The line between two peers is not a line 同层之间那条线，其实一根都没有

Every book about layered protocols says the transaction layer “talks to” the transaction layer on the other side. Take that literally and you go looking for its wire. There is none. Only the bottom layer has one. 讲分层协议的书都会写「传输层和对方的传输层通信」。照字面理解，你就会去找传输层那根线——找不到。横向那条线一根都没有，真线只有最底下一根。

NIC CPU PCIe
The card touches the board in one place: the slot. Every byte that leaves the card leaves through those contacts — there is no second path kept aside for “the upper layers”. 卡和主板只有一处接触：那道插槽。卡上出去的每一个字节都从那排金手指出去——没有第二条路留给「上面几层」走。

Why the sentence misleads 那句话为什么会误导人

“Talks to” is a statement about rules, not about wiring. Both sides agree on what a packet looks like, what an acknowledgement means, when to resend. Agreeing is not connecting. 「通信」说的是规矩，不是接线。两边说好包长什么样、确认是什么意思、什么时候重发——说好，不等于连上。

You post a letter. Inside it you wrote to your friend — that is the layer that “talks to” them. But the letter goes into a courier bag, the bag goes onto a truck, and the truck is the only thing that actually moves. Your friend unloads, unwraps, then reads, in the opposite order. You and your friend never touched. Neither did the two bags. 你寄一封信。信里写的是给朋友的话——那一层「和他通信」。可信要装进快递袋，袋要上卡车，真正在动的只有卡车。朋友那边按相反的顺序来：先卸车，再拆袋，最后读信。你和朋友从头到尾没碰过面，两个快递袋也没有。

What it looks like drawn honestly 老实画出来是这样

Draw the agreement as what it is — a name both sides know — and the picture stops lying. Then follow one transfer through it. 把「约定」按它本来的样子画出来——一个两边都认的名字——图就不再说假话了。然后跟着一笔传输走一遍。

传输层 Transaction 数据链路层 Data link 物理层 Physical TLP TLP DLLP DLLP 128b/130b 128b/130b PCIe lane 说好的约定，不是线 an agreement, not a wire 只有这一根是真的 only this one is real
Three layers, a block on each side of every one. The top two are joined by nothing — a broken hairline and a bubble carrying the layer name, which is what an agreement looks like when you draw it honestly. Only the bottom pair has a real line: it drops out of the left block, runs across under the yellow band, and climbs into the right one. The violet arrow is one transfer: down every layer, across that single line, up every layer. That is why a protocol analyser clips onto the lane and never onto the transaction layer — there is nothing up there to clip to. 三层，每层左右各一块。上面两层之间什么都没有——一条断开的细虚线，加一个写着层名的气泡，这就是「约定」被老实画出来的样子。只有最下面那一对之间有真线：它从左边那块底下落下来，横过黄条，再升进右边那块。紫箭头是一笔传输：一层层下去，过那唯一一根线，再一层层上来。协议分析仪之所以只能夹在 lane 上、夹不到传输层，就是因为上面根本没有东西可夹。