Subject: [RRG] APT security and other matters

From: Robin Whittle <[email-masked]>
Date: Wed, 12 Mar 2008 23:44:00 +1100

Here are some comments on:

     http://www.cs.ucla.edu/~meisel/apt-rrg.pdf
&    http://cs.ucla.edu/~meisel/draft-apt-incremental-00.txt

  Path MTU and Fragmentation?
  ITR and ETR placement
  Load Sharing
  Failure Detection and Recovery
  Anycast and multicast
  DMs collating mapping information
  Recursive Application
  Detecting bogus MapSets - after the fact
  Changing providers
  Decentralised authentication of end-users credentials
  Charging for mapping changes

I am mainly pointing out things I am critical of.  The basic
architecture of DMs as local query servers with other ITRs being
caching ITRs is a really good one, I think.

  - Robin

Path MTU and Fragmentation?
---------------------------

APT makes no mention of coping with the PMTU and fragmentation
problems which I think any map-encap scheme needs to cope with.

TRRP doesn't have an approach to this either.  Recent discussions on
the list have been aimed at improving LISP's approach to PMTUD and
fragmentation.

Ivip's approach will be based on:

  http://www.firstpr.com.au/ip/ivip/pmtud-frag/

and Fred Templin's work:

  http://tools.ietf.org/html/draft-templin-seal

hopefully without any extra headers.

ITR and ETR placement
---------------------

apt-rrg.pdf:

I note that APT places the ITR and ETR functions (typically in the
same device) at a very specific location: the Provider Edge routers
of ISPs.  This contrasts with what Dino wrote a few days ago - that
with LISP the ITR and ETR functions would be in Customer Edge routers:

  http://psg.com/lists/rrg/2008/msg00687.html

     For Loc/ID purposes (there are several other reasons to use
     LISP than what is intended by this venue), we want to strongly
     suggest that xTRs be placed on CE (CPE) routers. We think that
     is the best balance of tradeoffs.

The Provider Edge location makes more sense to me.  With Ivip, they
can be placed anywhere, but the Provider Edge would be a good place.

I figure your Default Mapper (DM) function is placed somewhere else,
such as in the ISP network somewhere, or at border routers.

apt-incremental:

Here you state that the ITR and ETR functions are performed at
Border Routers:

   APT islands configure their border routers as "tunnel
   routers" (TRs) so that their customers' data packets will be
   encapsulated and decapsulated as they enter and exit the AS.

which seems to contradict the text in apt-rrg.pdf:

   APT places TRs at the provider-side of the link between edge
   networks and their providers ...

However, you do need ITRs at the border routers if APT networks are
to receive and correctly tunnel packets sent by hosts in non-APT
networks which are addressed to APT-mapped addresses.

Load Sharing
------------

The DM makes a decision about which ETR to use every time it gets a
mapping request (in the form of a traffic packet) from an ITR.  I
assume that where there are multiple ETRs which it considers
reachable at the same priority level, it uses a pseudo-random number
to choose between them according to their weights.

The DM tunnels the packet to that ETR and tells the ITR to use that
ETR.

I note that the DM uses that ITR's address in the outer header, so
the ETR has the impression it was tunnelled by the ITR.  This seems
good to me, but breaks a convention of using the genuine tunnel
start point address in source address of the outer header.  I
completely break this convention with Ivip - by using the sending
host's address - so I am interested to see the convention slightly
broken in APT.

I understand that the ITR keeps this one ETR address and tunnels all
packets there.  So if the ITR has one or more sending hosts sending
packets to a particular edge prefix (prefix with an APT-mapped
address) then it will keep sending all those packets to one ETR.
This is more likely to keep them in order at the receiving host than
if the ITR sent some packets to one ETR and others to another ETR.

This also simplifies the ITR, compared to LISP or TRRP, which I
think is a very good thing.

Failure Detection and Recovery
------------------------------

APT detects 3 kinds of failure - in all cases without any explicit
probing or ETR -> ITR communication.

I am not sure this is a complete list of failure modes in a
multihoming situation.  With Ivip, failure detection is left to the
end-user's choice of system - so they can use whatever techniques
they like.

LISP, APT and TRRP hard-wire the failure detection system into the
protocols and ITRs, ETRs (and DMs for APT), so it had better be
complete and robust.  Even then, it is fixed in its functionality
and I think a major advantage of Ivip is its modular nature - that
it lets the user do whatever they like to detect failure and decide
which ETR traffic should be tunnelled to.

1 - The transit prefix that contains the ETR becomes unreachable.

      I think you mean by this that the BGP system does not provide
      a route to this prefix, in which case the ITR (or DM) finds
      this out either directly from its BGP activities, or via the
      internal routing system.

