Network Working Group E. Duros
Internet-Draft W. Dabbous
April 2000 INRIA Sophia-Antipolis
H. Izumiyama
N. Fujii
WIDE
Y. Zhang
HRL
A Link Layer Tunneling Mechanism for Unidirectional Links
<draft-ietf-udlr-lltunnel-04.txt>
Status of this Memo
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Abstract
This document describes a mechanism to emulate bidirectional
connectivity between nodes that are directly connected by a
unidirectional link. The "receiver" uses a link layer tunneling
mechanism to forward datagrams to "feeds" over a separate
bidirectional IP network. As it is implemented at the link layer,
protocols in addition to IP may also be supported by this mechanism.
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1. Introduction
Internet routing and upper layer protocols assume that links are
bidirectional, i.e., directly connected hosts can communicate with
each other over the same link.
This document describes a link layer tunneling mechanism that allows
nodes which are directly connected by a unidirectional link (feeds
and receivers, see Section 2 for terminology) to send datagrams as if
they were connected to a bidirectional link. We present a generic
topology with a tunneling mechanism that supports multiple feeds and
receivers.
The tunneling mechanism requires that all nodes have an additional
interface to an IP interconnected infrastructure.
The tunneling mechanism is implemented at the link layer of the
interface of every node connected to the unidirectional link. The aim
is to hide from higher layers, i.e. the network layer and above, the
unidirectional nature of the link. The tunneling mechanism also
includes an automatic tunnel configuration protocol that allows nodes
to come up/down at any time.
Generic Routing Encapsulation [rfc2784] is suggested as the tunneling
mechanism as it provides a means for carrying IP, ARP datagrams, and
any other layer-3 protocol between nodes.
The tunneling mechanism described in this document was discussed and
agreed upon by the UDLR working group.
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [rfc2119].
2. Terminology
Unidirectional link (UDL): A one way transmission link, e.g. a
broadcast satellite link.
Receiver: A router or a host that has receive-only connectivity to a
UDL.
Send-only feed: A router that has send-only connectivity to a UDL.
Receive capable feed: A router that has send-and-receive connectivity
to a UDL.
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Feed: A send-only or a receive capable feed.
Node: A receiver or a feed.
3. Topology
Feeds and receivers are connected via a unidirectional link. Send-
only feeds can only send data over this unidirectional link, and
receivers can only receive data from it. Receive capable feeds have
both send and receive capabilities.
This mechanism has been designed to work with any topology with any
number of receivers and one or more feeds. However, it is expected
that the number of feeds will be small. In particular, the special
case of a single send-only feed and multiple receivers is among the
topologies supported.
A receiver has several interfaces, a receive-only interface and one
or more additional bidirectional communication interfaces.
A feed has several interfaces, a send-only or a send-and-receive
capable interface connected to the unidirectional link and one or
more additional bidirectional communication interfaces. A feed MUST
be a router.
Tunnels are constructed between the bidirectional interfaces of
nodes, so these interfaces must be interconnected by an IP
infrastructure. In this document we assume that that infrastructure
is the Internet.
Figure 1 depicts a generic topology with several feeds and several
receivers.
Unidirectional Link
---->---------->------------------->------
| | | |
|f1u |f2u |r2u |r1u
-------- -------- -------- -------- ----------
|Feed 1| |Feed 2| |Recv 2| |Recv 1|---|subnet A|
-------- -------- -------- -------- ----------
|f1b |f2b |r2b |r1b |
| | | | |
----------------------------------------------------
| Internet |
----------------------------------------------------
Figure 1: Generic topology
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f1u (resp. f2u) is the IP address of the 'Feed 1' (resp. Feed 2)
send-only interface.
f1b (resp. f2b) is the IP address of the 'Feed 1' (resp. Feed 2)
bidirectional interface connected to the Internet.
r1u (resp. r2u) is the IP address of the 'Receiver 1' (resp. Receiver
2) receive-only interface.
r1b (resp. r2b) is the IP address of the 'Receiver 1' (resp. Receiver
2) bidirectional interface connected to the Internet.
Subnet A is a local area network connected to recv1.
Note that nodes have IP addresses on their unidirectional and their
bidirectional interfaces. The addresses on the unidirectional
interfaces (f1u, f2u, r1u, r2u) will be drawn from the same IP
network. In general the addresses on the bidirectional interfaces
(f1b, f2b, r1b, r2b) will be drawn from different IP networks, and
the Internet will route between them.
