Reviewer: David Black Review result: On the Right Track TSVART Review of: DLEP Credit-Based Flow Control Messages and Data Items draft-ietf-manet-dlep-credit-flow-control-09 DLEP DiffServ Aware Credit Window Extension draft-ietf-manet-dlep-da-credit-extension-12 DLEP Traffic Classification Data Item draft-ietf-manet-dlep-traffic-classification-06 Reviewer: David L. Black Date: November 19, 2021 This is a combined review of all three drafts. These three drafts specify a flow control mechanism between a "router" and a "modem" in the sending direction (router to modem) to prevent overrun of buffers in the "modem". The mechanism is designed to provide flow control for multiple queues in the "modem" that are independently protected, with the "modem" in control of how much data the router is allowed to send - to a first approximation, there is a separate instance of flow control, called a Credit Window, for each queue. Both Diffserv (DSCP) and Ethernet Priority (PCP) values can be used to classify traffic to determine which flow control instance (Credit Window) applies to each packet being sent to the "modem" by the "router". Each of the drafts is reasonably well written, but there are some difficulties in understanding the combination of the three drafts, which have to be used together in order to implement this flow control mechanism. That's one of a number of issues that deserve attention. ****** Major Issues: -- [1] -- Number of Documents I understand and see the merit in specifying the flow control mechanism (-credit-flow-control draft) independently of traffic classification (-traffic-classification draft). In contrast, I do not see a strong rationale for the separate DA credit extension draft (-da-credit-extension), which has very little content (about a page of actual protocol specification plus one addition to an IANA registry). I strongly suggest folding the DA credit extension draft into the traffic classification draft, as the resulting two-draft structure will be easier to explain, and more importantly, should be easier for implementers to understand. -- [2] -- Flow Match Criteria The Traffic Classification draft does not explain how traffic classification is actually performed, i.e., how the applicable FID is determined for a packet, particularly when more than one FID is possible. For example, suppose a packet carries a DSCP that matches FID 5 and a PCP that matches FID 7 - does that packet consume FID 5 credits or FID 7 credits? If the router and modem make different decisions, the result may be undesirable. This example is within the same TID - the situation appears to be worse across TIDs for the same destination, because the same match criteria could appear in multiple TIDs, e.g., same DSCP could match different FIDs under different TIDs. Clarity is needed here so that the router and modem agree on which FID's credits are consumed by each packet, and on a related note, the DA credit extension draft does not prohibit use of Ethernet traffic classification - perhaps it should. -- [3] -- Router MAY ignore flow control The Management Considerations section in both the traffic classification and DA credit extension drafts contains this text: When modem-provided credit window information exceeds the capabilities of a router, the router MAY use a subset of the provided credit windows. In other words, the router MAY ignore any credit windows that exceed the routers capabilities ... which in practice could amount to "MAY ignore any credit windows that the router implementer views as inconvenient" ... with the router proceeding to overrun the corresponding modem queues. That doesn't seem right, so if this is intended, some serious rationale/explanation ought to be provided. -- [4] -- Association Not Well-Specified The overall association structure is that flow control instances are identified by FIDs which are associated with TIDs which are associated with destinations. That structure is described towards the end of Section 2 of the flow control draft, but the details of how it is done are mostly missing - the flow control draft ought to state that: i) FIDs are associated with TIDs via traffic classification information specified in the traffic classification draft. ii) TIDs (and their associated FIDs) are associated with destinations via use of the Credit Window Association data item in DLEP Destination Up and Destination Update messages. The first item (i) can be mostly inferred from the last few paragraphs in section 2 of the flow control draft (and needs to be more explicit), but the second item (ii) is missing because the flow control draft does not specify how to determine the "destination" with which a TID is associated by a Credit Window Association data item (in a DLEP Destination Up or Destination Update) message. I would expect that the destination is a relatively obvious field in those messages, but the draft needs to specify that field. This should be easy to fix, although it made the drafts much more difficult to understand. -- [5] -- Initialization vs. in-flight traffic The Credit Window Initialization data item appears to be intended to establish common state for a Credit Window (e.g., size, number of credits) across the router and modem ... but ... that data item appears to be allowed to be sent while there's traffic in flight. The result would be that the modem would count in-flight traffic against the initialized Credit Window, but the router would not. The resulting inconsistency deserves discussion - it may be acceptable if the amount of traffic in flight is miniscule by comparison to both the Credit Window size and initial credit balance. -- [6] -- Security Considerations Although this is a Transport review, I found a security issue that would be better dealt with now before the security directorate points it out ;-) - the security considerations sections in each of the 3 drafts claims that adding credit window control and flow functionality to DLEP does not introduce any new security considerations (vulnerabilities). That's a nice try, but it's incorrect. These drafts specify a new resource (credits in a credit window) that is subject to resource exhaustion attacks that could cause denial-of-service. For example, suppose an attacker injects a Credit Window Initialization