From: David Ankers (d.ankers@xxxxxxxxx)
Date: Tue Feb 13 2001 - 20:16:36 GMT-3
The quote below from Drew is completely false, I have no idea why he posted
it as it misleads people although I'm sure it was done in good faith.
I'll answer why it's wrong with a question:
What happens if a packet gets fragmented and the fragments take different
paths? If the router with the access list is in the middle of the path there
is a possiblility that it won't get all the packets, so how does can it
specifically it re-assemble them? TCP/IP doesn't have such a design flaw.
Which system is the only system that we can be sure will get all the packets
given that no loss occurs? The system with the destination address.
>From a programmers point of view having intermediate systems perform
re-assembly would be a nightmare for two reasons, 1, buffer alocation and 2,
it would cause deadlocks.
I don't like sending "you are wrong mails" but I know a couple of the guys
here are trying to get an understanding of this topic, sorry Drew.
D.
On Tuesday 13 February 2001 19:47, you wrote:
> Maybe this means that an interface with an access list must re-assemble the
> fragmented packet before it can run it through the access-list? If I get a
> chance I'll try it in the lab.
>
> Chuck Church
> CCNP, CCDP, MCNE, MCSE
> Sr. Network Engineer
> Magnacom Technologies
> 140 N. Rt. 303
> Valley Cottage, NY 10989
> 845-267-4000 x218
>
>
> -----Original Message-----
> From: Shawn Bowen [mailto:shawn@bowen.com]
> Sent: Tuesday, February 13, 2001 2:06 PM
> To: Ccielab@Groupstudy. Com
> Subject: RE: Fragmentation Concepts
>
>
> What about a router that's in the middle of the network that the stream of
> packets is going across? How can the middle router effectively use an
> extended access-list if the port number is only contained in the first
> fragment?
>
> Thanks,
> Shawn
>
> -----Original Message-----
> From: Maness, Drew [mailto:drew.maness@veritect.com]
> Sent: Tuesday, February 13, 2001 10:41 AM
> To: 'Tariq Sharif'; Ccielab@Groupstudy. Com
> Subject: RE: Fragmentation Concepts
>
> The router will need to combine the fragmentations or (frames) to form the
> whole packet.
>
> -----Original Message-----
> From: Tariq Sharif [mailto:tariq_sharif@btinternet.com]
> Sent: Monday, February 12, 2001 1:14 PM
> To: Tariq Sharif; Priscilla Oppenheimer; David Ankers; Chuck Larrieu;
> Choon, Raymond (); Ccielab@Groupstudy. Com
> Subject: RE: Fragmentation Concepts
>
>
> Digging a bit deeper...
>
> When IP carrying UDP/TCP is fragmented, only 1st fragment has layer 4 info
> (i.e. port numbers). How do access lists handle this? Will a router open a
> port (say 69) for the 1st segment & looking at the ID/offset it keeps the
> port open for the rest of segments (which do not carry port #)?
>
> Hope you understand what I'm asking.
>
> Many thanks & regards.
>
> Tariq Sharif
>
>
> -----Original Message-----
> From: Tariq Sharif [mailto:tariq_sharif@btinternet.com]
> Sent: 07 February 2001 19:06
> To: Priscilla Oppenheimer; David Ankers; Chuck Larrieu; Choon, Raymond
> (); 'Tariq Sharif'; Ccielab@Groupstudy. Com
> Subject: RE: Fragmentation Concepts
>
>
> Hi Priscilla
>
> Great to have you onboard! Thanks to everyone for a mighty discussion!
>
> Many thanks & regards.
>
> Tariq Sharif
>
>
>
> -----Original Message-----
> From: Priscilla Oppenheimer [mailto:cilla@priscilla.com]
> Sent: 07 February 2001 18:37
> To: David Ankers; Chuck Larrieu; Choon, Raymond (); 'Tariq Sharif';
> Ccielab@Groupstudy. Com
> Subject: Re: Fragmentation Concepts
>
>
> Hi David,
>
> I'm not on the ccie groupstudy list, but it appears that my answer got
> there anyway. That's too bad, because it was wrong! &;-) Sorry.
