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diff --git a/src/plugins/srv6-am/am_plugin_doc.md b/src/plugins/srv6-am/am_plugin_doc.md index d5d18cf99f6..11aad855408 100644 --- a/src/plugins/srv6-am/am_plugin_doc.md +++ b/src/plugins/srv6-am/am_plugin_doc.md @@ -1,91 +1,100 @@ # SRv6 endpoint to SR-unaware appliance via masquerading (End.AM) {#srv6_am_plugin_doc} -## Overview - -The "Endpoint to SR-unaware appliance via masquerading" (End.AM) is a two-parts -function for processing SRv6 **inserted** traffic on behalf of an SR-unaware -appliance. The first part decrements the Segments Left value and **replaces the -IPv6 Destination Address with the last segment in the SRH**, while the second -restores the IPv6 Destination Address with the active segment in the traffic -coming back from the appliance. - -In this scenario, we assume that the appliance can only inspect, drop or perform -limited changes to the packets. In particular, the appliance must not change the -IP Destination Address of the packet, terminate a transport connection nor -generate arbitrary packets. For example, Firewalls, Intrusion Detection Systems, -Deep Packet Inspectors are among the appliances that can be supported in this -scenario. +The masquerading proxy is an SR endpoint behavior for processing SRv6 traffic on +behalf of an SR-unaware SF. This proxy thus receives SR traffic that is formed +of an IPv6 header and an SRH on top of an inner payload. The masquerading +behavior is independent from the inner payload type. Hence, the inner payload +can be of any type but it is usually expected to be a transport layer packet, +such as TCP or UDP. + +A masquerading SR proxy segment is associated with the following mandatory +parameters: + +- S-ADDR: Ethernet or IPv6 address of the SF +- IFACE-OUT: Local interface for sending traffic towards the SF +- IFACE-IN: Local interface receiving the traffic coming back from the SF + +A masquerading SR proxy segment is thus defined for a specific SF and bound to a +pair of directed interfaces or sub-interfaces on the proxy. As opposed to the +static and dynamic SR proxies, a masquerading segment can be present at the same +time in any number of SR SC policies and the same interfaces can be bound to +multiple masquerading proxy segments. The only restriction is that a +masquerading proxy segment cannot be the last segment in an SR SC policy. + +The first part of the masquerading behavior is triggered when the proxy node +receives an IPv6 packet whose Destination Address matches a masquerading proxy +segment. The proxy inspects the IPv6 extension headers and substitutes the +Destination Address with the last segment in the SRH attached to the IPv6 +header, which represents the final destination of the IPv6 packet. The packet is +then sent out towards the SF. + +The SF receives an IPv6 packet whose source and destination addresses are +respectively the original source and final destination. It does not attempt to +inspect the SRH, as RFC8200 specifies that routing extension headers are not +examined or processed by transit nodes. Instead, the SF simply forwards the +packet based on its current Destination Address. In this scenario, we assume +that the SF can only inspect, drop or perform limited changes to the packets. +For example, Intrusion Detection Systems, Deep Packet Inspectors and non-NAT +Firewalls are among the SFs that can be supported by a masquerading SR proxy. + +The second part of the masquerading behavior, also called de- masquerading, is +an inbound policy attached to the proxy interface receiving the traffic +returning from the SF, IFACE-IN. This policy inspects the incoming traffic and +triggers a regular SRv6 endpoint processing (End) on any IPv6 packet that +contains an SRH. This processing occurs before any lookup on the packet +Destination Address is performed and it is sufficient to restore the right +active segment as the Destination Address of the IPv6 packet. + +For more information, please see +[draft-xuclad-spring-sr-service-chaining](https://datatracker.ietf.org/doc/draft-xuclad-spring-sr-service-chaining/). + +## CLI configuration + +The following