Re: [alto] ALTO Draft ReCharter WG review Fri, 05 March 2021 03:03 UTC

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Subject: Re: [alto] ALTO Draft ReCharter WG review
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Hi Qin,

Thanks for the comments. A quick summary of my response is

1. "Pub/sub" means different things in different contexts and I think we must clarify what it means in the context of distributing ALTO information.

2. There are two ways of realizing complex "pub/sub" of ALTO information but I think they are fundamentally different deployment settings for one generic framework (whose details are, unfortunately, not thought through yet).

Please see the details inline.



-----Original Messages-----
From:"Qin Wu" <>
Sent Time:2021-03-04 22:21:06 (Thursday)
To: "" <>
Cc: "" <>, "" <>, "IETF ALTO" <>
Subject: Re: [alto] ALTO Draft ReCharter WG review


发件人: []
发送时间: 2021年3月3日 21:40
收件人: Qin Wu <>
抄送: IETF ALTO <>;;
主题: Re: [alto] ALTO Draft ReCharter WG review


Dear all,


Below are some comments on the 2nd item in the recharter text.


As far as I know, the ALTO incremental update extension (RFC 8895) already provides a mechanism to enable the "pub-sub" of ALTO information, using Server-Sent Events (SSE). I see there are multiple directions indicated by the new charter item:

[Qin]: Thanks for clarifying the difference between SSE and pub sub proposed in the new proposed charter item.

1. Decouple the "pub-sub" protocol with the underlying mechanism.


Besides SSE, other mechanisms can also be used to realize the "pub-sub" of ALTO information, such as HTTP/2, HTTP/3 or the methods mentioned in the charter text. Thus, a direct extension is to define the abstract format of control messages and data messages (i.e., WHAT information should be provided but not HOW), and allow different underlying protocols to use protocol-specific encodings.


For example, SSE encodes the metadata (e.g., content-type and stream id) and the content of an event using "event:" and "data:" prefixes at the beginning of each line, and uses empty lines to indicate the end of a message, while HTTP/2 (RFC 7540) may encode the metadata and the content of an event using PUSH_PROMISE/HEADERS and DATA frame .

[Qin]: Good analysis, I think we need to decide whether we should define generic pub sub mechanism or transport specific pub sub mechanism. Do you have any suggestion on this?

[KAI]: I think the generic pub-sub mechanism (or maybe the term framework is more appropriate) is more important at this point, which should also cover the direction of providing more fine-grained control. One thing that just strikes me after taking a quick look at rabbitMQ is that "pub/sub" means different things in different contexts. It is important we understand what are the requirements of generic pub/sub in the ALTO framework.

[KAI]: When we discuss "pub/sub" with SSE and HTTP/2, which is a one-to-one client-server communication, the focus of the "pub/sub" here is simple: what are the messages and how the client can control the subscribed information. However, with message queues (e.g., rabbitMQ), the communication pattern may be more complex: a message can be sent to multiple queues without knowing exactly who is subscribing. I see two ways to realize the more complex "pub/sub" requirement for ALTO information.


[KAI]: CASE A: First, the client application may use deploy its own "pub/sub" system, and the ALTO client simply serves as a producer by forwarding the ALTO messages to the "pub/sub" system. In this way, the problem is reduced to the one-to-one "pub/sub" problem.

[KAI]: CASE B: Second, ALTO servers may natively support the "pub/sub" of ALTO information. In this case, an ALTO server may need to handle events such as join/leave of clients that subscribe to the same ALTO information. For example, for a client that just subscribes to a network map, the server should send the whole map instead of incremental updates.

[KAI]: Both approaches have pros and cons. The first is simple on the server side but may be less efficient (because of triangle routing) and complex on the client side (client must handle data consistency to support dynamic subscribers). I think the generic framework should contain two aspects:

[KAI] 1. Control of ALTO information: a server-client protocol which is similar to RFC 8895 but maybe with some extended capabilities.

[KAI] 2. Distribution of ALTO information: a MQ-like protocol that controls how the ALTO information can be efficiently and consistently delivered to subscribers.

[KAI] I think the connection between these two aspects is a logical entity called ALTO Exchange (following the term used by rabbitMQ). This entity can be operated by an application provider (as in CASE A) or by a network operator (as in CASE B). However, the detailed responsibilities of this entity may still need some investigation.

I think this requirement may help integrating ALTO in network management platforms (such as OpenDaylight, Kubernetes, and ETSI ZSM*) which design their own pub-sub systems for reasons such as consistency or ease of development. It would be great if there is an interest in this direction from companies/organizations.

[Qin]: I can see The 3GPP has defined a Service-Based Architecture (SBA), whereby the control plane functionality and common data repositories of a 5G network are delivered by way of a set of interconnected Network Functions (NFs),pub sub mechanism has been well adopted in 3GPP interface.

Also in the public cloud, popular pub/sub implementations has been widely deployed,e.g., rabbitMQ (AMQP), mosquitto (MQTT), ejabberd (XMPP), and ZeroMQ. We also see many pub sub mechanism or extension has been developed in IETF, e.g., YANG Push, draft-ietf-dots-telemetry,draft-ietf-ace-mqtt-tls-profile.

* The integration fabric of ETSI ZSM provides pub-sub support but ZSM also allows services to use their own pub-sub mechanisms.


