Re: RtgDir review: draft-ietf-rtgwg-bgp-pic-00
Hi Ahmed,
Many thanks for the updated version and your below answers.
Looks good to me.
Please find below 2 minor typos
Thanks.
Bruno
:s/unreacreachable/unreachable
:s/hierarchal/hierarchical
*From:*rtgwg [mailto:[email protected]] *On Behalf Of *Ahmed
Bashandy (bashandy)
*Sent:* Tuesday, June 21, 2016 6:36 PM
*To:* [email protected]
*Cc:* [email protected]; [email protected]
*Subject:* Re: RtgDir review: draft-ietf-rtgwg-bgp-pic-00
Hi,
Thanks a lot for the detailed review.
I submitted version 01. I CCed Jeff and Rob Shakir, who volunteered
to be a shepherd (Thanks a lot):):)
I have restructured the document to address your comment. Besides I
went over all the "Minor issues" as well as "nits" and corrected them,
except 1-2 comments which I explained why I did not make the
modifications.
Version 01 is modified and now it contains an "overview" section.
Hence all sections have been shifted by one. So for example, the
comment about section 2.3.3 now applies to section 3.2
Please see my reply inline starting with "#Ahmed"
Thanks
Ahmed
On 4/20/2016 6:53 AM, [email protected]
<mailto:[email protected]> wrote:
Hello,
I have been selected as the Routing Directorate reviewer for this
draft. The Routing Directorate seeks to review all routing or
routing-related drafts as they pass through IETF last call and
IESG review, and sometimes on special request. The purpose of the
review is to provide assistance to the Routing ADs. For more
information about the Routing Directorate, please see
http://trac.tools.ietf.org/area/rtg/trac/wiki/RtgDir
Although these comments are primarily for the use of the Routing
ADs, it would be helpful if you could consider them along with any
other IETF Last Call comments that you receive, and strive to
resolve them through discussion or by updating the draft.
Document: draft-ietf-rtgwg-bgp-pic-00
Reviewer: Bruno Decraene
IETF LC End Date: “QA review” pre WG LC
Intended Status: Informational
*Summary:*
I have some minor concerns about this document that I think should
be resolved before publication.
*Comments:*
- Document is interesting. Document is relatively clear but
sometime it feels like there is a little room for some
reformulation/edition to improve fluidity. In particular, the
learning curve is a bit steep at the beginning of the doc as most
of the concepts are introduced in 3 pages (pages 4-6) in the form
of a list of terminology and a pseudo code. I would find useful to
have an overview section just after the introduction, with a high
level view of the solution with a limited number of new terms.
- The text feels like authoritative, while probably many terms are
implementation specific. A priori, I would not expect all
implementation of BGP PIC to use the same terms, and possibly not
the same data structure. May be the text could be generalized to
cover multiple implementations; or modified to describe a
generalized concept (i.e. data-structure designed to share as much
data as possible between elements, at the cost of additional
indirections); or the document could state that it describes a
specific implementation with implementations specific terminology,
data structure, and specifics. Or a combination of both (e.g.
adding a section being both generalized and describing the
concept, and then the existing sections after stating that they
are specific to one implementation).
#Ahmed: I have restructured the document to make much more general.
Please take a look. All feedback is most welcomed.
*Minor Issues:*
- I find figure 2 very useful to understand the data-structure. I
would move it sooner in the doc, somewhere before §2.2. (with its
subsequent text below) e.g. a new §2.2 "FIB data-structure"
It would need to be generalized i.e. example non-specific. I could
think of:
IP Leaf: Pathlist: IP Leaf: Pathlist:
-------- --------- ------- --------
BGP NLRI ---> BGP NH1 ----> IGP IP1 (BGP NH1) ---> IGP NH1, I1 --->
Adjacency1
BGP NHi --... IGP NHi, Ii --..
| |
| |
| |
v v
OutLabel Array: OutLabel Array:
-------------- --------------
L (NLRI, NH1) L (IP1, NH1)
L (NLRI, NHi) L (IP1, NHi)
- Figure 1 could be enhanced with IGP-NH1, IGP-NH2, I1 and I2.
