Asking for WAN Trouble

In Jeffrey Fritz's article ``Digital Remote Access'' (September), he illustrates an example of split routing from Washington, D.C., to New York City. Many implementations of TCP/IP would render this scenario deadly. To improve throughput, NFS (Network File System) mounts typically use the UDP packet, leaving reordering or retransmission requests to the higher levels of the protocol stack. If the vendor also eliminates the out-of-order checking (as is frequently the case), then the described link would lock up the workstation, cause it to spontaneously reboot, or present lots of nasty messages to the user.

Robert Gammon

rgammon@rgam.micro.ti.com

Houston, TX

The fact that split routes can and do occur in ISDN multichannel connections was exactly the point I was trying to make. Split routing can cause packets to arrive out of order causi ng grief for a number of protocols. Generally, TCP/IP can reorder out-of-sequence packets. However, if TCPs that leave resequencing to higher levels (e.g., UDP) are used, things can get interesting. Fortunately, most ISDN remote-access vendors include packet resequencing in their products, and the network protocols are not required to deal with out-of-sequence packets. At worst, they will see short delays occasionally; these are typical and should not cause any major problems.

--Jeffrey Fritz