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Do not fragment

 
 
KAL
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      03-08-2006
What happens if a receiver gets a packet with DF set to 1 but can not
process it? What will it notify the sender?


Also can somebody tell me the size of a UDP frame?

Thanks.

 
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Dan C
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      03-08-2006
On Tue, 07 Mar 2006 16:46:25 -0800, KAL wrote:

> What happens if a receiver gets a packet with DF set to 1 but can not
> process it? What will it notify the sender?


> Also can somebody tell me the size of a UDP frame?


Do your own homework.
Ask your teacher.

--
If you're not on the edge, you're taking up too much space.
Linux Registered User #327951

 
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noEMA
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      03-08-2006
On Tue, 07 Mar 2006 16:46:25 -0800, KAL wrote:

> What happens if a receiver gets a packet with DF set to 1 but can not
> process it? What will it notify the sender?


- First : fragmentation :
Not all networking technology have the same frame size.
Case in point Ethernet and Token Ring.
The MSS (Maximum Segment Size) of an Ethernet network is 1500 Bytes.
The MSS of a Token Ring network is 4500.
The MSS of some other networking technology is only 476 bytes.

So a router transferring a Token Ring frame to an Ethernet network need to
cut that packet in roughly 3 parts. This is why you need to do
fragmentation. Now, since transferring a TCP packet onto that frame
require some headers the number are actually smaller.

Routers will fragment packets too big for the next step on the network
path. Only the destination host will (except for IDS/IPSes) have to
reassemble all the part to make it whole and ordered again. In the case
where the network is not very efficient and loose packets badly, you will
miss parts and your communication will have to manage timeouts and ask for
retries.

All that take time and computer resources. In some cases, it's better if
there is never any fragmentation.

As per in our example, if the 4500 bytes Token ring packet goes to a
router with the DF flag set, then the router will send back to the
originating host an ICMP type 3 with code 4 (Destination Unreachable ) and
(Frag needed and DF set.)




> Also can somebody tell me the size of a UDP frame?


The maximum size of a TCP segment or UDP datagram is 65565 bytes
minus the header size of the IP packet header and the header of the
TCP/UDP part.

Since TCP is more complex than UDP its header take more space.


All of that is is clearly written in the specifications of the protocols
in documents called RFCs. All RFCs can be found at :

http://www.rfc-editor.org

RFC 791 id for IP
RFC 768 is for UDP
and
RFC 793 is for TCP.


Well I hope it help.



>
> Thanks.


 
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noEMA
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      03-08-2006
On Tue, 07 Mar 2006 16:46:25 -0800, KAL wrote:

> What happens if a receiver gets a packet with DF set to 1 but can not
> process it? What will it notify the sender?
>
>
> Also can somebody tell me the size of a UDP frame?
>
> Thanks.



By the way if I may correct the vocabulary for
the name of a header and a data payload at OSI layers

At layer 1 (physical) it's called a bit stream... or serial comm.
At layer 2 (data) it's called a "frame" (Ethernet)
At layer 3 (network) it's called a "packet" (IP)
At layer 4 (transport) it's a "segment"(TCP) or a "datagram"(UDP)
Want me to go further ?

And at layer 8 (human!) it's called correct vocabulary...
 
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Rick Jones
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      03-08-2006
In comp.os.linux.networking KAL <(E-Mail Removed)> wrote:
> What happens if a receiver gets a packet with DF set to 1 but can
> not process it? What will it notify the sender?


What exactly do you mean by "process it" and by receiver do you mean
the actual end destination, or an intermediate router?

Details about IP and ICMP processing can be found in their respective
RFC's which are archived at www.ietf.org.

> Also can somebody tell me the size of a UDP frame?


Approximately the size of New Jersey, or the length of a piece of
string. The UDP RFC(s) are also at www.ietf.org if your question
was meant to be "the maximum size of a UDP datagram"

stricly speaking (at least as strictly as I can recall)

Application "packets" are called messages
TCP "packets" are called segments
UDP "packets" are called datagrams
IP "packets" are called datagrams
Ethernet "packets" are called frames

Sooo we can have for example an application-level message carried in a
UDP datagram that is carried in some number of IP datagram fragments,
each of which are carried in separate Ethernet frames. If that is
then set over an ATM network those frames are further bend, folded,
spindled and mutilated into cells.

rick jones
--
portable adj, code that compiles under more than one compiler
these opinions are mine, all mine; HP might not want them anyway...
feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...
 
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Rick Jones
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      03-08-2006
In comp.os.linux.networking noEMA <(E-Mail Removed)> wrote:
> By the way if I may correct the vocabulary for
> the name of a header and a data payload at OSI layers


> At layer 1 (physical) it's called a bit stream... or serial comm.
> At layer 2 (data) it's called a "frame" (Ethernet)
> At layer 3 (network) it's called a "packet" (IP)


I've always understood that IP packets were called datagrams.

> At layer 4 (transport) it's a "segment"(TCP) or a "datagram"(UDP)
> Want me to go further ?


> And at layer 8 (human!) it's called correct vocabulary...


We were all new at this once. Just that as we get old at it it
becomes harder to remember what it was like to be new. To layer is
human, to tolerate divine

Oh, and layer 8 is actually the Financial layer:

https://secure.isc.org/index.pl?/store/t-shirt/

followed by layer 9, the Political layer.

rick jones
--
denial, anger, bargaining, depression, acceptance, rebirth...
where do you want to be today?
these opinions are mine, all mine; HP might not want them anyway...
feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...
 
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