Re: [tsvwg] Comments on draft-ietf-tsvwg-transport-encrypt-14

["Black, David" <[e-mail]>]

The text on ports is in Section 3.1.1 of the draft - it makes a lot of sense to refer to it rather than cover the same territory again, so mea culpa for overlooking that text.  Also, in 20/20 hindsight, “more effort” was not the right word choice to convey “more involved” or “more complex” – besides, it’s better to simply point out that the transport protocol has to be identified in order to use its headers.

Trying again on the new paragraph, much of it can be replaced by just citing


   Network-layer optional headers explicitly indicate the information

   that is exposed, whereas use of exposed transport header information

   requires identifying the transport protocol. See Section 3.1.1

   for further discussion of transport protocol identification.


That’s shorter, crisper text that shifts the discussion to Section 3.1.1, where a reference to RFC 7605 makes a lot of sense to me, along with a minor edit to this paragraph:


   In some uses, a low-numbered (well-known) transport port number can

   identify the protocol.  However, port information alone is not

   sufficient to guarantee identification.  Applications can use

   arbitrary ports, multiple sessions can be multiplexed on a single

   port, and ports can be re-used by subsequent sessions.  UDP-based

   protocols often do not use well-known port numbers.  Some flows can

   be identified by observing signalling protocol data (e.g., [RFC3261],

   [I-D.ietf-rtcweb-overview]) or through the use of magic numbers

   placed in the first byte(s) of the datagram payload [RFC7983].

OLD

   UDP-based protocols often do not use well-known port numbers.
NEW

   UDP-based protocols often do not use well-known port numbers,

   and use of a well-known port number is not limited to the

   protocol for which the port is well known [RFC7605].


This approach looks significantly better than what I originally wrote.

Thanks, --David (still as draft shepherd)

From: Gorry Fairhurst <[e-mail]>
Sent: Saturday, April 4, 2020 3:13 AM
To: Black, David; Tom Herbert
Cc: tsvwg
Subject: Re: [tsvwg] Comments on draft-ietf-tsvwg-transport-encrypt-14


[EXTERNAL EMAIL]

See below on (2).
On 04/04/2020 01:35, Black, David wrote:

Tom,



Ok, 1 issue settled, 1 more to go ...



[1]

Suggested revised text:



   The usefulness of this information would be enhanced if the exposed

   information could be verified to match the protocol's actual behavior,

   e.g., by observing whether the network traffic sent by the protocol

   is consistent with the exposed information in that traffic.



But then that would be akin to making inferences encrypted data, and

preventing such inferences in the network is precisely one of the

reasons the transport header is being encrypted by the end hosts.



I think that response is overdone for several reasons:



1.  A decision to expose some transport header information is a decision to allow this sort of inference from observations (sorry, there's no free lunch available here).



2.  What is being inferred is protocol functional behavior, not the specific contents of the encrypted headers that the protocol is using as part of producing that behavior, so characterizing this as somehow reversing part of the encryption is a stretch.



3.  Beyond that, if one is determined to prevent all inference of protocol functional behavior, not exposing any transport protocol information does not suffice to frustrate all traffic analysis (e.g., set a few CE marks on packets inbound to a protocol receiver that supports ECN, sit back and watch what happens).  There's been extensive security work on traffic analysis countermeasures which is (IMHO) not germane to this draft.



[2]

Add this paragraph:



   Network-layer optional headers explicitly indicate the information

   that is exposed, whereas more effort may be required for a network

   device to determine whether a packet contains a partially encrypted

   transport header.  A particular concern is UDP-encapsulated protocols

   because the UDP ports do not definitively indicate which protocol has

   been encapsulated, even though some protocols are the predominant

   usage of specific UDP destination ports (e.g., a packet sent to UDP

   port 4500 is highly likely to contain UDP-encapsulated IKE [RFC3948]

   or IKEv2 [RFC7296]).



Would that work?



That's good, but I think there should be a reference to RFC7605 also.



Ok, I think the factual statement suffices, but I won't object to a reference to RFC7605.



Thanks, --David

The phrase "whereas more effort may be required" is speculation. It could be that hardware in routers, etc has been designed and optimised for network-header extensions to allow metrics to be extracted, or it could be that it hasn't, or that this is only true from some vendors or some models of equipment. It's possible to say that about many things. I don't think we should speculate here, unless we can point to references that have observed this.

I think we already have covered the ground for the second part of the additonal text.The current text on ports is:

   In some uses, a low-numbered (well-known) transport port number can

   identify the protocol.  However, port information alone is not

   sufficient to guarantee identification.  Applications can use

   arbitrary ports, multiple sessions can be multiplexed on a single

   port, and ports can be re-used by subsequent sessions.  UDP-based

   protocols often do not use well-known port numbers.  Some flows can

   be identified by observing signalling protocol data (e.g., [RFC3261],

   [I-D.ietf-rtcweb-overview]) or through the use of magic numbers

   placed in the first byte(s) of the datagram payload [RFC7983].



I do agree with Tom that this could benefit from the addition of a reference to RFC7605!



I do not personally buy this: "A particular concern is UDP-encapsulated protocols".

To me, this has little to do with encryption, but we could say something more here

if there is a need to say more. This seems true of other protocols. They can

be layered also, we see many services over HTTP/TCP and people shouldn't

disect traffic based only on a TCP port.



