INTERNET-DRAFT Matt Crawford
Fermilab
<draft-ietf-ipngwg-esd-analysis-01.txt> Allison Mankin
ISI
Thomas Narten
IBM
John W. Stewart, III
ISI
Lixia Zhang
UCLA
July 30, 1997
Separating Identifiers and Locators in Addresses:
An Analysis of the GSE Proposal for IPv6
<draft-ietf-ipngwg-esd-analysis-01.txt>
Status of this Memo
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Distribution of this memo is unlimited.
This Internet Draft expires January 30, 1997.
Abstract
On February 27-28, 1997, the IPng Working Group held an interim
meeting in Palo Alto, California to consider adopting Mike O'Dell's
'GSE - An Alternate Addressing Architecture for IPv6' proposal [GSE].
In GSE, 16-byte IPv6 addresses are split into three portions: a
globally unique End System Designator (ESD), a Site Topology
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Partition (STP) and a Routing Goop (RG) portion. The STP corresponds
(roughly) to a site's subnet portion of an IPv4 address, whereas the
RG identifies the attachment point to the public Internet. Routers
use the RG+STP portions of addresses (called 'Routing Stuff' in this
document) to route packets to the link to which the destination is
directly attached; the ESD is used to deliver the packet across the
last hop link. An important idea in GSE is that nodes within a site
do not know the RG portion of their addresses. A border router at the
site's Internet connect point would dynamically replace the RG part
of source addresses of all outgoing IP datagrams and the RG part of
destination addresses on incoming traffic.
This document provides a detailed analysis of the GSE plan. Much of
the analysis presented here is an expansion of official meeting
minutes, though it also includes issues uncovered by the authors in
the process of fully fleshing out the analysis. In summary, the
working group eventually decided that the full addresses of nodes
within a site should not be hidden from those nodes, so as a result
it is not necessary for routers to rewrite the Routing Goop portion
of addresses. However, other parts of the GSE plan were adopted
(e.g., having 64-bit interface identifiers with an option for
specifying them as globally unique and easing the renumbering of the
high-order portion of addresses within DNS).
In addition to analyzing the GSE proposal in particular, the document
also studies the general issue of separating network layer addresses
into two separate values satisfying location and identification
purposes, respectively.
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Contents
Status of this Memo.......................................... 1
1. Introduction............................................. 4
2. Addressing and Routing in IPv4........................... 5
2.1. The Need for Aggregation............................ 7
2.2. The Pre-CIDR Internet............................... 7
2.3. CIDR and Provider-Based Addressing.................. 8
2.4. Multi-Homing and Aggregation........................ 11
3. GSE Background........................................... 14
3.1. Motivation For GSE.................................. 14
3.2. GSE Address Format.................................. 15
3.3. Routing Stuff (RG and STP).......................... 15
3.4. End-System Designator............................... 17
3.5. Address Rewriting by Border Routers................. 18
3.6. Renumbering and Rehoming Mid-Level ISPs............. 19
3.7. Support for Multi-Homed Sites....................... 20
3.8. Explicit Non-Goals for GSE.......................... 21
4. Analysis of GSE's Advantages and Disadvantages........... 21
4.1. End System Designator............................... 21
4.1.1. Uniqueness Enforcement in the IPv4 Internet.... 21
4.1.2. Overloading Addresses: Network Layer Issues.... 23
4.1.3. Overloading Addresses: Transport Layer Issues.. 24
4.1.4. Potential Benefits of Globally Unique ESDs..... 25
4.1.5. ESD: Network Layer Issues...................... 26
4.1.6. ESD: Transport Layer Issues.................... 28
4.1.7. On The Uniqueness Of ESDs...................... 34
4.1.8. DNS PTR Queries................................ 35
4.1.9. Reverse Mapping of ESDs........................ 37
4.1.10. Reverse Mapping of Complete GSE Addresses..... 38
4.1.11. The ICMP "Who Are You" Message................ 39
4.2. Renumbering and Domain Name System (DNS) Issues..... 40
4.2.1. How Frequently Can We Renumber?................ 40
4.2.2. Efficient DNS support for Site Renumbering..... 41
4.2.3. Two-Faced DNS.................................. 42
4.2.4. Bootstrapping Issues........................... 43
4.2.5. Renumbering and Reverse DNS Lookups............ 44
4.3. Address Rewriting Routers........................... 44
4.3.1. Load Balancing................................. 45
4.3.2. End-To-End Argument: Don't Hide RG from Hosts.. 45
4.4. Multi-Homing........................................ 46
5. Results.................................................. 48
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6. Security Considerations.................................. 49
7. Acknowledgments.......................................... 49
8. References............................................... 49
9. Authors' Addresses....................................... 51
1. Introduction
In October of 1996, Mike O'Dell published an Internet-Draft (dubbed
"8+8") that proposed significant changes to the IPv6 addressing
architecture. The 8+8 proposal was the topic of considerable
discussion at the December 1996 IETF meeting in San Jose. Because the
proposal offered both potential benefits (e.g., enhanced routing
scalability) and risks (e.g., changes to the basic IPv6
architecture), the IPng Working Group held an interim meeting on
February 27-28, 1997 to consider adopting the 8+8 proposal. The
meeting, at which over 45 persons attended, was held at Sun
Microsystems' PAL1 facility in Palo Alto, CA.
Shortly before the interim meeting, an updated version of the
Internet-Draft was produced, in which the name of the proposal was
changed from "8+8" to "GSE," to identify the three separate
components of the address: Global, site and End-System Designator.
This last version of the GSE proposal was published as an
Informational RFC [GSE] for historical purposes.
The purpose of the meeting was to evaluate the GSE proposal and
decide whether to adopt it in whole or in part or to reject it.
The well-attended meeting generated high caliber, focused technical
discussions on the issues involved, with participation by almost all
of the attendees. By the middle of the second day there was unanimous
agreement by the attendees that the GSE proposal as written presented
too many risks and should not be adopted as the basis for IPv6.
However, the attendees also concluded that some of the issues
discussed in the GSE proposal were equally applicable to the current
IPv6 provider-based addressing plan and had enough benefit to warrant
further consideration apart from the GSE address format. These
changes include:
1) Making changes to the IPv6 provider-based addressing document to
facilitate increased aggregation.
2) Creating hard boundaries in IPv6 addresses to clearly
distinguish between the portions used for identifying hosts and
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for routing.
3) Having an option to indicate that the low-order 8 bytes of an
IPv6 address is a globally unique End System Designator (ESD).
This change has potential benefits to future transport protocols
(e.g., TCPng).
4) Making a clear distinction between the "locator" part of an
address and the "identifier" part of the address. The former is
used to route a packet to its end-point, the latter is used to
identify an end-point, independent of the path used to deliver
the packet.
5) Making changes to the way AAAA records are stored within the
DNS, so that renumbering a site (e.g., when a site changes ISPs)
requires few changes to the DNS database in order to effectively
change all of a site's address AAAA RRs.
While this document does contain an analysis of the specific
mechanisms of the GSE proposal, much of document's analysis applies
to any proposal in which the identifying and locating properties of
an address (which are combined in IPv4) are split apart into
separable pieces.
2. Addressing and Routing in IPv4
Before dealing with details of GSE, we present some background about
how routing and addressing works in "classical IP" (i.e., IPv4). We
present this background because the GSE proposal proposes a fairly
major change to the base model. In order to properly evaluate the
benefits of GSE, one must understand what problems in IPv4 it alleges
to improve or fix.
The structure and semantics of a network layer protocol's addresses
are absolutely core to that protocol. Addressing substantially
impacts the way packets are routed, the ability of a protocol to
scale and the kinds of functionality higher layer protocols can
provide. Indeed, addressing is intertwined with both routing and
transport layer issues; a change in any one of these can impact
another. Issues of administration and operation (e.g., address
allocation and required renumbering), while not part of the pure
exercise of engineering a network layer protocol, turn out to be
critical to the scalability of that protocol in a global and
commercial network. The interaction between addressing, routing and
especially aggregation is particularly relevant to this document, so
some time will be spent describing it.
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Addresses in IPv4 serve two purposes:
1) Unique identification of an interface. An IP address by itself
identifies which interface a packet should be delivered to.
2) Location information of that interface. Routers extract location
information from a packet's destination address in order to
route it towards its ultimate destination. That is, addresses
identify "where" the intended recipient is located within the
Internet topology.
For scalability, the location information contained in addresses
must be aggregatable. In practice, this means nodes
topologically close to each other (e.g., connected to the same
link, residing at the same site, or customers of the same ISP)
must use addresses that share a common prefix.
What is important to note is that these identification and location
requirements have been met through the use of the same value, namely
the IP address. As will be noted repeatedly in this document, the
"over-loading" of IPv4 addresses with multiple semantics has some
undesirable implications. For example, the embedding of IPv4
addresses within transport protocol addresses that identify the end-
point of a connection couples those transport protocols with routing.
This entanglement is inconsistent with a strictly layered model in
which routing would be a completely independent function of the
network layer and not directly impact the transport layer.
Combining locator and identifier functions also has the practical
impact of complicating the support for mobility. In a mobile
environment, the location of an end-station may change even though
its identity stays the same; ideally, transport connections should be
able to survive such changes. In IPv4, however, one cannot change the
locator without also changing the identifier. Consequently,
conventional wisdom for some time has been that having separate
values for location and identification could be of significant
benefit. The GSE proposal attempts to make such a separation.
This document frequently uses mobility as an example to demonstrate
the pros and cons of separating the identifier from the locator.
However, the reader should note the fundamental equivalence between
the problems faced by mobile hosts and the problem faced by sites
that change providers yet don't want to be required to renumber their
network. When a site changes providers, it moves (topologically) in
much the same way a mobile node does when it moves from one place to
another. Consequently, techniques that help (or hinder) mobility are
often relevant to the issue of site renumbering.
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2.1. The Need for Aggregation
IPv4 has seen a number of different addressing schemes. Since the
original specification, the two major additions have been subnetting
and classless routing. The motivation for adding subnetting was to
allow a collection of networks located at one site to be viewed from
afar as being just one IP network (i.e., to aggregate all of the
individual networks into one bigger network). The practical benefit
of subnetting was that all of a site's hosts, even if scattered among
tens or hundreds of LANs, could be represented via a single routing
table entry in routers located far from the site. In contrast, prior
to subnetting, a site with ten LANs would advertise ten separate
network entries, and all routers would have to maintain ten separate
entries, even though they contained redundant information..
The benefits of aggregation should be clear. The amount of work
involved in computing forwarding tables from routing tables is
dependent in part on the number of network routes (i.e.,
destinations) to which best paths are computed. If each site has 10
internal networks, and each of those networks is individually
advertised to the global routing subsystem, the complexity of
computing forwarding tables can easily be an order of magnitude
greater than if each site advertised just a single entry that covered
all of the addresses used within the site.
2.2. The Pre-CIDR Internet
In the early days of the Internet, the Internet's topology and its
addressing were treated as orthogonal. Specifically, when a site
wanted to connect to the Internet, it approached a centralized
address allocation authority to obtain an address and then approached
a provider about procuring connectivity. This procedure for address
allocation resulted in a system where the addresses used by customers
of the same provider bore little relation to the addresses used by
other customers of that provider. In other words, though the topology
of the Internet was mostly hierarchical (i.e., customers connected to
only one provider and the same path was used to reach all customers
of the same provider), the addressing was not, and little aggregation
of routes took place. An example of such a topology and addressing
scheme shown in Figure 1.
