This describes the Apache JServ Protocol version 1.3 (hereafter ajp13). There is, apparently, no current documentation of how the protocol works. This document is an attempt to remedy that, in order to make life easier for maintainers of mod_jk, and for anyone who wants to port the protocol somewhere (into jakarta 4.x, for example).
Who Am I?
I am not one of the designers of this protocol -- I believe that Gal
Shachor was the original designer. Everything in this document is derived
from the actual implementation I found in the tomcat 3.x code. I hope it
is useful, but I can't make any grand claims to perfect accuracy. I also
don't know why certain design decisions were made. Where I was able, I've
offered some possible justifications for certain choices, but those are
only my guesses. In general, the C code which Shachor wrote is very clean
and comprehensible (if almost totally undocumented). I've cleaned up the
Java code, and I think it's reasonably readable.
Design Goals
According to email from Gal Shachor to the jakarta-dev mailing list,
the original goals of mod_jk (and thus ajp13) were to extend
mod_jserv and ajp12 by (I am only including the goals which
relate to communication between the web server and the servlet container):
isSecure() and
geScheme() will function correctly within the servlet
container. The client certificates and cipher suite will be
available to servlets as request attributes.
Overview
The ajp13 protocol is packet-oriented. A binary format was
presumably chosen over the more readable plain text for reasons of
performance. The web server communicates with the servlet container over
TCP connections. To cut down on the expensive process of socket creation,
the web server will attempt to maintain persistent TCP connections to the
servlet container, and to reuse a connection for multiple request/response
cycles.
Once a connection is assigned to a particular request, it will not be used for any others until the request-handling cycle has terminated. In other words, requests are not multiplexed over connections. This makes for much simpler code at either end of the connection, although it does cause more connections to be open at once.
Once the web server has opened a connection to the servlet container, the connection can be in one of the following states:
Once a connection is assigned to handle a particular request, the basic request informaton (e.g. HTTP headers, etc) is sent over the connection in a highly condensed form (e.g. common strings are encoded as integers). Details of that format are below in Request Packet Structure. If there is a body to the request (content-length > 0), that is sent in a separate packet immediately after.
At this point, the servlet container is presumably ready to start processing the request. As it does so, it can send the following messages back to the web server:
There is a bit of an XDR heritage to this protocol, but it differs in lots of ways (no 4 byte alignment, for example).
Byte order: I am not clear about the endian-ness of the individual bytes. I'm guessing the bytes are little-endian, because that's what XDR specifies, and I'm guessing that sys/socket library is magically making that so (on the C side). If anyone with a better knowledge of socket calls can step in, that would be great.
There are four data types in the protocol: bytes, booleans, integers and strings.
strlen. This is a touch
confusing on the Java side, which is littered with odd autoincrement
statements to skip over these terminators. I believe the reason this was
done was to allow the C code to be extra efficient when reading strings
which the servlet container is sending back -- with the terminating \0
character, the C code can pass around references into a single buffer,
without copying. If the \0 was missing, the C code would have to copy
things out in order to get its notion of a string.
Packet Size
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.
Packet Headers
The servlet container can send the following types of messages to the web
server:
Each of the above messages has a different internal structure, detailed below.
For messages from the server to the container of type "Forward Request":
Details of above
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
The
The
The
Beyond this list of basic attributes, any number of other attributes can
be sent via the
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 Packets as a whole.
For messages which the container can send back to the server.
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, whch is a body packet with a payload length of 0.
What happens if the request headers > max packet size? There is no
provision to send a second packet of request headers in case there are more
than 8K (I think this is correctly handled for response headers, though I'm
not certain). I don't know if there is a way to get more than 8K worth of
data into that initial set of request headers, but I'll bet there is
(combine long cookies with long ssl information and a lot of environment
variables, and you should hit 8K easily). I think the connector would just
fail before trying to send any headers in this case, but I'm not certain.
What about authentication? There doesn't seem to be any authentication
of the connection between the web server and the container. This strikes
me as potentially dangerous.
Packets sent from the server to the container 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
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:
Packet Format (Container->Server)
Byte
0
1
2
3
4...(n+3)
Contents
A
B
Data Length (n)
Data
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.
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.
Request Packet Structure
AJP13_FORWARD_REQUEST :=
prefix_code 2
method (byte)
protocol (string)
req_uri (string)
remote_addr (string)
remote_host (string)
server_name (string)
server_port (integer)
is_ssl (boolean)
num_headers (integer)
request_headers *(req_header_name req_header_value)
?context (byte string)
?servlet_path (byte string)
?remote_user (byte string)
?auth_type (byte string)
?query_string (byte string)
?jvm_route (byte string)
?ssl_cert (byte string)
?ssl_cipher (byte string)
?ssl_session (byte string)
?attributes *(attribute_name attribute_value)
request_terminator (byte)
req_header_name :=
sc_req_header_name | (string) [see below for how this is parsed]
sc_req_header_name := 0xA0 (byte)
req_header_value := (string)
attribute_name := (string)
attribute_value := (string)
request_terminator := 0xFF
Not that the all-important header is "content-length', because it
determines whether or not the container looks for another packet
immediately.
The server can also send a
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
accept 0xA001
accept-charset 0xA002
accept-encoding 0xA003
accept-language 0xA004
authorization 0xA005
connection 0xA006
content-type 0xA007
content-length 0xA008
cookie 0xA009
cookie2 0xA00A
host 0xA00B
pragma 0xA00C
referer 0xA00D
user-agent 0xA00E
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.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.?
(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:
context 1 [Not currently implemented]
servlet_path 2 [Not currently implemented]
remote_user 3
auth_type 4
query_string 5
jvm_route 6
ssl_cert 7
ssl_cipher 8
ssl_session 9
req_attribute 10
terminator 0xFF
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.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. query_string, ssl_cert,
ssl_cipher, and ssl_session refer to the
corresponding pieces of HTTP and HTTPS.jvm_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. req_attribute code (10). 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. shutdown packet. 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.
Response Packet Structures
AJP13_SEND_BODY_CHUNK :=
prefix_code 3
chunk_length (integer)
chunk *(byte)
AJP13_SEND_HEADERS :=
prefix_code 4
http_status_code (integer)
http_status_msg (string)
num_headers (integer)
response_headers *(res_header_name header_value)
res_header_name :=
sc_res_header_name | (string) [see below for how this is parsed]
sc_res_header_name := 0xA0 (byte)
header_value := (string)
AJP13_END_RESPONSE :=
prefix_code 5
reuse (boolean)
AJP13_GET_BODY_CHUNK :=
prefix_code 6
requested_length (integer)
Details:
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.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.request_length, the maximum send body size (XXX), and the
number of bytes actually left to send from the request body.
Questions I Have