Internet DRAFT - draft-penno-sfc-packet
draft-penno-sfc-packet
SFC R. Penno
Internet-Draft C. Pignataro
Intended status: Standards Track C. Yen
Expires: October 31, 2016 E. Wang
K. Leung
Cisco Systems
D. Dolson
Sandvine
April 29, 2016
Packet Generation in Service Function Chains
draft-penno-sfc-packet-03
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 October 31, 2016.
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Copyright Notice
Copyright (c) 2016 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
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3
3. Definitions and Acronyms . . . . . . . . . . . . . . . . . . 3
4. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Service Function Behavior . . . . . . . . . . . . . . . . . . 5
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 . . . . . . . . . . . . . . . 9
5.3.2. Symmetric Service Paths, Optimized . . . . . . . . . 13
5.3.3. Analysis . . . . . . . . . . . . . . . . . . . . . . 15
5.4. Algorithmic Reversed Path ID Generation . . . . . . . . . 16
5.4.1. Same Path-ID and Disjoint Index Spaces . . . . . . . 16
5.4.2. Flip Path-Id and Index High Order bits . . . . . . . 17
6. Asymmetric Service Paths . . . . . . . . . . . . . . . . . . 18
7. Metadata . . . . . . . . . . . . . . . . . . . . . . . . . . 21
7.1. Service-Path-Invariant Metadata . . . . . . . . . . . . . 21
7.2. Service-Path-Default Metadata . . . . . . . . . . . . . . 21
7.3. Bidirectional Clonable Metadata . . . . . . . . . . . . . 22
7.4. Unidirectional Clonable Metadata . . . . . . . . . . . . 22
7.5. Service-Function-Mastered Metadata . . . . . . . . . . . 23
7.6. Metadata from Reclassification . . . . . . . . . . . . . 23
8. Other solutions . . . . . . . . . . . . . . . . . . . . . . . 23
9. Implementation . . . . . . . . . . . . . . . . . . . . . . . 24
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
11. Security Considerations . . . . . . . . . . . . . . . . . . . 24
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 24
13. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
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14. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
14.1. Normative References . . . . . . . . . . . . . . . . . . 24
14.2. Informative References . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
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. In
some cases, devices generate packets without any in-process packet.
Packet generation is a basic intrinsic functionality and therefore
needs to be supported in a service function chaining deployment.
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 to 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,
alternatively, indicate reverse path for the service path ID in
the original packet
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]
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4. Assumptions
We make the following assumption throughout 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 Figure 1 scenario is not permitted. (SFF2
cannot complete the forward path which contains SFF3 and
potentially SFs connected to SFF3.)
.-. .-.
/ \ / \
( SF1 ) ( SF2 )
\ / \ / \
`+' `+' \
| | \
| | \
+--+---+ +--+---+ \+------+
...---+ SFF1 +------+ SFF2 | | SFF3 +---...
+------+ +--+---+ +------+
|
|
+-----...
RSFP Forward -> SFF1 : SF1 : SFF1 : SFF2 : SF2 : SFF3 : ...
Figure 1: Arrangement not supported
3. Forward and reverse paths may be required to utilize different
service function forwarders. In the Figure 2 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
Figure 2: Supported SFF arrangement
Assumption #2 allows an SF to always bounce a packet back to the SFF
that originally sent the packet. 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 are free of the constraints in #3, which will
simplify the SF logic in handling generated traffic.
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
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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:
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
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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)
Ver: 1
OAM Bit: 1
Length: 6
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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.
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.
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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 cannot 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.
5.3.1. Symmetric Service Paths
Figure 3 below shows a simple SFC with symmetric service paths
comprising three SFs.
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.....................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 3: 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,
"service-function-forwarder-locator": "eth0",
"service-function-name": "SF3",
"service-function-forwarder": "SFF3"
},
{
"hop-number": 1,
"service-index": 254,
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"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",
"service-function-forwarder": "SFF3"
}
],
"symmetric-path-id": 3
}
]
}
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}"""
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
The following tables in Figure 4 summarize the service indexes as
calculated by each SF in the forward and reverse paths respectively.
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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 4: Service indexes generated by each SF in the symmetric
forward and reverse paths
5.3.2. Symmetric Service Paths, Optimized
This approach is effectively the same as Section 5.3.1, but with
redundant information removed such that the reverse-path information
can be packed into 32 bits. This approach is obtained by observing
that the same arithmetic is always done on the same constants of
starting_index, symmetric_starting_index and symmetric_number_hops.
