Network Working Group B. Carpenter
Request for Comments: 3234 IBM Zurich Research Laboratory
Category: Informational S. Brim
February 2002

Middleboxes: Taxonomy and Issues

Status of this Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2002). All Rights Reserved.

Abstract

This document is intended as part of an IETF discussion about
"middleboxes" - defined as any intermediary box performing functions
apart from normal, standard functions of an IP router on the data
path between a source host and destination host. This document
establishes a catalogue or taxonomy of middleboxes, cites previous
and current IETF work concerning middleboxes, and attempts to
identify some preliminary conclusions. It does not, however, claim
to be definitive.

Table of Contents

1. IntrodUCtion and Goals......................................... 3
1.1. Terminology.................................................. 3
1.2. The Hourglass Model, Past and Future......................... 3
1.4. Goals of this Document....................................... 4
2. A catalogue of middleboxes..................................... 5
2.1 NAT........................................................... 6
2.2 NAT-PT........................................................ 7
2.3 SOCKS gateway................................................. 7
2.4 IP Tunnel Endpoints........................................... 8
2.5. Packet classifiers, markers and schedulers................... 8
2.6 Transport relay............................................... 9
2.7. TCP performance enhancing proxies............................ 10
2.8. Load balancers that divert/munge packets..................... 10
2.9. IP Firewalls................................................. 11
2.10. Application Firewalls....................................... 11
2.11. Application-level gateways.................................. 12
2.12. Gatekeepers/ session control boxes.......................... 12
2.13. Transcoders................................................. 12
2.14. Proxies..................................................... 13
2.15. Caches...................................................... 14
2.16. Modified DNS servers........................................ 14
2.17. Content and applications distribution boxes................. 15
2.18. Load balancers that divert/munge URLs....................... 16
2.19. Application-level interceptors.............................. 16
2.20. Application-level multicast................................. 16
2.21. Involuntary packet redirection.............................. 16
2.22. Anonymisers................................................. 17
2.23. Not included................................................ 17
2.24. Summary of facets........................................... 17
3. Ongoing work in the IETF and elsewhere......................... 18
4. Comments and Issues............................................ 19
4.1. The end to end principle under challenge..................... 19
4.2. Failure handling............................................. 20
4.3. Failures at multiple layers.................................. 21
4.4. Multihop application protocols............................... 21
4.5. Common features.............................................. 22
5. Security Considerations........................................ 22
6. Acknowledgements............................................... 23
7. References..................................................... 23
Authors' Addresses................................................ 26
Acknowledgement................................................... 26
Full Copyright Statement.......................................... 27

1. Introduction and Goals

1.1. Terminology

The phrase "middlebox" was coined by Lixia Zhang as a graphic
description of a recent phenomenon in the Internet. A middlebox is
defined as any intermediary device performing functions other than
the normal, standard functions of an IP router on the datagram path
between a source host and destination host.

In some discussions, especially those concentrating on HTTP traffic,
the Word "intermediary" is used. For the present document, we prefer
the more graphic phrase. Of course, a middlebox can be virtual,
i.e., an embedded function of some other box. It should not be
interpreted as necessarily referring to a separate physical box. It
may be a device that terminates one IP packet flow and originates
another, or a device that transforms or diverts an IP packet flow in
some way, or a combination. In any case it is never the ultimate
end-system of an applications session.

Normal, standard IP routing functions (i.e., the route discovery and
selection functions described in [RFC1812], and their equivalent for
IPv6) are not considered to be middlebox functions; a standard IP
router is essentially transparent to IP packets. Other functions
taking place within the IP layer may be considered to be middlebox
functions, but functions below the IP layer are excluded from the
definition.

There is some discrepancy in the way the word "routing" is used in
the community. Some people use it in the narrow, traditional sense
of path selection based on IP address, i.e., the decision-making
action of an IP router. Others use it in the sense of higher layer
decision-making (based perhaps on a URL or other applications layer
string). In either case it implies a choice of outbound direction,
not the mere forwarding of a packet in the only direction available.
In this document, the traditional sense is always qualified as "IP
routing."

1.2. The Hourglass Model, Past and Future

The classical description of the Internet architecture is based
around the hourglass model [HOURG] and the end-to-end principle
[Clark88, Saltzer]. The hourglass model depicts the protocol
architecture as a narrow-necked hourglass, with all upper layers
riding over a single IP protocol, which itself rides over a variety
of hardware layers.

The end-to-end principle asserts that some functions (such as
security and reliability) can only be implemented completely and
correctly end-to-end, with the help of the end points. The end-to-
end principle notes that providing an incomplete version of such
functions in the network itself can sometimes be useful as a
performance enhancement, but not as a substitute for the end-to-end
implementation of the function. The references above, and [RFC
1958], go into more detail.

In this architecture, the only boxes in the neck of the hourglass are
IP routers, and their only function is to determine routes and
forward packets (while also updating fields necessary for the
forwarding process). This is why they are not classed as
middleboxes.

Today, we observe deviations from this model, caused by the insertion
in the network of numerous middleboxes performing functions other
than IP forwarding. Viewed in one way, these boxes are a challenge
to the transparency of the network layer [RFC2775]. Viewed another
way, they are a challenge to the hourglass model: although the IP
layer does not go away, middleboxes dilute its significance as the
single necessary feature of all communications sessions. Instead of
concentrating diversity and function at the end systems, they spread
diversity and function throughout the network.

This is a matter of concern for several reasons:

* New middleboxes challenge old protocols. Protocols designed
without consideration of middleboxes may fail, predictably or
unpredictably, in the presence of middleboxes.

