NS2运行TCL脚本报sendmsg() for UDP should not be -1错误

运行环境为Ubuntu 20.04，使用NS2与NAM组件编写TCL网络仿真脚本，仿真需求如下：

• 创建8个网络节点，通过带宽可自定义的duplex-link完成节点链路互联
• 在节点0、1部署TCP Agent，节点2部署UDP Agent
• 为TCP Agent绑定CBR流量源，为UDP Agent绑定FTP流量源

执行ns file.tcl命令运行脚本时，触发Error: sendmsg() for UDP should not be -1报错，以下为完整脚本代码与报错调用栈信息，需定位问题诱因并提供修复方案。

问题复现TCL脚本 #

set val(stop)   10.0;


set ns [new Simulator]

#Open the NS trace file
set tracefile [open out.tr w]
$ns trace-all $tracefile

#Open the NAM trace file
set namfile [open out.nam w]
$ns namtrace-all $namfile

#===================================
#        Nodes Definition        
#===================================
#Create 8 nodes
set n0 [$ns node]
set n1 [$ns node]
set n2 [$ns node]
set n3 [$ns node]
set n4 [$ns node]
set n5 [$ns node]
set n6 [$ns node]
set n7 [$ns node]

$ns duplex-link $n0 $n3 20.0Mb 10ms DropTail
$ns duplex-link $n1 $n3 20.0Mb 10ms DropTail
$ns duplex-link $n3 $n2 20.0Mb 10ms DropTail
$ns duplex-link $n3 $n4 20.0Mb 10ms DropTail
$ns duplex-link $n4 $n6 20.0Mb 10ms DropTail
$ns duplex-link $n4 $n5 20.0Mb 10ms DropTail
$ns duplex-link $n4 $n7 20.0Mb 10ms DropTail

$ns duplex-link-op $n0 $n3 orient right-down
$ns duplex-link-op $n1 $n3 orient right
$ns duplex-link-op $n3 $n2 orient left-down
$ns duplex-link-op $n3 $n4 orient right
$ns duplex-link-op $n4 $n6 orient right
$ns duplex-link-op $n4 $n5 orient right-up
$ns duplex-link-op $n4 $n7 orient right-down

#===================================
#        Agents Definition        
#===================================

set tcp [new Agent/TCP]
$ns attach-agent $n0 $tcp
$ns attach-agent $n1 $tcp

set udp [new Agent/UDP]
$ns attach-agent $n2 $udp

set cbr [new Application/Traffic/CBR]
$cbr attach-agent $tcp
$cbr set packet_size_ 1500

set ftp [new Application/FTP]
$ftp attach-agent $udp
$ftp set type_ FTP

# null & sink

set null [new Agent/Null]
$ns attach-agent $n6 $null
$ns connect $udp $null

set sink [new Agent/TCPSink]
$ns attach-agent $n6 $sink
$ns connect $tcp $sink


proc finish {} {
    global ns tracefile namfile
    $ns flush-trace
    close $tracefile
    close $namfile
    exec nam out.nam &
    exit 0
}

$ns at 0.5 "$ftp start"
$ns at 5.0 "$ftp stop"

$ns at 6.0 "finish"
$ns run

报错调用栈信息 #

Error:  sendmsg() for UDP should not be -1
ns: _o180 stop: 
    (_o177 cmd line 1)
    invoked from within
"_o177 cmd advance 0"
    invoked from within
"catch "$self cmd $args" ret"
    invoked from within
"if [catch "$self cmd $args" ret] {
set cls [$self info class]
global errorInfo
set savedInfo $errorInfo
error "error when calling class $cls: $args" $..."
    (procedure "_o177" line 2)
    (SplitObject unknown line 2)
    invoked from within
"[$self agent] advance 0"
    (procedure "_o180" line 2)
    (Application/FTP stop line 2)
    invoked from within
"_o180 stop"

报错由3处违反NS2运行规则的编写错误共同导致：

1. Agent实例重复挂载：单个TCP Agent实例被先后绑定到n0、n1两个节点，NS2中单个Agent实例仅能归属一个节点，第二次attach-agent会覆盖前序绑定关系，造成Agent状态异常。
2. 流量源与Agent类型不匹配：FTP是基于TCP连接的应用层流量，无法在无连接的UDP Agent上运行；CBR为恒定速率无连接流量，通常搭配UDP Agent使用，原脚本反向绑定直接触发协议栈逻辑错误。
3. 流量调度逻辑缺失：原脚本仅启动了错误绑定到UDP的FTP流量，绑定到TCP的CBR流量未配置启停调度，同时双TCP节点的接收端配置不符合要求。

按照NS2语法规则修正对应逻辑即可：

• 为n0、n1分别创建独立的TCP Agent实例，各自绑定对应的TCPSink接收端
• 调整流量源绑定关系：CBR绑定到UDP Agent，FTP绑定到TCP Agent
• 补全所有流量的启停调度命令，保证Agent与节点一一对应

修正后的核心代码段参考：

# 为n0、n1分别创建独立TCP实例
set tcp0 [new Agent/TCP]
$ns attach-agent $n0 $tcp0
set tcp1 [new Agent/TCP]
$ns attach-agent $n1 $tcp1

set udp [new Agent/UDP]
$ns attach-agent $n2 $udp

# 修正流量源与Agent的绑定匹配关系
set cbr [new Application/Traffic/CBR]
$cbr attach-agent $udp
$cbr set packet_size_ 1500

set ftp0 [new Application/FTP]
$ftp0 attach-agent $tcp0
set ftp1 [new Application/FTP]
$ftp1 attach-agent $tcp1

# 配置各协议对应的接收端
set null [new Agent/Null]
$ns attach-agent $n6 $null
$ns connect $udp $null

set sink0 [new Agent/TCPSink]
$ns attach-agent $n5 $sink0
$ns connect $tcp0 $sink0

set sink1 [new Agent/TCPSink]
$ns attach-agent $n7 $sink1
$ns connect $tcp1 $sink1

# 补全所有流量的启停调度
$ns at 0.5 "$ftp0 start"
$ns at 0.5 "$ftp1 start"
$ns at 0.5 "$cbr start"
$ns at 5.0 "$ftp0 stop"
$ns at 5.0 "$ftp1 stop"
$ns at 5.0 "$cbr stop"

内容的提问来源于stack exchange，提问作者VenoM