Subject: Dumb question

From: tom <[email-masked]>
Date: Thu, 10 Jun 1999 20:12:58 -0700

Hi all,

I have a really dumb question.  What happens to a IP packet when the
don't fragment bit is set to "1" and travels over a wide area
connection?  For instaance, the packet is 1518 Ethernet and must travel
over a T1 with HDLC encapsulation.

If my guess is correct, HDLC has a 512 packet size and will discard all
packets exceeding that with the don't fragment bit set.

Thanks

TW

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From: Peter Dawson <[email-masked]>
Date: Sat, 12 Jun 1999 18:35:14 +0400

Tom,

1) The network layer, SHULD silently discard the datagram.
(RFC791-section 3.2)
2) The Router MUST send an ICMP-Destination Unreachable;
    and SHOULD respond  with  code 4 (Fragmentation needed
    and DF set). (RFC1812- section 5.2.7.1))
3) The responsiblity of correct datagram length is on the source
    host. This is done by MTU. (RFC1812- section 4.2.3.3)

/pete

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From: Peter Dawson <[email-masked]>
Date: Sat, 12 Jun 1999 18:35:14 +0400

Tom,

/pete

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From: "Michel Gilbert" <[email-masked]>
Date: Mon, 14 Jun 1999 16:06:08 -0400

Tom -

You are correct that the router would discard all such packets in the
scenario you describe. This is, however, an IP function, not an HDLC
function. IP knows the MTU supported on the various interfaces of the
router and simply discards a packet that is too large for the link on which
it is to be forwarded.

As for HDLC, the HDLC packet size does not default to 512 octets. The
default maximum transmission unit (MTU) on a point-point circuit
implementing HDLC is usually a parameter specified by the vendor of the
router, and can vary widely. A number between 1500 and 2000 octets is more
common as a default in the products I have seen. Some products also
automatically set the default MTU on the WAN link to be the largest
supported MTU on the enabled LAN interfaces.

/Michel

>packets exceeding that with the don't fragment bit set.

=====================
Michel Luca Gilbert
Director of Technical Staff Development
Hill Associates, Inc.
17 Roosevelt Hwy
Colchester, VT 05446

Cell: (802) 233-1638
Main: (802) 655-0940
Direct: (802) 655-8621
FAX : (802) 655-7974
EMAIL: [email-masked]
WWW: http://www.hill.com
=====================

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From: "Michel Gilbert" <[email-masked]>
Date: Mon, 14 Jun 1999 16:06:08 -0400

Tom -

/Michel

=====================
Michel Luca Gilbert

Cell: (802) 233-1638
Main: (802) 655-0940
Direct: (802) 655-8621
FAX : (802) 655-7974