Architecture for Chaining Legacy Layer 4-7 Service Functions
draft-dunbar-sfc-legacy-l4-l7-chain-architecture-00
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| Last updated | 2013-10-16 | ||
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draft-dunbar-sfc-legacy-l4-l7-chain-architecture-00
Network working group L. Dunbar
Internet Draft Huawei
Intended status: Informational I. Smith; S. Majee
Expires: January 2014 F5 Networks
N. So
Tata Communications
Donald Eastlake
Huawei
October 16, 2013
Architecture for Chaining Legacy Layer 4-7 Service Functions
draft-dunbar-sfc-legacy-l4-l7-chain-architecture-00.txt
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Abstract
This draft analyzes the issues associated with chaining existing
Layer 4-7 service functions that are not aware of service
encapsulation layers. This draft also examines the network
architecture for chaining existing L4-L7 service functions. The
intent is to identify and describe gaps that have not been
addressed by other SFC drafts.
Conventions used in this document
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 0.
The term "traffic steering" and "traffic forwarding" are used
interchangeably in this draft.
Table of Contents
1. Introduction.................................................3
2. Terminology..................................................3
3. Layer 4-7 Service Functions and Chaining.................... 4
3.1. Layer 4-7 Service Functions ............................4
3.2. Service Functions for Wireless network ................4
3.3. Proxy Node for Layer 4-7 Service Functions ...........5
4. Architecture for Chaining Legacy Layer 4-7 Service Functions.7
4.1. L4-L7 nodes connection to Service Chain Proxy Nodes ....7
4.2. Traffic Steering at Proxy Nodes.........................7
5. Challenges of Chaining L4-L7 Service Function ...............9
5.1. Challenge of Multiple Instances of a Service Function ..9
5.2. Challenges of Layer 4-7 traffic Steering ...............10
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5.3. Challenges of Service Chain Classification .............11
6. Challenge of Service Chain from the Layer 7 Perspective .....12
7. Conclusion and Recommendation ............................. 13
8. Manageability Considerations .............................. 13
9. Security Considerations ................................... 13
10. IANA Considerations ...................................... 13
11. Acknowledgments.......................................... 14
12. References............................................... 14
Authors' Addresses........................................... 15
Intellectual Property Statement ................................15
Disclaimer of Liability ...................................... 16
1. Introduction
This draft focuses on the issues associated with chaining common
Layer 4-7 Service functions that are not aware of new service
encapsulation layers. Some examples of service function chaining
in the mobile network environment are studied in this draft. The
intent is to demonstrate gaps that haven't been addressed by
other Service Function Chaining (SFC) drafts.
There have been many drafts submitted on the topic of SFC.
However, a number of issues associated with chaining existing
Layer 4-7 service functions that don't support Service
Encapsulation header have not been described well.
2. Terminology
DPI Deep Packet Inspection
FW Firewall
Layer 4-7 Service Function: a functional module that performs
certain action to the packets belonging to an end-to end flow.
The action can change the behavior of the packet flow and/or the
packets themselves, including the packet header and packet
payload. Exemplary functional modules include Firewall, Deep
Packet Inspection (DPI), Encryption, Packet De-duplication,
Compression, TCP Acceleration, NAT, and etc. They are similar to
the Service Functions defined in [SFC-Problem], except they are
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not aware of any new service header encapsulations. Many of
existing Layer 4-7 service functions fall into this category.
Layer 4-7 service functions can be instantiated on a standalone
physical or virtual device, which is called "Service Node" by
[SFC-Problem]. Layer 4-7 functions can also be embedded in
another device, such as router/switch or other devices.
3. Layer 4-7 Service Functions and Chaining
3.1. Layer 4-7 Service Functions
A Layer 4-7 service function performs certain action to the
packets belonging to an end-to end flow. The implementation of
such service function can be either Proxy based or Packet Based,
or a hybrid of both when more than one function is performed to
the same packet flow. Multiple service functions can be
instantiated on a single service node as defined by [SFC-ARCH],
or embedded in a L2/L3 network node.
