From: Tariq Sharif (tariq_sharif@xxxxxxxxxxxxxx)
Date: Wed Feb 07 2001 - 07:15:45 GMT-3
Many thanks & regards to all the respondents!
Tariq Sharif
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
David Ankers
Sent: 07 February 2001 04:05
To: Chuck Larrieu; Choon, Raymond (); 'Tariq Sharif';
Ccielab@Groupstudy. Com
Subject: Re: Fragmentation Concepts
Time to put the thread to bed....
I've written IP stack code, I kinda know a little about this low level
stuff,
not a lot, but enough.
Even when a router has to fragment fragments (sub fragmentation), the end
host is the only one that will reassemble them, ever. (there is an exception
where routers can do it called intranet fragmentation which is done via
mutual agreement between the routers, don't bother about it, even RFC 791
doesn't).
Sub fragments look exactly the same as the orginal (except for size), they
are also not reassembled as a seperate set of fragments by the end host as
they contain their orginal position in the orginal fragment. So it's not a
hierarchical scheme as you might sumize.
>"It would be highly inefficient for the routers along the path to have to
>examine each packet with the purpose of determining if the packets had been
>fragmented and could be consolidated".
Agreed, Fragmentation is a waste of bandwidth, and this is why PMTU
discovery
(by the sending host) is the default on IPV6....
D.
On Tuesday 06 February 2001 23:56, Chuck Larrieu wrote:
> Original question:
>
> Host A sends 1500 bytes long packet to e0 of R1 (destined for Host B).
> R1 s0 has MTU set to 750 bytes. So R1 fragments packet into 2.
> R2 has MTU of 375 bytes on s1, R2 fragments packet into 2
> R3 has MTU of 1500 on Ethernet LAN B, so it reassembles the fragments into
> one 1500 byte long packet & hands it to Host B??
>
> Long winded response:
>
> Another way to look at this is in terms of protocol behaviour at the
> various layers.
>
> Application / layer 7 is the interface between the rest of the protocol
> stack and the user application
>
> TCP / layer 4 is responsible for reliable end to end reliable transfer of
> data.
>
> IP / layer 3 is responsible for path determination and packet forwarding
>
> Frame/ layer 2 is where the physical limitations of packet size occur.
E.g.
> token ring MTU versus ethernet MTU
>
> If a router receives a valid packet, and has a route to the destination
> contained within that packet, there is nothing for the router ( layer 3
> device ) to do other than to forward it.
>
> In terms of the following model:
>
>
Host_1----segment_1----Router_1------segment_2------Router_2----segment_3--
>- ------Host_2
>
> ( I'm going to assume that the OSI model doesn't exactly fit here in the
> real world of computer processor operations, and that in terms of actual
> stack operation, there is interaction up and down )
>
> Layer 7 - app sends 2 meg file to host_2
> Layer 4 - TCP prepares the TCP packet, based on what it knows of the
> transmission requirements, including MTU
> Layer 3 - IP interacts with layer 2 data link - to get the proper size
> frame to the next point in the path
> Layer 3 - IP - router receives packet, forwards it. If the outbound
> interface segment requires a smaller packet size, ip fragments it.
> Layer 2 - data link - rules of the data link segments determine MTU
>
> Segment 1 is a token ring with an MTU of 4480, segment 2 is an ethernet
> with an MTU 1500, and segment 3 is another token ring, with an MTU of
> 17000.
>
> Host 1 is sending a 2 meg file to host 2. TCP breaks that file into
smaller
> chunks, based on the segment MTU. IP fragments further based on segment
MTU
> at the next lower level. The data itself, i.e. the 2 meg file, is of use
to
> host 2 only as a single file. The protocol stack on host 2 is responsible
> for receiving and checking the data, based on information handed up from
> the lower layers.
>
> It would be highly inefficient for the routers along the path to have to
> examine each packet with the purpose of determining if the packets had
been
> fragmented and could be consolidated, not to mention the nature of the
> fragmentation. Layer 3 devices just send packets on their merry way
without
> any concern as to whether or not there are better ways to do this.
>
> Hhmmmm.... This could be better written. To bad Priscilla Oppenheimer
> isn't on this list. This deserves a good writer :->
>
> Chuck
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> Choon, Raymond ()
> Sent: Tuesday, February 06, 2001 10:23 AM
> To: 'Tariq Sharif'; Ccielab@Groupstudy. Com
> Subject: RE: Fragmentation Concepts
>
> Routers don't reassemble packets. Host B receives packets with MTU of 375
> bytes and host B will reassemble packets to 1500 bytes because this is the
> original MTU from host A.
>
> I do not have any references for that. This is from my memory when I ran
> into doing troubleshooting before. Correct me if I'm wrong.
>
> Raymond Choon
>
> -----Original Message-----
> From: Tariq Sharif [mailto:tariq_sharif@btinternet.com]
> Sent: Tuesday, February 06, 2001 1:01 PM
> To: Ccielab@Groupstudy. Com
> Subject: Fragmentation Concepts
>
>
> Need some help on understanding IP fragmentation. In the setup below, are
> the steps below correct?
>
>
> LAN A |
>
> Host A_| Serial line 1 Serial line 2
>
> | LAN B
> |
> |______e0 R1 s0-------------------s0 R2
>
> s1---------------------------s0 R3 e0 -----------|
>
> |-------Host B
>
> Host A sends 1500 bytes long packet to e0 of R1 (destined for Host B).
> R1 s0 has MTU set to 750 bytes. So R1 fragments packet into 2.
> R2 has MTU of 375 bytes on s1, R2 fragments packet into 2
> R3 has MTU of 1500 on Ethernet LAN B, so it reassembles the fragments into
> one 1500 byte long packet & hands it to Host B??
>
> Any references to sites/books will be greatly appreciated.
>
> Many thanks & regards.
>
> Tariq Sharif
>
>
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