#Ahmed: Corrected
- Example 3 does not use the same naming convention than examples 1
and 2, this make it harder to follow for a priori no reason. e.g. VPN
labels are named VPN-L11 in examples 1 and 2, but are named
VPN-PE21(P1) in exmaple 3; transport labels are named LDP-L12 in
exmaples 1 and 2, but LASBR11(PE22) and L11 in figure 3.
#Ahmed: I renamed the VPN labels to conform to the convention used in
Examples 1 and 2. Also All IGP labels were renamed to IGP-Lij to make
them applicable to both LDP and Segment routing.
For ASBR labels, we need to differentiate between ASBR labels and IGP
labels. Hence the labels advertised by ASBRs are named differently.
- §2.3.3
"The local labels of the next hops".
- All labels are locally assigned. So what do you mean by "local"
- "next-hop" sometimes refers to IGP/connected next-hop (a priori the
case here) and sometimes to BGP next-hop. I find it hard to follow. I
rather use a different name (e.g; connected next-hop vs BGP next-hop)
#Ahmed: Corrected.
- §3
"the hashing at the BGP level yields path 0 while the hashing at the
IGP level yields path 1. In that case, the packet will be sent out of
interface I1 with the label stack "LDP-L12,VPN-L21".
Does not seem to match my understanding. For "LDP-L12,VPN-L21" I would
assume BGP used path index 1 and IGP used path index 0.
#Ahmed: Corrected.
IMHO:
OLD: "Hence ASBR22 swaps "LASBR22(PE22)" with the LDP/SR label of
PE22, pushes the label of the next-hop towards PE22 in domain 2, and
sends the packet along the shortest path towards PE22."
NEW: "Hence ASBR22 swaps "LASBR22(PE22)" with the LDP/SR label for
PE22 advertised by the next-hop towards PE22 in domain 2, and sends
the packet along the shortest path towards PE22."
(in all cases "swaps" then "pushes" would increase the label stack by
1, which is not the case.)
#Ahmed: Corrected
§4.1
"the useable paths in the loadinfo"
loadinfo is a proprietary FIB datastructure which has not been
introduced/defined. You need to either remove that term (if possible)
or define it somewhere.
#Ahmed: Corrected. "loadinfo" is replaced with "pathlist", which is
the intended term
"Hence traffic restoration occurs within the time frame of IGP
convergence,"
agree.
..."and, for local link failure, within the timeframe of local
detection. Thus it is possible to achieve sub-50 msec convergence as
described in [10] for local link failure"
IMO, this is restricted to specific cases. e.g. external (eBGP) link
failure, ECMP case, possibly IP FRR. So possibly
OLD: for local link failure, within the timeframe of local detection.
Thus it is possible to achieve sub-50 msec convergence as described in
[10] for local link failure
NEW: for local link failure, assuming a backup path has been
precomputed, within the timeframe of local detection (e.g. 50ms).
Example of solutions precomputing a backup path are IP FRR [LFA],
[RLFA], [MRT], [TI-LFA] or eBGP path having a backup path [10].
#Ahmed: Corrected
§4
I would find useful to indicate, for each type of failure, the number
of data-structure that need to be updated.
#Ahmed: In general it is not always possible to know the number of
updated data structures due to a topology change because it depends on
the topology itself. For example, in Section 5.1 (The section about
BGP-PIC core), it is not possible to outline the datastructures that
are modified for a remote link failure. However for local link
failure, we added a statement in second paragraph of Section 5.1
indicating that only pathlists with paths using the local link need to
be updated. We also indicated in the 3rd paragraph that datastructures
are modified starting from the IGP leaves without walking back to BGP
pathlists or data structures
---
§4.2.2
"To avoid loops, ePE2 MUST treat any core facing path as a backup
path, otherwise ePE2 may redirect traffic arriving from the core
back to ePE1 causing a loop."
