Russell Heilling's questions

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.

I am often told by Sales Engineers and Product managers that a Layer 2 Ethernet service is not fit for purpose if the maximum supported frame size is in the 1518-1522 range (enough to support standard Ethernet frames or VLAN tagged frames).

Or in other words: an MTU of 1500 is not enough (see this blog post for my definition of MTU and a short rant on how the term is often misused)

I am never able to get any details from them on:

a) What proportion of customers (Enterprise / SMB) require Jumbo frames

b) What are typical expectations on frame size for Jumbos on WAN links? 1600, 2000, 9k, etc...

I know that in Telco and Data Centre environments Jumbos are pretty common, but I am after some insight as to how common this is within Enterprise and SMB networks.

As WAN bandwidths increase default network settings in older operating systems are less and less able to efficiently use the network.

A description of the problem with high bandwidth-delay product networks (also known as the long fat pipe problem) can be found here: http://www.psc.edu/networking/projects/tcptune/

Recent Linux releases do a good job of automatically enabling large windows and selective ACKs allowing better performance out of the box; however I have not seen any data on how recent Windows (or OSX) releases perform out of the box.

Does anyone know what the defaults for enhancements such as receive window size, window scaling and selective ACKs are in Windows 7?

As a secondary point of interest I would also like to know how Vista and recent OSX releases behave.

Juniper list price for 1GB of CF is around 100 times what I would expect to pay for a third party card.

The J-Series release notes have a list of supported third party parts, but there is no equivalent list for the M-Series.

J-Series Supported 3rd party hardware

My questions are:

Does anyone have any experiences (good or bad) about using 3rd party flash in an M-Series routing engine?

Should anything on the J-Series supported list work?

Has anyone had JTAC refuse to support a problem on a router with 3rd party flash installed?