Transport WG A. Knutsen
Internet-Draft Blue Coat Systems
Intended status: Informational A. Ramaiah
Expires: September 1, 2012 Cisco
February 29, 2012
TCP option for transparent Middlebox discovery
draft-ananth-middisc-tcpopt-00.txt
Abstract
This document describes a TCP option for use by middleboxes to
facilitate transparent detection of other middleboxes along the path
of the TCP connection during the connection initiation phase. The
option has no effect if an appropriate middlebox is not present on
the path. The TCP end hosts are unaware of this option and its usage
is strictly for use by TCP middleboxes. Multiple vendors of WAN
optimization products have used similar (but incompatible and
proprietary) mechanisms since at least 2004. Those existing vendor
systems use multiple TCP option code points, all of which are
officially unassigned by IANA. The goal of this memo is to
standardize a single TCP option code point for this functionality.
This memo is the product of discussion among some of the vendors
currently using incompatible proprietary TCP options for middlebox
discovery. It is a non-goal of this memo to achieve interoperability
of middlebox discovery between multiple vendors. It needs to be
noted that an earlier document
[I-D.knutsen-tcpm-middlebox-discovery], is re-written as the current
document after agreement from multiple vendors.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 1, 2012.
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Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Requirements of the TCP-MDO . . . . . . . . . . . . . . . . . 5
4. Description . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. TCP-MDO option format . . . . . . . . . . . . . . . . . . . . 7
6. TCP option interoperability . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 11
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
11.1. Normative References . . . . . . . . . . . . . . . . . . 13
11.2. Informative References . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]
Middlebox
"Middleboxes: Taxonomy and Issues" [RFC3234] defines a middlebox
as follows:
"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."
Proxy
HTTP1.1 [RFC2616] defines a 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." Proxies exist for
many protocols, such as HTTP, CIFS, MAPI and streaming. Since
they act as both server and client, they have separate TCP
connections to the original client and the actual server (also
referred to as the "Original Content Server"). Proxies are often
implemented on middleboxes. Proxies fall into two general
categories: "Explicit" and "Transparent". The client must be
configured to connect to an explicit proxy; it then passes the
server address to it using an application protocol, such as HTTP.
Transparent proxies require no client configuration; they
intercept the client connection to the server, speaking to the
client on its behalf, and make a separate connection to the server
without the knowledge of the client. This memo deals exclusively
with cooperating transparent proxies.
Peer
Two or more middleboxes with an effective association or
relationship are peers. For example, one middlebox might compress
data while another middlebox decompresses; neither middlebox can
correctly manipulate traffic unless they both establish the
existence of the other and coordinate their actions. The most
common peering relationships are two-way (like the compression
example) in which one middlebox performs a transformation that the
other middlebox inverts. However, two-way peering that does not
involve an invertible transformation is also found, as are n-way
peering relationships where the "intermediate" peers are simply
operating in a pass-through or failover mode.
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2. Introduction
For middleboxes that operate on TCP-based application protocols, it
is highly desirable for discovery information to be carried within
packets containing valid TCP protocol data. One significant kind of
service offered by such middleboxes is application acceleration, in
which there is an intrinsic requirement to be efficient and avoid
network round trips. A middlebox-discovery mechanism that imposes
additional round trips could defeat the purpose of such middleboxes.
Middlebox discovery on a per-connection basis allows for significant
advantages in scale and flexibility. Middlebox-based services can be
invoked dynamically, using the appropriate peers, without any need
for statically identifying middleboxes to end hosts or identifying
middleboxes to each other. Dynamic discovery means that there is no
overlay routing required for middlebox-based services, where such
overlay routing could potentially clash with the underlying IP
routing. For all of these reasons, multiple vendors have implemented
TCP-option-based discovery mechanisms; and for similar reasons, this
memo requests a TCP option for middle box discovery.
