Transmission of IPv6 Packets over AERO LinksBoeing Research & TechnologyP.O. Box 3707SeattleWA98124USAfltemplin@acm.orgI-DInternet-DraftThis document specifies the operation of IPv6 over tunnel virtual
Non-Broadcast, Multiple Access (NBMA) links using Asymmetric Extended
Route Optimization (AERO). Nodes attached to AERO links can exchange
packets via trusted intermediate routers on the link that provide
forwarding services to reach off-link destinations and/or redirection
services to inform the node of an on-link neighbor that is closer to the
final destination. Operation of the IPv6 Neighbor Discovery (ND)
protocol over AERO links is based on an IPv6 link local address format
known as the AERO address.This document specifies the operation of IPv6 over tunnel virtual
Non-Broadcast, Multiple Access (NBMA) links using Asymmetric Extended
Route Optimization (AERO). Nodes attached to AERO links can exchange
packets via trusted intermediate routers on the link that provide
forwarding services to reach off-link destinations and/or redirection
services to inform the node of an on-link neighbor that is closer to the
final destination. This redirection provides a route optimization
capability that addresses the requirements outlined in .Nodes on AERO links use an IPv6 link-local address format known as
the AERO Address. This address type has properties that statelessly link
IPv6 Neighbor Discovery (ND) to IPv6 routing. The AERO link can be used
for tunneling to neighboring nodes on either IPv6 or IPv4 networks,
i.e., AERO views the IPv6 and IPv4 networks as equivalent links for
tunneling. The remainder of this document presents the AERO
specification.The terminology in the normative references applies; the following
terms are defined within the scope of this document:a Non-Broadcast, Multiple Access
(NBMA) tunnel virtual overlay configured over a node's attached IPv6
and/or IPv4 networks. All nodes on the AERO link appear as
single-hop neighbors from the perspective of IPv6.a node's attachment to an AERO
link. The AERO interface Maximum Transmission Unit (MTU) is less
than or equal to the AERO link MTU.an IPv6 link-local address
assigned to an AERO interface and constructed as specified in
Section 3.6.a node that is connected to an AERO
link and that participates in IPv6 Neighbor Discovery over the
link.a node that
configures either a host interface or a router interface on an AERO
link.a node that
configures a router interface on an AERO link over which it can
provide default forwarding and redirection services for other AERO
nodes.a node that
relays IPv6 packets between Servers on the same AERO link, and/or
that forwards IPv6 packets between the AERO link and the IPv6
Internet. An AERO Relay may or may not also be configured as an AERO
Server.an AERO
interface endpoint that injects tunneled packets into an AERO
link.an AERO
interface endpoint that receives tunneled packets from an AERO
link.a connected IPv6 or IPv4
network routing region over which AERO nodes tunnel IPv6
packets.an AERO node's interface
point of attachment to an underlying network.an IP address assigned to
an AERO node's underlying interface. When UDP encapsulation is used,
the UDP port number is also considered as part of the underlying
address. Underlying addresses are used as the source and destination
addresses of the AERO encapsulation header.the same as defined for
"underlying address" above, and formed from the concatenation of the
UDP port number and underlying address as specified in Section
3.3.an IPv6 address used as
the source or destination address of the inner IPv6 packet
header.an IPv6 network
attached to a downstream interface of an AERO Client (where the AERO
interface is seen as the upstream interface).Throughout the document, the simple terms "Server" and "Relay"
refer to "AERO Server" and "AERO Relay", respectively. Capitalization is
used to distinguish these terms from DHCPv6 server and DHCPv6 relay.
This is an important distinction, since an AERO Server may be a DHCPv6
relay, and an AERO Relay may be a DHCPv6 server.The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in .The following sections specify the operation of IPv6 over Asymmetric
Extended Route Optimization (AERO) links:AERO Relays relay packets between nodes connected to the same AERO
link and also forward packets between the AERO link and the native
IPv6 network. The relaying process entails re-encapsulation of IPv6
packets that were received from a first AERO node and are to be
forwarded without modification to a second AERO node.AERO Servers configure their AERO interfaces as router interfaces,
and provide default routing services to AERO Clients. AERO Servers
configure a DHCPv6 relay or server function and facilitate DHCPv6
Prefix Delegation (PD) exchanges. An AERO Server may also act as an
AERO Relay.AERO Clients act as requesting routers to receive IPv6 prefixes
through a DHCPv6 PD exchange via an AERO Server over the AERO link.
(In Server-less environments an alternate AERO link prefix delegation
authority may be necessary, but out of scope for this document.) Each
AERO Client receives at least a /64 prefix delegation, and may receive
even shorter prefixes.AERO Clients that act as routers configure their AERO interfaces as
router interfaces and sub-delegate portions of their received prefix
delegations to links on EUNs.AERO Clients that act as ordinary hosts configure their AERO
interfaces as host interfaces and assign one or more IPv6 addresses
taken from their received prefix delegations to the AERO interface but
DO NOT assign the delegated prefix itself to the AERO interface.
