INTERNET DRAFT Jeffrey Lo
Expires November 1999 NEC USA
Michael Borella
David Grabelsky
3Com Corp
Realm Specific IP: A Framework
<draft-ietf-nat-rsip-framework-01.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
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Abstract
This document examines the general framework of Realm Specific IP
(RSIP). All RSIP solutions must solve the same set of problems, and
all RSIP-related proposals to date are similar in many ways. We
attempt to enumerate the similarities and differences of these
proposals, and expand the scope of RSIP to include several other
possible mechanisms. We do not advocate any one RSIP solution over
the other; instead, we present these solutions in the hope to clarify
RSIP issues and generate further discussion towards adoption of RSIP.
1. Introduction
While NAT has become a popular mechanism of sharing IP addresses
amongst a number of hosts, it suffers from a lack of flexibility. In
particular, a NAT router must examine and change the network layer,
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and possibly the transport layer, headers of each packet to or from
the NAT subnet(s) sharing its IP address(es). This causes NAT to
break the end-to-end nature of Internet connectivity, and disrupts
protocols requring end-to-end connectivity, such as the network
security protocols which embody IPSEC. Furthermore, any application
that transmits IP address or port content, such as FTP, will require
a proxy module (application layer gateway) within the NAT router.
Given these limitations of NAT, RSIP has emerged as an attempt to
remedy them.
RSIP is based on the concept of granting host from one realm (e.g.,
privately addressed realm) a presence in another realm (e.g.,
publicly addressed realm) by granting it resources from the second
realm. While this document is limited to the discussion of IPv4
networks, RSIP is general and may be applied beyond the limitations
of IPv4 networks and used as a means of IPv4 public address
assignment to IPv6 subnets. In this document we discuss the
approaches of several possible RSIP systems and address the issues
that any RSIP solution must face.
2. Terminology
Private Realm
A routing realm that uses private IP addresses from the ranges
(10/8, 172.16/12, 192/16) specified in [RFC1918], or addresses
that are non-routable from the Internet.
Public Realm
A routing realm with unique network addresses assigned by the
Internet Assigned Number Authority (IANA) or an equivalent address
registry.
RSIP Server
A router situated on the boundary between a private realm and a
public realm and owns one or more public IP addresses. An RSIP
server is responsible for public parameter management and
assignment to RSIP clients. An RSIP server may act as a normal
NAT box for hosts within the private realm that are not RSIP
enabled.
RSIP Client
A host within the private realm that assumes publicly unique
parameters from an RSIP server through the use of RSIP.
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RSA-IP: Realm Specific Address IP
An RSIP method in which each RSIP client is allocated a unique IP
address from the public realm. Dicussed in detail in [NAT-TERM].
RSAP-IP: Realm Specific Address and Port IP
An RSIP method in which each RSIP client is allocated an IP
address (possibly shared) and some number of per-address unique
ports from the public realm. Dicussed in detail in [NAT-TERM].
All other terminology found in this document is consistent with that
of [NAT-TERM].
3. Specification of Requirements
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
documents are to be interpreted as described in [RFC2119].
4. Architecture
In a typical scenario where RSIP is deployed, there are some number
of hosts within one addressing realm connected to another addressing
realm by the RSIP server. This model is diagrammatically represented
as follows:
RSIP Client RSIP Server Host
Xa Na Nb Yb
[X]------( Addr sp. A )----[N]-----( Addr sp. B )-------[Y]
( Network ) ( Network )
Hosts X and Y belong to different addressing realms A and B,
respectively, and N is an RSIP server (which may also act as a NAT).
N has two addresses: Na on address space A, and Nb on address space
B. N may have a pool of addresses in address space B which it can
assign to or lend to X and other hosts in addressing realm A. These
addresses are not shown above, but they can be denoted as Nb1, Nb2,
Nb3 and so on. The hosts within address space A are likely to use
private addresses while the RSIP server is multi-homed with one or
more private addresses in addition to it's public addresses.
Using the public parameters assigned by the RSIP server, RSIP clients
route (usually tunnel) data packets to the RSIP server within address
space A. If tunneling is used, the RSIP server acts as the end point
of such tunnels, stripping off the outer headers and routing the
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inner packets onto the public realm. An RSIP server maintains a
mapping of the assigned public parameters as demultiplexing tuples
for uniquely mapping them to RSIP client private addresses. When a
packet from the public realm arrives at the RSIP server and it
matches a given set of demultiplexing tuples, then the RSIP server
will tunnel it to the appropriate RSIP client.
5. RSIP Fundamentals
This section discusses the issues that all RSIP schemes must address.
Note that these issues are not orthogonal; thus, by addressing one,
in some cases another issue is also addressed sufficiently.
