我有一个支持站点到站点 IPSec 的无线网关。我在 Ubuntu Server 12.04（隧道左侧）上配置了 openswan，配置如下/etc/ipsec.conf：
version 2.0
config setup
nat_traversal=yes
virtual_private=%v4:10.0.0.0/8,%v4:192.168.0.0/16,%v4:172.16.0.0/12
oe=off
protostack=netkey
conn net-to-net
authby=secret
left=192.168.0.11
leftsubnet=10.1.0.0/16
leftsourceip=10.1.0.1
right=%any
rightsubnet=192.168.127.0/24
rightsourceip=192.168.127.254
auto=add
我已经以类似的方式配置了无线网关（隧道右侧）。我正在使用主模式（而非攻击模式）IKE 第 1 阶段，但看起来它在进入第 2 阶段之前就失败了。您知道原因吗？
以下是 的输出tcpdump：
# tcpdump -vv -i eth0 udp port 500 or udp port 4500
tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
00:11:26.042928 IP (tos 0x0, ttl 235, id 39655, offset 0, flags [none], proto UDP (17), length 196)
74.198.87.62.44248 > 192.168.0.11.isakmp: [udp sum ok] isakmp 1.0 msgid 00000000 cookie 7d88f683ff25b40a->0000000000000000: phase 1 I ident:
(sa: doi=ipsec situation=identity
(p: #1 protoid=isakmp transform=1
(t: #1 id=ike (type=enc value=aes)(type=keylen value=0100)(type=hash value=md5)(type=auth value=preshared)(type=group desc value=modp1536)(type=lifetype value=sec)(type=lifeduration len=4 value=00015180))))
(vid: len=16)
(vid: len=16)
(vid: len=16)
(vid: len=16)
00:11:26.044511 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto UDP (17), length 172)
192.168.0.11.isakmp > 74.198.87.62.44248: [bad udp cksum 0x6361 -> 0x70a3!] isakmp 1.0 msgid 00000000 cookie 7d88f683ff25b40a->6569330ef208fe4d: phase 1 R ident:
(sa: doi=ipsec situation=identity
(p: #1 protoid=isakmp transform=1
(t: #1 id=ike (type=enc value=aes)(type=keylen value=0100)(type=hash value=md5)(type=auth value=preshared)(type=group desc value=modp1536)(type=lifetype value=sec)(type=lifeduration len=4 value=00015180))))
(vid: len=12)
(vid: len=16)
(vid: len=16)
00:11:26.683227 IP (tos 0x0, ttl 235, id 39656, offset 0, flags [none], proto UDP (17), length 312)
74.198.87.62.44248 > 192.168.0.11.isakmp: [udp sum ok] isakmp 1.0 msgid 00000000 cookie 7d88f683ff25b40a->6569330ef208fe4d: phase 1 I ident:
(ke: key len=192)
(nonce: n len=16 data=(7d88f683ff25b40ac37b...00000014471b8027b9d2d76240f4329456eb22db))
(pay20)
(pay20)
00:11:26.688968 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto UDP (17), length 312)
192.168.0.11.isakmp > 74.198.87.62.44248: [bad udp cksum 0x63ed -> 0x4fc7!] isakmp 1.0 msgid 00000000 cookie 7d88f683ff25b40a->6569330ef208fe4d: phase 1 R ident:
(ke: key len=192)
(nonce: n len=16 data=(aaeeb24d4daa1e07f949...00000014637d4a0b75f97e0f771664cd9ac0d818))
(pay20)
(pay20)
00:11:36.704351 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto UDP (17), length 312)
192.168.0.11.isakmp > 74.198.87.62.44248: [bad udp cksum 0x63ed -> 0x4fc7!] isakmp 1.0 msgid 00000000 cookie 7d88f683ff25b40a->6569330ef208fe4d: phase 1 R ident:
(ke: key len=192)
(nonce: n len=16 data=(aaeeb24d4daa1e07f949...00000014637d4a0b75f97e0f771664cd9ac0d818))
(pay20)
(pay20)
00:11:56.165304 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto UDP (17), length 312)
192.168.0.11.isakmp > 74.198.87.62.44248: [bad udp cksum 0x63ed -> 0x4fc7!] isakmp 1.0 msgid 00000000 cookie 7d88f683ff25b40a->6569330ef208fe4d: phase 1 R ident:
(ke: key len=192)
(nonce: n len=16 data=(aaeeb24d4daa1e07f949...00000014637d4a0b75f97e0f771664cd9ac0d818))
(pay20)
(pay20)
隧道的右侧正在尝试使用主模式 IKE 第 1 阶段启动隧道。主模式涉及三次双向交换。OpenSWAN 响应前两次交换，但出于某种原因，右侧没有启动第三次交换。在这个过程的这个阶段，可能出现什么问题？