Remember that MRU is the maximum receive unit. The device sending it is
saying how big packets that it receives can be. The other side could have a
different MRU.
There's no need for them to negotiate it. They don't have to agree.
RFC 1661 also implies that even if a device advertises a value that is less
than 1500, it must still be able to receive 1500 bytes. So maybe that's why
you still see 1500.
But it may just be that you aren't looking at it as two flows. Just like the
TCP MTU option on a SYN packet, the PPP MRU only requests a max size for
packets received. The max size for sending on that device is based on the
max size that the other side can receive.
Priscilla
paul dong so wrote:
Hi All,
I have a question but can't find an answer from RFC 1661
During ppp negotiation, if A advertises MRU 1440, B advertises
MRU 1460,
do they have to re-negotiate to agree with a MRU? If so, should
it be
the lower MRU? If they don't need to re-negotiate, what MRU is
actually
being used? Is there any guideline for this?
I observed a ppp nego debug between cisco 7200 and an adsl
modem, the
result appears to be if one end advertises 1500, it becomes the
one
regardless what MRU the other end advertises.
Mar 7 03:25:28.768: ppp1152 PPP: Authorization required
Mar 7 03:25:28.768: ppp1152 PPP: Phase is ESTABLISHING
Mar 7 03:25:28.768: ppp1152 PPP: Authorization required
Mar 7 03:25:28.768: ppp1152 LCP: O CONFREQ [Closed] id 1 len 14
Mar 7 03:25:28.768: ppp1152 LCP:AuthProto PAP (0x0304C023)
Mar 7 03:25:28.768: ppp1152 LCP:MagicNumber 0x8261E624
(0x05068261E624)
Mar 7 03:25:28.796: ppp1152 LCP: I CONFREQ [REQsent] id 2 len
14
Mar 7 03:25:28.796: ppp1152 LCP:MRU 1454 (0x010405AE)
Mar 7 03:25:28.796: ppp1152 LCP:MagicNumber 0x6DB9FEC2
(0x05066DB9FEC2)
Mar 7 03:25:28.796: ppp1152 LCP: O CONFNAK [REQsent] id 2 len 8
Mar 7 03:25:28.796: ppp1152 LCP:MRU 1500 (0x010405DC)
Mar 7 03:25:28.800: ppp1152 LCP: I CONFACK [REQsent] id 1 len
14
Mar 7 03:25:28.800: ppp1152 LCP:AuthProto PAP (0x0304C023)
Mar 7 03:25:28.800: ppp1152 LCP:MagicNumber 0x8261E624
(0x05068261E624)
Mar 7 03:25:28.816: ppp1152 LCP: I CONFREQ [ACKrcvd] id 3 len
10
Mar 7 03:25:28.816: ppp1152 LCP:MagicNumber 0x6DB9FEC2
(0x05066DB9FEC2)
Mar 7 03:25:28.816: ppp1152 LCP: O CONFACK [ACKrcvd] id 3 len
10
Mar 7 03:25:28.816: ppp1152 LCP:MagicNumber 0x6DB9FEC2
(0x05066DB9FEC2)
Mar 7 03:25:28.816: ppp1152 LCP: State is Open
Mar 7 03:25:28.816: ppp1152 PPP: Phase is AUTHENTICATING, by
this end
Thanks
Paul
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