有一个简单的网络模型。在主机之间传输文件。有多种路由器具有不同的MTU.这是完整的OTcL代码：
####################################################################
# #
# PRESETS #
# #
####################################################################
# new simulator object
set ns [new Simulator]
# trace file
set allchan [open /ns2/examples/data.tr w]
$ns trace-all $allchan
# for nam
set visio [open /ns2/examples/data.nam w]
$ns namtrace-all $visio
#procedure triggered at finish time
proc finish {} {
global ns visio allchan
$ns flush-trace
close $visio
close $allchan
exec nam /ns2/examples/data.nam &
exit 0
}
####################################################################
# #
# LINKS &NODES #
# #
####################################################################
set god [create-god 6]
for {set i 0} {$i < 6} {incr i} {
set node_($i) [$ns node]
}
for {set i 0} {$i < 5} {incr i} {
$ns duplex-link $node_($i) $node_([expr $i+1]) 2Mb 10ms DropTail
}
#============================= hosts ===============================
$node_(0) color Blue
$node_(5) color Blue
#============================ routers ==============================
$node_(1) color Red
$node_(2) color Red
$node_(3) color Red
$node_(4) color Red
####################################################################
# #
# AGENTS #
# #
####################################################################
set host1 [new Agent/TCP]
$ns attach-agent $node_(0) $host1
$host1 set fid_ 0
$host1 set mtu_ 1500
set router1 [new Agent/TCP]
$ns attach-agent $node_(1) $router1
$router1 set fid_ 1
$router1 set mtu_ 1000
set router2 [new Agent/TCP]
$ns attach-agent $node_(2) $router2
$router2 set fid_ 2
$router2 set mtu_ 800
set router3 [new Agent/TCP]
$ns attach-agent $node_(3) $router3
$router3 set fid_ 3
$router3 set mtu_ 700
set router4 [new Agent/TCP]
$ns attach-agent $node_(4) $router4
$router4 set fid_ 4
$router4 set mtu_ 600
set host2 [new Agent/TCPSink]
$ns attach-agent $node_(5) $host2
$host2 set mtu_ 1500
$ns connect $host1 $host2
####################################################################
# #
# APPLICATIONS #
# #
####################################################################
set ftp [new Application/FTP]
$ftp attach-agent $host1
$ftp set type FTP
####################################################################
# #
# START SIM #
# #
####################################################################
$ns at 0.1 "$ftp start"
$ns at 10.0 "$ftp stop"
$ns at 11.0 "finish"
$ns run
我想查看数据包的碎片，但它没有显示。这可能是什么原因？
我怀疑我无法设置MTU。
它说这里那：
ns2 中 MTU（最大传输单元）的默认值一般为 1500。现在要根据您的研究更改此值，只需转到文件“mac-802.3.h”并更改“IEEE_8023_MAXFRAME”的值并重新编译即可。您可以在以下位置找到该文件
/home/用户名/ns-allinone-2.xx/ns-2.xx/mac
一部分数据.tr:
+ 0.1 0 1 tcp 40 ------- 0 0.0 5.0 0 0
- 0.1 0 1 tcp 40 ------- 0 0.0 5.0 0 0
