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Do any of the ITU DSL standards limit the maximum MTU that can be supported?

I know that RFC2516 (PPPoE) limits the MTU to 1492 bytes; however when PPPoE is not used this limit should not exist.

The PPP, L2TP and ATM AAL5 standards all have 16-bit length fields allowing for packets up 64k in length.

I have frequently seen wholesale carrier hand offs for ADSL and G.SHDSL with limits on MTU supported anywhere between 1460 and 1500 bytes; however I am not aware of any carriers advertising capabilities above 1500 bytes.

Is there any standards based limitation on DSL connections restricting the packet sizes on DSL lines, or is it simply arbitrary limits from vendors and carriers that are restricting the availability of large frame support in the market?

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2 Answers

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MTU of higher than 1500 is generally not used on internet connections so as to avoid black hole conditions.

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I know that sending packets of more than 1500 bytes across the public internet is uncommon; however I use a lot of DSL lines in VPN deployments which go nowhere near the public internet and am curious as to why the MTU is still restricted in this situation. – Russell Heilling Jul 6 '10 at 10:00
I don't know of any company that provides point to point DSL connections except via their normal internet connecting service. To offer jumbo frames on some lines and not others would leave a mismatch in their own network. – JamesRyan Jul 6 '10 at 12:32
Pretty much any incumbent telecoms company will do this, e.g. British Telecom, Deutsche Telecom, France Telecom. The delivery mechanism varies and can include direct connection to an end-to-end ATM PVC, or a dedicated IP port delivering PPP over L2TP (often over ethernet with an MTU in the 2k-9k range). – Russell Heilling Jul 6 '10 at 13:54
With all due respect if you ask them they will probably give you the same answer I did. It is one of those cases where a practical limit is more restrictive than the theoretical one. – JamesRyan Jul 6 '10 at 14:05
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kitz.co.uk/adsl/BTCentrals.htm So assuming the handoff at the LTS is PPPoA then it seems to me that both the LTS and the edge router would need to support it – JamesRyan Jul 8 '10 at 11:29
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up vote -1 down vote

1500 is the default for Ethernet which what a lot of networked devices are utilizing these days... Here's some other default windowing sizes - http://support.microsoft.com/kb/314496

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