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Request SHOULD NOT be sent

MUST be sent specifying a different Magic-Number value. A new

Configure- Request SHOULD NOT be sent to the peer until normal

processing would cause it to be sent (that is, until a Configure-

Nak is received or the Restart timer runs out).

Reception of a Configure-Nak with a Magic-Number different from

that of the last Configure-Nak sent to the peer proves that a link

is not looped-back, and indicates a unique Magic-Number. If the

Magic-Number is equal to the one sent in the last Configure-Nak,

the possibility of a looped-back link is increased, and a new

Magic-Number MUST be chosen. In either case, a new Configure-

Request SHOULD be sent with the new Magic-Number.

If the link is indeed looped-back, this sequence (transmit

Configure-Request, receive Configure-Request, transmit Configure-

Simpson [Page 45]

RFC 1661 Point-to-Point Protocol July 1994

Nak, receive Configure-Nak) will repeat over and over again. If

the link is not looped-back, this sequence might occur a few

times, but it is extremely unlikely to occur repeatedly. More

likely, the Magic-Numbers chosen at either end will quickly

diverge, terminating the sequence. The following table shows the

probability of collisions assuming that both ends of the link

select Magic-Numbers with a perfectly uniform distribution:

Number of Collisions Probability

-------------------- ---------------------

1 1/2**32 = 2.3 E-10

2 1/2**32**2 = 5.4 E-20

3 1/2**32**3 = 1.3 E-29

Good sources of uniqueness or randomness are required for this

divergence to occur. If a good source of uniqueness cannot be

found, it is recommended that this Configuration Option not be

enabled; Configure-Requests with the option SHOULD NOT be

transmitted and any Magic-Number Configuration Options which the

peer sends SHOULD be either acknowledged or rejected. In this

case, looped-back links cannot be reliably detected by the

implementation, although they may still be detectable by the peer.

If an implementation does transmit a Configure-Request with a

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