec...)

The router cannot resend the packet. Once a packet leaves the router,
that's it. The router doesn't hold X number of packets just in case it
needs to resend the packet. It's up to the end stations to retransmit.

Again this is correct. Yannic, what you are thinking of is a proxy.
But in terms of the OSI reference model a proxy is nothing else than a
host. It stores information to shorten the distance between sender and
reciever.
Routing facilities may have a proxy implemented but then they are no
longer only routers, but rather more proxy hosts providing routing
functionality. The router itself does only forwarding of the "Missing
In Action" notifications and the recovery-resends of the upper layers.
Those "Missing In Action" events are caused by lower layers silently
discarding damaged PDUs.

regards
André

Regarding who can 're-send,' the CSMA/CD standard for 1/2 dux ethernet does
specify how
the MAC sublayer can detect a collision, and attempt to re-access the medium
to retransmit the frame.
So, an ethernet NIC 'can' retransmit frames. It does not retransmit due to
detecting corrupted frames / CRC events,
but will retransmit in the event of contention for media access. Even a
switch port operating in 1/2 duplex mode
will retransmit when detecting collisions when attempting to access the
medium.

So the question of "who retransmits?" is ........"it depends."
One cannot answer the question accurately unless the 'scenario-specific'
context is being taken into consideration,
IE - what protocols are being used, and what event "caused" the failure of
the first transmission attempt.

-ja

YannickC
Guest

Posted: Thu Aug 28, 2003 1:06 am Post subject: Re: Who resend ?

André Franke <dump.it@digital-filestore.de> wrote in message news:<cluokvk7taiiviic5a6rml23jtqinnr0cv@4ax.com>...

Quote:

named CSMA/CD, it tries up to 16 times, truncated binary exponential
back-off algorithm.
André could you give me the RFC number please.

Quote:

It depends on the protocol used, PPP detects errors (LQM) but does not
resend.
In the case of HDLC, this "High Level Data Link Connection" protocol
manages errors and resends. there are sequence and acknowledgement
numbers. X25 uses it.

mjm
Guest

Posted: Thu Aug 28, 2003 4:20 am Post subject: Re: Who resend ?

"YannickC" <yannick@mail.opt.pf> wrote in message
news:f4d6f831.0308271206.61dd337d@posting.google.com...

Quote:

André Franke <dump.it@digital-filestore.de> wrote in message
news:<cluokvk7taiiviic5a6rml23jtqinnr0cv@4ax.com>...
Hansang Bae <uonr@alp.ee.pbz> wrote:

In article <f4d6f831.0308261643.611e618b@posting.google.com>,
yannick@mail.opt.pf says...
I would modify my answer and say that is the responsability of the
sender to resend a bad frame to the receiver to inform the sender if
the layer 2 supports error recovery and dectection.
For ethernet (connectionless)the sender detect its own bad frame and
resends, the receiver do nothing

The NIC does not resend anything. If it detects a bad CRC on
*receiving* the frame, it'll throw it away. But when you send, it
doesn't do any error checking a la ACKs or NACKs.

That's correct. Within the according RFCs the term "silently discard"
is used. Since each layer participating in true encapsulation is
unable to calculate the header and trailer of the next higher layer,
it can only see if the header and trailer of its corresponding
layer(peer) is ok. Just how you would check a parcel for damaged
packing and if the weight is the same as written on it(CRC).You can't
really tell if the china vase inside doesn't have a crack, while you
are not allowed to open the parcel.
And since there is not such thing like an ID number on a bit, a frame
or a packet, neither of physical, datalink and network layer could
tell the sender which of the recieved PDUs was damaged.
The first layer numbering it's PDUs is the transport layer using
reliable mechanisms like those of TCP or SPX sequencing.

Ethernet resends in case of error (collision detection) that why it is
named CSMA/CD, it tries up to 16 times, truncated binary exponential
back-off algorithm.
André could you give me the RFC number please.