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+----------------+
| |------- Customer1 (192.2.2.0)
| |------- Customer2 (128.128.0.0)
| Provider A |------- Customer3 (18.0.0.0)
| |------- Customer4 (193.3.3.0)
| |------- Customer5 (194.4.4.0)
+----------------+
|
|
|
|
+----------------+
| Provider B |
+----------------+
Figure 1
Figure 1 shows Provider A having 5 customers, each with their own
independently obtained network addresses. Providers A and B connect
to each other. In order for Provider B to be able to send traffic to
Customers1-5, Provider A must announce each of the 5 networks to
Provider B. That is, the routers within Provider B must have explicit
routing entries for each of Provider A's customers, 5 separate routes
in Figure 1.
Experience has shown that this approach scales very poorly. In the
Default-Free Zone (DFZ) of the Public Internet, where routers must
maintain routing entries for all reachable destinations, the cost of
computing forwarding tables quickly becomes unacceptably large. A
large part of the cost is related to the seemingly redundant
computations that must be made for each individual network, even
though the reality is that many reside in the same topological
location (e.g., the same provider). Looking at Figure 1, the problem
is that provider B performs 5 separate calculations to construct the
routing tables needed to reach each of A's customers.
2.3. CIDR and Provider-Based Addressing
One of the reasons Classless Inter-Domain Routing (CIDR) and its
associated provider-assigned address allocation policy were
introduced was to help reduce the size of and cost of computing
forwarding tables. CIDR reduces the cost of computing forwarding
tables by aggressively aggregating addresses. Aggregating addresses
means structuring them in such a way that the location of the nodes
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having those addresses can be represented by a single routing entry.
In CIDR, this means that addresses share a common prefix. The common
prefix provides location information for all addresses sharing that
same prefix.
In CIDR, sites that want to connect to the Internet approach a
provider to procure both connectivity and a network address;
individual providers have a large block of address space covered by
one prefix and assign pieces of their space to customers.
Consequently, customers of the same provider have addresses that
share the same prefix. Note that CIDR started the use of the term
"prefix" to refer to a Classless network. The combination of CIDR and
provider-based addressing results in the ability for a provider to
address many hundreds of sites while introducing just *one* network
address into the global routing system, i.e., aggregating all of its
customers addresses under one prefix. An example of such a topology
and addressing scheme is shown in Figure 2.
+----------------+
| |------- Customer1 (204.1.0.0/19)
| |------- Customer2 (204.1.32.0/23)
| Provider A |------- Customer3 (204.1.34.0/24)
| |------- Customer4 (204.1.35.0/24)
| |------- Customer5 (204.1.36.0/23)
+----------------+
|
| A announces
| 204.1/16 to B
|
+----------------+
| Provider B |
+----------------+
Figure 2
In Figure 2, Provider A has been assigned the classless block, or
"aggregate," 204.1.0.0/16 (i.e., a network prefix with 16 bits for
the network part and 16 bits for local use). Provider A has 5
customers, each of which has been assigned a prefix subordinate to
the aggregate. In order for Provider B to be able to reach
Customers1-5, Provider A need only announce a single prefix,
204.1.0.0/16, because that prefix covers all of its customers. The
benefit for Provider B is that its routers need only a single routing
table entry to reach all of Provider A's customers. Note the
difference between the cases described in Figures 1 and 2. The
important difference in the two Figures is that the latter example
uses fewer slots in the routing table to reach the same number of
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destinations.
CIDR was a critical step for the Internet: in the early 1990s the
size of default-free routing tables required to support the Classful
Internet was almost more than the commercially-available hardware and
software of the day could handle. The introduction of BGP4's
classless routing and provider-based address allocation policies
resulted in an immediate relief. Having said that, however, there are
some weaknesses of the system. First, the Internet addressing model
shifted from one of "address owning" to "address lending." In pre-
CIDR days sites acquired addresses from a central authority
independent of who their network provider was, and a site could
assume it "owned" the address it was given. Owning addresses meant
that once one had been given a set of network addresses, one could
always use them and assume that no matter where a site connected to
the Internet, the prefix for that network could be injected into the
public routing system. Today, however, it is simply no longer
possible for each individual site to have its own private prefix
injected into the DFZ; there would simply be too many of them.
Consequently, if a site decides to change providers, then it needs to
number itself out of space given to it by the new provider and give
its old address back to the old provider. To understand this,
consider if, from Figure 2, Customer3 changes its provider from
Provider A to Provider C, but does not renumber. The picture would be
as follows:
+----------------+
| |---- Customer1 (204.1.0.0/19)
| |---- Customer2 (204.1.32.0/23)
| Provider A |
+---------------| |---- Customer4 (204.1.35.0/24)
| A announces | |---- Customer5 (204.1.36.0/23)
| 204.1/16 to B +----------------+
| |
+----------------+ |
| Provider B | |
+----------------+ |
| |
| C announces |
| 204.1.34/24 |
| to B +----------------+
+---------------| Provider C |---- Customer3 (204.1.34.0/24)
+----------------+
Figure 3
In Figure 3, each of Provider A, B and C are directly connected to
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each other provider. In order for Provider B to reach Customers 1, 2,
4 and 5, Provider A still only announces the 204.1.0.0/16 aggregate.
However, in order for Provider B to reach Customer 3, Provider C must
announce the prefix 204.1.34.0/24. Prefix 204.1.34.0/24 is called a
"more-specific" of 204.1.0.0/16; another term used is that Customer3
and Provider C have "punched a hole in" Provider A's block. The
result of this is that from Provider B's view, the address space
underneath 204.1.0.0/16 is no longer cleanly aggregated into a single
prefix and instead the aggregation has been broken because the
addressing is inconsistent with the topology; in order to maintain
reachability to Customer3, Provider B must carry two prefixes where
it used to have to carry only one.
The example in Figure 3 explains why sites must renumber if existing
levels of aggregation are to be maintained. While it is certainly
clear that one or two "exceptions" to the ideal case can be
tolerated, the reality in today's Internet is that there are
thousands of providers, many with thousands of individual customers.
It is generally accepted that some renumbering of sites is essential
for maintaining sufficient aggregation.
The empirical cost of renumbering a site in order to maintain
aggregation has been the subject of much discussion. The practical
reality, however, is that forcing all sites to renumber is difficult
given the size and wealth of companies that now depend on the
Internet for running their business. Thus, although the technical
community came to consensus that address lending was necessary in
order for the Internet to continue to operate and grow, the reality
has been that some of CIDR's benefits have been lost because sites
refuse to renumber.
One unfortunate characteristic of CIDR at an architectural level is
that the pieces of the infrastructure which benefit from the
aggregation (i.e., the providers whose major headache is managing
routing table growth in the DFZ) are not the pieces that incur the
cost (i.e., the end site). The logical corollary of this statement is
that the pieces of the infrastructure which do incur cost to achieve
aggregation (e.g., sites which renumber when they change providers)
don't directly see the benefit. (The word "directly" is used here
because one could claim that the continued operation of the Internet
is a benefit, though it is an indirect benefit and requires
selflessness on the part of the site in order to recognize it.)
2.4. Multi-Homing and Aggregation
As sites become more dependent on the Internet, they have begun to
install additional connections to the Internet to improve robustness
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and performance. Such sites are called "multi-homed." Unfortunately,
when a site connects to the Internet at multiple places, the impact
on routing can be much like a site that switches providers but
refuses to renumber.
In the pre-CIDR days, multi-homed sites were typically known by only
one network prefix. When that site's providers announced the site's
network into the global routing system, a "shortest path" type of
routing would occur so that pieces of the Internet closest to the
first provider would use the first provider while other pieces of the
Internet might use the second provider. This allowed sites to use the
routing system itself to load balance traffic across their multiple
connections. This type of multi-homing assumes that a site's prefix
can be propagated throughout the DFZ, an assumption that is no longer
universally true.
With CIDR, issues of addressing and aggregation complicate matters
significantly. At the highest levels, there are three possible ways
to deal with multi-homed sites. The first approach is for multi-
homed sites to receive address space directly from a registry,
independent of its providers. The problem with this approach is
that, because the address space is obtained independent of either
provider, it is not aggregatable and therefore has a negative impact
on the scaling of global routing.
The second approach is for a multi-homed site to receive an
allocation from one of its providers and just use that single prefix.
The site would advertise its prefix to all of the providers to which
it connects. Their are two problems with this is approach. First,
although the prefix is aggregatable by the provider which made the
allocation, it is not aggregatable by the other providers. To the
other providers, the site's prefix poses the same problem as a
provider-independent address would. This has a negative impact on
the scaling of global routing. Second, due to CIDR's longest-match
routing rules, it turns out that the site's prefix is not always
aggregable in practice by the provider that made the allocation.
Consider Figure 4. Provider C has two paths for reaching customer 1.
Provider A advertises 204.1/16, which includes customer 1. But
Provider C will also receive an advertisement for prefix 204.1.0/19
from Provider B, and because the prefix match through B is longer, C
will choose that path. In order for Provider C to be able to choose
between the two paths, Provider A would also have to advertise the
longer prefix for 204.1.0/19 in addition to the shorter 204.1/16. At
this point, from the routing perspective, the situation is very
similar to the general problem posed by the use of provider-
independent addresses.
It should be noted that the above example simplifies a very complex
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issue. For example, consider the example in Figure 4 again. Provider
A could choose *not* to propagate a route entry for the longer
2.4.1.0/19 prefix, advertising only the shorter 204.1/16. In such
cases, provider C would always select Provider B. Internally,
Provider A would continue to router traffic from its other customers
to customer 1 directly. If Provider A had a large enough customer
base, effective load sharing would achieved.
+------------+ +------------+
_____| Provider A |---| Provider C |
/ +------------+ +------------+
/ 204.1/16 /
/ /
Customer 1 --- / B advertises 204.1.0/19 to C
204.1.0.0/19 | /
| +------------+
----- | Provider B |
+------------+
Figure 4
The third approach is for a multi-homed site to receive an allocation
from each of its providers. This approach has advantages from the
perspective of route scaling because both allocations are
aggregatable. Unfortunately, the approach doesn't necessarily meet
the demands of the multi-homed site. A site that has a prefix from
each of its providers has a number of choices about how to use that
address space. Possibilities include:
1) The site can number a distinct set of hosts out of each of the
prefixes. Consider a configuration where a site is connected to
ISP-A and ISP-B. If the link to ISP-A goes down, then unless the
ISP-A prefix is announced to ISP-B (which breaks aggregation),
the hosts numbered out of the ISP-A prefix would be unreachable.
2) The site could assign each host multiple addresses (i.e., one
address for each ISP connection). There are two problems with
this. First, it accelerates the consumption of the address
space. Second, when the connection to ISP-A goes down,
addresses numbered out of ISP-A's space become unreachable.
Remote peers would have to have sufficient intelligence to use
the second address. For example, when initiating a connection to
a host, the DNS would return multiple candidate addresses.
Clients would need to try them all before concluding that a
destination is unreachable (something not all hosts currently
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do). In addition, a site's hosts would need a significant amount
of intelligence for choosing the source addresses they use. A
host shouldn't choose a source address corresponding to a
addresses that are not reachable from the Public Internet. At
present, hosts do not have such sophistication.
In summary, how best to achieve multi-homing with IPv4 in the face of
CIDR is an unsolved problem. There is a delicate balance between the
scalability of routing versus the site's requirements of robustness
and load-sharing. At this point in time, no solution has been
discovered that satisfies the competing requirements of route scaling
and robustness/performance. It is worth noting, however, that some
people are beginning to study the issue more closely and propose
novel ideas [BATES].
3. GSE Background
This section provides background information about GSE with the
intent of making this document stand-alone with respect to the GSE
"specification." Additional details on GSE can be found in [GSE].
We begin by reviewing the motivation for GSE. Next we review the
salient technical details, and we conclude by listing the explicit
non-goals of the GSE proposal.
