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Author Topic: BRAS IPv4 address  (Read 1783 times)

Weaver

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BRAS IPv4 address
« on: May 05, 2018, 01:43:16 AM »

Until some months back I used to see entries in the BRAS column of AA’s clueless.aa.net control server which were names of the relevant BRAS that each of my three links was ultimately connected to. Then it changed, the names are now gone and all three show 213.1.180.152.

Apart from the obvious, what does that IPv4 address mean? (https://apps.db.ripe.net/db-web-ui/#/query?bflag&searchtext=213.1.180.152&source=RIPE#resultsSection  https://myip.ms/view/ip_addresses/3573658624/213.1.180.0_213.1.180.255)

I think it is a box in Edinburgh. If memory serves there used to be some box in Falkirk that was mentioned at one time, perhaps that was in the 20CN days.

If that box is on the public internet then I find that surprising. Perhaps it is so heavily firewalled off that they have no worries.

Is there a chance that that IP address is used in (PPP over) L2TP over UDP over IPv4 then, to the ISP? So the ISP needs IPv4 at their end of the tunnel.

And I don't see it as a hop because the PPP tunnel I live in goes straight (ish) through it?
« Last Edit: May 05, 2018, 02:00:06 AM by Weaver »
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Ixel

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Re: BRAS IPv4 address
« Reply #1 on: May 05, 2018, 10:51:23 AM »

Interesting, well I guess there's a good reason why it's changed.

For comparison, mine are showing up on control.aa.net.uk for the two bonded FTTC lines as:
- LTS2.VDSL.THW.TTNEW
- LTS2.VDSL.HEX.TTNEW

Which should both be in London, two different datacenters, Telehouse West and Harbour Exchange.
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Weaver

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Re: BRAS IPv4 address
« Reply #2 on: May 05, 2018, 01:45:11 PM »

Ixel, those are names, textual then?
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CarlT

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Re: BRAS IPv4 address
« Reply #3 on: May 05, 2018, 03:19:52 PM »

Don't need firewalls. Can just not advertise those prefixes to public Internet, only to kit that needs to connect to it.

IP addresses are required to transport L2TP if there's no ATM network to carry the stuff or other encapsulation. Carried over UDP port 1701.
« Last Edit: May 05, 2018, 03:23:32 PM by Ignitionnet »
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Weaver

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Re: BRAS IPv4 address
« Reply #4 on: May 05, 2018, 03:42:12 PM »

I'm on 21CN ADSL2+ now, so as I understand it that means no more ATM between DSLAM / MSAN (now) and BRAS so ethernet and possibly VLANs in it? And what, L2TP over UDP over say IPv4 over ethernet over something optical - just guessing - from DSLAM / MSAN to 21CN BRAS ?

Is that right? It's not just the BRAS-to-ISP side that uses L2TP perhaps then?

I've never managed to find out too much about the MSAN to BRAS path. Burakkucat and I were talking about it a good while back and I collected a list of every mention I could find, every scrap of info about the protocol stack along that stretch.

These are the notes I put together back then :


Exchange backhaul protocols BT 21CN MSAN

* BT 21CN: SIN466 says Ethernet, no mention of ATM at all.

* BT 21CN: SAMKnows states Ethernet replaced ATM
    https://www.samknows.com/broadband/exchanges/21cn_overview
But perhaps SAMKnows doesn't know :-)

* BT 21CN So does this : http://blog.farnz.org.uk/2010/02/on-pppoa-pppoe-atm-and-adsl.html

Q: Is WBC 21CN? - If so, SIN 472 v2, sec 3.7 says “WBC uses ATM over DSL for the link between the end user and the MSAN but Ethernet between the MSAN and the BRAS.”

Q: Is 21CN TR-101 a description of BT 21CN ?

This page hints that it is, not v explicit -     https://support.zen.co.uk/kb/Knowledgebase/Zen-ADSL-Broadband-Products-Line-Data

Also, this post claims it is - http://forums.thinkbroadband.com/zen/4075096od-tr101.html?fpart=all&vc=1

* If that's so, TR-101 (2006) talks unambiguously about the total replacement of ATM with Ethernet, and hallelujah that spec has explicit protocol stacks! Yay!

* Weak support: 21CN - the following doc makes no mention of ATM for 21CN, only ethernet
        http://www.edinburgh.bcs.org/events/2007-08/080109.ppt

Vvery weak hint: VDSL2 :- Paper by Eriksson and Odenhammar: Ericsson Review no 1 (2006) - "VDSL2", p. 38 - “The elimination of ATM in the first mile means the access architecture can be simplified into an end-to-end Ethernet access architecture that uses virtual local area networks (VLAN) as the service-delivery mechanism across the entire access network.” - does this apply to 21CN?

