The full SSH_MSG_CHANNEL_DATA packet looks like this:
uint32 packet length
byte SSH_MSG_CHANNEL_DATA
uint32 recipient channel
uint32 nr data bytes
byte[] data
OpenSSH_6.1p1 considers that the 'maximum packet size' from
SSH_MSG_CHANNEL_OPEN or SSH_MSG_CHANNEL_OPEN_CONFIRMATION impacts only the
'byte[] data' field and not the entire message (headers included).
Section 5.1 from RFC 4254 (which talks about this limitation) leaves room for
interpretation when it comes to the 'maximum packet size' because it
"specifies the maximum size of an individual data packet that can be sent
to the sender" without pointing out if specific headers are included or
not.
This is equivalent to a 32768+13 byte packet for the lower layer, the binary
packet protocol.
There's another section which somewhat clears things out - section 6.1 from
RFC4253. Each SSH Connection Protocol packet (RFC 4254) is encapsulated entirely
(headers included) in the binary packet protocol (bpp) payload from the SSH
Transport Layer (RFC4253). And the above mentioned section stipulates all
implementations must support an uncompressed payload length of 32768 for a bpp
packet. Of course, implementations may support even bigger payload lengths and
the means to announce it to a partner is through the identification string. In
the absence of a specific identification string, a minimal SSH implementation
should be assumed, and as such the SSH_MSG_CHANNEL_DATA packet should not
surpass 32678 bytes in total.
I searched the mailing list for similar topics, but couldn't find any. Was
this subject discussed somewhere? Is it considered a known issue, maybe?
Thank you,
Marian STANCIU
