(Read more: Hedge fund Elliott offers to buy Riverbed, higher bid expected)
The letter, written by portfolio manager Jesse Cohn, also said Elliott hoped Riverbed would either accept its buyout offer or shop around for others. The hedge fund's proposal includes a so-called "go shop" provision, meaning Riverbed is encouraged to seek other offers.
"We are aware that numerous parties have expressed acquisition interest in Riverbed, and this structure guarantees that the company will secure a healthy premium for its stockholders while holding open the opportunity to obtain an even higher premium," Cohn wrote.
"By any measure, we believe our proposal represents a compelling opportunity that your stockholders will find extremely attractive," he added.
(Read more: Hedge fund Elliott takes stake in "undervalued" Riverbed)
Riverbed's relatively constructive response echoes a previous comment on an "open dialogue with our shareholders" the company made Nov. 8 after Elliott disclosed a 9 percent stake and called the company "significantly undervalued."
San Francisco-based Riverbed has also responded to Elliott's earlier advance with several safety measures. It instituted a stockholder rights plan or so-called "poison pill" to make it more difficult for Elliott to gain a controlling stake in the company. Riverbed also hired Goldman Sachs as an advisor. That move was to defend against a hostile takeover, not to explore a potential sale, according to a person familiar with the situation.
While hedge funds rarely own entire companies, the move is not a first for Elliott. The hedge fund firm recently attempted to buy another tech company, Compuware, but backed off when it sold parts of its business. Elliott has also successfully asked for other companies to sell themselves, including Novell and BMC Software.
A spokesman for Elliott had no comment beyond the letter. The firm's flaghsip hedge fund gained 11.8 percent net of fees in 2013, according to investor documents obtained by CNBC.com.
Analyst Daniel Ives of FBR & Co. viewed the Elliott offer positively and said other offers for the company were likely.
"Other strategic and financial buyers could enter the Riverbed bidding process over the coming months as this remains a core strategic asset within the broader technology food chain," Ives wrote in a client note Wednesday. "In our opinion, if not Elliott, we see a financial/private equity buyer as the most likely bidder."
Added Ives: "We believe it represents the first step in this "game of high stakes poker", and that ultimately Elliott's activism will lead to higher shareholder value for investors."