* Middleboxes introduce new failure modes; rerouting of IP packets
around crashed routers is no longer the only case to consider.
The fate of sessions involving crashed middleboxes must also be
considered.

* Configuration is no longer limited to the two ends of a session;
middleboxes may also require configuration and management.

* Diagnosis of failures and misconfigurations is more complex.

1.4. Goals of this Document

The principle goal of this document is to describe and analyse the
current impact of middleboxes on the architecture of the Internet and
its applications. From this, we attempt to identify some general
conclusions.

Goals that might follow on from this work are:

* to identify harmful and harmless practices,

* to suggest architectural guidelines for application protocol and
middlebox design,

* to identify requirements and dependencies for common functions in
the middlebox environment,

* to derive a system design for standardisation of these functions,

* to identify additional work that should be done in the IETF and
IRTF.

An implied goal is to identify any necessary updates to the
Architectural Principles of the Internet [RFC1958].

The document initially establishes a catalogue of middleboxes, and
cites previous or current IETF work concerning middleboxes, before
proceeding to discussion and conclusions.

2. A catalogue of middleboxes

The core of this document is a catalogue of a number of types of
middlebox. There is no obvious way of classifying them to form a
hierarchy or other simple form of taxonomy. Middleboxes have a
number of facets that might be used to classify them in a
multidimensional taxonomy.

DISCLAIMER: These facets, many of distinctions between different
types of middlebox, and the decision to include or exclude a
particular type of device, are to some extent subjective. Not
everyone who commented on drafts of this document agrees with our
classifications and descriptions. We do not claim that the following
catalogue is mathematically complete and consistent, and in some
cases purely arbitrary choices have been made, or ambiguity remains.
Thus, this document makes no claim to be definitive.

The facets considered are:

1. Protocol layer. Does the box act at the IP layer, the transport
layer, the upper layers, or a mixture?

2. EXPlicit versus implicit. Is the middlebox function an explicit
design feature of the protocol(s) in use, like an SMTP relay? Or
is it an add-on not foreseen by the protocol design, probably
attempting to be invisible, like a network address translator?

3. Single hop versus multi-hop. Can there be only one box in the
path, or can there be several?

4. In-line versus call-out. The middlebox function may be executed
in-line on the datapath, or it may involve a call-out to an
ancillary box.

5. Functional versus optimising. Does the box perform a function
without which the application session cannot run, or is the
function only an optimisation?

6. Routing versus processing. Does the box simply choose which way
to send the packets of a session, or does it actually process them
in some way (i.e., change them or create a side-effect)?

7. Soft state versus hard state. If the box loses its state
information, does the session continue to run in a degraded mode
while reconstructing necessary state (soft state), or does it
simply fail (hard state)?

8. Failover versus restart. In the event that a hard state box
fails, is the session redirected to an alternative box that has a
copy of the state information, or is it forced to abort and
restart?

One possible classification is deliberately excluded: "good" versus
"evil". While analysis shows that some types of middlebox come with
a host of complications and disadvantages, no useful purpose would be
served by simply deprecating them. They have been invented for
compelling reasons, and it is instructive to understand those
reasons.

The types of box listed below are in an arbitrary order, although
adjacent entries may have some affinity. At the end of each entry is
an attempt to characterise it in terms of the facets identified
above. These characterisations should not be interpreted as rigid;
in many cases they are a gross simplification.

Note: many types of middlebox may need to perform IP packet
fragmentation and re-assembly. This is mentioned only in certain
cases.

2.1 NAT

Network Address Translator. A function, often built into a router,
that dynamically assigns a globally unique address to a host that
doesn't have one, without that host's knowledge. As a result, the
appropriate address field in all packets to and from that host is

translated on the fly. Because NAT is incompatible with application
protocols with IP address dependencies, a NAT is in practice always
accompanied by an ALG (Application Level Gateway - see below). It
also touches the transport layer to the extent of fixing up
checksums.

NATs have been extensively analysed in the IETF [RFC2663, RFC2993,
RFC3022, RFC3027, etc.]

The experimental RSIP proposal complements NAT with a dynamic tunnel
mechanism inserting a stateful RSIP server in place of the NAT
[RSIP].

{1 IP layer, 2 implicit, 3 multihop, 4 in-line, 5 functional, 6
processing, 7 hard, 8 restart}

2.2 NAT-PT

NAT with Protocol Translator. A function, normally built into a
router, that performs NAT between an IPv6 host and an IPv4 network,
additionally translating the entire IP header between IPv6 and IPv4
formats.

NAT-PT itself depends on the Stateless IP/ICMP Translation Algorithm
(SIIT) mechanism [RFC2765] for its protocol translation function.
In practice, SIIT and NAT-PT will both need an associated ALG and
will need to touch transport checksums. Due to the permitted absence
of a UDP checksum in IPv4, translation of fragmented unchecksummed
UDP from IPv4 to IPv6 is hopeless. NAT-PT and SIIT also have other
potential fragmentation/MTU problems, particularly when dealing with
endpoints that don't do path MTU discovery (or when transiting other
middleboxes that break path MTU discovery). ICMP translation also
has some intractable difficulties.

NAT-PT is a Proposed Standard from the NGTRANS WG [RFC2766]. The
Dual Stack Transition Mechanism adds a second related middlebox, the
DSTM server [DSTM].

{1 IP layer, 2 implicit, 3 multihop, 4 in-line, 5 functional, 6
processing, 7 hard, 8 restart}

2.3 SOCKS gateway

SOCKSv5 [RFC1928] is a stateful mechanism for authenticated firewall
traversal, in which the client host must communicate first with the
SOCKS server in the firewall before it is able to traverse the
firewall. It is the SOCKS server, not the client, that determines
the source IP address and port number used outside the firewall. The

client's stack must be "SOCKSified" to take account of this, and
address-sensitive applications may get confused, rather as with NAT.
However, SOCKS gateways do not require ALGs.