Instead, the host assigns the delegated prefix to a "black hole" route
so that unused portions of the prefix are nullified.End system applications on AERO hosts bind directly to the AERO
interface, while applications on AERO routers (or IPv6 hosts served by
an AERO router) bind to EUN interfaces.An AERO address is an IPv6 link-local address assigned to an AERO
interface and with an IPv6 prefix embedded within the interface
identifier. The AERO address is formatted as:fe80::[IPv6 prefix]Each AERO Client configures an AERO address based on the prefix it
has received from the AERO link prefix delegation authority (e.g., the
DHCPv6 server). The address begins with the prefix fe80::/64 and
includes in its interface identifier the base /64 prefix taken from
the Client's delegated IPv6 prefix. The base prefix is determined by
masking the delegated prefix with the prefix length. For example, if
an AERO Client has received the prefix delegation:2001:db8:1000:2000::/56it would construct its AERO address as:fe80::2001:db8:1000:2000The AERO address remains stable as the Client moves between
topological locations, i.e., even if its underlying address
changes.AERO interfaces use IPv6-in-IPv6 encapsulation to exchange tunneled packets with AERO neighbors
attached to an underlying IPv6 network, and use IPv6-in-IPv4
encapsulation to exchange tunneled packets
with AERO neighbors attached to an underlying IPv4 network. AERO
interfaces can also use secured tunnel types such as IPsec or TLS in environments
where strong authentication and confidentiality are required. When NAT
traversal and/or filtering middlebox traversal is necessary, a UDP
header is further inserted immediately above the outer IP
encapsulation header.Servers assign the link-local address 'fe80::0' to their AERO
interface; this provides a handle for Clients to insert into a
neighbor cache entry for their current Server. Servers and Relays also
configure administratively-assigned link-local addresses on their AERO
interfaces to support the operation of the inter-Server/Relay routing
system (see: ).Clients initially use a "temporary" IPv6 link-local address in the
DHCPv6 PD exchanges used to receive an IPv6 prefix and derive an AERO
address. If the Client is pre-provisioned with an IPv6 prefix
associated with the AERO service, it SHOULD use the AERO address
derived from the prefix as the temporary address. Otherwise, the
Client SHOULD use "fe80::1" as the temporary address since this
address will not conflict with any valid AERO addresses and will thus
not be used in any AERO neighbor discovery messaging. After the Client
receives a prefix delegation, it assigns the corresponding AERO
address to the AERO interface. DHCPv6 is therefore used to bootstrap
the assignment of link-local addresses on the AERO link.AERO interfaces maintain a neighbor cache and use an augmentation
of standard unicast IPv6 ND messaging. AERO interfaces use Redirect,
Neighbor Solicitation (NS) and Neighbor Advertisement (NA) Router
Solicitation (RS) and Router Advertisement (RA) messages the same as
for any IPv6 link. Finally, AERO links use link-local-only addressing;
hence, Clients MUST ignore any Prefix Information Options (PIOs) they
may receive in RA messages.AERO Redirect messages include a TLLAO the same as for any IPv6
link. The TLLAO includes the link-layer address for the target node,
which is formed from the concatenation of the 2-octet UDP port number
used by the target when it sends UDP-encapsulated packets over the
AERO interface (or 0 when the target does not use UDP encapsulation)
followed by the 16-octet IP address. The TLLAO format is shown in
:(Note that in the above TLLAO format, the IP address is formed as
an IPv4-compatible IPv6 address (see: ) when
the encapsulation IP address family is IPv4. Note also that more than
one TLLAO option may appear in a Redirect message, e.g., if the target
node has multiple link-layer addresses.)AERO NS, NA, RS and RA messages do not include Source/Target Link
Layer Address Options (S/TLLAOs). Instead, AERO nodes determine the
link-layer addresses of neighbors by examining the link-layer source
address of any NS/NA/RS/RA messages they receive and ignore any
S/TLLAOs included in these messages. This is vital to the operation of
AERO links for which neighbors are separated by Network Address
Translators (NATs) (either IPv4 or IPv6) since the source may have no
way of knowing what its translated address will be and hence may not
be able to supply the correct values in a S/TLLAO.Finally, AERO interface NS/NA messages only update existing
neighbor cache entires and do not create new neighbor cache entries,
whereas Redirect, RS and RA messages both update and create neighbor
cache entries. This represents a departure from the normal operation
of IPv6 ND over common link types, but is consistent with the spirit
of IPv6 over NBMA links as discussed in . Note
however that this restriction may be relaxed and/or redefined on AERO
links that participate in a fully distributed mobility management
model (i.e., a "Client-only" AERO link) coordinated in a manner
outside the scope of this document.Nodes on AERO interfaces use a simple data origin authentication
for encapsulated packets they receive from other nodes. In particular,
AERO Clients accept encapsulated packets with a link-layer source
address belonging to their current AERO Server. AERO nodes also accept
encapsulated packets with a link-layer source address that is correct
for the network-layer source address.The AERO node considers the link-layer source address correct for
the network-layer source address if there is an IPv6 forwarding table
entry that matches the network-layer source address as well as a
neighbor cache entry corresponding to the next hop that includes the
link-layer address. An exception is that neighbor discovery messages
may include a different link-layer address than the one currently in
the neighbor cache, and the new link-layer address updates the
neighbor cache entry.Each AERO node maintains a per-AERO interface conceptual neighbor
cache that includes an entry for each neighbor it communicates with on
the AERO link, the same as for any IPv6 interface (see ). Neighbor cache entries are created and maintained
as follows:When an AERO Server relays a DHCPv6 Reply message to an AERO
Client, it creates or updates a neighbor cache entry for the Client
based on the information in the IA-PD option.When an AERO node receives a valid RS/RA message, it creates or
updates a neighbor cache entry the same as described in .When an AERO Client receives a valid Predirect message (See Section
3.10.5) it creates or updates a neighbor cache entry for the Predirect
target L3 and L2 addresses, and also creates an IPv6 forwarding table
entry for the Predirected (source) prefix. The node then sets an
ACCEPT timer and uses this timer to validate any messages received
from the Predirected neighbor.When an AERO Client receives a valid Redirect message (see Section
3.10.7) it creates or updates a dynamic neighbor cache entry for the
Redirect target L3 and L2 addresses, and also creates an IPv6
forwarding table entry for the Redirected (destination) prefix. The
node then sets a FORWARD timer and uses this timer to determine
whether packets can be sent directly to the Redirected neighbor. The
node also maintains a constant value MAX_RETRY to limit the number of
keepalives sent when a neighbor has gone unreachable.It is RECOMMENDED that FORWARD_TIME be set to the default constant
value 30 seconds to match the default REACHABLE_TIME value specified
for IPv6 neighbor discovery .It is RECOMMENDED that ACCEPT_TIME be set to the default constant
value 40 seconds to allow a 10 second window so that the AERO
redirection procedure can converge before the ACCEPT_TIME timer
decrements below FORWARD_TIME.It is RECOMMENDED that MAX_RETRY be set to 3 the same as described
for IPv6 neighbor discovery address resolution in Section 7.3.3 of
.Different values for FORWARD_TIME, ACCEPT_TIME, and MAX_RETRY MAY
be administratively set, if necessary, to better match the AERO link's
performance characteristics; however, if different values are chosen,
all nodes on the link MUST consistently configure the same values.
