[policedemil]

Thank you! This is starting to make a lot more sense! I guess what's still confusing to me has to do with the seriality (continuity?) of the process vs the OSI model, which seems like there are more discrete stages.

Does each stage of the model need to complete for the whole packet before moving on to the next stage? For example, does A/D conversion take place until all of the packet information is converted, then the whole binary blob is enframed as a header and a packet... then we filter for MAC address and move on up the stack in discrete and consecutive stages? Or are the voltages read off the line and once there is enough information to construct the header, compare it, then choose to continue reading the rest from the line or stop the A/D conversion because it is just a waste of energy? The latter makes a lot more sense to me.

EDIT: words

[mschuster91]

> Does each stage of the model need to complete for the whole packet before moving on to the next stage?

Usually, not. Massive core switches could not work if they had to wait for every frame being fully in the buffer before beginning to transmit it to the correct out port. All a core switch needs to look at is the destination MAC address.

Simple math explains why: MTU (max packet size) is usually 1500 bytes (due to most packets originating in Ethernet systems). The dstmac in the Ethernet frame is bytes 9 through 16, which means it would be an absolute waste of time to wait with forward transmission until the remaining 1484 bytes are in the buffer.

Let's calculate this with your ordinary 100 MBit/s home connection (to keep the numbers in reasonable magnitudes). 100 MBit/s means: 0.00000001 s/bit (or 0.01 us/bit, or 0.08 us/byte). With retransmit start after the first 16 bytes, the delay introduced by the equipment is 1.28 us (and needs, basically, 9 bytes of buffering capacity from start of packet to end of packet). Waiting for the full 1500 bytes would introduce 120 us (or 0.12ms) of latency, as well as require 1500 bytes of buffer during the transmission time.

[policedemil]

Excellently put! That's exactly what I was looking for! Thank you!

[mschuster91]

If you want to read further, this is a part of network delay (https://en.wikipedia.org/wiki/Network_delay). A highly interesting field.

By the way, one thing I have forgotten: an instant-forward has much less latency (obviously), but it cannot retract packets that were corrupted during receiving at the ingress port - simply because the checksum can only be calculated when the whole packet is in the buffer.

So basically you choose between safety (corrupted packets do not travel as long, because they don't even reach the final station) and latency (e.g. 10 hops a 0.12ms = 1.20ms delay on 100MBit/s), and also for the cost in buffer memory.

[dom0]

> For example, does A/D conversion take place until all of the packet information is converted, then the whole binary blob is enframed as a header and a packet

The "A/D stage" only gives off a stream of bytes, so something similar to a state machine will be used to separate it into packets that are then further processed.

That doesn't mean you have to wait for a whole packet to come through, though. E.g. a switch (or router) could start to forward the packet as soon as it sees the destination address.