AS inventory
Serbia has 162 autonomous systems visible in the global routing table. Of those, 161 (99 %) currently announce at least one IPv4 prefix to the global routing table, and 31 (19 %) currently announce at least one IPv6 prefix. That means 19 % of the IPv4-active ASes have followed through and turned on IPv6.
Operators - PA (Provider Aggregatable) holders
PA-holding ASes are LIRs with their own RIR allocation - they are the operators running parts of Serbia’s internet (transits, ISPs, content networks with infrastructure of their own). Serbia has 108 PA-holding operator ASes registered with the RIR. Of those, 108 (100 %) announce IPv4 today and 23 (21 %) announce IPv6.
End-sites - PI (Provider Independent) holders
PI-holding ASes hold address space directly from the RIR for their own use - typically enterprises, content sites running their own AS for routing diversity, government / academic single-site deployments. They’re not “operators” in the LIR sense, but they show up in BGP and their v6 readiness still matters. Serbia has 48 PI-holding end-site ASes. Of those, 47 (98 %) announce IPv4 and 2 (4 %) announce IPv6.
Country totals: 162 routed ASes / 161 announce IPv4 / 31 announce IPv6. 6 ASes are unclassified (RIPE inet*num lookup didn’t return a clear status). PA / PI counts above include the “mixed” bucket on the PA side since a mixed AS holds at least one PA prefix and operationally functions as an LIR.
Executive summary
We measure how 31 Serbian autonomous systems that announce 74 IPv6 prefixes treat ICMPv6 in practice - Echo, Type 2 (Packet Too Big), Time-Exceeded - using eight ICMPv6 vantage points and a 12-node global yarrp mesh, plus the SOX route-server BGP table as ground truth. Generated 2026-09-23 11:15 UTC.
Adoption
According to RIPEstat, 31 ASes registered to Serbia announce at least one IPv6 prefix to the global routing table, advertising 74 distinct prefixes between them. The 12-vantage-point mesh observed 60 distinct ASes (Serbian + foreign transits) and 84 AS-AS adjacencies along paths into Serbia.
ICMPv6 behaviour at the destination
Each AS gets a single classification per vantage point by combining the small Echo, the an unfragmented 1500-byte Echo, traceroute reachability, and a TCP liveness check. Aggregated across all 8 vantage points, the SRB population looks like this:
- open - 17 ASes (55%)
- echo only - 1 AS (3%)
The open bucket means every vantage point received Echo Replies for both small and 1500-byte probes; this is what every SRB network should look like. echo_only means small ping passed but 1500-byte packets did not - usually either a path-MTU constraint downstream of us, or a real ICMPv6 Type 2 (Packet Too Big) filter en route. The two unreachable buckets cover ASes whose AS edge filters all incoming ICMPv6.
Weekly-probe schedule: 13 of SRB’s 31 IPv6-announcing ASes are on a weekly probe cycle today because they’ve produced a stably-unreachable / no-host result for 14+ consecutive daily runs. Today’s measurements therefore cover the 18 ASes probed this run; the skipped 13 are re-tested weekly. (Tuneable in scripts/19_update_skip_list.py -- raise STREAK_TO_SKIP to probe more aggressively, lower WEEKLY_PROBE_DAYS to re-test sooner.)
RFC 4890 compliance (active Type 2 test)
RFC 4890 gives operators the option to drop ICMPv6 Echo at the firewall, but says Type 2 (Packet Too Big) must remain permitted end-to-end - otherwise PMTUD is broken and large flows hang silently. We test this directly: for every reachable AS we open an active flow (TCP/53 DNS, TCP/443 TLS, ICMPv6 Echo, or HTTP/80 - whichever applies), forge an ICMPv6 Type 2 with MTU=1280, and observe whether the destination acts on it.
- RFC 4890 ✓ - 14 ASes proven compliant: AS8400, AS13004, AS15958, AS31042, AS35779, AS41897
- RFC 4890 ✗ - 3 ASes proven non-compliant (PMTUD will break for users): AS6700, AS44143, AS208398
- The remaining 1 are unknown - typically because their network blocks Echo entirely (we cannot directly probe Type 2 acceptance from outside) or because the active test was inconclusive on this run.
Active-test method breakdown across all targets: honored = 165, not_honored = 121, no_echo = 115, inconclusive = 7.
SOX peering
The Serbian Open eXchange (SOX) currently has 43 peering members reachable via its route servers. Of those, 11 are Serbian-registered ASes; the rest are international peers that serve Serbian traffic via SOX (common examples: Hurricane Electric, Cloudflare, PCH).
Top SOX-internal transit relationships (BGP-truth)
For the full members list with live route-server session state, see the dedicated SOX page.
Path-MTU and Type 2 evidence
The active Type 2 test produced definitive verdicts for 23 unique destination IP addresses across vantage points.