ACCEPT_TIME SHOULD further be set to a value that is sufficiently
longer than FORWARD_TIME to allow the AERO redirection procedure to
converge.The AERO link Maximum Transmission Unit (MTU) is 64KB minus the
encapsulation overhead for IPv4 and 4GB minus
the encapsulation overhead for IPv6 . This is
the most that IPv4 and IPv6 (respectively) can convey within the
constraints of protocol constants, but actual sizes available for
tunneling will frequently be much smaller.The base tunneling specifications for IPv4 and IPv6 typically set a
static MTU on the tunnel interface to 1500 bytes minus the
encapsulation overhead or smaller still if the tunnel is likely to
incur additional encapsulations on the path. This can result in path
MTU related black holes when packets that are too large to be
accommodated over the AERO link are dropped, but the resulting ICMP
Packet Too Big (PTB) messages are lost on the return path. As a
result, AERO nodes use the following MTU mitigations to accommodate
larger packets.AERO nodes set their AERO interface MTU to the larger of the
underlying interface MTU minus the encapsulation overhead, and 1500
bytes. AERO nodes optionally cache other per-neighbor MTU values in
the underlying IP path MTU discovery cache initialized to the
underlying interface MTU.AERO nodes admit packets that are no larger than 1280 bytes minus
the encapsulation overhead (*) as well as packets that are larger than
1500 bytes into the tunnel without fragmentation, i.e., as long as
they are no larger than the AERO interface MTU before encapsulation
and also no larger than the cached per-neighbor MTU following
encapsulation. For IPv4, the node sets the "Don't Fragment" (DF) bit
to 0 for packets no larger than 1280 bytes minus the encapsulation
overhead (*) and sets the DF bit to 1 for packets larger than 1500
bytes. If a large packet is lost in the path, the node may optionally
cache the MTU reported in the resulting PTB message or may ignore the
message, e.g., if there is a possibility that the message is
spurious.For packets destined to an AERO node that are larger than 1280
bytes minus the encapsulation overhead (*) but no larger than 1500
bytes, the node uses outer IP fragmentation to fragment the packet
into two pieces (where the first fragment contains 1024 bytes of the
fragmented inner packet) then admits the fragments into the tunnel. If
the outer protocol is IPv4, the node admits the packet into the tunnel
with DF set to 0 and subject to rate limiting to avoid reassembly
errors . For both IPv4
and IPv6, the node also sends a 1500 byte probe message (**) to the
neighbor, subject to rate limiting. To construct a probe, the node
prepares an ICMPv6 Neighbor Solicitation (NS) message with trailing
padding octets added to a length of 1500 bytes but does not include
the length of the padding in the IPv6 Payload Length field. The node
then encapsulates the NS in the outer encapsulation headers (while
including the length of the padding in the outer length fields), sets
DF to 1 (for IPv4) and sends the padded NS message to the neighbor. If
the neighbor returns an NA message, the node may then send whole
packets within this size range and (for IPv4) relax the rate limiting
requirement.AERO nodes MUST be capable of reassembling packets up to 1500 bytes
plus the encapsulation overhead length. It is therefore RECOMMENDED
that AERO nodes be capable of reassembling at least 2KB.(*) Note that if it is known without probing that the minimum Path
MTU to an AERO node is MINMTU bytes (where 1280 < MINMTU < 1500)
then MINMTU can be used instead of 1280 in the fragmentation threshold
considerations listed above.(**) It is RECOMMENDED that no probes smaller than 1500 bytes be
used for MTU probing purposes, since smaller probes may be fragmented
if there is a nested tunnel somewhere on the path to the neighbor.
