RFC 3408 (rfc3408) - Page 2 of 7
Zero-byte Support for Bidirectional Reliable Mode (R-mode) in Extended Link-Layer Assisted RObust Header Compression (ROHC) Profile
Alternative Format: Original Text Document
RFC 3408 0-byte Support for R-mode December 2002
For simplification, this profile is defined in the form of the
additions and exceptions to [RFC 3242] that are required to extend the
RFC 3242 profile with zero-byte support for R-mode. All terminology
used in this document is the same as in [RFC 3242].
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 BCP 14, RFC 2119
[RFC 2119].
2. Extensions to the assisting layer (AL) interface
This section describes additions (some are optional) to the assisting
layer interface as defined in [RFC 3242, section 4.2].
2.1. Additional parameters to the compressor to AL interface
- Mode, indicating the mode in which the compressor is operating.
The AL has slightly different logic depending on the mode value.
- SN_ACKed, indicating the latest RTP SN that has been acknowledged.
It is used only when Mode value = R-mode.
Note that these two parameters MUST always be attached to every
packet delivered to the AL.
2.2. Additional interface, assisting layer to compressor
To improve the compression efficiency of this profile in some
specific cases, e.g., when the AL operates in such a way that it
often becomes unsafe to send NHPs, it is RECOMMENDED to implement
this additional interface. Here, the word "unsafe" means that the
compressor allows the AL to send NHP but the AL cannot guarantee that
the RTP SN of the NHP will be correctly decompressed at the receiving
side. The interface is used to carry update_request as described in
section 3. Note that this interface is not required in the sense
that the impossibility of implementing such an interface should not
be an obstacle to implement this profile over a specific link.
3. R-mode operation
For the R-mode, this profile extends ROHC RTP by performing a mapping
of the R-0 packet to the NHP packet. Note that R-0 is the only type
of packets in R-mode that can be replaced with NHP.
On the receiving side, the RTP SN of an NHP is determined by the
decompressor as = SN_Ref_D + Offset_D, where SN_Ref_D is the RTP SN
of the last update packet received by the decompressor, and Offset_D
Liu & Le Standards Track