something about PPPoE MTU smells wrong

[legumo]

Hello everyone, i need to ask a question about MTU on a PPPoE connection and if you don't mind i'd like a serious reply.

ill refer you here to take a look : http://www.dslreports.com/faq/695

my question :

we do know PPPoE uses a maximum of 1492 MTU which you find easily by using the Ping command : "ping -f -l 1464 google.com"
so 1464 + 28 (icmp and headers) give you 1492 MTU correct?

here's the deal, the MSS of the PPPoE connection is 1452 which you would add 40 bytes and get 1492......this is a mistake! imho.
40 bytes are for PPPoA connections, not PPPoE.

im saying so because if you refer to the website i gave above you would notice it says : "48 bytes -The sum of IP, TCP and PPPoE headers

it seems PPPoE adds 8 bytes over the typical 40 bytes and make the total 48bytes...and if this is so you would end up with a PPPoE connection with MTU 1500 which is same as PPPoA and you would get Fragmentations (1452 + 48 = 1500).

this is why i think setting your MTU in your router to 1484 instead of 1492 for PPPoE connections is the correct value because a PPPoE MSS of 1444 + the 48 bytes = 1492 MTU.

correct me if i am wrong but i feel what i said makes sense.

[trogers]

it seems PPPoE adds 8 bytes over the typical 40 bytes and make the total 48bytes...and if this is so you would end up with a PPPoE connection with MTU 1500 which is same as PPPoA and you would get Fragmentations (1452 + 48 = 1500).

this is why i think setting your MTU in your router to 1484 instead of 1492 for PPPoE connections is the correct value because a PPPoE MSS of 1444 + the 48 bytes = 1492 MTU.

correct me if i am wrong but i feel what i said makes sense.

You have double counted. PPPoE has an additional 8 bytes of overheads and that is why its MTU is 1492 instead of 1500 in the first place.

From this base MTU, PPPoA at 1500 and PPPoE at 1492, you deduct the other 40 bytes to get MSS.

So why would you deduct 8 bytes again from 1492?

[legumo]

So why would you deduct 8 bytes again from 1492?

1st of all thank you very much for the quick reply

i'm not deducting.....i'm adding...

you said the 8 bytes are the reason why the MTU is 1492 for PPPoE correct?

good, this means the MTU is initially 1500 and 8 bytes are substracted to transform it to the "usable" 1492 on PPPoE connections right?

DSLREPORTS says : For PPPoE, your MaxMTU should be no more than 1492 to (allow) space for the 8 byte PPPoE "wrapper.

does that means when i send something on the net is sends at 1500 initially but then 8 bytes are removed and 1492 is what the websites will see?

this is why i recommended 1484 MTU, maybe by setting the MTU a bit lower what is sent/received is less than the maximum. you lose 1-2 kb/s of speed but gain faster surfing especially in cases where your Latency is highish (150-300ms) because it's less work.

think of Bittorrent, if you had a 2mbit/512k connection and upload at 60kb/s (maximum) you will be unable to even reach 100kb/s (max download should be 220kb/s) because you hogged the connection to the point of not being able to send anything else......HOWEVER, if you set your upload to 45k you then have a bit more room for your connection to breathe and you will achieve higher download speeds.

so reducing the MTU to 1484 allows more room to breathe when traveling over your ISP network and into the INTERNET, you are not requesting the maximum allowed for PPPoE, this is very beneficial during Peak Times where congestion is more likely and latency may increase.

when i lowered my MTU to 1484 i did noticed faster web pages loading from 1:00 till 4:00 PM (peak times for me).

this is why i suspect Microsoft set PPPoE dial connections to an unchangeble MTU of 1480 rather than 1492. it's a good explanation imho.

i could be wrong, probably am, but it does work.

[trogers]

DSLREPORTS says : For PPPoE, your MaxMTU should be no more than 1492 to (allow) space for the 8 byte PPPoE "wrapper.

What this means is your comp sends out a packet at size 1492 and your ISP will add an additional 8 bytes to it to make it 1500.

You may have found setting MTU at 1484 gives you a slightly higher speed.

Here is a link that argues the most efficient MTU for PPPoE is at 1454:

http://www.mynetwatchman.com/kb/adsl/pppoemtu.htm

[Andrzej]

..."You may have found setting MTU at 1484 gives you a slightly higher speed"...
heheheheheh ya - 'never ending story'

IMVHO you are wrong
unfortunately above for PPPoE
is only a few years old urban myths (UK origin)

BTW from above link http://www.mynetwatchman.com/kb/adsl/pppoemtu.htm
..."When a packet is converted to an ATM cell stream
it is broken down into multiple cells of 53bytes each
(48 bytes of user data and bytes of ATM header information)"...
it means PPPoA

FYI reduction MTU for PPPoA
can speed up connections >>10Mbps PPPoA only
but optimal MTU depends on ISP (different overheaders)