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\title{DHCPv6 Options for SIP Servers}
\author{H. Schulzrinne/B. Volz}
\shorttitle{}
\filename{draft-ietf-sip-dhcpv6-01}
\wg{SIP WG}
\address{Columbia University/Ericsson}
\pubdate{\today}
\expires{April 2003}

\begin{document}
\maketitle

\begin{abstract}
This document defines a DHCPv6 option that contains a list of domain
names or IPv6 addresses that can be mapped to one or more SIP outbound
proxy servers.  This is one of the many methods that a SIP client can
use to obtain the addresses of such a local SIP server.
\end{abstract}

\section{Terminology}
\label{sec:terminology}

This document uses the DHCP terminology defined in
\citenorm{draft-ietf-dhc-dhcpv6}.

A SIP server is defined in RFC 3261 \citenorm{rfc3261}.  This server {\MUST}
be an outbound proxy server, as defined in \citenorm{rfc3263}. 
In the context of this document, a SIP server refers to the host the SIP
server is running on.

A SIP client is defined in RFC 3261 \citenorm{rfc3261}.  The client can be a
user agent client or the client portion of a proxy server.  In the
context of this document, a SIP client refers to the host the SIP client
is running on.

In this document, the key words ``{\MUST}'', ``{\MUSTNOT}'',
``{\REQUIRED}'', ``{\SHALL}'', ``{\SHALLNOT}'', ``{\SHOULD}'',
``{\SHOULDNOT}'', ``{\RECOMMENDED}'', ``{\MAY}'', and ``{\OPTIONAL}''
are to be interpreted as described in RFC 2119 \citenorm{rfc2119}.

\section{Introduction}

The Session Initiation Protocol (SIP) \citenorm{rfc3261} is an
application-layer control protocol that can establish, modify and
terminate multimedia sessions or calls.  A SIP system has a number of
logical components:  user agents, proxy servers, redirect servers and
registrars.  User agents {\MAY} contain SIP clients, proxy servers
always do.

This draft specifies two DHCPv6 options \citenorm{draft-ietf-dhc-dhcpv6}
that allow SIP clients to locate a local SIP server that is to be used
for all outbound SIP requests, a so-called outbound proxy server.  (SIP
clients {\MAY} contact the address identified in the SIP URL directly,
without involving a local SIP server.  However in some circumstances,
such as when firewalls are present, or local dialing plans, local emergency
and other services need to be provided, SIP clients need to use a local
server for outbound requests.) This is one of many possible solutions
for locating the outbound SIP server; manual configuration is an example
of another.

\section{SIP Server DHCPv6 Option}
\label{sec:sipn}

This document defines two DHCPv6 options that describe a local outbound
SIP proxy:  one carries a list of domain name (Section~\ref{sec:dnl},
the other a list of 128-bit (binary) IPv6 addresses
(Section~\ref{sec:ipl}).

\motivation{Since DHCPv6 does not suffer from a shortage of option
codes, we avoid the encoding byte found in the IPv4 DHCP option for SIP
servers \citenonnorm{rfc3361}.  This makes the option shorter,
easier to parse, simplifies appropriate word alignment for the numeric
addresses and allows the client to request either numeric or domain name
options using the ``option request option''.}

An implementation implementing this specification {\MUST} support both
options.

\subsection{SIP Servers Domain Name List}
\label{sec:dnl}

The option length is followed by a sequence of labels, encoded according
to Section 3.1 of RFC 1035 \citenorm{rfc1035}, quoted below:

\begin{quotation}
"Domain names in messages are expressed in terms of a sequence of labels. 
Each label is represented as a one octet length field followed by that
number of octets.  Since every domain name ends with the null label of
the root, a domain name is terminated by a length byte of zero.  The
high order two bits of every length octet must be zero, and the
remaining six bits of the length field limit the label to 63 octets or
less.  To simplify implementations, the total length of a domain name
(i.e., label octets and label length octets) is restricted to 255 octets
or less."
\end{quotation}

\motivation{RFC 1035 encoding was chosen to accommodate future
internationalized domain name mechanisms.}

The option {\MAY} contain multiple domain names, but these {\SHOULD}
refer to different NAPTR records, rather than different A records.  The
client {\MUST} try the records in the order listed, applying the
mechanism described in Section 4.1 of RFC 3263 \citenorm{rfc3263} for each. 
The client only resolves the subsequent domain names if attempts to
contact the first one failed or yielded no common transport protocols
between client and server or denote a domain administratively prohibited
by client policy.  Domain names {\MUST} be listed in order of
preference.

\motivation{Use of multiple domain names is not meant to replace NAPTR
or SRV records, but rather to allow a single DHCP server to indicate
outbound proxy servers operated by multiple providers.}

The DHCPv6 option has the format shown in Fig.~\ref{fig:domainlist}.

