[477] in linux-net channel archive
Re: CONFIG_INET_SNARL: What for?
daemon@ATHENA.MIT.EDU (C. Harald Koch)
Tue Jun 13 17:53:28 1995
To: urlichs@smurf.noris.de (Matthias Urlichs)
cc: submit-linux-dev-net@ratatosk.yggdrasil.com
In-reply-to: Your message of "Tue, 13 Jun 1995 05:35:09 -0400".
<3rjm4e$tlk@smurf.noris.de>
Date: Tue, 13 Jun 1995 16:25:59 -0400
From: "C. Harald Koch" <harald@enfm.utcc.utoronto.ca>
> CONFIG_INET_SNARL selects whether to use the netmask of a device (set by
> ifconfig) or the IP-class-default netmask (i.e. 255.255.255.0 for a class C
> address) for figuring out which MTU to use for TCP.
>
> I wonder what the reason for this is. I can't think of any correctly-
> configured machine where the latter behavior would make any more sense than
> the former, yet the latter is the default (and more expensive to compute).
> On the other hand, for networks with non-default netmasks, I can think of a
> few cases where the former behavior would make much more sense.
The acronym stands for "SubNets ARe Local", if I understand correctly.
Internet standards specify that when you're talking to a remote network (via
a gateway), you must use an MTU of 576 bytes.
However, when you've got a bunch of local Ethernets (for example) all
connected by a router, using 576 bytes is inefficient, since you could be
using the 1500 byte Ethernet MTU instead.
SNARL tells the kernel that it should assume that subnets of a locally
attached, class-full network should be considered "local" networks, and
should use the default MTU of the local interface.
[ SGI's kernel also has an "allnetsarelocal" config paramater, which is
useful for local internets. It does the same thing, but uses the local
interface MTU for *all* outbound traffic. ]
Now, SNARL is a good hack, but the real answer is to use MTU Discovery.
--
C. Harald Koch | University of Toronto Computing & Communications
harald@canet.ca | Network & Operations Services
+1 416 978 0992 (voice) | External Network Facilities Managment
+1 416 978 6620 (fax) | 4 Bancroft Ave., Rm 101, Toronto, ON M5S 1C1