r 0.11016 0 1 tcp 40 ------- 0 0.0 5.0 0 0
+ 0.11016 1 2 tcp 40 ------- 0 0.0 5.0 0 0
- 0.11016 1 2 tcp 40 ------- 0 0.0 5.0 0 0
r 0.12032 1 2 tcp 40 ------- 0 0.0 5.0 0 0
+ 0.12032 2 3 tcp 40 ------- 0 0.0 5.0 0 0
- 0.12032 2 3 tcp 40 ------- 0 0.0 5.0 0 0
r 0.13048 2 3 tcp 40 ------- 0 0.0 5.0 0 0
+ 0.13048 3 4 tcp 40 ------- 0 0.0 5.0 0 0
- 0.13048 3 4 tcp 40 ------- 0 0.0 5.0 0 0
r 0.14064 3 4 tcp 40 ------- 0 0.0 5.0 0 0
+ 0.14064 4 5 tcp 40 ------- 0 0.0 5.0 0 0
- 0.14064 4 5 tcp 40 ------- 0 0.0 5.0 0 0
r 0.1508 4 5 tcp 40 ------- 0 0.0 5.0 0 0
+ 0.1508 5 4 ack 40 ------- 0 5.0 0.0 0 1
- 0.1508 5 4 ack 40 ------- 0 5.0 0.0 0 1
r 0.16096 5 4 ack 40 ------- 0 5.0 0.0 0 1
+ 0.16096 4 3 ack 40 ------- 0 5.0 0.0 0 1
- 0.16096 4 3 ack 40 ------- 0 5.0 0.0 0 1
r 0.17112 4 3 ack 40 ------- 0 5.0 0.0 0 1
+ 0.17112 3 2 ack 40 ------- 0 5.0 0.0 0 1
- 0.17112 3 2 ack 40 ------- 0 5.0 0.0 0 1
r 0.18128 3 2 ack 40 ------- 0 5.0 0.0 0 1
+ 0.18128 2 1 ack 40 ------- 0 5.0 0.0 0 1
- 0.18128 2 1 ack 40 ------- 0 5.0 0.0 0 1
r 0.19144 2 1 ack 40 ------- 0 5.0 0.0 0 1
+ 0.19144 1 0 ack 40 ------- 0 5.0 0.0 0 1
- 0.19144 1 0 ack 40 ------- 0 5.0 0.0 0 1
r 0.2016 1 0 ack 40 ------- 0 5.0 0.0 0 1
+ 0.2016 0 1 tcp 1040 ------- 0 0.0 5.0 1 2
- 0.2016 0 1 tcp 1040 ------- 0 0.0 5.0 1 2
+ 0.2016 0 1 tcp 1040 ------- 0 0.0 5.0 2 3
- 0.20576 0 1 tcp 1040 ------- 0 0.0 5.0 2 3
r 0.21576 0 1 tcp 1040 ------- 0 0.0 5.0 1 2
+ 0.21576 1 2 tcp 1040 ------- 0 0.0 5.0 1 2
- 0.21576 1 2 tcp 1040 ------- 0 0.0 5.0 1 2
r 0.21992 0 1 tcp 1040 ------- 0 0.0 5.0 2 3
+ 0.21992 1 2 tcp 1040 ------- 0 0.0 5.0 2 3
- 0.21992 1 2 tcp 1040 ------- 0 0.0 5.0 2 3
r 0.22992 1 2 tcp 1040 ------- 0 0.0 5.0 1 2
+ 0.22992 2 3 tcp 1040 ------- 0 0.0 5.0 1 2
- 0.22992 2 3 tcp 1040 ------- 0 0.0 5.0 1 2
r 0.23408 1 2 tcp 1040 ------- 0 0.0 5.0 2 3
+ 0.23408 2 3 tcp 1040 ------- 0 0.0 5.0 2 3
- 0.23408 2 3 tcp 1040 ------- 0 0.0 5.0 2 3
r 0.24408 2 3 tcp 1040 ------- 0 0.0 5.0 1 2
+ 0.24408 3 4 tcp 1040 ------- 0 0.0 5.0 1 2
- 0.24408 3 4 tcp 1040 ------- 0 0.0 5.0 1 2
r 0.24824 2 3 tcp 1040 ------- 0 0.0 5.0 2 3
+ 0.24824 3 4 tcp 1040 ------- 0 0.0 5.0 2 3
- 0.24824 3 4 tcp 1040 ------- 0 0.0 5.0 2 3
r 0.25824 3 4 tcp 1040 ------- 0 0.0 5.0 1 2
+ 0.25824 4 5 tcp 1040 ------- 0 0.0 5.0 1 2
- 0.25824 4 5 tcp 1040 ------- 0 0.0 5.0 1 2
r 0.2624 3 4 tcp 1040 ------- 0 0.0 5.0 2 3
+ 0.2624 4 5 tcp 1040 ------- 0 0.0 5.0 2 3
- 0.2624 4 5 tcp 1040 ------- 0 0.0 5.0 2 3
r 0.2724 4 5 tcp 1040 ------- 0 0.0 5.0 1 2
+ 0.2724 5 4 ack 40 ------- 0 5.0 0.0 1 4
- 0.2724 5 4 ack 40 ------- 0 5.0 0.0 1 4
r 0.27656 4 5 tcp 1040 ------- 0 0.0 5.0 2 3
...
本文说，（NS-2和NS-3模拟隐藏终端问题的比较研究）：
核心数据包数据结构不适合仿真，并且不支持分段。
如何模拟碎片？我可以将其建模吗ns-2？
如果我使用TCP代理在哪里MTU？
一部分TCP协议:
...