3.1. Motivation For GSE
The primary motivation for GSE is the fact that the chief IPv6 global
unicast address structure, provider-based [RFC 2073], is
fundamentally the same as IPv4 with CIDR and provider-based
aggregation. Provider-based addressing requires that sites renumber
when they switch providers, so that sites are always aggregated
within their provider's prefix. In practice, the cost of renumbering
(which can only grow as a site grows in size and becomes more
dependent on the Internet for day-to-day business) is high enough
that an increasing number of sites refuse to renumber. This cost is
particularly relevant in cases where end-users are asked to renumber
because an upstream provider has changed its transit provider (i.e.,
the end site is asked to renumber for reasons outside of its control
and for which it sees no direct benefit). Consequently, The GSE
draft asserts that IPv4 with CIDR has not achieved the aggressive
aggregation required for the route computation functions of the
default-free zone of the Internet to scale for IPv4, and that the
larger addresses of IPv6 simply exacerbate the problem.
The GSE proposal does not propose to eliminate the need for
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renumbering. Indeed, it asserts that end sites will have to be
renumbered more frequently in order to continue scaling the Internet.
However, GSE proposes to make the cost of such a renumbering so
small, that sites could be renumbered at essentially any time with
only minor disruption to the site.
Finally, GSE deals significantly with sites that have multiple
Internet connections. In some addressing schemes (e.g., CIDR), this
"multi-homing" can create exceptions to the aggregation and result in
poor scaling. That is, the public routing infrastructure needs to
carry multiple distinct routes for the multi-homed site, one for each
independent path. GSE recognizes the "special work done by the global
Internet infrastructure on behalf of multi-homed sites," [GSE] and
proposes a way for multi-homed sites to gain some benefit without
impacting global scaling. This includes a specific mechanism that
providers could use to support multi-homed sites, presumably at a
cost that the Site would consider when deciding whether or not to
become multi-homed.
3.2. GSE Address Format
The key departure of GSE from classical IP addressing (both v4 and
v6) is that rather than over-loading addresses with both locator and
identifier purposes, it splits the address into two elements: the
high-order 8 bytes for routing (called "Routing Stuff" throughout the
rest of this document) and the low-order 8 bytes for unique
identification of an end-point. The structure of GSE addresses is:
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| Routing Goop | STP| End System Designator |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
6+ bytes ~2 bytes 8 bytes
Figure 5
3.3. Routing Stuff (RG and STP)
The Routing Goop (RG) identifies the place in the Public Internet
topology where a Site connects and is used to route datagrams to the
Site. RG is structured as follows:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| xxx | 13 Bits of LSID | Upper 16 bits of Goop |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3 4
2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bottom 18 bits of Routing Goop |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6
The RG describes the location of a Site's connection by identifying
smaller and smaller regions of topology until finally it identifies a
single link to which the site. Before interpreting the bits in the
RG, it is important to understand that routing with GSE depends on
decomposing the Internet's topology into a specific graph. At the
highest level, the topology is broken into Large Structures (LSs). An
LS is basically a region that can aggregate significant amounts of
topology. Examples of potential LSs are large providers and exchange
points. Within an LS the topology is further divided into another
graph of structures, with each LS dividing itself however it sees
fit. This division of the topology into smaller and smaller
structures can recurse for a number of levels, where the trade-off is
"between the flat-routing complexity within a region and minimizing
total depth of the substructure." [ESD]
Having described the decomposition process, we can now examine the
bits in the RG. After the 3-bit prefix identifying the address as
GSE, the next 13 bits identify the LS. By limiting the field to 13
bits, a ceiling is defined on the complexity of the top-most routing
level. In the next 34 bits, a series of subordinate structure(s) are
identified until finally the leaf subordinate structure is
identified, at which point the remaining bits identify the individual
link within that leaf structure. The remaining 14 bits of the Routing
Stuff comprise the STP and are used for routing structure within a
Site, similar to subnetting with IPv4, though these bits are *not*
part of the Routing Goop. The distinction between Routing Stuff and
Routing Goop is that RG controls routing in the Public Internet,
while Routing Stuff includes the RG plus the Site Topology Partition
(STP). The STP is used for routing structure within a Site.
The GSE proposal formalizes the ideas of sites and of public versus
private topology. In the first case, a Site is a set of hosts,
routers and media which have one or more connections to the Internet.
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A Site can have an arbitrarily complicated topology, but all of that
complexity is hidden from everyone outside of the Site. A Site only
carries packets which originated from, or are destined to, that Site;
in other words, a Site cannot be a transit network. A Site is private
topology, while the transit networks form the public topology.
A datagram is routed through public topology using just the RG, but
within the destination Site routing is based on the Site Topology
Partition (STP) field.
3.4. End-System Designator
The End-System Designator (ESD) is an unstructured 8-byte field that
uniquely identifies that interface from all others. The most
important feature of the ESD is that it alone identifies an end
point; the Routing Stuff portion of an address, although used to help
deliver a packet to its destination, is not used to actually identify
an end point. End-points of communication care about the ESD; as
examples, TCP peers could be identified by the source and destination
ESDs alone (together with port numbers), checksums would exclude the
RG (the sender doesn't know its RG, so can't include it in the
checksum), and on receipt of a datagram only the ESD would be used in
testing whether a packet is intended for local delivery.
The leading contender for the role of a 64-bit globally unique ESD is
the recently defined "EUI-64" identifier [EUI64]. These identifiers
consist of a 24-bit "company_id" concatenated with a 40-bit
"extension." (Company_id is just a new name for the Organizationally
Unique Identifier (OUI) that forms the first half of an 802 MAC
address.) Manufacturers are expected to assign locally unique values
to the extension field, guaranteeing global uniqueness for the
complete 64-bit identifier.
A range of the EUI-64 space is reserved to cover pre-existing 48-bit
MAC addresses, and a defined mapping insures that an ESD derived from
a MAC address will not duplicate the ESD of a device that has a
built-in EUI-64.
In some cases, interfaces may not have access to an appropriate MAC
address or EUI-64 identifier. A globally unique ESD must then be
obtained through some alternate mechanism. Several possible
mechanisms can be imagined (e.g., the IANA could hand out addresses
from the company id assigned it has been allocated), but we do not
explore them in detail here.
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3.5. Address Rewriting by Border Routers
GSE Site border routers rewrite addresses of the packets they forward
across the Site/Public Topology boundary. Within a Site, nodes need
not know the RG associated with their addresses. They simply use a
designated "Site-Local RG" value for internal addresses. When a
packet is forwarded to the Public Topology, the border router
replaces the Site-Local RG portion of packet's source address with an
appropriate value. Likewise, when a packet from the Public Topology
is forwarded into a Site, the border router replaces the RG part of
the destination address with the designated Site-Local RG.
To simplify discussion, the following discussion uses the singular
term RG as if a site could have only one RG value (i.e., one
connection to the Public Internet). Of course, a site could have
multiple Internet connections and consequently multiple RGs.
Having border routers rewrite addresses obviates the need to renumber
devices within sites because of changing providers --- GSE's approach
isn't so much to ease renumbering as to make it transparent to end
sites. To achieve transparency, the RG by which a Site is known is
hidden (i.e., kept secret) from hosts or routers within that Site.
Instead, the RG for the Site would be known only by the exit router,
either through static configuration or through a dynamic protocol
with an upstream provider.
Because end-hosts don't know their RG, they don't know their entire
16-byte public address, so they can't specify the full address in the
source fields of packets they originate. Consequently, when a
datagram leaves a Site, the egress border router fills in the high-
order portion of the source address with the appropriate RG.
The point of keeping the RG hidden from nodes within the core of a
Site is to insure the changeability of this value without impacting
the Site itself. It is expected that the RG will need to change
relatively frequently (e.g., several times a year) in order to
support scalable aggregation as the topology of the Public Internet
changes. A change to a Site's RG would only require a change at the
Site's egress point (or points, in the case of a multi-homed Site);
and it's well possible that this change would be accomplished through
a dynamic protocol with the upstream provider.
Hiding a Site's RG from its internal nodes does not, however, mean
that changes to RG have no impact on end sites. Since the full 16-
byte address of a node isn't a stable value (the RG portion can
change), a stored address may contain invalid RG and be unusable if
it isn't "refreshed" through some other means. For example, opening a
TCP connection, writing the address of the peer to a file and then
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later trying to reestablish a connection to that peer is likely to
fail. For intra-Site communication, however, it is expected that
only the Site-Local RG would be used (and stored) which would
continue to work for intra-Site communication regardless of changes
to the Site's external RG. This has the benefit of shielding a site's
internal traffic from the affects of renumbering changes outside of
the site.
In addition to rewriting source addresses upon leaving a Site,
destination addresses are rewritten upon entering a Site. To
understand the motivation behind this, consider a Site with
connection to three Internet providers. Because each of those
connections has its own RG, each destination within the Site would be
known by three different 16-byte addresses. As a result, intra-Site
routers would have to carry a routing table three times larger than
expected. Instead, GSE proposes replacing the RG in inbound packets
with the special "Site-local RG" value to reduce intra-Site routing
tables to the minimum necessary.
In summary, when a node initiates a flow to a node in another Site,
the initiating node knows the full 16-byte address for the
destination through some mechanism like a DNS query. The initiating
node places the full 16-byte address in the destination address field
of the datagram, and that field stays intact through the first Site
and through all of the Public Topology. When the datagram reaches
the exit border router, the router replaces the RG of the packet's
source address. When the datagram arrives at entry router at the
destination Site, the router replaces the RG portion of the
destination address with the distinguished "Site-Local RG" value.
When the destination host needs to send return traffic, that host
knows the full 16-byte address for the destination because it
appeared in the source address field of the arriving packet.
3.6. Renumbering and Rehoming Mid-Level ISPs
One of the most difficult-to-solve components of the renumbering
problem is that of renumbering mid-level service providers.
Specifically, if SmallISP1 changes its transit provider from BigISP1
to BigISP2 (in the CIDR model), then all of SmallISP1's customers
would have to renumber into address space covered by an aggregate of
BigISP2 (if the overall size of routing tables is to stay the same).
GSE deals with this problem by handling the RG in DNS with
indirection. Specifically, a Site's DNS server specifies the RG
portion of its addresses by referencing the *name* of its immediate
provider, which is a resolvable DNS name (this obviously implies a
new Resource Record type). That provider may define some of the low-
order bits of the RG and then reference its immediate provider. This
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chain of reference allows mid-level service providers to change
transit providers, and the customers of that mid-level will simply
"inherit" the change in RG.
3.7. Support for Multi-Homed Sites
GSE defines a specific mechanism for providers to use to support
multi-homed customers that gives those customers more reliability
than singly-homed sites, but without a negative impact on the scaling
of global routing. This mechanism is not specific to GSE and could be
applied to any multi-homing scenario where a site is known by
multiple prefixes (including provider-based addressing). Assume the
following topology:
Provider1 Provider2
+------+ +------+
| | | |
| PBR1 | | PBR2 |
+----x-+ +-x----+
| |
RG1 | | RG2
| |
+--x-----------x--+
| SBR1 SBR2 |
| |
+-----------------+
Site
Figure 7
PBR1 is Provider1's border router while PBR2 is Provider2's border
router. SBR1 is the Site's border router that connects to Provider1
while SBR2 is the Site's border router that connects to Provider2.
Imagine, for example, that the line between Provider1 and the Site
goes down. Any already existing flows that use a destination address
including RG1 would stop working. In addition, any DNS queries that
return addresses including RG1 would not be viable addresses. If PBR1
and PBR2 knew about each other, however, then in this case PBR1 could
tunnel packets destined for RG1-prefixed addresses to PBR2, thus
keeping the communication working. (Note that true tunneling, i.e.,
re-encapsulation, is necessary since routers between PBR1 and PBR2
would forward RG1 addresses towards PBR1.)
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3.8. Explicit Non-Goals for GSE
It is worth noting explicitly that GSE does not attempt to address
the following issues:
1) Survival of TCP connections through renumbering events. If a
Site is renumbered, TCP connections using a previous address
will continue to work only as long as the previous address still
works (i.e., while it is still "valid" using RFC 1971
terminology). No attempt is made to have existing connections
switch to the new address.