For serial link between router, routers detect provide error
detection
and recovery (PPP, HDLC, ATM, connected protocols) the sending router
resends the frame not the host as it dectects it (time out, bad
sequence), the receiveing router informs the sending router in case
of
error (time out, bad sequencing, bad crc, fcs, hec...)

The router cannot resend the packet. Once a packet leaves the router,
that's it. The router doesn't hold X number of packets just in case it
needs to resend the packet. It's up to the end stations to retransmit.

Again this is correct. Yannic, what you are thinking of is a proxy.
But in terms of the OSI reference model a proxy is nothing else than a
host. It stores information to shorten the distance between sender and
reciever.
Routing facilities may have a proxy implemented but then they are no
longer only routers, but rather more proxy hosts providing routing
functionality. The router itself does only forwarding of the "Missing
In Action" notifications and the recovery-resends of the upper layers.
Those "Missing In Action" events are caused by lower layers silently
discarding damaged PDUs.

regards
André

It depends on the protocol used, PPP detects errors (LQM) but does not
resend.
In the case of HDLC, this "High Level Data Link Connection" protocol
manages errors and resends. there are sequence and acknowledgement
numbers. X25 uses it.

Just to through some more confusion into the mix..

If you have Host A that is connected to Switch A, and Host B that is
connected to Switch B. Switch A and B are cross connected to each other and
configured for store and forward. If Host A sent a packet to Host B at the
exact same time that Host B sent a packet to Host A. The packets would be
recieved by their corisonding switches at the same time. The switches would
then try to transmit the packet over the cross connect at the same time. A
collision would be detected. Now as the switches are store and forware,
would they do their own retransmision because the cross connect is in it's
own collision domain?

Hansang Bae
Guest

Posted: Thu Aug 28, 2003 4:39 am Post subject: Re: Who resend ?

In article <ssmdnUaEypseYtGiU-KYuA@wideopenwest.com>,
j_agosta@wideopenwest.kom says...

Quote:

But no one uses it so I left it out. I had to learn LLC/2 because of
the stupid CNX cert! smile

There was a web page that explained it in
painful detail!

Quote:

But the problem is that "retransmision" in this cconversation is
actually resending the entire package. If a collision is detected, the
NIC stops transmitting the actual frame. Since it never finished
sending it in the first, place, you can't really say it's
"re"transmitting. But the original sender wasn't going into this level
of detail so I guess it's an academic exercise only! :)

--

hsb

"Somehow I imagined this experience would be more rewarding" Calvin
*************** USE ROT13 TO SEE MY EMAIL ADDRESS ****************
********************************************************************
Due to the volume of email that I receive, I may not not be able to
reply to emails sent to my account. Please post a followup instead.
********************************************************************

Hansang Bae
Guest

Posted: Thu Aug 28, 2003 4:40 am Post subject: Re: Who resend ?

In article <f4d6f831.0308271206.61dd337d@posting.google.com>,
yannick@mail.opt.pf says...

Quote:

Ethernet resends in case of error (collision detection) that why it is
named CSMA/CD, it tries up to 16 times, truncated binary exponential
back-off algorithm.

But a collision isn't an error condition. It's just how it works.

--

hsb

"Somehow I imagined this experience would be more rewarding" Calvin
*************** USE ROT13 TO SEE MY EMAIL ADDRESS ****************
********************************************************************
Due to the volume of email that I receive, I may not not be able to
reply to emails sent to my account. Please post a followup instead.
********************************************************************

John Agosta
Guest

Posted: Thu Aug 28, 2003 7:09 am Post subject: Re: Who resend ?

"Hansang Bae" <uonr@alp.ee.pbz> wrote in message
news:MPG.19b70c9a6e444cfc989a88@news-server.nyc.rr.com...

Quote:

In article <ssmdnUaEypseYtGiU-KYuA@wideopenwest.com>,
j_agosta@wideopenwest.kom says...

