SFC working group H. Song
Internet-Draft J. You
Intended status: Informational L. Yong
Expires: February 20, 2016 Y. Jiang
L. Dunbar
Huawei
N. Bouthors
Qosmos
D. Dolson
Sandvine
August 19, 2015
SFC Header Mapping for Legacy SF
draft-song-sfc-legacy-sf-mapping-06
Abstract
A Service Function Chain (SFC) defines a set of abstract Service
Functions (SF) and ordering constraints that must be applied to
packets and/or frames selected as a result of classification. One
assumption of this document is that legacy service functions can
participate in service function chains without having support for the
SFC header, or even being aware of it. This document provides a
mechanism between an SFC proxy and an SFC-unaware service function
(herein termed "legacy SF"), to identify the SFC header associated
with a packet that is returned from a legacy SF, without an SFC
header being explicitly carried in the wired protocol between SFC
proxy and legacy SF.
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 February 20, 2016.
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Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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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 . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. For Transparent Service Functions . . . . . . . . . . . . 6
3.1.1. Layer 2 MAC Address . . . . . . . . . . . . . . . . . 6
3.1.2. VLAN . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1.3. QinQ . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1.4. VXLAN . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1.5. 5-tuple . . . . . . . . . . . . . . . . . . . . . . . 12
3.2. For Non-transparent Service Functions . . . . . . . . . . 12
4. Operation Considerations . . . . . . . . . . . . . . . . . . 13
4.1. Metadata Consideration . . . . . . . . . . . . . . . . . 15
5. Security Considerations . . . . . . . . . . . . . . . . . . . 15
6. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 15
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
7.1. Normative References . . . . . . . . . . . . . . . . . . 15
7.2. Informative References . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction
A Service Function Chain (SFC) [I-D.ietf-sfc-architecture] defines a
set of abstract service functions and ordering constraints that must
be applied to packets and/or frames selected as a result of
classification. One assumption of this document is that some service
functions may remain as legacy implementations, and they neither have
to be aware of the SFC header, nor interpret it. It is a
straightforward function for an SFC proxy to remove an SFC header to
send a packet to a legacy SF, but it is not obvious what SFC header
should be added to packets arriving at the SFC proxy from the legacy
SF.
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This document provides a mechanism between an SFC proxy and a legacy
SF, to identify the SFC header associated with a packet that is
returned from a legacy SF, without anything in the SFC header being
explicitly carried in the wired protocol between SFC proxy and legacy
SF. The motivation for supporting legacy SF is that existing service
functions don't need to be upgraded to support SFC, removing one
barrier to wide adoption of SFC.
+----------------+
|SFC-unaware |
|Service Function|
| (Legacy SF) |
+----+----+------+
^ |
| |
+----+----+------+
| Switch |
+----+----+------+
| |
(2)| |(3)
| |
+----+----V--------+
(1) | SFC | (4)
-------->| Proxy +------->
+------------------+
Figure 1: Procedure of a packet processed by a legacy SF
The legacy service function (i.e., "SFC-unaware Service Function" in
the Figure 1) only handles packets without SFC header, because it
does not understand the SFC header. Note that different classes of
legacy SF may have varying support for different types of packets
with respect to parsing and semantics (e.g., some classes of legacy
SF may accept VLAN-tagged traffic; others may not.).
This document focuses heavily on legacy SFs that are transparent at
layer 2. In particular, we assume the following about layer-
2-transparent legacy SFs:
1. Traffic is forwarded between pairs of interfaces, such that
packets received on the "left" are forwarded on the "right" and
vice versa.
2. A packet is forwarded between interfaces without modifying the
layer 2 header; i.e., neither source MAC nor destination MAC is
modified.
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3. When supported, VLAN-tagged or Q-in-Q packets are forwarded
with the original VLAN tag(s) intact (S-tags and C-tags).
4. Traffic may be discarded by some functions (e.g., by a
firewall).
5. Traffic may be injected in either direction by some functions
(e.g., extra data coming from a cache, or simply TCP
retransmissions). We assume injected traffic relates to a layer 3
or layer 4 flow, and the SF clones layer 2 headers from exemplar
packets of the same flow.
6. Traffic may be modified by some functions at layer 3 (e.g.,
DSCP marking) or higher layers (e.g., HTTP header enrichment or
anonymization). Note that modification can be considered a
special case of discarding following by injection.
7. Traffic may be reordered by some functions (e.g., due to
queuing/scheduling).
We leave the legacy SFs which modify the original layer 2 packet
headers as an open issue for further study.
