ACR X DSCNT THROUGH OCCUPIED ALT TCASII RA HAD LTSS FROM MLT. SYS ERROR. EVASIVE ACTION TAKEN.

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ACR X INBOUND TO BOSTON FROM ORLANDO ON THE NORWICH 1 ARR. FO FLYING. WERE CLRED TO 11000 FT WHICH WAS READ BACK TO ATC AND SET/VERBALIZED INTERNALLY BY BOTH PLTS. TFC CALLED BY ATC FROM THE L AT 13000 FT. WE HAD BOTH VISUAL AND TCASII CONTACT WITH THIS TARGET; AN MLT; BOUND FOR OTIS AFB. WE SLOWED OUR RATE OF DSCNT AT THIS TIME (AT 13800 FT). ATC ASKED OUR ALT AS MLT PASSED AFT OF US. ATC SAID TO CLB TO 14000 FT. WE HAD AN RA ON THE TCASII WHICH INDICATED NO CHANGE TO WHAT WE WERE ALREADY DOING (I.E.; SLOW DSCNT). LATER; PHONE CONVERSATION WITH ZBW CONFIRMED WE HAD BEEN CLRED TO 11000 FT. BOTH ACFT WERE IN VISUAL CONTACT WITH ONE ANOTHER THROUGHOUT. CONSIDERATIONS: 1) VERY BUSY FREQ/SECTOR. ATC CLRED US BELOW 14000 FT; THEN A LITTLE LATER CALLED THE TFC AT 13000 FT. I FELT THERE WAS A PLAN; I.E.; THE MLT WAS TO PASS WELL TO THE REAR OF US; SO I WAS NOT UNDULY ALARMED WHEN; PERHAPS; I SHOULD HAVE BEEN. THE VISUAL CONTACT WITH THE MLT REINFORCED THIS CONCLUSION. LESSON: ASSUME NOTHING AND ANALYZE EVERY STEP TO BE TAKEN IN THIS BUSY PHASE OF FLT. WE MUST CONSTANTLY EXTRAPOLATE INFO FROM A CLUTTERED BACKGND AND THIS IS TRUE FOR THE PEOPLE AT ATC AS WELL AS THOSE AIRBORNE.