G
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Archived from groups: comp.dcom.lans.ethernet (More info?)
Hi,
I have come across a hypothetical situation in which the content of
the pad bytes after a short packet will change through a network.
1. Generate short packet over 802.2 LLC SNAP (which requires that we
put the length in the length/type field).
2. Pad packet (with pad#1) to 64 bytes, send over LAN.
3. Add VLAN tag (32 bits total). If I've interpreted 802.1Q
correctly, the equipment doing this is allowed to remove pad so that
the frame is still only 64 bytes long (rather than 68). It can tell
pad from packet because of the length field.
4. Strip VLAN tag. The frame is now short, so add another 4 bytes of
pad (pad#2) and send over LAN.
The initial frame had the packet followed by pad#1. The final frame
has the same packet followed by pad#2.
The standard doesn't require the pad to be any particular value, so
pad#2 isn't necessarily the same as pad#1.
Does anyone know if this scenario actually ever happens in real life?
It requires
(a) pad values to be different (every implementation I've seen uses
zero) and
(b) the device adding the VLAN tag needs to strip pad (does this
happen?) and
(c) we have to put a length in the length/type field, which happens
when we use SNAP.
I guess that last one would also include SAP, but I think that's less
common than SNAP.
TIA,
Allan.
Hi,
I have come across a hypothetical situation in which the content of
the pad bytes after a short packet will change through a network.
1. Generate short packet over 802.2 LLC SNAP (which requires that we
put the length in the length/type field).
2. Pad packet (with pad#1) to 64 bytes, send over LAN.
3. Add VLAN tag (32 bits total). If I've interpreted 802.1Q
correctly, the equipment doing this is allowed to remove pad so that
the frame is still only 64 bytes long (rather than 68). It can tell
pad from packet because of the length field.
4. Strip VLAN tag. The frame is now short, so add another 4 bytes of
pad (pad#2) and send over LAN.
The initial frame had the packet followed by pad#1. The final frame
has the same packet followed by pad#2.
The standard doesn't require the pad to be any particular value, so
pad#2 isn't necessarily the same as pad#1.
Does anyone know if this scenario actually ever happens in real life?
It requires
(a) pad values to be different (every implementation I've seen uses
zero) and
(b) the device adding the VLAN tag needs to strip pad (does this
happen?) and
(c) we have to put a length in the length/type field, which happens
when we use SNAP.
I guess that last one would also include SAP, but I think that's less
common than SNAP.
TIA,
Allan.