Mathish

the two halves of my tasty brain

STOMP 1.1

Every post I make to the stomp-spec group that isn’t a “+1” or “me, too!” is often followed up with a thought of “you’re a moron” about 10 minutes later. I’m going to improve this situation by spending some time hashing out my thoughts here, and if they still look good a couple hours later, then I’ll post them.

STOMP 1.0 does not allow the octets ‘:’ and LF to appear in header names. It also does not allow the LF octet in a header value. These conditions make sense, ‘:’ serves as a delimiter between a header name and its value, and LF serves as a delimiter between headers. However, this means certain characters cannot be used within headers, and that can be a problem, for example, if you want to stuff a JSON encoded string into a header value.

STOMP 1.1 rectifies this by specifying escape sequences for LF and ‘:’, ‘\n’ (two octets, not the single character \n) and ‘\c’ respectively. Naturally, as we use ‘' as an escape sequence indicator, we need an escape sequence for literal ‘' octets, and thus have ‘\’.

The problem is that until the protocol version is fully negotiated, header escaping/unescaping may produce undesirable results. That is where the CONNECT and CONNECTED frames come in to play.

Scenario 1

A client could send a CONNECT frame with escaped headers. Eg:

CONNECT
passcode:s6842FW!\c4284\\\$
accept-version:1.1

^@


The client has indicated that it will only accept a 1.1 connection and has used escaping appropriate for that version. The server, however, does not know that the client will only accept version 1.1 until it has already read the escaped header from the stream.

Scenario 2

A client could send a CONNECT frame that accepts either version 1.0 or 1.1, and use only STOMP 1.0 compliant headers. However, even before the server issues a CONNECTED frame, it already knows which version of the spec is going to be used. The client, however, does not. If the server decides on STOMP 1.1, and sends a CONNECTED frame to the client such as:

CONNECTED
session:D\chostname-63348-1297283114292-4\c0
version:1.1

^@


The server already knows we’re using STOMP 1.1, but the client will not figure that out until it reads the ‘version’ header.

Resolutions

1. Header processing of CONNECT frames by the server has to be deferred until ‘accept-version’ is read. If list of versions include 1.0, it would be bad form if the client escaped headers according to 1.1, so no unescaping need be done. However, if the client is only version 1.1, the server may have to unescape headers. Similarly, a client will have to defer processing of all headers of a received CONNECTED frame until ‘version’ is read. From the value given, the client can then unescape headers and values (1.1), or leave them untouched (1.0). In either case, the spec may benefit from an explicit statement regarding the treatment of CONNECT/CONNECTED headers.
2. Essentially follow Resolution 1, but require the first header of CONNECT to be ‘accept-version’ and the first header of CONNECTED to be ‘version’, this allows servers and clients to decide on the escaping rules to follow after reading the first header.
3. Require that CONNECT/CONNECTED frames follow STOMP 1.0 header rules, regardless of the desired protocol version (ie: no escape sequences get generated, no unescaping gets done, LF and ‘:’ are not allowed in header names, LF is not allowed in header values)

Of these, Resolution 3 is the most appealing to me, because it doesn’t require me to do anything else. Both Resolution 1 and Resolution 2 will require some changes, though both are obviously doable. Resolution 1 doesn’t require any further changes to the spec, except perhaps a warning about deferring header decoding until you know what version is running. Resolution 2 is only helpful because of how I process headers on the stream, and is probably just selfish.

Found the following in the Protocol Negotiation section.

The protocol that will be used for the rest of the session will be the highest protocol version that both the client and server have in common.

All that really remains then is clarifying when the “rest of the session” begins (with the CONNECTED frame or after the CONNECTED frame.)