- Proxy based service functions: these service functions
terminate original packets, may reassemble multiple packets,
reopen a new connection, or formulate new packets based on
the received packets.
- Packet based service functions: these service functions
maintain original packets, i.e. they don't make changes to
packets traversed through except possibly to metadata or
packet outer header fields.
3.2. Service Functions for Wireless network
[SFC-MobileNetwork] and [SFC-use-cases] have provided examples of
service chain in mobile network. In particular, the P-GW/PCEF
determines the desired service treatment, i.e. desired sequence
of service functions, to specific flows based on the policies
from PCRF.
Here are some of the issues of service chain in wireless network:
- large number of permutations of service functions to be
chained together
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- The sequence of services functions applied to selective
flows can change. New service functions may need to be
inserted; existing service functions may need to be removed
or changed.
- The criteria for applying service functions can take
combination of application classification, packet headers,
and/or other factors. For example, the factors that have
direct association with the packet flow include packet
source address, destination address, TCP ports. The factors
that do not have association with the packet flow include
user/source location, account status, time of day, network
condition, and etc.
| Mobile backhaul Network
+-----+ | +---+---+
|PCRF | | |Network|
| | < ---- > |Ctrller|
+-----+ | +----+--+
| |
| |
+---------+ | +--------+ +----+ +---------+
-- > | P-GW | --> |LB |---| FW |--> | Web | ------>
| | | | | | | | Proxy |
--->| | | +--------+ +----+ +---------+
--->| | | +---------+ +----+
-- > | | --> |Video |---| FW |--> ----------- ------>
| [PCEF]| | |Optimizer| | |
| | | +---------+ +----+
--->| | | +--------+ +-----+
-- > | | --> |SBC |---| ACL |--> ----------- ------>
| | | | | | |
+---------+ | +--------+ +-----+
Figure 1: Service Chain for Mobile Network
3.3. Proxy Node for Layer 4-7 Service Functions
Section 3.4 of [NSC-Header] describes a scenario that the service
nodes are not aware of service encapsulation header. The term
"proxy node" is introduced by [NSC-header] as a gateway to remove
and insert service layer encapsulation on behalf of service nodes
that are not aware service layer encapsulation.
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Even though Layer 4-7 Service nodes can be instantiated anywhere
in the network, it is not uncommon to have more than one service
nodes located in close vicinity, or one to two links away from
the service chain's "proxy node". The following figure depicts
typical network architecture for chaining those Layer 4-7 service
nodes that are not aware of service layer encapsulation.
|1 ----- |n |21 ---- |2m
+---+---+ +---+---+ +-+---+ +--+-----+
| Ad | |Content| |Video| |Security|
|Insert | | Opt | | Opt | | App |
+---+---+ +---+---+ +--+--+ +--+--+--+
: : : : :
: : : : :
\ / \ /
+--------------+ +--------+ +---------+
-- > | Chain | --> | Proxy |---------> | Proxy | ------>
|classification| |Node-1 | | Node-i |
+--------------+ +----+---+ +----+--+-+
|-- | |
V V +--->
+--------+
| Proxy |
| -j |----->
+--------+
Figure 2: Chaining existing Layer 4-7 service nodes
When there are more than one service functions or nodes located
in the vicinity of the service chain "proxy nodes", the Proxy
nodes need to steer designated traffic to the designated service
functions.
The service chain proxy nodes can be standalone devices, or can
be embedded with routers or switches in the network. Overlay
tunnels are expected to connect the "proxy nodes" together.
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4. Architecture for Chaining Legacy Layer 4-7 Service Functions
4.1. L4-L7 nodes connection to Service Chain Proxy Nodes
L4-L7 Service nodes can be connected to Service Chain proxy nodes
in various ways. The topology could be bump in a wire or one
armed topology.
- A service function can be embedded in a Service Chain proxy
node (i.e. embedded in a router or a switch).
