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Hi Donald,
After having read your response I think there is an important question about the applicability of this document that affects several of the issues below and what solution you need. That is the question of what type of paths one expect to get Trill over IP working over. Because if the target is general Internet and also through middleboxes such as NATs and Firewall (Not intending to block) then there are a lot more work to ensure this. If you for example changes the applicability to require any on path middleboxes to fulfill certain requirements things can be more easily addressed. Den 2017-06-26 kl. 02:07, skrev Donald Eastlake: Hi Magnus, Thanks for the extensive review. See my responses below. On Thu, Jun 15, 2017 at 1:32 PM, Magnus Westerlund <magnus.westerlund@xxxxxxxxxxxx> wrote: I would note that the choice of encapsulation here do becomes important. Your's and Joe Touch's observation that for TCP, you can only have a single DSCP marking per TCP connection for example. For others, see the discussion in Section 5.1 of https://datatracker.ietf.org/doc/rfc7657/ on this issue. David Black also raised an important question if one should treat this as a tunnel with a single predictable behavior or let the inner networks marking show through. Establishing a tunnel with a single PHB has less risk of running into issues than multiple different markings.
David Black provide a good answer on this. MTU and Fragmentation --------------------- I think there are two main issue here. The first one is MTUD discovery of the actual IP path MTU between the ports. That will be needed to prevent a lot of traffic going into MTU black holes. Especially as TRILL requries 1470 byte support which is likey above a lot of paths.Seems like it would depend on the environments where TRILL was used. For example, I do not think 1470 would be a problem in most Data Center or Internet Exchange point uses, for example. Data Centers sometimes support 9K jumbo frames and the like. In fact, it is probably bad to focus too much on 1470 -- that is a required minimum to be sure that reasonable size link state PDUs can be successfully flooded through the TRILL campus so that routing will work. However, it would commonly be the case that, for the TRILL campus to be useful in a particular case, links need to be able to carry the expected size TRILL Data packets. For example, if there were two parts of a TRILL campus connected by one or a few TRILL over IP links and the end stations in each part were assuming they could use 1500 byte Ethernet packets, then the TRILL over IP links would need to support an MTU based on 1500 + TRILL Header + IP and TRILL over IP encapsulation. And more if security was being used or there were any other reasons for additional headers/encapsulation... Yes, and over general Internet you should be happy if you get 1500 bytes of IP MTU, it may easily be lower with a couple of additional tunnel headers. Thus, what you say is the goal is not feasible without a solution that supports fragmentation and reassembly, enabling one TRILL packet to be sent in multiple IP packets. The re-assembly do requires buffering and not something to easily perform on a router fast path. And attempting to use IP fragmentation is likely doomed if you have any type of NAT or Firewall in the way. This points to a dedicated solution or using a transport protocol that supports carrying arbitrary data sizes, like TCP or SCTP. And you need to use the byte-stream API of TCP to achieve this. Section 8.4: Path MTU discovery [RFC4821] should be useful in determining the IP MTU between a pair of RBridge ports with IP connectivity. The issue with RFC4821 is that it has requirements on the packetization layer. Trill appears to have several components that are useful. However, it will require a specification of the procedure to result in a useful tool.See below.Section 8.4: TRILL IS-IS MTU PDUs, as specified in Section 5 of [RFC6325] and in [RFC7177], can be used to obtain added assurance of the MTU of a link. Yes, that can confirm working MTUs that are at 1470 or above, but appears prevented from working below 1470?While there is a minimum size for TRILL IS-IS MTU PDUs, determined by header size, it is well below 1470, probably (depending on whether secuirty is in use, etc.) below 150 bytes. Okay, if you say so, it was not obvious from the spec that is was allowed to probe for paths with lesser MTUs than 1470. Thus, it appears that there is a lack of mechanism here to actually get a valid and functional MTU from TRILL in the cases where the Path MTU is below 1470. If I am wrong good, but I think this is an important piece for how to handle the next main issue.How about referencing Section 3 of https://tools.ietf.org/html/draft-ietf-trill-mtu-negotiation-05 which is currently in IETF Last Call? (The wording of that section is probably going to be improved based on an OPS review by Brian Carpenter.) I looked at this, and it appears to have the same issue, that it can't probe for MTU values below 1470. 2) RB1 tries to send an MTU-probe padded to the size 1470.
a) If RB1 fails to receive an MTU-ack from RB2 after k tries, RB1
sets the "failed minimum MTU test" flag for RB2 in RB1's Hello
and stop.
But, the algorithm clearly performs a binary search for the MTU. If
one look at RFC 4821 one will notice that there are some additional considerations
there how to make the probing better and robuster. But, cleary Trill has some other
criterias for what is a success. Verification that Sz works appears sufficient,
and there are no need to probe further upwards.
