Re: [Softwires] [Int-area] Is it feasible to perform fragmentation on UDP encapsulated packets.

[Xuxiaohu <[e-mail]>]

Hi Tom,

> -----Original Message-----
> From: Tom Herbert [mailto:[e-mail]]
> Sent: Friday, May 27, 2016 11:06 PM
> To: Xuxiaohu
> Cc: [e-mail]; Softwires WG; [e-mail]; [e-mail];
> [e-mail]
> Subject: Re: [Int-area] Is it feasible to perform fragmentation on UDP
> encapsulated packets.
> 
> > The possible side-effect of performing fragmentation on UDP encapsulated
> packets is to worsen the reassembly burden on tunnel egress since fragments of
> UDP encapsulated packets are more likely to be forwarded across different
> paths towards the tunnel egress than those of IP or GRE encapsulated packets.
> >
> 
> Xiaohu,
> 
> I don't understand why UDP encapsulation would make things worse than other
> encapsulations. Fragmentation would be needed at tunnel ingress when packet

When performing fragmentation after UDP encapsulation (a.k.a., outer fragmentation), the first fragment (still a UDP packet) and the subsequent fragments (not a UDP packet anymore) of a given UDP encapsulated packet are more likely to be routed over different paths in contrast to IP or GRE encapsulated packets. As a result, the reordering issue on the tunnel egress which is required to do reassembly would become even worse.

> size exceeds MTU of the tunnel. RFC4459 describes the issues and general
> solutions that apply to the different techniques of IP tunneling.

Partially agree. The use of UDP as a tunneling protocol was not popular at the time when RFC4459 was published. Otherwise, the worse recording issue associated with the outer fragmentation of UDP encapsulated packets may need more attention in that RFC.

By the way, RFC4459 has illustrated the risk of performing [outer] fragmentation on tunneled packets clearly and said clearly that "...   So, if reassembly could be made to work sufficiently reliably, this would be one acceptable fallback solution but only for IPv6." However, I just occasionally noticed that in section 3.3.2.1 (Tunneling GRE over IPv4) of RFC7588, it said "By default, the GRE ingress node does not fragment delivery packets. However, the GRE ingress node includes a configuration option that allows delivery packet fragmentation." Does it mean there is a conflict between those two RFCs regarding whether outer fragmentation is needed for GRE over IPv4?

> > It seems that most X-over-UDP proposals choose to prohibit the tunnel ingress
> from performing fragmentation on UDP encapsulated packets. See the following
> quoted text regarding fragmentation from those X-over-UDP drafts:
> >
> Please look a draft-herbert-gue-fragmentation-02. This is a method where the
> fragmentation/reassembly is performed in the encapsulation layer instead of
> (outer or inner) IP layer.

It's an interesting idea. However, since it needs a dedicated fragmentation field in the GUE header, it seems that this approach is not suitable for the compressed GUE packet (a.k.a., in the same format of IP-in-UDP) where the fragmentation field and even the whole GUE header is gone.

Best regards,
Xiaohu

> Tom