def f3(a,b,c,d):
    if(a.split('.')[1]!='pcap'):
        print('Le format est incorrect')
        return
    e=rdpcap(a)
    for p in e:
        if(str(b) in p):
            if(str(c) in p[str(b)].fields.keys()):
                p[str(b)].fields[str(c)]=d
                p.show2()
    f=a.split('.')[0]
    wrpcap(f+'_modified.pcap',p)