RFC3426 - General Architectural and Policy Considerations

Network Working Group S. Floyd, Editor

Request for Comments: 3426 Internet Architecture Board

Category: Informational November 2002

General Architectural and Policy Considerations

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.

Abstract

This document suggests general architectural and policy questions

that the IETF community has to address when working on new standards

and protocols. We note that this document contains questions to be

addressed, as opposed to guidelines or architectural principles to be

followed.

1. IntrodUCtion

This document suggests general architectural and policy questions to

be addressed in our work in the IETF. This document contains

questions to be addressed, as opposed to guidelines or architectural

principles to be followed. These questions are somewhat similar to

the "Security Considerations" currently required in IETF documents

[RFC2316].

This document is motivated in part by concerns about a growing lack

of coherence in the overall Internet architecture. We have moved

from a world of a single, coherent architecture designed by a small

group of people, to a world of complex, intricate architecture to

address a wide-spread and heterogeneous environment. Because

individual pieces of the architecture are often designed by

sub-communities, with each sub-community having its own set of

interests, it is necessary to pay increasing attention to how each

piece fits into the larger picture, and to consider how each piece is

chosen. For example, it is unavoidable that each of us is inclined

to solve a problem at the layer of the protocol stack and using the

tools that we understand the best; that does not necessarily mean

that this is the most appropriate layer or set of tools for solving

this problem in the long-term.

Our assumption is that this document will be used as suggestions (but

not a checklist!) of issues to be addressed by IETF members in

chartering new working groups, in working in working groups, and in

evaluating the output from other working groups. This document is

not a primer on how to design protocols and architectures, and does

not provide answers to anything.

2. Relationship to "Architectural Principles of the Internet"

RFC1958 [RFC1958] outlines some architectural principles of the

Internet, as "guidelines that have been found useful in the past, and

that may be useful to those designing new protocols or evaluating

such designs." An example guideline is that "it is also generally

felt that end-to-end functions can best be realized by end-to-end

protocols." Similarly, an example design issue from [RFC1958] is that

"heterogeneity is inevitable and must be supported by design."

In contrast, this document serves a slightly different purpose, by

suggesting additional architectural questions to be addressed. Thus,

one question suggested in this document is the following: "Is this

proposal the best long-term solution to the problem? If not, what

are the long-term costs of this solution, in terms of restrictions on

future development, if any?" This question could be translated to a

roughly equivalent architectural guideline, as follows: "Identify

whether the proposed protocol is a long-term solution or a short-term

solution, and identify the long-term costs and the exit strategy for

any short-term solutions."

In contrast, other questions are more open-ended, such as the

question about robustness: "How robust is the protocol, not just to

the failure of nodes, but also to compromised or malfunctioning

components, imperfect or defective implementations, etc?" As a

community, we are still learning about the degree of robustness that

we are able to build into our protocols, as well as the tools that

are available to ensure this robustness. Thus, there are not yet

clear architectural guidelines along the lines of "Ensure that your

protocol is robust against X, Y, and Z."

3. Questions

In this section we list some questions to ask in designing protocols.

Each question is discussed more depth in the rest of this paper. We

aren't suggesting that all protocol design efforts should be required

to eXPlicitly answer all of these questions; some questions will be

more relevant to one document than to another. We also aren't

suggesting that this is a complete list of architectural concerns.

DESIGN QUESTIONS:

Justifying the Solution:

* Why are you proposing this solution, instead of proposing something

else, or instead of using existing protocols and procedures?

Interactions between Layers:

* Why are you proposing a solution at this layer of the protocol

stack, rather than at another layer? Are there solutions at other

layers of the protocol stack as well?

* Is this an appropriate layer in terms of correctness of function,

data integrity, performance, ease of deployment, the diagnosability

of failures, and other concerns?

* What are the interactions between layers, if any?

Long-term vs. Short-term Solutions:

* Is this proposal the best long-term solution to the problem?

* If not, what are the long-term costs of this solution, in terms of

restrictions on future development, if any? What are the

requirements for the development of longer-term solutions?

The Whole Picture vs. Building Blocks:

* Have you considered the larger context, while appropriately

restricting your own design efforts to one part of the whole?

* Are there parts of the overall solution that will have to be

provided by other IETF Working Groups or by other standards bodies?

EVALUATION QUESTIONS:

Weighing Benefits against Costs:

* How do the architectural benefits of a proposed new protocol

compare against the architectural costs, if any? Have the

architectural costs been carefully considered?

Robustness:

* How robust is the protocol, not just to the failure of nodes, but

also to compromised or malfunctioning components, imperfect or

defective implementations, etc?

* Does the protocol take into account the realistic conditions of the

current or future Internet (e.g., packet drops and packet corruption;

packet reordering; asymmetric routing; etc.)?

Tragedy of the Commons:

* Is performance still robust if everyone is using this protocol?