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As before, we require symmetric paths, meaning there are two paths
that are exactly the reverse of each other. We assume that the
classifier at each end has available the following information:
o symmetric path-id (24 bits)
o starting index (8 bits)
o symmetric starting index (8 bits)
o symmetric number of hops, which is the same in both directions (8
bits)
The classifier computes, for each path, a "reverse service offset":
# Compute using 8-bit, two's-complement arithmetic:
# (Overflow or underflow are okay)
reverse_service_offset = symmetric_starting_index
+ starting_index
- symmetric_number_of_hops
This reverse_service_offset is an 8-bit value that is encoded in
metadata along with the 24 bits of reverse_path_id.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Reverse |
| Reverse Path ID | Service |
| | Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Metadata format of reverse_info_metadata (32 bits)
We'll refer to the 32-bit value as reverse_info_metadata. Any
Service Function may compute the NSH fields of a reverse packet as
follows from the NSH fields of a forward packet.
reverse.NSH.Service_Path_ID =
forward.NSH.reverse_info_metadata.Reverse_Path_ID
# Compute using 8-bit two's-complement arithmetic:
# (Overflow or underflow are okay)
reverse.NSH.Service_Index :=
forward.NSH.reverse_info_metadata.Reverse_Service_Offset
- forward.NSH.Service_Index - 1
reverse.NSH.reverse_info_metadata.Reverse_Service_Offset =
forward.NSH.reverse_info_metadata.Reverse_Service_Offset
reverse.NSH.reverse_info_metadata.Reverse_Path_ID =
forward.NSH.Service_Path_ID
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As you can see, this approach has the convenient property that the
reverse_info_metadata can be determined by a Service Function while
being agnostic about both forward and reverse paths.
Using the example of Section 5.3.1, these values are used for the
SFP2 Forward path:
o starting_index=253
o symmetric_starting_index=255
o symmetric_number_of_hops=3
o reverse_service_offset=(253+255-3)=249 in 8-bit two's complement
arithmetic
At SF2 on the SFP2 Forward path, where the service index is 251 after
decrementing the index, the reverse service index is calculated as:
o reverse_service_index = 249-251-1 = 253 using 8-bit two's
complement arithmetic
This is the correct index to forward to SF1 on SFP3.
5.3.3. Analysis
Advantages of encoding information in the NSH frame:
o SF does not need to request SFF cooperation or contact controller
o No SFF performance impact
Disadvantages:
o Metadata overhead in case MD-Type 2 is used or use of a metadata
slot in case MD-Type 1 is used.
o Relies on classifier to encode metadata information
o Requires perfectly symmetrical paths. E.g., one direction cannot
have more SFs than the other direction.
o If classifier will encode information it needs to receive and
process rendered service path information
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5.4. Algorithmic Reversed Path ID Generation
In these proposals 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.
5.4.1. Same Path-ID and Disjoint Index Spaces
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
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.
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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.
Note that this mechanism is a special case of Section 5.3.2 wherein
Reverse_Path_ID is the forward path ID and
Reverse_Service_Offset=255.
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.
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 keep reverse path symmetric as described
in section 6 can be applied in this method as well.
5.4.2. Flip Path-Id and Index High Order bits
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
134
This approach is fully compatible with the current NSH protocol
standard and provides a fully deterministic way of determining
reverse paths. It is the recommended approach.
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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 Assumption that the reverse path Service Functions are the same as
forward path, only in reverse
o Forward and Reverse Path IDs are algorithmically linked and can
not be chosen arbitrarily.
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 5 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.
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.......... .........
. . . .
. 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 5: 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:
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
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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.
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 |
+-------+--------+--------+--------+--------+--------+
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This symmetrization of asymmetric paths could be performed by a
controller during path creation.
7. Metadata
A crucial consideration when generating a packet is which metadata
should be included in the context headers. In some scenarios if the
metadata is not present the packet will not reach its intended
destination. Although one could think of many different ways to
convey this information, we believe the solution should be simple and
require little or no new Service Function functionality.
We assume that a Service Function normally needs to know the
semantics of the context headers in order to perform its functions.
But clearly knowing the semantics of the metadata is not enough. The
issue is that although the SF knows the semantics of the metadata
when it receives a packet, it might not be able to generate or
retrieve the correct metadata values to insert in the context headers
when generating a packet. It is usually the classifier that inserts
the metadata in the context headers.
7.1. Service-Path-Invariant Metadata
In order to solve this problem we propose the notion of service-path-
invariant metadata. This is metadata that is the same for all
packets traversing a certain path. For example, if all packets
exiting a service-path need to be routed to a certain VPN, the VPN id
would be a path-invariant metadata.
To implement this, the controller needs to configure appropriate
fixed values of the metadata present in the context headers for each
path identifier in each Service Function that needs to inject
packets. The Service Function must store this information so that
when the Service Function generates a packet it can insert the
minimum required metadata for a packet to reach its destination.
