Using XML in the MASP Client-Server Protocol

Mark A. Jones, Tony L. Hansen
AT&T Labs - Research, Florham Park, NJ; AT&T Labs - Lincroft, NJ


The strength of ASCII protocols for network services such as SMTP [1], NNTP [2], and IMAP [3], is their relative simplicity for debugging, trussing, etc. On the other hand, an undesirable hallmark is their invention of unique syntaxes for specifying requests and replies -- particularly in their conventions for quoting metacharacters, dealing with line continuations, encoding binary data, handling error conditions, etc. XML (Extensible Markup Language 1.0, [4]) is often viewed as encoding domain-specific data payloads over a protocol such as HTTP [6] [7] [8], but not as the protocol substrate itself. This paper presents our experience with MASP (Mediated Attribute Store Protocol), a simple, synchronous, fully XML, client-server protocol.

The MASP Protocol

The important states in MASP are:

  1. the initiation and successful completion of a connection from the client to the server to form a session
  2. the repeated submission of a client requests for service and server responses
  3. the termination of the session and the connection between the client and the server

From the client side, the protocol document begins:

<?xml version="1.0"?>

The server side is similar. A session is closed with the appropriate </client-session> and </server-session> end tags. Although arbitrary markup can represent the requests and responses, we have found the following conventions to be valuable:

  1. Each client request tag such as <search> is paired with either a server response tag such as <search-response> or by an <error-response> tag.
  2. Each client request tag includes a unique id attribute which is also carried in the corresponding server response. The id provides greater security in associating responses, even in a synchronous protocol.
  3. The XML mechanism of "CDATA sections" can handle arbitrary character data. For binary data, the MASP EDATA tag was introduced with an encoding attribute (base64, quoted-printable, url and hex).

The following is an example of a client search request and a successful server response. Note the use of attribute value indexing. The ix attribute references previous name attributes by index.

<search id='1'>                <!-- client request -->
 <typedecl>user$u</typedecl> <filter><![CDATA[(last_name[$u]='Burnes')]]></filter>
 <select name='face[$u]'/>

<search-response id='1'>       <!-- server response -->
  <results count='2'>
    <ids> <id>hermod0000000102</id> </ids>
    <attrvals> <val ix='0' name='face[$u]'><EDATA encoding='qp'>GIF87a=01=00=01=00=80=00=00=95=76=81=00
=00=00=2c=00=00=00=00=01=00=01=00=00=02=02=44=01=00=3b=00</EDATA></val> </attrvals>
    <ids> <id>hermod0000000324</id> </ids>
    <attrvals> <val ix='0'><EDATA encoding='qp'>GIF87a=01=00=01=00=80=00=00=95=76=81=00=00=00=4e=00
=00=00=00=01=00=01=00=00=02=02=25=09=00=3b=00</EDATA></val> </attrvals>

MASP also supports complex multi-turn protocols such as SASL [5] authentication mechanisms. XML debugging comments can be observed with a tool such as the Unix truss utility without affecting the protocol operations. Syntax errors, semantic errors, resource failures, etc. cause the server to return an appropriate <error-response>, which includes a indication of permanence, an errorcode, and an error message. For example:

<error-response id='1' permanence='permanent' errorcode='5'>
<![CDATA[Error parsing : parse error, column 22: '!'Bur...']]>


MASP is an entirely XML-based client-server protocol whose extensions and conventions form a very useful protocol substrate. XML offers a standard set of mechanisms for representing structured data, and there are many high-quality XML parsers that are now available. DTD's (or XML schemas) present a clear picture of the client and server protocol syntax, and, especially with a validating parser, can enforce very precise syntactic requirements. Modifying the DTD's, changing a dispatch table in the code, and testing a new feature/command is easier than modifying ad hoc parsing code or a YACC grammar.

Most of the features that we have described for turn-taking, escaping and encoding mechanisms, error handling, attribute indexing, debugging and session management would be generally useful for many protocols. A longer version of this paper can be found at


  1. Simple Mail Transfer Protocol, RFC 821,
  2. Network News Transfer Protocol, RFC 977,
  3. Internet Message Access Protocol, RFC 2060,
  4. Extensible Markup Language (XML) 1.0,
  5. Simple Authentication and Security Layer (SASL), RFC 2222,
  6. The Information and Content Exchange (ICE) Protocol,
  7. XML-RPC,
  8. SOAP: Simple Object Access Protocol,


Mark Jones is a researcher at AT&T Labs. He works on information modeling, artificial intelligence, natural language processing and machine learning, particularly as these fields apply to messaging systems. Tony Hansen is a developer at AT&T Labs. He works on messaging systems, web server systems and Internet standards.