SOCKS is maintained by the AFT (Authenticated Firewall Traversal) WG.

{1 multi-layer, 2 explicit, 3 multihop, 4 in-line, 5 functional, 6
routing, 7 hard, 8 restart}

2.4 IP Tunnel Endpoints

Tunnel endpoints, including virtual private network endpoints, use
basic IP services to set up tunnels with their peer tunnel endpoints
which might be anywhere in the Internet. Tunnels create entirely new
"virtual" networks and network interfaces based on the Internet
infrastructure, and thereby open up a number of new services. Tunnel
endpoints base their forwarding decisions at least partly on their
own policies, and only partly if at all on information visible to
surrounding routers.

To the extent that they deliver packets intact to their destinations,
tunnel endpoints appear to follow the end-to-end principle in the
outer Internet. However, the destination may be completely different
from what a router near the tunnel entrance might expect. Also, the
per-hop treatment a tunneled packet receives, for example in terms of
QoS, may not be what it would have received had the packet traveled
untunneled [RFC2983].

Tunnels also cause difficulties with MTU size (they reduce it) and
with ICMP replies (they may lack necessary diagnostic information).

When a tunnel fails for some reason, this may cause the user session
to abort, or an alternative IP route may prove to be available, or in
some cases the tunnel may be re-established automatically.

{1 multi-layer, 2 implicit, 3 multihop, 4 in-line, 5 functional, 6
processing, 7 hard, 8 restart or failover}

2.5. Packet classifiers, markers and schedulers

Packet classifiers classify packets flowing through them according to
policy and either select them for special treatment or mark them, in
particular for differentiated services [Clark95, RFC2475]. They may
alter the sequence of packet flow through subsequent hops, since they
control the behaviour of traffic conditioners.

Schedulers or traffic conditioners (in routers, hosts, or specialist
boxes inserted in the data path) may alter the time sequence of
packet flow, the order in which packets are sent, and which packets
are dropped. This can significantly impact end-to-end performance.
It does not, however, fundamentally change the unreliable datagram
model of the Internet.

When a classifier or traffic conditioner fails, the user session may
see any result between complete loss of connectivity (all packets are
dropped), through best-effort service (all packets are given default
QOS), up to automatic restoration of the original service level.

{1 multi-layer, 2 implicit, 3 multihop, 4 in-line, 5 optimising, 6
processing, 7 soft, 8 failover or restart}

2.6 Transport relay

Transport relays are basically the transport layer equivalent of an
ALG; another (less common) name for them is a TLG. As with ALGs,
they're used for a variety of purposes, some well established and
meeting needs not otherwise met. Early examples of transport relays
were those that ran on MIT's ITS and TOPS-20 PDP-10s on the ARPANET
and allowed Chaosnet-only hosts to make outgoing connections from
Chaosnet onto TCP/IP. Later there were some uses of TCP-TP4 relays.
A transport relay between IPv6-only and IPv4-only hosts is one of the
tools of IPv6 transition [TRANS64]. TLGs are sometimes used in
combination with simple packet filtering firewalls to enforce
restrictions on which hosts can talk to the outside world or to
kludge around strange IP routing configurations. TLGs are also
sometimes used to gateway between two instances of the same transport
protocol with significantly different connection characteristics; it
is in this sense that a TLG may also be called a TCP or transport
spoofer. In this role, the TLG may shade into being an optimising
rather than a functional middlebox, but it is distinguished from
Transport Proxies (next section) by the fact that it makes its
optimisations only by creating back-to- back connections, and not by
modification or re-timing of TCP messages.

Terminating one TCP connection and starting another mid-path means
that the TCP checksum does not cover the sender's data end-to-end.
Data corruptions or modifications may be introduced in the processing
when the data is transferred from the first to the second connection.
Some TCP relays are split relays and have even more possibility of
lost data integrity, because the there may be more than two TCP
connections, and multiple nodes and network paths involved. In all
cases, the sender has less than the expected assurance of data
integrity that is the TCP reliable byte stream service. Note that

this problem is not unique to middleboxes, but can also be caused by
checksum offloading TCP implementations within the sender, for
example.

In some such cases, other session layer mechanisms such as SSH or
HTTPS would detect any loss of data integrity at the TCP level,
leading not to retransmission as with TCP, but to session failure.
However, there is no general session mechanism to add application
data integrity so one can detect or mitigate possible lack of TCP
data integrity.

{1 Transport layer, 2 implicit, 3 multihop, 4 in-line, 5 functional
(mainly), 6 routing, 7 hard, 8 restart}

2.7. TCP performance enhancing proxies

"TCP spoofer" is often used as a term for middleboxes that modify the
timing or action of the TCP protocol in flight for the purposes of
enhancing performance. Another, more accurate name is TCP
performance enhancing proxy (PEP). Many TCP PEPs are proprietary and
have been characterised in the open Internet primarily when they
introduce interoperability errors with standard TCP. As with TLGs,
there are circumstances in which a TCP PEP is seen to meet needs not
otherwise met. For example, a TCP PEP may provide re-spacing of ACKs
that have been bunched together by a link with bursty service, thus
avoiding undesireable data segment bursts. The PILC (Performance
Implications of Link Characteristics) working group has analyzed
types of TCP PEPs and their applicability [PILCPEP]. TCP PEPs can
introduce not only TCP errors, but also unintended changes in TCP
adaptive behavior.