5.1. Negotiation and/or determination of Demultiplexing Fields
Assume that an RSIP client within a private realm has transmitted
a request to a public server within a public realm, and the server
has sent a response packet that successfully arrived at the RSIP
server. Based on a pre-arranged mapping, the RSIP server must be
able to determine the private IP address of the packet's
destination; i.e., the RSIP client. The only information that the
RSIP server may use is what is already contained within the
headers of the inbound data packet. We will refer to these header
fields as the "demultiplexing fields" as they are used to spread
the incoming streams of packets to multiple destinations within
the private realm.
Depending on the type of mapping used by the RSIP server,
demultiplexing parameters could either be public IPv4 addresses,
TCP/UDP ports, IPSEC Security Payload Indexes (SPIs), ISAKMP
initiator cookies, some combination of the above, or some other
field(s). Such demultiplexing of incoming traffic resembles a
decision tree, which could be represented as follows:
- A unique public IP address is mapped to each RSIP client.
- If the same IP address is used for more than one RSIP client,
then subsequent headers must have at least one field that will
be assigned a unqieu value per client so that it is usable as a
demultiplexing field.
- If the subsequent header is TCP or UDP, then destination port
number can be used. Otherwise, there must exist another field
usable as a demultiplexing field.
- If the TCP or UDP port number is the same for more than one RSIP
client, the payload section of the packet must contain a
demultiplexing field that is guaranteed to be different for each
RSIP client.
In general, it is desirable for all demultiplexing fields to occur
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in well-known fixed locations so that an RSIP server can mask out
and examine the appropriate fields on incoming packets. In some
cases, for example, where different RSIP methods (RSA-IP, RSAP-IP,
etc.) are used by the same RSIP client using just one IP address,
the decision tree approach would be beneficial.
Demultiplexing of incoming streams of packet requires pre-
assignment of the demultiplexing fields to RSIP clients. Hence
there exists a requirement for a negotiation process that enables
these parameters to be negotiated between RSIP server and RSIP
clients. Such a negotiation process can be based on the following
approaches.
- As an extension of current host configuration or policy protocols
such as DHCP, COPS, RADIUS, DIAMETER, or SOCKS.
- During tunnel establishment, for example as an extension to L2TP
parameter negotiation.
- As an RSIP specific protocol such as described in [RSIP-PROTO].
5.2. Determination of other RSIP parameters
Apart from negotiation of demultiplexing fields, other information
pertaining to the assignment of those fields may also need to be
negotiated. Examples of such parameters are:
A binding identifier may be assigned for each public parameter
assignment. The binding identifier serves to uniquely identify
the resource(s) that has been allocated by an RSIP server. It
may also be used during lookup to efficiently index existing
bindings.
A time duration (lease) may be associated with each bind of
public parameters to an RSIP client.
RSIP clients may require that the RSIP server specify how it
allocates address and port resources (referred to as the RSIP
method). RSIP servers may only allocate a public IP address to
each unique host, known as RSA-IP. Or, RSIP-servers may
distribute a (potentially shared) public IP address and a
unique port range per that IP address to each host, termed
RSAP-IP.
The negotiation and assignment mechanism SHOULD be extensible
and facilitate vendor specific parameters.
5.3. Tunnel Use and Establishment
Once the public demultiplexing fields have been allocated by the
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RSIP server, RSIP clients will be able to use them freely.
However, RSIP implementations generally requires data packets to
be tunnelled between the RSIP client and server within the private
realm since public routing information is not advertised in the
private realm. While it is possible to imagine an RSIP
implementation that does not require tunneling, it seems that
tunneling is a flexible method for solving address ambiguity
problems. The type of tunnel may be IP-IP, GRE, IPSEC tunnel
mode, L2TP, or another form of tunnel.
There is a disadvantage to not using a tunnel between the RSIP
client and the RSIP server. It is likely that an RSIP server will
also act as a firewall or packet filter for the private network.
In this case, it publically addressed packets are transmitted on
the private network, the RSIP server may consider these packets
tobe part of an attack.
Tunnels may be established statically or dynamically between RSIP
clients and servers. A static tunnel is established at host boot
and remains in service until the host is no longer using the
network. A dynamic tunnel is established at the beginning of a
session or flow and exists only for the lifetime of the session.
Both types of tunnels may allow for on-the-fly re-negotiation of
demultiplexing fields and re-assignment of parameters to RSIP
clients. If tunneling is used to route the publicly addressed
packet within private realm, public parameter negotiation could be
associated with tunnel establishment mechanisms. Alternatively, a
negotiation protocol may enable the negotiation of tunnel type as
well.
6. Miscellaneous Issues
The resolution of a number of RSIP issues are still open. Although
solutions may exist for these problems, they may have unattractive
side effects. In this section we discuss several such issues.
