From mboxrd@z Thu Jan 1 00:00:00 1970 From: erik quanstrom Date: Wed, 23 Jun 2010 08:55:08 -0400 To: 9fans@9fans.net Message-ID: In-Reply-To: References: MIME-Version: 1.0 Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit Subject: Re: [9fans] Multicast example Topicbox-Message-UUID: 36769c5a-ead6-11e9-9d60-3106f5b1d025 > However, I'm not quite sure about receiving. > I see an "addmulti" in ip(3) but I don't quite understand the comment > and I couldn't penetrate the source. what comment? unfortunately, where ip(3) says "media address", i think it really means ip address. the code says it accepts addmulti $rxaddr $multiaddr or addmulti $multiaddr # assume $rxaddr = normal rec. addr ip(3) is quite unclear (to me, anyway) about the differences between the ip stack's notions and the interface' notions. perhaps i notice because we send a lot of non-ip traffic. in the case of multicast mac addresses, ip tries to bridge the gap. but i don't think that current hardware works this way. modern hardware tends to hash multicast *ethernet* addresses, but not ip addresses. and that's the way that addmulti/remmulti are implemented in netif.c and /sys/src/9/pc/ether[a-z]*.c:/^[a-z0-9]+multicast mind the gap! also for mtu, ip assumes that it is fixed per medium, and not per hardware device. this bit from ip(3) is misleading. - The - .I mtu - is the size in bytes of the largest packet that this interface can send. the mtu for ip(3) is the largest packet that ip tries to send. this may be larger (that is, misconfigured), smaller or the same as the interface mtu. in 9atom, the interface mtu may be set by echo mtu 9000 > /net/ether0/clone and the /net/ether0/mtu file has the mintu mtu and maxtu. i have been meaning to let ip discover the current mtu by reading it from the device, but i need a solution that will keep the mtu high for aoe even if ip wants it at 1514 to allow ip to make sure that its idea of the interface (media) mtu isn't wrong. i'm also somewhat uncomfortable with managing the interface via an ip proxy. we routinely mix 8228-byte aoe packets with 1514-byte ip packets. the interface's mtu is set to 9000, while the ip stack's mtu is set to 1514. to sum up, i think there is a little confusion in the code. i'd be interested in your results. - erik