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TCP/IP

Posted on 2004-09-14
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there is something confusing me here. if a packet is passed in the internet layer and if the palket is too big, what will happen?? will the packet get discarded, or will it be split in smaller pieces. can someone explain to me what really happen???
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Question by:pritam123456
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Microtech earned 25 total points
ID: 12054234
Hi pritam123456,
the maximum size data that can be carried in an ethernet frame is 1500 bytes (headers are 28bytes) if the frame is over 1500 bytes it will fragment it and then reassemble it at the other end. The data is only reassembled at the destination host and can be received in any order. each segment is identified by a sequence number in the tcp header. During the session setup, both sides inform each other what their starting sequence numbers will be. the session startup is indicated in the SYN flag in the tcp header. each time a segment is transmitted, the sequence number is incremented by the number of data octets transmitted.

Hope This helps
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by:Microtech
ID: 12054244
octect = 8 bits (byte)
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by:pseudocyber
pseudocyber earned 25 total points
ID: 12054253
IP is designed to fragment and reassemble the packet on the other end.  So, the answer is split into smaller pieces - known as fragmentation.

This page illustrates and explains the process nicely:  http://www.tcpipguide.com/free/t_IPMessageFragmentationProcess.htm

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by:pritam123456
ID: 12054297
sorry i did not ask the question well. suppose the packet is in the internet layer, it is too big and it cant be passed onto the network. what will happen to the packet???
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by:pseudocyber
ID: 12054396
The data link layer (which I assume you're calling the "network") would see it's a "giant" and drop it.
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by:pritam123456
ID: 12054466
actually i am reading about TCP/IP and it all about layering.  let say suppose the packet has already passed to the application layer and trasport layer and now it is in the internet layer. from the internet layer it need to be send to the network layer. what will happen if the packet is too big?? thats my question
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by:PsiCop
ID: 12054517
The answer to your Question is --> it depends.

One router might be configured to fragment and pass along a too-large packet. The next router in the link might decide you're launching a DOS attack and simply discard your packet. It all depends on the routers involved in the communications and how those routers have been configured.
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by:pseudocyber
ID: 12054597
pritam123456  - are you talking about the DoD 4 layer model or the OSI Seven Layer Model?

You can try this out yourself.  

C:\WINDOWS>ping -l 2000 127.1.0.1

Pinging 127.1.0.1 with 2000 bytes of data:

Reply from 127.0.0.1: bytes=2000 time<1ms TTL=128
Reply from 127.0.0.1: bytes=2000 time<1ms TTL=128
Reply from 127.0.0.1: bytes=2000 time<1ms TTL=128
Reply from 127.0.0.1: bytes=2000 time<1ms TTL=128

Ping statistics for 127.1.0.1:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
    Minimum = 0ms, Maximum = 0ms, Average = 0ms

C:\WINDOWS>ping -l 2000 -f 127.1.0.1

Pinging 127.1.0.1 with 2000 bytes of data:

Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Ping statistics for 127.1.0.1:
    Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),
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by:crazijoe
ID: 12055232
Is this homework?
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by:pritam123456
ID: 12055327
no its not homework. am a help desk and i just want to want to know more about TCP/IP.
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by:crazijoe
ID: 12055355
Good enough. Just asking. It's hard to tell the men from the boys.
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by:pritam123456
ID: 12055406
well this does not answer my question. anyway thanks for trying to help me
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by:PsiCop
ID: 12055447
I suggest that we HAVE answered your Question, it just wasn't the Answer you thot you'd get.
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by:pseudocyber
ID: 12055456
PsiCop answered it with - it depends on the equipment it's traversing.

I asked you if you're referring to the 4 layer DoD model or the 7 layer OSI model.
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by:Spankyxeon
ID: 12055459
here you go pretty simple really,

it will split into smaller pieces and then reassemble it at the other end. The data is only reassembled at the destination host and can be received in any order.
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by:pritam123456
ID: 12055469
am referring to the 7 layer OSI model
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by:pseudocyber
ID: 12055472
>>can be received in any order.

That really depends on the application - video and voice isn't really happy with being fragmented and reassembled in any order ... ;)
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by:Spankyxeon
ID: 12055502
I believe he is looking for a SIMPLE understanding, if you want indepth take a course, CNA and MS Networking are two pretty good course so you can understand it in depth.

Always some a$$clown trying to out do the next guy and confusing the original question
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by:crazijoe
ID: 12055515
I asked the question about homework because you have asked another question basically on the same topic.
http://www.experts-exchange.com/Networking/Q_21130223.html 
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by:pseudocyber
ID: 12055533
Ok.  

>>trasport layer and now it is in the internet layer. from the internet layer it need to be send to the network layer. what will happen if the packet is too big?? thats my question

Let's say the frame makes it from the transport layer (layer 4 TCP) to the network layer (layer 3 IP) down to the data link (layer 2 ethernet) and it's still too big?  Then the ethernet/data link layer will see a GIANT frame, record it as an error and drop it ... UNLESS ... it's configured to support "jumbo frames" for GigE.

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by:PennGwyn
ID: 12059812
The internet layer configuration includes a variable called MTU (Maximum Transmission Unit).  If it receives a packet passed down from layers above which exceeds this size, it will break it into chunks no bigger than this, and insert header information which allows the internet layer on the receiving end to reassemble the pieces before passing it to the transport layer.

This should not normally be necessary, since the transport layer includes similar code, and also looks at the MTU value.  But sometimes things like VPNs, etc, insert padding between when the packet is encapsulated by the transport layer and when it is received by the Internetwork layer.

Note that the MTU size varies with the interface type.  So a router's Internetwork layer may receive a packet from one interface which is too large to forward on another interface without fragmenting it....

This is all, though, governed by the "DF" flag in the transport packet header.  If this bit is set, the packet is not fragmented and instead an ICMP error packet is returned to the sender.

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by:PsiCop
ID: 12636366
*shrug* Whatever
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