4. Problems related to unidirectional links
Receive-only interfaces are "dumb" and send-only interfaces are
"deaf". Thus a datagram passed to the link layer driver of a
receive-only interface is simply discarded. The link layer of a
send-only interface never receives anything.
The network layer has no knowledge of the underlying transmission
technology except that it considers its access as bidirectional.
Basically, for outgoing datagrams, the network layer selects the
correct first hop on the connected network according to a routing
table and passes the packet(s) to the appropriate link layer driver.
Referring to Figure 1, Recv 1 and Feed 1 belong to the same network.
However, if Recv 1 initiates a 'ping f1u', it cannot get a response
from Feed 1. The network layer of Recv 1 delivers the packet to the
driver of the receive-only interface, which obviously cannot send it
to the feed.
Many protocols in the Internet assume that links are bidirectional.
In particular, routing protocols used by directly connected routers
no longer behave properly in the presence of a unidirectional link.
5. Emulating a broadcast bidirectional network
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The simplest solution is to emulate a broadcast capable link layer
network. This will allow the immediate deployment of existing higher
level protocols without change. Though other network structures, such
as NBMA, could also be emulated, a broadcast network is more
generally useful. Though a layer 3 network could be emulated, a link
layer network allows the immediate use of any other network layer
protocols, and most particularly allows the immediate use of ARP.
A link layer (LL) tunneling mechanism which emulates bidirectional
connectivity in the presence of a unidirectional link will be
described in the next Section. We first consider the various
communication scenarios which characterize a broadcast network in
order to define what functionalities the link layer tunneling
mechanism has to perform in order to emulate a bidirectional
broadcast link.
Here we enumerate the scenarios which would be feasible on a
broadcast network, i.e. if feeds and receivers were connected by a
bidirectional broadcast link:
Scenario 1: A receiver can send a packet to a feed (point-to-point
communication between a receiver and a feed).
Scenario 2: A receiver can send a broadcast/multicast packet on the
link to all nodes (point-to-multipoint).
Scenario 3: A receiver can send a packet to another receiver (point-
to-point communication between two receivers).
Scenario 4: A feed can send a packet to a send-only feed (point-to-
point communication between two feeds).
Scenario 5: A feed can send a broadcast/multicast packet on the link
to all nodes (point-to-multipoint).
Scenario 6: A feed can send a packet to a receiver or a receive
capable feed.
These scenarios are possible on a broadcast network. Scenario 6 is
already feasible on the unidirectional link. The link layer tunneling
mechanism should therefore provide the functionality to support
scenarios 1 to 5.
Note that regular IP forwarding over such an emulated network (i.e.
using the emulated network as a transit network) works correctly; the
next hop address at the receiver will be the unidirectional link
address of another router (a feed or a receiver) which will then
relay the packet.
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6. Link layer tunneling mechanism
This link layer tunneling mechanism operates underneath the network
layer. Its aim is to emulate bidirectional link layer connectivity.
This is transparent to the network layer: the link appears and
behaves to the network layer as if it was bidirectional.
Figure 2 depicts a layered representation of the link layer tunneling
mechanism in the case of Scenario 1.
Send-only Feed Receiver
decapsulation encapsulation
/-----***************----\ /-->---***************--\
| | | |
| | | |
--|---------------------- | | ---------------------|---
| | f1b | f1u | | | | x r1u | r1b | |
| | | ^ | | IP | | | | v |
| ^ | | | v | | | | | |
| | | | | | | | v | | |
|-|---------|-------|---| | | |----|------|--------|--|
| | | | | | ^ | | | | |
| | | | | | LL | | | | | |
| | | | | | | | | | | |
| | | O------/ \<------O | | |
|-|---------|-----------| |-----------|--------|--|
| | | | | | | |
| | | | PHY | | | |
| | | | | | v |
| | | | | | | | | |
--|-----------|---------- ----------|----------|---
| Bidir | Send-Only Recv-Only | Bidir |
^ Interf | Interf UDL Interf | Interf |
| \------------>------->------------/ |
\----------------------<------------------------<--------/
Bidirectional network
x : IP layer at the receiver generates a datagram to be forwarded
on the receive-only interface.
O : Entry point where the link layer tunneling mechanism is
triggered.
