Sec Form 4 Filing - CHUNG PETER Y @ Casa Systems Inc - 2017-12-19

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person CHUNG PETER Y 2. Issuer Name and Ticker or Trading Symbol Casa Systems Inc [ CASA] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) C/O SUMMIT PARTNERS, 222 BERKELEY STREET, 18TH FLOOR 3. Date of Earliest Transaction (MM/DD/YY) 12/19/2017 (Street) BOSTON, MA02116 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) _____ Form filed by One Reporting Person __ X __ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock 12/19/2017 C 38,592,000 A 38,592,000 ( 2 ) I See Footnotes. ( 3 ) ( 4 ) Common Stock 12/19/2017 P 250,000 ( 5 ) A $ 13 38,842,000 ( 6 ) I See Footnotes. ( 3 ) ( 4 )

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares Series C Convertible Preferred Stock ( 1 ) 12/19/2017 C 3,859,200 ( 1 ) ( 1 ) Common Stock 38,592,000 ( 2 ) $ 0 ( 1 ) 0 I See Footnotes. ( 3 ) ( 4 )

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other CHUNG PETER Y C/O SUMMIT PARTNERS 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 X MANNION MARTIN J C/O SUMMIT PARTNERS 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 X Signatures Robin W. Devereux, Power of Attorney for Peter Y. Chung 12/19/2017 Signature of Reporting Person Date Robin W. Devereux, POA for Martin J. Mannion 12/19/2017 Signature of Reporting Person Date

Explanation of Responses:

( 1 )The Series C Convertible Preferred Stock automatically converted into common stock of Casa Systems, Inc. (the "Issuer") on a ten-for-one basis upon the closing of the Issuer's initial public offering. The Series C Convertible Preferred Stock had no expiration date. ( 2 )Consists of 24,052,910 shares of common stock issued upon the conversion of Series C Convertible Preferred Stock held by Summit Partners Private Equity Fund VII-A, L.P., 14,446,570 shares of common stock issued upon the conversion of Series C Convertible Preferred Stock held by Summit Partners Private Equity Fund VII-B, L.P., 82,110 shares of common stock issued upon the conversion of Series C Convertible Preferred Stock held by Summit Investors I, LLC and 10,410 shares of common stock issued upon the conversion of Series C Convertible Preferred Stock held by Summit Investors I (UK), L.P. ( 3 )Summit Partners, L.P. is the managing member of Summit Partners PE VII, LLC, which is the general partner of Summit Partners PE VII, L.P., which is the general partner of each of Summit Partners Private Equity Fund VII-A, L.P. and Summit Partners Private Equity Fund VII-B, L.P. Summit Master Company, LLC is the managing member of Summit Investors Management, LLC, which is the manager of Summit Investors I, LLC, and the general partner of Summit Investors I (UK), L.P. Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated investment decisions, including voting and dispositive power, to Summit Partners, L.P. and its investment committee responsible for voting and investment decisions with respect to the Issuer. ( 4 )Summit Partners, L.P., through a three-person investment committee responsible for voting and investment decisions with respect to the Issuer, currently comprised of Peter Y. Chung, Bruce R. Evans and Martin J. Mannion, has voting and dispositive power over the shares held by each of these entities and therefore may be deemed to beneficially own such shares. Each of the Summit entities and persons mentioned in this footnote disclaims beneficial ownership of the shares, except for those shares held of record by such entity, and except to the extent of their pecuniary interest therein. ( 5 )Consists of 155,816 shares of common stock purchased by Summit Partners Private Equity Fund VII-A, L.P., 93,585 shares of common stock purchased by Summit Partners Private Equity Fund VII-B, L.P., 532 shares of common stock issued purchased by Summit Investors I, LLC and 67 shares of common stock purchased by Summit Investors I (UK), L.P. ( 6 )Consists of 24,208,726 shares of common stock held by Summit Partners Private Equity Fund VII-A, L.P., 14,540,155 shares of common stock held by Summit Partners Private Equity Fund VII-B, L.P., 82,642 shares of common stock held by Summit Investors I, LLC and 10,477 shares of common stock held by Summit Investors I (UK), L.P.

Remarks: Exhibit 24.1 Power of Attorney

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

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