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PPP
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... ISDN,
ADSL, etc.) and then runs PPP over that connection. In such a
configuration, the L2 termination point and PPP ...
... PPP over that connection. In such a
configuration, the L2 termination point and PPP session endpoint
...
... L2TP extends the PPP model by allowing the L2 and PPP endpoints to
reside on different devices interconnected by a packet-switched
network ...
... DSLAM, etc.), and the
concentrator then tunnels individual PPP frames to the NAS. This
allows the actual processing of PPP ...
... PPP frames to the NAS. This
allows the actual processing of PPP packets to be divorced from the
termination of the L2 circuit.
...
... connection may terminate at a (local)
circuit concentrator, which then extends the logical PPP session over
a shared infrastructure such as frame relay ...
... L2TP may also solve the multilink hunt-group splitting problem.
Multilink PPP [RFC1990] requires that all channels composing a
...
... Network Access Server (NAS).
Due to its ability to project a PPP session to a location other than
the point at which it was physically received, L2TP ...
... Challenge Handshake Authentication Protocol [RFC1994], a PPP
cryptographic challenge/response ...
... LAC). The LNS is the
logical termination point of a PPP session that is being tunneled
from the remote system ...
... LNS and vice versa. When used in context
with PPP, a peer is either side of the PPP connection.
...
... LAC and LNS when an end-to-end PPP
connection is established between a Remote System and the LNS.
...
... L2TP scenario. The goal is to
tunnel PPP frames between the Remote System or LAC Client and an LNS ...
... connection to the public Internet. A "virtual" PPP connection is then
created and the local L2TP ...
... Data messages are used to
encapsulate PPP frames being carried over the tunnel. Control
messages utilize a reliable Control Channel ...
... L2TP Protocol Structure
Figure 3.0 depicts the relationship of PPP frames and Control
Messages over the L2TP Control and Data Channels ...
... Control
Messages over the L2TP Control and Data Channels. PPP Frames are
passed over an unreliable Data Channel encapsulated ...
... LAC based on LNS policy, for instance,
the existence of multilink PPP bundles.
8 - Session or tunnel ...
...
The Framing Type is a 32-bit mask, which indicates the type of PPP
framing requested for an OCRQ, or the type of PPP ...
... PPP
framing requested for an OCRQ, or the type of PPP framing
negotiated for an OCCN or ICCN ...
... OCCN or ICCN. The framing type MAY be used as an
indication to PPP on the LNS as to what link options to use for
...
... the remote system and LAC ultimately negotiated, as well as PPP
authentication information sent and received by the LAC ...
... authentication information sent and received by the LAC. This
information may be used to initiate the PPP LCP and authentication
...
... authentication
systems on the LNS, allowing PPP to continue without renegotiation
of LCP. Note that the LNS ...
... LNS with the Initial CONFREQ received by the LAC from
the PPP Peer.
The Attribute Value field ...
... AVP, Attribute Type 31, specifies the
challenge sent by the LAC to the PPP Peer, when using proxy
authentication ...
... Proxy Authen ID AVP, Attribute Type 32, specifies the ID value
of the PPP Authentication that was started between the LAC and the
...
... Authentication that was started between the LAC and the
PPP Peer, when proxy authentication is being used.
...
... Proxy Authen Response AVP, Attribute Type 33, specifies the
PPP Authentication response received by the LAC from the PPP ...
... PPP Authentication response received by the LAC from the PPP Peer,
when proxy authentication ...
... Reserved - Not used, MUST be 0
CRC Errors - Number of PPP frames received with CRC errors
since call was established
...
... CRC errors
since call was established
Framing Errors - Number of improperly framed PPP packets
received
Hardware ...
... Session MUST be established before L2TP can begin to tunnel PPP
frames. Multiple Sessions may exist across a single Tunnel ...
... L2TP Session=============* |
PPP +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ |
| | | |
[Remote] | | | |
...
... L2TP Session=============* |
PPP +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ |
| | | |
| |~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~| |
...
... Forwarding PPP Frames ...
...
Once tunnel establishment is complete, PPP frames from the remote
system are received at the LAC, stripped of CRC ...
... L2TP packet, and processes
the encapsulated PPP frame as if it were received on a local PPP
interface ...
... Tunnel ID header for all outgoing messages. In
this manner, PPP frames are multiplexed and demultiplexed over a
single tunnel between a given LNS ...
... sequence numbers always be enabled during the initial negotiation
stages of PPP and disabled only when and if the risk is considered
acceptable. For example, if the PPP session ...
... stages of PPP and disabled only when and if the risk is considered
acceptable. For example, if the PPP session being tunneled is not
utilizing any stateful compression ...
... encryption protocols and is
only carrying IP (as determined by the PPP NCPs that are
established), then the LNS ...
... LAC may
answer the call, negotiate LCP and PPP authentication, and use the
information gained to choose the LNS ...
... error conditions (conditions that
occur on the interface supporting PPP). The counters in this message
are cumulative. This message should only be sent when an error
...
... LNS
to the LAC to set PPP-negotiated options. These options can change
at any time during the life of the call, thus the LAC MUST be able to
...
... L2TP-over-UDP/IP tunnel, the PPP link has
the characteristic of being able to reorder or silently drop packets.
...
... the characteristic of being able to reorder or silently drop packets.
The former may break non-IP protocols being carried by PPP,
especially LAN-centric ones such as bridging ...
... L2TP data
message sequence numbers if any protocol being transported by the PPP
tunnel cannot tolerate reordering. The sequence dependency
...
...
Allowing packets to be dropped silently is perhaps more problematic
with some protocols. If PPP reliable delivery [RFC1663] is enabled,
...
... delivery [RFC1663] is enabled,
no upper PPP protocol will encounter lost packets. If L2TP sequence
numbers ...
... packet loss. In the case of
an LNS, the PPP and L2TP stacks are both present within the LNS, and
...
... with a CRC error. Where the LAC and PPP stack are co-resident, this
technique also applies. Where the LAC and PPP ...
... PPP stack are co-resident, this
technique also applies. Where the LAC and PPP client are physically
distinct, the analogous signaling ...
... signaling MAY be accomplished by sending a
packet with a CRC error to the PPP client. Note that this would
greatly increase the complexity of debugging client ...
... transport is
an unreliable transport. As with any PPP media that is subject to
loss, care should be taken when using protocols ...
... transport operates on the entire L2TP packet
and is functionally independent of PPP and the protocol being carried
by PPP. As such, L2TP ...
... and is functionally independent of PPP and the protocol being carried
by PPP. As such, L2TP is only concerned with confidentiality,
...
... packet stream via a secure transport does, in
turn, also protect the data within the tunneled PPP packets while
transported from the LAC to the LNS ...
... tunneling model, analogous
filtering is logically performed at the PPP layer or network layer ...
... authorization features based
upon the authenticated PPP user, or at the network layer itself by
...
... AVPs that MAY be exchanged during session establishment
to provide forwarding of PPP authentication information obtained at
the LAC ...
... proxy authentication, it MUST be able to be
configured off, requiring a new round a PPP authentication initiated
by the LNS ...
... Sklower, K., Lloyd, B., McGregor, G., Carr, D. and T. Coradetti, "The PPP Multilink Protocol (MP)", RFC 1990draft ...