TR Forums

Does anyone know if Comcast broadband supports jumbo frames? This whole jumbo frame thing is all new to me even though supposedly from what other people are saying online that it's been around for a long time now.

Comcast isn't offering gigE to the home AFAIK, so it's a moot point. If you're asking about support at the LAN level for the network switch that might be a feature of some of their cable modems, it would depend on the model (though I can't remember seeing any that offer gigE I haven't looked at Comcast's offerings recently).

There's no way you are going to get jumbo frames across the Internet. You'll even be lucky to get full 1518 frames to some end points given that they might be on DSL with PPP encapsulation stealing bytes. Jumbo frames is for LANs where you are maxing out a GigE or higher and you want less overhead to give your CPU a break. There's less need for it these days with NICs including interrupt moderation and checksum offloading.

Omniman wrote:

Does anyone know if Comcast broadband supports jumbo frames? This whole jumbo frame thing is all new to me even though supposedly from what other people are saying online that it's been around for a long time now.

Jumbo frames increases the size of the packets sent through the network, for the purpose of reducing overhead. I don't recall the exact numbers, but a normal frame is of size a, and a jumbo frame has size b >> a. Each packet has some amount of information about where it's from, where it's going, etc of size c in both. With jumbo frames, that c is less of the overall network traffic, so you can push more live data.

In the case of internet connection speeds, there's not much point as you're probably not even maxing out a 100 mbit/s connection.

Jumbo frames are an ethernet standard which stops right at your modem. Remember IP packets can be as large as 64KB regardless of frame sizes.

http://en.wikipedia.org/wiki/Ethernet_II_framing
http://en.wikipedia.org/wiki/Jumbo_frame

So, if you look at the diagram on the Ethernet II framing article, you'll see the "data" part of the frame is limited to 1500 bytes. Basically it makes it so that you can increase your data load to 9000 bytes or so (or some other limit), thus making it so that you can send about 6 times more data per packet before sending the next segment of data. This means that the calculation of each packet's CRC checksum, generate each checksum on the Ethernet side of it as well as the TCP packet checksum's generation and checking is reduced greatly.

hmmm...very interesting...thanks everyone!

notfred wrote:

There's no way you are going to get jumbo frames across the Internet. You'll even be lucky to get full 1518 frames to some end points given that they might be on DSL with PPP encapsulation stealing bytes. Jumbo frames is for LANs where you are maxing out a GigE or higher and you want less overhead to give your CPU a break. There's less need for it these days with NICs including interrupt moderation and checksum offloading.

Nitrodist wrote:

http://en.wikipedia.org/wiki/Ethernet_II_framing
http://en.wikipedia.org/wiki/Jumbo_frame

So, if you look at the diagram on the Ethernet II framing article, you'll see the "data" part of the frame is limited to 1500 bytes. Basically it makes it so that you can increase your data load to 9000 bytes or so (or some other limit), thus making it so that you can send about 6 times more data per packet before sending the next segment of data. This means that the calculation of each packet's CRC checksum, generate each checksum on the Ethernet side of it as well as the TCP packet checksum's generation and checking is reduced greatly.

And here I thought the maximum was 1500 and not 1518. For the record, 1492 is the largest frame I've ever got to traverse the Internet.

shank15217 wrote:

Jumbo frames are an ethernet standard which stops right at your modem. Remember IP packets can be as large as 64KB regardless of frame sizes.

Unless I'm mistaken, 64KiB is the largest frame size or packet supported by IPv6. IPv4 is a bit more limited. I have to read that again in one of my networking books to be sure.

Speaking of IPv6, that's going to be one serious benefit: Auto-detection of the maximum frame size supported in a given path. I can't remember if that's least-common-denominator style where the lowest frame size supported in all paths is the one that determines the size of all frames leaving the client or if it varies based on path.

titan wrote:

shank15217 wrote:

Jumbo frames are an ethernet standard which stops right at your modem. Remember IP packets can be as large as 64KB regardless of frame sizes.

Unless I'm mistaken, 64KiB is the largest frame size or packet supported by IPv6. IPv4 is a bit more limited. I have to read that again in one of my networking books to be sure.

So, I was a bit off. IPv4 can support a 64KiB payload, but it is fragmented into approximately 45 packets.

IPv6 can support a payload as large as 4GiB. Fragmentation will still occur here as well, but the number of packets is dependent on the capabilities of the connection.

titan wrote:

And here I thought the maximum was 1500 and not 1518. For the record, 1492 is the largest frame I've ever got to traverse the Internet.

Yup, 1500 bytes of data and 18 bytes of Ethernet encap - 6 bytes DA, 6 bytes SA, 2 bytes size or type and 4 bytes FCS on the end. Note that this doesn't count the 7 bytes of preamble, 1 byte Start of Frame and 12 bytes (standard, non-standard 16, variable with a WIS layer on 10GE) of Inter Packet Gap. If you are on DSL running PPPoE then you lose 8 bytes to the PPPoE so that brings your data down from 1500 to 1492.

I used to write the software for the 10GE interfaces for the Cisco CRS-1 so I know my way around an Ethernet frame.

notfred wrote:

titan wrote:

And here I thought the maximum was 1500 and not 1518. For the record, 1492 is the largest frame I've ever got to traverse the Internet.

Yup, 1500 bytes of data and 18 bytes of Ethernet encap - 6 bytes DA, 6 bytes SA, 2 bytes size or type and 4 bytes FCS on the end. Note that this doesn't count the 7 bytes of preamble, 1 byte Start of Frame and 12 bytes (standard, non-standard 16, variable with a WIS layer on 10GE) of Inter Packet Gap. If you are on DSL running PPPoE then you lose 8 bytes to the PPPoE so that brings your data down from 1500 to 1492.

I used to write the software for the 10GE interfaces for the Cisco CRS-1 so I know my way around an Ethernet frame.

Neat!

Now that I think about it, when I had cable Internet I did have 1500 there. I've been on DSL in so many other places that I've omitted that from my recall.