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Re: [BEHAVE] Fwd: IPv6 hosts sending <1280 byte packets

Iljitsch van Beijnum <iljitsch@muada.com> Tue, 09 February 2010 18:01 UTC

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From: Iljitsch van Beijnum <iljitsch@muada.com>
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Date: Tue, 09 Feb 2010 19:01:34 +0100
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References: <4B6F08CC.2070900@wand.net.nz> <063A973F-EBC3-4CD0-B5B6-B0FB42A8 593D@muada.com><00f201caa8da$b78e3e90$c4f0200a@cisco.com><4B704153.2020007@ it.uc3m.es><015801caa8e6$9b72fff0$c4f0200a@cisco.com><75A95C0D-E2CC-4FD6-B1 1A-5C772FCD0F5C@muada.com><02cd01caa90e$dde921c0$c4f0200a@cisco.com><B50C7F 0A-19DB-4C63-9F72-867B5C2D4841@muada.com><02f101caa916$712d0170$c4f0200a@ci sco.com><E1829B60731D1740BB7A0626B4FAF0A64951038053@XCH-NW-01V.nw.nos.boein g.com><032d01caa91e$343c71d0$c4f0200a@cisco.com> <E1829B60731D1740BB7A0626B4FAF0A64951038079@XCH-NW-01V.nw.nos.boeing.com> <035601caa925$800f2240$c4f0200a@cisco.com> <E1829B60731D1740BB7A0626B4FAF0A649510381E7@XCH-NW-01V.nw.nos.boeing.com>
To: "Templin, Fred L" <Fred.L.Templin@boeing.com>
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Cc: "behave@ietf.org" <behave@ietf.org>, Dan Wing <dwing@cisco.com>
Subject: Re: [BEHAVE] Fwd: IPv6 hosts sending <1280 byte packets
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On 9 feb 2010, at 18:51, Templin, Fred L wrote:

> So, e.g., if the translator receives a 1500byte
> IPv4 packet and has to translate it into a 1520byte IPv6
> packet, there is a chance that it may be dropped due to a
> link MTU restriction somewhere in the IPv6 network and an
> ICMPv6 PTB sent back toward the translator. Presumably, the
> translator would translate that into an ICMPv4 PTB message
> that it forwards on to the IPv4 host.

If the IPv6 host is connected through a link with a 1500-byte MTU, it will advertise a maximum segment size (MSS option) in its TCP SYN packet of 1500 - 40 (IPv6 header) - 20 (TCP header) = 1440. So the IPv4 host will send 1480-byte IPv4 packets which are then translated into 1500-byte IPv6 packets and there are no issues.

Only when the IPv6 host has an MTU larger than 1500 but the translator doesnt the IPv4 PTMUD black holes will be triggered.

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