A disadvantage to path-invariant metadata is that it is a type of
metadata that adds no information beyond the information available in
the path identifier itself. The corollary is that if different
metadata is required, a different service paths must be created.
7.2. Service-Path-Default Metadata
We also propose the notion of service-path-default metadata. This is
metadata that could vary for different packets on a path but has a
default value acceptable for any packet injected onto a certain path.
For example, metadata might indicate a quality-of-service (QoS)
treatment, and an operator considers it acceptable for injected
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packets to have a default QoS treatment. It might also be considered
acceptable to not send a particular type of metadata.
To implement this, the controller configures appropriate default
metadata values for each path identifier in Service Functions that
need to inject packets. The controller may also indicate a
particular type may be omitted. The Service Function must store this
information so that it can insert the minimum required metadata for a
packet to reach its destination.
The disadvantage of this approach is that it relies on the assumption
that there is a meaningful default metadata value, which may not
exist.
7.3. Bidirectional Clonable Metadata
Some types of metadata may use values applicable to both directions
of traffic. An example is routing domain, for which an identifier
indicates a private network such that the value is the same for both
directions of traffic and may be copied from one packet to another.
To implement this, the controller must indicate to each Service
Function that a particular metadata type is bidirectional-clonable.
The Service Function can therefore clone the metadata value from one
packet to a new packet that it creates, even in the reverse
direction. For this type, it is also considered safe to save a copy
of metadata for the transport flow. (E.g., to retransmit a TCP
packet using metadata cloned from another TCP packet of the same
connection.)
Note that the Service Function need not know the meaning of the
metadata; it just needs to know it is safe to clone in this manner.
7.4. Unidirectional Clonable Metadata
Some types of metadata may use values applicable to only one
direction of traffic, but a value may be cloned from one packet to
another in the same direction. An example is a destination
identifier, in which meatadata indicates a network egress point.
Another example is metadata indicating a property of either the
source or destination end-point of the packet.
To implement this, the controller must indicate to each Service
Function that a particular metadata type is unidirectional-clonable.
A transport-layer-stateful Service Function can therefore save away
metadata values that it has witnessed. An injected packet can
therefore be assigned a clone of metadata taken from an earlier
packet going in the same direction. For example, a Service Function
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can send a TCP packet using metadata cloned from another TCP packet
of the same connection and direction.
Note that the Service Function need not know the meaning of the
metadata; it just needs to know it is safe to clone in this manner.
A disadvantage of unidirectional clonable metadata is that a device
cannot respond to a packet unless it has previously witnessed a
packet for the same connection in the opposite direction. For
example, a firewall cannot respond to the first packet of a
connection (since both directions have not been witnessed). However,
having seen a full hand-shake, a cache or optimizing proxy can inject
or retransmit packets.
7.5. Service-Function-Mastered Metadata
The easiest case to reason about is a type of metadata for which the
Service Function can provide the appropriate values: specifically the
metadata that it would be responsible for inserting for all packets
as part of packet processing. We can assume this is configured by
Service-Function-Specific methods.
7.6. Metadata from Reclassification
Finally if the packet needs crucial metadata values that cannot be
supplied by the methods above then a reclassification is needed.
This reclassification would need to be done by the classifier that
would normally process packets in the reverse path or a SFF that had
the same rules and capabilities. Ideally the first SFF that
processes the generated packet.
If a packet needs to be sent to classifier then it should be carried
inside a NSH OAM packet that in turn is tunneled with a protocol such
as VXLAN-GPE with the classifier as its tunnel endpoint.
8. Other solutions
We explored other solution that we deemed too 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
symmetric counterpart) might change and SF would not be notified,
therefore caching benefits will be limited.
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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.
9. Implementation
The solutions "Flip Path-Id and Index High Order bits" and "SF
receives Reverse Forwarding Information" were implemented in
Opendaylight.
10. IANA Considerations
TBD
11. Security Considerations
Service Functions must be trusted entities, being permitted to
rewrite service path headers.
12. Acknowledgements
Paul Quinn, Jim Guichard
13. Changes
14. References
14.1. 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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14.2. Informative References
[I-D.ietf-nvo3-vxlan-gpe]
Kreeger, L. and U. Elzur, "Generic Protocol Extension for
VXLAN", draft-ietf-nvo3-vxlan-gpe-02 (work in progress),
April 2016.
[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-04 (work in progress), March 2016.
[I-D.penno-sfc-trace]
Penno, R., Quinn, P., Pignataro, C., and D. Zhou,
"Services Function Chaining Traceroute", draft-penno-sfc-
trace-03 (work in progress), September 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-14 (work in progress), January 2016.
[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
David Dolson
Sandvine
408 Albert Street
Waterloo, ON N2L 3V3
Canada
Phone: +1 519 880 2400
Email: ddolson@sandvine.com
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