Fast-Slow Retransmission Timeout and Congestion Control Algorithm for CoAP

FASOR is composed of three key components: RTO computation, Slow RTO, and novel retransmission timeout back off logic.

The FASOR algorithm measures the RTT for an CoAP message exchange over an end-to-end path and computes the RTO value using the TCP RTO algorithm specified in . We call this normal RTO or FastRTO. In contrast to the TCP RTO mechanism, FASOR SHOULD NOT use 1 second lower-bound when setting the RTO because RTO is only a backup mechanisms for loss detection with TCP, whereas with CoAP RTO is the primary and only loss detection mechanism. A lower-bound of 1 second would impact timeliness of the loss detection in low RTT environments. The RTO value MAY be upper-bounded by at least 60 seconds. A CoAP sender using the FASOR algorithm SHOULD set initial RTO to 2 seconds. The computed RTO value as well as the initial RTO value is subject to dithering; they are dithered between RTO + 1/4 x SRTT and RTO + SRTT. For dithering initial RTO, SRTT is unset; therefore, SRTT is replaced with initial RTO / 3 which is derived from the RTO formula and equals to a hypothetical initial RTT that would yield the initial RTO using the SRTT and RTTVAR initialization rule of RFC 6298. That is, for initial RTO of 2 seconds we use SRTT value of 2/3 seconds. FastRTO is updated only with unambiguous RTT samples. Therefore, it closely tracks the actual RTT of the network and can quickly trigger a retransmission when the network state is not dubious. Retransmitting without extra delay is very useful when the end-to-end path is subject to losses that are unrelated to congestion. When the first unambiguous RTT sample is received, the RTT estimator is initialized with that sample as specified in except RTTVAR that is set to R/2K.

We introduce Slow RTO as a safe way to ensure that only a unique copy of message is sent before at least one RTT has elapsed. To achieve this the sender must ensure that its retransmission timeout is set to a value that is larger than the path end-to-end RTT that may be inflated by the unnecessary retransmission themselves. Therefore, whenever a message needs to be retransmitted, we measure Slow RTO as the elapsed time required for getting an acknowledgement. That is, Slow RTO is measured starting from the original transmission of the request message until the receipt of the acknowledgement, regardless of the number of retransmissions. In this way, Slow RTO always covers the worst-case RTT during which a number of unnecessary retransmissions were made but the acknowledgement is received for the original transmission. In contrast to computing normal RTO, Slow RTO is not smoothed because it is derived from the sending pattern of the retransmissions (that may turn out unnecessary). In order to drain the potential unnecessary retransmissions successfully from the network, it makes sense to wait for the time used for sending them rather than some smoothed value. However, Slow RTO is multiplied by a factor to allow some growth in load without making Slow RTO too aggressive (by default the factor of 1.5 is used). FASOR then applies Slow RTO as one of the backed off timer values used with the next request message. Slow RTO allows rapidly converging towards stable operating point because 1) it lets the duplicate copies sent earlier to drain from the network reducing the perceived end-to-end RTT, and 2) allows enough time to acquire an unambiguous RTT sample for the RTO computation. Robustly acquiring the RTT sample ensures that the next RTO is set according to the recent measurement and further unnecessary retransmissions are avoided. Slow RTO itself is a form of back off because it includes the accumulated time from the retransmission timeout back off of the previous exchange. FASOR uses this for its advantage as the time included into Slow RTO is what is needed to drain all unnecessary retransmissions possibly made during the previous exchange. Assuming a stable RTT and that all of the retransmissions were unnecessary, the time to drain them is the time elapsed from the original transmission to the sending time of the last retransmission plus one RTT. When the acknowledgement for the original transmission arrives, one RTT has already elapsed, leaving only the sending time difference still unaccounted for which is at minimum the value for Slow RTO (when an RTT sample arrives immediately after the last retransmission). Even if RTT would be increasing, the draining still occurs rapidly due to exponentially backed off frequency in sending the unnecessary retransmissions.