Probe sizes larger than 1500 bytes MAY be used, but may be unnecessary
since original sources are expected to implement when sending large
packets.AERO interfaces encapsulate IPv6 packets according to whether they
are entering the AERO interface for the first time or if they are
being forwarded out the same AERO interface that they arrived on. This
latter form of encapsulation is known as "re-encapsulation".AERO interfaces encapsulate packets per the specifications in
except that the interface copies the "Hop Limit", "Traffic Class" and
"Congestion Experienced" values in the inner IPv6 header into the
corresponding fields in the outer IP header. For packets undergoing
re-encapsulation, the AERO interface instead copies the "TTL/Hop
Limit", "Type of Service/Traffic Class" and "Congestion Experienced"
values in the original outer IP header into the corresponding fields
in the new outer IP header (i.e., the values are transferred between
outer headers and *not* copied from the inner network layer
header).When UDP encapsulation is used, the AERO interface inserts a UDP
header immediately above the outer IP header. The AERO interface sets
the UDP source port to a constant value that it will use in each
successive packet it sends, sets the UDP checksum field to zero (see:
) and sets the UDP
length field to the length of the inner packet plus 8 bytes for the
UDP header itself. For packets sent via a Server, the AERO interface
sets the UDP destination port to 8060 (i.e., the IANA-registerd port
number for AERO). For packets sent to a neighboring Client, the AERO
interface sets the UDP destination port to the port value stored in
the neighbor cache entry for this neighbor.The AERO interface next sets the outer IP protocol number to the
appropriate value for the first protocol layer within the
encapsulation (e.g., IPv6, UDP, IPsec, etc.). When IPv6 is used as the
outer IP protocol, the ITE then sets the flow label value in the outer
IPv6 header the same as described in . When
IPv4 is used as the outer IP protocol, the AERO interface sets the DF
bit as discussed in Section 3.6.AERO interfaces decapsulate packets destined either to the node
itself or to a destination reached via an interface other than the
receiving AERO interface per the specifications in . When the encapsulated
packet includes a UDP header, the AERO interface examines the first
octet of data following the UDP header. If the most significant four
bits of the first octet encode the value '0110', the inner header is
an IPv6 header; otherwise, the packet is discarded. During the
decapsulation, the AERO interface records the UDP source port in the
neighbor cache entry for this neighbor then discards the UDP
header.Note that AERO messaging and addressing can also be used in
conjunction with other tunnel types such as IPsec and TLS . In that case, the
native encapsulation format of the tunnel is used, and the AERO
messaging and addressing mechanisms are applied as a layered
extension. All other aspects of AERO neighbor coordination are
as-specified in this document. depicts the AERO reference
operational scenario. The figure shows an AERO Server('A'), two AERO
Clients ('B', 'D') and three ordinary IPv6 hosts ('C', 'E', 'F'):In , AERO Server ('A')
connects to the AERO link and connects to the IPv6 Internet, either
directly or via an AERO Relay (not shown). Server ('A') assigns the
address fe80::0 to its AERO interface with link-layer address L2(A).
Server ('A') next arranges to add L2(A) to a published list of valid
Servers for the AERO link.AERO Client ('B') registers the IPv6 prefix 2001:db8:0::/48 in a
DHCPv6 PD exchange via AERO Server ('A') then assigns the address
fe80::2001:db8:0:0 to its AERO interface with link-layer address
L2(B). Client ('B') configures a default route via the AERO interface
with next-hop address fe80::0 and link-layer address L2(A), then
sub-delegates the prefix 2001:db8:0::/48 to its attached EUNs. IPv6
host ('C') connects to the EUN, and configures the address
2001:db8:0::1.AERO Client ('D') registers the IPv6 prefix 2001:db8:1::/48 in a
DHCPv6 PD exchange via AERO Server ('A') then assigns the address
fe80::2001:db8:1:0 to its AERO interface with link-layer address
L2(D). Client ('D') configures a default route via the AERO interface
with next-hop address fe80::0 and link-layer address L2(A), then
sub-delegates the prefix 2001:db8:1::/48 to its attached EUNs. IPv6
host ('E') connects to the EUN, and configures the address
2001:db8:1::1.Finally, IPv6 host ('F') connects to an IPv6 network outside of the
AERO link domain. Host ('F') configures its IPv6 interface in a manner
specific to its attached IPv6 link, and assigns the address
2001:db8:2::1 to its IPv6 link interface.AERO Clients observe the IPv6 node requirements defined in . AERO Clients first discover the link-layer
address of an AERO Server via static configuration, or through an
automated means such as DNS name resolution. In the absence of other
information, the Client resolves the Fully-Qualified Domain Name
(FQDN) "linkupnetworks.domainname", where "domainname" is the DNS
domain appropriate for the Client's attached underlying network. The
Client then creates a static neighbor cache entry with fe80::0 as
the network-layer address and the discovered address as the
link-layer address then creates a static default IPv6 forwarding
table entry with fe80::0 as the next hop address.Next, the Client acts as a requesting router to request an IPv6
prefix through DHCPv6 PD via the AERO
Server using a temporary link-local address (see: Section 3.3) as
the IPv6 source address and fe80::0 as the IPv6 destination address.