- Honored (destination accepted our forged Packet-Too-Big and shrank its next response): 16 ASes - AS8400, AS13004, AS13092, AS15958, AS31042, AS35779, AS41897, AS43191 (+8 more)
- Not honored (destination ignored the PTB or the PTB was filtered en route): 7 ASes - AS6700, AS13004, AS15958, AS44143, AS47267, AS60733, AS208398
Global BGP visibility (RIPE RIS)
For every SRB prefix we asked all RIPE RIS collectors what AS-paths they currently see. That gives us each AS' direct upstream(s) according to the global BGP table.
RPKI & ASPA
RPKI ROAs cryptographically bind a prefix to its origin AS, so origin hijacks
of that prefix can be filtered. (Route leaks, where the origin is legitimate but the
AS path is not, are what ASPA below addresses.) We validate every announced SRB IPv6 prefix
against the local Routinator (rpki-lju.6connect.com).
Of 31 SRB ASes announcing IPv6:
- 27 (87%) have at least one valid ROA
- 26 (84%) have all their IPv6 prefixes covered by valid ROAs
- 4 (13%) have not signed any of their prefixes
- 0 have at least one invalid prefix (origin or maxLength mismatch)
Per prefix: 69 valid, 0 invalid, 5 not-found (of 74 total).
Top unsigned ASes (no ROAs)
- AS35779 MCLOUD-AS mCloud doo (1 prefix)
- AS49167 COMUTEL-AS COMUTEL d.o.o. (1 prefix)
- AS57722 ATI-KOS-AS N.SH.T "ATI-KOS" sh.p.k (1 prefix)
- AS201236 NETCAST-OBLAK-DOO NETCAST OBLAK d.o.o. Beograd (1 prefix)
ASPA (Autonomous System Provider Authorization)
ASPA is a much newer RPKI object (still an IETF SIDROPS draft, not yet a ratified RFC): it lets a customer AS list which upstream providers may legitimately propagate routes from it. Routes from that AS arriving via any other path are then ASPA-invalid.
1 of 31 SRB ASes have published an ASPA record:
- AS208398 TELETECH Edge Technology Plus d.o.o. Beograd - provider(s): AS174, AS1299, AS3491, AS6762
Path asymmetry
For every SRB AS we compared where our outgoing traffic enters their network (the AS just upstream of them on traceroute paths from any of our 8 ICMPv6 vantage points or the 12-node yarrp mesh, 20 sources in total) to which AS the global BGP table says is their direct upstream (RIS view). When these disagree, the network is usually multi-homed and traffic in and out takes different paths - normal in well-peered networks but worth surfacing.
14 SRB ASes flagged as asymmetric this run. Most asymmetric examples (low symmetry score):
- AS207604 UNITED United Internet Ltd. - symmetry score 0.00; RIS sees upstreams [AS12459] but our paths went via [AS174, AS9002, AS13004].
- AS208398 TELETECH Edge Technology Plus d.o.o. Beograd - symmetry score 0.07; RIS sees upstreams [AS1299, AS3491, AS6762, AS6939] but our paths went via [].
- AS31042 SERBIA-BROADBAND-AS Serbia BroadBand-Srpske Kablovske mreze d.o.o. - symmetry score 0.12; RIS sees upstreams [AS6057, AS6939, AS7195, AS8218] but our paths went via [AS1299, AS55211].
- AS21215 UGNI UNITED GROUP NETWORK INFRASTRUCTURE d.o.o. Beograd - symmetry score 0.12; RIS sees upstreams [AS174, AS6939, AS8218, AS9002] but our paths went via [AS51988].
- AS47267 RNIDS-AS Serbian National Internet Domain Registry Foundation - symmetry score 0.20; RIS sees upstreams [AS2107, AS8400, AS13004, AS15958] but our paths went via [].
- AS44143 A1SERBIA-AS A1 Srbija d.o.o. Beograd - symmetry score 0.22; RIS sees upstreams [AS13004] but our paths went via [AS5603, AS6939, AS24961, AS50629].
- AS43191 providus-as Providus d.o.o. - symmetry score 0.25; RIS sees upstreams [AS1299, AS8400, AS9125] but our paths went via [].
- AS15958 CETIN_doo_AS CETIN Ltd. Belgrade - symmetry score 0.26; RIS sees upstreams [AS6777, AS8218, AS12779, AS13004] but our paths went via [AS24961, AS41327, AS60733, AS203993].
Bottom line
- 55% of SRB IPv6 networks accept ICMPv6 Echo unrestricted from external probes.
- 0% of SRB IPv6 networks drop all ICMPv6 at their AS edge - a meaningful fraction of the SRB population that some traffic types may struggle to debug against.
- Of the 17 ASes where our active Type 2 test reached a verdict, 14 (82%) proved RFC 4890-compliant. The rest break PMTUD for any flow that needs a smaller path-MTU than 1500.
- The SOX route servers carry the BGP-truth backbone of Serbian IPv6: transit relationships visible on rs1 and rs2 today, with the country's largest ISPs dominating the inbound transit graph and a long tail of small ISPs and one-AS enterprises forming the customer base.
This report is regenerated daily at 12:00 Europe/Belgrade from
data/analyzed.json, data/six_lg.json, data/global_topology.json,
data/ris_bgp.json, data/asymmetry.json and data/type2_results_<vantage>.json.
The text is deterministic - no manual editorial pass.