[zeromq-dev] Difficulty/Confusion in understanding ZMTP

[Jeremy Richemont]

If you'd read a little further you'd have found that the value 0xFF (255)
is reserved for saying "a second body part follows". Therefore length can
only be between 0 (probably pointless) and 254 for a single body. I agree
that the wording of "For frames with a length of 1 to 254 octets, the
length SHOULD BE encoded as a single octet" is unclear whether it is
referring to the entire packet (254 - 1 (flag) - 1 (length) == 252 bytes
max body) or just the body part. On the other hand, reading even further
gives:

The specification of the length field is surprising (and has confused even
> experts reading the spec). It should not include the flag field, nor other
> possible header fields. This would allow a zero-length body to be specified
> with a length of zero.


So we can assume the length field is only the body length and does not
include metadata. Since already stated that 255 is reserved, the value of
length can be zero to 254 or an array of [0..253] which is length-1.

Jeremy



On 2 November 2015 at 17:49, Indradhanush Gupta <
indradhanush.gupta at gmail.com> wrote:

> Hi,
>
> I'm trying to understand the ZMTP protocol, so I was going through the RFC
> for version 1.0 [0]. It states that:
>
>> The following diagram shows the layout of a frame with a length of 1 to
>> 254 octets:
>>
>>             +----------------+
>>  Octet 0    | Length         |
>>             +----------------+
>>  Octet 1    | Flags          |
>>             +----------------+- ... ---------------------+
>>  Octets 2+  | Body                     Length - 1 octets |
>>             +------------------ ... ---------------------+
>>
>> But, what I don't understand is that if the total length is that of *254
> Octets, *and the first and second octets are occupied by the *Length *and
> the *Flags *part of the message respectively, how can the *Body *be from *Octet
> 2 to (Length - 1)* ?  Shouldn't it be *(Length - 2) *, since the first
> two octets are occupied already?
>
> The RFC also says that,
>
> For frames with a length of 1 to 254 octets, the length SHOULD BE encoded
>> as a single octet.
>
>
> The length being *1 Octet *itself, can store a max value of *255. *So my
> question is, why is the max length of a frame, *254 *and not
>
> *255 ? *
> What am I missing here?
>
> [0] - http://rfc.zeromq.org/spec:13
>
> Thanks,
> --
> Indradhanush Gupta
>
>
> _______________________________________________
> zeromq-dev mailing list
> zeromq-dev at lists.zeromq.org
> http://lists.zeromq.org/mailman/listinfo/zeromq-dev
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.zeromq.org/pipermail/zeromq-dev/attachments/20151103/43cf0036/attachment.htm>