SFC R. Penno
Internet-Draft C. Pignataro
Intended status: Standards Track C. Yen
Expires: March 26, 2016 E. Wang
K. Leung
Cisco Systems
September 23, 2015
Packet Generation in Service Function Chains
draft-penno-sfc-packet-01
Abstract
Service Functions (e.g., Firewall, NAT, Proxies and Intrusion
Prevention Systems) generate packets in the reverse flow direction to
the source of the current in-process packet/flow. In this document
we discuss and propose how to support this required functionality
within the SFC framework.
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].
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 March 26, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3
3. Definitions and Acronyms . . . . . . . . . . . . . . . . . . 3
4. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 3
5. Service Function Behavior . . . . . . . . . . . . . . . . . . 6
5.1. SF receives Reverse Forwarding Information . . . . . . . 6
5.2. SF requests SFF cooperation . . . . . . . . . . . . . . . 7
5.2.1. OAM Header . . . . . . . . . . . . . . . . . . . . . 7
5.2.2. Service Function Forwarder Behavior . . . . . . . . . 8
5.2.3. Reserved bit . . . . . . . . . . . . . . . . . . . . 9
5.3. Classifier Encodes Information . . . . . . . . . . . . . 9
5.3.1. Symmetric Service Paths . . . . . . . . . . . . . . . 10
5.3.2. Analysis . . . . . . . . . . . . . . . . . . . . . . 13
5.4. Reversed Path derived using Forward Path ID and Index
Method . . . . . . . . . . . . . . . . . . . . . . . . . 14
6. Asymmetric Service Paths . . . . . . . . . . . . . . . . . . 16
7. Other solutions . . . . . . . . . . . . . . . . . . . . . . . 18
8. Implementation . . . . . . . . . . . . . . . . . . . . . . . 19
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
10. Security Considerations . . . . . . . . . . . . . . . . . . . 19
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
12. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
13.1. Normative References . . . . . . . . . . . . . . . . . . 19
13.2. Informative References . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
Service Functions (e.g., Firewall, NAT, Proxies and Intrusion
Prevention Systems) generate packets in the reverse flow direction
destined to the source of the current in-process packet/flow. This
is a basic intrinsic functionality and therefore needs to be
supported in a service function chaining deployment.
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2. Problem Statement
The challenge of this functionality in service chain environments is
that generated packets need to traverse in the reverse order the same
Service Functions traversed by original packet that triggered the
packet generation.
Although this might seem to be a straightforward problem, on further
inspection there are a few interesting challenges that need to be
solved. First and foremost a few requirements need to be met in
order to allow a packet to make its way through back to its source
through the service path:
o A symmetric path ID needs to exist. Symmetric path is discussed
in [SymmetricPaths]
o The SF needs to be able encapsulate such error or proxy packets in
a encapsulation transport such as VXLAN-GPE
[I-D.ietf-nvo3-vxlan-gpe] + NSH header [I-D.ietf-sfc-nsh]
o The SF needs to be able to determine, directly or indirectly, the
symmetric path ID and associated next service-hop index or
indicate reverse path for the service path ID in the original
packet (TBD: verify or part)
3. Definitions and Acronyms
The reader should be familiar with the terms contained in
[I-D.ietf-sfc-nsh] ,[I-D.ietf-sfc-architecture] and
[I-D.ietf-nvo3-vxlan-gpe]
4. Assumptions
We make the following assumption thorughout this document
1. An SF could be connected to more than one SFF directly. In other
words, a SF can be multi-homed and each connection can use
different encapsulations.
2. After forwarding a packet to an SF, the SFF always has
connectivity to the next hop SFF to complete the path. This
means the following scenario is not possible (SFF2 cannot
complete the forward path which contains SFF3 and potentially SFs
connected to SFF3
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.-. .-.
/ \ / \
( SF1 ) ( SF2 )
\ / \ / \
`+' `+' \
| | \
| | \
+--+---+ +--+---+ \+------+
...---+ SFF1 +------+ SFF2 | | SFF3 +---...
+------+ +--+---+ +------+
|
|
+-----...
RSFP Forward -> SFF1 : SF1 : SFF1 : SFF2 : SF2 : SFF3 : ...
3. In the figure below, if SF2 is directly connected to SFF2A and
SFF2B, there could be a case that SFF2A only has the forwarding
rules for the forward path, and SFF2B only has the forwarding
rules for the reverse path
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.-. .-. .-.