{1 Transport layer, 2 implicit, 3 multihop, 4 in-line, 5 optimising,
6 routing, 7 hard, 8 restart}

2.8. Load balancers that divert/munge packets.

There is a variety of techniques that divert packets from their
intended IP destination, or make that destination ambiguous. The
motivation is typically to balance load across servers, or even to
split applications across servers by IP routing based on the
destination port number. Except for rare instances of one-shot UDP
protocols, these techniques are inevitably stateful as all packets
from the same application session need to be directed to the same
physical server. (However, a sophisticated solution would also be
able to handle failover.)

To date these techniques are proprietary and can therefore only be
applied in closely managed environments.

{1 multi-layer, 2 implicit, 3 single hop, 4 in-line, 5 optimising, 6
routing, 7 hard, 8 restart}

2.9. IP Firewalls

The simplest form of firewall is a router that screens and rejects
packets based purely on fields in the IP and Transport headers (e.g.,
disallow incoming traffic to certain port numbers, disallow any
traffic to certain subnets, etc.)

Although firewalls have not been the subject of standardisation, some
analysis has been done [RFC2979].

Although a pure IP firewall does not alter the packets flowing
through it, by rejecting some of them it may cause connectivity
problems that are very hard for a user to understand and diagnose.

"Stateless" firewalls typically allow all IP fragments through since
they do not contain enough upper-layer header information to make a
filtering decision. Many "stateful" firewalls therefore reassemble
IP fragments (and re-fragment if necessary) in order to avoid leaking
fragments, particularly fragments that may exploit bugs in the
reassembly implementations of end receivers.

{1 IP layer, 2 implicit, 3 multihop, 4 in-line, 5 functional, 6
routing, 7 hard, 8 restart}

2.10. Application Firewalls

Application-level firewalls act as a protocol end point and relay
(e.g., an SMTP client/server or a Web proxy agent). They may

(1) implement a "safe" subset of the protocol,

(2) perform extensive protocol validity checks,

(3) use an implementation methodology designed to minimize the
likelihood of bugs,

(4) run in an insulated, "safe" environment, or

(5) use some combination of these techniques in tandem.

Although firewalls have not been the subject of standardisation, some
analysis has been done [RFC2979]. The issue of firewall traversal
using HTTP has been discussed [HTTPSUB].

{1 Application layer, 2 implicit, 3 multihop, 4 in-line, 5
functional, 6 processing, 7 hard, 8 restart}

2.11. Application-level gateways

These come in many shapes and forms. NATs require ALGs for certain
address-dependent protocols such as FTP; these do not change the
semantics of the application protocol, but carry out mechanical
substitution of fields. At the other end of the scale, still using
FTP as an example, gateways have been constructed between FTP and
other file transfer protocols such as the OSI and DECnet (R)
equivalents. In any case, such gateways need to maintain state for
the sessions they are handling, and if this state is lost, the
session will normally break irrevocably.

Some ALGs are also implemented in ways that create fragmentation
problems, although in this case the problem is arguably the result of
a deliberate layer violation (e.g., mucking with the application data
stream of an FTP control connection by twiddling TCP segments on the
fly).

{1 Application layer, 2 implicit or explicit, 3 multihop, 4 in-line,
5 functional, 6 processing, 7 hard, 8 restart}

2.12. Gatekeepers/ session control boxes

Particularly with the rise of IP Telephony, the need to create and
manage sessions other than TCP connections has arisen. In a
multimedia environment that has to deal with name lookup,
authentication, authorization, accounting, firewall traversal, and
sometimes media conversion, the establishment and control of a
session by a third-party box seems to be the inevitable solution.
Examples include H.323 gatekeepers [H323], SIP servers [RFC2543] and
MEGACO controllers [RFC3015].

{1 Application layer, 2 explicit, 3 multihop, 4 in-line or call-out,
5 functional, 6 processing, 7 hard, 8 restart?}

2.13. Transcoders

Transcoders are boxes performing some type of on-the-fly conversion
of application level data. Examples include the transcoding of
existing web pages for display on hand-held wireless devices, and
transcoding between various audio formats for interconnecting digital
mobile phones with voice-over-IP services. In many cases, such
transcoding cannot be done by the end-systems, and at least in the
case of voice, it must be done in strict real time with extremely
rapid failure recovery.

Not all media translators are mandatory. They may simply be an
optimisation. For example, in the case of multicast, if all the
low-bandwidth receivers sit in one "corner" of the network, it would
be inefficient for the sender to generate two streams or send both
stream all the way across the network if the "thin" one is only
needed far away from the sender. Generally, media translators are
only useful if the two end systems don't have overlapping codecs or
if the overlapping set is not a good network match.

{1 Application layer, 2 explicit or implicit, 3 single hop, 4 in-
line, 5 functional, 6 processing, 7 hard?, 8 restart or failover}

2.14. Proxies

HTTP1.1 [RFC2616] defines a Web proxy as follows:

"An intermediary program which acts as both a server and a client
for the purpose of making requests on behalf of other clients.
Requests are serviced internally or by passing them on, with
possible translation, to other servers. A proxy MUST implement
both the client and server requirements of this specification. A
"transparent proxy" is a proxy that does not modify the request or
response beyond what is required for proxy authentication and
identification. A "non-transparent proxy" is a proxy that
modifies the request or response in order to provide some added
service to the user agent, such as group annotation services,
media type transformation, protocol reduction, or anonymity
filtering."

A Web proxy may be associated with a firewall, when the firewall does
not allow outgoing HTTP packets. However, HTTP makes the use of a
proxy "voluntary": the client must be configured to use the proxy.

Note that HTTP proxies do in fact terminate an IP packet flow and
recreate another one, but they fall under the definition of
"middlebox" given in Section 1.1 because the actual applications
sessions traverse them.