The Client includes a DHCPv6 Unique Identifier (DUID) in the Client
Identifier option of its DHCPv6 messages
and includes any additional authenticating information necessary to
authenticate itself to the DHCPv6 server. (Note that other DUID
formats such as DUID-UUID may also be
used.) If the Client is pre-provisioned with an IPv6 prefix
associated with the AERO service, it MAY also include the prefix in
an IA-PD option in its DHCPv6 Request command to indicate its
preferred prefix to the DHCPv6 server.After the Client receives its prefix delegation, it assigns the
link-local AERO address taken from the prefix to the AERO interface
(see: Section 3.3) and sub-delegates the prefix to nodes and links
within its attached EUNs (the AERO link-local address thereafter
remains stable as the Client moves). The Client further renews its
prefix delegation via standard DHCPv6 procedures by sending DHCPv6
Renew messages with its AERO address as the IPv6 source address,
fe80::0 as the IPv6 destination address and the same DUID value in
the Client Identifier option.The Client then sends an RS message to the Server to receive an
RA message with a default router lifetime and other configuration
information. The Client ignores any Prefix Information Options
(PIOs) included in the RA message, since the AERO link is
link-local-only. The Client further ignores any RS messages it might
receive, since only Servers may process RS messages. The Client can
then send periodic NS messages to the Server to obtain new NA
messages for Neighbor Unreachability Detection (NUD) and to refresh
any network state, and can send periodic RS messages to obtain new
RA messages in order to update the default router lifetimes. (The
Client can alternately use RS messages for both purposes, but NS/NA
exchanges are the standard method for performing NUD.) The Client
can also forward IPv6 packets destined to networks beyond its local
EUNs via the Server as an IPv6 default router. The Server may in
turn return a Redirect message informing the Client of a neighbor on
the AERO link that is topologically closer to the final destination
as specified in .Note that, since the Client's AERO address is configured from the
unique prefix delegation it receives via the Server, there is no
need for Duplicate Address Detection (DAD) on AERO links. Other
nodes maliciously attempting to hijack an authorized Client's AERO
address will be denied due to an unacceptable link-layer address
and/or security parameters (see: Security Considerations).AERO Servers observe the IPv6 router requirements defined in
and further configure a DHCPv6 relay
function on their AERO links. When the AERO Server relays a Client's
DHCPv6 PD messages to the DHCPv6 server, it wraps each message in a
"Relay-forward" message per and includes a
DHCPv6 Interface Identifier option that encodes a value that
identifies the AERO link to the DHCPv6 server.The AERO Server then includes the Client's link-layer address in
a Client Link Layer Address Option (CLLAO)
with the link-layer address format shown in , i.e., a 2-octet UDP port number followed by a
16-octet IP address. The Server sets the CLLAO 'option-length' field
to 20 (2 plus the length of the link-layer address) and sets the
'link-layer type' field to TBD (see: IANA Considerations). The
Server finally includes a DHCPv6 Echo Request Option (ERO) that encodes the option code for the CLLAO in a
'requested-option-code-n' field. The CLLAO information will
therefore subsequently be echoed back in the DHCPv6 Server's
"Relay-reply" message.When the DHCPv6 server issues the IPv6 prefix delegation in a
"Relay-reply" message via the AERO Server (acting as a DHCPv6
relay), the AERO Server obtains the Client's link-layer address from
the echoed CLLAO option and obtains the Client's delegated prefix
from the included IA_PD option. The Server then creates a static
neighbor cache entry for the Client's AERO address (see: Section
3.3) with the Client's link-layer address as the link-layer address
for the neighbor cache entry. The Server also configures an IPv6
forwarding table entry that lists the Client's AERO address as the
next hop toward the delegated IPv6 prefix with a lifetime derived
from the DHCPv6 lease lifetime. The AERO Server finally injects the
Client's prefix as an IPv6 route into the inter-Server/Relay routing
system (see: ) then relays the DHCPv6 message
to the Client while using fe80::0 as the IPv6 source address, the
link-local address found in the "peer address" field of the
Relay-reply message as the IPv6 destination address, and the
Client's link-layer address as the destination link-layer
address.Servers respond to RS/NS messages from Clients on their AERO
interfaces by returning an RA/NA message. When the Server receives
an RS/NS message, it updates the neighbor cache entry using the
network layer source address as the neighbor's network layer address
and using the link-layer source address of the RS/NS message as the
neighbor's link-layer address. The Server SHOULD NOT include PIOs in
any RA messages it sends to Clients, since the Client will ignore
any such options.Servers ignore any RA messages they may receive from a Client.
Servers MAY examine RA messages they may receive from other Servers
for consistency verification purposes.When the Server forwards a packet via the same AERO interface on
which it arrived, it initiates an AERO route optimization procedure
as specified in . describes the AERO reference
operational scenario. We now discuss the operation and protocol
details of AERO Redirection with respect to this reference
scenario.With reference to , when the
IPv6 source host ('C') sends a packet to an IPv6 destination host
('E'), the packet is first forwarded via the EUN to AERO Client
('B'). Client ('B') then forwards the packet over its AERO interface
to AERO Server ('A'), which then re-encapsulates and forwards the
packet to AERO Client ('D'), where the packet is finally forwarded
to the IPv6 destination host ('E'). When Server ('A')
re-encapsulates and forwards the packet back out on its advertising
AERO interface, it must arrange to redirect Client ('B') toward
Client ('D') as a better next-hop node on the AERO link that is
closer to the final destination. However, this redirection process
applied to AERO interfaces must be more carefully orchestrated than
on ordinary links since the parties may be separated by potentially
many underlying network routing hops.Consider a first alternative in which Server ('A') informs Client
('B') only and does not inform Client ('D') (i.e., "classical
redirection"). In that case, Client ('D') has no way of knowing that
Client ('B') is authorized to forward packets from the claimed
source address, and it may simply elect to drop the packets. Also,
Client ('B') has no way of knowing whether Client ('D') is
performing some form of source address filtering that would reject
packets arriving from a node other than a trusted default router,
nor whether Client ('D') is even reachable via a direct path that
does not involve Server ('A').Consider a second alternative in which Server ('A') informs both
Client ('B') and Client ('D') separately, via independent
redirection control messages (i.e., "augmented redirection"). In
that case, if Client ('B') receives the redirection control message
but Client ('D') does not, subsequent packets sent by Client ('B')
could be dropped due to filtering since Client ('D') would not have
a route to verify the claimed source address. Also, if Client ('D')
receives the redirection control message but Client ('B') does not,
subsequent packets sent in the reverse direction by Client ('D')
would be lost.Since both of these alternatives have shortcomings, a new
redirection technique (i.e., "AERO redirection") is needed.Again, with reference to ,
when source host ('C') sends a packet to destination host ('E'), the
packet is first forwarded over the source host's attached EUN to
Client ('B'), which then forwards the packet via its AERO interface
to Server ('A').Server ('A') then re-encapsulates and forwards the packet out the
same AERO interface toward Client ('D') and also sends an AERO
"Predirect" message forward to Client ('D') as specified in . The Predirect message includes Client
('B')'s network- and link-layer addresses as well as information
that Client ('D') can use to determine the IPv6 prefix used by
Client ('B') . After Client ('D') receives the Predirect message, it
process the message and returns an AERO Redirect message destined
for Client ('B') via Server ('A') as specified in . During the process, Client ('D') also creates
or updates a dynamic neighbor cache entry for Client ('B'), and
creates an IPv6 forwarding table entry for Client ('B')'s IPv6
prefix.When Server ('A') receives the Redirect message, it
re-encapsulates the message and forwards it on to Client ('B') as
specified in . The message includes
Client ('D')'s network- and link-layer addresses as well as
information that Client ('B') can use to determine the IPv6 prefix
used by Client ('D'). After Client ('B') receives the Redirect
message, it processes the message as specified in . During the process, Client ('B') also
creates or updates a dynamic neighbor cache entry for Client ('D'),
and creates an IPv6 forwarding table entry for Client ('D')'s IPv6
prefix.Following the above Predirect/Redirect message exchange,
forwarding of packets from Client ('B') to Client ('D') without
involving Server ('A) as an intermediary is enabled. The mechanisms
that support this exchange are specified in the following
sections.AERO Redirect/Predirect messages use the same format as for
ICMPv6 Redirect messages depicted in Section 4.5 of , but also include a new "Prefix Length" field
taken from the low-order 8 bits of the Redirect message Reserved
field (valid values for the Prefix Length field are 0 through 64).