      However, perhaps the network in which the ETR is located is
      unreachable, but this has not yet propagated through BGP to
      the APT system with the ITR and DM.

      So I think this does not rapidly or reliably detect all actual
      failures of this nature.

2 - The ETR itself becomes unreachable.

      You are relying on the ETR in the destination network's ISP
      going missing from the internal routing system, and so having
      the tunnelled packets arrive at a DM there.  This would
      capture some faults, but what if the ETR was simply dead or
      not working properly?  That wouldn't necessarily show up in
      the internal routing system.

      This system assumes that there is a DM there which is working,
      and that it can send messages to the DM in the sending ISP.
      But perhaps some kinds of failure would disrupt this, and so
      not be visible to the DM in the sending network's ISP's
      network.

3 - ETR can't reach the destination network.

      This seems more robust, since you state somewhere that the
      ETR has a direct connection to the destination network.

      Again, it depends on a DM in the destination ISP network,
      but that is less likely to be affected by whatever clobbered
      the link to the edge network's CE router.

How does the DM in the ETR-end network send a message to the DM in
the sending network's ISP's network?  This is supposedly specified
in section 4.5.2.

A DM in ISP-B needs to know the address of a DM in ISP-A's network,
based only on the ITR address in ISP-A - which is all it knows about
the source of the tunnelled packet.  4.5.2 doesn't mention this.

I guess the DM in ISP-A uses the web of trust and certificates to
check the validity of the failure report from the DM in ISP-A.

Since ISP-A could have hundreds or thousands of ITRs, and dozens or
hundreds of DMs, is it OK for the ISP-B DM to choose any one of the
DMs in ISP-A?  What if that DM is dead?  How does it choose another?

How could ISP-A in some way publish all its DM addresses, without
this being a major storage burden for all other APT networks, which
would presumably have to remember this stuff in preparation for one
of their DMs sending a failure message?  This brute force approach
would have every APT network knowing the address of every DM in
every other APT network.

As with LISP and TRRP, you rely on each ITR (or in this case ITR DM
combination) to find out in their own way about failures which would
mean they need to select another ETR.  With Ivip, some external
system finds this out, makes a decision, and tells all the ITRDs and
QSDs the new ETR address.  The QSDs immediately notifies any ITR
which needs to know of the new ETR address.

I understand how the DM in a sending network's ISP network can be
informed of failures which would require a new decision about which
ETR to tunnel the packets to.  I can't see in apt-rrg.pdf how the DM
tells the ITR about this, which it needs to for failure types 2 and
3.  I recall there was some kind of notification system, but I can't
easily see this in draft-jen-apt-01.

Anycast and multicast
---------------------

The DMs within an APT network are supposed to share information with
each other via an internal anycast address DMany and that there is
also an internal DMall multicast group.  I am not sure how this
could be achieved reliably.

How do you use either anycast or multicast to reliably send
information to or between a group of devices?

An external anycast address DMany_ext is initially (p3) mentioned as
an option, but later (p5) assumed to exist for all APT networks.

If you have an external anycast address which all DMs respond to,
then if you have multiple border routers and multiple DMs, unless
there are packet losses, the incoming packet will go to one DM.
This means you can't use TCP etc. but must use simple UDP message
and response arrangements.  You also have to assume that the one DM
which gets the message can get it to all the others in the network.

I think this inter-DM traffic could become a scaling problem in the
really large networks.  They all need to respond as if they were one
system, since any one of them is supposed to accept a message for
all of them, and acknowledge its receipt.

DMs collating mapping information
---------------------------------

DMs have a lot of work to do.  If you have hundreds of millions of
edge prefixes (EID prefixes in LISP terminology = micronets for
Ivip) and each one has two or three ETRs, then the DM will get two
or three sets of mapping for each edge prefix, one from each ETR's
ISP's DM.

(I think only Ivip will get the really large numbers of micronets,
EID prefixes etc. because it is the only one which provides a new
form of mobility.  Nonetheless, other schemes such as ALT and TRRP
supposedly can cope with billions of EIDs etc.  They can't do
mobility either, but their proponents criticise APT and Ivip for
being less scalable in this regard.  So I will continue to assume
APT could be called upon to do the hundreds of millions of edge
prefixes the ALT and TRRP folks talk about.)

This is a lot of data to hold and store.  When a mapping request
(traffic packet) arrives, it has to look at the two or three or
whatever sets of mapping information for each prefix.  Fortunately,
the DM is an intensive device and only does this for fresh flows.
However, with P2P traffic, there are going to be lots of fresh
flows!  Worm activity is even worse.  So I think the DM has a lot of
work to do - compared to an Ivip full database ITR or query server
(ITRD or QSD) which simply looks up the micronet and finds a single
ETR address there.