Figure 2: Scenario 1 using the LL Tunneling Mechanism
6.1. Tunneling mechanism on the receiver
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On the receiver, a datagram is delivered to the link layer of the
unidirectional interface for transmission (see Figure 2). It is then
encapsulated behind a MAC header corresponding to the unidirectional
link. This packet cannot be sent directly over the link, so it is
then processed by the tunneling mechanism.
The packet is encapsulated behind an IP header whose destination is
the IP address of a feed bidirectional interface (f1b or f2b). This
destination address is also called the tunnel end-point. The
mechanism for a receiver to learn these addresses and to choose the
feed is explained in Section 7. The type of encapsulation is
described in Section 8.
In all cases the packet is encapsulated, but the tunnel end-point (an
IP address) depends on the encapsulated packet's destination MAC
address. If the destination MAC address is:
1) the MAC address of a feed interface connected to the
unidirectional link (Scenario 1). The datagram is encapsulated,
the destination address of the encapsulating datagram is the
feed tunnel end-point (f1b referring to Figure 2).
2) a MAC broadcast/multicast address (Scenario 2). The datagram is
encapsulated, the destination address of the encapsulating
datagram is the default feed tunnel end-point. See Section 7.4
for further details on the default feed.
3) a MAC address that belongs to the unidirectional network but is
not a feed address (Scenario 3). The datagram is encapsulated,
the destination address of the encapsulating datagram is the
default feed tunnel end-point.
The encapsulated datagram is passed to the network layer which
forwards it according to its destination address. The destination
address is a feed bidirectional interface which is reachable via the
Internet. In this case, the encapsulated datagram is forwarded via
the receiver bidirectional interface (r1b).
6.2. Tunneling mechanism on the feed
A feed processes unidirectional link related packets in two different
ways:
- packets generated by a local application or packets routed as
usual by the IP layer may have to be forwarded over the
unidirectional link (Section 6.2.1)
- encapsulated packets received from another receiver or feed need
tunnel processing (Section 6.2.2).
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A feed cannot directly send a packet to a send-only feed over the
unidirectional link (Scenario 4). In order to emulate this type of
communication, feeds have to tunnel packets to send-only feeds. A
feed MUST maintain a list of all other feed tunnel end-points. This
list MUST indicate which are send-only feed tunnel end-points. This
is configured manually at the feed by the local administrator, as
described in Section 7.
6.2.1. Forwarding packets over the unidirectional link
When a datagram is delivered to the link layer of the unidirectional
interface of a feed for transmission, its treatment depends on the
packet's destination MAC address. If the destination MAC address is:
1) the MAC address of a receiver or a receive capable feed
(Scenario 6). The packet is sent over the unidirectional link.
This is classical "forwarding".
2) the MAC address of a send-only feed (Scenario 4). The packet is
encapsulated and sent to the send-only feed tunnel end-point.
The type of encapsulation is described in Section 8.
3) a broadcast/multicast destination (Scenario 5). The packet is
sent over the unidirectional link. Concurrently, a copy of this
packet is encapsulated and sent to every feed of the list of
send-only feed tunnel end-points. Thus the broadcast/multicast
will reach all receivers and all send-only feeds.
6.2.2. Receiving encapsulated packets
Feeds listen for incoming encapsulated datagrams on their tunnel end-
points. Encapsulated packets will have been received on a
bidirectional interface, and traversed their way up the IP stack.
They will then enter a decapsulation process (See Figure 2).
Decapsulation reveals the original link layer packet. Note that this
has not been modified in any way by intermediate routers; in
particular, the original MAC header will be intact.
Further actions depend on the destination MAC address of the link
layer packet, which can be:
1) the MAC address of the feed interface connected to the
unidirectional link, i.e. own MAC address (Scenarios 1 and 4).
The packet is passed to the link layer of the interface
connected to the unidirectional link which can then deliver it
up to higher layers. As a result, the datagram is processed as
if it was coming from the unidirectional link, and being
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delivered locally. Scenarios 1 and 4 are now feasible, a
receiver or a feed can send a packet to a feed.
2) a receiver address (Scenario 3). The packet is passed to the
link layer of the interface connected to the unidirectional
link. It is directly sent over the unidirectional link, to the
indicated receiver. Note, the packet must not be delivered
locally. Scenario 3 is now feasible, a receiver can send a
packet to another receiver.