The Redirect/Predirect messages are formatted as shown in :When an AERO Server forwards a packet out the same AERO interface
that it arrived on, the Server sends a Predirect message forward
toward the AERO Client nearest the destination instead of sending a
Redirect message back to AERO Client nearest the source.In the reference operational scenario, when Server ('A') forwards
a packet sent by Client ('B') toward Client ('D'), it also sends a
Predirect message forward toward Client ('D'), subject to rate
limiting (see Section 8.2 of ). Server ('A')
prepares the Predirect message as follows:the link-layer source address is set to 'L2(A)' (i.e., the
underlying address of Server ('A')).the link-layer destination address is set to 'L2(D)' (i.e.,
the underlying address of Client ('D')).the network-layer source address is set to fe80::0 (i.e., the
link-local address of Server ('A')).the network-layer destination address is set to
fe80::2001:db8:1:0 (i.e., the AERO address of Client ('D')).the Type is set to 137.the Code is set to 1 to indicate "Predirect".the Prefix Length is set to the length of the prefix to be
applied to Target address.the Target Address is set to fe80::2001:db8:0::0 (i.e., the
AERO address of Client ('B')).the Destination Address is set to the IPv6 source address of
the packet that triggered the Predirection event.the message includes one or more TLLAOs set to 'L2(B)' and
any other underlying address(es) of Client ('B').the message includes a Redirected Header Option (RHO) that
contains the originating packet truncated to ensure that at
least the network-layer header is included but the size of the
message does not exceed 1280 bytes.Server ('A') then sends the message forward to Client ('D').When Client ('D') receives a Predirect message, it accepts the
message only if it has a link-layer source address of the Server,
i.e. 'L2(A)'. Client ('D') further accepts the message only if it is
willing to serve as a redirection target. Next, Client ('D')
validates the message according to the ICMPv6 Redirect message
validation rules in Section 8.1 of .In the reference operational scenario, when the Client ('D')
receives a valid Predirect message, it either creates or updates a
dynamic neighbor cache entry that stores the Target Address of the
message as the network-layer address of Client ('B') and stores the
link-layer address(es) found in the TLLAO(s) as the link-layer
address(es) of Client ('B'). Client ('D') then applies the Prefix
Length to the Interface Identifier portion of the Target Address and
records the resulting IPv6 prefix in its IPv6 forwarding table.After processing the message, Client ('D') prepares a Redirect
message response as follows:the link-layer source address is set to 'L2(D)' (i.e., the
link-layer address of Client ('D')).the link-layer destination address is set to 'L2(A)' (i.e.,
the link-layer address of Server ('A')).the network-layer source address is set to 'L3(D)' (i.e., the
AERO address of Client ('D')).the network-layer destination address is set to 'L3(B)'
(i.e., the AERO address of Client ('B')).the Type is set to 137.the Code is set to 0 to indicate "Redirect".the Prefix Length is set to the length of the prefix to be
applied to the Target and Destination address.the Target Address is set to fe80::2001:db8:1::1 (i.e., the
AERO address of Client ('D')).the Destination Address is set to the IPv6 destination
address of the packet that triggered the Redirection event.the message includes one or more TLLAOs set to 'L2(D)' and
any other underlying address(es) of Client ('D').the message includes as much of the RHO copied from the
corresponding AERO Predirect message as possible such that at
least the network-layer header is included but the size of the
message does not exceed 1280 bytes.After Client ('D') prepares the Redirect message, it sends the
message to Server ('A').When Server ('A') receives a Redirect message, it accepts the
message only if it has a neighbor cache entry that associates the
message's link-layer source address with the network-layer source
address. Next, Server ('A') validates the message according to the
ICMPv6 Redirect message validation rules in Section 8.1 of . Following validation, Server ('A')
re-encapsulates the Redirect then relays the re-encapsulated
Redirect on to Client ('B') as follows.In the reference operational scenario, Server ('A') receives the
Redirect message from Client ('D') and prepares to re-encapsulate
and forward the message to Client ('B'). Server ('A') first verifies
that Client ('D') is authorized to use the Prefix Length in the
Redirect message when applied to the AERO address in the
network-layer source of the Redirect message, and discards the
message if verification fails. Otherwise, Server ('A')
re-encapsulates the message by changing the link-layer source
address of the message to 'L2(A)', changing the network-layer source
address of the message to fe80::0, and changing the link-layer
destination address to 'L2(B)' . Server ('A') finally relays the
re-encapsulated message to the ingress node ('B') without
decrementing the network-layer IPv6 header Hop Limit field.While not shown in , AERO
Relays relay Redirect and Predirect messages in exactly this same
fashion described above. See in
Appendix A for an extension of the reference operational scenario
that includes Relays.When Client ('B') receives the Redirect message, it accepts the
message only if it has a link-layer source address of the Server,
i.e. 'L2(A)'. Next, Client ('B') validates the message according to
the ICMPv6 Redirect message validation rules in Section 8.1 of . Following validation, Client ('B') then
