SFC working group H. Song Internet-Draft J. You Intended status: Informational L. Yong Expires: March 10, 2017 Y. Jiang L. Dunbar Huawei N. Bouthors Qosmos D. Dolson Sandvine September 6, 2016 SFC Header Mapping for Legacy SF draft-song-sfc-legacy-sf-mapping-08 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 supporting the SFC header, or even being aware of it. This document provides some of the mechanisms 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 March 10, 2017. Song, et al. Expires March 10, 2017 [Page 1] Internet-Draft Legacy SF Mapping September 2016 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 . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.1. For Transparent Service Functions . . . . . . . . . . . . 5 3.1.1. VLAN . . . . . . . . . . . . . . . . . . . . . . . . 5 3.1.2. VXLAN . . . . . . . . . . . . . . . . . . . . . . . . 6 3.1.3. Ethernet MAC Address . . . . . . . . . . . . . . . . 6 3.1.4. 5-tuple . . . . . . . . . . . . . . . . . . . . . . . 6 3.2. For Non-transparent Service Functions . . . . . . . . . . 7 4. Operation Considerations . . . . . . . . . . . . . . . . . . 8 4.1. Examplar Mechanisms . . . . . . . . . . . . . . . . . . . 8 4.2. Challenges to Support Legacy SF . . . . . . . . . . . . . 8 4.3. Metadata . . . . . . . . . . . . . . . . . . . . . . . . 10 5. Security Considerations . . . . . . . . . . . . . . . . . . . 10 6. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 10 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 7.1. Normative References . . . . . . . . . . . . . . . . . . 10 7.2. Informative References . . . . . . . . . . . . . . . . . 11 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11 1. Introduction A Service Function Chain (SFC) [RFC7665] 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, i.e. SFC-unaware SFs. The SFC proxy is proposed to act as a gateway between the SFC encapsulation and SFC- unaware SFs. The SFC proxy removes the SFC header and then sends the packet to a legacy SF for processing, but how to associate the Song, et al. Expires March 10, 2017 [Page 2] Internet-Draft Legacy SF Mapping September 2016 original SFC header with the packet returned from the legacy SF needs to be considered. This document describes some of the mechanisms between an SFC proxy and a legacy SF, to identify the SFC header associated with a packet that is returned from a legacy SF. The benefit for supporting legacy SF is that SFC-unaware SFs can exist in the SFC-enabled domain. An SFC proxy allows a legacy SF to function in the SFC-enabled domain without modification of the legacy SF. +----------------+ |SFC-unaware | |Service Function| | (Legacy SF) | +----+----+------+ ^ | | | +----+----+------+ |Switch(optional)| +----+----+------+ | | (2)| |(3) | | +----+----V--------+ (1) | SFC | (4) -------->| Proxy +-------> +------------------+ Figure 1: Procedure of a packet processed by a legacy SF Different classes of legacy SF may have variable 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), usually depending on device configuration. For example, by creation of VLANs, traffic is steered through a firewall. This document focuses heavily on legacy SFs that are transparent at layer 2. In particular we assume the following conditions apply in the class of legacy SF we are considering proxying: 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. Song, et al. Expires March 10, 2017 [Page 3] Internet-Draft Legacy SF Mapping September 2016 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 followed 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 (1) from an SFF, and removes its SFC header, which may optionally contain metadata, and store the SFC header locally, and then (2) sends the de-encapsulated packet to the SF. After the SF processes the packet, the packet will be sent back (3) 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, returning the packet to an SFF (4). Song, et al. Expires March 10, 2017 [Page 4] Internet-Draft Legacy SF Mapping September 2016 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 layer 2/3/4 packet header sent to it, but it may drop the packet. Non-transparent SF: A service function that changes some bits of the layer 2/3/4 packet header sent to it. SFC Proxy: Removes and inserts SFC encapsulation on behalf of an SFC-unaware service function. SFC proxies are logical elements. 3. Mechanisms The mapping mechanisms between the SFC proxy and the transparent or non-transparent legacy SFs are discussed in this section. The mechanisms used in this document require that each forwarding entity (i.e., SFC proxy) and its connected service functions are in the same layer 2 network. The detailed definitions of SFC proxy and SFC- unaware SFs is discussed in [RFC7665]. 