Internet DRAFT - draft-ozegovic-rohc-fn-rtp
draft-ozegovic-rohc-fn-rtp
Network Working Group Julije Ozegovic, University of Split
INTERNET-DRAFT Mario Mornar, Croatian Telecom
Expires: January 1, 2004
July 1, 2003
RTP Profile in ROHC-FN
draft-ozegovic-rtp-rohc-fn-00.txt
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This draft describes a profile for compressing RTP/IP using the
Robust Header Compression Formal Notation.
The RObust Header Compression [1] scheme is designed to compress
packet headers over error prone channels. It is built around an
extensible core framework that can be tailored to compress new
protocol stacks by adding additional ROHC profiles.
This draft describes a modification of previously published RTP
profiles for ROHC to conform to ROHC-FN [2] and Prolog.
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Note
This is a preliminary profile. It is documented in this draft for
two main reasons.
1. To encourage discussion regarding the compression specification
for RTP in ROHC-FN
2. To explore the range of protocols that the Formal Notation can be
used to compress
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . 3
3. RTP compression in ROHC formal notation. . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . 7
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . 7
References . . . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . 8
Full Copyright Statement . . . . . . . . . . . . . . . . 9
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1. Introduction
This document presents a profile for compressing RTP/UDP/IP. The
Real Time Protocol [3] is in focus of interest for header compression
and its compression profile is elaborated in [1] using text and box
notation. The profile proposed here is based on [4], where RTP profile
for EPIC [5] was described.
The behavior of RTP/UDP/IP fields is described in details in [1] and
[4].
Presented profile is adopted for ROHC-FN and Prolog language for
verification.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119.
Control field
Control fields are transmitted from a ROHC compressor to a ROHC
decompressor, but are not part of the uncompressed protocol header
itself. An example is a checksum field over the header to ensure
robustness against bit errors and dropped packets.
Encoding method
Encoding methods are functions that can be applied to compress
fields in a protocol header. ROHC-FN includes a library of
commonly used encoding methods.
Field
ROHC-FN divides the protocol to be compressed into a set of
contiguous bit patterns known as fields.
Library of encoding methods
The library of encoding methods contains a number of commonly used
encoding methods for compressing header fields.
Profile
A ROHC [RFC-3095] profile is a description of how to compress a
certain protocol stack over a certain type of link. Each profile
includes packet formats to compress the headers and a state machine
to control the actions of each endpoint.
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3. RTP compression in ROHC formal notation
RTP profile presented is adapted to the possibilities of current Prolog
test program. The minor drawbacks are:
- UDP length is not inferred
- NBO is not used
- CRC is not implemented, zero bits inserted
- scale uses explicit scale factor (160)
- list is shortened for 8 CSRCs, to avoid 64 bit order field
rtp_udp_ip_header --> ip_header++
udp_header++
rtp_header++
master_seq_number++
header_crc.
ip_header --> inferred_ip_checksum++
inferred_size(16,-16)++
ip_version++
ip_header_length++
ip_tos++
ip_length++
ip_id++
ip_reserved++
ip_dont_fragment++
ip_more_fragments++
ip_offset++
ip_time_to_live++
ip_protocol++
ip_checksum++
ip_source_address++
ip_dest_address.
udp_header --> %inferred_size(16,-32)++
udp_source_port++
udp_dest_port++
udp_size++
udp_checksum.
rtp_header --> rtp_version++
rtp_padding++
rtp_extension++
rtp_csrc_count++
rtp_marker++
rtp_payload++
rtp_sn++
rtp_ts++
rtp_ssrc++
rtp_csrc.
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master_seq_number --> next_field(msn)++
lsb(2,0) #90 / irregular(16) #10.
header_crc --> map(current_field,nil,0)++
irregular(3).
%*******************************************************************************
% IP header
%*******************************************************************************
ip_version --> value(4,4).
ip_header_length --> value(4,5).
ip_tos --> static // irregular(8).
ip_length --> skip(16).
% NBO is not implemented yet
ip_id --> lsb(8,0) #90 / irregular(16) #10.
% With NBO the field should be compressed like below
%
% nbo (16)++
% next_field(nbo_flag)++
% static // irregular(1)++
% next_field(nbo_field)++
% static / lsb(5,-1) // irregular(16).
ip_reserved --> value(1,0).
ip_dont_fragment --> static #99 / irregular(1) #1.
ip_more_fragments --> value(1,0).
ip_offset --> value(13,0).
ip_time_to_live --> static #99 / irregular(8) #1.
ip_protocol --> value(8,17).
ip_checksum --> skip(16).
ip_source_address --> static // irregular(32).
ip_dest_address --> static // irregular(32).