3) a broadcast/multicast address, this corresponds to Scenarios 2
and 5. We have to distinguish two cases, either (i) the
encapsulated packet was sent from a receiver or (ii) from a feed
(encapsulated broadcast/multicast packet sent to a send-only
feed). These cases are distinguished by examining the source
address of the encapsulating packet and comparing it with the
configured list of feed IP addresses. The action then taken is:
i) the feed was designated as a default feed by a receiver to
forward the broadcast/multicast packet. The feed is then in
charge of sending the multicast packet to all nodes. Delivery
to all nodes is accomplished by executing all 3 of the
following actions:
- The packet is encapsulated and sent to the list of send-
only feed tunnel end-points.
- Also, the packet is passed to the link layer of the
interface which forwards it directly over the
unidirectional link (all receivers and receive capable
feeds receive it).
- Also, the link layer delivers it locally to higher layers.
Caution: a receiver which sends an encapsulated
broadcast/multicast packet to a default feed will receive
its own packet via the unidirectional link. Correct
filtering as described in [rfc1112] must be applied.
ii) the feed receives the packet and keeps it for local
delivery. The packet is passed to the link layer of the
interface connected to the unidirectional link which delivers
it to higher layers.
Scenario 2 is now feasible, a receiver can send a
broadcast/multicast packet over the unidirectional link and it
will be heard by all nodes.
7. Dynamic Tunnel Configuration Protocol (DTCP)
Receivers and feeds have to know the feed tunnel end-points in order
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to forward encapsulated datagrams (e.g. Scenarios 1 and 4).
The number of feeds is expected to be relatively small (Section 3),
so at every feed the list of all feeds is configured manually. This
list should note which are send-only feeds, and which are receive
capable feeds. The administrator sets up tunnels to all send-only
feeds. A tunnel end-point is an IP address of a bidirectional link on
a send-only feed.
For scalability reasons, manual configuration cannot be done at the
receivers. Tunnels must be configured and maintained dynamically by
receivers, both for scalability, and in order to cope with the
following events:
1) New feed detection.
When a new feed comes up, every receiver must create a tunnel to
enable bidirectional communication with it.
2) Loss of unidirectional link detection.
When the unidirectional link is down, receivers must disable
their tunnels. The tunneling mechanism emulates bidirectional
connectivity between nodes. Therefore, if the unidirectional
link is down, a feed should not receive datagrams from the
receivers. Protocols that consider a link as operational if they
receive datagrams from it (e.g. the RIP protocol [rfc2453])
require this behavior for correct operation.
3) Loss of feed detection.
When a feed is down, receivers must disable their corresponding
tunnel. This prevents unnecessary datagrams from being tunneled
which might overload the Internet. For instance, there is no
need for receivers to forward a broadcast message through a
tunnel whose end-point is down.
The DTCP protocol provides a means for receivers to dynamically
discover the presence of feeds and to maintain a list of operational
tunnel end-points. Feeds periodically announce their tunnel end-point
addresses over the unidirectional link. Receivers listen to these
announcements and maintain a list of tunnel end-points.
7.1. The HELLO message
The DTCP protocol is a 'unidirectional protocol', messages are only
sent from feeds to receivers.
The packet format is shown in Figure 3. Fields contain binary
integers, in normal Internet order with the most significant bit
first. Each tick mark represents one bit.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vers | Com | Interval | Sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| res |F|IP Vers| Tunnel Type | Nb of FBIP | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Feed BDL IP addr (FBIP1) (32/128 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ..... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Feed BDL IP addr (FBIPn) (32/128 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Packet Format
Every datagram contains the following fields, note that constants are
written in uppercase and are defined in Section 7.5:
Vers (4 bit unsigned integer): DTCP version number. MUST be
DTCP_VERSION.
Com (4 bit unsigned integer): Command field, possible values are
1 - JOIN A message announcing that the feed sending this message
is up and running.
2 - LEAVE A message announcing that the feed sending this message
is being shut down.
Interval (8 bit unsigned integer): Interval in seconds between HELLO
messages for the IP protocol in "IP Vers". Must be > 0. The
recommended value is HELLO_INTERVAL. If this value is increased,
the feed MUST continue to send HELLO messages at the old rate for
at least the old HELLO_LEAVE period.
Sequence (16 bit unsigned integer): Random value initialized at boot
time and incremented by 1 every time a value of the HELLO message
is modified.
res (3 bits): Reserved/unused field, MUST be zero.
