关于LLC协商，协议中一段话该如何理解？

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6.4.1.6            Exchange Identification (XID) command/response

LLC参数协商中，LLC层参数N201-I,mD，mU，kD，kU在44.064协议的table 6中都是向下协商的，就是表示协商值要小于原来的参数，但是后面又说，In ABM, N201-I,mD，mU，kD，kU can only be negotiated to the same or higher values as previously used. 那在ABM模式下，到底是向下协商还是向上协商呢，这里不是矛盾的吗？ 求助艾版主。

N201‑I (maximum information field length for  I frames)	6	2	00000bbb   bbbbbbbb	140 through 1 520	octets	down
mD  (I frame buffer size in the downlink direction)	7	2	0bbbbbbb   bbbbbbbb	0, 9 through 24 320	16 octets	down
mU  (I frame buffer size in the uplink direction)	8	2	0bbbbbbb   bbbbbbbb	0, 9 through 24 320	16 octets	down
kD (window  size in the downlink direction)	9	1	bbbbbbbb	1 through 255	frames	down