SIP proxies [RFC2543] also raise some interesting issues, since they
can "bend" the media pipe to also serve as media translators. (A
proxy can modify the session description so that media no longer
travel end-to-end but to a designated intermediate box.)

{1 Application layer, 2 explicit (HTTP) or implicit (interception), 3
multihop, 4 in-line, 5 functional, 6 processing, 7 soft, 8 restart}.

Note: Some so-called Web proxies have been implemented as
"interception" devices that intercept HTTP packets and re-issue them
with their own source address; like NAT and SOCKs, this can disturb
address-sensitive applications. Unfortunately some vendors have
caused confusion by mis-describing these as "transparent" proxies.
Interception devices are anything but transparent. See [WREC] for a
full discussion.

2.15. Caches

Caches are of course used in many shapes and forms in the Internet,
and are in principle distinct from proxies. Here we refer mainly to
content caches, intended to optimise user response times. HTTP makes
provision for proxies to act as caches, by providing for both
expiration and re-validation mechanisms for cached content. These
mechanisms may be used to guarantee that specific content is not
cached, which is a requirement for transient content, particularly in
transactional applications. HTTP caching is well described in
Section 13 of [RFC2616], and in the HTTP case caches and proxies are
inextricably mixed.

To improve optimisation, caching is not uniquely conducted between
the origin server and the proxy cache directly serving the user. If
there is a network of caches, the nearest copy of the required
content may be in a peer cache. For this an inter-cache protocol is
required. At present the most widely deployed solution is Internet
Cache Protocol (ICP) [RFC2186] although there have been alternative
proposals such as [RFC2756].

It can be argued that caches terminate the applications sessions, and
should not be counted as middleboxes (any more than we count SMTP
relays). However, we have arbitrarily chosen to include them since
they do in practice re-issue the client's HTTP request in the case of
a cache miss, and they are not the ultimate source of the application
data.

{1 Application layer, 2 explicit (if HTTP proxy caches), 3 multihop,
4 in-line, 5 functional, 6 processing, 7 soft, 8 restart}

2.16. Modified DNS servers

DNS servers can play games. As long as they appear to deliver a
syntactically correct response to every query, they can fiddle the
semantics. For example, names can be made into "anycast" names by
arranging for them to resolve to different IP addresses in different
parts of the network. Or load can be shared among different members
of a server farm by having the local DNS server return the address of

different servers in turn. In a NAT environment, it is not uncommon
for the FQDN-to-address mapping to be quite different outside and
inside the NAT ("two-faced DNS").

Modified DNS servers are not intermediaries in the application data
flow of interest. They are included here because they mean that
independent sessions that at one level appear to involve a single
host actually involve multiple hosts, which can have suBTle effects.
State created in host A.FOR.EXAMPLE by one session may turn out not
to be there when a second session apparently to the same host is
started, because the DNS server has directed the second session
elsewhere.

If such a DNS server fails, users may fail over to an alternate DNS
server that doesn't know the same tricks, with unpredicatble results.

{1 Application layer, 2 implicit, 3 multihop, 4 in-line (on DNS query
path), 5 functional or optimising, 6 processing, 7 soft, 8 failover}

2.17. Content and applications distribution boxes

An emerging generalisation of caching is content distribution and
application distribution. In this model, content (such as static web
content or streaming multimedia content) is replicated in advance to
many widely distributed servers. Further, interactive or even
transactional applications may be remotely replicated, with some of
their associated data. Since this is a recent model, it cannot be
said that there is an industry standard practice in this area. Some
of the issues are discussed in [WREC] and several new IETF activities
have been proposed in this area.

Content distribution solutions tend to play with URLs in one way or
another, and often involve a system of middleboxes - for example
using HTTP redirects to send a request for WWW.EXAMPLE.COM off to
WWW.EXAMPLE.NET, where the latter name may be an "anycast" name as
mentioned above, and will actually resolve in DNS to the nearest
instance of a content distribution box.

As with caches, it is an arbitrary choice to include these devices,
on the grounds that although they terminate the client session, they
are not the ultimate origin of the applications data.

{1 Application layer, 2 implicit or explicit, 3 multihop, 4 in-line
or call-out, 5 optimising, 6 routing or processing, 7 soft, 8
restart?}

2.18. Load balancers that divert/munge URLs

Like DNS tricks, URL redirects can be used to balance load among a
pool of servers - essentially a local version of a content
distribution network. Alternatively, an HTTP proxy can rewrite HTTP
requests to direct them to a particular member of a pool of servers.

These devices are included as middleboxes because they divert an
applications session in an arbitrary way.

{1 Application layer, 2 explicit, 3 single hop, 4 in-line, 5
functional, 6 routing, 7 soft, 8 restart}

2.19. Application-level interceptors

Some forms of pseudo-proxy intercept HTTP packets and deliver them to
a local proxy server instead of forwarding them to the intended
destination. Thus the destination IP address in the packet is
ignored. It is hard to state whether this is a functional box (i.e.,
a non-standard proxy) or an optimising box (i.e., a way of forcing
the user to use a cache). Like any non-standard proxy, it has
undefined consequences in the case of dynamic or non-cacheable
content.

{1 Application layer, 2 implicit, 3 single hop, 4 in-line, 5
functional or optimising, 6 routing, 7 hard, 8 restart}

2.20. Application-level multicast

Some (mainly proprietary) applications, including some approaches to
instant messaging, use an application-level mechanism to replicate
packets to multiple destinations.

An example is given in [CHU].

