KVC@ENGVAX.SCG.HAC.COM (Kevin Carosso) (06/27/87)
When a SYN is sent to a Berkeley UNIX TCP/IP implementation (4.3 or 4.2)
it appears that UNIX can send the SYN-ACK without having to ARP for the
host it's ACKing. Does it indeed add an entry to the ARP table when an
ethernet IP packet comes in? I'll buy that anything to cut down
on ethernet broadcasts is a good thing...
This may seem like a dumb question, but the code is a little opaque and
I'm not a UNIX kernel hack. I have a hypothesis which explains some behaviour
I'm seeing with the TCP I maintain (CMU/Tek for VAX/VMS) and UNIX must be
doing this if my theory's any good.
Thanks,
/Kevin Carosso kvc@engvax.scg.hac.com
Hughes Aircraft Co. kvc%engvax@oberon.usc.eduDCP@QUABBIN.SCRC.SYMBOLICS.COM.UUCP (06/29/87)
Date: Fri, 26 Jun 87 19:06 PDT
From: Kevin Carosso <@YMIR.BITNET:KVC@ENGVAX.SCG.HAC.COM>
When a SYN is sent to a Berkeley UNIX TCP/IP implementation (4.3 or 4.2)
it appears that UNIX can send the SYN-ACK without having to ARP for the
host it's ACKing. Does it indeed add an entry to the ARP table when an
ethernet IP packet comes in? I'll buy that anything to cut down
on ethernet broadcasts is a good thing...
This may seem like a dumb question, but the code is a little opaque and
I'm not a UNIX kernel hack. I have a hypothesis which explains some behaviour
I'm seeing with the TCP I maintain (CMU/Tek for VAX/VMS) and UNIX must be
doing this if my theory's any good.
That can't really work in the face of gateways, since on input the
source IP address does not correspond to the hardware source address of
the gateway, and on output routing is taking place so that the packet is
sent to the gateway's IP address, which the sender doesn't necessarily
have a translation for and may not have it handy.karels%okeeffe@UCBVAX.BERKELEY.EDU.UUCP (06/29/87)
Of course a Berkeley host can send a SYN-ACK in response to an incoming SYN without doing an ARP request! If you got the SYN to it, you must have sent an ARP request at some time (all of this assuming the hosts are on the same Ethernet). When the ARP request was received and processed, the hardware address for your machine was added to the ARP cache. Mike
louie@trantor.UMD.EDU (Louis A. Mamakos) (06/29/87)
What probably happens is that who every sent the original packet with a SYN segment first had to ARP for your address. The 4.x BSD ARP module caches the address of the host requesting your address; thus when you want to reply, the ARP mapping is already present. Louis A. Mamakos WA3YMH Internet: louie@TRANTOR.UMD.EDU University of Maryland, Computer Science Center - Systems Programming
ron@TOPAZ.RUTGERS.EDU (Ron Natalie) (06/30/87)
I can't say if BSD is picking up on Ethernet hardware addresses from the incoming connection request, but there is no reason why this can't happen. The internet address of remote end would be entered into the arp cache with the gateway's hardware address. This will work. This is how gateways on Ethernet subnets trick hosts that don't know about subnets into working properly. The gateway sends arp answers for IP addresses off the local subnet that it will route for. Nearly every device I've come across (including BSD UNIX systems and other things for which we don't have source) will allow manyone mappings of IP to Ethernet address. =Ron