The TCP middlebox discovery option (TCP-MDO) allows a source node
(initiating middlebox) on the initiating path of a TCP connection to
request a response from other middleboxes with a matching capability
closer to the destination host. In addition, it allows the
initiating middlebox to provide information to the other middleboxes
which they may need to decide whether to respond to this request. A
middlebox MAY examine TCP packets with the SYN bit set to determine
if the associated TCP connection qualifies for the middlebox-provided
service. If so, the middlebox MAY insert the TCP-MDO into the packet
header before sending it on. A middlebox MAY examine packets
containing the TCP-MDO to determine if the associated TCP connection
qualifies for the middlebox-provided service. If so, the middlebox
MAY take additional actions to coordinate with the initiating
middlebox. Such actions MAY include acknowledging the SYN packet to
intercept the connection; originating a separate connection to the
client; or perhaps notifying a management station.
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3. Requirements of the TCP-MDO
The following are the requirements of TCP-MDO :
1) The TCP-MDO MUST be variable length to accommodate multiple vendor
option formats.
2) The TCP-MDO MUST have a vendor ID which can identify the specific
vendor as implied by 1)
3) The TCP option space limitation puts a burden on how flexible the
option can be. Please refer section 6 below.
4) TCP option numbers already in use by proprietary systems SHOULD
NOT be reused for TCP-MDO since it would create confusion. (These
option numbers would get eventually retired when all vendors
migrate to the newly allocated TCP-MDO option)
5) The TCP-MDO SHOULD be used for middleboxes only. The hosts are
expected to silently ignore this option.
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4. Description
The TCP-MDO MAY be included in the TCP handshake (SYN and SYN+
ACKpackets). The TCP-MDO contained in the SYN packet is used to
discover peer middleboxes along the path to the server. The TCP-MDO
option MAY be present in the TCP SYN+ACK and other TCP data packets
as well.
It should be noted that a common use of middleboxes is to set up
cooperating peer proxies. One example is to implement a feature
which optimizes the WAN traffic, like a compression protocol. In
these cases, the option is used by the device nearer to the client to
discover a possible device nearer to the server. Thus the client and
server application are not aware of the option. Figure 1 illustrates
the text above.
CLIENT ----- Middlebox1 ================ Middlebox2 ------ SERVER
WAN link
------SYN------->|---- SYN, TCP-MDO ----->|----SYN--->
<---SYN+ACK---|<---SYN+ACK, TCP-MDO----|<---SYN+ACK----
Figure 1: TCP-MDO option insertion during TCP handshake
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5. TCP-MDO option format
The following is the agreed-upon option format for TCP-MDO.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind = xx | Length | Vendor = YY | Vendor |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Payload data dependent on Vendor typecode |
| |
| (Variable Length) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
(One tick mark represents one bit)
Figure 2 : Format of TCP-MDO
Even after 8 years of commercial development, there are only a
handful of vendors with different option-based discovery schemes.
Accordingly, 8 bits is likely to be more than sufficient for vendor
identification. Ideally, future work would define a single
interoperable middlebox discovery scheme to replace the multiple
vendor-specific schemes.
The PEN (32 bit Private Enterprise numbers) is an existing mechanism
which could be used to identify various vendors
(http://www.iana.org/assignments/enterprise-numbers). Because of the
very limited amount of TCP option space, using 32 bits for an
enterprise number is impractical. The next section talks more about
the TCP option space issue.
Other formats for TCP-MDO have been previously considered, including
an approach in which the option contains a Type code followed by
Vendor ID. That design had the advantage of supporting an extension
into 32-bit enterprise numbers. But no meaningful commonality could
be identified among the different vendor schemes; at any level beyond
the use of a TCP option, each functions quite differently and there
was no clear basis for choosing a standardized type scheme.
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6. TCP option interoperability
This section touches upon the interoperability issues of TCP-MDO with
other TCP options.