UDP encapsulation and IP fragments.----------------------------------I see it as a big issue that UDP encapsulation is the native one, and that relies on IP fragmentation despite the need for reliable fragmentation. With the setup of having to support 1470 MTU on TRILL level some packets will be fragmented in many environments. That will lead to a lot of losses, and as discussed below a very big problem with middleboxes. The main problem here is that if one tries to rely on IP fragments one will have issues with packets ending up in black holes. And different problems depending on IPv4 or IPv6. IPv6 is lilkely the lesser problem assuming that one have working PMTUD. There are several ways out of this. 1. Detect issues and use TCP encapsulation with correctly set MSS to not get IP fragements 2. Determine MTU and implement an fragmentation mechanism on top of UDP.So, I don't see that much problem with UDP being the general default consistent with the TRILL philosophy of defaulting to need zero or minimal configuration. The default should be to use multicast Hellos for discovery of neighbors which sure points at UDP to me. Having to traverse a NAT should be a rare case. Since, in the NAT case, you have to configure things related to the static binding and the IP address(es) of peer(s) anyway you can also configure to use a different encapsulation than UDP, such as TCP, at the same time. I don't see it as much of a problem if, by default, TRILL won't operate through a NAT. If you are using UDP and it fragments and fragments are dropped at a NAT, probably you can't exchange Hellos so you will not form an adjacency and anything on the other side of the NAT will not be visible. Yes, but this is the issue of applicability and documenting that applicability. I don't know what goals and requirements that exist for Trill. If the WG are fine with some restrictions, then document them and focus on solving the issues that must be solved. You can clearly choose to require TCP for cases where the IP MTU is insufficient for carrying the Sz sized trill packets between the RBs using UDP. Zero Checksum: -------------- Section 5.4: UDP Checksum - as specified in [RFC0768] Considering the fast path encapsulation desire, I am surprised to not see any mentioning of use of zero checksum here. Raising the zero checksum and forward reference would be good I think. And then Section 8.5: The requirements for the usage of the zero UDP Checksum in a UDP tunnel protocol are detailed in [RFC6936]. These requirements apply to the UDP based TRILL over IP encapsulations specified herein (native and VXLAN), which are applications of UDP tunnel. If you actually intended to allow zero checksum, then you actually should document that Trill fulfills the requirements that the applicability statement raises. I have not analyzed how well it meets these requirements. Please review Section 6.2 of RFC 8086 for example how that can be done.OK. We'll look into it.TCP Encapsulation issue ----------------------- Section 5.6: The TCP encapsulation appear to be missing an delimiter format allowing each individual TRILL packet/payload to be read out of the TCP's byte stream. In other words, a normal implementation has no way of ensuring that the TCP payload starts with the start of a new TRILL payload. Multiple small TRILL payloads may be included in the same TCP payload, and also only parts as TCP is one way of dealing with TRILL packets that are larger than the IP+Encapsulation MTU that actually will work. This comment is based on that there appear to be no length fields included in the TRILL header. The most straight forward delimiter is a 2-byte length field for the TRILL payload to be encapsulated.Right. It might also be useful to include some sort of check field, as is done in BGP, to detect if you are out of sync in parsing the TCP stream. As you need to actually perform re-assembly, the solution is to use the byte stream semantics the TCP API provides and have a framing for each packet. Another point is that, while with UDP it seems fine to send packets with assorted QoS, you don't want to encourage re-ordering of TCP packets in a stream. So if TCP encapsulation is being used, you want to use the same DSCP value for the packets in a particular TCP stream. So, generally, you need to have a TCP connection per priority handling category. Mapping the 8 priority levels into a smaller number of handling categories is a normal thing to do so you certainly don't necessarily need 8 TCP connections. Adding material on this should not be too hard. Yes, agreed it is a possibility and points into possible considerations that David raised. Section 5.6: TCP endpoint requirements. I do wonder if an application like TRILL actual would need to discuss performance impacting implementation choices or limitations. For example use of NAGLE, the requirements on buffer sizes in relation to Bandwidth delay products, as buffer memory in a RBridge will impact performance.Well, I'm not sure how deeply this document should get into such performance issues. What about just saying something about consideration being given to tuning TCP for performance and pointing to one or a few other RFCs that talk about this? As Joe said, these are important considerations. If your intention is to enable this to run at substantial fractions of line rates of the interfaces. Then this do require considerations. Congestion Control ------------------ First thanks for the effort here.You're welcome.8.1.2 In Other Environments Where UDP based encapsulation headers are used in TRILL over IP in environments other than those discussed in Section 8.1.1, specific congestion control mechanisms are commonly needed. However, if the traffic being carried by the TRILL over IP link is already congestion controlled and the