command instantiates a new End.AM segment that sends masqueraded +traffic on interface `IFACE-OUT` towards an appliance at address `S-ADDR` and +restores the active segment in the IPv6 header of the packets coming back on +interface `IFACE-IN`. -## Pseudo-code +``` +sr localsid address SID behavior end.am nh S-ADDR oif IFACE-OUT iif IFACE-IN +``` -When instantiating an End.AM SID, the following parameters are required: +For example, the below command configures the SID `1::A1` with an End.AM +function for sending traffic on interface `GigabitEthernet0/8/0` to the +appliance at address `A1::`, and receiving it back on interface +`GigabitEthernet0/9/0`. -- APP-ADDR: IP or Ethernet address of the appliance -- IFACE-OUT: local interface for sending traffic towards the appliance -- IFACE-IN: local interface receiving the traffic coming back from the appliance +``` +sr localsid address 1::A1 behavior end.am nh A1:: oif GigabitEthernet0/8/0 iif GigabitEthernet0/9/0 +``` -Packets can be sent to and received from an appliance on the same interface -(IFACE-IN = IFACE-OUT). +## Pseudocode ### Masquerading -Upon receiving a packet destined to S, where S is a local End.AM SID, a node N -does: - - IF NH=SRH & SL > 0 THEN ;; Ref1 - Decrement SL - Write the last SID in the DA - Forward the packet on IFACE-OUT - ELSE - Drop the packet +Upon receiving a packet destined for S, where S is an IPv6 masquerading proxy +segment, a node N processes it as follows. -**Ref1:** an End.AM must not be the last SID. +``` +IF NH=SRH & SL > 0 THEN + Update the IPv6 DA with SRH[0] + Forward the packet on IFACE-OUT +ELSE + Drop the packet +``` ### De-masquerading -Upon receiving a non-link-local IPv6 packet on IFACE-IN, a node N does: - - IF NH=SRH THEN - Replace IP DA with SRH[SL] - Lookup DA in the appropriate table and proceed accordingly - -De-masquerading is a policy attached to IFACE-IN that intercepts all packets -coming back from the appliance and restores the destination address. This -occurs before any lookup on the packet destination address (e.g. in "My Local -SIDs" table or in the FIB) is performed. - -## Benefits - -The End.AM masquerading function brings the following benefits: - -1. The appliance receives a packet with the source and destination addresses -respectively set as the original source and the final destination. -2. The appliance does not try and inspect the SRH, as RFC2460 specifies that -routing extension headers are not examined or processed by transit nodes. - -## Limitations - -An End.AM SID may be present in any number of segment lists at the same time. - -However, since the returning traffic from the appliance is processed based on -the receiving interface (IFACE-IN), this interface may only be bound to a single -End.AM SID at a time. - -In the case of a bi-directional service chain, the same End.AM SID and receiving -interface (IFACE-IN) may be used in both directions. - -## Configuration - -The following CLI instantiates a new End.AM segment that sends masqueraded -traffic on interface `IFACE-OUT` towards an appliance at address `APP-ADDR` and -restores the active segment in the IPv6 header of the packets coming back on -interface `IFACE-IN`. - - sr localsid address SID behavior end.am nh APP-ADDR oif IFACE-OUT iif IFACE-IN - -For example, the following command configures the SID `1::A1` with an End.AM -function for sending traffic on interface `GigabitEthernet0/8/0` to the appliance at -address `A1::`, and receiving it back on interface `GigabitEthernet0/9/0`. +Upon receiving a non-link-local IPv6 packet on IFACE-IN, a node N processes it +as follows. - sr localsid address 1::A1 behavior end.am nh A1:: oif GigabitEthernet0/8/0 iif GigabitEthernet0/9/0 +``` +IF NH=SRH & SL > 0 THEN + Decrement SL + Update the IPv6 DA with SRH[SL] ;; Ref1 + Lookup DA in appropriate table and proceed accordingly +``` +**Ref1:** This pseudocode can be augmented to support the Penultimate Segment +Popping (PSP) endpoint flavor. The exact pseudocode modification are provided in +[draft-filsfils-spring-srv6-network-programming](https://datatracker.ietf.org/doc/draft-filsfils-spring-srv6-network-programming/). |