Diagram of different frame types & other stuff

[Poontos]

Can someone please explain the following:

1.) Does a packet only contain a frame? If so, do you have any diagrams which so different types of frames and there contents?

2.) Are bridges commonly used in todays IT network infastructures? If so, what are some common brands/models?

3.) Besides the physical layer, what other layers do not modify the contents of a frame?

4.) What devices on a network are able to read a frame?


Thanks! I think that is it for now

 

[spidey07]

Can someone please explain the following:

1.) Does a packet only contain a frame? If so, do you have any diagrams which so different types of frames and there contents?
- A frame is a complete data chunk on the wire. This includes all 7 layers of information. For example...a HTTP packet would include the IP header, the TCP header and the HTTP application layer but not underlying layer two and one (physical and MAC)

2.) Are bridges commonly used in todays IT network infastructures? If so, what are some common brands/models?
- Are bridges used commonly in todays IT infrastructure? You bet, that is what an ethernet switch is pure and simple. Whether it be a linksys 4 port switch or a cisco 6500 loaded with 384 ports of 10/100, they are still bridges.

3.) Besides the physical layer, what other layers do not modify the contents of a frame?
Hmmmm, the only device that really modifies a frame are routers. They change layer2 information and layer3 info like TTL. A translational bridge does modify the frame (like ethernet frame to FDDI frame). Kindof confusing question. A layer does not modify a frame - depending on the device the frame passes through certain layers can be affected. hope that make sense.

4.) What devices on a network are able to read a frame?
- A frame is just the bits on the wire with its associated layers (hence FRAMED data). All devices on a network read a frame if it is delivered to them...and just how it is delivered to them is completely upto the network (switches, routers, bridges, hubs). Generally you think of a frame as layer2 and up. A packet is layer4 and up. Doesn't matter much, hardcore network guys think frames...others think in terms of chunks of data (packets)


Thanks! I think that is it for now

 

[Poontos]

<<
1.) Does a packet only contain a frame? If so, do you have any diagrams which so different types of frames and there contents?
- A frame is a complete data chunk on the wire. This includes all 7 layers of information. For example...a HTTP packet would include the IP header, the TCP header and the HTTP application layer but not underlying layer two and one (physical and MAC) >>



Thanks Scott, we would be lost without you! You don`t happen to have any diagrams showing various types of frames, comparing them and the like, do ya?



<<
2.) Are bridges commonly used in todays IT network infastructures? If so, what are some common brands/models?
- Are bridges used commonly in todays IT infrastructure? You bet, that is what an ethernet switch is pure and simple. Whether it be a linksys 4 port switch or a cisco 6500 loaded with 384 ports of 10/100, they are still bridges. >>


What about the ones that send out BPDU's, calculate spanning tree, and the root bridge, for example? I cannot see my 4 port Netgear switch doing this?



<< Besides the physical layer, what other layers do not modify the contents of a frame?
Hmmmm, the only device that really modifies a frame are routers. They change layer2 information and layer3 info like TTL. A translational bridge does modify the frame (like ethernet frame to FDDI frame). Kindof confusing question. A layer does not modify a frame - depending on the device the frame passes through certain layers can be affected. hope that make sense. >>


I think so... what exactly is TTL?



<< What devices on a network are able to read a frame?
- A frame is just the bits on the wire with its associated layers (hence FRAMED data). All devices on a network read a frame if it is delivered to them...and just how it is delivered to them is completely upto the network (switches, routers, bridges, hubs). Generally you think of a frame as layer2 and up. A packet is layer4 and up. Doesn't matter much, hardcore network guys think frames...others think in terms of chunks of data (packets) >>


So the final product down the layers would be a packet made up of a frame or frames? Or the other way around?

I know the "Final" product would be electrical signals, and 01010101's before that, but at layer three I guess I would be talking about.

Thanks again!

 

[ScottMac]

You might be thinking in reverse here....if I recall from the books (being old & feeble as I am)...

Layer 4 creates data SEGMENTS and passes it to layer three...

Layer 3 breaks it up and puts it into protocol PACKETS (high-level addressing) then passes it to layer two....

Layer 2 encapsulates the packets into FRAMES (low-level / MAC adressing) and passes it to layer one

Layer 1 encodes the frames using the proper signalling and sends it down the media.....

Other than Token Ring and maybe FDDI, you don't see/hear much about straight bridges these days. DEC, ACSYS, Synoptics (Bay, Nortel), Allied Telesys, and a mess of others all made and sold a sh*tload of bridges in their time. I'm not talking about switches (which are realy just multi-port bridges)...I'm talking regular old traditional bridges.

The "inside" data of the segment/packet never really gets changed in "normal" handling. As the data decends the stack, the next layer of formatting/addressing is added around the outside of the information ("stuffing into another envelope").

As the data ascends the stack on the receiving system, each layer peels off a layer of information, looks at the instructions (addresses/protocols/ports) and send the information to the next appropriate process in the next-higher layer ("Removes the envelope from the bigger envelope").

If I'm correct in the above information, then only a layer two device (of the appropriate protocol) can look into the frame. You wouldn't be able to look into an Ethernet frame with a protocol analyzer set up for Frame-Relay...even though they're both layer-two critters.

Analyzers (like a "Sniffer") look at each layer as it's peeled off and presents the decode layer-by-layer.

