Re: [trill] Tsvart early review of draft-ietf-trill-over-ip-10

[Donald Eastlake <[e-mail]>]

Hi Magnus,

Thanks for the extensive review. See my responses below.

On Thu, Jun 15, 2017 at 1:32 PM, Magnus Westerlund
<[e-mail]> wrote:
>
> Reviewer: Magnus Westerlund
> Review result: Not Ready
>
> Early review of draft-ietf-trill-over-ip-10
> Reviewer: Magnus Westerlund
> Review result: Not Ready
>
> TSV-ART review comments:
>
> I have set this to not ready as there are several issues, some significant that
> could affect the protocol realization significantly. Some may be me missing
> things in TRILL, I was not that familiar with it before this review and I have
> only tried looking up things, not reading the whole earlier specifications. So
> don't hesitate to push back and provide pointers to things that can resolve
> issues. The authors and the WG clearly have thought about a lot of issues and
> dealt with much already.

OK. Hopefully we can resolve these one way or the other.

> Diffserv usage
> --------------
>
> Section 4.3:
>
>    TRILL over IP implementations MUST support setting the DSCP value in
>    the outer IP Header of TRILL packets they send by mapping the TRILL
>    priority and DEI to the DSCP. They MAY support, for a TRILL Data
>    packet where the native frame payload is an IP packet, mapping the
>    DSCP in this inner IP packet to the outer IP Header with the default
>    for that mapping being to copy the DSCP without change.
>
> I think it is fine to require that implementations are capable  of setting
> DSCP values on the outer IP header. However, I fail to see any discussion of
> the potential issues with actually setting the DSCP values. It is one thing to
> do this in an IP back bone use case where one can know and have control over
> the PHB that the DSCP values maps to. But otherwise, over general internet the
> behavior is not that predictable. One can easily be subject to policers or
> remapping. Also as the actual DSCP code point usage is domain specific this is
> difficult. Priority reversal is likely the least of the problems that this can
> run into over general Internet.

It sounds like appropriate discussion and warnings about these issues
would resolve the above comment.

> Section 4.3:
>
>    The default TRILL priority and DEI to DSCP mapping, which may be
>    configured per TRILL over IP port, is an follows. Note that the DEI
>    value does not affect the default mapping and, to provide a
>    potentially lower priority service than the default priority 0,
>    priority 1 is considered lower priority than 0. So the priority
>    sequence from lower to higher priority is 1, 0, 2, 3, 4, 5, 6, 7.
>
>       TRILL Priority  DEI  DSCP Field (Binary/decimal)
>       --------------  ---  -----------------------------
>                   0   0/1  001000 / 8
>                   1   0/1  000000 / 0
>                   2   0/1  010000 / 16
>                   3   0/1  011000 / 24
>                   4   0/1  100000 / 32
>                   5   0/1  101000 / 40
>                   6   0/1  110000 / 48
>                   7   0/1  111000 / 56
>
> This appear to be an problematic mapping. At least for prio 0 and 1. As
> priority 1 appears to be intended to be higher than priority 0, it is
> interesting that it is mapped to CS1, which to quote
> https://datatracker.ietf.org/doc/rfc7657/:
>
> CS1 ('001000') was subsequently designated as the recommended
>       codepoint for the Lower Effort (LE) PHB [RFC3662].
>
> So what is proposed can in a network using default mapping, result in that you
> get priority 0 to be lower priority than 1. Plus that in some networks this can
> also results in strange remapping that results in a different PHB for CS1 than.

The intent in the draft is to reflect the default relative priority of
the different priority code points in IEEE Std 802.1Q where priority 1
is lower than priority 0. At a quick look, it appears to me that RFC
2474 requires that 0x001000 be handled as being of a priority not
lower than the priority with which 0x000000 is handled. Yet RFC 3662,
which you point to, seems to suggest using 0x001000 as a lower
priority code point than 0x000000. Given that 3662 not only does not
update 2474 but is only Informational while 2474 is Standards Track, I
would say that 2474 dominates and that this draft makes the best
assumptions it can about default behavior...

> 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...

> 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.

> 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.)

> 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.

> 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.

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.

> 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?

> 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.

> 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.

> 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.

> 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.

Thanks,
Donald
===============================
 Donald E. Eastlake 3rd   [phone] (cell)
 155 Beaver Street, Milford, MA 01757 USA
 [e-mail]

> Cheers
>
> Magnus Westerlund