Clients {\MUST} support compression according to the encoding in Section
4.1.4 of ``Domain Names - Implementation And Specification''
\citenorm{rfc1035}. The offset here specifies an offset from the start of
the SIP Server Domain List (see Fig.~\ref{fig:domainlist}):

\begin{figure}[htpb]
\verbatimfile{domainlist.txt}
\caption{DHCPv6 option for SIP Server Domain List}
\label{fig:domainlist}
\end{figure}

\begin{description}
\item[option-code:] OPTION\_SIP\_SERVER\_D (TBD)

\item[option-length:] Length of the 'SIP Server Domain Name List' field 
in octets; variable.

\item[SIP Server Domain List:] The domain names of the SIP outbound
proxy servers for the client to use.  The domain names are encoded as
specified in section ``Representation and use of domain names'' of the
DHCPv6 specification \citenorm{draft-ietf-dhc-dhcpv6}.

\end{description}

\subsection{SIP Servers IPv6 Address List}
\label{sec:ipl}

This option specifies a list of IPv6 addresses indicating SIP outbound
proxy servers available to the client.  Servers {\MUST} be listed in
order of preference.

\begin{verbatim}
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      OPTION_SIP_SERVER_A      |           option-len          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                   SIP server (IP address)                     |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                   SIP server (IP address)                     |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\end{verbatim}

\begin{description}
\item[option-code:] OPTION\_SIP\_SERVER\_A (TBD)

\item[option-length:] Length of the 'options' field in octets; must be a
multiple of 16.

\item[SIP server:] IPv6 address of a SIP server for the client to use. 
The servers are listed in the order of preference for use by the client.
\end{description}

\section{Client Operation}
\label{sec:client}

A client may request either or both of the SIP Servers Domain Name
List and SIP Servers IPv6 Address List options in an Options Request
Option (ORO) as described in \citenorm{draft-ietf-dhc-dhcpv6},

If a client receives both the SIP Servers Domain Name List and SIP
Servers IPv6 Address List options, it {\SHOULD} use the SIP Servers
Domain Name List option.  Only if no server in the SIP Servers Domain
Name List can be resolved or reached, the client {\MAY} use the SIP
Servers IPv6 Address List option.

\section{Server Operation}
\label{sec:server}

A server {\MAY} send a client one or both of the SIP Servers Domain Name
List and SIP Servers IPv6 Address List options. 

If a client requests both options and the server is configured for both,
the server {\MAY} send a client only one of these options and {\SHOULD}
send the SIP Servers Domain Name List. 

A server configured with the SIP Servers IPv6 Address List option
{\MUST} send a client the SIP Servers IPv6 Address List option if that
client requested the SIP Servers IPv6 Address List option and not the
SIP Servers Domain Name List option in an ORO (see
\citenorm{draft-ietf-dhc-dhcpv6}).

The following table summarizes the server's response: 

\begin{flushleft}
\begin{tabular}{lll}
Client sends in ORO           & Domain Name List & IPv6 Address List\\
\hline
Neither option                & {\SHOULD} & {\MAY}\\
SIP Servers Domain Name List  & {\SHOULD} & {\MAY}\\
SIP Servers IPv6 Address List & {\MAY} & {\MUST}\\
Both options                  & {\SHOULD} & {\MAY}\\
\end{tabular}
\end{flushleft}

\section{Security Consideration}
\label{sec:security}

The security considerations in RFC XXXX
\citenorm{draft-ietf-dhc-dhcpv6}, RFC 3261 \citenorm{rfc3261} and RFC
3263 \citenorm{rfc3263} apply.  If an adversary manages to modify the
response from a DHCP server or insert its own response, a SIP user agent
could be led to contact a rogue SIP server, possibly one that then
intercepts call requests or denies service.  A modified DHCP answer
could also omit host names that translated to TLS-based SIP servers,
thus facilitating intercept.

\section{IANA Considerations}

IANA has assigned a DHCPv6 option number of TBD for the ``SIP Servers
Domain Name List'' and the DHCPv6 option number of TBD for the ``SIP
Servers IPv6 Address List'' defined in this document.

\section{Acknowledgements}

Erik Nordmark and Alex Zinin provided helpful comments.

\section{Authors' Addresses}
\begin{flushleft}
Henning Schulzrinne\\
Dept. of Computer Science\\
Columbia University\\
1214 Amsterdam Avenue, MC 0401\\
New York, NY 10027\\
USA\\
electronic mail: \textsf{schulzrinne@cs.columbia.edu}\\
\end{flushleft}

\begin{flushleft}
Bernie Volz\\
Ericsson\\
959 Concord Street\\
Framingham, MA  01701\\
USA\\
electronic mail: \textsf{Bernie.Volz@am1.ericsson.se}\\
\end{flushleft}

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