2) It is not known how mobility can be made to work under GSE.
3) It is not known how multicast can be made to work under GSE.
4) The performance impact of having routers rewrite portions of the
source and destination address in packet headers requires
further study.
That GSE doesn't address the above does not mean they cannot be
solved. Rather the issues haven't been studied in sufficient depth.
4. Analysis of GSE's Advantages and Disadvantages
This section contains the bulk of the GSE analysis and the analysis
of the general locator/identifier split.
4.1. End System Designator
4.1.1. Uniqueness Enforcement in the IPv4 Internet
As described earlier, in the IPv4 Public Internet, IP addresses
contain two pieces of information: a unique identifier and a locator.
Embedding location information within an address has the side-effect
of helping insure that all addresses are globally unique. If
interfaces on two different nodes are assigned the same unicast
address, the routing subsystem will (generally) deliver packets to
only one of those nodes. The other node will quickly realize that
something is wrong (since communication using the duplicate address
fails) and take corrective action (e.g., obtain a proper address).
This is important for two reasons. It helps detect misconfigurations
(use of the wrong address prevents communication from taking place),
and helps thwart intruders.
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In IPv4, communication usually fails quickly when addresses are not
unique. There are two cases to consider, depending on whether the two
interfaces assigned duplicate addresses are attached to the same or
to different links.
When two interfaces on the same link use the same address, a node
(host or router) sending traffic to the duplicate address will in
practice send all packets to one of the nodes. On Ethernets, for
example, the sender will use ARP (or Neighbor Discovery in IPv6) to
determine the link layer address corresponding to the destination
address. When multiple ARP replies for the target IP address are
received, the most recently received response replaces whatever is
already in the cache. Consequently, the destinations a node using a
duplicate IP address can communicate with depends on what its
neighboring nodes have in their ARP caches. In most cases, such
communication failures become apparent relatively quickly, since it
is unlikely that communication can proceed correctly on both nodes.
It is also the case that a number of ARP implementations (e.g., BSD-
derived implementations) log warning messages when an ARP request is
received from a node using the same address as the machine receiving
the ARP request.
When two interfaces on different links use the same address, the
routing subsystem will generally deliver packets to only one of the
nodes because only one of the links has the right "prefix" or "subnet
part" corresponding to the IP address. Consequently, the node using
the address on the "wrong" link will generally never receive any
packets sent to it and will be unable to communicate with anyone. For
obvious reasons, this condition is usually detected quickly.
An important observation is that, with classical IP, when different
nodes mistakenly assign the same IP address to different interfaces,
problems become apparent relatively quickly because communication
with several (if not all) destinations fails. In contrast, failure
scenarios differ when globally unique ESDs are assumed, but two nodes
mistakenly select the same one.
Embedding location information within an address also provides some,
though not much, protection from forged addresses. Although it is
trivial to forge a source address in today's Internet, the routing
subsystem will in most cases forward any return traffic sent to that
address to its proper destination --- not to an arbitrary node
masquerading as someone else. To masquerade as someone else requires
subverting the routing subsystem, placing the intruder somewhere on
the normal routing path between the masqueraded host and its peer,
etc.
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4.1.2. Overloading Addresses: Network Layer Issues
At the network layer, a node compares the destination address of
received packets against the addresses of its attached interfaces.
Only if the addresses of received packets match are packets handed up
to higher layer protocols. In IPv4, the entire address must match.
Otherwise, the packet is assumed to be intended for some other node
and forwarded on (if received by a router) or silently discarded (if
received by a host). This has subtle but significant implications:
1) If a receiving host has multiple interfaces, it has multiple IP
addresses. When a packet addressed to a multi-homed host is
received on an interface other than the one to which a packet is
addressed, the host may reject (i.e., silently discard) the
packet, if it implements the "Strong ES Model" defined in
[RFC1122].
2) In recent IPv4 stacks, an interface may have more than one
unicast IP address assigned to it. Indeed, one way to renumber
an end site is to phase out an address (i.e., "deprecate" it
using RFC 1971 terminology) while simultaneously phasing in a
new one. Once the deprecated address becomes invalid, packets
sent to the invalid address will no longer be accepted by the
node, even though the packet may have intuitively reached its
intended recipient. Thus, even if a packet sent to an invalid
address is somehow delivered to the intended recipient (e.g.,
via tunneling), the receiver would reject the packet because the
address it was sent to no longer belongs to any of the node's
interfaces. Consequently, any communication using the invalid
address will fail (e.g., new and existing TCP connections).
Anyone wishing to communicate with the node must learn and
switch to the new address.
3) Because an address also indicates "where" the destination
resides within the Internet, a mobile node that moves from one
part of the Internet to another must obtain a new address that
reflects its new location. Moreover, the routing subsystem will
continue to forward packets sent to the mobile node's previous
address to the node's previous point of attachment where they
are likely be discarded. That is, even if a mobile node is
willing to continue accepting packets addressed to one its
previous addresses, it is unlikely that they will be received
(in the absence of something like Mobile IP [RFC2002]).
4) A multi-homed host has multiple interfaces, each with its own
address(es). If one of its interfaces fails, packets could, in
theory, be delivered to one of the host's other interfaces. In
practice, however, the routing subsystem has no way of knowing
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that the interface to which a packet is addressed has failed and
what alternate interface addresses the packet could be delivered
to. Consequently, packets sent to a failed interface of a
multi-homed host won't be delivered, even though the node is
reachable through alternate interfaces.
Note that the above problems fall into two general categories:
1) Today's routing subsystem is unable to automatically deliver a
packet to a host's "alternate" addresses (if the host is multi-
homed) or a new address (if the host moves), should there be a
problem delivering a packet to the destination address listed in
the packet. It is possible to imagine, however, future routing
advances addressing this problem (e.g., Mobile IP).
2) Even if a packet is delivered to its intended destination, the
packet may still be rejected because the packet's destination
address does not match any of the addresses assigned to
destination's interfaces. This problem does not appear to be
insurmountable and could be rectified (for example) by having a
host remember its previous addresses.
4.1.3. Overloading Addresses: Transport Layer Issues
The problems discussed previously create particular complications at
the transport level. Transport protocols such as TCP and UDP use
embedded IP addresses to identify the end-points of a transport
connection. Specifically, the communicating end-points of a transport
connection are uniquely identified by the sender's source IP address
and source port number together with the recipient's destination IP
address and port number. Once a connection has been established, the
IP addresses can not change. In particular, if a mobile host moves to
a new location and obtains a new address, packets intended for a TCP
connection created prior to the move cannot use the new address. TCP
will treat any packets sent to the new address as belonging to a
different TCP connection.
It is possible to imagine changes to TCP that might allow connections
to change the addresses they are using mid-connection without
breaking the connection. However, some subtle issues arise:
1) Packets intended for a pre-existing connection must be
demultiplexed to that connection as part of any negotiation to
change the addresses that identify that transport end-point.
However, because the demultiplexing operation uses the transport
addresses of the pre-existing TCP connection (which is based on
the previous address), TCP packets sent to a new address won't
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be delivered to the desired transport end-point (which still
uses the previous address). Consequently, packets would need to
be sent to the previous address. However, by the time a mobile
node has moved and knows its new address, packets sent to the
previous address may no longer be delivered (i.e., they may not
be forwarded to the mobile host's new location).
2) When a mobile host moves, it could inform its TCP peers that it
has a new address. However, such a message could not be
delivered to the remote TCP connection if it was sent using its
new address for its source address. Just as above, such packets
would not be demultiplexed to the correct TCP connection. On the
other hand, it is infeasible to send packets using its previous
address from its new location. Because of the danger of spoofing
attacks, routers are now encouraged to actively look for, and
discard traffic from, a source address that does not match known
addresses for that region of the Internet [CERT]. Consequently,
such packets cannot be expected to be delivered.
Although the previous discussion used mobile nodes as an example, the
same problem arises in other contexts. For example, if a site is
being renumbered in IPv6, it may have two addresses, a previous
(i.e., deprecated) one being phased out and a new (i.e., preferred)
one being phased in. At the transport level, the problem of switching
addresses is similar in many respects to the mobility problem.
4.1.4. Potential Benefits of Globally Unique ESDs
Having a clear separation between the Routing Stuff and the ESD
portion of an address gives protocols some additional flexibility. At
the network layer, for example, recipients can examine just the ESD
portion of the destination addresses when determining whether a
packet is intended for them. This means that if a packet is delivered
to the correct destination node, the node will accept the packet,
regardless of how the packet got there, i.e., without regard to the
Routing Stuff of the address, which interface it arrived on, etc.
Such packets would then be delivered and accepted by the target host.
The idea of using addresses that cleanly separate the Routing Stuff
from an ESD is not new [references XXX]. However, there are several
different flavors. In its pure form, a sender would only need to know
the ESD of an end-point in order to send packets to it. When
presented with a datagram to send, network software would be
responsible for finding the Routing Stuff associated with the ESD so
that the packet can be delivered. A key question is who is
responsible for finding the Routing Stuff associated with a given
ESD? There are a number of possibilities:
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1) The network layer could be responsible for doing the mapping.
The advantage of such a system is that an ESD could be stored
essentially forever (e.g., in configuration files), but whenever
it is actually used, network layer software would automatically
perform the mapping to determine the appropriate Routing Stuff
for the destination. Likewise, should an existing mapping become
invalid, network layer software could dynamically determine the
updated quantity. Unfortunately, building such a mapping
mechanism that is scalable is a hard problem.
2) The transport layer could be responsible for doing the mapping.
It could perform the mapping when a connection is first opened,
periodically refreshing the binding for long-running
connections. Implementing such a scheme would change the
existing transport layer protocols TCP and UDP significantly.
3) Higher-layer software (e.g., the application itself) could be
responsible for performing the mapping. This potentially
increases the burden on application programmers significantly,
especially if long-running connections are required to survive
renumbering and/or deal with mobile nodes.
It should be noted that the GSE proposal does not embrace the general
model. Indeed, it proposes the last. The network layer (and indeed
the transport layer) is always presented both the Routing Stuff (RG +
STP) and the ESD together in one IPv6 address. It is not the network
(or transport) layer's job to determine the Routing Stuff given only
the ESD or to validate that the Routing Stuff is correct. When an
application has data to send, it queries the DNS to obtain the IPv6
AAAA record for a destination. The returned AAAA record contains both
the Routing Stuff and the ESD of the specified destination. While
such an approach eliminates the need for the lower layers to be able
to map ESDs into corresponding Routing Stuff, it also means that when
presented with an address containing an incorrect (i.e., no longer
valid) Routing Stuff, the network is unable to deliver the packet to
its correct destination. It is up to applications themselves to deal
with such failures. Note that addresses containing invalid Routing
Stuff will result any time cached addresses are used after the
Routing Stuff of the address becomes invalid. This may happen if
addresses are stored in configuration files, or with long-running
communication.
4.1.5. ESD: Network Layer Issues
Along with the flexibility offered by separating the ESD from the
Routing Stuff come additional considerations that must be considered
at the network layer:
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1) Addresses must have a locator embedded within them. It is not
feasible to route packets solely on an ESD; doing so would make
it impossible to aggregate routing information in a scalable
way. The GSE proposal assumes that the locator part of an
address is filled with an appropriate value by higher layers
(i.e., the transport or application layer).
2) If a receiver observes that recent packets are arriving with a
different Routing Stuff in the source address than before, it
may want to send return traffic using the new Routing Stuff.
However, such information should not be accepted without
appropriate authentication of the new Routing Stuff, otherwise
it would be trivial to hijack existing transport connections.
Always using the most recently received Routing Stuff of an
address to send return traffic without appropriate
authentication leads to a vulnerability that is equivalent in
potential danger to "reversing and using an unauthenticated
received source route."
