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GPSD Server?

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Mohamed RACHID edited this page Feb 19, 2017 · 5 revisions

As explained in the How does it work? section of this wiki, the GPSD server is an application or a daemon running on a GPS device.

Protocols

GPSD servers usually provide TCP or UDP connectivity, depending on their design.

The Android application Share GPS provides a GPSD server available in TCP or UDP.

TCP

The TCP (Transmission Control Protocol) is a protocol based on handshake establishment. Packets of data are acknowledged, so the packets order is guaranteed. This means that it is reliable but heavy.

The TCP_NODELAY socket option allows to send packets without waiting for a send buffer to be full. It is used by this program. But if the GPSD server does not use it, useless latency is introduced.

UDP

The UDP (User Datagram Protocol) is a protocol simply transmits datagrams with no check. There is no actual so-called connection. The packets order is not guaranteed, so the network can change the order of packets. This means that it is light but not so reliable.

Normal UDP

In general, a client bounds outgoing connection with a random local UDP port, let's say 53964 (normal UDP).

Let's say that the GPSD server is set to use the default UDP port 2947.

Here is a small exchange:

  • Request: source port 53964 & destination port 2947
  • Reply: source port 2947 & destination port 53964

The GPSD client receives the answer on its bound port. It can treat the answer.

Symmetric UDP

Well, this is not so usual but it is possible. What I call Symmetric UDP is UDP where the server replies with a destination port that is the same as its local port. For instance, Share GPS uses symmetric UDP.

Let's see what happens with a symmetric UDP server, using a normal UDP client:

  • Request: source port 53964 & destination port 2947
  • Reply: source port 2947 & destination port 2947

The GPSD client does not receive the answer. The operating system finds no program bound to the local UDP port 2947 when it receives the packet: it gets destroyed.

The GPSD server ignores the source port of the request. We need to force the source port on the client instead of using a random number. That is what I call symmetric UDP.

Let's see how it works now with a symmetric UDP client:

  • Request: source port 2947 & destination port 2947
  • Reply: source port 2947 & destination port 2947
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