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Author Topic: TR-101  (Read 1886 times)

Weaver

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TR-101
« on: May 07, 2018, 12:38:25 AM »

Is TR-101 usable as a vague description of BT21CN-type architecture? I like it because it has real explicit architecture diagrams and explicit protocol stacks in it.
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burakkucat

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Re: TR-101
« Reply #1 on: May 07, 2018, 05:09:12 PM »

I had to lookup TR-101 --

Broadband Forum, Technical Report, TR-101, "Migration to Ethernet-Based DSL Aggregation", Issue: 2, July 2011.

Having done that and read the relevant document I am still unable to answer your query. Sorry.  :(
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Weaver

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Re: TR-101
« Reply #2 on: May 07, 2018, 11:43:00 PM »

I was in a quest to find concise architectural diagrams and especially protocol stacks for 21CN. Kitz herself has dealt with the former at length.
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niemand

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Re: TR-101
« Reply #3 on: May 08, 2018, 05:22:31 PM »

It's kinda close to what happens with the VDSL products. What are you hoping to get from the protocol stacks?

For information, your traffic when it reaches the ISP is IP over PPP over L2TP over UDP over IP over Ethernet. Just before it reaches your ISP there may also be MPLS encapsulation, with the label removed before presentation to the ISP.

Your PPP session leaves your modem running as IP over PPP over Ethernet over xDSL, the xDSL encapsulation is removed at the MSAN, the PPP session sent to a BRAS via IP, where it is enclosed in L2TP for transport to the ISP. On the way it may be decapsulated and re-encapsulated by an arbitrary number of L2TP Access Concentrators.

You see none of these hops as your visibility ends with the BRAS terminating your PPP at that level and the IP traffic is never native so at no point is the hop counter decremented, hence no signs of hops in traceroutes - these rely on setting ascending hop counts starting at 1 and nodes sending ICMP time expired messages when TTL, a counter that decrements at each IP hop, hits zero. While being transported to the ISP the IP traffic is always encapsulated in something.

Obvious question - how does the BRAS know where to send you? It pre-auths you based on the domain in your username and then forwards the full authentication material to the ISP's authentication servers. The ISP servers can populate Attribute Value pairs in the response that tell the BRAS which LNS the ISP wants your traffic to go to, and can use round-robin DNS to balance both authentication and LNS too.

This was all from memory. It may be wrong. I just got out of an MSc exam in Digital Forensics so brain is somewhat cooked.
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Weaver

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Re: TR-101
« Reply #4 on: May 08, 2018, 11:35:50 PM »

Good for you about the MSc, well done. If I were in better health I would like to continue my education. Indeed I left work and went back into full-time education (at smo.uhi.ac.uk) twenty years ago, that is why I am living here now.

Many thanks for your generous explanation. That confirms my guesswork from bits of rumour I have gleaned over the years but without real sources.

> What are you hoping to get from the protocol stacks?

Just self-education, very keen to understand. I did this kind of thing for a living (software) many years ago.

You mentioned that “encapsulation is removed at the MSAN, the PPP session sent to a BRAS via IP” - what is the full protocol stack at that point for 21CN?
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niemand

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Re: TR-101
« Reply #5 on: May 09, 2018, 02:33:07 PM »

Thanks!

I believe the MSAN feeds the MSE with PPPoE, riding over L2TP, UDP and IP so that it can use a routed network to reach the MSE. Without L2TP the MSAN and MSE would have to be on the same layer 2 network, and there are most definitely not MSEs in each exchange where there are MSANs. The MSE then does its pre-auth and packages the data in L2TP for delivery to ISPs via LACs, arriving at the ISP LNS.

It would be somewhat weird however BTW could also transport the PPPoE over a layer 2 MPLS VPN. Google MPLS pseudowire for more on this. I am reasonable sure it's using IP rather than this but you might be interested in the above, it allows extension of layer 2 networks across layer 3 boundaries transparently to hosts.
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