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diff --git a/content/en/blog/20220520-oping-flm.md b/content/en/blog/20220520-oping-flm.md new file mode 100644 index 0000000..eb9e25a --- /dev/null +++ b/content/en/blog/20220520-oping-flm.md @@ -0,0 +1,104 @@ +--- +date: 2022-05-20 +title: "What is there to learn from oping about flow liveness monitoring?" +linkTitle: "cleaning up flows" +author: Thijs Paelman +--- + +### Cleaning up flows + +While I was browsing through some oping code +(trying to get a feeling about how to do [broadcast](https://ouroboros.rocks/blog/2021/04/02/how-does-ouroboros-do-anycast-and-multicast/#broadcast)), +I stumbled about the [cleaner thread](https://ouroboros.rocks/cgit/ouroboros/tree/src/tools/oping/oping_server.c?id=bec8f9ac7d6ebefbce6bd4c882c0f9616f561f1c#n54). +As we can see, it was used to clean up 'stale' flows (sanitized): + +```C +void * cleaner_thread(void * o) +{ + int deadline_ms = 10000; + + while (true) { + for (/* all active flows i */) { + + diff = /* diff in ms between last valid ping packet and now */; + + if (diff > deadline_ms) { + printf("Flow %d timed out.\n", i); + flow_dealloc(i); + } + } + sleep(1); + } +} +``` + +But we have since version 19.x flow liveness monitoring (FLM), which does this for us! +So all this code could be thrown away, right? + +Turns out I was semi-wrong! +It's all about semantics, or 'what do you want to achieve'. + +If this thread was there for cleaning up flows from which the peers stopped their flow (and stopped sending keep-alives), +then we could throw it away by all means! Because FLM does that job. + +Or was it there to clean up valid flows, but from which the peers didn't send any ping packets anymore (they *do* send keep-alives, otherwise FLM kicks in)? +Then we should of course keep it, because this is a server-side decision to cut those peers off. +This might protect for example against client implementations which connect, send a few pings, but then leave the flow open. +Or a better illustration of the 'cleaner' thread might be to cut off peers after a 100 pings, +showing that this decision to 'clean up' has nothing to do with flow timeouts. + +### Keeping timed-out flows + +On the other side of the spectrum, we have those flows that are timing out (no keep-alives are coming in anymore). +This is my proposal for the server side parsing of messages: + +```C +while(/* get next fd on which an event happened */) { + msg_len = flow_read(fd, buf, OPING_BUF_SIZE); + if (msg_len < 0) { + /* if-statement is the only difference with before */ + if (msg_len == -EFLOWPEER) { + fset_del(server.flows, fd); + flow_dealloc(fd); + } + continue; + } + /* continue with parsing and responding */ +} +``` + +We can see here that the decision is taken to 'clean up' (= `flow_dealloc`) those flows that are timing out. +But, as we can see, it's an application decision! +We might as well decide to keep it open for another 10 min to see if the client (or the network in between) recovers from interruptions, e.g.. + +### Assymetrical QoS +There will probably follow some more [discussion](https://ouroboros.rocks/community/) +on assymetric QoS[^1], but here we're only talking about assymetric FLM. + +When I was working on the server code, I tried to set a FLM of 10 sec on the server side, +such that it would time-out (and [remove](#keeping-timed-out-flows)) +flows from unresponsive clients. +I then discovered that `flow_accept(qosspec_t * qs, const struct timespec * timeo)` only uses qs to write to, +not to read the server QoS expectations. +Thus, at the moment, only the client sets the QoS (if the layer can give it). +This will change, such that the server sends its QoS to the client too. + +When we're talking about FLM, this might result in the server saying something like: +> I don't trust you, I'm gone if I don't hear from you in 4 minutes, and if you want to want to wait for me 2 days, suits me just fine + +where the server sets a FLM of 4 min and the client sets a FLM of 2 days. +In this scenario, the client has to send[^2] every minute a keep-alive to keep the server interested ([unless it cleans up anyway](#cleaning-up-flows)). +The server, on the other hand, only needs to send a keep-alive every 12 hours to keep the flow open (assuming no other traffic at all). + +There might be cases where you want to sync this timeout (like taking the smallest value), +but it still needs to be determined if this should be done at the application level or ouroboros and if so, how. +But for a 'raw' flow, we go with the 'none-of-your-business, just send in time' principle. + +Excited for my first blog post & always learning, + +Thijs + +[^1]: like for wireless links, with a potential different BER or loss in each direction + or even IPsec, where you can have encryption in one direction, but not the other + +[^2]: fixed at 1/4 of the time-out period at the moment, see [previous post](https://ouroboros.rocks/blog/2022/02/28/application-level-flow-liveness-monitoring/) |