Troubleshooting - UDP a fragmentacia paketov eXplorer - 13.06.2011 - 22:19 Post subject: UDP a fragmentacia paketov
Potreboval by som donutit linuxovy router/firewall, aby fragmentoval UDP pakety na mensie ako 1400 B.
Nastavil som MTU na oboch stranach spoja - na routri kam je pripojeny uplink z linuxu, aj na samotnom linuxe (prepoj je metalicky patchcord, ziadne aktivne prvky), ale linux na mna prdi a veselo posiela 1500B UDP pakety aj nadalej (aj ked nemaju DF bit). S TCP-ckom problem nemam ale udp protokol je tak tupy ...
Otazka teda znie, co este okrem MTU na interfaci (ifcfg-eth0, MTU=1400) musim nastavit ? asi som lama lukic - 19.06.2011 - 14:15 Post subject:
In path MTU discovery, a TCP stack sets a special Don’t Fragment (DF) bit in the IP datagrams. Routers which cannot forward the datagram without fragmenting it are supposed to drop it and send an ICMP “Fragmentation-Required” datagram to the originating host.
The originating host then tries a lower MTU value. Unfortunately, many routers are anti-social and do not generate the fragmentation-required datagrams. Many firewalls are equally anti-social and drop all ICMP datagrams.
Cize zacni dropovat vacsie ako 1400byte pakety s ICMP odkazom fragmentation required. Ono totiz mensie UDP musi robit povodny odosielatel, kedze skladanie nie je mozne (hlavicka nie je TCP like).
http://en.wikipedia.org/wiki/User_Datagram_Protocollukic - 19.06.2011 - 14:58 Post subject:
este potom pamataj, ze ak na trase k povodcovi UDP paketov (jedna abo druha strana) je nieco, co dropuje ICMP, tak si dropnes cely UDP traffic ktory ma viac ako 1400 byte.
