From: Chuck Larrieu (chuck@xxxxxxxxxxxxx)
Date: Tue Feb 06 2001 - 20:56:40 GMT-3
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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