Internet DRAFT - draft-kumar-sfc-sfp-optimization
draft-kumar-sfc-sfp-optimization
Service Function Chaining S. Kumar
Internet-Draft J. Guichard
Intended status: Informational P. Quinn
Expires: April 4, 2015 Cisco Systems, Inc.
J. Halpern
Ericsson
Oct 2014
Service Function Path Optimization
draft-kumar-sfc-sfp-optimization-01
Abstract
Service Function Chaining (SFC) enables services to be delivered by
selective traffic steering through an ordered set of service
functions. Once classified into an SFC, the traffic for a given flow
is steered through all the service functions of the SFC for the life
of the traffic flow even though this is often not necessary.
Steering traffic to service functions only while required and not
otherwise, leads to optimal SFCs with improved latencies, reduced
resource consumption and better user experience.
This document describes the rationale, techniques and necessary
protocol extensions to achieve such optimization.
Status of this Memo
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This Internet-Draft will expire on April 4, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Definition Of Terms . . . . . . . . . . . . . . . . . . . . . 3
3. Service Function Path Optimization . . . . . . . . . . . . . . 4
3.1. Bypass . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Simple Offload . . . . . . . . . . . . . . . . . . . . . . 6
3.2.1. Stateful SFF . . . . . . . . . . . . . . . . . . . . . 7
3.2.2. Packet Re-ordering . . . . . . . . . . . . . . . . . . 7
3.2.3. Policy Implications . . . . . . . . . . . . . . . . . 7
3.2.4. Capabilities Exchange . . . . . . . . . . . . . . . . 8
4. Methods For SFP Optimization . . . . . . . . . . . . . . . . . 8
4.1. Hop-by-hop Offload . . . . . . . . . . . . . . . . . . . . 9
4.1.1. Progression Of SFP Optimization . . . . . . . . . . . 11
4.2. Service Controller Offload . . . . . . . . . . . . . . . . 12
5. Offload Data-plane Signaling . . . . . . . . . . . . . . . . . 12
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
8. Security Considerations . . . . . . . . . . . . . . . . . . . 14
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
9.1. Normative References . . . . . . . . . . . . . . . . . . . 14
9.2. Informative References . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Introduction
Service function chaining involves steering traffic flows through a
set of service functions in a specific order. Such an ordered list
of service functions is called a Service Function Chain (SFC). The
actual forwarding path used to realize an SFC is called the Service
Function Path (SFP).
Service functions forming an SFC are hosted at different points in
the network, often co-located with different types of service
functions to form logical groupings. Applying a SFC thus requires
traffic steering by the SFC infrastructure from one service function
to the next until all the service functions of the SFC are applied.
Service functions know best what type of traffic they can service and
how much traffic needs to be delivered to them to achieve complete
delivery of service. As a consequence any service function may
potentially request, within its policy constraints, traffic no longer
be delivered to it or its function be performed by the SFC
infrastructure, if such a mechanism is available.
This document outlines mechanisms to not steer traffic to service
functions, on request, while still ensuring compliance to the
instantiated policy that mandates the SFC.
1.1. Requirements Language
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 RFC 2119 [RFC2119].
2. Definition Of Terms
This document uses the following terms. Additional terms are defined
in [I-D.ietf-sfc-problem-statement], [I-D.ietf-sfc-architecture] and
[I-D.quinn-sfc-nsh]. Some are reproduced here only for convenience
and the reader is advised to consult the referenced documents.
Service Function (SF): A function that is responsible for specific
treatment of received packets. A Service Function can act at the
network layer or other OSI layers. A Service Function can be a
virtual instance or be embedded in a physical network element.
One of multiple Service Functions can be embedded in the same
network element. Multiple instances of the Service Function can
be enabled in the same administrative domain. A non-exhaustive
list of Service Functions includes: firewalls, WAN and
application acceleration, Deep Packet Inspection (DPI), server
load balancers, NAT44 [RFC3022], NAT64 [RFC6146], HOST_ID
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injection, HTTP Header Enrichment functions, TCP optimizer, etc.
Service Node (SN): A virtual or physical device that hosts one or
more service functions, which can be accessed via the network
location associated with it.
Service Function Forwarder (SFF): A service function forwarder is
responsible for forwarding traffic along the service path, which
includes delivery of traffic to the connected service functions.
Network Forwarder (NF): The entity, typically part of the network
infrastructure, responsible for performing the transport function
in traffic forwarding.
