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osmo-sgsn: Explain TCP/IP header compression 

The does not mention TCP/IP header compression yet. This
commit adds some info about it

Change-Id: I98408e72020a474d378e39263a933eb7567de146
This commit is contained in:
Philipp Maier 2017-03-09 12:28:44 +01:00 committed by Harald Welte
parent dd412cb33c
commit 826c6eb086
1 changed files with 80 additions and 0 deletions
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OsmoSGSN/chapters/configuration.adoc
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@ -223,3 +223,83 @@ individual fields of each CSV line.
|pdp-terminate|Forced PDP context termination during MM context release
|pdp-free|Release of the PDP context memory
|===
=== User traffic compression
In order to save optimize GPRS bandwith, OsmoSGSN implements header and data
compression schemes. The compression will reduce the packet length in order
to save radio bandwith.
==== Header compression
On TCP/IP connections, each packet is prepended with a fairly long TCP/IP
header. The header contains a lot of static information that never changes
throughout the connection. (source and destination address, port numbers etc.)
OsmoSGSN implements a TCP/IP header compression scheme called RFC1144, also
known as SLHC. This type of header compression removes the TCP/IP header
entirely and replaces it with a shorter version, that only contains the
information that is absolutely necessary to identify and check the packet.
The receiving part then restores the original header and forwards it to higher
layers.
*compression rfc1144 passive*::
TCP/IP header compression has to be actively requested by the modem. The
network will not promote compression by itself. This is the recommended mode
of operation.
*compression rfc1144 active slots <1-256>*::
TCP/IP header compression is actively promoted by the network. Modems may still
actively request different compression parameters or reject the offered
compression parameters entirely. The number of slots is the maximum number
of packet headers per subscriber that can be stored in the codebook.
.Example: Accept compression if requested:
----
sgsn
compression rfc1144 passive
----
.Example: Actively promote compression:
----
sgsn
compression rfc1144 active slots 8
----
NOTE: The usage of TCP/IP options may disturb the RFC1144 header compression
scheme. TCP/IP options may render RFC1144 ineffective if variable data is
encoded into the option section of the TCP/IP packet. (e.g. TCP option 8,
Timestamp)
==== Data compression
Data compression works on the raw packet data, including the header part of the
packet. If enabled, header compression is applied before first data compression
is applied. OsmoSGSN implements the V.42bis data compression scheme.
*compression rfc1144 passive*::
V42bis data compression has to be actively requested by the modem. The network
will not promote compression by itself. This is the recommended mode of
operation.
*compression v42bis active direction (ms|sgsn|both) codewords <512-65535> strlen <6-250>*::
V42bis data compression is actively promoted by the network. Modems may still
actively request different compression parameters or reject the offered
compression parameters entirely. The direction configures which sides are
allowed to send compressed packets. For most cases, compressing 'both'
directions will be the preferred option. The following to parameters configure
the codebook size by the maxium number ('codewords') and size ('strlen') of
entries.
.Example: Accept compression if requested:
----
sgsn
compression v42bis passive
----
.Example: Actively promote compression:
----
sgsn
compression v42bis active direction both codewords 512 strlen 20
----