The first layer that "numbers" its PDUs is not always layer 4.
For example, LLC Type 2 will includes NS and NR sequencing, even when
operating on an Ethernet medium.
LLC type 2 isn't used for IP transmission, but it "does" exist for other
protocols.

But no one uses it so I left it out. I had to learn LLC/2 because of
the stupid CNX cert! smile

There was a web page that explained it in
painful detail!

Regarding who can 're-send,' the CSMA/CD standard for 1/2 dux ethernet
does
specify how
the MAC sublayer can detect a collision, and attempt to re-access the
medium
to retransmit the frame.
So, an ethernet NIC 'can' retransmit frames. It does not retransmit due
to
detecting corrupted frames / CRC events,
but will retransmit in the event of contention for media access. Even a
switch port operating in 1/2 duplex mode
will retransmit when detecting collisions when attempting to access the
medium.

But the problem is that "retransmision" in this cconversation is
actually resending the entire package. If a collision is detected, the
NIC stops transmitting the actual frame. Since it never finished
sending it in the first, place, you can't really say it's
"re"transmitting. But the original sender wasn't going into this level
of detail so I guess it's an academic exercise only! :)

I guess we are picking nits here, but the IEEE document uses the term
"retransmission" with
respect to recovering from collisions, even though the initial transmission
attempt was "terminated"
before completion.....But like I said, this is just picking nits.

My main point was to state that it is not always Layer 4 that retransmits,
and not to disagree with your
(as usual) valid observations......

So, HSB, when are you gonna email me with your info so I can send you that
CD?

-ja

André Franke
Guest

Posted: Thu Aug 28, 2003 9:31 am Post subject: Re: Who resend ?

"John Agosta" <j_agosta@wideopenwest.kom> wrote:

Quote:

I find this discussion very interesting.
Some thoughts are inserted, in-line....

This is indeed very interesting, especially since there are aspects we

didn't cover before, which seem to show that the point where error
recovery occures is not always the same.

Quote:

"André Franke" <dump.it@digital-filestore.de> wrote in message
news:cluokvk7taiiviic5a6rml23jtqinnr0cv@4ax.com...
Hansang Bae <uonr@alp.ee.pbz> wrote:

In article <f4d6f831.0308261643.611e618b@posting.google.com>,
yannick@mail.opt.pf says...
[sender detects errors and resends, reciever does nothing]

[NIC does CRC check but no ACK or NACK]

[(ACK) transport layer is the first to sequence its PDUs]

The first layer that "numbers" its PDUs is not always layer 4.
For example, LLC Type 2 will includes NS and NR sequencing, even when
operating on an Ethernet medium.
LLC type 2 isn't used for IP transmission, but it "does" exist for other
protocols.
Also, IP itself at layer 3 identifies each datagram with a number. Although
the datagram ID is not used for
error correction purposes - it is used for re-assembling fragmented
datagrams. So within the context of
(bit) error detection and correction, IP does not use sequence numbers for
that purpose. However, IP
does indeed number its PDUs for other purposes.

I agree that IP numbers its datagrams to be able to reassemble

fragments to transport layer segments. As you stated, again the
transport layer has to send a NACK if a segment can't be restored due
to lost IP packets resulting in missing fragments.
I further agree that LLC2 uses indeed ACKs and NACKs and it does so
even for error recovery. It just looks as if I'm mistaken.
But if you take a closer look at LLC2 you'll notice that this flow
control, error detection and recovery is kept on the LLC sublayer of
the datalink layer, namely for control purposes not for the host to
host communication but rather more to control the connection used for
host to host communication.
Within that context you could even say there is some kind of flow
control, error detection and recovery on the physical layer since a
delimiter is send as a row of bits to mark the start of a
transmission.