/ \ / \ / \
( SF1 ) ( SF2 ) ( SF3 )
\ /\ \ /\ \ /\
`+' \ `+' \ `+' \
| \ | \ | \
| | | | | |
+---+---+ | +-------+ | +---+---+ |
...---+ SFF1A +-|-----+ SFF2A +-|-----+ SFF3A +-|---...
+-------+ | +-------+ | +-------+ |
| | |
+---+---+ +---+---+ +---+---+
...---+ SFF1B +-------+ SFF2B +-------+ SFF3B +-----...
+-------+ +-------+ +-------+
Symmetric Paths:
RSFP Forward -> SFF1A : SF1 : SFF1A : SFF2A : SF2 :
SFF2A : SFF3A : SF3 : SFF3A ...
RSFP Reverse <- SFF1B : SF1 : SFF1B : SFF2B : SF2 :
SFF2B : SFF3B : SF3 : SFF3B
Asymmetric Paths (skipping SF2 on reverse):
RSFP Forward -> SFF1A : SF1 : SFF1A : SFF2A : SF2 : SFF2A :
SFF3A : SF3 : SFF3A ...
RSFP Reverse <- SFF1B : SF1 : SFF1B : SFF2B :
SFF3B : SF3 : SFF3B
4.
Assumption #2 allows an SF to always bounce a packet back to the SFF
where the packet came from.
Due to #3, an SF has to determine which SFF to send the generated
packet to. It cannot treat generated packet the same way as
forwarded packet, as in #2.
These assumptions make sense for certain implementation. However,
some implementations may not have the constraints in #3, which will
simplify the SF logic in handling generated traffic. The 3
assumptions can be illustrated below. The SFF "A"s only have
knowledge for the forward path, and SFF "B"s only have knowledge for
the reverse path. When SF2 generates a packet in the reverse
direction, SF2 must determine which SFF ('A' or 'B') to send to.
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5. Service Function Behavior
When a Service Function wants to send packets to the reverse
direction back to the source it needs to know the symmetric service
path ID (if it exists) and associated service index. This
information is not available to Service Functions since they do not
need to perform a next-hop service lookup. There are four
recommended approaches to solve this problem and we assume different
implementations might make different choices.
1. The SF can receive service path forwarding information in the
same manner a SFF does.
2. The SF can send the packet in the forward direction but set
appropriate bits in the NSH header requesting a SFF to send the
packet back to the source
3. The classifier can encode all information the SF needs to send a
reverse packet in the metadata header
4. The controller uses a deterministic algorithm when creating the
associated symmetric path ID and service index.
We will discuss the ramifications of these approaches in the next
sections.
5.1. SF receives Reverse Forwarding Information
This solution is easy to understand but brings a change on how
traditionally service functions operate. It requires SFs to receive
and process a subset of the information a SFF does. When a SF wants
to send a packet to the source, the SF uses information conveyed via
the control plane to impose the correct NSH header values.
Advantages:
o Changes are restricted to SF and controller, no changes to SFF
o Incremental deployment possible
o No protocol between SF and SFF, which avoids interoperability
issues
o No performance penalty on SFF due to in or out-of-band protocol
Disadvantages:
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o SFs need to process and understand Rendered Service Path messages
from controller
This solution can be characterized by putting the burden on the SF,
but that brings the advantage of being self-contained (as well as
providing a mechanism for other features). Also, many SFs have
policy or classification function which in fact makes them a
classifier and SF combination in practice.
5.2. SF requests SFF cooperation
These solutions can be characterized by distributing the burden
between SF and SFF. In this section we discuss two possible in-band
solutions: using OAM header and using a reserved bit 'R' in the NSH
header.
5.2.1. OAM Header
When the SF needs to send a packet in the reverse direction it will
set the OAM bit in the NSH header and use an OAM protocol
[I-D.penno-sfc-trace] to request that the SFF impose a new, reverse
path NSH header. Post imposition, the SFF forwards the packet
correctly.
SF Reverse Packet Request
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+\
|Ver|1|C|R|R|R|R|R|R| Length | MD-type=0x1 | OAM Protocol | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Service Path ID | Service Index | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Mandatory Context Header | |S
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |F
| Mandatory Context Header | |C
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Mandatory Context Header | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Mandatory Context Header | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ <
|Rev. Pkt Req | Original NSH headers (optional) | |O
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |A
|M
/
(postamble)
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Ver: 1
OAM Bit: 1
Length: 6
MD-Type: 1
Next Protocol: OAM Protocol
Rev. Pkt Req: 1 Reverse packet request
Advantages:
o SF does not need to process and understand control plane path
messages.
o Clear division of labor between SF and SFF.
o Extensible
o Original NSH header could be carried inside OAM protocol which
leaves metadata headers available for SF-SFF communication.