Note also that in the GSE proposal, since a sender does not know
its own RG, it is not possible for the sender to compute an
Authentication Header via IPSec that covers the RG portion of an
address. Thus, a recipient of new RG would need to authenticate
the received information via some alternate (undefined)
mechanism.
Finally, receipt of packets from different Routing Stuff than
before does not necessarily indicate a permanent change. In the
GSE proposal, for example, when a Site is multi-homed, some of
its packets may exit via one egress router while other packets
exit via a different egress router. Even packets originated from
the same source may exit through multiple egress routers.
Consequently, a node may receive traffic from the same sender in
which the Routing Stuff part changes on every packet.
3) In general, whenever an address is embedded within a packet
(including within data), one must consider whether all the bits
in the address should be used in computations, or whether just
the ESD portion should be used. Examples where such decisions
would need to be made include, but are not limited to, Neighbor
Discovery packets containing Neighbor Solicitations and
Responses [RFC 1970], IPSec packets being demultiplexed to their
appropriate Security Association, IP deciding whether to accept
an IP datagram (before reaching the transport level), the
reassembly of fragments, transport layer demultiplexing of
received packets to end-points, etc.
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4.1.6. ESD: Transport Layer Issues
Previous sections have made clear that the embedding of full IPv6
addresses (i.e., Routing Stuff) within transport connection end-point
identifiers poses problems for mobility and site renumbering. This
section discusses an alternate approach, in which transport end-point
identifiers use ESDs rather than full addresses (with embedded
Routing Stuff).
In the following discussion, it should be kept in mind that the IPng
Recommendation [RFC 1752] states that a transition to IPv6 cannot
also require deployment of a "TCPng." In addition, although we focus
on TCP, UDP-based protocols also depend on the Routing Stuff in
similar ways, e.g., starting with the UDP checksum of the peers'
addresses. Indeed, we believe that TCP is the "easy" case to deal
with, for two reasons. First, TCP is a stateful protocol in which
both ends of the connection can negotiate with each other. Some UDP-
based protocols are stateless, and remember nothing from one packet
to the next. Consequently, changing UDP-based protocols may require
the introduction of "session" features, perhaps as part of a common
"library", for use by applications whose transport protocol is
relatively stateless. Second, changes to UDP-based protocols in
practice mean changing individual applications themselves, raising
deployability questions.
4.1.6.1. Demultiplexing Packets to Transport Endpoints
Connections in GSE are identified by the ESDs rather than full IPv6
addresses (with embedded Routing Stuff). That is:
unique IPv4 TCP connection: srcaddr dstaddr srcport destport
unique GSE TCP connection: srcESD dstESD srcport dstport
Consequently, with GSE, when demultiplexing incoming packets, TCP
would ignore the Routing Stuff portions of addresses when delivering
packets to their proper end-point.
Although there are potential benefits to this approach (discussed
below), demultiplexing on ESDs alone without the RS is, in fact,
required with GSE. If a site is multi-homed, the packets it sends may
exit different egress border routers during the lifetime of a
connection. Because each border router will place its own RG into the
source addresses of outgoing packets, the receiving TCP must ignore
(at least) the RG portion of addresses when demultiplexing received
packets. The alternative would be to make TCP less robust with
respect to changes in routing, i.e., if the path changed, packets
delivered correctly would be discarded by the receiving TCP rather
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than processed.
4.1.6.2. Pseudo-Header Checksum Calculations
Having routers rewrite the RG portion of addresses means that TCP
cannot include the RG in its checksum calculation; the sender does
not know its own RG. Consequently, upon receipt of a TCP segment, the
receiver has no way of determining whether the RG portion of an
address has been corrupted (or modified) in transit (the implications
of this are discussed below).
4.1.6.3. RG Selection When Sending Packets
When a host has a packet to send, there are three cases for deciding
what RG to use in the destination. If the host is performing an
"active open", it queries the DNS to obtain the destination address,
which contains appropriate RG. If the host is responding to an active
open from a remote peer, the source address of packets from that peer
contains usable RG. Note that assuming that the RG on an incoming TCP
connection is "correct" needs qualification. It is "correct" in the
sense that it corresponds to the site originating the connection.
Whether the ESD paired with the RG is actually located at that site
cannot be assumed. The issue of spoofing is discussed in more detail
later. The last (and most interesting) case is when RG changes mid-
connection. Although, the GSE proposal calls for always using the
first RG learned (and then never switching), we explored the
possibility of doing so in order to better understand the issues.
4.1.6.4. Mid-Connection RG Changes
During a connection, the RG appearing on subsequent packets is
susceptible to change through renumbering events, and indeed more
frequently, to change through Site-internal routing changes that
cause the egress point for off-Site traffic to change. It is even
possible (in the worst case) that traffic-balancing schemes could
result in the use of two egress routers, with roughly every other
packet exiting through a different egress router. Consequently it may
be desirable to switch to the just-received RG, as the old RG may no
longer be valid (e.g., a border router has failed), but care must be
taken not to thrash. Moreover, simply using the most-recently-
received RG makes it trivial for an intruder to hijack connections.
Because TCP under GSE demultiplexes packets using only ESDs, packets
will be delivered to the correct end-point regardless of what source
RG is used. However, return traffic will continue to be sent via the
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"old" RG, even though it may have been deprecated or become less
optimal because the peer's border router has changed. It would seem
highly desirable for TCP connections to be able to survive such
events. However, the completion of renumbering events (so that an
earlier RG is now invalid) and certain topology changes would require
TCP to switch sending to a new RG mid-connection. To explore the
whole space, we considered ways of allowing this mid-connection RG
change to happen.
If TCP connection identifiers are based on ESDs rather than full
addresses, traffic from the same ESD would be viewed as coming from
the same peer, regardless of its source RG. This makes it trivial for
any Internet host to impersonate another, and have such traffic be
accepted by TCP. Because this vulnerability is already present in
today's Internet (forging full source addresses is trivial), the mere
delivery of incoming datagrams with the same ESD but a different RG
does not introduce new vulnerability to TCP. In today's Internet,
any node can already originate FINs/RSTs from an arbitrary source
address and potentially or definitely disrupt the connection.
Therefore, changing RG for acceptance, or acceptance of traffic
independent of its source RG, does not appear to significantly worsen
existing robustness.
We also considered allowing TCP to reply to each segment using the RG
of the most recently-received segment. Although this allows TCP to
survive some important events (e.g., renumbering), it also makes it
trivial to hijack connections, unacceptably weakening robustness
compared with today's Internet. A sender simply needs to guess the
sequence numbers in use by a given TCP connection [Bellovin 89] and
send traffic with a bogus RG to hijack a connection to an intruder
at an arbitrary location.
Providing protection from hijacking implies that the RG used to send
packets must be bound to a connection end-point (e.g., it is part of
the connection state). Although it may be reasonable to accept
incoming traffic independent of the source RG, the choice of sending
RG requires more careful consideration. Indeed, any subsequent change
in what RG is used for sending traffic must be properly authenticated
using cryptographic means. In the GSE proposal, it is not clear how
to authenticate such a change, since the remote peer doesn't even
know what RG it is using! Consequently, the only reasonable approach
in GSE is to send to the peer at the first RG used by the peer for
the entire life of a connection. That is, always continue to use the
first RG seen.
In summary, changing the RG dynamically in a safe way for a
connection requires that an originator of traffic be able to
authenticate a proposed change in the RG before sending to a
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particular ESD via that RG. Such a mechanism would need to be
invented, as the TCP/IP suite has no obvious candidate that operates
at or below the transport layer (using the DNS, an application
protocol that resides above IP, would be problematic due to layering
circularity considerations).
4.1.6.5. Passive Opens
One question that arises is what impact corrupted RG would have on
robustness. Because the RG is not covered by any checksums, it would
be difficult to detect such corruption. Moreover, once a specific RG
is in use, it does not change for the duration of a connection. The
interesting case occurs on the passive side of a TCP connection,
where a server accepts incoming connections from remote clients. If
the initial SYN from the client includes corrupted RG, the server TCP
will create a TCP connection (in the SYN-RECEIVED state) and cache
the corrupted RG with the connection. The second packet of the 3-way
handshake, the SYN-ACK packet, would be sent to the wrong RG and
consequently not reach the correct destination. Later, when the
client retransmits the unacknowledged SYN, the server will continue
to send the SYN-ACK using the bad RG. Eventually the client times
out, and the attempt to open a TCP connection fails. Figure 8 shows
the details.
TCP A TCP B
1. CLOSED LISTEN
2. SYN-SENT --> <SRC RG=BITERR><SEQ=100><CTL=SYN> --> SYN-RECEIVED
3. <-- <DST RG=BITERR><SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
4. SYN-SENT --> <SRC RG><SEQ=100><CTL=SYN> --> SYN-RECEIVED
5. <-- <DST RG=BITERR><SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
... TCP A times out
Figure 8
We next consider relaxing the restriction on switching RGs in an
attempt to avoid the previous failure scenario. The situation is
complicated by the fact that the RG on received packets may change
for legitimate reasons (e.g., a multi-homed site load-shares traffic
across multiple border routers). The key question is how can one
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determine which RG is valid and which is not. That is, for each of
the RGs a sender attempts to use, how can it determine which RG
worked and which did not? Solving this problem is more difficult than
first appears, since one must cover the cases of delayed segments,
lost segments, simultaneous opens, etc. If a SYN-ACK is retransmitted
using different RGs, it is not possible to determine which of those
RGs worked correctly. We conclude that the only way TCP could
determine that a particular RG was used to deliver segments was if it
received an ACK for a specific sequence number in which all
transmissions of that sequence number used the same RG (a non-trivial
addition to TCP).
We analyze multiple cases of RG changing within the time of the
opening handshake. One example is diagrammed in Figure 9, and it and
two others are summarized in Table 1. We observe that RG flap and
large numbers of passive opens may coincide, for instance, when a
power failure at a server farm affects both internal routers and
servers.
time TCP A time TCP B
t0 --> <SRC RG=M><SEQ=100><SYN> t1
t3 <-- <DST RG=M><SEQ=300><ACK=101><SYN,ACK> t1
TCP B's SYN,ACK is delayed and crosses with retransmit of TCP A's
SYN on which RG has changed from M to N
t2 --> <SRC RG=N><SEQ=100><SYN> t3
t4 --> <SRC RG=N><SEQ=101><ACK=301> t3 ESTABLISHED
TCP B decides to use DST RG=M for TCP A, because it heard from
RG=M and was ACK'd on a send to RG=M
Figure 9
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SYNFROM SYNACKTO ACKFROM SELECT
W W X W
------------------------------------
W
X W X W
------------------------------------
W W
X X Y ??
Table 1
At best, an RG selection algorithm for TCP would be relatively
straightforward but would require new logic in implementations of
TCP's opening handshake --- a significant transition issue. We are
not certain that a valid algorithm is attainable, however. RG changes
would have to be handled in all cases handled by the opening
handshake: delayed segments, lost segments, undetected bit errors in
RG, simultaneous opens, old segments and so on.
In the end, we conclude that although the corrupted SYN case of
Figure 8 was a potential problem, the changes that would need to be
made to TCP to robustly deal with such corruption would be
significant, if tractable at all. This would result in transition to
GSE needing a significant TCPng transition.
Our final conclusion is that transport protocol end-points must make
an early, single choice of the RG to use when sending to a peer and
stick with that choice for the duration of the connection.
Specifically:
1) The demultiplexing of arriving packets to their transport end
points should use only the ESD, and not the Routing Stuff.
2) If the application chooses an RG for the remote peer (i.e., an
active open), use the provided RG for all traffic sent to that
peer, even if alternative RGs are received on subsequent
incoming datagrams from the same ESD.
3) For all other cases, use the first RG received with a given ESD
for all sending. We recommend that a means be found for RGs to
be checksummed if the GSE address structure is used.