Service Controller (SC): The entity responsible for managing the
service chains, including create/read/update/delete actions as
well as programming the service forwarding state in the network -
SFP distribution.
Classifier (CF): The entity, responsible for selecting traffic as
well as SFP, based on policy, and forwarding the selected traffic
on the SFP after adding the necessary encapsulation. Classifier
is implicitly an SFF.
Offload: A request or a directive from the SF to alter the SFP so as
to remove the requesting SF from the SFP while maintaining the
effect of the removed SF on the offloaded flow.
Un-offload: A request or directive to cancel the effect of Offload -
leads to altering the SFP so as to insert the requesting SF back
into the SFP and steer the flow to it.
3. Service Function Path Optimization
The packet forwarding path of a SFP involves the classifier, one or
more SFFs and all the SFs that are part of the SFP. Packets of a
flow are forwarded along this path to each of the SFs, for the life
of the flow, whether SFs perform the full function in treating the
packet or reapply the cached result, from the last application of the
function, on the residual packets of the flow. In other words, every
packet on the flow incurs the same latency and the end-to-end SFP
latency remains more or less constant subject to the nature of the
SFs involved. If an SF can be removed from the SFP, for a specific
flow, traffic steering to the SF is avoided for that flow; thus
leading to a shorter SFP for the flow. When multiple SFs in a SFP
are removed, the SFP starts to converge towards the optimum path,
which in its best case starts and terminates at the classifier itself
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without incurring any latency associated with traversing the SFP.
Although SFs are removed from the SFP, the corresponding SFC is not
changed - this is subtle but an important characteristic of this
mechanism. In other words, this mechanism does not alter the SFC and
still uses the SFP associated with the SFC.
There are two primary approaches to removing an SF from the SFP.
Namely,
o Bypass: Mechanism that alters the SFC. Described in this draft
for completeness.
o Simple Offload: Mechanism that alters the SFP alone, does not
affect the SFC. This is the primary focus of this draft.
3.1. Bypass
Many service functions do not deliver service to certain types of
traffic. For instance, typical WAN optimization service functions
are geared towards optimizing TCP traffic and add no value to non-TCP
traffic. Non-TCP traffic thus can bypass such a service function.
Even in the case of TCP, a WAN optimization SF may not be able to
service the traffic if the corresponding TCP flow is not seen by it
from inception. In such a situation a WAN optimization SF can avoid
the overhead of processing such a flow or reserving resources for it,
if it had the ability to request such flows not be steered to it. In
other words such service functions need the ability to request they
be bypassed for a specified flow from a certain time in the life of
that flow.
A seemingly simple alternative is to require service functions pre
specify the traffic flow types they add value to, such as the one-
tuple: IP protocol-type described above. A classifier built to use
such data exposed by SFs, may thus enable bypassing such SFs for
specific flows by way of selecting a different SFC that does not
contain the SF being removed.
Although knowledge of detailed SF profiles helps SFC selection at the
classifier starting the SFC, it leads to shortcomings.
o It adds to the overhead of classification at that classifier as
all SF classification requirements have to be met by the
classifier.
o It leads to conflicts in classification requirements between the
classifier and the SFs. Classification needs of different SFs in
the same SFC may vary. A classifier thus cannot classify traffic
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based on the classification of one of the SFs in the chain. For
instance, even though a flow is uninteresting to one SF on an SFC,
it may be interesting to another SF in the same SFC.
o The trigger for bypassing an SF may be dynamic as opposed to the
static classification at the classifier - it may originate at the
SFs themselves and involve the control and policy planes. The
policy and control planes may react to such a trigger by
instructing the classifier to select a different SFC for the flow,
thereby achieving SF bypass.
3.2. Simple Offload
Service delivery by a class of service functions involves inspecting
the initial portion of the traffic and determining whether traffic
should be permitted or dropped. In some service functions, such an
inspection may be limited to just the five tuple, in some others it
may involve protocol headers, and in yet others it may involve
inspection of the byte stream or application content based on the
policy specified. Firewall service functions fall into such a class,
for example. In all such instances, servicing involves determining
whether to permit the traffic to proceed onwards or to deny the
traffic from proceeding onwards and drop the traffic. In some cases,
dropping of the traffic may be accompanied with the generation of a
response to the originator of traffic or to the destination or both.