Are there other potential impacts of widespread deployment that need

to be considered?

Protecting Competing Interests:

* Does the protocol protect the interests of competing parties (e.g.,

not only end-users, but also ISPs, router vendors, software vendors,

or other parties)?

Designing for Choice vs. Avoiding Unnecessary Complexity:

* Is the protocol designed for choice, to allow different players to

express their preferences where appropriate? At the other extreme,

does the protocol provide so many choices that it threatens

interoperability or introduces other significant problems?

Preserving Evolvability?

* Does the protocol protect the interests of the future, by

preserving the evolvability of the Internet? Does the protocol

enable future developments?

* If an old protocol is overloaded with new functionality, or reused

for new purposes, have the possible complexities introduced been

taken carefully into account?

* For a protocol that introduces new complexity to the Internet

architecture, how does the protocol add robustness and preserve

evolvability, and how does it also introduce new fragilities to the

system?

Internationalization:

* Where protocols require elements in text format, have the possibly

conflicting requirements of global comprehensibility and the ability

to represent local text content been properly weighed against each

other?

DEPLOYMENT QUESTIONS:

* Is the protocol deployable?

Each of these questions is discussed in more depth in the rest of

this paper.

4. Justifying the Solution

Question: Why are you proposing this solution, instead of proposing

something else, or instead of using existing protocols and

procedures?

4.1. Case study: Integrated and Differentiated Services

A good part of the work of developing integrated and differentiated

services has been to understand the problem to be solved, and to come

to agreement on the architectural framework of the solution, and on

the nature of specific services. Thus, when integrated services were

being developed, the specification of the Controlled Load [RFC2211]

and Guaranteed [RFC2212] services each required justification of the

need for that particular service, of low loss and bounded delay

respectively.

Later, when RFC2475 on "An Architecture for Differentiated Services"

proposed a scalable, service differentiation architecture that

differs from the previously-defined architecture for integrated

services, the document also had to clearly justify the requirements

for this new architecture, and compare the proposed architecture to

other possible approaches [RFC2475]. Similarly, when the Assured

Forwarding [RFC2597] and Expedited Forwarding [RFC3246] Per-Hop

Behaviors of differentiated services were proposed, each service

required a justification of the need for that service in the

Internet.

5. Interactions between Layers

Questions: Why are you proposing a solution at this layer of the

protocol stack, rather than at another layer? Are there solutions at

other layers of the protocol stack as well?

Is this an appropriate layer in terms of correctness of function,

data integrity, performance, ease of deployment, the diagnosability

of failures, and other concerns?

What are the interactions between layers, if any?

5.1. Discussion: The End-to-End Argument

The classic 1984 paper on "End-To-End Arguments In System Design"

[SRC84] begins a discussion of where to place functions among modules

by suggesting that "functions placed at low levels of a system may be

redundant or of little value when compared with the cost of providing

them at that low level. Examples discussed in the paper include bit

error recovery, security using encryption, duplicate message

suppression, recovery from system crashes, and delivery

acknowledgement. Low level mechanisms to support these functions are

justified only as performance enhancements." The end-to-end

principle is one of the key architectural guidelines to consider in

choosing the appropriate layer for a function.

5.2. Case study: Endpoint Congestion Management

The goal of the Congestion Manager in Endpoint Congestion Management

is to allow multiple concurrent flows with the same source and

destination address to share congestion control state [RFC3124].

There has been a history of proposals for multiplexing flows at

different levels of the protocol stack; proposals have included

adding multiplexing at the HTTP (WebMux) and TCP (TCP Control Blocks)

layers, as well as below TCP (the Congestion Manager) [Multiplexing].

However, the 1989 article on "Layered Multiplexing Considered

Harmful" suggests that "the extensive duplication of multiplexing

functionality across the middle and upper layers is harmful and

should be avoided" [T89]. Thus, one of the key issues in providing

mechanisms for multiplexing flows is to determine which layer or

layers of the protocol stack are most appropriate for providing this

functionality. The natural tendency of each researcher is generally

to add functionality at the layer that they know the best; this does

not necessarily result in the most appropriate overall architecture.

5.3. Case study: Layering Applications on Top of HTTP

There has been considerable interest in layering applications on top

of HTTP [RFC3205]. Reasons cited include compatibility with widely-

deployed browsers, the ability to reuse client and server libraries,

the ability to use existing security mechanisms, and the ability to

traverse firewalls. As RFC3205 discusses, "the recent interest in

layering new protocols over HTTP has raised a number of questions

when such use is appropriate, and the proper way to use HTTP in

contexts where it is appropriate." Thus, RFC3205 addresses not only

the benefits of layering applications on top of HTTP, but also

evaluates the additional complexity and overhead of layering an

application on top of HTTP, compared to the costs of introducing a

special-purpose protocol.