In this case, the service function doesn't need an address
to receive data packets. The forwarding entity can send
packets that meet the steering criteria directly to the
service function regardless of the destination addresses in
the packets. The Service function always sends the processed
packets back to the forwarding entity regardless of the
destination addresses in the packets.
- A service node can be one hop away from a service chain
proxy node
The one hop between the Service Chain proxy node and the
service node can be a physical link (e.g. Ethernet link).
Under this scenario, there would be a Link Header, i.e. an
outer MAC header, added to the data packets that meet the
steering criteria.
The one hop link can be a transparent link, i.e. no link
address is added to the data packets on the link between the
Service Chain proxy node and Service node. I.e. the service
nodes are set to promiscuous mode that can apply treatment
to data frames arrived at the ingress port regardless of the
Link Destination address. This scenario is considered the
same as a service function being embedded in the Service
Chain proxy node.
- A service node can be multiple hops away, such as when a
service is deployed in an on-net or private *aaS offering.
4.2. Traffic Steering at Proxy Nodes
Some service chains may require steering to Layer 4-7 functions
based on Layer 2 or 3 fields of the data packets. Some service
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chains may require steering traffic to Layer 4-7 service
functions based on some higher layer fields in the data packets,
i.e. Layer 4 to Layer 7 fields.
The policies for Proxy Nodes to steer traffic to its connected
service functions or service function instances can change. Those
policies can be carried by the "metadata" introduced by [SFC-
ARCH], or can be dynamically updated by network controller to the
proxy nodes.
There are multiple types of traffic steering:
- Fixed header based forwarding: traffic steering based on
header fields that have fixed position in the data
packets:
?Forwarding based on Layer 2-3 header fields, such as
MAC or IP Destination Address, Source Addresses, MPLS
label, VLAN ID, or combination of multiple Layer 2-3
header fields.
?Forwarding based on Layer 4 header (TCP or UDP).
?QoS header based forwarding.
- Layer 7 based forwarding: traffic steering (or forwarding)
based on the payload (L7) of data packets.
Multiple data packets may carry some meaningful data, like
one HTTP message. Under this scenario, multiple data
packets have to be examined before meaningful data can be
extracted for making Layer 7 based forwarding decision.
- Metadata based steering: traffic steering (or forwarding)
based on the identity of the initiating user, the UE model
or type, the site name or FQDN, or network conditions
(congestion, utilization, etc.).
However those metadata might not necessarily be carried by
each data packet due to extended bits required that can
cause high probability of packet fragmentation. Those
metadata can be dynamically passed down to steering nodes
in some forms of steering policies from network
controller(s).
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5. Challenges of Chaining L4-L7 Service Function
From user's perspective, the service chain is a sequence of
service functions, such as Chain#1 {s1, s4, s6}, Chain#2{s4, s7}
applied to a data flow. A flow is loosely used in this document
to refer to a selective of packets that meet certain criteria,
e.g. IP flows, 5 tuple based flow or a HTTP transaction. Some
users might not care at which points in the network the selected
flow is steered to those service functions as long as the
sequence of the service functions is correct.
5.1. Challenge of Multiple Instances of a Service Function
From the traffic flow perspective, a Service Chain guarantees
that specific data flows go through a specific sequence of
service functions' instances.
For each service function, there could be multiple instances,
with some in close proximity and others far apart being connected
by different service chain proxy nodes. With Network Function
Virtualization (NFV) initiative, more and more service functions
have virtualized instances instantiated on servers or virtual
machines. NFV imposes higher chance of service function instances
being changed due failure or load shifting.
When instances change, the management system or the Service Chain
controller can update the steering policies to proxy nodes,
dynamically, to steer the selected flows to different instances.
Alternatively, a proxy node may employ a method for selecting a
particular service instance for a flow or group of related flows.
The selection could be stateful like basic round-robin,
connection count etc. or stateless e.g hash based or random.
