SCP v. SFTP

[Chris Rapier]

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