On Fri, May 12, 2017 at 10:05 AM, Teco Boot <teco@xxxxxxxxxx> wrote: > IP MTU and L2 MTU are different animals. > > IMHO IP MTU is for fragmentation at sender of a link. There is no need dropping IP packets at receiver with size > configured IP MTU. IP packets with size > receiver L2 MTU will be dropped at sub-IP layer. > First, thanks for putting words on the different MTU:s (L2 vs IP MTU) I agree and don't understand why we are dropping packets due to receiver IP MTU at all and would not mind removing that test altogether, at least for veth. /Fredrik > For this patch: if veth has some notion on L2 MTU (e.g. buffer size limits), there has to be checks for it. I don't know why configuring MRU helps, more config, more mistakes. If there is no need for dropping the packet: don't. > > Teco > > >> Op 11 mei 2017, om 21:10 heeft Fredrik Markström <fredrik.markstrom@xxxxxxxxx> het volgende geschreven: >> >> On Thu, May 11, 2017 at 6:01 PM, Stephen Hemminger >> <stephen@xxxxxxxxxxxxxxxxxx> wrote: >>> On Thu, 11 May 2017 15:46:27 +0200 >>> Fredrik Markstrom <fredrik.markstrom@xxxxxxxxx> wrote: >>> >>>> From: Fredrik Markström <fredrik.markstrom@xxxxxxxxx> >>>> >>>> is_skb_forwardable() currently checks if the packet size is <= mtu of >>>> the receiving interface. This is not consistent with most of the hardware >>>> ethernet drivers that happily receives packets larger then MTU. >>> >>> Wrong. >> >> What is "Wrong" ? I was initially skeptical to implement this patch, >> since it feels odd to have different MTU:s set on the two sides of a >> link. After consulting some IP people and the RFC:s I kind of changed >> my mind and thought I'd give it a shot. In the RFCs I couldn't find >> anything that defined when and when not a received packet should be >> dropped. >> >>> >>> Hardware interfaces are free to drop any packet greater than MTU (actually MTU + VLAN). >>> The actual limit is a function of the hardware. Some hardware can only limit by >>> power of 2; some can only limit frames larger than 1500; some have no limiting at all. >> >> Agreed. The purpose of these patches is to be able to configure an >> veth interface to mimic these different behaviors. Non of the Ethernet >> interfaces I have access to drops packets due to them being larger >> then the configured MTU like veth does. >> >> Being able to mimic real Ethernet hardware is useful when >> consolidating hardware using containers/namespaces. >> >> In a reply to a comment from David Miller in my previous version of >> the patch I attached the example below to demonstrate the case in >> detail. >> >> This works with all ethernet hardware setups I have access to: >> >> ---- 8< ------ >> # Host A eth2 and Host B eth0 is on the same network. >> >> # On HOST A >> % ip address add 1.2.3.4/24 dev eth2 >> % ip link set eth2 mtu 300 up >> >> % # HOST B >> % ip address add 1.2.3.5/24 dev eth0 >> % ip link set eth0 mtu 1000 up >> % ping -c 1 -W 1 -s 400 1.2.3.4 >> PING 1.2.3.4 (1.2.3.4) 400(428) bytes of data. >> 408 bytes from 1.2.3.4: icmp_seq=1 ttl=64 time=1.57 ms >> >> --- 1.2.3.4 ping statistics --- >> 1 packets transmitted, 1 received, 0% packet loss, time 0ms >> rtt min/avg/max/mdev = 1.573/1.573/1.573/0.000 ms >> ---- 8< ------ >> >> >> But it doesn't work with veth: >> >> ---- 8< ------ >> # veth0 and veth1 is a veth pair and veth1 has ben moved to a separate >> network namespace. >> % # NS A >> % ip address add 1.2.3.4/24 dev veth0 >> % ip link set veth0 mtu 300 up >> >> % # NS B >> % ip address add 1.2.3.5/24 dev veth1 >> % ip link set veth1 mtu 1000 up >> % ping -c 1 -W 1 -s 400 1.2.3.4 >> PING 1.2.3.4 (1.2.3.4) 400(428) bytes of data. >> >> --- 1.2.3.4 ping statistics --- >> 1 packets transmitted, 0 received, 100% packet loss, time 0ms >> ---- 8< ------ >> >> -- >> /Fredrik > -- /Fredrik
