The broadband protocol stacks
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The Broadband Forum as a lot of technical reports about the xDSL architecture but it is not so easy to find a good description of the global architecture. Those are ASCII-art protocol stack I inferred from those documents. What is in there may be wrong, feel free to correct me.
You can find relevant diagrams in TR-025 (figure 4, for end-to-end ATM network; figure 5 for L2TP; figure 6 for PPP (Point to Point) termination at the access node), in TR-101 (figure 4 for the U interface; figure 7 for the V interface), TR0-59 (figure 11).
Table of content
Classic POTS access with PPP
[IP ]<-------------------------------------->[IP ] [PPP ]<-------------------------------------->[PPP ] [RS-232]<->[RS-232|V.92]<------>[V.92|RS-232]<->[RS-232] Client Modem Modem RAS
The link between the client and the Remote Access Server (RAS) is a point-to-point link. The PPP link-layer protocol is used. It provides:
- multiplexing of multiple network protocols (IPv6, IPv4, IPX, etc);
- configuration of the network protocol parameters (IP address, netmask, IPv6 delegated prefix, etc.) using dedicated Network Control Protocols (NCPs);
- session establishment;
- authentication (using PAP, CHAP, EAP, etc.),
- compression, encryption;
- etc.
Basic example: PPPoA
PPPoE PPPoA L2TP
<-------------> <------------------------------> <---------->
[IP ]<->[IP ]<------------------------------------------------->[IP ]
[PPP ]<---------------------------------/PPP \-->[PPP ]
[PPPoE]<->[PPPoE|(NLPID)]<-------------->[(NLIPD)|L2TP]<->[L2TP]
[(LLC )]<-------------->[(LLC) |UDP ]<->[UDP ]
[AAL5 ]<-------------->[AAL5 |IP ]<->[IP ]
[Eth]<->[Eth|Eth ]<->[Eth |ATM ]<->[ATM ]<->[ATM | ] … [ ]
[PHY]<->[PHY|PHY ]<->[PHY |xDSL ]<->[xDSL|PHY]<->[PHY | ] … [ ]
S T U V A10
Router B-NT/xDSL modem AN/DSLAM LAC/BRAS … LNS
Explanations:
- A PPP session is first established between the user router (usually this will be the user modem-router/box) to the L2TP Access Concentrator (LAC).
- This session is transported over Ethernet in the local network and then over ATM: the xDSL modem change the encapsulation of the PPP session from PPPoE (PPP over Ethernet) to PPPoA (PPP over AAL5). Alternatively, the modem could encapsulate the Ethernet frames over ATM (PPPoEoA). On new deployments, it is recommended to use Ethernet instead of ATM for the aggregation.
- Based on authentication information supplied by the user router, the LAC will choose the suitable ISP, authenticate the user of this ISP RADIUS server. If the authentication succeeds, the RADIUS server will ask the LAC to extend the PPP session over L2TP (Layer 2 Tunneling Protocol) over UDP over IP to the ISP LNS (L2TP Network Server).
Variations
- The modem and the router are usually combined in a single modem-router (the “box”).
- The modem can be connected to the router with USB. In this case, what is used? ATM-over-USB? Ethernet over USB (CDC or RNDIS)?
- RFC 2684 defines two encapsulation methods over AAL5.
- IPsec can be used to secure the L2TP session.
- Ethernet is supposed to be used instead of ATM for aggregation in new deployments.
- Instead of converting from PPPoE to PPPoA, the modem can encapsulate the Ethernet frame over ATM behaving like a simple bridge. In this, case we have PPP over Ethernet over ATM (PPPoEoA). This is usually called PPPoE in the DSL context.
- Interworking functions can be used at the AN in order to transition between the ATM layer (user side) and the Ethernet layer (network side).
- SLIP (Serial Line Internet Protocol) is an old protocol, similar to PPP, for encapsulation of IP over a serial line.
- QinQ at the V interface for tagging the different users: this way it is possible to provide L2 VPN with VLAN support.
- Instead of extending the PPP, the BNC can terminate it and transport the IP packets directly.