To support this class of legacy SF, if the payload in the SFC
encapsulation is layer 3 traffic, the SFC proxy will extract the
layer 3 payload from SFC encapsulation and prepend a new layer 2
header before sending the packet to the SF. However if the payload
in the SFC encapsulation is layer 2 traffic, the SFC proxy may
extract the layer 2 packet from SFC encapsulation, modify the
original source MAC address and use the new source MAC address for
mapping to the stored SFC and layer 2 headers when the packets are
returned to the SFC proxy. This will not impact the SF processing.
The SF will send the traffic back after processing.
As shown in Figure 1, there are four steps. The SFC proxy receives a
packet, and removes its SFC header, which may optionally contain
metadata, and store the SFC header locally, and then sends the de-
encapsulated packet to the SF. After the SF processes the packet,
the packet will be sent back to the SFC proxy. The SFC proxy
retrieves the pre-stored SFC header accordingly, determines the SFC
header for the next stage of the path and encapsulates the packet
with the next SFC header.
The key problem contemplated in this document is: what layer 2 header
should be put on the packets sent to a legacy SF such that packets
returned from the legacy SF can be mapped to the original SFC header?
We need to consider the relationship between an SFC path and flows
within the path. Should the path act as a qualifier to the flow, or
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should a flow be allowed to change paths? Below, we assume flows can
change path; this means that a given legacy SF cannot handle traffic
from more than one routing domain. (Private IP addresses cannot be
qualified by the SFC header; different VPNs must use different legacy
SFs.)
Because we've assumed that a flow can be on multiple paths, or change
paths, or if metadata can vary during the life of a flow, we need to
ask to what extent packet accuracy matters. If the SFC header used
with a flow is changed from one path to another by the classifier,
does it matter if packets retain exactly the original SFC header? If
the change is to handle routing updates or fail-over then it would be
acceptable to put all packets returning from the legacy SF onto the
most recently updated header. If metadata is changed, can that
update be applied to all packets of a flow, or does it apply to a
specific packet?
In the case that changes to paths and metadata are considered updates
to the flow vs. packet properties, the SFC proxy can find the SFC
header based on flow (e.g., the 5-tuple of the returning IP packet).
If, in contrast, packet accuracy of SFC headers does matter, (e.g.,
the metadata says something about the specific packet associated with
it), then some form of per-packet bookkeeping must be done by the SFC
proxy and the 5-tuple cannot be used for the mapping to retrieve the
original SFC header.
When packet accuracy does matter, packets injected by the legacy SF
pose a fundamental problem. Is there any correct SFC header that can
be added? Observation: the same problem exists for a normal (not
legacy) SF that wishes to modify or inject a packet.
When metadata is sent without any associated payload (congruent
metadata) and the associated service function is a legacy one, then
SFF MUST relay the metadata to the next hop SFF, without sending the
metadata to SFC proxy. For some types of metadata, the metadata
should be saved in case it needs to be added to packets injected by
the legacy SF.
Because the SFC proxy needs to keep dynamic state by storing packet
headers, an expiration time should be used for each mapping entry in
the SFC proxy. If the SFC header in that entry has not been
witnessed or retrieved after the expiration time, the entry will be
deleted from the entry table.
Observation: if metadata is not used, the number distinct SFC headers
is known at configuration time, equivalent to the number of paths
configured to pass through the SF. The mappings between SFC headers
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and layer 2 encodings could be configured at this time vs. at run
time. However, if metadata is used, a combinatorial explosion of
distinct SFC headers may result, which is a problem for any device
attempting to store them for later retrieval.
2. Terminology
The terminology used in this document is defined below:
Legacy SF: A conventional service function that does not support
SFC header, i.e. SFC-unaware SF.
Transparent SF: A service function that does not change any bit of
the original service packet header (Layer 2, layer 3, and layer 4)
sent to it, but it may drop packets.
Non-transparent SF: A service function that changes some part of
the original service packet header sent to it.
Original Service Packet: The payload in an SFC encapsulation
packet or a packet constructed based on the original payload.
SFC Proxy: A network function that operates as an SF node within
the SFC architecture while delegating application functions to one
or more attached Legacy SFs by acting as an adapter or bridge
between the SFC protocol and SF wire protocols understood by the
legacy SF.
3. Mechanisms
The mechanisms used in this document require that each forwarding
entity and its connected service functions in the same layer 2
network. The following are considerations mainly for transparent
SFs. If the original payload packet is a layer 2 packet, and the
mapping method used is layer 2 MAC address, then the assumption is
that the SF does not need to look into the layer 2 header. If it
does, other mechanisms should be used.