The web page on "References on Layering and the Internet

Architecture" has pointers to additional papers discussing general

layering issues in the Internet architecture [Layering].

6. Long-term vs. Short-term Solutions

Questions: Is this proposal the best long-term solution to the

problem?

If not, what are the long-term costs of this solution, in terms of

restrictions on future development, if any? What are the

requirements for the development of longer-term solutions?

6.1. Case study: Traversing NATs

In order to address problems with NAT middleboxes altering the

external address of endpoints, various proposals have been made for

mechanisms where an originating process attempts to determine the

address (and port) by which it is known on the other side of a NAT.

This would allow an originating process to be able to use address

data in the protocol exchange, or to advertise an external address

from which it will receive connections.

The IAB in [RFC3424] has outlined reasons why these proposals can be

considered, at best, short-term fixes to specific problems, and the

specific issues to be carefully evaluated before standardizing such

proposals. These issues include the identification of the

limited-scope problem to be fixed, the description of an exit

strategy for the short-term solution, and the description of the

longer-term problems left unsolved by the short-term solution.

7. Looking at the whole picture vs. making a building block

For a complex protocol which interacts with protocols from other

standards bodies as well as from other IETF working groups, it can be

necessary to keep in mind the overall picture while, at the same

time, breaking out specific parts of the problem to be standardized

in particular working groups.

Question: Have you considered the larger context, while restricting

your own design efforts to one part of the whole?

Question: Are there parts of the overall solution that will have to

be provided by other IETF Working Groups or by other standards

bodies?

7.1. Case Study: The Session Initiation Protocol (SIP)

The Session Initiation Protocol (SIP) [RFC2543], for managing

connected, multimedia sessions, is an example of a complex protocol

that has been broken into pieces for standardization in other working

groups. SIP has also involved interaction with other standardization

bodies.

The basic SIP framework is being standardized by the SIP working

group. This working group has focused on the core functional

features of setting up, managing, and tearing down multimedia

sessions. Extensions are considered if they relate to these core

features.

The task of setting up a multimedia session also requires a

description of the desired multimedia session. This is provided by

the Session Description Protocol (SDP). SDP is a building block that

is supplied by the Multiparty Multimedia Session Control (MMUSIC)

working group. It is not standardized within the SIP working group.

Other working groups are involved in standardizing extensions to SIP

that fall outside of core functional features or applications. The

SIPPING working group is analyzing the requirements for SIP applied

to different tasks, and the SIMPLE working group is examining the

application of SIP to instant messaging and presence. The IPTEL

working group is defining a call processing language (CPL) that

interacts with SIP in various ways. These working groups

occasionally feed requirements back into the main SIP working group.

Finally, outside standardization groups have been very active in

providing the SIP working group with requirements. The Distributed

Call Signaling (DCS) group from the PacketCable Consortium, 3GPP, and

3GPP2 are all using SIP for various telephony-related applications,

and members of these groups have been involved in drafting

requirements for SIP. In addition, there are extensions of SIP which

are under consideration in these standardization bodies. Procedures

are under development in the IETF to address proposed extensions to

SIP, including a category of preliminary, private, or proprietary

extensions, and to provide for the safe management of the SIP

namespace [MBMWOR02].

8. Weighing architectural benefits against architectural costs

Questions: How do the architectural benefits of a proposed new

protocol compare against the architectural costs, if any? Have the

architectural costs been carefully considered?

8.1. Case Study: Performance-enhancing proxies (PEPs)

RFC3135 [RFC3135] considers the relative costs and benefits of

placing performance-enhancing proxies (PEPs) in the middle of a

network to address link-related degradations. In the case of PEPs,

the potential costs include disabling the end-to-end use of IP layer

security mechanisms; introducing a new possible point of failure that

is not under the control of the end systems; adding increased

difficulty in diagnosing and dealing with failures; and introducing

possible complications with asymmetric routing or mobile hosts. RFC

3135 carefully considers these possible costs, the mitigations that

can be introduced, and the cases when the benefits of

performance-enhancing proxies to the user are likely to outweigh the

costs.

8.2. Case Study: Open Pluggable Edge Services (OPES)

One of the issues raised by middleboxes in the Internet involves the

end-to-end integrity of data. This is illustrated in the recent

question of chartering the Open Pluggable Edge Services (OPES)

Working Group. Open Pluggable Edge Services are services that would

be deployed as application-level intermediaries in the network, for

example, at a web proxy cache between the origin server and the

client. These intermediaries would transform or filter content, with

the explicit consent of either the content provider or the end user.

One of the architectural issues that arose in the process of

chartering the OPES Working Group concerned the end-to-end integrity

of data. As an example, it was suggested that "OPES would reduce

both the integrity, and the perception of integrity, of

communications over the Internet, and would significantly increase

uncertainly about what might have been done to content as it moved

through the network", and that therefore the risks of OPES outweighed

the benefits [CDT01].

