Hi Joe, thank you for your review. Responses inline.
David
On Fri, Apr 14, 2023 at 8:29 PM Joseph Touch via Datatracker <noreply@xxxxxxxx> wrote:
Reviewer: Joseph Touch
Review result: Not Ready
This review focuses on the behavior of the tunnel from an IP perspective. The
HTTP aspects are not considered.
In summary, the document presents a method for tunneling IP packets over an
HTTP connection. Its terminology and discussion is confusing, being presented
largely from the perspective of the HTTP mechanism and not sufficiently from
the perspective of the resulting IP tunnel that is provided. Details are
provided below.
--
The terminology is confusing. Because HTTP is a client/server protocol, it
makes sense to refer to a web (or HTTP) proxy, a single entity that acts as
both server and client, relaying requests received on the server side and
initiating them on the client side. An IP proxy would relay datagrams received
on one IP address and issue them with at least some new addressing. What
appears to be described here is way to create an IP tunnel over a web
connection, which is not a proxy. It is (and should be called) just an IP
tunnel. The web client and server are not co-located as they are in a proxy;
the term proxy is nonsensical in this context.
The target audience for this document is HTTP implementers, since you need
an HTTP stack in order to implement this. Because of this, it is more helpful
to use HTTP terminology instead of tunneling terminology.
Sec 4.6 indicates that ipproto can be any number in [IANA-PN]; this is not
consistent with the document title or remainder of the document description.
That list is of protocols that can be payloads of IP packets, not of IP
protocols - e.g., IPv6 is not 6 but 41, and when it is 41 it describes IPv6 as
a payload of an outer IP packet, not an IPv6 packet. There does not appear to
be a way to indicate the outermost IP packet protocol version, e.g.,
https://www.iana.org/assignments/version-numbers/version-numbers.xhtml. The
claim that implementations MUST walk the chain of extensions to find the
protocol is unboundeded as stated; it needs to be limited, e.g., it needs to
stop at the first non-IP protocol (e.g., it should not “find” TCP in IP in UDP
in IP, if limited to protocol=6).
I don't understand what you mean here. IPv6 extension headers all contain the
Next Header field, but IP protocols like TCP and UDP do not. So the scenario
that you describe of "UDP in TCP" isn't possible given the wire format.
Note that this also raises a further concern, that of the utility of an IP
tunnel. The Internet is composed of links and tunnels emulate links. This
document appears to describe tunnels that can be limited to specific types of
IP packets, but it overlooks the outermost IP packet version. It also overlooks
support for non-IP protocols that are critical to IP operation, e.g., for IPv4,
these include ICMP and ARP. If an “IP proxy” were limited to protocol 6 (TCP),
it would block ICMP (which protocol 1) and ARP (which isn’t an IP packet at
all).
ARP is not required to use an IP tunnel. The tunnel endpoints do not have MAC
addresses, so there is no need for ARP's translation of IP to MAC.
Note that this is also inconsistent with the ROUTE_ADVERTISEMENT capsule
as described in Sec 4.7.3, which asserts that ICMPs are exceptions to the
indicated protocol (at a minimum, this needs to be noted here as well).
Good catch! This is somewhat mentioned in security considerations but I added a
sentence to the Scoping section:
Sec 4.7, although defining new capsules, is really about tunnel configuration.
It should be moved to a top-level section and presented as such.
Except for Sec 4.7, which explains how IP addresses and routing can be obtained
over the HTTP connection, the document views the necessary behavior from only
the perspective of the tunnel ingress as an HTTP client and the tunnel egress
as an HTTP server.
That's not right, most of the document is written in terms of "IP proxying endpoints"
because most of the mechanisms are symmetric. Do you have a specific example?
It is missing the way in which these ingress/egress
components are viewed a their endpoints, e.g., to be useful as an IP tunnel,
these need to appear as attached to (possibly virtual) network interfaces,
i.e., to appear as a link, which allows them to then be used for local
processes (via sockets), packet forwarding, etc.