Quote:

[recieving router on serial links informs sending router, which resends]

[routers don't resend, they don't store info, endstations do recovery]

[(ACK) resending routers are proxies w/ routing, proxies are hosts]

Regarding who can 're-send,' the CSMA/CD standard for 1/2 dux ethernet does
specify how
the MAC sublayer can detect a collision, and attempt to re-access the medium
to retransmit the frame.
So, an ethernet NIC 'can' retransmit frames. It does not retransmit due to
detecting corrupted frames / CRC events,
but will retransmit in the event of contention for media access. Even a
switch port operating in 1/2 duplex mode
will retransmit when detecting collisions when attempting to access the
medium.

This one is easy. If we agree that error recovery describes a process

in communication and communication involves at least two communicating
partners, a retry after backoff time in a CSMA/CD environment is not
an error recovery at all. The reciever didn't even notice that there
was some sender trying to address him. This is just as if you wanted
to tell me something but there is too much noise so that I don't even
notice you said something and you stop and try again and again without
speaking louder and without any chance to change the noise level.

Quote:

have to consider all the aspects of a problem. Your best bet is to
keep your terms straight and defined, so that not too much chances for
confusion are left over.
But remember: Doubts are friends, because if you have doubts if what
you seem to know is true, you can try to clear up what's unclear and
either strengthen your theories or refine or revise them to come
closer to the truth.

I really enjoy this discussion. Please don't stop.

regards
André

Ivan Ostres
Guest

Posted: Thu Aug 28, 2003 11:43 am Post subject: Re: Who resend ?

"mjm" <no_spam@sympatico.ca> wrote in message
news:k1b3b.10677$Cg2.845844@news20.bellglobal.com...

Quote:

If you have Host A that is connected to Switch A, and Host B that is
connected to Switch B. Switch A and B are cross connected to each other
and
configured for store and forward. If Host A sent a packet to Host B at
the
exact same time that Host B sent a packet to Host A. The packets would be
recieved by their corisonding switches at the same time. The switches
would
then try to transmit the packet over the cross connect at the same time.
A
collision would be detected. Now as the switches are store and forware,
would they do their own retransmision because the cross connect is in it's
own collision domain?

Hm.... I expect full duplex when cross-connecting switches, so... no
collision at all :-).

Ivan

John Agosta
Guest

Posted: Thu Aug 28, 2003 9:26 pm Post subject: Re: Who resend ?

"André Franke" <dump.it@digital-filestore.de> wrote in message
news:bpsqkvgqk39cdhs3vi63p0i72mku08ih4g@4ax.com...

Quote:

"John Agosta" <j_agosta@wideopenwest.kom> wrote:

I find this discussion very interesting.
Some thoughts are inserted, in-line....

This is indeed very interesting, especially since there are aspects we
didn't cover before, which seem to show that the point where error
recovery occures is not always the same.

"André Franke" <dump.it@digital-filestore.de> wrote in message
news:cluokvk7taiiviic5a6rml23jtqinnr0cv@4ax.com...
Hansang Bae <uonr@alp.ee.pbz> wrote:

In article <f4d6f831.0308261643.611e618b@posting.google.com>,
yannick@mail.opt.pf says...
[sender detects errors and resends, reciever does nothing]

[NIC does CRC check but no ACK or NACK]

[(ACK) transport layer is the first to sequence its PDUs]

The first layer that "numbers" its PDUs is not always layer 4.
For example, LLC Type 2 will includes NS and NR sequencing, even when
operating on an Ethernet medium.
LLC type 2 isn't used for IP transmission, but it "does" exist for other
protocols.
Also, IP itself at layer 3 identifies each datagram with a number.
Although
the datagram ID is not used for
error correction purposes - it is used for re-assembling fragmented
datagrams. So within the context of
(bit) error detection and correction, IP does not use sequence numbers
for
that purpose. However, IP
does indeed number its PDUs for other purposes.