Consequently, there does not appear to be a straightforward way to
use ESDs in conjunction with mobility or site renumbering (in which
existing connections survive the renumbering).
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4.1.6.6. Summary: ESD and RG Not Strictly Independent
We cannot emphasize enough that the use of an ESD independent of an
associated RG can be very dangerous. That is, communicating with a
peer implies that one is always talking to the same peer for the
duration of the communication. But as has been described in previous
sections, such assurance can only take place if there are assurances
that only properly authenticated RG is used.
We conclude that the rules for transport processing when ESDs are
present differ from classical IP. Specifically:
1) The demultiplexing of packets to transport connection end-points
should use ESDs, but should not use the Routing Stuff part of
addresses. This insures that packets are delivered to their
intended destination independent of RG.
2) Once a packet has been delivered to its transport end-point, a
separate (i.e., distinct) decision should be made concerning
whether and how to act upon the received packet. Such a decision
would be transport-protocol specific. A protocol could chose to
completely ignore the packet, it could selectively use parts of
the packet (e.g., to attempt out-of-band authentication of the
RG), or it could process the packet in its entirety. It must
not, however, use the received RG to send subsequent return
traffic without first authenticating the RG.
4.1.7. On The Uniqueness Of ESDs
The uniqueness requirements for ESDs depends on what purpose they
serve. In GSE, ESDs identify end systems, requiring that they be
globally unique. It does not make sense for two different end systems
to use the same ESD; every end system must have its own ESD to
distinguish from other end systems.
If ESDs are only used to identify session endpoints, the situation
becomes more complex. At first glance it might appear that two nodes
using the same ESD cannot communicate. However, this is not
necessarily the case. In the GSE proposal, for example, a node
queries the DNS to obtain an IPv6 address. The returned address
includes the Routing Stuff of an address (the RG+STP portions). Since
the sending host transmits packets based on the entire destination
IPv6 address, the sender may well forward the packet to a router that
delivers the packet to its correct destination (using the information
in the Routing Stuff). It is only on receipt of a packet that a node
would extract the ESD portion of a datagram's destination address and
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ask "is this for me?"
A more problematic case occurs if two nodes using the same ESD
communicate with a third party. To the third party, packets received
from either machine might appear to be coming from the same machine
since they are both using the same ESD. Consequently, at the
transport level, if both machines choose the same source and
destination port numbers (one of the ports --- a server's well-known
port number will likely be the same), packets belonging to two
distinct transport connections will be demultiplexed to a single
transport end-point.
When packets from different sources using the same source ESD are
delivered to the same transport end-point, a number of possibilities
come to mind:
1) The transport end-point could accept the packet, without regard
to the Routing Stuff of the source address. This may lead to a
number of robustness problems, if data from two different
sources mistakenly using the same ESD are delivered to the same
transport or application end-point (which at best will confuse
the application).
2) The transport end-point could verify that the Routing Stuff of
the source address matches one of a set of expected values
before processing the packet further. If the Routing Stuff
doesn't match any expected value, the packet could be dropped.
This would result in a connection from one host operating
correctly, while a connection from another host (using the same
ESD) would fail.
3) When a packet is received with an unexpected Routing Stuff the
receiver could invoke special-purpose code to deal with this
case. Possible actions include attempting to verify whether the
Routing Stuff is indeed correct (the saved values may have
expired) or attempting to verify whether duplicate ESDs are in
use (e.g., by inventing a protocol that sends packets using both
Routing Stuff and verifies that they are delivered to the same
end-point).
4.1.8. DNS PTR Queries
IPv4 uses the domain "IN-ADDR.ARPA" to hold PTR Resource Records. PTR
RRs allow a client to map IP addresses back into the domain name
corresponding to that address. IPv4 addresses can be put into the DNS
because they have hierarchical structure -- the same hierarchy used
to aggregate routes.
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The ability to map an IP address into its corresponding DNS name is
used in several contexts:
1) Network packet tracing utilities (e.g., tcpdump) display the
contents of packets. Printing out the DNS names appearing in
those packets (rather than dotted IP addresses) requires access
to an address-to-name mapping mechanism.
2) Some applications perform "cheap" authentication by using the
DNS to map a source address of a peer into a DNS name. Then, the
client queries the DNS a second time, this time asking for the
address(es) corresponding to the peer's DNS name. Only if one of
the addresses returned by the DNS matches the peer address of
the TCP connection is the source of the TCP connection accepted
as being from the indicated DNS name.
It is important to note that although two DNS queries are made
during the above operation, it is the second one --- mapping the
peer's DNS name back into an IP address --- that provides the
authentication property. The first transaction simply obtains
the peer's DNS name, but no assumption is made that the returned
DNS name is correct. Thus, the first DNS query could be
replaced by an alternate mechanism without weakening the already
weak authentication check described above. One possible
alternate mechanism, an ICMP "Who Are You" message, is described
in Section 4.1.11.
3) Applications that log all incoming network connections (e.g.,
anonymous FTP servers) may prefer logging recognizable DNS names
to addresses.
4) Network administrators examining logs or other trace data
containing addresses may wish to determine the DNS name of some
addresses. Note that this may occur sometime after those
addresses were actually used.
Although DNS PTR records have proven useful in several contexts,
there is also widespread agreement that, in practice, many IP
addresses in use today are not properly registered in the IN-
ADDR.ARPA namespace. Consequently, PTR queries frequently fail to
return usable information. Thus, the overall utility of PTR records
is questionable.
It is also worth noting that the primary reason that so few addresses
are properly registered in the PTR space is the absence of incentive
for doing so. With no key piece of the Internet infrastructure
depending on such mappings being in place or correct, there is little
practical harm in failing to keep it up-to-date.
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Finally, it might appear at first glance that secure DNS [RFC2065]
provides a means for cryptographically signing a PTR record and
thereby providing authentication. Things are not so simple, however.
The signature on a PTR record indicates that the entity owning an
address has given it a DNS name. It does not mean that the owner of
the address is authorized to use that specific name. For example,
anyone owning an address can set up a PTR record indicating that the
address corresponds to the name "www.ietf.org". However, the name
"www.ietf.org" belongs to only one entity, regardless of how many PTR
records indicate otherwise.
4.1.9. Reverse Mapping of ESDs
It is reasonable to ask if it is necessary or desirable to be able to
map an ESD (alone) into some other meaningful quantity, such as a
fully qualified domain name. The benefits of being able to perform
such a mapping are analogous to those described in the preceding
section.
The primary difficulty with constructing such a mapping is that it
requires that ESDs have sufficient structure to support the
delegating mechanism of a distributed database such as DNS. The sorts
of built-in identifiers now found in computing hardware, such as
"EUI-48" and "EUI-64" addresses [IEEE802, IEEE1212], do not have the
structure required for this delegation. Hence, stateless
autoconfiguration [RFC1971] cannot create addresses with the
necessary hierarchical property.
Another possibility would be to define ESDs with sufficient structure
to permit the construction of a mapping mechanism. However, analysis
performed during the IPng deliberations concluded that close to 48-
bits of hierarchy were needed to identify all the possible sites
30-40 years from now. That would leave only 2 bytes for host
numbering at a site, a number clearly incompatible with stateless
autoconfiguration [RFC1971].
There are several arguments against having a global ESD-lookup
capability. Adding sufficient structure to an 8-byte ESD would be
incompatible with stateless autoconfiguration, which already uses 6
bytes for its token; two additional bytes for hierarchy are clearly
insufficient. In addition, experience with the IN-ADDR.ARPA domain
suggests that the required databases will be poorly maintained.
Finally, imposing a required hierarchical structure on ESDs would
also introduce a new administrative burden and a new or expanded
registry system to manage ESD space. While the procedures for
assigning ESDs, which need only organizational and not topological
significance, would be simpler than the procedures for managing IPv4
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addresses (or DNS names), it is hard to imagine such a process being
universally well-received or without controversy; it seems a laudable
goal to avoid the problem altogether if possible.
4.1.10. Reverse Mapping of Complete GSE Addresses
Although it seems infeasible to have a global scale, reverse mapping
of ESDs, within a Site, one could imagine maintaining a database
keyed on unstructured 8-byte ESDs. However, it is a matter of debate
whether such a database can be kept up-to-date at reasonable cost,
without making unreasonable assumptions as to how large sites are
going to grow, and how frequently ESD registrations will be made or
updated. Note that the issue isn't just the physical database itself,
but the operational issues involved in keeping it up-to-date. For the
rest of this section, however, let us assume such a database can be
built.
A mechanism supporting a lookup keyed on a flat-space ESD from an
arbitrary Site requires having sufficient structure to identify the
Site that needs to be queried. In practice, an ESD will almost always
be used in conjunction with Routing Stuff (i.e., a full 16-byte
address). Since the Routing Stuff is organized hierarchically, it
becomes feasible to maintain a DNS tree that maps full GSE addresses
into DNS names, in a fashion analogous to what is done with IPv4 PTR
records today.
It should be noted that a GSE address lookup will work only if the
Routing Stuff portion of the address is correctly entered in the DNS
tree. Because the RG portion of an address is expected to change over
time, this assumption will not be valid indefinitely. As a
consequence, a packet trace recorded in the past might not contain
enough information to identify the off-Site sources of the packets in
the present. This problem can be addressed by requiring that the
database of RG delegations be maintained for some period of time
after the RG is no longer usable for routing packets.
Finally, it should be noted that the problem where an address's RG
"expires" with the implication that the mapping of "expired"
addresses into DNS names may no longer hold is not a problem specific
to the GSE proposal. With provider-based addressing, the same issue
arises when a site renumbers into a new provider prefix and releases
the allocation from a previous block. The authors are aware of one
such renumbering in IPv4 where a block of returned addresses was
reassigned and reused within 24 hours of the renumbering.
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4.1.11. The ICMP "Who Are You" Message
Although there is widespread agreement on the utility of being able
to determine the DNS name one is communicating with, there is also
widespread concern that repeating the experience of the "IN-
ADDR.ARPA" domain is undesirable. Consequently, an old proposal to
define an ICMP "Who Are You?" message was resurrected [RFC1788]. A
client would send such a message to a peer, and that peer would
return an ICMP message containing its DNS name.
Asking a remote host to supply its own name in no way implies that
the returned information is accurate. However, having a remote peer
provide a piece of information that a client can use as input to a
separate authentication procedure provides a starting point for
performing strong authentication. The actual strength of the
authentication depends on the authentication procedure invoked,
rather than the untrustable piece of information provided by a remote
peer.
Reconsidering the "cheap" authentication procedure described in
Section 4.1.9, the ICMP "Who Are You" replaces the DNS PTR query used
to obtain the DNS name of a remote peer. The second DNS query, to map
the DNS name back into a set of addresses, would be performed as
before. Because the latter DNS query provides the strength of the
authentication, the use of an ICMP "Who Are You" message does not in
any way weaken the strength of the authentication method. Indeed, it
can only make it more useful in practice, because virtually all hosts
can be expected to implement the "Who Are You" message.
The "Who Are You" message could contain an identifier for matching
replies to requests, and perhaps a nonce value to provide resistance
to spoofing. In order to minimize the number of WRU packets on the
Internet, the WRU messages should be sent by DNS servers who would
then cache the answers. This has the pleasant side-effect of reducing
the impact on existing applications (i.e., they would continue to
look up addresses using the same API as before). In many cases there
is a natural TTL that the target node can provide in its reply:
either the remaining lifetime of a DHCP lease or the remaining valid
time of a prefix from which the address was derived through stateless
autoconfiguration.
