Code change:
2.1.2. <ins> A dynamic table entry is considered blocking and cannot be evicted until its insertion has been acknowledged and there are no outstanding unacknowledged references to the entry.  In particular, a dynamic table entry that has never been referenced can still be blocking. A blocking entry is unrelated to a blocked stream, which is a stream that a decoder cannot decode as a result of references to entries that are not yet available.  Any encoder that uses the dynamic table has to keep track of blocked entries, whereas blocked streams are optional. </ins> An encoder MUST NOT insert an entry into the dynamic table (or <del> duplicate an existing entry) if doing so would evict an entry with unacknowledged references.  For header blocks that might rely on the newly added entry, the encoder can use a literal representation. </del> <ins> duplicate an existing entry) if doing so would evict a blocking entry.  In this case, the encoder can send literal representations of header fields. </ins> To ensure that the encoder is not prevented from adding new entries, the encoder can avoid referencing entries that are close to eviction.

Related discussion:
text: A dynamic table entry that has not been acknowledged by the decoder must not be evicted by the encoder, even if there has never been any references to it. This is already alluded to in the current spec at URL by the phrase "acknowledged by the decoder", but since "acknowledged" is used interchangeably for acknowledging the insertion instruction and acknowledging references, a reader could understand this to mean "all references acknowledged". In addition, at three other places eviction of entries with unacknowledged references are banned, with unambigously no ban on eviction of unacknowledged entries with no references. I argue that more explicit language would be beneficial. This issue has already been discussed at URL and below. Note that avoiding this kind of corruption could also be achieved by banning an encoder from inserting entries that it does not immediately reference, but this is probably way too restrictive. For example, an encoder might wish to achieve best latency and speculatively improve future compression ratio at the expense of potentially wasting current bandwidth by encoding header fields as literals while inserting them at the same time for potiential future use. This is already alluded to in the spec at multiple places. Below is a specific scenario when this issue might cause corruption: Suppose MaxEntries is 5. Suppose that there is a fresh connection, with an empty dynamic table, and there is a header list to be sent that has two header fields which fit in the dynamic table together. Suppose one encoder implementation inserts these two header fields into the dynamic table, and encodes the header list using two Indexed Header Field instructions, referring to absolute indices 1 and 2. This header block will have an abstract Largest Reference value of 2, which is encoded as 3 on the wire. A spec compliant decoder should have no difficultly correctly decoding this header block. Suppose another encoder implementation starts by adding 10 unique entries, all different from the two header fields in the header list to be sent, to the dynamic table, but does not emit references to any of them. Of course in the end only the last couple will stay in the dynamic table, the rest will be evicted. This can be considered spec compliant depending on how one interprets the spec, see above.