6.1. Policy and Accounting
All RSIP-clients SHOULD have a mechansims of authenticating
themselves to RSIP-servers, in order to alleviate possible denial
of service attacks in which a malicious RSIP client attempts to
utilize the resources assigned to a different RSIP client.
Any RSIP implementation SHOULD implement accounting of irregular
event seen by the RSIP-server. Events such as denial of service
attacks, illegal use of resources (traffic without bindings or
after binding expirations) and public resource depletion SHOULD be
logged.
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6.2. Contacting Internal Servers
In order for an RSIP implementation to allow private hosts to run
servers that can be contacted from the public network, these
servers must be registered with the RSIP server. Registration of
servers with unique and/or well known listen ports may be limited
to one per private realm unless other means beyond port number are
used for demultiplexing (e.g. multiple WWW domains may be
disambiguated by looking into the HTTP headers).
6.3. Determining Locality of Destinations
In general, an RSIP client must know, for a particular IP address,
whether it should transmit the packet normally for local delivery,
or tunnel the packet to the RSIP server. Since more than one
subnet may be behind an RSIP server, looking at a local subnet
mask will not always work. We'd rather not have to propagate
routing tables to all RSIP clients. A simple alternative,
proposed in [RSIP-PROTO], that will solve this problem is to
require that the RSIP server knows all of the subnets that are on
the private network. This information can be manually entered
because it is not expected to change often. Then, if an IP
address in question is not on a host's local subnet, the host can
query the server with the address. The RSIP server will return a
simple "yes or "no" answer - yes, this address is local, or no, it
is not. As proposed in [RSIP-PROTO], a queried RSIP server may
respond with the list of subnets supported. An RSIP client may
cache this information. However, in large enterprise networks, an
RSIP server may not be aware of all private subnets.
Alternatively, RSIP-clients could send all packets for
destinations without an explicit static route to the RSIP server.
If they arrive at the RSIP server, it informs the host that it
should instead tunnel the packet. The host then acquires the
necessary public parameters and tunnels the packet, to the RSIP
server. This approach may require further changes to the TCP/IP
stack at the host, since, in the case of TCP traffic, a half-open
TCP socket must be discarded. Likewise, the RSIP client could at
first tunnel the packets to the RSIP server. If the server
determines that the destination is local, it would inform the host
of this fact and the host could then transmit the packet in the
standard fashion. Regardless of the solution chosen, RSIP clients
caching the "locality" of recently-contacted IP addresses may be
beneficial.
6.4. Implementing RSIP Client Deallocation
As currently defined in [RSIP-PROTO], an RSIP client MAY free
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resources that it has determined that it no longer requires. For
example, on a large RSAP-IP subnet with a limited number of public
IP addresses, locally-unique port numbers may become scarse.
Thus, if RSIP clients are able to deallocate ports that they no
longer need, RSIP will be more scalable.
However, this functionality may require significant modifications
to a vanilla TCP/IP stack in order to implement properly. The
RSIP client must be able to determine which TCP or UDP sessions
are using RSIP resources. If those resources are unused for a
period of time, then the RSIP client may deallocate them. When an
open socket's resources are deallocated, it will cause some
associated applications to fail. An analogous case would be TCP
and UDP sessions that must terminate when a PPP interface that
they are using loses connectivity.
On the other hand, this issue can be considered a resource
allocation problem. It is not recommended that a large number
(hundreds) of hosts share the same IP address, for performance
purposes. Even if, say, 100 hosts each are allocated 100 ports,
the total number of ports in use by RSIP would be still less than
one-sixth the total port space for an IP address. If more hosts
or more ports are needed, more IP addresses should be used. Thus,
it is reasonable, that in many cases, RSIP clients will not have
to deallocate ports for the lifetime of their activity.
Similarly, it is non-trivial for an RSIP client to know when to
allocate ports. It will have to detect activity on a socket,
determine if the destination host is local or external, and then
request the appropriate resources. In cases when the allocation
requires multiple rounds, for example when more than one public
resources are to be allocated and multiple assignment requests are
issued or a request gets denied a number of times, delays may be
introduced by the resource allocation process.
7. Cascaded RSIP
It is possible for RSIP to allow for cascading of RSIP-servers. For
example, consider an ISP that uses RSIP for address sharing amongst
its customers. It might assign resources (e.g., IP addresses and
ports) to a particular customer. This customer may further subdivide
the port ranges and address(es) amongst individual end hosts. A
reference architecture is depicted below.