Recursive Application
---------------------

I couldn't envisage this section in a concrete manner.

Detecting bogus MapSets - after the fact
----------------------------------------

This seems to be the most striking security weakness in APT.

The DM accepts all MapSets which are correctly signed by any one of
the tens or hundreds of thousands of APT ISPs.  Assuming an attacker
convinces any one of these ISPs that it is the legitimate "owner" of
some edge prefix 22.22.22.0/20, then the attacker can have a MapSet
for this prefix propagated around the world, directing traffic to
its own ETR.

You state (p7) that the only protection against this is the
legitimate owner monitoring the APT mapping traffic, detecting the
bogus MapSet, and then to "take action".  By then, it would be too
late, since the bogus MapSet would be used by some DMs and their
ITRs, probably for quite a while longer, since I don't know how a DM
can tell an ITR to change its mapping.

This seems an onerous task for every end-user network, though I
suppose it could be handled efficiently by a suitably equipped
company.  That company could set up servers all around the Net,
sucking in the APT mapping traffic at various points, and checking
it for validity against its customer's specifications.

But how to take action?  You have to find out which ISP signed the
MapSet and contact them by some presumably non-automated means.
Then you need to prove your bona-fides better than the attacker
proved themselves to be the legitimate owner.  Then the ISP has to
withdraw the MapSet and presumably block the traffic packets being
sent to the attacker's ETR, which is presumably in one of its
ordinary prefixes.  Meanwhile, packets are being snarfed by the
attacker . . .

Changing providers
------------------

In order to change providers, an end-user network needs to get the
old provider to stop issuing MapSets.  I guess this is the same as
with ISPs advertising PI space now.  However, it is not a problem
with Ivip, since the system which controls the mapping information
has nothing necessarily to do with whatever ISPs the end-user
selects for connectivity.

Decentralised authentication of end-users credentials
-----------------------------------------------------

Ivip's approach is pretty simple.  The Replicator system fans out
signed updates for each Mapped Address Block.  A separate server
system provides snapshots for boot time, also signed.  The mapping
for each MAB is controlled by an RUAS - Root Update Authorisation
System.  The ITRDs and QSDs basically trust the updates and
snapshots they get, which are signed by one of these few dozen RUASes.

The RUASes make sure that only the end-user can give commands to
their system to change the mapping.

This makes life relatively easy for the ITRDs and QSDs which receive
the full stream of mapping updates.

This has absolutely nothing to do with ETRs or with whichever ISPs
the end-user selects for connectivity.  So the ISPs don't do
anything, and can't do anything, to hasten or thwart a change of
service from one ISP to another.   Also, they can't do anything,
prompted by an attacker, to change the mapping information.

With APT, you rely on each ISP somehow deciding that the entity
which purports to be the rightful owner of edge network X really is
that entity.  Once an ISP accepts this, that entity can have the ISP
propagate a MapSet all over the world.

LISP-ALT is probably similar in this regard.

I think it is much better to have a centralised system, unrelated to
ISPs and ETRs, by which the end-user controlling the mapping.

Charging for mapping changes
----------------------------

Like today's BGP system, APT has a completely distributed system by
which end-users inject their changes into the global system.  Every
such change propagates and costs resources in some or all the
networks in the global system.

Because there is no central registry of what is legitimate or not,
and no gateway, there is no way of preventing someone changing their
mapping ever 20 minutes or whatever.  This is true for BGP and APT.

With Ivip, the end-user pays the RUAS for propagating each mapping
change, and possibly for the rental of their address space - with
the RUAS running "anycast ITRs in the core/DFZ" for their address
space, and contributing to the cost of the global "Replicator"
fast-push network.

So Ivip contains a revenue generation system - the RUAS charging per
update - by which the cost of the fast-push system can be recovered.
 This ensures that no matter what the demand and whatever the cost
of supplying the push service, there will be some price set which
makes the system profitable, or at least sustainable.

With BGP and ALT, each injected change is a burden to all others -
and there is no way of collecting fees or stopping an "unreasonable
number" of changes without also disrupting "legitimate" changes
(however judged).

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---

From: Scott Brim <[email-masked]>
Date: Wed, 12 Mar 2008 09:29:21 -0400

> can be placed anywhere, but the Provider Edge would be a good place.

In order for a site to be able to coordinate behavior of its xTRs, to
any degree and in any way it chooses, it is good to have them under
the site's direct control.  I don't believe we can predict or should
control how a site's xTRs work together.  I hope to be surprised at
sites' innovation.  So it's good if we take that out of the realm of
inter-provider cooperation and put it under the site's control.