3.1. For Transparent Service Functions
If the service function is transparent to packet headers, the
following methods can be used for SFC header mapping.
3.1.1. Layer 2 MAC Address
The layer 2 MAC address is used to associate a SFC header between SFC
proxy and SF; i.e., each SFC header will be assigned a source MAC
address on the SFC proxy. If SFC header can be changed per packet,
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then SFC proxy assigns a new source MAC address for each packet it
received, otherwise, it assigns a new MAC address for each unique SFC
header that must be applied to returning packets. (It is not
necessary to have a unique MAC address for each flow received.)
When SFC proxy received the returned packet from the SF, it retrieves
the packet's original SFC header by using the source MAC address as a
key. And then it encapsulates the packet with that SFC header and
sends to the next hop.
Open issue: usually the MAC address table size in a switch is no more
than 16K. When there is a requirement that per packet metadata needs
to be restored to each packet after the packet returns from the SF
instance, it may require more MAC addresses than the MAC table size
in the switch. This may overflow the MAC table, thus the packet
cannot route back to the SFC proxy correctly.
An issue with the source-MAC address approach is that there is not
symmetry between packets going left-to-right with packets going
right-to-left. Such symmetry might be assumed by some legacy SFs.
For example, if a layer-2-transparent SF responds to a TCP SYN with a
TCP RST, it might do so by reversing the source and destination of
the layer 2 header. Such a packet received by the SFC proxy would
not result in finding of the correct SFC header. A variation that is
symmetric assigns a unique source/destination pair for each unique
SFC header.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
Outer Ethernet Header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SF Destination MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SF Destination MAC Address | SFC Proxy Source MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SFC Proxy Source MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ethertype = 0x0800 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Original IP Payload:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Original Payload |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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3.1.2. VLAN
If the network between the SFC proxy and SF is a layer 2 network, and
in the case that an SF needs to look into the MAC address of the
packet, then VLAN can be used for the mapping between them. The SFC
proxy removes the SFC header and sends the packet to the SF, with
encapsulating a certain VLAN ID. It is a new encapsulation,
supposing that the legacy App can be configured to accept VLAN-tagged
packets and to send them back on the same VLAN. It is assumed that
the receiving service function host/VM can support multiple VLANs.
The SFC proxy locally maintains the mapping between VLAN ID/direction
and the SFC header.
When it gets the returned packet from the SF, the SFC proxy removes
the VLAN part from the packet and retrieves the corresponding SFC
header according to the VLAN ID and the direction of packet travel,
and then encapsulates SFC header into that packet before sending to
the next service function. Packet direction is required because the
SFC header for left-to-right packets is different than the SFC header
for right-to-left packets.
If metadata is not used, the number of VLAN tags required is exactly
the number of SFC paths that pass through the SF, and it can be known
at configuration time how many are required.
[I-D.dolson-sfc-vlan] describes an approach for service function
chaining by using the input interface and VLAN number to select the
next output interface and new VLAN number. SF devices that work with
the dolson-sfc-vlan scheme will work with the VLAN scheme described
here.
One open issue with VLAN tag is that if the use case requires per
packet metadata, then the address space of VLAN digits cannot be
enough.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
Outer Ethernet Header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SFI Destination MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SF Destination MAC Address | SFC Proxy Source MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SFC Proxy Source MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|OptnlEthtype = C-Tag 802.1Q |Outer.VLAN Tag Information |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ethertype = 0x0800 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Original IP Payload:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Original Payload |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.1.3. QinQ
If the network between the SFC proxy and SF is already a VLAN
network, and the SF needs to look into the MAC address, then QinQ is
used for the communication between SFC proxy and SF. The SFC proxy
removes the SFC header and sends the original traffic to legacy SF
with a certain outer VLAN ID. It locally maintains the mapping
between outer VLAN ID and the SFC header.
If the network between SFC proxy and SF is not a VLAN network, then
QinQ can be used for either per flow mapping or per packet mapping,
using two layer VLAN fields. Because of the increase in address
space, QinQ can be used in two-layer VLAN: outer VLAN-id per flow,
and inner VLAN-id per packet. If the network between SFC proxy and
SF is a VLAN network, then QinQ can only be used for per flow
mapping, using one VLAN field.
