Re: routing & conversion between 2 networks using different packet sizes (1500 & 9000 bytes)

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This is a classic MTU mismatch scenario. A la 1500 B Ethernet II frame vs 40=
00 B Token Ring frame.

I would assume and expect that the routing stack would handle the 9000 B to 1=
500 B just the same as it would 4000 B to 1500 B.  I think this means that t=
he TCP packet in the 9000 B Jumbo Ethernet frame would be fragmented and sen=
t as multiple 1500 B Ethernet frames. Assuming that the Don=E2=80=99t Fragme=
nt bit is not set. If it were set, I=E2=80=99d expect the router to return a=
n ICMP error indicating that fragmentation is needed.

I would be shocked if routing from 9000 B to 1500 B didn=E2=80=99t just work=
.

N.B. I=E2=80=99m assuming that you=E2=80=99re using standard CIDR routing an=
d not trying to use anything like proxy ARP. Though even that theoretically s=
hould work in that the P.A. router will frag / ICMP need frag as necessary t=
o send TCP packets that will fit in the 1500 B frame.

I think ~> expect.



--=20
Grant. . . .
unix || die

> On Nov 23, 2022, at 5:15 PM, L A Walsh <[email protected]> wrote:
>=20
> =EF=BB=BFMy computers and switches handle 9k jumbo packets and file transf=
ers go faster
> between 2 computers using them, however, ran into a slight snag -- IOT (In=
ternet of Things): things like media devices, TV, receiver, disc player -- a=
ll talk ethernet
> @ 1.5k packet size.
>=20
> So I'm wondering, if I setup the devices on a separate subnet and route tr=
affic from
> my desktop into my server, and have a route to the devices that uses 1.5k p=
ackets, will the networking stack automatically marshal the 9k into 1.5k pac=
kets for data that gets sent to the 1.5k subnet?  I'm guessing it won't auto=
matically do the reverse, but that's not so important, since sending 1.5k pa=
ckets to a 9k destination will just use the smaller packets -- its only when=
 I send from a 9k packet enabled
> network port to a 1.5k-talking device that I think problems occur (occasio=
nal random data loss and connection resets).   Does it sound like that would=
 work?
>=20
> I currently have a bridge on the server between 2 networks, one w/9k packe=
ts and the other with 1.5k packets and that seems to work fine -- I'm hoping=
 that's not a coincidence(?)
>=20
> Thanks!
>=20
>=20
>=20

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