Incorrect Source Routing Bit Causes NetBEUI Network Storm (105282)

SYMPTOMS

Receiving a frame on an Ethernet with the source routing bit enabled can cause a machine running NetBEUI to propagate a network storm: entering a loop of transmitting invalid frames to other machines on the net. This seems to occur mostly with 3Com Etherlink3 network interface cards (NICs).

CAUSE

NOTE: SABME and UA are defined below.

When a link is established and the SABME frame has the source routing bit set erroneously, the routing length field is accounted for in the link table entry as well as in the original routing header info.

This causes all subsequent frames on this link (including the following UA) to contain the original source routing info, but the source routing bit is ignored on the receive end, causing the frame to be misinterpreted. When the original frame contains a SABME with the source routing bit set incorrectly, the two machines enter a loop broadcasting SABME frames back and forth.

STATUS

A fix has been made to NetBEUI, causing it to refuse to accept frames containing source routing information when operating on an Ethernet network.

DEFINITIONS

SABME stands for "set asynchronous balanced mode extended." It is a frame sent from one logical link control (LLC) layer to another requesting establishment of a link connection between them.
UA stands for "unnumbered acknowledgment." It is a frame sent in response to a SABME that accepts the request and establishes the link connection.