SCP v. SFTP

[Chris Rapier]

Okay, so I believe I figured out why this is happening but I still have 
a couple of questions I'm hoping someone can answer. So I believe the 
extra data is from the SFTP messages requesting data (in chunk sizes 
determined by -B) and the subsequent acknowledgment of the write to the 
file system.

So the larger you set -B (up to 256K) the fewer messages will be passed 
back and forth and so less extra data. From my tests it seems that this 
step is actually blocking so fewer messages not only decreases the total 
amount of data but it also keeps the link from sitting idle as often.

If I am correct in this I was wondering why its set up this way? Is it 
to prevent writing to a full file system or some place where the user 
doesn't have appropriate permission?

Thanks!



Chris Rapier wrote:
> I was comparing some traces from SCP and SFTP when transferring the same 
> file 200MB file between the same host pairs. Even when I put SFTP in 
> batch mode I noticed that I saw 403208 bytes from the receiver in 
> comparison to 3368 bytes with SCP. I've attached the relevant output 
> from tcptrace below (the b->a column is the return side of the trace). 
> Mostly I'm just curious as to what is generating so much return traffic 
> for SFTP? Actually, now that I think about it, why almost an additional 
> 9000 packets* for SFTP on the sending side as well?
> 
> Is it just some artifact of my network that I didn't pick up on or does 
> the sftp mechanism require this?
> 
> 
> Thanks!
> Chris Rapier
> 
> 
> 
> *I didn't disable TSO before taking these dumps so these pakets size 
> don't necessarily correspond to the actual MTU.
> 
> Ostermann's tcptrace -- version 6.6.7 -- Thu Nov  4, 2004
> 
> 155626 packets seen, 155626 TCP packets traced
> elapsed wallclock time: 0:00:00.722889, 215283 pkts/sec analyzed
> trace file elapsed time: 0:00:19.943138
> TCP connection info:
> 1 TCP connection traced:
> TCP connection 1:
>          host a:        delta
>          host b:        echo
>          complete conn: yes
>          first packet:  Tue Apr 17 16:27:22.204583 2007
>          last packet:   Tue Apr 17 16:27:42.147722 2007
>          elapsed time:  0:00:19.943138
>          total packets: 155626
>          filename:      sftp2echo200mb.dump
>     a->b:                              b->a:
>       total packets:         74547           total packets: 
> 81079
>       ack pkts sent:         74546           ack pkts sent: 
> 81079
>       pure acks sent:           34           pure acks sent: 
> 74806
>       sack pkts sent:            0           sack pkts sent: 
> 0
>       dsack pkts sent:           0           dsack pkts sent: 
> 0
>       max sack blks/ack:         0           max sack blks/ack: 
> 0
>       unique bytes sent: 205796103           unique bytes sent: 
> 403208
>       actual data pkts:      74511           actual data pkts: 
> 6271
>       actual data bytes: 205848615           actual data bytes: 
> 403208
>       rexmt data pkts:          22           rexmt data pkts: 
> 0
>       rexmt data bytes:      52512           rexmt data bytes: 
> 0
>       zwnd probe pkts:           0           zwnd probe pkts: 
> 0
>       zwnd probe bytes:          0           zwnd probe bytes: 
> 0
>       outoforder pkts:           0           outoforder pkts: 
> 0
>       pushed data pkts:      18049           pushed data pkts: 
> 6271
>       SYN/FIN pkts sent:       1/1           SYN/FIN pkts sent: 
> 1/1
>       req 1323 ws/ts:          Y/Y           req 1323 ws/ts: 
> Y/Y
>       adv wind scale:            9           adv wind scale: 
> 7
>       req sack:                  Y           req sack: 
> Y
>       sacks sent:                0           sacks sent: 
> 0
>       urgent data pkts:          0 pkts      urgent data pkts:          0
> 
> 
> 
> ================================
> TCP connection 2:
>          host c:        delta
>          host d:        echo
>          complete conn: yes
>          first packet:  Tue Apr 17 13:40:48.350911 2007
>          last packet:   Tue Apr 17 13:40:55.841408 2007
>          elapsed time:  0:00:07.490497
>          total packets: 140843
>          filename:      scp2echo200mb.dump
>     c->d:                              d->c:
>       total packets:         65970           total packets: 
> 74873
>       ack pkts sent:         65969           ack pkts sent: 
> 74873
>       pure acks sent:           20           pure acks sent: 
> 74837
>       sack pkts sent:            0           sack pkts sent: 
> 0
>       dsack pkts sent:           0           dsack pkts sent: 
> 0
>       max sack blks/ack:         0           max sack blks/ack: 
> 0
>       unique bytes sent: 205256455           unique bytes sent: 
> 3368
>       actual data pkts:      65948           actual data pkts: 
> 34
>       actual data bytes: 205364239           actual data bytes: 
> 3368
>       rexmt data pkts:          37           rexmt data pkts: 
> 0
>       rexmt data bytes:     107784           rexmt data bytes: 
> 0
>       zwnd probe pkts:           0           zwnd probe pkts: 
> 0
>       zwnd probe bytes:          0           zwnd probe bytes: 
> 0
>       outoforder pkts:           0           outoforder pkts: 
> 0
>       pushed data pkts:      10483           pushed data pkts: 
> 34
>       SYN/FIN pkts sent:       1/1           SYN/FIN pkts sent: 
> 1/1
>       req 1323 ws/ts:          Y/Y           req 1323 ws/ts: 
> Y/Y
>       adv wind scale:            9           adv wind scale: 
> 7
>       req sack:                  Y           req sack: 
> Y
>       sacks sent:                0           sacks sent: 
> 0
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