F (1 bit): bit indicating the type of feed:
0 = Send-only feed
1 = Receive-capable feed
IP Vers (4 bit unsigned integer): IP protocol version of the feed
bidirectional IP addresses (FBIP):
4 = IP version 4
6 = IP version 6
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Tunnel Type (8 bit unsigned integer): tunneling protocol supported by
the feed; receivers MUST use this form of tunnel encapsulation when
tunneling to the feed.
47 = GRE [rfc2784] (recommended)
Other values may be used, but their interpretation is not specified
here.
Nb of FBIP (8 bit unsigned integer): Number of bidirectional IP feed
addresses which are enumerated in the HELLO message
reserved (8 bits): Reserved/unused field, MUST be zero.
Feed BDL IP addr (32 or 128 bits). The bidirectional IP address feed
is the IP address of a feed bidirectional interface (tunnel end-
point) reachable via the Internet. A feed has 'Nb of FBIP' IP
addresses which are operational tunnel end-points. They are
enumerated in preferred order. FBIP1 being the most suitable tunnel
end-point.
7.2. DTCP on the feed: sending HELLO packets
The DTCP protocol runs on top of UDP. Packets are sent to the "DTCP
announcement" multicast address over the unidirectional link on port
HELLO_PORT with a TTL of 1.
The source address of the HELLO packet is set to the IP address of
the feed interface connected to the unidirectional link. In the rest
of the document, this value is called FUIP (Feed Unidirectional IP
address).
The process in charge of sending HELLO packets fills every field of
the datagram according to the description given in Section 7.1.
As long as a feed is up and running, it periodically announces its
presence to receivers. It MUST send HELLO packets containing a JOIN
command every HELLO_INTERVAL over the unidirectional link.
Referring to Figure 1 in Section 3, Feed 1 (resp. Feed 2) sends HELLO
messages with the FBIP1 field set to f1b (resp. f2b).
When a feed is about to be shut down, or when routing over the
unidirectional link is about to be intentionally interrupted, it is
recommended that feeds:
1) stop sending HELLO messages containing a JOIN command.
2) send a HELLO message containing a LEAVE command to inform
receivers that the feed is no longer performing routing over the
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unidirectional link.
7.3. DTCP on the receiver: receiving HELLO packets
Based on the reception of HELLO messages, receivers discover the
presence of feeds, maintain a list of active feeds, and keep track of
the tunnel end-points for those feeds.
For each active feed, and each IP protocol supported, at least the
following information will be kept:
FUIP - feed unidirectional link IP address
FUMAC - MAC address corresponding to the above IP
address
(FBIP1,...,FBIPn) - list of tunnel end-points
tunnel type - tunnel type supported by this feed
Sequence - "Sequence" value from the last HELLO received
from this feed
timer - used to timeout this entry
The FUMAC value for an active feed is needed for the operation of
this protocol. However, the method of discovery of this value is not
specified here.
Initially, the list of active feeds is empty.
When a receiver is started, it MUST run a process which joins the
"DTCP announcement" multicast group and listens to incoming packets
on the HELLO_PORT port from the unidirectional link.
Upon the reception of a HELLO message, the process checks the version
number of the protocol. If it is different from HELLO_VERSION, the
packet is discarded and the process waits for the next incoming
packet.
After successfully checking the version number further action depends
on the type of command:
- JOIN:
The process verifies if the address FUIP already belongs to the
list of active feeds.
If it does not, a new entry, for feed FUIP, is created and added
to the list of active feeds. The number of feed bidirectional IP
addresses to read is deduced from the 'Nb of FBID' field. These
tunnel end-points (FBIP1,...,FBIPn) can then be added to the new
entry. The tunnel Type and Sequence values are also taken from the
HELLO packet and recorded in the new entry. A timer set to
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HELLO_LEAVE is associated with this entry.
If it does, the sequence number is compared to the sequence number
contained in the previous HELLO packet sent by this feed. If they
are equal, the timer associated with this entry is reset to
HELLO_LEAVE. Otherwise all the information corresponding to FUIP
is set to the values from the HELLO packet.
Referring to Figure 1 in Section 3, both receivers (recv 1 and
recv 2) have a list of active feeds containing two entries: Feed 1
with a FUIP of f1u and a list of tunnel end-points (f1b); and Feed
2 with a FUIP of f2u and a list of tunnel end-points (f2b).
