cpj@SUN.COM (Chuck Jerian) (04/29/89)
Its a host requirment that broadcasts not be fragmented! --cpj
slevy@POINCARE.GEOM.UMN.EDU ("Stuart Levy") (04/30/89)
> Its a host requirment that broadcasts not be fragmented!
But what's the technical reason for requiring that?
Stuartmogul@decwrl.dec.com (Jeffrey Mogul) (05/02/89)
slevy@POINCARE.GEOM.UMN.EDU ("Stuart Levy") writes: >> Its a host requirement that broadcasts not be fragmented! > >But what's the technical reason for requiring that? I don't know the reasoning behind the prohibition in the Host Requirements document, but think about what kinds of problems fragmented broadcasts might cause. If a fragment is lost at the transmitter, for example, then every IP host on the cable has reassembly buffer space tied up, until the TTL expires. Also think about what one is trying to accomplish with huge broadcasts. Some broadcast protocols (e.g., rwhod) are simply a rotten idea to start with, and so "my rwhod packets are too big" is not an excuse. Most appropriate uses of broadcasting fall into two categories: trying to locate something for which you don't know an address, and trying to spread some information to all (or most) hosts on the cable. The former should never require large packets. If you are stuck with an application that really does require broadcasting large amounts of information (such as a routing table protocol), then you are better off breaking the information into packet-sized chunks at the application level, not the IP (fragmentation) level. This allows the receiving application to compensate for lost packets (for example, to request retransmission, and/or to combine packets from several transmissions); fragmentation does not allow this, since the receiving application never sees the partial data if a fragment is lost. Note that what I say here about broadcasting probably applies as well to multicasting. -Jeff
ron@ron.rutgers.edu (Ron Natalie) (05/02/89)
Host requirement? Am I missing something I've looked through the Host Requirements doc of April 6, 89, the last I retrieved and I see no prohibition on fragmented broadcasts (not that it isn't a bad idea). I looked through the Internet layer especially the sections on Fragmentation, reassembly and Broadcasts and I can't find it. -Ron