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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?

EDIT: Note that this question is asking about what standards apply to the MTU over DSL connections, not about what common conventions are in place.

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3 Answers 3

There is an important distinction that needs to be made here. Traffic that flows out of a DSL connection changes form a few times along the way. When it leaves your computer, travels through your network and hits the modem, it is Ethernet traffic. Most consumer grade equipment defaults to 1500 and the modem your ISP sends you is also probably defaulted at 1500. If you were to change the MTU, ALL interfaces between your PC NIC and the modem (including the modem) would need to be changed.

Once it leaves the modem, it is officially on the ISP network and is running as ATM traffic. This interface, and the rest of the path, you will have no control over. ATM traffic runs at a different MTU typically, but it depends on the equipment and the network. For example, cisco ATM equipment runs at 4470. An ISP network's ATM cloud may consist of Juniper, Cisco, Alcatel, Nortel, Fujitsu, Adtran or any of many other vendors. It may also interface with other provider's networks and their equipment. In other words, you have no way of knowing what will happen to packets once they leave the prem - and your ISP may not even have an idea of the entire journey of a packet either.

Once it reaches the other side and becomes Ethernet again, the MTU on the other network will then be a factor. If you don't know anything about the network you're sending to, it is best to assume it is 1500.

Also, ATMs most likely will not be set to block fragmented packets. All that will happen if a packet too large for the interface's MTU comes through is that the packet will be broken down in to smaller chunks and passed on. This may not be the case with the network at the other end.

If you send packets that are equal to or lower than the Maximum MTU on the chain, they'll get through without a problem. If you send higher, there is a chance they'll fragment or even be discarded.

What it comes down to is that you really should be aware of the MTU across the entire path if you are going to use anything higher than 1500, otherwise it is safest to just default to 1500.

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A mostly correct answer, but not to the question asked. I am a little wary about resurrecting this 4 year old question, as I am no longer involved in DSL service provision; however MTU and large scale misunderstanding of it is still a personal hobby horse of mine. You are correct to state that 4470 is often used as a default when running IP over AAL5 on ATM; however it is a convention not a standard. My question is whether the ITU standards define a maximum MTU for DSL, not what common conventions are used. –  Russell Heilling Feb 21 at 10:49
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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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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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