- LEAVE:
The process checks if there is an entry for FUIP in the list of
active feeds. If there is, the timer is disabled and the entry is
deleted from the list. The LEAVE message provides a means of
quickly updating the list of active feeds.
A timeout occurs for either of two reasons:
1) a feed went down without sending a LEAVE message. As JOIN
messages are no longer sent from this feed, a timeout occurs at
HELLO_LEAVE after the last JOIN message.
2) the unidirectional link is down. Thus no more JOIN messages are
received from any of the feeds, and they will each timeout
independently. The timeout of each entry depends on its
individual HELLO_LEAVE value, and when the last JOIN message was
sent by that feed, before the unidirectional link went down.
In either case, bidirectional connectivity can no longer be ensured
between the receiver and the feed (FUIP): either the feed is no
longer routing datagrams over the unidirectional link, or the link is
down. Thus the associated entry is removed from the list of active
feeds, whatever the cause. As a result, the list only contains
operational tunnel end-points.
The HELLO protocol provides receivers with a list of feeds, and a
list of usable tunnel end-points (FBIP1,..., FBIPn) for each feed. In
the following Section, we describe how to integrate the HELLO
protocol into the tunneling mechanism described in Sections 6.1 and
6.2.
7.4. Tunneling mechanism using the list of active feeds
This Section explains how the tunneling mechanism uses the list of
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active feeds to handle datagrams which are to be tunneled. Referring
to Section 6.1, it shows how feed tunnel end-points are selected.
The choice of the default feed is made independently at each
receiver. The choice is a matter of local policy, and this policy is
out of scope for this document. However, as an example, the default
feed may be the feed that has the lowest round trip time to the
receiver.
When a receiver sends a packet to a feed, it must choose a tunnel
end-point from within the FBIP list. The 'preferred FBIP' is
generally FBIP1 (Section 7.1). For various reasons, a receiver may
decide to use a different FBIP, say FBIPi instead of FBIP1, as the
tunnel end-point. For example, the receiver may have better
connectivity to FBIPi. This decision is taken by the receiver
administrator.
Here we show how the list of active feeds is involved when a receiver
tunnels a link layer packet. Section 6.1 listed the following cases,
depending on whether the MAC destination address of the packet is:
1) the MAC address of a feed interface connected to the
unidirectional link: This is TRUE if the address matches a FUMAC
address in the list of active feeds. The packet is tunneled to
the preferred FBIP of the matching feed.
2) the broadcast address of the unidirectional link or a multicast
address:
This is determined by the MAC address format rules, and the list
of active feeds is not involved. The packet is tunneled to the
preferred FBIP of the default feed.
3) an address that belongs to the unidirectional network but is not
a feed address:
This is TRUE if the address is neither broadcast nor multicast,
nor found in the list of active feeds. The packet is tunneled to
the preferred FBIP of the default feed.
In all cases, the encapsulation type depends on the tunnel type
required by the feed which is selected.
7.5. Constant definitions
DTCP_VERSION is 1.
HELLO_INTERVAL is 5 seconds.
"DTCP announcement" multicast group is 224.0.1.124.
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HELLO_PORT is 652. It is a reserved system port, no other traffic
must be allowed.
HELLO_LEAVE is 3*Interval, as advertised in a HELLO packet, i.e. 15
seconds if the default HELLO_INTERVAL was advertised.
8. Tunnel encapsulation format
The tunneling mechanism operates at the link layer and emulates
bidirectional connectivity amongst receivers and feeds. We assume
that hardware connected to the unidirectional link supports broadcast
and unicast MAC addressing. That is, a feed can send a packet to a
particular receiver using a unicast MAC destination address or to a
set of receivers using a broadcast/multicast destination address. The
hardware (or the driver) of the receiver can then filter the incoming
packets sent over the unidirectional links without any assumption
about the encapsulated data type.
In a similar way, a receiver should be capable of sending unicast and
broadcast MAC packets via its tunnels. Link layer packets are
encapsulated. As a result, after decapsulating an incoming packet,
the feed can perform link layer filtering as if the data came
directly from the unidirectional link (See Figure 2).
Generic Routing Encapsulation (GRE) [rfc2784] suits our requirements
because it specifies a protocol for encapsulating arbitrary packets,
and allows use of IP as the delivery protocol.