The "Who Are You?" (WRU) message described in Section 4.1.10 is
robust against renumbering, since it follows the paths of valid
routable prefixes. Essentially, it uses the Internet routing system
in place of the DNS delegation scheme. It is attractive in the
context of GSE-style renumbering, since no host or DNS server needs
to be updated after a renumbering event for WRU-based lookups to
work. It has advantages outside the context of GSE as well, including
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a more decentralized, and hence more scalable, administration and
easier upkeep than a DNS reverse-lookup zone. It also has drawbacks:
it requires the target node to be up and reachable at the time of the
query and to know its fully qualified domain name. It is also not
possible to resolve addresses once those addresses become unroutable.
In contrast, the DNS PTR mirrors, but is independent of, the routing
hierarchy. The DNS can maintain mappings long after the routing
subsystem stops delivering packets to certain addresses.
The requirement that the target node be up and reachable at the time
of the query makes it very uncertain that one would be able to take
addresses from a packet log and translate them to correct domain
names at a later date. This is a design flaw in the logging system,
as it violates the architectural principle, "Avoid any design that
requires addresses to be ... stored on non-volatile storage."
[RFC1958] A better-designed system would look up domain names
promptly from logged addresses. Indeed, one of the authors is pleased
to be able to state that his site has been doing that for some years.
(Speculative note: Proxy servers to answer WRU queries are possible.
If the boundary between the global and site portions of addresses are
fixed and/or the boundary between the routing and the end-node
portions are fixed, then one could define a well-known anycast
address for proxy WRU service per site and/or per subnet. The low-
order portion of this address would presumably be created from the
IANA's IEEE OUI. The WRU client-side interface would have to be
defined to try this address after or before sending a query to the
target address itself. Nodes answering to this anycast address could
reply to WRU queries using a database maintained by private means.
By carrying a /128 route site-wide or in the site's provider, these
servers need not even be located within the subnet or site they
serve. Co-location of the proxy WRU servers with some DNS servers is
a natural choice in some scenarios.)
4.2. Renumbering and Domain Name System (DNS) Issues
4.2.1. How Frequently Can We Renumber?
One premise of the GSE proposal [GSE] is that an ISP can renumber the
Routing Goop portion of a Site's addresses transparently to the Site
(i.e., without coordinating the change with the Site). This would
make it possible for backbone providers to aggressively renumber the
Routing Goop part of addresses and achieve a high degree of route
aggregation. On closer examination, frequent (e.g., daily)
renumbering turns out to be difficult in practice because of a
circular dependency between the DNS and routing. Specifically, if a
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Site's Routing Stuff changes, nodes communicating with the Site need
to obtain the new Routing Stuff. In the GSE proposal, one queries the
DNS to obtain this information. However, in order to reach a Site's
DNS servers, the pointers controlling the downward delegation of
authoritative DNS servers (i.e., DNS "glue records") must use
addresses (with Routing Stuff) that are reachable. That is, in order
to find the address for the web server "www.foo.bar.com", DNS queries
might need to be sent to a root DNS servers, as well as DNS servers
for "bar.com" and "foo.bar.com". Each of these servers must be
reachable from the querying client. Consequently, there must be an
overlap period during which both the old Routing Stuff and the new
Routing Stuff can be used simultaneously. During the overlap period,
DNS glue records would need to be updated to use the new addresses
(including Routing Stuff). Only after all relevant DNS servers have
been updated and older cached RRs containing the old addresses have
timed out can the old address be deleted.
An important observation is that the above issue is not specific to
GSE: the same requirement exists with today's provider-based
addressing architecture. When a site is renumbered (e.g., it switches
ISPs and obtains a new set of addresses from its new provider), the
DNS must be updated in a similar fashion.
4.2.2. Efficient DNS support for Site Renumbering
When a site renumbers to satisfy its ISP, only the site's routing
prefix needs to change. That is, the prefix reflects where within the
Internet the site resides. Although some sites may also change the
numbering of their internal topology when switching providers, this
is not a requirement. Rather, it may be a convenient time to also
perform any desired internal renumbering since in practice that any
address renumbering tends to cause disruptions.
In the current Internet, when a site is renumbered, the addresses of
all the site's internal nodes change. This requires a potentially
large update to the RR database for that site. Although Dynamic DNS
[DDNS] could potentially be used, the cost is likely to be large due
to the large number of individual records that would need to be
updated. In addition, when DHCP and DDNS are used together [DHCP-
DDNS], it may be the case that individual hosts "own" their own A or
AAAA records, further complicating the question of who is able to
update the contents of DNS RRs.
One change that could reduce the cost of updating the DNS when a site
is renumbered is to split addresses into two distinct portions: a
Routing Goop that reflects where a node attaches to the Internet and
a "site internal part" that is the site-specific part of an address.
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During a renumbering, only the Routing Goop would change; the "site
internal part" would remain fixed. Furthermore, the two parts of the
address could be stored in the DNS as separate RRs. That way,
renumbering a site would only require that the Routing Goop RR of a
site be updated; the "site-internal part" of individual addresses
would not change.
To obtain the address of a node from the DNS, a DNS query for the
name would return two quantities: the "site internal part" and the
DNS name of the Routing Stuff for the site. An additional DNS query
would then obtain the specific RR of the site, and the complete
address would be synthesized by concatenating the two pieces of
information.
Implementing these DNS changes increases the practicality of using
Dynamic DNS to update a site's DNS records as it is renumbered. Only
the site's Routing Goop RRs would need updating.
Finally, it may be useful to divide a node's AAAA RR into the three
logical parts of the GSE proposal, namely RG, STP and ESD. Whether or
not it is useful to have separate RRs for the STP and ESD portions of
an address or a single RR combining both is an issue that requires
further study.
If AAAA records are comprised of multiple distinct RRs, then one
question is who should be responsible for synthesizing the AAAA from
its components: the resolver running on the querying client's machine
or the queried name server? To minimize the impact on client hosts
and make it easier to deploy future changes, it is recommended that
the synthesis of AAAA records from its constituent parts be done on
name servers rather than in client resolvers.
4.2.3. Two-Faced DNS
The GSE proposal attempts to hide the RG part of addresses from nodes
within a Site. If the nodes do not know their own RG, then they can't
store or use them in ways that cause problems should the Site be
renumbered and its RG change (i.e., the cached RG become invalid). A
Site's DNS servers, however, will need to have more information about
the RG its Site uses. Moreover, the responses it returns will depend
on who queries the server. A query from a node within the Site should
return an address with an RG portion equal to "Site local," whereas a
query for the same name from a client located at a different Site
would return the appropriate RG portion. This facilitates intra-site
communication to be more resilient to failures outside of the site.
Such context-dependent DNS servers are commonly referred as "two-
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faced" DNS servers.
Some issues that must be considered in this context:
1) A DNS server may recursively attempt to resolve a query on
behalf of a requesting client. Consequently, a DNS query might
be received from a proxy rather than from the client that
actually seeks the information. Because the proxy may not be
located at the same Site as the originating client, a DNS server
cannot reliably determine whether a DNS request is coming from
the same Site or a remote Site. One solution would be to
disallow recursive queries for off-Site requesters, though this
raises additional questions.
2) Since cached responses are, in general, context sensitive, a
name server may be unable to correctly answer a query from its
cache, since the information it has is incomplete. That is, it
may have loaded the information via a query from a local client,
and the information has a Site-local prefix. If a subsequent
request comes in from an off-Site requester, the DNS server
cannot return a correct response (i.e., one containing the
correct RG).
4.2.4. Bootstrapping Issues
If Routing Stuff information is distributed via the DNS, key DNS
servers must always be reachable. In particular, the addresses
(including Routing Stuff) of all root DNS servers are, for all
practical purposes, well-known and assumed to never change. It is not
uncommon for the addresses of root servers to be hard-coded into
software distributions. Consequently, the Routing Stuff associated
with such addresses must always be usable for reaching root servers.
If it becomes necessary or desirable to change the Routing Stuff of
an address at which a root DNS server resides, the routing subsystem
will likely need to continue carrying "exceptions" for those
addresses. Because the total number of root DNS servers is relatively
small, the routing subsystem is expected to be able to handle this
requirement.
All other DNS server addresses can be changed, since their addresses
are typically learned from an upper-level DNS server that has
delegated a part of the name space to them. So long as the delegating
server is configured with the new address, the addresses of other
servers can change.
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4.2.5. Renumbering and Reverse DNS Lookups
It is certain that many sites will, from time to time, undergo a
renumbering event, either through the mechanisms proposed for GSE or
using the facilities already specified for IPv6. It would be useful
to an outside node corresponding with such a site to be able to
distinguish a legitimate renumbering from an attempt to impersonate
the site. We claim that the DNS IP6.INT zone, without security
extensions [RFC2065], is of no use in making this determination and
that even a completely secured IP6.INT zone is of little use compared
with the "forward" DNS zone.
The first half of the claim is almost self-evident. An impersonator
can set up an insecure zone at some point in the IP6.INT hierarchy
and load it with any desired data. This is the reason that current
applications doing minimal access control follow a reverse lookup
with a forward lookup.
With a secured reverse zone, the problem of verifying an apparent
renumbering of a site can still be quite complex in the general case,
and will certainly be outside the scope of a transport protocol, if
survival of long-running sessions is contemplated. Under provider-
based addressing [RFC2073], renumbering is expected to occur due to a
change in network topology (e.g., a change in a provider relationship
at some point in the address aggregation tree). This alters the
global prefixes in use below the point of the change, and
correspondingly alters the chain of delegations of the DNS reverse-
mapping tree. And, although operational experience with secure DNS is
quite limited, it seems likely that there would also be a change in
the chain of certifications of the signing key of the leaf zone
representing the site. It is then problematic to translate
established trust in the old reverse mapping zone into trust in the
new zone. Certainly it's simpler to rely on the forward zone only.
The only function of the reverse zone, then, is to suggest an entry
point to the forward zone's database. It is this function which we
propose to achieve by means of a new ICMP message exchange.
4.3. Address Rewriting Routers
One of the most novel pieces of GSE is the rewriting of addresses as
datagrams enter and leave sites. If only a small number of routers
know the RG portion of the addresses, then the operational impact of
renumbering a Site would be small. In fact, assuming that the
critical security issues are dealt with, one could imagine a dynamic
protocol that a Site uses with its upstream provider to be told what
RG to use, so it might even be possible to renumber a Site
transparently.
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GSE's ability to insure that the RG portion of a Site's addresses
reflect the actual location of that Site within the Public Internet
means that very aggressive aggregation (i.e., better route scaling)
can be achieved. Both GSE and other route-scaling approaches that use
provider-based addressing depend on aggressive aggregation, but while
other schemes rely largely on operational policies, GSE attempts to
include mechanisms in its core to insure that aggressive aggregation
happens in practice.
GSE has an advantage over other provider-based addressing schemes
like IPv4's CIDR with respect to the "fair distribution of work."
CIDR addresses the scaling of routing in DFZ portions of the
Internet, but the cost of carrying out the renumbering to maintain
the aggregation falls on the shoulders of subscribers who are far
away from the DFZ; in other words, subscribers must do the work of
renumbering so that their provider (or possibly even their provider's
provider) sees better aggregation. With GSE, the majority of the cost
required to make the routing scale would be incurred by the parties
who reap the benefits.
4.3.1. Load Balancing
While not considered a major advantage, with GSE, multi-homed sites
can more easily achieve symmetry with respect to which of their links
is used for a given flow. With GSE, if HostA in multi-homed Site1
initiates a flow to HostB in Site2, then when the initial packet
leaves Site1 the source address will be rewritten with an RG that
identifies the egress link used. As a result, when HostB needs to
send return traffic, it will use the full 16-byte address from the
arriving packet and this necessarily means that traffic for this flow
coming into Site1 will use the same circuit that outgoing traffic for
that flow took. In contrast, if the source address (i.e., Routing
Stuff) is fixed by the sending host, the same return path is used for
return traffic coming back to a site, regardless of which egress
router packets traverse when leaving that site.