Modem-router
The modem and the router are often merged in a modem-router:
PPPoA L2TP
<-------------------------------><---------->
[IP ]<->[IP ]<------------------------------->[IP ]
[PPP ]<---------------/PPP \-->[PPP ]
[(NLPID)]<-------------->[(NLIPD)|L2TP]<->[L2TP]
[(LLC) ]<-------------->[(LLC) |UDP ]<->[UDP ]
[AAL5 ]<-------------->[AAL5 |IP ]<->[IP ]
[Eth]<->[Eth|ATM ]<->[ATM ]<->[ATM | ] … [ ]
[PHY]<->[PHY|xDSL ]<->[xDSL|PHY]<->[PHY | ] … [ ]
T U V A10
Modem-Router AN/DSLAM LAC/BRAS … LNS
PPPoEoA
Instead of converting between PPPoE to PPPoA, the modem can encapsulate PPPoE over ATM (PPPoEoA). The modem can be seen as an Ethernet bridge. This solution is often called PPPoE because ATM was previously always used for aggregation.
PPPoE L2TP
<----------------------------------------------> <---------->
[IP ]<->[IP ]<------------------------------------------------->[IP ]
[PPP ]<---------------------------------/PPP \-->[PPP ]
[PPPoE]<-------------------------------->[PPPoE |L2TP]<->[L2TP]
[Eth]<->[Eth|Eth ]<->[Eth ]<-------------->[Eth |UDP ] … [UDP ]
[ |AAL5 ]<-------------->[AAL5 |IP ] … [IP ]
[ |ATM ]<->[ATM ]<->[ATM | ] … [ ]
[PHY]<->[PHY|PHY ]<->[PHY |xDSL ]<->[xDSL|PHY]<->[PHY | ] … [ ]
S T U V A10
Router B-NT/xDSL modem AN/DSLAM LAC/BRAS … LNS
Ethernet aggregation
In this example, the aggregation network is Ethernet based. This is recommended for new deployments. PPPoE (without ATM) is used instead of PPPoA. As before, the modem can be seen as an Ethernet switch.
PPPoE L2TP
<-----------------------------------------------> <---------->
[IP ]<->[IP ]<-------------------------------------------------->[IP ]
[PPP ]<----------------------------------/PPP \-->[PPP ]
[PPPoE]<--------------------------------->[PPPoE |L2TP]<->[L2TP]
[ |UDP ]<->[UDP ]
[QinQ]<->[QinQ |IP ]<->[IP ]
[Eth]<->[Eth|Eth ]<->[Eth ]<->[Eth ]<->[Eth | ] … [ ]
[PHY]<->[PHY|PHY ]<->[PHY |xDSL ]<->[xDSL|PHY ]<->[PHY | ] … [ ]
S T U V A10
Router B-NT/xDSL modem AN/DSLAM LAC/BRAS … LNS
IPoE
Ethernet aggregation (no ATM) without PPP.
[IP ]<->[IP ]<--------------------------------->[IP ]<->[IP ]
[QinQ]<->[QinQ | ] … [ ]
[Eth]<->[Eth|Eth ]<->[Eth ]<->[Eth ]<->[Eth | ] … [ ]
[PHY]<->[PHY|PHY ]<->[PHY |xDSL ]<->[xDSL|PHY ]<->[PHY | ] … [ ]
S T U V A10
Router B-NT AN BNG …
(xDSL modem) (DSLAM)
Interfaces
Details of the interfaces can be found in TR-059 page 9.
- The S interface;
- The T interface, between the user premises and the DSL modem/RG (Routing Gateway). The RG may initiate the PPP session itself or transport PPPoE sessions initiated from the local network.
- The U interface, between the B-NT (xDSL modem) and the Access Node (DSLAM);
- The V interface;
- The A10 interface, between the Regional Broadband Network and the NSP/ASP/ISP POP.
The T interface
[ IP ]
[ PPP ]
[ IP ] [ PPPoE ]
[ Eth ] [ Eth ]
[ PHY ] [ PHY ]
The U Interface
The U interface is the interface between the B-NT (the xDSL modem) and the Access Node (DSLAM):
[IP ]
[PPP ]
[IP ] [PPPoE ] [IP ]
[Eth ] [Eth ] [IP ] [PPP ]
[(SNAP) ] [(SNAP) ] [(NLPID)] [(NLPID)] [IP ]
[(LLC) ] [(LLC) ] [(LLC) ] [(LLC) ] [PPP ]
[AAL5 ] [AAL5 ] [AAL5 ] [AAL5 ] [IP ] [PPPoE]
[ATM ] [ATM ] [ATM ] [ATM ] [Eth ] [Eth ] [Eth]
[PHY ] [PHY ] [PHY ] [PHY ] [PHY ] [PHY ] [PHY]
IPoEoA PPPoEoA IPoA PPPoA IPoE PPPoE Eth
a b c d e f g
TR-043 compares the different ATM-based solutions.