{1 Application layer, 2 explicit, 3 multihop, 4 in-line, 5
functional, 6 routing, 7 hard, 8 restart}

2.21. Involuntary packet redirection

There appear to be a few instances of boxes that (based on
application level content or other information above the network
layer) redirect packets for functional reasons. For example, more
than one "high speed Internet" service offered in hotel rooms
intercepts initial HTTP requests and diverts them to an HTTP server
that demands payment before opening Access to the Internet. These
boxes usually also perform NAT functions.

{1 multi-layer, 2 implicit, 3 single hop, 4 call-out, 5 functional, 6
routing, 7 hard, 8 restart}

2.22. Anonymisers

Anonymiser boxes can be implemented in various ways that hide the IP
address of the data sender or receiver. Although the implementation
may be distinct, this is in practice very similar to a NAT plus ALG.

{1 multi-layer, 2 implicit or explicit, 3 multihop, 4 in-line, 5
functional, 6 processing, 7 hard, 8 restart}

2.23. Not included

Some candidates suggested for the above list were excluded for the
reasons given below. In general, they do not fundamentally change
the architectural model of packet delivery from source to
destination.

Bridges and switches that snoop ARP, IGMP etc. These are below the
IP layer, but use a layer violation to emulate network layer
functions. They do not change IP layer functions.

Wiretaps and snoopers in general - if they are working correctly,
they have no impact on traffic, so do not require analysis.

Mobile IP home agents are intended to assist packet delivery to the
originally desired destination, so they are excluded on the same
grounds as standard routers.

Relays in interplanetary networks - although these would certainly
appear to be middleboxes, they are not currently deployed.

2.24. Summary of facets

By tabulating the rough classifications above, we observe that of the
22 classes of middlebox described:

17 are application or multi-layer
16 are implicit (and others are explicit OR implicit)
17 are multi-hop
21 are in-line; call-out is rare
18 are functional; pure optimisation is rare
Routing & processing are evenly split
16 have hard state
21 must restart session on failure

We can deduce that current types of middlebox are predominantly
application layer devices not designed as part of the relevant
protocol, performing required functions, maintaining hard state, and
aborting user sessions when they crash. Indeed this represents a
profound challenge to the end-to-end hourglass model.

3. Ongoing work in the IETF and elsewhere

Apart from work cited in references above, current or planned work in
the IETF includes:

MIDCOM - a working group with focus on the architectural framework
and the requirements for the protocol between a requesting device and
a middlebox and the architectural framework for the interface between
a middlebox and a policy entity [MIDFRAME, MIDARCH]. This may
interact with session control issues [SIPFIRE].

Work is also proceeding outside the MIDCOM group on middlebox
discovery [MIDDISC].

WEBI (Web Intermediaries) - a working group that addresses specific
issues in the world wide web infrastructure (as identified by the
WREC working group), by providing generic mechanisms which are useful
in several application domains (e.g., proxies, content delivery
surrogates). Specific mechanisms will be Intermediary Discovery and
Description and a Resource Update Protocol.

Intermediaries are also an important focus in the development of XML
Protocol by the World-Wide Web Consortium, who have published an
interesting analysis [XMLPI].

OPES (Open Pluggable Extension Services) - a proposed working group
whose output will enable construction of services executed on
application data by participating transit intermediaries. Caching is
the most basic intermediary service, one that requires a basic
understanding of application semantics by the cache server.

CDI (Content Distribution Internetworking) is a potential working
group for allowing cooperation between different Content Distribution
Networks and cache clusters [CDNP].

RSERPOOL (Reliable Server Pooling) is a working group that will
define architecture and requirements for management and access to
server pools, including requirements from a variety of applications,
building blocks and interfaces, different styles of pooling, security
requirements and performance requirements, such as failover times and
coping with heterogeneous latencies.

4. Comments and Issues

A review of the list in Section 2 suggests that middleboxes fit into
one or more of three broad categories:

1) mechanisms to connect dissimilar networks to enable cross-protocol
interoperability;

2) mechanisms to separate similar networks into zones, especially
security zones;

3) performance enhancement.

As observed in [RFC2775], the rise of middleboxes puts into question
the general applicability of the end-to-end principle [RFC1958].
Middleboxes introduce dependencies and hidden points of failure that
violate the fate-sharing ASPect of the end-to-end principle. Can we
define architectural principles that guarantee robustness in the
presence of middleboxes?

4.1. The end to end principle under challenge

Many forms of middlebox are explicitly addressed at the IP level, and
terminate a transport connection (or act as a final destination for
UDP packets) in a normal way. Although they are potential single
points of failure, they do not otherwise interfere with the end to
end principle [RFC1958]. (This statement does not apply to
transport relays or TCP spoofers; they do not terminate a transport
connection at the expected destination in the normal way.)

However, there is a general feeling that middleboxes that divert an
IP packet from its intended destination, or substantively modify its
content on the fly, are fundamentally different from those that
correctly terminate a transport connection and carry out their
manipulations at applications level. Such diversion or modification
violates the basic architectural assumption that packets flow from
source to destination essentially unchanged (except for time-to-live
and QOS-related fields). The effects of such changes on transport
and applications is unpredictable in the general case. Much of the
analysis that applies to NAT [RFC2993, RFC3027] will also apply to
RSIP, NAT-PT, DSTM, SOCKS, and involuntary packet reDirectors.
Interception proxies, anonymisers, and some types of load balancer
can also have subtle effects on address-sensitive applications, when
they cause packets to be delivered to or from a different address.
Transport relays and TCP spoofers may deceive applications by
delivering an unreliable service on a TCP socket.

