Re: [IPsec] Mirja Kühlewind's Discuss on draft-ietf-ipsecme-tcp-encaps-09: (with DISCUSS)

[Tommy Pauly <[e-mail]>]

> On Apr 25, 2017, at 5:48 AM, Mirja Kühlewind <[e-mail]> wrote:
> 
> Mirja Kühlewind has entered the following ballot position for
> draft-ietf-ipsecme-tcp-encaps-09: Discuss
> 
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> Please refer to https://www.ietf.org/iesg/statement/discuss-criteria.html
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> The document, along with other ballot positions, can be found here:
> https://datatracker.ietf.org/doc/draft-ietf-ipsecme-tcp-encaps/
> 
> 
> 
> ----------------------------------------------------------------------
> DISCUSS:
> ----------------------------------------------------------------------
> 
> This draft suggests that ports that are assigned to other services can
> simply be used. This is not okay. If a port is assigned to a certain
> service, this service and/or the respective RFC defines how this port is
> used. Simply changing the specified behavior by requiring a check for a
> magic number cannot be done without updating the RFC that the port
> assignment belongs to. Also for the use of 4500/tcp RFC3948 as well as
> the IANA registry would need to be updated.

At this point, the only portion of the document that mentions a port other than 500 and 4500 is the appendix. We recommend that 4500 is used as the port for TCP encapsulation. The IANA registry for 4500/tcp is already assigned to IPSec NAT Traversal in RFC 3947; that document does not explain how TCP is to be used, so the reference should be updated to point to the document on TCP encapsulation.

We can soften the references in the appendix to the fact that other ports may, in fact, be used. As for the configuration, it should state 4500 as the default, but allow peers to configure something else out-of-band if they want to modify behavior (which is standard even in UDP implementations of IKE).

> 
> Further, as also mentioned in the tsv-art review (Thanks Wes!), this
> draft does not sufficiently handle the case of TCP in TCP encapsulation.
> Here a copy of the tsv-art review:
> 
> Reviewer: Wesley Eddy
> Review result: On the Right Track
> 
> This document is clear and well-written.  It can easily be implemented
> based on the description.
> 
> There are a few additional issues that should be considered with
> advice to implementers in Section 12 on performance considerations:
> 1) Invisibility of packet loss - Inner protocols that require packet
> losses as a signal of congestion (e.g. TCP) will have a challenge due
> to not being able to see any packet losses since the outer TCP will
> repair them (unless sending into a full outer TCP socket buffer shows
> up back to the inner TCP as a packet loss?).

Yes, this is definitely true. We try to capture that with the line: "This will make loss-
   recovery of the inner TCP traffic less reactive and more prone to
   spurious retransmission timeouts."

However, this can certainly be expanded upon.

> 2) Nesting of ECN -  Inner TCP connections will not be able to use
> effectively ECN on the portion of the path covered by the outer TCP
> connection.

Generally, IPsec tunnels will apply RFC 6040 for translating ECN markings between inner/outer packets. Since TCP encapsulation places the inner IP packets in a stream, there isn't a direct mapping.

An implementation could try to roughly map, but it may be best to suggest that the ECN markings for inner and outer packets be left separate. What is your opinion?

> 3) Impact of congestion response on aggregate - The general "TCP in
> TCP" problem is mentioned, and is mostly appropriate for a single
> flow.  If an aggregate of flows is sharing the same outer TCP
> connection, there may be additional concerns about how the congestion
> response behavior impacts an aggregate of flows, since it may cause a
> shared delay spike even to low-rate flows rather than distributing
> losses proportional to per-flow throughput.

Indeed. We can add further comments to that effect.

> 4) Additional potential for bufferbloat - Since TCP does not bound
> latency, some applications in the IPsec-protected aggregate could
> drive latency of the shared connection up and impact the aggregate of
> flows that may include real-time applications.  The socket buffer for
> the outer TCP connection might need to be limited in size to ensure
> some bounds?

We can add a comment to suggest that the buffering should be limited on the outer connection if possible.

> 
> Not addressing these could lead to poor experiences in deployment, if
> implementations make wrong assumptions or fail to consider them.

I do think all of these concerns go back to the overall recommendation of "use direct ESP or UDP Encapsulation whenever possible". Anything to help back up that point is great!

Thanks,
Tommy
> 
> In the security considerations section, there are several RFCs on
> mechanisms to increase robustness to RST attacks and SYN floods that
> could be mentioned if it's worthwhile.
> 
> 
> 
> 
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