Notes:
- When talking about DSL, “PPPoE” was often used over ATM. In this context, PPPoE is often used for PPP over Ethernet over ATM (PPPoEoA, option b). Sometimes, the name “PPPoEoE” is used for PPP over Ethernet without ATM (option f).
The V Interface
[IP ]
[IP ] [PPP ] [PPP ]
[802.1ad] [802.1ad] [802.1ad] [AAL5]
[Eth ] [Eth ] [Eth ] [ATM ]
[PHY ] [PHY ] [PHY ] [PHY ]
IPoE PPPoE
a b c
More stuff
Multiplexing over AAL5
RFC 2684 defines two methods of protocol multiplexing over AAL5:
- Virtual Circuit multiplexing (VC mux): a different ATM VC is used for each encapsulated protocol;
- LLC encapsulation: LLC headers are used which includes SAP fields (both SSAP and DSAP) for protocol multiplexing. An intermediate NLPID field (for routed PDU in NLPID format) or a SNAP frame can be used.
[.... / ... ]
[SNAP / NLPID]
[ ] [LLC ]
[AAL5] [AAL5 ]
[ATM ] [ATM ]
VC Multiplexing LLC Encapsulation
L2TP and RADIUS message exchange
ASCII art diagram
[Client] [BRAS] [LNS] | | [RADIUS Proxy]| [RADIUS] | | | | | | | | | | | | | | | I] Initial challenge | | | | | |<------| | | | CHAP Challenge |------>| | | | CHAP Response | |------>|------------>| RADIUS Access-Request | |<------|<------------| RADIUS Access-Accept | | | | | Tunnel-Type=L2TP | | | | | Tunnel-Medium-Type=IPv4 | | | | | Tunnel-Server-Endpoint=lns.example.com | | | | | Tunnel-Password=potato |<------| | | | CHAP Success | | | | | | | | | | | | | | | II] Tunnel establishment | | | | | | |-------------->| | L2TP Start-Control-Connection-Request | |<--------------| | L2TP Start-Control-Connection-Reply | |-------------->| | L2TP Start-Control-Connection-Connected | |<--------------| | L2TP Zero-Length Body Ack. | | | | | | | | | | III] Call establishment | | | | | | |-------------->| | L2TP Incoming-Call-Request | |<--------------| | L2TP Incoming-Call-Reply | |-------------->| | L2TP Incoming-Call-Connected | |<--------------| | L2TP Zero-Length Body Ack. | | | | | | | | | | | | | | | IV] New challenge | | | | | |<----------------------| | CHAP Challenge |---------------------->| | CHAP Response | | | |---->| RADIUS Access-Request | | | |<----| RADIUS Access-Accept |<----------------------| | CHAP Success | | | | |
References
- RFC 1661, PPP
- RFC 2364, PPPoA
- RFC 2516, PPPoE
- RFC 2661, L2TP
- RFC 3931, L2TPv3
- RFC 5072, IPv6 over PPP
- RFC 1334, PPP Authentication Protocols,
- RFC 1994, CHAP
- RFC 3748, EAP
- RFC 1055, SLIP
- 802.2, LLC
- RFC 2684, Multiprotocol over AAL5
- Broadband Services Architecture for Access to Legacy Data Networks over ADSL, Issue 1, TR-012, recommendation of PPP over ATM at the U interface for ATM end-to-end network.
- Core Network Architecture Recommendations for Access to Legacy Data Networkds over ADSL, TR-025
- Protocols at the U interface for Accessing Data Networks using ATM/DSL, TR-043, describes PPPoEoA, IPoEoA, IPoA. A this time ATM was always used so PPPoEoA was named PPPoE and IPoEoA was named IPoE.
- DSL Evolution - Architecture Requirements for the Support of QoS-Enabled IP Services, TR-059
- Ethernet based aggregation, TR-101 issue 2
- IPv6 for PPP Broadband Access, TR-187, suggests at least /60 IPv6 delegated prefix for home network and recommends /56. Suggests up to /48 for large organisations.
- Whats is the correct VP/VC to use?, in this page “PPPoE” is PPP over Ethernet over ATM (PPPoEoA).
- An Overview of Point to Point Protocol Architectures in Broadband Access Networks
- ATM over ADSL
- RFC 2868, RADIUS Attributes for Tunnel Protocol Support
- RFC 7542, The Network Access Identifier (NAI)