Once the service function determines the result - permit or deny (or
drop), it simply applies the same result to the residual packets of
the flow by caching the result in the flow state.
In essence, the effect of service delivery is a PERMIT or a DENY
action on the traffic of a flow. This class of service functions can
avoid all the overhead of processing such traffic at the SF, by
simply requesting another entity in the SFP, to assume the function
of performing the action determined by the service function. Since
PERMIT and DENY are very simple actions other entities in the SFP are
very likely to be able to perform them on behalf of the requesting
SF. A service function can thus offload simple functions to other
entities in the SFP.
Since SFF is the one steering traffic to the SFs and hence is on the
SFP, is a natural entity to assume the offload function. An SF not
interested in traffic being steered to it can simply perform a simple
offload by indicating a PERMIT action along with an OFFLOAD request.
The SFF responsible for steering the traffic to the SF takes note of
the ACTION and offload request. The OFFLOAD directive and the ACTION
received from the requesting SF are cached against the SF for that
flow. Once cached, residual packets on the flow are serviced by the
cached directive and action as if being serviced by the corresponding
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SF.
3.2.1. Stateful SFF
SFFs are the closest SFC infrastructure entities to the service
functions. SFFs may be state-full and hence can cache the offload
and action in both of the unidirectional flows of a connection. As a
consequence, action and offload become effective on both the flows
simultaneously and remain so until cancelled or the flow terminates.
SFFs may not always honor the offload requests received from SFs.
This does not affect the correctness of the SFP in any way. It
implies that the SFs can expect traffic to arrive on a flow, which it
offloaded, and hence must service them, which may involve requesting
an offload again. It is natural to think of an acknowledgement
mechanism to provide offload guarantees to the SFs but such a
mechanism just adds to the overhead while not providing significant
benefit. Offload serves as a best effort mechanism.
3.2.2. Packet Re-ordering
Offload mechanism creates short time-windows where packet re-ordering
may occur. While SFs request flows be offloaded to SFFs, packets may
still be in flight at various points along the SFP, including some
between the SFF and the SF. Once the offload decision is received
and committed into the flow entry at the SFF, any packets arriving
after and destined to the offloading SF are treated to the offload
decision and forwarded along (if it is a PERMIT action). Inflight
packets to the offloading SF may arrive at the SFF after one or more
packets are already treated to the offload decision and forwarded
along.
This is a transitional effect and may not occur in all cases. For
instance, if the decision to offload a flow by an SF is based on the
first packet of TCP flow, a reasonable time window exists between the
offload action being committed into the SFF and arrival of subsequent
packet of the same flow at that SFF. Likewise, request/response
based protocols such as HTTP may not always be subject to the re-
ordering effects.
3.2.3. Policy Implications
Offload mechanism may be controlled by the policy layer. The SFs
themselves may have a static policy to utilize the capability offered
by the SFC infrastructure. They could also be dynamic and controlled
by the specific policy layer under which the SFs operate.
Similarly, the SFC infrastructure, specifically the classifiers and
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the SFFs, may be under the SFC infrastructure control plane policy
controlling the decision to honor offloads from an SF. This policy
in turn may be coarse-grain, at the SF level, and hence static. It
can also be fine grain and hence dynamic but it adds to the overhead
of policy distribution.
Policy model related to offloads is out of scope of this document.
3.2.4. Capabilities Exchange
Simple offloads can be exposed and negotiated a priori as a
capability between the SFFs and the SFs or the corresponding control
layers. In the simplest of the implementations, this is provided by
the SFC infrastructure and the SFs are statically configured to
utilize them without capabilities negotiation, within the constraints
of the SF specific policies.
Capabilities exchange is outside the scope of this document.
4. Methods For SFP Optimization
There are a number of different models that may be used to facilitate
shortest SFP realization. We present two here.
The shortest SFP methods discussed in the following sections require
signaling among the participant components to communicate offload and
permit/deny actions. The signaling may be performed in the data-
plane or in the control plane.
a. Data-plane: An SFC specific communication channel is needed for
SNs to communicate the offload request along with the SF treated
packet. [NSH] defines a header specifically for carrying SFP
along with metadata and provides such a channel for use with
offloads. Necessary bits need to be allocated in NSH to convey
the action as well as the offload directive. This signaling may
be limited to SN and SFF or may continue from one SFF to another
SFF or the classifier. It may also involve signaling directly
from the SF to the classifier.
b. Control-plane: Messages are required between the SN and the
service controller as well as between the SFF and the control
plane. Service controller messaging is out of scope of this
document and it is assumed to be service controller specific.
