Re: Path MTU for draft-ietf-v6ops-mech-v2-00.txt

["Fred L. Templin" <ftemplin@IPRG.nokia.com>]

Itojun,

itojun@iijlab.net wrote:
> 	and, i don't think you have convinced me on the technical benefits
> 	of 1380 (the particular value).
The Internet cell size is pretty much 1500 bytes these days, so an
IPv6/IPv4 tunnel MTU of 1500 - 20 = 1480 might seem like a
natural choice. However, link layer encapsulations below the IPv6/IPv4
tunnel may occur, e.g., when the tunnel is carried over a VPN interface.

Microsoft, cisco and others use 1400 bytes as the default MTU for VPN
interfaces in order to allow enough room for link layer encapsulations
(e.g., IPsec, L2TP, PPPoE, etc.) without exceeding the 1500 byte budget
and incurring fragmentation. So, the default IPv6/IPv4 tunnel MTU should
be set to at most 1400 - 20 = 1380.

There are two reasons to set the default MTU as large as 1380 bytes:

 1) 1380 provides a 7.8% efficiency gain over 1280.
 2) 1380 allows 100 bytes for an additional layer of
    IPv6 encapsulation above the IPv6/IPv4 tunnel.

Fred
ftemplin@iprg.nokia.com