size and volatility of the TRILL IS-IS link state database is limited, then specific congestion control may not be needed. See [RFC8085] Section 3.1.11 for further guidance. This is correct, however my question is if the RBridges have any way of knowing which traffic is actually congestion controlled, considering that TRILL provides an layer 2 abstraction. I wonder if there should be any type of white list of the types of layer 2 payloads that can be assumed to be congestion controlled, and thus okay to forward over IP paths? I am worried that without any recommendation to prevent traffic that is not controlled to be forwarded, can lead to congestion issues. The other issue I think may exist is the issue serial unicast emulation of broadcast/multicast creates. As this amplifies the outgoing packet rate with a factor of how many addresses are configured for serial unicast this can be significant traffic expansion. Thus, I think additional considerations are needed here, and maybe rate limiting of the amount of traffic to be multicasted.OK. We can think about those issues.Flow and ECMP ------------- Section 8.3: For example, for TRILL Data, this entropy field could be based on some hash of the Inner.MacDA, Inner.MacSA, and Inner.VLAN or Inner.FGL. I would appreciate clearer references to what these fields are.In a TRILL Data packet, the payload after the TRILL Header looks like an Ethernet frame except that there is always either a VLAN tag or, alternatively, where the VLAN tag would be, a Fine Grained Label [RFC7172]. (The preceding is the view in the TRILL RFCs, but there is an equivalent and equally valid view in which all the fields through and including the VLAN or FGL tag are part of the TRILL Header.) The TRILL base protocol specification focuses on Ethernet as a link technology between TRILL switches, in which case there will be a link header including an Outer.MacDA and Outer.MacSA fields and possibly an Outer.VLAN, all before the TRILL Header. See Figure 1 and Figure 2 in RFC 7172. Some of the above could be added to the draft for clarity.If I understand this correctly, the idea here is to look into the inner layer 2 frames, and use the flow equivalents that exists on that level and hash that into value that maps the flows onto the source port range.Yes.I think this text should include a summary of the principle and ensure to note the important requirement that what is considered flows in the inner must not result in being striped over multiple source ports as this may lead to reordering issues due to packets taking different paths.Well, we can add some text. But when would the relative ordering matter for two TRILL Data packets where the two inner native payloads have different values for any one or more of these three fields (Inner.MacDA, Inner.MacSA, and inner VLAN/FGL tag) ? If any of those fields are different, you are talking about different streams. Okay, then this is very straightforward. NAT and TRILL over IP: Section 8.5: If one like to use TRILL over IP through a NAT, then there are some very important considerations that are missing. First the need for static binding configurations or the need for determining ones external address(es) and be able to communicate that to the peer RBridges, and in addition ensure that one has keep-alives to that the NAT binding never times out.I think those are good points. There is an additional problem that TRILL Hellos detect neighbors with which they have 2-way connectivity by indicating, inside the Hellos that are sent, from what neighbors Hellos have been received on that port. If a NAT is involved, these neighbor addresses inside Hellos need to be mapped. Yes, and the question is how that can be handled, by the receiver of the packet, or if the sender needs to determine what address it uses and provide that in the HELLOs. If the first is possible that can simplify a lot. Next is the issue that there is almost zero chance of getting a IP/UDP encapsulation TRILL payload through the NAT if it results in IP fragmentation, as NATs don't do defragment and refragmented on the internal side, and an IP fragment lacks UDP port and thus can't be matched to binding.So perhaps the recommendation should be to configure the port to use TCP if there will be fragmentation. Yes, I think that are likely the simplest solution for you. Also if you like to run IP/ESP through a NAT, then you most likely need the IP/UDP/ESP encapsulation (https://tools.ietf.org/html/rfc3948). Note that this will restrict the MTU even further and thus ensure that the 1470 requirement cannot be fulfilled even without additional tunnels over an 1500 bytes MTU Ethernet infrastructure. I would note that also firewalls likely have issues with IP fragments for the same reason, they require significant amount of state to be verified if they should be let through. In general I think you should create a configuration that has chance to work through most middleboxes, but I think you should require static bindings. I think that configuration is, and don't laugh now, but IP/UDP/ESP/TCP/TRILL, otherwise you will not be able to have both security and reliable fragmentation of TRILL packets.OK. Thanks again for this review. It has pointed out a number of problems and in thinking about those, I believe a couple of further problems have come to mind that I mentioned above. We'll work on a revised draft. Cheers Magnus Westerlund ---------------------------------------------------------------------- Media Technologies, Ericsson Research ---------------------------------------------------------------------- Ericsson AB | Phone +46 10 7148287 Torshamnsgatan 23 | Mobile +46 73 0949079 SE-164 80 Stockholm, Sweden | mailto: magnus.westerlund@xxxxxxxxxxxx ---------------------------------------------------------------------- |