In article ,
K. Jennings 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

On Mon, 05 Nov 2007 23:23:22 +0000, Ben Harris wrote:

> In article , K. Jennings
> 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.

Thanks. I just wonder why the standard is so vague here?