Other encapsulations are possible, such as directly encapsulating a
MAC level packet within an IP datagram.
The feed's local administrator decides what encapsulation it will
demand that receivers use, and sets the tunnel type field in the
HELLO message appropriately. The value 47 (decimal) indicates GRE.
Other values can be used, but their interpretation must be agreed
upon between feeds and receivers. Such usage is not defined here.
8.1. Generic Routing Encapsulation on the receiver
A GRE packet is composed of a header in which a type field specifies
the encapsulated protocol (ARP, IP, IPX, etc.). See [rfc2784] for
details about the encapsulation. In our case, only support for the
MAC addressing scheme of the unidirectional link MUST be implemented.
A packet tunneled with a GRE encapsulation has the following format:
the delivery header is an IP header whose destination is the tunnel
end-point (FBIP), followed by a GRE header specifying the link layer
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type of the unidirectional link. Figure 4 presents the entire
encapsulated packet.
----------------------------------------
| IP delivery header |
| destination addr = FBIP |
| IP proto = GRE (47) |
----------------------------------------
| GRE Header |
| type = MAC type of the UDL |
----------------------------------------
| Payload packet |
| MAC packet |
----------------------------------------
Figure 4: Encapsulated packet
8.2. Encapsulation of UDL MAC level packets
An alternative is to encapsulate the MAC level packet within IP. The
protocol field in the IP datagram is then set to the MAC type of the
unidirectional link. Figure 5 presents the entire encapsulated
packet.
----------------------------------------
| IP delivery header |
| destination addr = FBIP |
| IP proto = MAC type of the UDL |
----------------------------------------
| Payload packet |
| MAC packet |
----------------------------------------
Figure 5: Encapsulated packet
9. Issues
9.1. Hardware address resolution
Regardless of whether the link is unidirectional or bidirectional, if
a feed sends a packet over a non-point-to-point type network, it
requires the data link address of the destination. ARP [rfc826] is
used on Ethernet networks for this purpose.
The link layer mechanism emulates a bidirectional network in the
presence of an unidirectional link. However, there are asymmetric
delays between every (feed, receiver) pair. The backchannel between a
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receiver and a feed has varying delays because packets go through the
Internet. Furthermore, a typical example of a unidirectional link is
a GEO satellite link whose delay is about 250 milliseconds.
Because of long round trip delays, reactive address resolution
methods such as ARP [rfc826] may not work well. For example, a feed
may have to forward packets at high data rates to a receiver whose
hardware address is unknown. The stream of packets is passed to the
link layer driver of the feed send-only interface. When the first
packet arrives, the link layer realizes it does not have the
corresponding hardware address of the next hop, and sends an ARP
request. While the link layer is waiting for the response (at least
250 ms for the GEO satellite case), IP packets are buffered by the
feed. If it runs out of space before the ARP response arrives, IP
packets will be dropped.
This problem of address resolution protocols is not addressed in this
document. An ad-hoc solution is possible when the MAC address is
configurable, which is possible in some satellite receiver cards. A
simple transformation (maybe null) of the IP address can then be used
as the MAC address. In this case, senders do not need to "resolve" an
IP address to a MAC address, they just need to perform the simple
transformation.
9.2. Routing protocols
The link layer tunneling mechanism hides from the network and higher
layers the fact that feeds and receivers are connected by a
unidirectional link. Communication is bidirectional, but asymmetric
in bandwidths and delays.
In order to incorporate unidirectional links in the Internet, feeds
and receivers might have to run routing protocols in some topologies.
These protocols will work fine because the tunneling mechanism
results in bidirectional connectivity between all feeds and
receivers. Thus routing messages can be exchanged as on any
bidirectional network.
The tunneling mechanism allows any IP traffic, not just routing
protocol messages, to be forwarded between receivers and feeds.
Receivers can route datagrams on the Internet using the most suitable
feed or receiver as a next hop. Administrators may want to set the
metrics used by their routing protocols in order to reflect in
routing tables the asymmetric characteristics of the link, and thus
direct traffic over appropriate paths.
Feeds and receivers may implement multicast routing and therefore
dynamic multicast routing can be performed over the unidirectional
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link. However issues related to multicast routing (e.g. protocol
configuration) are not addressed in this document.
