Understanding of AMQP Basic.Publish frame format

[Daron Burleson]

Hello everyone,

I am trying to parse the "arguments" field in the payload of AMQP Basic.Publish method According to the specifications, the three first parameters have always the same size:

• Consumer-tag: (shortsrt) 31 bytes
• Delivery-tag: (longlong) 8 bytes
• Redelivered: (1 bit) packed into 1 byte as far I understand from the pcap file

The problem comes with the two following parameters, "exchange" and "routing-key" parameters, as they have variable size and sometimes even the "exchange" field is empty (and I don't meant that is a empty string, what I mean is that the actual bytes are not sent into the wire). Even though I know the total payload size, I cannot detected where the "exchange" parameter ends and the "routing-key" parameter starts

My doubt is wow can I know where each fields starts in order to parse them. Wireshark is able to parse them correctly, but I don't know where it gets that info.

Any help is much appreciated!

Cheers,

Daron

[Wesley Peng]

Maybe you look through the C library source who can give you glues.

https://github.com/alanxz/rabbitmq-c

Thanks 

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[Dominic Morneau]

There's a couple string types defined in the AMQP spec, quote follows:

• Short strings, stored as an 8-bit unsigned integer length followed by zero or more octets of data. Short strings can carry up to 255 octets of UTF-8 data, but may not contain binary zero octets.
  
• Long strings, stored as a 32-bit unsigned integer length followed by zero or more octets of data. Long strings can contain any data.

The fields you're referring to are listed as short strings in the AMQP reference page on the RabbitMQ website. 

Dominic

2021年2月7日日曜日 22:24:18 UTC+9 Daron Burleson:

[Daron Burleson]

Thanks Dominic and Wesley for the hint!

I've been digesting the C library source ,which has been quite useful, and I understand that the length of both fields are stored in variable of this type

typedef struct amqp_envelope_t_ {

    ... other fields ..

    amqp_bytes_t exchange;

    amqp_bytes_t routing_key;
 } amqp_envelope_t;

Field type of both parameters is amqp_bytes_t that has just two fields:

      size_t  len

   void *  bytes

but the thing is that I cannot understand/find/understand the logic that computes its length (size_t len)

Any help is much appreciated!

-- Daron

[Gavin M. Roy]

You might find Chapter 2 of my book useful: https://livebook.manning.com/book/rabbitmq-in-depth/chapter-2/26

--

[Daron Burleson]

 I have just understood how the string encoding works here... The first byte in the wire accounts for the field lenght, that's all :)

Thanks!