>
> The host (end node) definitely reassembles, as you say, but I was worried
> about fragments of fragments, which could get kind of ugly. But right after
> sending my message I decided that they probably aren't any different than
> normal fragments. I checked RFC 791 and it doesn't talk about fragments of
> fragments, and, in fact, I couldn't find any mention in any documents of
> what a router does when it receives a fragment that it can't forward
> because the MTU for the outgoing interface is too small. But that's
> probably because it's pretty straight-forward.
>
> Say, the first router receives a 4000-byte packet that it needs to divide
> into 1000-byte frames.
>
> First fragment: ID = 4567, offset = 0, More fragments = 1 (true)
> Second fragment: ID = 4567, offset = 1000, More fragments = 1
> Third fragment ID = 4567, offset = 2000, More fragments = 1
> Fourth fragment ID = 4567, offset = 3000, More fragments = 0
>
> So, now think about an intermediate router further down the path that
> receives the first fragment. Say that router needs to forward the frame to
> a network with an MTU of 500. What should that router do? It should just
> keeps subdividing.
>
> Receive first fragment
> Divide it up:
> ID = 4567, offset = 0, More fragments = 1
> ID = 4567, offset = 500, More fragments = 1
>
> Receive next fragment
> Divide it up:
> ID = 4567, offset = 1000, More fragments = 1
> ID = 4567, offset = 1500, More fragments = 1
>
> Etc.
>
> I think that must be what it does, because the fragments could take
> different paths so the router can't just sit there and wait for them.
>
> The last fragment worries me a bit, because the more fragments bit would be
> 0, which is also the value for non-fragmented packets. But the offset would
> be non-zero, so I guess the recipient would figure it out.
>
> Anyway, thanks for your info. I will join the list now. I passed the
> written (though to be honest I have to take it again, because I let it
> expire before taking the lab.)
>
> Priscilla
>
> At 03:51 AM 2/7/01, you wrote:
> >Is this the Priscilla? Author of top down network design, I guess so as
>
> there
>
> >won't be than many Priscilla Oppenheimers around. Great book, thank you.
> >
> >The list you came across is the group study list for the ccie lab exam. It
>
> a
>
> >bunch of people that are going for the lab in the near future most are
>
> fairly
>
> >cluey but there are expections (me for instance :-). The list is run by
> >groupstudy and the joining info is here:
> >
> >http://www.groupstudy.com/list/CCIELab.html
> >
> >About the fragmentation.... 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,
>
> >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
>
> might
>
> >sumize. Trust me on this, I've written IP stack code to do it!
> >
> >I can't wait to get to IP version 6, PMTU discovery is the default there!
> >
> >Kind regards,
> >
> >David.
> >
> >On Wednesday 07 February 2001 00:41, you wrote:
> > > What list did this come from and how do I join?
> > >
> > > Usually when a host or router fragments an IP datagram, it is not
> > > reassembled until it reaches the final destination. Each fragment has
>
> the
>
> > > same ID in the IP header. The offset field in the header allows the
> > > recipient to put the fragments back in order for delivery to the next
>
> layer
>
> > > up.
> > >
> > > What you are talking about in this devious example is fragmentation of
> > > fragments. In that case, I think the router would have to wait for all
>
> the
>
> > > original fragments to arrive, reassemble them, and refragment them for
>
> the
>
> > > new MTU. Obviously this should be avoided if at all possible.
> > >
> > > Applications can do an MTU discovery by sending varying sized frames
>
> with
>
> > > the Don't Fragment bit set until a good frame size is found.
> > >
> > > Priscilla
> > >
> > > At 03:56 PM 2/6/01, 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
> > > >
> > > >
This archive was generated by hypermail 2.1.4 : Thu Jun 13 2002 - 10:28:48 GMT-3