| > In the asn1 readme, and in some comments in these threads you mention the perils of the tag-length-value scheme, but you never seemed to explain whats wrong with it? Not OP, but one of the challenges is that definite-length encodings like DER have to be encoded in a non-intuitive way. Values must be encoded prior to lengths (because the length is unknown), and the values can be nested. Therefore you have to encode a message essentially backwards when using definite-length encodings. This can potentially require a great deal of memory and can increase latency because streaming the data is hard. Indefinite lengths (BER has this option, CER requires it) can help avoid this problem, but then you lose the benefit of skipping elements (which you allude to in your next paragraph). > Do you feel that the same doesn't apply to serialisation formats? How are the non-tlv binaries encoded then? Just implied offsets according to the schema? Can you then evolve the schema at all, or do you feel that both producer and consumer should have always access to the full schema, and flexiblity here is a non-feature? You've hit the tradeoffs pretty well in the question, I think. The nice thing about TLV is that you can decode without a schema and potentially work with the contents: it's a relatively simple format to decode and validate even if it's not necessarily great for the encoder. ASN.1 supports schema-informed packed encodings that place greater demands on both the encoder and decoder. The main advantage is that they greatly reduce message overhead, but it requires a lot of bit-twiddling for presence/absence, default values, and, in unaligned variants, everything else, too. It's impossible, generally, to decode everything without the schema. PER has rules that disambiguate the values (e.g., they have to be ordered in a particular way, so you know what's coming next), and this mitigates some of the problems of TLV-style encodings. The tradeoffs are worth it when your pipes are small. 3GPP and LTE messages are largely encoded in PER. The people playing in that world usually have plenty of money to spend on commercial solutions and have bandwidth to roll their own, too. That's a bit different than smaller shops who are looking for convenient automated serialization formats. |