As one consequence of this debate, the IAB wrote a document on "IAB

Architectural and Policy Considerations for OPES", considering both

the potential architectural benefits and costs of OPES [RFC3238].

This document did not recommend specific solutions or mandate

specific functional requirements, but instead included

recommendations of issues such as concerns about data integrity that

OPES solutions standardized in the IETF should be required to

address.

9. General Robustness Questions

Questions: How robust is the protocol, not just to the failure of

nodes, but also to compromised or malfunctioning components,

imperfect or defective implementations, etc?

Does the protocol take into account the realistic conditions of the

current or future Internet (e.g., packet drops and packet corruption,

packet reordering, asymmetric routing, etc.)?

9.1. Discussion: Designing for Robustness

Robustness has long been cited as one of the overriding goals of the

Internet architecture [Clark88]. The robustness issues discussed in

[Clark88] largely referred to the robustness of packet delivery even

in the presence of failed routers; today robustness concerns have

widened to include a goal of robust performance in the presence of a

wide range of failures, buggy code, and malicious actions.

As [ASSW02] argues, protocols need to be designed somewhat

defensively, to maximize robustness against inconsistencies and

errors. [ASSW02] discusses several approaches for increasing

robustness in protocols, such as verifying information whenever

possible; designing interfaces that are conceptually simple and

therefore less conducive to error; protecting resources against

attack or overuse; and identifying and exposing errors so that they

can be repaired.

Techniques for verifying information range from verifying that

acknowledgements in TCP acknowledge data that was actually sent, to

providing mechanisms for routers to verify information in routing

messages.

Techniques for protecting resources against attack range from

preventing "SYN flood" attacks by designing protocols that don't

allocate resources for a single SYN packet, to partitioning resources

(e.g., preventing one flow or aggregate from using all of the link

bandwidth).

9.2. Case Study: Explicit Congestion Notification (ECN)

The Internet is based on end-to-end congestion control, and

historically the Internet has used packet drops as the only method

for routers to indicate congestion to the end nodes. ECN [RFC3168]

is a recent addition to the IP architecture to allow routers to set a

bit in the IP packet header to inform end-nodes of congestion,

instead of dropping the packet.

The first, Experimental specification of ECN [RFC3168] contained an

extensive discussion of the dangers of a rogue or broken router

"erasing" information from the ECN field in the IP header, thus

preventing indications of congestion from reaching the end-nodes. To

add robustness, the standards-track specification [RFC3168] specified

an additional codepoint in the IP header's ECN field, to use for an

ECN "nonce". The development of the ECN nonce was motivated by

earlier research on specific robustness issues in TCP [SCWA99]. RFC

3168 explains that the addition of the codepoint "is motivated

primarily by the desire to allow mechanisms for the data sender to

verify that network elements are not erasing the CE codepoint, and

that data receivers are properly reporting to the sender the receipt

of packets with the CE codepoint set, as required by the transport

protocol." Supporting mechanisms for the ECN nonce are needed in the

transport protocol to ensure robustness of delivery of the ECN-based

congestion indication.

In contrast, a more difficult and less clear-cut robustness issue for

ECN concerns the differential treatment of packets in the network by

middleboxes, based on the TCP header's ECN flags in a TCP SYN packet

[RFC3360]. The issue concerns "ECN-setup" SYN packets, that is, SYN

packets with ECN flags set in the TCP header to negotiate the use of

ECN between the two TCP end-hosts. There exist firewalls in the

network that drop "ECN-setup" SYN packets, others that send TCP Reset

messages, and yet others that zero ECN flags in TCP headers. None of

this was anticipated by the designers of ECN, and RFC3168 added

optional mechanisms to permit the robust operation of TCP in the

presence of firewalls that drop "ECN-setup" SYN packets. However,

ECN is still not robust to all possible scenarios of middleboxes

zeroing ECN flags in the TCP header. Up until now, transport

protocols have been standardized independently from the mechanisms

used by middleboxes to control the use of these protocols, and it is

still not clear what degree of robustness is required from transport

protocols in the presence of the unauthorized modification of

transport headers in the network. These and similar issues are

discussed in more detail in [RFC3360].

10. Avoiding Tragedy of the Commons

Question: Is performance still robust if everyone is using the new

protocol? Are there other potential impacts of widespread deployment

that need to be considered?

10.1. Case Study: End-to-end Congestion Control

[RFC2914] discusses the potential for congestion collapse if flows

are not using end-to-end congestion control in a time of high

congestion. For example, if a new transport protocol was proposed

that did not use end-to-end congestion control, it might be easy to

show that an individual flow using the new transport protocol would

perform quite well as long as all of the competing flows in the

network were using end-to-end congestion control. To fully evaluate

the new transport protocol, it is necessary to look at performance

when many flows are competing, all using the new transport protocol.