4.3.2. End-To-End Argument: Don't Hide RG from Hosts
Despite these significant advantages, however, the overwhelming
consensus was that address rewriting by routers should not be pursued
as part of the current standardization effort. Although hiding RG
knowledge from hosts has advantages in some scenarios, that lack of
knowledge also makes it difficult to solve important problems.
For example, a host in a multi-homed site is known by multiple
addresses, but without knowing its address the host can play no role
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in the source address selection; instead, the host relies on the
routing infrastructure to magically select the right one, i.e., by
selecting the egress router closest to the sender. For many sites,
this is the desired behavior. For others, this is not the desired
behavior. In those cases, the historically difficult-to-solve problem
of source address selection is made more difficult by moving it from
an intra-host decision to a distributed one. Now a site's internal
routers would have to have sufficient knowledge to decide which
egress router to forward traffic to, perhaps on a source-by-source
(or worse) basis.
Another end-to-end problem resulting from address rewriting has to do
with how transport connections should deal with the RG portion of the
address in incoming packets, particularly when authenticating the RG
changes. The sections on transport issues deal with the subject in
much more detail.
Interesting questions arise about address rewriting when dealing with
tunnels. Any node that acts as a tunnel for which the other end
resides in a different Site must be able to behave as a Site border
router and do address rewriting. This means that the RG may need to
be configured in more than just a Site's egress router, thus making
renumbering more problematic.
Another problem related to both performance and "architectural
cleanliness" has to do with IPv6's Routing Headers. It may be
necessary for addresses other than just the simple source and
destination to be rewritten. And again, this rewriting would need to
be done by both egress routers and nodes which terminate tunnels that
go to other sites.
4.4. Multi-Homing
Multi-Homing can mean many things. In the context of GSE, multi-
homing refers to a Site having more than one connection to the
Internet and therefore being known by multiple RGs. In many ways this
is close to multi-homing with IPv6 provider-based addressing. It is
hard to make comparisons to IPv4 because multi-homing has
traditionally been done in an ad hoc fashion.
With GSE, the ability of a Site to control the load-sharing over its
multiple links is not clear, partially because there is little
operational experience with multi-homed sites known by multiple
prefixes (with IPv4 the site is generally only known by a single
prefix). The following analysis is relevant to any scheme where an
Internet-connected site is known by multiple prefixes. For flows that
the multi-homed site initiates, load-sharing is impacted by the
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source address used because that is the address that the remote site
will use for return traffic. If we assume the model of routers
rewriting source addresses, then the outgoing link selected
determines the load-sharing because that also determines what RG is
contained in the source address. If the routers do not rewrite source
addresses, then the end-host itself will have to make the source
address selection, and the optimal choice may require knowledge of
the topology. For flows initiated by someone outside of the multi-
homed site, the load-sharing is dependent on the destination address
specified, so the DNS has a large impact on load-sharing. There is
some amount of operational experience in using DNS to control load on
servers (e.g., having a Web server resolve to multiple addresses),
though that is load-sharing of a different resource and at a
different scope and scale. It is also worth noting that the selection
of the optimal outgoing link may well depend on the destination,
which has particularly interesting results on the DNS understanding
topology (and brings up the question of whether the DNS servers or
the resolvers are responsible for knowing the topology).
One advantage that GSE has for multi-homed sites is symmetry. Because
the source address is selected based on the outgoing link, and that
source address is what determines the return path, flows initiated by
the Site will be symmetric with respect to which of the Site's links
is used.
The multi-homing mechanism described in Section 3.7 has some
weaknesses and complexities. First, the mechanism only supports
healing a failed link and not a router; in other words, referencing
Figure 7, from Section 3.7, if PBR1 were not up at all, then it could
not tunnel the packets anywhere. One could imagine ways of
distributing PBR1's knowledge of PBR2 to other routers within
Provider1 to add more reliability, though this makes the problem
distributed rather than point-to-point and therefore more difficult.
Second, in the general case, static identification of PBR2 to PBR1,
and vice-versa, is not adequate. Imagine, for example, that the link
to PBR1 is much faster than the link to PBR2. In this case, it's
possible that packets whose destination addresses contain RG1 might
normally transit PBR2 without going directly to the Site. So there
seems to be a need for a dynamic protocol between PBR1 and PBR2 to
notify when PBR2, for example, should forward RG1-prefaced
destinations directly to the Site as opposed to forwarding it towards
PBR1.
Another note about multi-homing is the potential impact of internal
topology changes in the face of address rewriting. Using the
previously referenced diagram, if a flow from a host within the Site
is leaving via SBR1, but then something happens such that SBR2
becomes the host's closest exit point, then the remote end-point of
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the flow will begin seeing different RG. Reasons such as this are why
the repercussions on the transport layer are so important (e.g.,
whether or not transport peers pay attention to the RG).
5. Results
This section summarizes the results of the GSE deliberations on the
IPv6 process.
1) Make changes to the IPv6 provider-based addressing document to
facilitate aggressive aggregation that is also operationally
realistic.
2) Create hard boundaries in IPv6 addresses to clearly distinguish
between the portions used to identify hosts, for routing within
a site, and for routing within the Public Internet.
3) Allow an option for the low-order 8 bytes of IPv6 addresses to
be designated as a globally unique End System Designator (ESD).
This change has potential benefits to future transport protocols
(e.g., TCPng).
4) Make a clear distinction between the "locator" part of an
address and the "identifier" part of the address. The former is
used to route a packet to its end-point, the latter is used to
identify an end-point, independent of the path used to deliver
the packet. Although this is a potentially revolutionary change
to IPv6 addressing model, existing transport protocols such as
TCP and UDP will not take advantage of the split. Future
transport protocols (e.g., TCPng), however, may.
5) Make changes to the way AAAA records are stored within the DNS,
so that renumbering a site (e.g., when a site changes ISPs)
requires few changes to the DNS database in order to effectively
change all of a site's address AAAA RRs.
6) Don't hide a node's full address from that node. In a scheme
where all nodes know their full address, address rewriting
should not be necessary.
7) Consider multi-homing and its effect on aggregation and route
scaling from the beginning. Have a goal of architecting a way to
do multi-homing that is both scalable and operationally
practical, and consider related issues such as load-sharing.
8) Consider the issue of subnetting. For example, how are point-
to-point links numbered? With IPv4, current practice is to
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number point-to-point links out of "/30" subnets. However, do
network masks longer than 64 bits make sense with the concept of
the low-order 8 bytes being a globally unique ESD? If not, then
is it acceptable to either leave point-to-point links un-
numbered or to use an entire subnet for each point-to-point
link? Will there need to be an exception for IPv6 host routes
(i.e., /128s) as a work-around for the bootstrapping issue of
addressing root DNS servers? If /128s are allowed, but not masks
between /65 and /127, inclusive, then a possible way to number
point-to-point links within a backbone is to dedicate a single
subnet to them and route them as /128s.
9) Search for ways to minimize the impact that renumbering has on
intra-site communication. Renumbering operations that change
only the RG portion of addresses should not impact existing
intra-site communication. One possible approach is to encourage
the use of site-local addresses for all intra-site
communication.
6. Security Considerations
The primary security consideration with GSE or, more generally, a
network layer with addresses split into locator and identifier parts,
is that of one node impersonating another by copying the
identification without the location.
7. Acknowledgments
Thanks go to Steve Deering and Bob Hinden (the Chairs of the IPng
Working Group) as well as Sun Microsystems (the host for the PAL1
meeting) for the planning and execution of the interim meeting.
Thanks also goes to Mike O'Dell for writing the 8+8 and GSE drafts.
By publishing these documents and speaking on their behalf, Mike was
the catalyst for some very valuable discussions that are expected to
result in improved IPv6 addressing. Special thanks to the attendees
of the meeting who carried on the high caliber discussions which were
the source for this document.
8. References
[BATES] Scalable support for multi-homed multi-provider
connectivity, Internet Draft, Tony Bates & Yakov Rekhter,
draft-bates-multihoming-01.txt.
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[Bellovin 89] "Security Problems in the TCP/IP Protocol Suite",
Bellovin, Steve, Computer Communications Review, Vol. 19,
No. 2, pp32-48, April 1989.
[CERT] CERT(sm) Advisory CA-96.21
(ftp://info.cert.org/pub/cert_advisories)
[DANVERS] Minutes of the IPNG working Group, April 1995.
ftp://ftp.ietf.cnri.reston.va.us/ietf-online-proceedings/
95apr/area.and.wg.reports/ipng/ipngwg/ ipngwg-minutes-
95apr.txt.
[DHCP-DDNS] Interaction between DHCP and DNS, Internet Draft, Yakov
Rekhtor, draft-ietf-dhc-dhcp-dns-04.txt.
[DDNS] "Dynamic Updates in the Domain Name System (DNS UPDATE)",
Paul Vixie (Editor), draft-ietf-dnsind-dynDNS-11.txt,
November, 1996.
[EUI64] 64-Bit Global Identifier Format Tutorial.
http://standards.ieee.org/db/oui/tutorials/EUI64.html.
Note: "EUI-64" is claimed as a trademark by an organization
which also forbids reference to itself in association with
that term in a standards document which is not their own,
unless they have approved that reference. However, since
this document is not standards-track, it seems safe to name
that organization: the IEEE.
[GSE] "GSE - An Alternate Addressing Architecture for IPv6", Mike
O'Dell, draft-ietf-ipngwg-gseaddr-00.txt.
[IEEE802] IEEE Std 802-1990, Local and Metropolitan Area Networks:
IEEE Standard Overview and Architecture.
[IEEE1212] IEEE Std 1212-1994, Information technology--
Microprocessor systems: Control and Status Registers (CSR)
Architecture for microcomputer buses.
[RFC1122] "Requirements for Internet hosts - communication layers",
R. Braden, 10/01/1989.
[RFC1715] The H Ratio for Address Assignment Efficiency. C.
Huitema.
[RFC1726] Technical Criteria for Choosing IP:The Next Generation
(IPng). F. Kastenholz, C. Partridge.
[RFC1752] "The Recommendation for the IP Next Generation Protocol,"
draft-ietf-ipngwg-esd-analysis-01.txt [Page 50]
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S. Bradner, A. Mankin, 01/18/1995.
[RFC1788] "ICMP Domain Name Messages", W. Simpson, 04/14/1995
[RFC1958] Architectural Principles of the Internet. B. Carpenter.
[RFC1971] IPv6 Stateless Address Autoconfiguration. S. Thomson, T.
Narten.
[RFC2002] "IP Mobility Support", 10/22/1996, C. Perkins.
[RFC2008] "Implications of Various Address Allocation Policies for
Internet Routing", Y. Rekhter, T. Li.
[RFC2065] Domain Name System Security Extensions. D. Eastlake, C.
Kaufman.
[RFC2073] An IPv6 Provider-Based Unicast Address Format. Y.
Rekhter, P. Lothberg, R. Hinden, S. Deering, J. Postel
9. Authors' Addresses
Matt Crawford John Stewart
Fermilab MS 368 USC/ISI
PO Box 500 4350 North Fairfax Drive
Batavia, IL 60510 USA Suite 620
Phone: 708-840-3461 Arlington, VA 22203 USA
EMail: crawdad@fnal.gov Phone: 703-807-0132
EMail: jstewart@isi.edu
Allison Mankin Lixia Zhang
USC/ISI UCLA Computer Science Department
4350 North Fairfax Drive 4531G Boelter Hall
Suite 620 Los Angeles, CA 90095-1596 USA
Arlington, VA 22203 USA Phone: 310-825-2695
EMail: mankin@isi.edu EMail: lixia@cs.ucla.edu
Phone: 703-807-0132
Thomas Narten
IBM Corporation
3039 Cornwallis Ave.
PO Box 12195 - F11/502
Research Triangle Park, NC 27709-2195
Phone: 919-254-7798
EMail: narten@raleigh.ibm.com
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