9.3. Scalability
The DTCP protocol does not generate a lot of traffic whatever the
number of nodes. The problem with a large number of nodes is not
related to this protocol but to more general issues such as the
maximum number of nodes which can be connected to any link. This is
out of scope of this document.
10. Security Considerations
Security in a network using the link layer tunneling mechanism should
be relatively similar to security in a normal IPv4 network. However,
as the link layer tunneling mechanism uses GRE[rfc2784], it is
expected that GRE authentication mechanism combined with a specific
link layer security mechanism on the back-channel will help to
enhance security in a unidirectional link environment.
In order to prevent unauthorised users from providing fake routing
information, routing protocols running on top of the link layer
tunneling mechanism MUST use authentication mechanisms when
available.
11. Acknowledgments
We would like to thank Tim Gleeson (Cisco Japan) for his valuable
editing and technical input during the finalization phase of the
document.
We would like to thank Patrick Cipiere (INRIA) for his valuable input
concerning the design of the encapsulation mechanism.
We would like also to thank for their participation: Akihiro Tosaka
(IMD), Akira Kato (Tokyo Univ.), Hitoshi Asaeda (IBM/ITS), Hiromi
Komatsu (JSAT), Hiroyuki Kusumoto (Keio Univ.), Kazuhiro Hara (Sony),
Kenji Fujisawa (Sony), Mikiyo Nishida (Keio Univ.), Noritoshi Demizu
(Sony CSL), Jun Murai (Keio Univ.), Jun Takei (JSAT) and Harri
Hakulinen (Nokia).
A. Conformance and interoperability
This document describes a mechanism to emulate bidirectional
connectivity between nodes that are directly connected by a
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unidirectional link. Applicability over a variety of equipment and
environments is ensured by allowing a choice of several key system
parameters.
Thus in order to ensure interoperability of equipment it is not
enough to simply claim conformance with the mechanism defined here. A
usage profile for a particular environment will require the
definition of several parameters:
- the MAC format used
- the tunneling mechanism to be used (GRE is recommended)
- the "tunnel type" indication if GRE is not used
For example, a system might claim to implement "the link layer
tunneling mechanism for unidirectional links, using IEEE 802 LLC, and
GRE encapsulation for the tunnels."
References
[rfc826] 'An Ethernet Address Resolution Protocol', David C. Plummer,
November 1982.
[rfc1112] 'Host Extensions for IP Multicasting', S. Deering, Stanford
University, August 1989
[rfc2119] 'Key words for use in RFCs to Indicate Requirement Levels',
S. Bradner, Harvard University, March 1997
[rfc2401] 'Security Architecture for the Internet Protocol', S. Kent,
BBN Corp, R. Atkinson, @Home Network
[rfc2402] 'IP Authentication Header', S. Kent, BBN Corp, R. Atkinson,
@Home Network
[rfc2453] 'RIP Version 2', G. Malkin, Bay Networks, November 1998
[rfc2784] 'Generic Routing Encapsulation (GRE)', D. Farinacci, T. Li,
Procket Networks, S. Hanks, Enron Communications, D. Meyer,
Cisco Systems, P. Traina, Juniper Networks, March 2000
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Author's address
Emmanuel Duros
INRIA Sophia Antipolis
2004, Route des Lucioles BP 93
06902 Sophia Antipolis
France
Phone : +33 4 92 38 79 42
Fax : +33 4 92 38 79 78
Email : Emmanuel.Duros@inria.fr
Walid Dabbous
INRIA Sophia Antipolis
2004, Route des Lucioles BP 93
06902 Sophia Antipolis
France
Phone : +33 4 92 38 77 18
Fax : +33 4 92 38 79 78
Email : Walid.Dabbous@inria.fr
Hidetaka Izumiyama
JSAT Corporation
Toranomon 17 Mori Bldg.5F
1-26-5 Toranomon, Minato-ku
Tokyo 105
Japan
Voice : +81-3-5511-7568
Fax : +81-3-5512-7181
Email : izu@jsat.net
Noboru Fujii
Sony Corporation
2-10-14 Osaki, Shinagawa-ku
Tokyo 141
Japan
Voice : +81-3-3495-3092
Fax : +81-3-3495-3527
Email : fujii@dct.sony.co.jp
Yongguang Zhang
HRL
RL-96, 3011 Malibu Canyon Road
Malibu, CA 90265,
USA
Phone : 310-317-5147
Fax : 310-317-5695
Email : ygz@hrl.com
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