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4.1. Hop-by-hop Offload
SNs receive traffic on an overlay from the SFF. SNs service the
traffic and turn them back to the SFF on an overlay or forward the
traffic on the underlay. In the former case, along with returning
the traffic to SFF, they can perform simple offload by signaling
OFFLOAD and ACTION to the SFF. SFF caches the OFFLOAD and ACTION
while forwarding the serviced packet onwards to the next service hop
on the SFP or dropping it. SFF can now enforce the OFFLOAD and
ACTION on the residual packets of the flow.
Additionally, SFF may choose to signal the upstream SFFs of the
OFFLOAD and ACTION received from an SF. This may continue
recursively until the first SFF is reached, which is the classifier
itself.
By performing such hop-by-hop offloads, SFP can be reduced to an
optimum one, steering traffic to only those SFs that really need to
see the traffic.
Figure 1 to Figure 3 show an example of SF and SFF performing an
offload operation and the effect thereafter on the SFP.
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SFID(1) SFID(2) SFID(3)
+------+ +------+ +------+
....| SF1 |.... ....| SF2 |.... ....| SF3 |....
. +------+ . . +------+ . . +------+ .
. | . . | . . | .
. +------+ . . +------+ . . +------+ .
. | SFF1 | . . | SFF2 | . . | SFF3 | .
. +------+ . . +------+ . . +------+ .
. | . . | . . | . +-+
+----+ . +------+ . . +------+ . . +------+ . |N|
| CF |------| NF1 |-----------| NF2 |-----------| NF3 |------|e|
+----+ . +------+ . . +------+ . . +------+ . |t|
. . . . . . +-+
SFP1 ... ..... ..... ... >
Service Function Chain, SFC1 = {SF1, SF2, SF3}
where SF1, SF2 and SF3 are three service functions.
Service Function Path SFP1 is the SFP for SFC1.
Classifier CF starts SFP1 based on policy.
Figure 1: SFC1 with corresponding SFP1
O
f
SFID(1) f +- SFID(2) SFID(3)
+------+ l | +------+ +------+
....| SF1 |.... o | | SF2 | ....| SF3 |....
. +------+ . a | +------+ . +------+ .
. | . d | | . | .
. +------+ . v +------+ . +------+ .
. | SFF1 | . ....| SFF2 |.... . | SFF3 | .
. +------+ . . +------+ . . +------+ .
. | . . | . . | . +-+
+----+ . +------+ . . +------+ . . +------+ . |N|
| CF |------| NF1 |-----------| NF2 |-----------| NF3 |-----|e|
+----+ . +------+ . . +------+ . . +------+ . |t|
. . . . . . +-+
SFP1 ... ..... ..... ... >
Figure 2: SFP1 after SFID(2) performs an Offload
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O
SFID(1) f SFID(2) SFID(3)
+------+ f +------+ +------+
....| SF1 |.... l | SF2 | ....| SF3 |....
. +------+ . o +------+ . +------+ .
. | . a | . | .
. +------+ . d +------+ . +------+ .
. | SFF1 | <-------- | SFF2 | . | SFF3 | .
. +------+ . +------+ . +------+ .
. | . | . | . +-+
+----+ . +------+ . +------+ . +------+ . |N|
| CF |------| NF1 |-----------| NF2 |-----------| NF3 |------|e|
+----+ . +------+ . +------+ . +------+ . |t|
. . . . +-+
SFP1 ... ........................ ... >
Figure 3: SFP1 after SFF2 propagates the Offload upstream to SFF1
4.1.1. Progression Of SFP Optimization
SFP optimization happens at two levels:
Level-1: Collapsing of the SFF-to-SF hops into the SFF or the SFC
infrastructure.
Level-2: Reduction of the (collapsed) SFP within the SFC
infrastructure to the shortest possible.
Figure 1 to Figure 3 show one sequence of offload events leading to a
shorter SFP. Although not shown in these figures, further offload
events ultimately lead to an optimum SFP.
Below steps show one such sequence of offload events that lead to
such an optimum SFP.
