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doc: clarify 'pcap snaplen' / 'max-snaplen'

Change-Id: Ic2c82173c12814d61f0ee9f7454b9e5dcb0c13f4
Related: SYS#8099
---
M doc/manuals/chapters/client.adoc
M doc/manuals/chapters/server.adoc
2 files changed, 15 insertions(+), 0 deletions(-)

git pull ssh://gerrit.osmocom.org:29418/osmo-pcap refs/changes/35/42835/1

diff --git a/doc/manuals/chapters/client.adoc b/doc/manuals/chapters/client.adoc
index 1769b60..d9885d1 100644
--- a/doc/manuals/chapters/client.adoc
+++ b/doc/manuals/chapters/client.adoc
@@ -49,6 +49,7 @@
  pcap device eth2 <3>
  pcap filter udp port 23000 <4>
  pcap detect-loop 1 <5>
+ pcap snaplen 9000 <6>
 ----
 <1> Prepare records in pcapng format.
 <2> The network device from which to obtain a capture.
@@ -57,6 +58,14 @@
 <5> Instruct osmo-pcap-client to automatically add a filter that prevents
     capturing the traffic between osmo-pcap-client and osmo-pcap-server,
     which would create a loop.
+<6> Maximum number of bytes captured per packet (snapshot length).
+
+Packets longer than the configured `snaplen` are captured truncated, and
+osmo-pcap-client logs an error for each such packet. Note that kernel receive
+offloading (GRO/LRO/GSO) can hand libpcap coalesced "super-frames" much larger
+than the link MTU; either raise `snaplen` accordingly or disable offloading on
+the captured interface (e.g. `ethtool -K <iface> gro off lro off gso off tso
+off`).
 
 Adding or removing new recording network devices during operation is not really
 supported, and a restart of osmo-pcap-client is expected for the new
diff --git a/doc/manuals/chapters/server.adoc b/doc/manuals/chapters/server.adoc
index e0d9ff4..5d6ed32 100644
--- a/doc/manuals/chapters/server.adoc
+++ b/doc/manuals/chapters/server.adoc
@@ -46,6 +46,12 @@
 <3> TCP port number to which to bind/listen
 <4> maximum pcap snapshot length (per packet, in bytes; default: 9000)
 
+The `max-snaplen` value acts as an upper bound (ceiling) for all connected
+clients: a client is accepted as long as its own configured `snaplen` does not
+exceed the server's `max-snaplen`. It is therefore perfectly fine to configure a
+larger `max-snaplen` on the server than the `snaplen` used by some clients; each
+client's trace is recorded using that client's own snaplen.
+
 The received packets are stored to a pcap file below the `base-path` using a filename
 encoding both the client name and the date/time at time of file creation.
 

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