一道IPSEC的习题解决办法

一道IPSEC的习题
这里是一道IPSEC的习题， 高手们能帮忙解答一下吗？万分感谢。急盼.....
Consider the IP network segment with two end nodes A and B and 3 routers, R1, R2, and R3. The corresponding IP addresses are shown the in the figure.
  R2（213.71.82.34）
  R1(129.111.2.47) R3(234.167.57.223)
A(204.171.2.3) B(192.168.2.66)

( 备注：以上是个树形结构图，小女不材，贴不上原图，又画不出新图:( ) 
Given:

• The MTU between R1 and R3 = 1024 bytes
• MTU between A and R1 = 2048 bytes

Consider IP packets of size 1280 bytes sent from A to B (A->R1->R2->R3->B). Now consider IPSec applied to the connection from A-B. Show header formats of the packets (fragments where applicable) highlighting the address fields for the following two scenarios.
a. Tunnel mode ESP between R1 and R3 (name them from left to right)
b. Tunnel mode ESP between R2 and R3, Tunnel AH between R1 and B and transport AH between A and B




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以前学过，现在有点忘记了，帮顶。
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贴不上图不好做啊