It is assumed that the receiving service function host/VM can support
multiple service VLAN IDs with multiple inner VLAN IDs.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
Outer Ethernet Header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SF Destination MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SF Destination MAC Address | SFC Proxy Source MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SFC Proxy Source MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|OptnlEthtype = S-Tag 802.1Q |Outer.VLAN Tag Information |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ethertype = C-Tag 802.1Q |Inner.VLAN Tag Information |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ethertype = 0x0800 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Original IP Payload:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Original Payload |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.1.4. VXLAN
If the SFC proxy and SF are already deployed in a QinQ network, then
VXLAN [RFC7348] can be used for the mapping, i.e. VNI can be used for
the mapping between them. This tunneling technology is only used
when the original packet type is at layer 2 and the SF has to look
into the layer 2 MAC header.
The drawback of this mechanism is that it requires both SFC proxy and
SF to support VXLAN.
This approach has similar features and drawbacks of the VLAN scheme,
but the number of possible VLANs is larger.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
Outer Ethernet Header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SF Destination MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|SFI Destination MAC Address | SFC Proxy Source MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SFC Proxy Source MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|OptnlEthtype = C-Tag 802.1Q |Outer.VLAN Tag Information |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ethertype = 0x0800 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Outer IP Header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| IHL |Type of Service| Total Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification |Flags| Fragment Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Time to Live |Protocol=17(UDP) | Header Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Outer Source IPv4 Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Outer Destination IPv4 Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Outer UDP Header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port = xxxx | Dest Port = VXLAN Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDP Length | UDP Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
VXLAN Header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R|R|R|R|I|R|R|R| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VXLAN Network Identifier (VNI) | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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3.1.5. 5-tuple
The 5-tuple of an SFC packet can be used as a key to associate an SFC
header in the SFC proxy when the 5-tuple is not modified by the
legacy SF. The SFC proxy maintains a mapping table for the 5-tuple
and the SFC header. When the packet returns from the SF instance,
the original SFC header for this packet can be retrieved by inquiring
the mapping table using 5-tuple as the key. However, this method may
not work in multi-tenant organizations, as such unicity could be
Valid only within the scope of a single tenant. So if the SFC is
provided as a multi-tenant service, this method would fail.
Another similar use case could be that a client and a server use http
80 port for transporting different types of data, and if each type
has its specific SFC header with metadata, then 5-tuple does not work
either.
This method cannot support per-packet metadata.
3.2. For Non-transparent Service Functions
Non transparent service functions including NAT (Network Address
Translation), WOC (WAN Optimization Controller) and etc, are more
complicated, as they may change any part of the original packet sent
to them. It is better to analyze case by case, to utilize a specific
field that the SF does not change for the mapping and retrieving the
SFC header. We would like to leave it for open discussion.
The Figure below shows an example that SFC proxy can learn the
behavior of the SF changing the packet. In this example, the
following method is used for SFC header mapping. The SF needs to
report its mapping rules (e.g. 5-tuple mapping rules) to the control
plane (e.g. by static configuration), and then the control plane can
notify the SFC proxy the mapping information (step 1) via interface
C4 [I-D.ww-sfc-control-plane]. According to the mapping information,
the SFC proxy can establish a mapping table for the SFC header, the
original header, and the processed header of the packet. After
receiving the packet from the SF (step 4), the SFC proxy retrieves
the SFC header from the mapping table by using the processed header
as a key.
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+-------------+
|Control Plane|
+--+----------+
^
|
| +----------------+
| |SFC-unaware |
(1)| |Service Function|
| +-----+---+------+
| (3)^ |(4)
+---------------+ | |
| | |
+--V---+---V-------+
(2) | SFC | (5)
--------->+ Proxy +------->
+------------------+
4. Operation Considerations
The following table shows all the methods and the conditions to use.