If all of the competing flows were using the more aggressive

transport protocol in a time of high congestion, the result could be

high steady-state packet drop rates and reduced overall throughput,

with busy links carrying packets that will only be dropped

downstream. This can be viewed as a form of tragedy of the commons,

when a practice that is productive if done by only one person (e.g.,

adding a few more sheep to the common grazing pasture) is instead

counter-productive when done by everyone [H68].

11. Balancing Competing Interests

Question: Does the protocol protect the interests of competing

parties (e.g., not only end-users, but also ISPs, router vendors,

software vendors, or other parties)?

11.1. Discussion: balancing competing interests

[CWSB02] discusses the role that competition between competing

interests plays in the evolution of the Internet, and takes the

position that the role of Internet protocols is to design the playing

field in this competition, rather than to pick the outcome. The IETF

has long taken the position that it can only design the protocols,

and that often two competing approaches will be developed, with the

marketplace left to decide between them [A02]. (It has also been

suggested that "the marketplace" left entirely to itself does not

always make the best decisions, and that working to identify and

adopt the technically best solution is sometimes helpful. Thus,

while the role of the marketplace should not be ignored, the

decisions of the marketplace should also not be held as sacred or

infallible.)

An example cited in [CWSB02] of modularization in support of

competing interests is the decision to use codepoints in the IP

header to select QoS, rather than binding QoS to other properties

such as port numbers. This separates the structural and economic

issues related to QoS from technical issues of protocols and port

numbers, and allows space for a wide range of structural and pricing

solutions to emerge.

There have been perceived problems over the years of individuals

adding unnecessary complexity to IETF protocols, increasing the

barrier to other implementations of those protocols. Clearly such

action would not be protecting the interests of open competition.

Underspecified protocols can also serve as an unnecessary barrier to

open competition.

12. Designing for Choice vs. Avoiding Unnecessary Complexity:

Is the protocol designed for choice, to allow different players to

express their preferences where appropriate? At the other extreme,

does the protocol provide so many choices that it threatens

interoperability or introduces other significant problems?

12.1. Discussion: the importance of choice

[CWSB02] suggests that "the fundamental design goal of the Internet

is to hook computers together, and since computers are used for

unpredictable and evolving purposes, making sure that the users are

not constrained in what they can do is doing nothing more than

preserving the core design tenet of the Internet. In this context,

user empowerment is a basic building block, and should be embedded

into all mechanism whenever possible."

As an example of choice, "the design of the mail system allows the

user to select his SMTP server and his POP server" [CWSB02]. More

open-ended questions about choice concern the design of mechanisms

that would enable the user to choose the path at the level of

providers, or to allow users to choose third-party intermediaries

such as web caches, or providers for Open Pluggable Edge Services

(OPES). [CWSB02] also notes that the issue of choice itself reflects

competing interests. For example, ISPs would generally like to lock

in customers, while customers would like to preserve their ability to

change among providers.

At the same time, we note that excessive choice can lead to "kitchen

sink" protocols that are inefficient and hard to understand, have too

much negotiation, or have unanticipated interactions between options.

For example, [P99] notes that excessive choice can lead to difficulty

in ensuring interoperability between two independent, conformant

implementations of the protocol.

The dangers of excessive options are also discussed in [MBMWOR02],

which gives guidelines for responding to the "continuous flood" of

suggestions for modifications and extensions to SIP (Session

Initiation Protocol). In particular, the SIP Working Group is

concerned that proposed extensions have general use, and do not

provide efficiency at the expense of simplicity or robustness.

[MBMWOR02] suggests that other highly extensible protocols developed

in the IETF might also benefit from more coordination of extensions.

13. Preserving evolvability?

Does the protocol protect the interests of the future, by preserving

the evolvability of the Internet? Does the protocol enable future

developments?

If an old protocol is overloaded with new functionality, or reused

for new purposes, have the possible complexities introduced been

taken into account?

For a protocol that introduces new complexity to the Internet

architecture, does the protocol add robustness and preserve

evolvability? Does it also introduce unwanted new fragilities to the

system?

13.1. Discussion: evolvability

There is an extensive literature and an ongoing discussion about the

evolvability, or lack of evolvability, of the Internet

infrastructure; the web page on "Papers on the Evolvability of the

Internet Infrastructure" has pointers to some of this literature

[Evolvability]. Issues range from the evolvability and overloading

of the DNS; the difficulties of the Internet in evolving to

incorporate multicast, QoS, or IPv6; the difficulties of routing in

meeting the demands of a changing and expanding Internet; and the

role of firewalls and other middleboxes in limiting evolvability.