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+-----------+
| |
| RSIP |
| server +---- 10.0.0.0/8
| B |
| |
+-----+-----+
|
| 10.0.1.0/24
+-----------+ | (149.112.240.0/25)
| | |
149.112.240.0/24| RSIP +--+
----------------+ server |
| A +--+
| | |
+-----------+ | 10.0.2.0/24
| (149.112.240.128/25)
|
+-----+-----+
| |
| RSIP |
| server +---- 10.0.0.0/8
| C |
| |
+-----------+
RSIP-server A is in charge of the IP addresses of subnet
149.112.240.0/24. It distributes these addresses to RSIP-clients and
RSIP-servers. In the given configuration, it distributes addresses
149.112.240.0 - 149.112.240.127 to RSIP-server B, and addresses
149.112.240.128 - 149.112.240.254 to RSIP-server C. Note that the
subnet broadcast address, 149.112.240.255, must remain unclaimed, so
that broadcast packets can be distributed to arbitrary hosts behind
RSIP-server A. Also, the subnets between RSIP-server A and RSIP-
servers B and C will use private addresses.
Due to the tree-like fashion in which addresses will be cascaded, we
will refer to RSIP-servers A as the 'parent' of RSIP-servers B and C,
and RSIP-servers B and C as 'children' of RSIP-servers A. An
arbitrary number of levels of children may exist under a parent RSIP-
server.
A parent RSIP-server will not necessarily be aware that the
address(es) and port blocks that it distributes to a child RSIP-
server will be further distributed. Thus, the RSIP-clients MUST
tunnel their outgoing packets to the nearest RSIP-server. This
server will then verify that the sending host has used the proper
address and port block, and then tunnel the packet on to its parent
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RSIP-server.
For example, in the context of the diagram above, host 10.0.0.1,
behind RSIP-server C will use its assigned external IP address (say,
149.112.240.130) and tunnel its packets over the 10.0.0.0/8 subnet to
RSIP-server C. RSIP-server C strips off the outer IP header. After
verifying that the source public IP address and source port number is
valid, RSIP-server C will tunnel the packets over the 10.0.2.0/8
subnet to RSIP-server A. RSIP-server A strips off the outer IP
header. After verifying that the source public IP address and source
port number is valid, RSIP-server A transmits the packet on the
public network.
While it may be more efficient in terms of computation to have a
RSIP-client tunnel directly to the overall parent of an RSIP-server
tree, this would introduce significant state and administrative
difficulties.
A RSIP-server that is a child MUST take into consideration the
parameter assignment constraints that it inherits from its parent
when it assigns parameters to its children. For example, if a child
RSIP-server is given a lease time of 3600 seconds on an IP address,
it MUST compare the current time to the lease time and the time that
the lease was assigned to compute the maximum allowable lease time on
the address if it is to assign the address to a RSIP-client or child
RSIP-server.
8. To Do
- RSIP impact on ALGs?
- Pros and cons of RSIP riding on top of existing protocols such as
DHCP, RADIUS, SOCKS, etc.
9. Changelog
00 to 01:
- Synched up terminology with the latest NAT terminology draft.
- Changed all instances of "global" to "public"
- Modified section on "Architecture"
- Added discussion of demultiplexing parameters tree to the
"Negotiation and assignment of demultiplexing fields" section
- Added discussion of subnet list query in "Determining Locality of
Destination" section
- Added "RSIP Client Deallocation" discussion section
- Added more explanation in "Tunnel Use and Establishment" section
10. Acknowledgements
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The authors would like to thank Gabriel Montenegro, Pyda Srisuresh,
Dan Nessett, Gary Jaszewski, and Rick Cobb for their input.
11. References
[RSIP-PROTO] Michael Borella, David Grabelsky, Jeffrey Lo and Kuni
Taniguchi, "Realm Specific IP: Protocol Specification,"
<draft-ietf-nat-rsip-protocol-01.txt>, work in progress, Apr. 1999.
[RFC2119] S. Bradner, "Key words for use in RFCs to indicate
requirement levels," RFC 2119, Mar. 1997.
[RFC1918] Y. Rekhter, B. Moskowitz, D. Karrenberg, G. J. de Groot, and
E. Lear, "Address Allocation for Private Internets," RFC 1918,
Feb. 1996.
[NAT-TERM] P. Srisuresh and Matt Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations,"
<draft-ietf-nat-terminology-02.txt>, work in progress, Apr. 1999.
12.
Authors' Addresses
Jeffrey Lo
NEC USA
C&C Research Labs.
110 Rio Robles
San Jose, CA 95134
(408) 943 3033
jlo@ccrl.sj.nec.com
Michael Borella
3Com Corp.
1800 W. Central Rd.
Mount Prospect IL 60056
(847) 842 6093
mike_borella@3com.com
David Grabelsky
3Com Corp.
1800 W. Central Rd.
Mount Prospect IL 60056
(847) 222 2483
david_grabelsky@3com.com
Copyright (c) The Internet Society (1999). All Rights Reserved.
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