-- TR-101 refs --
2006:
a. Sec 2 "migration process from an ATM based aggregation network to an Ethernet based aggregation network."

b. sec 2.3 (p 21);
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Ixel

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Re: BRAS IPv4 address
« Reply #5 on: May 05, 2018, 09:51:19 PM »

Ixel, those are names, textual then?

Indeed, that's how it appears on the main page at control.aa.net.uk for me. I can see the IP address of either if I want to however.
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CarlT

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Re: BRAS IPv4 address
« Reply #6 on: May 06, 2018, 12:50:40 PM »

On 21CN an MSE in an exchange is usually your BRAS. Other concentrators / LACS may be used on the way to the ISP LNS but that's likely what you're seeing.

21CN uses an MPLS network. Hops are hidden by L2TP and indeed PPP. Your data doesn't have any TTL drop on its way to the ISP as there are no points where the IP traffic inside your PPP session is used or decapsulated.

Your very weak hint paper mentions the access network. FTTC does indeed use VLANs, as does FTTP. 21CN does not in the same way. It uses L2TP riding over IP to segment traffic, with individual PPP sessions inside the L2TP tunnels. VLANs are used to split customers, backhaul uses VLANs but these are not expressed to customers and aren't actually required, but make management, QoS etc easier as these backhauls carry a variety of traffic with varying priorities and service levels, not just residential broadband. You'll have seen mention of SVLANs.

FTTC delivers subscribers to CPs via nested VLANs. SVLANs / Service VLANs to individual CPs inside which are CVLANs, Customer VLANs.
« Last Edit: May 06, 2018, 04:38:11 PM by burakkucat »
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Terrydaktal

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Re: BRAS IPv4 address
« Reply #7 on: November 28, 2018, 04:14:24 AM »

On 21CN an MSE in an exchange is usually your BRAS. Other concentrators / LACS may be used on the way to the ISP LNS but that's likely what you're seeing.

21CN uses an MPLS network. Hops are hidden by L2TP and indeed PPP. Your data doesn't have any TTL drop on its way to the ISP as there are no points where the IP traffic inside your PPP session is used or decapsulated.

Your very weak hint paper mentions the access network. FTTC does indeed use VLANs, as does FTTP. 21CN does not in the same way. It uses L2TP riding over IP to segment traffic, with individual PPP sessions inside the L2TP tunnels. VLANs are used to split customers, backhaul uses VLANs but these are not expressed to customers and aren't actually required, but make management, QoS etc easier as these backhauls carry a variety of traffic with varying priorities and service levels, not just residential broadband. You'll have seen mention of SVLANs.

FTTC delivers subscribers to CPs via nested VLANs. SVLANs / Service VLANs to individual CPs inside which are CVLANs, Customer VLANs.

So to get this straight, I'm really not sure at all about the protocol stacks here so I'm going to make a wild guess. I'd appreciate correcting so I fully know the whole process

Traffic from ADSL2+ customers will leave the modem as IP over PPP over PPPoE over Ethernet over AAL5 over ATM over ADSL2+, it will go through the MDF,HDF to the CMSAN, CMSAN removes ADSL2+, ATM and AAL5 and frames the Ethernet frame in GFP and SDH and it is sent to the FMSAN and then to the Tier 1 MSAN which performs WDM and it is received by the Metro EEA which demultiplexes? and adds MPLS label and it goes to the bRAS which terminates the PPPoE session and places the PPP session in the L2TP tunnel over UDP over IP over MPLS then it goes to the AP port on the BEA, then through the MSIL onto the core network using MPLS to route to the core node next to the ISP and the ISP edge router functions as the PPP terminator which removes PPP, L2TP, UDP and the IP packet travels raw over the ISP network

Traffic from FTTC customers will leave the modem as IP over PPP over PPPoE over Ethernet?PTM? over VDSL2, the DSLAM in the cabinet removes VDSL2 and encapsulates the Ethernet in GFP, SDH and it continues through the E side ducts, the OCR, the OLT and the L2 switch and then to the F.MSAN? and then to the Tier 1 MSAN and so on as above

Traffic from legacy 20C customers will leave the modem as IP over PPP over PPPoA over ATM over ADSL and go to the DSLAM in the exchange not sure whether it keeps ADSL on or strips it off and replaces with another PHY layer, but it goes through the virtual path and the ATM MSiP backhaul to the bRAS which terminates the PPPoA, strips off the ADSL and ATM layer and encapsulates the IP packet in L2TP over UDP over IP over MPLS over Ethernet and travels to the EEA it is connected to which aggregates it with other customers to travel over the MSIL pipe to the ISP LNS which terminates the L2TP tunnel

Of course. There are many inaccuracies here and I would love for them to be corrected so I get a better understanding. A further question I'd like to ask is where Virgin comes into the BTw network and also how the BTw network is connected to Tier 1 ISPs like Deutsche Telekom
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Weaver

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Re: BRAS IPv4 address
« Reply #8 on: November 28, 2018, 05:42:47 AM »

Superb piece of work, fantastic, thank you. Regarding FTTC in the CPE modem, I would think it has to be IP over PPP over PPPoE over Ethernet over PTM over VDSL2, as you say.