[CWSB02] suggests that among all of the issues of evolvability,

"keeping the net open and transparent for new applications is the

most important goal." In the beginning, the relative transparency of

the infrastructure was sufficient to ensure evolvability, where a

"transparent" network simply routes packets from one end-node to

another. However, this transparency has become more murky over time,

as cataloged in [RFC3234], which discusses the ways that middleboxes

interact with existing protocols and increase the difficulties in

diagnosing failures. [CWSB02] realistically suggests the following

guideline: "Failures of transparency will occur - design what happens

then," where examples of failures of transparency include firewalls,

application filtering, connection redirection, caches, kludges to

DNS, and the like. Thus, maintaining evolvability also requires

mechanisms for allowing evolution in the face of a lack of

transparency of the infrastructure itself.

One of the ways for maintaining evolvability is for designers of new

mechanisms and protocols to be as clear as possible about the

assumptions that are being made about the rest of the network. New

mechanisms that make unwarranted assumptions about the network can

end up placing unreasonable constraints on the future evolution of

the network.

13.2. Discussion: overloading

There has been a strong tendency in the last few years to overload

some designs with new functionality, with resulting operational

complexities. Extensible protocols could be seen as one of the tools

for providing evolvability. However, if protocols and systems are

stretched beyond their reasonable design parameters, then scaling,

reliability, or security issues could be introduced. Examples of

protocols that have been seen as either productively extended, or as

dangerously overloaded, or both, include DNS [K02,RFC3403], MPLS

[A02a], and BGP [B02]. In some cases, overloading or extending a

protocol may reduce total complexity and deployment costs by avoiding

the creation of a new protocol; in other cases a new protocol might

be the simpler solution.

We have a number of reusable technologies, including component

technologies specifically designed for re-use. Examples include

SASL, BEEP and APEX. TCP and UDP can also be viewed as reusable

transport protocols, used by a range of applications. On the other

hand, re-use should not go so far as to turn a protocol into a Trojan

Horse, as has happened with HTTP [RFC3205].

13.3. Discussion: complexity, robustness, and fragility

[WD02] gives a historical account of the evolution of the Internet as

a complex system, with particular attention to the tradeoffs between

complexity, robustness, and fragility. [WD02] describes the

robustness that follows from the simplicity of a connectionless,

layered, datagram infrastructure and a universal logical addressing

scheme, and, as case studies, describes the increasing complexity of

TCP and of BGP. The paper describes a complexity/robustness spiral

of an initially robust design and the appearance of fragilities,

followed by modifications for more robustness that themselves

introduce new fragilities. [WD02] conjectures that "the Internet is

only now beginning to experience an acceleration of this

complexity/robustness spiral and, if left unattended, can be fully

expected to experience arcane, irreconcilable, and far-reaching

robustness problems in the not-too-distant future." Citing [WD02],

[BFM02] focuses on the ways that complexity increases capital and

operational expenditures in carrier IP network, and views complexity

as the primary mechanism that impedes efficient scaling.

14. Internationalization

Where protocols require elements in text format, have the possibly

conflicting requirements of global comprehensibility and the ability

to represent local text content been properly weighed against each

other?

14.1. Discussion: internationalization

RFC1958 [RFC1958] included a simple statement of the need for a

common language:

"Public (i.e. widely visible) names should be in case-independent

ASCII. Specifically, this refers to DNS names, and to protocol

elements that are transmitted in text format."

The IETF has studied character set issues in general [RFC2130] and

made specific recommendations for the use of a standardized approach

[RFC2277]. The situation is complicated by the fact that some uses

of text are hidden entirely in protocol elements and need only be

read by machines, while other uses are intended entirely for human

consumption. Many uses lie between these two extremes, which leads

to conflicting implementation requirements.

For the specific case of DNS, the Internationalized Domain Name

working group is considering these issues. As stated in the charter

of that working group, "A fundamental requirement in this work is to

not disturb the current use and operation of the domain name system,

and for the DNS to continue to allow any system anywhere to resolve

any domain name." This leads to some very strong requirements for

backwards compatibility with the existing ASCII-only DNS. Yet since

the DNS has come to be used as if it was a Directory service, domain

names are also expected to be presented to users in local character

sets.

This document does not attempt to resolve these complex and difficult

issues, but simply states this as an issue to be addressed in our

work. The requirement that names encoded in a text format within

protocol elements be universally decodable (i.e. encoded in a

globally standard format with no intrinsic ambiguity) does not seem

likely to change. However, at some point, it is possible that this

format will no longer be case-independent ASCII.

15. Deployability

Question: Is the protocol deployable?

15.1. Discussion: deployability

It has been suggested that the failure to understand deployability

considerations in the current environment is one of the major

weakness of the IETF. As examples of deployment difficulties, RFC

2990 [RFC2990] discusses deployment difficulties with Quality of

Service (QoS) architectures, various documents of the MBONE

Deployment Working Group address deployment problems with IP

Multicast, and the IPv6 Working Group discusses a wealth of issues

related to the deployment of IPv6. [CN02] discusses how the

deployment of Integrated Services has been limited by factors such as

the failure to take into account administrative boundaries.

