[ns] packet size in the trace file

["Danaus" <danausli@263.net>]

Hi:

For test DSR in NS2, I setup a two wireless nodes top, say node0 and node1.
Infact, in my scenario these two nodes are actionless and connected by 
a FTP traffic source because I just want to see if it can work.

The following is a segment of my trace file 
(line number is added by me for explaining easily):
---------
1:  M 0.0 nn 2 x 670 y 670 rp DSR
2:  M 0.0 sc ./scene/dsr-scen-2 cp ./scene/dsr-cp-2 seed 0.0
3:  M 0.0 prop Propagation/TwoRayGround ant Antenna/OmniAntenna
4:  Sconfig 0.00000 using MOBICACHE
5:  s 0.000000000 _1_ AGT  --- 0 tcp 40 [0 0 0 0] ------- [1:0 0:0 32 0] [0 0] 0 0
6:  r 0.012924572 _0_ AGT  --- 0 tcp 68 [a2 0 1 800] ------- [1:0 0:0 32 0] [0 0] 1 0
7:  s 0.012924572 _0_ AGT  --- 3 ack 40 [0 0 0 0] ------- [0:0 1:0 32 0] [0 0] 0 0
8:  r 0.014406572 _1_ AGT  --- 3 ack 68 [a2 1 0 800] ------- [0:0 1:0 32 1] [0 0] 1 0
9:  s 0.014406572 _1_ AGT  --- 4 tcp 1000 [0 0 0 0] ------- [1:0 0:0 32 0] [1 0] 0 0
10: s 0.014406572 _1_ AGT  --- 5 tcp 1000 [0 0 0 0] ------- [1:0 0:0 32 0] [2 0] 0 0
11: r 0.019868572 _0_ AGT  --- 4 tcp 1028 [a2 0 1 800] ------- [1:0 0:0 32 0] [1 0] 1 0
12: s 0.019868572 _0_ AGT  --- 6 ack 40 [0 0 0 0] ------- [0:0 1:0 32 0] [1 0] 0 0
13: r 0.024971239 _0_ AGT  --- 5 tcp 1028 [a2 0 1 800] ------- [1:0 0:0 32 0] [2 0] 1 0
14: s 0.024971239 _0_ AGT  --- 7 ack 40 [0 0 0 0] ------- [0:0 1:0 32 0] [2 0] 0 0
15: r 0.026153239 _1_ AGT  --- 6 ack 68 [a2 1 0 800] ------- [0:0 1:0 32 1] [1 0] 1 0
16: s 0.026153239 _1_ AGT  --- 8 tcp 1000 [0 0 0 0] ------- [1:0 0:0 32 0] [3 0] 0 0
17: s 0.026153239 _1_ AGT  --- 9 tcp 1000 [0 0 0 0] ------- [1:0 0:0 32 0] [4 0] 0 0
18: r 0.031175239 _0_ AGT  --- 8 tcp 1028 [a2 0 1 800] ------- [1:0 0:0 32 0] [3 0] 1 0
19: s 0.031175239 _0_ AGT  --- 10 ack 40 [0 0 0 0] ------- [0:0 1:0 32 0] [3 0] 0 0
20: r 0.032537239 _1_ AGT  --- 7 ack 68 [a2 1 0 800] ------- [0:0 1:0 32 1] [2 0] 1 0
21: s 0.032537239 _1_ AGT  --- 11 tcp 1000 [0 0 0 0] ------- [1:0 0:0 32 0] [5 0] 0 0
22: s 0.032537239 _1_ AGT  --- 12 tcp 1000 [0 0 0 0] ------- [1:0 0:0 32 0] [6 0] 0 0
23: r 0.037659239 _0_ AGT  --- 9 tcp 1028 [a2 0 1 800] ------- [1:0 0:0 32 0] [4 0] 1 0
24: s 0.037659239 _0_ AGT  --- 13 ack 40 [0 0 0 0] ------- [0:0 1:0 32 0] [4 0] 0 0
25: r 0.039081239 _1_ AGT  --- 10 ack 68 [a2 1 0 800] ------- [0:0 1:0 32 1] [3 0] 1 0
...
---------

From the file we can see that node1 send a packet to node0 (line 5) and 
then node0 receive this packet (line 6).

*MY QUESTION* is why the packet size in line 5 and line 6 is different? 
In line 5 is 40 bytes and in line 6 is 68 bytes.

Similar phenomena can be seen at other places in this file.
(line 10 and 11, line 22 and 23 ...) 
It seems that the packet is 28 bytes larger when it is received.
Where are these bytes come from?

Is this a bug in ns?

My system is NS2.1b8a + Redhat 7.1? Could any one give me some advice?

Thanks!

Danaus