We conclude that:

Although the rise of middleboxes has negative impact on the end to
end principle at the packet level, it does not nullify it as a
useful or desirable principle of applications protocol design.
However, future application protocols should be designed in
recognition of the likely presence of network address translation,
packet diversion, and packet level firewalls, along the data path.

4.2. Failure handling

If a middlebox fails, it is desirable that the effect on sessions
currently in progress should be inconvenient rather than
catastrophic. There appear to be three approaches to achieve this:

Soft state mechanisms. The session continues in the absence of
the box, probably with reduced performance, until the necessary
session state is recreated automatically in an alternative box (or
the original one, restarted). In other words the state
information optimises the user session but is not essential. An
example might be a true caching mechanism, whose temporary failure
only reduces performance.

Rapid failover mechanisms. The session is promptly redirected to
a hot spare box, which already has a copy of the necessary session
state.

Rapid restart mechanisms. The two ends of the session promptly
detect the failure and themselves restart the session via a spare
box, without being externally redirected. Enough session state is
kept at each end to recover from the glitch.

It appears likely that "optimising" middleboxes are suitable
candidates for the soft state approach and for non-real-time data
streams, since the consequence of failure of the box is not
catastrophic for the user. (Configured HTTP proxies used as caches
are an awkward case, as their failure causes client failure.) On the
other hand, "functional" middleboxes must be present for the session
to continue, so they are candidates for rapid failover or rapid
restart mechanisms. We conclude that:

Middlebox design should include a clear mechanism for dealing with
failure.

4.3. Failures at multiple layers

Difficulties occur when middlebox functions occur at different
layers, for example the following situation, where B and C are not in
the same physical box:

Apps layer: A ------------------------> C ------> D

Lower layer: A -----> B -------------------------> D

When all is well, i.e., there is an IP path from A to B to C to D and
both B and C are working, this may appear quite workable. But the
failure modes are very challenging. For example, if there is a
network failure between C and D, how is B instructed to divert the
session to a backup box for C?. Since C and B function at different
protocol layers, there is no expectation that they will have
coordinated failure recovery mechanisms. Unless this is remedied in
some general way, we conclude that

Middlebox failure recovery mechanisms cannot currently assume they
will get any help from other layers, and must have their own means
of dealing with failures in other layers.

In the long term future, we should be able to state clearly for
each middlebox function what it expects from its environment, and
make recommendations about which middlebox functions should be
bound together if deployed.

4.4. Multihop application protocols

We can also observe that protocols such as SMTP, UUCP, and NNTP have
always worked hop-by-hop, i.e., via multiple middleboxes. Nobody
considers this to be an issue or a problem. Difficulties arise when
inserting a middlebox in an application protocol stream that was not
designed for it. We conclude that:

New application protocol designs should include explicit
mechanisms for the insertion of middleboxes, and should consider
the facets identified in Section 2 above as part of the design.

A specific challenge is how to make interactive or real-time
applications ride smoothly over middleboxes. This will put
particular stress on the failure handling aspects.

4.5. Common features

Given that the IP layer - the neck of the hourglass - is no longer
alone in its role supporting end-to-end connectivity, it would be
desirable to define requirements and features that are common to
middlebox intermediaries. It would then be possible to implement
middleboxes, and in particular the protocols that communicate with
them, fully from the stance of supporting the end-to-end principle.
Conceptually, this would extend the neck of the hourglass upwards to
include a set of common features available to all (or many)
applications. In the context of middleboxes and multihop protocols,
this would require common features addressing at least:

Middlebox discovery and monitoring
Middlebox configuration and control
Call-out
Routing preferences
Failover and restart handling
Security, including mutual authentication

As far as possible, the solutions in these areas being developed in
the IETF and W3C should be sufficiently general to cover all types of
middlebox; if not, the work will be done several times.

5. Security Considerations

Security risks are specific to each type of middlebox, so little can
be said in general. Of course, adding extra boxes in the
communication path creates extra points of attack, reduces or
eliminates the ability to perform end to end encryption, and
complicates trust models and key distribution models. Thus, every
middlebox design requires particular attention to security analysis.
A few general points can be made:

1. The interference with end-to-end packet transmission by many types
of middlebox is a crippling impediment to generalised use of IPSEC
in its present form, and also invalidates transport layer security
in many scenarios.

2. Middleboxes require us to move definitively from a two-way to an
N-way approach to trust relationships and key sharing.

3. The management and configuration mechanisms of middleboxes are a
tempting point of attack, and must be strongly defended.

These points suggest that we need a whole new approach to security
solutions as the middlebox paradigm ends up being deployed in lots of
different technologies, if only to avoid each new technology

designing a end-to-end security solution appropriate to its
particular impact on the data stream.

Additionally, content caches and content distribution mechanisms
raise the issue of access control for content that is subject to
copyright or other rights. Distributed authentication, authorisation
and accounting are required.

6. Acknowledgements

Steve Bellovin, Jon Crowcroft, Steve Deering, Patrik Faltstrom,
Henning Schulzrinne, and Lixia Zhang all gave valuable feedback on
early versions of this document. Rob Austein and Allison Mankin
drafted the text on transport relays and TCP spoofers, and Rob
Austein made other substantial contributions. Participants in the
MIDTAX BOF at the 50th IETF and on the MIDTAX mailing list, including
Harald Alverstrand, Stanislav Shalunov, Michael Smirnov, Jeff Parker,
Sandy Murphy, David Martin, Phil Neumiller, Eric Travis, Ed Bowen,
Sally Floyd, Ian Cooper, Mike Fisk and Eric Fleischman gave
invaluable input. Mark Nottingham brought the W3C work to our
attention. Melinda Shore suggested using a facet-based
categorization. Patrik Faltstrom inspired section 4.3.