processes the message as follows.In the reference operational scenario, when Client ('B') receives
the Redirect message, it either creates or updates a dynamic
neighbor cache entry that stores the Target Address of the message
as the network-layer address of Client ('D') and stores the
link-layer address(es) found in the TLLAO(s) as the link-layer
address(es) of Client ('D'). Client ('B') then applies the Prefix
Length to the Interface Identifier portion of the Target Address and
records the resulting IPv6 prefix in its IPv6 forwarding table.Now, Client ('B') has an IPv6 forwarding table entry for
Client('D')'s prefix, and Client ('D') has an IPv6 forwarding table
entry for Client ('B')'s prefix. Thereafter, the clients may
exchange ordinary network-layer data packets directly without
forwarding through Server ('A').Each AERO node uses its delegated prefix to create an AERO address
(see: Section 3.3). It can then send unicast NS messages to elicit NA
messages from other AERO nodes the same as described for Neighbor
Unreachability Detection (NUD) in
except that the messages do not include S/TLLAOs. When an AERO node
sends an NS/NA message, it MUST use its AERO address as the IPv6
source address and the AERO address of the neighbor as the IPv6
destination address. When an AERO node receives an NS/NA message, it
accepts the message if it has a neighbor cache entry for the neighbor;
otherwise, it ignores the message.When a source Client is redirected to a target Client it SHOULD
test the direct path to the target by sending an initial NS message to
elicit a solicited NA response. While testing the path, the source
Client SHOULD continue sending packets via the Server until target
Client reachability has been confirmed. The source Client SHOULD
thereafter continue to test the direct path to the target Client (see
Section 7.3 of ) in order to keep dynamic
neighbor cache entries alive. In particular, the source Client sends
NS messages to the target Client subject to rate limiting in order to
receive solicited NA messages. If at any time the direct path appears
to be failing, the source Client can resume sending packets via the
Server which may or may not result in a new redirection event.When a target Client receives an NS message from a source Client,
it resets the ACCEPT_TIME timer if a neighbor cache entry exists;
otherwise, it discards the NS message.When a source Client receives a solicited NA message form a target
Client, it resets the FORWARD_TIME timer if a neighbor cache entry
exists; otherwise, it discards the NA message.When both the FORWARD_TIME and ACCEPT_TIME timers on a dynamic
neighbor cache entry expire, the Client deletes both the neighbor
cache entry and the corresponding IPv6 forwarding table entry.If the source Client is unable to elicit an NA response from the
target Client after MAX_RETRY attempts, it SHOULD consider the direct
path unusable for forwarding purposes. Otherwise, the source Client
may continue to send packets directly to the target Client and SHOULD
thereafter process any link-layer errors as a hint that the direct
path to the target Client has either failed or has become
intermittent.When a Client needs to change its link-layer address (e.g., due to
a mobility event, due to a change in underlying network interface,
etc.), it sends an immediate NS message forward to any of its
correspondents (including the Server and any other Clients) which then
discover the new link-layer address.If two Clients change their link-layer addresses simultaneously,
the NS/NA messages may be lost. In that case, the Clients SHOULD
delete their respective dynamic neighbor cache and IPv6 forwarding
table entries, and allow packets to again flow through their Server(s)
which MAY result in a new AERO redirection exchange.When a Client needs to change to a new AERO Server, it issues a new
DHCPv6 Request message via the new AERO Server as the DHCPv6 relay.
The new AERO Server then relays the message to the DHCPv6 server and
processes the resulting exchange the same as described in Section
3.9.2. After the Client receives the resulting DHCPv6 Reply message,
it sends an RS message to the new Server to receive a new RA message
and update its neighbor cache entry for fe80::0.After conducting the DHCPv6 exchange via the new AERO Server, the
Client then sends a "terminating NS" message to the old AERO Server.
The terminating NS message is prepared exactly the same as for an
ordinary NS message, except that the Code field contains the value
'1'. When the old Server receives the terminating NS message, it
withdraws the IPv6 route from the routing system and deletes the
neighbor cache entry and IPv6 forwarding table entry for the Client.
The old Server then returns an NA message which the Client can use to
verify that the termination signal has been processed. (Note that the
old Server can impose a small delay before deleting the neighbor cache
and IPv6 forwarding table entries so that any packets already in the
system can still be delivered to the Client.)An alternative to sending a "terminating NS" message would be for
the Client to somehow perform a DHCPv6 exchange with the DHCPv6 relay
function on the old AERO Server but without involving the DHCPv6
server. This would be insecure because the Client only has a DHCPv6
security association with the DHCPv6 server and not the DHCPv6 relay.
Conversely, the AERO Client and Server already require an authentic
exchange of IPv6 Neighbor Discovery messages.A source Client may connect only to an IPvX underlying network,
while the target Client connects only to an IPvY underlying network.
