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Versions: 00 01                                                         
INTERNET-DRAFT                                      Iljitsch van Beijnum
Jul 11, 2005

                        Shim6 Reachability Detection

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      Copyright (C) The Internet Society (2005).  All Rights Reserved.


The shim6 working group is developing a mechanism that allows
multihoming by using multiple addresses. When communication between
the initially chosen addresses for a transport session is no longer
possible, a "shim" layer makes it possible to switch to a different
set of addresses without breaking current transport protocol
assumptions. This draft discusses the issues of detecting failures
in a currently used address pair between two hosts and picking a
new address pair to be used when a failure occurs. The input for
these processes are ordered lists of local and remote addresses
that are reasonably likely to work. (I.e., not include addresses
that are known to be unreachable for local reasons.) These lists
must be available at both ends of the communication, although the
ordering may differ. Building these address lists from locally
available information and synchronizing them with the remote end
are outside the scope of this document.

This text is for the most part based on discussions on the multi6
list, several multi6 design team lists and the shim6 list, with
notable contributions from Erik Nordmark, Marcelo Bagnulo and Jari
Arkko. Suggestions and additions are more than welcome.

1 Introduction

A naive implementation of an (un)reachability detection mechanism
could just probe all possible paths between two hosts periodically.
A "path" is defined as a combination of a source address for host A
and a destination address for host B. In hop-by-hop forwarding the
source address doesn't have any effect on reachability, but in the
presence of filters or source address based routing, it may. And
although links almost always work in two directions, routing
protocols and filters only work in one direction so unidirectional
reachability can happen. Without additional mechanisms, the
practice of ingress filtering by ISPs makes unidirectional
connectivity likely. Being able to use the working leg in a
unidirectional path is useful, it's not an essential requirement.
It is essential, however, to avoid assuming bidirectional
connectivity when there is in fact a unidirectional failure.

Exploring the full set of communication options between two hosts
that both have two or more addresses is an expensive operation as
the number of combinations to be explored increases very quickly
with the number of addresses. For instance, with two addresses on
both sides, there are four possible address pairs. Since we can't
assume that reachability in one direction automatically means
reachability for the complement pair in the other direction, the
total number of two-way combinations is eight. (Combinations = nA *
nB * 2.)

An important observation in multihoming is that failures are
relatively infrequent, so that a path that worked a few seconds ago
is very likely to work now as well. So it makes sense to have a
light-weight protocol that confirms existing reachability, and only
invoke the much heavier protocol that can determine full
reachability when a there is a suspected failure.

2 Determining reachability for the current pair

Reachability for the currently used address pair in a shim context
is determined by making sure that whenever there is data traffic in
one direction, there is also traffic in the other direction. This
can be data traffic as well, but also transport layer
acknowledgments or a shim reachability keepalive if there is no
other traffic. This way, it is no longer possible to have traffic
in only one direction, so whenever there is data traffic going out,
but there are no return packets, there must be a failure, so the
full path exploration mechanism is started.

A more detailed description of the current pair reachability
evaluation mechanism:

1. The base timing unit for this mechanism is named ShimKeepT.
   Until a negotiation mechanism to negotiate different values for
   ShimKeepT becomes available, a value of 10 for ShimKeepT MUST be

2. Whenever outgoing packets are generated that are part of a shim
   context, one of two timestamps belonging to the shim context is
   updated: the timestamp for outgoing data packets, or the timestamp
   for outgoing non-data packets. The difference between the two is
   that data packets are packets that should generate return traffic.
   The host should use the information available to it to determine
   whether a packet is a data or a non-data packet. Examples of
   non-data packets are TCP ACKs and shim keepalive packets. If there
   is any doubt, a packet should be considered a data packet.

3. Whenever incoming packets are received that are part of a shim
   context, one of two timestamps belonging to the shim context is
   updated: the timestamp for incoming data packets, or the timestamp
   of incoming non-data packets. For incoming packets, it's less
   critical that packets are labeled as data or non-data correctly. In
   the absence of better information, hosts may assume that any IPv6
   packet with a total length field with a value of 20 or lower is a
   non-data packet.