Stage-1: Prior to any offloads, service function path SFP1
(corresponding to SFC1) has the following actual forwarding
path as shown in Figure 1:
CF ->
NF1 -> SFF1 -> SF1 -> SFF1 -> NF1 ->
NF2 -> SFF2 -> SF2 -> SFF2 -> NF2 ->
NF3 -> SFF3 -> SF3 -> SFF3 -> NF3 ->
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Stage-2: After SF2 performs a simple offload, which is signaled to
upstream SFFs - SFF1, Classifier, SFP1 forwarding path
changes to the below as shown in Figure 3:
CF ->
NF1 -> SFF1 -> SF1 -> SFF1 -> NF1 ->
NF3 -> SFF3 -> SF3 -> SFF3 -> NF3 ->
Stage-3: After SF3 performs simple offload, which is signaled to
upstream SFFs - SFF2, SFF1, and Classifier, SFP1 forwarding
path changes to the below (figure not shown) />:
CF ->
NF1 -> SFF1 -> SF1 -> SFF1 -> NF1 ->
Stage-4: After SF1 performs simple offload, which is signaled to
upstream SFFs - Classifier, SFP1 forwarding path changes to
the below (figure not shown):
CF ->
4.2. Service Controller Offload
Each SN signals the service controller of the OFFLOAD and ACTION via
control plane messaging for a specific flow. The service controller
then signals the appropriate SFFs to offload the requested SFs, there
by achieving the hop-by-hop offload behavior.
The service controller has full knowledge of all the SFs of the SFP
offloading the flow and hence can determine the optimum SFP within
the Service Controller and program the appropriate SFFs to achieve
SFP optimization.
5. Offload Data-plane Signaling
Since Offload and action are signaled at the time of returning the
traffic to SFF, post servicing the traffic, such signaling can be
integrated into the service header of the packet. Figure 4 shows the
bits necessary to achieve the signaling using the SFC encapsulation
as described in [I-D.quinn-sfc-nsh].
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Base Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Path Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Platform Context |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F|D| Network Shared Context |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Platform Context |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Shared Context |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Optional Variable Length Context Headers ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
F : Offload bit; indicates offload request when F is set to 1
D : Drop (or Deny) bit, drop when D=1 and permit when D=0;
Drop bit is valid only when F is set to 1
Figure 4: NSH Offload and ACTION Bits
Although SFs can signal SFFs by piggy backing on the serviced packet,
SFFs cannot signal in a similar fashion. This is because the traffic
is forwarded along the SFP to the next or downstream SFF. The
offload signaling has to go to the upstream SFFs, in the opposite
direction. SFFs have a choice: perform out-of-band signaling towards
the upstream SFFs or wait until traffic arrives on the opposite flow
on the reverse SFP, where SFPs are symmetric. SFFs can then
piggyback the offload signaling on the reverse traffic towards the
upstream SFFs. The actual method employed to signal offload between
the SFFs is implementation specific.
6. Acknowledgements
The authors would like to thank Nagaraj Bagepalli for his review
comments.
7. IANA Considerations
This memo includes no request to IANA.
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8. Security Considerations
Security of the offload signaling mechanism is very important. This
document does not advocate any additional security mechanisms other
the data plane and control plane signaling security mechanisms.
9. References
9.1. Normative References
[I-D.ietf-sfc-architecture]
Halpern, J. and C. Pignataro, "Service Function Chaining
(SFC) Architecture", draft-ietf-sfc-architecture-02 (work
in progress), September 2014.
[I-D.quinn-sfc-nsh]
Quinn, P., Guichard, J., Fernando, R., Surendra, S.,
Smith, M., Yadav, N., Agarwal, P., Manur, R., Chauhan, A.,
Elzur, U., Garg, P., McConnell, B., and C. Wright,
"Network Service Header", draft-quinn-sfc-nsh-03 (work in
progress), July 2014.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9.2. Informative References
[I-D.ietf-sfc-problem-statement]
Quinn, P. and T. Nadeau, "Service Function Chaining
Problem Statement", draft-ietf-sfc-problem-statement-10
(work in progress), August 2014.
[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022,
January 2001.
[RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers", RFC 6146, April 2011.
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Authors' Addresses
Surendra Kumar
Cisco Systems, Inc.
170 W. Tasman Dr.
San Jose, CA 95134
Email: smkumar@cisco.com
Jim Guichard
Cisco Systems, Inc.
Email: jguichar@cisco.com
Paul Quinn
Cisco Systems, Inc.
Email: paulq@cisco.com
Joel Halpern
Ericsson
Email: joel.halpern@ericsson.com
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