According to much of the code, the max packet size is 8 * 1024 bytes (8K). The actual length of the packet is encoded in the header.

Packets sent from the server to the container begin with 0x1234. Packets sent from the container to the server begin with AB (that's the ASCII code for A followed by the ASCII code for B). After those first two bytes, there is an integer (encoded as above) with the length of the payload. Although this might suggest that the maximum payload could be as large as 2^16, in fact, the code sets the maximum to be 8K.

Packet Format (Server->Container)
Byte	0	1	2	3	4...(n+3)
Contents	0x12	0x34	Data Length (n)		Data

Packet Format (Container->Server)
Byte	0	1	2	3	4...(n+3)
Contents	A	B	Data Length (n)		Data

For most packets, the first byte of the payload encodes the type of message. The exception is for request body packets sent from the server to the container -- they are sent with a standard packet header (0x1234 and then length of the packet), but without any prefix code after that (this seems like a mistake to me).

The web server can send the following messages to the servlet container:

Code	Type of Packet	Meaning
2	Forward Request	Begin the request-processing cycle with the following data
7	Shutdown	The web server asks the container to shut itself down.
8	Ping	The web server asks the container to take control (secure login phase).
10	CPing	The web server asks the container to respond quickly with a CPong.
none	Data	Size (2 bytes) and corresponding body data.

To ensure some basic security, the container will only actually do the Shutdown if the request comes from the same machine on which it's hosted.

The first Data packet is send immediatly after the Forward Request by the web server.

The servlet container can send the following types of messages to the web server:

Code	Type of Packet	Meaning
3	Send Body Chunk	Send a chunk of the body from the servlet container to the web server (and presumably, onto the browser).
4	Send Headers	Send the response headers from the servlet container to the web server (and presumably, onto the browser).
5	End Response	Marks the end of the response (and thus the request-handling cycle).
6	Get Body Chunk	Get further data from the request if it hasn't all been transferred yet.
9	CPong Reply	The reply to a CPing request

Each of the above messages has a different internal structure, detailed below.

For all requests, this will be 2. See above for details on other prefix codes.

The HTTP method, encoded as a single byte:

Command Name	Code
OPTIONS	1
GET	2
HEAD	3
POST	4
PUT	5
DELETE	6
TRACE	7
PROPFIND	8
PROPPATCH	9
MKCOL	10
COPY	11
MOVE	12
LOCK	13
UNLOCK	14
ACL	15
REPORT	16
VERSION-CONTROL	17
CHECKIN	18
CHECKOUT	19
UNCHECKOUT	20
SEARCH	21
MKWORKSPACE	22
UPDATE	23
LABEL	24
MERGE	25
BASELINE_CONTROL	26
MKACTIVITY	27

Later version of ajp13, when used with mod_jk2, will transport additional methods, even if they are not in this list.

These are all fairly self-explanatory. Each of these is required, and will be sent for every request.

The structure of request_headers is the following: First, the number of headers num_headers is encoded. Then, a series of header name req_header_name / value req_header_value pairs follows. Common header names are encoded as integers, to save space. If the header name is not in the list of basic headers, it is encoded normally (as a string, with prefixed length). The list of common headers sc_req_header_nameand their codes is as follows (all are case-sensitive):

Name	Code value	Code name
accept	0xA001	SC_REQ_ACCEPT
accept-charset	0xA002	SC_REQ_ACCEPT_CHARSET
accept-encoding	0xA003	SC_REQ_ACCEPT_ENCODING
accept-language	0xA004	SC_REQ_ACCEPT_LANGUAGE
authorization	0xA005	SC_REQ_AUTHORIZATION
connection	0xA006	SC_REQ_CONNECTION
content-type	0xA007	SC_REQ_CONTENT_TYPE
content-length	0xA008	SC_REQ_CONTENT_LENGTH
cookie	0xA009	SC_REQ_COOKIE
cookie2	0xA00A	SC_REQ_COOKIE2
host	0xA00B	SC_REQ_HOST
pragma	0xA00C	SC_REQ_PRAGMA
referer	0xA00D	SC_REQ_REFERER
user-agent	0xA00E	SC_REQ_USER_AGENT

The Java code that reads this grabs the first two-byte integer and if it sees an '0xA0' in the most significant byte, it uses the integer in the second byte as an index into an array of header names. If the first byte is not '0xA0', it assumes that the two-byte integer is the length of a string, which is then read in.