4. ShimKeepT seconds after the last data packet has been received
   for a context, and if no other packet has been sent within this
   context since the data packet has been received, a shim keepalive
   packet is generated for the context in question and transmitted to
   the correspondent. The shim keepalive packet consists of an IPv6
   header and a shim header containing the context tag, but no
   subsequent headers. Intermediate headers may be present between the
   IPv6 and shim headers. A host may send the shim keepalive after
   fewer than ShimKeepT seconds if implementation considerations
   warrant this. The average time after which shim keepalives are sent
   must be at least ShimKeepT / 2 seconds. After potentially sending a
   single shim keepalive, no additional shim keepalives are sent until
   a data packet is received within this shim context. If the shim
   keepalive wasn't sent because a data or non-data packet was sent
   since the last received data packet, no shim keepalives are sent.

5. When after a timeout period since the last transmission of a
   data packet no packets were received from the correspondent within
   this context, a full reachability exploration is started. The
   timeout period is ShimKeepT seconds plus additional time to
   accommodate for a round trip and regular variations in
   network-related functions. In the absence of better information, a
   timeout of at least ShimKeepT + 2 seconds but no more than
   ShimKeepT + 5 seconds is recommended.

3 Address pair exploration

In its essence, address pair exploration is very simple: just send
probes using every possible address pair, wait for something to
come back and possibly consider the round trip time. In practice,
testing the full combination of all source addresses and all
destination addresses is very undesirable because of the large
number of packets involved. This can be especially harmful when a
lot of hosts on a link start doing this for many of their
correspondents at the same time when there is a failure further

In order to arrive at a desired outcome more quickly and with less
packets, and also to accommodate traffic engineering needs, we'll
assume a model where each address (source or destination) has two
preference values: p1 and p2. Addresses within the same set (source
or destination) are ranked by their p1 value, where a higher p1
means that the address is more preferred. When there are multiple
addresses with the same p1 value, an address is selected at random
from the group with the same p1 value, where the likelihood of
selecting any given address is relative to its p2 value compared to
the sum of all p2 values. So if addresses A, B and C have the same
p1 value and p2 values of 10, 30 and 60 for a total of 100, the
chance that A is selected is 10%, the chance that B is selected is
30% and the chance that C is selected is 60%.

Note that preference information may be related to type of service.
So different context with different type of service requirements
may see different p1 and p2 values for a given address.

When a host suspects that there is a failure for a context, it
gathers the set of possible source addresses and the set of
possible destination addresses. Both sets are ordered such that
each next address has an equal or lower p1 value. Addresses with
the same p1 value are further ordered as per any heuristics that
the host may employ, such as longest prefix matches on known
working and/or known not working addresses along with the p2 value.
The p2 value is considered relatively weak, and breaking p2
ordering is allowed if there is a sufficient reason for this.
However, in the absence of other information, p2 ordering should be
used. P1 ordering overrules any other information except a recent
reachability failure for the address in question. In addition to
this, the most recently used address is put in front of the list.

From the lists of eligible source and destination addresses, the
host creates a list of source/destination address pairs, along with
a combined preference value for this address pair. The calculation
of the preference value is implementation specific, with the only
requirement being that when one address pair has a higher p1 for
both the source and destination address than another pair, the pair
with the higher p1 values also has a higher combined pair
preference value.

The list of address pairs from different contexts is combined into
a host-wide list of address pairs. The preference values are
updated to take into consideration the number of contexts that is
interested in the pair. The specifics of calculating the resulting
host-wide preference value are left upto the implementation, but
implementations SHOULD try, within reason, to avoid using address
pairs with lower p1 values when pairs with higher p1 values are
available for a context. Context-specific address pair preferences
may be normalized prior to calculating host-wide address pair
preference values. (So when context A has pairs P and Q with p1
values 10 and 1, while context B has pairs R and S with p1 values 7
and 4, the values for P and R are changed to 2 and the values for Q
and S to 1.)

The host now starts probing address pairs, in order from the pair
with the highest pair preference to the pair with the lowest pair
preference. When all address pairs have been tested, testing
restarts from the pair with the highest preference. New pairs that
become available are put in the list before pairs that have been
probed already, regardless of the preference values. However, both
the group of address pairs that haven't been probed and the group
of address pairs that have may be reordered to reflect the
preference values, as long as reordering is done such that
starvation doesn't occur.