Disadvantages:
o SFFs need to process and understand a new OAM message type
o Possible interoperability issues between SF-SFF
o SFF Performance penalty
5.2.2. Service Function Forwarder Behavior
In the case where the SF has all the information to send the packet
back to the origin no changes are needed at the SFF. When an SF
requests SFF cooperation the SFF MUST be able to process the OAM
message used to signal reverse path forwarding.
o Process/decode OAM message
o Examine and act on any metadata present in the NSH header
o Examine its forwarding tables and find the reverse path-id and
index of the next service-hop
The reverse path can be found in the Rendered Service Path Yang model
[RSPYang] that conveyed to the SFF when a path is constructed.
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If a SFF does not understand the OAM message it just forwards the
packet based on the original path-id and index. Since it is a
special OAM packet, it tells other SFFs and SFs that they should
process it differently. For example, a downstream intrusion
detection SF might not associate flow state with this packet.
5.2.3. Reserved bit
In this solution the SF sets a reversed bit in the NSH that carries
the same semantic as in the OAM solution discussed previously. This
solution is simpler from a SF perspective but requires allocating one
of the reserved bits. Another issue is that the metadata in the
original packet might be overwritten by SFs or SFFs in the path.
When a SFF receives a NSH packet with the reversed bit set, it shall
look up a preprogrammed table to map the Service Path ID and Index in
the NSH packet into the reverse Service Path ID and Index. The SFF
would then use the new reverse ID and Index pair to determine the SF/
SFF which is in the reverse direction.
Advantages:
o No protocol header overhead
o Limited performance impact on SF
Disadvantages:
o Use of a reserved bit
o SFF Performance penalty
o Not extensible
5.3. Classifier Encodes Information
This solution allows the Service Function to send a reverse packet
without interactions with the controller or SFF, therefore it is very
attractive. Also, it does not need to have the OAM bit set or use a
reserved bit. The penalty is that for a MD Type-1 packet a
significant amount of information (48 bits) need to be encoded in the
metadata section of the packet and this data can not be overwritten.
Ideally this metadata would need to be added by the classifier.
The Rendered Service Path yang model [RSPYang] already provides all
the necessary information that a classifier would need to add to the
metadata header. An explanation of this method is better served with
an examples.
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5.3.1. Symmetric Service Paths
The figure below shows a simple SFC with symmetric service paths
comprising three SFs.
(preamble)
.....................SFP2 Forward........................>
Forward SI 253 252 251
+---+ .-. .-. .-. +---+
| | / \ / \ / \ | |
| A +-------( SF1 )------( SF2 )------( SF3 )----------+ B |
| | \ / \ / \ / | |
+---+ `-' `-' `-' +---+
Reverse SI 253 254 255
<....................SFP3 (Reverse of SFP2).........................
SFP2 Forward -> SF1 : SF2 : SF3
SFP3 Reverse <- SF1 : SF2 : SF3
RSP2 Forward -> SF1 : SF2 : SF3
RSP3 Reverse <- SF1 : SF2 : SF3
Figure 1: SFC example with symmetric path
Below we see the JSON objects of the two symmetric paths depicted
above.
RENDERED_SERVICE_PATH_RESP_JSON = """
{
"rendered-service-paths": {
"rendered-service-path": [
{
"name": "SFC1-SFP1-Path-2-Reverse",
"transport-type": "service-locator:vxlan-gpe",
"parent-service-function-path": "SFC1-SFP1",
"path-id": 3,
"service-chain-name": "SFC1",
"starting-index": 255,
"rendered-service-path-hop": [
{
"hop-number": 0,
"service-index": 255,
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"service-function-forwarder-locator": "eth0",
"service-function-name": "SF3",
"service-function-forwarder": "SFF3"
},
{
"hop-number": 1,
"service-index": 254,
"service-function-forwarder-locator": "eth0",
"service-function-name": "SF2",
"service-function-forwarder": "SFF2"
},
{
"hop-number": 2,
"service-index": 253,
"service-function-forwarder-locator": "eth0",
"service-function-name": "SF1",
"service-function-forwarder": "SFF1"
}
],
"symmetric-path-id": 2
},
{
"name": "SFC1-SFP1-Path-2",
"transport-type": "service-locator:vxlan-gpe",
"parent-service-function-path": "SFC1-SFP1",
"path-id": 2,
"service-chain-name": "SFC1",
"starting-index": 253,
"rendered-service-path-hop": [
{
"hop-number": 0,
"service-index": 253,
"service-function-forwarder-locator": "eth0",
"service-function-name": "SF1",
"service-function-forwarder": "SFF1"
},
{
"hop-number": 1,
"service-index": 252,
"service-function-forwarder-locator": "eth0",
"service-function-name": "SF2",
"service-function-forwarder": "SFF2"
},
{
"hop-number": 2,
"service-index": 251,
"service-function-forwarder-locator": "eth0",
"service-function-name": "SF3",
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"service-function-forwarder": "SFF3"
}
],
"symmetric-path-id": 3
}
]
}
}"""
We will assume the classifier will encode the following information
in the metadata:
o symmetric path-id = 2 (24 bits)
o symmetric starting index = 253 (8 bits)
o symmetric number of hops = 3 (8 bits)
o starting index = 255 (8 bits)
In the method below we will assume SF will generate a reverse packet
after decrementing the index of the current packet. We will call
that current index.