Additional papers on difficulties in the evolution of the Internet

architecture are available from [Evolvability].

Issues that can complicate deployment include whether the protocol is

compatible with pre-existing standards, and whether the protocol is

compatible with the installed base. For example, a transport

protocol is more likely to be deployable if it performs correctly and

reasonably robustly in the presence of dropped, reordered,

duplicated, delayed, and rerouted packets, as all of this can occur

in the current Internet.

16. Conclusions

This document suggests general architectural and policy questions to

be addressed when working on new protocols and standards in the IETF.

The case studies in this document have generally been short

illustrations of how the questions proposed in the document have been

addressed in working groups in the past. However, we have generally

steered away from case studies of more controversial issues, where

there is not yet a consensus in the IETF community. Thus, we

side-stepped suggestions for adding a case study for IKE (Internet

Key Exchange) as an possible example of a protocol with too much

negotiation, or of adding a case study of network management

protocols as illustrating the possible costs of leaving things to the

marketplace to decide. We would encourage others to contribute case

studies of these or any other issues that may shed light on some of

the questions in this document; any such case studies could include

a careful presentation of the architectural reasoning on both sides.

we would conjecture that there is a lot to be learned, in terms of

the range of answers to the questions posed in this document, by

studying the successes, failures, and other struggles of the past.

We would welcome feedback on this document for future revisions.

Feedback could be send to the editor, Sally Floyd, at floyd@icir.org.

17. Acknowledgements

This document has borrowed text freely from other IETF RFCs, and has

drawn on ideas from [ASSW02], [CWSB02], [M01] and elsewhere. This

document has developed from discussions in the IAB, and has drawn

from suggestions made at IAB Plenary sessions and on the ietf general

discussion mailing list. The case study on SIP was contributed by

James Kempf, an early case study on Stresses on DNS was contributed

by Karen Sollins, and Keith Moore contributed suggestions that were

incorporated in a number of places in the document. The discussions

on Internationalization and on Overloading were based on an earlier

document by Brian Carpenter and Rob Austein. We have also benefited

from discussions with Noel Chiappa, Karen Sollins, John Wroclawski,

and others, and from helpful feedback from members of the IESG. We

specifically thank Senthilkumar Ayyasamy, John Loughney, Keith Moore,

Eric Rosen, and Dean Willis and others for feedback on various stages

of this document.

18. Normative References

19. Informative References

[A02] Harald Alvestrand, "Re: How many standards or

protocols...", email to the ietf discussion mailing

list, Message-id: <598204031.1018942481@localhost>,

April 16, 2002.

[A02a] Loa Andersson, "The Role of MPLS in Current IP

Network", MPLS World News, September 16, 2002. URL

"http://www.mplsworld.com/archi_drafts/focus/analy-

andersson.htm".

[ASSW02] T. Anderson, S. Shenker, I. Stoica, and D. Wetherall,

"Design Guidelines for Robust Internet Protocols",

HotNets-I, October 2002.

[BFM02] Randy Bush, Tim Griffin, and David Meyer, "Some

Internet Architectural Guidelines and Philosophy",

Work in Progress.

[B02] Hamid Ould-Brahim, Bryan Gleeson, Peter Ashwood-Smith,

Eric C. Rosen, Yakov Rekhter, Luyuan Fang, Jeremy De

Clercq, Riad Hartani, and Tissa Senevirathne, "Using

BGP as an Auto-Discovery Mechanism for Network-based

VPNs", Work in Progress.

[CDT01] Policy Concerns Raised by Proposed OPES Working Group

Efforts, email to the IESG, from the Center for

Democracy & Technology, August 3, 2001. URL

"http://www.imc.org/ietf-openproxy/mail-

archive/msg00828.Html".

[Clark88] David D. Clark, The Design Philosophy of the DARPA

Internet Protocols, SIGCOMM 1988.

[CN02] Brian Carpenter, Kathleen Nichols, "Differentiated

Services in the Internet", Technical Report, February

2002, URL "http://www.research.ibm.com/resources/

paper_search.shtml".

[CWSB02] Clark, D., Wroslawski, J., Sollins, K., and Braden,

R., "Tussle in Cyberspace: Defining Tomorrow's

Internet", SIGCOMM 2002. URL

"http://www.acm.org/sigcomm/sigcomm2002/papers/

tussle.html".

[Evolvability] Floyd, S., "Papers on the Evolvability of the Internet

Infrastructure". Web Page, URL

"http://www.icir.org/floyd/evolution.html".

[H68] Garrett Hardin, "The Tragedy of the Commons", Science,

V. 162, 1968, pp. 1243-1248. URL

"http://dieoff.org/page95.htm".

[K02] John C. Klensin, "Role of the Domain Name System",

Work in Progress.

[Layering] Floyd, S., "References on Layering and the Internet

Architecture", Web Page, URL

"http://www.icir.org/floyd/layers.html".