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%*******************************************************************************
% UDP header
%*******************************************************************************
udp_source_port --> static // irregular(16).
udp_dest_port --> static // irregular(16).
%inferred size works properly at the beginning of the header only
udp_size --> irregular(16).
%udp_size --> skip(16).
udp_checksum --> value(16,0) #99 / irregular(16) #1.
%*******************************************************************************
% RTP header
%*******************************************************************************
rtp_version --> value(2,2).
rtp_padding --> static // irregular(1).
rtp_extension --> static // irregular(1).
rtp_csrc_count --> label(4,csrc_count).
rtp_marker --> irregular(1).
rtp_payload --> static // irregular(7).
rtp_sn --> label(16,msn).
% Scale ***** version with FIXED SCALE FACTOR *******!
rtp_ts --> scale(32,160)++
next_field(scaled_field)++
rtp_scaled++
next_field(remainder_field)++
rtp_reminder.
rtp_scaled --> lsb(2,0) // irregular(32).
rtp_reminder --> static // irregular(32).
%profile builder not powerful enough to chew this:
%rtp_scaled --> lsb(2,0) #90 / irregular(32) #10.
%rtp_reminder --> static #90 / irregular(32) #10.
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rtp_ssrc --> static // irregular(32).
rtp_csrc --> list(csrc_count,1,32,0,
[optional(csrc_entry),
optional(csrc_entry),
optional(csrc_entry),
optional(csrc_entry),
optional(csrc_entry),
optional(csrc_entry),
optional(csrc_entry),
% optional(csrc_entry),
% optional(csrc_entry),
% optional(csrc_entry),
% optional(csrc_entry),
% optional(csrc_entry),
% optional(csrc_entry),
% optional(csrc_entry),
% optional(csrc_entry),
optional(csrc_entry)])++
csrc_options_order++
csrc_options_presence.
% 16 bit presence field means a 64 bit order field. We have 32 bits in Prolog
csrc_options_order --> next_field(order)++
% static // irregular(64).
static // irregular(24).
csrc_options_presence --> next_field(presence)++
% static // irregular(16).
static // irregular(8).
csrc_entry --> irregular(32).
4. Security Considerations
This draft describes a profile for the compression and decompression
of SCTP/IPv4 headers.
See [1] for the security issues raised by robust header compression.
See [2] for the security issues raised by the use of the Formal
Notation.
5. Acknowledgements
The authors would like to acknowledge the many people who have helped
with issues relating to SCTP, ROHC and the formal notation. These
stalwarts include Carsten Bormann, Max Riegel, Christian Schmidt,
Michael Tuexen, Richard Price, Rob Hancock, and Stephen McCann.
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References
[1] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H.,
Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le, K., Liu,
Z., Martensson, A., Miyazaki, A., Svanbro, K., Wiebke, T.,
Yoshimura, T. and H. Zheng, "RObust Header Compression (ROHC):
Framework and four profiles: RTP, UDP, ESP, and uncompressed",
RFC 3095, July 2001.
[2] Price, R., Surtees, A. and M. West, "Formal Notation for Robust
Header Compression ROHC-FN", draft-ietf-rohc-formal-notation-
01.txt (work in progress), March 2003.
[3] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP:
A Transport Protocol for Real-Time Applications", RFC 1889,
January 1996.
[4] A. Surtees, M. West, "RTP Profile for EPIC",
draft-surtees-rtp-epic-00.txt, February 2002.
[5] Price, R., Hancock, R., Ollis, P., Surtees, A. and M. West, "Framework
for EPIC-lite", draft-ietf-rohc-epic-lite-01.txt, February 2002.
Authors' Addresses
Julije Ozegovic
University of Split
FESB Split
R. Boskovica bb
21000 SPLIT
Croatia
Phone: +385 (0)21 305725
EMail: julije@fesb.hr
Mario Mornar
Croatian telecom, TKC Split
Sinjska 5
21000 SPLIT
Croatia
Phone: +385 (0)21 351063
EMail: mario.mornar@ht.hr
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Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
This Internet-Draft will expire on January 1, 2004.
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