If SF1 wants to generate a reverse packet it can find the appropriate
index by applying the following algorithm:
current_index = 252
remaining_hops = symmetric_number_hops - starting_index - current_index
remaining_hops = 3 - (255 - 252) = 0
reverse_service_index = symmetric_starting_index - remaining_hops - 1
reverse_service_index = next_service_hop_index = 253 - 0 - 1 = 252
The "-1" is necessary for the service index to point to the next service_hop.
If SF2 wants to send reverse packet:
current index = 253
remaining_hops = 3 - (255 - 253) = 1
reverse_service_index = next_service_hop_index = 253 - 1 - 1 = 251
IF SF3 wants to send reverse packet:
current index = 254
remaining_hops = 3 - (255 - 254) = 2
reverse_service_index = next_service_hop_index = 253 - 2 - 1 = 250
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The following tables summarize the service indexes as calculated by
each SF in the forward and reverse paths respectively.
(preamble)
Fwd SI = forward Service Index
Cur SI = Current Service Index
Gen SI = Service Index for Generated packets
RSFP1 Forward -
Number of Hops: 3
Forward Starting Index: 253
Reverse Starting Index: 255
+-------+--------+--------+--------+
| SF | SF1 | SF2 | SF3 |
+-------+--------+--------+--------+
|Fwd SI | 253 | 252 | 251 |
+-------+--------+--------+--------+
|Cur SI | 252 | 251 | 250 |
+-------+--------+--------+--------+
|Gen SI | 252 | 253 | 254 |
+-------+--------+--------+--------+
RSFP1 Reverse -
Number of Hops: 3
Reverse Starting Index: 255
Forward Starting Index: 253
+-------+--------+--------+--------+
| SF | SF1 | SF2 | SF3 |
+-------+--------+--------+--------+
|Rev SI | 253 | 254 | 255 |
+-------+--------+--------+--------+
|Cur SI | 252 | 253 | 254 |
+-------+--------+--------+--------+
|Gen SI | 252 | 251 | 250 |
+-------+--------+--------+--------+
Figure 2: Service indexes generated by each SF in the symmetric
forward and reverse paths
5.3.2. Analysis
Advantages:
o SF does not need to request SFF cooperation or contact controller
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o No SFF performance impact
Disadvantages:
o Metadata overhead in case MD-Type 2 is used
o Relies on classifier or SFF to encode metadata information
o If classifier will encode information it needs to receive and
process rendered service path information
o SFF needs to decrement NOP associated indexes
5.4. Reversed Path derived using Forward Path ID and Index Method
In this simplified model, no extra storage is required from the NSH
and SFF does not need to know how to handle the reversed packet nor
does it know about it. Reverse Path is programmed by Orchestrator
and used by SF having the need to send upstream traffic.
Instead of defining a new Service Path ID, the same Service Path ID
is used. The Orchestrator must define the reverse chain of service
using a different range of Service Path Index. It is also assumed
that the reverse packet must go through the same number of Services
as its forward path. It is proposed that Service Path Index (SPI)
1..127 and 255..129 are the exact mirror of each other.
Here is an example: SF1, SF2, and SF3 are identified using Service
Path Index (SPI) 8, 7 and 6 respectively.
Path 100 Index 8 - SF1
Path 100 Index 7 - SF2
Path 100 Index 6 - SF3
Path 100 Index 5 - Terminate
At the same time, Orchestrator programs SPI 248, 249 and 250 as SF1,
SF2 and SF3. Orchestrator also programs SPI 247 as "terminate".