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Table 1: Operation Consideration
+-----------+--------+-------------------------------+-------------------+
| |Methods | Stored Key-Value |Application |
| | | |Scenario |
+-----------+--------+-------------------------------+-------------------+
| |MAC | (Source MAC Address, SFC |L2 header won't |
|For Trans- |Address | header) |be modified by the |
|parent SF | | |SF. |
| | |e.g. assign a source MAC | |
| | |address per packet or path ID | |
| +--------+-------------------------------+-------------------+
| |VLAN | (Direction, VLAN ID, |L2 header won't |
| | | SFC header) |be modified by the |
| | |e.g. assign a VLAN ID per |SF. |
| | |bidirectional path-pair | |
| +--------+-------------------------------+-------------------+
| |QinQ | (Direction, Outer VLAN ID, |The SF is required |
| | | SFC header) |to support QinQ. |
| | |e.g. assign an outer VLAN ID |L2 header won't |
| | |per bidirectional path-pair |be modified by |
| | | |the SF. |
| +--------+-------------------------------+-------------------+
| |VXLAN | (Direction, VNI, SFC header) |The SF is required |
| | |e.g. assign a VNI per |to support VXLAN. |
| | |bidirectional path-pair |VNI is not modified|
| | | |by the SF. |
| +--------+-------------------------------+-------------------+
| |5-tuple |(5-tuple, SFC header) |5-tuple is not |
| | | |modified by the |
| | |The SFC proxy maintains the |SF. |
| | |mapping table for 5-tuple and | |
| | |the SFC header. | |
| | |Note: an SFC header for each | |
| | |direction of a TCP flow. | |
+-----------+--------+----------------- -------------+-------------------+
| |TBD |Mapping rules: |The SFC proxy is |
|For | |e.g. 5-tuple -> 5-tuple' |configured or is |
|Non-trans- | | |able to obtain the |
|parent SF | |SFC Proxy: |mapping rules of |
| | |5-tuple -> 5-tuple' |the SF. The SF |
| | |5-tuple'-> SFC header |modifies the |
| | | |5-tuple based on |
| | | |the mapping |
| | | |rules. |
+-----------+--------+---------------------------------------------------+
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4.1. Metadata Consideration
Some classes of SF may need to inject new packets, for example a
transparent cache sending content from its disk. The legacy SF
usually encapsulates the new packets with the same encapsulation with
the related received packets, e.g. with the same 5-tuple, or V-LAN
ID. The SFC proxy would associate the new packet with the
corresponding SFC header based on the mechanisms discussed in
Section 3. However, per-packet metadata should be prohibited for
this case.
Some classes of SF may need to inject a packet in the opposite
direction of a received packet, for example a firewall responding to
a TCP SYN with a RST. If the RST generator is VLAN-type legacy, it
may know what VLAN to use; then the SFC proxy would translate VLAN
into a reverse SFP and attach a corresponding SFC header insetad of
the original SFC header. In this case, the SFC proxy should be
configured with the bidirectional SFP, i.e. SFC proxy needs to be
designed according to the properties of the SF. Similarly, packet-
specific metadata is not recommended to be used.
We leave the metadata model as an open issue that will be documented
in other documents. In some cases this information will also assist
normal (non-legacy) SFs that wish to modify or inject packets.
5. Security Considerations
When the layer 2 header of the original packet is modified and sent
to the SF, if the SF needs to look into the layer 2 header, it may
cause security threats. It also provides diagrams of the main
entities that the information model is comprised of.
6. Acknowledgement
The authors would like to thank Ron Parker and Joel Halpern for their
comments.
7. References
7.1. Normative References
[RFC7348] Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger,
L., Sridhar, T., Bursell, M., and C. Wright, "Virtual
eXtensible Local Area Network (VXLAN): A Framework for
Overlaying Virtualized Layer 2 Networks over Layer 3
Networks", RFC 7348, DOI 10.17487/RFC7348, August 2014,
<http://www.rfc-editor.org/info/rfc7348>.
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Internet-Draft Legacy SF Mapping August 2015
7.2. Informative References
[I-D.dolson-sfc-vlan]
Dolson, D., "VLAN Service Function Chaining", draft-
dolson-sfc-vlan-00 (work in progress), February 2014.
[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.ww-sfc-control-plane]
Li, H., Wu, Q., Boucadair, M., Jacquenet, C., Haeffner,
W., Lee, S., Parker, R., Dunbar, L., Malis, A., Halpern,
J., Reddy, T., and P. Patil, "Service Function Chaining
(SFC) Control Plane Components & Requirements", draft-ww-
sfc-control-plane-06 (work in progress), June 2015.
Authors' Addresses
Haibin Song
Huawei
101 Software Avenue, Yuhuatai District
Nanjing, Jiangsu 210012
China
Email: haibin.song@huawei.com
Jianjie You
Huawei
101 Software Avenue, Yuhuatai District
Nanjing, 210012
China
Email: youjianjie@huawei.com
Lucy Yong
Huawei
5340 Legacy Drive
Plano, TX 75025
U.S.A.
Email: lucy.yong@huawei.com
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Yuanlong Jiang
Huawei
Bantian, Longgang district
Shenzhen 518129
China
Email: jiangyuanlong@huawei.com
Linda Dunbar
Huawei
1700 Alma Drive, Suite 500
Plano, TX 75075
U.S.A.
Email: ldunbar@huawei.com
Nicolas Bouthors
Qosmos
Email: nicolas.bouthors@qosmos.com
David Dolson
Sandvine
408 Albert Street
Waterloo, ON N2L 3V3
Canada
Email: ddolson@sandvine.com
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