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|
+---+------+ +---+---+ +--+-----+
|controller| |Service| |Service |
| | |Func-1 | |Func- m |
+---+------+ +----+--+ +--+--+--+
/ \ \ : /
/ \ +-------------+ : /
/ \ \ : /
+--------------+ +--------+ +---------+
-- > | Service | --> |Proxy |---------> | Proxy | ------>
| node | |Node-1 | | Node-2 |
+--------------+ +--------+ +---------+
Figure 3: Service Chain from Traffic Steering Point of View
Some service functions make changes to data packets, such as NAT
changing the address fields. If any of those fields are used in
traffic steering along the service chain, the criteria can be
different before and after those the service functions.
5.2. Challenges of Layer 4-7 traffic Steering
Very often the criteria for steering flows to service functions
are based on higher layer headers, such as TCP header, HTTP
header, etc.
Most of deployed switches/routers are very efficient in
forwarding packets based on Layer 2 or Layer 3 headers, such as
MAC/IP destination addresses, or VLAN/MPLS labels but have
limited capacity for forwarding data packets based on higher
layer header. As of today, differentiating data packets based on
higher layer headers depends on ACLs (Access Control List field
matching) or DPI, both of which are relatively expensive and
extensive use of such facilities may limit the bandwidth of
switches/routers.
The Service Chain classification node introduced by [Boucadair-
framework] and [SFC-ARCH] can alleviate the workload on large
number of nodes in the network, including service chain proxy
nodes, from steer traffic based on higher layer fields.
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|1 ----- |n |21 ---- |2m
+---+---+ +---+---+ +-+---+ +--+-----+
| Ad | |Content| |Video| |Security|
|Insert | | Opt | | Opt | | App |
+---+---+ +---+---+ +--+--+ +--+--+--+
: : : : :
: : : : :
\ / \ /
+--------------+ +--------+ +---------+
-- > | Chain | --> | Proxy |---------> | Proxy | ------>
|classification| |Node-1 | | Node-2 |
+--------------+ +--------+ +---------+
Figure 4: Service Chain Marking At Ingress
A Service Chain Classification node can associate a unique
Service Chain Label (e.g. Layer 2 or 3 Label) to the packets in
the flow. Such a Layer 2 or 3 Label makes it easier for
subsequent nodes along the flow path to steer the flow to the
service functions specified by the flow's service chain.
The network elements that have the Service Chain Classification
Function are most likely network ingress edge nodes, such as
Wireless Packet Gateway, Broadband Network Gateways, Cell Site
Gateways, etc.
In some situations, like service chain for wireless subscribers,
many flows (i.e. subscribers) have common service chain
requirements. Under those situations, the Service Chain
classification Functional can mark multiple flows with the same
service chain requirement using the same Layer 2 or 3 Label,
which effectively aggregates those flows into one service chain.
For service chains that are shared by a great number of flows,
they can be pre-provisioned. For example, if VLAN ID=10 is the
service chain that need to traverse "Service-1" at Proxy Node #1
and "Service-3" at Proxy Node #2, the steering policy for VLAN
ID=10 can be dynamically changed by controllers.
5.3. Challenges of Service Chain Classification
The policy for associating flows with their service chains can be
complicated and could be dynamic due to different behavior
associated with chains, balancing load among multiple instances
for one service function, and instance failure.
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For a chain of {FW, Header_enrichment, smart_node, Video_opt,
Parental Control}, the video optimizer really needs to work on
the response path. It may also use completely different
encapsulation e.g. ICAP for example. There could be Smart-Node to
further classify a particular part of the flow and bypass
something, say the video_opt. Therefore, the classification done
by the service chain classification nodes at the network entrance
can't completely dictate the exact sequence of service functions.
The Service Chain Classification node can encounter flows that
don't match with any policies. There is a default policy that
applies all statutorily required policies to the unknown flows.
Multiple flows can share one service chain. The criteria to
select flows to be associated with their service chain could be
different. For example, for one service chain "A" shared by Flow
X, Y, Z:
- Criteria for Flow X to the Service Chain "A" are TCP port
- Criteria for Flow Y to the Service Chain "A" are Destination
Address
- Criteria for Flow Z to the Service Chain "A" are MPLS label.