FASOR uses normal RTO as the base for binary exponential back off when no retransmission were needed for the previous CoAP message exchange. When retransmission were needed for the previous CoAP message exchange, the algorithm rules, however, are more complicated than with the traditional RTO back off because Slow RTO is injected into the back off series to reduce high impact of using Slow RTO. FASOR logic chooses from three possible back off series alternatives: FAST back off: Perform traditional RTO back off with the normal RTO as the base. Applied when the previous message was not retransmitted. FAST_SLOW_FAST back off: First perform a probe using the normal RTO for the original transmission of the request message to improve cases with losses unrelated to congestion. If the probe for the original transmission of the request message is successful without retransmissions, continue with FAST back off for the next message exchange. If the request message needs to be retransmitted, continue by using Slow RTO for the first retransmission in order to respond to congestion and drain the network from the unnecessary retransmissions that were potentially sent for the previous exchange. If still further RTOs are needed, continue by backing off the normal RTO further on each timeout. FAST_SLOW_FAST back off is applied just once when the previous request message using FAST back off required one or more retransmissions. SLOW_FAST back off: Perform Slow RTO first for the original transmisssion to respond to congestion and to acquire an unambiguous RTT sample with high probability. Then, if the original request needs to be retransmitted, continue with the normal RTO-based RTO back off serie by backing off the normal RTO on each timeout. SLOW_FAST back off is applied when the previous request message using FAST_SLOW_FAST or SLOW_FAST back off required one or more retransmissions. Once an acknowledgement for the original transmission with unambigous RTT sample is received, continue with FAST back off for the next message exchange. For the initial message, FAST is used with INITIAL_RTO as the FastRTO value. From there on, state is updated when an acknowledgement arrives. Following unambiguous RTT samples, FASOR always uses FAST. Whenever retransmissions are needed, the back off series selection is first downgraded to FAST_SLOW_FAST back off and then to SLOW_FAST back off if further retransmission are needed in FAST_SLOW_FAST. When Slow RTO is used as the first RTO value, the sender is likely to acquire unambiguous RTT sample even when the network has high delay due to congestion because Slow RTO is based on a very recent measurement of the worst-case RTT. However, using Slow RTO may negatively impact the performance when losses unrelated to congestion are occurring. Due to its potential high cost, FASOR algorithm attempts to avoid using Slow RTO unnecessarily. The CoAP protocol is often used by devices that are connected through a wireless network where non-congestion related losses are much more frequent than in their wired counterparts. This has implications for the retransmission timeout algorithm. While it would be possible to implement FASOR such that it immediately uses Slow RTO when a dubious network state is detected, which would handle congestion very well, it would do significant harm for performance when RTOs occur due to non-congestion related losses. Instead, FASOR uses first normal RTO for one transmission and only responds using Slow RTO if RTO expires also for that request message. Such a pattern quickly probes if the losses were unrelated to congestion and only slightly delays response if real congestion event is taking place. To ensure that an unambiguous RTT sample is also acquired on a congested network path, FASOR then needs to use Slow RTO for the original transmission of the subsequent packet if the probe was not successful.

FASOR consists of the three states discussed above while making retranmission decisions, FAST, FAST_SLOW_FAST and SLOW_FAST. The state machine of the FASOR algorithm is depicted in .

FAST_SLOW_FAST-----a----->SLOW_FAST--+ | ^ ^ | ^ | | | | | | | +-b-+ +------b------+ +-a-+ a: retransmission acknowledged, ambiguous RTT sample acquired; b: no retransmission, umambiguous RTT sample acquired; ]]> In the FAST state, if the original transmission of the message has not been acknowledged by the receiver within the time defined by FastRTO, the sender will retransmit it. If there is still no acknowledgement of the retransmitted packet within 2*FastRTO, the sender performs the second retransmission and if necessary, each further retransmission applying binary exponential back off of FastRTO. The retransmission interval in this state is defined as FastRTO, 2^1 * FastRTO, ..., 2^i * FastRTO. When there is an acknowledgement after any retransmission, the sender will calculate SlowRTO value based on the algorithm defined in . When these is an acknowledgement after any retransmission, the sender will also switch to the second state, FAST_FLOW_FAST. In this state, the retransmission interval is defined as FastRTO, Max(SlowRTO, 2*FastRTO), FastRTO * 2^1, ..., 2^i * FastRTO. The state will be switched back to the FAST state once an acknowledgement is returned within FastRTO, i.e., no retransmission happens for a message. This is reasonable because it shows the network has recovered from congestion or bloated queue. If some retransmission has been made before the acknowledged arrives in the FAST_SLOW_FAST state, the sender updates the SlowRTO value, and moves to the third state, SLOW_FAST. The retransmission interval in the SLOW_FAST state is defined as SlowRTO, FastRTO, FastRTO * 2^1, ..., 2^i * FastRTO. In SLOW_FAST state, the sender switches back to the FAST state if an unambiguous acknowledgement arrives. Otherwise, the sender stays in the SLOW_FAST state if retransmission happens again.