Reverse-SPI = 256 - SPI.
Path 100 Index 247 - Terminate
Path 100 Index 248 (256 - 8) - SF1
Path 100 Index 249 (256 - 7) - SF2
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Path 100 Index 250 (256 - 6) - SF3
If SF3 needs to send the packet in reverse direction, it calculates
the new SPI as 256 - 6 (6 is the SPI of the packet) and obtained 250.
It then subtract the SPI by 1 and send the packet back to SFF
Subsequently, SFF received the packet and sees the SPI 249. It then
diverts the packet to SF2, etc. Eventually, the packet SPI will drop
to 247 and the SFF will strip off the NSH and deliver the packet.
The same mechanism works even if SF1 later decided to send back
another upstream packet. The packet can ping-pong between SF1 and
SF3 using existing mechanism.
Advantages:
o No precious NSH area is consumed
o SF self-contained solution
o No SFF performance impact and no cooperation needed
o No Special Classification required
Disadvantages:
o SPI range is reduced and may become incompatible with existing
topology
o Assumption that the reverse path Service Functions are the same as
forward path, only in reverse
o Reverse paths need to use Service Index = 128 for loop detection
instead of SI = 0.
An alternative to reducing Service Path Index range is to make use of
a different Service Path ID, e.g. the most significant bit. The bit
can be flipped when the SF needs to send packet in reverse. However,
the negation of the SPI is still required, e.g. SPI 6 becomes SPI
250
In either case, the SF must have the knowledge through Orchestrator
that the reverse path has been programmed and the method (SPI only or
SPI + SPID bit) to use.
The symmetrization mechanism ikeep reverse path symmetric as
described in section 6 can be applied in this method as well.
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6. Asymmetric Service Paths
In real world the forward and reverse paths can be asymmetric,
comprising different set of SFs or SFs in different orders. The
following figure illustrates an example. The forward path is
composed of SF1, SF2, SF4 and SF5, while the reverse path skips SF5
and has SF3 in place of SF2.
.......... .........
. . . .
. 249 . . 246 .
. . . .
. .-. .. .-. .
.............. / \ / \ ....SFP1 Forward....>
( SF2 ) 247 ( SF5 )
Forward SI 250 / \ / \ / \ /\
/ `-' \ / `-' \
/ \ / \
+---+ .-./ `-./ \ +---+
| | / \ / \ \ | |
| A +-------( SF1 )----------( SF4 )-------------+-------------+ B |
| | \ / \ / | |
+---+ `-'\ ,-' +---+
\ /
\ .-. /
Reverse SI 251 \ / \ / 254
<........... ( SF3 ) .................SFP2 Reverse.....
. \ / .
. `-' .
. .
. .
. 253 .
..............
SFP1 Forward -> SF1 : SF2 : SF4 : SF5
SFP2 Reverse <- SF1 : SF3 : SF4
Figure 3: SFC example with asymmetric paths
An asymmetric SFC can have completely independent forward and reverse
paths. An SF's location in the forward path can be different from
that in the reverse path. An SF may appear only in the forward path
but not reverse (and-vice-versa). In order to use the same algorithm
to calculate the service index generated by an SF, one design option
is to insert special NOP SFs in the rendered service paths so that
each SF is positioned symmetrically in the forward and reverse
rendered paths. The SFP corresponding to the example above is:
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SFP1 Forward -> SF1 : SF2 : NOP : SF4 : SF5
SFP2 Reverse <- SF1 : NOP : SF3 : SF4 : NOP
The NOP SF is assigned with a sequential service index the same way
as a regular SF. The SFF receiving a packet with the service path ID
and service index corresponding to a NOP SF should advance the
service index till the service index points to a regular SF.
Implementation can use a loopback interface or other methods on the
SFF to skip the NOP SFs.
Once the NOP SF is inserted in the rendered service paths, the
forward and reverse paths become symmetric. The same algorithm can
be applied by the SFs to generate service indexes in the opposite
directional path. The following tables list the service indexes
corresponding to the example above.
