Re: [sfc] TTL field within the NSH base header

[Dave Dolson <[e-mail]>]

Ron,
Decrementing from zero to reach 63, and checking for zero after decrementing provides a degree of backwards compatibility with the prior header format.
This permits packets on the wire to have 0 in the bits that were previously "reserved".

-Dave


From: sfc [mailto:[e-mail]] On Behalf Of Ron Parker
Sent: Thursday, April 27, 2017 3:21 PM
To: James N Guichard; [e-mail]
Subject: Re: [sfc] TTL field within the NSH base header

I had to really think about the language around decrementing by 1 from 0 and reaching 63.   This would only occur in an error scenario where a preceding SFF misbehaved, wouldn't it?   Why are we insistent that the test for 0 happen after decrement, thereby creating this problem.     I suspect that this has been already discussed and I apologize if I missed it, but if the language were simplified to say an SFF that receives an NSH with TTL=1 shall not forward it to another SFF (e.g., allowing it to engage its local SF instances, still).   Otherwise, it must decrement by 1 before forwarding to another SFF.

An exact wording along these lines might be something like:

TTL: Service plane time-to-live. An SFF MUST test the TTL before forwarding to another SFF for a given Service Function Chain.  If the received TTL value is 1, the SFF MUST drop packets that would otherwise have been forwarded to another SFF, but SHALL send such packets to attached service functions if the SFF terminates the Service Function Chain.   If the TTL value is greater than 1, the SFF must decrement the TTL by 1 before forwarding to another SFF.   The default for originating an NSH packet is a TTL value of 63.



From: sfc [mailto:[e-mail]] On Behalf Of James N Guichard
Sent: Thursday, April 27, 2017 2:54 PM
To: [e-mail]<mailto:[e-mail]>
Subject: [sfc] TTL field within the NSH base header

Dear WG:

Having reviewed all of the email discussion on the mailing list it appears to the chairs that we have consensus to add a TTL field to the NSH base header. We would like to propose the following changes:

Section 3.2:
Update figure 2 as follows:

     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|O|R|    TTL    |   Length  |R|R|R|R|MD Type| Next Protocol |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Add the following text after figure 2:

TTL: Service plane time-to-live. An SFF MUST decrement the TTL by a value of 1 for all NSH packets it receives. Decrementing by a value of 1 from 0 shall result in a TTL value of 63. The default for originating an NSH packet is a TTL value of 63. The decrement SHALL occur before testing for 0. After decrement, if the TTL is 0, the NSH packet MUST NOT be forwarded.

Section 3.4:
Update figure 4 to reflect the new base header format as per section 3.2 base header.

Section 3.5:
Update figure 5 to reflect the new base header format as per section 3.2 base header.

Section 12.2.1:
Current text is as follows:

   There are ten bits at the beginning of the NSH Base Header.  New bits
   are assigned via Standards Action [RFC5226].

   Bits 0-1 - Version
   Bit 2 - OAM (O bit)
   Bit 3 - Critical TLV (C bit)
   Bits 4-9 - Reserved

Replace entire text as follows:

   There are eight reserved bits in the NSH Base Header. New bits
   are assigned via Standards Action [RFC5226].

   Bits 0-1 - Version
   Bit 2 - OAM (O bit)
   Bit 3 - Reserved
   Bits 16-19 - Reserved

Section 12.2.3:
Current text has the MD-type as 8-bit values.

Update text for this section and table 1 to reflect 4-bit values *not* 8-bit values.

Please review carefully and indicate support for these changes (or any changes to the suggested text).

Thanks,

Jim & Joel