http://www.faqs.org/rfcs/rfc2364.html
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4. Multi-Protocol Encapsulation
This specification uses the principles, terminology, and frame
structure described in "Multiprotocol Encapsulation over ATM
Adaptation Layer 5" [4].
The purpose of this specification is not to document what is already
standardized in [4], but to specify how the mechanisms described in
[4] are to be used to map PPP onto an AAL5-based ATM network.
Section 1 within [4] defines the two mechanisms for identifying the
Protocol Data Unit (PDU) payload field's protocol type: virtual
circuit based multiplexing, and Logical Link Control (LLC)
encapsulation. In the former technique, the payload's protocol type
is implicitly agreed to by the end points for each virtual circuit
using provisioning or control plane procedures. When using the LLC
encapsulation technique, the payload's protocol type is explicitly
identified on a per PDU basis by an in-band LLC header, followed by
the payload data.
When transporting a PPP payload over AAL5, an implementation:
1. MUST support virtual circuit multiplexed PPP payloads as
described in section 5 below by mutual configuration or
negotiation of both end points. This technique is referred to
as "VC-multiplexed PPP".
2. MUST support LLC encapsulated PPP payloads on PVCs as
described in section 6 below by mutual configuration or
negotiation of both end points. This technique is referred to
as "LLC encapsulated PPP".
3. For SVC set up, an implementation MUST negotiate using the
Q.2931 [9] Annex C procedure, encoding the Broadband Lower Layer
Interface (B-LLI) information element to signal either VC-
multiplexed PPP or LLC encapsulated PPP. The details of this
control plane procedure are described in section 7.
If an implementation is connecting through a Frame Relay/ATM FRF.8
[7] service inter-working unit to an RFC 1973 [6] end point, then it
MUST use LLC encapsulated PPP payloads. Frame Relay/ATM FRF.8
inter-working units are exempted from the requirement to support VC-
multiplexed PPP. This exemption allows the FR/ATM IWU to remain
compliant with FRF.8 when the PPP over AAL5 end point is inter-
operating with an RFC 1973 end point.
int rand(void);
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