I agree that IP numbers its datagrams to be able to reassemble
fragments to transport layer segments. As you stated, again the
transport layer has to send a NACK if a segment can't be restored due
to lost IP packets resulting in missing fragments.

TCP does not use negative acknowledgements. (NACK)
TCP transmitters use a timeout feature, and if a positive acknowledgement
is not received within a window of time that exists, the transmitter will
automatically re-transmit the 'segment.'

Quote:

I further agree that LLC2 uses indeed ACKs and NACKs and it does so
even for error recovery. It just looks as if I'm mistaken.
But if you take a closer look at LLC2 you'll notice that this flow
control, error detection and recovery is kept on the LLC sublayer of
the datalink layer, namely for control purposes not for the host to
host communication but rather more to control the connection used for
host to host communication.
Within that context you could even say there is some kind of flow
control, error detection and recovery on the physical layer since a
delimiter is send as a row of bits to mark the start of a
transmission.

I guess you 'could' say that, although I wouldn't.
I think debating that thought would just be nit-picking
so I'll stay away from that.

Quote:

[recieving router on serial links informs sending router, which
resends]

[routers don't resend, they don't store info, endstations do recovery]

[(ACK) resending routers are proxies w/ routing, proxies are hosts]

Regarding who can 're-send,' the CSMA/CD standard for 1/2 dux ethernet
does
specify how
the MAC sublayer can detect a collision, and attempt to re-access the
medium
to retransmit the frame.
So, an ethernet NIC 'can' retransmit frames. It does not retransmit due
to
detecting corrupted frames / CRC events,
but will retransmit in the event of contention for media access. Even a
switch port operating in 1/2 duplex mode
will retransmit when detecting collisions when attempting to access the
medium.

This one is easy. If we agree that error recovery describes a process
in communication and communication involves at least two communicating
partners, a retry after backoff time in a CSMA/CD environment is not
an error recovery at all. The reciever didn't even notice that there
was some sender trying to address him.

Error recovery, even 'bit error' recovery is not always part of a two-way
dialog. Simplex, uni-directional systems often incorporate Foward Error
Correction
(FEC) mechanisms allowing the receiver to detect and then correct bit errors
encountered during transmission. Some examples include early 3 out of 5
telex numbering,
LRC/VRC, and Reed-Solomon encoding, quite frequently used in RF / cellular
environments.

Quote:

This is just as if you wanted
to tell me something but there is too much noise so that I don't even
notice you said something and you stop and try again and again without
speaking louder and without any chance to change the noise level.

I am not saying that a collision is an "error" event.
I am simply stating that the MAC functions do allow for 'retransmissions'
when contention on the medium (collisions) is detected. So I'm not debating
your point on this one. To do so would be nit picking, and besides, I tend
to
agree with what you said anyway given the context of the discussion.

Quote:

So the question of "who retransmits?" is ........"it depends."
One cannot answer the question accurately unless the 'scenario-specific'
context is being taken into consideration,
IE - what protocols are being used, and what event "caused" the failure
of
the first transmission attempt.

There are no questions that can be answered straightly. You always
have to consider all the aspects of a problem. Your best bet is to
keep your terms straight and defined, so that not too much chances for
confusion are left over.
But remember: Doubts are friends, because if you have doubts if what
you seem to know is true, you can try to clear up what's unclear and
either strengthen your theories or refine or revise them to come
closer to the truth.

I really enjoy this discussion. Please don't stop.

regards
André

André Franke
Guest

Posted: Thu Aug 28, 2003 9:56 pm Post subject: Re: Who resend ?

yannick@mail.opt.pf (YannickC) wrote:

Quote:

André could you give me the RFC number please.

I'm sorry if I confused you. I don't know of an RFC describing
CSMA/CD. The right source for CSMA/CD documents would be the
IEEE 802.3 working group.
http://grouper.ieee.org/groups/802/3/
I just wanted to point out that "silently discarded" is the RFC-term
related to what happens on lower layers, if errors are detected

regards
André

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