TcpAgent::TcpAgent() : Agent(PT_TCP),
t_seqno_(0), t_rtt_(0), t_srtt_(0), t_rttvar_(0),
t_backoff_(0), ts_peer_(0),
rtx_timer_(this), delsnd_timer_(this),
burstsnd_timer_(this),
dupacks_(0), curseq_(0), highest_ack_(0), cwnd_(0), ssthresh_(0),
count_(0), fcnt_(0), rtt_active_(0), rtt_seq_(-1), rtt_ts_(0.0),
maxseq_(0), cong_action_(0), ecn_burst_(0), ecn_backoff_(0),
ect_(0), lastreset_(0.0),
restart_bugfix_(1), closed_(0), nrexmit_(0),
first_decrease_(1), qs_requested_(0), qs_approved_(0)
{
#ifdef TCP_DELAY_BIND_ALL
#else /* ! TCP_DELAY_BIND_ALL */
// not delay-bound because delay-bound tracevars aren't yet supported
bind("t_seqno_", &t_seqno_);
bind("rtt_", &t_rtt_);
bind("srtt_", &t_srtt_);
bind("rttvar_", &t_rttvar_);
bind("backoff_", &t_backoff_);
bind("dupacks_", &dupacks_);
bind("seqno_", &curseq_);
bind("ack_", &highest_ack_);
bind("cwnd_", &cwnd_);
bind("ssthresh_", &ssthresh_);
bind("maxseq_", &maxseq_);
bind("ndatapack_", &ndatapack_);
bind("ndatabytes_", &ndatabytes_);
bind("nackpack_", &nackpack_);
bind("nrexmit_", &nrexmit_);
bind("nrexmitpack_", &nrexmitpack_);
bind("nrexmitbytes_", &nrexmitbytes_);
bind("necnresponses_", &necnresponses_);
bind("ncwndcuts_", &ncwndcuts_);
#endif /* TCP_DELAY_BIND_ALL */
}
...
如果我使用SCTP代理，我可以设置不同的MTU：
部分sctp.cc
...
void SctpAgent::delay_bind_init_all()
{
delay_bind_init_one("debugMask_");
delay_bind_init_one("debugFileIndex_");
delay_bind_init_one("associationMaxRetrans_");
delay_bind_init_one("pathMaxRetrans_");
delay_bind_init_one("changePrimaryThresh_");
delay_bind_init_one("maxInitRetransmits_");
delay_bind_init_one("heartbeatInterval_");
delay_bind_init_one("mtu_");
delay_bind_init_one("initialRwnd_");
delay_bind_init_one("initialSsthresh_");
delay_bind_init_one("ipHeaderSize_");
delay_bind_init_one("dataChunkSize_");
delay_bind_init_one("numOutStreams_");
delay_bind_init_one("useDelayedSacks_");
delay_bind_init_one("sackDelay_");
delay_bind_init_one("useMaxBurst_");
delay_bind_init_one("initialCwnd_");
delay_bind_init_one("initialRto_");
delay_bind_init_one("minRto_");
delay_bind_init_one("maxRto_");
delay_bind_init_one("fastRtxTrigger_");
delay_bind_init_one("numUnrelStreams_");
delay_bind_init_one("reliability_");
delay_bind_init_one("unordered_");
delay_bind_init_one("rtxToAlt_");
delay_bind_init_one("dormantAction_");
delay_bind_init_one("routeCacheLifetime_");
delay_bind_init_one("routeCalcDelay_");
/* PN: 5/07. Simulate send window */
delay_bind_init_one("initialSwnd_");
/* PN: 5/07. NR-Sacks */
delay_bind_init_one("useNonRenegSacks_");
delay_bind_init_one("trace_all_");
delay_bind_init_one("cwnd_");
delay_bind_init_one("rwnd_");
delay_bind_init_one("rto_");
delay_bind_init_one("errorCount_");
delay_bind_init_one("frCount_");
delay_bind_init_one("timeoutCount_");
delay_bind_init_one("rcdCount_");
Agent::delay_bind_init_all();
}
...
..但碎片仍然没有得到 -
[root@localhost ...]# ns /ns2/examples/mathm2.tcl
num_nodes is set 6
SCTP ERROR: DATA chunk size (1468) too big!
SCTP/IP header = 32
Control chunk reservation = 0
MTU = 600