[Multiplexing] S. Floyd, "Multiplexing, TCP, and UDP: Pointers to the

Discussion", Web Page, URL

"http://www.icir.org/floyd/tcp_mux.html".

[MBMWOR02] Mankin, A., Bradner, S., Mahy, R., Willis, D., Ott, J.

and B. Rosen, "Change Process for the Session

Initiation Protocol (SIP)", BCP 67, RFC3427, November

2002.

[M01] Tim Moors, A Critical Review of End-to-end Arguments

in System Design, 2001. URL

"http://uluru.poly.edu/~tmoors/".

[P99] Radia Perlman, "Protocol Design Folklore", in

Interconnections Second Edition: Bridges, Routers,

Switches, and Internetworking Protocols, Addison-

Wesley, 1999.

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

Internet", RFC1958, June 1996.

[RFC2211] Wroclawski, J., "Specification of the Controlled Load

Quality of Service", RFC2211, September 1997.

[RFC2212] Shenker, S., Partridge, C., and R. Guerin,

"Specification of Guaranteed Quality of Service", RFC

2212, September 1997.

[RFC2316] Bellovin, S., "Report of the IAB Security Architecture

Workshop", RFC2316, April 1998.

[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang,

Z. and W. Weiss, "An Architecture for Differentiated

Services", RFC2475, December 1998.

[RFC2543] Handley, M., Schulzrinne, H., Schooler, B. and J.

Rosenberg, "SIP: Session Initiation Protocol", RFC

25434, March 1999.

[RFC2597] Heinanen, J., Baker, F., Weiss, W. and J. Wroclawski,

"Assured Forwarding PHB Group", RFC2597, June 1999.

[RFC2990] Huston, G., "Next Steps for the IP QoS Architecture",

RFC2990, November 2000.

[RFC3124] Balakrishnan, H. and S. Seshan, "The Congestion

Manager", RFC3124, June 2001.

[RFC3135] Border, J., Kojo, M., Griner, J., Montenegro, G. and

Z. Shelby, "Performance Enhancing Proxies Intended to

Mitigate Link-Related Degradations", RFC3135, June

2001.

[RFC3168] Ramakrishnan, K. K., Floyd, S. and D. Black, "The

Addition of Explicit Congestion Notification (ECN) to

IP", RFC3168, September 2001.

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

56, RFC3205, February 2002.

[RFC3221] Huston, G., "Commentary on Inter-Domain Routing in the

Internet", RFC3221, December 2001.

[RFC3234] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and

Issues", RFC3234, February 2002.

[RFC3238] Floyd, S. and L. Daigle, "IAB Architectural and Policy

Considerations for Open Pluggable Edge Services", RFC

3238, January 2002.

[RFC3246] Davie, B., Charny, A., Bennet, J. C. R., Benson, K.,

Le Boudec, J. Y., Courtney, W., Davari, S., Firoiu, V.

and D. Stiliadis, "An Expedited Forwarding PHB (Per-

Hop Behavior)", RFC3246, March 2002.

[RFC3360] Floyd, S., "Inappropriate TCP Resets Considered

Harmful", BCP 60, RFC3360, August 2002.

[RFC3403] Mealling, M., "Dynamic Delegation Discovery System

(DDDS) Part Three: The Domain Name System (DNS)

Database", RFC3403, October 2002.

[RFC3424] Daigle, L., "IAB Considerations for UNilateral Self-

Address Fixing (UNSAF)", RFC3424, November 2002.

[SCWA99] Stefan Savage, Neal Cardwell, David Wetherall, Tom

Anderson, "TCP Congestion Control with a Misbehaving

Receiver", ACM Computer Communications Review, October

1999.

[SRC84] J. Saltzer, D. Reed, and D. D. Clark, "End-To-End

Arguments In System Design", ACM Transactions on

Computer Systems, V.2, N.4, p. 277-88. 1984.

[T89] D. Tennenhouse, "Layered Multiplexing Considered

Harmful", Protocols for High-Speed Networks, 1989.

[WD02] Walter Willinger and John Doyle, "Robustness and the

Internet: Design and Evolution", draft, March 2002,

URL "http://netlab.caltech.edu/internet/".

20. Security Considerations

This document does not propose any new protocols, and therefore does

not involve any security considerations in that sense. However,

throughout this document there are discussions of the privacy and

integrity issues and the architectural requirements created by those

issues.

21. IANA Considerations

There are no IANA considerations regarding this document.

Authors' Addresses

Internet Architecture Board

EMail: iab@iab.org

IAB Membership at time this document was completed:

Harald Alvestrand

Ran Atkinson

Rob Austein

Fred Baker

Leslie Daigle

Steve Deering

Sally Floyd

Ted Hardie

Geoff Huston

Charlie Kaufman

James Kempf

Eric Rescorla

Mike St. Johns

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