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Fwd SI = forward Service Index
Cur SI = Current Service Index
Gen SI = Service Index for Generated packets
RSP1 Forward -
Number of hops: 5
Forward Starting Index: 250
Reverse Starting Index: 255
+-------+--------+--------+--------+--------+--------+
| SF | SF1 | SF2 | NOP | SF4 | SF5 |
+-------+--------+--------+--------+--------+--------+
|Fwd SI | 250 | 249 | 248 | 247 | 246 |
+-------+--------+--------+--------+--------+--------+
|Cur SI | 249 | 248 | 247 | 246 | 245 |
+-------+--------+--------+--------+--------+--------+
|Gen SI | 250 | 251 | N/A | 253 | 254 |
+-------+--------+--------+--------+--------+--------+
RSP1 Reverse -
Number of hops: 5
Reverse Starting Index: 255
Forward Starting Index: 250
+-------+--------+--------+--------+--------+--------+
| SF | SF1 | NOP | SF3 | SF4 | NOP |
+-------+--------+--------+--------+--------+--------+
|Rev SI | 251 | 252 | 253 | 254 | 255 |
+-------+--------+--------+--------+--------+--------+
|Cur SI | 250 | 251 | 252 | 253 | 254 |
+-------+--------+--------+--------+--------+--------+
|Gen SI | 249 | N/A | 247 | 246 | N/A |
+-------+--------+--------+--------+--------+--------+
This symmetrization of asymetric paths could be performed by a
controller during path creation.
7. Other solutions
We explored other solution that we deemed to complex or that would
bring a severe performance penalty:
o An out-of-band request-response protocol between SF-SFF. Given
that some service functions need to be able to generate packets
quite often this will would create a considerable performance
penalty. Specially given the fact that path-ids (and their
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symmetric counterpart) might change and SF would not be notified,
therefore caching benefits will be limited.
o An out-of-band request-response protocol between SF-Controller.
Given that admin or network conditions can trigger service path
creation, update or deletions a SF would not be aware of new path
attributes. The controller should be able to push new information
as it becomes available to the interested parties.
o SF (or SFF) punts the packet back to the controller. This
solution obviously has severe scaling limitations.
8. Implementation
The solutions "Reversed Path derived using Forward Path ID and Index
Method" and "SF receives Reverse Forwarding Information" were
implemented in Opendaylight
9. IANA Considerations
TBD
10. Security Considerations
11. Acknowledgements
Paul Quinn, Jim Guichard
12. Changes
13. References
13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[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,
DOI 10.17487/RFC2616, June 1999,
<http://www.rfc-editor.org/info/rfc2616>.
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13.2. Informative References
[I-D.ietf-nvo3-vxlan-gpe]
Quinn, P., Manur, R., Kreeger, L., Lewis, D., Maino, F.,
Smith, M., Agarwal, P., Yong, L., Xu, X., Elzur, U., Garg,
P., and D. Melman, "Generic Protocol Extension for VXLAN",
draft-ietf-nvo3-vxlan-gpe-00 (work in progress), May 2015.
[I-D.ietf-sfc-architecture]
Halpern, J. and C. Pignataro, "Service Function Chaining
(SFC) Architecture", draft-ietf-sfc-architecture-11 (work
in progress), July 2015.
[I-D.ietf-sfc-nsh]
Quinn, P. and U. Elzur, "Network Service Header", draft-
ietf-sfc-nsh-01 (work in progress), July 2015.
[I-D.penno-sfc-trace]
Penno, R., Quinn, P., Pignataro, C., and D. Zhou,
"Services Function Chaining Traceroute", draft-penno-sfc-
trace-02 (work in progress), March 2015.
[I-D.penno-sfc-yang]
Penno, R., Quinn, P., Zhou, D., and J. Li, "Yang Data
Model for Service Function Chaining", draft-penno-sfc-
yang-13 (work in progress), March 2015.
[RSPYang] Opendaylight, , "Rendered Service Path Yang Model",
February 2011,
<https://github.com/opendaylight/sfc/blob/master/sfc-
model/src/main/yang/rendered-service-path.yang>.
[SymmetricPaths]
IETF, , "Symmetric Paths", February 2011,
<https://tools.ietf.org/html/draft-ietf-sfc-architecture-
11#section-2.2>.
Authors' Addresses
Reinaldo Penno
Cisco Systems
170 West Tasman Dr
San Jose CA
USA
Email: repenno@cisco.com
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Carlos Pignataro
Cisco Systems
170 West Tasman Dr
San Jose CA
USA
Email: cpignata@cisco.com
Chui-Tin Yen
Cisco Systems
170 West Tasman Dr
San Jose CA
USA
Email: tin@cisco.com
Eric Wang
Cisco Systems
170 West Tasman Dr
San Jose CA
USA
Email: ejwang@cisco.com
Kent Leung
Cisco Systems
170 West Tasman Dr
San Jose CA
USA
Email: kleung@cisco.com
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