I don’t think that it is correct about PPPoA vs PPPoEoA being linked with 20CN vs 21CN is it? Are the two not just independent? 21CN ADSL exchanges will support two types of CPE which will speak either PPPoEoA over ADSL or PPPoA over ADSL.

To be completely accurate when talking about ADSL we would have to say “PPPoE over Ethernet over RFC2684 over AAL5 over ATM”, rather than just “PPPoE over Ethernet over AAL5 over ATM” for the case of PPPoEoA, and also similarly add “RFC2684 over (AAL5)” in the case of PPPoA.
« Last Edit: November 28, 2018, 06:00:32 AM by Weaver »
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CarlT

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Re: BRAS IPv4 address
« Reply #9 on: November 29, 2018, 09:19:30 AM »

Will come back to this later but nowhere is PPPoE over Ethernet used. PPPoE doesn't need another Ethernet encapsulation it already has one, the E part in PPPoE. Also 21CN uses PPPoA, the ATM is stripped at the MSAN.

The 21CN MSANs for most subscribers feed into an MSE, Multi Service Edge, which terminates the PPP session then re-encapsulates. None of this kit does WDM, it is not transmission equipment.

The cabinet MSANs do not do SDH. Ethernet only. GTP is not used here at all, just Ethernet, with 802.1ad.

The transmission equipment that has to worry about SDH is separate kit. Wholesale buy circuits from Openreach, Openreach's problem how to transport them.
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j0hn

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Re: BRAS IPv4 address
« Reply #10 on: November 29, 2018, 02:04:16 PM »

Quote
Traffic from FTTC customers will leave the modem as IP over PPP over PPPoE over Ethernet?PTM? over VDSL2

Not necessarily.
Talktalk use IPoE instead of PPPoE
Sky do similar with DHCP Option 61
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CarlT

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Re: BRAS IPv4 address
« Reply #11 on: November 29, 2018, 03:10:52 PM »

May come back to this again but multiple MSANs aren't used as a general rule. These are for terminating broadband lines and those are already terminated by the MSAN. FTTC has no MSAN after the cabinet, and there's no mention of CableLinks. L2S and OLT are actually the same thing in at least some cases now. A big converged edge box, probably a Huawei 5600T in at least some cases, with some line card slots populated with OLT cards and others with Ethernet ports of 1G or 10G variety for L2S duty.

Having DSLAMs retain xDSL as a physical layer for presentation deeper into the network isn't going to happen. DSL is there to provide transport over the copper telephone network.

Reading a subsequent post I see another mention of PPPoE being a separate layer to PPP or Ethernet. It's not - it is PPP over Ethernet. Running a PPP session inside another PPP session or encapsulating Ethernet frames inside Ethernet frames is crazy. There's enough overhead there already without adding more.

Suggest reviewing the 'basics' of how this stuff works and the protocols are layered. Without that I could recite pretty much the components and where they fit in but it wouldn't make sense.
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Weaver

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Re: BRAS IPv4 address
« Reply #12 on: November 29, 2018, 03:59:46 PM »

Carl, I ought to confess - it’s a matter of terminology. You and I are in agreement, in fact. I was talking about individual headers, so counting PPPoE as three things not one, because of the individual headers. You are of course quite correct that PPPoE implies PPP and Ethernet that’s what it’s for, to carry the one over the other ultimately. And I agree about ATM being stripped at the MSAN in 21CN, I was arguing for this in an earlier post. I think I also confused you because in that recent post I was talking about the link between CPE and MSAN, not further upstream, and that was not helpful. I didn’t mean to imply that xdsl was going further into the network. Sorry for the confusion. I was uncertain about the stuff further upstream than the MSAN and was trying to work it out based on a lot of vague articles I’ve read. But I have very good, detailed references for the variety on first-mile protocol stacks in use.
« Last Edit: November 29, 2018, 04:15:56 PM by Weaver »
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Terrydaktal

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Re: BRAS IPv4 address
« Reply #13 on: November 30, 2018, 01:33:58 AM »

Hey guys, thanks for this helpful information. With regards to PPPoEoE, I thought that's just PPPoE in an Ethernet frame, the PPPoE encapsulation containing the PPP payload, length, session ID, code, type and VER, which in itself is the Ethernet payload. The below image I just found should help
« Last Edit: November 30, 2018, 01:49:09 AM by Terrydaktal »
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CarlT

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Re: BRAS IPv4 address
« Reply #14 on: November 30, 2018, 02:17:17 AM »

Please see https://tools.ietf.org/html/rfc2516

PPPoE is PPP transported over Ethernet. I guess they included redundant information in the diagram as an attempt at extra clarity.

Weaver: I was referring back to an earlier post rather than answering yours, Sir, please excuse my lack of clarity.
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