7. References

[RFC1812] Baker, F., "Requirements for IP Version 4 Routers", RFC
1812, June 1995.

[RFC1928] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D. and
L. Jones, "SOCKS Protocol Version 5", March 1996.

[RFC1958] Carpenter, B., "Architectural Principles of the Internet",
RFC1958, June 1996.

[RFC2186] Wessels, D. and K. Claffy, "Internet Cache Protocol (ICP),
version 2", RFC2186, September 1997.

[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.
and W. Weiss, "An Architecture for Differentiated
Service", RFC2475, December 1998.

[RFC2543] Handley, M., Schulzrinne, H., Schooler, E. and J.
Rosenberg, "SIP: Session Initiation Protocol", RFC2543,
March 1999.

[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P. and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC2616, June 1999.

[RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations", RFC
2663, August 1999.

[RFC2756] Vixie, P. and D. Wessels, "Hyper Text Caching Protocol
(HTCP/0.0)", RFC2756, January 2000.

[RFC2766] Tsirtsis, G. and P. Srisuresh, "Network Address
Translation - Protocol Translation (NAT-PT)", RFC2766,
February 2000.

[RFC2775] Carpenter, B., "Internet Transparency", RFC2775, February
2000.

[RFC2979] Freed, N., "Behavior of and Requirements for Internet
Firewalls", RFC2979, October 2000.

[RFC2983] Black, D., "Differentiated Services and Tunnels", RFC
2983, October 2000.

[RFC2993] Hain, T., "Architectural Implications of NAT", RFC2993,
November 2000.

[RFC3015] Cuervo, F., Greene, N., Rayhan, A., Huitema, C., Rosen, B.
and J. Segers, "Megaco Protocol 1.0", RFC3015, November
2000.

[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC3022, January
2001.

[RFC3027] Holdrege, M. and P. Srisuresh, "Protocol Complications
with the IP Network Address Translator", RFC3027, January
2001.

[CHU] Y. Chu, S. Rao, and H. Zhang, A Case for End System
Multicast, SIGMETRICS, June 2000.
http://citeseer.nj.nec.com/chu00case.Html

[CLARK88] The Design Philosophy of the DARPA Internet Protocols,
D.D.Clark, Proc SIGCOMM 88, ACM CCR Vol 18, Number 4,
August 1988, pages 106-114 (reprinted in ACM CCR Vol 25,
Number 1, January 1995, pages 102-111).

[CLARK95] "Adding Service Discrimination to the Internet", D.D.
Clark, Proceedings of the 23rd Annual Telecommunications
Policy Research Conference (TPRC), Solomons, MD, October
1995.

[CDNP] M. Day, et al., "A Model for Content Internetworking
(CDI)", Work in Progress.

[DSTM] J. Bound, L. Toutain, F. Dupont, O. Medina, H. Afifi, A.
Durand, "Dual Stack Transition Mechanism (DSTM)", Work in
Progress.

[H323] ITU-T Recommendation H.323: "Packet Based Multimedia
Communication Systems".

[HOURG] "Realizing the Information Future: The Internet and
Beyond", Computer Science and Telecommunications Board,
National Research Council, Washington, D.C., National
Academy Press, 1994. However, the "hourglass" metaphor was
first used by John Aschenbrenner in 1979, with reference
to the ISO Open Systems Interconnection model.

[HTTPSUB] Moore, K., "On the use of HTTP as a Substrate", BCP 56,
RFC3205, February 2002.

[MIDARCH] E. Lear, "A Middlebox Architectural Framework", Work in
Progress.

[MIDDISC] E. Lear, "Requirements for Discovering Middleboxes", Work
in Progress.

[MIDFRAME] P. Srisuresh, J. Kuthan, J. Rosenberg, A. Molitor, A.
Rayhan, "Middlebox Communication: Framework and
Requirements", Work in Progress.

[PILCPEP] Border, J., Kojo, M., Griner, J., Montenegro, G. and Z.
Shelby, "Performance Enhancing Proxies Intended to
Mitigate Link-Related Degradations", RFC3135, June 2001.

[RSIP] Borella, M., Lo, J., Grabelsky, D. and G. Montenegro,
"Realm Specific IP: Framework", RFC3102, October 2001.

[SALTZER] End-To-End Arguments in System Design, J.H. Saltzer,
D.P.Reed, D.D.Clark, ACM TOCS, Vol 2, Number 4, November
1984, pp 277-288.

[SIPFIRE] S. Moyer, D. Marples, S. Tsang, J. Katz, P. Gurung, T.
Cheng, A. Dutta, H. Schulzrinne, A. Roychowdhury,
"Framework Draft for Networked Appliances Using the
Session Initiation Protocol", Work in Progress.

[SOCKS6] Kitamura, H., "A SOCKS-based IPv6/IPv4 Gateway Mechanism",
RFC3089, April 2001.

[TRANS64] "Overview of Transition Techniques for IPv6-only to Talk
to IPv4-only Communication", Work in Progress.

[WREC] Cooper, I, Melve, I. and G. Tomlinson, "Internet Web
Replication and Caching Taxonomy", RFC3040, January 2001.

[XMLPI] Intermediaries and XML Protocol, Mark Nottingham, Work in
Progress at http://lists.w3.org/Archives/Public/xml-dist-
app/2001Mar/0045.html

Authors' Addresses

Brian E. Carpenter
IBM Zurich Research Laboratory
Saeumerstrasse 4 / Postfach
8803 Rueschlikon
Switzerland

EMail: brian@hursley.ibm.com

Scott W. Brim
146 Honness Lane
Ithaca, NY 14850
USA

EMail: sbrim@cisco.com

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