In that case, the source Client has no means for reaching the target
directly (since they connect to underlying networks of different IP
protocol versions) and so must ignore any Redirects and continue to
send packets via the Server.When the underlying network does not support multicast, AERO nodes
map IPv6 link-scoped multicast addresses (including
"All_DHCP_Relay_Agents_and_Servers") to the underlying IP address of
the current AERO Server.When the underlying network supports multicast, AERO nodes use the
multicast address mapping specification found in for IPv4 underlying networks and use a direct
multicast mapping for IPv6 underlying networks. (In the latter case,
"direct multicast mapping" means that if the IPv6 multicast
destination address of the inner packet is "M", then the IPv6
multicast destination address of the encapsulating header is also
"M".)In some AERO link scenarios, there may be no Server on the link
and/or no need for Clients to use a Server as an intermediary trust
anchor. In that case, Clients can then establish dynamic neighbor
cache entries and IPv6 forwarding table entries by performing direct
Client-to-Client Predirect/Redirect exchanges, and some other form of
trust basis must be applied so that each Client can verify that the
prospective neighbor is authorized to use its claimed prefix.When there is no Server on the link, Clients must arrange to
receive prefix delegations and publish the delegations via a secure
alternate prefix delegation authority through some means outside the
scope of this document.IPv6 hosts serviced by an AERO Client can reach IPv4-only services
via a NAT64 gateway within the IPv6
network.AERO nodes can use the Default Address Selection Policy with DHCPv6
option the same as on any IPv6 link.All other (non-multicast) functions that operate over ordinary IPv6
links operate in the same fashion over AERO links.An early implementation is available at:
http://linkupnetworks.com/aero/aerov2-0.3.tgz.The IANA is instructed to assign a new 2-octet Hardware Type number
for AERO in the "arp-parameters" registry per Section 2 of . The number is assigned from the 2-octet Unassigned
range with Hardware Type "AERO" and with this document as the
reference.AERO link security considerations are the same as for standard IPv6
Neighbor Discovery except that AERO improves on
some aspects. In particular, AERO is dependent on a trust basis between
AERO Clients and Servers, where the Clients only engage in the AERO
mechanism when it is facilitated by a trust anchor.AERO links must be protected against link-layer address spoofing
attacks in which an attacker on the link pretends to be a trusted
neighbor. Links that provide link-layer securing mechanisms (e.g., WiFi
networks) and links that provide physical security (e.g., enterprise
network wired LANs) provide a first line of defense that is often
sufficient. In other instances, securing mechanisms such as Secure
Neighbor Discovery (SeND) , IPsec or TLS may be necessary.AERO Clients MUST ensure that their connectivity is not used by
unauthorized nodes on EUNs to gain access to a protected network. (This
concern is no different than for ordinary hosts that receive an IP
address delegation but then "share" the address with unauthorized nodes
via an IPv6/IPv6 NAT function.)On some AERO links, establishment and maintenance of a direct path
between neighbors requires secured coordination such as through the
Internet Key Exchange (IKEv2) protocol to
establish a security association.Discussions both on the v6ops list and in private exchanges helped
shape some of the concepts in this work. Individuals who contributed
insights include Mikael Abrahamsson, Fred Baker, Stewart Bryant, Brian
Carpenter, Wojciech Dec, Brian Haberman, Joel Halpern, Sascha Hlusiak,
Lee Howard, Joe Touch and Bernie Volz. Members of the IESG also provided
valuable input during their review process that greatly improved the
document. Special thanks go to Stewart Bryant, Joel Halpern and Brian
Haberman for their shepherding guidance.This work has further been encouraged and supported by Boeing
colleagues including Keith Bartley, Dave Bernhardt, Cam Brodie,
Balaguruna Chidambaram, Wen Fang, Anthony Gregory, Jeff Holland, Ed
King, Gen MacLean, Kent Shuey, Mike Slane, Julie Wulff, Yueli Yang, and
other members of the BR&T and BIT mobile networking teams.Earlier works on NBMA tunneling approaches are found in .The Internet Routing Overlay Network (IRON)Since the Internet must continue to support escalating growth
due to increasing demand, it is clear that current routing
architectures and operational practices must be updated. This
document proposes an Internet Routing Overlay Network (IRON)
architecture that supports sustainable growth while requiring no
changes to end systems and no changes to the existing routing
system. In addition to routing scaling, IRON further addresses
other important issues including mobility management, mobile
networks, multihoming, traffic engineering, NAT traversal and
security. While business considerations are an important
determining factor for widespread adoption, they are out of scope
for this document. depicts a reference AERO
operational scenario with a single Server on the AERO link. In order to
support scaling to larger numbers of nodes, the AERO link can deploy
multiple Servers and Relays, e.g., as shown in .In this example, AERO Client ('B') associates with AERO
Server ('C'), while AERO Client ('F') associates with AERO Server ('E').
Furthermore, AERO Servers ('C') and ('E') do not associate with each
other directly, but rather have an association with AERO Relay ('D')
(i.e., a router that has full topology information concerning its
associated Servers and their Clients). Relay ('D') connects to the AERO
link, and also connects to the native IPv6 Internetwork.When host ('A') sends a packet toward destination host ('G'), IPv6
forwarding directs the packet through the EUN to Client ('B'), which
forwards the packet to Server ('C') in absence of more-specific
forwarding information. Server ('C') forwards the packet, and it also
generates an AERO Predirect message that is then forwarded through Relay
('D') to Server ('E'). When Server ('E') receives the message, it
forwards the message to Client ('F').After processing the AERO Predirect message, Client ('F') sends an
AERO Redirect message to Server ('E'). Server ('E'), in turn, forwards
the message through Relay ('D') to Server ('C'). When Server ('C')
receives the message, it forwards the message to Client ('B') informing
it that host 'G's EUN can be reached via Client ('F'), thus completing
the AERO redirection.The network layer routing information shared between Servers and
Relays must be carefully coordinated in a manner outside the scope of
this document. In particular, Relays require full topology information,
while individual Servers only require partial topology information
(i.e., they only need to know the EUN prefixes associated with their
current set of Clients). See for an architectural
discussion of routing coordination between Relays and Servers.