When retransmissions are needed to deliver a CoAP message, it is not possible to measure RTT for the RTO computation as the RTT sample becomes ambiguous. Therefore, it would be beneficial to be able to distinguish whether an acknowledgement arrives for the original transmission of the message or for a retransmission of it. This would allow reliably acquiring an RTT sample for every CoAP message exchange and thereby compute a more accurate RTO even during periods of congestion and loss. The Retransmission Count Option is used to distinguish whether an Acknowledgement message arrives for the original transmission or one of the retransmissions of a Confirmable message. However, the Retransmission Count Option cannot be used with an Empty Acknowledgement (or Reset) message because the CoAP protocol specification does not allow adding options to an Empty message. Therefore, Retransmission Count Option is useful only for the common case of Piggybacked Response. In case of Empty Acknowledgements the operation of FASOR is the same as without the option. This restriction with Empty Acknowledgements may limit the usefulness of the Retransmission Count Option in deployment scenarios where the receiver is a proxy that will typically respond with an Empty Acknowledgement when it receives a request message. No. C U N R Name Format Length Default TBD X Rexmit-Cnt uint 0-1 0 C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable Implementation of the Retransmission Count option is optional and it is identified as elective. However, when it is present in a CoAP message and a CoAP endpoint processes it, it MUST be processed as described in this document. The Retransmission Count option MUST NOT occur more than once in a single message. The value of the Retransmission Count option is a variable-size (0 to 1 byte) unsigned integer. The default value for the option is the number 0 and it is represented with an empty option value (a zero-length sequence of bytes). However, when a client intends to use Retransmit Count option, it MUST reserve space for it by limiting the request message size also when the value is empty in order to fit the full-sized option into retransmissions. The Retransmission Count option can be present in both the request and response message. When the option is present in a request it indicates the ordinal number of the transmission for the request message. If the server supports (implements) the Retransmission Count option and the option is present in a request, the server MUST echo the option value in its Piggybacked Response unmodified. If the server replies with an Empty Acknowledgement the server MUST silently ignore the option and MUST NOT include it in a later separate response to that request. When Piggybacked Response carrying the Retransmission Count option arrives, the client uses the option to match the response message to the corresponding transmission of the request. In order to measure a correct RTT, the client must store the timestamp for the original transmission of the request as well as the timestamp for each retransmission, if any, of the request. The resulting RTT sample is used for the RTO computation. If the client retransmitted the request without the option but the response includes the option, the client MUST silently ignore the option. The original transmission of a request is indicated with the number 0, except when sending the first request to a new destination endpoint (i.e., an endpoint not already in the memory). The first original transmission of the request to a new endpoint carries the number 255 (0xFF) and is interpreted the same as an original transmission carrying the number 0. Once the first Piggybacked Response from the new endpoint arrives the client learns whether or not the other endpoint implements the option. If the first response includes the echoed option, the client learns that the other endpoint supports the option and may continue including the option to each retransmitted request. From this point on the original transmissions of requests implicitly include the option number 0 and a zero-byte integer will be sent according to the CoAP uint-encoding rules. If the first Piggybacked Response does not include the option, the client SHOULD stop including the option into the requests to that endpoint. Retransmissions, if any, carry the ordinal number of the retransmission. That is, the client increments the retransmission count by one for each retransmission of the message. When the Retransmission Count option is in use, the client bases the retransmission timeout for the normal RTO in the back off series as follows: max(RTO, Previous-RTT-Sample) Previous-RTT-Sample is the RTT sample acquired from the previous message exchange. If no RTT sample was available with the previous message exchange (e.g., the server replied with an Empty Acknowledgement), RTO computed earlier is used like in case the Retransmission Count option is not in use.

An alternative way of exchanging the retransmission count information between a client and server is to encode it in the Token. The Token is a client-local identifier and a client solely decides how it generates the Token. Therefore, including a varying Token value to retransmissions of the same request is all possible as long as the client can use the Token to differentiate between requests and match a response to the corresponding request. The server is required to make no assumptions about the content or structure of a Token and always echo the Token unmodified in its response. How exactly a client encodes the retransmission count into a Token is an implementation issue. Note that the original transmission of a request may carry a zero-length Token given that the rules for generating a Token as specified in RFC 7252 are followed. This allows reducing the overhead of including the Token into the reguests in such cases where Token could otherwise be omitted. However, similar to Retransmit Count option the maximum request message size MUST be limited to accommodate the Token with retransmit count into the retransmissions of the request.