6. Challenge of Service Chain from the Layer 7 Perspective
From the Layer 7 perspective, the service chain can be much more
complex. As shown in the figure below, the service functions to
be chained can depend on the HTTP message request and reply. The
service chain classification nodes may have to examine the whole
HTTP message to determine the specific sequence of service
functions for the flows. The HTTP message might have to be
extracted from multiple data packets. Sometimes, the logic to
steer traffic to chain of service functions might depend on the
data retrieved from a database based on messages constructed from
packets. The decision may depend on the HTTP response rather than
the request, or it may depend on a particular sequence of
request-response messages. The message handler may also alter the
Layer 7 service chain based on hints or modification done by
previous service function. HTTP based service function may insert
HTTP header to add further criterion for service selection in the
next round of classification.
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+----------+
Client --------->( Layer 7 )---------> Internet
<---------( Message )<---------
( Handler )
_______( )________
/ +----------+ \
/ / \ \
|1 |2 |3 |4
+---+---+ +---+---+ +-+---+ +--+-----+
| Ad | |Content| |Video| |Security|
|Insert | | Opt | | Opt | | App |
+---+---+ +---+---+ +--+--+ +--+--+--+
: : : : :
: : : : :
Figure 5: Layer 7 Service Chain Complexity
7. Conclusion and Recommendation
There are many service functions being deployed already in the
network. Many of them are not capable to adapt to new service
chain encapsulation layer.
This document provides architecture framework for chaining those
Layer 4-7 service functions that are not aware of new service
layer encapsulation.
8. Manageability Considerations
There currently exists no single management methodology to
control the L2-4 packet-based forwarding device, the L4-7 service
delivery device, and the L7+ application server. Such unified
management of configuration state is required for service
function chaining to be a practical solution.
9. Security Considerations
TBD.
10. IANA Considerations
This document requires no IANA actions. RFC Editor: Please remove
this section before publication.
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11. Acknowledgments
This draft has taken input from "Application Layer SDN"
presentation given by John Giacomoni of F5 at Layer 123
conference. Thanks to Huang Shi Bi and Li Hong Yu for the
valuable comments and suggestions.
This document was prepared using 2-Word-v2.0.template.dot.
12. References
[Boucadair-framework] M. Boucadair, et al, "Differentiated
Service Function Chaining Framework", < draft-
boucadair-service-chaining-framework-00>; Aug 2013
[SFC-Problem] P. Quinn, et al, "Network Service Chaining Problem
statement", <draft-quinn-nsc-problem-statement-03>,
Aug, 2013
[SFC-Arch] P. Quinn, et al, "Service Function Chaining (SFC)
Architecture", < draft-quinn-nsc-arch-00>, Sept 2013
[NSH-Header] P. Quinn, et al, "Network Service Header", <
draft-quinn-nsh-01>, July 12, 2013
[SC-MobileNetwork] W. Haeffner, N. Leymann, "Network Based
Services in Mobile Network", IETF87 Berlin, July 29 2013
[Application-SDN] J. Giacomonni, "Application Layer SDN", Layer
123 ONF Presentation, Singapore, June 2013
[SC-Use-Case] Liu, et, al., "Service Chaining Use Cases", <
draft-liu-service-chaining-use-cases-00>, Sept, 2013
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Authors' Addresses
Linda Dunbar
Huawei Technologies
1700 Alma Drive, Suite 500
Plano, TX 75075, USA
Phone: (469) 277 5840
Email: ldunbar@huawei.com
Ian Smith
F5 Networks
Email: I.Smith@F5.com
Sumandra Majee
F5 Networks
Email: S.Majee@F5.com
Ning So
Tata Communications
Email: Ning.So@tatacommunications.com
Donald Eastlake
Huawei Technologies
155 Beaver Street
Milford, MA 01757 USA
Phone: 1-508-333-2270
Email: d3e3e3@gmail.com
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