When a probe is answered by the correspondent, the context that use
the address pair in question are informed so they can start
remapping address is outgoing packets to the pair in question. (All
of this also happens when there is a working pair but an address
pair with at least one address with a higher preference is
determined to work.) At this point, the context updates its list of
address pairs to probe by removing all pairs where either the
source address has a lower p1 value than the p1 value of the now
working source address, or the destination address has a lower p1
value than the p1 value of the now working destination address.
Additionally, all address pairs where the p1 values for the source
and destination addresses match the respective p1 values of the
source and destination addresses in the now working pair are
removed from the list. The host-wide list of address pair to probe
is updated to reflect the removal of lower or equal priority
addresses, so probing will only continue for pairs where at least
one address has a higher p1 than the currently working pair.

The time between probes (ShimProbeT) must be chosen such that the
number of probes is limited to 60 per 300 second period. When no
probes have been sent for some time, an implementation may send the
initial group of probes at a fairly aggressive rate. For instance,
when no probes have been sent for 60 seconds, a host may send a
second probe 200 ms after the first one, and increase the
ShimProbeT by a factor 1.25 after every probe, until ShimProbeT
reaches 5 seconds. This results in sending 5 probes in the first 2
seconds and/or 14 probes within the first 20 seconds after a
failure. After that, there is one probe every 5 seconds.

When a context didn't see any outgoing data packets (see section 2)
for four minutes, it removes all its address pairs from the
host-wide list of address pairs.

4 Address pair exploration packet format

The address pair exploration packet may be encapsulated in
different ways. An obvious way is inside a shim header. The address
pair exploration packet contains the following information:

- A type field that is at least 8 bits long
- An 8 bit "number of probes sent" field
- An 8 bit "number of probes received" field
- An 8 bit "options length" field
- One or more sent probes (see below)
- Zero or more received probes (see below)
- Zero or more bytes of option data

There is currently one bit in the type field defined: the reply
requested bit. If this bit is set, the other side should send a
probe in reply to this probe.

The option data contains zero or more options in the following

- An 8 bit option type
- An 8 bit option length
- Zero or more bytes of data in this option

Sent and received probes contain data in the following format:

- Source locator/address (128 bits)
- Destination locator/address (128 bits)
- Sent timestamp (32 bits in ms resolution relative to private epoch)
- Time between reception and retransmission (32 bits in ms resolution,
  0 on first transmission)
- Nonce (32 bits)
- Sequence number (32 bits)

The first and only mandatory sent probe structure contains the
addresses that are present in the current IPv6 packet along with a
timestamp for the current time. Additional probe structures contain
copies of earlier probes, presumably toward different addresses,
with the appropriate field indicating how long ago the probe in
question was sent. The received probes are copies of the last seen
probes from the other side.

Note that an application must be able to infer which addresses
belong to the same host in order to perform this probing correctly

5 NAT and firewall considerations

Since shim6 is chartered for IPv6 solutions only, and NAT
compatibility is not expected, and by most people, not desired in
IPv6, there is no requirement for this protocol to pass through
Network Address Translation devices. However, the protocol may be
applicable outside shim6, making NAT compatibility desirable.

It is absolutely essential that the shim6 negotiations and the
reachability detection packets are passed through filters or
firewalls wherever application packets are passed through. If the
shim6 negotiation and reachability detection packets are filtered
out, shim6 can't be used.

A more complex situation arises when the shim6 negotiation packets
pass through a firewall, but the reachability detection packets are
blocked. To avoid this complexity, it's highly desirable to make
the shim6 negotiation and reachability detection part of the same
protocol, so either both are allowed through or both are blocked.
However, the same is true if this reachability detection mechanism
is used in other protocols. This makes it desirable to define the
reachability detection protocol such that it can be embedded in
other protocols.

Since firewalls are in wide use, it's important to consider whether
a new protocol will be able to pass through most firewalls without
requiring changes to the filter configuration. On the other hand,
it may not be possible to come up with a protocol that would be
allowed through a large percentage of all firewalls without
changes, so extra effort in this area may produce limited results.
Also, in the long run firewall configuration will presumably be
changed, so any compromises would only have short term benefits but
long term downsides.

6 Security considerations

To avoid exposing information (even if it's just the fact that an
address is reachable), hosts will probably want to limit themselves
to taking part in reachability detection with known correspondents.
This means that there must be identifying information and a nonce
that is at least hard to guess but easy to check in all
reachability detection packets.

4 Document and author information

This document expires April, 2006. The latest version will always
be available at http://www.muada.com/drafts/. Comments are welcome

    Iljitsch van Beijnum

    Email: iljitsch@muada.com

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Van Beijnum             Expires January 11, 2006                [Page 6]