Sec Form 4 Filing - SUMMIT PARTNERS L P @ Casa Systems Inc - 2018-04-30

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 SUMMIT PARTNERS L P 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) X __ Other (specify below) Manager of GP of 10% owner (Last) (First) (Middle) 222 BERKELEY STREET, 18TH FLOOR 3. Date of Earliest Transaction (MM/DD/YY) 04/30/2018 (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 04/30/2018 S 4,717,520 ( 1 ) ( 2 ) D $ 25 34,124,480 ( 3 ) I See Footnotes ( 4 ) ( 5 )

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

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other SUMMIT PARTNERS L P 222 BERKELEY STREET 18TH FLOOR BOSTON, MA02116 X Manager of GP of 10% owner SUMMIT MASTER COMPANY, LLC 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 X Manager of Manager of 10% owne Summit Partners PE VII, L.P. 222 BERKELEY STREET 18TH FLOOR BOSTON, MA02116 X GP of 10% owner. Summit Partners PE VII, LLC 222 BERKELEY STREET 18TH FLOOR BOSTON, MA02116 X GP of GP of 10% owner Summit Investors Management, LLC 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 X Manager and GP of 10% Owner Summit Partners Private Equity Fund VII-A, L.P. 222 BERKELEY STREET 18TH FLOOR BOSTON, MA02116 X Summit Partners Private Equity Fund VII B L P 222 BERKELEY STREET 18TH FLOOR BOSTON, MA02116 X Summit Investors I, LLC 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 X Summit Investors I (UK), L.P. 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 X Signatures Summit Partners, L.P., by Summit Master Company, LLC, its GP, by /s/ Robin W. Devereux, Member 04/30/2018 Signature of Reporting Person Date Summit Master Company, LLC, by Robin W. Devereux, Member 04/30/2018 Signature of Reporting Person Date Summit Partners PE VII, L.P., by Summit Partners PE VII, LLC, its GP, by Summit Partners, L.P., its Manager, Summit Master Company, LLC, by Robin W. Devereux, Member 04/30/2018 Signature of Reporting Person Date Summit Partners PE VII, LLC, by Summit Partners, L.P., its Manager, Summit Master Company, LLC, by Robin W. Devereux, Membe 04/30/2018 Signature of Reporting Person Date Summit Investors Management, LLC, by Summit Master Company, LLC, its Managing Member, by Robin W. Devereux, Member 04/30/2018 Signature of Reporting Person Date Summit Partners Private Equity Fund VII-A, L.P., by Summit Partners PE VII, L.P., its GP, by Summit Partners PE VII, LLC, its GP, by Summit Partners, L.P., its Managing Member, Summit Master Company, LLC, by Robin W. Devereux, Member 04/30/2018 Signature of Reporting Person Date Summit Partners Private Equity Fund VII-B, L.P., by Summit Partners PE VII, L.P., its GP, by Summit Partners PE VII, LLC, its GP, by Summit Partners, L.P., its Managing Member, Summit Master Company, LLC, by Robin W. Devereux, Member 04/30/2018 Signature of Reporting Person Date Summit Investors I, LLC, by Summit Investors Management, LLC, its Manager, by Summit Master Company, LLC, its Managing Member, by Robin W. Devereux, Member 04/30/2018 Signature of Reporting Person Date Summit Investors I (UK), L.P., by Summit Investors Management, LLC, its General Partner, by Summit Master Company, LLC, its Managing Member, by Robin W. Devereux, Member 04/30/2018 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Consists of 2,940,250 shares of common stock sold by Summit Partners Private Equity Fund VII-A, L.P., 1,765,961 shares of common stock sold by Summit Partners Private Equity Fund VII-B, L.P., 10,037 shares of common stock sold by Summit Investors I, LLC and 1,272 shares of common stock sold by Summit Investors I (UK), L.P. ( 2 )Of the shares sold by the reporting persons mentioned in footnote (1), the sale of 250,000 shares is matchable under Section 16(b) of the Securities Exchange Act of 1934, as amended, with the purchase of 250,000 shares on December 19, 2017 at $13.0 per share. The reporting persons mentioned in footnote (1) have paid to the issuer an aggregate of $2,721,875.00, representing the full amount of the profit realized in connection with the short-swing transaction, less transaction costs. ( 3 )Consists of 21,268,476 shares of common stock held by Summit Partners Private Equity Fund VII-A, L.P., 12,774,194 shares of common stock held by Summit Partners Private Equity Fund VII-B, L.P., 72,605 shares of common stock issued held by Summit Investors I, LLC and 9,205 shares of common stock held by Summit Investors I (UK), L.P. ( 4 )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. ( 5 )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.

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

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).

** Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).

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