Early Review of draft-ietf-rtgwg-bgp-pic-00
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.
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Reviewer: Bruno Decraene
IETF LC End Date: “QA review” pre WG LC
Intended Status: Informational
I have some minor concerns about this document that I think should be resolved before publication.
- 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).
- 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 --..
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.
- 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.
"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)
"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.
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.)
"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.
"Hence traffic restoration occurs within the time frame of IGP convergence,"
..."and, for local link failure, within the timeframe of local detection. Thus it is possible to achieve sub-50 msec convergence as described in  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  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 .
I would find useful to indicate, for each type of failure, the number of data-structure that need to be updated.
"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?
" 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
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)
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)....
"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)
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)
"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)
"The objective is achieved through organizing the forwarding chains"
"chain" does not self self explicit to me. what about :s/chains/data structure"
what do you mean? transparency to what / from who?
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
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.
"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?
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.
- 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.
As a side note, the previous definition assume that there were no Route Relfector (the iBGP peer is the BGP Next Hop)
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.
OLD: ePE2 constructs the forwarding chain depicted in Figure 1
NEW: ePE2 constructs the forwarding chain depicted in Figure 3
OLD: can reach ASBR1
NEW: can reach ASBR11
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
OLD: The labels for advertised to iPE by ASBR11 using BGP-LU
NEW: The labels advertised by ASBR11 to iPE using BGP-LU
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.
(somewhat guesssing. But in all cases, there is no figure 1 in section 2.3.1)
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.
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