Editing XMODEM (section)

===SEAlink===
One of the first third-party mailers for the [[FidoNet]] system was '''SEAdog''', written by the same author as the then-popular [[.arc]] [[data compression]] format. SEAdog included a wide variety of improvements, including [[SEAlink]], an improved transfer protocol based on the same sliding window concept as WXmodem.{{sfn|SEAlink|1987}} It differed from WXmodem mostly in details.

One difference is that SEAlink supported the "zero packet" introduced by TeLink, which is needed in order to operate as a drop-in replacement for TeLink in FidoNet systems where the header was expected. <code>ACK</code>s and <code>NAK</code>s were extended to three-byte "packets", starting with the <code>ACK</code> or <code>NAK</code>, then the packet number, then the complement of the packet number, in the same fashion as the original XMODEM packet header. The window size was normally set to six packets.{{sfn|SEAlink|1987}}

SEAlink was not expected to operate over X.25 or similar links, and thus did not perform escaping. This was also needed so the zero packet would work properly, as this standard used the <code>SYN</code> character that WXmodem had re-purposed.{{sfn|SEAlink|1987}} On top of these changes, it added an "Overdrive" mode for half duplex links. This suppressed ACKs for packets that were successfully transferred, in effect making the window of infinite size. This mode was indicated by a flag in the zero block.{{sfn|SEAlink|1987}}

SEAlink later added a number of other improvements and was a useful general-purpose protocol. However, it remained rare outside the FidoNet world, and was rarely seen in user-facing software.