Looks a bit under-described to me. Could you please elaborate a bit?
In particular:
- if 2 PE (PE1, PE2) are connected in U to 2 P (P1, P2)
(P1-PE1-PE2-P2), PE1 being nominal and PE2 only used in backup, in the
nominal situation, if the core network sends the trafic to PE1 via PE2
(used as a P/transit), how does PE2 know that it must send this
traffic to PE1? (rather than CE2)
- this behavior looks like an additional specific feature. How doew
ePE1 knows that ePE2 have this feature?
---
#Ahmed: The statement is removed. As mentioned in Section 6.1 how to
avoid loops in case of CE node failure is a different topic and needs
to be addressed separately
§4.3
" Hence if the platform supports the "unflattened" forwarding chain,
then a single pathlist needs to be updated while if the platform
supports a shallower forwarding chain, then two pathlists need to be
updated."
IINM "single" and "two" pathlist applies to the specific example. In
this last sentence/summary, I'd prefer a more general statement. A
priori, without digging too much in this most complex use case, it
seems like :s/single/o(1) :s/two/o(PE) . The former looks close
(single vs o(1)) but IMHO there is a significant difference between 2
and o(PE) (i.e. 100s)
#Ahmed: Agreed. The last paragraph is generalized and modified to
indicate possible outcomes of flattenning forwarding chains
---
§5.1
Good paragraph. It's quite clear that the convergence time does not
depend on the number of BGP prefixes, which is good. For the benefit
of the reader, it would be even more interesting if, for each type of
failure, the text could indicate on what it depends. e.g. o(1),
o(connected interfaces), o(PE), o(PEnominal*PEbackup)....
#Ahmed: Each subsection indicates the convergence time dependency. For
example, in Section 6.1.1, the last paragraph says that the dependence
is on the IGP convergence time only
--
§7
"No additional security risk is introduced by using the mechanisms
proposed in this document"
In general, with such a sentence, it's difficult to evaluate whether
the authors have very quickly evaluated the risk or if this evaluation
has been performed in details. So in general, some more text detailing
which aspects have been evaluated is interesting for the reader (yet
painful for the authors).
As the document describe an internal box behavior, this is difficult
to evaluate and discuss. But from a bad experience, I fear that there
may be an impact. Indeed, with such structure, the FIB
structure/memory is typically different between BGP prefixes and IGP
prefixes. In general, the implementation is designed to support the
"right" numbers of both. But assuming an accident or an attack, the
numbers may not be "right". e.g. one upon a time, someone has
redistributed the BGP table into the IGP. In this case, the total
number of IP prefixes in the FIB is exactly the same. But as the data
structure used in the FIB was different between BGP and IGP prefixes,
the FIB ran out of memory and the line card crashed (well actually
only the IP FIB, so IS-IS hello packet were still correctly sent and
forwarded. As a result, traffic was permanently black holed)
#Ahmed: I added a statement indicating that the behavior is internal
to the router.
On another front, although the scenario that you mentioned is related
to reliability rather than security, it indicates that BGP-PIC
actually improves the reliability of FIB because it greatly reduces
the use of hardware and software memory
---
§ 9
OLD: that allows achieving prefix independent convergence
NEW: that allows achieving BGP prefixes independent convergence
(it's still depend on the number of IGP prefixes and/or BGP pathlist)
#Ahmed: Corrected
*Nits:*
Abstract
"via more than one path."
In this 1rst sentence, it's not clear what path really means. (e.g cf
the terminology section where you have more than one). I guess that
you mean "BGP path". (as there are also typically multiple IGP path to
reach each BGP Next Hop)
#Ahmed: modified to next-hop to indicate BGP next-hops
"The objective is achieved through organizing the forwarding chains"
"chain" does not self self explicit to me. what about :s/chains/data
structure"
#Ahmed: Agreed, changed to "data structures"
"complete transparency"
what do you mean? transparency to what / from who?