-----Original Message-----

From: Tom Herbert <[e-mail]><mailto:[e-mail]>

Sent: Friday, April 3, 2020 8:04 PM

To: Black, David

Cc: tsvwg

Subject: Re: [tsvwg] Comments on draft-ietf-tsvwg-transport-encrypt-14





[EXTERNAL EMAIL]



On Fri, Apr 3, 2020 at 1:06 PM Black, David <[e-mail]><mailto:[e-mail]> wrote:



Hi Tom,

[writing as draft shepherd]



[1]

I don't understand this statement from the draft:



"The value of this information would be enhanced if the exposed

information could be verified to match the internal state of the

transport by observing the transport behaviour."



I assume this means that the network nodes would need to understand

the internal state of transport protocols. How would this work?



Oops, that's not a good assumption - in 20/20 hindsight, the quoted

text should focus on protocol behavior rather than internal state.



Suggested revised text:



   The usefulness of this information would be enhanced if the exposed

   information could be verified to match the protocol's actual behavior,

   e.g., by observing whether the network traffic sent by the protocol

   is consistent with the exposed information in that traffic.



But then that would be akin to making inferences encrypted data, and

preventing such inferences in the network is precisely one of the

reasons the transport header is being encrypted by the end hosts.



Beyond that, one could (partially) infer protocol state from observed

traffic behavior, but I don't think it's important to say that.



[2]

>From the draft:



"An endpoint/protocol could choose to expose transport header

information to optimise the benefit it gets from the network

[RFC8558]."



There is also the possibility that the endpoints didn't expose

transport layer information, but the network incorrectly thinks it

did.  The network may simply misinterpret bits in packets as being

transport layer information when in fact the data can be something

completely different and unrelated. The canonical example of this is

QUIC or any transport encapsulated in UDP payload.



That's actually an observation about transport-layer vs. network-layer

information exposure that would be better addressed slightly earlier in

the draft where both of those layers are discussed.



After the first paragraph in Section 6.1 (Exposing Transport Information

in Extension Headers) looks like a good place to make that observation.



After this paragraph:



   At the network-layer, packets can carry optional headers (similar to

   Section 5) that may be used to explicitly expose transport header

   information to the on-path devices operating at the network layer

   (Section 3.1.3).  For example, an endpoint that sends an IPv6 Hop-by-

   Hop option [RFC8200] can provide explicit transport layer information

   that can be observed and used by network devices on the path.



Add this paragraph:



   Network-layer optional headers explicitly indicate the information

   that is exposed, whereas more effort may be required for a network

   device to determine whether a packet contains a partially encrypted

   transport header.  A particular concern is UDP-encapsulated protocols

   because the UDP ports do not definitively indicate which protocol has

   been encapsulated, even though some protocols are the predominant

   usage of specific UDP destination ports (e.g., a packet sent to UDP

   port 4500 is highly likely to contain UDP-encapsulated IKE [RFC3948]

   or IKEv2 [RFC7296]).



Would that work?



That's good, but I think there should be a reference to RFC7605 also.



Tom

Thanks, --David



-----Original Message-----

From: tsvwg <[e-mail]><mailto:[e-mail]> On Behalf Of Tom Herbert

Sent: Friday, April 3, 2020 12:49 PM

To: tsvwg

Subject: [tsvwg] Comments on draft-ietf-tsvwg-transport-encrypt-14





[EXTERNAL EMAIL]



Hi, a few comments.



I don't understand this statement from the draft:



"The value of this information would be enhanced if the exposed

information could be verified to match the internal state of the

transport by observing the transport behaviour."



I assume this means that the network nodes would need to understand

the internal state of transport protocols. How would this work?



If the idea is that the exposed information is somehow verified with

the endpoints that securely provide the information then maybe I

understand that. But, if the idea is that intermediate nodes need to

autonomously deduce the internal state themselves, I think that is

problematic. Aside from all the known problems that stateful network

devices have caused, there seems to be a circular dependency here.

AFAIK the only way to deduce the internal state of a transport

connection in the network would be by inspecting the exposed transport

information, but this statement seems to be saying the exposed

information can't be trusted unless the internal state has been

deduced. Am I missing something?



>From the draft:



"An endpoint/protocol could choose to expose transport header

information to optimise the benefit it gets from the network

[RFC8558]."



There is also the possibility that the endpoints didn't expose

transport layer information, but the network incorrectly thinks it

did. The network may simply misinterpret bits in packets as being

transport layer information when in fact the data can be something

completely different and unrelated. The canonical example of this is

QUIC or any transport encapsulated in UDP payload. Per, RFC7605,

transport port numbers only have meaning at end points. So for example

a network device may think a packet with UDP destination port 80 is

QUIC, when in fact it is something completely different, hence the

network device may misinterpret the payload as being QUIC. I suspect

it's unlikely that this situation will benefit the user, and more

likely the network would not treat such packets well. There's been

some work on this problem, like magic numbers in SPUD or analysis to

mitigate the effects of misinterpretation like done for QUIC spin bit,

but I don't believe anyone has proposed a general solution (other than

moving the necessary information into the network layer and have

intermediate nodes stop doing DPI).  I believe this problem should be

mentioned in the draft with a reference to RFC7605 .



Tom