data item that contains almost no credits and/or specifies a ridiculously tiny Window (Max) Size. I expect that the protocol contains mechanisms to counter this and related attacks on credit resources (e.g., if something looks wrong, the modem reinitializes the Credit Window), but the current text incorrectly asserts the non-existence of such attacks. These sorts of attacks definitely exist - I am aware of a (subsequently fixed) resource exhaustion problem in another credit-based flow control mechanism caused by an unanticipated environmental "attack" on signal integrity of credit exchange messages, resulting in message discard and credit loss. ***** Minor Issues: [A] Packets consume credits: Section 2 of the flow control draft needs to say somewhere early in the section that each packet that a router sends consumes a credit for each octet in the packet. This is to be found at the end of the second paragraph in Section 2.1, but ought to be stated much earlier, e.g., at the end of second paragraph in Section 2. [B] Bidirectional messages: Both the Credit Control Message and its response can be sent in both directions (i.e., by both modems and routers). Has the possibility of using different message types in each direction been considered? That might help out people who have to read packet traces/dumps. NOTE: "Yes, that was carefully considered and the WG decided not to do that." is a fine response. [C] Data item usage: It's not clear which data items are required, allowed or prohibited in which DLEP messages. It appears that any data item MAY be used in any DLEP message, which is surely not what was intended. Both the flow control and traffic classification drafts ought to spell out these details, including what happens when the rules are violated, e.g., data item shows up in a message where it's not supposed to be used - is the data item ignored or does it cause an error? [D] Uninitialized window: The second sentence of Section 2.3.1 in the flow control draft begins with: This Data Item SHOULD be included in any Session Initialization Response Message that also ... What happens if that SHOULD is not adhered to? Please explain. [E] Credit Window Size: In Section 2.3.1 of the flow control draft (Credit Window Initialization), what happens if the Credit Value exceeds the Credit Window Size? I also strongly suggest renaming Credit Window Size to Credit Window Max Size or something similar to avoid possible confusion of this concept with current credit balance (Credit Value) [F] TID association with destination: Section 2.3.2 of the flow control draft (Credit Window Association) says that a the traffic classification information associated with a TID "MUST be associated with the destination" - what does that mean? Specifically: - Does that traffic classification information replace any existing traffic classification information, e.g., if the info associated with that TID has been updated since that TID was most recently associated with that destination? - Is there any limit on the number of TIDs or amount of traffic classification information that can be associated with a destination? If so, what happens if that limit is exceeded? Also note that "the destination" is not currently defined, see [4] above. [G] Bad Credit Window math: The next to last paragraph in Section 2.3.3 (Credit Window Grant) of the flow control draft contains this text about adding credit to a window: If the increase results in a window overflow, i.e., the size of the credit window after the increase is smaller than the original credit window size, the Credit Window must be set to its maximum (0xFFFFFFFFFFFFFFFF). That's doubly wrong: - consider a window max size of 10, a current window size of 3 and an addition of 15 credits. Overflow occurs, but the window size increases, as the new window size is 8. - The maximum size of the credit window may be considerably smaller than 0xFFFFFFFFFFFFFFFF. For the first item, just use the concept of overflow in modular arithmetic. For the second item, remove that all-F's constant and refer back to the Credit Window Initialization material for max size. This text is also an example of why [E] strongly suggests renaming Credit Window Size to Credit Window Max Size or somthing similar. [H] Amount of imprecision: Section 2.3.4 of the flow control draft discusses modem comparison of internal values with values received from the router, and contains this text: If the values significantly differ, i.e., greater than can be accounted for based on observed data frames, That needs some guidance on what a significant difference is and how much of a "greater than" difference ought to trigger the consequences, accompanied by a warning against precise (== vs. !=) comparisons (e.g., courtesy of [5] above). [I] Update means what?: The last paragraph of Section 2.1 of the traffic classification draft (Traffic Classification Data Item) contains: the router MUST update the information using the values carried in the Data Item. What does "update" mean, e.g., "replace", "merge"? [J] Error behavior: Section 2.1.1 of the traffic classification draft leaves error behavior as a "go fish" exercise for the reader: Any errors or inconsistencies encountered in parsing Sub-Data Items are handled in the same fashion as any other Data Item parsing error encountered in DLEP. That doesn't tell an implementer what to do - this needs to cite a reference that specifies the applicable error behavior. [K] RFC 2474 CU field: Section 2.2 of the traffic classification draft defines the DS Field as an octet and reproduces the definition from RFC 2474 that includes the DSCP and CU fields. Unfortunately, this does not reflect updates to RFC 2474 - those two bits are now the ECN Field, which may be non-zero. The fix for this is simple - use the 6 bit DSCP field from RFC 2474 and replace the CU field with two zero bits to produce an octet. [L] 802.1Q DEI field: This has a similar problem to the CU field ... and a similar solution - replace the DEI field with a zero bit. ****** Nits: FID paragraph in section 2.3.5 of the flow control draft: The FID also uniquely identifies a credit window s/identifies/indicates/ for consistency. End of Section 2 of the traffic classification draft: TID and FID values is a modem-local scope. s/is a/have/