#Ahmed: Removed the word "transparency
§1
OLD: to allow for more than one path for a given prefix
NEW: to allow for BGP to advertise more than one path for a given prefix
#Ahmed: Modified as suggested
OLD: Another more common and widely deployed scenario is L3VPN with
multi-homed VPN sites
NEW: Another more common and widely deployed scenario is L3VPN with
multi-homed VPN sites with unique Route Distinguisher.
#Ahmed: Modified as suggested
---
§1.2
"Pathlist: It is an array of paths"
"OutLabel-Array: The OutLabel-Array is a list of one or more outgoing
labels "
So a list is defined as an array and the array is defined as a list :-).
What about using the same term, e.g. a list?
#Ahmed: Corrected. Both are now pathlist and OutLabel-List:)
--
The OutLabel-Array is a list of one or more
outgoing labels and/or label actions where each label or label
action has 1-to-1 correspondence to a path in the pathlist. It
is possible that the number of entries in the OutLabel-array is
different from the number of paths in the pathlist and the ith
Outlabel-Array entry is associated with the path whose path-
index is "i".
- I don't see how one can have a 1-to-1 correspondance if the number
of elements is not the same.
#Ahmed Corrected.
- Last sentence could be splitted in 2.
--
Since the term ingres PE is defined, you could also detine the term
egress PE. Possibly in the same sentence.
OLD: "Ingress PE, "iPE": It is a BGP speaker that learns about a
prefix through another IBGP peer and chooses that IBGP peer as
the next-hop for the prefix
NEW: "Ingress PE, "iPE": It is a BGP speaker that learns about a
prefix through a IBGP peer and chooses an egress PE as the
next-hop for the prefix.
#Ahmed: Modified as suggested
As a side note, the previous definition assume that there were no
Route Relfector (the iBGP peer is the BGP Next Hop)
#Ahmed: Correct. I did not want to unnecessarily complicate the
discussion with information
--
§2.3
Figure 1 represents a VPN network with 3 PE and a CE. In this context,
"VPN-P1" sounds a bit like a P router. What about :s/VPN-P1/VPN-IP1 ?
Same comment for IGP-P1.
#Ahmed: Agreed. All modified as suggested
--
§2.3.2
OLD: ePE2 constructs the forwarding chain depicted in Figure 1
NEW: ePE2 constructs the forwarding chain depicted in Figure 3
#Ahmed: Corrected
OLD: VPL-L11
NEW: VPN-L11
#Ahmed: Corrected
§2.3.3
OLD: can reach ASBR1
NEW: can reach ASBR11
#Ahmed: Corrected
OLD: The label for advertised by ASBR11 to iPE
NEW: The label advertised by ASBR11 to iPE
OLD: The labels for advertised by ASBR12 to iPE
NEW: The labels advertised by ASBR12 to iPE
#Ahmed: Corrected
OLD: The labels for advertised to iPE by ASBR11 using BGP-LU
NEW: The labels advertised by ASBR11 to iPE using BGP-LU
#Ahmed: Corrected
---
§3
OLD: Let's applying the above forwarding steps to the example
described in Figure 1 Section 2.3.1.
OLD: Let's applying the above forwarding steps to the example
described in Figure 2 Section 2.3.1.
#Ahmed: Corrected
(somewhat guesssing. But in all cases, there is no figure 1 in section
2.3.1)
---
§4.1
IMO
OLD: As soon as the IGP convergence is effective for the BGP nhop
entry, the new forwarding state is immediately available to all
dependent BGP prefixes.
NEW: As soon as the IGP convergence is effective for a BGP next-hop
entry, the new forwarding state is immediately available to all
dependent BGP prefixes.
more generally
:s/nhop/next-hop
#Ahmed: Replaced nhop with next-hop in the entire document
---
§4.3
:s/PE222/PE22
#Ahmed: Corrected
Best regards,
Bruno
**
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