3.1. For Transparent Service Functions 3.1.1. VLAN If the service function is transparent to packet headers, for example, layer-2-transparent SF, then VLAN can be used for mapping between the SFC proxy and SF. It is assumed that the switch between the SFC proxy and SF delivers traffic for all VLANs, or the SFC proxy and SF may be directly connected. The SFC proxy removes the SFC header and sends the packet to the SF, with encapsulating a certain VLAN ID that can represent the SFC header. The legacy SF is supposed to accept VLAN-tagged packets and send them back on the same VLAN. It is assumed that the SF is able to process Ethernet packets with VLAN tags and also accept a wide range of VLAN tags. The SFC proxy locally maintains the mapping between VLAN ID/direction and the SFC header. When receiving 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 Song, et al. Expires March 10, 2017 [Page 5] Internet-Draft Legacy SF Mapping September 2016 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. 3.1.2. VXLAN If the SFC proxy and SF are already deployed in a nested VLAN network, the VLAN mapping method is not applicable. Then VXLAN [RFC7348] can be used for the mapping, i.e. VNI can be used for the mapping between them. VXLAN is a Layer 2 overlay scheme over a Layer 3 network. It uses MAC Address-in-User Datagram Protocol (MAC-in- UDP) encapsulation. 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 VNIs is larger. 3.1.3. Ethernet MAC Address The MAC address also can be used to associate an SFC header between the SFC proxy and SF; i.e., each SFC header will be assigned a source MAC address on the SFC proxy. When the SFC proxy receives 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. 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. It is assumed that the SF passes the MAC header through without even reversal. A variation that is symmetric assigns a unique source/destination pair for each unique SFC header. 3.1.4. 5-tuple The 5-tuple of a packet carried within SFC encapsulation can be used by the SFC proxy as a key to associate an SFC header 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 scenario, as such uniqueness could be valid only within the scope of a single tenant. Song, et al. Expires March 10, 2017 [Page 6] Internet-Draft Legacy SF Mapping September 2016 So if the SFC is provided as a multi-tenant service, this method would fail. 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 procedure 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.ietf-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. +------------------------+ | | | SFC Control Plane | | | +---^------------^-------+ | | |C2 | (1) | | +---V----+ |C4 -----+ SFF +------ | | | | +----+---+ | | +---V----+ +-------+ SFC | (2) | Proxy | +---^----+ | (3)|(4) +------V---------+ |SFC-unaware | |Service Function| +----------------+ Song, et al. Expires March 10, 2017 [Page 7] Internet-Draft Legacy SF Mapping September 2016 4. Operation Considerations 4.1. Examplar Mechanisms The following table gives some examplar methods and the conditions to use. Table 1: Mapping Examples +-----------+--------+-------------------------------+-------------------+ | |Methods | Stored Key-Value |Application | | | | |Scenario | +-----------+--------+-------------------------------+-------------------+ | |VLAN | (Direction, VLAN ID, |L2 header won't | | For Trans-| | SFC header) |be modified by the | | parent SF | |e.g., assign a VLAN ID per |SF. | | | |bidirectional path-pair | | | +--------+-------------------------------+-------------------+ | |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. | | +-----------+--------+----------------- -------------+-------------------+ | |Case-by-|Mapping rules: |The SFC proxy is | |For |case |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. | +-----------+--------+---------------------------------------------------+ 4.2. Challenges to Support Legacy SF The key problem contemplated in this document is: what packet 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 Song, et al. Expires March 10, 2017 [Page 8] Internet-Draft Legacy SF Mapping September 2016 within the path. Should the path act as a qualifier to the flow, or should a flow be allowed to change paths? 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. 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 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. Song, et al. Expires March 10, 2017 [Page 9] Internet-Draft Legacy SF Mapping September 2016 4.3. Metadata 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 make use of the layer 2 header, it may cause security threats. There may be security issues with state exhaustion on the SFC proxy, e.g., exhausting VLAN IDs, or exhausting 5-tuple state memory. 6. Acknowledgement The authors would like to thank Ron Parker and Joel Halpern for their valuable 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, . Song, et al. Expires March 10, 2017 [Page 10] Internet-Draft Legacy SF Mapping September 2016 [RFC7665] Halpern, J., Ed. and C. Pignataro, Ed., "Service Function Chaining (SFC) Architecture", RFC 7665, DOI 10.17487/RFC7665, October 2015, . 7.2. Informative References [I-D.ietf-sfc-control-plane] Boucadair, M., "Service Function Chaining (SFC) Control Plane Components & Requirements", draft-ietf-sfc-control- plane-07 (work in progress), August 2016. 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 Yuanlong Jiang Huawei Bantian, Longgang district Shenzhen 518129 China Email: jiangyuanlong@huawei.com Song, et al. Expires March 10, 2017 [Page 11] Internet-Draft Legacy SF Mapping September 2016 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 Song, et al. Expires March 10, 2017 [Page 12]