This works on the assumption that no header names will have length greater than 0x9999 (==0xA000 - 1), which is perfectly reasonable, though somewhat arbitrary. (If you, like me, started to think about the cookie spec here, and about how long headers can get, fear not -- this limit is on header names not header values. It seems unlikely that unmanageably huge header names will be showing up in the HTTP spec any time soon).

Note: The content-length header is extremely important. If it is present and non-zero, the container assumes that the request has a body (a POST request, for example), and immediately reads a separate packet off the input stream to get that body.

The attributes prefixed with a ? (e.g. ?context) are all optional. For each, there is a single byte code to indicate the type of attribute, and then a string to give its value. They can be sent in any order (thogh the C code always sends them in the order listed below). A special terminating code is sent to signal the end of the list of optional attributes. The list of byte codes is:

Information	Code Value	Note
?context	0x01	Not currently implemented
?servlet_path	0x02	Not currently implemented
?remote_user	0x03
?auth_type	0x04
?query_string	0x05
?route	0x06
?ssl_cert	0x07
?ssl_cipher	0x08
?ssl_session	0x09
?req_attribute	0x0A	Name (the name of the attribut follows)
?ssl_key_size	0x0B
?secret	0x0C
?stored_method	0x0D
are_done	0xFF	request_terminator

The context and servlet_path are not currently set by the C code, and most of the Java code completely ignores whatever is sent over for those fields (and some of it will actually break if a string is sent along after one of those codes). I don't know if this is a bug or an unimplemented feature or just vestigial code, but it's missing from both sides of the connection.

The remote_user and auth_type presumably refer to HTTP-level authentication, and communicate the remote user's username and the type of authentication used to establish their identity (e.g. Basic, Digest). I'm not clear on why the password isn't also sent, but I don't know HTTP authentication inside and out.

The query_string, ssl_cert, ssl_cipher, and ssl_session refer to the corresponding pieces of HTTP and HTTPS.

The route, as I understand it, is used to support sticky sessions -- associating a user's sesson with a particular Tomcat instance in the presence of multiple, load-balancing servers. I don't know the details.

Beyond this list of basic attributes, any number of other attributes can be sent via the req_attribute code (0x0A). A pair of strings to represent the attribute name and value are sent immediately after each instance of that code. Environment values are passed in via this method.

Finally, after all the attributes have been sent, the attribute terminator, 0xFF, is sent. This signals both the end of the list of attributes and also then end of the Request Packet.

The chunk is basically binary data, and is sent directly back to the browser.

The status code and message are the usual HTTP things (e.g. "200" and "OK"). The response header names are encoded the same way the request header names are. See above for details about how the the codes are distinguished from the strings. The codes for common headers are:

Name	Code value
Content-Type	0xA001
Content-Language	0xA002
Content-Length	0xA003
Date	0xA004
Last-Modified	0xA005
Location	0xA006
Set-Cookie	0xA007
Set-Cookie2	0xA008
Servlet-Engine	0xA009
Status	0xA00A
WWW-Authenticate	0xA00B

After the code or the string header name, the header value is immediately encoded.

Signals the end of this request-handling cycle. If the reuse flag is true (==1), this TCP connection can now be used to handle new incoming requests. If reuse is false (anything other than 1 in the actual C code), the connection should be closed.

The container asks for more data from the request (If the body was too large to fit in the first packet sent over or when the request is chuncked). The server will send a body packet back with an amount of data which is the minimum of the request_length, the maximum send body size (8186 (8 Kbytes - 6)), and the number of bytes actually left to send from the request body.
If there is no more data in the body (i.e. the servlet container is trying to read past the end of the body), the server will send back an "empty" packet, which is a body packet with a payload length of 0. (0x12,0x34,0x00,0x00)