Re: [OSPF] [IPv6 SR] Regarding 128 bits IPv6 address in Segment List of SRH

"Stefano Previdi (sprevidi)" <> Fri, 10 February 2017 09:39 UTC

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From: "Stefano Previdi (sprevidi)" <>
To: Veerendranatha Reddy Vallem <>
Thread-Topic: [IPv6 SR] Regarding 128 bits IPv6 address in Segment List of SRH
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Date: Fri, 10 Feb 2017 09:31:51 +0000
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Subject: Re: [OSPF] [IPv6 SR] Regarding 128 bits IPv6 address in Segment List of SRH
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Hi Veerendranath,

yes, an SR-IPv6 SID is a 128-bit IPv6 addresses. 

The semantic associated to the SID is given by the control plane. We have already documented the signaling of the SIDs in ISIS, OSPF and BGP. Currently we have defined Node-SIDs (representing a node) and Adjacency-SIDs (instruction to forward out to the interface the SID is allocated to).

In the IPv6 dataplane a SID being an IPv6 address, it makes the SID a global IPv6 address (even in the case of Adj-SIDs). This of course is orthogonal to the control plane that may or may not advertise such address.

I.e., you may have an Adj-SID as a global IPv6 address that it is not advertised by any routing protocol in the network (which implies of course that the packet will have to first reach the node using a node-SID).

The use of LL addresses as SID has not been contemplated for the simple reason that a router may well allocated the same address to all links so it is not a reliable mechanism for forwarding. This will be fixed in the next revision of the ospfv3 draft (isis draft is ok).


> On Feb 10, 2017, at 7:37 AM, Veerendranatha Reddy Vallem <> wrote:
> Dear Previdi,
> Thanks for your reply. 
> As per first version of draft, Adj-SID is also  IPv6 prefix. 
> 4.2.2.  Adjacency-SID
>   The Adjacency-SID identifies a given interface.  In the SR
>   architecture a node may advertise one or more Adj-SIDs allocated to a
>   given interface so to force the forwarding of the packet (when
>   received with that particular Adj-SID) into the interface, regardless
>   the routing entry for the packet destination.  The same is defined
>   for SR-IPv6: a node may advertise a given IPv6 prefix which is
>   associated to the SR semantic of "send out the packet to the
>   interface this prefix is allocated to".  Here also, the SID is in
>   fact the IPv6 prefix.
> As per my understanding  "segment list in SRH is global IPv6 prefixes, If we need to include Adj SID to segment list then Adj-SID is also global IPv6 prefix". 
> Please correct me if my understanding is wrong.
> Thanks and Regards,
> Veerendranath
> -----Original Message-----
> From: Stefano Previdi (sprevidi) [] 
> Sent: 09 February 2017 22:41
> To: Veerendranatha Reddy Vallem <>
> Cc:;;;
> Subject: Re: [IPv6 SR] Regarding 128 bits IPv6 address in Segment List of SRH
> Hi,
> the first version of the draft (draft-previdi-6man-segment-routing-header) had a description of Node-SID and Adj-SID. Later, in order to simplify the document, we removed the descriptions and focused the document into the SRH format.
> I think it will be helpful to re-introduce a section on the two main SID types (Node, Adjacency). I’m working on an update for the next version.
> Thanks.
> s.
>> On Feb 9, 2017, at 5:02 PM, Veerendranatha Reddy Vallem <> wrote:
>> Dear Authors,
>> I am requesting your clarification regarding usage of Adj-SID in SRH header