Routers can see the Layer-three information, because the packets have been addressed to the routers in Layer-two (MAC), which is discovered through the ARP (IP-->MAC addressing).

So, my guess would be that no device looks into the frame, without actually passing the frame (or a copy of it) up the stack according to its corresponding protocol (so that each layer can pass its "instructions" to that layer's processes).

There are, of course, some exceptions given the occasional proprietary "features" of some products. For example, Cisco's Multi-Layer Switching (MLS). With MLS active between a router-switch pair, the first packet of a stream is sent to the router for examination and evaluation. The router makes whatever decision regarding the forwarding of the packet, and ultimately that information is sent and stored in a cache in the switch. When the next packet of that session comes along, the switch compares it to the cached information; if it knows about this particular packet's sesion (a cache Hit), then it switches the packet without further intervention from the router (and all subsequent packets of this specific session/stream).

So there ya go. That's the way I remember it...hopefully it's pretty close to target.

Edit: Spidey got his post in a few milliseconds ahead of me, it seems.....damn he's quick...

FWIW

Scott

 

[spidey07]

who's scott??!!! you mean macintosh boy?

Any bridge should and must participate in spanning tree, thus generating BDPUs. in the earlier expample of a 384 port 6500 would indeed send out a bdpu every two seconds on ever single port. Now imagine if every port was trunking 100 VLANS. That would mean every port would send out 100 bdpus every two seconds. multiply by 384 and you get 38,400 bdpus every two seconds. yuck. I'm gonna run a quick trace on my linksys router and see if it generates bdpus. my guess is it would have two because it is a switch (BRIDGE - same thing...exact same thing)

BRB

WOWOWOWOWOWWOOWWOWOWOWWOW. That little SOB does NOT send out bdpus and does not participate in spanning-tree. What does this mean???????? This means that you can create a loop in your network that sends frames (L2 and above) forever looping and never dying.

shame, shame, shame on linksys. This breaks so many network DON'Ts it is no wonder why we've pulled them out from PC support's desk and smashed them on the floor right in front of them. you have got to be kidding me....a switch that doesn't send bdpus!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

I must be capturing wrong cause this just isn't possible. more to follow. SCOTTMAC WHERE YOU AT!!!??? Major discovery here.

 

[ScottMac]

I figured it didn't send BPDUs...how badly can a bridge loop affect a home user? So what if the network fills up with perpetually circulating data? Something goes wrong, the home user unplugs something....

I mean, EVEN CISCO lets you shut down Spanning Tree if you're confident enough in your network architecture (and it's a end(L)user-free environment....)

I'm just not surprised at all....I mean, really, how bad can a cascade of four-port switches really get?

*shrug*

Scott (McIntosh Boy???? sheesh...it's a good thing I don't respect your opinion, otherwise I'd be offended

)

 

[spidey07]

<< I mean, really, how bad can a cascade of four-port switches really get? >>



you know exactly how bad it can get.

Respect my opinion??? hell, I'm just a plumber anyway. I am a bit surprised that something so simple, tried and true would be left out of the linksys gear. again, kindof like my proxy-arp problem. I'm gonna make a loop tomorrow and see if it truly will foward frames untill that little blue bastard smokes himself into oblivion.

 

[ScottMac]

...Don't forget the PICs!!!

It might be more famous than Tom's Intel/Athon barbecue.....

From what I could tell (from previous browsings), none of the home/SOHO/consumer switches support STP. I believe it takes, OH, maybe a second to build to critical mass, then the XXXBase-T spec kicks in, shuts down the offending port for a while, then brings it up, then shuts it down (repeat Ad Lib)....

...and I was only foolin' about the respect thing. But Gee Whiz,...McIntosh Boy....(!) If you ever walk through a Marketing/Art department and see a bunch of Macs with wooden stakes through the floppy drive, you'll know I've been there.

Now, CP/M, that's a different story. My old IMSAI is in the (if it still exists) DEC museum in Boston. And I still have a H/Z 100 S100 all-in-one on the shelf (loaded with Wordstar, SuperCalc, and some other vital programs) for my backup machine. I still have the 64K Static RAM board I built; out in the garage in the same box with my 3C501, 3C2012, and S100 PROM burner. I think there's still a Tarbell tape controller in there, maybe a cassette interface too...I haven't been able to find my old PUN/RDR unit...my Diablo 630 is out on loan.....those were the days *sigh*.

Well, the sit-down timer has expired...the Sciatic is starting to glow again.

Have fun, get some Pics. The Swich vs. Hub data is still out on www.scottmac.net...maybe we can embellish the data with some pics showing which unit (switches or hubs) smoke better (I'm thinking switches, 'cause most of 'em have plastic cases).

G'night

Scott

 

[Poontos]

God you guys are hilarious! Thanks for all the info! spidey, I want PICS of the Linksys after your experiment!

Thanks guys! spidey, my bad... I scanned your icon too fast and assumed it was Scott, lack of sleep for me too, I guess.

Sorry about that mon!

 

[Poontos]

What about diagrams for the different frame types and the like?

 

[Poontos]

^^^

 

[Nutz]

<< What about diagrams for the different frame types and the like? >>



Frame images

I have printouts of these taped to my locker and desk at work.

 

[Poontos]

Thanks Nutz!