Re: Maximum packet size

In article <pan.2007.11.05.17.58.39@xxxxxxxxxxxxxx>,
K. Jennings <kjennings@xxxxxxxxxxxxxx> wrote:
In version 2 of the SSH protocol, how is the value of the maximum
packet size M in an SSH_MSG_CHANNEL_OPEN packet to be understood? I see
two possibilities:

1) M is the maximum value admissible for the packet length field
in a packet; maximum value as far as the party who has sent the
SSH_MSG_CHANNEL_OPEN packet is concerned.

2) M is the value in the packet length field, plus the length of
that field (always 4), plus the length of the MAC field. That is, M is
the total length of the data actually exchanged when a packet is sent/
received.

Neither. It's the maximum amount of channel data in a packet. i.e.,
it's the maximum value for the length field of the string embedded in an
SSH_MSG_CHANNEL_DATA or SSH_MSG_CHANNEL_EXTENDED_DATA. The RFC is not
well-worded here, but the giveaway is in section 5.2:

The maximum amount of data allowed is determined by the maximum
packet size for the channel, and the current window size, whichever
is smaller.

Since the window size must (to be meaningful) only cover channel data,
to make this comparison meaningful the maximum packet size must also.

More pragmatically, this is what PuTTY and OpenSSH do.

--
Ben Harris
.

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