2. Enable more fine-grained control of pub-sub.


In RFC 8895, there are two types of commands which only defines WHAT information to subscribe:


- add: Make one or more new requests to receive the incremental updates.

- remove: Terminate the subscription of one or more previously-made requests.


In the meantime, the updates will be continuously sent to the client whenever a server sees fit.


The charter text proposes to enable ALTO clients to request and receive "a diverse types (such as event-triggered/sporadic, continuous), continuous, customized feed of publisher-generated information". It seems to me that the new extension wants to allow clients to specify not only WHAT information to be subscribed but also WHEN/HOW the information should be delivered (e.g., Notify me the latest value every 5 second.).

[Qin]:Good point, I think fine grained control of pub-sub allows not only periodical subscription, but also on demand subscription, which is the missing piece in the existing SSE incremental update.

Personally I find both directions to be interesting and useful. It would be great if they can be supported by real use cases.


Just my two cents.





-----Original Messages-----
From:"Qin Wu" <>
Sent Time:2021-02-22 21:50:44 (Monday)
To: "IETF ALTO" <>
Cc: "" <>, "" <>
Subject: [alto] ALTO Draft ReCharter WG review

Hi, :

We have requested one hour session for ALTO WG meeting in the upcoming IETF 110, which is arranged on Friday, March 12, 14:30-15:30(UTC).

The goal is to boil down ALTO recharter and have consensus on charter contents in IETF 110.

To get this goal, an updated inline draft charter text for ALTO has just been posted to this list,

This charter has received a couple of rounds of informal review from WG members, chairs and our Ads from brief to deep thorough, 5 new chartered items have been listed.

We would like to solicit feedback on these new chartered items and your use case, deployment, idea corresponding to these new chartered items.

Sharing your past deployment story will also be appreciated.



The ALTO working group was established in 2008 to devise a request/response protocol to allow a host to benefit from a server that is more cognizant of the network infrastructure than the host is.


The working group has developed an HTTP-based protocol and recent work has reported large-scale deployment of ALTO based solutions supporting applications such as content distribution networks (CDN).


ALTO is now proposed as a component for cloud-based interactive applications, large-scale data analytics, multi-cloud SD-WAN deployment, and distributed

computing. In all these cases, exposing network information such as abstract topologies and network function deployment location helps applications.


To support these emerging uses, extensions are needed, and additional functional and architectural features need to be considered as follows:


o Protocol extensions to support a richer and extensible set of policy attributes in ALTO information update request and response. Such policy attributes may indicate information dependency (e.g., ALTO path-cost/QoS properties with dependency on real-time network  indications), optimization criteria (e.g., lowest latency/throughput network performance objective), and constraints (e.g., relaxation bound of optimization criteria, domain or network node to be traversed, diversity and redundancy of paths).


o Protocol extensions for facilitating operational automation tasks and improving transport efficiency. In particular, extensions to provide "pub/sub" mechanisms to allow the client to request and receive a diverse types (such as event-triggered/sporadic, continuous), continuous, customized feed of publisher-generated information. Efforts developed in other working groups such as MQTT Publish / Subscribe Architecture, WebSub, Subscription to YANG Notifications will be considered, and issues such as scalability (e.g., using unicast or broadcast/multicast, and periodicity of object updates) should be considered.


o The working group will investigate the configuration, management, and operation of ALTO systems and may develop suitable data models.


o Extensions to ALTO services to support multi-domain settings. ALTO is currently specified for a single ALTO server in a single administrative domain, but a network may consist of

multiple domains and the potential information sources may not be limited to a certain domain. The working group will investigate extending the ALTO framework to (1) specify multi-ALTO-server protocol flow and usage guidelines when an ALTO service involves network paths spanning multiple domains with multiple ALTO servers, and (2) extend or introduce ALTO

services allowing east-west interfaces for multiple ALTO server integration and collaboration. The specifications and extensions should use existing services whenever possible. The specifications and extensions should consider realistic complexities including incremental deployment, dynamicity, and security issues such as access control, authorization (e.g., an ALTO server provides information for a network that the server has no authorization), and privacy protection in multi-domain settings.


o The working group will update RFC 7971 to provide operational considerations for recent protocol extensions (e.g., cost calendar, unified properties, and path vector) and new extensions that the WG develops. New considerations will include decisions about the set of information resources (e.g., what metrics to use), notification of changes either in proactive or reactive mode (e.g., pull the backend, or trigger just-in-time measurements), aggregation/processing of the collected information  (e.g., compute information and network information )according to the clients’ requests, and integration with new transport mechanisms (e.g., HTTP/2 and HTTP/3).


When the WG considers standardizing information that the ALTO server could provide, the following criteria are important

to ensure real feasibility:


- Can the ALTO server realistically provide (measure or derive) that information?


- Is it information that the ALTO client cannot find easily some other way?


- Is the distribution of the information allowed by the operator of the network? Does the exposure of the information introduce privacy and information leakage concerns?


Issues related to the specific content exchanged in systems that make use of ALTO are excluded from the WG's scope, as is the issue of dealing with enforcing the legality of the content. The WG will also not propose standards on how congestion is signaled, remediated, or avoided.


-Qin Wu (on behalf of chairs)