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cholla

packet or datagram size

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I was just wanting to know if anyone knew why 1500 bytes was decided on to be the MTU or general size of a packet or datagram.At least for most broadband.To me 1024 bytes seems a more logical size given it fits KB.MB ,etc.

Heres the chart & an explanation for why these are 1024.

1 bit = a single digit, either 1 or 0

8 bits = 1 byte, a combination of 1's and 0's

1024 Bytes = 1 KB (kilobyte)

1024 Kilobytes = 1 MB (megabyte)

1024 Megabytes = 1 GB (gigabyte)

Now to explain how they get that magic number 1024.

Because the binary code system has only 2 numbers, powers of 2 plays an important role.

Numbers always have to be 2 to the power of ?

They take 2 to the 10th power to get the number 1024.

2 is the 1st power

2 X 2 = 4  (the 2nd power)

2 X 4 = 8  (the 3rd power)

2 X 8 = 16  (the 4th power)

2 X 16 = 32  (the 5th power)

2 X 32 = 64  (the 6th power)

2 X 64 = 128  (the 7th power)

2 X 128 = 256  (the 8th power)

2 X 256 = 512  (the 9th power)

2 X 512 = 1024  (the 10th power)

And that's how you get 1 KB.

So  1500 -1024 = 476

I could understand if they had used 1536 becaus that would have a packet of 1 & a half KB.

So any explanations?

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Maximum transmission unit (Wikipedia)

. A higher MTU brings higher bandwidth efficiency. However large packets can block up a slow interface for some time' date=' increasing the lag on other packets. For example a 1500 byte packet, the largest allowed by Ethernet (and hence most of the Internet), would block up a 14.4k modem for about one second.[/quote']

Basicly if the MTU was set any higher than that, a 14.Kb modem would pause for a few seconds as it can only handle upto 14.4 Kb/s.  You have to have the packets small enough for the slowest link.

Also, I found this.  Why An MTU Of !500 For Ethernet Networks (SECOND RESPONSE) I don't go there, I just Yahoo!'ed it.

It seems there is still a lot of old hardware still out there. So until enough of it get's upgrated, 1500 will have to do.

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netmasta;First thanks for replying I had wanted this to generate more discussion from the testmy think tank.I did read the second link & I had already read the Wikipedia link.

So here is why Wikipedia used a 14.Kb modem as an example.14.Kb  is the minimum the FCC requires a phone company to have over a POTS voice line.This is the line dial-up of all speeds uses.

Next I doubt there are many 14K modems being used today & most are 56K which is limited to 53 K. So I'm not sure how much a 1500 byte packet slows a 56K modem if it falls in line with the 14k one then 1/3 of a second.So do you want your 1500 byte packet smaller so my 56K modem will be faster?I'm saying a 1536 packet (some more binary based) would be about the same to the slower modems. On dial-up you get used to slow anyway.I should know.

But really the next step would probably produce the best results for broadband with minimum increase in delay for dial-up modems. That would be a MTU of 2048.

This falls in line with binary & the AFD parameters XP uses.

Here's my theory on where this would actually help slower connections.Since this should have less retransmits on packets for broadband & the broadband would be faster less congestion would result making the dial-up work better.

I think the real resistence to this is the higher lever hardware that can't handle faster than 1500 byte packets.

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i agree with you cholla, i think its about time with IPv6 and the deployment of ADSL2+ and fiber that they increase the MTU

if we keep on looking back to the poor guys on 56k (like me in the summer) then we will never move forward, with an MTU of 2048 i think we still give people with 56k less then 1sec black out, so this is reasonable enough for now.

maybe we should go talk to the FCC or IEEE about it

put a whitepaper forward

only if u knew anything of what I'm talking about LOL

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just- ;Since I tweak I have thrown about every setting at my 56K.The recomended MTU of 576 for 56K is slower than when I use 1500.So I use 1500 for my MTU & adjust the MSS & Rwin to that as well.So for a 56 K if there is any slowdown I think it will be slight.& maybe faster because of less congestion.Also my ISP uses a MTU of 1500 for my dial-up connection.I think I still have a screen shot of the DUN box that shows this.I would do another one to post but this test is a little tricky to get it to work.

IPv4IPv6ADSL2+FCCIEEE looks forboding all lumped together opps did I add one.  :haha:

I have been helping a new member named mrproper get his ADSL2+ speed up.It's on PPPoA

Which could use an MTU of  9180 bytes max if the rest of the internet could handle it.

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