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squircle

join:2009-06-23
Oakville, ON
reply to dsiemon

Re: [Internet] Per packet overhead on Bell's VDSL? ATM based?

said by dsiemon:

let me clarify that I'm trying to figure out the number of bytes added to an IP packet.

Well then, let's go question-by-question:

said by dsiemon:

More questions:
- Does payload rate include 802.3 overhead or just the encoding on the line? I'm guessing the later based on some experiments I've done.

It includes the full 802.3 overhead; payload in this case is IP packets in 802.3 frames.

said by dsiemon:

- Is the full 802.3 header transmitted over the VDSL link?
- 7 bytes preamble
- 1 byte start of frame
- 6 byte D MAC
- 6 byte S MAC
- 2 byte Ethertype
- 2 byte 802.1Q tag it's 4 and always will be
- 4 byte FCS
- 12 byte Interframe gap
Total: 40 bytes

+ 8 for PPPoE gives me a total per packet overhead of 48 bytes.

You're comparing apples to oranges. 8 bytes for PPPoE gives you a total packet overhead of 8 bytes. That's it. Nothing more. At all. Period. There are 40 bits added to the frame, but you also seem to be under the impression that the frame is subject to the IP MTU which it is not. Wireline frames don't care about what protocol is running atop them.

This means your 1492 + 8 byte IP packet becomes (1500+7+1+6+6+2+4+12=) 1538 bytes on the wire.

So if you're looking for per-packet overhead, it's 8 bytes, just like I said in the beginning. The rate of 1492-byte IP packets on the WAN is given to you by the payload rate of your modem. When you're doing calculations for shaping, ignore 802.3 overhead (as most guides from commercial appliances will tell you). Shaping is done a layers higher than data link, acknowledging that framing will always be there and doesn't actually add anything to the packet. Nothing useful comes from caring about 802.3 overhead (not even for small packets).

I talked to the person who managed our massive Sandvine boxes here, and he said that you're wasting your time worrying about 802.3 stuff for small-packet performance; he says to "just treat them like any other packet because that's what every other device does."

HTH.

dsiemon

join:2012-09-26
Does payload rate include 802.3 overhead or just the encoding on the line? I'm guessing the later based on some experiments I've done.
I wasn't precise enough with this question. What I should have asked is does the difference between the sync rate and the payload rate include the 802.3 overhead. From your response I gather you agree that it doesn't. The alternative doesn't make sense because the amount of overhead has to vary by the size of the packet. There is a fixed 802.3 header added to a variably sized IP packet. Unless the payload value is an average or something like that it cannot reflect 802.3 overhead without assuming a fixed IP packet size (802.3 header / IP packet size is not constant).

Ooops, you are right about the 802.1Q field size. So 42 for 802.11 and 8 for PPPoE for a total of 50.

You're comparing apples to oranges. 8 bytes for PPPoE gives you a total packet overhead of 8 bytes. That's it. Nothing more. At all. Period. There are 40 bits added to the frame
From the perspective of the PPPoE interface on my home router everything beyond the IP packet is overhead. The bit rate limit is applied to the PPPoE interface. Since the overhead bytes are transmitted on the wire they affect the total 'real' bandwidth available and must be taken into account for optimal shaping. Otherwise the shaper will overshoot the bandwidth available.

I've always seen the term frame used to talk about the Ethernet header + payload so your comment about 40 bytes added to the frame doesn't make sense to me.

but you also seem to be under the impression that the frame is subject to the IP MTU which it is not.
Not at all.

This means your 1492 + 8 byte IP packet becomes (1500+7+1+6+6+2+4+12=) 1538 bytes on the wire.
Aren't you missing the 802.1Q bytes there? Ignoring that, the above calculation is 46 bytes of overhead on top of each IP packet.

To summarize:
- The difference between sync rate and payload rate is DSL error correction overhead.
- The payload is the entire 802.3 frame including FCS, inter frame gap etc.