It needs to be noted that the TCP-MDO can be inserted only if the TCP
option space permits. Given that there is a maximum of 40 bytes for
TCP options, A SYN (with MSS, window scale, SACK permitted, and
timestamp options) leaves 16 bytes spare (if the options are word-
aligned) or 21 bytes spare (if the options are not word-aligned)
which may just make it. In particular, it is not typically possible
to insert TCP-MDO if a vendor-proprietary option has already been
inserted, nor vice-versa. Strictly speaking, this conflict is not a
new problem but the attempt at standardization may mean it occurs
more often. It is already true in most cases that it is not possible
to insert vendor A's option if vendor B's option has previously been
inserted. This memo does not attempt to define rules on precedence
or priority of such options, nor does it define circumstances in
which a proprietary option may safely be replaced by TCP-MDO.
Multipath TCP ([I-D.ietf-mptcp-multiaddressed]) would have more
options sent in the SYN, therby restricting the TCP option space that
can be used for TCP-MDO.
The TCP-MDO option is incompatible with TCP options which aim to
protect the integrity of the TCP SYN. An example of such an option
is the TCP MD5 signature option[RFC2385]. Such TCP options are not
commonly seen by current WAN optimization systems, so the restriction
should not pose any worries.
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7. IANA Considerations
This section is interpreted according to [RFC5226])
This document needs a new TCP option to be allocated by IANA for the
TCP-MDO option from the "TCP Option Kind Numbers" registry maintained
at http://www.iana.org
IANA is also requested to assign an 8 bit vendor code that can be
used to differentiate multiple vendors and to maintain an associated
registry.
It needs to be noted that at the time of writing this document, the
following vendors have used the following TCP option numbers for TCP
middlebox auto-discovery.
Vendor TCP option number
--------------------------------------
Bluecoat 253
Cisco 33
Figure 3: Current TCP auto-discovery option numbers used by various vendors.
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8. Security Considerations
The TCP-MDO option is incompatible with any TCP option which aims to
protect the integrity of the TCP SYN, such as the TCP MD5 signature
option[RFC2385]. In addition, this memo's approach to middlebox
discovery introduces two security issues. First, the option-based
paradigm intrinsically involves modifying traffic. Some traffic may
be modified even though it cannot or should not receive the
middlebox-based service. Secondly, there is no authentication
mechanism defined in this memo to ensure that middleboxes communicate
only with well-behaved and trusted potential peers. A rogue
middlebox can potentially insert itself into a functioning community
of middleboxes, disrupting the service provided.
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9. Contributors
The following individuals contributed immensely to this document :
Ron Fredrick, Blue Coat.
Mani Ramaswamy, Cisco.
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10. Acknowledgements
Thanks to Lars Eggert for forming the middisc alias which was
responsible for discussions leading to the current document. Thanks
to Wes Eddy and David Harrington for the constant encouragement in
getting this document written.
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11. References
11.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
11.2. Informative References
[I-D.ietf-mptcp-multiaddressed]
Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
"TCP Extensions for Multipath Operation with Multiple
Addresses", draft-ietf-mptcp-multiaddressed-06 (work in
progress), January 2012.
[I-D.knutsen-tcpm-middlebox-discovery]
Knutsen, A., Frederick, R., Mahdavi, J., Li, Q., and W.
Yeh, "TCP Option for Transparent Middlebox Discovery",
draft-knutsen-tcpm-middlebox-discovery-04 (work in
progress), May 2010.
[RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385, August 1998.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3234] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and
Issues", RFC 3234, February 2002.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
July 2003.
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Authors' Addresses
Andrew Knutsen
Blue Coat Systems
420 North Mary Ave
Sunnyvale, CA 94085-4121
USA
Phone: +1 (408) 220-2250
Email: andrew.knutsen@bluecoat.com
Anantha Ramaiah
Cisco
170 Tasman Drive
San Jose, CA 95134
USA
Phone: +1 (408) 525-6486
Email: ananth@cisco.com
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