That's an implementation detail that doesn't belong in this document. Most
implementations will indeed use virtual TUN interfaces, but it's not a requirement.
There is a known implementation with transport protocols in userspace that doesn't
do what you describe.
E.g., the mechanism in Sec 4.7
is only one way that endpoints may be configured; others should be discussed –
including using DHCP or IPv6 RAs over the tunnel itself.
We support assigning addresses out of band (e.g., in a config file) but don't see
the need for DHCP or RAs through the tunnel.
Sec 7 explains many aspects of IP packet handling that are already sufficiently
described in RFCs 1122 and 1812 (for IPv4) and 8200 (for IPv6). That section
unnecessarily repeats that detail and is also vague as to where particular
behaviors are to be realized. I.e., parsing the IP header, hopcount processing,
and packet forwarding. The document should just clearly state that tunnels
behave as links (as explained in draft-ietf-intarea-tunnels) and the rest of
processing happens *outside* the HTTP server as defined for a host or router.
Where the code lives is an implementation detail that we don't need to constrain.
It seems incorrect to design these tunnels as something less than a typical IP
interface, esp. for IPv6 (autoconfig, neighbor discovery). Doing so only serves
to invite incompatibility and/or undermine existing mechanisms that are useful
in detecting misconfiguration (e.g., duplicate address detection). If there is
believed to be a technical justification for this limitation, the argument
needs to be presented and its implications reviewed (e.g., an IPv6 tunnel that
isn’t a true IPv6 interface may not be useful as an IPv6 tunnel).
The WG members who are implementing this don't have a need for a true IPv6
interface. IKEv2 made the same design choice and it's been working fine.
The end of section 7 on link MTUs needs to address the additional requirement
of IPv6 EMTU_R of 1500 bytes. Source fragmentation and reassembly need to be
addressed for both IPv4 and IPv6, and path-fragmentation for IPv4 (unless it is
not supported, which needs to be noted).
I'm not sure we need to go into these details that are well covered by IP RFCs.
Sec 8 should be clear on what entity is responsible for ICMP packet generation
and receipt. This presumably should be the HTTP endpoint device, not the HTTP
client or server. I.e., the tunnel transits ICMP packets but tunnel endpoints
should neither generate nor consume them, just like any other link.
Similar to above, where the code lives is an implementation detail.
As other reviewers have noted, Sec 10 on nested congestion control is quite
thin. The current statement is equivalent to “if you KNOW congestion is nested,
turn it off” – it should be the opposite, i.e., “turn congestion ON only if you
KNOW congestion is NOT nested”.
Fair enough. We're tweaking that section to be more permissive:
Section 11.1 refers to fragmented packets; it should refer to them as not being
able to be “re-fragmented”; source-generated fragments are still fragmented and
can cross the tunnel subject to the tunnel MTU.
The use of "fragmented" in that section refers to QUIC datagram frames, which
cannot be fragmented - this isn't about IP fragmentation.
Section 11.1 should require that the IP tunnel be created with a given MTU and
to indicate that to the endpoint where the client/servers reside; issuing ICMP
PTBs is the responsibility of those endpoints when they are seeking to use
those tunnel endpoints, NOT of the tunnel endpoint itself. Tunnels are links;
links do not issue ICMPs.
This use of ICMP is a pragmatic solution to ensure that the link doesn't violate
the minimum IPv6 link MTU. Some working group members felt strongly that
we needed a solution for this.
Sec 12 should indicate how access to the HTTP proxy itself should be access
limited, i.e., to avoid presenting an arbitrary traffic injection point.
Are you talking about things like <<don't set your root SSH password to
"password">>?
It should also address the potential use of HTTP-level encryption (e.g., TLS,
DTLS) to protect the tunnel contents and tunnel configuration exchanges.
We require the scheme to be https, which means that QUIC or TLS is mandatory.
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