Except you can’t actually do that. ‘\044' becomes ‘,' on the first pass if you parse it as a character string first. The ONLY way this works is if you remember which commas are escaped or not (\044 or \, vs ,). It’s dead easy to split it into alpn-id as you unescape the string.
Mark > On 18 Jun 2020, at 23:53, [email protected] wrote: > > OK, I think I now understand the intent, and refactored my code accordingly, > and it is now simpler and cleaner. Yay. > > I think it would be clearer to implementers if section 2.1.1 said that all > values are initially parsed as character-strings (allowed to exceed 255 > characters), and then further parsed by SvcParamKey-specific parsing which > may, for example, split on comma. I think the current text isn't entirely > clear on the functional separation between generic parsing and key-specific > parsing. > > - lc > > >> On Jun 15, 2020, at 22:04, Mark Andrews <[email protected]> wrote: >> >> >> >>> On 14 Jun 2020, at 05:01, Larry Campbell >>> <[email protected]> wrote: >>> >>> I think there's an implementation difficulty. Consider: >>> >>> 1. alpn=h2 ; clear enough >>> 2. alpn="h2" ; should be equivalent >>> 3. alpn=\h\2 ; should also be equivalent >>> 4. alpn=h2,h3 ; ok (two values) >>> 5. alpn="h2","h3" ; should be equivalent >> >> No, as it is key=quoted-string as per 2.1.1 not >> key=quoted-string(,quoted-string\)* >> >>> 6. alpn="h2,h3" ; malformed? or a single alpn value of h2,h3? or two >>> three-character values, "h2 and h3”? >> >> this is correct >> >>> 7. alpn=h2\,h3,h4 ; how should this be parsed? >> >> 0x05 0x68 0x32 0xc2 0x68 0x33 0x02 0x68 0x34 >> >>> Section 2.1.1 tempts one to build the obvious implementation of using one's >>> existing character-string parser, and then passing the parsed >>> character-string to the individual handler for each key type. The alpn and >>> ipv*hint handlers are going to want to split that character-string on >>> comma. That would treat #6 as two two-character values (h2,h3). But #7 is >>> problematic: the generic character-string parser would remove the >>> backslash, and then the alpn handler would treat this as three alpn values >>> when you probably wanted just two >> >> When you are also parsing domain names you have to deal with \. being a >> literal period not a domain separator. >> exa\.mple.com and “exa\.mple.com” aree being two labels ‘exa.mple’ and >> ‘com’. This is not really different. >> >> That said we do need to address this issue. >> >> In BIND we extract quoted-string preserving the escapes (except for ‘\”’) >> then pass the token to a domain name parser or a text parser. Having ‘key=‘ >> preceding the quoted-string is more of a issue and we have to shift modes >> mid-token. >> >>> We could make a special character-string parser for alpn and ipv*hint, that >>> handles commas, but it feels odd to have to use a special parser just for >>> certain key types. However, if we must allow commas in alpn names, then we >>> have no choice. >> >> You need to reparse value for port, alpn, ipv*hint, >> >>> Perhaps it would be clearer to simply remove the three paragraphs of >>> section 2.1.1 beginning with "The presentation for for SvcFieldValue is..." >>> and ending with "...not limited to 255 characters.)". Since the previous >>> paragraph says "Values are in a format specific to the SvcParamKey", >>> perhaps it would be best to leave the description of each value format in >>> the appropriate part of section 6 and for section 2.1.1 to discuss only how >>> to represent and parse unrecognized keys. >> >> >>> >>> To keep the implementation simple, the alpn value could be defined as a >>> comma-separated list of sequences of printing ASCII characters, with >>> embedded comma represented as \, backslash as \\, and all nonprinting and >>> non-ASCII characters reprsented as \nnn. (In other words, the full >>> generality of character-string, particularly double-quotes, is not needed >>> here. >>> >>> The other comma-separated value types -- ipv4hint and ipv6hint -- do not >>> have this difficulty; they also don't need the full generality of >>> character-string handling, because the individual values can contain only >>> hex digits, periods, and colons, so their specification and implementation >>> can be much simpler. >>> >>> And I think section 2.1.1 would be clearer if >>> >>> using decimal escape codes (e.g. \255) when necessary >>> >>> were replaced by >>> >>> using decimal escape codes (e.g. \255) for all nonprinting and non-ASCII >>> characters, and using \\ to represent backslash >>> >>> - lc >>> >>> >>>> On Jun 13, 2020, at 11:25, Ben Schwartz >>>> <[email protected]> wrote: >>>> >>>> Larry, >>>> >>>> I think that's the intent of the current text, especially the ABNF for >>>> "element". If you think it's unclear, we should adjust it. Please >>>> suggest text! >>>> >>>> --Ben Schwartz >>>> >>>> On Sat, Jun 13, 2020, 10:53 AM Larry Campbell >>>> <[email protected]> wrote: >>>> Seciont 6.1 says: >>>> >>>>> The presentation value of "alpn" is a comma-separated list of one or more >>>>> "alpn-id"s. Any commas present in the protocol-id are escaped by a >>>>> backslash: >>>>> >>>>> escaped-octet = %x00-2b / "\," / %x2d-5b / "\\" / %x5D-FF >>>>> escaped-id = 1*(escaped-octet) >>>>> alpn-value = escaped-id *("," escaped-id) >>>> >>>> If I read this correctly, the presentation value is allowed to contain >>>> nulls and control characters. This seems likely to make such records very >>>> difficult to edit. Wouldn't it be better to require these to be encoded as >>>> \nnn? >>>> >>>> - lc >>>> >>>> _______________________________________________ >>>> DNSOP mailing list >>>> [email protected] >>>> https://urldefense.proofpoint.com/v2/url?u=https-3A__www.ietf.org_mailman_listinfo_dnsop&d=DwIFaQ&c=96ZbZZcaMF4w0F4jpN6LZg&r=gc4HNe2gylF-6x1tOpS9Zq70q_kVFHKTtJkp1pJY_D4&m=kf9220DuFaSJ-dcBUyvrvUHI9A9wneAvcmzLgZgs8ok&s=xlHdRU6fzrAQDx2lgeob7c2tR-iF311nphkHB_GHcU0&e= >>>> >>> >>> _______________________________________________ >>> DNSOP mailing list >>> [email protected] >>> https://urldefense.proofpoint.com/v2/url?u=https-3A__www.ietf.org_mailman_listinfo_dnsop&d=DwIFaQ&c=96ZbZZcaMF4w0F4jpN6LZg&r=gc4HNe2gylF-6x1tOpS9Zq70q_kVFHKTtJkp1pJY_D4&m=kf9220DuFaSJ-dcBUyvrvUHI9A9wneAvcmzLgZgs8ok&s=xlHdRU6fzrAQDx2lgeob7c2tR-iF311nphkHB_GHcU0&e= >>> >> >> -- >> Mark Andrews, ISC >> 1 Seymour St., Dundas Valley, NSW 2117, Australia >> PHONE: +61 2 9871 4742 INTERNET: [email protected] -- Mark Andrews, ISC 1 Seymour St., Dundas Valley, NSW 2117, Australia PHONE: +61 2 9871 4742 INTERNET: [email protected] _______________________________________________ DNSOP mailing list [email protected] https://www.ietf.org/mailman/listinfo/dnsop
