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

Magnus Westerlund <magnus.westerlund@ericsson.com> Thu, 15 June 2017 17:32 UTC

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

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.

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.

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.

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.

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?

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.

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.

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.

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.

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.

Congestion Control
------------------
First thanks for the effort here.

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 anyway 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.

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.

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.

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.

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.

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.

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.

Cheers

Magnus Westerlund