\section{Compact Form}
\label{sec:compact}

When SIP is carried over UDP with authentication and a complex session
description, it may be possible that the size of a request or response is
larger than the MTU.  To address this problem, a more compact form of SIP
is also defined by using abbreviations for the common header fields
listed below:

\begin{flushleft}
\begin{tabular}{lll}
short field name & long field name & note\\
\header{c} & \header{Content-Type}     & \\
\header{e} & \header{Content-Encoding} & \\
\header{f} & \header{From}             & \\
\header{i} & \header{Call-ID}          & \\
\header{k} & \header{Supported}        & from ``know''\\
\header{l} & \header{Content-Length}   & \\
\header{m} & \header{Contact}          & from ``moved''\\
\header{s} & \header{Subject}          & \\
\header{t} & \header{To}               & \\
\header{v} & \header{Via}              & \\
\end{tabular}
\end{flushleft}

Thus, the message in section \ref{sec:invite-example} could also be
written:

\begin{verbatim}
  INVITE sip:bob@example.com SIP/2.0
  v:SIP/2.0/UDP 131.215.131.131;maddr=239.128.16.254;ttl=16
  v:SIP/2.0/UDP 216.112.6.38
  f:sip:alice@wonderland.com
  t:sip:bob@example.com
  m:sip:alice@mouse.wonderland.com
  i:62729-27@216.112.6.38
  c:application/sdp
  CSeq: 4711 INVITE
  l:187

  v=0
  o=user1 53655765 2353687637 IN IP4 128.3.4.5
  s=Mbone Audio
  i=Discussion of Mbone Engineering Issues
  e=mbone@somewhere.com
  c=IN IP4 224.2.0.1/127
  t=0 0
  m=audio 3456 RTP/AVP 0
  a=rtpmap:0 PCMU/8000
\end{verbatim}

Clients {\MAY} mix short field names and long field names within the
same request.  Servers {\MUST} accept both short and long field names
for requests.  Proxies {\MAY} change header fields between their long
and short forms, but this {\MUSTNOT} be done to fields following an
\header{Authorization} header.
